Patrick Masset

Abstract:

Titanium and its alloys with aluminum are lightweight structural materials, which find ever-increasing use in a number of advanced aerospace, automotive and power generation applications. These materials, however, are limited in applicability by their inadequate oxidation resistance at elevated temperatures (> 500A^oC for Ti, and > 750A^oC for TiAl). This talk reviews recent advances in using state-of-the-art techniques for surface engineering of Ti, Ti-base alloys and I³-TiAl intermetallics, with a view to rendering them resistant to high-temperature environmental oxidation and oxygen embrittlement. The first part of the talk covers the surface modification of Ti and low-Al-content Ti-base alloys by using combined techniques involving either aluminization followed by plasma immersion ion implantation (PIII) of fluorine or formation of a surface barrier coating by magnetron sputter co-deposition of Ti and Al followed by vacuum annealing and PIII of F. The second part focuses on the direct surface treatment of I³-TiAl by PIII of F. Such type of fluorination enables the F-implanted alloy surface to develop a stable, adherent and highly protective alumina scale upon subsequent oxidation in air at temperatures in excess of 1000A^oC for extended exposure times. The last part deals with the fabrication of protective TiAl coatings using a two-step coating scheme. First, an Al-rich TiAl layer is formed on the I³-TiAl alloy by either MO-CVD, PVD or thermal spraying. Then the TiAl layer is treated by PIII of F. The resulting coatings are tested for oxidation resistance, oxygen embrittlement, and retention of mechanical properties. A combination of an Al-rich CVD coating and treatment by PIII of F gives the best results. An example is also given of a thermal barrier coating whose structure comprises, instead of a bond coat, a thin alumina layer formed by PIII of F and subsequent high-T oxidation. The results of these studies have been helpful in understanding the oxidation behavior of the surface-engineered alloys from both a scientific and a technological standpoint.