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The effect of heat treatment on the microstructure of electroless Ni–P coatings containing SiC particles
Thin Solid Films 416 (2002) 31–37
The effect of heat treatment on the microstructure of electroless Ni–P coatings containing SiC particles C.K. Chen*, H.M. Feng, H.C. Lin, M.H. Hon Department of Materials Science and Engineering, National Cheng Kung University, Tainan 70101, Taiwan, ROC Received 3 August 2001; received in revised form 14 June 2002; accepted 4 July 2002
Abstract Electroless Ni–P coatings containing SiC particles were co-deposited on SKD61 tool steel substrate. The effect of heat treatment on the microstructure of Ni–P–SiC composite coatings was investigated by X-ray diffraction and transmission electron microscopy. The presence of SiC particles did not affect the microstructure of the Ni–P alloy matrix when annealing temperature was below 400 8C. However, by increasing annealing temperature to 450 8C, SiC particles decomposed and reacted with nickel to form gNi5Si2 and b1-Ni3Si phases with a consequent free carbon precipitation. The structure of carbon was crystalline graphite with (0 0 0 2) preferred orientation and tended to aggregate while the amount of free carbon increased. On further annealing at 500 8C, phosphorus was incorporated into the Ni5Si2 lattice, forming a Ni5(Si1yx, Px)2 solid solution. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Ni; SiC; Composite electroless plating; Heat treatment
1. Introduction Current trends of coating techniques involve composite coatings, such as multilayer or multiphase coatings, which are expected to have tailor-made properties for some specific applications. Recent progress in electroless plating is the co-deposition of solid particles into coatings, although electroless Ni–P coatings have been widely used in industry during the past 20 years for wear and corrosion protection. Consequently, functional composite coatings with highly specific characteristics can easily be produced by choosing suitable particulate materials. These solid particles can be hard materials (such as SiC, Al2O3 and diamond) w1–3x to enhance the hardness andyor wear resistance of the deposits, or can be dry lubricants (such as MoS2, PTFE and graphite) w4–6x to impart lubricity and reduce the coefficient of friction. Among the particulate materials used for reinforcement, SiC is the most frequently studied and applied. Broszeit w7x found that mechanical properties, such as *Corresponding author. Tel.: q886-6-2380208; fax: q886-62380208. E-mail address: [email protected] (C.K. Chen).
hardness, strength and elastic modulus can be increased with increasing content of SiC particles in the composite coating. Xinmin and Zongang w8x suggested that the SiC particles can increase the hardness of a composite coating and improve the resistance to abrasion, but a hard and stable matrix is necessary to support them. Unfortunately, the high temperature application of Ni– SiC composite coatings is limited by the thermal decomposition of SiC particles in the nickel matrix at approximately 500 8C w7x. Pan and Baptista w9x established that the nickel silicide, which is thermodynamically more stable than SiC, makes SiC unstable. The chemical instability of SiC in presence of nickel at high temperature results in uncontrollable mechanical properties of the material and limits the application of Ni– P–SiC composite coatings. Microscopically, what really happens to the coating matrix and the particles is not well understood and needs further investigation. In this paper, an attempt was made to incorporate SiC particles into a Ni–P alloy matrix by electroless plating. The purpose of this work was to study the effect of heat treatment on microstructural changes of electroless Ni– P–SiC composite coatings by X-ray diffractometry (XRD) and transmission electron microscopy (TEM).
0040-6090/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 2 . 0 0 6 2 8 - 4
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C.K. Chen et al. / Thin Solid Films 416 (2002) 31–37
Fig. 1. Schematic diagram of plating equipment showing: (a) substrate; (b) plating bath; (c) thermal bath; (d) stirrer.
