Deposition of Hybrid Organic–Inorganic Composite Coatings Using an Atmospheric Plasma Jet System

The objective of this study is to investigate the influence of alcohol addition on the incorporation of metal oxide nanoparticles into nm thick siloxane coatings. Titanium oxide (TiO₂) nanoparticles with diameters of 30–80 nm were incorporated into an atmospheric plasma deposited tetramethylorthosilicate (TMOS) siloxane coating. The TMOS/TiO₂ coating was deposited using the atmospheric plasma jet system known as PlasmaStream^™. In this system the liquid precursor/nanoparticle mixture is nebulised into the plasma. It was observed that prior to being nebulised the TiO₂ particles agglomerated and settled over time in the TMOS/TiO₂ mixture. In order to obtain a more stable nanoparticle/TMOS suspension the addition of the alcohols methanol, octanol and pentanol to this mixture was investigated. The addition of each of these alcohols was found to stabilise the nanoparticle suspension. The effect of the alcohol was therefore assessed with respect to the properties of the deposited coatings. It was observed that coatings deposited from TMOS/TiO₂, with and without the addition of methanol were broadly similar. In contrast the coatings deposited with octanol and pentanol addition to the TMOS/TiO₂ mixture were significantly thicker, for a given set of deposition parameters and were also more homogeneous. This would indicate that the alcohol precursor was incorporated into the plasma polymerised siloxane. The incorporation of the organic functionality from the alcohols was confirmed from FTIR spectra of the coatings. The difference in behaviour with alcohol type is likely to be due to the lower boiling point of methanol (65 °C), which is lower than the maximum plasma temperature measured at the jet orifice (77 °C). This temperature is significantly lower than the 196 °C and 136 °C boiling points of octanol and pentanol respectively. The friction of the coatings was determined using the Pin-on-disc technique. The more organic coatings deposited with octanol and pentanol exhibited friction values of 0.2, compared with approx. 0.8 for the coatings deposited from TMOS/TiO₂ mixture (with and without methanol). Wear performance comparison between the two types of coating again indicated a significant organic component in the coatings deposited from the higher boiling point alcohols.