Superhydrophobicity produced by vapor deposition of a hydrophobic layer onto fine protrusions formed by sputter-etching of steels

Argon ion sputter-etching was applied to W–Mo–Cr–V high speed steel, Cr–Mo–V die steel, and type 304 and 316 austenitic stainless steels. Vapor-deposition of the polyethylene (PE) or 1H, 1H, 2H, 2H, -Perfluorodecyltriethoxysilane (PFDS) was carried out on the sputter-etched surface. The hydrophobicity was evaluated by measuring the contact angle (CA) of a water droplet on the surface. The sputter-etching formed fine and dense pillar- or cone-shaped protrusions with a diameter of 100 nm to 1 μm on the tool steels and the type 316 steel but cone-shaped protrusions with a diameter of more than 1 μm on the type 304 steel. The surfaces of all the steels just after the sputter-etching revealed CAs smaller than 4.5°, or superhydrophilicity. On the other hand, all the steels that were sputter-etched and vapor-deposited with PE or PFDS produced CAs of more than 150°, or superhydrophobicity, except that the CA of the type 304 steel with micron-sized protrusions was 146°. The adhesion of the PFDS layer to the protrusions is much stronger than the PE layer, i.e. it is difficult to remove the PFDS layer by heating at 473 K and ultrasonic cleaning in acetone.