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Electroless deposition of NiP alloys...1. CHAPTER 1 1.1 From electroplating to electroless...

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POLITECNICO DI MILANO School of Industrial and Information engineering Master of Science in Materials Engineering and Nanotechnology Electroless deposition of NiP alloys Author: Supervisor: Filippo Mariani Prof. Ing. Luca Magagnin ID: 837140 Advisor: Simona IEFFA Academic year 2016/2017
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Page 1: Electroless deposition of NiP alloys...1. CHAPTER 1 1.1 From electroplating to electroless deposition Surface engineering is a very large and important sub-discipline of material science

POLITECNICO DI MILANO

School of Industrial and Information engineering

Master of Science in

Materials Engineering and Nanotechnology

Electroless deposition of NiP alloys

Author: Supervisor:

Filippo Mariani Prof. Ing. Luca Magagnin

ID: 837140 Advisor:

Simona IEFFA

Academic year 2016/2017

Page 2: Electroless deposition of NiP alloys...1. CHAPTER 1 1.1 From electroplating to electroless deposition Surface engineering is a very large and important sub-discipline of material science

Contents

Abstract……………………………………………………………………………………………...4

Abstract (Italiano)…………………………………………………………………………………..5

1. Chapter 1………………………………………………………………………………………….6

1.1 From electroplating to electroless deposition…………………………………………6

1.2 Differences between the two techniques……………………………………………….7

1.3 Typology of Nickel baths………………………………………………………………..8

1.4 Electroless Nickel baths……………………………………………………………...….8

1.4.1 Acidic baths…………………………………………………………………....9

1.4.2 Alkaline baths………………………………………………………………...10

1.5 Components of the bath……………………………………………………………….11

1.5.1 Reducing agents……………………………………………………………...13

1.5.1.1 Sodium hypophosphite…………………………………………….13

1.5.1.2 Aminoborane baths………………………………………………...14

1.5.1.3 Sodium borohydride baths………………………………………...14

1.5.2 Complexing agents…………………………………………………………...14

1.6 Deposit characteristics…………………………………………………………………15

1.7 Applications…………………………………………………………………………….18

1.8 Electroless codeposition of particles in NiP matrix…………………………………..19

1.8.1 Carbides………………………………………………………………………20

1.8.1.1 Silicon carbide……………………………………………………...20

1.8.1.2 Boron carbide………………………………………………………22

1.8.1.3 Tungsten carbide…………………………………………………...23

1.8.2 Ceramics……………………………………………………………………...24

1.8.2.1 Titanium oxide……………………………………………………...24

1.8.2.2 Zirconia……………………………………………………………..28

1.8.2.3 Aluminum oxide……………………………………………………28

1.8.2.4 Silica…………………………………………………………………30

1.8.2.5 Ceria………………………………………………………………...32

1.8.3 Lubricants…………………………………………………………………….33

1.8.3.1 PTFE………………………………………………………………...33

1.8.3.2 Molibdenum sulfide………………………………………………...35

1.8.3.3 Tungsten sulfide…………………………………………………….36

1

Page 3: Electroless deposition of NiP alloys...1. CHAPTER 1 1.1 From electroplating to electroless deposition Surface engineering is a very large and important sub-discipline of material science

1.8.4 Allotropics form of carbon…………………………………………………..37

1.8.4.1 Graphite…………………………………………………………….37

1.8.4.2 Carbon nanotubes (CNTs)…………………………………………37

1.8.4.3 Diamonds…………………………………………………..………..39

1.8.5 Rare Earth elements………………………………………………..………..41

2. Chapter 2………………………………………………………………………………………...42

2.1 Characterization techniques…………………………………………………………..42

2.1.1 Mechanical Polishing for Optical Microscopy (OM)………………………42

2.1.2 Optical Microscope……………………………………………..……………42

2.1.3 Scanning electron microscope (SEM) and EDX………………...………….43

2.1.4 XRF………………………………………………………………..………….47

2.1.5 XRD…………………………………………………………………….……..48

2.1.6 Microdurometer………………………………………………...……………49

3. Chapter 3………………………………………………………………………………………...50

3.1 Procedures……………………………………………………………………………...50

3.1.1 Brass pretreatment…………………………………………………………..50

3.1.2 Preparation of CTAB and DTAB solutions………………………………...50

3.1.3 Preparation of the bath for CTAB and DTAB tests………………………..50

3.1.4 Addition of particles………………………………………………………….51

3.2 Results…………………………………………………………………………..………51

3.2.1 SEM-EDX, XRD and Optical Microscope analysis………………..………52

3.2.1.1 NiP first formulation (NiP0)……………………………..…….…..52

3.2.1.2 NiP0(CTAB)……………………………………………….………..53

3.2.1.3 NiP0(DTAB)………………………………………………….……..55

3.2.1.4 NiP0/W………………………………………………………………56

3.2.1.5 NiP0/W(CTAB)……………………………………….…………….58

3.2.1.6 NiP0-SiC………………………………………………………...…..59

3.2.1.7 NiP0-SiC/B4C………………………………………………….……61

3.2.1.8 NiP second formulation (NiPA)………………………………..…..63

3.2.1.9 NiPA/W (4g/L)………………………………………………...…….65

3.2.1.10 NiPA/W (6 g/L)…………………………………………………….67

3.2.1.11 NiPA/W(18 g/L)…………………………………………..………..69

3.2.1.12 NiP third formulation (NiPB)…………………………….………71

3.2.1.13 NiPB(CTAB) ………………………………………………………73

2

Page 4: Electroless deposition of NiP alloys...1. CHAPTER 1 1.1 From electroplating to electroless deposition Surface engineering is a very large and important sub-discipline of material science

3.2.1.14 NiPB(DTAB)…………………………………………………….....75

3.2.1.15 NiPB/W……………………………………………………...……..77

3.2.1.16 NiPB/W(CTAB)………………………………………..…………..79

3.2.1.17 NiPB/W(DTAB)……………………………………………………81

3.2.1.18 NiPB/W(CTAB)-SiC………………………………………………83

3.2.1.19 NiPB/W(DTAB)-SiC………………………………………………85

3.2.1.20 NiPB/W(CTAB)-TiO2......................................................................87

3.2.1.21 NiPB/W(DTAB)-TiO2…………………………………………..…89

3.2.1.22 NiPB/W(CTAB)-SiC/B4C………………………………………....91

3.2.1.23 NiPB/W(DTAB)-SiC/B4C…………………………………………93

3.2.2 Microhardness………………………………………………………………..95

4. Chapter 4……………………………………………………………………………………...…97

4.1 Conclusions and future developments………………………………………………..97

Index of figures………………………………………………………………………………...…..98

Index of tables…………………………………………………………………………………….103

References…………………………………………………………………………...…………….104

Acknowledgments……………………………………………………………………………..….110

3

Page 5: Electroless deposition of NiP alloys...1. CHAPTER 1 1.1 From electroplating to electroless deposition Surface engineering is a very large and important sub-discipline of material science

Abstract

This thesis presents the electroless deposition of Nickel-Phosphorus (NiP) with tecnoplate3000®

solutions on brass.

The aim is improving and enhancing the mechanical properties of some mechanical parts.

After the pretreatment of the substrate, we tried three formulations to obtain the best performance:

NiP0, NiPA, NiPB; they vary mainly for the different concentration of additives. The selected one is

NiPB a stable, monophase solution.

At the end we added in this solution particles of carbides and ceramics to produce a hard and with

high wear resistance coating.

A characterization of the samples obtained was performed by optical microscope analysis, scanning

electron microscope analysis (SEM), X-ray diffraction technique (XRD) and microhardness test.

4

Page 6: Electroless deposition of NiP alloys...1. CHAPTER 1 1.1 From electroplating to electroless deposition Surface engineering is a very large and important sub-discipline of material science

Abstract (Italiano)

In questa tesi viene presentata la deposizione electroless di Nichel-Fosforo (NiP) su ottone avvenuta

grazie alla soluzione tecnoplate3000®.

L’obiettivo è quello di aumentare le proprietà meccaniche e quindi le prestazioni di alcuni

componenti utilizzati in meccanica.

Dopo il pretrattamento del substrato, abbiamo provato tre diverse formulazioni per ottenere le

migliori perfromance: NiP0, NiPA, NiPB; esse variano soprattuto per la concentrazione degli additivi.

La soluzione migliore selezionata è NiPB che risulta stabile e monofasica. Alla fine in questa

soluzione abbiamo aggiunto particelle di carburi e ceramici per produrre un rivestimento che fosse

duro e resistente all’usura.

Sono state infine effettuate prove di caratterizzazione sui provini ottenuti attraverso analisi al

microscopio elettronico (SEM), diffrazione ai raggi X (XRD) e test di microdurezza.

5

Page 7: Electroless deposition of NiP alloys...1. CHAPTER 1 1.1 From electroplating to electroless deposition Surface engineering is a very large and important sub-discipline of material science

1. CHAPTER 1

1.1 From electroplating to electroless deposition

Surface engineering is a very large and important sub-discipline of material science and deals with

the surface of solid matter. In particular, it tries to alter and improve the properties of the surface

phase in order to reduce the degradation over time. This aspect is reached thanks to some useful

techniques such plating and emerging nanotechnologies.

Nowadays one of the most used process to ensure a long life to a material surface is

electrodeposition.

This technique needs the use of an external current (direct, alternate or pulsed), applied on an

electrolytic cell, to reduce some metal cations to form a coherent thin coating on an electrode.

The electrolytic cell is formed by a soluble or insoluble anodes, a cathode which acts like a

substrate all immersed in an electrolytic solution where metal salts, that we want to deposit, are

dissolved.

The technique was developed in the XIX century, although undergoing subtle changes through the

time, to improve environment sustainability replacing toxic chemical compounds.

The most important researches were about the efficiency and the way to ensure the metallization of

other substrates, for example polymers. In 1844 Wurtz firstly reported an autocatalytic substrate

surface[1]

. This discovery led to the most important innovation in electrochemistry field: electroless

deposition[2]

. This new technique didn’t need an external current to deposit metal ions on a

substrate[3]

. In an electroless deposition system there is no generator or anode and so the depositions

are made only under the controlled chemical reduction reactions.

6

Page 8: Electroless deposition of NiP alloys...1. CHAPTER 1 1.1 From electroplating to electroless deposition Surface engineering is a very large and important sub-discipline of material science

1.2 Differences between the two techniques

There are some differences between the two processes: as we have just seen the main difference is

the lack of the external power in electroless deposition; in electroplating it isn’t necessary to use

reducing agents or surfactants; electroless can deposit also onto non metallic substrates, wherever

the plates touch the solution; by the proper choice of the solution composition, pH, and the

operating temperatures, the rate of deposition can be seen to be as high as 20 to 25 mm/h, which is

sufficiently fast for industrial applications, finally electroless is more expensive.

Table1. 1 Differences between electrodeposition and electroless deposition

[5]

Electroplating Electroless deposition

Need external power No external power

Reducing agents not necessary Need reducing agents

Only conductive substrates Also non conductive substrates

Medium costs High costs

Other more important peculiarities of the two processes: electroless deposition has more uniform,

less porous and better abrasion and wear resistance deposit, but it is more brittle, the temperature is

higher and the life of the bath is shorter than electrodeposition[4]

. Below a table which summarizes

advantages and disadvantages of this technique.

Table1. 2: Advantages and disadvantages of electroless deposition

[5]

Advantages Disadvantages

More uniform deposit More brittle deposit

Less porous Shorter bath life

Better abrasion and wear resistance Higher bath temperature

Plates where the part is wetted More chemical control

Our main porpurs is obteining a better abrawion and wear resitence, thus we select to procede with

the electroless deposition. In particular we focus on the NiP electroless deposition baths, being

higkly stabile, easy to reproduce and ablee to make amorphous and hard coatings.

