A minimized and efficient low temperature loading device for indentation

A minimized and efficient low temperature loading device cooling with Peltier coolers for indentation test is developed. Both specimen and indentation tip are immersed in 50% methanol-water solution, totally eliminating the contact thermal drift problem. Low temperature indentation tests down to 253.8 K can be realized within 10 min. The size of the device is miniaturized within 50 × 40 × 30 mm³, and no vacuum environment is required. Monocrystalline copper is tested to perform the feasibility of the device. Based on the simple structure and stable experimental effect, the developed device can be integrated into various types of current indentation devices to attach low temperature testing ability.

Understanding Adsorption Behaviors of Organic Friction Modifiers on Hydroxylated SiO2 (001) Surfaces: Effects of Molecular Polarity and Temperature

Molecular dynamics simulations are used to investigate the physisorption of organic friction modifiers (OFMs) lubricated by 1-decene trimer (PAO4) representing a base oil and confined between hydroxylated SiO₂ (001) surfaces. The results indicate that OFM molecules form dense, tendentiously vertical monolayer films at low temperature but loose adsorption layers at high temperature, particularly for R-NH₂ with weaker molecular polarity. The structural information is quantitatively clarified by mass density profiles, radial distribution function, and probability distributions of an end-to-end distance at a perpendicular-to-surface direction. The movement performance of lubricant, reflected by the thickness of the organic part and radius of gyration of PAO4 molecules, strongly depends on temperature. The adsorption amount of OFM molecules decreases dramatically with lowering OFM polarity and increasing temperature above the critical desorption temperatures of about 320, 373, and 453 K for amine (R-NH₂), alcohol (R-OH), and acid (R-COOH), respectively. The interaction energies of the OFM-surface decrease continuously for the R-NH₂ system with temperature and decrease rapidly as temperature exceeds a critical value for both R-OH and R-COOH systems. The single-molecule geometry optimization validates the significant role of the electrostatic and hydrogen-bond attractions in molecular adsorption. Therefore, the OFMs with stronger polarity (like R-COOH) present stronger adsorption and better temperature resistance. The findings in this work are of particular value and provide a guideline in designing and engineering novel OFM additives for extreme lubrication conditions.

The Influence of Lubrication and the Solid-Fluid Interaction on Thermodynamic Properties in a Nanoscopic Scratching Process

Liquid lubricants play an important role in contact processes; for example, they reduce friction and cool the contact zone. To gain better understanding of the influence of lubrication on the nanoscale, both dry and lubricated scratching processes in a model system are compared in the present work using molecular dynamics simulations. The entire range between total dewetting and total wetting is investigated by tuning the solid-fluid interaction energy. The investigated scratching process consists of three sequential movements: A cylindrical indenter penetrates an initially flat substrate, then scratches in the lateral direction, and is finally retracted out of the contact with the substrate. The indenter is fully submersed in the fluid in the lubricated cases. The substrate, the indenter, and the fluid are described by suitably parametrized Lennard-Jones model potentials. The presence of the lubricant is found to have a significant influence on the friction and on the energy balance of the process. The thermodynamic properties of the lubricant are evaluated in detail. A correlation of the simulation results for the profiles of the temperature, density, and pressure of the fluid in the vicinity of the chip is developed. The work done by the indenter is found to mainly dissipate and thereby heat up the substrate and eventually the fluid. Only a minor part of the work causes plastic deformation of the substrate.