Solid interfaces are ubiquitous in machinery, materials, and devices. From nanoscale to macroscale, friction, wear, and fracture at such interfaces contribute significantly to energy loss and component failure. On the other hand, unique interfacial phenomena observed on nano-/microscales offer exciting opportunities for unexpected discoveries and technologies addressing global challenges in energy efficiency, environmental sustainability, materials manufacturing, and innovative nano-/microdevices. In this talk, I will present three interrelated topics from my recent research: (1) structural superlubricity (achieving near-zero friction without the need for lubricants), including how interesting mechanical behaviors emerge on the microscale when friction is nearly zero, how edges limit superlubricity, and how defects cause its failure; (2) mechanochemistry (driving and controlling chemical reactions with interfacial forces), including a new insight from combining mechanochemistry with contact mechanics, how it enables reliable reaction kinetics measurements, and its application to in-situ formation of anti-wear coatings and to chemical reactions that constrain the lifetime of nano-electro-mechanical switches; and (3) nanofracture of layered materials, studied using atomic force microscopes, revealing graphene’s fracture anisotropy and mechanochemically-governed fracture initiation. Finally, I will discuss the implications from these fundamental studies, the limitations of the current work, and challenges to address in future research.

The fracture properties of graphene are critical for applications that require robust mechanical properties such as low-friction coatings, but conflicting results on fracture anisotropy and limited work on fracture initiation remain challenges. We developed an AFM-based method to determine graphene's fracture anisotropy and studied the kinetics of fracture initiation by sliding the tip against atomic step edges on graphite. Using naturally-formed atomic steps from exfoliating graphene, this method enables precise, high-throughput measurements. We show that zigzag (ZZ) direction has slightly lower fracture toughness than the armchair (AC) direction, with an anisotropy factor of 0.971. The dependence of fracture initiation rate on applied normal and shear stresses and the temperature agrees with stress-assisted thermal activation kinetics, as described by the Eyring model. This is used to determine the activation energy and activation volume for the fracture initiation process.

Activation volume is the key variable in mechanochemistry describing the effect of stress on reaction rate. However, its physical interpretation remains uncertain and significant discrepancies exist in recent tribology experiments. Here, we analyze the contact mechanics of the standard stress-assisted thermal activation model and find that, in some cases, a large correction is needed. We consider the force-dependent contact area and the nonuniform stress distribution, which were previously overlooked, leading to a correction function. For validation, we study the formation of antiwear tribofilms from zinc dialkyldithiophosphates (ZDDP). Combining colloidal-probe and regular AFM, we show that these and prior literature results, which are widely scattered if treated with the standard model, are in excellent agreement with our corrected model. This provides an accurate method for determining activation volumes, and provides insights for interpretating them for elucidating tribochemistry.

Nanoelectromechanical systems (NEMS) switches, a candidate for next-generation electronics for their negligible leakage and low operation voltage, suffer from poor reliability featured by various failure modes during cyclic operation. In this work, the durability of electrical contact materials is studied by scanning probe microscopy (SPM) under NEMS switch-like conditions, with the goal of understanding the tribo-electro-mechanical mechanisms leading to failure. We use an SPM-based methodology for high-throughput assessment of candidate contact materials, with a Pt/Pt interface studied as a prototypical demonstration. The evolution of interfacial properties is measured for millions to billions of contact cycles. The accumulation of insulating tribopolymers resulting from applied stress and bias to adsorbed airborne contaminants is investigated. Measurement on the tribopolymer growth rate and its dependence on contact stress supports a stress-assisted thermal activation model.