Study On The High-Temperature Oxidation Resistance Of SiO₂-Coated TiAl-Based Alloy

TiAl-based alloy was taken as an ideal candidate material for high temperature structural application in aerospace and automobile indursity,owing to its low density,high strength at elevated temperatures and good creep resistance.The problem of poor ductility of these materials at room temperature has been mostly solved by alloying and microstructure controlling.Thus the problem of further improving high temperature oxidation resistance has become one of the most important research subjects.Sol-gel derived SiO₂ was spin coated on TiAl based alloy in this research. The coated TiAl based samples were embedded in active carbon powder with a slow heating to 1000℃,then holded at this temperature for 4h.Four types of TiAl based alloy was once-,twice-,tri-,quartic-spin coated,respectively,then followed by heat treatment to obtain a composite coating.The four kinds of samples suffered a circulating oxidation test at 900℃under a static air atmosphere.The effect of coating times on the antioxidative properties of TiAl based alloy was discussed at elevated teperature.The forming mechanism of coat and its antioxidative mechanism were also opened out.The results show that coatings,on TiAl based alloy could promote its circulating antioxidative abilities at 900℃under static air,among which the tri-spin coated TiAl sample has a prime antioxidative properties:it only gained 0.2143g/cm² at 900℃after a circulating oxidation for 504h,without scaling off.The surface of samples formed into a composite layer after calcinaing the coted samples.From outer to inner,the composite layer was consisted of SiO₂ layer,Ti-Si compound layer,Ti₂Al layer and lamellar TiAl matrix.At a lower temperature,the SiO₂ sol coated on TiAl based alloy transformed into SiO₂ gel, then it chapped into noncrystallic SiO₂ layer at 500℃,while at a higher temperature,SiO₂,remains of O₂ and the matrix reactioned into composite coating:O₂+TiAl+SiO₂→Al₂O₃+Ti₅Si₃+TiSi₂ and TiAl+O₂→Al₂O₃+Ti₂Al.The composite layer gained after a circulating oxidation at 900℃.The surface morphology and constituent of non-defect SiO₂ area were the same as that of non-heat treatment owing to the existence of SiO₂ layer,the TiAl sample was first oxidized into protective Al₂O₃,while in defect area,it was oxidized into TiO₂.The tri-spin coated sample has the best oxidation resistence properties.The surface structure have not changed before and after oxidation,meanwhile,the Al₂O₃ also has prevented the TiAl base from oxidazing even if the generation of tiny amount of TiO₂.Moreover,the TiAl layer next to the TiAl base has a relative excellent oxidation resistence itself and slows the therm stress between matrix and coating,which avoids the scaling of SiO₂ layer and Al₂O₃ layer.