A REVIEW ON ADHESION STRENGTH OF PEO COATINGS BY SCRATCH TEST METHOD · 2018. 12. 4. · by Scratch...

A REVIEW ON ADHESION STRENGTH OF PEOCOATINGS BY SCRATCH TEST METHOD

HOSSEIN SHARIFI*, MAHMOOD ALIOFKHAZRAEI†,§

and GHASEM BRATAI DARBAND‡

Department of Materials Engineering,Faculty of Engineering, Tarbiat Modares University,

P.O. Box: 14115-143, Tehran, Iran*[email protected]

†[email protected]; [email protected]‡[email protected]

SUMAN SHRESTHAKeronite International Ltd,

Haverhill, CB9 8PJ, United [email protected]

Received 31 October 2016Revised 5 July 2017

Accepted 17 July 2017Published 25 August 2017

Adhesion strength is one of the important properties that re°ects the quality of a plasma electrolyticoxidation (PEO) coating. Scratch testing can be considered as an appropriate technique to evaluatethe adhesion strength of PEO coatings on magnesium, titanium, and aluminum substrates. Thescratch test is usually performed either under a constant or a progressively increasing normal load,where the critical load is used as a measure of adhesion strength of the coatings. In this review paper,the e®ect of di®erent factors such as duration of coating processing, electrolyte composition, andprocessing current density, as well as di®erent additives to the electrolyte bath, was studied on theadhesion strength of PEO coatings formed on magnesium, titanium, and aluminum substrates. It isunderstood that an optimum increase in process time and input energy leads to a correspondingincrease in thickness of the PEO dense oxide layer and, consequently, an increase in critical load andadhesion strength. Moreover, the electrolyte composition and additives were found to a®ect thecoating microstructure and composition and, subsequently, the coating adhesion strength.

Keywords: Adhesion strength; scratch test; plasma electrolytic oxidation.

1. Introduction

1.1. Why PEO coating

Light alloys (such as magnesium, aluminum, and ti-

tanium) are widely used in aerospace, automotive,

and medical applications because of their lightness,

high strength-to-weight ratio, good thermal and

electrical conductivities, and biocompatibility.1–5

Meanwhile, application of these alloys is limited by

some shortcomings such as poor tribological and

corrosion resistance and lack of performance data in

compliance with living tissue.6–8 To surmount these

§Corresponding author.

Surface Review and Letters, Vol. 25, No. 7 (2018) 1830004 (24 pages)°c World Scienti¯c Publishing CompanyDOI: 10.1142/S0218625X18300046

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http://dx.doi.org/10.1142/S0218625X18300046

shortcomings and improve the performance of the

surface of these alloys, various coating methods (e.g.,

ion implantation, laser surfacing, di®usion treatment,

thermal spraying, physical vapor deposition (PVD),

chemical vapor deposition (CVD) and conversion

coatings such as anodizing) are implemented.9–11

Plasma electrolytic oxidation (PEO), also known

as micro-arc oxidation (MAO) or micro-plasma oxi-

dation (MPO), is a high-voltage anodizing process

used for magnesium,12–16 aluminum,17–21 and titani-

um22–26 alloys. The mechanism of PEO ceramic

coating formation is controlled by complex chemical,

electrochemical and plasma reactions. During the

process, plasma discharge causes the formation of a

highly adhesive and dense oxide layer onto the sub-

strate.27–29 The developed oxide coating often has

good corrosion resistance,30 high hardness,31 and

good wear resistance.32 Typically, durability of the

coatings is dependent on their adhesion to the sub-

strate. Higher the adhesion strength of the coating to

the substrate, the greater the durability of the coat-

ing. An advantage of the PEO method in improving

the coating performance compared to other methods

(e.g. thermal spray and sol–gel) is the higher adhesion

strength between coating and substrate.33–38

1.2. Application of scratch test forevaluating adhesion strength

Based on the above-mentioned points, applying

a proper assessment method of coating adhesion

strength is necessary. There are several approaches

to determine the coating adhesion strength.

An assessment method of coating adhesion strength

depends on the type of coating and the substrate.39–41

Among the various methods, scratch test is one of the

quickest and most e®ective ways of determining the

adhesion strength of coatings. This method is based

on the development, progression and evaluation of a

scratch created on a coated specimen using an in-

denter with a known size and geometry. The intender

moves on a surface of the sample at a constant rate

and concurrently applies the vertical force (constant

or progressively increasing load) through an indenter

onto the coating surface in plane. This vertical force

that produces a certain amount of damage is named

as \scratch critical load" (LC). In a substrate/coating

system, one or several scratch critical load (LCN) is

de¯ned for the progressive damage surfaces of the

coating. The values of critical load can be considered

as an adhesion of the coating's internal and external

layers \cohesive strength" and adhesion of the coat-

ing with the substrate \adhesion strength".42–47

The measured coating adhesion strength is con-

trolled by the interaction between the properties of

the coating/substrate (such as hardness, fracture

strength, modulus of elasticity, microstructure, com-

position, and thickness) and test parameters (in-

cluding the properties of indenter geometric shape,

loading rate, and displacement rate). The scratch

adhesion test method can be employed to a wide

range of hard ceramic coatings such as carbides,

nitrides, oxides, diamond, and diamond-like carbon

(DLC) developed using the PVD, CVD, and oxida-

tion over the metal or ceramic.48–51 Ceramic coatings

produced in this way could be either amorphous or

crystalline but they all have generally a relatively

high density and limited porosity (

maximum force. The critical scratch load at which

certain coating damage occurs depends on the inter-

action between the properties of coating substrate

and parameters of the testing conditions.53,54

The complexity of the interaction between Rock-

well C diamond intender slipping under vertical load

and the coated sample is shown in Fig. 1. The ¯gure

demonstrates the stress types and the actual geo-

metric relationship between the diamond indenter

and a 5�m thickness coating. A careful look at this

image reveals that the scratch test is a localized

forming operation between the indenter and the

coating, leading to the creation of a compressive

Fig. 2. Main failure mechanisms and correlated types ofdamage.55

Fig. 3. (Color online) Common scratch patterns and scratch path generated on the PEO coating.

A Review on Adhesion Strength of PEO Coatings

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stress in front of the diamond and a tensile stress in its

rear side. The shear strain caused by the friction be-

tween the indenter and the coating emerges at the

contact surface. A bending stress is developed in the

margin between the deformed zones and non-deformed

ones.

Major failure mechanisms from the scratch test

are by loss of continuity in the coating and by loss of

adhesion between the coating and the substrate.

Figure 2 indicates the relationship between these

mechanisms and the damage caused by cracks and

delamination.55

Scratch atlas is used as a framework for evaluation

and analyzing di®erent damage characteristics on the

coating after the scratch test. A full description of the

scratch atlas for hard ceramic coating is given in

ASTM-C 1624. Here, an example of a PEO coating

damage feature is presented in Fig. 3 which describes

various forms of cracks on the coating surface.