7

Page 9: Electroless deposition of NiP alloys...1. CHAPTER 1 1.1 From electroplating to electroless deposition Surface engineering is a very large and important sub-discipline of material science

1.3 Typology of electroless baths

Most applications of the electroless coating are based on their wear and corrosion resistance.

However, characteristic like luminescence has a great potential in defense and aerospace

applications[3]

.

Electroless coatings can be divided, essentially, into three main categories:

Alloy coatings

Composite coatings

Metallic coatings

In particular NiP with or without particles is considered alloy or composite coating.

1.4 Electroless Nickel Baths

Electroless nickel plating is carried out by the immersion of objects, with a surface wetted and

activated by a catalyst, in a solution containing nickel ions and a suitable reducing agents, which

may include hypophosphite, borohydride, aminoboranes, hydrazine, etc. at temperatures above

90 °C. Further, some organic complexing agents for nickel ions, buffers, stabilizers, accelerators,

etc. are also present.

The compositions of chemical nickel-plating solutions used by Brenner and Riddell[2]

have certain

advantages over other formulations and, hence, are most popular. The compositions are more stable

since there is no loss of the complexants by evaporation. The properties of the electroless nickel-

phosphorous alloy can be regulated easily by controlling the amount of phosphorous in the deposit.

Hence, the acid solutions are generally preferred in many applications. Reactions occurring in

electroless nickel deposition with hypophosphite ion as the reducing agent may be represented as[4]

:

Ni2+

+ H2PO2- + H2O Ni + 2H

+ + H(HPO3)

-

HPO2- + H2O H(HPO3)

- + ½H2

Mainly two types of baths have been used for depositing alloys, These also include acidic and

alkaline baths[3]

.

8

Catalytic active

Catalytic active

surface

surface

Page 10: Electroless deposition of NiP alloys...1. CHAPTER 1 1.1 From electroplating to electroless deposition Surface engineering is a very large and important sub-discipline of material science

1.4.1 Acidic Baths

Firstly acidic baths, that represents almost the whole of the process used nowadays, are described.

Once the substrate is immersed into the bath, the reaction proceeds forward due to the following

factors: reduction in nickel ion concentration, conversion of the hypophosphite to phosphate, then

the consequence is the increase in hydrogen ion concentration, and adsorption of this gas by the

deposit.

Aminoboranes are also used as reductants instead of hypophosphite in electroless nickel deposition

from acid solutions[4]

.

The coatings obtained from this formulation have a better quality.

Table1. 3 Example of acid bath (all the values represent the concentration of the compounds)

[3]

Component Parameter 1 Parameter 2 Parameter 3 Parameter 4

Nickel chloride 30 g/L 30 g/L 30 g/L -

Nickel sulphate - - - 30 g/L

Sodium

hypophosphite

10 g/L 10 g/L 10 g/L 10 g/L

Sodium glycolate 50 g/L 10 g/L - -

Sodium acetate - - - 10 g/L

Sodium citrate - - 10 g/L -

General

appearance of the

coating

Semi-bright Semi-bright Semi-bright Coarse uneven

9

Figure1. 1 Some mechanical parts in an electroless bath of NiP[6]

Page 11: Electroless deposition of NiP alloys...1. CHAPTER 1 1.1 From electroplating to electroless deposition Surface engineering is a very large and important sub-discipline of material science

1.4.2 Alkaline baths

The second and less used type of electroless nickel baths consists in solutions with alkaline features.

The reduction of nickel in these solutions follows the same pattern as in acid ones unfortunately

they have some problems.

The main disadvantage of the alkaline solutions are their high instability at higher temperature, in

particular greater than 90°C, due to the loss of ammonia, which is useful to raise the bath pH at that

temperature. Another difference from the previous solutions is that the nickel’s rate of deposition

increases with hypophosphite concentrations; unfortunately very high concentrations of this element

make the bath unstable due to homogeneous deposition in the bulk. Also temperature influences the

rate of deposition in the same way as in acid solutions. The control of pH is very difficult due to

higher temperature than 90°C. Borohydrides are used as reducing agents in alkaline electroless

nickel-plating baths. They operate at low temperatures, i.e., from 20 °C to 97 °C, the rate of

deposition being lower at low temperature. In table 4-5 examples of alkaline are shown[7-22]

.

Table1. 4 First example of alkaline bath (all the values represent the concentration of the compounds)

[4]

Component Parameter 1 Parameter 2

Nickel chloride NiCl2 * 6H2O 20 g/L 24 g/L

Ethylene diamine (98 pct) 45 g/L -

Sodium hydroxide NaOH 40 g/L 120 g/L

Sodium borohydride NaBH4 0.67 g/L 0.4 g/L

pH 11 to 12 11 to 12

Temperature 97 °C 60 °C

Rate of deposition 7 to 8 to 9.2 mgm/cm2/h 0 to 12 mgm/cm

2/h

Table1. 5 Another example of alkaline bath (all the values represent the concentration of the compounds)

[3]

Components Parameter 1 Parameter 2 Parameter 3

Nickel chloride 30 g/L 30 g/L 30 g/L

Sodium hypophosphite 10 g/L 10 g/L 10 g/L

Ammonium chloride 50 g/L 100 g/L -

Sodium citrate 100 g/L - 100 g/L

Appearance of the

coating

Medium dark Bright Bright

10

Page 12: Electroless deposition of NiP alloys...1. CHAPTER 1 1.1 From electroplating to electroless deposition Surface engineering is a very large and important sub-discipline of material science

1.5 Components of Ni bath

Firstly the most important component is represented by the metal ions which constitutes the source

of metal that we want to deposit. The reducing agents are another fundamental element because

they reduce metal ions and allow them to deposit on the substrate. Complexant agents are used in

order to prevent excess of free Nickel ions concentration, so they stabilize the Nickel phosphate

precipitation and act also like pH buffers. Exultants, known as accelerators, are useful to increase

the deposition rate and are the exact opposite of complexants and stabilizers. These last prevent the

breakdown of the solution by shielding catalytically eventual active nuclei.

Buffers are used for long-term pH control instead pH regulators are used for subsequent pH

adjustment. Finally wetting agents increase the wettability of the surfaces that has to be coated.

It follows a list where are described the components, seen before, and some examples of them.

Metal ions: nickelchloride, nickel sulfate, nickel acetates

Reducing agents: sodium hypophosphite, amineborane, sodium borohydride, and hydrazine

Complexants: monocarboxylic acids, dicarboxlyic acids, hydroxycarboxylic acids,

ammonia, alkanolamines, etc.

Accelerators or exultants: anions of some mono- and di-carboxlyic acids, fluorides, borates

Stabilizers: lead, tin, arsenic, molibdenum, cadmium or thorium ions, thioures, etc.

Buffers: Sodium salt of certain complexants, choice depends on pH range used

pH regulators: sulfuric and hydrochloric acids, soda, caustic soda, ammonia

Wetting agents: ionic and nonionic surfactants

11

Page 13: Electroless deposition of NiP alloys...1. CHAPTER 1 1.1 From electroplating to electroless deposition Surface engineering is a very large and important sub-discipline of material science

Table1. 6 General components of a standard Ni bath[3]

Component Function Example

Metal ions Source of metal Nickel Chloride, Nickel Sulfate,

Nickel acetates

Hypophosphite ions Reducing agent Sodium hypophosphite

Complexants From Ni complexes, prevent

excess free Ni ion

concentration so stabilizing and

preventing

Ni phosphate precipitation; also

act as pH buffers

Monocarboxylic acids,

Dicarboxlyic acids,

Hydroxycarboxylic acids,

Ammonia,

alkanolamines, etc.

Accelerators (exultants) Active reducing agent and

accelerate deposition;

mode of action opposes

stabilizers and complexants

Anions of some mono- and di-

carboxlyic acids,

fluorides, borates

Stabilizers (inhibitors) Prevent solution breakdown by

shielding catalytically

active nuclei

Pb, Sn, As, Mo, Cd, or Th ions,

thioures, etc.

Buffers For long-term pH control Sodium salt of certain

complexants, choice depends

on pH range used

PH regulators For subsequent pH adjustment Sulfuric and hydrochloric acids,

Soda, caustic soda,

ammonia

Wetting agents Increase wettability of surfaces

to be coated

Ionic and nonionic surfactants

12

Page 14: Electroless deposition of NiP alloys...1. CHAPTER 1 1.1 From electroplating to electroless deposition Surface engineering is a very large and important sub-discipline of material science

1.5.1 Reducing agents

Several reducing agents have been used in electroless coating of alloys. Four types of reducing

agent have been used for electroless nickel bath including sodium hypophosphite, aminoboranes,

sodium borohydride, and hydrazine.

1.5.1.1 Sodium hypophosphite

More than 70% of electroless nickel is deposited from solutions reduced by this compound. The

main advantage of these solutions over those reduced by borohydride or hydrazine includes lower

costs, greater ease of process control. A lot of researches have been made to explain the mechanisms

involving the chemical reactions that occur in hypophosphite reduced electroless nickel plating

solutions. Most widely accepted theories are illustrated by the following equations[23,24]

.

1) Electrochemical mechanism, where catalytic oxidation of the hypophosphite yields electrons at

the catalytic surface which in turn reduces nickel and hydrogen ions is illustrated below:

H2PO2- + H2O → H2PO3

- + 2H

+ + 2e

- (1)

Ni2+

+ 2e- → Ni, (2)

2H+ + 2e

- → H2 (3)

H2PO2- + 2H

+ → P + 2H2O (4)

2) Atomic hydrogen mechanism, where atomic hydrogen is released as the result of the

catalytic dehydrogenation of hypophosphite molecule adsorbed at the surface is illustrated

below:

H2PO2- + H2O → HPO3

2- + H

+2Hads (5)

2Hads + Ni2+

→ Ni + 2H+

(6)

H2PO2 + Hads → H2O + OH- + P (7)

The adsorbed active hydrogen, (6) then reduces nickel at the surface of the catalyst.

(H2PO2)2-

+ H2O → H+ + (HPO3)

2- + H2 (8)

Simultaneously, some of the absorbed hydrogen reduces a small amount of the hypophosphite

at the catalytic surface to water, hydroxyl ion and phosphorus (7). Most of the hypophosphite

present is catalytic, which is oxidized to orthophosphite and gaseous hydrogen, (8), causing

low efficiency of electroless nickel solutions for alloy coating while the deposition of nickel

and phosphorus continues. For example, usually 5 kg of sodium hypophosphite are required to

reduce 1 kg of nickel, for an average efficiency of 37%[25,26]

. The amount of hypophosphite varies a

little with the nature of buffer additive; the highest one was observed in the solution containing

sodium acetate, the lowest is the one with sodium citrate. The main characteristic of the process is

the change in the composition of the solution. The physical process that occurs is that the

concentration of nickel salt and hypophosphite decreases and the concentration of acid increases

during the progress of deposition. This is the main factor that causes the lowering of deposition

rate[25]

.

13

Page 15: Electroless deposition of NiP alloys...1. CHAPTER 1 1.1 From electroplating to electroless deposition Surface engineering is a very large and important sub-discipline of material science

1.5.1.2 Aminoborane baths

Use of aminoboranes in electroless Ni plating solutions have been limited to two compounds:

N-dimethylamine borane (DMAB)–(CH3)2NHBH3, and H-diethylamine borane

(DEAB)–(C2H5)2NHBH3[26-28]

. DEAB is used primarily in European establishments, whereas

DMAB is used generally in USA. DMAB is readily soluble in aqueous solutions while DEAB

should be mixed with aliphatic alcohol such as ethanol, before being added to plating bath.

The aminoboranes are effective reducing agents over a wide range of pH but due to evolution

of hydrogen, although a lower limit of pH for the plating process exists[27]

. Nickel in the deposit

increases following the increasing of the bath’s pH. Usually, the aminoborane baths have a pH

range from 6 to 9. Operating temperatures for these baths range from 50 to 80°C, however, they can

be used at temperature as low as 30°C. Accordingly, aminoborane baths are very useful for plating

non-catalytic surfaces such as plastics, nonmetals, which are their primary applications. The rate of

depositions varies with pH and temperature, but it usually varies from 7 to 12 µm/h.

