Surface protection of titanium against degradation in high-temperature oxidizing environments

Titanium exhibits an extraordinary combination of desirable properties that make it an important engineering material. However, a fundamental limitation of Ti is its high affinity for oxygen at temperatures of 500°-600°C, which results in oxygen embrittlement and reduces the maximum use temperature to below 500°C.
It is now well-established that a γ-TiAl alloy containing Al between 45-55 at.% may be rendered oxidation resistant at temperatures above 700°C by introducing fluorine into its near-surface region (the so-called fluorine effect). Upon subsequent high-temperature oxidation, a γ-TiAl alloy modified in such a way acquires a stable, adherent and highly protective alumina scale.
We have developed a protective γ-TiAl-based coating for α-Ti that serves as an efficient barrier to the oxygen in-diffusion, thereby preventing embrittlement of the material and raising its maximal service temperature. The coating is accomplished by using a three-step process, namely (i). magnetron sputter co-deposition of Ti and Al onto the Ti substrate; (ii) vacuum annealing resulting in the formation of a γ-phase TiAl layer; and (iii) introduction of fluorine by plasma immersion ion implantation.
Analytical techniques such as XTEM/EELS, XRD, ERDA and EDX have been used for materials characterization. Oxidation testing in air at temperatures of 600° to 700°C indicates that the coating provides adequate oxidation protection of α-Ti due to the alumina scale formation on the coating surface.