Abrasive Wear Resistance of Electroless Ni–P Coated Aluminium After Post Treatment

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Surface & Coatings Technology 205 (2010) 766772

Contents lists available at ScienceDirect

Surface & Coatin

l sfunctional and quality aspects.The surface roughness of the coating after deposition depends on

the roughness of the surface coated, on the total coating thickness andon the type of coating applied. In general, the surface of the coatingwill have at least the same roughness as the initial coated surface; theroughness will increase with coating thickness.

The wear resistance of a coated component is mainly determined bythe coating as long as it covers the contact area. As soon as the coating ispartly worn through, or the substrate is exposed due to adhesive failure

alloys. Cast AlSi alloys having excellent castability are suitable forproduction of large series of complex-shape components, such as engineblocks, pistons, cylinder liners, cylinder heads, and wheels. In someapplications, however, they suffer from insufcient wear resistance.

Generally, past studies were made on heat treatment of ENcoating. Here, an attempt is made to study the effect of post treatment(heat treatment and lapping with two different surface textures) onthe wear resistance of EN coated aluminium. In addition to the wearperformance, this work also analyses the surface morphology featuresor cracking and spalling, the wear resistancebecomes important. Two main categories candominated by coating detachment and wear caof coating material. The latter often involves m

Corresponding author. Tel.: +44 28 90974017; fax:E-mail address: [email protected] (W. Sha).

0257-8972/$ see front matter 2010 Elsevier B.V. Adoi:10.1016/j.surfcoat.2010.07.124it is sometimes benecial,vents them from weldingis important in terms of

cast iron, have been increasingly used in automotive engine cylinderblocks. The eutectic AlSi alloy with high percentages of alloyingelements has better scufng resistance than the hypereutectic AlSias it allows surfaces to trap lubricants and pretogether. Hence, controlling surface roughness1. Introduction

Surfaces produced by various procestexture. These differences make it possturned,milled, or ground surfaces to betexture of a critical surface of a part inufatigue, to assist or destroy effective lubits friction and abrasive action on otherwell as affect many other properties thconditions [1]. Roughness is sometimemay cause friction, wear, drag and fatiguibit distinct differences inhoned, lapped, polished,entied. Variations in theability to resist wear and

n, to increase or decreaseand to resist corrosion, asbe critical under certaindesirable property, as it

Electroless nickel (EN) coating is a well established surfaceengineering process widely used in automotive and aerospace industriesas it provides high hardness and excellent resistance to wear, abrasionand corrosion [2]. The auto industry takes particular advantage of theuniformity of the electroless nickel deposit on irregular surfaces, directdeposition on surface activated non-conductors and the formation of lessporous, more corrosion resistant deposits [3].

AlSi alloys, with lighter weight and better heat conductivity thanof the substrate materialbe distinguished: wearused by gradual removalild wear due to abrasion,

of the EN coatin

2. Experimental

2.1. Surface prep

The samples wSi alloy casting b

+44 28 90974278.

ll rights reserved.ing wear of homogeneous materials.

erosion, chemical dissolution, etc., and does not deviate from theAbrasive wear resistance of electroless Ni

R. Rajendran a, W. Sha b,, R. Elansezhian c

a School of Mechanical and Building Sciences, B. S. Abdur Rahman University, GST Road, Vb School of Planning, Architecture and Civil Engineering, Queen's University Belfast, Belfastc Department of Mechanical Engineering, Pondicherry Engineering College, Pondicherry 60

a b s t r a c ta r t i c l e i n f o

Article history:Received 28 June 2010Accepted in revised form 29 July 2010Available online 10 August 2010

Keywords:Amorphous materialsCoating materialsElectroless platingPhase transitionsScanning electron microscopySurface morphology

