Study On High Temperature Stability Of Ionic Coatings By Metal Organic Chemical Vapor Deposition

Iridium (Ir) having a high melting point (2454 ℃), low oxygen permeability, high temperature and high mechanical strength at high temperature. It can serve as a super high temperature anti-oxidation coating to protect Re/Ir composite nozzle which maximum working temperature is up to 2200 ℃ and does not need to additional fuel for cooling. According to currently available literature, the use of metal organic chemical vapor deposition (MOCVD) can be prepared to meet the requirements of the Ir coating, but currently only the United States holds its preparation process.In this paper, coating of iridium is prepared by MOCVD on the molybdenum (Mo) substrate, with acetylacetonate iridium as precursor, argon (Ar) as the carrier gas, hydrogen (H2) or H2 added water vapor as mixing reaction atmosphere. Through the study of the as-deposit characteristics of iridium coatings prepared under different conditions and the changes at high temperature under different roasting atmosphere, the effect of deposition temperature and water vapor on the stability of the coatings is analyzed. The mechanism of water in the deposition process of iridium coating is discussed.Studies have shown that MOCVD iridium coatings are two kinds of structure:a "fine grain zone" closed to substrate and cluster structure far away from substrate. The presence of internal MOCVD Ir coating near the base of the "fine grain zones" and two structures tufted area away from the substrate, the coating surface there are many projections, water vapor is added during the preparation of the coating surface is more smooth, the internal two structures become more pronounced.Thermal analysis showed that the coating is oxidized to iridium dioxide (IrO2) with increasing temperature in air atmosphere weight gain phenomenon occurring. When the temperature is higher than 987.5 ℃ IrO2 further oxidation of gaseous iridium oxide (IrO3) and Ir is directly oxidized to IrO3 appearing weightlessness. At 1036.2 ℃ weightlessness is fastest. When the temperature is higher than 1065.5 ℃ weight loss is caused by Ir directly oxidized to IrO3.High temperature experiments show that raising temperature and added water vapor as a co-reactive atmosphere can reduce the internal pores proportion. Water vapor also acts as a grain refinement to original coating.