In this study, we describe an experimental setup and a new approach for operando investigation of structural evolution of materials during wear and friction. The setup is particularly suited for ...testing various friction pairs, including those in which both rubbing bodies are made of metals. The developed device allows circumventing the problems related to significant scattering of X-rays produced by metals and makes it possible using “real samples” in synchrotron beamlines operating in reflection mode. To demonstrate the capabilities of the device and the proposed new approach, an iron-based massive sample was subjected to thousands of friction cycles using a cemented carbide pin. The material was probed with synchrotron X-ray radiation within a few milliseconds after leaving the friction zone. The results of the microstructural and structural analysis, as well as results obtained from diverse mathematical models, allowed us to evaluate several features, including gradual accumulation of defects, microstructural refinement, dislocation density changes, surface layer oxidation, as well as several other phenomena caused by the dry sliding friction process. Mainly, it was possible to conclude that the process of wear occurred due to the cooperative action of oxidation and plastic deformation, which began during the first cycle of frictional interaction and was manifested in increasing the dislocation density, whose type was changed gradually during testing. The number of defects quickly reached a threshold value and subsequently fluctuated around it due to periodically repeated processes of defect accumulation and stress relaxation resulting from material wear. It was also observed that friction led to the quick formation of a mechanically mixed layer, consisting of the sample material and a mixture of two types of iron oxide – hematite and magnetite. The delamination of this layer was probably the primary wear mechanism.
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Heterogeneous gradient nanostructure has been evidenced to significantly reduce the coefficient of friction and wear loss. However, the tribological performance and underlying microscopic mechanisms ...of gradient nanograined metals containing stable nanotwins with superior strength-ductility synergy are still unclear. In this work, an ultra-precision machining technique named single point diamond turning was developed to fabricate a stable gradient nanograined-nanotwinned layer on Cu-4.5%Al alloy (in which stacking fault energy was tailored through alloying of Al). The reciprocal dry-sliding tests under loads of 10 N and 35 N at room temperature were conducted on gradient nanograined-nanotwinned specimen and five Cu counterparts (uniformly refined grain sizes, nanotwins, and gradient nanograins). The gradient nanograined-nanotwinned specimen exhibits the unprecedented low steady-state coefficient of frictions (0.18–0.24), demonstrating further improvement in comparison with the gradient nanograined Cu (0.31–0.35). Moreover, scanning electron microscope, white light interferometer, and high-resolution transmission electron microscope observations were mainly employed to evaluate the microstructural evolutions of worn surfaces and subsurface layers. It reveals that the worn subsurface of stable gradient nanograined-nanotwinned specimen consists of grain refinement, slightly grain coarsening and dynamic recrystallization, and deformation layers along the depth direction, showing further effectively suppressed grain coarsening process relative to the gradient nanograined Cu. Accordingly, a twin boundary migration plus grain rotation-mediated grain coalescing process in the worn subsurface is elucidated to be responsible for such mechanical-structural stability of the gradient nanotwinned structure towards the superior tribological properties.
•A gradient nanograined-nanotwinned surface layer is achieved on Cu-4.5%Al alloy by an ultra-precision machining technique.•Gradient nanograined-nanotwinned Cu-Al alloy exhibits the unprecedented low coefficient of frictions of 0.18–0.24.•HRTEM observations reveal the microstructural evolutions of worn surfaces and subsurface layers.•The gradient nanograined-nanotwinned structure could effectively suppress the grain coarsening in the worn subsurface.•A twin boundary migration plus grain rotation-mediated grain coalescing mechanism in the worn subsurface is elucidated.
When the Kuroshio passes Green Island off Taiwan, well-organized wakes are formed by the interaction between the Kuroshio and the island. The vertical mixing in the wake produces cold water, which ...entrains from the mixed layer below the surface and results in relatively colder and saltier waters with a higher chlorophyll-a concentration on the sea surface to the lee of the island. The meander of the Kuroshio maximum velocity axis has a major influence on the island wake. The shift of the Kuroshio in front of Green Island not only weakens the flow velocity but also affects the development of the wake. In the Green Island wake, the density overturns with a Thorpe scale between 2.9 m and 20.5 m and the turbulent kinetic energy dissipation rate is 0.2 × 10−6–8.5 × 10−5 W kg−1, which corresponds to an eddy diffusivity in the range of 0.01–0.23 m2 s−1. The spatial distribution patterns of the sea surface temperature (SST) in the island wake are classified into four distinct types, as obtained from moderate-resolution imaging spectroradiometer (MODIS) SST images. The most frequently occurring type is of the wake alone, which accounts for 86.7% of the island wake patterns. The other three types are a wake with a tail stretching downstream (4.0%), a wake with a small cyclonic cold core but no tail stretching downstream (6.8%) and an S-shaped meandering wake (2.5%).
•The vertical structure of Kuroshio-induced Green Island wake is presented.•Four distinct types of surface pattern of the island wake are identified.•Wind force has an obvious impact on the spatial size of the island coldwake.
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•Pb and alkali depletion is shown on the polished surface of lead crystal.•The Pb leaching kinetic is rapidly limited by diffusion through a barrier layer.•In acetic acid, the silanol ...condensation explains the formation of the barrier layer.•Successive leaching and storage reduce the Pb release from lead crystal glass.
