Biplav Acharya

Nanoparticles (NPs) can be highly beneficial as additives to lubricating fluids, and the tribotronic response of charged NPs tuned by external fields represents an area of great technological potential. Tribotronic response, however, is expected to be highly size dependent, which represents a significant design challenge. To explore this issue, quartz crystal microbalance and cyclic voltammetry were employed to characterize nanotribological and electrochemical behavior of platinum–nanofluid interfaces formed by aqueous suspensions of different-sized negatively charged titanium dioxide (TiO2) NPs. Suspensions of 5, 40, and 100 nm NPs were all observed to reduced interfacial frictional drag forces upon introduction into pure water in zero field conditions, with reductions for the 40 nm NPs about twice those of 5 nm particles at comparable concentrations. Suspensions of 100 nm NPs produced even greater reductions, but rapidly precipitated from the suspension when left unstirred. NPs were also driven to and from Pt electrode surfaces by applying external electric fields with varying amplitudes and modulation frequencies. For electric fields of sufficient amplitude and duration, the 40 nm TiO2 nanosuspension exhibited tribological properties consistent with a reversible electrophoretic deposition of the NPs, accompanied by changes in the electrochemical attributes and increasing interfacial drag. The 5 nm NP properties were consistent with progressive reductions in interfacial drag forces at the NP–suspension interface linked to field-induced increases in concentration.

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.

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.

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.

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.

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.