Wettability, Spreading, and Interfacial Phenomena in High-Temperature Coatings
spreading, interface evolution, and adhesion. The wettability of a solid by a liquid is characterized in terms of the angle of contact that the liquid makes on the solid.1,2 The contact angle, q, is obtained from a balance of interfacial tensions (Figure 1) and is defined from Young's equation, according to which
slv.cosq + sls = ssv
where slv, sls, and ssv are the interfacial tensions at the boundaries between liquid (l), solid (s), and vapor (v). Here, s represents the force needed to stretch an interface by a unit distance (or, equivalently, energy required to create a unit surface area of a given interface, provided that, in the case of ssv, mechanical distortion and strains are negligible). The condition q < 90° indicates that the solid is wet by the liquid, and q > 90° indicates nonwetting, with the limits q = 0 and q = 180° defining complete wetting and complete nonwetting, respectively. This article reviews the status of high-temperature wettability as manifested in the contact-angle phenomena, with a focus on the behavior of metallic coatings in contact with liquid metals
Young's equation predicts a unique value of the equilibrium angle, q, in terms of thermodynamic quantities (s's) without regard to the presence of external fields, such as gravitation. This is in contrast to the common observations of shape distortion of droplets on an inclined plane in the earth's gravity. Theories and experiments3-5 have been advanced both to challenge and support Young's equation.
Gravity also influences gas adsorption at nonwetting rough surfaces;6 it opposes adsorption because the gas has to locally lift the liquid to enter into surface troughs. Gas adsorption effectively smoothens the surface because the liquid contacts only a portion of the asperities. However, adsorption creates additional solid-gas and liquid-gas interfaces, and will be energetically favored in a gravitational field if the decrease in ssl due to reduced roughness is greater than the energy increase due to the creation of additional surface.6
Precursor Films and Microscopic Angles
In many solid-liquid systems, the liquid front is spearheaded by a thin foot or a precursor film that forms when the liquid develops a finite, non-zero curvature near its periphery due to local intermolecular interactions.1,10 A sharp distinction between solid, liquid, and vapor phases can no longer be assumed as the liquid is gradually thinned, yielding a microscopic angle that does not obey Young's equation.11 This is because the continuum concept of surface energies to represent the local molecular interactions becomes increasingly tenuous for precursor films approaching molecular thicknesses.
Most solid surfaces (with the exception, perhaps, of single crystals) are seldom consistent and clean, with different surface domains possessing different chemistries and wetting properties. Such chemical inhomogeneity could result from oxidation, corrosion, coatings, multiple phases (e.g., eutectic), adsorbed films, ledges, kinks, dislocations and grain-boundary intersections with the surface, and crystallographic anisotropy (planes having different packing density and exposing different molecular groups). Wettability and spreading are sensitive to such chemical and structural inhomogeneity.1,17,20-28 Carefully prepared single crystals are closest to idealized surfaces, and several studies have focused on the wetting behavior of single crystals by metallic melts.29-33
The wettability on chemically inhomogeneous surfaces is conveniently characterized in terms of an effective contact angle. The effective equilibrium contact angle on a composite surface constituted by chemical phases having different area coverages and contact angles is the area-fraction-weighted angle,20 provided there is no hysteresis. Like Wenzel's equation for roughness, the weighted-average rule applies to the equilibrium state rather than to metastable states. Note also that chemical reactions are often accompanied by reconstruction of the solid's surface, which could promote the chemical inhomogeneity and roughness. For example, in the SiO2-Al system, the reduction of silica by aluminum leads to a 38% volume reduction, which causes cavities to develop ahead of the contact line, thereby enhancing the roughness and hampering the spreading.15
WETTABILITY AND COATING PROCESSES
Wettability is manifested in numerous forms in a variety of coating processes. In many industrial processes, the substrate is immersed in a liquid coating material, then withdrawn to leave a liquid film on the substrate. The film (coating) thickness depends upon the surface tension, withdrawal speed, substrate geometry, roughness, and melt viscosity.34-36
The dispersion of fine, granular solids in a liquid vehicle is a basic step in preparing paints and other coating materials and involves particle transfer across a gas-liquid interface. The transfer of nonwettable solids into liquids requires the solid to overcome a surface energy barrier at the liquid-gas interface,37-44 and energy must be expended to assist the transfer of nonwettable solids. Once the solid enters the liquid, the capillary (attractive) forces and gas bridges between solids control such phenomena as agglomeration, dispersion, and air entrapment.45-51
The interparticle forces between dispersed solids are due to liquid surface tension and pressure difference across the curved liquid-vapor boundary between contacting solids. The maximum interparticle force, F, due to capillary forces between two touching spheres is49
F = 2(2)1/2slv cosq/Rwhere R is the radius of the sphere. The force increases with increasing liquid surface tension and decreasing contact angle and particle radius. These forces affect the viscosity, density, and sedimentation behavior of the suspension and the properties of the coating deposited using the suspension. Other coating processes, such as thermal spray, which atomize and spray molten or semimolten coating materials, also involve surface energetic considerations. These considerations become important in the shredding of droplets during flight and upon impact on the substrate, as well as in the engulfment of fine particulates into atomized droplets during flight in the case of composite coatings.52,53
- . .A.W. Neumann and R.J. Good, Surface and Colloid Science, vol. II, ed. R.J. Good and R.R. Stromberg (New York: Plenum Press, 1979).
- L.E. Murr, Interfacial Phenomena in Metals and Alloys (New York: Addison Wesley Publ. Company, 1974).
3 B.A. Pethica, J. Colloid Interface Sci., 62 (3) (1977), p. 567.
4. J.P. Garandet, B. Drevet, and N. Eustathopoulos, Scripta Mater., 38 (9) (1998), p. 1391.
5. Y. Liu and R.M. German, Acta Mater., 44 (4) (1996), p. 1657.
6. H. Sakai and T. Fujii, J. Colloid Interface Sci., 210 (1999), p. 152.
7. G.R. Wickham and S.D.R. Wilson, J. Colloid Interface Sci., 51 (1) (1975), p. 189.
8. G.R. Lester, J. Colloid Sci., 16 (1961), p. 315.
9. M.A. Fortes, J. Colloid Interface Sci., 100 (1) (1984), p. 17.
10. P.G. de Gennes, Rev. Modern Phys., Part I, 57 (July 1985), p. 827.