Sputtered intermetallic Ti–Al–X coatings: phase formation and oxidation behavior

Intermetallic Ti–Al–X (X=Cr, Nb) coatings were deposited on conventional near-α titanium alloy TIMETAL 1100 and on γ-TiAl based Ti–48Al–2Cr–2Nb by magnetron sputtering for oxidation protection. Low temperature deposition process leads to a metastable coating structure which transforms into a two-phase microstructure during high temperature exposure. Oxidation behavior was tested in interrupted weight gain tests in laboratory air at 750°C for TIMETAL 1100 and at 900°C for Ti–48Al–2Cr–2Nb. Oxidation behavior of the coatings shows the same tendency for both substrates and oxidation temperatures. Nb-containing coatings exhibited only poor protection due to partial spallation of the coatings during cooling. Maximum oxidation resistance was achieved by Ti–63Al–7Cr coatings on both substrates. For the γ-TiAl based Ti–Al–Cr coatings oxidation resistance is improved with increasing Cr content. Despite high oxidation resistance of the Cr-containing coatings rutile was always found in addition to protective alumina in the oxide scale.