Memorandum on “chemical conversion coatings improve wear and lubrication of titanium” : Titanium Metallurgical Laboratory, Battelle Memorial Institute, Columbus 1, Ohio

Abstract

The oxidation of Ta and Ta-10%W hot-dipped in aluminum and Ta hotdipped in Al-Cr alloys has been studied in the temperature range 800–1600°C and at oxygen pressures ranging from 0.1 torr to 1 atm O2. The experimental studies comprised thermogravimetric measurements of oxidation, and X-ray diffraction studies, metallographic investigations, and electron-microprobe analyses on reacted specimens.The unalloyed coatings on Ta and Ta-10%W gave at 0.1 torr O2 excellent oxidation resistance at temperatures above 1100°C, but at 800–1000°C breakaway oxidation occurred. The protectivity is provided by an α-Al2O3 layer. At 1 atm O2 the unalloyed coatings failed at all temperatures. The interdiffusion of aluminum and tantalum leads to the formation of TaAl3, Ta3Al, the Ta-Al σ-phase with approximately 7–9 wt.%Al, and a Ta-Al phase with approximately 20 wt.%Al.The Al-Cr coatings exhibited significantly improved performance compared with the pure aluminide coatings. At 0.1 torr O2, Al-20% Cr and Al-30%Cr coatings on Ta provided excellent oxidation protection above 1000°C and at 800–1000°C breakaway oxidation was significantly retarded. Increasing concentrations of Cr also reduced failures at 1 atm O2, and good protection could be achieved with Al-30%Cr coatings in 1 atm O2 at 1500°C. The protectivity is provided by a layer of the α-Al2O3 phase (Al2O3-Cr2O3 solid solution).