1.5.1.3 Sodium borohydride baths

The borohydride ion is the most powerful reducing agent available for electroless nickel plating.

Any water-soluble borohydride can be used; however, for optimum results sodium borohydride is

preferred[25]

. In acid or neutral solutions, hydrolysis of borohydride ions is very rapid. In the

presence of nickel ions nickel boride may form spontaneously. If the pH of the solutions is

maintained between 12 and 14 the formation of nickel boride is suppressed and the reaction product

is principally elemental nickel.

1.5.2 Complexing agents

One of the difficulties of reduction reactions or chemical plating is the maintenance of the bath

composition. As the plating proceeds, continuous lowering of the rate of reduction of nickel occurs.

The solutions cannot be replenished due to the formation of nickel phosphite. If nickel phosphite is

diminished in the bath, the surface quality of coating deteriorates resulting in rough and dark

coatings. Moreover, the nickel concentration in the solution also decreases and the bath goes to the

verge of total decomposition. Sodium citrate reduces the formation of nickel phosphite and reduces

the rate of deposition[29,30]

. The ability to form nickel complexes has been attributed to some of the

proposed additives like salts of glycolic, succinic or malonic acids, however, these fail to stop the

precipitation of nickel phosphite.

The best results are obtained when the sodium citrate concentration is about 30 g/l. It helps

in checking the coating from becoming porous and dull. Because of the reduction in rate

of deposition, accelerators like salts of carbonic acids, soluble fluorides and inhibitors like

thiourea can also be added to avoid the total decomposition of the bath.

Bi, Pb, Cd and Te additions act as bath stabilizers[31]

. Bismuth and Tellurium seem to be less

effective than Pb and Cd in bath stabilization. These stabilizers are added in concentrations of only

a few parts per million.

14

Page 16: Electroless deposition of NiP alloys...1. CHAPTER 1 1.1 From electroplating to electroless deposition Surface engineering is a very large and important sub-discipline of material science

1.6 Deposit characteristic

Nickel is one of the most used metals in the electroless deposition; usually its properties are

increased through adding a certain number of other elements, phosphorus is added to the solution in

order to form also a composite matrix of NiP. We will discuss how the amount of phosphorus,

mixed to the solution as phosphoric acid, can influence the crystalline structure.

During the deposition of electroless nickel films, the growth of the film starts at isolated

locations on the substrate. Then the whole surface is covered by lateral growth. The alloys

containing lower phosphorus concentrations (low P, 1-4%) are characterized by the presence of

crystalline and microcrystalline nickel, which indicates that the numbers of phosphorus atoms are

not sufficient to distort the nickel lattice.

However, some very small pockets of amorphous nickel have been observed in coatings having

10.8% P (medium-high P)[32-34]

. As the phosphorus content increases to 23.4% P (high P), the

amorphous nickel region increases, due to larger lattice distortion caused by interstitial phosphorus

atoms position[35,36]

. In such deposited films, several crystalline non-equilibrium phases like Ni,

Ni3P, Ni5P12 and others have been observed.

The most stable of these phases is Ni3P which acts like the others as a reinforcement of the structure,

giving higher hardness values than the original film.

The variation of the reinforcement mechanisms shows a direct dependence of the annealing

temperature as we can see in the figures[37]

.

However, the precipitates other than Ni3P are metastable transition phases because on continued

heating, they disappear or become balances Ni3P[38-40]

.

The presence of inhomogenities in electroless films may, energetically as well as kinetically, favor

different local balances in different regions of the film.

The width and the shift of the rings, obtained for different polycrystalline electroless NiP

deposit with varying phosphorus contents, have been determined from the selected area diffraction

(SAD) patterns of transmission electron microscope (TEM) by the travelling microscope and are

reported in the table below along with the shift of these rings compared to pure nickel[32,41]

.

Table1. 7 SAD patterns of TEM

[32,41]

% P content in the

sample

Δ2θ θ shift % shift

14,3 0.07 0.07 14.92

15,3 0.09 0.08 17.63

17,8 0.12 0.07 15.59

19,8 0.16 0.07 15.13

22,4 0.21 0.11 22.01

23,4 0.22 0.1 21.91

15

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Figure1. 2 Film reinforcement mechanism evolution vs. phosphorus content

[37]

Figure1. 3 NiP film Hardness variation vs. phosphorus content

[37]

The relationship between wear resistance and friction coefficient vs. phosphorus content have the

following values[42]

.

16

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Figure1. 4 Friction coefficient in a high phosphorus content film (b) and low content (c)

[42]

Figure1. 5 Wear resistance vs. phosphorus content

[42]

17

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1.7 Applications

There are a lot of NiP coating applications due to its high wear and corrosion resistance; we

concentrate our studies onto the improvement and enhancement of mechanical properties of some

parts. Below you can see a table where some of the most important applications of NiP are listed.

Table1. 8 Example of NiP coatings applications

[3]

Application avenue Components Coating

thickness (μm)

Automotive Heat sinks, carburetor components, fuel injection, ball

studs, differential pinion ball shafts, disk brake pistons and

pad holders, transmission thrust washers, synchromesh

gears, knuckle, pins, exhaust manifolds and pipes,

mufflers, shock absorbers, lock components, hose

couplings, gear and gear assemblies. Fuel

pump motors, aluminum wheels, water pump,

components, steering column wheel components, air bag

hardware, air conditioning, compressor components,

decorative plastics and slip yokes

2-38

Air craft/aerospace Bearing journals, servo valves, compressor blades, hot

zone hardware, pistons heads, engine main shafts and

propellers, hydraulic actuator splines, seal snaps and

spacers, landing gear components, pilot tables, gyro parts,

engine mounts, oil nozzle components, turbine front

bearing cases, engine mount insulator housing, flanges,

sun gears, breech caps, shear bolts, engine oil feed

tubes, flexible bearing supports, break attach bolts,

antirotational plates, wing flap universal joints and

titanium thruster tracks

10-50

Chemical &

petroleum

Pressure vessels, reactors, mixer shafts, pumps and

impellers, heat exchangers, filters and components, turbine

blades and rotor assembles, compressor blades and

impellers, spray nozzles, valves: ball, gate, plug, check

and butterfly, stainless steel valves, chokes and control

valves, oil field tools, oil well packers and equipment, oil

well turbine and pumps, drilling mud pumps, hydraulic

systems actuators and blowout preventers

25-125

Electronics Head sinks, computer drive mechanisms, chassis memory

drums and discs, terminals of lead wires, connectors, diode

and transistor, cans, interlocks, junction fittings and PCB

2-25

Material handling Hydraulic cylinders and shafts, extruders, link drive belts,

gears and clutches

12-75

Medical &

pharmaceutical

Disposable surgical instruments and equipment, sizering

screens, pill sorters and feed screws and extruders

12-25

Mining Hydraulic systems, jetting pump heads, mine engine

components, piping connections, framing hardware

30-60

18

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1.8 Electroless codeposition of particles in NiP matrix

In addition to the classical electrodeposition or electroless deposition, there is the codeposition

process which forms faster a film, giving new and better characteristics; for example if we

separately process a substrate with the two methods for the same time we’ll obtain a higher

thickness of the coating for the one processed with the codeposition. Furthermore the codeposition

method can be a very effective alternative to the post-electroless deposition treatments for the

optimization of the surface characteristics. Adding micro or nanoparticles in the NiP matrix,

increases the hardness, wear and corrosion resistance and the general structural cohesion. There is a

large amount of materials that can be codeposited, the most used are divided as:

1) Carbides

2) Ceramics

3) Allotropics form of carbon

4) Lubricants

5) Rare earth elements

19

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1.8.1 Carbides

Carbides are used to reinforce NiP matrix thanks to their very high hardness and very high wear

resistance. Another important property is the very good thermal and chemical stability because of

their very high melting point. All these properties make carbides the most used in electroless

codeposition with NiP; in literature, the most selected are: silicon carbide (SiC, or Carborundum),

tungsten carbide (WC), boron carbide (B4C) or a mixture of them.

1.8.1.1 Silicon carbide

Silicon carbide are codeposited to strenghten NiP matrix and to improve its mechanical peculiarities,

in particular wear resistance and hardness and the corrosion resistance of the coating that is

influenced by the inglobation of SiC particles. After the codeposition, an annealing treatment is

necessary to reach very high values of all the properties, and obtain the best performances.

The mechanical properties of the particles, hardness, work as a rigid bond which increases the

elastic response of the material and delays the plastic deformation. Both annealed and not annealed

coatings experiment the optimal concentration of the particles in the bath near the agglomeration

limit; if this is exceeded there’ll be a very bad dispersion causing a decreasing of performances[43]

.

Figure1. 6 SiC content effect on (1) micro-hardness and (2) wear loss of mass[43]

Table1. 9 Micro-hardness vs. annealing temperature of NiP-SiC[44]

Type of NiP-SiC coating Micro-hardness (HV)

As-plated 957.4

Heat treatment at 200°C 1017.7

Heat treatment at 400°C 1289.3

Heat treatment at 600°C 1325.3

Thermal treatment leads to the formation of secondary precipitated phases, different from

traditional NixPy, which contributes to the matrix reinforcement as shown by XRD analysis[44]

(figure1. 7)

20

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Figure1. 7 NiP-SiC XRD spectrum variation vs. annealing temperature

[44]

The presence of SiC in the NiP matrix improves the corrosion resistance of the composite coating,

although the durability considerably decreases when silicon carbide exceeds 9%, and the corrosion

resistance is worse or equal as having only NiP coating[44]

.

21

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1.8.1.2 Boron carbide

This compound is very important due to its extreme hardness (9.3 in the Mohs Scale), high

chemical stability, high melting point, low density and its very high neutrons absorption coefficient

which gives an optimal shielding ability to the coatings[45]

.

After the thermal treatment, NiP-B4C composites show remarkable mechanical characteristics

which attest how the incorporation of this carbide nanoparticles is useful to the microstructure

without changing the performance. Specifically, hardness values higher than 1000 HV can be

obtained[46]

.

Figure1. 8 Annealing temperature and B4C concentration influence on micro-hardness of NiP-B4C

composite[45]

Adding B4C particles into the metallic matrix of NiP increases the wear resistance in terms of

removed mass and friction coefficient, obtaining a coating with high performances like

electrodeposited chromium[47,48]

.

Figure1. 9 Variation of wear mass loss and friction coefficient of NiP-B4C film

[45]

22

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1.8.1.3 Tungsten carbide

This compound is one of the hardest material that we can select and is used into the codeposition of

NiP to improve some mechanical properties like: hardness, friction coefficient, wear and corrosion

resistance. The presence of the WC particles doesn’t influence the physical phases, before or after

annealing, but they make thinner grains due to the increasing of the available nucleation sites[49]

. As

for SiC particles, an annealing treatment can bring the performances to very competitive values.

We can see typical values of hardness in the figure below.

Figure1. 10 Microhardness of (a) NiP, (b) NiP-WC and NiP-WC annealed at 200, 400 and 600 °C (c)-(d)

[50]

Regarding the wear resistance, WC plays a fundamental role: these types of nanoparticles can be

anchored very well to the metallic matrix and increase the cohesion forces of the coating. As we

have seen before, WC also prevents the plastic deformation of the film, it is capable of resisting to

high stress values and the high stripping coefficient of the particles makes the coating very resistant

to slip wear. After removing WC, the wear mechanism becomes mainly abrasive[50]

.