2. Experimental Tool steel JIS SKD61 specimens with a thickness of 2 mm and a diameter of 1.5 mm were used as the substrate material. The specimens were ground and then surface polished with 1 mm alumina powder. Before electroless plating, the specimens were degreased and ultrasonically cleaned in a dilute hydrochloric acid solution. The Ni–P plating and Ni–P–SiC composite plating were carried out in a beaker heated by a
Fig. 2. SEM micrographs of as-deposited coatings, (a) Ni–P; (b) Ni– P–SiC (100 gyl SiC suspension).
thermostatically controlled bath. The substrates were vertically positioned as illustrated in Fig. 1 in the plating bath which was a commercial electroless nickel solution (Nickora, product of Schering Company). The b-SiC powder with an average particle size of 0.84 mm was added to the bath to produce Ni–P–SiC composite coating. Three concentrations of SiC particles: 0, 10 and 100 gyl were used to obtain coatings with different SiC contents. To keep the particles in suspension, the solutions were mechanically agitated. The bath was kept constantly at pH 5.0 and a temperature of 90 8C. In order to investigate the microstructural stability, the specimens were heat-treated in a vacuum (26.6 Pa) chamber at 350, 400, 450 and 500 8C for 1 h prior to furnace cooling. The composition of the deposited films was determined with a glow discharge optical spectrometer (GDOS; Model LECD GDS-750 QDP). The crosssection morphology was observed by scanning electron microscopy (SEM; Model JOEL JSM-5200). Film structure was analyzed by XRD (Model Rigaku DyMax-IV) with a CuKa X-ray source. Additional structure characterization was performed by energy-dispersive spectroscopy (EDS) and selected-area electron diffraction (SAD) mode of a TEM (Model JEOL JEM-3010) at 300 kV. 3. Results and discussion The cross-sectional SEM micrographs of the deposited Ni–P and composite coatings in Fig. 2 show that the SiC particles are uniformly distributed in the entire Ni– P film matrix. The thicknesses of the Ni–P and Ni–P– SiC coatings are approximately 15 and 20 mm, respectively. The composition of the deposited Ni–P and composite coatings measured by GDOS in Fig. 3 indicates that SiC concentration in the composite coating increases with increasing SiC concentration in the plating bath. Phosphorus concentration in these coatings falls in the range between 6.2 and 6.8 wt.%. Backovic´ et al. w10x indicated that the as-deposited Ni–P alloy forms a metallic glass when the phosphorus content exceeds 6 wt.%. According to the low temperature phase diagram of Ni–P w11x, the structure of as-deposited Ni– P coating consists of microcrystalline b-phase and amorphous g-phase for a phosphorus content between 4.5 and 11 wt.%. The XRD patterns of the as-deposited Ni–P in Fig. 4a show a single broad, diffuse peak which is identified as an amorphous phase. The XRD patterns of the as-deposited Ni–P–SiC composite coatings are similar to that of Ni–P with the exception of a peak at 2us35.68 corresponding to SiC(1 1 1) which becomes stronger with increasing SiC concentration in the specimen, hence the plating bath as determined by GDOS analysis. For the as-deposited films, it can be deduced that both the Ni–P matrix and SiC particles in the
C.K. Chen et al. / Thin Solid Films 416 (2002) 31–37
Fig. 3. Composition of films deposited with different SiC concentrations in the plating bath.
composite coating conserve their original structures. The presence of added SiC particles did not change the structure of the Ni–P matrix. The XRD patterns of the heat-treated Ni–P and composite coatings at various temperatures of 350, 400, 450 and 500 8C for 1 h are shown in Fig. 4b and c, respectively. Duncan w11x indicated that the b-phase converts to a-nickel at 250–290 8C, whilst g-phase converts to Ni3P and a-nickel at 310–330 8C. In this study, the XRD analysis confirms the above results by confirming the transformation from an amorphous structure to crystalline nickel and Ni3P at 350 8C. With a further increase in temperature, the peak intensities of nickel and Ni3P increase without any other phase being detected, which is also consistent with Refs. w11,12x. Comparing with Ni–P coatings, the structure of the Ni– P matrix in the Ni–P–SiC