Novel Ir-X thermal protection coatings designed for extreme aerodynamic heating environment

Due to the rapid evaporation of SiO2 protective layer, most Si-containing oxidation resistant coatings could not withstand a temperature above 1800℃, which is not enough for hypersonic voyage in upper atmosphere. With a higher melting point (2440℃) and lower oxygen permeability（10-20g·m-1·s-1）, iridium is supposed to be a promising coating material for ultra-high temperature applications. However, Iridium has a low emissivity ε(0.017 for 2.5-25μm) and high recombination coefficient γ(0.64 at 1200℃) of atomic oxygen, resulting in a much higher thermal response compared with the ceramic materials under the same aerodynamic environment. To solve this problem, elements such as Al, Cr, Zr etc. were selected to modify pure Ir to form Ir-X (X=Al, Cr or Zr) coating. The modification element X in Ir-X coating forms high emissivity and low recombination coeffcient oxide on Ir, which meanwhile prevents the Ir from atomic oxygen. It was found that Ir-Al, Ir-Cr, Ir-Ti, Ir-Zr, Ir-Ta and Ir-Hf diffusion coating could be prepared via pack cementation. The recombination coefficient and emissivity of as-oxidized Ir-Al were changed to 0.0089 and 0.723, respectively. Please click Additional Files below to see the full abstract