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Journal of Alloys and Compounds 437 (2007) 289–297

The heat treatment effects on the structure and wear behaviorof pulse electroforming Ni–P alloy coatings

Kung-Hsu Hou a, Ming-Chang Jeng a, Ming-Der Ger b,∗a Department of Mechanical Engineering, National Central University, Chung-Li, Taiwan, 320, ROC

b Department of Applied Chemistry, Chung Cheng Institute of Technology, National Defense University, Tahsi, Taiwan, 335, ROC

Received 4 June 2006; received in revised form 22 July 2006; accepted 24 July 2006Available online 18 September 2006

bstract

The Ni–P alloy coatings with phosphorous content from 8.7 to 13.9 wt.% were prepared in this study by pulse current electroforming. Theffects of operating conditions of electroformed Ni–P coatings on the phosphorous contents and micro-hardness of the deposits were investigated.he heat treatment effects on the morphology and grain size was evaluated by X-ray diffraction. The effects of heat treatment on the hardness andear resistance were also investigated. Our results show that the internal stress within the pc Ni–P deposited coating is much lower than that of dci–P deposited coating and the phosphorus content profile is also more stable than that of dc Ni–P deposited coating. The grain size of as-platedi–P deposited coating is smaller than 1.5 nm, but as the phosphorus content increased, both the grain size and microhardness decrease. Aftereat-treated at 400 ◦C, the hard Ni3P phase precipitates and the grain size become gross, however, the grain size is still smaller than 5 nm. Theardness of heat-treated coating can be enhanced efficiently and is well above 1000 HV. It leads to a lower wear rate for heat-treated coating. The

ear resistance of heat-treated coating can be as high as 2.5 times that of as-plated coating. In addition, the wear resistance and hardness increasesith the increasing of grain size for both as-plated and heat-treated coatings. It suggests that the strength and grain size of the Ni–P coating withigh phosphorus content obeys the inverse Hall–Petch relationship.

2006 Elsevier B.V. All rights reserved.

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eywords: Pulse current electroforming; Ni–P coatings; Wear resistance

. Introduction

In recent years, due to the development and progress in scien-ific technology that the mechanism structures and componentsre in favor of small size such as in micron, sub-micron and evenano-meter. In view of nano- or micro-structural technology, theequirement in the system regulation of lithography electroform-ng micro-molding (LIGA) and micro-electro mechanical sys-em (MEMS) is that the Ni-based micro-electroforming methodse given utmost importance, say for instance in Ni-based micro-rinding tool [1,2], mini-motor revolving shaft and bearing [3],icro-gear train, piston-cylinder burner or mini-turbine [4]. The

ontact and friction characteristics of micro-tools, power devicend transmission element are generally subjected to severelyigh temperatures, high-pressured environment and strict con-

∗ Corresponding author. Fax: +886 3 3900714.E-mail address: mdger@ccit.edu.tw (M.-D. Ger).

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925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.jallcom.2006.07.120

act friction mechanism [4]. However, to this day there has noteen obtained a major breakthrough that will resolve difficultribological issues.

Based on the micro-component with host and passive con-act, it is imperative to have a suitable tribological properties,ood anti-oxidize and anti-corrosion ability, as well as highhermal stability in nickel electroforming coatings with binaryevelopment, such as Ni–P, Ni–Co and Ni–Fe coatings [2,5,6].lthough Ni–Fe and Ni–Co alloys have good hardness at room

emperature, their softening phenomenon are evident when theemperature is rising, the hardness of Ni–P alloy, however,an be enhanced by precipitate strengthened effect which is aesult of heat treatment and it promotes the hardness signifi-antly [7]. Among these, Ni–P alloy possesses high hardness,igh strength and other superior mechanical properties; so that,

lectroless Ni–P alloy or composites have been approved thathey are suitable used in automobiles, aviation, food, printing,hemical, and other instruments [8–12]. Unfortunately, the oper-ting conditions of electroless technique are difficult to alter

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nd are unsuitable for thick film coating deposition. However,hese characteristics of Ni–P alloy as well as its wear and cor-osion resistances are attractive that making it has potentialo become a more valuable material in the development ofano-meter sized frictional components. Nevertheless, knowns having strong internal stress of direct current (dc) electro-eposited Ni–P alloys, there is no way of satisfying most of thelectroforming requirement. The internal stress, perhaps causedy the hydrogen-permeated in coatings [13,14], shows a closeorrelation with electroforming conditions. Thus, before electro-eposited Ni–P alloy can be used, the problem of its internaltress must be solved firstly.

