Effects of furnace annealing on in situ reacted Ti2AlC MAX phase composite coatings deposited by laser cladding

Abstract

The ability to deposit mm-scale coatings that are metallurgically bonded to substrates shows great promise for the fabrication of components coated with expensive materials, for their resistance to heat, radiation damage or oxidation in extreme environments. Although considered excellent candidates, in situ laser clad MAX phase coatings on metal substrates have found limited success due to the low proportion of MAX phase in the products. In this work, low-purity laser clad MAX phase coatings were subjected to post-deposition furnace annealing at 1350 °C to improve the Ti2AlC purity. Aside from a thin oxide layer on the coating surface, a pure Ti2AlC layer was formed immediately below. In total, the coatings were found to exist as seven microstructurally unique sub-layers due to equilibration of supersaturated phases formed during the laser cladding process. No delamination between any layers was observed. The phase identification and microstructure, as determined using XRD, SEM and EDS, are described in detail. Some unique microstructures were observed, including dendritic Ti2AlC MAX phase grains produced from a topochemical reaction between TiCx and TiyAlz regions, and α-Ti supersaturated with up to 33 at.% C. The kinetically trapped phases produced within the coating using this process may offer a strong combination of material properties which could be advantageous for use in extreme environments.