It should be noted that scratch atlas is not gen-

erally enough for describing all forms of damages that

occur during scratch tests. To identify the critical

load in the scratch test, patterns from the current

scratch in references with the images of scratch path

generated on the samples were compared. Then, the

location of coating damage was ¯gured out eventually

by its superimposition with the required force to

damage and the amount of applied load equivalent to

the critical load was determined.44

In order to investigate the e®ect of each parameter,

the best way is to identify the controlling factors of

these parameters. Determining the in°uencing factors

allows examining the changes occurred in the men-

tioned parameter and its behavior with other condi-

tions as variables. This study was conducted to

investigate the adhesion strength of the oxide coatings

obtained by PEO on magnesium, aluminum, and

titanium substrates. To this goal, parameters a®ecting

the adhesion strength ofmagnesium, including coating

hardness, microstructure, composition, and thickness

were examined. Next, these factors are varied under

di®erent conditions of the PEO coating process

in order to measure the changes in their adhesion

strength. Factors a®ecting adhesion strength during

PEO processing are process time, electrolyte, additive,

voltage, and applied current. However, in some cases,

secondary processes after the coating such as thermal

treatment might a®ect the adhesion strength of coat-

ings. In this work, the e®ect of these factors on thePEO

process and the resulting e®ect on the adhesion

strength of the PEO coating on magnesium, alumi-

num, and titanium alloys are investigated.

2. Adhesion Strength of PEO Coatingby Scratch Test

2.1. A®ecting parameter on adhesionstrength of PEO coating onmagnesium

2.1.1. Processing duration

Durdu et al.56 investigated the e®ect of processing

time on the adhesion strength of PEO coatings on a

magnesium alloy AZ31. The electrolyte used in the

process consists of sodium silicate and potassium

hydroxide as key ingredients. To study the e®ect of

time, magnesium alloy was subjected to the PEO

process at the current density of 0.085A/cm2 over the

time intervals of 15, 30, 45, and 60min. After for-

mation of the oxide coating, a scratch test was per-

formed to evaluate the adhesion strength of the

specimens. It was concluded that the coating thick-

ness increases as a result of the increase in duration of

coating time that consequently leads to the increase

in critical load and ultimately the adhesion strength.

The maximum critical load was obtained at 60min

for the corresponding load of 140.82N.

2.1.2. Electrolyte

Pan et al.57 studied the e®ect of electrolyte formula-

tion on the adhesion strength of the PEO oxide

coating on the substrate of ZK60 magnesium alloy.

The applied voltage and frequency in PEO process

were maintained at 600V and 400Hz, respectively.

To investigate the e®ect of electrolyte constituents

on adhesion strength, three di®erent electrolytes

with varying compositions, each containing primarily

phosphate compounds of disodium hydrogen phos-

phate dodecahydrate (Na2HPO4�12H2O), sodiumphosphate (Na3PO4�H2O), and sodium hexameta-phosphate ((NaPO3Þ6), were used. A scratch test wasundertaken to evaluate the adhesion strength.

Loading was performed up to the maximum load of

40N with the loading rate of 4N/min over the scratch

path having a constant rate of 4mm/min. The coat-

ing thicknesses prepared in the electrolytes of Na2H-

PO4�12H2O, Na3PO4�H2O, and (NaPO3Þ6 were 22,

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25, and 27�m, respectively. Adhesion strengths

of the coatings corresponding to these samples

(Na2HPO4 � 12H2O, Na3PO4 �H2O and (NaPO3)6) are87.5, 111.4, and 127.3MPa, respectively. The coating

adhesion strength increased with increasing coating

thickness.

2.1.3. Additives

Mandelli et al.58 investigated the e®ect of additives

such as Al2O3, ZrO2, and TiO2 into the electrolyte,

on the adhesion strength of the coatings resulted

from the PEO process for AM60B magnesium alloy.

Alkaline electrolytes containing either sodium phos-

phate, sodium borate, or sodium metasilicate were

used as a primary constituent. To evaluate the e®ect

of additives, the presence and absence of additives

such as Al2O3, ZrO2 or TiO2 in the amount 4.0 g/L

was studied. PEO process was conducted in the form

of voltage control in the range of 60–160V and

a constant temperature. The process time lasted for

10 min for all samples. After obtaining the ¯nal

coating, scratch testing was carried out to evaluate

the coating adhesion strength. A load range of 0.3–

30N and Rockwell C indenter with a radius of 200�m

were applied. A 3mm long scratch was created on the

surface at the rate of 1.26mm/min. Acoustic emission

signal and SEM micrographs were analysed to eval-

uate the scratch path and determine the critical load.

Figure 4 shows the scratches developed on three

specimens. The values of critical load were calculated

based on the location of ductility developed on the

coating during the scratch path. These values for the

electrolyte containing TiO2 were signi¯cantly lower

compared with those of ZrO2 and Al2O3. The coating

adhesion strength changed accordingly with an in-

crease in coating hardness.

Moreover, in another research Pan et al.59 inves-

tigated the e®ect of KF, NH4HF2, C3H8O3, and H2O2additives on the adhesion strength of an oxide coating

obtained from the PEO process in the electrolyte

containing these additives on ZK60 magnesium alloy.

The electrolyte used in this process consisted of the

solution of sodium silicate and potassium hydroxide

as the base electrolyte. After preparation of the ¯nal

coating, the scratch test was implemented to evaluate

the e®ect of an additive on the adhesion strength.

Loading was applied to maximum 25N with a prog-

ress rate of 10mm/min over the scratch path and a

speed of 2N/min. The results of this study revealed

that, by increasing the concentration of KF, NH4HF2,

C3H8O3, and H2O2 additives in the electrolyte, criti-

cal load, and thereby adhesion strength increase be-

cause of the increase in the coating thickness. Here,

the largest critical load obtained is related to the

sample coated in the electrolyte containing the

highest concentration of NH4HF2.

