The Use of Plasma Immersion Ion Implantation in the High-Temperature Oxidation Protection of Low-Al-Content Ti-Base Alloys and TiAl Intermetallics

Low-Al content Ti-base alloys and TiAl intermetallics are attractive lightweight materials for advanced medium-temperature (500°-750°C) structural applications including components such as jet engine and industrial gas turbine blades, turbocharger rotors and automotive engine valves. However, envisaged service temperatures are in the range of 750° to 1050°C at which these alloys are prone to both destructive oxidation and oxygen embrittlement. Therefore, development of surface-engineering techniques for preventing high-T environmental damage is critical in exploiting the advantages of the TiAl alloys to their fullest extent.
We propose two techniques for protecting candidate Ti-base and TiAl alloys from high-temperature (>750°C) oxidation environments. The first technique involves a single step, namely treating the alloys directly by plasma immersion ion implantation (PIII) of fluorine using a mixture of CH2F2+6.25% Ar as the precursor gas. This technique is applicable to TiAl alloys of an Al content of ~ 45 to 55 at.%. The F implant dose has been found to depend critically on the gas flow rate ratio (GFRR, i.e. CH2F2/Ar) while the resulting F depth profiles show dependence on both the GFRR and the alloy material. Optimum implantation conditions have been established under which the F-implanted alloy surface is able to form a highly protective Al2O3 film upon subsequent oxidation in air. Oxidation resistance has been evaluated by thermal gravimetric analysis (TGA) at temperatures as high as 1050°C for extended exposure times.
The alternative technique is applicable to low-Al-content Ti-base alloys (< 40 at.% Al). It involves the fabrication of a barrier coating in a three-step process, namely formation of a Ti+Al layer by magnetron co-sputtering of Ti and Al followed by vacuum annealing to form a gamma-TiAl coating and, finally, PIII of fluorine. The coating so formed has been shown to prevent further oxidation of the base material at elevated temperatures.