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Characterisation and mechanical properties of electroless NiP–ZrO2 coatings
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Surface & Coatings Technology 202 (2007) 1167 – 1171 www.elsevier.com/locate/surfcoat
Characterisation and mechanical properties of electroless NiP–ZrO2 coatings P.-A. Gay a,⁎, J.M. Limat a , P.-A. Steinmann a , J. Pagetti b a
b
University of Applied Sciences, HE-ARC, Avenue de l'Hôtel-de-Ville, 7, CH-2400 Le Locle, Switzerland Institut des Traitements de Surface de Franche-Comté, Equipe Corrosion Traitements de Surface et Systèmes électrochimiques, Université de Franche-Comté, 16, route de Gray, F-25030 Besançon Cedex, France Available online 2 June 2007
Abstract The paper describes the deposition of the composite NiP–ZrO2 coatings by electroless process, and the characterisation of the coating mechanical and tribological properties. The process parameters, namely deposition temperature, particle concentration in the bath and stirring rate, were systematically investigated with respect to the incorporation rate (Vp) of the zirconia particles in the NiP coatings. The mechanical and tribological properties of the composite coatings are strongly influenced by the incorporation rate. In fact, the values of hardness, friction coefficient and wear resistance of the coatings increased with the increase of the Vp and the temperature of the post-deposition heat treatment. © 2007 Elsevier B.V. All rights reserved. Keywords: NiP; Electroless plating; ZrO2; Hardness; Composite
1. Introduction Incorporation of finely dispersed particles into a metallic matrix, achieved by the co-deposition process, led to a new generation of composites, presenting particular chemical and physical properties. Various theories describe the mechanism of particle co-deposition [1–11].Most investigations have been made from a qualitative point of view. Gugliemi, in electroplating, quantitatively investigated the mechanism of particle co-deposition with Langmuir's adsorption model [12–15].Celis et al. [4] studied the influence of some parameters (e.g. hydrodynamics) on the particle incorporation, and several papers have been published on this topic [11,16–18]]. Recently, the growing interest to composite materials brought researchers to incorporate fine solid particles in a metallic matrix [16,19–21]. In this way, interesting characteristics of the plated coatings, showing homogeneous particle dispersion within the composite, have been obtained. The use of ceramic particles offers many advantages in terms of hardness, friction coefficient, chemical inertness, and thermal stability of the resulting composite coatings. Electroless NiP coatings show better corrosion resistance compared to electrodeposited Ni coatings because of the pre⁎ Corresponding author. Tel.: +41 32 930 13 86; fax: +41 32 930 13 14. E-mail address: [email protected] (P.-A. Gay). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.05.081
sence of phosphorus. After heat treatment and crystallisation of NiP matrix, the hardness of the NiP coatings can be further increased. Composite NiP–ZrO2 coatings show even better mechanical performance due to the presence of hard zirconia particles [21]. The aim of this work is to produce NiP–ZrO2 coatings by electroless plating process, and to investigate the influence of the incorporation rate of ZrO2 particles on the mechanical and tribological properties of the composite coatings. 2. Experimental details The plating solution was an electroless nickel (EN) bath containing NiSO4 30 g l− 1, NaH2PO2 H2O 38 g l− 1, lactic acid (85%) 15 ml l− 1, malic acid 15 g l− 1, citric acid 30 g l− 1, succinic acid 5 g l− 1, propionic acid 5 g l− 1, MoO3 5 g l− 1, and fluorinated surfactant 1.5 g l− 1. The concentration of ZrO2 particles (C) was varying from 0 to 90 g l− 1. Typical properties of ZrO2 particles, having an average size of 0.9 μm, are described in [22]. The depositions were carried out at pH 4.8 ± 0.5 at three different temperatures (70, 80 and 90 °C), in an aerated solution stirred with a magnetic bar at a speed varying from 100 to 1300 rpm. During the deposition, the particles were kept suspended in the bath by a conventional mechanically controlled
