Microstructural requirements for alumina SiC nanocomposites

The relative ease with which alumina/SiC nanocomposites polish, compared with monolithic alumina, has been noted in the literature and taken to be an example of the beneficial 'nanocomposite effect' stemming from an apparent strengthening of the grain boundaries. The percentage of 'polished' surface, as opposed to areas dominated by grain pullout or fracture, following grinding or polishing therefore provides an easily obtainable figure of merit for different nanocomposite materials. A variety of alumina/SiC nanocomposites have been fabricated by co-milling commercially available powders and hot pressing in an inert atmosphere. Using variations in starting powder size and heating schedules, it has been possible to produce nanocomposites with similar matrix grain size but with different proportions of particles on the grain boundaries relative to those within the grains. The microstructures were examined using TEM and FEG-SEM, and the effect of microstructural variations on the mechanical properties of the materials was investigated. Fracture surfaces were examined, and a study was made of the relative amounts of polished surface and grain pullout after both grinding and polishing the materials using standard metallographic techniques. The performance of the nanocomposites was compared with that of a pure alumina of the same grain size. The results indicate that the SiC has a direct effect on crack initiation and propagation in alumina, and that it is the particles on the grain boundaries which are responsible for the beneficial effect of the SiC additions. If sufficient SiC is present on the grain boundaries, crack initiation during abrasive wear can be prevented, resulting in a smoother, plastically deformed surface.