Effects of yttria stabilized zirconia on transformation toughening of molybdenum disilicide composites

The effects of yttria stabilization (0-6 mol%) on the fracture toughness of molybdenum disilicide composites reinforced with 20 vol.% yttria partially stabilized zirconia (Y-PSZ) particles are presented. It is found that the fracture toughness increased from 3.8 MPa{radical}m for the monolithic MoSi{sub 2} to 4.8-5.7 MPa{radical}m for the Y-PSZ reinforced composites. Examinations of the fractured specimens, using optical interference microscopy and laser Raman spectroscopy, showed that the stress-induced martensitic transformation of the Y-PSZ particles occurred only in the plastic wake of the composite stabilized with 2 mol.% yttria. However, the fracture toughness of the 2 mol.% Y-PSZ reinforced composite was in the same range as those of the other composites which did not exhibit evidence of transformation toughening. The results are rationalized using micromechanical models for the assessment of transformation toughening and microcrack shielding mechanisms.