Tribological characterization of heat-treated Ni-P and Ni-P-Al2O3 composite coatings by reciprocating sliding tests

Abstract

Linear reciprocating-ball-on-flat sliding wear tests were performed to evaluate the tribological performance of electroless Ni-P and Ni-P-Al2O3 composite coatings in the as-deposited and thermal-treated conditions for reinforcement contents of 8.3 and 18.2 vol% alumina particles. The tests were performed under dry conditions at loads of 5, 10 and 15 N to samples annealed in the range 100–600 °C. The effect of alumina content on the heat treatment response was studied and related with the microstructure and wear behavior. Analysis of variance (ANOVA) was used to study the effect of process parameters on the wear rate. A Taguchi L27 orthogonal array design was applied to evaluate the influence of annealing temperature, amount of reinforcement, load and sliding distance on wear rate. It was found that all the coatings decreased the wear rate of the steel substrates in more than one order of magnitude. The composite coatings are hardened by the presence of the Al2O3 ceramic particles that act as a barrier to the plastic deformation of the Ni-P matrix. With temperature, a more homogeneous texture and densification of deposits was observed, and the appearance of hard Ni-P precipitates was detected, which decreased slightly the coefficient of friction of the ternary deposits. The best behavior was observed on heat treated samples at temperature of 400 °C which corresponds to the temperature at which crystallization of hard precipitates (Ni3P) occurred. Above that temperature, the hardness and wear resistance were decreased by the growth of the phosphides and thickening of nickel grain due to overaging of the matrix. The Taguchi method confirmed an optimal combination of parameters 18.2 vol% Al2O3, 400 °C, 10 N load and 500 m sliding distance for minimum wear rate. From ANOVA analysis, annealing temperature and amount of reinforcement were identified as the highly contributing parameters over the wear behavior of electroless coatings. It was observed plastic deformation of the matrix but smother scars in the composite coatings as well as the presence of tribochemical layers. The inherent variability in wear performance under linear reciprocating sliding is correlated to the wear mechanisms involved, abrasive wear and adhesion wear. In fact, a tribochemical mixed-layer is formed, which involves a complex mechanism with wear debris from both surfaces, oxidation, comminution and formation of new sublayers.