Generally, when nickel coating is subjected to same peakurrent density conditions, the internal stress of its pulse currenteposition (pc) is much lower than the internal stress of its directurrent deposition (dc) [15]. Based on literatures, pc as comparedo dc electroforming mode is more capable of reducing coatingorosity and intensifying toughness [16]. The pc system is per-aps comparatively better than dc system in terms of suitabilityith MEMS particularly in the precise electroforming of Ni–P

oating. Nonetheless, it seems that systematic researches on theirect correlation of the tribological or wear qualities of pc Ni–Poatings have seldom been published in literatures.

In this research, we focused on develop a new material ofi–P alloy with nano-grain size structure that produced by pc

lectroforming. The pc electroforming procedures were con-ucted with fixed electrolyte chemical composition while theperating parameters of pulsed power supply were adjusted.urthermore, optima conditions for the Ni–P coatings that withharacteristics such as low internal stress, high phosphorus con-ent and a specific structure would be selected in this study. The

ain objectives of this paper are to discuss how the coatingstructure and properties are influenced by the pulse conditionsnd heat treatment at 400 ◦C, and related wear characteristicsould be also presented.

. Experimental details

.1. Electrodeposition experiments and analysis

This research uses nickel sulfamate (320 g/L), added nickel chloride hex-hydrate (10 g/L), boric acid (40 g/L), phosphorous acid (10 g/L) to make upL of electrolyte. The composition of the electrolytes can be seen in Table 1.he electrolytes were placed inside a glass chamber and were stirred until thor-ughly mixed. The electrolyte temperature was adjusted to 60 ◦C and its pH

able 1hemical composition of Ni–P electrolyte and pulse current electroformingperating conditions

i (NH3SO3)2 320 g/LiCl2 10 g/L

3BO3 40 g/L

3PO3 10 g/LH 1.5eak current density 5–20 A/dm2

uty cycle 0.1–0.5requency 10–1000 Hztirring rate 250 rpmemperature 60 ◦C

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Compounds 437 (2007) 289–297

alue was recorded at 1.5, with the magnetic stirrer circulated at a speed of50 rpm. Rectangle wave was adopted for the pulse current electroform in thisxperiment primarily because of its provide more finer grain in plating [17]. Theulse current was provided with a model of HC-113, HOKUTO DENKO powerenerator, and the current regulates the on–off period that yielding symmetricalr asymmetrical electric cycle rectangle waves, in plating. Most pulse currentarameters include: peak current density ip, charging time (Ton), rest time (Toff),uty cycle (Ton/(Ton + Toff)), and pulse frequency (1/(Ton + Toff)). The electrode-osition conditions were scope with peak current density of 5–20 A/dm2, dutyycle of 0.1–0.5 and pulse frequency of 10–1000 Hz.

The internal stress of coating was measured using the dual bent copper striptress testing method by a 683 EC deposit stress analyzer. The Ni–P depositsas coated on one side of copper strip’s bands with a non-conductive varnish

nd plate the exposed surface to the desired thickness (approximately 3 �m) inparticular plating bath. After plating, the test strip is supported in the testing

tand, which measures the distance that the test strip’s bands have spread. Thenternal stress was calculated based on the distance of the test strip bands, filmhickness, density of deposit film and calibrated constant of copper strip. Thebove-mentioned information are then inserted into a calculated formula thatas provided by manufacturer. In this research, the reported internal stress was

n average stress that represented with three measurements.The analytical used Ni–P coatings were deposited approximately 200 �m

hickness on FC25 gray cast iron disk (same as for wear test substrate) with32 mm diameter and 5 mm thickness. The iron disk deposited surface wererst rubbed in the polish machine causing the superficial roughing of the surfaceefore using the chemical oil eliminator, followed by the use of de-ionized waterinse to clean it and placed as the cathodes. Each deposition condition has haveeen carry out three replicate tests.

