Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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energies. The surface energy consists of the potential energy of the molecules or atoms on a surface (specific surface energy). The energy results from the ratio of work per surface increase W to the surface growth A. For liquids, this surface energy equals the surface tension.10,11 = W/A [N/m or J/m2 ] Thomas Young established the relationship between the free surface energy s of a solid, the interfacial energy sl of the solid and the suspended drop, the sur- face tension sl of the liquid and the contact angle () between the vectors sl and sl (see Fig. 1).12 Young's formula can be described as follows: cos = s –sl/ l (Indices s and l represent "solid" and "liquid," respectively). The most stable thermodynamic state of a system is that of lowest (free) ener- gy. Therefore, each system strives to avoid surfaces possessing high surface ener- gy or tending to reduce surface contact. It is for this reason that materials are slight- ly wetted with materials of a low surface energy. The wetting angle can be within the following limits: 0°< <180°. A solid can be wetted by a liquid if the contact angle is <90°. A pure copper surface is nearly completely wetted by water. Figure 3 shows such a wetting with a very small contact angle that lies outside measurement accura- cy. In this case, the surface energy of the copper (1.85 J/m2 s er than the energy of the water (0.05 J/m2): l << .13 Surface energy is also influenced by surface preparation. The sample shown in Figure 2 was activated (i.e., all oxides were removed from the surface prior to mea- surement). For an inactivated copper surface the contact angle increases to approximately 60°. An oxide layer, therefore, leads to a more hydrophobic cop- per surface. For meaningful measurements and to remove the aforementioned strong influence, all post-dip treated metal combinations on top of the copper substrate (including copper, nickel, gold, and a combined nickel/gold layer) were activat- ed (removal of the oxide layer) prior to treatment with Betatec. The copper substrate standard immersed in Betatec post-dip provided an increased contact angle of approximately 76° (see Fig. 3). Tests were also carried out for a nickel surface (surface energy of nickel is 2.45 J/m2) with a measured contact angle of approximately 92° after treatment with the post-dip.14 Hence, despite nickel possessing considerably higher surface energy, thereby making it more hydrophilic, Betatec post-dip treatment was very effective at imparting hydrophobic surface properties (see Fig. 4). Similar testing was undertaken with pure gold surfaces (surface energy of 1.5 J/m2) that were also made water-repellent, achieving a contact angle of 87°.15 In these cases, an almost complete wetting of the gold surface was achieved without the post-dip treatment. For the final contact angle measurements, copper sub- 149 ) is significantly high-

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