Processing - microstructure relationships of chemically vapor deposited zirconia fiber coating for environmentally durable silicon carbide/silicon carbide composites

In SiC/SiC ceramic matrix composites, toughness is obtained by adding a fiber coating which provides a weak interface for crack deflection and debonding between the fiber and the matrix. However, the most commonly used fiber coatings, carbon and boron nitride, are unstable in oxidative environments. In the present study, the feasibility of using a chemically vapor deposited zirconia (CVD-ZrO ₂) fiber coating as an oxidation-resistant interphase for SiC/SiC composites was investigated. The feasibility of the CVD-ZrO₂ coating as a useful interphase for SiC/SiC composites was investigated with emphasis on developing critical processing-microstructure relationships.; A study of morphological evolution in the CVD-ZrO₂ coating suggested that a size-controlled displacive phase transformation from tetragonal ZrO₂ (t-ZrO₂) to monoclinic ZrO₂ (m-ZrO₂) was the key mechanism responsible for the weak interface behavior exhibited by the ZrO₂ coating. The pre-delamination occurred as a result of (i) continuous formation of t-ZrO₂ nuclei on the deposition surface; (ii) martensitic transformation of the tetragonal phase to a monoclinic phase upon reaching a critical grain size; and (iii) development of significant compressive hoop stresses due to the volume dilation associated with the transformation.; We also discovered that low oxygen partial pressure in the CVD reactor was required for the nucleation of t-ZrO₂ and was ultimately responsible for the delamination behavior. The effects of oxygen partial pressure on the nucleation behavior of the CVD-ZrO₂ coating was systematically studied by intentionally adding the controlled amount of O₂ into the CVD chamber. Characterization results suggested that the number density of t-ZrO₂ nuclei apparently decreased with increasing the oxygen partial pressure from 0.004 to 1.6 Pa. Also, the coating layer became more columnar and contained larger m-ZrO₂ grains. The observed relationships between the oxygen partial pressure and the morphological characteristics of the ZrO ₂ coating were explained in the context of the grain size and oxygen deficiency effects which have been previously reported to cause the stabilization of the t-ZrO₂ phase in bulk ZrO₂ specimens.