2.1.4. Applied current density

Durdu et al.60 addressed simultaneously the e®ect of

an applied current density during the PEO process

and the type of electrolyte on the adhesion strength of

coatings obtained from this process on commercially

pure (CP) magnesium. Silicate solution electrolyte

comprised sodium silicate and potassium hydroxide,

while the phosphate solution electrolyte contained

sodium phosphate and potassium hydroxide. The

PEO process in both electrolytes was carried out with

three di®erent current densities 0.060, 0.085, and

0.14A/cm2, respectively. The scratch test was per-

formed on the coated Mg specimens to examine the

e®ect of the current density on the coating adhesion

strength. Loading was applied during the scratch test

Fig. 4. (Color online) SEM Image of tracks of anodic oxide produced in Al2O3, ZrO2, and TiO2 containing solution.58


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in the range of 1–175N and along a 5mm long scratch

path. The results of this study indicated that the in-

crease in the value of applied current density culmi-

nates in the rise of critical load values in both

electrolytes. In addition, the highest critical load

obtained in the phosphate-based electrolyte with the

current density of 0.140A/cm2 was 85.01N. The

values of critical load were calculated based on the

position of the damages occurred over the scratch

path and their proportion with the required force

regarding the damages. Figure 5 shows the scratch

path formed on the samples tested. According to the

obtained results, the increase in the applied current

density gives rise to the enhanced coating thickness

followed by a rise in adhesion strength of the coating-

to-the-substrate. It has to be noted that the critical

load indicates the adhesion strength between the

substrate and the coating.

The results concerning the e®ect of various factors

on the adhesion strength of PEO coating on magne-

sium substrate are summarized in Table 1.

2.2. A®ecting parameter on adhesionstrength of PEO coating onaluminum alloy

2.2.1. Duration of coating

Nie et al.71 studied the e®ect of PEO duration on

adhesion strength of coatings on 6082 aluminum

alloy. Oxide coating was prepared using the PEO

process in the range of 400–600V with a constant

frequency of 50Hz to a constant current density. The

electrolyte of the process was based on distilled water,

sodium silicate, and other additives. To obtain dif-

ferent coating thicknesses, three process times were

used in the ¯nally prepared coatings with a nominal

thickness of 100, 150, and 250�m on the surface. The

scratch test was used to study the adhesion strength

of the coatings with di®erent thicknesses. The loading

was applied at a rate of 100N/min up to a maximum

load of 100N and during the scratch path with a ¯xed

speed of 10mm/min. The results of the scratch test

showed that the critical load and accordingly adhe-

sion strength of coatings increase with a correspond-

ing increase in coating time because of the rise in

coating thickness.

2.2.2. Electrolyte

Polat et al.72 studied the e®ect of PEO electrolyte

composition on the adhesion strength of PEO coating

on 2017A aluminum alloy substrate. PEO process

was performed using an alternating current and

100 kW power supply. The electrolyte utilized in this

process is composed of potassium hydroxide and three

(0, 4, and 8 g/L) concentrations of sodium silicate.

After coating, adhesion strength was investigated

using the scratch test. In this regard, the scratch test

Fig. 5. (Color online) Optical micrograph of the coatings produced in silicate electrolyte for 30min (a) 0.060A/cm2,(b) 0.085A/cm2, (c) 0.140A/cm2 and in phosphate electrolyte for 30min by MAO method (d) 0.060A/cm2, (e) 0.085A/cm2,(f) 0.140A/cm2 after scratch test.60

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Tab

le1.

Theresultsconcerningthee®

ectof

variousfactorson

thead

hesionstrengthof

PEO

coatings

onmag

nesium

substrate.

Alloy

Objectives

ofthestudy

Con

ditionof

scratchtest

Resultsof

scratchtest

Electrolyte

Thickness(�m)

Phases

Ref.

AZ31

E®ectof

processingtime

(900

,18

00,27

00,an

d36

00s)

Progressiveload

Thecritical

load

ofcoating

increase

(from

LC¼

129:47–LC¼

140:82

N)byincreasingthe

processingtime(from

900s

to36

00s).

Sod

ium

silicate

andpotassium

hydroxide

17–56

MgO

,Mg2SiO

456

ZK60

E®ectof

di®erent

phosphateelectrolyte

(Na2HPO

4�12

H2O

,Na3PO

4�H

2O,an

d(N

aPO

3Þ 6Þ

Progressiveload

,max

imum

load

¼40

N,load

ing

rate

¼0.06

N/s.

Thebestad

hesionstrengthof

thecoatingisform

edin

(NaP

O3Þ 6

containing

electrolyte

(127

.3�1.4MPa).

Calcium

acetate

mon

ohydrate

anddi®erent

phosphates

22–27

MgO

,MgF2,

ZnO,ZnF2,

CaO

,CaF2,

Ca3(P

O4Þ 2

57

AM60

Bmag

nesium

E®ectof

thead

ditionof

ZrO

2,TiO

2,an

dAl 2O

3

Progressiveload

,0.3–30

N,

applied

scratchwith

3mm

length

Thecritical

load

forad

dition

ofTiO

2(L

C¼

15N)islower

than

additionof

Al 2O

3

(LC¼

21N)an

dZrO

2

(LC¼

22N)dueto

inferior

hardnessof

TiO

2.

Sod

ium

metasilicate

5–12

forZrO

2an

dTiO

2,7–

18forAl 2O

3

—58

ZK60

E®ectof

di®erent

additives

such

asKF,

NH

4HF2,H

2O

2,an

dC

3H

8O

3

Progressiveload

,max

imum

load

¼40

N,load

ing

rate

¼0.06

N/s

Increasingtheconcentration

ofeach

additive,

lead

toincreasingthead

hesion

strengthof

coating.

Sod

ium

silicate

andpotassium

hydroxide

15–32

MgO

,Mg2SiO

4,

MgS

iO3,

SiO

2

59

Pure mag

nesium

E®ectof

applied

current

density

(60,

850,

and

1400

A/m

2)

Progressiveload

,1–

30N

Thecritical

load

ofthecoating

increased(L

C(silicate

solution

)from

58.8Nto

83.3Nan

dLC(phosphate

solution

)from

72.3Nto

85.0N)withan

increaseinthe

applied

currentdensity

(from

0.06

A/cm

2to

0.14

A/cm

2).

Sod

ium

silicate

andsodium

phosphate

27–48

for

silicate

solution

,45–

75for

phosphate

solution

MgO

,Mg2SiO

4,

Mg3(P

O4Þ 2

60

AZ91

Investiga

tion

ofad

hesion

strengthof

PEO

coating

Progressiveload

,Rockwell

Cindenter

Based

onscratchdepth

curve

andmatchingwiththe

distance

critical

load

was

obtained

1.02

N.

—14

.7—

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Tab

le1.

(Continued

)

Alloy

Objectives

ofthestudy

Con

ditionof

scratchtest

Resultsof

scratchtest

Electrolyte

Thickness(�m)

Phases

Ref.

AZ61

Investiga

tion

ofad

hesion

strengthof

PEO

coating

Progressiveload

,1–4N,

loadingrate

¼0.06

N/s,

applied

scratchwith

6�10

�3m

length

Coh

esivestrengthbetweenthe

outeran

dinner

layersof

the

coating(criticalload

)was

estimated

to2.96

N.

