Influence of α-Al2O3 (0 0 0 1) surface reconstruction on wettability of Al/Al2O3 interface: A first-principle study

Influence of α-Al2O3 (0 0 0 1) surface reconstructions on the wettability of Al/α-Al2O3 system was theoretically examined by employing density functional theory (DFT). The ( 31 × 31 ) R9° reconstructed surface was modeled by removing two outmost oxygen layers from the (1 × 1) stoichiometric α-Al2O3 (0 0 0 1) surface while the unreconstructed surface with Al termination was simulated for contrast. Interface structure, electronic analysis, work of adhesion (Wad) and deduced contact angle θ of the interface systems were presented. We found that though the (1 × 1) stoichiometric α-Al2O3 (0 0 0 1) surface is most stable, agreeing with other theoretical studies and experimental evidence, interfacial systems of Al/α-Al2O3 with ( 31 × 31 ) R9° reconstruction has the largest work of adhesion (2.05 J/m2), far larger than that of the Al/α-Al2O3 system with (1 × 1) stoichiometric α-Al2O3 (0 0 0 1) surface (1.03 J/m2). For unoxidized condition at 1073 K, the contact angle θ deduced from the calculated Wad for the interface system with (1 × 1) stoichiometric α-Al2O3 (0 0 0 1) surface was around 95°, while that of the interfacial system with ( 31 × 31 ) R9° reconstruction was only about 38°. If oxidation of liquid Al occurs, the contact angle for both systems will be further decreased. The significant wettability improvement occurs with surface reconstructions of α-Al2O3 results from the formation of a metallic Al layer on the reconstructed α-Al2O3 surface. Our calculation results provide theoretical verifications for the improved wettability observed experimentally in Al/α-Al2O3 systems with surface reconstructions.