Figure1. 11 Friction coefficient evolution vs. wear test duration[50]

23

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1.8.2 Ceramics

Micro or nanoparticles of ceramics, metal oxides, are used as reinforcement for the matrix of NiP

due to their hardness and their capacity of improving the wear resistance of the coating. Particles

are introduced in NiP through the codeposition after being added to the bath which can be

electroplating or electroless. The mainly used materials are titanium oxide (TiO2), zirconia (ZrO2),

alumina (Al2O3) and quartz (SiO2).

1.8.2.1 Titanium oxide

Titania, like other ceramic materials, are used as reinforcement phase in NiP matrix obtained

through classic electroplating or electroless deposition in order to improve mechanical

properties.The insertion of TiO2 to the baths produces advantages in terms of increasing in

hardness[51]

, which reaches and passes 1000 HV after thermal treatment, better wear resistance

capacity, showing a less damaged surface, reduction of surface friction coefficient, which depends

on the concentration of the particles dispersed in the coating. However, we obtain different

characteristics of the films through codeposition which can be caused by the micro or nano particles

synthesis method. As a matter of fact, dimensions and the capacity of being dispersed into the

matrix directly influence mechanical properties which are optimized by the presence of particles

themselves. For example, if we make the synthesis of the particles using the precursor TiCl3 we’ll

have particles with very small dimensions, about 12 nm, and after the codeposition these ones

produce an enhancement of the hardness, over 900 HV, and a very sensible improvement of the

friction coefficient[51]

. The obtained peculiarities can be summarized in the figure.

Figure1. 12 Hardness and friction coefficient evolution of NiP-TiO2 with chemical reduced Titanium [51]

24

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If we consider Titania powder obtained by ball milling after 40 hours we’ll have particles with

higher dimensions than before, between 33 and 35 nm. These method produces a coating with

hardness above 1000HV, very good friction coefficient and wear resistance. In general, the

mechanical properties will be higher than those samples which have TiO2 precipitates[52]

.

Figure1. 13 Microhardness, wear and friction coefficient of NiP-TiO2 with ball milling Titania[51-52]

25

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As shown the use of smaller particles for the reinforcement leads to better final results.

The main problem is that the smaller dimensions gets the more superficial area of particles becomes,

consequently, the ability of each other forming clusters that prevents good dispersion of the

reinforcement elements of the matrix. A proposed solution for this problem is represented by an

alternative synthesis method of TiO2 nanoparticles, which exploit a sol-gel process in order to have

a reduction of the dimensions and a very good dispersion caused by the impossibility of the

interactions[53]

.

This is possible due to the presence of polymeric elements which acts like hindrance for the

formation of the unwanted clusters during the codeposition. A properly solution is added to the bath

instead of micro or nanopowders. This type of treatment leads to higher crystallinity of the coating

structure which means higher hardness[54]

.

Figure1. 14 Microhardness of NiP-TiO2 film. (a) NiP, (b) conventional NiP-TiO2, (c) NiP-TiO2 with Titania

obtained from sol-gel[54]

A further characteristic is that the Titania codeposition can give to NiP coatings the antibacteric

ability. In fact antiseptic properties of TiO2 are largely known, when it is exposed to UV light, and

also in nanocomposites of NiP-TiO2 this kind of peculiarity is present. So, these coatings can have

new fields of application that we don’t consider before[55]

.

26

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Figure1. 15 Comparison among antibacterial properties of stainless steel, NiP and NiP-TiO2[55]

27

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1.8.2.2 Zirconia

The reinforcement of the NiP matrix through zirconia leads an improvement of a lot of

characteristic rather than film without particles: these coatings show a better hardness and wear

resistance also deposition rate is higher, without negatively influencing the structural composition

of the deposit. The amount of P in the matrix is slightly influenced by the concentration variations

of the zirconia leaving the properties related to the percentage of P unchanged, as corrosion, fatigue

and abrasion resistance. Moreover, useful for our research is the fact that the zirconia particles in

electroless deposition show catalyst properties also on substrates without the PdCl2 activator

pretreatment[53,56]

.

Table1. 10 Composition and physical-chemical properties of the composite NiP-ZrO2

[53]

1.8.2.3 Aluminum oxide

Aluminum oxide (Al2O3) shows similar characteristics to the other described metallic oxides in

terms of improvement of hardness ad wear resistance[57,58]

. As well as the other codeposited

particles, the process influences superficial morphology of the coating, which becomes more rough,

and the crystallinity without Al2O3 particles being involved to the phase transformation during the

thermal treatment[59]

. In the figure1. 16 and figure1. 17 show the hardness and wear resistance

behaviour.

Figure1. 16 Annelaing treatment effect on microhardness of NiP and NiP-Al2O3 coatings[59]

28

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Figure1. 17 Annealing treatment effect on wear resistance of NiP-Al2O3

[59]

Similarly to data for the other ceramic materials, the annealing is the fundamental step which allows

very good improvements and in the case of alumina it is found that temperature of annealing and

corrosion resistance are closely related, as shown in the diagram[60]

.

Figure1. 18 Corrosion current density variation vs. annealing temperature for NiP and NiP-Al2O3 at

different pH[60]

29

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1.8.2.4 Silica

The codeposition of SiO2 shows an excellent potential to reinforce classic NiP coatings due to its

ceramic characteristic. As we said before, the most important data on mechanical properties are

collected after thermal treatment which allows the modification of NiP structures from amorphous

to crystalline thanks to the precipitation of Ni3P which reinforces considerably the coating giving it

a very thin and coherent grains structure.

In the case of silica deposition if the annealing is done in oxidant atmosphere we’ll see the

formation of a new phase (NiO) in the matrix.

Also for quartz particles dimensions added to the solution and their dispersion coefficient

considerably influence the properties of the coating.

In fact making a comparison between silica particles’ data with an average diameter of 10-20 nm

and particles of 40-50 nm we can observe how smaller dimensions cause batter mechanical

performances[61,62]

.

Figure1. 19 Microhardness and friction coefficient of NiP-SiO2 with particles dimension about 10-20 nm

[62]

Figure1. 20 Microhardness and friction coefficient of NiP-SiO2 with particles dimension about 40-50 nm[61]

30

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As we have seen for alumina, also silica contributes to improve film behavior in corrosive

environment in function of the concentration in bath, figure1. 21 refers to corrosion current of 40-50

nm SiO2 particles[63]

.

Figure1. 21 Corrosion current density variation vs. SiO2 content

[63]

31

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1.8.2.5 Ceria

An alternative material to the more classical ceramics already considered is cerium oxide (CeO2)

which is used to increase the resistance of the NiP coating in a corrosive environment due to the

structural changes caused by the codeposition.

The presence of ceria in the bath influences the structural morphology of the NiP matrix increasing

the amorphous character. This is caused by the cerium ions inclusions which prevents the formation

of a well-ordered lattice.

Differently from the use of rare earth in chloride form, actually cerium oxide is codeposited to

modify the film structure. Ceria is scarcely used in NiP nanocomposite so its behavior in the matrix,

increasing the temperature, is investigated through differential calorimetry scansion (DSC),

showing that above 665°C occurs a solid phase reaction between nickel and cerium leading to a

formation of a new phase NiCe2O4[64]

.

The effects of ceria addition, on the corrosion resistance, are visible from the SEM images which

shows that structural integrity is maintained[65]

.

Figure1. 22 SEM morphology of (a) NiP and (b) NiP-CeO2 under corrosion for 48h in acid solution

[64]

32

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1.8.3 Lubricants

Another class of interesting materials useful for codeposition in NiP matrix is chemical compounds

which have lubricating properties. One of the requested parameter for NiP coatings is to have a low

friction coefficient. This aspect is very important when the coating is functionally used so the

metallic surface treatments aimed to industrial use as machinery parts.

Main used materials for this purpose are PTFE (polytetrafluoroethylene better known as Teflon®),

MoS2 (molibdenum sulphide) and WS2 (tungsten sulphide).

1.8.3.1 PTFE

Polytetrafluoroethylene (PTFE or Teflon®) is a great material to have been codeposited in NiP

matrix to improve mechanical performance for its idrophobic characteristics, low friction

coefficient and low adhesion. Being a polymeric material, its incorporation during the deposition

shows some problems. Due to water repellency when it is introduced in the aqueous solution, it

tends to form clusters and precipitate. This fact affects the good dispersion of PTFE particles and

leads to their matrix incorporation into too high dimensions.

The result is an inhomogeneous coating with too rough superficial morphology, opposing the first

aim of the PTFE codeposition[66]

.

The solution is represented using surfactants which leads to steric and ionic interactions among

PTFE particles causing a well-done dispersion without the formation of clusters. Cationic

surfactants are preferred because they increase the charge of the particles towards the positive zone

stimulating the incorporation at the cathode. (figure1. 23)

Figure1. 23 Codeposited volumetric percentage of PTFE vs. concentration and type of surfactant

[66]

The presence of well dispersed particles in the coating leads to friction advantages and

improvement of corrosion resistance after a thermal treatment. (figure1. 24) This last feature seems

to be due to that the codeposition of PTFE modifies the NiP structure making it more crystalline,

which should favor the corrosion especially at low P content.

However, a homogeneous superficial layer is created which prevents from the corrosive ions. That

layer improves the porosity of the composite NiP-PTFE because it allows to expel rapidly hydrogen

formed during the deposition[67]

.

33

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Figure1. 24 Evolution of friction coefficient

[68]

As the diagram shows (figure1. 25), NiP-PTFE-SiC quaternary compounds can be considered

because the addition of PTFE influences system microhardness, which results considerably lower

than traditional NiP films[68]

.

Figure1. 25 Film NiP-PTFE microhardness vs. annealing temperature

[68]

34

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1.8.3.2 Molibdenum sulfide

Transition metal sulphides are known for their lubricant characteristics at solid state when they are

disposed with a multiple layer structure, known as 2H. However, due to dangling bonds, atoms of a

materials not stabilized by surface bonds, atmospheric conditions influence characteristics of 2H-

MoS2 going to smother lubricant properties in oxidant environment.

So, a form of molibdenum sulphide has been codeposited in NiP matrix: IF-MoS2 (inorganic

fullerene-like MoS2). This type of particles has a spherical structure, without dangling bonds, and

more indicated for NiP reinforcement applications. The incorporation of that particles have a

positive effect on the deposition rate and micro hardness, so we can obtain higher values than

1300HV for concentrations equal to 2 g/L corresponding to 5% of MoS2 in the coating[69]

(table1.

11).

Table1. 11 Chemical and physical properties of NiP-MoS2

[69]

Concentration of IF-

MoS2 (g/L)

Concentration of IF-

MoS2 (%)

Plating speed (μm/h) Microhardness of

coating (50HV)

0 0 12.0 795

0.4 2.2 13.3 1117

0.8 2.9 13.5 1229

1.2 3.4 13.8 1304

1.6 4.9 14.1 1320

2.0 5.0 15.0 1335

To evaluate the difference between 2H-MoS2 and IF-MoS2 we can compare the data of the friction

coefficient and wear rate when there is 5% of molibdenum sulphide in aqueous environment under

vacuum (table1. 12)

Table1. 12 Chemical and physical properties of NiP-MoS2 film

[69]

As we can notice from the previous table both MoS2 forms have a positive effect on the friction

coefficient and on wear resistance, however the IF form is less influenced by environmental

conditions, making this material more desirable to codeposit in NiP matrix. Although positive

results obtained with the use of this type of solid lubricant, there is the possibility that the thermal

treatment, made to achieve a crystalline coating, has a negative effect on the mechanical

performance due to the generation of a MoO3 phase.

35

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1.8.3.3 Tungsten sulfide

The structure of WS2 is almost the same as MoS2, but the fullerene-like arrangement is more stable.

Also, tungsten sulphide has more stability at high temperature than MoS2 so this fact makes

impossible the undesired secondary compounds during the annealing after the deposition[70]

.

the codeposition of WS2, differently from molibdenum sulphide, has a negative effect on

microhardness, which results considerably inferior than in only NiP. So, as for PTFE, there must be

quaternary composites with very hard particles like carbides. (figure1. 26)

Figure1. 26 Thermal treatment effect on microhardness of NiP and NiP-WS2 film

[70]

Although hardness reduction of about 60%, this material improves friction coefficient and loss mass

rate during the wear test, making the surface suitable for abrasion and adhesion resistance. However,

wear resistance is slightly less than traditional NiP, graphics show the evolution of the discussed

values[70]

.

