INFLUENCE OF HEAT TREATMENT ON TRIBOLOGICAL PROPERTIES OF Ni-P ELECTROLESS COATINGS. Michal Novák a Dalibor Vojt ch a Michala Zelinková a Tomá Vít b

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1 INFLUENCE OF HEAT TREATMENT ON TRIBOLOGICAL PROPERTIES OF Ni-P ELECTROLESS COATINGS Michal Novák a Dalibor Vojt ch a Michala Zelinková a Tomá Vít b a Department of Metals and Cor rosion Engineering, ICT Prague, Technická 5, Prague 6, Czech Republic, novakm@vscht.c z b Department of Physics, CTU in Prague, Technická 3, Prague 6, Czech Republic Abstract The aim of this work was to describe evolution of tribological properties of electroless Ni-P based coatings during heat treatment. Coatings were prepared using nickel lactate- hypophosphite bath, conventional AlSi10Mg0.3 cast alloy was used as a substrate. Coated samples were heat treated at C for 1 8 hours. Adherence of coatings to the substrate was estimated from the scratch test. To determine wear resistance, chosen samples were subjected to pin-on-disc test. It was found that intermetallic phases formed during heat treatment due to the diffusion of nickel into the substrate severely decrease wear resistance of coatings. Keywords: electroless Ni-P coatings, tribology, heat treatment 1. INTRODUCTION Due to their good mechanical, electric, corrosion and tribological properties, electroless Ni-P coatings are frequently used in engineering and chemical industry or in electronics. Various materials can be used as a substrate and it is possible to coat complex-shape components. In contrast to the galvanic coating, Ni-P coating is formed on the substrate as a result of an autocatalytic reaction without use of electric current. Electroless nickeling bath usually contains aqueous solution of metal ions, complexing agents, reducing agents and ph stabilisers. Rate of deposition is influenced by various factors such as metal ions concentration, type and concentration of complexing agent, temperature, ph etc. Structure and properties of prepared coatings depend significantly on phosphorus content [1-3] and on subsequent heat treatment regime [4-7]. Generally acknowledged optimal heat treatment regime is 400 C/1 h [6, 8], when fine phosphide particles precipitate. Higher annealing temperatures and longer times are not used since they result in coating hardness decrease due to the coarsening of nickel grains and of phosphide particles. It should be noted that some engine components made of aluminium alloys may be exposed to elevated temperatures even for longer periods. For this reason, it is necessary to describe evolution of Ni-P coatings during heat treatment using other regimes than the above sai d optimum. 2. EXPERIMENT Commercial AlSi10Mg0.3 (wt. %) alloy was used as a substrate. The material provided by an industrial supplier was remelted in an electric resistance furnace and cast into metal mould. Samples of 10 mm in thickness were cut out from cylindrical 1

2 ingots having 20 mm in diameter and length of 200 mm. Samples were progressively ground using P60 P1200 SiC papers and ultrasonically degreased for 15 minutes in acetone. Befor e being transported into the plating bath, samples were etched for 60 s in solution containing 5 ml HNO 3, 2 ml HF and 93 ml H 2 O. Electroless plating conditions are summarized in Table 1. Table 1. Conditions used for electroless deposition nickel lactate 30 g/l bath composi tion nickel hypophosphite 20 g/l lactic acid 10 ml/l 3 x 5 ml 1 M NaOH sol ution ph adjustment (at the start and subseq. aft er 40 minutes of plating) bath temperature 90±2 C bath volume 250 ml coating time 120 minutes Prepared samples were subsequently heat treated in an electric resistance furnace under protective atmosphere (Ar, flow rate 0.5 l/min) at temperatures ranging C for 1 8 hours. Cooling down to the room temperature was performed in air. Prior to tribological tests, coating adherence was proved by means of the scratch test. Coatings that showed very low critical loads (Lc < 20N) were immediately eliminated due to their tribological instability risks. Tribological properties of Ni-P based coatings were studied using a high temperature pin-on-disc CSM Tribometer. The tests were carried out under conditions specified in Table 2. Table 2. Conditions of tribological tests normal load 5.0 N linear sliding velocity 0.05 m.s -1 number of cycles 5000 temperature room temperature (approx. 298 K) testing counter part 440C steel ball (d = 6.0 mm) track radius 5.0 mm lubricant unlubricated, dry ambient environment (RH approx. 40±5 %) Coating wear volumes were evaluated by means of the multiple cross-section area surface profilometry. Diamond stylus profilometer Alpha-Step with maximum scan length 5 mm was used. The width of wear tracks usually did not exceed 1 mm. The maximum surface roughness (hills-valleys distance) range was 0.5 µm with a detection accuracy of 2 nm. Using the wear volumes, the wear rates of coatings K were calculated by a following formula: K V = W s [m 3.N -1.m -1 ], (1) 2

3 , Hradec nad Moravicí METAL 2009 where V is the wear volume (m3) of coating, W is the normal load (N), and s is the total sliding distance (m). 3. RESULTS 3.1 Coating adhesion Coating adhesion to the substrate was estimated from the scratch test with initial load of 8.80 N. The load was gradually increased five times by 8.80 N. Fig. 1 shows samples after scratch-test with the initial load. As it was expected, the best results were obtained in case of as-coated sample (Fig. 1a). The scratch is even and no cracks were observed in its vicinity. Slight decrease of adhesion was observed in case of sample annealed at 400 C/1 h. Small areas where partial delamination of coating occurred were identified in the vicinity of the scratch (Fig. 1b). Nevertheless, the coating adhesion is still satisfactory. Fig. 1c and Fig. 1d show results of scratch test on sampl es annealed at 450 C/8 h and at 550 C/1 h. In the vi cinity of the scratch there are small areas where partial delamination of coating and formation of cracks occurred. Nevertheless, coating is still compact. During annealing at 550 C/8 h, the adhesion of coating decreases critically and the coating is totally destructed during scratch test with the initial load of 8.80 N (Fig. 2). For this reason the coating annealed at 550 C/8 h was not subjected to the pin-on-disc test. The character of the scratch showed no visible evolution with increasing load; even the maximal load of 44.0 N did not result in total destruction of the coating. a b c d 3

