Duc-Cuong Pham

Hydrophobicity and micro-/nanotribological properties of polymeric nanolines

Abstract This paper presents an investigation of the effects of a topographical modification, namely, nanolines, on the hydrophobicity and micro-/nanotribological properties of poly(methyl methacrylate) surfaces. The polymeric line patterns were fabricated on the poly(methyl methacrylate) films by using the capillary force lithography technique. Examinations of the water contact angle revealed that the line patterns exhibited increased hydrophobicity compared to the flat film, along with an anisotropic wetting. It was observed that the presence of the nanolines greatly reduced the adhesion and micro-/nanofriction. Furthermore, the friction behaviour varied depending upon the sliding direction as the counter bodies slid over the line structure. It was also observed that the shape of the top of the nanolines noticeably influenced nanoscale adhesion and friction. Both the flat film and the nanolines were damaged in the microscale tests; however, the nanolines exhibited less damage than the film, presumably due to their enhanced adhesion and friction properties.

Tribochemical Interactions of Si-doped DLC Film Against Steel in Sliding Contact

This study concerns the effects of tribochemical interactions at the interface of Si-DLC (silicon-doped diamond-like carbon) film and steel ball in sliding contact on tribological properties of the film. The Si-DLC film was over-coated on pure DLC coating by radio frequency plasma-assisted chemical vapor deposition (r.f. PACVD) with different Si concentration. Friction tests against steel ball using a reciprocating type tribotester were performed in ambient environment. X-Ray photoelectron spectroscopy (XPS) and auger electron spectroscopy (AES) were used to study the chemical characteristics and elemental composition of the films and mating balls after tests. Results showed a darkgray film consisting of carbon, oxygen and silicon on the worn steel ball surface with different thickness. On the contrary, such film was not observed on the surface of the ball slid against pure DLC coating. The oxidation of Si-DLC surface and steel ball was also found at particular regions of contact area. This demonstrates that tribochemical interactions occurred at the contact area of Si-DLC and steel ball during sliding to form a tribofilm (so called transfer film) on the ball specimen. While the pure DLC coating exhibited high coefficient of friction (∼0.06), the Si-DLC film showed a significant lower coefficient of friction (∼0.022) with the presence of tribofilm on mating ball surface. However, the Si-DLC film possesses a very high wear rate in comparison with the pure DLC. It was found that the tribochemical interactions strongly affected tribological properties of the Si-DLC film in sliding against steel.

Solutions for friction reduction at nano/microscale for MEMS actuators-based devices

Micro-Electro-Mechanical Systems (MEMS) are miniaturized devices built at nano/micro-scale. MEMS are fabricated from semiconductor silicon. At nano/micro-scale, capillary force along with asperity deformation induces friction at the surfaces of moving elements of actuators-based devices. The magnitude of friction force so generated is of the same order as that of the forces driving the moving elements. Friction therefore resists the relative mechanical motion between moving elements and reduces the useful operating lifetime of actuator devices. Solutions to reduce friction at nano/micro-scale in silicon by surface modification include: (i) chemical modification i.e. thin lubricant films/coatings, (ii) topographical modification i.e. surface patterning and (iii) hybrid modification, i.e. combination of topographical and chemical modifications. Friction behavior at nano/micro-scale of silicon and modified silicon surfaces is presented and discussed in this paper. Amongst the different surface modification approaches, the hybrid modification shows the best results in terms of friction reduction.

Nano-Scale Tribological Properties of Silicon Pillars With the Variation in Pitch

Topographical modification of silicon (100) wafers was performed by the creation of nano-structures that consists of pillars with different pitch. The modified surfaces were investigated for their tribological characteristics at nano-scale in comparison with those of the bare silicon surface. Results showed that the modified surfaces have superior nano-tribological properties, as they reduce adhesion and friction forces significantly when compared to the bare silicon surface. Among the nano-structures, adhesion and friction forces reduce with the pitch. The topographical modification of silicon surfaces provides a promising solution to improve the tribological properties of miniaturized devices such as microelectromechanical systems (MEMS) in which silicon is a typically used material.Copyright

The Effect of Channelling a Polymeric Surface on Its Adhesion and Friction Characteristics

In recent years, a new approach towards enhancing tribological properties at small-scale by the fabrication of nano/micro-scale polymeric patterns using soft lithography has gained popularity. These patterned surfaces provide promising solutions to the tribological issues in small-scale devices such as micro/nano-electromechanical systems (MEMS/NEMS). In the present study, we investigate the nano-scale tribological properties of a novel topography, namely submicrometer-scale channels fabricated on thin polymeric films using a soft lithographic technique. Results show that these surfaces have superior nano-tribological properties, as they reduce adhesion and friction forces to a significant extent when compared to silicon wafer and non-patterned polymeric thin film.Copyright