Electroless nickel (EN) coataerospace industries. In thiLM24 (Al9 wt.% Si alloy) atreatments included heat tmicroscopy (SEM), energy dwere used to analyse motreatment signicantly impwear resistance. Microhardnthe formation of Ni3P after

j ourna l homepage: www.ecoated aluminium after post treatment

alur, Chennai 600 048, India1NN, UK

4, India

s are recognised for their hardness and wear resistance in automotive andork, electroless NiP coatings were deposited on aluminium alloy substratethe effect of post treatment on the wear resistance was studied. The posttment and lapping with two different surface textures. Scanning electronersive spectrometry (EDS), X-ray diffraction (XRD) and micro-abrasion testerology, structure and abrasive wear resistance of the coatings. Post heated the coating density and structure, giving rise to enhanced hardness andof electroless NiP coatings with thickness of about 15 m increased due tot treatment.

2010 Elsevier B.V. All rights reserved.

gs Technology

ev ie r.com/ locate /sur fcoatg.

aration

ere cut into required size of 36253 mm fromAly electrical discharge machining (EDM). The step-by-

step cleaning procedure employed prior to plating consists ofultrasonic degreasing in acetone, rinsing with distilled water,deoxidizing in 10% HCl acid, rinsing with distilled water, pickling in10% H2SO4, and rinsing in distilled water followed by a methanolwash. It is believed that Al alloys can also be treated using NaOH andHNO3, which is perhaps a more common option.

Usually, aluminium alloys are pre-treated by a zincate etch bathprior to electroless nickel-plating. Zincating process was not used inthis research experimental work, potentially an advantage becausethis eliminates a process, reduces the use of chemicals and thus mayhave environmental advantages. However, that should be a subject ofseparate studies, and is not the objective of the research shown here.

SLb4

=kskckskc

t4b2Rt

2

b4

!+

1ks

64R

2

where b is the outer diameter of the wear crater, ks and kc are thesubstrate and the coating wear coefcients, respectively, and t is thecoating thickness. Both substrate and coating wear coefcients can beobtained from a linear plot of SL/b4 versus t /4b2Rt2/b4; thesubstrate wear coefcient, ks, is obtained from the intercept of the

Table 2Test conditions for wear tests.

Parameters Value

Load 0.5 NSpeed 0.1 m/sBall material Stainless steel AISI 440-CBall diameter 25.4 mmBall nish Conditioned using run-in procedureAbrasive material SiC 3 mFluid carrier WaterFeed rate Keep wetAbrasive concentration 20 vol.%Test duration 400, 600, 900, 1200, 1500, 2000 ball-revolutions

767R. Rajendran et al. / Surface & Coatings Technology 205 (2010) 7667722.2. Plating bath and operating conditions

The composition of the plating bath for electroless NiP depositionhad nickel chloride as the source of nickel, sodium hypophosphite asthe reducing agent, and sodium citrate as the complexing agent. Thebath compositions and plating conditions used are given in Table 1.Temperature of the plating bath was maintained at 851 C. The pHof the bath was maintained between 9 and 10 by addition of sufcientquantity of ammonia solution. Due to the small volume of the platingbath and the experimental (manual operation) rather than thecommercial nature of the plating process, a stricter limit in thevariation of pH within 0.2 was not achieved in this work. This mighthave resulted in variations of the amount of phosphorus in thecoatings, but this is not expected to have affected the mainconclusions from the research. The electrolyte was heated indirectlyby an electrically heated water bath. The temperature of theelectrolyte was monitored using a thermometer. The coating wasfor a period of 30 min with total volume of the plating bath restrictedto 500 ml.

The heat treatment of some coating specimens was conducted inair. Although some believe that oxidation rate of EN coating is fast asthe temperature is above 200 C, surface morphology observation andX-ray diffraction results shown in later sections do not reveal anyoxidation product, and thus oxidation during the heat treatment inthis work is considered insignicant.

After this treatment and direct coating on Al surface without undercoating layer, the adhesion of the coated layer was found to be morethan satisfactory, as revealed by later wear testing results.

