Applied Surface Science 257 (2011) 10421–10426 Contents lists available at ScienceDirect Applied Surface Science j our nal ho me p age: www.elsevier.com/loc ate/apsusc Superhydrophobic sol–gel nanocomposite coatings with enhanced hardness R.V. Lakshmi ∗ , T. Bharathidasan, Bharathibai J. Basu Surface Engineering Division, CSIR – National Aerospace Laboratories, Bangalore-560 017, India a r t i c l e i n f o Article history: Received 9 December 2010 Received in revised form 22 June 2011 Accepted 24 June 2011 Available online 30 July 2011 Keywords: Superhydrophobic Sol–gel Colloidal silica Hardness Roughness a b s t r a c t Sol–gel superhydrophobic coatings with improved hardness were prepared by embedding fumed silica nanoparticles in a partially condensed hybrid sol of methyltriethoxysilane (MTEOS) and colloidal silica. Fumed silica particles of size 25–30 nm were incorporated in the sol and the mixture was spray-coated on glass substrate. Water contact angle (WCA) of the composite coating increased with increase in silica content of the sol mixture. The concentration of silica in the sol mixture was optimized to obtain robust superhydrophobic coatings with a WCA of 162.5 ◦ and a pencil hardness of 5H. The wetting state of water droplet on the sol–gel composite coatings was analysed with both Wenzel and Cassie-Baxter models. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Superhydrophobic surfaces with water contact angle (WCA) above 150 ◦ and a sliding angle below 5 ◦ have been actively pur- sued by both the scientific and engineering communities in the past few decades. It is the low contact area of water drops with the surface, which makes it self-cleaning and hence has a lot of technological applications like inhibition of corrosion, self-cleaning windshields of automobiles, solar cells and optical glasses. Numer- ous works and studies have revealed that superhydrophobicity is a result of micro-nano scale roughness along with a low sur- face energy material [1,2]. Various techniques have been followed to impart roughness to the solid surfaces and also to induce low surface energy to the materials [3–6]. Of the different techniques available, sol–gel process is widely recognized as a useful and prac- tical method for preparing thin films on various substrates [7–10]. This is because of its advantages like low temperature synthesis, high purity products, simple and easy technique and its ability to provide uniform films on large substrate area. Although there have been several reports on the fabrication of stable superhydropho- bic coatings [9–13], very few reports discuss on their mechanical properties [14–18]. It has been reported that incorporation of colloidal silica increases the hardness of sol–gel coatings [19–22]. Wu et al. [22] have prepared hard hydrophobic sol–gel coating with WCA of 123 ◦ ∗ Corresponding author. E-mail address: lakshmi rv@nal.res.in (R.V. Lakshmi). using colloidal silica. Colloidal silica has also been used in obtain- ing organic-inorganic hybrid films exhibiting superhydrophobicity [23–25]. Nakajima et al. [23] have prepared roughened silica films using colloidal silica and were made superhydrophobic by deposit- ing an ethanolic solution of heptadecafluorodecyltrimethoxysilane (FAS-17). Hikita et al. [24] have prepared super-liquid-repellent surfaces using colloidal silica nanoparticles and FAS-17. The coat- ings have shown good repellency for both water and dodecane. Bravo et al. [25] have created transparent superhydrophobic films using colloidal SiO 2 nanoparticles of different size and a fluorosi- lane for the surface modification to obtain WCA > 150 ◦ . However, there was no discussion on the mechanical properties of the coat- ings. In the present work, a simple method to fabricate hard super- hydrophobic sol–gel nanocomposite coatings using hydrophilic silica particles is described. Fumed silica particles of size 25–30 nm were incorporated in a hybrid sol of nano-sized colloidal silica and methyltriethoxysilane (MTEOS) to create appropriate roughness on the surface of the coatings. The prepared superhydrophobic coatings were investigated for its self-cleaning and mechanical properties. 2. Experimental 2.1. Materials Colloidal silica [LUDOX ® LS, solid contents 30 wt%, aqueous dis- persion, particle size 12 nm, pH 8.2] was procured from Sigma Aldrich and was used as received. MTEOS was purchased from Fluka 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.06.122