Surface & Coatings Technology 200 (2006) 5836–5842www.elsevier.com/locate/surfcoat

Electroless Ni–P–SiC composite coatings with superfine particles

Gao Jiaqiang ⁎, Liu Lei, Wu Yating, Shen Bin, Hu Wenbin

State Key Lab of MMCs, Shanghai Jiao Tong University, Shanghai, 200030, P.R. China

Received 17 May 2005; accepted in revised form 27 August 2005Available online 19 October 2005

Abstract

Superfine silicon carbide (SiC) particles reinforced nickel–phosphorus (Ni–P) matrix composite (Ni–P–SiC) coatings were prepared byelectroless plating. The morphology and structure as well as the phase transformation of the composite coatings with three sizes of SiC particleswere studied by scanning electron microscopy (SEM), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and transmission electronmicroscopy (TEM), respectively. It was shown that SiC particles co-deposited homogeneously, and the structure of Ni–P–SiC composite coatingsas deposited was amorphous. After certain heat treatment, the matrix of composite coatings crystallized into nickel crystal and nickel phosphide(Ni3P). At the higher temperature nickel reacted with SiC, and nickel silicides with free carbon were produced. The reaction temperature inelectroless composites coatings decreased with the decrease in the size of SiC particles. Microhardness of electroless Ni–P–SiC compositecoatings increased due to the existence of particles, and reached to the maximum value after heat treatment at 400 °C for 1 h.© 2005 Elsevier B.V. All rights reserved.

Keywords: Electroless Ni–P–SiC; Superfine particles; Crystallization and reaction; Microhardness

Table 1Main solution components and experimental conditions

Component and condition Concentration

NiSO4•6H2O 15∼20 g/LNaH2PO2•H2O 20∼25 g/LCH3CHOHCOOH 88% (lactic acid) 30∼33 ml/L

1. Introduction

Since the invention of electroless plating technology in 1946by A. Brenner and G. Riddell, electroless nickel coatings havebeen used in many fields due to its unique properties. Thestructure of electroless Ni–P as deposited is amorphous withhigh phosphorus content. However, this amorphous structure ismetastable and undergoes a crystalline transition with tempe-rature increase [1–6]. After adequate heat treatment, the coatingturned to crystal and its hardness and anti-wear ability improvedgreatly.

Embedding particles in electroless deposited metals is aconvenient method of preparing composite coatings, and theparticles increase its mechanical and physical properties [7].There are wear-resistant composite coatings with SiC, Al2O3 andso on. SiC is a kind of useful electronic material, and has highstrength ceramic material with excellent corrosion and erosionresistance. So electroless Ni–P–SiC composite coatings areattractive and have been investigated before [8,9].

The sedimentation velocity of particles in electroless bath isslow with small size, and superfine particles can be dispersed

⁎ Corresponding author. Tel.: +86 21 62933585; fax: +86 21 62822012.E-mail address: jdgjq@yahoo.com.cn (G. Jiaqiang).

0257-8972/$ - see front matter © 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.surfcoat.2005.08.134

and suspended in electroless bath for its small-scale-effect andlarge-specific-area. Up to now, few studies were reported oncrystallization and reaction behavior of electroless compositecoatings influenced by superfine particles in details [10,11]. Inthe present work, electroless Ni–P–SiC composite coatings withsuperfine particles were prepared and the structures as well asthe phase transformation of composite coatings were studiedlater.

2. Experimental

An optimized electroless nickel (EN) bath had been developedin our laboratory, and the main solution components and someexperimental conditions are summarized in Table 1. All chemicals

Superfine SiC particles 5∼15 g/LTemperature 90±1 °CpH 4.3∼4.5

50nm

a b

Fig. 1. (a) SEM of 110 nm SiC and (b) TEM of 50 nm SiC particles, respectively.

5837G. Jiaqiang et al. / Surface & Coatings Technology 200 (2006) 5836–5842

usedwere of analytic grade. In addition, a small count of stabilizeragents and complex non-ionic surfactant agents containingpolyethylene glycol were introduced into the bath.

