Abstract
Alumina is a candidate material for the reinforcement of molybdenum disilicide, potentially offering improvement in both the low and high temperature mechanical properties. In this work, emphasis has been placed on the influence of alumina on the microstructure of powder processed MoSi2 and the effect of the reinforcement on the room temperature properties. A range of MoSi2 matrix composites containing 5, 10, 15 and 20 vol% alumina, were fabricated by hot-pressing at both 1550°C and 1680°C along with monolithic MoSi2. Powder processed MoSi2 generally results in the inclusion of silica in the microstructure as a consequence of powder oxidation during handling and processing. Chemical analysis using a scanning electron microscope has shown that the alumina additions react with the silica to form stoichiometric mullite. It was determined that 15 vol% alumina was required to convert all of the free silica to mullite. Equilibrium conditions were not obtained in any of the composites; residual silica was present in the composites consisting of 5 and 10 vol % alumina, and an amorphous silicon rich phase was present in the composites of 15 and 20 vol% alumina. Increasing the processing temperature or annealing of the samples resulted in a greater volume fraction of the silica being converted to mullite. Transmission electron microscopy identified that the reinforcement phases consisted of crystalline mullite coated alumina particles, discreet mullite particles and a residual silicon rich, aluminium containing glass. From these microstructures the nucleation and growth mechanisms of mullite in the composite materials were formulated. The fracture toughness of these materials, measured using an indentation fracture technique and a double cantilever beam technique, was found to be approximately 3 MPa m½ and it was observed that reinforcement phases did not significantly alter the fracture behaviour of MoSi2. MoSi2 was observed to fracture predominantly in a transgranular fashion, which was attributed to its crystal structure. Residual stress calculations have shown that mullite will have a similar influence on the fracture behaviour of MoSi2 as silica. From three-point bending tests of small samples, the strength of these materials was observed to be around 520 MPa. The flaw size was estimated to be on a similar scale to the second phase inclusions, suggesting that these are the failure origins. Whilst the secondary phases have done little to improve the room temperature properties, it is envisaged that the resultant microstructure, in which the silica has been removed, should be more creep resistant than monolithic powder processed MoSi2.