Reactive spreading: Adsorption, ridging and compound formation

Reactive spreading, in which a chemically active element is added to promot e wetting of noble metals on nonmetallic materials, is evaluated mechanisti cally. Theories for the energetics and kinetics of the steps involved in sp reading are outlined to permit comparison to the steps in the compound form ation that typically accompanies reactive wetting. These include: fluid flo w, active metal adsorption. including nonequilibrium effects, and triple li ne ridging. They can all be faster than compound nucleation under certain c onditions. This analysis plus assessment of recently reported experiments o n metal/ceramic systems lead to a focus on those conditions under which spr eading proceeds ahead of the actual formation of a new phase at the interfa ce. This scenario may be more typical than commonly believed, and perhaps i s the most effective situation leading to enhanced spreading. A rationale f or the slow spreading rates plus the pervasive variability and hysteresis o bserved during high-temperature wetting also emerges. (C) 2000 Elsevier Sci ence Ltd. Published by Elsevier Science Ltd. All rights reserved.