Shock-enhanced alpha to beta phase transformation in Si3N4 powders

Shock-compression was used to make Si3N4 powder compacts (80–85% theoretical density) in order to investigate its effect on the α–β phase transformation occurring during post-shock thermal treatments. Crystallite size reduction to half of the starting powder size was attained, and residual strains in crystallites in excess of 10−3–10−2, corresponding to dislocation densities of the order of 1015–1017 cm−2, were generated due to shock compression. Transmission electron microscopy showed evidence of defects in the form of irregular striations (fringes), hexagonal dislocation arrays, sub-grains, and vacancy clusters in the interiors of the individual crystallites, with contrast-free amorphous regions along interparticle boundaries. Upon subsequent annealing (sintering) of the shock-densified compacts, the α–β phase transformation was observed to occur at temperatures of ≈200°C below that in unshocked powders, with no obvious change in density. The amount of α–β phase transformation increased with increasing temperature and time, with up to 87% conversion to β-phase in the compact heated for 5 h at 1650°C. The apparent activation energy for the α–β phase transformation in the shock-compressed silicon nitride powder compacts was determined based on the measured fraction transformed as a function of time and temperature, and was found to be in the range of 154–286 kJ mol−1. The results provide evidence that shock compression activates and enhances the reactivity of Si3N4 powders, by creating defect sites for heterogeneous nucleation of the β-phase, and thereby causing the α–β phase transformation to occur in the solid state and at reduced temperatures.