α-Al2O3 and Ti2O3 protective coatings on aluminide substrates

A process for forming a specific reactive element bilayer barrier on a titanium aluminide substrate, the process comprising:

• creating a gaseous hydrogen/water vapor atmosphere with a concentration of water vapor below 750 ppm at a temperature above about 550° C. contiguous to a surface of the titanium aluminide substrate on which the barrier layer is to be formed;
• while maintaining the temperature above about 550° C. and water vapor concentration below 750 ppm, reacting the gaseous hydrogen/water vapor atmosphere with specific reactive elements at the titanium aluminide substrate surface and with until:
• α-Al2O3 is formed directly on the substrate surface in preference to titanium oxide to form an α-Al2O3 layer at a substrate/barrier layer interface;
• titanium atoms from titanium oxide are reduced with aluminum and the titanium atoms difuse outward through the α-Al2O3 layer;
• and
• the titanium atoms that have diffused through the α-Al2O3 layer are oxidized to form a Ti2O3 layer at a barrier layer/gas interface.