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glass blade. Deposition time was adjusted between 45 and 90 min. Thickness of the coatings was controlled by a system based on X-ray fluorescence measurement (Fisherscope X-Ray 1600). The incorporation rate of zirconia particles included in the metallic matrix was determined with a Scanning Electron Microscope (SEM Jeol model JSM 5600) and Energy Dispersive X-Ray Spectroscopy (EDS Fondis). The incorporation rate Vp was calculated in wt.%. The experimental device was made of an electrolytic cell, containing the plating bath and the immersed cathode. The cathode was a 10 cm2 brass plate placed vertically. Before plating, the brass plates were first degreased in alkaline bath, rinsed with deionised water and etched in acidic bath. Hardness measurements were performed using a Schimadzu microhardness tester, calibrated with a load of 100 g, allowing to obtain a reproducible imprint area in order to improve the measuring accuracy. To avoid the influence of the substrate, the Jönsson-Hogmark method [23] was applied. Friction coefficient and wear resistance of the composite coatings were evaluated from pin-on-disc tests. During the test, the sample was rotated with an angular velocity of 14 rad s− 1. The counterpart was steel 100Cr6 ball (10 mm in diameter) loaded at 2 N. The pin-on-disc test was carried out over 5000 cycles. The friction coefficient was calculated by averaging over the whole test duration. In order to evaluate the wear resistance of coatings, profilometry analysis of the wear scars was done. The wear scar depth and width were measured. 3. Results and discussion 3.1. Deposition rate The deposition rate of the NiP coatings is influenced by the deposition temperature, increasing from 7 μm h− 1 (at 70 °C) to 20 μm h− 1 (at 90 °C), because the temperature influences the oxidation–reduction of hypophosphite. Fig. 1 shows the deposition rate versus particles concentration in the bath. For the concentrations higher than 30 g l− 1 the deposition rate decreases. It can be explained by the reduction of NiP on the surface of the ZrO2 particles, and, at the same time, the modification of the substrate area occurring during the co-
Fig. 1. Deposition rate versus particle concentration in the bath of the NiP–ZrO2 coatings deposited at different deposition temperatures.
Fig. 2. The relationship between particle concentration in the bath and Vp for three deposition temperatures. The stirring rate was 400 rpm.
deposition process. This result is different from that of Shibli [21], most likely due to the differencies in the bath composition and the deposition temperature. 3.2. Factors influencing the incorporation rate of the particles 3.2.1. Temperature and particle concentration in the bath The relationship between particle concentrations in the bath and Vp for three deposition temperatures is shown in Fig. 2. This relation is non-linear, exhibiting a maximum of Vp for the particle concentration of about 30 g l− 1 for all studied temperatures. The maximum values of Vp are related to the deposition temperature, and are equal to 2.1, 3.4 and 7.4% for T = 70, 80 and 90 °C, respectively. These results differ from the results of Shibli et al. [21] as they found Vp = 7.4% for the particle concentration of 12 g l− 1. Such a discrepancy can be explained by different nature and size of zirconia particles. In this study we used smaller particles compared to [21] (about 0.9 μm). The studies dealing with electroless coatings containing ZrO2, Al2O3, CaF2 and SiC particles show similar phenomenon [16,17,24]. 3.2.2. Stirring rate Fig. 3 shows the relationship between stirring rate and Vp for different deposition temperatures. This relationship is again temperature-sensitive and exhibits a complex behaviour.
Fig. 3. Effect of the stirring rate on Vp for three different temperatures. C =30 g l− 1.
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due to their high specific gravity (5.75 g cm− 3); as a consequence, the Vp is very weak (1%). The reduction of incorporation rate is also due to the collision factor [24]. These results agree with literature data [17,25–27] showing the effect of stirring rate on the incorporation of the fine particles in a metal matrix. A maximum stirring rate, for which all particles are kept in suspension in the plating bath, was determined. Up to this value, a decrease of the incorporation rate due to the collision factor was observed. As a general rule, the co-deposition of fine particles is obtained by suspending all particles in the electrolyte. The suspension stability depends on the particle size, particle density and the stirring rate. Williams and Martin [26] reported that high stirring rates lead to a decrease in particle co-deposition, because the particles are swept away from the cathode surface before they are built in. 3.3. Properties of the NiP–ZrO2 composite coatings 3.3.1. Morphology observations SEM photograph of the surface morphology of a particlefree NiP coating deposited at 90 °C and stirring rate of 800 rpm reveals a white smooth surface (Fig. 4a). Fig. 4b shows the effect of zirconia particles on the surface morphology of the coatings (for a maximum Vp = 7.4%). The surface of the composite coatings exhibits ZrO2 particles of an irregular shape, occasionally assembled in small aggregates. SEM cross-section image of a NiP–ZrO2 coating (Fig. 5) shows the presence of zirconia particles homogeneously dispersed in the NiP matrix. EDS analysis confirms the chemical composition of the particles and of the matrix.
