On the P-T conditions of wet silicon carbide tribo-corrosion

Using a Bassett-type hydrothermal diamond anvil cell (HDAC) the corrosive behaviour of undoped single-crystal silicon carbide was studied. For this, temperatures up to 800°C, corresponding pressures of ~ 2 GPa and durations up to 60 hours were chosen as experimental parameters. Comprised between two diamond anvils the cylindrical sample chamber (Vsample ~ 0.1 mm3) introduced into a rhenium gasket was filled with distilled water along with the sample. Heating the sample chamber via a resistive heating put around the diamond anvil mountings an isostatic pressure on the sample was created as a function of temperature. Water as a pressure-mediating media in the sample chamber allowed us to calculate the pressure by the equations of state supposing an isochoric sample chamber volume. Analytica l methods were applied in-situ and ex-situ. While in-situ optical inspection of the sample was possible using a polarization microscope equipped with a video camera, each sample was studied via micro-Raman-spectroscopy, micro-XRD², SEM and EDX after hydrothermal treatment. The goal of our experiments was to examine the basics of wear situations, in which water is sealed in openings (semi-open pores, surface roughness, pits, etc.) and heated locally by friction. In contrast to earlier studies we did not find a passive oxidation to take place as the silicon carbide oxidation products were instantly dissolved into the surrounding hydrothermal fluid. Both, increased surface roughness and a locally deposited precipitate were found after hydrothermal exposure. Latter was found to be of silica-li ke composition and seems to be a process of dissolution and reprecipitation. The observation of a very low friction coefficient of SiC under wet lubrication conditions therefore cannot only be due to the formation of a reaction layer (i.e. passive oxidation), but must be a more complex process of structural deterioration, oxidation, dissolution and precipitation of silica from the cooling fluid