composite coating was changed from amorphous to crystalline at 350 8C, as shown in Fig. 4c. The amount of nickel and Ni3P phases increased further at 400 8C. When the temperature was increased to 450 8C, the diffraction peaks of nickel and SiC phases decreased and the nickel peak shifted to a higher angle. Furthermore, a new peak at approximately 2us47.28 was observed indicating that phase transformation occurred, which became more obvious at 500 8C annealing. Moreover, it can be observed that the new peak slightly shifted to a higher angle, but the peak attributed to SiC eventually disappeared, as shown in Fig. 4d. The above observation from XRD results implies that nickel reacted with SiC to form the new phase, when the annealing temperature exceeded 450 8C, which is somewhat lower than Broszeit’s w7x findings by thermal differential analysis that chemical reac-
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tion between nickel and SiC took place at 580 8C for the Ni–P–SiC composite coating. The new phases resulting from this chemical reaction appeared by optical micrograph observation to be Ni3Si and carbon. Pan and Baptista w9x also indicated that SiC was not stable in the presence of nickel in the temperature range 1127– 1727 8C and the resulting reaction produced nickel silicide and free carbon. According to the International Center for Diffraction Data (ICDD) card file w13x and the Ni–Si phase diagram w14x, the structure of b1-Ni3Si phase is face-centeredcubic with the lattice constant slightly smaller than that of nickel. It may therefore be presumed that the reaction between nickel and SiC produced b1-Ni3Si phase, and resulted in a peak shifting to a higher angle. The extra peak at approximately 2us47.28 might be corresponding to g-Ni5Si2(3 0 0). Canali et al. w15x proposed that the compound formation in a Ni–Si system is driven towards the phases that are richer with the remaining element. In this study, the amount of Si was far lower than that of Ni. In other words, when silicon was consumed the compound formation was driven towards g-Ni5Si2, and subsequently b1-Ni3Si. As a result, the peak at approximately 2us47.28 shifted to a higher angle, while the annealing temperature was increased to above 450 8C. It is then strongly suggested that the products were not only g-Ni5Si2 and b1-Ni3Si, but also another phase. Conclusive identification requires work with TEM, because there was only one peak in the XRD patterns obtained. TEM bright field image of a Ni–P–7.0 wt.% SiC composite coating annealed at 450 8C for 1 h is exhibited in Fig. 5a. From the EDS spectra (in Fig. 5b and c) and SAD patterns (not shown here), it is clear that the phases in regions A and B are Ni3P and Ni, respectively. Also, it was found that nickel particles precipitated in a continuous matrix of Ni3P, as previously reported w16,17x. The composition in region C consists of nickel and silicon as indicated in Fig. 5d. The corresponding SAD patterns, as illustrated in Fig. 6 show that the two adjacent big spots correspond to a superlattice. The big spots of these SAD patterns correspond to the NaCl type crystal structure with Bw1¯ 1 1x, Bw1¯ 1 2x, Bw0 1 1x and Bw0 0 1x zone axes, respectively, which are similar to that of nickel. These superlattice spots confirm a sixfold symmetry in Fig. 6a, a twofold symmetry in Fig. 6b and c, and a fourfold symmetry in Fig. 6d. That is exactly a pattern of superlattice of cubic structure, i.e. the relationship between superlattice and NaCl type reflections is cube–cube orientation. In other words, this compound possesses a NaCl structure with ordered phase, which should be the Ni3Si with ordered L12 structure, as determined by XRD analysis. It is assumed that nickel reacted with SiC to produce the Ni3Si (L12) phase, whose amount increased with increasing anneal-
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C.K. Chen et al. / Thin Solid Films 416 (2002) 31–37
Fig. 4. XRD patterns of (a) as-deposited Ni–P coatings with different SiC contents; (b) Ni–P coatings annealed at different temperatures for 1 h; (c) Ni–P–7.0 wt.% SiC composite coatings annealed at different temperatures for 1 h; and (d) Ni–P coatings with different SiC contents annealed at 500 8C for 1 h.
