The sulfidation and oxidation behavior of sputter-deposited Al-Nb-Mo alloys

Al-Nb-Mo ternary alloys of about 5 μm thickness have been deposited on quartz substrates using a sputter deposition technique, and their sulfidation and oxidation behavior has been compared with the Al-Nb, Al-Mo and Nb-Mo binary alloys. The sulfidation of the ternary alloys follows two-stage parabolic kinetics at temperatures of 1073-1273 K in sulfur pressure of 10³ Pa. The steady-state sulfidation is significantly slower than the initial sulfidation. An interesting fact is that the steady-state sulfidation rates of the ternary alloys are further lower than those of the Al-Nb and Al-Mo binary alloys. In particular, an increase in niobium content in the ternary alloys leads to a decrease in the sulfidation rate. Similarly to the Al-Nb and Al-Mo binary alloys, apparently two-layered scales are formed on the ternary alloys, consisting of an outer Al₂S₃ layer and an inner refractory metal sulfide layer. The detailed analysis of the inner layer reveals that the inner barrier layer is not a single layer but consisting of an outer thin NbS₂ layer and inner major layer of MoS₂ containing Nb₃S₄. The formation of the inner barrier layer consisting of two layers in which diffusing matters and directions are different is attributed to the higher sulfidation resistance of the ternary alloys than those of Al-Nb and Al-Mo binary alloys. The steady-state sulfidation rates of the ternary alloys are also lower than those of the Nb-Mo binary alloys, because the two layered niobium sulfide-molybdenum sulfide doped with Al³⁺ ions is more protective than the individual NbS₂ and MoS₂ sulfides doped with Al³⁺ ions. The oxidation resistance of the aluminum-rich Al-Nb-Mo alloys is better than the Al-Nb and Al-Mo alloys, although the resistance of the former alloys is still not sufficient.