Tribochemical synergy between phosphate-intercalated layered double hydroxide additives and super high oleic safflower oil on sliding contacts.

In this study, the tribological properties of a series of layered double hydroxides (LDHs) intercalated by various phosphate anions (hydrogen phosphate, orthophosphate, pyrophosphate, and hexametaphosphate) were investigated by ball-on-disc testing in safflower oil which has a super high content of oleic acid (OA). The Mg-Al-NO3- LDH precursor was first synthesized by a coprecipitation method, and then nitrate anions were substituted by different phosphate anions via anion exchange. The successful intercalations were confirmed by X-ray powder diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy, and scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS) analysis. The tribological results indicate that the antiwear properties increased linearly with the basic strength of the phosphate anions. Stronger bases, such as orthophosphate (PO43-) and pyrophosphate (P2O74-), encourage the dissociation of OA to form a protective oleate tribofilm on steel surfaces, leading to more than 40% wear reduction compared to the base oil. The tribological performance of the best antiwear candidate, PO43--LDH, was studied further under different loads and concentrations. A comparison between PO43--LDH and other nanomaterials was also carried out with PO43--LDH exhibiting a comparable performance against hexagonal boron nitride (BN).

Intrinsic Effect of Nanoparticles on the Mechanical Rupture of Doubled-Shell Colloidal Capsule via In Situ TEM Mechanical Testing and STEM Interfacial Analysis.

The discovery of Pickering emulsion templated assembly enables the design of a hybrid colloidal capsule with engineered properties. However, the underlying mechanisms by which nanoparticles affect the mechanical properties of the shell are poorly understood. Herein, in situ mechanical compression on the transmission electron microscope and aberration-corrected scanning transmission microscope are unprecedentedly implemented to study the intrinsic effect of nanoparticles on the mechanical properties of the calcium carbonate (CaCO3 )-decorated silica (SiO2 ) colloidal capsule. The stiff and brittle nature of the colloidal capsule is due to the interfacial chemical bonding between the CaCO3 nanoparticles and SiO2 inner shell. Such bonding strengthens the mechanical strength of the SiO2 shell (166 ± 14 nm) from the colloidal capsule compared to the thicker single SiO2 shell (310 ± 70 nm) from the silica hollow sphere. At elevated temperature, this interfacial bonding accelerates the formation of the single calcium silicate shell, causing shell morphology transformation and yielding significantly enhanced mechanical strength by 30.9% and ductility by 94.7%. The superior thermal durability of the heat-treated colloidal capsule holds great potential for the fabrication of the functional additives that can be applied in the wide range of applications at elevated temperatures.

Effect of Temperature on Deformation and Fracture Behaviour of Nanostructured Polycrystalline Ni Under Tensile Hydrostatic Stress by Molecular Dynamics Simulation.

Real materials have structural defects that are normally brought in during the processes of manufacturing and storage and often have a structure with abundant grains, as well as being subjected to multi-directional force conditions. The study of temperature's effect on plastic deformation mechanisms in polycrystalline materials bathed by a multi-axial force is still very rare and not clear. Therefore, we conducted very large-scale molecular dynamics simulations to study the deformation and fracture behaviour of nanostructured polycrystalline Ni under a pre-existing external tensile hydrostatic stress with various temperatures. By characterizing the deformation and fracture mechanisms at an atomic scale, our results elucidate the effect of temperature on brittle versus ductile fracture behaviour by analysing the local stresses for void nucleation and crack propagation and the associated interplays of grain boundary, dislocation/twin and void/crack activities. The lower temperature results in a more brittle fracture manner. This is because the decreasing temperatures contribute to more sources of local stress concentrators for void/crack nucleation and propagation, and suppress the plastic deformation achieved by the activities of grain boundary, twin and dislocation. Our findings shed a light on a fundamental understanding of polycrystalline Ni metals subjected to complex working environments.

A crystal plasticity FEM study of through-thickness deformation and texture in a {112} <111> aluminium single crystal during accumulative roll-bonding.

In this study, a crystal plasticity finite element method (CPFEM) model was used to study the deformation behaviour in an aluminium single crystal (1 1 2)[1 1 -1] processed by accumulative roll-bonding (ARB) up to 9 cycles. The simulation followed the real ARB process based on the developed finite element model. The predicted through-thickness texture matches well with the experimental observations. The deformation behaviours, in terms of crystal rotation, shear strain and slip system activation, in the first and second cycles (conventional rolling) were unidirectional, but the deformation was altered after ARB was applied from the third cycle onwards. Such alteration was found to be caused by the thickness position change and deformation discontinuity at interfaces, which were investigated in detail. The role that interfaces play became dominant over thickness position change as increasing ARB cycles.

