The improvement in mechanical properties of MoSi2 through in situ crystallization of grain boundary silica glass by the additions of refractory oxides

The improvement in the mechanical properties of powder processed MoSi2, by the additions of 3 mol.% Y2O3 stabilized ZrO2, (3Y–ZrO2) and unstabilized HfO2, are discussed. Microstructure of the nominal MoSi2/3Y–ZrO2, composites consisted of MoSi2 matrix, Mo≤5Si3C≤1, well-crystallized ZrSiO4, and unreacted tetragonal ZrO2 (t-ZrO2). The glassy SiO2 phase was converted into more thermodynamically stable crystalline ZrSiO4 (zircon) by the 3Y–ZrO2 addition. MoSi2 with only 1 mol.% 3Y–ZrO2 showed excellent fracture strength attributed to the crystallization of glassy SiO2. MoSi2 with higher content of 3Y–ZrO2 additive possessed various improved mechanical properties at room temperature, arising from the combined effects of the crystallization of glassy SiO2 and the transformation toughening by t-ZrO2. MoSi2 with 2 mol.% HfO2 additive also showed improved fracture strength. MoSi2 with 15–20 mol.% HfO2 exhibited obvious hardening, perhaps due to residual stress field formation resulting from the thermal expansion anisotropy.