Static Fatigue Limit for Sintered Silicon Carbide at Elevated Temperatures

Abstract

The modified static loading technique for estimating static fatigue limits was used to study the effects of oxidation and temperature on the static fatigue limit, K,,, for crack growth in sintered silicon carbide. For as-machined, unoxidized sintered silicon carbide with a static load time of 4 h, KIo ~2 . 2 5 MPa * m'" at 1200" and ~1 . 7 5 at 1400°C. On oxidation for 10 h at 12OO0C, Kro drops to ~1 . 7 5MPam"' at 1200" and -1.25 at 1400°C when tested in a nonoxidizing ambient. Similar results were obtained at 1200°C for tests performed in air. A tendency for strengthening below the static fatigue limit appears to result from plastic relaxation of stress in the crack-tip region by viscous deformation involving an oxide grain-boundary phase for oxidized material and, possibly, diffusive creep deformation in the case of unoxidized material.

Atomistic Aspects of Brittle Fracture

“…26 It is obvious that fracture of multicomponent materials in corrosive environments may be associated with several such rate-limiting processes. 34 Finally, the fracture energies of silicate glasses 20 are far above the surface energies measured at high temperatures and even well above estimates of the lowtemperature surface energy from simulations. The effects of impurities on the detailed processes are largely not understood.…”

Ceramic Femoral Heads for Total Hip Arthroplasty

“…[13,14] Similar results were reported for sintered silicon carbide by Minford. [15] If one transfers these results to alumina ceramics, the value of K Io would be about 1 MPa m 1/2 . This approach is used here since it seems plausible, a hypothesis supported by our examination of explanted femoral heads.…”