Improving the damage tolerance of Si₃N₄ by forming laminate composites with refractory metals

The objective of this research was to demonstrate that the damage tolerance of Si₃N₄ could be significantly improved by forming laminate composites with refractory metals, providing materials that undergo graceful failure, rather than the fast-fracture mechanism exhibited by monolithic Si₃N₄. A damage tolerant Si₃N₄ could be used as a ring material in an all-ceramic bearing, decreasing the chance for catastrophic failure if the ring is stressed in tension during operation. The technical approach formed a laminate composite material using alternating Si₃N₄-metallic layers, with both outer layers being Si₃N₄ to take advantage of its greater wear resistance, chemical stability, and thermal stability. The metallic layers are designed to arrest any cracks in the outer layers, thus producing a toughened Si₃N₄ and avoiding the catastrophic failure behavior exhibited by monolithic ceramics. The laminate composites were fabricated using a combination of tape-casting Si₃N₄ and metals from slurries, as well as metal foils, followed by hot pressing at 1500°C. The metallic materials employed were chromium, titanium, and tantalum. Analysis confirmed that the interfaces were well formed, and the laminates with chromium and titanium formed intermetallic compounds more readily than the composites with tantalum. The Si₃N₄-Ta laminates demonstrated crack deflection and bridging behavior during failure and flexural strength of 800–900 MPa. The hardness and elastic modulus of Si₃N₄-Ta laminates measured by nanoindentation were similar to those reported in literature. The hardness across the interface of the Si₃N₄-Ta composite varied according to the composition of the interface, which displayed a profile indicative of a diffusion bond.