Low modulus, oxidation-resistant interface coatings for SiC/SiC composites

A novel material, (Ca_0.6,Mg_0.4)Zr₄(PO₄)₆ (CMZP), was evaluated as a weak interface coating for SiC/SiC composites. A procedure was developed to put down uniform and crack-free CMZP coatings on Nicalon cloth and tows using sol-gel and metal organic deposition (MOD). The coated Nicalon cloth samples and tows were infiltrated with SiC matrix using Chemical Vapor Infiltration (CVI). Bars were cut for flexure testing from the infiltrated composite containing Nicalon cloth samples that had been coated using sol-gel. These composites failed gracefully, 1.e., there was fiber pullout and debonding probably at the matrix-coating interface. Minicomposites that contained tows coated using MOD were too weak to be tested for tensile strength. This necessitated the deposition of a thin (~ 30 nm) layer of carbon both on the tows before depositing CMZP coating to protect the fibers as well as on the CMZP coating to protect the coating. Minicomposites that contained these tows, coated using sol-gel and MOD, demonstrated extensive pullout and debonding. The composite behavior could not have been due to the carbon alone as there was very less (~ 60-80 nm) present. Thus, the CMZP coating was responsible, probably in addition to the carbon layers, for the composite behavior.