Research Status and Prospects of Ir and Ir-Alloy Coatings
At first,Tanant found iridium when separating platinum.The iridium has the melting point as high as 2440 ℃ with excel-lent corrosion resistance and extremely low high temperature oxygen permeability,and cannot react with carbon even in the high tem-perature of 2280 ℃.The iridium possesses a very high application rate in the field of high-temperature protective coatings,especially as the temperature exceeds 1800 ℃.Therefore,Ir is considered to be one of the most promising high temperature oxidation resistant protective materials.The Ir is widely used as a protective coating for high-temperature structural components,such as satellites,liquid rocket engines,leading edges and nose of hypersonic aircraft.There are many kinds of preparation techniques for Ir coatings,includ-ing frequently-used chemical vapor deposition(CVD),metalorganic chemical vapor deposition(MOCVD),double glow plasma(DGP)and electrodeposition(ED),as well as less-applied radio frequency magnetron sputtering(RFMS),ion beam assisted deposition(IB AD),pulsed laser deposition(PLD),laser-induced chemical vapour decomposition(LCVD)and atomic layer deposition(ALD),with ad-vantages and disadvantages.For example,CVD method is simple to operate,but the precursor of reactant is toxic.The coating pre-pared by MOCVD is smooth,but the preparation process is too cumbersome.Thus,the new concept of composite preparation technolo-gy is introduced,which can complement each other's strengths in different preparation technologies and make multilayer coatings with better performance to meet the higher demand for Ir coatings in the future.In the present work,the microstructure of Ir coating was in-troduced in detail.First,the crystal orientation of Ir coating was introduced.The orientation of coatings prepared by different prepara-tion technologies was different.For body centered cubic metals,the coatings formed by MOCVD,PVD and ED showed<111>orienta-tion.However,the orientation of Ir coating formed by direct current radio frequency magnetron sputtering(DCMS),RFMS and MOCVD was(111)crystal plane.The different crystal orientations determined different crystal structures,such as equiaxed grains and columnar grains.Moreover,different deposition conditions could also affect the crystal structure of the coatings,and different crys-tal structures exerted different effects on the performance of the coatings.Then the influence of microstructure on the mechanical prop-erties and oxidation resistance of Ir coatings was elaborated.For example,the larger the grain size,the smaller the hardness of the crystal material.Therefore,after finding out these rules,the optimal preparation method and process according to the required perfor-mance could be selected,which greatly reduced the time costs.There were many problems for a single Ir coating,such as high-temper-ature oxidation volatilization,micropore diffusion and grain boundary segregation,which would affect the service life of the coating.The surface alloying of Ir was also described.In general,The oxidation resistance of the coating could be improved by combining Ir with other metal elements,such as Zr,Hf and Al.The binary alloy coating systems and multicomponent alloy coating systems were al-so summarized.For binary alloy systems,Ir-Hf,Ir-Al,Ir-Zr,Ir-Ta and Ir-Pt systems were introduced.Ir-Hf and Ir-Al systems were fo-cused with oxidation resistance as well as the effect of element addition on the coating performance.In terms of ternary alloy systems,Ir-Hf-Al and Ir-Hf-Zr alloy systems were mainly introduced.This multi-element alloy modified coating was the future development trend and could meet the increasing demand for high temperature protection.Finally,the development direction of Ir and Ir alloy coat-ings was summarized,and its future development trend was prospeced:(1)There was little research on the combination of Ir coating and substrate,this paper proposed some new combination methods through finishing preparation technology,hoping to make the con-nection between coating and substrate closer.(2)The texture of Ir coating affected its microstructure and mechanical properties,and the texture evolution of the coating was also affected and controlled by the preparation process.The growth mechanism of Ir coating pro-duced by different deposition processes was different,so the process should be selected according to the requirements of the intended application.The transformation of columnar crystals to equiaaxial crystals made the arrangement of microcrystals more stable.After high temperature treatment,the micropores in Ir coating spread outward due to the extrusion effect of grain growth.Therefore,the dense,thick Ir coating could improve the service life of engineering components in high temperature environment.Currently,there were limited reports on determining the deposition mode of iridium coating by determining the micromorphology.In the future,the mi-cromorphology could be determined first and then the deposition mode of the coating according to the required properties could be de-termined,and the rule,which would lay a solid foundation for the future application of iridium coating in the field of extreme environ-ment could be summaried.(3)The types of Ir alloy coating could be more abundant,and more high-melting point metal elements could be added,similar to the formation of high-entropy alloy.From scratch,high entropy alloy had become an extremely important al-loy.When different elements were added into the alloy,it would have different properties and characteristics.(4)At present,the prep-aration matrix of iridium coating was mostly graphite or Re,and there were no reports on other substrates.Therefore,more experiments should be conducted to explore the morphology,performance and adhesion parameters of iridium coating on different substrates,so as to open up more directions for the field of high-temperature protective coating in the future.
iridiumiridium alloycoatingelectrodeposition in molten salt
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