Electroless composite bath contained particles pretreated byhydrofluoric acid. Three kinds of SiC particles were used and themean sizes are 1000, 110 and 50 nm, respectively. The bath wasair-agitated and the agitationmagnitude was adjusted for differentSiC particles during plating. Copper coupons (40×40×0.1 mm3)were used as substrate and pre-treated before immersed in thebath. And the pretreatment procedure of substrates can be shownas below (rinsing samples with de-ionized water):

Polish→ rinsing→oil removing (sodium hydroxide solu-tion, 30 min)→ rinsing→activation (diluted hydrochloric acid,60 s)→ rinsing.

(1000nm SiC) (110nm

5μm

5μm

a1 b1

a2 b2

Fig. 2. (a1, b1, c1) Surface and (a2, b2, c2) cross-section morphology of electroless Ndots in figures are SiC particles).

The chemical composition of coatings was determined byenergy dispersive spectrum (EDS) and chemical titration, andthe average values were adopted. Specimens of heat treatmentwere heated and cooled in air atmosphere. Philips Sirion 200field emission scanning electron microscope (SEM), HitachiS-520 SEM, Philips CM-12 transmission electron microscope(TEM, 100 kV) and JEOL JEM-2010 F field emission high-resolution electronic microscope (HREM, 200 kV) were usedto show the structure of particles and coatings. Crystallizationand reaction processes were examined by NETZSCH 404differential scanning calorimetry (DSC) and Rigaku X-raydiffractometer (XRD) with Cu Kα radiation. The microhardnessof coatings was measured by HX-100 Vickers hardness tester atthe load of 50 g for 15 s.

SiC) (50nm SiC)

5μm 5μm

1μm 1μm

c1

c2

i–P–SiC coatings with three sizes of superfine particles, respectively (the black

Table 2Heat of formation and Gibbs free energy of SiC and several nickel compounds[13,14]

Phase ΔHf (kJ/mol) ΔGf (kJ/mol)

SiC −62.8 –Ni3Si −148.8 −86.0Ni5Si2 −301.4 −175.8Ni2Si −131.7 −68.9Ni3Si2 −223.6 −98.0NiSi −85.3 −22.5NiSi2 −86.5 39.1Ni3C 67.3 130.1

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3. Results and discussion

3.1. Morphology of electroless Ni–P–SiC composite coatings

Two kinds of superfine SiC particles are shown in Fig. 1, andthey can be suspended in the bath for a long time. With propersurfactant agents, three sizes of SiC particles suspended inelectroless bath by air agitation, and three kinds of particlesreinforced Ni–P matrix composite coatings were prepared,respectively.

Superfine SiC particles were successfully co-deposited inNi–P alloy matrix by electroless plating. With the increase ofSiC concentration in the bath, the content of particles in thecomposite coating increased, but the plating rate decreased. Theaverage deposition rate of composite coatings with 110 nm SiCparticles was about 10 μm/h at the first metal turnover (5 h). Thesuperfine particles content was about 4.3 wt.%, and thephosphorus content of the composite coating decreased slightlyto 10.5 wt.% compared with that of electroless Ni–P coating (11wt.% P), and the atomic ratio of Ni with P (4 :1) did not change.The particles contents were 3.8 and 3.6 wt.% of compositecoatings with 1000 nm SiC and 50 nm SiC, respectively, and theatomic ratios of Ni with P in composite coatings were the sameas that in electroless Ni–P alloys.

Observed by the naked eye, the composite coatings aresmooth and semi-bright as deposited. It is shown in Fig. 2(a1,b1 and c1) that the composite coatings with superfine SiCparticles are smooth, and the morphology of coatings waschanged slightly. Fig. 2(a2, b2 and c2) shows that superfine SiCparticles disperse homogeneously and the content of particlesare at high levels.

