Wear Characteristics of Thermally Sprayed Diamond-Reinforced Ni-P Coatings

In this experimental study, diamond-reinforced Ni-P coatings were developed by flame and high-velocity air fuel (HVAF) spraying techniques using Ni-P capped diamond powder. Further, effect of heat-treatment on microstructure, structural, hardness and high temperature wear characteristics of the above coatings was investigated. After heat-treatment, high hardness was observed in HVAF coating compared to flame sprayed which is attributed to the high porosity of the latter as evident from the microstructure. Extensive diamond particle fragmentation was observed in the HVAF sprayed coating, providing motivation for including the lower velocity flame spraying in this work. It is interesting to note from the wear tests that coatings deposited by flame spraying exhibited superior wear resistance and low friction coefficient at high temperature, i.e., under dominated oxidative wear conditions, which is attributed to the soft matrix leading to diamond particles' exposure and graphitization. However, hard and dense heat-treated HVAF sprayed coatings exhibited highest wear resistance in room temperature tests dominated by abrasive wear mechanism as evident from the wear track morphology. Raman spectroscopy and energy dispersive spectroscopy analysis (EDS) confirmed the graphitization for the flame sprayed coatings and formation of oxides in the wear tracks.