As the same time, all samples were sorted, selected and using the electricalire machine cut into small area with 1 cm2, each one condition has prepared

our samples. Half of samples take and than work with heat treatment at 400 ◦Cnd hold 1 h. Before and after heat-treated samples followed by hot compressionounting, mechanical grinding, polishing, and the measurement of the phospho-

us content and hardness were detected with coating’s cross-section layer. Thelectron probe X-ray micro-analyzer (EPMA) determines the phosphorus con-ent as well as the desired mean values using three points. On the other hand,he mean value for the hardness is calculated after 10 tests, loading with 100 gnd 15 s.

X-ray diffraction (XRD) measurements were carried out by a MAC Science,XP 18 diffractometer using Cu K� radiation. Diffraction patterns were used

o investigate the structural evolution of Ni–P coatings before and after heatreatment.

.2. Wear test of Ni–P alloy coatings

The friction and wear tests were carried out on a rotational wear test machine18] using a ring on disk pair (same as thrust washer adapter). The steel ring wasade by JIS SKD-11 steel and thermal treated to achieve average hardness HRC

2 ± 1. The stationary iron disk was coated with Ni–P deposit with thicknessbout 240 �m on the counter surface.

Before wear test, machined contact surfaces of friction pairs were throughubbed with #600, #800, #1200 SiC abrasive sheets, and than polished with.25 �m alumina powder using a low speed polish machine. The resultingverage surface roughness values (Ra) of the Ni–P coatings was 0.1 �m approxi-ately. For each deposition conditions, five as-plated and five heat treated (heat

reatment at 400 ◦C, 1 h) samples were prepared and then stand by for wearest.

Each friction pair was cleaned by ultrasonically washed in acetone before andfter each test. The weight loss of the Ni–P coatings was obtained by measuringhe disk weight before and after each wear test, which were using an electricalalance with 0.01 mg weight scale accuracy. During the tests, the steel ringotating at a constant velocity was pressed against the stationary Ni–P coatingisk. All the experiments were carried out in ambient condition of temperature

5 ± 3 ◦C and 55 ± 5% relative humidity. No lubrication was used during wearests. The tests were carried out at a speed of 0.27 m/s and under a contact pressuref 0.7 MPa, and the sliding distances was 1000 m. Three replicate friction andear tests were carried out so as to minimize data scattering, and the average of

he three replicate test results are reported in this work. Wear rate was defined as

s and Compounds 437 (2007) 289–297 291

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he weight loss of Ni–P coatings divide by the test length and the wear resistanceas defined as the reciprocal of wear rate.

The morphologies of the wear traces were observed using a scanning electronicroscope (SEM), with energy-dispersive X-ray analysis (EDX) attached to theEM.

. Results and discussion

The internal stress, phosphorus content as well as hardnessre compared firstly between pc and dc Ni–P deposited coatingsn this study. The optima operation parameters for pc electro-orming to form the Ni–P deposited coating with high phospho-us content and low internal stress are identified and used forlectroforming later on to study the effects of heat treatment onardness, structure and wear rate. Our results and discussion areresented as follows.

.1. The effects of operation parameters of electroformingn the Ni–P deposited coating

Fig. 1 presents various peak current densities and com-ares the internal stresses associated with pc and dc electro-orming Ni–P alloy coatings. This illustration shows that thenternal stresses value of dc electroforming coatings increasesith the current density, at internal stresses from 115 to25 MPa. However, pc electroforming has duty cycle = 0.1, fre-uency = 1000 Hz, and the internal stress of the coating increasedery slightly with the current density. The range of internaltresses was 53–60 MPa. Evidently, the internal stress of pc elec-roforming coatings is much lower than that of dc electroformingoatings. The surface morphology of newly plated coatings isuch that an internal stress of over 250 MPa makes the dc electro-orming coatings layer extremely brittle such that it fractures ands likely to separate from the base material. In this research, thenternal stress values of the pc Ni–P coatings are all lower thanhose of the dc Ni–P coatings, primarily causing by the electricurrent has regulated on and off periods, in the pc electroforming

rocessing. During the “on” periods, the reduction rates of Ni2+

nd H+ ions in the surface of the cathode greatly exceeded thoseuring the “off” periods. Therefore, during the “off” period, theydrogen gas bubbles generated on the cathode surface experi-

ig. 1. The internal stress of Ni–P coatings varies with the current density forirect current deposition and pulse current deposition, respectively.

