Resume

• 2014

  Doctor of Philosophy - PhD

  Physics

  North Carolina State University

• 2011

  National School of Sciences Lainchour

  North Carolina State University

  Kathmandu

  Lecturer

  National School of Sciences Lainchour

• 2009

  Master's degree

  Physics

  Tribhuvan University

• Tribology

  Matlab

  Atomic Force Microscopy

  Lubrication

  Ionic Liquids

  LabVIEW

  Nanoparticles

  Material Characterization

  OriginLab

  Additives

  TOF-SIMS

  Surface Science

  Contact Mechanics

  Scanning Electron Microscopy

  Undergraduate Teaching

  Nanotribological Performance Factors for Aqueous Suspensions of Oxide Nanoparticles and Their Relation to Macroscale Lubricity

  Quartz crystal microbalance (QCM) measurements of nanotribological properties of statistically diverse materials combinations of nanoparticles and substrate electrodes in aqueous suspensions are reported and compared to macroscale measurements of the same materials combinations for a subset of the nanoparticle combinations. Four ceramic nanoparticles

  TiO2

  SiO2

  Al2O3

  and maghemite (γ-Fe2O3) and ten substrate materials (Au

  Al

  Cr

  Cu

  Mo

  Ni

  Pt

  SiO2

  Al2O3

  and SS304) were studied. The QCM technique was employed to measure frequency and motional resistance changes upon introduction of nanoparticles into the water surrounding its liquid-facing electrode. This series of experiments expanded prior studies that were often limited to a single nanoparticle - solid liquid combination. The variations in QCM response from one nanoparticle to another are observed to be far greater than the variation from one substrate to another

  indicating that the nanoparticles play a larger role than the substrates in determining the frictional drag force levels. The results were categorized according to the direction of the frequency and motional resistance changes and candidate statistical performance factors for the datasets were generated. The performance factors were employed to identify associations between the QCM atomic scale results and the macroscale friction coefficient measurements. Macroscale measurements of friction coefficients for selected systems document that reductions (increases) in motional resistance to shear

  as measured by the QCM

  are linked to decreases (increases) in macroscale friction coefficients. The performance factors identified in the initial study therefore appear applicable to a broader set of statistically diverse samples. The results facilitate full statistical analyses of the data for identification of candidate materials properties or materials genomes that underlie the performance of nanoparticle systems as lubricants.

  Nanotribological Performance Factors for Aqueous Suspensions of Oxide Nanoparticles and Their Relation to Macroscale Lubricity

  The temperature at which TCP forms a thermal reaction film with Fe and 304SS surfaces has been measured in real time

  in situ liquid environments utilizing a QCM immersed in a synthetic dibasic ester base stock containing 5% TCP

  and observed to be 210 °C. The thermal reaction films were characterized by AFM

  and observed to be uniform on Fe and non-uniform and nodular in nature on 304SS substrates

  with the thicknesses in the range of 60–100 nm. The chemical composition of the thermal reaction films was studied with EDS. The methods employed here demonstrate an effective means for screening of reaction film formation

  and are extendable to systems involving multiple additives and/or complex metal composite materials.

  In situ

  real time studies of thermal reaction film formation temperatures for iron and 304SS surfaces immersed in 5% tricresyl phosphate in base oil

  Nanodiamonds are known to improve tribological performance when added to lubricants

  but their impact on additives that may already be present in the lubricant is poorly documented. Here

  we report on a study of their effects on thermal reaction films formed from tricresyl phosphate (TCP) on Fe substrates immersed in a dibasic ester basestock when blended with TCP. Thermal reaction film formation temperatures were recorded in-situ by monitoring the reaction film formation on both Fe and air baked Fe surfaces using a quartz crystal microbalance (QCM). The nanodiamonds were found to raise the thermal reaction film formation temperature by 18 °C

  possibly by raising the activation energy for the reaction

  but they were not observed to affect the thickness or rate of formation of the films. The nanodiamonds

  moreover

  were observed to trigger thermal reaction film formation on air baked Fe surfaces that otherwise were highly resistance to reaction film formation. The surface morphology

  roughness

  and thickness of the thermal reaction films

  as measured by atomic force microscopy (AFM)

  are reported as well as their chemical compositions

  as studied with Electron Dispersive X-ray Spectroscopy (EDS). The coefficients of friction measured on the thermal reaction films during dry solid–solid contact are also reported.

  Synergistic Effect of Nanodiamond and Phosphate Ester Anti-Wear Additive Blends

  Sliding friction levels of thin (1–2 monolayers) and thick (~10 monolayers) oxygen films adsorbed on nickel and gold at 47.5 K have been measured by means of a quartz crystal microbalance (QCM) technique. Friction levels for the thin (thick) films on nickel in the presence of a weak magnetic field were observed to be approximately 30% (50%) lower than those recorded in the absence of the external field. Friction levels for thin films on gold were meanwhile observed to be substantially increased in the presence of the field. Magnetically-induced structural reorientation (magnetostriction) and/or realignment of adlayer spins

  which respectively reduce structural and magnetic interfacial corrugation and commensurability

  appear likely mechanisms underlying the observed field-induced reductions in friction for the nickel samples. Eddy current formation in the gold substrates may account for the increased friction levels in this system. The work demonstrates the role of magnetic effects in model systems that are highly amenable to theoretical studies and modeling.

