INFLUENCE OF HEAT TREATMENT ON TRIBOLOGICAL PROPERTIES OF Ni-P-Al 2 O 3 ELECTROLESS COATINGS

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1 INFLUENCE OF HEAT TREATMENT ON TRIBOLOGICAL PROPERTIES OF Ni-P-Al 2 O 3 ELECTROLESS COATINGS Michal NOVÁK a, Dalibor VOJTĚCH a, Tomáš VÍTŮ b a Department of Metals and Corrosion Engineering, ICT Prague, Technická 5, Prague 6, Czech Republic, novakm@vscht.cz b Department of Physics, CTU in Prague, Technická 3, Prague 6, Czech Republic Abstract The aim of this work is to describe the evolution of tribological properties of electroless Ni-P based coatings reinforced with Al 2 O 3 fibres. Coatings were prepared using nickel lactate-hypophosphite bath, conventional AlSi10Mg0.3 cast alloy was used as a substrate. To prepare composite coatings commercial Saffil fibres (Al 2 O 3 with 4wt.% of SiO 2 ) were added to the plating bath. Coated samples were heat treated in a resistance furnace at C for 1-8 hours. The adherence of coatings was estimated from the scratch-test. Chosen samples were subjected to the "pin-on-disc" test to determine coating wear resistance. Having superior hardness, alumina fibres significantly increase coating wear resistance compared to that of the nonreinforced Ni-P coatings. As it was already described in case of Ni-P coatings, the intermetallic phases formed during heat treatment decrease wear resistance of coatings. However, the presence of Al 2 O 3 fibres partially suppresses this negative effect and, thus, increases the coating's performance. Keywords: electroless Ni-P coatings, composite coatings, tribology, heat treatment 1. INTRODUCTION Electroless Ni-P coatings are frequently used in engineering and chemical industry or in electronics, as various materials can be used as a substrate and it is possible to coat complex-shape components. Ni-P coating is formed on the substrate as a result of the autocatalytic reaction without use of electric current. Electroless nickeling bath usually consists of aqueous solution of metal ions, complexing agents, reducing agents and ph stabilisers (buffers). The rate of deposition depends on metal ions concentration, type and concentration of complexing agent, temperature, stabiliser concentration and ph. Structure and properties of prepared coatings depend significantly on phosphorus content [1-2] and on the heat treatment regime [3]. Generally, heat treatment at 400 C fo 1 h is used, as it yields maximal coating hardness as a result of the precipitation of fine phosphide particles [4, 5]. When higher temperatures and longer times are applied, the hardness of the coating progressively decreases. For this reason, higher temperatures are not used for the heat treatment of electroless Ni-P coatings. However, thermally-loaded engine components may be exposed to higher temperatures even for longer periods. Besides a reduction of hardness, solid state reactions may also occur on the substrate coating interface. The formed phases may significantly reduce coating adhesion to the substrate and, therefore, shorten the component lifetime. Various reinforcements, such as Al 2 O 3 or SiC fibres may be used to compensate the effect of the above mentioned phenomena. For these reasons, the purpose of this work is to determine the influence of heat treatment on tribological properties of fibrereinforced electroless Ni-P-Al 2 O 3 coatings. 2. EXPERIMENT Commercial Al-Si10-Mg0.3 alloy (all concentration in wt. %, unless stated otherwise) was used as a substrate for the electroless deposition. The alloy provided by an industrial supplier was remelted in an

2 electric resistance furnace and cast into metal mould. Samples of 10 mm in thickness were cut out from cylindrical 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. Before being transported into the plating bath, samples were dezoxidized for 60 s in solution containing 5 ml HNO 3, 2 ml HF and 93 ml H 2 O. The conditions of electroless plating conditions are summarized in Table 1. Table 1. Conditions used for electroless deposition nickel lactate 30 g/l bath composition nickel hypophosphite 20 g/l lactic acid 10 ml/l 3 x 5 ml 1 M NaOH solution ph adjustment (at the start and subseq. after 40 and 80 minutes of plating) bath temperature 90±2 C bath volume 250 ml reinforcement 2.5 g/l (Saffil fibres) stirring magnetic stirrer, 400 rpm coating time 120 minutes After the electroless coating, samples were heat treated in an electric resistance furnace under protective argon atmosphere (flow rate 0.5 l/min) at temperatures between 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. Only samples showing sufficient critical loads (Lc > 20N) were subjected to the pin-on-disc tests. Tribological properties of Ni-P based coatings were studied using a pin-on-disc CSM Tribometer. The tests conditions are summarised 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 counterpart 440C steel ball (d = 6.0 mm) track radius 9.0 mm lubricant unlubricated, dry ambient environment (RH approx. 40±5 %) Coating wear volumes were evaluated using the multiple cross-section area surface profilometry. Diamond stylus profilometer Alpha-Step was used. 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) where V is the wear volume (m 3 ) of coating, W is the normal load (N), and s is the total sliding distance (m). 3. RESULTS 3.1 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 load of N). As it was expected, the best results were obtained in case of as-coated sample (Fig. 1a). The scratch is

