THE INFLUENCE OF METALLIC COATINGS ON THE STRUCTURE, WETTING, AND MECHANICAL STRENGTH OF CERAMIC/METAL INTERFACES

摘要

The paper reviews and previews recent research on the role of metallic films in modifying the wettability, interface structure and bonding in Al/Al_2O_3 and Al/graphite couples. Contact angles from the sessile-drop test and interface bond strength from an improved "push-off test that allows shearing of solidified sessile drops with less than 90°contact angle, were characterized, and the wettability-bond strength relationship was examined in light of the microstructure of the joint. In particular, the effects of coating type, temperature, contact time, and alloying were evaluated. A significant outcome is the observation that rapid dissolution of thin (< 1 μm) metallic films in the droplet yields transient lowering of the contact angle; however, partial dewetting could occur upon complete dissolution of films (e.g., Ni and Cu) if no chemical interaction takes place with the 'undressed' substrate. In such a case, stable wetting is attained only if the 'undressed' substrate has a favorable surface structure (e.g., through prior activation) to counter the dewetting effects of film dissolution. In contrast, thin films of reactive metals (e.g. Ti films on Al_2O_3) yield low contact angles and high bond strength because of chemical interaction with the substrate.