High-Temperature Deformation and Fracture Behavior of Al2O3–Y2O3 Doped Silicon Nitride

In order to clarify the high-temperature deformation and fracture behavior of Al2O3–Y2O3 doped Si3N4, four-point bending tests were conducted at temperatures from room temperature to 1790 K and at strain rates from 1.5×10−6s−1 to 3×10−2s−1 in a nitrigen gas of 1.013×105Pa (1 atm). As a result, the mechanical equation of state at yield stress was obtained as ε=A(σy⁄E)mexp(−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 Si3N4 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 Si3N4 was discussed on the basis of m value and fracture mode.