| In order to improve the ability of?-TiAl alloy to resist corrosion,high temperature oxidation,thermal shock and molten salt,micro-arc oxidation technique was used to prepare a rough and porous ceramic coating on the surface of the substrate in this paper.Subsequent sealing treatment was performed by magnetron sputtering technique and Al/Al2Ti O5 composite coatings were prepared.The methods of SEM,XRD,XPS,TEM,FIB and JB-6C were used to study the microstructure and phase composition of the composite coatings.The effects of micro-arc oxidation electrolyte concentration,voltage and magnetron sputtering power on the surface morphology,corrosion resistance,and high-temperature oxidation performance of the coatings were studied.The thermal shock,molten salt corrosion,high temperature oxidation tests were employed to investigate the thermal shock performance,molten salt corrosion performance and high temperature oxidation performance of coated and uncoated?-TiAl alloy.The high-temperature oxidation mechanism of?-TiAl alloy composite coatings was also discussed.Except the Ti3Al and TiAl substrate phases,the micro-arc oxidation treated sample is mainly composed of Al2O3 phaseand Al2Ti O5 phase in the sodium aluminate electrolyte system.The silicate electrolyte system is mainly composed of Al2Ti O5 phase and amorphous Si O2 phase.At the silicate electrolyte voltage of 400V,a large number of white convex granular materials(about 5?m in size)and randomly distributed micropores with a diameter of 500nm are on the sample surface.When the treatment voltage is too high,the surface of the MAO sample is too rough,and the micropores distributed over the coating surface will facilitate the invasion of oxygen.While the coating prepared with too low voltage is too thin to prevent the invasion of oxygen.The coating prepared with 400V is relatively dense and has relatively good oxidation resistance.The micro-arc oxidation samples prepared in the sodium aluminate and sodium silicate electrolyte systems were oxidized at 850°C for 100h.The weight gain percentages were 43.69%and 17.84%of the substrate oxidation weight,respectively.The corrosion current density of the 400V MAO treated sample in silicate electrolyte system is 1.93×10-8 A/cm2,one order of magnitude lower than that of the uncoated?-TiAl alloy,indicating that the corrosion resistance of?-TiAl alloy is greatly improved.When the magnetron sputtering power is 120W and the composite coatings are oxidized at 800°C,850°C and 900°C,the weight gain after oxidizing for 100h is only 0.21 mg/cm2?0.51mg/cm2?1.50 mg/cm2,which is 3.16%,2.67%and 6.16%of the substrate weight gain at the same temperature respectively.It shows that the composite coatings greatly improve the high temperature oxidation resistance of ?-TiAl alloy.Thermal shock at 900°C for 100 times,the oxide layer of the substrate completely peeled off,but the composite coating sample is intact,indicating that the composite coatings have strong thermal shock resistance.During the molten salt erosion process at800°C,the oxidation thickness of the composite coating sample is much smaller than that of the substrate and micro-arc oxidation sample,and its weight gain is only 14.05%of the substrate weight gain.This indicates that the composite coatings can prevent aggressive substances from penetrating inward and improve the resistance of the substrate to molten salt erosion and oxidation.After oxidation at 850°C for 100h,the high activity of Al in the composite coatings with the magnetron sputtering power of 120W has a higher affinity for oxygen,which also promotes the formation of protective Al2O3 in the middle.The rapid diffusion of oxygen ions inward and metal ions is inhibited,so the high temperature oxidation resistance of the composite coating sample is greatly improved compared to the substrate. |
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