Efficient, durable protection of the Ti6242S titanium alloy against high-temperature oxidation through MOCVD processed amorphous alumina coatings

With their exceptional strength-to-weight ratio, titanium alloys find applications in numerous key enabling technologies. However, their implementation in harsh environments comes up against their limited resistance to high-temperature oxidation. To face this problem, in this work dense, amorphous alumina, Al₂O₃ coatings are applied on the surface of Ti6242S alloy by metalorganic chemical vapor deposition, MOCVD, from aluminum triisopropoxide, ATI and from dimethylaluminum isopropoxide, DMAI. Isothermal oxidation tests show that the parabolic rate constants of the mass gain of the coated Ti6242S coupons are reduced up to two orders of magnitude compared with the bare material. 5000 h long oxidation of DMAI Al₂O₃ coated alloy at 600 °C results in 0.180 mg cm⁻² weight gain to be compared with 1.143 mg cm⁻² for the bare alloy. In these conditions, an interfacial layer is formed, containing the complex Ti₃(Al_0.5Sn_0.5) (or (Ti,Sn)₂N) phase. Cyclic oxidation consisting of eighty, 1 h cycles between 50 and 600 °C show null mass gain of the coated sample. Finally, the hardness profiles determined on cross sections of oxidized coupons reveal a very limited oxygen dissolution for the coated alloy. MOCVD coatings of amorphous Al₂O₃ have great potential for efficient, durable protection against oxidation of Ti6242S alloys.