Fig. 4. SEM micrographs of the surface morphology of the a) NiP coating deposited at 90 °C and b) a NiP–ZrO2 composite coating deposited at T = 90 °C (C = 30 g l− 1; Vp = 7.4%).
At low deposition temperature (70 °C), the incorporation rate is weak (b1%) and slightly influenced by stirring rate up to 700 rpm. For higher stirring rates Vp increased to a maximum of 2% at 800 rpm, and then decreased to its lowest value (b 1%). At 80 °C, the behaviour is similar but the maximum of Vp appeared at the stirring rate of 700 rpm. Finally at 90 °C, Vp increased for stirring rates from 350 to 800 rpm, and the incorporation rate of zirconia particles is the highest (about 7.4%). At stirring rates between 850 and 900 rpm the incorporation rate decreased down to 1%, then showed a second maximum of 4% at about 1000 rpm and, finally, at higher stirring rates (N1200 rpm), Vp decreased again. This phenomenon can be explained by the volume of aggregated particles at high stirring rate. For low rates (b500 rpm), a considerable amount of the particles can be observed at the bottom of the electrolytic cell
3.3.2. Hardness The hardness of the NiP–ZrO2 coatings, deposited at 90 °C using different particle concentrations in the bath, and heat treated after deposition for 1 h is shown in Fig. 6. The hardness of all coatings strongly influenced by the concentration of the incorporated ZrO2 particles. For the as-deposited NiP coating, hardness is about 520 kg mm− 2. The hardness of as-deposited zirconia-containing coatings increased up to about 630 kg mm− 2.
Fig. 5. SEM cross-section micrograph of the NiP–ZrO2 composite coating: T = 90 °C, C = 30 g l− 1; Vp = 7.4 wt.%.
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P.-A. Gay et al. / Surface & Coatings Technology 202 (2007) 1167–1171 Table 1 Wear resistance of the NiP–ZrO2 coatings: wear scar width and depth after pinon-disc test
Fig. 6. Hardness of the NiP–ZrO2 coatings deposited with different particle concentration in the bath versus temperature of post-deposition heat treatment (deposition temperature was 90 °C).
Lower hardness values (about 530 kg mm− 2) were observed for the coating deposited at the particle concentration in the bath of 90 g l− 1. Similar influence of ZrO2 particles on composite hardness was observed in [21,22]. After heat treatment at 400 °C the hardness of the coatings increased up to about 1000–1150 kg mm− 2, except for the coating elaborated at particle concentration of 90 g l− 1. Such an exceptional behaviour can be explained by the agglomeration of the ZrO2 particles on the surface of this coating. 3.3.3. Friction and wear resistance Fig. 7 shows the evolution of friction coefficient, μ, of the coatings for different particle concentrations in the bath. The average friction coefficient of all NiP–ZrO2 and NiP coatings is about 0.7. After the heat treatment the friction coefficient of the coatings decreased to about 0.6. The lowest values of friction coefficient (μ = 0.52) were measured for the coating deposited with the particle concentration of 30 g l− 1 and heat treated at 400 °C. Contrary to the Ag–ZrO2 composite coatings, where the friction coefficient decreases with increasing Vp due to significant change in the surface morphology, in case of NiP– ZrO2 coatings, there is no similar decrease of μ. ZrO2 is not a lubricant phase, therefore its incorporation in the metallic matrix will not reduce μ. In case of the Ag–ZrO2 coatings, the surface
Fig. 7. Friction coefficient of the coatings (as-deposited and heat-treated at different temperatures) as a function of the particle concentration in the bath.
