STRENGTH CHARACTERIZATION OF YTTRIA-DOPED SINTERED SILICON-NITRIDE

The flexural strength of yttria-doped sintered silicon nitride was evaluated as a function of temperature (20 to 1300 degree C in air environment), applied stress and time. Two mechanistic regimes were manifest in the temperature dependence of the fracture stress. A temperature-independent region of fast fracture (catastrophic crack extension) existed up to 900 degree C, in which the mode of crack propagation was primarily transgranular. Above 1000 degree C, the strength (fracture stress) decreased considerably due to the presence of subcritical or slow crack growth which occurred intergranularly. This material did not show a static oxidation problem in short-term tests in the low-temperature regime (600 to 1000 degree C) as has been observed in other yttria-doped silicon nitrides. Flexural-stress rupture testing in the temperature range 800 to 1200 degree C in air indicated the material's susceptibility to time-dependent failure, and outlines safe applied stress levels for a given temperature.