Integrated QCM-Microtribometry: Friction of Single-Crystal MoS₂ and Gold from μm/s to m/s

Abstract

Two opposing microtribometry approaches have been developed over the past decade to help connect the dots between fundamental and practical tribology measurements: spring-based (e.g., AFM) approaches use low speed, low stiffness, and long relative slip length to quantify friction, while quartz crystal microbalance (QCM)based approaches use high speed, high stiffness, and short relative slip length. Because the friction forces generated in these experiments are attributed to entirely different phenomena, it is unclear if or how the resulting friction forces are related. This study aims to resolve this uncertainty by integrating these distinct techniques into a single apparatus that allows two independent measurements of friction at a single interface. Alumina microspheres were tested against single-crystal MoS 2 , a model nominally wear-free solid lubricant, and gold, a model metal control, at loads between 0.01 and 1 mN. The combined results from both measurement approaches gave friction coefficients (mean ± standard error) of 0.087 ± 0.007 and 0.27 ± 0.02 for alumina-MoS 2 and alumina-gold, respectively. The observed agreement between these methods for two different material systems suggests that friction in microscale contacts can be far less sensitive to external effects from compliance and slip speed than currently thought. Perhaps more importantly, this Article describes and validates a novel approach to closing the "tribology gap" while demonstrating how integration creates new opportunities for fundamental studies of practical friction.

Robust vibration-activated lubricity

“…Finished MoS 2 samples were ~ 200 nm thick and 2 nm rough. AFM pro lometry revealed that these surfaces were basally oriented and atomically smooth with occasional ~ 0.7 nm high steps re ecting subsurface edge sites [39].…”

“…Two friction measurement devices, a custom microtribometer and an AFM, were used to vary the resonant properties of the system. The microtribometer comprised a high resolution nanopositioning stage (PI P-628.1CD) with a range of 800 µm and a resolution of < 2 nm for lateral reciprocation, a vertical nanopositioning stage (PI Q-545.240) with a range of 26 mm and a resolution of 6 nm, and two orthogonal capacitive displacement sensors (Lion Precision CPL290, C3S) for measuring normal and friction forces on cantilevers of varying stiffness (190-1200 N/m normal stiffness, 80-700 N/m lateral stiffness and 200-1000 Hz normal resonant frequencies) [39,40]. The AFM used in this study is the Dimension 3100V.…”

Charting New Depths for Understanding Friction in Micromachines

“…Professor Brian Borovsky, Associate Professor in the Physics Department at St. Olaf College in Northfield, MN, has been researching micro/nanotribology for over two decades [1][2][3][4]. He has pioneered friction research as applied to very tiny micromechanical machines, having developed state-of-the-art instrumentation and a process that tests frictional properties of surfaces coated with ultrathin lubricants.…”

“…He has pioneered friction research as applied to very tiny micromechanical machines, having developed state-of-the-art instrumentation and a process that tests frictional properties of surfaces coated with ultrathin lubricants. His is one of the few labs that can measure friction of micromachine surfaces sliding past each other at very high speeds that approach 1m/s [1].…”

Traceable Lateral Force Calibration (TLFC) for Atomic Force Microscopy

“…This section compares quantitative friction results from the AFM to those of a directly calibrated microtribometer of known uncertainty under matched conditions. Nominally identical 50 µm alumina colloids were mounted to the microtribometer cantilever (using methods described previously [21]) and a 40 N/m (normal stiffness) AFM cantilever (BudgetSensors Tap300Al-G) at a distance of 42 microns from the cantilevered end; based on reported dimensions and the measured length, the expected range for lateral stiffness was 150-750 N/m [19]. The microtribometer force sensor was calibrated directly with the microbalance (the gold standard).…”