Magnetron-sputtered Ti-Cr-Al coatings for oxidation protection of titanium alloys

Ti-Cr-Al coatings with a nominal composition of Ti-51Al-12Cr (in at.%) were deposited on the near-alpha titanium alloy TIMETAL 1100 (Ti-6A1-2.75Sn-4Zr-0.4Mo-0.45Si, in wt.%) by dual-source magnetron sputtering. Substrate bias voltage between 0 and 80 V was applied during coating deposition. The microstructure of the coating changes from fine columnar to a nearly structureless morphology by increasing the substrate bias voltage. Due to the specific shape of the electrical field around the samples the morphology of the as-coated samples is divided into three characteristic zones, the width of which are essentially dependent on the height of the bias voltage. All-around coated disk-shaped samples were isothermally tested in a thermobalance at 750 degrees C in dry air to investigate their oxidation kinetics. Oxidation resistance of the coatings was maximal at zero bias voltage and slightly decreases up to 40 V. For those coatings deposited at 60 and 80 V strong spallation was observed, which is caused by bubble formation on heating due to argon incorporated during deposition. (C) 1998 Elsevier Science S.A. All rights reserved.