Journal of Alloys and Compounds 636 (2015) 156–164

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Journal of Alloys and Compounds

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Microstructure and micromechanical properties of electrodepositedZn–Mo coatings on steel

http://dx.doi.org/10.1016/j.jallcom.2015.02.1650925-8388/� 2015 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +48 12 2952812; fax: +48 12 2952804.E-mail address: h.kazimierczak@imim.pl (H. Kazimierczak).

Honorata Kazimierczak a,⇑, Piotr Ozga a, Katarzyna Berent b, Marcin Kot c

a Institute of Metallurgy and Material Science, Polish Academy of Sciences, 30-059 Krakow, Reymonta 25, Polandb Academic Centre for Materials and Nanotechnology, AGH University of Science and Technology, 30-059 Krakow, Mickiewicza Av. 30, Polandc Faculty of Mechanical Engineering and Robotics, AGH University of Science and Technology, 30-059 Krakow, Mickiewicza Av. 30, Poland

a r t i c l e i n f o

Article history:Received 3 October 2014Received in revised form 9 February 2015Accepted 22 February 2015Available online 26 February 2015

Keywords:Zinc–molybdenum alloyCoating materialsElectrodepositionMicrostructureTopographyMicrohardness

a b s t r a c t

The aim of the work was to characterise the new coating material based on zinc with the addition ofmolybdenum, electrodeposited on steel substrate from nontoxic, citrate based electrolytes. The surfacecomposition of deposits was ascertained by chemical analysis (WDXRF). The morphology of coatingswas studied by SEM. The surface morphology and roughness of Zn–Mo coatings on steel was investigatedby AFM. The microhardness and Young modulus were determined by indentation technique, whereas thecoating adhesion to the substrate was examined by means of scratch test. The optimal ranges of elec-trodeposition parameters, enabling the preparation of good quality coatings (i.e. uniform, compact, withgood adhesion to the substrate), was specified. The morphology of deposits depends significantly on thecontent of molybdenum and on the thickness of electrodeposited layer. The microhardness of Zn–Mocoating increases with the increase of molybdenum content up to 3 wt.% and then reaches about3.5 GPa, which is almost five times that of the value of the microhardness of the Zn coating studied.

� 2015 Elsevier B.V. All rights reserved.

1. Introduction

Durability and reliability of constructions and parts of machinesand devices in operation are closely related to the gradually pro-gressing processes of corrosion damage and mechanical wearwhich take place on the subsurface areas of materials. An effectiveway of reducing materials destruction is covering them with layerof a different material with suitable properties. By a proper selec-tion of the coating material advantageous mechanical propertiesand corrosion protection ability can be provided.

Protective layers based on zinc are one of the cheapest and wide-ly used protective coatings. They are used in the automotive indus-try, machine construction, building industry, household appliances,and many other areas, with zinc and zinc alloys being used mainlyas anticorrosive coatings on steel. The additional significantincrease in the corrosion resistance of zinc based layers was widelyachieved by applying chromate conversion layers based on Cr(VI),on such coatings [1–3]. Hexavalent chromium is unfortunatelyhighly toxic material, which may have genotoxic effects and couldlead to carcinogenesis. An interesting material with improved cor-rosion resistance are zinc based coatings with the additives of

molybdenum uniformly distributed in the whole volume of the lay-er [4,5], which may be an alternative to chromate films on zinc,because molybdenum is much less toxic and there are no genotoxiceffect associated with its use [6]. Moreover, it is commonly knownthat the addition of molybdenum improves abrasion hardness,toughness and corrosion resistance of alloys [7]. It has already beenproved that the addition of Mo has a great effect on the corrosionbehaviour of zinc and zinc alloys [8–12]. The protective action ofsuch coatings lasted a minimum of approximately twice as longin comparison with ordinary zinc coatings.

