Oxidation protection coatings for titanium, titanium-base alloys and titanium aluminides in high-temperature oxidizing environments

Titanium and its alloys with aluminum have been widely used as engineering materials for a number of advanced technical applications, and particularly in aeronautics, because of their high strength-to-weight ratio and good oxidation resistance in the medium temperature range (up to 500°C for Ti and ~ 750°C for TiAl). However, environmental durability at higher temperatures is presently still a concern due to insufficient oxidation and embrittlement resistance. Therefore, oxidation protection coatings are needed to fulfill the high temperature structural potential of these materials.
This work consists of two parts. The first part deals with the development of an oxygen barrier coating to prevent oxidation of Ti and low-Al-content Ti-base alloys (< 10 at.% Al) at temperatures not exceeding 600°C. The surface modification process has involved magnetron co-sputtering of Ti and Al onto the base alloy material followed by vacuum annealing and plasma immersion ion implantation (PIII) of fluorine to activate the so-called halogen effect. Oxidation exposure in air at 600°C for 100 h has shown that the coating is able to form a protective Al2O3-containing scale and, moreover, exhibits good substrate compatibility.
The second part of the work has focused on the development of an oxidation resistant coating for γ-TiAl alloys (Ti-48Al-2Cr-2Nb). The fabrication process has involved formation of an Al-rich (50-60 at.%) TiAl overlayer by chemical vapor deposition followed by PIII of fluorine. Subsequent oxidation exposures of the coated γ-TiAl alloy to air at 900°C for 350 h have shown that the coating prevents effectively both oxidation and embrittlement to the baseline material while preserving 90% of its initial mechanical properties.