Fig. 2. The phosphorus contents and hardness of Ni–P coatings related withpulse current deposition conditions. (a) Corresponding to different current den-sdi

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ity at duty cycle: 0.2, and frequency: 1000 Hz. (b) Corresponding to differentuty cycle at current density: 5 A/dm2, and frequency: 1000 Hz. (c) Correspond-ng to different frequency at current density: 10 A/dm2, and duty cycle: 0.2.

nce the fluid dynamic shear force, facilitating their departurerom the deposition surface, without their being entrapped inhe coating layer. This fact may explain why the pc of the Ni–Poatings had lower internal stress than dc Ni–P coatings. More-ver, that the excess of hydrogen was co-deposited in the coating

urface is the primary cause of hydrogen embrittlement.

Fig. 2 displays the relationship between the content of phos-horus as well as hardness of Ni–P deposit coating and theperating parameters of pc electroforming. With the duty cycle

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f 0.2 and the frequency of 1000 Hz, Fig. 2(a) illustrates that thehosphorus content of deposited coating tends to decrease as theurrent density of pc increased, but the micro-hardness showsgentle enhancement. This result implies that the reductions

f nickel ions are much easier with the raising current density,t inhibits the deposition of phosphorus and, consequently, the

icro-hardness of deposited coating is enhanced. On the otherand, the effect of phosphorus element on the grain refinements lower since the alloying elements act as the nucleation siteso refine the grain size [19].

Meanwhile, Fig. 2(b) shows the duty cycle effects on thehosphorus content and micro-hardness of deposited coating athe pc of 5 A/dm2 and the frequency of 1000 Hz. As the dutyycle increased (reduction of off time), the phosphorus contents decreased significantly while the micro-hardness of depositedoating increases. Concentration of the reacting species at thenterface of cathode and electroplating bath is increased as theuty cycle is decreased due to an increase of rest time duringhe electroforming process leading to a sufficient time to theccurrence of the mass transfer. Hence, the concentrations ofhe reacting species at the interface, thus, are maintained, andt increases the phosphorus content of the deposited coating.s a matter of fact, a pc electroforming with a higher duty cycleehaviors like dc electroforming, it is not surprised that the phos-

horus content of deposit is less than that of a lower duty cycle 1.ith the current density of 10 A/dm2 and the duty cycle of 0.2,

ig. 2(c) shows that the phosphorus content increased from about.3–12.6 wt.% as the frequency increased from 100 to 1000 Hz.

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ig. 3. The macro-surface morphologies of Ni–P coatings in which produced using0 A/dm2; (d) dc, 20 A/dm2.

Compounds 437 (2007) 289–297

he micro-hardness of the deposit decreased slightly as the fre-uency increased, however, the micro-hardness is still higherhan 700 HV. It seems that the phosphorus content is increasedue to the increase of the number of disturbance (1000 Hz) onhe diffusion layer leading to an effective enhancement of trans-ort of ions to the interface. Meanwhile, it inhibits the growthf grain and leads to the decrease of hardness.

Fig. 3 shows the photographs of as-plated Ni–P coatings onhe FC25 gray iron disk. As shown in Fig. 3(a) and (b), the sur-aces of pc coatings are smooth, but Fig. 3(c) and (d) presentshat the dc coatings are brittle and fractures layers are formedeading to the separation of coatings from the substrate alonghe circular edge. Therefore, it is regarded that the dc Ni–P coat-ngs are completely unsuitable for using as the wear-resisting

aterials, as their internal stresses are too high.The phosphorus content varies with the thickness of Ni–P

eposit coatings. As shown in Fig. 4, the phosphorus con-ents decrease as the thickness of coating increased up to about00 �m for both pc and dc Ni–P deposited coatings, but thehosphorus content of pc Ni–P deposited coating is less affectedy the thickness than that of dc Ni–P deposited coating does.his figure reveals that the phosphorus content profile for pceposited coating is more stable than that of dc deposited coat-ng. It is evident that Ni–P deposited coating formed by pc

lectroforming is a promising method for producing the MEMSarts with high aspect ratio.

