Tribology of silica nanoparticle-reinforced, hydrophobic sol-gel composite coatings

Hydrophobically functional coatings can be used to protect surfaces and therefore improve the performance and lifetime of a broad range of applications such as optoelectronics and touchscreens. Organic-inorganic hybrid materials such as silica sol-gel coatings are particularly effective for this purpose, but the functional molecules in these coatings are susceptible to abrasive wear and thus lose their performance over time in the harsh environments typically encountered. To combat these problems, a silica nanoparticle-reinforced matrix was developed to increase hardness and wear resistance of the overall coating. This study involved the abrasive wear analysis of fluorinated composite silica particle reinforced sol-gel silica coatings dip-coated on glass substrates. Varying amounts of silica nanoparticles from 0.5 to 10 wt.% of the precursor weight were added to examine the structural dependence of abrasive wear mechanisms to elucidate strengthening mechanisms that could lead to improvements of coating properties. Abrasion was conducted using an in-house built reciprocating polishing wear apparatus. Characterization of the water contact angle of the coating was conducted to determine the hydrophobic functionality after wear cycles. Atomic force microscopy, lateral force microscopy, nanoindentation, nano-scratch, contact angle goniometry, and optical microscopy were performed at intervals of abrasive wear testing to characterize these wear mechanisms and the functional degradation of the coating. It was generally found that, among other possible factors, the increased indentation hardness of coatings led to a decreased wear rate. Additionally, an optimal amount of added colloidal silica of 1-2 wt.% of the precursors provided the best overall mechanical, tribological, and functional performance. (C) 2014 Elsevier B.V. All rights reserved.