3 even and no cracks were observed in its vicinity. In case of sample annealed at 400 C / 1 h no adhesion decrease was observed (Fig. 1b). Slight decrease of adhesion was observed in case of the samples annealed at 450 C / 8 h and at 550 C / 1 h. Few small areas where delamination occurred are visible. Nevertheless, the coating adhesion is still satisfactory, as the delamination affects the vicinity of the scratch and there are no visible cracks indicating larger failure of the coating (Fig. 1c and Fig. 1d). During annealing at 550 C / 8 h, the adhesion of coating decreases critically due to the formation of intermetallic phases on the substrate-coating boundary. Delamination of the coating affects large areas in the vicinity of the scratch. For this reason the coating annealed at 550 C / 8 h was not subjected to the pin-on-disc test. Nevertheless, due to the presence of Safiil fibres, the coating failure is not as ultimate as in the case of Ni-P coating [6]. a b c d Fig.1. Tracks after scratch tests with load of 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, e) heat treated at 550 C / 8 h e

4 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 of approximately 500 cycles. If we consider only the values of friction coefficient during the steady-state period, the average value is ranging from 0.75 (400 C / 1 h) to 0.91 (450 C / 8 h). Lower values of the friction coefficient in case of the as coated sample and the sample annealed at 400 C / 1 h may be attributed to the nearly homogeneous structure of the coating, on the contrary, the structure inhomogeneities, such as precipitated phosphides and formed intermetallic phases, cause the increase of the friction coefficient. Fig. 3. Typical friction curves of Ni-P-Al 2 O 3 coatings against 440C steel ball Results of the wear tests are presented in the Fig. 4. According to the expectations, the wear rate of the optimally heat treated coating (i.e. 400 C / 1 h) was the lowest one. 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. Nevertheless, the decrease due to the annealing at higher temperatures is not as significant as in the case of non-reinforced Ni-P coating [6]. The presence of hard Saffil fibres significantly improves coating wear resistance. Furthermore, negative influence of the coarsening of the nickel grains and of the phosphide precipitates 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 even higher than that of the optimally heat treated sample (400 C/1 h) [7]. It should be mentioned, that calculated wear rates of heat treated Ni-P-Al 2 O 3 coatings were seemingly higher than wear rates of non-reinforced Ni-P coatings [6]. This is caused mainly by the systematic error of multiple cross section area profilometry. Because the Ni-P-Al 2 O 3 coatings surface roughness is significantly higher than that of the non-reinforced Ni-P coatings, it is not possible to determine precisely the wear track edge and, thus, determine the exact reference level for depth measures. However, as the surface roughness does not evolve during heat treatment, it is still possible to compare wear rates of identically coated samples after different heat treatment.

5 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 results in an increase of the wear rate. This is partly compensated by the 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 Fig. 4. Coating wear rates of Ni-P-Al thermal expansion (CTE) difference, the 2 O 3 coatings for different heat treatments procedures adhesion and subsequently the wear resistance of the coating is decreased. However, the presence of Saffil fibres significantly compensates the decrease, as the fibres effectively redistribute inner stress and prevent it from concentrating above the critical value. 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 ). 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] APACHITEI, I., DUSZCZYK, J. Autocatalytic nickel coatings on aluminium with improved abrasive wear resistence. Surface and Coatings Technology 132 (2000) p [4] APACHITEI, I. et al. Electroless Ni P Composite Coatings: The Effect of Heat Treatment on the Microhardness of Substrate and Coating. Scripta Materialia 38 (1998) p [5] GROSJEAN, A. et al. Hardness, friction and wear characteristics of nickel-sic electroless composite deposits. Surface and Coatings Technology 137 (2001) p [6] NOVÁK, M. et al. Influence of heat treatment on tribological properties of Ni-P electroless coatings. Proceedings of Metal 2009, Hradec nad Moravicí, 2009, CD [7] BRUNELLI, K., DABALA, M. Surface hardening of Al7075 alloy by diffusion treatment of electrolytic Ni coatings. Proceedings of 2nd International Conference on Heat Treatment and Surface Engineering in Automotive Applications, Riva del Garda, 2005, CD