The above results indicate that the amorphous Ni–P depositedoating with low internal stress can be produced by a pulsed

different deposition conditions: (a) pc, 10 A/dm2; (b) pc, 20 A/dm2; (c) dc,

K.-H. Hou et al. / Journal of Alloys and Compounds 437 (2007) 289–297 293

Ft0

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where λ is the radiation wavelength, β the half-maximumwidth, and θ is the diffraction angle of the main peak. The calcu-lated crystallite sizes, listed in Table 2, showed that the crystallite

Table 2Phosphorous content, and average grain size for both as-plated amorphous Ni–Pcoatings and coatings that treated at 400 ◦C for 1 h

P (wt.%) Grain size (nm)

As-plated Heat-treated

8.7 1.57 4.35

ig. 4. Phosphorous contents of Ni–P coatings were distributed along the filmhickness direction: (a) pc 5 A/dm3, 0.1, 1000 Hz; P-13.9 wt.%; (b) pc 10 A/dm3,.2, 10 Hz; P-8.7 wt.%; (c) dc 5 A/dm3; P-5.8 wt.%; (b) dc 10 A/dm3; P-4.6 wt.%.

ower electroforming operation. The phosphorus content andardness of Ni–P deposited coatings can be varied by this elec-roforming process without adjustment of H3PO3 concentrationithin the electroplating bath. The properties of the deposited

oatings are influenced directly by the operating parameters ofhe pulsed electroforming process, the hardness of the depositedoatings decreases as the phosphorus content increased whichs related to the structure of the coating (crystallite size). Onhe other hand, it is reasonable to assume the strength of thec Ni–P deposited coating is only determined by its propertynd structure, such as phosphorus content and crystallite size,ased on the observation that the pc Ni–P deposited coatingsn our study all have low internal stress and stable phosphorusistribution.

The XRD diffraction diagram shown in Fig. 5(a) indicatedhat the microstructures of Ni–P deposit coating depend onhe phosphorus content, the relative strength of Ni(1 1 1)eak becomes weak and broad as the phosphorus content isncreasing. In addition, the half width ratio of diffraction peak isarger at the higher phosphorus content. It implies that the nickelrystallite lattice sizes decrease with increasing phosphorusontent. These results might be ascribed to the increasing ofhe numbers of phosphorus atoms within the fcc Ni lattice, itauses the increase of lattice strain which will further reducehe growth rate of crystallite. Consequently, it leads to a smallerrystallite size [20].

Fig. 5(b) presents the X-ray diffraction patterns of heat-reated deposit coatings. Compared with Fig. 5(a), it is obvi-us that the Ni(1 1 1) peak in X-ray diffraction pattern for theeat-treated deposit coating is sharper than that of as-depositedample. The diffraction peak corresponding to Ni3P was alsoeen found. This could be an indication that the morphologyransits from a single Ni phase to a mixture which is combined

f bct Ni3P and fcc Ni phase after heat-treated at a temperaturef 400 ◦C. The Ni3P precipitates are preferentially located at therain boundaries and triple junctions [21]. Since the hardnessf Ni3P precipitate is higher than that of Ni phase, the Ni3P

1111

ig. 5. X-ray diffraction pattern of pc Ni–P coatings: (a) as-plate and (b) heat-reated coatings.

recipitates provide an additional barrier to the movement ofislocation.

The crystallite sizes of Ni–P deposit coatings with differenthosphorus contents are calculated by Scherrer equation [22]:

= 0.89λ

β cos θ(1)

0.3 1.21 4.231.4 0.63 3.932.6 0.56 3.643.9 0.49 3.12

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94 K.-H. Hou et al. / Journal of Alloy

izes of as-deposited coatings all are less than 1.6 nm and theirrain sizes are finer at the higher phosphorus contents. This isn agreement with observation of Lewis [20] who stated that therystallite size is less than 1.5 nm when the phosphorus contentxceeds 10 wt.% and the higher the phosphorus content is, thener the crystallite. However, they did not investigate the hard-ess and wear behavior of the Ni–P deposited coating in thataper.

