Wettability coatings: lubricant

The strong bonding of the fluorine ligand to most other atoms has given rise to a major interest in fluoro-coatings for surfaces and this has in turn fostered much work on the effective use of plasmas in which SFg or CF4 other kinetically inert (and hence rather safe reagents) are used as the source of F atoms. Such fluorocoatings can profoundly affect not only the susceptibility of the material to chemical attack but also the physical behavior, such as wettability and lubricity. 

Plasma processing technologies ate used for surface treatments and coatings for plastics, elastomers, glasses, metals, ceramics, etc. Such treatments provide better wear characteristics, thermal stability, color, controlled electrical properties, lubricity, abrasion resistance, barrier properties, adhesion promotion, wettability, blood compatibility, and controlled light transmissivity. 

Control of fiber friction is essential to the processing of fibers, and it is sometimes desirable to modify fiber surfaces for particular end-uses. Most fiber friction modifications are accomplished by coating the fibers with lubricants or finishes. In most cases, these are temporary treatments that are removed in final processing steps before sale of the finished good. In some cases, a more permanent treatment is desired, and chemical reactions are performed to attach different species to the fiber surface, e.g. siliconized slick finishes or rubber adhesion promoters. Polyester s lack of chemical bonding sites can be modified by surface treatments that generate free radicals, such as with corrosive chemicals (e.g. acrylic acid) or by ionic bombardment with plasma treatments. The broken molecular bonds produce more polar sites, thus providing increased surface wettability and reactivity. 

It is possible to get much thicker surface layers by plasma deposition of a polymer-like layer or by treating the surface to make it wettable and bondable. That plasma generated surface can then be dip or spray-coated with a coating of the desired properties (such as an insulator, barrier coating, paint, lubricant, etc.). 

Apart from wettability and the related spreading of hquids, chemically heterogeneous surface composition has a profound impact on adhesion and its respective failure mechanisms and defects [24]. Further relevant areas comprise release surfaces, corrosion, lubrication, as well as chemical surface functionalization in coatings, sensors and biomedical apphcations (stealth surfaces) [18]. These important phenomena together with related appUca-tions in miniaturized devices, where the tolerances for defects and lateral heterogeneities are rapidly decreasing, are in the focus of the microscopic techniques reviewed in this article. 

Superhydrophobic materials have surfaces that are extremely difficult to wet, with water contact angles in excess of 150° or even greater, see Fig. 20.6 shows that surfaces with ultrahydrophobicity have aroused much interest with their potential applications in self-cleaning coatings, microfluidics, and biocompatible materials and so on. Many physical-chemical processes, such as adsorption, lubrication, adhesion, dispersion, friction, etc., are closely related to the wettability of materials surfaces [52, 53]. Examples of hydrophobic molecules include alkanes, oils, fats, wax, and greasy and organic substances with C, N, O, or F as the key constituent element. 

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