Fabrication of Cr2AlB2 and Cr4AlB4 MAB Phase Coatings by Magnetron Sputtering and Post-Annealing

Cr2AlB2 and Cr4AlB4 are members of the MAB phases that exhibit unique properties of both metals and ceramics. However, despite these unique characteristics, Cr2AlB2 and Cr4AlB4 phase coatings have not been widely investigated. In this study, Cr2AlB2 and Cr4AlB4 MAB phase coatings were fabricated by magnetron sputtering at room temperature and post-annealing. A composite target, consisting of a phase-pure disc-shaped CrB target overlapped by uniformly dispersed fan-shaped Al slices, was placed parallel to the substrates. The Al content of the coatings was adjusted by altering the areal proportion of the Al slices. MAB phases have crystallized upon post-annealing the as-deposited coatings on Al2O3(0001) substrates in Ar. The phase compositions and morphologies of the crystalline coatings were found to be dependent on the Al content and the annealing temperature. As-deposited coatings with a Cr:Al:B ratio close to 2:1:2 could crystallize as pure and dense Cr2AlB2 phases within the temperature range of 650–800 °C; higher annealing temperatures resulted in the decomposition of Cr2AlB2, while crystallization at lower temperatures was not evident from X-ray diffraction. As-deposited coatings with a Cr:Al:B ratio close to 3:1:3, despite containing a relatively higher Al content than required by the stoichiometry of Cr4AlB4, exhibited insufficient crystallization of Cr4AlB4 with unknown phases below 840 °C. Higher annealing temperatures resulted in the coexistence of Cr4AlB4 and CrB, indicating that achieving phase-pure and well-crystallized Cr4AlB4 coatings proved challenging, possibly due to the inevitable loss of Al during annealing. The configuration of the composite target and the substrates provides a promising strategy for fabricating phase-pure and dense Cr2AlB2 coatings.