Silicon carbide ceramic (SiC) is widely used as face seal material for water related equipment due to its good corrosion and wear resistance performance. Additionally, with water lubricated, ultra-low friction coefficient (around 0.01) can be achieved between sliding self-mated SiC with proper parameters. Until now, the consensus for its mechanism focuses on a kind of tribo-chemical reaction occurring between SiC and water to generate SiO2, which is much softer than SiC and has good water solubility. As a result, the lubricant is not only the deionized water, but together with dissolved SiO2 and possible suspended wear debris.
To get a better understanding of this process, a surface contacting oscillation friction test is used to simulate under worst lubrication conditions of SiC in engineering application: changing velocity, elevated temperatures, mixed-lubrication and wear debris are involved. Deionized water as the lubricant is pumped with higher pressure (2 bars from gauge) into the gap between frictional SiC surfaces, which can be heated at controllable temperatures. Friction tests at different temperatures (from 40 °C to 120 °C) and two surface roughnesses (Ra = 0.39 µm and 0.05 µm) are performed.
From the frictional results, surfaces with higher roughness appear to perform better than the ones with lower value. When the heating temperature is lower or equal than 100 °C, all rough surfaces can achieve stable ultra-low friction status at the end of each test, of which friction coefficients are around 0.01, lowest at 0.005. On the other hand, surfaces with lower roughness can only get such ultra-low friction status under certain conditions.
Research on scars’ matching condition between worn samples shows that there are tolerances between the corresponding abrasive scars when ultra-low friction happens, but asperities of abrasive wear scar insert into each other when final friction coefficient is higher and unstable. We assume that the formation mechanisms for these different worn structures are attributed to third-body wear that is from wear debris.
The lubrication mechanism for ultra-low friction status together with abrasive wear scars is stated, and potential methods to prevent abrasive wear before ultra-low friction status are discussed.