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Explorations for fundamental mechanisms of friction and wear can lay foundation for their real-world applications, which are hindered by the complex contacting surfaces at the macroscale under various environmental conditions. In situ transmission electron microscope (TEM) techniques demonstrate the ability to conduct frictional studies for single-asperity contacts at the microscale and to link frictional behaviors with dynamically physical events near the interface, providing well-defined interface under well-defined conditions. In this work, we conducted systematically investigations on friction and wear mechanisms for submicron-sized single-asperity metallic contacts via in situ TEM triboprobes. First, we developed an in situ TEM approach to conduct friction tests. Taking advantage of the magnetic field inside TEM, a unique Lorentz-force-actuated method was developed via a commercial electromechanical holder. The friction force was obtained by tracking the elastic deflection of the fabricated cantilever from in situ video, with the normal force controlled by the built-in transducer of the holder. The transition processes from static to dynamic friction involved the elastic deformation, plastic deformation, and fracture near the interface for the elastic contact (W-W), the elastic-plastic contact (Ag-W), and the elastic-plastic contact with strong adhesion (Al-W), respectively. Experimental evidences demonstrated that the relative motion at the interface was not always commencing with the maximum friction force. And we propose that the arrival of maximum friction force was accompanied by the interfacial failure. Then, we conducted friction tests for single-asperity worn contacts to explore effects of adhesion and contact-area evolution on the friction law (the linear relationship between friction force and external normal force). For the elastic contact (W-W pair), the static coefficient of friction (SCOF) increased from 0.1 to 0.4 with the gradual wear of the nat

In situ bending tests of amorphous Si nanowires (a-Si NWs) found different elastic behavior depending on whether they were straight or curved to begin with. The axially straight NWs exhibit pure elastic deformation; however, the axially curved NWs exhibit obvious anelastic behavior when they are bent in the direction of original curvature. On the basis of STEM-EELS analysis, we propose that the underlying mechanism for this anelastic behavior is a bond-switching assisted redistribution of the nonuniform density (structure) in the curved NWs under the inhomogeneous stress field. This mechanism was further supported by the fact that the originally straight a-Si NWs also display similar anelasticity with the as-grown curved NWs after focused ion beam irradiation that can cause nonuniform structure distribution. As compared to what has been reported in other 1D materials, the anelasticity of a-Si NWs can be tuned by modifying their morphology, controlling the loading direction, or irradiating them via ion beam. Our findings suggest that a-Si NWs could be a promising material in the nanoscale damping systems, especially the semiconductor nanodevices. Copyright © 2019 American Chemical Society.

Despite their energy-efficient merits as promising light-weight structural materials, magnesium (Mg) based alloys suffer from inadequate corrosion resistance. One primary reason is that the native surface film on Mg formed in air mainly consists of Mg(OH)2 and MgO, which is porous and unprotective, especially in humid environments. Here, we demonstrate an environmentally benign method to grow a protective film on the surface of Mg/Mg alloy samples at room temperature, via a direct reaction of already-existing surface film with excited CO2. Moreover, for samples that have been corroded obviously on surface, the corrosion products can be converted directly to create a new protective surface. Mechanical tests show that compared with untreated samples, the protective layer can elevate the yield stress, suppress plastic instability and prolong compressive strains without peeling off from the metal surface. This environmentally friendly surface treatment method is promising to protect Mg alloys, including those already-corroded on the surface., China. Ministry of Science and Technology. National Key Research and Development Program (No. 2017YFB0702001), National Natural Science Foundation of China (51621063), National Natural Science Foundation of China (51601141), National Natural Science Foundation of China ( 51401239), Shaanxi Sheng (China). Science and Technology Department (2016KTZDGY-04-03), Shaanxi Sheng (China). Science and Technology Department (2016KTZDGY-04-04), China Postdoctoral Science Foundation (2016M600788), China University of Petroleum. Science Foundation (No. 2462018BJC005), China University of Petroleum. Science Foundation (C201603), National Science Foundation (U.S.) (ECCS-1610806)

In situ tensile tests show atypical defect motions in the brittle Na2Ti3O7 (NTO) nanowire (NW) within the elastic deformation range. After brittle fracture, elastic recovery of the NTO NW is followed by reversible motion of the defects in a time-dependent manner. An in situ cyclic loading–unloading test shows that these mobile defects shift back and forth along the NW in accordance with the loading–unloading cycles and eventually restore their initial positions after the load is completely removed. The existence of the defects within the NTO NWs and their motions does not lead to plastic deformation of the NW. The atypical defect motion is speculated to be the result of the glidibility of the TiO6 layers, where weakly bonded cation layers are in between. Exploration of the above novel observation can establish new understandings of the deformation behavior of superlattice nanostructures.

Due to its high diffusivity, hydrogen is often considered a weak inhibitor or even a promoter of dislocation movements in metals and alloys. By quantitative mechanical tests in an environmental transmission electron microscope, here we demonstrate that after exposing aluminium to hydrogen, mobile dislocations can lose mobility, with activating stress more than doubled. On degassing, the locked dislocations can be reactivated under cyclic loading to move in a stick-slip manner. However, relocking the dislocations thereafter requires a surprisingly long waiting time of ∼10[superscript 3] s, much longer than that expected from hydrogen interstitial diffusion. Both the observed slow relocking and strong locking strength can be attributed to superabundant hydrogenated vacancies, verified by our atomistic calculations. Vacancies therefore could be a key plastic flow localization agent as well as damage agent in hydrogen environment.

Particulate pollution has raised serious concerns regarding its potential impacts on human health in developing countries. However, much less attention has been paid to the threat of haze particles to machinery and industry. By employing a state-of-the-art in situ scanning electron microscope compression testing technique, we demonstrate that iron-rich and fly ash haze particles, which account for nearly 70% of the total micron-sized spherical haze particles, are strong enough to generate abrasive damage to most engineering alloys, and therefore can generate significant scratch damage to moving contacting surfaces in high precision machineries. Our finding calls for preventive measures to protect against haze related threat., National Basic Research Program of China (973 Program) (Grant 2012CB619402), National 111 Project of China (Grant B06025), National Science Foundation (U.S.) (Grants DMR-1120901 and DMR-1410636), National Natural Science Foundation (China) (Grants 51231005, 51471128 and 51321003)

“Smaller is stronger.” Nanostructured materials such as thin films, nanowires, nanoparticles, bulk nanocomposites, and atomic sheets can withstand non-hydrostatic (e.g., tensile or shear) stresses up to a significant fraction of their ideal strength without inelastic relaxation by plasticity or fracture. Large elastic strains, up to ∼10%, can be generated by epitaxy or by external loading on small-volume or bulk-scale nanomaterials and can be spatially homogeneous or inhomogeneous. This leads to new possibilities for tuning the physical and chemical properties of a material, such as electronic, optical, magnetic, phononic, and catalytic properties, by varying the six-dimensional elastic strain as continuous variables. By controlling the elastic strain field statically or dynamically, a much larger parameter space opens up for optimizing the functional properties of materials, which gives new meaning to Richard Feynman’s 1959 statement, “there’s plenty of room at the bottom.”, National Science Foundation (U.S.) (DMR-1240933), National Science Foundation (U.S.) (DMR-1120901)