Static and dynamic friction and wear of cobalt-based coatings at elevated temperatures

Advanced ultra-supercritical power plants, in which steam temperatures exceed 700 degrees C, demand the development of protective coatings, particularly for valves governing the steam flow where sealing surfaces are subjected to high static tribological solicitations. In this context, we systematically investigated the friction (static and dynamic) and wear of two cobalt-based coatings and one nickel-based substrate. Results indicate that oxidation kinetics exhibits a double-edged sword effect in terms of tribological properties at elevated temperatures. Fast oxidation kinetics promotes the sintering of oxides, generating a smoother oxide tribofilm and lowering the dynamic coefficient of friction (COF). However, this can also significantly increase the static COF over loading time due to welding/sintering of oxidized asperities, leading to increased torque requirements for valve operation.