Mullite-whisker reinforced molybdenum disilicide composites

Molybdenum disilicide (MoSi;Mechanical property predictions were made for the proposed composite material. The toughening mechanisms examined were crack bridging, pullout, crack deflection and microcracking. For the bridging model alone, a doubling of the fracture toughness was expected for a 40 percent mullite whisker volume. The creep models examined were the isostress, isostrain, shear-lag and self-consistent scheme. The shear-lag model predicted a factor of five decrease in the creep rate compared to pure MoSi;Composites of MoSi;The resulting materials were subjected to mechanical testing. At room temperature, indentation testing was used to determine the toughness and modulus of the composites. Indented beams were subjected to four-point bending until failure to determine the toughness. The maximum fracture toughness measured was 1.7 MPa;The composites were examined by means of scanning and transmission electron microscopy (SEM and TEM). SEM examination of the as-processed composites revealed extensive sintering of the mullite reinforcements, which was postulated to affect the sliding creep resistance of these materials. TEM diffuse dark-field imaging revealed the presence of a thin (less than 5nm) film of glass at the two-phase interface. The interface is evidently strong enough at room temperature to prevent debonding of the interface which might lead to extensive toughening. At elevated temperature, the glass layer softens, reducing the calculated improvement in creep rate.