Improvement of mechanical properties after superplastic deformation of silicon nitride

Fine and uniform powder of beta-silicon nitride was liquid phase sintered by hot-pressing at 1700 degrees C. Deformation behavior was investigated under compressive stress at 1500 - 1600 OC. The material deformed at a strain rate, 1-7 x 10(-4) /s, under a pressure of 10-100 MPa at 1550 degrees C. Strain hardening was not observed even at slow deformation rate because of the absence of appreciable dynamic 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), rtspectively. Further Improvement was achieved by the annealing at 1700 degrees 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.