However, because of great differences in the melting and boil-ing temperatures of zinc and molybdenum, and also the very lim-ited solubility of molybdenum in zinc [13], the preparation of Zn–Mo alloys by conventional thermal methods is very difficult. Henceelectrodeposition can be considered as a way to obtain such alloys,and it is a relatively simple and low-cost method of producingcoatings. Moreover, the electrochemical deposition is one of themost commonly used methods for metal and metallic alloy coatingpreparation in many technological processes. This method useselectrical current to reduce dissolved metal cations so that theyform a coherent layer. While, the method of coating formation isthe basic factor determining its structure and properties. Animportant characteristic of any coating is also the distribution ofthe film over the substrate, and its adherence to the substrate,

Table 1Chemical composition of electrolytic baths used.

No. Electrolyte composition, pH = 5 + 0.5 g/dm3 PEGb + 0.05 g/dm3 SDSc

C6H5Na3O7�2H2O (M) a ZnSO4�7H2O (M) Na2MoO4�2H2O (M)

A1 0.25 0.16 0A2 0.25 0.16 0.02A3 0.25 0.16 0.03A4 0.25 0.16 0.05A5 0.25 0.16 0.07A6 0.25 0.16 0.09A7 0.25 0.16 0.24

a Denoted in text as Na3Hcit (where cit = C6H4O7).b PEG-3000 – polyethylene glycol 3000 (molecular mass between 2700 and

3300).c SDS – dodecyl sulphate sodium salt.

H. Kazimierczak et al. / Journal of Alloys and Compounds 636 (2015) 156–164 157

which is governed by the chemical composition and microstruc-ture of the coating and the substrate [14]. The coating applicationby electrodeposition method is carried out at or near room tem-perature. This ensures that the properties of substrates are notchanged during the process, and intermetallic compounds are notformed at the substrate-coating interface. Thus it can be statedgenerally that electrodeposited coatings have lower adhesivestrength than the coatings produced by hot dipping. However theelectrodeposition method offers a series of advantages. The mainadvantage of electrolytic coatings of zinc and zinc alloys is thatcoatings of uniform and precisely defined thickness can be appliedon one or both sides of the product. Hence the electrodepositedzinc-based coatings may be applied instead of hot dip galvanisedlayers to improve surface finishing and for finer control of steelproduct dimensions [15,16].

However, it is known that molybdenum cannot be electrode-posited in a pure state from an aqueous solution. It requires thepresence of another metal, which causes molybdenum co-deposi-tion [17]. Various hypotheses have been proposed to explain thisprocess [18]. In all of these theories iron group metals play animportant role, as they may act as catalysts in the formation of

Fig. 1. Effect of total charge passed during electrodeposition, on the content of molybdenfor their co-deposition, (b) current efficiency of the electrodeposition process and (c) MElectrolyte ‘A2’, ‘A4’, ‘A6’, ‘A7’, E = �1.4 V vs. SCE (‘A2’, ‘A4’), E = �1.3 V vs. SCE (‘A6’, ‘A7

intermediate products. Nevertheless, the possibility of zinc elec-trodeposition with small quantities of molybdenum has beenproved [9,19–23] but the maximal content of molybdenum in thealloy was significantly lower than in deposits of molybdenum withiron-group metals, and it was suggested that molybdenum addi-tives occur in the form of oxides in the deposits. Moreover themicrostructure and micromechanical properties of such Zn–Molayers have not been described. Except the work of Vyacheslavov[24] which claimed that microhardness of the Zn–Mo depositincreases from 50 to 320 MPa when the Mo content is increasedfrom 0% to 9%. Recently the work of Kazimierczak and Ozga provedthat zinc actually induces metallic molybdenum co-deposition innontoxic citrate electrolytes [4,25]. The ranges of electrodepositionparameters, enabling the preparation of coatings with various con-tent of molybdenum have been determined [5]. The mechanism ofthe co-deposition of molybdenum with zinc has been described.XPS measurements confirm the presence of metallic molybdenumin obtained deposits [4]. XRD analysis indicated that two Zn–Mophases are formed in deposits, a hexagonal phase with a lowmolybdenum content (less than 1 wt.%), and an amorphous ornanocrystalline phase, enriched in molybdenum [5]. However thedetailed study of the surface morphology and the basic microme-chanical properties of the electrolytic Zn–Mo coatings have notso far been described.

