Abstract
Pressureless sintered Si{sub 3}N{sub 4} with devitrified grain-boundary phases was investigated for thermal expansion, fracture toughness, and strength behavior. An extended hysteresis of the heating and cooling dilatation curves was observed for the {alpha}ss/{beta}-eucryptite materials. Fracture toughness decreased in the {alpha}-ss/{beta}-eucryptite but increased from 4.2 to 5.6 MPa{center dot}m{sup 1/2} in the {beta}-ss/{beta}-spodumene microstructures. Fracture surfaces reveal extended crack deflection in the grain-boundary-toughened {beta}-ss/{beta}-spodumene materials compared with the glass-containing materials. Residual grain-boundary stresses resulting from thermal expansion mismatch are supposed to account for the deflection of crack path and a substantial toughness increment in the {beta}-ss/{beta}-spodumene microstructures.
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