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

Three powder compositions near the β'‐sialon phase (Si 6‐ z Al 2 O z N 8‐ 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‐Si 3 N 4 , Al 2 O 3 , 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‐σ 0 ) 2 , where σ 0 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‐σ 0 ) 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.