High-Temperature Deformation and Fracture Behavior of Al<SUB>2</SUB>O<SUB>3</SUB>–Y<SUB>2</SUB>O<SUB>3</SUB> Doped Silicon Nitride

Abstract

In order to clarify the high-temperature deformation and fracture behavior of Al 2 O 3 -Y 2 0 3 doped Si 3 N 4 , four-point bending tests were conducted at temperatures from room temperature to 1790K and at strain rates from 1.5x10 -6 s -1 to 3x10 -2 s -1 in a nitrigen gas of 1.013x10 5 Pa (1 atm). As a result, the mechanical equation of state at yield stress was obtained as e=A(σy/E) m exp(-Q/RT), where m =3.9 in the low temperature range or high strain-rate range, where m =1.7 in the high temperature range or low strain-rate range. The critical strain rate at which m changed shifted to high strain-rate side as temperature rises. It was suggested that plastic deformation of the Si 3 N 4 was controlled by grain boundary sliding with cavity formation in the range of m=1.7, and dislocation motion and micro-crack propagation in the range of m=3.9. The activation energy for deformation Q was almost independent of strain rate when m=3.9, while Q tended to increase with decreasing strain rate when m=1.7. The relationship between deformation and fracture in Si 3 N 4 was discussed on the basis of m value and fracture mode.