The purpose of this work was to investigate the effect of elec-trodeposition parameters and molybdenum content in Zn–Mo lay-ers on their microstructural and micromechanical properties, andhence to determine the optimal conditions enabling the prepara-tion of good quality coatings (i.e. uniform, compact, with goodadhesion to the substrate).

2. Experimental

2.1. Electrolytic bath

Zn–Mo coatings with various molybdenum content were electrodepositedfrom aqueous citrate baths. The plating baths were prepared by dissolving ofsodium citrate in deionised water (18.2 MX cm�1), followed by the addition of

um in deposits. (a) The partial current densities of molybdenum, zinc and hydrogeno content in deposits as a function of total charge passed during electrodeposition.’), x = 16 rad/s, T = 20 �C.

x 10 000 x 50 000

(a) Zn

(b) 0.5% Mo

(c) 1.0% Mo

(d) 1.5% Mo

(e) 2.5% Mo

(f) 3.1%Mo

(g) 6.9%Mo

Fig. 2. SEM images (SE) of Zn–Mo coatings with various content of molybdenum deposited on steel substrate from electrolytes: ‘A1–A7’, E = �1.3 V vs. SCE, Q = 50 C,x = 16 rad/s, T = 20 �C. Thickness (b–g) from 3 to 4 lm and (a) 0.2 lm.

158 H. Kazimierczak et al. / Journal of Alloys and Compounds 636 (2015) 156–164

zinc sulphate, sodium molybdate, polyethylene glycol (PEG 3000) as a levellingagent and sodium dodecyl sulphate (SDS) as an agent improving the electrolyteswettability. The solution pH was adjusted to 5 by the addition of sulphuric acid.All chemicals used in this work were of analytical grade. The chemical composi-tions of the electrolytes studied are given in Table 1.

2.2. Electrochemical cell

Electrochemical deposition of Zn and Zn–Mo layers was carried out in a 50 cm3

cell, in a system with a rotating disc electrode (RDE) to ensure constant and con-trolled hydrodynamic conditions. A low-carbon steel disc was the working elec-trode (surface area 2.83 cm2) and a Pt sheet was used as the counter electrode(surface area 3.5 cm2). The working electrode potentials were referred to thesaturated calomel electrode (SCE) and were corrected for ohmic drop (CI method).

2.3. Substrate preparation

Steel electrodes were chemically polished using a solution of oxalic acid and a30% solution of hydrogen peroxide (mixture of 14 ml oxalic acid 100 g/dm3 with2 cm3 30% H2O2 and 40 cm3 H2O in 40 �C).

2.4. Chemical composition study

The composition of deposits was determined by wavelength dispersive X-rayfluorescence (WDXRF). The WDXRF analysis was determined via a Rigaku Priminispectrofluorimeter using scintillation counters (LiF crystal).

(a) (b) (c)

BSE

SE

Fig. 3. Comparison of BSE and SE SEM images of selected Zn–Mo coatings presented on previous figure. (a) 1.0% Mo, (b) 1.5% Mo and (c) 3.1% Mo.

H. Kazimierczak et al. / Journal of Alloys and Compounds 636 (2015) 156–164 159

2.5. Morphology characterisation

The morphology of the resulting coatings was studied by scanning electronmicroscopy (SEM). The SEM analysis of the surface of the coatings was performedusing the FEI Quanta 3D FEG scanning electron microscope with acceleration volt-age 20 kV.

2.6. Topography and roughness

Topography and roughness of selected Zn–Mo coatings were studied by the tap-ping mode AFM technique, using an Innova Atomic Force Microscope (AFM),equipped with a scanner of maximum ranges of 90 lm in the x and y directionsand 8 lm in the z direction. Antimony doped silicon probes were used (nominalprobe curvature radius of 10 nm, force constant of 42 N/m). Images were acquiredat a resolution of 512 � 512 points and were subjected to first-order flattening. Thesoftware package NanoScope Analysis was used for AFM image processing.