2.3. Micro-abrasion test

In conditions of mild abrasion, the coatingmaterial may determinethe wear resistance of a coating composite. Standard abrasive weartests are usually too coarse to be useful for measuring the intrinsicwear resistance of thin coatings. With the crater grinder method as ameans for coating wear evaluation, it is possible to distinguish theabrasive wear resistance of a thin coating material from that of thesubstrate, also in conditions where the coating is worn through [4,5].

A Plint TE66 micro-scale abrasion tester was used to evaluate thecoating abrasive wear resistance. During the test, a directly driven ball

Table 1Composition and conditions of plating bath for electroless deposition.

Particulars Value

Nickel chloride 30 g/lSodium hypophosphite 40 g/lSodium citrate 25 g/lAmmonium chloride 50 g/lTemperature 85 C (1 C)pH 910Bath volume 500 mlDeposition time 30 minHeat treatment 330 C/1 his rotated against the specimen, which is mounted on a dead-weightload lever, in the presence of slurry of SiC abrasive particles. The slurryis drip fed onto the contact between the specimen and the ball (Fig. 1)and the wear scar produced on the specimen surface is assumed toreproduce the spherical geometry of the ball [6]. By making a series ofthese craters and measuring the size of the scar dimensions, bothcoating and substrate wear coefcients (kc and ks) can be simulta-neously determined from the test [6,7]. For bulk materials, theequation that is assumed to describe the abrasive wear is [69]

SL =Vk=

1k

b4

64R

!forb WW R 1

where S is the distance slid by the ball, L is the normal force on thesample, V is the wear volume, k is the wear coefcient, b is thediameter of the crater, and R is the radius of the ball. Two completeseries of six tests with different durations as in Table 2 wereconducted.

Eq. (1) was extended to a model that combines the wear in thecoating and substrate, providing both coating and substrate wearcoefcients [6,7]

Fig. 1. Schematic diagram of the micro-abrasion apparatus.

linear plot and the coating wear coefcient, kc, is calculated from the

structure of materials. Diffraction patterns contain informationshowing various phases of a material and residual stresses within a

0

0.1

0.2

0.3

0.4

0.5

0.6

0

Wea

r Vol

ume,

mm

3

Sliding Distance, m

Al

EN Al

HT EN Al

20015010050

Fig. 2. Wear volume as a function of sliding distance of aluminium (Al), electrolessnickel coated aluminium (EN Al) and heat treated electroless nickel coated aluminium(HT EN Al).

Table 3Surface roughness value for aluminium, electroless nickel coated aluminium and heattreated electroless nickel coated aluminium (with and without lapping).

Specimen Surface nish (m)

Ra Rz

Al 4.04 24.63Al S 1.98 11.45EN Al 4.72 26.03EN Al S 1.87 11.92HT EN Al 4.63 23.32HT EN Al S 1.73 10.13

S indicates lapped surface.

768 R. Rajendran et al. / Surface & Coatings Technology 205 (2010) 766772gradient of the plot.The wear constants kc and ks are essentially a measure of the

abrasive wear rate; the lower the wear constant, the better theresistance to abrasive wear. After measuring the inner and outerdiameters of the wear craters, regression analyses, according toEqs. (1) and (2), were carried out in order to determine both kc and ksfor each sample.

A Nikon Eclipse ME600D optical microscope with a JVC digitalcamera was used for measurement of the scar diameter and to checkthe roundness of the scar. The analySIS Auto-version 3.0 (Soft ImagingSystem GmbH) was used to digitally monitor and store the imagesfrom the camera attached to the optical microscopy with a computer.The morphology of wear scars and debris was observed by opticalmicroscopy and scanning electron microscopy (SEM).

