High temperature oxidation and corrosion resistant nanocrystalline coatings

Traditional high temperature oxidation resistant coatings are divided into three types: aluminide or modified aluminide coatings such as Cr-Al, Si-Al and Pt-Al; MCrAlY(M=Fe,Co and/or Ni) overlayer coatings and MCrAlY+Y2O3/ZrO2 thermal barrier coatings. Characteristic of these coatings is that their compositions are very different from their substrate alloys. Protection comes from the higher aluminium 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, a new kind of coating has been developed: high temperature oxidation resistant and hot corrosion resistant nanocrystalline coating, the composition of which 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 aluminium. content, but from the grain boundary diffusion. The main advantages of this kind of coating are that neither interdiffusion nor thermal mismatch exist between coating and substrate. The present paper reviews the high temperature oxidation and hot corrosion resistant nanocrystalline coating with particular reference to the authors' own results.