Wear characteristics of water-lubricated SiC journal bearings in intermittent motion

In water-lubricated journal bearing geometries, silicon carbide (SiC) offers a load-carrying capability that is higher than that of most other structural ceramics. This superiority derives from the high hardness of SiC, its good thermal conductivity and its ability to form water-soluble wear products and smooth surfaces when sliding in water. Once the smooth sliding surfaces have been formed, the subsequent sliding procedure will rely on a water film, and the wear of SiC terminates. However, in many situations it is important to know the wear characteristics of SiC materials in repeated start-up events in which the conditions in the sliding interface evolve from static friction through boundary and mixed lubrication, comprising some wear, into an equilibrium of hydrodynamic lubrication. For studying the wear characteristics of water-lubricated SiC journal bearings in intermittent motion, a series of journal bearing tests were carried out. During each test, 10 000 start-up procedures and a total sliding distance of 100 km were used. The normal forces were 2.5 and 4 kN, and the maximum sliding velocity was 1 m s⁻¹. During the tests, the bearing temperature and the friction force were recorded, while the wear was determined after the tests. All the materials studied were worn mainly by tribochemical polishing, which resulted in smooth surfaces with distributed tiny scratches. All wear rates were very low, and the wear rates of the SiC materials at the respective loads were all within a quite narrow range. Due to stronger tribochemical wear of the Si phase, the SiSiC material experienced slightly more wear than the SiC materials. The C-SiSiC material studied was worn as little as the SiC materials, regardless of its soft graphite phase. This study shows that SiC-based materials can well be utilized for demanding journal bearing solutions operating under conditions of water-lubrication.