ing temperature, as well as SiC content in the Ni–P– SiC composite coating. Therefore, the peak in the XRD pattern would be a superimposed one contributed by nickel and Ni3Si phase, and will gradually shift to a higher angle as temperature and SiC content increase. TEM bright field image of Ni–P–3.7 wt.% SiC composite coating annealed at 500 8C for 1 h, Fig. 7a, shows that there is a long and narrow strip (A). EDS spectrum of the composition in region A as exhibited in Fig. 7b shows exclusively carbon. The corresponding SAD pattern in Fig. 7c indicates a crystalline graphite with (0 0 0 2) preferred orientation. Crystalline graphite could also be found in the Ni–Si reaction area (B in Fig. 7a). The phases in the Ni–Si reaction area consist ¨ of nickel silicide and graphite. Gulpen et al. w18x pointed out that when nickel silicide forms, carbon appears as a
separate phase (graphite) since it cannot dissolve in any silicide. Comparing Fig. 5a and Fig. 7a, it appears that the graphite precipitates in the Ni–Si reaction area when the amount of free carbon is low, whereas, it tends to aggregate with increasing amount of free carbon. Fig. 8a shows the TEM bright field image from another arbitrary area of Ni–P–3.7 wt.% SiC composite coating annealed at 500 8C for 1 h and Fig. 8b shows the corresponding SAD pattern which is close to that of g-Ni5Si2 with Bw0 1 1¯ 0x zone axis. From EDS spectrum, Fig. 8c, it appears that this area consists of nickel, silicon and a small amount of phosphorus. Thus, the phase might consist nickel, silicon and phosphorus, presumably which is incorporated into the Ni5Si2 lattice to form a Ni5(Si1yx, Px)2 solid solution, with the value of x between 0.04 and 0.26. Because the ionic radius of
C.K. Chen et al. / Thin Solid Films 416 (2002) 31–37
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Fig. 5. (a) TEM bright field image of Ni–P–7.0 wt.% SiC composite coatings annealed at 450 8C for 1 h, and EDS spectra of regions: (b) A; (c) B and (d) C.
Fig. 6. SAD patterns of region C (in Fig. 5a) along various zone axes for (a) w1¯ 1 1x; (b) w1¯ 1 2x; (c) w0 1 1x and (d) w0 0 1x.
C.K. Chen et al. / Thin Solid Films 416 (2002) 31–37
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Fig. 7. (a) TEM bright field image of Ni–P–3.7 wt.% SiC composite coating annealed at 500 8C for 1 h; (b) EDS spectrum and (c) SAD pattern of region A.
Fig. 8. (a) TEM bright field image of Ni–P–3.7 wt.% SiC composite coating annealed at 500 8C for 1 h; (b) EDS spectrum and (c) SAD pattern of (a).
P3y (0.212 nm) is smaller than that of Si4y (0.271 nm) w19x a substitution of phosphorus atom into silicon lattice leads to a reduction in Ni5Si2 lattice constant, with a shift of the Ni5Si2(3 0 0) peak in XRD pattern to a higher angle, as indicated in Fig. 4c. This evidence for phosphorus substitution in the Ni5Si2 lattice agrees with the results of XRD analysis of the sample annealed at 500 8C for 1 h. Results on the heat-treated Ni–P–SiC composite coating were not so clearly reported before. Also, phases determined in this study may be helpful to tribological applications. Graphite, as an example, is expected to have a great influence on the friction coefficient.
formly distributed in a Ni–P alloy matrix by the electroless composite plating method. The SiC content in the composite coating increased with increasing SiC powder concentration in the plating bath. Heat treatment changed the structure of composite coatings. The structure of Ni–P–SiC composite coatings was similar to that of Ni–P coatings in both the as-deposited and the annealed below 400 8C states. During annealing above 450 8C, the nickel reacted with SiC to produce gNi5Si2, Ni3Si (L12) and graphite precipitate. On further annealing at 500 8C, the phosphorus was incorporated into the Ni5Si2 lattice, forming a Ni5(Si1yx, Px)2 solid solution.
4. Summary
Acknowledgments
SiC particles were successfully incorporated and uni-
The authors wish to thank the National Science
C.K. Chen et al. / Thin Solid Films 416 (2002) 31–37
Council of Taiwan, ROC for financial support under contract ‘NSC89-2218-E-006-017’. References w1x M.R. Kalantary, K.A. Holbrook, P.B. Wells, Trans. Inst. Metal Finish. 71 (1993) 55. w2x K.L. Lin, P.J. Lai, Plat. Surf. Finish. 76 (1989) 48. w3x N. Feldstein, T. Lancsek, R. Barras, R. Spencer, N. Bailey, Prod. Finish. 44 (1980) 65. w4x S.M. Moonir-Vaghefi, A. Saatchi, J. Hejazi, Metal Finish. 95 (1997) 46. w5x L.G. Yu, X.S. Zhang, Thin Solid Films 245 (1994) 98. w6x M. Izzard, J.K. Dennis, Trans. Inst. Metal Finish. 65 (1987) 85. w7x E. Broszeit, Thin Solid Films 95 (1982) 133. w8x H. Xinmin, D. Zongang, Trans. Inst. Metal Finish. 70 (1992) 84.
