Preparation And Properties Of Al₂O₃/Al Hot Protective Coating With Gradient Structure On γ-TiAl Alloy

TiAl based intermetallics are of great interest for application in aeroengine materials at high performance because of their high strength weight ratio, superior balanced mechnanical properties and excellent creep properties. However, the further exploitation has been somewhat limited by environmental and tribological degradation at elevated temperatures. Therefore, it is very important to improve the corrosion resistance, the wear resistance and the oxidation resistance of Ti Al based intermetallics at high temperature.Due to occurring in the surface of metal, it is a good way to improve the corrosion resistance, the wear resistance and the oxidation resistance of Ti Al based intermetallics at high temperature by surface modification without breaking the balance of their mechnanical properties.In this study, an Al₂O₃/Al high temperature protective coating with a gradient structure was deposited on the γ-TiAl alloy by magnetron sputtering. The micro-structure and mechanical properties of Al₂O₃/Al coating were detected. The virgin surface of the as-deposited Al₂O₃/Al coating was very uniform and compact without crack and pores. A dense composite layers structure with a thickness of ²0μm which consists of an Al₂O₃ top-layer, an Al-rich interlayer and a diffusion layer could be found in the cross-section SEM figure. The micro-hardness, nano-hardness and elastic modulus of γ-Ti Al alloy were enhanced by this coating, and the Al₂O₃/Al coating displayed good adhesion with substrate which was tested by the scratch test, tensile experiment and thermal shock test.The thesis studied the corrosion behaviors and mechanisms of the Al₂O₃/Al gradient high temperature protective coating in molten salt environment of 100 wt.% Na2SO4 at 850 and 900℃. The Al₂O₃/Al coating displayed a needle-like surface micro-structure, and no cracks and spallation could be found in the coating after the corrosion test at 850℃. The Al₂O₃, TiO2 and TiAl3 were identified as the main phase structure of the corroded coating surface. All these evidences indicated that the Al₂O₃/Al coating exhibited excellent corrosion resistance at 850℃. However, the spallation and though-walk cracks could be observed in the Al₂O₃/Al coating after the corrosion test at 900℃. The molten Na2SO4 eroded the Al₂O₃ top-layer and broke the protection of Al₂O₃/Al coating. Due to the poor adhesion with substrate, the products of the corrosion reaction, such as gas, oxide and sulfide, generated the cracks in the coating which was the main reason of the coating spallation. Therefore, the further efforts should be carried out to improve the corrosion resistance of Al₂O₃/Al coating at 900℃.The thesis studied the friction wear behaviors and mechanisms of the Al₂O₃/Al gradient high temperature protective coating under different loads, sliding velocities and temperatures. The wear rates and friction coefficients of Al₂O₃/Al coatings at all test conditions were much less than those of substrates. The abrasive and oxidative wear were considered as the main wear mechanism of substrates, while abrasive, adhesive and slight oxidative wear to Al₂O₃/Al coatings. The anti-wear behavior of Al₂O₃/Al coating was attributable synergistically to its high hardness, excellent elastic modulus and self-lubrication by Al₂O₃ particles.The thesis studied the oxidation behaviors and mechanisms of the Al₂O₃/Al gradient high temperature protective coating at 700, 800, 900 and 1000℃. The results suggested that the Al₂O₃/Al coating improved the oxidation resistance of γ-TiAl alloy more and more obviously as the test temperature increasing. No spallation or crack was observed in the oxide scale of coated specimen which was composed of α-Al₂O₃ and Ti-Al intermetallics after the oxidation test. As a main component of coating phase structure, α-Al₂O₃ phase had great contribution to the oxidation resistance of coating. The mass gain of Al₂O₃/Al coated specimen was only 1.43mg?cm-2 at the highest test temperature 1000℃ in this thesis. The outward Al diffusion from interlayer provided sufficient Al source for the formation of Al₂O₃ layer in the coating surface during the high temperature oxidation test. As an active intermetallic element with Ti, Al suppressed the outward of Ti effectively due to the formation of Ti-Al intermetallics. Due to inward and outward diffusion, the Al interlayer was consumed after oxidation test and a Ti-Al interdiffusion zone was formed which was beneficial for the adhesion between coating and substrate.