Microstructure and mechanical properties of low-pressure injection-moulded reaction-bonded alumina ceramics

Reaction-bonded alumina was fabricated using standard powder preparation methods and the low-pressure injection moulding (LPIM) forming technique, followed by reaction sintering. The feasibility of LPIM was investigated in terms of the compounding ability of a highly agglomerated mechanically alloyed powder in a non-polar organic vehicle, and the microstructural homogeneity and resulting reliability of sintered LPIM parts. The green density of LPIM parts after debinding, roughly corresponding to the solids loading in the LPIM feedstock, was in the range of fractional density achieved by dry pressing, although the powder packing and aluminium particle deformation during forming were not the same. LPIM forming and debinding induced microstructural inhomogeneities (i.e. larger voids due to trapped air and density fluctuations) which were reflected in a slightly lower Weibull modulus, while the average strength did not differ significantly from the values obtained with dry pressed samples. The microstructure and mechanical properties of sintered parts were also related to the purity of the starting powders. The presence of impurities in the starting aluminium powder resulted in a somewhat coarser microstructure, characterized by a broader Al2O3 grain-size distribution, as well as in the presence of a thin glassy phase on the grain boundaries and in partial destabilization of dispersed tetragonal (Y2O3-stabilized) ZrO2 particles. In spite of a less favourable microstructure, the room-temperature strength and Weibull modulus were still comparable to those obtained from high-purity starting powder.