2.4. Analysis of the deposits

Microhardness of the EN deposits was estimated using MitutoyoHM124 hardness testing machine, with a Vickers indenter, 50 g load,and 10 s loading time. Surface roughness of electroless nickel depositswas measured using a TESA Rugosurf 10G surface roughness tester,cut-off length 0.8 mm. The surface morphology of the electrolessnickel deposits was observed using a Jeol 6400 high-resolutionscanning electron microscope. The amount of phosphorus and nickelcontents on the EN deposits were analysed using Jeol 6400 SEM EDAXattachment. X-ray diffraction patterns were obtained from experi-ments using a computer controlled Philips X'Pert Pro X-ray diffrac-tometer using Cu K radiation of wavelength 1.54060 in the scanrange of 20 to 90. X-ray diffraction is widely used to determine the0.0

0.1

0.2

0.3

0.4

0.5

0.6

0

Wea

r Vol

ume,

mm

3

Sliding Distance, m

Al SEN Al SHT EN Al S

20015010050

Fig. 3. Wear volume as a function of sliding distance of lapped aluminium (Al S),electroless nickel coated aluminium (EN Al S) and heat treated electroless nickel coatedaluminium (HT EN Al S).coating.

3. Results and discussion

3.1. Micro-abrasion

Fig. 2 shows the wear volume of aluminium (Al), electroless nickelcoated aluminium (EN Al), and heat treated electroless nickel coatedaluminium (HT EN Al). It can be seen that the wear volume increaseswith increase in the number of revolutions (sliding distance). Whenthe wear volume is high, the penetration is deeper. However, it isfound that in most of the samples, the penetration depth exceeds thedepth of coatings into the substrate. Therefore, the abrasion tests weredone up to 2000 revolutions in order to determine the strength of thesamples.

Fig. 3 shows the wear volume of aluminium, electroless nickelcoated aluminium, and heat treated electroless nickel coatedaluminium under lapped condition. The measured surface roughnessfor the coating under different conditions is given in Table 3. The wearvolume of the lapped samples is increased when compared to thesamples without lapping. If the surface was perfectly smooth thenseizure would occur due to difculty of maintaining the lubricating oillm. The rates of wear are proportional to the surface areas in contactand the load per unit area.

It should be noted that the aim of this work is not to simulateactual cylinder liner wear, but to establish a test for evaluating thesurface characteristics of EN coated aluminium that may be related toliner wear performance. Therefore, the ball material need not reectthe properties of the counter face in actual cylinder liners.

The kc and ks wear coefcients were obtained from the linearregression analyses as shown in Figs. 4 and 5. It should be notedthat the x-axis in these two gures is intended to be as shown, t /4b2Rt2/b4, with a unit of m1. These diagrams show the

4.5E+12

y = 11.1E+11x + 3.86E+11

R2 = 0.987

y = 8.62E+11x + 1.23E+11R2 = 0.996

0.0E+005.0E+111.0E+121.5E+122.0E+122.5E+123.0E+123.5E+124.0E+12

0 1 2 3 4

EN AlHT EN Al

SL/b

4 , N

/m3

t/4b2Rt2/b4, m-1

Fig. 4. Linear regression analysis of electroless nickel coated aluminium (EN Al) andheat treated electroless nickel coated aluminium (HT EN Al).

lapping, with 20% SiC and nominal 0.5 N load, after 4002000revolutions. For the crater on the lapped substrate, it was not possible

y = 4.04E+11x + 9.65E+11R2= 0.994

y = 3.34E+11x + 6.68E+11R2= 0.993

0.0E+005.0E+111.0E+121.5E+122.0E+122.5E+123.0E+123.5E+124.0E+124.5E+12

EN Al SHT EN Al S

SL/b

4 , N

/m3

ear

Res

istan

ce (k

c-1)

1012

N

m/m

3

1

1.5

2

2.5

769R. Rajendran et al. / Surface & Coatings Technology 205 (2010) 766772correlation between the y-axis function and the x-axis function. Thiswas explained in the paragraph below Eq. (2). The curve tparameters are summarised in Table 4. The normal probability plotsof residuals, which were obtained for the regression in all samples,indicated that the residuals had a normal distribution and theirvariance was constant. The regression also indicated high values ofR2 and adequacy of t in all systems, conrming that the modelsleading to Eqs. (1) and (2) are successful in describing theexperimental data [10].