Figure1. 27 Evolution of friction coefficient and mass loss vs. creep distance, before and after thermal

treatment[70]

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36

1.8.4 Allotropics form of carbon

In addition to other materials that we’ve said before, also carbon in its various forms is used as

reinforcement element in metallic NiP matrix. The effects of the addition are different due to the

nature of the used compounds, we can distinguish: graphite, carbon nanotubes (CNTs) and diamond.

1.8.4.1 Graphite

For the use of carbon as graphite as reinforcement particles, we’ll have the same observation made

for the other “soft” materials like PTFE and WS2. We’ll have a performances improvement in wear

resistance and friction coefficient, but lower microhardness of the composite coating rahter than

simple NiP. Similarly, to other libricants, it is desirable the synthesis of a quaternary film compound

with hard particles which make up the lack of NiP-Cg coatings[71-73]

.

Figure1. 28 Evolution of the friction coefficient and microhardness for NiP-Cg film

[71]

1.8.4.2 Carbon nanotubes (CNTs)

The use of carbon nanotubes, with multiple walls (MWNTs), it is desirable for tribological

properties and corrosion improvement for many reasons, among them we have hardness, toughness,

flexibility, resiliency, conductivity and chemical stability[74]

.

Optimal dilution is very difficult due to the high length/diameter rate which can lower the film

properties after the deposition. Therefore, CNTs powders are subjected to a ball milling treatment

which allows the reduction of rate and obtain a better dispersion during the codeposition[75]

.

The incorporation of CNTs in NiP makes a variation of film microstructure which results more

crystalline due to pauperization of P and a different superficial morphology. (figura1. 29)

37

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Figure1. 29 NiP surface (a) and NiP-CNTs (b) taken with AFM (atomic force microscope)

[74]

After annealing treatment, the most important mechanical properties values are found; these are

generally improved as shown in the graphs below. (figure1. 30)

Figure1. 30 Microhardness of NiP and NiP-CNTs before and after annealing and variation of friction

coefficient[74]

The improvement of wear resistance is principally due to that the addition of CNTs contrasts the

brittle nature of Ni3P, which is formed after thermal treatment, in fact, nanotubes and its debris

contribute to create a lubricant layer which maintains the friction coefficient low and prevents the

excessive embrittlement due to mechanical stresses cracks[76]

.

A furthermore result is that NiP-CNTs has better corrosion resistance due to low chemical reactivity

and so are less susceptible to the activation of the corrosion process[77]

.

38

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1.8.4.3 Diamonds

The use of diamond, as reinforcement element for metallic matrix in codeposition, is justified by its

incredible mechanical properties and chemical stability. The incorporation of diamond nanoparticles

(Nds) to form a nanocomposite (NiP-ND) leads to a very remarkable improvement of the

performance of the traditional NiP coating, both for mechanical and corrosion tests.

As well as the other “hard” particles, hardness, corrosion resistance and friction coefficient values

are higher than NiP coatings. The friction behaviour is influenced by the partial release, in time, of

nanoparticles at the surface, which have lubricant effect due to high hardness and spheric form.

Similarly, to the previous cases, the most remarkable data are described below (table1. 13) and are

referred to annealed films[78]

.

Table1. 13 Annealing temperature effect on micro-hardness, wear rate and friction coefficient

[78]

Composite coatings Hardness (HV)

Carbon steel (base) 424.23

Ni-P 673 K 873.52

As-prepared Ni-P-ND 614.79

Ni-P-ND 573 K 983.42

Ni-P-ND 673 K 1315.81

Ni-P-ND 773 K 1203.62

Composite coatings Wear volume/ x 10-4

mm3 Friction coefficient

Carbon steel (base) 21.6 0.64

Ni-P 10.2 0.58

As-prepared Ni-P-ND 5.6 0.46

Ni-P-ND 200°C 5.8 0.47

Ni-P-ND 400°C 3.4 0.36

Ni-P-ND 600°C 3.9 0.39

The presence of NDs modifies the film structure making thinner grains and more compact due to

nanoparticles in intergranular spaces. Moreover, the good dispersion of the particles into the matrix

influences both mechanical properties isotropy and corrosion mechanisms[79]

.

Infact, although particles can give surperficial preferential sites for corrosion phenomena,

proceeding with the codeposition and the incorporation of NDs in the NiP structure corrosion

resistance improves because the nanoparticles inside the film break the linearity of the corrosive

paths which are more hindered and disadvantaged[80]

.

39

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Figure1. 31 Inglobation effect of diamond nanoparticles on corrosion preferential paths

[80]

40

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1.8.5 Rare earth elements

The use of rare earth as reinforcement elements in NiP matrix shows a very good potential in terms

of compatibility and improvement of deposited coating characteristics. These materials are

particularly studied for their large atomic radius and quite low electronegativity which makes them

very active in the process without being co-deposited and improving considerably corrosion

resistance. Specifically, it has been noticed that there are bonds between phosphorus and RE

elements which allows high structural cohesion. Then supported by the fact that ionic behaviour

shown by solution compounds like Ce-P or Nd-P (Δχ ≈ 1). The most used rare earth elements (RE)

are cerium (Ce), lanthanum (La), neodimium (Nd) and ytterbium (Yb).

Table1. 14 Chemical and physical properties of rare earth elements used in deposition of composites

Cerium Lanthanum Neodimium Ytterbium

Atomic radius 181.8 pm 187 pm 181 pm 176 pm

Electronegativity

(χ)

1.12 1.1 1.14 1.1

Configuration [Xe]4f15d

16s

2 [Xe]5d

16s

2 [Xe]4f

46s

2 [Xe]4f

146s

2

In the case of medium-low phosphorus coatings (P<8%) chlorates Ce and La precursors generate

substantial modify of the whole film structure. As a matter of fact, XRD analysis shows how NiP-

RE peaks are considerably larger than only NiP, this fact giustifies the presence of a higher

percentage of amorphous phase. The importance of amorphous phase increase is that, not being the

structure partially composed by thin crystalline grains with particles precipitates, preferential sites

of corrosion have been removed, giving to the deposit a better resistance in corrosive

environment[81]

.

Some experimental results show that RE elements, used for NiP film treatment, don’t take part in

the codeposition and don’t influence the amount of P, otherwise other researches tell how the RE

action can be compatible with electrodeposition and electroless and can modify phosphorus

percentage.

The increase of P was found because of amorphous phase increase, these materials can be a very

good promoter of this process, allowing to create a classification of RE based on the amount of

amorphous structure is caused by each of them can create acting on NiP matrix[82]

.

For what concern ytterbium (Yb), its use can be very interesting due to its capacity to make thinner

crystalline grains and low possibility to be codeposited, this is caused by higher atomic radius and

lower electronegativity than other RE considered before. Some analysis on the concentration of

ytterbium show how there are saturation limit for deposition rate and for corroded surface

appearance time[83]

.

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CHAPTER 2

2.1 Characterization techniques

2.1.1 Mechanical Polishing for OM analysis

In order to do the right observation at the microscope, samples must be treated as showed below.

Firstly they are cut by a circular saw then we obtain the cross sections, these are embedded inside a

transparent polyurethane resin (PRESI – Métallographie ™)[84]

which guarantees more stability for

the mechanical polishing.

Passages are listed below:

sandpaper 100 to sandpaper 1200

diamond paste from 6 to 1 μm

After this treatment samples can be observed with OM.

2.1.2 Optical Microscope

XRD analyzer has a thickness limit, so to eventually measure thickness and observe the surface of

the samples, it has been used pictures taken by optical microscope. Such device exploits the light

refraction to reach magnification till 1000X, this value is obtained multiplying the ocular and the

lens magnification. However, the most important value of the OM is the resolution that is the ability

to distinguish two points as close as possible to each other.

The following law shown how varies this parameter:

λ is the wavelength, n is the refraction index of the medium, α is the angle between the lens and the

sample.

Figure2. 1 Optical microscope in the laboratory

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To obtain a good image is important the focus of the device, which occurs through two small

levers[85]

.

The microscope used in laboratory is a LEICA DLMN; linked to OM there’s a camera LEICA

model DFC290 as shown in figure.

After centering the sample, thanks to other two levers which allow the movement of the observation

plate along the x and y axis, pictures of the surface, and if we want pictures of the thickness, are

taken.

2.1.3 Scanning electron microscope (SEM) and EDX analysis

SEM is an electronic microscope used to analyze many kinds of materials. It uses a focused beam of

high-energy electrons to generate a variety of signals at the surface of the specimen. These signals

can give us some information:

- the topography which includes surface peculiarity of an object and textures which have a direct

correlation with the main properties of materials.

- the morphology, namely the shape and dimensions of the particles which make up the sample in

direct relation with ductility, strength, reactivity, defects, etc.

- the composition that is the elements and compounds making up the object and their relative rates

tightly related to melting point, reactivity, hardness, etc. (it is used in the study of biphasic systems:

metallic alloys or composites).

This type of tool was developed in the early ‘40 to overcome the limitations of the optical

microscope (OM), it has a very huge depth of field, which manages to focus at the same time almost

all the sample and gives us a characteristic three-dimensional appearance useful for understanding

the surface of the object. The resolution is better than a simple optical microscope as a matter of

fact two very near spots are distinguished also with very high magnification. All these elements

make SEM the most used tool in research and in industry.

Appendix: magnification, resolution and depth of field/focus of SEM

Firstly, any type of microscope must have the possibility to enlarge an object. Magnification is

defined as follows:

However, magnification can sometimes compromise another fundamental parameter that is the

resolution. This one is defined as the minimal distance between two points, which are seen through

the optical system as two different entities, direct consequence of the Rayleigh’s criterion:

where λ is the wavelength of the sources, μ is the index of refraction of the medium between the

lens and sample, α is the aperture half angle and NA is the numerical aperture.

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The numerical aperture, referred to objective, is the measure of the capacity of collecting light and

resolving the small details of the sample at a fixed distance.

Since the aperture limits the resolution, it is demonstrated that an object can be also put not exactly

in the microscope focus, to be resolved. So are defined depth of field and focus:

Depth of focus increases decreasing the aperture, increasing the distance from the object and finally

increasing focal length. Approximately, to obtain this measure is necessary to multiply focal length

and the f-stop value (diaphragm) then divided by 1000. The formula to calculate depth of field,

considering the numerical aperture (A), the magnification (M) and the work distance (W), is:

Figure2. 2 Scanning electron microscope (SEM) of the laboratory

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The experimental apparatus is composed firstly by an electrons source; they can be extracted

principally in two ways: thermoionic effect or field effect emission.

In the first case a low work function metallic wire, usually tungsten (W) or lanthanum boride

(LaB6), is heated at high temperature (2700K for W and 2000K for LaB6). This process permits to

overcome the electrons work function that comes out from the wire and they are accelerated by a

voltage around 1-50kV. To avoid an excessive oxidation a pressure between 10-3

-10-4

Pa is applied.

The second way uses a very tiny tungsten tip (radius < 0.1 μm, sometime coated by low work

function material as zirconium oxide) that in presence of strong electric field (> 107 V/cm),

intensified by the tip itself, causes the charges to be escaped. This operation must be under UHV

(Ultra High Vacuum, pressure < 10-6

Pa) conditions to avoid contaminants; very concentrated beam

can be obtained (diameter < 1nm).

The second part of the microscope is formed by the lenses system: a capacitor determining the

current which flows on the sample and a lens determining the beam dimensions and so the

resolution; between them there is a diaphragm controlling the aperture.