Particle concentration, g l− 1
Wear scar width, μm
Wear scar depth, μm
0 5 15 30 60
798.2 718.8 654.2 439.2 713.5
15.4 12.1 14.8 4 15.2
morphology (e.i. formation of compact Ag crystallites incorporated between ZrO2 agglomerates) results in better sliding conditions [22]. Table 1 presents the values of depth and width of the wear scars to compare the wear resistance of the coatings. We can observe that wear resistance of the composite coatings increased with the particle concentration in the bath, or, in other words, it is strongly related to the concentration of zirconia particles embedded in the coating, Vp. The lowest wear was observed for the coating deposited with particle concentration of 30 g l− 1 (Vp = 7.4%). In this case the volume of material removed during the wear test was ten times smaller than that of a pure NiP coating. These results show that the addition of hard zirconia phase significantly improves wear resistance of the composite coatings. Similar observations were made by Shibli et al [21]. 4. Conclusions NiP–ZrO2 composite coatings were elaborated by an electroless plating process using a NiP bath. The incorporation of zirconia particles in the NiP coatings depends on the deposition temperature, particle concentration in the electrolyte and stirring rate. Setting the deposition temperature at 90 °C, the particle concentration in the bath at 30 g l− 1 and changing the stirring rate between 700 and 900 rpm, the incorporation rate of zirconia particles varied between 4 and 7.4%. With addition of ZrO2 particles in the bath, the surface morphology of the composite coatings changes from smooth to rough, exhibiting embedded zirconia particles of irregular shape. NiP–ZrO2 coatings show interesting mechanical properties. The hardness of as-deposited coatings increased slightly with the particle concentration in the coatings. A considerable increase of hardness was observed for the coatings after heat treatment of 400 °C. The hardness reached a value of 1150 kg mm− 2 for the coating containing 7.4% of ZrO2. The lowest friction coefficient of 0.52 was measured on the NiP–ZrO2 coating with maximum particle concentration (7.4%) after heat treatment at 400 °C. The wear resistance of the composite coatings can be considerably increased as compared to pure NiP coating. Again, the best tribological performance in terms of wear resistance was found for the coating containing 7.4% of ZrO2. In conclusion, the composite NiP–ZrO2 coating elaborated with Vp of 7.4% presents high hardness, low friction coefficient and good wear resistance.
P.-A. Gay et al. / Surface & Coatings Technology 202 (2007) 1167–1171
Acknowledgement The authors acknowledge the Centre Technique de l'Industrie Horlogère (CETEHOR), Besançon, France for support and technical help in this research. References [1] A. Grosjean, M. Rezrazi, P. Bercot, M. Tachez, Metal Finish. 96 (1998) 14. [2] G.R. Lakshminnarayanan, E.S. Chen, F.K. Sautter, J. Electrochem. Soc., 122 (1975) 1589. [3] D.W. Snaith, P.D. Groves, Trans. Inst. Met. Finish. 50 (1972) 95. [4] J.P. Celis, J.R. Roos, C. Buelens, J. Electrochem. Soc. 134 (1987) 1402. [5] A.M.J. Kariaper, J. Foster, Trans. Inst. Met. Finish. 54 (1974) 87. [6] D.W. Snaith, P.D. Groves, Trans. Inst. Met. Finish. 50 (1972) 95. [7] E.C. Kedward, Metallurgia 6 (1969) 221. [8] J.R. Roos, J.P. Celis, J. Fransaer, C. Buelens, J. Met. 42 (1960) 60. [9] J.P. Celis, J.R. Roos, C. Buelens, J. Fransaer, Trans. Inst. Met. Finish. 69 (1991) 133. [10] J. Fransaer, J.P. Celis, J.R. Roos, Met. Finish. 91 (1993) 97. [11] A. Hovestad, L.J.J. Janssen, J. Appl. Electrochem. 25 (1995) 519.
