Superplastic behavior of low-doped silicon nitride

Fully dense, fine-grained (0.55 μm) and equiaxed silicon nitride has been produced by hot isostatic pressing performed at 1740 °C under 160 MPa. The starting α-Si3N4 powder was mixed with 0.5 wt.% Y2O3 and 0.5 wt.% Al2O3 used as sintering additives. Prior to deformation a very small amount of glassy phase was detected by transmission electron microscopy at the two-grain and three-grain junctions. In order to study the superplastic deformation of this material, compression tests at constant crosshead rates were conducted up to about 0.5 strain, in nitrogen, from 1600 to 1700 °C. The mean stress exponent of the power law was determined to be n = 1 below a transition stress σ* = 20 MPa and n = 0.5 above this stress. Deformation proceeded via diffusion controlled grain boundary sliding with an apparent activation energy Q = 614 kJ mol−1. The observation of strain whorls, the shear-thickening phenomenon and the transition from a mild to a strong strain hardening, at high stresses, were attributed to the occurrence of rigid and dry grain-to-grain contacts under the dewetting of the intergranular liquid film above the previous transition stress.