Wettability and surface energy of oxidized and hydrogen plasma-treated diamond films

Abstract

The wettability of CVD diamond films with liquids of different physico-chemical natures (water, glycerin, tin melt) was investigated by measuring the contact angles using the sessile drop method. The translucent diamond films of 0.5-mm thickness were grown in a microwave plasma CVD using CH4–H2 mixtures as the source gas, and separated from Si substrates. The growth surfaces have been polished to a roughness of Ra≤10 nm, and then exposed to microwave hydrogen plasma or thermal oxidation in air at 500 °C. Raman and Auger/XPS spectroscopies, optical and atomic force microscopies have been used to characterize the diamond films. Based on the wettability data, the variations in surface energy induced by hydrogenation and oxidation of polycrystalline diamond have been evaluated by the Fowkes equation. The hydrogenation of the films in the plasma essentially increases hydrophobic properties (contact angle for water increases up to θ=93°) as compared to oxidation (θ=32°). This is attributable to a reduction in a polar component of the surface energy due to hydrogen adsorption-induced reconstruction of the film surface. For comparison, the wettability of HPHT diamond single crystal, diamond ceramics and pyrolytic graphite have been measured and the differences observed will be discussed in this paper.