[12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
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N. Guglielmi, J. Electrochem. Soc., 119 (1972) 1009. M. Ruimi, K.V. Quang, Technique de l'Ingénieur M 1626 (1990) 10. J.P. Celis, J.R. Roos, C. Buelens, J. Electrochem. Soc., 124 (1977) 1508. Y.S. Huang, X.T. Zeng, I. Annergren, F.M. Liu, Surf. Coat. Technol. 167 (2003) 207. Y.L. Wang, Y.Z. Wan, Sh. M. Zhao, H.M. Tao, X.H. Dong, Surf. Coat. Technol. 106 (1998) 162. C.F. Malfatti, J. Zoppas Ferreira, C.B. Santos, B.V. Souza, E.P. Fallavena, S. Vaillant, J.P. Bonino, Corros. Sci. 47 (2005) 567. J. Foster, B. Cameron, Trans. Inst. Met. Finish. 54 (1988) 178. A. Grosjean, M. Rezrazi, P. Berçot, Surf. Coat. Technol. 130 (2000) 252. E. Pena, Munoz, P. Berçot, A. Grosjean, M. Rezrazi, J. Pagetti, Surf. Coat. Technol. 107 (1998) 85. S.M.A. Shibli, V.S. Dilimon, T. Deepthi, Appl. Surf. Sci. 253 (2006) 2189. P.A. Gay, P. Berçot, J. Pagetti, Surf. Coat. Technol. 140 (2001) 147–154. D. Chicot, J. Lesage, Matér. Tech. 1011 (1994) 19. C.F. Malfatti, H.M. Veit, T.L. Menezes, J. Zoppas Ferreira, J.S. Rodriguês, J.P. Bonino, Surf. Coat. Technol. 201 (2007) 6318. M. Ruimi, K.V. Quang, Techniques de l'Ingénieur M 1626 (10) (1990) 2. R.V. Williams, P.W. Martin, Trans. Inst. Met. Finish. 42 (1964) 182. C.C. Lee, C.C. Wan, J. Electrochem. Soc., 135 (1988) 1930.
Surface & Coatings Technology 202 (2007) 1167 – 1171 www.elsevier.com/locate/surfcoat
Characterisation and mechanical properties of electroless NiP–ZrO2 coatings P.-A. Gay a,⁎, J.M. Limat a , P.-A. Steinmann a , J. Pagetti b a
b
University of Applied Sciences, HE-ARC, Avenue de l'Hôtel-de-Ville, 7, CH-2400 Le Locle, Switzerland Institut des Traitements de Surface de Franche-Comté, Equipe Corrosion Traitements de Surface et Systèmes électrochimiques, Université de Franche-Comté, 16, route de Gray, F-25030 Besançon Cedex, France Available online 2 June 2007
Abstract The paper describes the deposition of the composite NiP–ZrO2 coatings by electroless process, and the characterisation of the coating mechanical and tribological properties. The process parameters, namely deposition temperature, particle concentration in the bath and stirring rate, were systematically investigated with respect to the incorporation rate (Vp) of the zirconia particles in the NiP coatings. The mechanical and tribological properties of the composite coatings are strongly influenced by the incorporation rate. In fact, the values of hardness, friction coefficient and wear resistance of the coatings increased with the increase of the Vp and the temperature of the post-deposition heat treatment. © 2007 Elsevier B.V. All rights reserved. Keywords: NiP; Electroless plating; ZrO2; Hardness; Composite
1. Introduction Incorporation of finely dispersed particles into a metallic matrix, achieved by the co-deposition process, led to a new generation of composites, presenting particular chemical and physical properties. Various theories describe the mechanism of particle co-deposition [1–11].Most investigations have been made from a qualitative point of view. Gugliemi, in electroplating, quantitatively investigated the mechanism of particle co-deposition with Langmuir's adsorption model [12–15].Celis et al. [4] studied the influence of some parameters (e.g. hydrodynamics) on the particle incorporation, and several papers have been published on this topic [11,16–18]]. Recently, the growing interest to composite materials brought researchers to incorporate fine solid particles in a metallic matrix [16,19–21]. In this way, interesting characteristics of the plated coatings, showing homogeneous particle dispersion within the composite, have been obtained. The use of ceramic particles offers many advantages in terms of hardness, friction coefficient, chemical inertness, and thermal stability of the resulting composite coatings. Electroless NiP coatings show better corrosion resistance compared to electrodeposited Ni coatings because of the pre⁎ Corresponding author. Tel.: +41 32 930 13 86; fax: +41 32 930 13 14. E-mail address: [email protected] (P.-A. Gay). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.05.081
