Preparation and tribological investigation of thin silicone films
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Polydimethylsiloxane (PDMS) films were prepared on silicon wafers by spin coating. The tribological properties of the PDMS films sliding against AISI-52100 steel ball were investigated with a dynamic static friction coefficient measurement apparatus in a ball-on-plate configuration. It was found that crosslinking, end grafting, and chemical characteristics of the substrate surfaces combined to play an important role in keeping the films’ stability and improving the friction-reduction and wear-resistant properties. It was also noted that crosslinked PDMS films on hydroxylated substrates exhibited the most stable friction coefficient and the longest antiwear life, while a friction coefficient below 0.01 was observed for the crosslinked PDMS films on hydrophobic substrates. The latter might be attributed to the weak interaction at the interface of PDMS layer and the self-assembled monolayer. The friction coefficient appeared to be velocity dependent. At higher sliding velocities, the films showed more stable and smaller friction coefficients; this might be attributed to the shear thinning effect.
I. INTRODUCTION
Siloxane polymers have long been widely used in many technical systems because of their unique properties including viscosity-temperature characteristics, low and high temperature stability, compressibility, low toxicity etc.1,2 Polydimethylsiloxane (PDMS) is the most widely used siloxane material due to its notable very low surface tension because of the flexible siloxane main chain and the alignment of pendant methyl groups.3 So, PDMS and also other siloxane polymers have potential applications as the adhesion-resistant and frictionreducing layer in microelectromechanical systems (MEMS), where the applied loads between sliding pairs are so small that the van der Waals interaction, the capillary interaction, and cohesive interaction will together determine the value of friction force, and the durability of sliding pairs should no longer be neglected.4 When PDMS films are used as friction-reduction coatings, the mechanical degradation and leakage of the coating material are unavoidable problems. This can partly be overcome by crosslinking, covalent attachment to substrates, or forming copolymer films with other polymers.5–8 However, the effect of the substrate surface characteristics as well as the chemical states (such as crosslinking, interaction with the substrate surface, etc.) of silicone films on the tribological properties has not yet been well elaborated. The chemical characteristics of substrate surface can be changed by incorporation of J. Mater. Res., Vol. 17, No. 9, Sep 2002
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self-assembling monolayers thereon,9,10 which will pose different interactions between the self-assembled monolayer (SAM) and the silicone layer. This article deals with the preparation of thin silicone films on silicon wafers by making use of divinyl-endgroup-functionalized PDMS and the investigation of the tribological properties of the films. The effect of the substrat
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