The alloy performance to resist high-temperature oxidation is mainly determined either by the formation of oxide scales or by the deposition of oxide coatings on the surface of alloys.In the former case,a protective oxide scale,such as Al_2O_3,Cr_2O_3 and SiO_2,can be selectively formed by adjusting the composition of alloys.Even if the oxide scale on the surface of alloys is spalled,new oxide can grow continuously,so the oxidation of alloys can be inhibited effectively.In the latter case,theoretically, depositing oxygen diffusion barrier oxide coatings,such as Al_2O_3,Cr_2O_3 and SiO_2,can prevent alloys from oxidation as well.However,the integrity of these oxide coatings can not maintain for a long time,owing to the thermal stress produced by the difference of the expansion coefficients between the oxide coatings and alloy substrates.Consequently,the ability to resist high-temperature oxidation by depositing such oxide coatings should be disappeared.Therefore,an oxide coating possessed both properties to act as an excellent oxygen diffusion barrier and match a suitable expansion coefficient with alloy substrate,is the basic conditions for the coating to maintain the ability to resist high-temperature oxidation of alloys. Based on above-mentioned idea,a novel composite oxide coating has been developed to obtain two kinds of functions,to act as an oxygen diffusion barrier and to adjust the expansion coefficient of coating as required.Figure 1 shows the schematic diagram of such novel oxide coating.This coating is composed of ZrO_2-8%Y_2O_3(YSZ) particles packaged by nano-Al_2O_3 film.The nano-Al_2O_3 film has a bubble-like structure,each YSZ particle is packaged in an Al_2O_3 bubble.Owing to YSZ is a good conductor for oxygen ions,so oxygen diffusion in this composite coating is determined by the walls of Al_2O_3 bubbles or the nano-Al_2O_3 film.If the thickness of all walls of Al_2O_3 bubbles in the direction perpendicular to the surface of alloy is over a critical value,the Al_2O_3 bubbles can act as an excelle
Al2O3/Au nano-laminated composite coatings were prepared by means of magnetron sputtering. The coating was compact and comprised of nano-laminated Al2O3 and Au layers. High temperature cyclic oxidation test was employed to investigate the oxidation resistance of the composite coatings. The results revealed that the applied Al2O3/Au nano-laminated composite coatings improved the oxidation and spallation resistance of the stainless steel substrate significantly. The mechanism accounting for oxidation resistance was related with the suppression of inward oxygen diffusion and selective oxidation of Cr in the substrate. The mechanism accounting for spallation resistance was attributed to the relaxation of thermal stress by the nano-laminated structure.
