Quality Investigation Of Ir-PyC-SiC Coating System

Re nozzle with Ir coating (Tr-Re system) can be used in liquid rocket motor at 2200 ℃ for the excellent oxidation resistance of Ir. However, high diffusion coefficient of Re into Ir often brought about failure of the whole nozzle, which were a big drawback difficult to overcome for its further development and final application. In this thesis, a new C/SiC composite with Ir-PyC-SiC coating system were studied systemically. The processing, microstructure, properties and quality parameter of the coating system were systematically investigated, also its potential in application was evaluated.Ir-PyC-SiC multilayer coatings with changing thickness were successfully deposited onto the C/SiC bulk composites by metalorganic chemical vapor deposition (MOCVD) in which PyC acted as reaction barrier layer to avoid the solid phase reaction between Ir and SiC and their reactants with low melting point.The dependence of coating quality on depositing parameters, including the distance between precursor and substrates, roughness of substrates, deposition temperature was studied. The densification and defect density of Ir coating were influenced by distance between the precursor source and the substrate. With the increasing of the distance from 250mm to 450mm, the size of Ir coating grain correspondingly decreased from 50nm to 20nm, which resulted in the enhancing of the densification degree and the reducing of the defect density. The defect density was also adjusted by the toughness of substrates. Those rough surfaces tended to cause more defects, and pores or cracks usually originate from the concave sites. The deposition yields increased with heighten of deposition temperature. The Ir coating deposited at 600℃ was approximately 4 times thicker than that one at 500℃.The layered crystalline structure of PyC of reaction barrier layer and the mismatch between the Ir coating and substrate.largely contributed to the weak bonding between the Ir coating and the substrate. Also the thickness of PyC changed the bond. On the one hand. PyC is too thin at the edges and corners of the substateand at the crack in the SiC coating to effectively prevent the contact of Ir with SiC and subsequent reaction, on the other hand, when the thichness of PyC rose from 100nm to 400nm, the bond strength lowered from 15N to 9N which measured by acoustic emission nick.The high-temperature stability of the Ir-PyC-SiC-C multilayer coatings system was low. When heat-treating at or below 1450℃ in Ar for 5min, the thermal mismatch between coating and substrate and the sintering shrinkage of Ir coating resulted in the form and growth of large quantity of defects. Simultaneously Ir reacted with SiC at the crack sites into Ir3Si which boiled at 1345℃, accelerating the failure of the coatings. With the rising of heat-treatment temperature and/or the time elongation, finally the Ir coatings system failured in whole and no residual Ir can be detected on the C/SiC surface.