sence of phosphorus. After heat treatment and crystallisation of NiP matrix, the hardness of the NiP coatings can be further increased. Composite NiP–ZrO2 coatings show even better mechanical performance due to the presence of hard zirconia particles [21]. The aim of this work is to produce NiP–ZrO2 coatings by electroless plating process, and to investigate the influence of the incorporation rate of ZrO2 particles on the mechanical and tribological properties of the composite coatings. 2. Experimental details The plating solution was an electroless nickel (EN) bath containing NiSO4 30 g l− 1, NaH2PO2 H2O 38 g l− 1, lactic acid (85%) 15 ml l− 1, malic acid 15 g l− 1, citric acid 30 g l− 1, succinic acid 5 g l− 1, propionic acid 5 g l− 1, MoO3 5 g l− 1, and fluorinated surfactant 1.5 g l− 1. The concentration of ZrO2 particles (C) was varying from 0 to 90 g l− 1. Typical properties of ZrO2 particles, having an average size of 0.9 μm, are described in [22]. The depositions were carried out at pH 4.8 ± 0.5 at three different temperatures (70, 80 and 90 °C), in an aerated solution stirred with a magnetic bar at a speed varying from 100 to 1300 rpm. During the deposition, the particles were kept suspended in the bath by a conventional mechanically controlled
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P.-A. Gay et al. / Surface & Coatings Technology 202 (2007) 1167–1171
glass blade. Deposition time was adjusted between 45 and 90 min. Thickness of the coatings was controlled by a system based on X-ray fluorescence measurement (Fisherscope X-Ray 1600). The incorporation rate of zirconia particles included in the metallic matrix was determined with a Scanning Electron Microscope (SEM Jeol model JSM 5600) and Energy Dispersive X-Ray Spectroscopy (EDS Fondis). The incorporation rate Vp was calculated in wt.%. The experimental device was made of an electrolytic cell, containing the plating bath and the immersed cathode. The cathode was a 10 cm2 brass plate placed vertically. Before plating, the brass plates were first degreased in alkaline bath, rinsed with deionised water and etched in acidic bath. Hardness measurements were performed using a Schimadzu microhardness tester, calibrated with a load of 100 g, allowing to obtain a reproducible imprint area in order to improve the measuring accuracy. To avoid the influence of the substrate, the Jönsson-Hogmark method [23] was applied. Friction coefficient and wear resistance of the composite coatings were evaluated from pin-on-disc tests. During the test, the sample was rotated with an angular velocity of 14 rad s− 1. The counterpart was steel 100Cr6 ball (10 mm in diameter) loaded at 2 N. The pin-on-disc test was carried out over 5000 cycles. The friction coefficient was calculated by averaging over the whole test duration. In order to evaluate the wear resistance of coatings, profilometry analysis of the wear scars was done. The wear scar depth and width were measured. 3. Results and discussion 3.1. Deposition rate The deposition rate of the NiP coatings is influenced by the deposition temperature, increasing from 7 μm h− 1 (at 70 °C) to 20 μm h− 1 (at 90 °C), because the temperature influences the oxidation–reduction of hypophosphite. Fig. 1 shows the deposition rate versus particles concentration in the bath. For the concentrations higher than 30 g l− 1 the deposition rate decreases. It can be explained by the reduction of NiP on the surface of the ZrO2 particles, and, at the same time, the modification of the substrate area occurring during the co-
Fig. 1. Deposition rate versus particle concentration in the bath of the NiP–ZrO2 coatings deposited at different deposition temperatures.
