Wear behaviour of α-alumina in hot steam at high contact pressure

The use of materials for tribological applications in hot steam environment becomes more important in the future. Unfortunately, experimental data recorded under hot steam conditions are scarce mainly due to suitable testing equipment. This work examines the wear behaviour of α-aluminium oxide by combining thermodynamic modelling with advanced wear testing as well as analytical methods to get a better understanding of this structural ceramic materials wear behaviour and its possible use in high temperature steam environment. Self-mated α-alumina sliding couples have been investigated under oscillating sliding conditions in air, in water and in hot steam up to temperatures of 300 °C and ambient pressures up to 4 bars. The applied normal load was 100 N corresponding to an initial contact pressure of 3.1 GPa. The results show that the wear behaviour is directly dependent on the amount of water in the environment. Two competing wear mechanisms are identified. Following the fragmentation of alumina grains, abrasion is predominant in the low water vapour regime, i.e. dry sliding in laboratory air. Tribochemical wear is predominant, if enough water is available to form aluminium hydroxide and a gel-type reaction layer, i.e. when sliding in water or in steam. This gel layer has been confirmed as a possible explanation by electron backscattered diffraction. However, severe fracture and fragmentation of grains is observed which is attributed to initial wear at high Hertzian contact pressure during the running-in period. Thermodynamic calculations for identification of the possible phase equilibria support the experimental finding by Raman spectroscopy that only diaspore as crystalline hydroxide phase is present in the wear scar.