Effect of substrate surface orientation on the wettability and adhesion of α–Al₂O₃ single crystals by molten Cu

Wetting of α–Al₂O₃ single crystals with different crystallographic orientations, R(0112), A(1120), and C(0001), by molten Cu at 1423–1673 K was studied using an improved sessile drop method mainly in a reducing Ar–3%H₂ atmosphere to determine the effect of the alumina surface orientation on the wettability and adhesion in this system. The contact angles were generally in the range of 110–117°, and the work of adhesion was between 0.7 and 0.8 J m⁻², without a significant dependence on the alumina surface orientation. This result was explained by the possibly close bond strengths of Cu–O at the oxygen-terminated Cu/[R(0112)] and Cu/[A(1120)] α-alumina interfaces and Cu–Al at the Al-terminated (or Al-rich) Cu/[C(0001)] α-alumina interface under high-temperature and low oxygen partial pressure conditions. Additionally, the effects of alumina surface dissolution in the region around the triple junction and H₂ in the atmosphere were examined. Some reasons for the controversy on the bonding nature at the Cu/α–Al₂O₃ interfaces, i.e., Cu–O or Cu–Al on earth, present in the literature were also addressed.