  Tuning Nanoscale Friction by Applying Weak Magnetic Fields to Reorient Adsorbed Oxygen Molecules

  Addition of nanoparticles to liquid lubricants often leads to a reduction in both friction and wear rates for a wide range of solid–liquid–nanoparticle combinations. While the lubricating properties of nanoparticles are well documented

  the detailed physical mechanisms remain to be fully explored. In a step toward such an understanding

  the nano-tribological properties of gold surfaces immersed in aqueous suspensions of negatively charged SiO2 nanoparticles were examined by means of Quartz Crystal Microbalance (QCM) and Atomic Force Microscopy methods. The SiO2 nanoparticles were found to reduce the resistance to shear motion at the QCM’s solid–liquid interface. The effect was observed to be concentration dependent

  with ca. 1.5 wt% yielding the maximum reduction in shear. An electrokinetic mechanism is proposed whereby the loosely bound nanoparticles roll and/or slide on the surface

  while upper layers of nanoparticles slip over the surface layer because of the repulsive electrostatic forces between the individual particles. The nanoparticles were observed to remove the electrode material from the gold surface and slightly increase the overall roughness with the major change happening within the first hour of the exposure. This study inherently provides insight into a complex interface of solid

  liquid and nanoparticles at a nanometer scale.

  A Combined QCM and AFM Study Exploring the Nanoscale Lubrication Mechanism of Silica Nanoparticles in Aqueous Suspension

  In this work

  the nano- and macroscale tribological properties of maghemite (γ-Fe2O3) nanoparticles in aqueous solution are compared and contrasted for alumina contacts as a function of nanoparticle concentration. A quartz crystal microbalance (QCM) technique was used for nanotribology measurements and a ball-on-disk method was used to measure macroscale friction coefficients. Statistical methods were employed to identify significant associations between the QCM and ball-on-disk measurements

  employing selected candidate performance factors for each system. In particular

  the macroscale response was parameterized by % reduction in the friction coefficient while candidate QCM “bulk” and “surface” performance factors were selected from functions of the frequency f and resistance Rm shifts upon addition of nanoparticles to the water surrounding the QCM. Incremental increases in concentration were performed and reductions in friction and drag forces were observed for concentrations up to 0.6 wt%

  after which further reductions were not observed. The factor δR_film/δf_film exhibited a linear correlation with the reduction in macroscale friction coefficient

  defined as the ratio of the shift in resistance to the shift in frequency attributable to interfacial effects and changes in the glide plane location. Atomic force microscopy was also utilized to both qualitatively and quantitatively determine surface roughness before and after particle uptake

  leading to the observations that particles are easily removed from the surface and do not significantly alter surface morphology.

  A Tribological Study of γ-Fe2O3 Nanoparticles in Aqueous Suspension

  We report an experimental Quartz Crystal Microbalance (QCM) study of tuning interfacial friction and slip lengths for aqueous suspensions of TiO2 and Al2O3 nanoparticles on planar platinum surfaces by external electric fields. Data were analyzed within theoretical frameworks that incorporate slippage at the QCM surface electrode or alternatively at the surface of adsorbed particles

  yielding values for the slip lengths between 0 and 30 nm. Measurements were performed for negatively charged TiO2 and positively charged Al2O3 nanoparticles in both the absence and presence of external electric fields. Without the field the slip lengths inferred for the TiO2 suspensions were higher than those for the Al2O3 suspensions

  a result that was consistent with contact angle measurements also performed on the samples. Attraction and retraction of particles perpendicular to the surface by means of an externally applied field resulted in increased and decreased interfacial friction levels and slip lengths. The variation was observed to be non-monotonic

  with a profile attributed to the physical properties of interstitial water layers present between the nanoparticles and the platinum substrate.

  Tuning friction and slip at solid-nanoparticle suspension interfaces by electric fields

  A design for a Quartz Crystal Microbalance (QCM) setup for use with viscous liquids at temperatures of up to 300 °C is reported. The system response for iron and gold coated QCM crystals to two common lubricant base oils

  polyalphaolefin and halocarbon

  is reported

  yielding results that are consistent with theoretical predictions that incorporate electrode nanoscale surface roughness into their analysis.

  Quartz crystal microbalance apparatus for study of viscous liquids at high temperatures

  Effective control of friction

  wear and adhesion has a vast range of applications

  including energy efficiency

  national security

  manufacturing

  pharmaceuticals and the environment. Existing lubrication technologies were developed in an era that focused on wear elimination over energy losses from friction

  with less consideration of environmental consequences. There is a pressing need for revolutionary new materials that would reduce friction and wear

  and also eliminate the harsh environmental impacts of current materials. This DMREF project seeks to develop and test a new approach for rational design of materialliquid-nanoparticulate systems to achieve superior tribological performance. Theory

  simulation

  statistics

  synthesis and characterization will be iteratively combined to develop design rules for controlling friction

  adhesion and wear at material-liquid-nanoparticulate interfaces.

  Biplav

  Acharya

  North Carolina State University