*** 9800946 Jue This Small Business Innovation Research Phase II project will demonstrate significant improvements in resistance to corrosion, erosion and fracture and service lifetimes for valves with the graded silicon carbide (SiC) components to be developed relative to the valves with sintered alpha-SiC components. SiC is a low-cost ceramic that possesses high hardness, good sliding wear resistance, and excellent corrosion and oxidation resistance. Because of these attributes it is used as mechanical face seals for water pumps in automobiles and as balls and seats in shut-off valves. SiC, however, has a low fracture toughness that renders it susceptible to fracture under impact and thermal-shock conditions and to surface damage and erosion in particle impact. During Phase I, the feasibility of toughening SiC was demonstrated. The approach used consisted of designing and fabricating a functionally-graded material where the outer layer was 'pure' beta-SiC deposited by chemical-vapor deposition (CVD) on a substrate made of liquid-phase sintered SiC-TiC particulate composite. The designed composition and thermal-expansion mismatch led to a residual surface compression in the CVD-SiC coating on cooling from the deposition temperature. Coating compressions as high as 1.1 GPa were developed by this approach depending on the substrate composition, coating thickness, and the geometry of the component. Surface compression in the graded-SiC increased rolling-contact-fatigue life by greater than a factor of 100. The Phase II research will apply this technology to balls and seats in a 1.5 in. ball valve manufactured and marketed by Valtek International, the Phase III partner. Development of a ceramic manufacturing process for making SiC-TiC balls and seats, optimization of a rapid chemical-vapor deposition/composite vapor coating (CVD/CVC) coating process, low-cost surface finishing and prototype and field testing of the valve components are the major tasks in the proposed research. Th e Phase II research project is aimed at introducing surface-compression-toughened SiC in ceramic ball valves. These valves accounted for $ 8 million in sales in North America in 1996. The technology can also be implemented in the ceramic bearing and the automobile industries , e.g., cam followers. ***
