Pure Ir coating was produced by double nism of the Ir coating was investigated. glow plasma technology. Growth mecha- The Ir coating was composed of irregular compacted columnar grains with lots of nanovoids appeared on the interface between the coating and the substrate. The Ir coating was polycrystalline with a preferred （220） orientation due to the initial nuclei with preferred growth on the surface of the substrate. The formation mechanism of the Ir coating depended on kinetic adsorp- tion and diffusion process with nucleation, coalescence and thickness growth. At the beginning of the deposition process, the growth mode of the coating was mainly con- trolled by the nucleation rate. Due to the low substrate temperature resulting in low mobility of the deposited atoms, some micropores and nanoviods were present at the interface. With the deposition process, the substrate temperature was increased and then kept steady. The growth of the coating was governed by the growth rate. The high substrate temperature supported enough energy to surface mobilitv of adatoms.
Oxidation resistance coatings of Ir-40 at.% Zr and Ir were produced onto Mo substrates by double glow plasma technology. The oxidation resistances of the coatings were evaluated at high temperature. Ir-Zr coating consisted of two layers： the primary layer close to the substrate was composed of dense columnar grains and the second layer was composed of dense grains of nanometric size. The mass gain of Ir coating above 800℃ was about 1.35% due to the formation of solid IrO2. The mass loss of Ir coating was about 5.3% due to the formation of gaseous oxide IrO3 when being held at 1227 ℃ for 30 min. The substrate was protected more effectively by multilayer than monolayer coating of Ir in oxidizing environment. The Ir-Zr coating was well bonded to the substrate after oxidation at 800℃. After oxidation at 1000℃, the Ir-Zr coating was poorly bonded to the substrate. The oxidation resistance of Ir-Zr coating was poor due to high content of Zr.
Multilayer iridium coating was manufactured on tungsten carbide substrates by a double glow plasma process.As comparison,monolayer was also produced.The microstructure and morphology were observed using scanning electron microscopy.Grain orientation and phase were determined using X-ray diffraction.The residual stress of the coating was studied by glancing incidence X-ray diffraction.The adhesive force of the coating was measured by a scratch tester.The results showed that both monolayer and multilayer had a polycrystalline phase with a strong（110） reflection.The coating had an excellent adhesion with no evidence of delamination.The adhesive force of the monolayer and multilayer was about 50 and 43 N,respectively.The interfacial reaction between the substrate and the layer occurred and a new WIr phase was found due to the high-temperature deposition process.The residual stress in the monolayer and multilayer was-1.6 and-1.1 GPa,respectively.
Ir coatings were deposited on the heat-treated C/C composites and graphite by double glow plasma. Microstructure and morphology of the coating and substrate were observed by SEM and TEM. The effect of the surface treatment for the carbon structural materials on the microstructure of the coating was investigated. Many large gaps and pores appeared on the surface of the substrates after heat treatment. The Ir coating did not fully covered on the surface of heat-treated C/C composite and graphite substrates because of the large gaps and pores on the surface of substrates. The Ir coating exhibited excellent ablation resistance at super-high temperature. After super-high temperature ablation, the coating kept the integrity, but the coating was weekly bonded to the substrates. Some microcracks and micropores appeared on the surface of as-ablated coating. The Ir coating would need thick enough to cover and fill the large microgaps and micropores on the surface of the heat-treated C/C and graphite substrates.
Pt and Ir coatings were produced by double glow plasma technology on the surface of Ti alloy substrates. The chemical compositions of the coatings were determined by X-ray diffraction and X-ray photoelectron spectroscopy. The microstructure and morphology of the coatings were observed by scanning electron microscopy. The hardness and elastic modulus of the coatings were estimated by nanoindentation. The measurements of adhesive forces of the coatings were performed with scratch tester. The results indicated that the Pt and Ir coatings displayed the preferred (220) orientation due to the initial nuclei with preferred growth on the surface of the substrates. The interface between the Pt coating and substrate exhibited no evidence of delamination. The Ir coating was composed of irregular columnar grains with many nanovoids at the interface between the coating and substrate. The mean values of hardness for Pt and Ir coatings were 0.9 GPa and 9 GPa, respectively. The elastic modulus of Pt and Ir coatings were 178 GPa and 339 GPa, respectively. The adhesive forces of the Pt and Ir coatings were about 66.4 N and 55 N, respectively. The Pt and Ir coatings adhered well to the Ti alloy substrates.
Double glow plasma technique has a high deposition rate for preparing iridium coating. However, the glow plasma can influence the structure of the coating at the single substrate edge. In this study, the iridium coating was prepared by double glow plasma on the surface of single niobium substrate. The microstructure of iridium coating at the substrate edge was observed by scanning electron microscopy. The composition of the coating was confirmed by energy dispersive spectroscopy and X-ray diffraction. There was a boundary between the coating and the substrate edge. The covered area for the iridium coating at the substrate edge became fewer and fewer from the inner area to the outer flange-area. The bamboo sprout-like particles on the surface of the substrate edge were composed of elemental niobium. The substrate edge was composed of the Nb coating and there was a transition zone between the Ir coating and the Nb coating. The interesting phenomenon of the substrate edge could be attributed to the effects of the bias voltages and the plasma cloud in the deposition chamber. The substrate edge effect could be mitigated or eliminated by adding lots of small niobium plates around the substrate in a deposition process.
