Improvement of Mechanical Properties after Superplastic Deformation of Silicon Nitride

Abstract

Fine and uniform powder of β-silicon nitride was liquid phase sintered by hot-pressing at 1700 °C. Deformation behavior was investigated under compressive stress at 1500 - 1600 C. The material deformed at a strain rate, 1-7 x 10 -4 /s, under a pressure of 10-100 MPa at 1550 °C. Strain hardening was not observed even at slow deformation rate because of the absence of appreciable dy namic grain growth. The strength and fracture toughness of as-hot-pressed materials was 793 MPa and 2. 8 MPa.m 1/2 , respectively. The properties were improved by the deformation to 912 MPa and 3.5 MPa.m 1/2 . respectively. Further improvement was achieved by the annealing at 1700 C for 30 min. The bending strength and fracture toughness increased to 982 MPa and 3.7 MPa.m 1/2 , respectively, by slight grain growth. Present work provides a process to produce complex-shaped components by superplastic deformation at low temperatures and to improve the mechanical properties by further heat treatment.