D. Smith, L. Patterson
Keywords: low surface energy, hydrophobicity, oleophobicity, bulk processing, CVD, thermal stability
Summary:There are myriad potential applications and a significant need for low energy surfaces. The gas-phase modification of three dimensional structures, such as fittings, tubing, valves, or manifold blocks, in high volume, to give hydrophobic and oleophobic surface properties would be of high value to the automotive, process, analytical, and tech industries, to name a few. A chemical vapor deposition (CVD) process has been developed to generate a layer consisting of amorphous silicon, oxygen and carbon atoms, with a fluorocarbon-functionalized surface (US Patent application nos. 14/381,616 and 14/538,021). By not employing plasma or other additive energies, the thermal CVD process lends itself to ease of processing, high volume scale up, and uniform deposition on to complex, three dimensional substrates with internal cavities, high aspect ratio features and blind holes. Secondary Ion Mass Spectroscopy (SIMS), was used to analyze elemental composition and depth. SIMS data confirms an amorphous array silicon, oxygen and carbon in the base layer. The elemental ratios are relatively consistent throughout the layer at 3:3:1.5:1 for Si:H:O:C. Typical deposition thicknesses may range from 200-1000nm. The deposition is further treated to generate a non-polymeric, covalently bonded fluorocarbon functionalization. Grazing angle FT-IR spectroscopy confirms the presence of functional C-F moieties with a strong peak at 1250cm-1. The performance of the functionalized deposition confirms a low energy surface with high water (avg. 115°) and hexadecane (avg. 68°) contact angles. One measure of the stability and robustness of this surface is its resistance to thermal oxidation. Coated 304 stainless steel coupons with a mirror finish were placed in an oven, with a room air atmosphere, at elevated temperatures for set periods of time. The coupons were then cooled, and contact angles were tested to measure any oxidative reduction in hydrophobicity or oleophobicity. The variation of hydrophobicity and oleophobicity after total thermal oxidative exposure periods of 30, 60 and 90 minutes at two different temperatures (350°C and 450°C) was measured. The short-term thermal oxidative stability is excellent, whereas the surface is highly stable at 350°C, but shows degradation at 450°C via a decrease in hexadecane contact angle after 30min exposure. The nature of this thermal-only CVD process well-lends itself to bulk processing and scale up, as the uniform distribution of thermal energy is a simpler task than for that of plasma energy. This has been demonstrated by processing several three dimensional parts simultaneously in one reaction vessel. The drawback to this approach is the relatively high processing temperatures (450°C max) eliminates the use of substrates such as plastics, fabrics, polymers and other low melting or outgassing materials.