Nanoscale Adhesion and Material Transfer at 2D MoS2-MoS2 Interfaces Elucidated by In Situ Transmission Electron Microscopy and Atomistic Simulations.

Low-dimensional materials, such as MoS2, hold promise for use in a host of emerging applications, including flexible, wearable sensors due to their unique electrical, thermal, optical, mechanical, and tribological properties. The implementation of such devices requires an understanding of adhesive phenomena at the interfaces between these materials. Here, we describe combined nanoscale in situ transmission electron microscopy (TEM)/atomic force microscopy (AFM) experiments and simulations measuring the work of adhesion (Wadh) between self-mated contacts of ultrathin nominally amorphous and nanocrystalline MoS2 films deposited on Si scanning probe tips. A customized TEM/AFM nanoindenter permitted high-resolution imaging and force measurements in situ. The Wadh values for nanocrystalline and nominally amorphous MoS2 were 604 ± 323 mJ/m2 and 932 ± 647 mJ/m2, respectively, significantly higher than previously reported values for mechanically exfoliated MoS2 single crystals. Closely matched molecular dynamics (MD) simulations show that these high values can be explained by bonding between the opposing surfaces at defects such as grain boundaries. Simulations show that as grain size decreases, the number of bonds formed, the Wadh and its variability all increase, further supporting that interfacial covalent bond formation causes high adhesion. In some cases, sliding between delaminated MoS2 flakes during separation is observed, which further increases the Wadh and the range of adhesive interaction. These results indicate that for low adhesion, the MoS2 grains should be large relative to the contact area to limit the opportunity for bonding, whereas small grains may be beneficial, where high adhesion is needed to prevent device delamination in flexible systems.

Autonomous healing of fatigue cracks via cold welding.

Fatigue in metals involves gradual failure through incremental propagation of cracks under repetitive mechanical load. In structural applications, fatigue accounts for up to 90% of in-service failure1,2. Prevention of fatigue relies on implementation of large safety factors and inefficient overdesign3. In traditional metallurgical design for fatigue resistance, microstructures are developed to either arrest or slow the progression of cracks. Crack growth is assumed to be irreversible. By contrast, in other material classes, there is a compelling alternative based on latent healing mechanisms and damage reversal4-9. Here, we report that fatigue cracks in pure metals can undergo intrinsic self-healing. We directly observe the early progression of nanoscale fatigue cracks, and as expected, the cracks advance, deflect and arrest at local microstructural barriers. However, unexpectedly, cracks were also observed to heal by a process that can be described as crack flank cold welding induced by a combination of local stress state and grain boundary migration. The premise that fatigue cracks can autonomously heal in metals through local interaction with microstructural features challenges the most fundamental theories on how engineers design and evaluate fatigue life in structural materials. We discuss the implications for fatigue in a variety of service environments.

A modified multibond model for nanoscale static friction.

Several key features of nanoscale friction phenomena observed in experiments, including the stick-slip to smooth sliding transition and the velocity and temperature dependence of friction, are often described by reduced-order models. The most notable of these are the thermal Prandtl-Tomlinson model and the multibond model. Here we present a modified multibond (mMB) model whereby a physically-based criterion-a critical bond stretch length-is used to describe interfacial bond breaking. The model explicitly incorporates damping in both the cantilever and the contacting materials. Comparison to the Fokker-Planck formalism supports the results of this new model, confirming its ability to capture the relevant physics. Furthermore, the mMB model replicates the near-logarithmic trend of increasing friction with lateral scanning speed seen in many experiments. The model can also be used to probe both correlated and uncorrelated stick slip. Through greater understanding of the effects of damping and noise in the system and the ability to more accurately simulate a system with multiple interaction sites, this model extends the range of frictional systems and phenomena that can be investigated. This article is part of the theme issue 'Nanocracks in nature and industry'.

Ultrahigh strength and shear-assisted separation of sliding nanocontacts studied in situ.

The behavior of materials in sliding contact is challenging to determine since the interface is normally hidden from view. Using a custom microfabricated device, we conduct in situ, ultrahigh vacuum transmission electron microscope measurements of crystalline silver nanocontacts under combined tension and shear, permitting simultaneous observation of contact forces and contact width. While silver classically exhibits substantial sliding-induced plastic junction growth, the nanocontacts exhibit only limited plastic deformation despite high applied stresses. This difference arises from the nanocontacts' high strength, as we find the von Mises stresses at yield points approach the ideal strength of silver. We attribute this to the nanocontacts' nearly defect-free nature and small size. The contacts also separate unstably, with pull-off forces well below classical predictions for rupture under pure tension. This strongly indicates that shearing reduces nanoscale pull-off forces, predicted theoretically at the continuum level, but not directly observed before.

Covalent Bonding and Atomic-Level Plasticity Increase Adhesion in Silicon-Diamond Nanocontacts.

Nanoindentation and sliding experiments using single-crystal silicon atomic force microscope probes in contact with diamond substrates in vacuum were carried out in situ with a transmission electron microscope (TEM). After sliding, the experimentally measured works of adhesion were significantly larger than values estimated for pure van der Waals (vdW) interactions. Furthermore, the works of adhesion increased with both the normal stress and speed during the sliding, indicating that applied stress played a central role in the reactivity of the interface. Complementary molecular dynamics (MD) simulations were used to lend insight into the atomic-level processes that occur during these experiments. Simulations using crystalline silicon tips with varying degrees of roughness and diamond substrates with different amounts of hydrogen termination demonstrated two relevant phenomena. First, covalent bonds formed across the interface, where the number of bonds formed was affected by the hydrogen termination of the substrate, the tip roughness, the applied stress, and the stochastic nature of bond formation. Second, for initially rough tips, the sliding motion and the associated application of shear stress produced an increase in irreversible atomic-scale plasticity that tended to smoothen the tips' surfaces, which resulted in a concomitant increase in adhesion. In contrast, for initially smooth tips, sliding roughened some of these tips. In the limit of low applied stress, the experimentally determined works of adhesion match the intrinsic (van der Waals) work of adhesion for an atomically smooth silicon-diamond interface obtained from MD simulations. The results provide mechanistic interpretations of sliding-induced changes and interfacial adhesion and may help inform applications involving adhesive interfaces that are subject to applied shear forces and displacements.

Sliding History-Dependent Adhesion of Nanoscale Silicon Contacts Revealed by in Situ Transmission Electron Microscopy.

Nanoscale asperity-on-asperity sliding experiments were conducted using a nanoindentation apparatus inside a transmission electron microscope, allowing for atomic-scale resolution of contact formation, sliding, and adhesive separation of two silicon nanoasperities in real time. The formation and separation of the contacts without sliding revealed adhesion forces often below detectable limits (ca. 5 nN) or at most equal to values expected from van der Waals forces. Lateral sliding during contact by distances ranging from 3.7 µm down to as little as 20 nm resulted in an average 19× increase in the adhesive pull-off force, with increases as large as 32× seen. Adhesion after sliding increased with both the sliding speed and the applied normal contact stress. Unlike cold welding, where irreversible material changes like flow occur, these effects were repeatable and reversible multiple times, for multiple pairs of asperities. We hypothesize that sliding removes passivating surface terminal species, most likely hydrogen or hydroxyl groups, making sites available to form strong covalent bonds across the interface that increase adhesion. Upon separation, repassivation occurs within the experimentally limited lower bound time frame of 5 s, with full recovery of low adhesion. The results demonstrate the strong sliding history-dependence of adhesion, which hinges on the interplay between tribologically induced removal of adsorbed species and repassivation of unsaturated bonds on freshly separated surfaces by dissociative chemisorption.