The EN coating had the highest wear resistance to abrasion. Thecoating wear resistance (kc1) is shown in Fig. 6 along with the wearresistance of the uncoated substrate for a direct comparison. Asexpected, the choice of the EN coating plays an important role inimproving the micro-abrasive wear resistance. In the micro-abrasivewear tests, SiC particles, with a mean size of 3 m, were used asabrasives. Their sizes are smaller than the thickness of coatings. Theindentation depths of abrasive particles on surfaces depend on theapplied load, surface hardness and abrasive hardness [11]. Micro-hardness measurements were made on the cross-sections of the plainelectroless NiP coatings in as-plated and heat treated conditions. It isevident from Table 5 that the hardness of the coating increased byabout 4.5 times the base hardness. The SiC hardness is expected to be21002600 HV0.05 [12]. Habrasive was taken as 2400 HV. Therefore, itcan be seen that coatings are softer than the SiC particles.

An important parameter in abrasive wear testing is the ratiobetween the hardness of the abrasive particles (Habrasive, or Ha) andthe hardness of the surface (Hs). Hard abrasion typically occurswhen the ratio Ha/Hs is higher than 1.2. In this condition, the abrasionwill lead to much greater wear rates [12].

Dependence between the kc values and the ratio Ha/Hs can beobserved for the coatings. A lower Ha/Hs ratio in general leads tohigher micro-abrasive wear resistance. The electroless nickel and the

0

t/4b2Rt2/b4, m-14321

Fig. 5. Linear regression analysis of lapped electroless nickel coated aluminium (EN AlS) and heat treated electroless nickel coated aluminium (HT EN Al S).heat treated electroless nickel coating had a lower Ha/Hs ratio than thesubstrate and displayed better micro-abrasive wear resistance.Although the hardness of the heat treated electroless nickel washigher than that of untreated electroless nickel, its relatively lowhardness, in comparison to that of SiC particles, in combination with a

Table 4Coating and substrate wear coefcients obtained from experimental results and linearregression analyses.

Specimen Model kc ks Slope Intercept(1012 m3/Nm) (1012 m3/Nm) (m3/Nm) (N/m3)

Al Eq. (1) 4.90 6.091011

EN Al Eq. (2) 0.52 1.60 11.11011 3.681011

EN Al S Eq. (2) 0.73 2.54 4.041011 9.651011

HT EN Al Eq. (2) 0.58 1.78 8.621011 1.231011

HT EN Al S Eq. (2) 0.85 2.87 3.341011 6.681011

*S indicates lapped surface.15 m lm thickness could not provide an adequate improvement interms of abrasive wear.

If the SiC particles deeply penetrated through the EN coatingbecause of its relatively low hardness, then the EN coating may havebecome partly delaminated or removed through a severe ploughing orcutting mechanism during the last stages of the test. This wouldexplain the high wear rates recorded for these coatings. The poorabrasive wear resistance recorded for the EN coatings could beattributed to the hardness being much lower than that of the SiCabrasive particles, which caused tearing of the coating withsubsequent delamination [13].

It is sometimes reported in the literature that a reduction inabrasive wear resistance occurs with increasing surface roughness.This effect was not observed in this work, since the (rougher) ENcoatings exhibited higher resistance to micro-abrasion than thelapped EN coatings. This is probably due to an increased resistanceto abrasivewear offered by the EN coating, beingmore important thanthe supposed negative effect of the increased surface roughness.