Pyrophosphate–

silicate–

°uoride

5–6

—62

AZ91

D,

AZ31

B,

AM60

B,

AM50

B

E®ectof

substrate

composition(A

Z91

D,

AZ31

B,AM60

B,

AM50

B)

Progressiveload

Thecoatingwhichdeposited

atAZ91

Dsubstrate

hav

ehighestad

hesionstrength

(106

MPa).

—10–14

MgO

,MgA

lPO

5,

AlPO

4,

MgA

l 2O

4

63

ZK60

E®ectof

di®erent

concentrationratioof

calcium

and

phosphorus(C

a/P:1,

Ca/

P:3,

andCa/

P:5)

Progressiveload

Byincreasingthe

concentrationratioof

calcium

andphosphorus,the

adhesionstrengthof

the

coatingincreased.

Phosphatebase

35–65

Mg,

MgF2,

CaF

2,CaO,

MgO

,Ca3

(PO

4Þ 2

64

Mag

nesium-

based

alloys

Evaluationof

adhesion

strengthof

PEO

coating

Con

stan

tload

Theap

plied

forcefor

scratchingthePEO

coating

tothemetal

variesfrom

14.5N

to18

N.

Silicatebase

—MgO

,Mg2SiO

465

AZ31

E®ectof

anod

izing

electrolyte

(phosphate;

phosphatean

daluminate;

phosphate

andsilicate;an

dphosphate,

silicate,

andtetrab

orate)

Con

stan

tload

Thean

odizingsolution

consistingof

phosphate,

tetrab

orate,

andsilicate

producesacoatingwith

higher

critical

load

(LC¼

15:2N).

Phosphatebase

withdi®erent

additives

(silicate,

aluminate,

and

tetrab

orate)

—MgO

,MgA

l 2O

4,

Na2MgSiO

4

66

ZK60

E®ectof

electrolyte

concentration

(calcium

acetatean

dsodium

dihydrogen

phosphate)

Con

stan

tload

Byincreasingtheelectrolyte

concentration,thead

hesion

strengthof

coatingincreased

upto

95.5MPa.

Calcium

acetate

mon

ohydrate

anddisod

ium

hydrogen

phosphate

dod

ecah

ydrate

30–37

MgO

,MgF2,

ZnF2,CaO,

CaF

2,and

�-C

a3

(PO

4Þ 2

67

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Tab

le1.

(Continued

)

Alloy

Objectives

ofthestudy

Con

ditionof

scratchtest

Resultsof

scratchtest

Electrolyte

Thickness(�m)

Phases

Ref.

E21

andW

E43

E®ectof

post-treatm

ent

process

(MoS

2top

coat)

Progressiveload

,0.2N–

30N

Formationof

MoS

2topcoat

increasedthead

hesion

strengthof

PEO

coating.

Sod

ium

silicate,

sodium

phosphate,

potassium

hydroxide,

andpotassium

°uoride

26.5–34

MgO

and

Mg2SiO

4

68

AZ31

E®ectof

di®erent

concentrationsof

Na2SiO

3�5H

2O

inelectrolyte

(4an

d8g/

L)

Progressiveload

Adhesionstrengthof

PEO

coatingdecreased

with

increasingsodium

metasilicateconcentration.

Potassium

hydroxidean

dsodium

metasilicate

pentahydrate

67.7–73

.3Mg2SiO

4and

MgO

69

AZ91

Dan

dAM60

BCom

parison

between

adhesionstrengthof

PEO

coatingon

AZ91

Dan

dAM60

B

Progressiveload

Coa

tings

form

edon

AZ91

Dsubstrate

exhibited

greater

adhesionstrengththan

AM60

B.

KOH–Al 2O

3

based

alkaline

electrolyte

21.6

for

AM60

Ban

d22

.4for

AZ91

D

MgO

and

MgA

l 2O

4

70


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with progressively increasing load and an indenter

with a radius of 200�m were employed. Moreover,

scratch development was carried out with a speed of

5mm/sec. The results of the scratch test showed that

with increasing the silicate concentration in the

electrolyte coating thickness increases, as well.

Thickness increase contributes in a corresponding rise

in the critical load and consequently increasing the

adhesion strength of coatings.

2.2.3. Additives

Li et al.73 studied the e®ect of di®erent concentrations

of TiO2 added to the PEO process electrolyte on

adhesion strength of the coating made from this

process on the substrate of 6063 aluminum alloy. The

electrolyte composition used in the PEO process was

KOH and Na2SiO3 dilute aqueous solutions. TiO2nanoparticles with an approximate size of 10 nm were

dissolved in the concentrations ranges of 0.8–4 g/l in

the electrolyte. The adhesion strength was evaluated

in TiO2 concentrations of 0, 0.8, 1.6, 2.4, 3.2, and

4.0 g/l. After the PEO process in certain conditions,

the resulting adhesion strength of the coatings was

assessed using the scratch test. For this purpose, the

loading was performed at the rate of 100N/min with

a constant speed of 4mm/min. Adhesion strength of

the coatings resulting from di®erent TiO2 contents

into the electrolyte is presented in Fig. 6.

Adding TiO2 nanoparticles to the electrolyte leads

to the signi¯cantly enhanced adhesion strength values.

By increasing the amount of TiO2 up to 3.2 g/l,

adhesion strength is also shown to rise, followed by

the drop in adhesion strength by the further increase

of TiO2 values. As shown in Fig. 7, such a behavior

might be attributed to the hardness changes.

2.2.4. Bipolar pulse currents

Yerokhin et al.74 investigated the e®ect of bipolar

pulse currents in the PEO process on the adhesion

strength of the oxide coating. They prepared alkaline

electrolyte solution and test samples of 2024 alumi-

num alloy. PEO process was performed in two dif-

ferent currents mode on two samples. For the

reference sample, constant power density AC with the

frequency of 50Hz was applied. For the second sam-

ple, a bipolar pulse current with a frequency range of

0.5–10 kHz was used. After terminating the PEO

process, the scratch test was utilized to evaluate the

adhesion strength. Thus, loading occurred with the

rate of 100N/min and up to a maximum load of 100N

and passing the scratch path with the constant speed

of 10mm/min. The results obtained from the scratch

test are presented in Fig. 8.

As shown in the ¯gure, adhesion strength values of

the coating's inner and outer layers (cohesion when

the coating layer starts to chip) of both samples are

almost identical, as they are represented by the

smaller amount of critical load LC1. However, the LC2indicates that the adhesion strength of the coating/

substrate interface is di®erent for two coatings. Ref-

erence sample at maximum load of 100 N leaves no

slightest e®ect of fracture of crack in the scratch path,

while for the coated specimen at bipolar pulse cur-

rent, failure occurred at lower loads due to the pres-

sure fracture-removal and in some cases ductility in

the scratch path.

Fig. 6. Adhesion value, average friction coe±cient andmass loss of ceramic coatings prepared with di®erent TiO2nano-additive concentrations.73

Fig. 7. Average value of micro-hardness of ceramic coatingsprepared with di®erent TiO2 nano-additive concentrations.

