Friction, adhesion and wear durability of an ultra-thin perfluoropolyether-coated 3-glycidoxypropyltrimethoxy silane self-assembled monolayer on a Si surface

Abstract

The tribological properties, such as coefficient of friction, adhesion and wear durability of an ultra-thin (<10 nm) dual-layer film on a silicon surface were investigated. The dual-layer film was prepared by dip-coating perfluoropolyether (PFPE), a liquid polymer lubricant, as the top layer onto a 3-glycidoxypropyltrimethoxy silane self-assembled monolayer (epoxy SAM)-coated Si substrate. PFPE contains hydroxyl groups at both ends of its backbone chain, while the SAM surface contains epoxy groups, which terminate at the surface. A combination of tests involving contact angle measurements, ellipsometry, atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) was used to study the physical and chemical properties of the film. The coefficient of friction and wear durability of the film were investigated using a ball-on-disk tribometer (4 mm diameter Si3N4 ball as the counterface at a nominal contact pressure of ∼330 MPa). AFM was used to investigate the adhesion forces between a sharp Si3N4 tip and the film. This dual-layer film had a very low coefficient of friction, adhesion and wear when compared to epoxy SAM-coated Si only or bare Si surface. The reasons for the improved tribological performance are explained in terms of the lubrication characteristics of PFPE molecules, low surface energy of PFPE, covalent bonding between PFPE and epoxy SAM coupled with reduced mobile PFPE. The low adhesion forces coupled with high wear durability show that the film has applications as a wear resistant and anti-stiction film for microcomponents made from Si.