Reactive synthesis of oxidation-resistant coatings on pure Ti substrates via an Al/porous TiO2/Ti precursory stack

A precursory stack, composed of magnetron-sputtered Al top-layer, microarc-oxidized TiO2 interlayer and pure Ti substrate as bottom-layer, was used to fabricate oxidation-resistant coatings via reactive synthesis method that involves a vacuum-diffusional annealing at 600 °C and a following pre-oxidation treatment at 900 °C. The compositional and structural evolution from precursory stack to oxidation-resistant coating was investigated by EDS, XRD and SEM, respectively. The anti-oxidation performance of the coating was evaluated by cyclic oxidation test at 900 °C. The results indicate that a Ti-Al gradient structure preliminarily generates at coating/substrate interface for decreased thermal mismatch during vacuum-diffusional annealing, because of the controllable diffusion of Al atoms from top-layer to Ti substrate via porous TiO2 interlayer. On the other hand, in the process of subsequent pre-oxidation treatment, an Al2O3 rich outer coating is formed accompanied with the consumption of TiO2 interlayer, while the Ti-Al gradient structure is eventually achieved in the reactively synthesized coating. The oxidation resistance of Ti samples with the coating is about 12 times higher than that of bare Ti, demonstrated by a 50 cycle test.