Surface engineering-controlled tribological behavior and adhesion strength of Ni-Cr coating sprayed onto carburized AISI 4340 steel substrate

In this study, a nickel-chromium (Ni-Cr) coating was sprayed onto carburized AISI 4340 (UNS G43400) steel substrate by high-velocity oxygen fuel (HVOF) method. As-sprayed Ni-Cr coating was subjected to ultrasonic nanocrystal surface modification (UNSM) technology to control the tribological performance and adhesion strength. The aim of the current study is to improve the durability of Ni-Cr coating by the application of UNSM technology. The UNSM technology reduced the surface roughness and increased the surface hardness of the as sprayed coating by about 64 and 25% that were associated with the elimination of high peaks and valleys, filling up micro-pores, respectively, where the unfilled open-pores and micro-cracks may be found as a stress concentration cite to suppress a crack initiation and crack propagation and a less number of open-pores may hinder the initiation and propagation of micro-cracks. The evaluation of the tribological performance of Ni-Cr coatings was evaluated using a wear tester in dry conditions, while the adhesion strength was obtained using a progressive scratch tester. As a result, the friction coefficient of the as-sprayed coating was reduced from about 1.1 to 0.75, and the adhesion strength was increased from about 3.3 to 6.1 N by the application of UNSM technology. The reduction in surface roughness of the UNSM-treated coating can be considered as the main parameter that influences on the frictional behavior, while increase in surface hardness determines the wear resistance and adhesion strength. It was found that the UNSM technology improved the tribological performance and adhesion strength of the coating.