Investigation Of W/Ir Composite Coating On C/C Composite Prepared By Double Glow Plasma And Its Microstructure

Noble metal iridium (Ir) has high melting temperature and excellent properties of oxidation resistance in high temperature, so Ir is the best candidate anti-oxidation zero-ablation coating for C/C composite. However, the bond strength and thermal-shock resistance of the single Ir coating is not so good. If the Tungsten (W) coating is used as the interface layer, we can obtain the W/Ir composite coating which has high bond strength and high thermal-shock resistance. In this dissertation, W coating and Ir coating were alternately prepared on C/C composite substrate by double glow plasma technique (DGP) and the W/Ir composite coating was obtained. The microstructure and the morphology of the coating system were systematically investigated by SEM and XRD. Also the ablation property of the coating by oxy-acetylene torch was evaluated. The morphology and the mechanical properties of the W coating prepared by DGP were better than that of the W coating prepared by brush-painting. The best DGP process parameters of the Ir coating prepared on W substrate were as follows: workpiece voltage -400V, source voltage -800V, working pressure 30Pa and the distance between the target and substrate 13mm. The Ir coating prepared by the best process parameters was integrated, compact and had no obvious micro-defects. The (220) crystal plane preferential growth of the Ir was obvious in the XRD pattern. The W/Ir composite coating and the single Ir coating prepared on C/C composite substrate by DGP both had the Ir (220) crystal plane preferential growth orientation, which was different from the (111) crystal plane preferential growth of the Ir coating prepared by traditional MOCVD, PVD. The single Ir coating was exfoliated from the substrate after oxy-acetylene torch ablation. However, the W/Ir composite coating withstanded the ablation and protected the C/C composite substrate effectively. The result indicated the W interface layer the thermal-shock resistance of the coating. The crystal plane preferential growth orientation of the coating was changed from (220) crystal plane to (111) crystal plane after ablation. After ablation, the surface of the composite coating was rough and the ablation holes appeared on the surface. The W interface layer under the ablation holes was partly oxidized. Micro-cracks appeared on the coating surface and a few white particles which had special structure appeared around the micro-cracks.