Fig. 7 shows the optical micrographs of craters formed withdifferent wear durations on aluminium, electroless nickel coatedaluminium, and heat treated electroless nickel coated aluminium,with 20% SiC and nominal 0.5 N load, after 4002000 revolutions. Thescar of the samples at shorter test duration is not spherical in shape.It was found that the size of the crater increased as the number ofrevolutions increased, but that the proportion of grooving wear thatoccurred was also increasing.

Fig. 8 shows the optical micrographs of craters formed withdifferent wear durations on aluminium, electroless nickel coatedaluminium, and heat treated electroless nickel coated aluminiumwith

EN A1 EN A1 S HT EN A1 HT EN A1 SCoating Material

Coat

ing

W

A10

0.5

Fig. 6. Coating wear resistance of aluminium, electroless nickel coated aluminium andheat treated electroless nickel coated aluminium (with and without lapping).to dene clearly the inner and outer crater diameter.Most of the electroless nickel samples that are examined show the

substrate as the penetration exceeds the coatings. When viewing thewear scars, two regions can be seen in the spherical cap. The darkerregion in the middle shows the substrate whereas the region outsidethe inner circle represents the coating.

It can be noticed that the wear pattern observed in the coatingregion of the wear crater is the same as that seen in the substrate

Table 5Vickers hardness of aluminium, electroless nickel coated aluminium and heat treatedelectroless nickel coated aluminium.

Specimen HV0.05 Habrasive/Hsurface

Al 90 27EN Al 400 6HT EN Al 650 4

2000 EN Al1500 EN Al1200 EN Al900 EN Al600 EN Al400 EN Al

2000 HT EN Al1500 HT EN Al1200 HT EN Al900 HT EN Al600 HT EN Al400 HT EN Al

2000 Al1500 Al1200 Al900 Al600 Al400 Al

(a)

(b)

(c)

Fig. 7. Optical micrographs of craters formed with different durations on (a) aluminium, (b) electroless nickel coated aluminium and (c) heat treated electroless nickel coatedaluminium with 20% SiC and nominal 0.5 N load. The number in the micrographs is the number of revolutions. The scale bar in each micrograph is 1 mm.

770 R. Rajendran et al. / Surface & Coatings Technology 205 (2010) 766772region. However, the worn coating area that remains after the test isconned to a quite thin ring outside the worn substrate area.

After a certain number of ball revolutions, the coating material is

worn through, exposing the substrate material. After this point, the

(a)

(b)

(c)

900 HT EN Al S600 HT EN Al S400 HT EN Al S

900 EN Al S600 EN Al S400 EN Al S

900 Al S600 Al S400 Al S

Fig. 8. Optical micrographs of craters formed with different durations on lapped (a) aluminiualuminium with 20% SiC and nominal 0.5 N load. The number in the micrographs is the numicro-abrasive wear resistance can be further increased by increasingthe substrate hardness. Although increased hardness does notnecessarily result in better abrasive wear resistance (i.e., abrasive

wear by brittle fracture), the present results indicate a strong

2000 HT EN Al S1500 HT EN Al S1200 HT EN Al S

2000 EN Al S1500 EN Al S1200 EN Al S

2000 Al S1500 Al S1200 Al S

m, (b) electroless nickel coated aluminium and (c) heat treated electroless nickel coatedmber of revolutions. The scale bar in each micrograph is 1 mm.

The micro-abrasion test can thus be seen to be a quick and

microstructure. Cross-section microscopy would be required foridentication of microstructure, but the microscopy shown in this

3.4. X-ray diffraction

Fig. 10a shows the X-ray diffraction (XRD) characteristics ofaluminium. For aluminium, ve strong peaks can be observed at 2 ofaround 38.5, 44.7, 65.2, 78.3 and 82.4, and for silicon, three strongpeaks can be seen at 2 of around 28.5, 47.4 and 56.3.

The widths of the diffraction peaks are related to the quality of thecrystallites. Larger and perfect crystallites have sharper diffraction

Fig. 9. SEMmicrographs of (a) aluminium, (b) electroless nickel coated aluminium and(c) heat treated (330 C/1 h) electroless nickel coated aluminium.