Moreover, to decrease the beam, electromagnetic lenses are used (cylindrical symmetry coil

developing a magnetic field which deflects the charges) and electrostatic lenses (Wehnelt cylinder).

The resulting beam proceeds to scan with an alternating movement along parallel and equidistant

lines so to cover a small part of the sample. Then the signal, modulated by an amplifier, is

transmitted to a pc monitor.

Figure2. 3 SEM working scheme

The resulting images from this particulr microscopy technique can be of two types, according to

which kind of electrons are used: back scattered electrons (BSE) and secondary (SE). The first ones

have an energy very similar to the source and they are sensible to the variations of the atomic

weight (Z) for this reason heavier elements will be brighter than lighter ones.

Instead the secondary electrons are dependant by the sample topography, due to low energy, and

give a tridimensional view of the image.

The last part is the detector (Everhart-Thornley is the standard) made by a material able to emit

electrons, a light guide and a photo-multiplier channel. It is equipped by a polarizable grid (from -

50 to +300 V) which allows to select the type of electrons.

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Another way is a solid-state detector, formed by one or more silicon diodes, in which electron-hole

pairs are generated from BSE having the right energy for this passage.

One of the advantages is that can be very thin and can be assembled onto the lens with a good

collecting angle; it allows also to compare the signal among detectors in order to distinguish

contrast contributions.

Table2. 1 A comparison among the microscopy techniques

OM SEM

Magnification 1-103 10-10

5

Resolution: ordinary/limit 5μm/0.1μm 50nm/<1nm

Depth of field 1 mm at 10x

0.1 mm at 100x

10 mm at 10x

1 mm at 100x

Technics Transmission (T), Reflection

(R) and others

T, R and others

Sample:

Preparation

Type

Dimensions

Environment

Simple

Almost no limits

Tens of cm

Atmosphere

Simple

Conductors

Tens of cm

Vacuum

Cost of tool Intermediate Intermediate-high

A technique implemented in the SEM apparatus is the energy-dispersive X-ray spectroscopy (EDS

or EDX). It is used to analyze chemical composition of the samples and it is based on the

characteristic emission of some determined X-rays. To stimulate this characteristic emission a

source of high energy electrons or photons or X-rays themselves is used; this source excites one of

the core electrons of the sample which are at ground state; this electron comes out and an outer

electron fills the hole left from the previous one and the difference in energy between the higher-

energy shell and the lower energy shell may be released in the form of X-ray.

The number and the energy of the X-rays emitted from the specimen can be measured by an energy-

dispersive spectrometer. This difference is peculiar for each atomic structure of emitting element;

EDS allows the elemental composition of the specimen to be measured.

The equipment is composed by: the excitation source, an X-rays detector, the pulse processor and

finally the analyzer.

Moreover, there are technological variants: maybe there is an excess of energy resulting from the

decay of the outer electron is transferred to a third electron from a further outer shell, prompting its

ejection. This kind of species is called Auger electron.

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2.1.4 XRF

An X-ray fluorescence (XRF) is a useful device used to determine the thickness and a rough

approximation of the composition of materials (rocks, minerals, alloys, metals and fluids). This

technique is based on the analysis of the x-rays emitted by the sample, placed in the machine, after

the irradiation with other energy known x-rays. After it starts, on the screen of a linked PC is

possible to see a diagram which represents the intensity peaks of the elements in correspondence

with values of energy. Studying this spectrum, it is possible to estimate the elements in the sample.

In our case, it was visible the peaks of Silicon wafer substrate and the Ruthenium layer. The

thickness is measured by the system making a comparison between the deposited layer spectrum

and the substrate spectrum only[86]

.

Figure2. 4 XRF apparatus in the laboratory

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2.1.5 XRD

This technique uses the X-ray diffraction and it is on of the most important tools to measure the

crystallinity of the thin films. A beam of x-rays is sent to the sample surface varying the incident

angles; if the resulting pattern is a large peak the phase is amorphous while there are a very large

number of peaks, one larger than the others, we’ll have crystalline phase. The diffraction technique

is the key in the study of thin films crystallinity determined by the Bragg’s law.

with n= 0, 1, 2, …

d is the distance between crystalline planes

θ is the diffraction angle

Figure2. 5 XRD apparatus of the laboratory

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2.1.6 Microdurometer

Another important instrument useful for our studies is the microdurometer. In the laboratory is

present a FISCHERSCOPE 57 HCV with a Vickers tip made with diamond.

Through this device is possible to determine the hardness of the film, defined as the resistance that

the material opposes to a penetration of unbendable object with a determined shape.

Then a microscope, which helps the positioning of the tip and measures the mark left on the sample,

is implemented in the laboratory apparatus.

Per standards, coating thickness can’t be less than 1.5 times the diagonal length of the tip, such

dimension allows the measure won’t be influenced by the substrate.

The diamond tip used for our purpose, has a square pyramidal shape with the top angle of 136°.

The law to determine the hardness for an instrumentation, such as in the laboratory, is the following:

with P equal to the applied load and d the mark diagonal.

The hardness measure must be independent from the applied load and it is obtained doing the

average of enough measurement made at a minimal distance each other[82]

.

Using the load-unload curve is possible to estimate the elastic modulus E of the thin film.

Figure2. 6 Microdurometer apparatus of the laboratory

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CHAPTER 3

3.1 Procedures

3.1.1 Brass pretreatment

Clean and prepare the substrates is the first important passage of the whole experiment: it allows to

obtain a perfect coating and avoid any undesired difects. First of all brass is rinsed in demineralized

water, cleaned with acetone in order to remove any organic trace, then rinsed again in demi water;

after these passages the substrate is immersed in a solution (1:1 sulphuric acid (98%), hydrogen

peroxide) for about one minute, to remove inorganic residues and again rinsed.

Later it is immersed in a solution with PdCl2, which acts as an activator of the surface (it can be

observed a slight blackening of the brass surface), another rinse with demi water finally immersion

in the NiP solution (Tecnoplate 3000 ® provided by Tecnochimica S.p.A.).

3.1.2 Preparation of CTAB and DTAB solutions

In order to have the maximal control of the bath, we need to prepare a solution with surfactants:

cetyl trimethylammonium bromide (CTAB) and dodecyl trimethylammonium bromide (DTAB).

The concentration of both surfactants must be 1mM.

3.1.3 Preparation of the bath for CTAB and DTAB tests

Two kind of components were provided: part A, containing the nickel salts and thus displaying their

distinctive green color; part B, a transparent solution containing the reducing agents. Moreover both

A and B solutions could be with and without additives.

Firstly we prepare the solution all without additives: NiP0. Although the results derived from this

kind of formulation weren’t satisfactory, so we have decided to take into account also part A and

part B with additives.

The second formulation (NiPA) was prepared with half amount of additives.

With this solution, the experiments with CTAB and DTAB results very bad: after about 45 minutes,

the solution in both cases became turbid due to tiny particles of nickel in suspension.

Finally the solution NiPB was formed adding ¼ amount.

All baths should have a desired pH around 4.8-5: according to the effective measured pH we added

a solution 1:4 sulfuric acid (98%) and water to lower the pH or an ammonia solution to increase the

pH.

The brass substrate, after the pretreatment explained before, is immersed in one of the above

depositing solutions and the temperature is brought to 88° C through a heater and then kept constant.

This thermal condition is crucial because at that value the autocatalysis of the electroless deposition

starts.

Table3. 1 Three different bath formulations used in the experiments

Baths A without

additives

A with additives B without

additives

B with additives

NiP0 60 ml/L / 180 ml/L /

NiPA 30 ml/L 30 ml/L 90 ml/L 90 ml/L

NiPB 45 ml/L 15 ml/L 135 ml/L 45 ml/L

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3.1.4 Addition of particles

Solution NiPB is chosen as the best because after some attempts, it results very stable. Then

tungsten has been added to this solution (9 g/L in form of sodium tungstate), measured the pH and

brought again between 4.8 and 5 (the change is caused by alkalinity of the salt). The codeposition

didn’t give notable problems so it is decided to make solutions with silicon carbide, titania and

mixed carbides (silicon and boron carbide), firstly only with CTAB then with only DTAB.

3.2 Results

The following results represent the studies of the samples, both morphological and chemical-

physical point of view. For what concern morphology studies it is used a scanning electron

microscope (SEM), X-rays diffraction (XRD) and an optical microscope. Chemical-physical

analysis was made by micro-hardness test and SEM-EDX technique.

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3.2.1 SEM-EDX, XRD and Optical Microscope analysis

First of all it will be shown pictures of all sample with the first formulation: A plus B without

additives.

3.2.1.1 NiP first formulation (NiP0)

Figure3. 1 Optical microscope images of NiP0: 20X of magnification (left), 100X of magnification (right)

In this case we can see a small globular growth for NiP especially in figure3. 1 with 100X of

magnification.

Figure3. 2 XRD spectrum of the NiP0

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3.2.1.2 NiP0(CTAB)

Figure3. 3 Optical microscope images of NiP0(CTAB): 20X of magnification (left), 100X of magnification

(right)

More evident the regular globular growth is shown for this sample due to the CTAB surfactant.

Figure3. 4 SEM image of NiP0(CTAB)

This sample has a Nickel content of 87.3% and 12.7% of Phosphorus with an average thickness of

about 11 μm.

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Page 55: Electroless deposition of NiP alloys...1. CHAPTER 1 1.1 From electroplating to electroless deposition Surface engineering is a very large and important sub-discipline of material science

Figure3. 5 XRD spectrum of the NiP0(CTAB)

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3.2.1.3 NiP0(DTAB)

Figure3. 6 Optical microscope images of NiP0(DTAB): 20X of magnification (left), 100X of magnification

(right)

The sample with DTAB surfactant has less regular gstructure than the previous one with CTAB,

infact we can see thinner texture more evident in figure3. 6.

Figure3. 7 XRD spectrum of the NiP0(DTAB)

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3.2.1.4 NiP0/W

Figure3. 8 Optical microscope images of NiP0/W: 20X of magnification (left), 100X of magnification (right)

The content of tungsten in this sample is 9 g/L. Growth is quite regular, globular structure is almost

absent.

Figure3. 9 SEM image of the NiP0/W (9 g/L)

This sample has 91% of Nickel content, 8,4% of Phosphorus and 0,6% of tungsten with an average

thickness of 6 μm.

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Figure3. 10 XRD spectrum of the NiP0/W

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3.2.1.5 NiP0/W(CTAB)

Figure3. 11 Optical microscope images of NiP0/W(CTAB): 20X of magnification (left), 100X of

magnification (right)

Also in this situation tungsten concentration is 9 g/L. The growth is quite regular and the globular

structure is thin (see figure3. 11)

Figure3. 12 XRD spectrum of the NiP0/W(CTAB)

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3.2.1.6 NiP0-SiC

Figure3. 13 Optical microscope images of NiP0-SiC: 20X of magnification (left), 100X of magnification

(right)

We notice immediately that the globular structure is very thin at the surface due to the incorporation

of the carbides; this fact produce a very high hardness NiP coatings rather than the others without

carbides.

Figure3. 14 SEM image of the NiP0-SiC

In this case the content of nickel is about 80%, 7.8% of phosphorus and 11.02% of silicon; the

average thickness is about 15 μm.

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Figure3. 15 XRD spectrum of the NiP0-SiC

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3.2.1.7 NiP0–SiC/B4C

Figure3. 16 Optical microscope images of NiP0-SiC/B4C: 20X of magnification (left), 100X of magnification

(right)

Also in this case the globular structure at the surface is thin, anyway less than the previous one. The

nickel content of this sample is about 89%, phosphorus 10% and silicon about 1%; the average

thickness is about 16 μm.