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w9x Y. Pan, J.L. Baptista, J. Am. Ceram. Soc. 79 (8) (1996) 2017. w10x N. Backovic, ´ M. Jancic, ˇ ´ Lj. Radonjic, ´ Thin Solid Films 59 (1979) 1. w11x R.N. Duncan, Plat. Surf. Finish. 83 (1996) 65. w12x S.H. Park, D.N. Lee, J. Mat. Sci. 23 (1988) 1643. w13x Powder Diffraction File, International Center for Diffraction Data, Newtown Square, PA, 1999, Card 6-690. w14x T.B. Massalski, L. Bennett, L.H. Murray, Binary Alloy Phase Diagrams, ASM International, Metals Park, OH, 1986, p. 1755. w15x C. Canali, G. Majni, G. Ottaviani, G. Celotti, J. Appl. Phys. 50 (1) (1979) 255. w16x H.G. Schenzel, H. Kreye, Plat. Surf. Finish. 77 (1990) 50. w17x A.H. Graham, R.W. Lindsay, H.J. Read, J. Electrochem. Soc. 112 (4) (1965) 401. w18x J.H. Gulpen, ¨ A.A. Kodentsov, F.J.J. van Loo, Mater. Sci. Forum 155y156 (1994) 561. w19x R.C. Evans, An Introduction to Crystal Chemistry, second ed., Cambridge University Press, New York, 1964, p. 36.
The effect of heat treatment on the microstructure of electroless Ni–P coatings containing SiC particles C.K. Chen*, H.M. Feng, H.C. Lin, M.H. Hon Department of Materials Science and Engineering, National Cheng Kung University, Tainan 70101, Taiwan, ROC Received 3 August 2001; received in revised form 14 June 2002; accepted 4 July 2002
Abstract Electroless Ni–P coatings containing SiC particles were co-deposited on SKD61 tool steel substrate. The effect of heat treatment on the microstructure of Ni–P–SiC composite coatings was investigated by X-ray diffraction and transmission electron microscopy. The presence of SiC particles did not affect the microstructure of the Ni–P alloy matrix when annealing temperature was below 400 8C. However, by increasing annealing temperature to 450 8C, SiC particles decomposed and reacted with nickel to form gNi5Si2 and b1-Ni3Si phases with a consequent free carbon precipitation. The structure of carbon was crystalline graphite with (0 0 0 2) preferred orientation and tended to aggregate while the amount of free carbon increased. On further annealing at 500 8C, phosphorus was incorporated into the Ni5Si2 lattice, forming a Ni5(Si1yx, Px)2 solid solution. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Ni; SiC; Composite electroless plating; Heat treatment
1. Introduction Current trends of coating techniques involve composite coatings, such as multilayer or multiphase coatings, which are expected to have tailor-made properties for some specific applications. Recent progress in electroless plating is the co-deposition of solid particles into coatings, although electroless Ni–P coatings have been widely used in industry during the past 20 years for wear and corrosion protection. Consequently, functional composite coatings with highly specific characteristics can easily be produced by choosing suitable particulate materials. These solid particles can be hard materials (such as SiC, Al2O3 and diamond) w1–3x to enhance the hardness andyor wear resistance of the deposits, or can be dry lubricants (such as MoS2, PTFE and graphite) w4–6x to impart lubricity and reduce the coefficient of friction. Among the particulate materials used for reinforcement, SiC is the most frequently studied and applied. Broszeit w7x found that mechanical properties, such as *Corresponding author. Tel.: q886-6-2380208; fax: q886-62380208. E-mail address: [email protected] (C.K. Chen).
hardness, strength and elastic modulus can be increased with increasing content of SiC particles in the composite coating. Xinmin and Zongang w8x suggested that the SiC particles can increase the hardness of a composite coating and improve the resistance to abrasion, but a hard and stable matrix is necessary to support them. Unfortunately, the high temperature application of Ni– SiC composite coatings is limited by the thermal decomposition of SiC particles in the nickel matrix at approximately 500 8C w7x. Pan and Baptista w9x established that the nickel silicide, which is thermodynamically more stable than SiC, makes SiC unstable. The chemical instability of SiC in presence of nickel at high temperature results in uncontrollable mechanical properties of the material and limits the application of Ni– P–SiC composite coatings. Microscopically, what really happens to the coating matrix and the particles is not well understood and needs further investigation. In this paper, an attempt was made to incorporate SiC particles into a Ni–P alloy matrix by electroless plating. The purpose of this work was to study the effect of heat treatment on microstructural changes of electroless Ni– P–SiC composite coatings by X-ray diffractometry (XRD) and transmission electron microscopy (TEM).
