Metal Finishing Guide Book


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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 DW to the surface growth DA. For liquids, this surface energy equals the surface tension.10,11 ␴ = ΔW/ΔA [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 surface 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) energy. Therefore, each system strives to avoid surfaces possessing high surface energy or tending to reduce surface contact. It is for this reason that materials are slightly 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 u <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 accuracy. In this case, the surface energy of the copper (1.85 J/m2) is significantly higher than the energy of the water (0.05 J/m2): ␴l << ␴s.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 measurement). For an inactivated copper surface the contact angle increases to approximately 60°. An oxide layer, therefore, leads to a more hydrophobic copper 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 activated (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 substrate was plated with 1.5 μm nickel (followed by 0.3 μm gold). The nontreated sample showed close to 100% complete wetting effect after activation. 234

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