3.2. Crystallization behavior of electroless Ni–P–SiC compositecoatings

DSC curves of electroless Ni–P–SiC composite coatingswith three sizes of superfine particles and Ni–P alloy atisochronal heating were shown in Fig. 3. Electroless Ni–P–SiCcomposite coatings as deposited are amorphous and turn tocrystal structure after annealed. There is only one exothermal

200 400 600 800-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5 894ºC

340ºC

Ni-P-1000nmSiC, 606ºC

Ni-P-110nmSiC, 579ºC

Hea

t flo

w,m

w/m

g

Temperature,ºC

Ni-P-50nmSiC, 551ºC

Ni-P

Fig. 3. DSC curves of electroless Ni–P and Ni–P–SiC composite coatings(heating rate: 10 °C/min).

peak of the electroless Ni–P alloy, and the temperature isconsidered as the crystallization temperature. It can be seen that,no matter which size of SiC particles are deposited into thecomposites, the crystallization and melting point hold the samevalves, respectively. It implies that the SiC particles did notchange the crystallization behavior of composite coatings.

There are the other exothermal peaks in three kinds ofcomposite coatings, which respond to the reaction between Niand SiC. It can be seen that the reaction temperature decreaseswith the decrease in the size of SiC particles, because nickelatoms diffuse into the finer SiC particles easily and will reactwith SiC at the lower temperature.

SiC particles are embedded into Ni–P alloy during electrolesscomposite plating, and the nickel atoms deposit as clusters on theparticles surface at first [12]. The nickel atom clusters arethermodynamically metastable and nickel atomswill diffuse intoSiC particles during heating. The Si–C bonds are broken and Niatoms displace the C, then nickel silicides (simply expressed asNixPy , standing for Ni3Si, Ni2Si, Ni5Si2, etc.) are produced.

Neglecting the influence of phosphorus in electroless Ni–P–SiC composite coatings, the change of Gibbs free energy duringthe reaction can be expressed by:

ΔG ¼ ΔGNixSiy þ yΔGC−xΔGNi−yΔGSiC: ð1Þ

The change of Gibbs energy can be calculated by:ΔG=ΔH−TΔS, where ΔH is the heat of formation, T is the

Ni-P

Ni-P-SiC

Ni-P-SiC,400ºC

Inte

nsity

(a.

u.)

2θ(º)

Ni-P-SiC,600ºC

NiNi3PSiCNixSiy

NiO

Fig. 4. XRD spectra of as-deposited electroless Ni–P and Ni–P–SiC coatingsafter heat treatment.

20 40 60 80

Ni3Si

NixSiy

(2 2

0)(2

0 0

)

(1 1

1)

Ni3Si

Inte

nsity

(a.

u.)

2θ(º)

SiC

Fig. 5. The XRD spectra of the residual of electroless Ni–P–SiC compositecoating after heat treatment and dissolved in HNO3 compared with that ofsuperfine SiC particles.

5839G. Jiaqiang et al. / Surface & Coatings Technology 200 (2006) 5836–5842

temperature and ΔS is the entropy change. Because ΔS ismostly small in solid-state reaction and can be neglected, ΔGbecomes equal to ΔH. To obtain the net gain in energy uponsilicide and carbide formation, the enthalpy of SiC should besubtracted. There are many kinds of Ni–Si compounds, suchas NiSi, Ni2Si, Ni3Si, and Ni5Si2. Several nickel silicides canbe obtained after the reaction of nickel with SiC, which isinfluenced by the local nickel concentration. The heat offormation and ΔG of several nickel compounds are shown inTable 2 calculated according to the references [13] and [14],which have some differences with that of reference [9]. Somenickel silicides can be obtained because of negative ΔG, whilenickel carbide was not observed for positive ΔG. Further-more, nickel element is rich in the electroless compositecoatings, and the final nickel silicide is Ni3Si after completereaction.

