Pressureless sintering of particle-reinforced metal–ceramic composites for functionally graded materials: Part I. Porosity reduction models

There is a need to fabricate particle-reinforced metal–ceramic composites through pressureless sintering for functionally graded materials (FGMs) for use in a variety of high-temperature and energy-absorbing applications. However, the pressureless sintering behavior of these composites over wide volume fraction ranges has not been modeled. In this investigation, metal–ceramic composites were prepared by sintering mixtures of nickel and alumina powders. The changes in porosity and shrinkage were measured, and models were developed to understand both the nature of the porosity during sintering and the sintering behavior of the composites. In Part I of the investigation, the development of porosity reduction models is described to understand microstructural effects on the sintering behavior. These models indicate that there is significant porosity associated with the agglomeration of particles. Particle agglomeration and porosity are confirmed by direct observation of the microstructure. The porosity reduction models can be used to assess the quality of particle-reinforced metal–ceramic composites formed by pressureless sintering and to predict changes that can be achieved in the porosity reduction through engineering of the particle dispersion for the processing of FGMs.