Tribology study of dual-layer ultrathin ionic liquid films with bonded phase: Influences of the self-assembled underlayer

Two kinds of ultrathin dual-layer films, which contain both bonded and mobile phases in ionic liquids layer, were fabricated on silicon substrates by a two-step process. As an anchor layer, (3-aminopropyl)triethoxylsilane (APS) and N-[3-(trimethoxylsilyl)propyl]ethylenediamine (DA) separately self-assembled on silicon surfaces, then a few ionic liquids molecules were chemically bonded to the silicon substrates modified by the self-assembled monolayers (SAMs) to form two-phase structure. The formation and surface properties of the films were analyzed by means of ellipsometric thickness measurement, water contact angle measurement, attenuated total reflection Fourier transform infrared (ATR-FTIR) spectrometry, multi-functional X-ray photoelectron spectra (XPS), and atomic force microscope (AFM), It is shown that DA form more densely packed films than APS, and the density of the amino-terminated underlayer largely affects the density of the subsequent bonded ionic liquids layer. The adhesive and nanotribological behaviors of the films were evaluated by a homemade colloidal probe. A ball-on-plate tribometer was used to test the microtribological performances of the films. As a result, the two kinds of dual-layer films show the improved tribological properties than the single-layer ionic liquids film deposited directly on the silicon surface, which is ascribed to synergic effect between flowable mobile phase and steady bonded phase. In particular, the dual-layer film containing DA-SAM possesses excellent micro/nanotribological properties characterized by lower friction and higher antiwear ability due to more densely packed bonded phase and a few mobile phases. By studying the influences of self-assembled underlayer on tribological properties of ultrathin dual-layer film, we might find the way to further improve tribological properties and acquire insights into their potential in resolving the tribological problems of micro-electromechanical systems (MEMS).