On the α to β phase transformation and grain growth during hot-pressing of Si3N4 containing MgO

The α/β phase transformation and correlated grain growth during hot-pressing of silicon nitride containing MgO has been studied. The reconstructive transformation appears to be reaction controlled with the rate dependent on the amount of pre-existing β phase and α/liquid/β interface formed during hot-pressing. The microstructure of the dense, hot-pressed product, i.e. grain size, shape, and distribution, is influenced by the starting α/β ratio and nucleation rate of the β phase. Different growth rates perpendicular and parallel to the crystallographic c-axis of β result in an elongated microstructure. Increasing the hot-pressing pressure increases the transformation rate as a consequence of the enhanced nucleation rate of β. This produces a fine, uniform microstructure with good mechanical strength. If hot-pressing time is extended beyond that required for α/β conversion, coarsening of the β grains will degrade mechanical properties.