| Titanium alloys, with respect to their relative low density and high strength, have been the subject of recent investigations for the compressor part of aero-engines. However, poor oxidation resistance and oxygen-induced embrittlement of Ti alloys are well known to strongly limit their applications at high temperatures. Up to now, many approaches have been adopted to improve the oxidation and corrosion resistance of titanium alloys, such as alloying, surface treatment and protective coatings. However, the brittleness of the Al2O3 or SiO2 coatings and mismatch of coefficients of thermal expansion between them and their substrates could be problems leading to cracking and spalling by traditional fabricating techniques. Thus, new technologies for fabricating SiO2 coating need to be developed in order to improve the oxidation resistance of titanium alloys.The aim of the present work is to study the effect of sol-gel derived SiO2 thin film to explore the feasibility of using such a film as an oxidation resistant coating, and to investigate the solid reactions between SiO2 and titanium alloys during oxidation. Meanwhile, the effects of the sputtered Ni-Si coating on the oxidation behavior of Ti6A14V at 650℃and H2O/NaCl-induced corrosion behavior at 600℃were studied.1. Sol-gel SiO2 thin film has been successfully applied on the surface of Ti6Al4V alloy using directly sinter process, the thickness of SiO2 thin film was about 10μm. The isothermal and cyclic oxidation behaviors of the uncoated and coated alloys in air at 700 and 800℃were investigated. The SiO2 thin film had beneficial effects on the oxidation resistance of the alloy. The oxidation rates were significantly reduced. Cracking and spallation of the oxide scales were not detected on the coated specimens. The SiO2 film affected the distribution of oxide scales and favored the formation of the alumina.2. The scales formed on the SiO2 coated Ti6A14V specimens were multilayered. Beneath the silica film, formation of a thick rutile titania layer followed by a thin aluminum-rich layer occurred. Above the silica film, alumina plus minor titania layer formed. It is deduced that the growth of the multilayered and mixed oxide scales was dominated by both outward diffusion of metal and inward diffusion of oxygen.3. The as-received Ni-Si coatings on Ti6Al4V prepared by magnetron sputtering from a Ni3Si target were mixtures of y-Ni3Si2 and f.c.c. Ni, which transformed into ordered Ni3Si during oxidation at 650℃or corrosion at 600℃.4. The Ni-Si coatings possessed excellent oxidation resistances. External NiO scales along with silicic oxide precipitates formed on the coating surfaces during oxidation at 650℃. It is suggested that the formation of silicic oxide disturbs the growth of NiO scale, leading the kinetics deviate from the parabolic law.5. In the case of H2O/NaCl-induced corrosion, the Ni-Si coatings offered better corrosion resistance at 600℃for 10h. The external scales showed alternative lamellar structures, which consisted of lamellae of NiO, silicic oxide and metallic Ni, might result from the combination effects of multi-step reactions between Ni and NaCl/H2O/O2, the low diffusivity of Si and a displacement reaction between Si and NiO.
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