Myo Minn
Email Address
mpemyom@nus.edu.sg
Organizational Units
18 results
Publication Search Results
Now showing 1 - 10 of 18
Publication DLC and UHMWPE as hard/soft composite film on Si for improved tribological performance(2008-04-25) Minn, M.; Sinha, S.K.; MECHANICAL ENGINEERINGThe main purpose of this study is to explore the advantages of using a composite thin film of ultra high molecular weight polyethylene (UHMWPE) on a hard diamond like carbon (DLC) coating deposited on Si, for high wear life and low coefficient of friction. The experiments are carried out using a ball-on-disc tribometer at a constant linear speed of 0.052 m/s. A 4 mm diameter silicon nitride ball with a normal load of 40 mN is used as the counterface. The tribological results are discussed on the basis of hardness, elastic modulus, contact area, contact pressure and optical images of surface films. As a result of higher load carrying capacity (high hardness and elastic modulus), the wear life of Si/DLC/UHMWPE coated layer is approximately five times greater than that of Si/UHMWPE. Looking at the film thickness effect, UHMWPE film shows maximum wear resistance when the film is of optimum thickness (6.2 μm-12.3 μm) on DLC. Wear mechanisms of different UHMWPE thicknesses for Si/DLC/UHMWPE film are explained using optical microscopy of worn surfaces. Further, the use of perfluoropolyether (PFPE) ultra-thin film as the top layer on the composite coatings reduces the coefficient of friction to very low values (0.06-0.07) and increases the wear life of the films by several folds. © 2008 Elsevier B.V. All rights reserved.Publication Tribological properties of ultra-thin functionalized polyethylene film chemisorbed on Si with an intermediate benzophenone layer(2011-01) Minn, M.; Soetanto, Y.S.G.; Sinha, S.K.; MECHANICAL ENGINEERINGIn this study, an ultra-thin (~20 nm) functionalized polyethylene (fPE) film is successfully attached to Si substrate via a reactive benzophenone (Ph 2CO) layer. The presence of fPE promotes wear durability of Si/Ph 2CO/fPE to 1,000 cycles compared with 100 cycles for Si/Ph 2CO and nearly zero wear life for bare Si in a ball-on-disk (4-mm-diameter Si 3N 4 ball) wear test under 40 mN applied normal load and 500 rpm sliding speed. As an enhancement to the wear life, perfluoropolyether (PFPE) is applied as a top mobile lubricant layer coated onto Si/Ph 2CO and Si/Ph 2CO/fPE. A significant improvement in the wear durability is observed as Si/Ph 2CO/PFPE fails at 250,000 cycles and Si/Ph 2CO/fPE/PFPE does not fail until one million cycles. Si/Ph 2CO/fPE/PFPE can withstand a minimum applied load of 150 mN at a sliding speed of 0.052 ms -1 without failure, providing a PV (pressure x velocity) limit of greater than 106.6 MPa ms -1. © 2010 Springer Science+Business Media, LLC.Publication Polymer jet printing of SU-8 micro-dot patterns on Si surface: Optimization of tribological properties(2011-05) Tay, N.B.; Minn, M.; Sinha, S.K.; MECHANICAL ENGINEERINGTribological properties of optimized SU-8 patterns (micro-dots with varying pitch) on Si (silicon) were evaluated using a ball-on-disk tribometer. Sliding tests on the patterns were conducted against a 2-mm diameter Si 3N 4 ball at varying normal loads and sliding velocities. It was observed that the pitch of the SU-8 pattern on Si substrate had a significant effect on the initial coefficient of friction and wear durability. Initial coefficient of friction studies have concluded that the SU-8 polymeric micro-dots improved the tribological properties by sharing the normal force and reducing the contact area. For the wear durability test, ultra-thin layer of perfluoropolyether was over-coated onto SU-8 micro-dot specimens, and the optimized pitch specimens have shown wear durability of more than 100,000 cycles at a normal load of 350 mN. © 2011 Springer Science+Business Media, LLC.Publication Friction and wear properties of DLC/UHMWPE composite film(2008) Minn, M.; Sinha, S.K.; MECHANICAL ENGINEERINGIn this work, we focus on the advantages of soft (ultra high molecular weight polyethylene, UHMWPE) and