Sinha,Sujeet Kumar
Email Address
mpesks@nus.edu.sg
Organizational Units
102 results
Publication Search Results
Now showing 1 - 10 of 102
Publication Carbon nanotube reinforced polyimide thin-film for high wear durability(2007-08) Satyanarayana, N.; Rajan, K.S.S.; Sinha, S.K.; Shen, L.; MECHANICAL ENGINEERINGIn this paper, the influence of single walled carbon nano tubes (SWCNTs) addition on the tribological properties of the polyimide (PI) films on silicon substrate was studied. PI films, with and without SWCNTs, were spin coated onto the Si surface. Coefficient of friction and wear durability were characterized using a ball-on-disk tribometer by employing a 4 mm diameter Si 3N 4 ball sliding against the film, at a contact pressure of ∼370 MPa, and a sliding velocity of 0.042 ms -1. Water contact angle, AFM topography, and nano-indentation tests were conducted to study the physical and mechanical properties of the films. SWCNTs marginally increased the water contact angle of PI film. The addition of SWCNTs to PI has increased the hardness and elastic modulus of pristine PI films by 60-70%. The coefficient of friction of PI films increased slightly (∼20%) after the addition of SWCNTs, whereas, there was at least two-fold increase in the wear life of the film based on the film failure condition of coefficient of friction higher than 0.3. However, the film did not show any sign of wear even after 100,000 cycles of rotation indicating its robustness. This increase in the wear durability due to the addition of the SWCNTs is believed to be because of the improvement in the load-bearing capacity of the composite film and sliding induced microstructural changes of the composite film. © Springer Science+Business Media, LLC 2007.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 Tribo-functionalizing Si and SU8 materials by surface modification for application in MEMS/NEMS actuator-based devices(2011-01-12) Singh, R.A.; Satyanarayana, N.; Kustandi, T.S.; Sinha, S.K.; MECHANICAL ENGINEERINGMicro/nano-electro-mechanical-systems (MEMS/NEMS) are miniaturized devices built at micro/nanoscales. At these scales, the surface/interfacial forces are extremely strong and they adversely affect the smooth operation and the useful operating lifetimes of such devices. When these forces manifest in severe forms, they lead to material removal and thereby reduce the wear durability of the devices. In this paper, we present a simple, yet robust, two-step surface modification method to significantly enhance the tribological performance of MEMS/NEMS materials. The two-step method involves oxygen plasma treatment of polymeric films and the application of a nanolubricant, namely perfluoropolyether. We apply the two-step method to the two most important MEMS/NEMS structural materials, namely silicon and SU8 polymer. On applying surface modification to these materials, their initial coefficient of friction reduces by ∼4-7 times and the steady-state coefficient of friction reduces by ∼2.5-3.5 times. Simultaneously, the wear durability of both the materials increases by >1000 times. The two-step method is time effective as each of the steps takes the time duration of approximately 1 min. It is also cost effective as the oxygen plasma treatment is a part of the MEMS/NEMS fabrication process. The two-step method can be readily and easily integrated into MEMS/NEMS fabrication processes. It is anticipated that this method will work for any kind of structural material from which MEMS/NEMS are or can be made. © 2011 IOP Publishing Ltd.Publication Effect of carbon embedding on the tribological properties of magnetic media surface with and without a perfluoropolyether (PFPE) layer(2011-08-10) Samad, M.A.; Yang, H.; Sinha, S.K.; Bhatia, C.S.; MECHANICAL ENGINEERINGCarbon embedding (≤1 nm) in the top surface of cobalt (∼100 nm) sputtered on a silicon surface is used as a surface modification technique to evaluate the tribological properties with or without an ultra-thin layer of perfluoropolyether (PFPE) lubricant. The carbon embedding is achieved using the filtered cathodic vacuum arc technique at an ion energy of 90 eV. Transport of ions in matter simulations, time-of-flight secondary ion spectroscopy, transmission electron microscopy and x-ray photoelectron spectroscopy (XPS) are used to study the carbon embedding profiles and surface chemical composition. The XPS results show that carbon embedding using the ion energy of 90 eV results in the formation of about 58 6% of tetrahedral (sp3) carbon hybridization. Furthermore, the XPS results also show that the carbon embedding is effective in improving the anti-oxidation resistance of cobalt. Ball-on-disk tribological tests are conducted at a contact pressure of 0.26 GPa on the modified cobalt surface with or without the PFPE layer. It is observed that the average coefficient of friction is reduced considerably from a value of approximately 0.7 to 0.42 after the surface modification. The coefficient of friction is further reduced to ∼0.26 after the deposition of an ultra-thin layer of PFPE over the modified surface, which is lower than a friction coefficient of 0.4 from commercial media. The modified cobalt surface also shows much better wear life than the present day commercial media. © 2011 IOP Publishing Ltd.Publication Nano-scratching as a means to study wear debris generation in polymers(2003) Sani, R.B.M.; Sinha, S.K.; Tan, J.P.Y.; Zeng, K.Y.; MECHANICAL ENGINEERINGScratches in the range of 1000 nin deep were produced on the surfaces of two polymers losing diamond tip indenter to study the material deformation and wear debris generation characteristics. Our results show that the process of wear debris generation depends upon the manner in which scratches are produced on the surface. Single track scratches do not produce wear debris despite several passes of the indenter tip whereas intersecting scratches accelerate the wear debris generation process. The results indicate that the material near the four comers of intersecting scratches experience extremely high strain leading to extrusion and eventual breakage from the bulk of the polymer forming a debris particle.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 Application of micro-ball bearing on Si for high rolling life-cycle(2010-01) Sinha, S.K.; Pang, R.; Tang, X.; MECHANICAL ENGINEERINGIn this paper, we introduce a new class of micro-ball bearing that can be applied between two Si surfaces in relative motion where wear is a problem. Wide channel was created on one Si plate for all the ball bearings to roll within this channel rather than in individual grooves. This type of micro-bearings can be applied as friction reducers to micro- and nano-machines. The tribometer set-up consisted of a top plate (Si wafer), which was connected to a conventional bearing, resting on a bottom plate (also Si wafer) that was rotated at 300 and 500 RPM by a DC motor. Borosilicate glass micro-spheres, 53±3.7 μm in diameter, were rotated between the two circular silicon plates (15 mm diameter) under a dead weight of 235 μN and in high-humidity conditions (75% relative humidity (RH)). Tests on the plate with wide channel consistently exceeded 1 million cycles of rotation without failure of the bearing. The main factors affecting the life-cycle are identified as the presence of a wide channel, ball dispersion, and alignment of the Si plates. © 2009 Elsevier Ltd. All rights reserved.