A THERMODYNAMIC CRITERION TO PREDICT WETTABILITY AT METAL-ALUMINA INTERFACES

Metals are known to wet ceramics by chemical bond formation. Existing theories, using reaction thermodynamics, can predict the relative wetting trends in wetting systems but fail to distinguish between the wetting and nonwetting systems. Wetting is considered as a surface phenomenon, and the spontaneity of wetting is controlled by DELTA-G(w), a thermodynamic term defined here as the Gibbs free energy of wetting. A model that treats wetting as a reaction between the surface phase of the ceramic and the molten metal is presented to calculate DELTA-G(w). The model is used to predict wetting tendencies of various molten metals on alpha-alumina surfaces. The predictions are compared with previously published results, as well as with the experimental results of this study. Experimental wettability parameters were measured using a capillary rise apparatus. Measurements were made for various metals wetting an aluminum-oxide surface. Based on this model, a thermodynamic wetting map that delineates wetting and nonwetting regimes is drawn. A map of this nature can be used advantageously in ceramic joining and metal-matrix composites applications.