Wear 252 (2002) 946–955 Wear stability of polymer nanocomposite coatings with trilayer architecture A. Sidorenko a , Hyo-Sok Ahn b , Doo-In Kim b , H. Yang a , V.V. Tsukruk a,∗ a Department of Materials Science and Engineering, Iowa State University, 3155 Gilman Hall, Ames, IA 50011, USA b Tribology Research Center, Korea Institute of Science and Technology, 39-1 Hawolgok-dong, Songbuk-gu, Seoul 136-791, South Korea Received 2 November 2001; received in revised form 12 March 2002; accepted 12 March 2002 Abstract A polymer trilayer (sandwiched) film with a thickness of 20–30 nm has been designed to serve as a wear resistant nanoscale coating for silicon surfaces. These surface structures are formed by a multiple grafting technique applied to self-assembled monolayers (SAM) and functionalized tri-block copolymer, followed by the photopolymerization of a topmost polymer layer. The unique design of this layer includes a hard-soft-hard nanoscale architecture with a compliant rubber interlayer mediating localized stresses transferred through the topmost hard layer. This architecture provides a non-linear mechanical response under a normal compression stress and allows additional dissipation of mechanical energy via the highly elastic rubber interlayer. At modest loads, this coating shows friction coefficient against hard steel below 0.06, which is lower than that for a classic molecular lubricant, alkylsilane SAM. At the highest pressure tested in this work, 1.2 GPa, the sandwiched coating possesses four times higher wear resistance than the SAM coating. The wear mechanism for this coating is stress and temperature induced oxidation in the contact area followed by severe plowing wear. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Wear stability; MEMS; Nanocomposite coatings; Boundary lubricants 1. Introduction The development of silicon-based micro-device technol- ogy has initiated interest in sophisticated coatings with a nanoscale thickness. The nano-mechanical behavior of such coatings plays an important role in the reliable operation of silicon surfaces under variable environmental conditions. The character of the micro-mechanical response at very small deformations (nanometer-scale indentation depths) is especially critical for nanoscale coatings in nano- and micro-electromechanical devices (NEMS and MEMS) [1]. For such devices, the dynamic state of mating surfaces crit- ically depends upon normal and shear stresses developed within a nanometer scale contact area [2–4]. Various ver- sions of protective compliant coatings have been proposed to reduce friction and adhesion between mating micro- scopic parts made mainly from silicon. Deposition of a molecularly thick organic layer via chemical self-assembly, so called self-assembled monolayers (SAMs), has been proposed and realized recently. Indeed, SAMs have dramatically reduced friction and adhesion and have found ∗ Corresponding author. Tel.: +1-515-294-6904; fax: +1-515-294-5444. E-mail address: vladimir@iastate.edu (V.V. Tsukruk). use in various MEMS devices [5–8]. However, alkyl chains do not sustain high compression and shear stresses, which significantly limits their useful life [9]. Recently, we have proposed that nanocomposite polymer layers capable of very large elastic deformation can exhibit superior nano- and micro-tribological properties [10,11]. Following this strategy, we fabricated a compliant layer of tri-block copolymer, grafted to a silicon surface using the anchoring properties of an epoxysilane surface [12]. We investigated both the chemical composition and micro-mechanical and tribological properties of the grafted SEBS layer [12–14]. This paper discusses further developments in the fabri- cation and micro-tribological studies of the nanocomposite coatings composed of a cross-linked polymer layer tethered to a compliant interlayer from a reinforced rubber phase. We report on a new design of this nanoscale coating with a total thickness below 30 nm, which demonstrates a non-linear nano-mechanical response and low friction as reported ear- lier [15]. The compliant rubber layer is grafted to a silicon surface via anchoring epoxy-silane SAM [16]. The rubber interlayer is covered by the tethered hard layer of a copoly- mer of methylmethacrylate (MMA) and 1,6-hexanediol dimethacrylate (HDM) as a cross-linker (Fig. 1). We choose the copolymer poly(methylmethacrylate–1,6-hexanediol dimethacrylate) (PMA) because of its high elastic modulus 0043-1648/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII:S0043-1648(02)00048-0