| Titanium alloy has been widely used as structural material in automobile, energy and aerospace due to its high strength, low density and excellent corrosion resistance. The development of the advanced materials requires that titanium alloy could show excellent properties under the service conditions at a higher temperature. However, the thermostability of conventional Ti-6A1-4V, the newly-developed y-TiAl and Ti2AlNb alloy could be affected greatly by the problems of oxidation and high-temperature oxygen brittle when they were used at high temperatures. One of the main reasons is the formation of a loose and porous TiO2layer instead of the formation of alumina scale. Moreover, oxygen-enriched layer caused the decrease in mechanical property of alloy owing to the great solid solubility of oxygen in the alloy. Nowadays, the approaches used to solve these problems were mainly alloying, surface modification and coating with high temperature corrosion-resistant. Because high temperature oxidations mainly occur at the surface of titanium alloy, while the mechanical properties of the alloy lie on the cross sections of the material. Thus, coating technology is probably one of the effective methods for improving the high temperature oxidation resistance of the titanium alloy and obtaining the optimized mechanical properties.The Al2O3and SiO2coating derived from sol-gel method on three kinds of typical titanium alloy surface were prepared in order to propose a simple and practical high-temperature coating of titanium alloy. Additionally, the silica composite coating was obtained through the combination of phosphate treatment and SiO2coating. Phase composition and morphology of the oxide film were characterized using XRD/EDS and SEM. The thermodynamics of oxidation behavior was discussed based on the mass change before and after oxidation. The result as following:(1) Aluminide coating at the surface of the y-TiAl alloy mainly consist of the γ-Al2O3. The oxidation kinetics of the blank sample at1000℃in atmosphere transferred from the cubic law to the linear law, whereas the coated one transferred from the linear law to the parabolic relationship. The oxidation rate of the Al2O3coated specimen decreased nearly90%in comparison with the reference sample. For the cyclic oxidation at1000℃in static atmosphere, the oxide film formed on the y-TiAl alloy without alumina coating begun to spall off after seven cycles. After30cycles, the mass gain became negative, suggesting that the oxide film spalled off greatly. On the contrary, the kinetic curve of the cyclic oxidation to the coated sample was similar to that of the isothermal oxidation, appearing the trend of the smooth mass gain. The oxide scale formed on the specimens with or without alumina coating was mainly comprised of rutile TiO2and α-Al2O3. Nevertheless, the volume fraction of α-Al2O3at the coated sample was greatly higher than that at the reference. The alumina thin film could act as a barrier to restrict the outward diffusion of Ti and inward diffusion of O, so the high temperature oxidation resistance of this alloy was improved.(2) The amorphous silica coating with a thickness of5μm was deposited at the surface of Ti-22Al-26Nb alloy. The as-fabricated coating was uniform and crackless. The oxidation rate constants of the blank specimen and coated one from the oxidation kinetics curves simulation at800℃were0.1758mg·cm-4·h-1and0.0303mg2·cm-4·-h-1, respectively, while those values at900℃were0.6215mg2·cm-4·h-1and0.2594mg2·cm-4·h-1, respectively. The silica film decreased greatly the oxidation rate and improved the activation energy. The calculated activation energy of the coated specimen was224.65kJ-mol"1. while that of the reference was132.43kJ·mol-1. The activation energy of the coated specimen was increased70%in comparison with the Ti-22Al-26Nb alloy. The formed oxide scale was mainly comprised of TiO2、AlNbO4and Nb2TiO7.(3) The amorphous Al2O3and SiO2coating was fabricated onto the surface of the Ti-6A1-4V alloy, respectively. The isothermal and cyclic oxidation test at600℃and700℃in atmosphere were conducted to investigate the effect of the two sol-gel derived coating. The coating is uniform and crackless. The results from the optimization of the coating thickness showed that the optimal thickness of the Al2O3coating to provide better protectiveness for Ti-6A1-4V alloy is about0.8μm. The formed oxide scale at the two cases was rutile TiO2. However, the oxide film formed on blank sample was multilayered and its thickness was much higher than the coating one. The available oxygen diffusion to the surface of the alloy was limited by Al2O3coating, so the access of oxygen to the surface of alloy was prohibited.The phase of the SiO2coating was amorphous and the thickness was around5μm. The SiO2coating decreases the rate of the isothermal oxidation effectively, so the spallation phenomenon of the oxide film was eliminated in the process of the cyclic oxidation. The main compositions of the oxide film formed on SiO2coated specimen were rutile TiO2and some Al2O3. The oxidation rate of the SiO2coating sample is lower than the blank sample. The thickness of the SiO2coating is larger than Al2O3coating. So it is more efficiency to limit the amount of the available oxygen to the surface of the alloy, showing better high temperature protectiveness than Al2O3coating.(4) The TiP2O7layer at the surface of the Ti-6Al-4V alloy was firstly formed by the H3PO4treatment. Subsequently, the amorphous SiO2coating was deposited by the sol-gel method. The average oxidation rate of the Ti-6A1-4V alloy treated by phosphorization and deposited by silica film at600℃after24h oxidation in air was decreased75%in comparison with the blank samples. The oxidation resistance was markedly improved. The average oxidation rate of the composite amorphous SiO2coating with the phosphoric acid-assisted treatment over100h at600℃was decreased by95%in comparison with blank sample. The oxidation resistance was better than the silica coated sample and the only-phosphorized sample. The compactness of the oxide film of the silica coating sample combining with the phosphoric acid-assisted treatment was increased and a good adhesion with the substrate was shown, meanwhile, the volume fraction of the TiO2in oxide scale decreased. The TiP2O7with the phosphoric acid-assisted treatment were stable in the oxide film. |
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