Understanding the surface adsorption and oxidation of cubic Cr_0.5Al_0.5N by first-principles calculations

Bulk and surface properties of oxygen in cubic c-Cr_0.5Al_0.5N were investigated through first-principles calculations based on density functional theory (DFT) in the present work. It was observed that the (0 0 1) surface of c-Cr_0.5Al_0.5N terminated with Cr-Al-N is energetically favorable. Calculations of oxygen adsorbed on the (0 0 1) surface show that O is bonding with Cr preferentially instead of Al due to the stronger Cr-O bond strength in comparison with that of Al-O. The difference charge density and electronic density of states indicate that the electrons shared between O and Cr and hybridization between the O-p and Cr-d orbitals contribute to the formation of strong Cr-O bonding on the surface. An oxidation mechanism is proposed, i.e. the Cr-O bond is first established, resulting in the formation of α-Cr₂O₃, and subsequent incorporation of α-Al₂O₃ promotes the formation of a mixed (Cr, Al)₂O₃ scale.