High temperature oxidation and corrosion resistant nanocrystalline coatings

Traditional high temperature oxidation-resistant coatings are divided into 1) aluminide or modified aluminide coatings such as Cr-Al, Si-Al and Pt-Al coatings; 2) MCrAIY(M=Fe,Co and/or Ni) over layer coatings and 3) MCrAIY + Y2O3/ZrO2 thermal barrier coatings. The characteristics of the above coatings are that the compositions of the coatings are very different from the substrate alloys. The protectiveness comes from the higher aluminum content in the coating to form Al2O3 at high temperature. However, these coatings may be harmful to the mechanical properties of the alloys as a result of the interdiffusion or thermal mismatch between the coating and substrate. Recently, we have developed a new kind of coatings, i.e. high temperature oxidation-resistant and hot corrosion-resistant nanocrystalline coatings. The composition of the coating is the same as the substrate. The only difference is that the grain size of the coating is several orders of magnitude smaller than that of the substrate. The formation of protective Al2O3 scale is not from the high aluminum content, but from the grain boundary diffusion. The main advantages of this kind of coatings are that neither interdiffusion nor thermal mismatch exists between the coating and substrate. This paper reviews the high temperature oxidation and hot corrosion-resistant nanocrystalline coatings with particular reference to the authors' own results.