Investigation On Structure And Performance Of <110>-textured Iridium Coating

Due to high specific strength at high temperature, it’s of viable significance to utilise refractorymetals in the field of aerospace application. However, the refractory metals are easily oxidized toform micropores, even to form volatile oxides in high-temperature environments, which results intheir service temperature and life being restricted extremely. Platinum group metals have anexcellent chemical compatibility and stability, especially, iridium (Ir) has extremely low oxygenpermeability and oxidation rate, which is one of the most promising candidates as super-hightemperature oxidation-resistant coating for refractory materials. In this dissertation, the Ir coatingand was prepared by double glow plasma surface metallurgy technology on the surface of refractorymetals. The main goal of this dissertation was to investigate the effect of substrates and depositionparameters on structure of the coating and analyze the formation processing of the interface betweenthe coating and the substrate, the formation mechanism of <110> texture of the coating and growthmechanism of the coating. It was to investigate the mechanical properties of the coating, such ashardness, elastic modulus, adhesive force and residual stress. It was to investigate the mechanism ofmicropore formation in the Ir coating after heat treatment, oxidation and ablation. In order to inhibitmicropore formation or micropores self-healing, the effect of doping Zr on structure andperformance of the Ir coating was investigated.The microstructure and morphology of the coating were observed and characterized usingscanning electron microscopy, transmission electron microscopy and atomic force microscopy. Thechemical composition of the coating was examined by X-ray energy dispersive spectroscopy, X-rayphotoelectron spectroscopy and electron probe micro analyzer. The micro-texture, grainmisorientation angle and grain size of the Ir coating were determined by electron backscatterdiffraction technique. The phase identification, grian size, macro-texture and residual stress of thecoating were identified by X-ray diffraction. The hardness and the elastic modulus of the coatingwere estimated by nanoindentation instrument. The adhesive force of the coating was performedwith scratch tester. Ir coating were heat-treated at1400℃for90min in Ar atmosphere, to evaluatethe thermal stability of the coating. The Ir and Ir-Zr coatings were oxidized at800℃and1000℃for1h in air, meanwhile, the reaction heat and weight change of the Ir coating were measured using aNETZSCH thermal analyzer, to assess oxidation resistance of the coating. The ablation resistance ofthe coating was tested at2000℃±100℃for35s in an oxyacetylene flame. The main conclusions and innovative results of this dissertation are drawn as follows:1. The research work of Ir coating by double glow plasma technology is systematicly carried out,we proposed the formation mechanism of <110> texture for Ir coating. The formation mechanism of<110> texture is that (110) grains easily grow over under the low mobility of adatoms at the begin ofthe deposition, and sputtering mechanism of channeling effect. Due to the anisotropy of sputteringyields for grains, the erosion of <110> crystal direction grains is smaller, which could result in theformation and growth of <110> texture grains. In contrast, the erosion of other crystal direction grainsis relatively large, which lead to the inhibited growth of other crystal direction grains.2. We propose growth mode of Ir coating, develop a model of interface formation process of Ircoating and research the growth mechanism of the Ir coating. The growth mode of Ir coating followsisland growth model. The growth mechanism of the Ir coating is that the growth mode of the coatingis mainly controlled by the nucleation rate at the beginning of the deposition. With the depositionprocess, the growth of the coating is governed by the grain growth rate.3. We reseach the formation mechanisms of the micropores on/in the Ir coating after hightemperature vacuum heat-treatment, oxidation and ablation. After heat treatment, the formationmechanism of micropores in the coating results from recrystallization of the coating, aggregation inthe development of birth defects in the coating and Kirkendall effect. After oxidation, lots of voidsand gaps are present on the surface of the coating due to the formation of volatile IrO3. After hightemperature ablation, the formation mechanism of the micropores on the surface of the coating is dueto volatile IrO3and O element diffusing quickly through intercrystalline boundaries to be oxidized toform gaseous MoO3.4. We propose a new plan that Zr element is doped in the Ir coating. The Zr in the Ir coating isoxidized to make the micropore of the Ir coating self-heal. The volume expansion of Zr changed toZrO2imporves the high-temperature thermal stability of the coating. The thermal stability ofIr-25at.%Zr coating is better than that of Ir-40at.%Zr coating. After oxidation at800℃, the surface ofIr-25at.%Zr coating keeps integrity and no micropores are found. The surface of Ir-40at.%Zr coatingshows some oxidized portion, and presents large particles of oxides. After oxidation at1000℃, thesurface of Ir-25at.%Zr coating appears microcracks, abnormal large particles and less micropores dueto unhomogeneous distribution of Zr element in the coating.