alpha'-SiAlON: Phase stability, phase transformations and microstructural evolutions.

摘要

Silicon nitride is a prominent member of a ceramic family developed for use in structural applications at ambient and elevated temperatures. It exists in two polymorphic forms, {dollar}alpha{dollar} and {dollar}beta.{dollar} The predominant form in use today is {dollar}beta{dollar}-Si{dollar}sb3{dollar}N{dollar}sb4.{dollar} The other form of silicon nitride, {dollar}alpha{dollar}-Si{dollar}sb3{dollar}N{dollar}sb4{dollar}, is not used even though it is much harder. This is because only {dollar}beta{dollar}-Si{dollar}sb3{dollar}N{dollar}sb4{dollar} can be made into a tough and strong ceramic by forming a microstructure that resembles a fiber-reinforced composite. {dollar}alpha{dollar}-Si{dollar}sb3{dollar}N{dollar}sb4{dollar}, which is unstable at high temperature, can be stabilized by forming solid solutions with Al, O, and interstitial cations, but it always appears with a microstructure of fine, equiaxed grains and has low toughness and strength.; Microstructural development of silicon nitride is closely related to phase transformations mediated by a liquid. To help guide the development of new silicon nitride alloys, various aspects of these transformations, and some reverse transformations among them, have been studied. This dissertation sheds light on such issues as the determination of equilibrium phase relations at different temperatures for different compositions, the kinetics of forward and reverse transformations, and the connection of these transformations to the microstructure. Among the outstanding issues that have been resolved by this work are the stability of {dollar}alphaspprime{dollar}-SiAlON at low temperature and the role of starting powders and rare-earth cations in determining the rate of phase transformations. A new class of {dollar}alphaspprime{dollar}-SiAlON, which combines the toughness of {dollar}beta{dollar}-Si{dollar}sb3{dollar}N{dollar}sb4{dollar} and the hardness of {dollar}alpha{dollar}-Si{dollar}sb3{dollar}N{dollar}sb4{dollar}, with a whisker-like microstructure, has also been discovered by exploiting the nucleation and growth kinetics of phase transformations.