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)Si-2 (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)Si-2 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)Si-2 nanocrystalline films decreased with increasing Cr content in the films. The oxidation behavior of the (Mo1-xCrx)Si-2 nanocrystalline films was investigated at 500 degrees 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)Si-2 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. (C) 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.