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 1790K and at strain rates from l.Sx 10^-6s^-1 to 3x10^-2s^-1 in a nitrigen gas of 1.01³x10⁵ Pa (1 aim). 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-rale 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.