Application of the duplex TiN coatings to improve the tribological properties of Electro Hydrostatic Actuator pump parts

In this paper, the effects of duplex treatment consisting of plasma nitriding and PVD hard coating on the surface properties of EHA pump parts were investigated. Two different coatings (single-layered TiN coating and duplex TiN coating) were deposited on AISI 4340 steel using reactive unbalanced magnetron sputtering method. A high-speed disk-to-disk type wear tests under a mineral oil environment were performed and the results from various surface conditions were compared with each other. The duplex TiN coating has the lowest static friction coefficient of 0.04, while others indicated the almost identical values between 0.08 and 0.09. Especially, much improvement of starting and slow-running behavior in the Stribeck curve was obtained from the duplex TiN coating. Wear rate of the duplex TiN coating was reduced by a factor of 1.8 and an order of degree, compared with those of single-layered TiN and plasma-nitrided specimen, respectively. The critical load of duplex TiN coating (Lc) was measured to be approximately 38 N, which is approximately 2.5 times higher than that of single-layered TiN coating of 15 N. In addition, application of the duplex treatment on the commercial cylinder barrel against bronze valve plate showed much reduced friction coefficient up to 1200 rpm.