Effects of LaB6 on the high-temperature oxidation behavior of TiC+TiBx reinforced titanium matrix composite coatings fabricated by laser cladding

Abstract

The effects of LaB6 addition on the oxidation behavior of TiC+TiBx reinforced titanium matrix composite coatings at the temperature of 600 °C were investigated. The results showed that severe oxidation occurred on the Ti-6Al-4V after oxidation at 600 °C for 50 h. The oxidation weight gain of the (TiC+TiBx)/Ti coating without LaB6 addition (2.43 mg/cm2) was reduced to 49.6% that of the Ti-6Al-4V substrate (4.9 mg/cm2). Though (TiC+TiBx)/Ti coatings improved the oxidation resistance for the surface of Ti-6Al-4V substrates, cracks were easily formed between the coarse rod-like TiB2 and the titanium matrix during the high temperature resulting in less high-temperature oxidation protection. When the LaB6 addition gradually increased from 0 wt% to 1.0 wt%, the oxidation weight gain decreased from 2.43 mg/cm2 to 1.30 mg/cm2, and the linear rate constant Kp1 decreased from 0.04752 to 0.02703. The oxidation law gradually changed from the linear law to the parabolic law when the LaB6 addition increased to 1.5 wt%, which indicated the oxide layer gradually became dense and protective. The oxidation weight gain decreased from 1.23 mg/cm2 to 0.87 mg/cm2 when the LaB6 addition increased from 1.5 wt% to 2.0 wt%, while increased instead when the LaB6 addition increased to 2.5 wt%. An appropriate amount of LaB6 (2.0 wt%) addition significantly improved the oxidation resistance of the coating with the lowest parabolic velocity constant Kp2 = 0.01418 due to the refinement of grain sizes of the coating, especially the refinement of the coarse rod-like TiB2 phase, which decreased the crack forming sensibility and increased the compactness and continuity of the oxidation layer. A small amount of in-situ formatted La2O3 at the grain boundaries acted as a barrier also blocked the ingress of oxygen.