0040-6090/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 2 . 0 0 6 2 8 - 4
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C.K. Chen et al. / Thin Solid Films 416 (2002) 31–37
Fig. 1. Schematic diagram of plating equipment showing: (a) substrate; (b) plating bath; (c) thermal bath; (d) stirrer.
2. Experimental Tool steel JIS SKD61 specimens with a thickness of 2 mm and a diameter of 1.5 mm were used as the substrate material. The specimens were ground and then surface polished with 1 mm alumina powder. Before electroless plating, the specimens were degreased and ultrasonically cleaned in a dilute hydrochloric acid solution. The Ni–P plating and Ni–P–SiC composite plating were carried out in a beaker heated by a
Fig. 2. SEM micrographs of as-deposited coatings, (a) Ni–P; (b) Ni– P–SiC (100 gyl SiC suspension).
thermostatically controlled bath. The substrates were vertically positioned as illustrated in Fig. 1 in the plating bath which was a commercial electroless nickel solution (Nickora, product of Schering Company). The b-SiC powder with an average particle size of 0.84 mm was added to the bath to produce Ni–P–SiC composite coating. Three concentrations of SiC particles: 0, 10 and 100 gyl were used to obtain coatings with different SiC contents. To keep the particles in suspension, the solutions were mechanically agitated. The bath was kept constantly at pH 5.0 and a temperature of 90 8C. In order to investigate the microstructural stability, the specimens were heat-treated in a vacuum (26.6 Pa) chamber at 350, 400, 450 and 500 8C for 1 h prior to furnace cooling. The composition of the deposited films was determined with a glow discharge optical spectrometer (GDOS; Model LECD GDS-750 QDP). The crosssection morphology was observed by scanning electron microscopy (SEM; Model JOEL JSM-5200). Film structure was analyzed by XRD (Model Rigaku DyMax-IV) with a CuKa X-ray source. Additional structure characterization was performed by energy-dispersive spectroscopy (EDS) and selected-area electron diffraction (SAD) mode of a TEM (Model JEOL JEM-3010) at 300 kV. 3. Results and discussion The cross-sectional SEM micrographs of the deposited Ni–P and composite coatings in Fig. 2 show that the SiC particles are uniformly distributed in the entire Ni– P film matrix. The thicknesses of the Ni–P and Ni–P– SiC coatings are approximately 15 and 20 mm, respectively. The composition of the deposited Ni–P and composite coatings measured by GDOS in Fig. 3 indicates that SiC concentration in the composite coating increases with increasing SiC concentration in the plating bath. Phosphorus concentration in these coatings falls in the range between 6.2 and 6.8 wt.%. Backovic´ et al. w10x indicated that the as-deposited Ni–P alloy forms a metallic glass when the phosphorus content exceeds 6 wt.%. According to the low temperature phase diagram of Ni–P w11x, the structure of as-deposited Ni– P coating consists of microcrystalline b-phase and amorphous g-phase for a phosphorus content between 4.5 and 11 wt.%. The XRD patterns of the as-deposited Ni–P in Fig. 4a show a single broad, diffuse peak which is identified as an amorphous phase. The XRD patterns of the as-deposited Ni–P–SiC composite coatings are similar to that of Ni–P with the exception of a peak at 2us35.68 corresponding to SiC(1 1 1) which becomes stronger with increasing SiC concentration in the specimen, hence the plating bath as determined by GDOS analysis. For the as-deposited films, it can be deduced that both the Ni–P matrix and SiC particles in the
C.K. Chen et al. / Thin Solid Films 416 (2002) 31–37
Fig. 3. Composition of films deposited with different SiC concentrations in the plating bath.
