Effect of Si doping on the wear properties of CrN coatings synthesized by unbalanced magnetron sputtering

In this study, Cr–Si–N coatings were deposited on the pump parts made of AISI 4340 steel in order to improve wear characteristics of electro hydrostatic actuator (EHA) hydraulic pump parts. Coatings with various Si contents were synthesized by unbalanced magnetron sputtering method and their chemical and mechanical properties were investigated by auger electron spectroscopy (AES), scanning electron microscope (SEM), X-ray diffraction (XRD), nano-indentation test and were compared with those of TiN coatings. As Si target power was increasing, the Si contents in the deposited thin films were increasing. While CrN coating without Si content shows a maximum hardness of about 18 GPa, the hardness of Cr–Si–N coatings was increased up to 24 GPa. The results from XRD analyses on Cr–Si–N coatings with various Si content showed that peak broadening and reduction in peak intensity was observed as Si content was increasing in the film, indicating a grain size reduction. From a Cr–Si–N coating with 9.8 at.% Si, a reduced static friction coefficient of about 0.04 under lubricant condition was observed against a plasma-nitrided counterpart and similarly improved wear behaviors were observed from a wear test against a bronze valve plate, which suggested a possibility of application of this coating to a real pump part for improved wear behavior.