ENVIRONMENTALLY COMPATIBLE DOUBLE COATING CONCEPTS FOR SAPPHIRE FIBER-REINFORCED GAMMA-TIAL

Environmentally compatible interfaces based on double coating concepts have been developed for sapphire fibers in gamma-TiAl matrix composites. The fiber coating design involves two layers with distinct functions: one in contact with the fiber which provides the debonding and sliding characteristics required for toughening, and a second one next to the matrix which protects the fiber and debond layer from diffusional interactions and mechanical degradation during processing. Three double coating systems were tested wherein the debond layer consisted of pyrolytic carbon, colloidal carbon or a mixture of carbon and alumina. The diffusion barrier was in all cases a dense alumina layer less-than-or-equal-to 5 mum thick. With the exception of the pyrolytic carbon, all other coatings were applied by slurry processing techniques. After composite consolidation, the coatings were evaluated using fiber push-out tests, whereupon each double coating was found to enable successful debonding and sliding of the sapphire fibers. The environmental compatibility of the coatings was tested by oxidizing the carbon in the debond layer at elevated temperatures. This produced measurable changes in the interfacial properties, but push-out tests confirmed that the coating design still allowed fiber debonding and sliding within the ranges desired for improving toughness.