2.7. Micromechanical properties of deposits

The microhardness and elasticity modulus of selected Zn–Mo layers was exam-ined by the instrumented indentation technique, according to the PN-EN ISO14577-1 standard using the Micro-Combi-Tester produced by CSEM Instruments.The measurements were performed with a Vickers diamond indenter (for eachcoating at least seven indentations) under the load of 20 mN applied at 40 mN/s.The duration at maximum load was five seconds.

2.8. Adhesion studies

The Zn–Mo coatings’ adhesion to steel substrate was studied by scratch testing,using Micro-Combi-Tester CSEM apparatus and a standard conical Rockwell Cindenter with a 200 lm tip radius. Scratches were performed with a maximal load30 N, 5 mm length and 5 mm/min scratch speed. Research and analysis of theresults was conducted in accordance with the PN-EN 1071-3 standard.

3. Results and discussion

3.1. The effect of charge passed

The extraction of partial polarisation curves for alloy compo-nents is a common method of studying alloy electrodeposition pro-cess. The total polarisation curve characterises a complex ofphenomena occurring at the cathode, and does not give informa-tion about the process of discharge of individual metal ions, hence,in the present work, the method for resolving the total polarisationcurve of alloy deposition into partial polarisation curves was used,which allows determining the rate of deposition of parent metalsand the rate of hydrogen evolution. For this purpose, the currentefficiency and partial current densities were calculated from che-mical analysis of deposits electroplated under selected conditions,the charge passed and the mass of deposits, using Faraday’s law.The values of current density are presented in accordance with

IUPAC convention (the increase or decrease of current density val-ue discussed in the present study refers to its modulus).

The effect of the total charge passed during electrodeposition onthe content of molybdenum in deposits was studied in the fourchosen electrolytes containing various levels of sodium molybdate.Fig. 1 shows the dependence of partial current of Zn, Mo and H2 fortheir co-discharge (a), current efficiency (b) and molybdenum con-tent in deposits (c) on the total charge passed during the electrode-position process. Zn–Mo coatings were deposited on steelsubstrates at two chosen potentials. The value of the charge passedduring the electrodeposition was varied from 15 C to 100 C.

The partial current densities of molybdenum reduction do notchange considerably when the charge passed increases (Fig. 1a).Only when considering electrodeposition from electrolyte ‘A7’,with a relatively high concentration of sodium molybdate, a slightincrease of partial current density values can be observed with theincrease of the value of charge passed. However, in such conditionszinc and hydrogen partial current also grow slightly with theincrease of total charge passed. Likewise, the partial current densi-ties of zinc reduction from all other types of electrolytes studiedincrease slightly when the charge passed is increased. No singlegeneral dependence of hydrogen partial current densities on thecharge passed during electrodeposition can be determined, andthe changes of H2 partial current observed when changes are madeto the charge passed are relatively very slight in for all four poten-tials applied.

Nevertheless, it can be easily noticed that the content of molyb-denum in deposits does not depend considerably on the chargepassed (Fig. 1a). It depends only on bath composition and on thepotential applied in the studied conditions. Neither does currentefficiency change significantly with the increase of the value ofcharge passed (Fig. 1b). Hence it can be stated that the depositionprocess of Zn–Mo layers from the investigated bath is relativelystable across the whole range of the applied charge duringdeposition.

3.2. The effect of Mo content in deposits on their morphology

Fig. 2 presents the SEM images of the morphology of the surfaceof Zn–Mo coatings electrodeposited on steel substrates, at thesame potential E = �1.3 V vs. SCE, but from the electrolytes con-taining various concentrations of molybdate ions: Each samplemorphology is observed at low magnification (10,000�) and athigh magnification (50,000�).

It can be seen that each deposit’s morphology depends sig-nificantly on the content of molybdenum. Zinc layer without

(a) Q=15C

(b) Q=30C

(c) Q=50C

(d) Q=100C

(e) Q=150C

(f) Q=200C

Fig. 4. SEM images (SE) of Zn–Mo coatings with content of molybdenum about 1.7 wt.%, deposited on steel substrate from electrolyte ‘A5’, at E = �1.3 V vs. SCE, with variousvalues of charge passed during electrodeposition, x = 16 rad/s, T = 20 �C.

