Ceramic Matrix Composite Material in Highly Loaded Journal Bearings

Conventional sintered silicon carbide (SSiC) has been applied in journal bearings of pumps for more than 20 years with the pumped medium itself being the lubricant. High corrosion and wear resistance of SSiC have contributed to this success. The brittle failure of that material quite often is a problem, though, and limits the application of SSiC in highly loaded bearings. In contrast, ceramic matrix composites (CMC) based on C- or SiC-fiber-reinforced SiC-ceramics show strongly improved fracture toughness on the level of cast iron and are applicable in cases where conventional ceramics due to their lack of reliability cannot be used. These CMC-materials have been developed in several programs primarily for space and military applications and are also beeing successfully used in journal bearings for pumps in power plants [1] and for tubular casing pumps [2]. In power plant pumps, low viscosity water of up to 160°C can be the lubricant. In tubular casing pumps quite often water loaded with abrasive sand particles lubricates the bearings. CMC-journal bearings for pumps in cryogenic rocket engines for reusable launch vehicles (RLVs), where the lifetime of mechanical components is a critical issue, are presently tested. Journal bearings of the type introduced in water pumps could replace ball bearings presently in use. Improved stiffness and damping properties, reduced wear, increased reliability and no limitations in speed times diameter would be some of the expected advantages [3]. Journal bearings for hot hinges in re-entry systems are foreseen for the space vehicle CRV (crew rescue vehicle) and have successfully been tested under close to real conditions. They are envisaged to be flight-tested on the experimental NASA vehicle X38. The bearing faces run under dry conditions and temperatures of more than 1600°C in air of about 50 mbar pressure [4]. Presently, only CMCs based on carbon fibers have potential to operate successfully under such conditions.