Figure3. 17 SEM image of the NiP0-SiC/B4C

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Figure3. 18 XRD spectrum of the NiP0-SiC/B4C

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3.2.1.8 NiP second formulation (NiPA)

Figure3. 19 Optical microscope images of NiPA: 20X of magnification (left), 100X of magnification (right)

It is evident that in this sample the globular growth is very regular and well-distributed, not thin due

to the presence of the additives. The content of nickel in this sample is 89%, phosphorus is 10% and

1% represents silicon; the average thickness is about 12 μm.

Figure3. 20 SEM image of the NiPA

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Figure3. 21 XRD spectrum of the NiPA

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3.2.1.9 NiPA/W (4 g/L)

Figure3. 22 Optical microscope images of NiPA/W (4 g/L): 20X of magnification (left), 100X of

magnification (right)

With a different formulation we can see a more regular NiP growth due to the presence of additives.

Figure3. 23 SEM image of the NiPA/W (4 g/L)

In this case the content of Nickel is about 89.4%, Phosphorus is 10 % and Tungsten is 0,6% and the

average thickness is 11 μm.

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Figure3. 24 XRD spectrum of the NiPA/W (4 g/L)

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3.2.1.10 NiPA/W (6 g/L)

Figure3. 25 Optical microscope images of NiPA/W (6 g/L): 20X of magnification (left), 100X of

magnification (right)

The addition of tungsten changes drastically the structure: it is thinner and less regular.

Figure3. 26 SEM image of the NiPA/W (6 g/L)

This sample presents a content of nickel about 91%, phosphorus 8% and tungsten 1% with an

average thickness of about 10 μm.

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Figure3. 27 XRD spectrum of the NiPA/W (6 g/L)

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3.2.1.11 NiPA/W (18 g/L)

Figure3. 28 Optical microscope images of NiPA/W (18 g/L): 20X of magnification (left), 100X of

magnification (right)

The second formulation with a more tungsten concentration seems to be more regular and globular

growth but thinner than the coating without sodium tungstate.

Figure3. 29 SEM image of the NiPA/W (18 g/L)

The content of nickel is about 90%, phosphorus 9% and tungsten 1% with an average thickness of

about 17 μm.

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Figure3. 30 XRD spectrum of the NiPA/W (18 g/L)

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3.2.1.12 NiP third formulation (NiPB)

Figure3. 31 Optical microscope images of NiPB: 20X of magnification (left), 100X of magnification (right)

This sample presents a globular structure but thinner than NiPA (see figure3. 31 the right one). The

content of nickel is about 88%, phosphorus 11% and silicon 1%; the average thickness is about 10

μm.

Figure3. 32 SEM image of the NiPB

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Figure3. 33 XRD spectrum of the NiPB

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3.2.1.13 NiPB(CTAB)

Figure3. 34 Optical microscope images of NiPB(CTAB): 20X of magnification (left), 100X of magnification

(right)

The sample with CTAB has a globular structure like the previous one but larger than the sample

with DTAB. The content of nickel is 87.5% and phosphorus 12.5%; the average thickness is about 9

μm.

Figure3. 35 SEM image of NiPB(CTAB)

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Figure3. 36 XRD spectrum of the NiPB(CTAB)

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3.2.1.14 NiPB(DTAB)

Figure3. 37 Optical microscope images of NiPB(DTAB): 20X of magnification (left), 100X of magnification

(right)

The sample with DTAB presents a thinner globular structure maybe due to the presence of the

shorter chain of the surfactant compared with CTAB. The nickel content is 86% and phosphorus is

14% with an average thickness of about12 μm.

Figure3.38 SEM image of the NiPB(DTAB)

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Figure3. 39 XRD spectrum of the NiPB(DTAB)

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3.2.1.15 NiPB/W (9 g/L)

Figure3. 40 Optical microscope images of NiPB/W (9 g/L): 20X of magnification (left), 100X of

magnification (right)

This sample has one of the thinnest globular structure. Nickel content is about 96%, phosphorus is

6% and tungsten 1%; it has an average thickness of about 8 μm.

Figure3. 41 SEM image of the NiPB/W (9 g/L)

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Figure3. 42 XRD spectrum of the NiPB/W (9 g/L)

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3.2.1.16 NiPB/W(CTAB)

Figure3. 43 Optical microscope images of NiPB/W(CTAB): 20X of magnification (left), 100X of

magnification (right)

This is another example of thinnest globular structure, it has a nickel content of about 86% and 14%

of phosphorus. The average thickness is about 8.4 μm.

Figure3. 44 SEM image of the NiPB(CTAB)

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Figure3. 45 XRD spectrum of the NiPB/W(CTAB)

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3.2.1.17 NiPB/W(DTAB)

Figure3. 46 Optical microscope images of NiPB/W(DTAB): 20X of magnification (left), 100X of

magnification (right)

Another sample with a very large globular growth, the nickel content is about 87% and phosphorus

is 13% with an average thickness of about 12 μm.

Figure3. 47 SEM image of the NiPB/W(DTAB)

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Figure3. 48 XRD spectrum of the NiPB/W(DTAB)

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3.2.1.18 NiPB/W(CTAB)-SiC

Figure3. 49 Optical microscope images of NiPB/W(CTAB)-SiC: 20X of magnification (left), 100X of

magnification (right)

The structure of this sample is considerably less thin than the others with particles. The content of

nickel is about 86.42%, phosphorus is 13.19% and silicon 0.27%; the average thickness is about 8

μm.

Figure3. 50 SEM image of the NiPB/W(CTAB)-SiC

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Figure3. 51 XRD spectrum of the NiPB/W(CTAB)-SiC

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3.2.1.19 NiPB/W(DTAB)-SiC

Figure3. 52 Optical microscope images of NiPB/W(DTAB)-SiC: 20X of magnification (left), 100X of

magnification (right)

The structure is thinner than the previous sample with CTAB, the content of nickel is about 72%,

phosphorus is 10.86% and silicon is 17.12%; average thickness is about 15 μm

Figure3. 53 SEM image of the NiPB/W(DTAB)-SiC

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Figure3. 54 XRD spectrum of the NiPB/W(DTAB)-SiC

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3.2.1.20 NiPB/W(CTAB)-TiO2

Figure3. 55 Optical microscope images of NiPB/W(CTAB)-TiO2: 20X of magnification (left), 100X of

magnification (right)

Also in this sample the structure is very thin due to the presence of TiO2 particles. The average

thickness is about 15 μm.

Figure3. 56 SEM image of the NiPB/W(CTAB)-TiO2

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Figure3. 57 XRD spectrum of the NiPB/W(CTAB)-TiO2

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3.2.1.21 NiPB/W(DTAB)-TiO2

Figure3. 58 Optical microscope images of NiPB/W(DTAB)-TiO2: 20X of magnification (left), 100X of

magnification (right)

This sample presents a slightly larger structure of the surface than the one with CTAB. The content

of nickel is about 89%, phosphorus 10% and titanium 1%; the average thickness is about 17 μm.

Figure3. 59 SEM image of the NiPB/W(DTAB)-TiO2

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Figure3. 60 XRD spectrum of the NiPB/W(DTAB)-TiO2

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3.2.1.22 NiPB/W(CTAB)-SiC/B4C

Figure3. 61 Optical microscope images of NiPB/W(CTAB)-SiC/B4C: 20X of magnification (left), 100X of

magnification (right)

Also in this case the structure is very thin due to the mix of carbides added to the bath. The content

of nickel is about 86%, phopshorus is 12.52% and silicon 1.48%; the average thickness is about 6

μm.

Figure3. 62 SEM image of the NiPB/W(CTAB)-SiC/B4C

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Figure3. 63 XRD spectrum of the NiPB/W(CTAB)-SiC/B4C

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3.2.1.23 NiPB/W(DTAB)-SiC/B4C

Figure3. 64 Optical microscope images of NiPB/W(DTAB)-SiC/B4C: 20X of magnification (left), 100X of

magnification (right)

For what concern the structure of the surface, this sample with DTAB is very similar to the previous

one. The content of nickel is about 88.56%, phosphorus is 8.59% and silicon 2.96%; the average

thickness is about 20 μm.

Figure3. 65 SEM image of the NiPB/W(DTAB)-SiC/B4C

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Figure3. 66 XRD spectrum of the NiPB/W(DTAB)-SiC/B4C

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3.2.2 Microhardness

Finally we did microhardness test on all the samples obtained: some results are quite similar to tool

steels hardness (690-840 HV).

Figure3. 67 Microhardness values for the first formulation NiP0

Figure3. 68 Microhardness values for the second formulation NiPA

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Figure3. 69 Microhardness values for the third formulation NiPB

For the first formulation (figure3. 67) the highest value is reached for samples containing silicon

carbides as we expect; otherwise the remaining samples are considerably lower.

Due to the presence of more compact globular structure, the samples obtained from the second

formulation are harder (figure3. 68) but there is a minimum represented by NiPA/W where the

concentration of tungsten is 4 g/L.

In the last case (figure3. 69) the highest value is represented by NiPB(DTAB) due to the presence of

globular thinner structure than the others that we have seen because DTAB leads to a more regular

and controlled growth. FInally there is a remarkable decrease in values for the samples with

particles.

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4. CHAPTER 4

4.1 Conclusions and future developments

According to the general analysis of the properties of NiP we can say that electroless deposition of

this type of coating can be a valid substitute to electrodeposition one. In particular some coatings

obtained by NiPA, NiPA/W (18 g/L), NiPB(DTAB) and NiPB/W(DTAB) present hardness and other

interesting properties. However the procedure to make this kind of coatings is very laborious and it

is necessary the presence of qualified operators, thus affecting potential application on a large

industrial scale. Anyway it can be a very effective method in a small industrial production.

In these years Significant forward steps in the formulation of solutions are taking place: more

efficient electrolytic solutions which are able to incorporate countless materials as an additional

particulate.

Finally the electroless coating chemistry has emerged as one of the leading areas in surface

engineering, metal finishing etc. and is estimated to grow at a rate of beyond fifteen percent per

annum: no other chemistry is growing at this rate[3]

.

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Index of figures

Figure1. 1 Some mechanical parts in an electroless bath of NiP…………………………...………..9

Figure1. 2 Film reinforcement mechanism evolution vs. phosphorus content……………...………16

Figure1. 3 NiP film Hardness variation vs. phosphorus content…………………………..………..16

Figure1. 4 Friction coefficient in a high phosphorus content film (b) and low content (c)………...17

Figure1. 5 Wear resistance vs. phosphorus content…………………………………………………17

Figure1. 6 SiC content effect on (1) micro-hardness and (2) wear loss of mass……………………20

Figure1. 7 NiP-SiC XRD spectrum variation vs. annealing temperature…………………………...21

Figure1. 8 Annealing temperature and B4C concentration influence on micro-hardness of NiP-B4C

composite……………………………………………………………………………………………22

Figure1. 9 Variation of wear mass loss and friction coefficient of NiP-B4C film…………………..22

Figure1. 10Microhardness of (a) NiP, (b) NiP-WC and NiP-WC annealed at 200, 400 and 600 °C

(c)-(d)………………………………………………………………………………………………..23

Figure1. 11 Friction coefficient evolution vs. wear test duration…………………………………...23

Figure1. 12 Hardness and friction coefficient evolution of NiP-TiO2 with chemical reduced

Titanium……………………………………………………………………………………………..24

Figure1. 13 Microhardness, wear and friction coefficient of NiP-TiO2 with ball milling Titania….25

Figure1. 14 Microhardness of NiP-TiO2 film. (a) NiP, (b) conventional NiP-TiO2, (c) NiP-TiO2 with Titania

obtained from sol-gel…………………………………………………………………………………………26

Figure1. 15 Comparison among antibacterial properties of stainless steel, NiP and NiP-TiO2.........27

Figure1. 16 Annelaing treatment effect on microhardness of NiP and NiP-Al2O3 coatings………..28

Figure1. 17 Annealing treatment effect on wear resistance of NiP-Al2O3.........................................29

Figure1. 18 Microhardness and friction coefficient of NiP-SiO2 with particles dimension about 10-

