Adhesion and friction properties of micro/nano-engineered superhydrophobic/hydrophobic surfaces
Introduction
Large adhesion and friction forces are known detrimental issues in micro-electro-mechanical system (MEMS) applications due to large surface area to volume ratio and minute spacing between structures in MEMS [1], [2]. These issues can be alleviated by either increasing surface roughness through surface texturing or improving surface hydrophobicity through applying low surface energy materials to MEMS structural surfaces [1], [2]. Surface texturing can reduce the contact area and thus decrease adhesion and friction forces between surfaces. Improving surface hydrophobicity can reduce water adsorption on the surface and therefore reduce meniscus-mediated adhesion and friction forces between contacting surfaces. Since surface hydrophobicity significantly affects surface adhesion and friction properties, this paper focuses on producing superhydrophobic/hydrophobic surfaces and characterizing the wetting, adhesion, and friction properties of such surfaces.
Surface hydrophobicity is directly reflected by the water contact angle (WCA) of a surface. Surfaces having WCAs below 90° are referred to as hydrophilic surfaces, and surfaces having WCAs above 90° and 150° are referred to as hydrophobic and superhydrophobic surfaces, respectively. Chemically modifying smooth silicon surfaces can only lead to hydrophobic surfaces having WCAs of up to 120° [3]. To achieve superhydrophobic surfaces, textured surfaces are necessary. The topography effect on WCA can be explained by the Cassie–Baxter model [4], which assumes that a water droplet contacts the top of a rough surface with air trapped between the rough asperities on the surface. The apparent WCA, θ*, is given bywhere θ is the WCA of a smooth surface that has the same chemistry as the rough surface, f1 is the surface area fraction of the solid, f2 is the surface area fraction that are voids, and f1 + f2 = 1. Thus, with proper surface texturing and surface chemical modification, it is possible to produce a surface having a θ* much larger than θ [5], [6], [7].
Even though numerous methods of creating superhydrophobic surfaces have been reported, including using nanofibers and aligned carbon nanotubes [8], employing aligned nanorods [9], dissolving polypropylene in solvents at low temperatures [10], etching through oxygen plasma [11], depositing electrochemically [12], and many others, each of these methods has advantages and limitations. For example, they are either not MEMS process compatible or not easy to be applied to MEMS structures or too complicated to fabricate.
Here we report a MEMS process-compatible method for generating superhydrophobic surfaces for the purpose of reducing adhesion and friction forces in MEMS. The method includes using aluminum-induced crystallization (AIC) of amorphous (a-Si) technique to produce silicon micro/nano-textured surfaces (MNTSs) on silicon substrates and applying octadecyltrichlorosilane (OTS) self-assembled monolayers (SAMs) on the textured surfaces to reduce surface energies of the textured surfaces.
OTS SAM has been studied extensively to improve tribological performances of smooth surfaces [13], [14], [15]. AIC of a-Si has been used to produce polycrystalline silicon films with large grains for electronic and photovoltaic applications [16], [17], [18], [19], [20], [21]. The technique, however, has not been used to produce MNTSs for tribological application by other researchers. This research integrates AIC of a-Si and OTS SAM to produce superhydrophobic/hydrophobic surfaces for the purpose of reducing surface adhesion and friction forces.
Section snippets
Fabrication of textured surfaces by AIC of a-Si
One-side polished silicon (100) wafers were selected as substrates for producing MNTSs using the AIC of a-Si technique. The silicon wafers were cleaned by acetone, isopropanol, and deionized (DI) water and then wet oxidized at 950 °C for 8 h to grow about 2 μm-thick silicon oxide layers. The silicon oxide layers were used to prevent the crystalline orientation of the substrates from affecting the crystallization of a-Si process.
Amorphous silicon films were then deposited on the oxidized wafers
Topographies of MNTSs
Fig. 1 shows representative SEM micrographs of an oxidized silicon wafer and a micro/nano-textured sample. Fig. 1(a), taken from the oxidized silicon wafer with magnification of 5000×, shows that the oxidized silicon wafer surface is smooth. Fig. 1(b), taken from the micro/nano-textured sample with magnification of 5000×, shows that, after the AIC of a-Si process, the surface of a silicon wafer was textured by irregularly-shaped micro- and nano-scale islands with various sizes. To obtain better
Conclusions
Micro/nano-engineered surfaces by AIC of a-Si and OTS SAM modification were fabricated. The effects of OTS SAM modification and surface micro/nano-texturing by AIC of a-Si on the wetting, adhesion and friction properties of silicon substrates were studied. The results show that the combination of OTS SAM modification and AIC of a-Si micro/nano-texturing can reduce the adhesion force by more than 90% and create superhydrophobic surfaces with a WCA of 155° and a sliding angle of smaller than 1°.
Acknowledgments
This material is based upon the work supported by the National Science Foundation under Grants CMMI-0645040 and DMR-0520550. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. Equipment funding supports from Arkansas Biosciences Institute and the Oak Ridge Associated Universities are greatly acknowledged.
References (26)
- et al.
Thin Solid Films
(2001) - et al.
Wear
(2006) - et al.
Thin Solid Films
(2005) - et al.
Sol. Energy Mater. Sol. Cells
(2001) - et al.
J. Non-Cryst. Solids
(2002) - et al.
Wear
(2003) J. Adhes. Sci. Technol.
(2003)- et al.
J. Vac. Sci. Technol. B
(1997) - et al.
Nanotechnol.
(2005) - et al.
Trans. Faraday Soc.
(1944)
Nano Lett.
Adv. Mater.
Nano Lett.
Cited by (86)
Anti-friction mechanism of textured artificial joint material under the walking conditions of human
2022, Journal of Materials Research and TechnologyTest parameter and material dependence of the frictional properties of core-shell nanostructure textured surfaces
2022, Tribology InternationalOne-pot synthesis and self-assembly of anti-wear octadecyltrichlorosilane/silica nanoparticles composite films on silicon
2020, Applied Surface ScienceCitation Excerpt :One approach is chemical modification with self-assembled monolayers (SAMs) [2], vapor phase lubricants [3] or mussel-inspired chemistry [4–7] to create hydrophobic surfaces, because most hydrophobic surfaces can reduce surface energy, which can eliminate capillary force and thus reduce adhesion and friction between contact surfaces [8,9]. The other is surface roughening to reduce effective contact area, which can further reduce adhesion [10,11]. However, the structures textured by surface roughening are easily damaged by abrading.
Tribological performance of surface with different wettability under ball-on-disc test
2020, Applied Surface Science