Colaborative Research: Wetting of Liquid Metals on Rough Surfaces

High/ultra-high temperature brazing and low temperature soldering are widely used methods of materials joining. The quality of the bond between materials created by the brazing/soldering process depends critically on the spreading kinetics of the molten metal filler. The tight time scheduling of the processing, combined with often dissimilar wetting properties of materials being joined, requires control, which in turn requires quantitative prediction tools. The existing models of wetting/spreading of liquid metals fail to predict experimental results in a satisfactory manner. The major difficulties include: (1) roughness of the spreading surface, (2) chemical reactions between the melt and the substrate, and, (3) inadequate models of the physical mechanism by which the triple line propagates. We propose to address the three major difficulties by integrated experimental and modeling strategy which includes: (i) Carefully designed experiments on characterized virgin and designed surfaces, with in situ monitoring and measurements of the triple line motion, (ii) Advances in theory and modeling based on the diffuse interface (phase-field) models, capable of representing propagation of phase- and chemical reaction fronts, as well as the diffusive nature of the triple line motion, and, (iii) Close integration of modeling and experiments, with benchmarking of the models at several levels. Broader impact. In addition to brazing and soldering, the impact of this investigation will be felt in much broader set of applications, related to industrial and natural processes. The transformative nature of the research is that it will results in the ability to effectively control wetting by surface alterations and the selection of liquid system and solid substrate. This will enable rational design of industrial processes which depend on liquid spreading, and products whose function depends on wetting. Our integrated research and education program includes active engagement of both undergraduate and graduate students in the common project and changes in the graduate engineering curriculum. Three features of our program stand out: (i) Mentoring of undergraduates by graduate students, (ii) Engagement of undergraduate students in modeling and simulations, and, (iii) Inter-university cooperation of students involved. Intellectual merit. The major experimental challenges are related to the fast evolution of the triple line at the micro scale and the presence of reactive substrates. We will achieve the required time resolution by developing hot stage microscopy techniques for in situ monitoring of the moving liquid front and the dynamics of the contact angle. Suppression of a chemical reaction will be accomplished by formation of intermetallics prior to spreading of a liquid metal. The major modeling challenges include: (a) Implementation of the surface diffusion kinetics governing the motion of the triple line for the non-reactive model into the finite element framework to model rough surfaces, and, (b) Formulation and implementation of the combined liquid-gas phase-field and the chemical reaction phase-field. We have developed the theoretical framework to address these challenges. Finally, the synergy of experimental and theoretical studies is novel and unique in the area of molten metal spreading applications. The experiments on designed surfaces, and the experimental separation of roughness and reaction effects, are carefully designed to benchmark the model, and to shed light on the micro-scale processes that govern the macroscopically observed spreading.