.2. The effect of crystallite size of deposit coating on theicro-hardness and the wear rate

Fig. 6 presents the phosphorus content effect on the crystalliteize and hardness of the deposited coating. As shown in Fig. 6(a)or as-received deposited coatings, there is a sudden drop inrystallite size from 1.57 to 0.49 nm as the phosphorus contentncreases from 8.7 to 13.9 wt.%. After annealed at 400 ◦C, theirrystallite sizes are coarsened to 4.35 and 3.12 nm, respectively.ig. 6(b) indicates that the hardness decreases as the phosphorus

ontent increased for as-received deposited coatings. However,he hardness for all samples with different phosphorus contentre promoted to over 945 HV. This hardness enhancement hasainly been attributed to the precipitation of Ni3P phase, which

ig. 6. The effects of phosphorous content that influence of (a) grain size andb) micro-hardness, for both as-plated and heat-treated Ni–P coatings.

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ig. 7. The effects of phosphorous content vs. micro-hardness and wear rate fora) as-plated and (b) heat-treated Ni–P coatings.

trengthens the structure of deposited coating. However, thearger crystallite size after annealing might have some effectsn it. Both the crystallite size and hardness of the amorphousi–P deposited coating are decreased with the increasing of thehosphorus content.

Fig. 7 displays the relationship between hardness and wearate with reciprocal square-root grain size (d−1/2). As shownn Fig. 7, the hardness always decreases as d−1/2 increasedgrain size decreased), while the wear shows an opposite trend.he wear rate increases with the increasing of d−1/2. It isvident from Fig. 7(a) and (b) that the heat treatment is anfficient method to reduce the wear rate. The wear rates foreat-treated coatings, as shown in Fig. 7(b), are from about 3.6o 7.1 × 10−3 mg/m, which are much lower than those for as-lated coating (8.5–15.7 × 10−3 mg/m shown in Fig. 7(a)). Ouresults show that the Ni–P coating with phosphorus content of.7 wt.% has best hardness and lowest wear rate and the wearate increased as the phosphorus content decreased. As a whole,he strength of amorphous Ni–P deposited coating with highhosphorus content increases with the increasing of crystallite

ize, which shows the opposite trend to the Hall–Petch relation-hip. This result is known as inverse Hall–Petch relationship (orreakdown), which can be ascribed to the absence of the dislo-ation accumulation, the occurrence of the creep diffusion, rapid

K.-H. Hou et al. / Journal of Alloys and

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traicifincrease as the phosphorus content decreased, it is believed thatthe wear resistance will be improved when the phosphorus con-tent of deposited coating is lower. As shown in Fig. 8(b), itappears that the worn surface of the as-plate deposited coat-

ig. 8. Worn surface morphology of pc Ni–P as-plated coatings with differenthosphorous contents: (a) 13.9 wt.% P, (b) 11.4 wt.% P, and (c) 8.7 wt.% P.

islocation annihilation at grain boundaries and softening causedy the presence of a significant amount of grain triple junctions21,23]. However, a complete theory is not fully developed toatisfactorily explain the experimental results.

.3. Wear characteristics and wear resistance

The worn surface morphologies for as-plated Ni–P depositedoating are shown in Fig. 8. For Ni–P coating with phospho-us content of 13.9 wt.%, Fig. 8(a) confirms the presence ofbrasion trace and the occurrence of coatings delaminated from

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Compounds 437 (2007) 289–297 295

he surface. At the commencement of the sliding, the micro-oughen peak of the counter-part presses onto as well as contactdhesively with the relevant deposited coating. Under these act-ng, deposited coating undergoes plastic deformation and shearut as a result of the adhesive and plowing effects. It resultsn the development of grooves and trace of indent on the sur-ace of deposited coating. Since the grain size and hardness

ig. 9. Worn surface morphology of pc Ni–P heat-treated coatings with differenthosphorous contents: (a) 13.9 wt.% P, (b) 11.4 wt.% P, and (c) 8.7 wt.% P.

