A 2-STEP METHOD TO OBTAIN SUPERPLASTIC SILICON-NITRIDE WITH HIGH THERMOMECHANICAL PROPERTIES

Due to its microstructure, composed of very fine equiaxial grains and some amorphous phase, hot pressed alpha-Si3N4 was superplastically def ormed by bending at relatively low temperature (1540 degrees C) and st ress (15 MPa) and at high strain rate (6 x 10(-2) h(-1)). Afterwards, a thermal treatment was applied to the deformed samples in order to pr omote the alpha to beta silicon nitride phase transformation and to re crystallise the residual amorphous phase. Such a treatment leads to a significant increase in the mechanical properties (sigma(F) = 770 --> 880 MPa; K-lC = 3 --> 5 MPa root m) resulting in the presence of newly formed acicular grains of beta-Si3N4. The modulus of rupture measured at high temperature (between 800 and 1200 degrees C), as well as the creep strength, are also significantly improved compared to a referenc e beta-Si3N4 material hot pressed with the same sintering aids (Al2O3/ Y2O3). Therefore, it can reasonably be assumed that the residual amorp hous phase allowing superplastic deformation to occur is almost fully recrystallised after annealing. (C) 1996 Elsevier Science Limited.