Mechanisms of Dentification During Reaction Hot‐Pressing in the System Si‐Al‐O‐N

Abstract

Three powder compositions near the β'‐sialon phase (Si6‐zAl2OzN8‐z) were hot‐pressed at 1963 K and pressures of ∼7 to 35 MPa. The compositions, having z values near 0.75, were prepared from mixtures of α‐Si3N4, Al2O3, and AIN. It is shown that two processes operate sequentially during hot‐pressing. For compositions with the most grain‐boundary phase, the densification rate at a given density, (dp/dt), increases linearly with the applied stress, cr. For the composition with the least grain‐boundary phase, the rate could be described by the equation (dp/dt)∞(σ‐σ0)2, where σ0 is a threshold stress which varies with density and the powder composition. For the composition with an intermediate amount of grain‐boundary phase, the rate varies as (σ‐σ0)2 below ∼20 MPa and increases linearly with σ above 20 MPa. The two processes are interpreted in terms of grain‐boundary sliding limited by a threshold stress and a liquid‐phase solution‐diffusion‐reprecipitation process.