Influence of heat treatment on tribological properties of electroless Ni–P and Ni–P–Al2O3 coatings on Al–Si casting alloy
Introduction
Nickel-based coatings are widely used in automotive and aerospace industry to improve mechanical properties and wear resistance of various components. Their suitability for plating of complex-shaped components, such as those with holes or internal surfaces, makes them preferable to the electrolytic coatings.
Ni–P coatings on aluminium-based alloys may be prepared using various procedures [1], [2]. Structure and properties are significantly dependent on phosphorus content, bath composition, temperature and pH. The hardness of as-deposited coatings reaches 500–800 HV and it can be further increased by appropriate heat treatment or by the co-deposition of hard particles or fibers [3], [4], [5], [6], [7]. Generally acknowledged optimal heat treatment regime is 400 °C/1 h as it results in maximal hardness. The hardness increase is attributed to the crystallization of nickel and to the precipitation of fine particles of Ni3P phase [3], [8]. Use of higher heat treatment temperatures and longer times leads to the progressive hardness decrease, which can be attributed to the nickel grains growth and to the phosphides coarsening. As annealing temperatures above 400 °C are generally not used for the Ni–P coatings, very little information is available on evolution of coating properties using these regimes. However, it should be noted that Al–Si engine components, such as pistons, etc., may be exposed to temperatures above 400 °C during operation. It was found that intermetallic phases formed during annealing at temperatures above 400 °C may severely decrease adhesion between coating and substrate. This may lead to the local scaling or even to the total destruction of the coating [9]. It is expected that by reinforcing the coating with Al2O3 fibers it is possible to significantly reduce the scaling and thus increase coating wear resistance. For this reason, the purpose of this work is to compare the influence of various regimes of high temperature exposition on tribological properties of the Ni–P and Ni–P–Al2O3 fiber-reinforced coatings on Al–Si alloy.
Section snippets
Experiment
Conventional casting Al–Si10–Mg0.3 alloy (all concentration in wt.%, unless stated otherwise) was used as a substrate for electroless deposition. Commercial alloy was remelted in an electric resistance furnace and cast into a cast-iron mould to prepare cylindrical ingots of approximately 20 mm in diameter and 200 mm in length. Disc-shaped samples of approximately 10 mm in thickness were cut out directly from the ingots. Samples were ground using P60–P1200 SiC paper to obtain surface of defined
Friction coefficients and wear resistance
Evolution of friction coefficients during the wear tests of both Ni–P and Ni–P–Al2O3 coatings are presented in Fig. 1. Regardless of tested sample, the friction coefficient exhibits slightly unstable evolution with the number of cycles mainly due to heterogeneities caused by the preparation method and also by the annealing process. The friction coefficient curves could be characterized by clearly distinguishable running-in period. The run-in period is shorter in case of as-deposited samples and
Conclusions
Wear resistance of Ni–P and Ni–P–Al2O3-coated aluminium casting alloy was tested. As it was expected both coatings significantly improve wear resistance. The best results (i.e. the lowest wear rate) were obtained for samples exposed to 400 °C/1 h. This regime is generally used as optimal being given it leads to the maximal hardness of coatings. When exposing the coating to the temperatures above 400 °C, wear resistance decreases with increasing temperature and annealing time. From the
Acknowledgements
Authors wish to thank the Czech Science Foundation (project no. 104/08/1102), the Ministry of Education, Youth and Sports of Czech Republic (project no. MSM 6046137302) for their financial support of this research.
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