Ceramic surface energy

Initially in ceramic powder processing, particle surfaces are created tliat increase tlie surface energy of tlie system. During shape fomiing, surface/interface energy and interiiarticle forces are controlled witli surface active additives. 

Ultimately, the surface energy is used to produce a cohesive body during sintering. As such, surface energy, which is also referred to as surface tension, y, is obviously very important in ceramic powder processing. Surface tension causes liquids to fonn spherical drops, and allows solids to preferentially adsorb atoms to lower tire free energy of tire system. Also, surface tension creates pressure differences and chemical potential differences across curved surfaces tlrat cause matter to move. 

In any brazing/soldering process, a molten alloy comes in contact with a surface of solid, which may be an alloy, a ceramic, or a composite material (see Ceramics Composite materials). For a molten alloy to advance over the soHd surface a special relationship has to exist between surface energies of the hquid—gas, soHd—gas, and Hquid—soHd interfaces. The same relationships should, in principle, hold in joining processes where a molten alloy has to fill the gaps existing between surfaces of the parts to be joined. In general, the molten alloy should have a lower surface tension than that of the base material. 

Table 7.2 Calculated surface energies of ceramic oxides...

L. Mascia and T. Tang, Ceramers based on crosslinked epoxy resins-silica hybrids low surface energy systems, J. Sol-Gel Sci. Technol., 1998, 13, 405. 

Atoms in the free surface of solids (with no neighbors) have a higher free energy than those in the interior and surface energy can be estimated from the number of surface bonds (Cottrell 1971). We have discussed non-stoichiometric ceramic oxides like titania, FeO and UO2 earlier where matter is transported by the vacancy mechanism. Segregation of impurities at surfaces or interfaces is also important, with equilibrium and non-equilibrium conditions deciding the type of defect complexes that can occur. Simple oxides like MgO can have simple anion or cation vacancies when surface and Mg + are removed from the surface,... 

Hasselman, D.P.H., Elastic energy at fracture and surface energy as design criteria for thermal shock ,/. Am. Ceram. Soc., 1963 46(11) 535-40. 

Most common adhesive liquids readily wet clean metal surfaces, ceramic surfaces, and many high-energy polymeric surfaces. However, epoxy adhesives do not wet low-energy surfaces such as polyethylene and fluorocarbons. The fact that good wetting requires the adhesive to have a lower surface tension than the substrate explains why organic adhesives, such as epoxies, have excellent adhesion to metals, but offer weak adhesion on many untreated polymeric substrates, such as polyethylene, polypropylene, and the fluorocarbons. 

T o identify the nature of predominant interactions at interfaces between non-reactive metal M and ionocovalent oxide AO, different attempts have been made to correlate the energetic properties of interfaces (work of adhesion, work of immersion) to the energy of formation of M oxide or other quantities characteristic of the contacting phases, such as the surface energy of the metal or the gap energy of the ceramic. Any successful correlation between an energetic quantity of interfaces and the formation energy or enthalpy of MO oxide indicates the occurrence of a chemical interaction between M and AO at the interface, even... 

Figure 6.28. a) Effect of small additions of an alloying element on the interfacial or liquid surface energies of a non-reactive binary alloy/ceramic system for very positive and very negative values of adsorption energy, b) A very negative value of the slope of 0 implies a negligible slope... 

Rapid fluid flow cannot be achieved with active metal brazes because of the need to form solid wettable reaction product layers for their liquid fronts to advance. Equations (10.1) to (10.2) relating liquid flow rates to the opposed effects of surface energy imbalances and of viscous drag are not relevant. Actual penetration rates are so slow, usually of the order of 1 pm.s, that the usual practice is to place the active metal braze alloy within the joints rather than expecting it to fill them, and, as explained already, gap width is not the dominant consideration when designing ceramic-metal joints. 

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