Deposition of polysiloxane-like nanofilms onto an aluminium alloy by plasma polymerized hexamethyldisiloxane: characterization by XPS and contact angle measurements

In this work, aluminium (Alclad 2024-T3) substrates were cleaned by an r.f. (13.56 MHz) plasma, using argon (Ar), oxygen (O-2) and a mixture of O-2/Ar (50: 50) gases. The effectiveness of plasma cleaning was checked in situ using X-ray photoelectron spectroscopy (XPS) and ex situ using water contact angle measurements. XPS O/Al surface atomic ratios are in excellent agreement with those of the crystalline boehmite and the pseudoboehmite. Oxygen O 1s peak-fitting was used to quantify the proportion of hydroxyl ions and the functional composition on the aluminium surface: the surface cleaned with O-2 plasma contains 50% of aluminium hydroxides, the ones cleaned with Ar plasma and with Ar/O-2 plasma contain, respectively, 25 and 37% hydroxyl ions. The binding energy separation between Al 2p and O 1s is characteristic of AlO(OH). Thin SiOx films were subsequently deposited from a mixture of hexamethyldisiloxane (HMDSO) and oxygen. In the absence of oxygen, a hydrophobic (Theta >= 100 degrees) film characteristic of polydimethylsiloxane (PDMS) is formed: polysiloxane-like thinner films (SiOx) are obtained with the introduction of oxygen. XPS and contact angle measurements confirmed both the composition and the structure of these films. More importantly, contact angle measurements using different liquids and interpreted with the van Oss-Good-Chaudhury theory allowed determination of the surface free energy of the deposited films: the calculated values of surface tension of the film formed from HMDSO/O-2: (50/50) are in excellent agreement with those of reference silica-based materials such as a silicon wafer and cleaned glass. Copyright (c) 2007 John Wiley & Sons, Ltd.