Surface tension of liquid metals and capillarity

A fundamental property of liquids is their tendency to contract so as to present the smallest possible surface area. This property is taken into account by the quantity surface tension, or by the equivalent quantity surface energy. Surface energy is an important parameter in many processes for the production and shaping of metallic materials. For example, the surface energy of a liquid influences its ability to wet the surface of a solid. Wetting phenomena are of great importance in the galvanizing of steels by immersion, and in the production of composite materials by infiltration techniques. These phenomena are also essential in the assembly of solid parts using brazing alloys, and directly affect interactions between molten metal and crucible or mold materials. Variations in surface tension as a function of temperature and/or liquid composition lead to convective movements in the liquid, known as Marangoni convection. This type of convection is involved in certain crystallogenesis processes and becomes predominant in the case of thin films, for example during welding. In addition, there is a whole range of processes in which a dispersion of a metal or alloy is created in the form of droplets (atomization techniques using gas jets, rotating discs, rotating consumable electrodes, etc.). Once formed, these droplets can either be solidified to obtain a powder, or sprayed onto a surface to create a coating. What all these processes have in common is the creation of a large amount of free surface for the liquid metal, and as such, surface energy is a key factor in droplet formation and size distribution.

This work is a review of experimental results in the literature relating to the surface tension and temperature coefficient of pure metals. Over the past thirty years, our knowledge of the energetic properties of liquid metal surfaces has progressed significantly, thanks mainly to the development of non-contact measurement methods. These methods, which eliminate pollution from the crucibles or substrates used in conventional methods, have been successfully applied to refractory metals in particular.

This article first describes the role of surface energy and capillarity in the various processes mentioned above (wetting, infiltration, Marangoni convection). After defining the quantities surface energy and surface tension and their equivalence, a thermodynamic model is presented for evaluating surface energy and its variation with temperature for pure metals. The effect of oxygen, present in virtually all liquid metal production and processing plants, is then detailed, as this impurity is known to have a strong influence on the surface energy of metals. In the majority of applications, metal phases are made up of several constituents, hence the interest in presenting...