Fig. 2. The relationship between particle concentration in the bath and Vp for three deposition temperatures. The stirring rate was 400 rpm.
deposition process. This result is different from that of Shibli [21], most likely due to the differencies in the bath composition and the deposition temperature. 3.2. Factors influencing the incorporation rate of the particles 3.2.1. Temperature and particle concentration in the bath The relationship between particle concentrations in the bath and Vp for three deposition temperatures is shown in Fig. 2. This relation is non-linear, exhibiting a maximum of Vp for the particle concentration of about 30 g l− 1 for all studied temperatures. The maximum values of Vp are related to the deposition temperature, and are equal to 2.1, 3.4 and 7.4% for T = 70, 80 and 90 °C, respectively. These results differ from the results of Shibli et al. [21] as they found Vp = 7.4% for the particle concentration of 12 g l− 1. Such a discrepancy can be explained by different nature and size of zirconia particles. In this study we used smaller particles compared to [21] (about 0.9 μm). The studies dealing with electroless coatings containing ZrO2, Al2O3, CaF2 and SiC particles show similar phenomenon [16,17,24]. 3.2.2. Stirring rate Fig. 3 shows the relationship between stirring rate and Vp for different deposition temperatures. This relationship is again temperature-sensitive and exhibits a complex behaviour.
Fig. 3. Effect of the stirring rate on Vp for three different temperatures. C =30 g l− 1.
P.-A. Gay et al. / Surface & Coatings Technology 202 (2007) 1167–1171
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due to their high specific gravity (5.75 g cm− 3); as a consequence, the Vp is very weak (1%). The reduction of incorporation rate is also due to the collision factor [24]. These results agree with literature data [17,25–27] showing the effect of stirring rate on the incorporation of the fine particles in a metal matrix. A maximum stirring rate, for which all particles are kept in suspension in the plating bath, was determined. Up to this value, a decrease of the incorporation rate due to the collision factor was observed. As a general rule, the co-deposition of fine particles is obtained by suspending all particles in the electrolyte. The suspension stability depends on the particle size, particle density and the stirring rate. Williams and Martin [26] reported that high stirring rates lead to a decrease in particle co-deposition, because the particles are swept away from the cathode surface before they are built in. 3.3. Properties of the NiP–ZrO2 composite coatings 3.3.1. Morphology observations SEM photograph of the surface morphology of a particlefree NiP coating deposited at 90 °C and stirring rate of 800 rpm reveals a white smooth surface (Fig. 4a). Fig. 4b shows the effect of zirconia particles on the surface morphology of the coatings (for a maximum Vp = 7.4%). The surface of the composite coatings exhibits ZrO2 particles of an irregular shape, occasionally assembled in small aggregates. SEM cross-section image of a NiP–ZrO2 coating (Fig. 5) shows the presence of zirconia particles homogeneously dispersed in the NiP matrix. EDS analysis confirms the chemical composition of the particles and of the matrix.
Fig. 4. SEM micrographs of the surface morphology of the a) NiP coating deposited at 90 °C and b) a NiP–ZrO2 composite coating deposited at T = 90 °C (C = 30 g l− 1; Vp = 7.4%).
At low deposition temperature (70 °C), the incorporation rate is weak (b1%) and slightly influenced by stirring rate up to 700 rpm. For higher stirring rates Vp increased to a maximum of 2% at 800 rpm, and then decreased to its lowest value (b 1%). At 80 °C, the behaviour is similar but the maximum of Vp appeared at the stirring rate of 700 rpm. Finally at 90 °C, Vp increased for stirring rates from 350 to 800 rpm, and the incorporation rate of zirconia particles is the highest (about 7.4%). At stirring rates between 850 and 900 rpm the incorporation rate decreased down to 1%, then showed a second maximum of 4% at about 1000 rpm and, finally, at higher stirring rates (N1200 rpm), Vp decreased again. This phenomenon can be explained by the volume of aggregated particles at high stirring rate. For low rates (b500 rpm), a considerable amount of the particles can be observed at the bottom of the electrolytic cell
3.3.2. Hardness The hardness of the NiP–ZrO2 coatings, deposited at 90 °C using different particle concentrations in the bath, and heat treated after deposition for 1 h is shown in Fig. 6. The hardness of all coatings strongly influenced by the concentration of the incorporated ZrO2 particles. For the as-deposited NiP coating, hardness is about 520 kg mm− 2. The hardness of as-deposited zirconia-containing coatings increased up to about 630 kg mm− 2.
Fig. 5. SEM cross-section micrograph of the NiP–ZrO2 composite coating: T = 90 °C, C = 30 g l− 1; Vp = 7.4 wt.%.