160 H. Kazimierczak et al. / Journal of Alloys and Compounds 636 (2015) 156–164

molybdenum additives is characterised by a needle-like structure(Fig. 2a). While, when molybdenum content is relatively low (from0.5 to 1.5 wt.%) the coating obtained are characterised by an angu-lar platelet structure, which is much finer when molybdenum con-tent is increased from 0.5 to 1.5 wt.%. Moreover, in the case of adeposits containing 1.0 wt.% and 1.5 wt.% of Mo, some relativelylarge, elongated plate particles appear on the surface of the fine-grained compact layer (Fig. 2c and d). When 2.5 wt.% of

molybdenum is present in deposit it appears even more fine-grained, but the coating surface becomes undulated (Fig. 2e).Next, in the case of electrodeposition from baths containing0.09 M Na2MoO4, the surface morphology consist of relatively bignodules, built of very fine grains, which grow on a flat, but muchmore coarse-grained surface (Fig. 2f). Next, in the case of coatingscontaining the highest molybdenum contents considered here, thedeposit exhibit a complex, granular microstructure.

0

-0,5-0,4-0,3-0,2-0,10,0

Charge (C)

i[A/d

m2] Z

n

-0,05-0,04-0,03-0,02-0,010,00

i[A/d

m2 ] M

o

-1,0-0,8-0,6-0,4-0,20,0

i[A/d

m2 ] H

20

50 100 150 200

50 100 150 2000,0

0,5

1,0

1,5

2,0

2,5

3,0

Charge (C)

Mo

cont

ent (

wt.%

) Mo content (wt.%) current efficiency (%)

0

20

40

60

80

100(a) (b)

Fig. 5. Effect of total charge passed during electrodeposition, on the content of molybdenum in deposits. (a) The partial current densities of molybdenum, zinc and hydrogenfor their co-deposition, (b) Mo content in deposits and current efficiency of the electrodeposition process as a function of total charge passed during electrodeposition.Electrolyte ‘A5’, x = 16 rad/s, T = 20 �C.

H. Kazimierczak et al. / Journal of Alloys and Compounds 636 (2015) 156–164 161

Moreover, the BSE SEM image (Fig. 3a–c) of the complex surfacemorphologies discussed above (Fig. 2c, d and f) shows that thereare no composition and materials density changes in the structuresobserved. The considerable contrast that can be noted on SE SEMimages when observing such structures is caused only by sig-nificant differences in the height of particles on deposits.

The changes in the structure and morphology of the Zn–Mocoatings surface, along with increasing molybdenum content, canbe explained by the fact that the process of molybdenum co-depo-sition with zinc does not always proceeds fully accordingly to themechanism described in previous work [4]. The proposed mechan-ism describes an ideal case in which molybdate ions are fullyreduced to metallic molybdenum. While in many instances, thepresence of molybdenum oxides in deposits containing more than3 wt.% Mo was also registered. Depending on the electrodepositionparameters, the formed Zn–Mo layers exhibit more metallic oroxide character. In general, it can be stated that the highest is con-centration of molybdenum ions in electrolytic bath, the highest iscontent of molybdenum deposited in Zn–Mo layer, from such bath.Coatings containing more than 3 wt.% of molybdenum are dull anddark grey, which fact is associated with the formation of mainlymolybdenum oxides in such coatings. During the process of elec-trocrystallization of Zn–Mo layers from a bath containing relativelyhigh concentration of molybdenum ions, the formation of molyb-denum oxides can occur more easily on the coating surface. Thisconsequently results in a partial blocking of the active surface areaof the cathode, resulting in decreased rate of electrodeposition onsuch areas, which next may result in a more developed surfacemorphology of the coatings containing relatively high concentra-tion of molybdenum. Hence the formation of a complex structureon the surface of sample Zn–Mo3.1% (Fig. 2f), may be explainedby the fact that the blocking of the parts of cathode surface causesa preferential deposition of Zn–Mo coating on the not blocked sur-face areas which leads to the formation of structure consisting ofthe large nodular particles spread on the relatively flat surface.Next, during the electrocrystallization from the bath containinghigher concentrations of molybdenum ions, and thus when coat-ings containing higher amounts of Mo are deposited, the cathodepartial blocking by molybdenum oxides occurs on the larger sur-face area, than in the case of electrodeposition of Zn–Mo3.1% layer.

