Highly wear resistant chemisorbed polar ultra-high-molecular-weight polyethylene thin film on Si surface for micro-syste
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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 mm 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. I. INTRODUCTION
Improvements in the tribological properties of the Si surface are important for applications in micro-electromechanical systems (MEMS) or microsystems. Si, without any modification, shows high friction, high adhesive force, and high wear. Organic ultra-thin films [such as self-assembled monolayers (SAMs) or polymer films] have shown good improvement in reducing the coefficient of friction, adhesive forces, and wear.1–5 Further developments in improving the lubrication properties of the Si surface (especially wear durability) are needed so that many novel and reliable microcomponents can be developed/produced from Si. In our recent studies, we have identified that thermoplastic polymers which have linear microstructure2 and with the addition of nano-filler materials6 show excellent tribological properties (low friction and low wear) when they are coated onto Si. Further, UHMWPE films dip-coated onto a Si surface (28 mm thick) have shown a wear life of 12,000 cycles when tested against a 4 mm diameter Si3N4 ball at a normal load of 70 mN and at a sliding velocity of 0.042 m/s.7 However, two limitations have been identified from that study: (i) the thickness of the polymer film is high (28 mm; especially for some micro-system applications) and (ii) the wear life is still low. Chemisorption of the UHMWPE film is selected to overcome the above
II. EXPERIMENTAL
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0397
Polished single crystal Si (100) wafers were used as the substrate. These Si wafers were cut into pieces of
J. Mater. Res., Vol. 24, No. 11, Nov 2009
http://journals.cambridge.org
two limitations because chemisorption improves adhesion between the Si surface and the UHMWPE film which eventually helps in improving wear durability. Chemisorption also helps in the control of fil
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