771R. Rajendran et al. / Surface & Coatings Technology 205 (2010) 766772section is from the coating surfaces, which shows the coatinggrowth morphology. SEM cross-sectional micrograph [1416] andEDX analysis [14,15,17] results are reported in previous publica-tions for identication of coating thickness [14], elementalcomposition [14,15,17], and diffusion of nickel into the substrate[14]. These results are included in a recent book on electrolessplating [18].

3.3. EDX analysis

Three different spots around the structures were analysed andthe average phosphorus content is 4.70.1 wt.%, balance nickel.There is a small amount of oxygen present on the surface, which ispossibly due to very small amount of oxidation soon after thesamples were prepared. Since no signicant oxygen or otherelement is found, it is reasonable to believe that the structureseen on the deposits consists of Ni and P with no contribution fromsensitive measure of the degradation of the electroless nickel coatedmaterials. Wear is in fact the most important property related to theperformance of these components in service. Although the mode ofwear in actual service is not the same as in themicro-abrasion test andwear rates will differ, both cylinder liner wear and micro-abrasionwould seem to depend on the large deformation mechanicalbehaviour of electroless nickel coating.

3.2. Surface morphology

Fig. 9a shows the SEM image of the aluminium substrate beforeelectroless plating. The image shows that the substrate has a rough,eroded like surface, providing good adhesion of the subsequent nickellm on the substrate.

Fig. 9b shows the surface morphology of the electroless nickeldeposit. A smooth surface with relatively rough grains is acquired.The round particles seen on the image are the random nickeldeposits in the solution attached to the surface of the sample. Largeamount of circular shaped grains can be seen on the images. Thesegrains are sized around 3 microns in diameter. The increased size ofgrains may be due to the reduction of the concentration of thechemicals in the plating solution in the last stage of plating, whichreduces the plating rate and enables the grains to grow moreextensively. The thicknesses of the coated-deposit on both sides ofthe substrate were remarkably uniform. The appearance of thedeposits was homogeneous.

The heat treated (330 C/1 h) sample (Fig. 9c) does not showsignicant change in surface morphology against the as-plated one.The spherical globules of nickel have been reduced. In the case ofelectroless nickel deposit, the dispersion of nickel particles seems tobe less prominent when compared to heat treated electroless nickelcoated aluminium resulting in uniform surface nish.

It may be noted that surface morphology is not the same asinuence of surface hardness on the micro-scale abrasive wearperformance, which suggests a mechanism of abrasive wear by plasticdeformation [12].

The presence of an electroless nickel deposited layer diminishesthe scratching action of the SiC abrasive particles, since the Ha/Hs ratiois lower for an electroless nickel coated aluminium in comparison tothe base material. Therefore, for a given sort of abrasive particles, themicro-abrasive wear resistance can be improved by increasing thesurface hardness, i.e., by depositing a harder coating than the abrasiveparticles.other elements. peaks. The XRD pattern of the as-deposited NiP in Fig. 10b shows a

772 R. Rajendran et al. / Surface & Coatings Technology 205 (2010) 766772broad peak (around 2=45) and some sharp small peaks. The wideangular range prole at the 2 position of 3555 indicates thepresence of a mixture of amorphous and microcrystalline phases inthe deposits. Though it was previously believed that phosphoruscontents of lower than 7% result in microcrystalline EN deposits [2],some careful XRD peak separation work has shown the overlapping ofamorphous and microcrystalline diffractions [1820]. As expected,diffraction peaks from the AlSi alloy substrate are visible, with muchreduced intensity compared to Fig. 10a, as the X-ray can penetratethrough the coating. Note that microscopy work shows that the as-deposited NiP coating having a thickness of about 15 m shows arelatively good adherence to the substrate and uniformity.