Therefore the smaller are the fully active areas, on which notimpaired electrodeposition process takes place. This leads to theformation of a granular structure which exhibit more ‘‘fine-grained’’ character (Fig. 2g) as compared to layer containing about3 wt.% of molybdenum.

3.3. The study of the layer growing

Fig. 4 shows the dependence of the surface morphology ofselected Zn–Mo layers on the total charge passed during the elec-trodeposition process. Deposits were obtained from bath ‘A5’, atthe same rotation rate (x = 16 rad/s), at potential E = –1.3 V vs.SCE. The value of the charge passed was varied from 15 C to200 C. For a thorough analysis, the surface of each sample is pre-sented at three magnifications (5000�, 25,000� and 100,000�).

It can be easily noticed that relatively thin coatings, obtainedwhen the charge passed did not exceed 30 C, are compact and uni-form, with a very fine-grained structure (Fig. 4a and b). Then, withan increase in the charge passed, some flake-like particles protrud-ing from the compact layer are formed (Fig. 4c). This flake-likestructure grows preferentially when the charge passed increasesfrom 50 C to 200 C, and it forms relatively big agglomerates of flakes(Fig. 4d–f). The formation of such flake-like structures and theirpreferential growth suggests that some partial blocking of the elec-trode surface occurs during the electrodeposition process, resultingin the formation of jutting flakes on non-occupied areas of the elec-trode. However, the smooth and fine-grained layer also grows andchanges its structure slightly as the charge passed is increased,which can be seen on the images with the highest magnification.

The analysis of the effect of total charge passed on themicrostructure of obtained coatings is further compared to thedependence of partial current of Zn, Mo and H2 for their co-dis-charge (Fig. 5a), current efficiency, and molybdenum content indeposits (Fig. 5b) on the total charge passed during the electrode-position of the studied layers. Hence it can be noticed that the par-tial current of zinc and molybdenum co-deposition decreases veryslightly with the increase of the total charge passed, while the H2

partial current density increases slightly. This is associated withthe decrease of current efficiency from 38% at 15 C to about 33%at 200 C. However, any changes in composition of the deposits, at

162 H. Kazimierczak et al. / Journal of Alloys and Compounds 636 (2015) 156–164

the macroscopic level, with an increase in the thickness and struc-ture changes, have not been observed. The content of molybdenumin deposits maintains a relatively constant level of 1.7 wt.%. Hence,in the studied case, only the surface morphology of the layer varieswith the increasing value of the charge passed, while the chemicalcomposition of deposits is independent of the charge passed in theinvestigated range. Moreover BSE SEM observation of the flake likeparticles formed on the surface of Zn–Mo samples with variousmolybdenum content, discussed above (Fig. 3) clearly confirmsthat there are no significant differences in contrast of chemicalcomposition and density of the ‘‘matrix’’ and flake-like particlesformed. However, the occurrence of the flake-like structures andtheir relatively rapid growth during the electrocrystallization pro-cess is unfavourable factor. Due to the different structures of the‘‘matrix’’ and the growing particles, the different properties of the-se two types of structure can be expected. Hence this results indi-cate that, at the current level of advancement of research, it ispossible to deposit a relatively smooth and homogeneous Zn–Mocoating only up to a certain thickness (ca. 3–6 lm), before the

(a) Zn (-1.3 V)

(b) Zn (-5A/dm2)

(c) 0.5% Mo

(d) 1.5%Mo

(e) 3.1%Mo

(f) 6.9%Mo

Fig. 6. AFM images of Zn–Mo coatings with various content of molybdenumdeposited on steel substrate from electrolytes with the addition of polyethyleneglycol. Electrolytes: ‘A1’, ‘A2’, ‘A4’, ‘A6’, ‘A7’, x = 16 rad/s, T = 20 �C, (a) and (c–f)E = �1.3 V vs. SCE, (b) i = 5 A/dm2.

flake-like particles begin to form. Thus, the steel substrate coatedwith Zn–Mo layers with thickness from 3 to 6 lm, were used forfurther studies, to exclude the influence of such distinct structurechanges on the layers properties.

