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 excellent oxygen diffusion barrier. Owing to YSZ has a high thermal expansion coefficient,so the thermal expansion coefficient of coatings can be easily increased by enhancing the ratio of YSZ to Al_2O_3 in these coatings. These composite coatings have been prepared by an improved sol-gel method.Firstly, YSZ particles packaged Al_2O_3 gel film were painted or deposited by electrophoresis on the alloy surface. Then,the specimens were treated by thermal pressure filtration to get primary coatings without cracks.Finally,the primary coatings were sintered in microwave furnace to obtain compact coatings. It is demonstrated from the result of cycling oxidation experiment that both of the hightemperature oxidation resistance and oxide spallation resistance are increased obviously with the ratio of YSZ to Al_2O_3 in the coatings,which are much superior to that of MCrAlY coatings.Therefore, these novel coatings can be used to protect various kinds of alloys from high-temperature oxidation,and can be also served as the interlayer in the thermal barrier coating system.
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