Variable cycle fretting conditions meditated wear behavior and mechanism in a self-lubricating composite coating on TC21 titanium alloy substrate

To simulate the variational working conditions of the titanium fastener in the aero-engine, variable cycle fretting wear tests were carried out for normal loading (80/40 N), frequency (100/30 Hz), displacement amplitude (80/40μm) and operating temperature (500/350°C) in a self-lubricating composite coating on TC21 titanium alloy substrate, and the fretting wear behavior and wear mechanism were thoroughly investigated. The results shows that the coefficient of friction (COF) and the wear volume present a significant difference under variable cycle fretting conditions. The specimen exhibits a relatively stable COF under variable operating temperature. Whereas for the specimen under variable displacement amplitude presents a lowest wear volume. The adhesive wear is mainly found in the specimens under variable normal loading and frequency. Whereas for the specimens under variable displacement amplitude and operating temperature, abrasive wear and oxidation wear is mainly dominated, respectively. The dissipated energy calculation results demonstrate that the variable displacement amplitude and operating temperature have greater impact on the fretting wear behavior. The microstructure analysis in the cross-section of specimens reveals that the sub-surface is composed of amorphous-nano crystalline layer, plastic deformation layer and the origin coating. Additionally, the wear mechanism of the specimens under variable fretting conditions were deeply revealed. This work can provide essential information on fretting behavior and mechanism under the variational working conditions.