Suppression of oxygen-induced embrittlement in Ti by plasma immersion ion implantation (PIII)-based processing

Titanium has a large strength-to-weight ratio and good corrosion resistance at moderate temperatures, which make it an important structural material in a number of advanced technical applications. A major problem with the use of Ti is its high affinity to oxygen giving rise to massive oxygen in-diffusion at temperatures of about 550° to 600°C. The presence of large amounts of oxygen renders the material brittle and deteriorates its mechanical properties. In this work, we describe an effective way of overcoming the oxygen-induced embrittlement problem by forming a surface barrier to the diffusion of oxygen. Surface processing has involved two steps, namely enrichment of the Ti near-surface region with Al, and introduction of fluorine. For the Al enrichment, a novel hybrid system has been developed to implant Al into Ti. The apparatus consists basically of a PIII chamber configured with two magnetrons having an Al target each, and facing the RF antenna. The magnetrons are synchronized with the bias applied to the sample holder in such a way that the accelerating high-voltage pulse is triggered with a certain delay, i.e. during the time when the Al plasma generated by the magnetrons is most dense, thereby minimizing deposition. Alternative aluminization techniques have involved either magnetron sputtering of Al onto Ti followed by a thermal drive-in step or pack processes. Fluorine has been subsequently introduced by PIII employing a mixture of CH2F2 and Ar as the precursor gas. A variety of analytical techniques such as elastic recoil detection analysis (ERDA), Rutherford backscattering spectrometry (RBS), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX) and x-ray diffraction (XRD) have been used for characterization. Optimized PIII-based processing has been found to produce a continuous, adherent alumina scale on the Ti surface. Thus, the resulting material is inherently resistant to oxygen absorption and is not embritteled upon extended exposure to oxygen-containing environments at elevated temperatures up to about 600° C.