There is one major and one weak broad-peak diffraction at the 2positions corresponding to Ni {111} and {200} planes, respectively,superimposedwith the amorphous diffraction in the XRD proles. Theresults agreed with the studies from other investigators showing thatthe as-deposited low to medium phosphorus deposits consisted of anamorphous structure [21,22].

Fig. 10c shows the XRD characteristics of heat treated electrolessnickel on aluminium. Heating to the temperature of 330 C hascaused the broad-peak reections that correspond to the 2positions of Ni {111} and {200} planes, to develop into the relativelybroad Ni {111} and {200} diffraction peaks, respectively, withlimited height (intensity). The intensity of amorphous proledecreased, and the Ni {220} reection was seen in the XRD prole.In addition, the intensity of substrate peaks has reduced, becausethe penetration power of X-ray through a crystalline structure islower than through an amorphous structure, as proved in previouswork [14,18].

Duncan indicated that the microcrystalline -phase converts to-nickel at 250290 C, whilst the amorphous -phase converts toNi3P and -nickel at 310330 C [23]. In this study, the XRDanalysis conrms the above results by conrming the transforma-tion from an amorphous structure to crystalline nickel. According tothe low temperature phase diagram of NiP, the structure of as-

Fig. 10. X-ray diffraction for (a) aluminium (b) electroless nickel coated aluminium and(c) heat treated electroless nickel coated aluminium.deposited NiP coating consists of microcrystalline -phase andamorphous -phase for a phosphorus content between 4.5 and11 wt.% [23].

This study was carried out to understand the changes in the XRDpatterns of electroless nickel deposits with annealing temperature.In general, the heat treatment process transforms the NiP phase toa mixture of hard nickel phosphides (Ni3P) and nickel. As expected,the as-deposited coating and coating heat treated at low tempera-tures show a broad peak of NiP phase and sharp peaks ofcrystalline aluminium and silicon from the substrate. The phasecomposition of NiP coatings was analysed by X-ray diffraction.Further investigation will be required to quantitatively explainmicro-abrasive wear resistance change by phase transitions of ENcoatings.4. Conclusions

In this paper, a micro-abrasive wear test was used to evaluate thewear resistance of electroless nickel coatings after post treatment. Theexperimental results indicated that:

(1) The microhardness of electroless NiP coating heat treated at330 C for 1 h was higher than that of NiP alloy coating, due tothe presence of Ni3P particles.

(2) The wear volume of the lapped samples is increased whencompared to the sampleswithout lapping. Thewearabrasionwasfound to be dependent on the ratio between the hardness of theSiC abrasive particles and the hardness of the surface (coating) orsubsurface. By decreasing this ratio, the ability of the SiC abrasiveparticles to scratch the coated surface was reduced.

(3) Electroless nickel coatings showed higher micro-abrasive wearresistance than the base material. Electroless nickel coatingswere effective in improving the micro-abrasive wear resistanceof the substrate, since their wear coefcients were lower thanthe substrate wear coefcients. The hardness of the electrolessnickel coatingwas found to bemuch lower than the hardness ofthe SiC abrasive particles. Consequently, these coatings seem tohave partly delaminated during the test due to the presence ofharder SiC abrasives, which caused tearing of the coating withsubsequent delamination.

(4) The deposition of thicker electroless nickel coatings may beexpected to yield an improvement in the abrasive wearresistance, since the SiC particles would not be able topenetrate heavily into the electroless nickel lm and coatingdelamination would be inhibited.

Acknowledgement

We hereby express our sincere thanks to the UK Royal Society forproviding the support for this project and Queen's University Belfast,in the form of an International Travel Grant, 2009R2 Travel forCollaboration.

References

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Abrasive wear resistance of electroless NiP coated aluminium after post treatmentIntroductionExperimentalSurface preparationPlating bath and operating conditionsMicro-abrasion testAnalysis of the deposits

Results and discussionMicro-abrasionSurface morphologyEDX analysisX-ray diffraction

ConclusionsAcknowledgementReferences