composite coating conserve their original structures. The presence of added SiC particles did not change the structure of the Ni–P matrix. The XRD patterns of the heat-treated Ni–P and composite coatings at various temperatures of 350, 400, 450 and 500 8C for 1 h are shown in Fig. 4b and c, respectively. Duncan w11x indicated that the b-phase converts to a-nickel at 250–290 8C, whilst g-phase converts to Ni3P and a-nickel at 310–330 8C. In this study, the XRD analysis confirms the above results by confirming the transformation from an amorphous structure to crystalline nickel and Ni3P at 350 8C. With a further increase in temperature, the peak intensities of nickel and Ni3P increase without any other phase being detected, which is also consistent with Refs. w11,12x. Comparing with Ni–P coatings, the structure of the Ni– P matrix in the Ni–P–SiC composite coating was changed from amorphous to crystalline at 350 8C, as shown in Fig. 4c. The amount of nickel and Ni3P phases increased further at 400 8C. When the temperature was increased to 450 8C, the diffraction peaks of nickel and SiC phases decreased and the nickel peak shifted to a higher angle. Furthermore, a new peak at approximately 2us47.28 was observed indicating that phase transformation occurred, which became more obvious at 500 8C annealing. Moreover, it can be observed that the new peak slightly shifted to a higher angle, but the peak attributed to SiC eventually disappeared, as shown in Fig. 4d. The above observation from XRD results implies that nickel reacted with SiC to form the new phase, when the annealing temperature exceeded 450 8C, which is somewhat lower than Broszeit’s w7x findings by thermal differential analysis that chemical reac-
33
tion between nickel and SiC took place at 580 8C for the Ni–P–SiC composite coating. The new phases resulting from this chemical reaction appeared by optical micrograph observation to be Ni3Si and carbon. Pan and Baptista w9x also indicated that SiC was not stable in the presence of nickel in the temperature range 1127– 1727 8C and the resulting reaction produced nickel silicide and free carbon. According to the International Center for Diffraction Data (ICDD) card file w13x and the Ni–Si phase diagram w14x, the structure of b1-Ni3Si phase is face-centeredcubic with the lattice constant slightly smaller than that of nickel. It may therefore be presumed that the reaction between nickel and SiC produced b1-Ni3Si phase, and resulted in a peak shifting to a higher angle. The extra peak at approximately 2us47.28 might be corresponding to g-Ni5Si2(3 0 0). Canali et al. w15x proposed that the compound formation in a Ni–Si system is driven towards the phases that are richer with the remaining element. In this study, the amount of Si was far lower than that of Ni. In other words, when silicon was consumed the compound formation was driven towards g-Ni5Si2, and subsequently b1-Ni3Si. As a result, the peak at approximately 2us47.28 shifted to a higher angle, while the annealing temperature was increased to above 450 8C. It is then strongly suggested that the products were not only g-Ni5Si2 and b1-Ni3Si, but also another phase. Conclusive identification requires work with TEM, because there was only one peak in the XRD patterns obtained. TEM bright field image of a Ni–P–7.0 wt.% SiC composite coating annealed at 450 8C for 1 h is exhibited in Fig. 5a. From the EDS spectra (in Fig. 5b and c) and SAD patterns (not shown here), it is clear that the phases in regions A and B are Ni3P and Ni, respectively. Also, it was found that nickel particles precipitated in a continuous matrix of Ni3P, as previously reported w16,17x. The composition in region C consists of nickel and silicon as indicated in Fig. 5d. The corresponding SAD patterns, as illustrated in Fig. 6 show that the two adjacent big spots correspond to a superlattice. The big spots of these SAD patterns correspond to the NaCl type crystal structure with Bw1¯ 1 1x, Bw1¯ 1 2x, Bw0 1 1x and Bw0 0 1x zone axes, respectively, which are similar to that of nickel. These superlattice spots confirm a sixfold symmetry in Fig. 6a, a twofold symmetry in Fig. 6b and c, and a fourfold symmetry in Fig. 6d. That is exactly a pattern of superlattice of cubic structure, i.e. the relationship between superlattice and NaCl type reflections is cube–cube orientation. In other words, this compound possesses a NaCl structure with ordered phase, which should be the Ni3Si with ordered L12 structure, as determined by XRD analysis. It is assumed that nickel reacted with SiC to produce the Ni3Si (L12) phase, whose amount increased with increasing anneal-
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C.K. Chen et al. / Thin Solid Films 416 (2002) 31–37
Fig. 4. XRD patterns of (a) as-deposited Ni–P coatings with different SiC contents; (b) Ni–P coatings annealed at different temperatures for 1 h; (c) Ni–P–7.0 wt.% SiC composite coatings annealed at different temperatures for 1 h; and (d) Ni–P coatings with different SiC contents annealed at 500 8C for 1 h.
