Myo Minn

Effects of interfacial energy modifications on the tribology of UHMWPE coated Si

This paper presents results on the tribological properties of ultra-high molecular weight polyethylene (UHMWPE) films on Si substrates modified by interfacial layers of different surface energies (hydrophobicity). Five different interfaces, namely bare Si (i.e. no interfacial modification), heated Si, Si/3-aminopropyltrimethoxysilane (Si/APTMS), hydrogenated Si (Si–H) and Si/octadecyltrichlorosilane (Si/OTS) were prepared on a Si substrate by physical and chemical methods that gave water contact angles of the interfaces as 21°, 46°, 52°, 71° and 104°, respectively. UHMWPE (6 µm thickness) was coated onto surface energy modified Si substrates by the dip-coating method. The tribological tests were conducted using a ball-on-disc tribometer with a 4 mm diameter silicon nitride ball as the slider. Bare Si, which was the most hydrophilic of all, and Si/OTS, the most hydrophobic interface, showed much lower wear resistance when compared with heated Si, Si/APTMS and Si–H interfaces, with Si–H interface giving the best result. The results of a scratch test for adhesion strength between the interface and UHMWPE were consistent with the tribological properties of the films. It is concluded that the wear performance of ultra-thin UHMWPE films on Si substrates could be extended by several orders of magnitude by suitably modifying the interfaces (different surface energies) of the silicon substrates. The coefficient of friction also follows a similar trend with Si–H interface giving the lowest value among all samples.

Molecular Simulation of the Frictional Behavior of Polymer-on-Polymer Sliding

Molecular simulations of the sliding processes of polymer-on-polymer systems were performed to investigate the surface and subsurface deformations and how these affect tribological characteristics of nanometer-scale polymer films. It is shown that a very severe deformation is localized to a band of material about 2.5 nm thick at the interface of the polymer surfaces. Outside of this band, the polymer films experience a uniform shear strain that reaches a finite steady-state value of close to 100%. Only after the polymer films have achieved this steady-state shear strain do the contacting surfaces of the films show significant relative slippage over each other. Because severe deformation is limited to a localized band much thinner than the polymeric films, the thickness of the deformation band is envisaged to be independent of the film thickness and hence frictional forces are expected to be independent of the thickness of the polymer films. A strong dependency of friction on interfacial adhesion, surface roughness, and the shear modulus of the sliding system was observed. Although the simulations showed that frictional forces increase linearly with contact pressure, adhesive forces contribute significantly to the overall friction and must therefore be accounted for in nanometer-scale friction. It is also shown that the coefficient of friction is lower for lower-density polymers as well as for polymers with higher molecular weights.

Friction and Wear Studies of Ultra-Thin Functionalized Polyethylene Film Chemisorbed on Si with an Intermediate Benzophenone Layer

A successful attachment of an ultra-thin (thickness approximately 20 nm) functionalized polyethylene (fPE) film onto Si substrate via a reactive benzophenone (Ph2CO) layer is presented in this chapter. The wear durability results obtained under 40 mN applied load and 0.052 m/s sliding speed show an improvement in the wear life of Si/Ph2CO/fPE by one order of magnitude over Si/Ph2CO. When perfluoropolyether (PFPE) is applied as a top mobile lubricant layer coated onto Si/Ph2CO and Si/Ph2CO/fPE, a significant improvement in the wear durability is observed as Si/Ph2CO/fPE/PFPE does not fail until one million cycles whereas Si/Ph2CO/PFPE has shown a wear life of 250,000 cycles. Si/Ph2CO/fPE/PFPE can withstand a minimum applied load of 150 mN at a sliding speed of 0.052 ms−1 without failure, providing a pressure × velocity (PV) limit of greater than 106.6 MPa ms−1.

Tribological Properties of Thin Functionalized Polyethylene Film on Si With Chemical Modifications

A very thin functionalized polyethylene (fPE) film is successfully attached to Si substrate via a reactive benzophenone (Ben) layer. The presence of fPE promotes the wear durability of Si/Ben/fPE to 1000 cycles compares with 100 cycles of Si/Ben under 40 mN applied load and 500 rpm sliding speed. As an enhancement, perfluoropolyether (PFPE) is applied as a top mobile lubricant layer coated onto Si/Ben and Si/Ben/fPE and their tribological properties are evaluated under the same experimental conditions. A significant improvement in the wear durability is observed as Si/Ben/PFPE fails at 250,000 cycles and Si/Ben/fPE/PFPE does not fail till one million cycles. Si/Ben/fPE/PFPE can withstand the maximum applied load of 150 mN that provides the PV limit of 106.6 MPa ms−1 .Copyright