The mechanism of simultaneous sintering and phase transformation in alumina

The sintering behaviour of boehmitic alumina gels during the transformation to the stable phase has been studied using dilatometry, transmission electron microscopy, X-ray analysis and differential thermal analysis. The specimens for transmission electron microscopy were prepared from gel specimens, sintered to various predetermined temperatures, using an ion-beam thinning technique. The transmission electron microscope study and X-ray analysis have revealed a characteristic sintering behaviour which is associated with the to phase transition. The transformation to the phase occurs by a nucleation and growth process. During the growth process considerable redistribution of the fine porosity existing within the transition alumina matrix occurs, in the form of large elongated interconnected pores trapped within the nucleating grains. These pores grow rapidly to a size approximately one hundred times that of the grains. This process results in a rapid fall-off in sintering rate at the end of the transformation. A study of the/ interphase interface by transmission electron microscopy has led to the development of a model that accounts satisfactorily for the redistribution of the porosity.