| Molybdenum disilicide (MoSi2) is a candidate material for high temperature structural applications due to its high melting point and excellent oxidation resistance. A major impediment to implementing this material is its poor room temperature fracture toughness. This study incorporates the microstructure-mechanical property relationship for MoSi2, MoSi 2 alloys, and composites in an effort to determine the feasibility for improving room temperature fracture toughness without undermining the beneficial oxidation and creep properties.; The fracture toughness values reported in the literature vary significantly, and the first part of this study examines the origins for these differences. A diverse array of samples was collected from laboratories involved in exploring MoSi2. These samples were investigated for their microstructural features and indentation response. The non-ideal indentation cracking observed with some MoSi2 samples is based on the anisotropic single crystal properties. The orientation dependency for indentation deformation and fracture was observed for the (110) plane of MoSi2. Fracture toughness values were determined from the damage zone surrounding the indent irrespective of the formation of the ideal radial/median cracking system. Adjustments to fracture toughness values are made to account for the elastic/plastic response. Results from this study indicate that a finely dispersed secondary phase enhances tortuous crack paths and microcracking.; The indentation analysis used for MoSi2 was also applied to MoSi2-Al2O3 and MoSi2-SiC samples. The Al2O3 addition increases the dihedral wetting angles of the triple junction SiO2 in MoSi2 and retards grain growth. These microstructural changes result in differences in the indentation cracking behavior. The fracture toughness improvement for the hot-pressed MoSi2-SiC sample, which is confirmed by four-point bend tests, may be attributed to higher residual stresses that aid in the tortuous crack behavior. Alloying approaches using Al, Ti, or Re to improve fracture toughness met with limited success. The crystal structure changes, lattice spacing modifications, and second phase additions were not helpful in preventing crack propagation.; In addition, isothermal oxidation results of MoSi2 indicate a microstructural effect on the rates of oxidation. The oxidation kinetics are temperature dependent, changing from linear to parabolic at 500°C, and this change is attributed to the volatization of MoO3. |