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96 K.-H. Hou et al. / Journal of Alloy

ng with phosphorus content of 11.4 wt.% displaying only somemooth and shallow sliding abrasion traces. As the phosphorusontent is further down to 8.7 wt.%, there is no plowing and plas-ic deformation appeared on the surface as shown in Fig. 8(c).

eanwhile, the red debris, which is identified as ferrous oxidey EDX, are distributed unifirmly on the worn surface of coat-ng. It is reasonable to consider that the iron is fragmented andransmitted to the rubbing surface from the SKD steel counter-art during the sliding period. It is, then, oxidized and adheredn the rubbing surface.

The change in the wear behavior as a result of heat treatmentan be noticed with the examination of the worn surface. Theicrographs shown in Fig. 9 indicate that after heat treatment,

he sliding abrasion trace and delaminating phenomenon do notormed any more. This is related to the precipitation of Ni3Phase on the Ni grain boundary. The disperse strengthen effect asresult of pin force generated by the Ni3P phase provides a resis-

ance to the pressure and shear cut caused by the micro-rougheneak of the counter-part. The presence of red ferrous oxideebris also been observed on these heat-treated samples. Thisas been related to the high hardness of heat-treated depositedoating. After heat-treated at 400 ◦C, the hardness of heat-treatedeposited coating is well above 900 HV, which is higher than thatf the relevant SKD11 counter-part. This means that the steelounter-surface has also been worn by the deposited coating dur-ng the sliding stage. Since the hardness increases with the heatreatment, it leads to the increase of the amount of debris on thei–P coating surface owing to the increasing of wear loss of the

ounter-surface as indicated in Fig. 9(a)–(c). It is obvious thathe wear rate of heat-treated coating is less than its relevant as-lated one and the abrasion wear phenomenon for heat-treatedoating is also lower. It shows in Fig. 8(c) that the debris uni-ormly distributed on the worn surface of the deposited coatinghen the hardness of the as-plated coating is above 700 HV.

owever, Fig. 9 indicates the coagulation of the debris. This

esult might be ascribed to the presence of ferromagnetic prop-rty for the heat-treated Ni–P coating [24] and, hence induceshe coagulation of the debris.

ig. 10. The relationships of wear resistance vs. micro-hardness for both as-lated and heat-treated pc Ni–P coatings.

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Compounds 437 (2007) 289–297

The hardness of material represents its capability to resist theccurrence of local plastic deformation under load. It is consid-red that the wear resistance is related to the hardness of theaterial. Our results do show the existence of such relationship.rom Fig. 10, it is clear that the wear resistance increases almost

inearly as hardness increased. It means the wear resistance wille enhanced if the material has a bigger grain size. However, theardness effect on wear resistance is more significant for heat-reated coatings than that for as-plated coating. This behavioran be attributed to the precipitation of hard Ni3P phase afternnealing which strengthens the structure of Ni–P matrix. Heatreatment increases the wear resistance of the coatings to about.1–2.5 times that of as-plated coatings.

. Conclusion

With comparison of their properties, structures as well asear behaviors between as-plated and heat-treated coatings, our

onclusion can be summarized as follow:

. The pc Ni–P deposited coatings in this study display theyhave low internal stress. It also shows that the phospho-rus content distribution along the depth direction that forpc Ni–P deposited coatings are more stable than those ofdc Ni–P deposited coatings. In addition, the grain sizes ofpc Ni–P deposited coatings are within to the nano-scale,and as the phosphorus content increases from 8.7 to13.9 wt.%, the grain size decreases from about 1.57–0.49 nmand the micro-hardness also decreases from 765 to573 HV.

. The precipitation of Ni3P compound within the Ni–P coat-ing with heat treatment temperature at 400 ◦C enhances thehardness of deposited coating well above 1000 HV as thegross grain is smaller than 5 nm. It results in decreasingof the wear rate. The wear resistance of heat-treated coat-ing can be increased up to about 2.5 times that of as-platedcoatings.

. The wear resistances and hardness for both as-plated andheat-treated coating increase with the increasing of grain size,it represents that the strength and grain size for the pc Ni–Pwith high phosphorus content follows the inverse Hall–Petchrelationship.

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