Understanding the Individual Role of Composition and Spark Plasma Sintering Temperature on the Microstructure and Hardness of Alumina–Zirconia Composites

In the present study, the objective is to examine the individual effect of varying the composition and SPS conditions on the final mechanical property (hardness) of alumina–zirconia (AZ) composites through their microstructural variations. To this end, AZ composites with varying zirconia (ZrO₂) content are prepared vis-à-vis monolithic alumina (Al₂O₃) and zirconia (ZrO₂) ceramics by spark plasma sintering (SPS). The microstructure evolution, phase formation, and hardness for the monolithic ceramics and AZ composites are systematically investigated and a correlation between them is established using various analytical modeling approaches. By altering the SPS temperature, significant variation in grain sizes were produced for the two monolithic ceramics and various AZ composites. In case of the latter, the size and volume fraction of secondary ZrO₂ particles vary according to the composition, i.e., higher the ZrO₂ content, coarser and numerous are the second phase particles. The monolithic ceramics exhibit comparatively lower hardness than the AZ composites at equivalent grain sizes. The hardness for the AZ composites and monolithic ceramics also vary marginally with the size of the matrix grains/crystallites. The variation in hardness is rather attributed to the volume fraction of the secondary ZrO₂ phase in case of the AZ composites. Altogether, the AZ composite with a finer grain size and low volume fraction of secondary ZrO₂ phase exhibits the highest hardness, while coarse-grained microstructure and high ZrO₂ content contributes to significantly lower hardness. The present study thus emphasizes the synergistic roles of SPS temperature and composite compositions in dictating their microstructure and mechanical response.