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.