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P.-A. Gay et al. / Surface & Coatings Technology 202 (2007) 1167–1171 Table 1 Wear resistance of the NiP–ZrO2 coatings: wear scar width and depth after pinon-disc test
Fig. 6. Hardness of the NiP–ZrO2 coatings deposited with different particle concentration in the bath versus temperature of post-deposition heat treatment (deposition temperature was 90 °C).
Lower hardness values (about 530 kg mm− 2) were observed for the coating deposited at the particle concentration in the bath of 90 g l− 1. Similar influence of ZrO2 particles on composite hardness was observed in [21,22]. After heat treatment at 400 °C the hardness of the coatings increased up to about 1000–1150 kg mm− 2, except for the coating elaborated at particle concentration of 90 g l− 1. Such an exceptional behaviour can be explained by the agglomeration of the ZrO2 particles on the surface of this coating. 3.3.3. Friction and wear resistance Fig. 7 shows the evolution of friction coefficient, μ, of the coatings for different particle concentrations in the bath. The average friction coefficient of all NiP–ZrO2 and NiP coatings is about 0.7. After the heat treatment the friction coefficient of the coatings decreased to about 0.6. The lowest values of friction coefficient (μ = 0.52) were measured for the coating deposited with the particle concentration of 30 g l− 1 and heat treated at 400 °C. Contrary to the Ag–ZrO2 composite coatings, where the friction coefficient decreases with increasing Vp due to significant change in the surface morphology, in case of NiP– ZrO2 coatings, there is no similar decrease of μ. ZrO2 is not a lubricant phase, therefore its incorporation in the metallic matrix will not reduce μ. In case of the Ag–ZrO2 coatings, the surface
Fig. 7. Friction coefficient of the coatings (as-deposited and heat-treated at different temperatures) as a function of the particle concentration in the bath.
Particle concentration, g l− 1
Wear scar width, μm
Wear scar depth, μm
0 5 15 30 60
798.2 718.8 654.2 439.2 713.5
15.4 12.1 14.8 4 15.2
morphology (e.i. formation of compact Ag crystallites incorporated between ZrO2 agglomerates) results in better sliding conditions [22]. Table 1 presents the values of depth and width of the wear scars to compare the wear resistance of the coatings. We can observe that wear resistance of the composite coatings increased with the particle concentration in the bath, or, in other words, it is strongly related to the concentration of zirconia particles embedded in the coating, Vp. The lowest wear was observed for the coating deposited with particle concentration of 30 g l− 1 (Vp = 7.4%). In this case the volume of material removed during the wear test was ten times smaller than that of a pure NiP coating. These results show that the addition of hard zirconia phase significantly improves wear resistance of the composite coatings. Similar observations were made by Shibli et al [21]. 4. Conclusions NiP–ZrO2 composite coatings were elaborated by an electroless plating process using a NiP bath. The incorporation of zirconia particles in the NiP coatings depends on the deposition temperature, particle concentration in the electrolyte and stirring rate. Setting the deposition temperature at 90 °C, the particle concentration in the bath at 30 g l− 1 and changing the stirring rate between 700 and 900 rpm, the incorporation rate of zirconia particles varied between 4 and 7.4%. With addition of ZrO2 particles in the bath, the surface morphology of the composite coatings changes from smooth to rough, exhibiting embedded zirconia particles of irregular shape. NiP–ZrO2 coatings show interesting mechanical properties. The hardness of as-deposited coatings increased slightly with the particle concentration in the coatings. A considerable increase of hardness was observed for the coatings after heat treatment of 400 °C. The hardness reached a value of 1150 kg mm− 2 for the coating containing 7.4% of ZrO2. The lowest friction coefficient of 0.52 was measured on the NiP–ZrO2 coating with maximum particle concentration (7.4%) after heat treatment at 400 °C. The wear resistance of the composite coatings can be considerably increased as compared to pure NiP coating. Again, the best tribological performance in terms of wear resistance was found for the coating containing 7.4% of ZrO2. In conclusion, the composite NiP–ZrO2 coating elaborated with Vp of 7.4% presents high hardness, low friction coefficient and good wear resistance.
P.-A. Gay et al. / Surface & Coatings Technology 202 (2007) 1167–1171
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