3.3. Structures of electroless Ni–P–SiC composite coatings

From the XRD spectra in Fig. 4, it can be found thatelectroless Ni–P–SiC composite coating is amorphous as

A

B

C

a

100nm

Fig. 6. (a) TEM BF image and (b) SAD of A district in electroless Ni

deposited. That is to say, the superfine particles do not changethe structure of the Ni–P alloy during electroless plating.

As shown in Fig. 4, the electroless composite coatingcrystallized into nickel crystal, nickel phosphide and nickelsilicides (NixPy) after heat treated at 400 °C for 1 h. After heattreated at 600 °C, there is no special diffraction peak of nickelsilicides except for Ni3Si, whose grain lattice approaches to thatof nickel [8].

To make sure the final products of the crystallization andreaction in electroless Ni–P–SiC composite coatings withsuperfine particles, one piece of composite coating with 110nm SiC was striped from the substrate and heat treated at 600°C for 4 h. Then, the sample was dissolved in diluted nitricacid (HNO3) solution for several hours. The residual productwas filtered, rinsed and dried, then was studied by XRD. Theresult of XRD spectrum of the residual is shown in Fig. 5compared with that of SiC added into the bath. Itdemonstrated that there was Ni3Si mainly without SiC in theresidual product. So it is concluded that the final products ofelectroless Ni–P–SiC composite coatings after completelyheat treatment are mainly Ni, Ni3P, Ni3Si and C (freecarbon).

Electroless Ni–P–SiC composite coatings containingsuperfine particles (selecting the coating with 110 nm SiC as anexample) after heat treatment at 400 °C for 1 h wereinvestigated, and Fig. 6(a) shows the TEM bright field (BF)image of electroless Ni–P–SiC composite coatings. We can findthat the particles disperse in the Ni–P matrix. The selected areadiffraction of C district in Fig. 6(a) is shown in Fig. 6(b), whichcorresponds to polycrystalline nickel. From the results of EDS,the object of A district in Fig. 6(a) is SiC and B district in Fig. 6(a) is Ni3P, respectively.

JEM 2010 F TEM BF images and local element distributionanalysis in electroless composite coatings with 110 nm SiCparticles after heat treatment are shown in Fig. 7. It can be seenthat the edge of SiC particles is ambiguous due to the reaction.The element distribution in Fig. 7(b) of C, Si, Ni and P are shownin Fig. 7(c), (d), (e) and (f), respectively. It can be seen that Celement diffused everywhere, which implies that nickel diffusedinto SiC particle and the Si–C bond was broken.

b

–P–SiC composite coating after heat treatment at 400 °C for 1 h.

C Si Ni P

ba

d fec

50 nm

280nm

200nm

Fig. 7. (a) TEM BF and (b) local element analysis district in electroless Ni–P–SiC composite coating after heat treatment, as well as the distribution of (c) C, (d) Si,(e) Ni and (f) P, respectively.

Table 3Microhardness of electroless Ni–P and Ni–P–SiC composite coatings after heattreatment (HV50)

Coatings 1000 nm SiC 110 nm SiC 50 nm SiC Ni–P

As-deposited 580 537 520 510300 °C×1 h 854 860 781 776400 °C×1 h 1206 1192 1186 1012500 °C×1 h 967 932 966 937600 °C×1 h 807 826 888 764

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Fig. 8 shows the JEM 2010F HREM image of Ni–P–SiCcontaining 110 nm SiC particles after heat treatment at 400°C for 1 h. The interfacial reaction of Ni and SiC was studiedin details in Fig. 8(a), and the arrows point to the reactedinterface. Fig. 8(b) shows modified image by fast Fouriertransform at the same position. From SAD of A districtshown in Fig. 8(c), it can be concluded that the main phase ofthe district is body-centered tetragonal Ni3P with the directionof [7 5̄ 5̄].