73

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The summarized results regarding the e®ect of

various factors on the adhesion strength of PEO

coatings on the aluminum substrate are shown in

Table 2.

2.3. A®ecting parameter on adhesionstrength of PEO coating ontitanium alloy

2.3.1. Duration of coating

Durd et al.85 studied the e®ect of PEO coating du-

ration on the adhesion strength of the oxide coating

obtained from this process on Ti–6Al–4V titanium

alloy substrate. Then, under certain conditions of

current, voltage, temperature, and electrolyte, sam-

ples were processed in four periods of 20, 40, 60, and

90-min. After preparation of the oxide coating, ad-

hesion strength of the coatings was examined aided

with scratch test and a subsequent optical microscope

image. The scratch test was performed with the

progressively increasing load, in the range of 0–30N,

and scratch formation with the length of 5mm.

Figure 9 presents the distance-load graph for a period

of 20, 40, 60, and 90min. The represented including

LC1, LC2 and LC3 indicate the initial fracture due to

initial cracking, the secondary cracking due to wide-

spread cracking and ¯nal fracture due to detachment

of coating, respectively.

Based on the obtained results, the increase in the

process duration leads to an increase in the thickness

of the oxidized layer followed by adhesion strength

enhancement. In addition, the increase in process

time will increase the amount of compounds a®ecting

the adhesion strength.

2.3.2. Electrolyte

Yerokhin et al.,86 investigated the e®ect of electrolyte

on the adhesion strength of PEO oxide coating on

Ti–6Al–4V titanium alloy substrate. The electrolyte

compositions used in the PEO process were

KOH, K2SO4, Na3PO4�12H2O, Na2SiO3, and KAlO2(Table 3). After conducting PEO process and fabri-

cation of oxide coating, the scratch test was applied to

evaluate the adhesion strength. Loading was done up

to the maximum load of 100N with a loading rate of

40N/min and 100N/min and traveling scratch path

with the speed of 10mm/min. The results of the

scratch test are shown in Fig. 10, where the LC2 values

represent the force required to fracture the coating

adhesive layer. The maximum value of LC2 suggests

the force required to fracture the adhesion strength of

the coating. The maximum value ofLC2 belongs to the

coating provided in aluminate-phosphate electrolyte

(LC2(Al–P) ¼ 96N) due to a combination of highhardness and thickness of this coating.

2.3.3. Heat treatment temperature

Cheng et al.87 studied the e®ect of post heat treat-

ment temperature on adhesion strength of the PEO

coating fabricated on pure Ti sample. Samples were

treated under the same circumstances of the coating

process in terms of current, voltage, time, and elec-

trolyte. Then, they were subject to heat treatment for

one hour under air atmosphere at temperatures of

600�C, 700�C, and 800�C. Similarly, the adhesionstrength of the coating was assessed using the scratch

test and SEM micrographs. The scratch test was

carried out with constant loads of 100mN and

200mN utilizing a diamond indenter with a radius of

6�m. Figure 11 presents SEM micrographs of the

scratch on the titanium surface and the coating

obtained from coating process after heat treatment

and applied load. As it is seen, the scratch developed

on the titanium is deep and inclined, whereas the one

on the samples exposed to heat treatment is not ca-

pable of forming fracture by increasing the tempera-

ture. In other words, the adhesion strength of

coatings increases by increasing the temperature of

heat treatment. Increased heat treatment tempera-

ture leads to increasing TiO2 content and will help

increase coating strength and bonding strength at the

interface.

Fig. 8. Results of scratch adhesion tests for the coatingsproduced using di®erent PEO processes.74


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Tab

le2.

Thesummarized

resultsregardingthee®

ectof

variousfactorson

thead

hesionstrengthof

PEO

coatings

onthealuminum

substrate.

Alloy

Objectives

ofthestudy

Con

ditionof

scratch

test

Resultsof

scratchtest

Electrolyte

Thickness(�m)

Phases

Ref.

A7(F

e,0.16

%,

Si,0.16

%,

Cu,0.01

%,

Zn,0.04

%,

Ti,0.02

%,

Al,

remainder)

Investiga

tion

ofad

hesion

strengthof

PEO

coating

Progressiveload

,Loa

ding

rate

¼0.06

N/s.

Based

onscratchtest

results,

coatinghav

ego

odad

hesion

strength.

Potassium

tartrate,

sodium

°uoride

15–30

�-A

l 2O

3;�-

Al 2O

3,and

�-A

l 2O

3

32

6082

E®ectof

operationtime

(proper

timeto

achieve

coatingthicknessof

100,

150,

and25

0�10

�6m)

Progressiveload

Increasingthecoatingtime

resultsin

anincrease

inthe

thicknessof

oxidelayer

and

consequentlyincrease

incritical

load

(LC>

100N

for

coatingwithhigher

thickness).

Silicatebase

50–25

0�-A

l 2O

3,

�-A

l 2O

3,

and

Al 6Si 2O

13

71

2017

AE®ectof

sodium

silicate

concentrationin

PEO

electrolyte

(4g/

Lan

d8g/

L)

Progressiveload

,Loa

ding

rate

¼0.06

N/s.

Byincreasingthesilicate

concentration,thethickness

ofcoatingisincreased,

consequently,thecritical

load

isalso

increasedfrom

127.76

Nto

198.54

N.

Sod

ium

silicate

74–14

4�-A

l 2O

3,

�-A

l 2O

3,

and3Al 2O

3�

2SiO

2

72

6063

E®ectof

TiO

2ad

ditivein

electrolyte

(0.8,1.6,

2.4,

3.2,

and4.0g/

L)

Progressiveload

,Loa

ding

rate

¼0.06

N/s.

IncreasingtheTiO

2

concentrationuntil2.3g/

Llead

sto

increase

incoating

adhesionstrengthan

dfurther

increase

lead

toa

decreasein

coatingad

hesion

strength.

Silicateelectrolytes

—�-A

l 2O

3,

�-A

l 2O

3,

andTiO

2

73

2024

E®ectof

pulsed

bipolar

and

constan

tcurrentdensity

Progressiveload

Adhesionstrengthof

coatingin

pulsed

bipolar

current

(LC�

60N)ob

tained

lower

than

coatingin

constan

tcurrentdensity

(LC>

100N).

Silicatean

dphosphatebase

50–70

�-A

l 2O

3,

�-A

l 2O

3,

and�-Al 2O

3

74

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Tab

le2.

(Continued

)

Alloy

Objectives

ofthestudy

Con

ditionof

scratch

test

Resultsof

scratchtest

Electrolyte

Thickness(�m)

Phases

Ref.