20 nm………………………………………………………………………………………………..29

Figure1. 19 Corrosion current density variation vs. annealing temperature for NiP and NiP-Al2O3 at

different pH………………………………………………………………………………………….30

Figure1. 20 Microhardness and friction coefficient of NiP-SiO2 with particles dimension about 40-

50 nm………………………………………………………………………………………………..30

Figure1. 21 Corrosion current density variation vs. SiO2 content…………………………………..31

Figure1. 22 SEM morphology of (a) NiP and (b) NiP-CeO2 under corrosion for 48h in acid

solution……………………………………………………………………………………………...32

Figure1. 23 Codeposited volumetric percentage of PTFE vs. concentration and type of

surfactant……………………………………………………………………………………………33

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Figure1. 24 Evolution of friction coefficient………………………………………………………..34

Figure1. 25 Film NiP-PTFE microhardness vs. annealing temperature…………………………….34

Figure1. 26 Thermal treatment effect on microhardness of NiP and NiP-WS2 film………………..36

Figure1. 27 Evolution of friction coefficient and mass loss vs. creep distance, before and after

thermal treatment……………………………………………………………………………………36

Figure1. 28 Evolution of the friction coefficient and microhardness for NiP-Cg film……………...37

Figure1. 29 NiP surface (a) and NiP-CNTs (b) taken with AFM (atomic force microscope)………38

Figure1. 30 Microhardness of NiP and NiP-CNTs before and after annealing and variation of

friction coefficient…………………………………………………………………………………..38

Figure1. 31 Inglobation effect of diamond nanoparticles on corrosion preferential paths………….40

Figure2. 1 Optical microscope in the laboratory……………………………………………………42

Figure2. 2 Scanning electron microscope (SEM) of the laboratory………………………………...44

Figure2. 3 SEM working scheme…………………………………………………………………...45

Figure2. 4 XRF apparatus of the laboratory………………………………………………………...47

Figure2. 5 XRD apparatus of the laboratory………………………………………………………..48

Figure2. 6 Microdurometer apparatus of the laboratory……………………………………………49

Figure3. 1 Optical microscope images of NiP0: 20X of magnification (left), 100X of magnification

(right)………………………………………………………………………………………………..52

Figure3. 2 XRD spectrum of the NiP0………………………………………………………………52

Figure3. 3 Optical microscope images of NiP0(CTAB): 20X of magnification (left), 100X of

magnification (right)………………………………………………………………………………...53

Figure3. 4 SEM image of NiP0(CTAB)……………………………………………………………..53

Figure3. 5 XRD spectrum of the NiP0(CTAB)……………………………………………………..54

Figure3. 6 Optical microscope images of NiP0(DTAB): 20X of magnification (left), 100X of

magnification (right)………………………………………………………………………………...55

Figure3. 7 XRD spectrum of the NiP0(DTAB)……………………………………………………..55

Figure3. 8 Optical microscope images of NiP0/W (9 g/L): 20X of magnification (left), 100X of

magnification (right)………………………………………………………………………………...56

Figure3. 9 SEM image of the NiP0/W (9 g/L)………………………………………………………56

Figure3. 10 XRD spectrum of the NiP0/W (9 g/L)………………………………………………….57

Figure3. 11 Optical microscope images of NiP0/W(CTAB): 20X of magnification (left), 100X of

magnification (right)………………………………………………………………………………...58

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Figure3. 12 XRD spectrum of the NiP0/W(CTAB)…………………………………………………58

Figure3. 13 Optical microscope images of NiP0-SiC: 20X of magnification (left), 100X of

magnification (right)………………………………………………………………………………...59

Figure3. 14 SEM image of the NiP0-SiC……………………………………………………………59

Figure3. 15 XRD spectrum of the NiP0-SiC………………………………………………………..60

Figure3. 16 Optical microscope images of NiP0-SiC/B4C: 20X of magnification (left), 100X of

magnification (right)………………………………………………………………………………...61

Figure3. 17 SEM image of the NiP0-SiC/B4C………………………………………………………61

Figure3. 18 XRD spectrum of the NiP0-SiC/B4C…………………………………………………...62

Figure3. 19 Optical microscope images of NiPA: 20X of magnification (left), 100X of magnification

(right)………………………………………………………………………………………………..63

Figure3. 20 SEM image of the NiPA………………………………………………………………..63

Figure3. 21 XRD spectrum of the NiPA…………………………………………………………….64

Figure3. 22 Optical microscope images of NiPA/W (4 g/L): 20X of magnification (left), 100X of

magnification (right)………………………………………………………………………………...65

Figure3. 23 SEM image of the NiPA/W (4 g/L)……………………………………..……………..65

Figure3. 24 XRD spectrum of the NiPA/W (4 g/L)…………………………………………………66

Figure3. 25 Optical microscope images of NiPA/W (6 g/L): 20X of magnification (left), 100X of

magnification (right)………………………………………………………………………………...67

Figure3. 26 SEM image of the NiPA/W (6 g/L)…………………………………….……………....67

Figure3. 27 XRD spectrum of the NiPA/W (6 g/L)…………………………………………………68

Figure3. 28 Optical microscope images of NiPA/W (18 g/L): 20X of magnification (left), 100X of

magnification (right)………………………………………………………………………………...69

Figure3. 29 SEM image of the NiPA/W (18 g/L)………………………………………………...…69

Figure3. 30 XRD spectrum of the NiPA/W (18 g/L)………………………………………………..70

Figure3. 31 Optical microscope images of NiPB: 20X of magnification (left), 100X of magnification

(right)………………………………………………………………………………………………..71

Figure3. 32 SEM image of the NiPB………………………………………………………………..71

Figure3. 33 XRD spectrum of the NiPB…………………………………………………………….72

Figure3. 34 Optical microscope images of NiPB(CTAB): 20X of magnification (left), 100X of

magnification (right)………………………………………………………………………………...73

Figure3. 35 SEM image of NiPB(CTAB)…………………………………………………………..73

Figure3. 36 XRD spectrum of the NiPB(CTAB)………………………………………………….74

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Figure3. 37 Optical microscope images of NiPB(DTAB): 20X of magnification (left), 100X of

magnification (right)………………………………………………………………………………...75

Figure3. 38 SEM image of the NiPB(DTAB)……………………………………………………….75

Figure3. 39 XRD spectrum of the NiPB(DTAB)……………………………………………………76

Figure3. 40 Optical microscope images of NiPB/W (9 g/L): 20X of magnification (left), 100X of

magnification (right)………………………………………………………………………………...77

Figure3. 41 SEM image of the NiPB/W (9 g/L)…………………………………...……...………...77

Figure3. 42 XRD spectrum of the NiPB/W (9 g/L)……………………………………………...….78

Figure3. 43 Optical microscope images of NiPB/W(CTAB): 20X of magnification (left), 100X of

magnification (right)………………………………………………………………………………...79

Figure3. 44 SEM image of the NiPB(CTAB)…………………………………..…………………...79

Figure3. 45 XRD spectrum of the NiPB/W(CTAB)………………………………………...………80

Figure3. 46 Optical microscope images of NiPB/W(DTAB): 20X of magnification (left), 100X of

magnification (right)………………………………………………………………………………...81

Figure3. 47 SEM image of the NiPB/W(DTAB)…………………………………...………………81

Figure3. 48 XRD spectrum of the NiPB/W(DTAB)………………………………………………...82

Figure3. 49 Optical microscope images of NiPB/W(CTAB)-SiC: 20X of magnification (left), 100X

of magnification (right)……………………………………………………………………………..83

Figure3. 50 SEM image of the NiPB/W(CTAB)-SiC……………………………………………….83

Figure3. 51 XRD spectrum of the NiPB/W(CTAB)-SiC……………………………………………84

Figure3. 52 Optical microscope images of NiPB/W(DTAB)-SiC: 20X of magnification (left), 100X

of magnification (right)……………………………………………………………………………..85

Figure3. 53 SEM image of the NiPB/W(DTAB)-SiC……………………………………………….85

Figure3. 54 XRD spectrum of the NiPB/W(DTAB)-SiC……………………………………………86

Figure3. 55 Optical microscope images of NiPB/W(CTAB)-TiO2: 20X of magnification (left), 100X

of magnification (right)……………………………………………………………………………..87

Figure3. 56 SEM image of the NiPB/W(CTAB)-TiO2……………………………………………..87

Figure3. 57 XRD spectrum of the NiPB/W(CTAB)-TiO2……………………………………..…...88

Figure3. 58 Optical microscope images of NiPB/W(DTAB)-TiO2: 20X of magnification (left), 100X

of magnification (right)……………………………………………………………………………..89

Figure3. 59 SEM image of the NiPB/W(DTAB)-TiO2……………………………………………..89

Figure3. 60 XRD spectrum of the NiPB/W(DTAB)-TiO2…………………………………………..90

Figure3. 61 Optical microscope images of NiPB/W(CTAB)-SiC/B4C: 20X of magnification (left),

100X of magnification (right)………………………………………………………………………91

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Figure3. 62 SEM image of the NiPB/W(CTAB)-SiC/B4C…………………………………………92

Figure3. 63 XRD spectrum of the NiPB/W(CTAB)-SiC/B4C………………………………………92

Figure3. 64 Optical microscope images of NiPB/W(DTAB)-SiC/B4C: 20X of magnification (left),

100X of magnification (right)………………………………………………………………………93

Figure3. 65 SEM image of the NiPB/W(DTAB)-SiC/B4C………………………………………….93

Figure3. 66 XRD spectrum of the NiPB/W(DTAB)-SiC/B4C………………………………..…..…94

Figure3. 67 Microhardness values for the first formulation NiP0………………………………….95

Figure3. 68 Microhardness values for the first formulation NiPA…………………………………95

Figure3. 69 Microhardness values for the first formulation NiPB…………………………………96

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Index of tables

Table1. 1 Differences between electrodeposition and electroless deposition………………………...7

Table1. 2 Advantages and disadvantages of electroless deposition…………………………………..7

Table1. 3 Example of acid bath………………………………………………………………………9

Table1. 4 Example of alkaline bath…………………………………………………………………10

Table1. 5 Another example of alkaline bath………………………………………………………...10

Table1. 6 General components of a standard Ni bath……………………………………………….12

Table1. 7 SAD patterns of TEM…………………………………………………………………….15

Table1. 8 Example of NiP coatings applications……………………………………………………18

Table1. 9 Micro-hardness vs. annealing temperature of NiP-SiC…………………………………..20

Table1. 10 Composition and physical-chemical properties of the composite NiP-ZrO2…………....28

Table1. 11 Chemical and physical properties of NiP-MoS2………………………………………..35

Table1. 12 Chemical and physical properties of NiP-MoS2 film…………………………………...35

Table1. 13 Annealing temperature effect on micro-hardness, wear rate and friction coefficient…...39

Table1. 14 Chemical and physical properties of rare earth elements used in deposition of

composites…………………………………………………………………………………………..41

Table2. 1 A comparison among the microscopy techniques……………………………………..….46

Table3. 1 Three different bath formulations used in the experiments………………………….…...50

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Acknowledgments

In questa sezione vorrei ringraziare tutte le persone che mi sono state vicine e mi hanno

accompagnato in questo lungo e faticoso viaggio accademico.

In primis il mio ringraziamento va ai miei genitori che mi hanno sempre supportato e incoraggiato

nei momenti difficili. È soprattutto grazie a loro se oggi ho raggiunto questo importante traguardo.

Continuando, un ringraziamento speciale va al prof. Magagnin e a Simona che mi hanno seguito

durante le esperienze di laboratorio.

Un grazie va anche alla mia compagna di vita, Rita, conosciuta proprio tra le mura dell’ateneo e ai

suoi genitori che mi hanno spronato al raggiungimento dell’obiettivo.

Infine vorrei omaggiare tutti i miei familiari e gli amici che ho conosciuto dentro e fuori i corsi di

studi in tutti questi anni.

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