3.4. Microhardness of electroless Ni–P–SiC composite coatings

Not only the reaction process of composite coatings ischanged by superfine particles, but also properties of coatingsare modified. Microhardness is an important property ofelectroless coatings, and the results of Ni–P and Ni–P–SiCcoatings as deposited and those after certain heat treatment areshown in Table 3. Microhardness values of electroless Ni–P–SiC composite coatings as deposited increase because of theintroduction of hard particles and dispersion strengthening. Itcan be seen in Table 3 that the microhardness of electroless Ni–P–SiC composite coating with 1000 nm particles increased 10%as high as that of electroless Ni–P alloy. With the fine SiCparticles, the electroless composite coatings have low micro-hardness values.

After crystallization, microhardness of coatings increasesgreatly because of the precipitation of hard intermetalliccompound Ni3P. It is shown in Table 3 that the microhardnessof electroless Ni–P–SiC composite coatings raise to themaximum value (about HV501200) after heat treatment at400 °C for 1 h. After heat treatment at high temperature(e.g. 500 and 600 °C), microhardness of composite coatings

decreases, because the grains grow and become coarse leadingto strength decrease after heat treatment at high temperaturemore than 400 °C [15].

From Table 3, we can find that the composite coatings withfiner particles have higher microhardness values after heated at600 °C. The existence of free carbon and the volume shrinkageafter reaction lead to the strength decrease of compositecoatings. When nickel is abundant, the following reaction takesplace:

3Ni þ SiC→Ni3Si þ C: ð2ÞThe volume variation of the composite coatings after

reaction is expressed as:

ΔV ¼ ðV ns þ V cÞ−ðV n þ V sÞ ð3Þwhere ΔV—volume change during reaction; Vn—volume ofnickel matrix needed to react completely with SiC; Vs—volumeof a SiC particle; Vns—volume of reaction product (Ni3Si);Vc—volume of carbon (graphite). According to the data inreferences [14] and [16], one can obtain that the decrease of

SiC

Ni

a

1 1 2

2 3 1

bc

A

5 nm

Fig. 8. (a) HREM fringe image of electroless Ni–P–SiC composite coating containing 110 nm SiC particles, (b) modified image and (c) SAD of A district. Indexing isfor Ni3P.

5841G. Jiaqiang et al. / Surface & Coatings Technology 200 (2006) 5836–5842

volume of the system is 1.244 cm3/mol. It indicates thatshrinkage, even porosity, will occur in electroless compositecoatings after reaction of Ni with SiC. The mechanicalproperties of Ni–P–SiC composites deteriorated and themicrohardness decreased after heat treatment at hightemperature. The composite coatings containing the finerparticles, there is the smaller porosity produced after completereaction. So electroless Ni–P–SiC composite coatingscontaining 50 nm SiC particles have higher strength after hightemperature treatment than that of composite coatings withlarger particles.

4. Conclusions

(1) Electroless Ni–P–SiC composite coatings with superfineSiC particles were prepared by electroless plating.Superfine SiC particles dispersed homogeneously and thecontent of particles was at high level in compositecoatings. The structure of the composite coatings asdeposited was amorphous.

(2) During heating, the amorphous alloy in electroless Ni–P–SiC composite coatings crystallized, and then reactedwith SiC at high temperature. The crystallizationtemperature of Ni–P–SiC composite coatings was notinfluenced by superfine particles obviously, but thereaction temperature of electroless composites coatingsdecreased with the decrease in the size of particles. The

final products of the crystallization and reaction ofcomposite coatings were Ni, Ni3P, Ni3Si and freecarbon.

(3) The microhardness of electroless Ni–P–SiC compositecoatings were higher than that of Ni–P alloy coatingdue to the existence of superfine particles, and themicrohardness were increased to the maximum value(about HV501200) after heat treatment at 400 °C for1 h.

Acknowledgements

This work has been funded by the Science and TechnologyCommission of Shanghai Municipal Government (0352 nm025and 035211037), for which the authors are grateful. Thanks arealso due to Dr. Gu Jiajun, Dr. Zhang Hongxiang and Prof. LuoShoufu for their help and advice.

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