Intermetallic oxidation-resistant coatings for titanium alloys

Intermetallic Ti-Al coatings were deposited onto near-a titanium alloy TIMETAL 1100 using magnetron sputtering. Two coating systems were investigated: gradient layers with increasing Al content towards the surface of the coatings and a multilayer system consisting of three single layers of Ti3Al, TiAl and TiAl3. The overall coating thickness was 4 mu m and 16 mu m for both systems. Isothermal oxidation tests at 750 degrees C revealed good oxidation resistance and effective oxygen prevention from the substrate by the coatings. Room temperature tensile tests after long-term exposure to air at 600 degrees C proved the beneficial influence of the coatings on the ductility of the base material. The coatings are highly ductile under creep conditions thus keeping oxygen away from the substrate alloy even at high straining. In some cases creep lifetime was considerably prolonged. No detrimental influence of the Ti-Al coatings on the fatigue properties of TIMETAL 1100 was found for the 4 mu m multilayer coatings, whereas fatigue limit under repeated strain was slightly decreased for the 16 mu m coatings.