Process development for iridium aluminide coated silicon carbide-carbon functionally graded material for the oxidation protection of graphite

Silicon carbide has long been used to protect carbonaceous materials such as graphite and carbon/carbon composites from the detrimental effects of high temperature oxidizing environments. However, the effectiveness of SiC is limited at elevated temperatures by coefficient of thermal expansion mismatch stresses and a breakdown of the protective SiO{dollar}sb2{dollar} layer which forms as SiC is oxidized. Functionally graded materials (FGM) represent a new class of materials specifically engineered to accommodate CTE mismatch stresses. The SiC-C FGM is one such system. Grading the interface between SiC and C produces a gradually changing composite material capable of accommodating the CTE mismatch stresses that arise as a result of high temperature service. Iridium is another material that has been used as a protective coating for carbonaceous materials, it is non-reactive with carbon, has an extremely high melting temperature, 2447{dollar}spcirc{dollar}C, and has the lowest known rate of oxygen diffusivity; however, at 900{dollar}spcirc{dollar}C Ir forms a volatile oxide. Ir can be alloyed with Al to form the intermetallic compound IrAl. IrAl has a melting temperature of 2120{dollar}spcirc{dollar}C, has been shown to be non-reactive with carbon, and upon oxidation forms a protective {dollar}rm Alsb2Osb3{dollar} layer that inhibits the oxidation and volatilization of Ir.; The aim of this research was to develop a process for the deposition IrAl onto SiC-C FGMs so that compatibility of the two coatings could be evaluated for potential high temperature use. It was found that dense SiC-C FGM coatings could be deposited on graphite using the SiC{dollar}sb4{dollar}-{dollar}rm Csb3Hsb8{dollar}-H{dollar}sb2{dollar} gas system. By increasing the H{dollar}sb2{dollar} flow rate in the reactant gas stream it was shown that the porosity of the coating could be essentially eliminated. High temperature chemical compatibility studies showed that IrAl thin films are poor high temperature oxidation barrier for SiC-C FGMs. The large CTE mismatch between IrAl {dollar}(8.8times10sp{lcub}-6{rcub}spcirc {lcub}rm C{rcub}sp{lcub}-1{rcub}){dollar} and SiC {dollar}(4.3times10sp{lcub}-6{rcub}spcirc{lcub}rm C{rcub}sp{lcub}-1{rcub}){dollar} combined with extensive interfacial chemical reactions cause the IrAl thin films to debond at temperatures as low as 700{dollar}spcirc{dollar}C. Given the poor performance of the IrAl coatings it is recommended that for the high temperature oxidation protection of graphite, only the SiC-C FGM should be used. Where additional oxidation protection is needed the SiC layer should be made thicker to compensate for the higher oxidation rate, rather than relying on an additional coating for protection.