Role of Cr additions in enhancing wear and oxidation resistance of (Mo1−xCrx)Si2 nanocrystalline films

In order to explore the effects of Cr alloying on the wear and oxidation resistance of MoSi2 films, four kinds of (Mo1−xCrx)Si2 (x=0, 0.05, 0.10 and 0.15) nanocrystalline films of differing Cr contents were fabricated onto Ti6Al4V substrates by a double cathode glow discharge apparatus. The microstructure features of the as-deposited films were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). The hardness and elastic modulus of the (Mo1−xCrx)Si2 nanocrystalline films were evaluated by nano-indentation. The dry sliding wear properties of the as-deposited films sliding against GCr15 rolling bearing steel were studied using a ball-on-disc type tribometer at room temperature. The results showed that the friction coefficients and specific wear rates of the (Mo1−xCrx)Si2 nanocrystalline films decreased with increasing Cr content in the films. The oxidation behavior of the (Mo1−xCrx)Si2 nanocrystalline films was investigated at 500 °C under cyclic oxidation conditions in air for exposure time of 100 h. The oxidation kinetics of the films was found to obey a subparabolic behavior, and three Cr-containing MoSi2 films showed a much lower weight gain and the oxidation rate constant as compared to the monolithic MoSi2 film during a 100 h exposure. Surface morphologies, phase constituents and electrochemical behavior of the oxide scales formed on the (Mo1−xCrx)Si2 nanocrystalline films were characterized using FESEM, XRD and electrochemical-impedance spectroscopy (EIS). The electrochemical results revealed that the structure of the oxide scales changed from a homogeneous and dense structure to a duplex structure consisting of a porous outer layer and a denser inner layer. The overall resistance of the oxide scales depended strongly on both the oxidation time and Cr content in the films.