hard (diamond like carbon, DLC) composite film over either soft or hard film alone for tribological performance. Thin hard tetrahedral amorphous carbon, ta-C (DLC) is deposited onto Si substrate. Micron scale thin soft UHMWPE is coated onto Si and Si/DLC films by simple dip coating method. In order to get higher tribological performances, perfluoropolyether (PFPE) is coated onto Si/UHMWPE and Si/DLC/UHMWPE. The friction and wear tests are carried out using a custom-built high speed tribometer at a constant speed of 500rpm. A 4mm diameter silicon nitride ball under a normal load of 4g is used as counterface. The nanoscratch tests show that the load carrying capacity of Si/DLC/UHMWPE film is superior to that of Si/UHMWPE. The coefficients of friction of UHMWPE film on Si and Si/DLC are 0.18 and 0.13, respectively. After applying PFPE on UHMWPE, the coefficients of friction drop to 0.06 and 0.07 for Si/UHMWPE and Si/DLC/UHMWPE respectively. Hard DLC film can provide high load carrying capacity, high hardness, high penetration resistance and low contact area for the soft polymer film. The wear durability of Si/DLC/UHMWPE is approximately five times higher than that of Si/UHMWPE. Overcoating of PFPE further extends wear durability by approximately five times for Si/UHMWPE and three times for Si/DLC/UHMWPE (experiments stopped). This study shows that the combination of soft and hard composite film is better than either soft or hard film alone for tribological properties. Copyright © 2007 by ASME.Publication A tribological study of SU-8 micro-dot patterns printed on Si surface in a flat-on-flat reciprocating sliding test(2011-11) Beng, N.; Minn, T.M.; Sinha, S.K.; MECHANICAL ENGINEERINGTribological properties of optimized SU-8 micro-dot patterns on Silicon (Si) were evaluated using a flat-on-flat tribometer. Sliding tests on the patterns were conducted against a SU-8 spin-coated 2×2 mm 2 Si substrate at varying normal loads at a fixed rotational speed. It was observed that the pitch of the SU-8 pattern on Si substrate had significant influence on the wear durability. Ultra-thin layer of perfluoropolyether was over-coated onto SU-8 micro-dot patterned specimens for enhanced wear durability, and the specimen of the optimized pitch 450 m has shown a wear life of more than 100,000 cycles at a normal load of 650 mN. © Springer Science+Business Media LLC 2011.Publication Effects of interfacial energy modifications on the tribology of UHMWPE coated Si(2008-03-07) Minn, M.; Jonathan, L.Y.; Sinha, S.K.; MECHANICAL ENGINEERINGThis 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 νm 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. © 2008 IOP Publishing Ltd.Publication Molecular orientation, crystallinity, and topographical changes in sliding and their frictional effects for UHMWPE film(2009-05) Minn, M.; Sinha, S.K.; MECHANICAL ENGINEERINGThis paper presents a study on the frictional anisotropy of semi-crystalline UHMWPE polymer film deposited on DLC-overcoated Si substrate. For UHMWPE film slid against a silicon nitride ball, there is a remarkable difference in the coefficient of friction between the forward and reverse directions after the slider has been initially slid against the film for certain number of cycles. The changes in the friction are greatly influenced by the initial number of sliding cycles. This frictional behavior is explained in terms of crystallinity change and molecular orientational effects on UHMWPE and micro-topographical effects due to the initial sliding. Nanoscratch test is conducted to understand the friction of the polymer film in the sliding track and the data are compared with the macroscale friction data. The results show that the friction in the reverse of the initial sliding direction is high in comparison to that in the forward direction and this behavior mainly depends upon the number of initial sliding cycles. The initial sliding cycles affect the crystallinity and molecular orientation of the film, as well as the film topography. This combined effect on the polymer film results in an anisotropic frictional