| In recent years, titanium alloys are widely used in the manufacturing of military products like aerospace, aircraft, ships, and weapons because of its high specific strength and stiffness, corrosion resistance, high temperature resistance, good fatigue and creep resistance characteristics, and they find great potential applications in the automotive, medical, chemical and energy industry. But the characteristic of bad oxidation resistance in high temperature restricted their wide applications. TiAl3 layer was conventionally formed by Ti-Al in-situ reaction. In this thesis, TiAl3 coating and TiAl3-Al composite coating are deposited on TC4 substrate by LT-HVOF process with TiAl3 powder and Al powder as raw powders, the study on high temperature protective performance of the coatings under 700℃ isothermal oxidation were performed, the structure and phase of the coatings were characterized by SEM, XRD, EDS. The failure mechanism of the coatings were also discussed. The effect of coatings on mechanical properties of substrate was also investigated after high temperature oxidationAl coating was deposited on TC4 substrate by LT-HVOF, after words vacuum heat treatment at 610℃ for 8 h was applied to achieve TiAl3-Al composite coating. The results after isothermal oxidation at 700℃ revealed that TiAl3-Al composite coating offers good oxidation resistance performance after 500h. The reason is for the protective layer formed composing of Al2O3-phase and small amount of TiO2-phase on the coating surfaceIn order to prevent from the diffusion reaction between coating and substrate, TiAl3 coating was prepared with TiAl3 powder as spray powder. In order to enhance the resistance to deformation of coating, a certain of Al powder was added to TiAl3 powder to enhance the ductility of the coating. The TiA13 coating with low oxygen content has a dense structure. After oxidation for 1000h, a dense needle Al2O3 oxide film containing a small amount of TiO2 particles was generated on coating surface. The TiAl3-Al composite coating with two different content of Al also have a dense structure and low oxygen content, however the aggregation of Al in the composite coatings was observed. The higher content of Al the coating is, the more aggressive the aggregation region is found in the coatings. The oxide film was formed by Al2O3 and TiO2 on TiAl3-8Al coating surface after 1000h isothermal oxidation. The oxide film composed by high amount of TiO2 and Al2O3 on TiAl3-20Al coating surface after 500h isothermal oxidation. The Al in TiAl3-Al composite coating will result in the faster oxidation rate of coating.The TiAl3 coating failure mechanism is mainly governed by the consumption of Al in the coating, thenTiO2 is gradually formed. With the oxidation continuing, Al2O3 oxide film is interrupted by TiO2, and surrounding O can enter into the internal coating, which finally leads to the increase oxide content and the coating flaking failure eventually. The TiAl3-Al composite coating failure mechanism is mainly attributed to the Al aggregation region leading to the formation of diffusion channels, and these channels could not completely covered by Al2O3 oxide film which hasten the diffusion of Al toward coating surface and O diffusion toward the internal coating, and the molten Al also accelerated the diffusion rate. Lot of oxides are generated due to the diffusion. Under the influence of the oxide growth stress, coating gradually spalling failure along the diffusion channels.The tensile test for Coated Ti alloy after isothermal oxidation at 700℃ for 120 h showed that the yield strength and tensile strength of coated samples was lower than uncoated samples, this is due to the increased surface roughness which caused by pre treatment of substrate, and TiAl3-8Al coating coated samples has a slightly higher yield strength and tensile strength than the in-situ TiAl3-Al composite coating samples, indicating the Ti-Al diffusion reaction will have an adverse effect on the mechanical properties of the substrate. During the test, spallation was observed on the in-situ TiAl3-Al composite coating, but the TiAl3-8Al coating was still well bonded with the substrate surface, verifying the good performance of deformation of TiAl3-8Al coating. |
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