Highly wear resistant chemisorbed polar ultra-high-molecular-weight polyethylene thin film on Si surface for micro-system applications

We report deposition and tribological studies of a chemisorbed UHMWPE (ultra-high-molecular-weight polyethylene) film on an Si surface. UHMWPE molecules containing carboxyl and hydroxyl chemical groups were chemisorbed onto an Si surface using an intermediate GPTMS SAM (glycidoxypropyltrimethoxy silane self-assembled monolayer) layer. The carboxyl and hydroxyl groups of UHMWPE molecules react with the terminal epoxy groups of GPTMS SAM during chemisorption. The resultant film (∼1.4 µm thick) has shown low coefficient of friction (∼0.1) and high wear life (exceeding 100,000 cycles) in a sliding test against a 4 mm diameter Si₃N₄ ball at a normal load of 0.3 N and a sliding velocity of 0.042 m/s measured on a micro-tribometer. In contrast, bare Si or GPTMS SAM modified Si has shown a higher coefficient of friction and failed within a few tens of sliding cycles. The high wear durability of the chemisorbed polymer film is attributed to the excellent adhesion of the UHMWPE film with the substrate due to chemisorption and to the good lubrication properties of UHMWPE molecules. This wear resistant film has potential applications in micro-electro-mechanical systems made of Si.