Molecular dynamics simulations demonstrate that adhesion strengths as a function of charge for aqueous nanodiamonds (NDs) interacting with a gold substrate result from an interdependence of electrostatics and surface functionalization. The simulations reveal a water layer containing Na counterions between a negative ND with surface -COO functional groups that is not present for a positively charged ND with -NH functional groups. The closer proximity of the positive ND to the gold surface and the lack of cancelation of electrostatic interactions due to counterions and the water layer lead to an electrostatic adhesion force for the positive ND that is nearly three times larger than that of the negative ND. Prior interpretations of experimental tribological studies of ND-gold systems suggested that electrostatics or surface functionalization could be responsible for observed adhesion strength differences. The present work demonstrates how these two effects work together in determining adhesion for this system.

The dynamics of cubo-octahedral nanodiamonds (NDs) with three different surface treatments and confined in aqueous environments between gold surfaces under shear and normal loading conditions have been characterized via molecular dynamics (MD) simulations. The treatments consisted of carboxyl (−COO–) or amino (−NH3 +) groups attached to the NDs, producing either negatively or positively charged NDs, respectively, and hydrogen-terminated surfaces producing neutral NDs. Simulations were performed in the presence and absence of induced image charges to explore the impact of electrostatic interactions on friction and surface deformation. Significant deformation of the gold surfaces was observed for negatively charged NDs placed between gold surfaces under external loads that were sufficient to displace water from the contact. Rolling and relatively high friction levels were also observed for the negatively charged NDs under the same conditions. In contrast, the neutral and positively charged NDs exhibited sliding behavior with only minor deformation of the gold surfaces. The results suggest that the size of the surface functional group plays a major role in determining whether NDs slide or roll on solid contacts. Higher friction levels were also observed in conjunction with induced image charges in the gold contacts. The results demonstrate how surface functionalization and surface-induced charges can work in combination to profoundly influence tribological performance.

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, TiO , 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.

In general, three different types of processes can be utilized to cut thick stainless-clad steel: waterjet cutting, mechanical cutting, and thermal cutting (plasma, flame, and EDM cutting). Because stainless steel contains a lot of Cr, Ni, and other elements with high ignition and melting temperatures, flame cutting is not recommended for direct cutting of stainless steel. Three different stainless-clad steel specimen thicknesses were prepared for this study, and cutting experiments were conducted utilizing the flame-cutting procedure. The tests demonstrate that using suitable flame-cutting parameters can yield the optimal cutting process parameters and successfully cut thick stainless-clad steel.