Correlation of high-temperature properties and interphase characteristics in oxide/oxide fibre-reinforced composites

Oxide/oxide fibre-reinforced composites are favorite candidates as liners for thermal protection of combustion chambers in gas turbines. This application requires damage tolerance over long terms at high temperatures. Damage tolerance in ceramic composites is achieved by energy dissipation e.g. due to fibre debonding and sliding. On the other hand, between the fibre and matrix, a good load transfer is requested to obtain high strength. Fibre/matrix interaction can be controlled by introducing interphases between the fibre and matrix. Mullite based fibre-reinforced composites having fibres with CVD-processed carbon (fugitive)-single layer and carbon (fugitive)/A12O3-double layer coatings were produced. Changes in microstructure and mechanical behavior due to constant and cyclic thermal loading were investigated. The oxide layer behind a carbon (fugitive) layer was found to increase the strength of the composite and maintain damage tolerance in contrast to only fugitive layer. Under cyclic heat treatment, composites showed significantly better damage tolerance than under constant thermal loading.