Microstructure-property relationships of chemically vapor deposited zirconia fiber coating for environmentally durable silicon carbide/silicon carbide composites.

摘要

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-ZrO₂) fiber coating as an oxidation-resistant interphase for SiC/SiC composites was investigated. 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-ZrO ₂) was the key mechanism responsible for the weak interface behavior exhibited by the ZrO₂ coating. It appeared that a low oxygen partial pressure in the CVD reactor chamber was essential for the nucleation of t-ZrO₂ and therefore was responsible for the delamination behavior.; With this understanding of the weak interface mechanism, minicomposite specimens containing various ZrO₂ fiber coating morphologies were fabricated and tested. A fractographic analysis showed that in-situ fiber strength and minicomposite failure loads were strongly dependent on the phase contents and microstructure of the ZrO₂ coating. We determined that an optimum microstructure of the ZrO₂ coating should contain a predelaminated interface surrounded by a dense outer layer. The outer layer was needed to protect the fiber from degradation during the subsequent SiC matrix infiltration procedure. A preliminary tensile stress-rupture study indicated that the ZrO₂ coating exhibited promising performance in terms of providing the weak interface behavior and maintaining the thermal and oxidative stability at elevated temperatures.