4 Fig.1. Tracks after scratch tests with load of 8.80 N (light micrograph): a) as-deposited, b) heat treated at 400 C/1 h, c) heat treated at 450 C/8 h, d) heat treated at 550 C/1 h, Fig. 2. Tracks after scratch tests with load of 8.80 N (light micrograph) - heat treated at 550 C/8 h 3.2 Wear resistance Fig. 3 shows the evolution of friction coefficients during the pin-on-disc test. Value of the friction coefficient (the ratio of tangential friction force to normal force) varies slightly, mainly due to the inhomogeneities caused by the coating preparation and by the heat treatment. Curves can be characterised by run-in period which duration increase with increasing temperature and time of annealing. In case of as-coated sample, the run-in period length is about 500 cycles. In case of anneal ed samples, its length is about 1500 cycles. If we consider only the values of friction coefficient during the steady-state period, the average value is ranging from 0.47 (as-coated sample) to 0.72 (550 C/1 h). Lower values of the friction coefficient may be attributed to the homogeneous structure of the unannealed coating, on the contrary, the structure inhomogeneities (precipitated phosphides, formed intermetallic phases) cause the increase of the friction coefficient. Fig. 3. Typical friction curves of Ni-P coatings against 440C steel balls 4

5 Results of the wear tests are presented in the Fig. 4. Measured values of wear rate are in accordance with results of other authors [9]. Although the friction coefficient of the as-coated sample showed the lowest value (see Fig. 3), the wear rate of this sample was the highest. According to the expectations, the wear rate of the optimally heat treated coating (i.e. 400 C/1 h) was the lowest. Annealing at higher temperature for longer periods leads to the progressive decrease of the wear resistance. This is caused by the coarsening of the nickel grains and of the phosphide precipitates. Negative influence of the coarsening is partially compensated by the formation of hard intermetallic phases on the substrate-coating interface. This effect is most significant in case of the coating annealed at 450 C/8 h. It was found that hardness of the formed Al 3 Ni phase is higher than that of the optimally heat treated sample (400 C/1 h) [10]. In case of this sample, the wear rate increase is slightly lower than it was expected. Significant increase of wear rate can be observed in case of the sample annealed at 550 C/1 h. Adhesion of the coating to the substrate decreases with increasing thickness of the formed intermetallic phases. Nevertheless, the wear rate of the sample annealed at 550 C/1 h is more than 40 times lower than that of the Al-Si substrate. Fig. 4. Coating wear rates for different heat treatments procedures 4. CONCLUSIONS Various intermetallic phases are formed on the substrate-coating boundary during annealing at higher temperatures. During annealing at 400 C/1 h, formerly amorphous nickel crystallises and Ni 3 P phosphides precipitate. This leads to significant increase of the coating wear resistance. If coating is heat treated at higher temperatures, coarsening of these phases r esults in increase of the wear rate. This is partly compensated by presence of hard intermetallic phases formed due to the nickel diffusion into the substrate. Thickness of the intermetallic phases formed during annealing at 550 C is too high; due to the coefficient of thermal expansion 5

6 (CTE) difference, the adhesion and subsequently the wear resistance of the coating is severely decreased. From the technological point of view, the important fact is that even longer heat treatment at C does not necessarily result in severe wear rate increase. ACKNOWLEDGEMENTS The research was supported by the Czech Science Foundation (project no. 104/08/1102), by the Ministry of Education, Youth and Sports of Czech Republic (project no. MSM ) and by the ICT Prague (VG 106/08/0015). LITERATURE [1] BERKH, O., ESKIN, S., ZAHAVI, J. Properties of electrodeposited NiP-SiC composite coatings. Metal Finishing 94 (1996) p [2] ALLEN, R. M., VANDERSANDE, J. B. The structure of electroless Ni---P films as a function of composition. Scripta Metallurgica 16 (1982) p [3] HUR, K. H., JEONG, J. H., LEE, D. N. Microstructures and crystallization of electroless Ni-P deposits. Journal of Materials Science 25 (1990) p [4] APACHITEI, I., DUSZCZYK, J. Autocatalytic nickel coatings on aluminium with improved abrasive wear resistence. Surface and Coatings Technology 132 (2000) p [5] KEONG, K. G., SHA, W., MALINOV, S. Hardness evolution of electroless nickel phosphorus deposits with thermal processing. Surface and Coatings Technology 168 (2003) p [6] APACHITEI, I. aj. Electroless Ni P Composite Coatings: The Effect of Heat Treatment on the Microhardness of Substrate and Coating. Scripta Materialia 38 (1998) p [7] GROSJEAN, A. aj. Hardness, friction and wear characteristics of nickel-sic electroless composite deposits. Surface and Coatings Technology 137 (2001) p [8] STAIA, M. H. aj. Wear performance and mechanism of electroless Ni-P coating. Surface and Coatings Technology (1996) p [9] SAHOO, P., PAL, S. K. Tribological performance optimization of electroless Ni P coatings using the Taguchi method and grey relational analysis. Tribology Letter s 28 (2007) p [10] BRUNELLI, K., DABALA, M. Surface hardening of Al7075 alloy by diffusion treatment of electrolytic Ni coatings. Sborník konference 2nd International Conference on Heat Treatment and Surface Engineering in Automotive Applications, Riva del Garda, 2005, CD 6