6082

Com

parison

ofad

hesion

strengthof

PEO

and

hardan

odized

coating

Progressiveload

,max

imum

load

¼30

N,

Loa

ding

rate

¼0.83

N/s

Thecoatingob

tained

byPEO

hav

emoread

hesionin

comparison

withhard

anod

ized

coating.

AlkaliKOH

base

37forPEO

coatingan

d42

forhard

anod

ized

coating

�-A

l 2O

3and

�-A

l 2O

3

75

5052

Adhesionstrengthof

PEO

coatingan

dhard

anod

ized

coating

Progressiveload

,max

imum

load

¼20

0N,Loa

ding

rate

¼1.66

N/s

ThePEO

coatingdisplays

critical

load

sgreaterthan

thean

odized

coating.

H2SO

4forhard

anod

ized

coating

andsilicate

base

electrolyte

forPEO

coating

45.3

forhard

anod

ized

coatingan

d75

.4forPEO

coating

—76

7075

-T6

E®ectof

frequency

(50Hz

and10

00Hz)

andduty

cycle(20%

and80

%)

Progressiveload

,1–

40N

Higher

frequency

andlower

duty

cycleshow

edthe

highestscratchresistan

ceat

LC¼

28:1N.

Sod

ium

silicate

and

potassium

hydroxide

Coa

ting

thicknessat

higher

frequency

andlower

duty

cycle:

15.1

�-A

l 2O

377

2219

Com

parison

ofad

hesion

strengthof

PEO

and

hardan

odized

coating

Progressiveload

,0–

30N.

PEO

coating(L

C¼

19N)

indicates

higher

adhesion

strengththan

thehard

anod

ized

coating

(LC¼

10N).

Sod

ium

silicate

and

potassium

hydroxide

30forPEO

coatingan

d23

forhard

anod

izing

coating

�-A

l 2O

3and

�-A

l 2O

3

78

7075

E®ectof

CeO

2ad

ditivein

PEO

electrolyte

Progressiveload

,1–

40N.

Thecritical

load

sof

Al 2O

3

coatingan

dAl2O3–CeO

2

compositecoatingare21

.7an

d31

.9N,respectively.

Thesevalues

indicatethat

thecompositecoating

providemoreresistan

ceag

ainst

thestylusmov

ement

into

thecoating.

Sod

ium

silicate,

potassium

hydroxide,

and

CeO

2

�17in

silicate

electrolyte

and�2

9in

silicate/

CeO

2

electrolyte

�-A

l 2O

3and

CeO

2

79

7020

-T6

Investiga

tion

ofad

hesion

strengthof

PEO

coating

Progressiveload

,1–

25N.

Scratch

test

indicated

thego

odad

herence

ofthecoating.

Sod

ium

silicate

and

potassium

hydroxide

30�-A

l 2O

3and

�-A

l 2O

3

80


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Tab

le2.

(Continued

)

Alloy

Objectives

ofthestudy

Con

ditionof

scratch

test

Resultsof

scratchtest

Electrolyte

Thickness(�m)

Phases

Ref.

Al–12

Si–Mg–

Cu–Ni

Con

centratione®

ectof

sodium

silicate

(1.5–

3.5g/

l)an

dpotassium

hydroxide(1.5–3.5g/

l)

Progressiveload

,0.03

–30

NThecritical

load

ofcoatingwas

increasedfrom

26.0N

to28

.8N

byincrease

the

concentrationof

sodium

silicate

andpotassium

hydroxide.

Sod

ium

silicate

and

potassium

hydroxide

46–50

—81

AMg3

E®ectof

duty

cycle(6%,

12%,an

d21

%)an

dcoatingduration

(120

0s

and36

00s)

Progressiveload

,1–

15N

Thecritical

load

ofcoating

increasedfrom

5.9–9.2N

by

anincrease

oftheduty

cycle

andcoatingduration

.

NaF

,C

4H

4O

6K

2�

0.5H

2O,

Na 2MoO

4�

2H

2O,

Na 2B

4O

7�10

H2O

andNa 3PO

4�12

H2O

�3–9

�-A

l 2O

3,

�-A

l 2O

3,

AlPO

4and

Al 2Mo3C

82

2A12

(Si,Fe

0.50

%,Cu

3.8–4.9%

,Mn0.30

–

0.90

%,Mg

1.20

–1.80

%,

Zn0.30

%,

balan

ceAl)

Evaluationof

adhesion

strengthof

PEO

coating

Progressiveload

Thecritical

load

ofcoating

increasedupto

100N

byan

increase

ofthecoating

thickness.

Silicatebase

40–18

0�-A

l 2O

3and

�-A

l 2O

3

83

7075

E®ectof

dyead

ditives

onad

hesionstrengthof

black

PEO

coating

Progressiveload

of1–

40N

andthe

scratchlengthof

5mm

ataconstan

tspeedof

2.5mm/

min

Thesamplestreatedwith

additives

exhibited

higher

critical

load

sdueto

their

higher

thickness,which

wou

ldprovidehigher

load

-carryingcapacity.

Silicate-

andKOH-

based

electrolyte

20–30

�-A

l 2O

384

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2.3.4. Applied voltage

Cheng et al.88 studied the e®ect of the applied

voltage in the PEO process on adhesion strength of

the oxide coating produced by this method on pure

titanium. In the following, the samples were taken

as a substrate of pure titanium with certain

dimensions to be treated under PEO process. To

investigate the e®ect of the applied voltage, voltages

of 200, 250, 300, 350, 400, and 450V with a ¯xed

frequency of 600Hz were applied for 5min to the

samples. After formation of the oxide coating from

the PEO process, adhesion strength of the coating

was evaluated using the scratch test. In this regard,

constant loads of 100N and 200N through a dia-

mond indenter were exerted to the specimens and

the e®ect of load on the scratch path was studied

by SEM microscope. Determination of adhesion

strength at constant load was comparatively per-

formed. Generally speaking, by developing a scratch

and reviewing its path, the adhesion strength of

(a) (b)

(c) (d)

Fig. 9. (Color online) The load–distance curves and optical micrograph imaging of the PEO coatings produced at di®erentduration times: (a) 20min, (b) 40min, (c) 60min, and (d) 90min.85

Table 3. Electrolyte compositions used in the PEO process.86

Sample code S Si P Al Al–S Al–Si Al–P

Electrolytecomposition(g/L)

K2SO4:10–12

KOH: 2–4Na2SiO3: 150

Na3PO4:13–15

KAlO2:10–15

K2SO4: 3–4KAlO2: 10–15

Na2SiO3:5 KAlO2:10–15

Na3PO4:4–5 KAlO2:

25–30

Fig. 10. Derived from scratch tests the values of uppercritical loads corresponding to the ¯lm adhesion failure.86


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coatings can be considered greater than constant

load in the absence of cracks along the scratch

path. Based on the results shown in Fig. 12, mi-

crograph illustrates scratch path with respect to the

specimens with diverse voltages at the constant load

of 100mN. The ¯gure also shows that the surface of

pure titanium has a relatively deep crack with steep

edges. Despite pure titanium, in the case of the

Fig. 11. SEM Micrographs of the surfaces of the substrate and the MAO coatings after di®erent heat treatmenttemperatures with scratch test under loads of 100mN and 200mN: (a) titanium, (c) 600�C, (e) 700�C, and (g) 800�C, afterscratch test under the load of 100mN, (b) titanium, (d) 600�C, (f) 700�C, and (h) 800�C after scratch test under the load of200mN.87

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samples coated with increasing applied voltage, the

scratch becomes less deep. In addition, just in the

case of the samples with voltage 200 and 250, a

number of cracks can be observed along the path.

