Phase Transformations in Pt-Aluminide Coatings and Their Effect on Oxidation Resistance

Diffusion coatings have been widely used as metallic bond coatings in thermal barrier systems to protect aeronautical turbine blades from detrimental oxidation and hot corrosion. A cooling scheme is one of most effective methods to protect a material surface exposed to a high temperature environment. However, fast cooling rate will not only generate thermal damages by high thermal stresses, but also affect the structure of coatings. An experimental program was undertaken to study the effects of cooling rates on the microstructure of Pt-aluminide coatings after diffusion treatment at high temperatures followed by furnace-cooling and water-quenching treatments. In order to further study the effects of the phase transformation on coatings, the Al-deposited coatings were prepared by the pack cementation process to increase the Al content in Pt-aluminide coatings and the microstructures of these coatings were also investigated. The composition (in wt%) of the packs was xAl-2NH4Cl-(98 − x)Al2O3 with different Al levels (x = 1, 4 and 6) and the Pt-aluminide coatings were transformed to ε-PtAl, ξ-PtAl2 or γ′-(Ni, Pt)3Al after Al deposition. Isothermal oxidation tests were performed in air at 1000 °C for up to 100 h. These oxidation tests indicated that parabolic scaling kinetics were established and that the coating formed in the packs containing 1 wt% Al exhibited slower scale growth rate. After oxidation, the oxidation scales, Al2O3 or Cr2O3, were formed above the coatings. The mechanisms of phase transformations in Pt-aluminide coatings after furnace-cooling and water-quenching treatments are discussed. In addition, the effects of these phase transformations on the oxidation resistance of the coatings are also discussed.