3.4. Surface topography and roughness

Surface topography and roughness was investigated by atomicforce microscopy (AFM). Each coating was electrodeposited onsteel substrate, at the same potential (E = �1.3 V vs. SCE), fromelectrolytes containing surface active additives, differing only insodium molybdate concentration. The analysed coatings were ofa similar thickness (3–4 lm). Only the zinc layer deposited poten-tiostatically from baths containing surfactants was significantlythinner (about 0.2 lm), because the current efficiency of zinc elec-trodeposition under the applied conditions was only about 2%.Therefore, zinc coatings deposited galvanostatically, under theoptimal conditions designed previously [26], with a thicknessabout 3.5 lm, were also analysed.

Fig. 6 presents the AFM images showing the topography of thesurface of Zn and Zn–Mo coatings with various molybdenum con-tent. They are compared with the average roughness (Ra) (Fig. 7),kurtosis (Sku) and skewness (Ssk) (Fig. 8) of the steel substrate,Zn, and Zn–Mo coatings with various molybdenum content.There is a clear difference in the topography of the thin zinc coatingdeposited potentiostatically, compared to the 3.5 zinc layerdeposited galvanostatically (Fig. 6a and b). The addition of molyb-denum to the deposit also makes clear changes to the topography(Fig. 6c–f). Moreover, the average surface roughness of zinc layersvaries, depending on the conditions of deposition (Fig. 7). The addi-tion of molybdenum in the range from 0.5 to 1.5 wt.% causes a sig-nificant reduction of Zn–Mo roughness in comparison to Zncoatings. However, further increase of the molybdenum contentin deposits leads to a distinct increase in roughness.

Moreover, the values of the roughness parameters, kurtosis andskewness, also indicate that the surface of Zn–Mo, in contrast tozinc layers, reproduce the character of the surface of the steel sub-strate, with some slight tendency of predominance of peaks in rela-tion to normal distribution (Fig. 8).

3.5. Micromechanical properties of deposits

The study of the micromechanical properties were performedon Zn and Zn–Mo coatings (thickness about 6 lm) on steel, withmolybdenum content varied from 0.5 to 6 wt.% (Fig. 9). The Zn–Mo layers were deposited in the same conditions (E = �1.3 V vs.SCE) from baths containing various concentrations of molybdate

Fig. 7. The values of the average surface roughness (Ra) of the steel substrate, Znand Zn–Mo coatings with various content of molybdenum deposited on steelsubstrate from electrolytes with the addition of polyethylene glycol. Electrolytes:‘A1’, ‘A2’, ‘A4’, ‘A6’, ‘A7’, E = �1.3 V vs. SCE or i = 5 A/dm2 (only for Zn(b)), x = 16 rad/s, T = 20 �C.

Fig. 8. Surface roughness characteristics, based on Ssk and Sku parameters, for steelsubstrate, Zn and Zn–Mo coatings with various content of molybdenum depositedon steel substrate from electrolytes: ‘A1’, ‘A2’, ‘A4’, ‘A6’, ‘A7’, E = �1.3 V vs. SCE (for1, 2a, 3, 4, 5, 6), i = 5 A/dm2 (for 2b) x = 16 rad/s, T = 20 �C.

Fig. 9. Results of indentation tests: (a) hardness HIT and (b) modulus of elasticityEIT. Zn and Zn–Mo coatings with various content of molybdenum deposited on steelsubstrate.

Fig. 10. Coating fracture caused by critical load. Zn and Zn–Mo coatings withvarious content of molybdenum deposited on steel substrate.

H. Kazimierczak et al. / Journal of Alloys and Compounds 636 (2015) 156–164 163

ions, whereas the zinc layers were deposited galvanostatically (inthe same conditions as the Zn coating discussed in Section 3.4.).The maximal load (20 mN), chosen in preliminary testing, resultsin a penetration depth in the range from 450 to 1000 nm, which

is a small fraction of the thickness of the coating, hence it can bestated that the properties of the substrate did not affect the mea-surement results.