However, in this case, the connection of coating

with substrate does not disappear completely yet.

With respect to the constant load of 200mN, Fig. 13

indicates that except for samples of 200V and 250V

Fig. 13. SEM micrographs of the surfaces of the titanium and MAO coatings formed at various voltages after scratch testwith a load of 200mN: (a) titanium, (b) 200V, (c) 250V, (d) 300V, (e) 350V, (f) 400V, and (g) 450V.88

Fig. 12. SEMmicrographs of the surfaces of titanium and MAO coatings formed at various voltages after scratch test with aload of 100mN: (a) titanium, (b) 200V, (c) 250V, (d) 300V, (e) 350V, (f) 400V, and (g) 450V.88


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Tab

le4.

Thesummarized

resultsrega

rdingthee®

ectof

variousfactorson

thead

hesionstrengthof

PEO

coatings

onthetitanium

substrate.

Alloy

Objectives

ofthestudy

Con

ditionof

scratch

test

Resultsof

scratchtest

Electrolyte

Thickness(�m)Phases

Ref.

Ti–6A

l–4V

E®ectof

PEO

(aluminate,

phosphate,

silicate,an

dmixed)electrolytes

Con

stan

tload

Thecoatingwhich

obtained

inaluminate

solution

hav

elow

adhesionstrength.

Aluminate,

phosphate,

silicate,

andcomplex

electrolyte

60for

aluminate,

13for

phosphate,

60for

silicate

and

35for

complex

electrolyte

TiO

2,Al 2O

3,and

TiA

l 2O

2for

aluminate,

TiO

2

andTiA

l 2O

2for

phosphate,

and

TiO

2an

dam

orphou

sphase

forsilicate

complex

electrolyte

23

Ti–6A

l–4V

E®ectof

treatm

enttime

(120

0,24

00,36

00,an

d54

00s)

Progressiveload

Increasingthetreatm

ent

timelead

toincrease

incoatingthickness

andthereforeincrease

inad

hesionstrength

ofcoating.

Calcium

acetatean

d�-calcium

glycerophosphate

28–52

TiO

2(rutile

and

anatase),TiP

2,

Ca3(P

O4Þ 2,

Hydroxyapatite,

andCaT

iO3

85

Ti–6A

l–4V

E®ectof

di®erentPEO

electrolyte

(sulfate,

silicate,phosphate,

aluminate,

aluminate/

sulfate,

aluminate/

silicate,aluminate/

phosphate)

Progressiveload

,max

imum

load

¼10

0N,

Loa

ding

rate

¼1.66

N/s.

Thehighestcritical

load

isob

tained

inaluminate/phosphate

(LC¼

96N)sample

dueto

compactPEO

coating.

Aluminate,

phosphate,

silicate,

andsulfatean

ions

50–60

inaluminate/

phosphate

electrolyte

and60–90

insilicate

electrolyte

TiO

2(anataseand

rutile)an

dAl 2TiO

5

86

CPTi

E®ectof

heattreatm

ent

temperature

(400

� C,

500� C

,60

0� C

,70

0� C

,an

d80

0� C

)

Con

stan

tload

Increasingtheheat

treatm

enttimelead

toincrease

inTiO

2

contentin

thecoating

andconsequently

increase

inad

hesion

strength.

Silicatebase

——

87

CPTi

E®ectof

applied

voltage

(200

,25

0,30

0,35

0,40

0,an

d45

0V)

Progressiveload

Byincreasingthe

applied

voltage,more

TiO

2isform

ed,

therefore,

adhesion

strengthisincreased.

Silicatebase

—TiO

2(anatase)

88

H. Shari¯ et al.

1830004-18

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Tab

le4.

(Continued

)

Alloy

Objectives

ofthestudy

Con

ditionof

scratch

test

Resultsof

scratchtest

Electrolyte


Ref.

Ti–6A

l–7N

bInvestiga

tion

ofad

hesion

strengthof

PEO

coating

Progressiveload

Thecritical

load

ofcoatingwas

obtained

14N.

(CH

3COO) 2CaH

2O

andsodium

phosphate

2.7–

32.6

TiO

2(anatase

and

rutile)

89

CPTi

(grade2)

E®ectof

immersion

inSBF

electrolyte

(0;6:048

�10

5,an

d

2:59

2�10

6s)

onad

hesionstrengthof

coating

Progressiveload

Adhesionstrengthis

decreased

by

increasingthe

immersion

timein

SBF.

Sod

ium

silicate,

Ca-�-

glycerophosphate,

Si(CH

3COO) 4,

andNaO

H

Upto

200

TiO

2(anatase

and

rutile)

90

Ti–6A

l–4V

Investiga

tion

ofad

hesion

strengthof

PEO

coating

Progressiveload

Thecritical

load

ofTiO

2

coatingwas

obtained

equal

to8N.

Phosphatebase

10–20

TiO

2(anatase)

91

TNZS

E®ectof

heattreatm

ent

(withan

dwithou

t)Progressiveload

Thecritical

load

was

obtained

equal

to18

.6N

beforeheat

treatm

entan

d21

Nafterheattreatm

ent.

Calcium

acetate

0.5–3.5

TiO

2(rutile

and

anatase)

and

Nb2O

5

92

Ti–6A

l–4V

Com

parison

between

adhesionstrengthof

DLC

coatingan

dDLC/

PEO

two-layer

coating

Progressiveload

Adhesionstrengthof

DLC/P

EO

coatingis

much

morethan

PEO

coating.

Sod

ium

silicate,

disod

ium

phosphate,

and

potassium

hydroxide

10TiO

2(rutile

and

anatase)

93

Ti6Al4V

Evaluationof

adhesion

strengthof

hydroxyap

atitecoating

onMAO

coating

Progressiveload

Thehighestcritical

load

(LC�

14N)was

obtained

atHA/P

EO

coatingwherethe

relativelydense

hydroxyap

atite

crystalswereform

ed.

