MSRI has developed a class of functionally-graded SiC composites that exhibit high strength, damage resistance and reliability. The composites consist of SiC-TiB2 or SiC-TiC substrates with a thin surface layer of pure a- or b- SiC. The pure SiC surface layer ensures good corrosion and sliding-wear resistance while the residual surface compression induced in the SiC layer (due to the thermal-expansion mismatch with the substrates) enhances strength, damage-resistance and reliability of the components.
Testing has demonstrated a doubling of strength, damage resistance, and erosion resistance for the functionally-graded composites (compared to monolithic SiC ceramics) while maintaining a corrosion resistance comparable to that of commercial-grade SiCs.
MSRI has fabricated a variety of engineering components including balls and seats for shut-off valves and roller bearings from the graded composites. Balls made of a SiC/SiC-15 vol% TiB2 composite withstood a torque of 6800 in.lbs in prototype destructive tests (ordinary SiC balls fail at about 3300 in.lbs). Rolling elements made from SiC/SiC-30 vol% TiC had a mean fatigue life that was two orders of magnitude greater than the life of ordinary monolithic SiC elements.
Functional Silicon Carbide
- Functionally graded silicon carbide or silicon nitride composites
- Enhanced fracture toughness by design of residual surface compression
- Applicable for long-life and low-cost tooling for friction stir welding of high strength steel alloys