A.-P. Nikkilä

Environmental effects on microstructure and strength of SiC-based hot gas filters

The aggressive process environment in advanced coal-fired power generation systems causes microstructural changes in the ceramic hot gas filters used to clean the fuel gas. Changes in microstructure and their effect on strength were studied for commercial SiC-based clay bonded hot gas filters exposed to high temperature, water vapour and gaseous sodium compound. Exposures caused significant crystallization of the binder phase and oxidation of SiC. Loss in strength also occurred. The microstructural changes and their effect on strength are discussed.

Influence of topography on the running-in of water-lubricated silicon carbide journal bearings

Abstract The study summarizes an experimental investigation on the running-in of water-lubricated journal bearings and shaft sleeves made of 10 different commercial silicon carbide materials. In each test the sliding surfaces became polished and the sliding conditions were transformed from boundary or mixed lubrication to full film lubrication within a rather short sliding distance. The initial surface roughness strongly influenced on the initial dynamic coefficient of friction, and on the running-in distance required to reduce the coefficient of friction from a level typical of boundary or mixed lubrication to a level typical of full film lubrication. The initial surface roughness of the shaft sleeve was more slowly removed than that of the bearing, and therefore the surface quality of the shaft rather than that of the bearing governed the running-in distance required for the polishing. The initial surface roughness of the bearing practically only influenced the dynamic coefficient of friction at the beginning of the running-in procedure. In comparison with the influence of the surface roughness variations, the material properties of the silicon carbides studied has a limited influence on the running-in behaviour,which was principally related to a tribochemical surface polishing process.

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

Abstract 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.