Calcium

acetate

mon

ohydrate

and

Calcium

�-

glycerophosphate

10TiO

2(rutile

and

anatase)

94

Ti

Evaluationof

adhesion

strengthof

PEO

coating

—Thecritical

load

value

variesfrom

27.8

to33

.2N.

Calcium

acetatebase

5–40

TiO

2(rutile

and

anatase)

andSiO

2

95


1830004-19

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Tab

le4.

(Continued

)

Alloy

Objectives

ofthestudy

Con

ditionof

scratch

test

Resultsof

scratchtest

Electrolyte


Ref.

Ti

Com

parison

between

adhesionstrengthof

shot

blastingþ

MAO

coating

andMAO

coating

Progressiveload

Thecritical

load

valuein

shot

blastingþ

MAO

coating(�

400mN)

was

threetimes

higher

than

inMAO

coating

(135

mN).

Calcium

acetate

mon

ohydrate

and

sodium

phosphate

dehydrate

0.67

–3.6

TiO

2(rutile

and

anatase)

and

Al 2O

3

96

Ti–6A

l–4V

E®ectof

charge

density

(15,

45,90

,an

d

135�10

5C/m

2)

Progressiveload

,0–

100N,aload

ing

rate

of10

0N/m

in

Thevalueof

critical

load

increase

with

increasingcharge

density.

Aluminatebase

38–65

for

max

imum

charge

density

Al 2TiO

5an

dTiO

2

(rutile

and

anatase)

97

Ti–15

Mo,

Ti–

13Nb-13Z

r,Ti–6A

l–7N

b

E®ectof

substrate

composition(T

i–15

Mo,

Ti–13

Nb–13

Zr,Ti–6A

l–7N

b)

Progressiveload

,0.03

–20

NCoa

tingfabricatedon

Ti–13

Nb–13

Zrhas

highestvalue

ofcritical

load

(�13

.10N).

Ca(H

2PO

2Þ 2

andCa3

(PO

4Þ 2

17.8–34

.9for

Ti-15

Mo,

18.9–25

.4for

Ti–13

Nb–

13Zran

d22

.7–28

.1for

Ti–6A

l–7N

b

TiO

2(anatase)and

hydroxyapatite

98

Ti6Al4V

and

Cp–Ti

E®ectof

post-treatm

ent

process

(MoS

2topcoat)

anddi®erentelectrolyte

(silicate–phosphatean

daluminate–phosphate)

Progressiveload

,0-30

N.

Max

imum

critical

load

ofcoatingwithou

ttop

coat

was

obtained

inaluminate–phosphate

solution

(LC¼

12N)

andcoatingwithtop

coat

was

obtained

insilicate–phosphate

solution

(LC>

30N).

Silicate–phosphate

andaluminate–

phosphate

14.1

and9.9for

coatings

obtained

inAluminate–

phosphate

andsilicate–

phosphate

electrolyte

respectively

TiO

2(rutile

and

anatase)

99

CPTi

Investiga

tion

ofad

hesion

strengthof

PEO

coating

andhydroxyap

atite

Progressiveload

,¯nal

load

of10

0N

Thecritical

load

ofap

atitelayer/

TiO

2layer

and

TiO

2layer/substrate

was

obtained

26.5N

and48

.5N,

respectively.

Calcium

acetatean

dmon

osod

ium

orthop

hosphate

7.71

forap

atite

layer

and

6.44

for

TiO

2layer

TiO

2(rutile

and

anatase)

and

hydroxyapatite

100

H. Shari¯ et al.

1830004-20

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se o

nly.

Tab

le4.

(Continued

)

Alloy

Objectives

ofthestudy

Con

ditionof

scratch

test

Resultsof

scratchtest

Electrolyte


Ref.

CPTi

E®ectof

frequency

(50Hz

and10

00Hz)

andduty

cycle(10%

and95

%)

Progressiveload

,1–

40N

PEO

coatingfabricating

under

highduty

and

highfrequency

show

edahigher

Lc

(26N).

Trisodium

orthop

hosphate

andpotassium

hydroxide

4–9

TiO

2(rutile

and

anatase)

101

CPtitanium

Investiga

tion

ofad

hesion

strengthof

PEO

coating

Progressiveload

,¯nal

load

50N

Thecritical

load

ofcoatingwas

obtained

equal

to36

N.

Asolution

ofthe

phosphatesalt

�20

TiO

2(rutile

and

anatase)

102

Ti–6A

l–4V

E®ectof

hydroxyap

atite

additivein

PEO

electrolyte

Progressiveload

,0.9–30

NThead

ditionof

hydroxyap

atitein

electrolyte

increased

thecoatings'scratch

resistan

cefrom

8.0N

to14

.3N.

Disod

ium

hydrogen

phosphatean

dhydroxyap

atite

7.5–9.5

TiO

2(rutile

and

anatase)

and

calcium

phosphate

103


1830004-21

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no crack or ductility is developed on the scratch path

of other specimens and minor damages on the scratch

path for the samples of 400 and 450 V are also dated

back to smooth surface of scratch path. Based on

the consequences, the increasing applied voltage in

PEO process would cause the enhanced adhesion

strength of coatings, as the voltage increase, signi¯-

cant amount of TiO2 are formed in the coating

composition.

The summary of the results concerning the impact

of various factors on the adhesion strength of PEO

coating on titanium substrate is shown in Table 4.

3. Conclusion

A summary of this review is as follows:

(1) Generally, the thickness of the oxide layer increases

with increasing the coating duration. The increase

in coating thickness results in a rise in critical load

and thus increasing the adhesion strength.

(2) The electrolyte composition andadditives a®ect the

coating structure. Coating structure also a®ects the

hardness and thereby its adhesion strength.

(3) In general, by varying the input energy, thickness

and composition the of coating were a®ected and

consequently, adhesion strength of the coating

was changed.

(4) Post-PEO treatment steps lead to the enhanced

adhesion strength by improving the microstruc-

ture of the coatings.

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A REVIEW ON ADHESION STRENGTH OF PEO COATINGS BY SCRATCH TEST METHOD1. Introduction1.1. Why PEO coating1.2. Application of scratch test for evaluating adhesion strength

2. Adhesion Strength of PEO Coating by Scratch Test2.1. Affecting parameter on adhesion strength of PEO coating on magnesium2.1.1. Processing duration2.1.2. Electrolyte2.1.3. Additives2.1.4. Applied current density

2.2. Affecting parameter on adhesion strength of PEO coating on aluminum alloy2.2.1. Duration of coating2.2.2. Electrolyte2.2.3. Additives2.2.4. Bipolar pulse currents

2.3. Affecting parameter on adhesion strength of PEO coating on titanium alloy2.3.1. Duration of coating2.3.2. Electrolyte2.3.3. Heat treatment temperature2.3.4. Applied voltage

3. ConclusionReferences

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