ing temperature, as well as SiC content in the Ni–P– SiC composite coating. Therefore, the peak in the XRD pattern would be a superimposed one contributed by nickel and Ni3Si phase, and will gradually shift to a higher angle as temperature and SiC content increase. TEM bright field image of Ni–P–3.7 wt.% SiC composite coating annealed at 500 8C for 1 h, Fig. 7a, shows that there is a long and narrow strip (A). EDS spectrum of the composition in region A as exhibited in Fig. 7b shows exclusively carbon. The corresponding SAD pattern in Fig. 7c indicates a crystalline graphite with (0 0 0 2) preferred orientation. Crystalline graphite could also be found in the Ni–Si reaction area (B in Fig. 7a). The phases in the Ni–Si reaction area consist ¨ of nickel silicide and graphite. Gulpen et al. w18x pointed out that when nickel silicide forms, carbon appears as a
separate phase (graphite) since it cannot dissolve in any silicide. Comparing Fig. 5a and Fig. 7a, it appears that the graphite precipitates in the Ni–Si reaction area when the amount of free carbon is low, whereas, it tends to aggregate with increasing amount of free carbon. Fig. 8a shows the TEM bright field image from another arbitrary area of Ni–P–3.7 wt.% SiC composite coating annealed at 500 8C for 1 h and Fig. 8b shows the corresponding SAD pattern which is close to that of g-Ni5Si2 with Bw0 1 1¯ 0x zone axis. From EDS spectrum, Fig. 8c, it appears that this area consists of nickel, silicon and a small amount of phosphorus. Thus, the phase might consist nickel, silicon and phosphorus, presumably which is incorporated into the Ni5Si2 lattice to form a Ni5(Si1yx, Px)2 solid solution, with the value of x between 0.04 and 0.26. Because the ionic radius of
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Fig. 5. (a) TEM bright field image of Ni–P–7.0 wt.% SiC composite coatings annealed at 450 8C for 1 h, and EDS spectra of regions: (b) A; (c) B and (d) C.
Fig. 6. SAD patterns of region C (in Fig. 5a) along various zone axes for (a) w1¯ 1 1x; (b) w1¯ 1 2x; (c) w0 1 1x and (d) w0 0 1x.
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Fig. 7. (a) TEM bright field image of Ni–P–3.7 wt.% SiC composite coating annealed at 500 8C for 1 h; (b) EDS spectrum and (c) SAD pattern of region A.
Fig. 8. (a) TEM bright field image of Ni–P–3.7 wt.% SiC composite coating annealed at 500 8C for 1 h; (b) EDS spectrum and (c) SAD pattern of (a).
P3y (0.212 nm) is smaller than that of Si4y (0.271 nm) w19x a substitution of phosphorus atom into silicon lattice leads to a reduction in Ni5Si2 lattice constant, with a shift of the Ni5Si2(3 0 0) peak in XRD pattern to a higher angle, as indicated in Fig. 4c. This evidence for phosphorus substitution in the Ni5Si2 lattice agrees with the results of XRD analysis of the sample annealed at 500 8C for 1 h. Results on the heat-treated Ni–P–SiC composite coating were not so clearly reported before. Also, phases determined in this study may be helpful to tribological applications. Graphite, as an example, is expected to have a great influence on the friction coefficient.
formly distributed in a Ni–P alloy matrix by the electroless composite plating method. The SiC content in the composite coating increased with increasing SiC powder concentration in the plating bath. Heat treatment changed the structure of composite coatings. The structure of Ni–P–SiC composite coatings was similar to that of Ni–P coatings in both the as-deposited and the annealed below 400 8C states. During annealing above 450 8C, the nickel reacted with SiC to produce gNi5Si2, Ni3Si (L12) and graphite precipitate. On further annealing at 500 8C, the phosphorus was incorporated into the Ni5Si2 lattice, forming a Ni5(Si1yx, Px)2 solid solution.
4. Summary
Acknowledgments
SiC particles were successfully incorporated and uni-
The authors wish to thank the National Science
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