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).

Ultrathin dual-layer ionic liquid lubrication film with bonded and mobile phases on SAM modified Si substrate.Figure optionsDownload as PowerPoint slideResearch highlights▶ Two kinds of dual-layer ultrathin ionic liquid lubrication films were formed on a silicon substrate by a two-step self-assembly process. The synergic effect between flowable mobile phase and steady bonded phase plays a significant role in improving tribological properties of the ionic liquid lubrication film. ▶ More densely packed SAMs produce more densely packed bonded phase and reduce meniscus effect resulting from excessive mobile molecules because more ionic liquid molecules are chemically bonded thereon, which would lead to low micro/nano-friction coefficient of dual-layer films. ▶ The improved durability of dual-layer films is closely related to high load-carrying capacity of more densely packed and ordered bonded phase.