Lead crystal glass (24 wt% PbO) is one of the tableware materials that can release lead into the environment. A deep knowledge of the release of lead (Pb) due to food contact is necessary to control and minimize the human exposure to this heavy metal. The Pb leaching was characterized by elemental analysis (ICP-AES) of the 4 vol% acetic acid solution to simulate the food contact versus leaching time. The analysis of the subsurface layer (SIMS, STEM) allows to understand the change of composition and morphology. After an initial period of rapid leaching, the diffusion of lead out of the glass slows down, due to the formation of a blocking barrier of diffusion in the subsurface layer of lead crystal glass. Experiments, carried out with a D218O solution, highlight the different mechanisms of hydrolysis, then condensation of the silanol groups, explaining the decrease of the diffusion coefficient of Pb2+ by the formation of a “passive” sub-layer. This protective sub-layer can be reinforced by successive leaching processes or by the storage of lead crystal glass at high temperature after leaching.
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•We used artificial neural networks to predict surface and subsurface soil moisture.•The effects of topography, climate, soil, land use, and rainfall were evaluated.•Artificial neural ...networks efficiently modelled soil moisture.•Climate-related variables are most important for the performance of the surface model.•Soil-related variables are most important for the performance of the subsurface model.
Soil moisture, which impacts various hydrological processes, can be estimated from point measurements in a watershed or via remote sensing. As both methods are expensive and complex, efforts have been made to develop empirical models based on data that affect the occurrence of soil moisture. However, these models have been based only on surface-layer data. We present an original approach for investigating regional empirical models of soil moisture, both for the surface (0–10 cm) and subsurface (10–20 cm) layers, and evaluate the main factors which affect the model results. Based on data about the climate, soil properties, topographic features and rainfall, we applied artificial neural network soil-moisture models for a small watershed, in southern Brazil. The models for each layer, with all selected variables, showed Nash-Suitcliffe coefficients of 0.870 and 0.893, respectively, for the surface and subsurface models. We then tested the effects of removing each variable or categories of variables. The most important variables for the surface model were the season, followed by exponential weighted moving average (EWMA) of rainfall. For the subsurface model, the most important variables were the season (although less so than for the surface model), followed by microporosity. All of the variable categories were important in the surface model. In the subsurface model, the soil-related variables were the most important, whereas the rainfall and topography variables were of little importance. It was possible to estimate soil moisture for both layers with good performance. The subsurface model, which used only the soil- and climate-related variables, explained more of the variance in soil moisture than the other models. The subsurface layer is easier to model, because the variation in moisture that is induced by recent climate and precipitation effects is attenuated by the physical features of the soil which control water infiltration.
Subsurface Layer Temperature Inversion (SLTI) is a prominent physical process occurring in the Bay of Bengal (BoB) during winter. BoB also witnesses intense cyclones during winter (post-monsoon). ...Sidr is a category-5 cyclone that occurred in the BoB during 11–15 November 2007. The present study emphasises bringing out the effect of SLTI on SST warming. This paper uses observations and modelling to present SLTI's role in surface layer post-cyclone warming. In the absence of SLTI, the cyclone induces surface cooling. However, in the case of a surface layer with SLTI, instead of cooling, sea surface warming of < 0.5 °C is observed near the head bay, where SLTI is prominent. Model results and observations suggest the role of entrainment and warming of the surface layer.
●Subsurface Layer Temperature Inversion (SLTI) is a significant large-scale process in the Bay of Bengal.●This paper uses observations and modelling to present SLTI's role in surface layer post-cyclone warming.●Model results and observations suggest the role of entrainment and warming of the surface layer.
The crystalline lattice reorientation in copper single crystals resulted from friction-induced subsurface deformation has been studied by means of electron back scattering diffraction as well as ...optical microscopy. The results have been compared to those obtained earlier in uniaxial compression. The copper single crystals oriented by their normal load forces either along 110 or 11¯1¯ have been subjected to sliding tests during which the friction force direction was varied to assess the friction force orientation dependence on the crystalline lattice reorientation and segmentation. The results of dry sliding tests showed that the plastic deformation in copper single crystals depended on the crystal's orientation with respect to both normal and friction forces. Crystalline lattice reorientation in wear lip formation has been studied including reorientation by ridge-kink formations.
•Orientation dependencies of single crystal deformation by sliding have been studied using both numerical and experimental loading.•Segmentation and reorientation of sample zones occurs gradually and with respect to initial orientation.•Segmented grains form a misoriented subsurface layer.•Orientation dependent wear lip formation and slip plane reorientation considered.•Ridge-kink formation on the wear lip surface observed and studied.
This study presents an approach to extra-hard material removal using "medium-frequency vibration-assisted grinding with self-sensing grinding force" for producing a microgroove array on ...monocrystalline diamond (MCD). Vibration mode at nano-amplitude is employed to vibrate the MCD workpiece against the polycrystalline composite diamond (PCD) wheel-tool during grinding. This approach is used to break the covalent atomic bonds of diamond, creating a "nanocracked subsurface layer" thereby reducing grinding resistance during material removal. The PZT stage’s grinding system is designed with a low center of gravity, bilateral symmetry, and an in-situ co-shaft grinding devise. This setup provides a well-defined nanoscale depth of grinding. Expentmental results show that a highly consistent, smooth crisscross microgroove array can be successfully produced on MCD with no apparent burrs and chipping. It is confirmed that "medium-frequency vibration-assisted grinding with self-sensing grinding force" remarkably reduces grinding resistance and improves the surface finish on a MCD workpiece. In addition, these experiements prove that diamond’s innate charateristic of "thin-brittle resistance" self-generates a "nanocrack growth resistance boundary" stopping further crack expansion. Using a "self-sensing grinding force" design allows the system to determine whether the "nanocracked subsurface layer" has been completely removed. This ensures high quality grinding, which produces a flat, solid microgroove surface on MCD. Furthermore, detailed discussions are conducted for the following aspects: effects of dressing using spark erosion, influences of amplitude and grinding mode, geometry and topography of microgrooves, PCD wheel tool wear, and an assesment criterion for available nasal-tip radius.
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