On the basis of the indentation tests, it was found that themicrohardness of the investigated coatings increases explicitlywith the increase of molybdenum content from 0.5 to 3 wt.%.The microhardness (HIT) reaches about 3.5 GPa for the Zn–Mocoating with 3 wt.% of molybdenum, which is almost five timesthat of the value of the microhardness of the Zn coating studied.However, further increase of molybdenum content in depositcauses a further decrease of microhardness (Fig. 9a). Moreover,the value of the elastic modulus (EIT) of Zn–Mo coatings is sig-nificantly higher than that of the Zn layer (Fig. 9b). It also growswith the increase of molybdenum content from about 110 GPa(for Zn–Mo0.5 wt.% sample) to 137 GPa (for the Zn–Mo3 wt.%sample). Hence the highest value of the elastic modulus isobserved for the coating with the highest noted microhardness.The decrease of the value of microhardness and elastic modulusfor samples with relatively high molybdenum content (ca.6 wt.%) may result from the significant changes of deposits mor-phology with the increase of molybdenum concentration. Asalready stated above, due to the relatively high content ofmolybdenum oxides and considerable development of the sur-face of coatings containing about 6 wt.% of molybdenum, theselayers exhibit lower mechanical properties than compact andmetallic Zn–Mo coatings containing lower amount ofmolybdenum.

3.6. Adhesion-scratch test

The deformation mechanism and adhesion to the steel sub-strate were investigated by scratch test (Figs. 10 and 11). No cohe-sive cracks were observed for the tested coatings. The first form ofdestruction observed was the abrasion of the coating and exposureof the substrate. This occurred at a depth in the range of 5–6 lm,which confirms the thickness of the applied coatings. However, itshould be noted that this abrasion was not accompanied by lossof adhesion of coatings to the substrate. Images of scratch tracksfor selected samples are shown in Fig. 11. Catastrophic destructionof the coating-substrate joint as a result of delimitation was notobserved, even at the maximum load applied (30 N). The abrasivewear mechanism was typical. The initial step of removing the coat-ing was accompanied by a significant increase in friction coeffi-cient as a result of indenter contact with the steel substrate. Itcan therefore be concluded that the investigated Zn–Mo coatingsexhibit an elastic-plastic deformation character typical of the puremetal coatings, and demonstrated good adhesion to the steelsubstrate.

Fig. 11. Images of the scratch tracks on samples under 30 N. (a) Zn–Mo0.5% – �200, (b) Zn–Mo0.5% – �50, (c) Zn–Mo3% – �200 and (d) Zn–Mo3% – �50.

164 H. Kazimierczak et al. / Journal of Alloys and Compounds 636 (2015) 156–164

4. Conclusions

� The Zn–Mo deposit composition is significant factor controllingthe coatings microstructure. The addition from 0.5 to 2.5 wt.% ofmolybdenum to zinc results in formation of more compact lay-ers which exhibits a finer-grained microstructure in comparisonto zinc coatings� The molybdenum addition in the range from 0.5 to 1.5 wt.%

causes a significant reduction of Zn–Mo roughness in compar-ison to pure Zn coatings. Moreover, the surface of Zn–Mo, incontrast to zinc layers, reproduces the character of the surfaceof the steel substrate with some slight tendency of pre-dominance of peaks in relation to normal distribution.� Microhardness of Zn–Mo coatings is significantly higher in com-

parison to microhardness of pure zinc coating electrodepositedfrom citrate electrolyte. It increases gradually, with the increaseof molybdenum content from 0.5 to 3 wt.%.� The investigated Zn–Mo coatings exhibit an elastic-plastic

deformation character, typical of the pure metal coatings anddemonstrated good adhesion to the steel substrate.

Acknowledgements

This work was supported by Project POKL-04.01.01-00-004/10.We acknowledge Dr. R. Kowalik and Dr. P. Zabinski for enabling

us to carry out WDXRF measurement at AGH University of Scienceand Technology, Faculty of Non-Ferrous Metals, Cracow, Poland.

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