behavior of the film. © 2009 Springer Science+Business Media, LLC.Publication Microtribological study of perfluoropolyether with different functional groups coated on hydrogen terminated Si(2012-01-15) Minn, M.; Satyanarayana, N.; Sinha, S.K.; Kondo, H.; MECHANICAL ENGINEERINGFriction and wear properties of different perfluoropolyether (PFPE) films with and without hydrogen termination on Si (Si-H) were studied using a ball-on-disk tribometer. The physical and chemical properties of the films were evaluated using contact angle measurement, atomic force microscopy and X-ray photoelectron spectroscopy. Coating of PFPEs onto bare Si has lowered the coefficient of friction (from 0.6 for Si to ∼0.05 with PFPE) and enhanced the wear durability (20,000 times) in comparison with those for bare Si which failed immediately. The introduction of hydrogen termination onto Si prior to PFPE coating has further increased the wear durability of PFPE with different functional groups several times (>5 times) under a normal load of 30 mN and a sliding speed of 0.052 m/s. © 2011 Elsevier B.V. All rights reserved.Publication Friction and wear durability studies on the 3D negative fingerprint and honeycomb textured SU-8 surfaces(2013) Myint, S.M.; Minn, M.; Yaping, R.; Satyanarayana, N.; Sinha, S.K.; Bhatia, C.S.; ELECTRICAL & COMPUTER ENGINEERING; MECHANICAL ENGINEERINGThe effects of two different textures (a 3D negative fingerprint texture and a honeycomb texture) on the tribological performance of SU-8 polymer surface have been investigated with a ball-on-disc tribometer. Friction and wear behaviors of the textured surfaces are conducted against a 4 mm diameter silicon nitride (Si3N4) ball counterface. The coefficient of friction for the negative fingerprint textured surface (μ=∼0.08) is much lower than that of the untextured surface (∼0.2) and the honeycomb textured surface (∼0.41) under a normal load of 100 mN and a rotational speed of 2 rpm. The coefficients of friction of the textured surfaces decrease with increasing normal loads between 100 mN and 300 mN. Above the normal load of 300 mN, the coefficient of friction of the negative fingerprint textured surface increases due to the occurrence of plastic deformation. The honeycomb textured surface has shown the highest coefficient of friction. The wear durability tests are also conducted at a normal load of 100 mN and a rotational speed of 500 rpm on the untextured/textured surfaces on SU-8 in the presence of an overcoat of a nano-lubricant, perfluoropolyether(PFPE). Six samples i.e. the untextured surface (Si/SU-8 and Si/SU-8/PFPE), the 3D negative fingerprint textured surface (Si/SU-8/FP and Si/SU-8/FP/PFPE) and the honeycomb textured surface (Si/SU-8/HC and Si/SU-8/HC/PFPE), each with and without PFPE nano-lubricant, have been investigated for their tribological behaviours. The negative fingerprint pattern on SU-8 with PFPE coating has shown the highest wear life of 60,000 cycles under a normal load of 100 mN. The reasons for excellent tribological performance of 3D fingerprinted SU-8 surface are analyzed using the Hertzian contact area calculation. © 2012 Elsevier Ltd.Publication The frictional behavior of UHMWPE films with different surface energies at low normal loads(2010-03-09) Minn, M.; Sinha, S.K.; MECHANICAL ENGINEERINGThe relationship between initial coefficient of friction, μi and surface energy (or attractive force) for ultra-high molecular weight polyethylene (UHMWPE) film is modeled and experimentally measured at low normal loads (15-75 mN). The results show that the initial coefficient of friction increases exponentially with increasing attractive force between a silicon nitride ball and visco-elastic soft UHMWPE film. There is a strong influence of the normal load on this relationship for the polymer surface in the above-mentioned load range. The magnitude of the initial coefficient of friction decreases for UHMWPE film in the higher normal load regime because of its visco-elastic and soft nature as earlier proposed by Bowden and Tabor. © 2009 Elsevier B.V. All rights reserved.