Understanding the tribological impacts of alkali element on lubrication of binary borate melt.

Melt lubricants have been regarded as an effective class to deliver lubrication on moving mechanical contacts at extreme temperatures. Among the elementary constituents, alkali elements play a critical role in governing the physical-chemical characteristics of the lubricant despite the obscurity regarding their intrinsic roles on the rubbing interfaces. The present study attempts to unfold the effects of sodium on the tribological responses of mating steel pair under borate melt lubrication. It has been found that the involvement of Na inspires a total reversal in lubricating potentials of the lone B2O3 melt manifested by remarkable friction reduction, wear inhibition and prolonged load-bearing capacity. These exceptional performances are attributed to the accretion of nanothin Na layers on the contact interfaces. The interfacial occurrences are interpreted from a physico-chemistry perspective while the influences of surface microstructure are also discussed in detail. Multiple characterizations are employed to thoroughly examine the sliding interfaces in multi-dimensions including Scanning Electron Microscopy (SEM), Scanning Transmission Electron Microscopy (STEM) and Atomic Force Microscopy (AFM). In addition, chemical fingerprints of relevant elements are determined by Energy Dispersive Spectroscopy (EDS) and Electron Loss Energy Spectroscopy (EELS).

Coupled grain boundary motion in aluminium: the effect of structural multiplicity.

The shear-induced coupled grain boundary motion plays an important role in the deformation of nanocrystalline (NC) materials. It has been known that the atomic structure of the grain boundary (GB) is not necessarily unique for a given set of misorientation and inclination of the boundary plane. However, the effect of the structural multiplicity of the GB on its coupled motion has not been reported. In the present study we investigated the structural multiplicity of the symmetric tilt Σ5(310) boundary in aluminium and its influence on the GB behaviour at a temperature range of 300 K-600 K using molecular dynamic simulations. Two starting atomic configurations were adopted in the simulations which resulted in three different GB structures at different temperatures. Under the applied shear deformation each GB structure exhibited its unique GB behaviour. A dual GB behaviour, namely the transformation of one GB behaviour to another during deformation, was observed for the second starting configuration at a temperature of 500 K. The atomistic mechanisms responsible for these behaviour were analysed in detail. The result of this study implicates a strong relationship between GB structures and their behaviour, and provides a further information of the grain boundary mediated plasticity in nanocrystalline materials.

The shear response of copper bicrystals with Σ11 symmetric and asymmetric tilt grain boundaries by molecular dynamics simulation.

Grain boundaries (GBs) are important microstructure features and can significantly affect the properties of nanocrystalline materials. Molecular dynamics simulation was carried out in this study to investigate the shear response and deformation mechanisms of symmetric and asymmetric Σ11<1 1 0> tilt GBs in copper bicrystals. Different deformation mechanisms were reported, depending on GB inclination angles and equilibrium GB structures, including GB migration coupled to shear deformation, GB sliding caused by local atomic shuffling, and dislocation nucleation from GB. The simulation showed that migrating Σ11(1 1 3) GB under shear can be regarded as sliding of GB dislocations and their combination along the boundary plane. A non-planar structure with dissociated intrinsic stacking faults was prevalent in Σ11 asymmetric GBs of Cu. This type of structure can significantly increase the ductility of bicrystal models under shear deformation. A grain boundary can be a source of dislocation and migrate itself at different stress levels. The intrinsic free volume involved in the grain boundary area was correlated with dislocation nucleation and GB sliding, while the dislocation nucleation mechanism can be different for a grain boundary due to its different equilibrium structures.

A combined experimental-numerical approach for determining mechanical properties of aluminum subjects to nanoindentation.

A crystal plasticity finite element method (CPFEM) model has been developed to investigate the mechanical properties and micro-texture evolution of single-crystal aluminum induced by a sharp Berkovich indenter. The load-displacement curves, pile-up patterns and lattice rotation angles from simulation are consistent with the experimental results. The pile-up phenomenon and lattice rotation have been discussed based on the theory of crystal plasticity. In addition, a polycrystal tensile CPFEM model has been established to explore the relationship between indentation hardness and yield stress. The elastic constraint factor C is slightly larger than conventional value 3 due to the strain hardening.

Gaussian filters and filter synthesis using a Hermite/Laguerre neural network.

A neural network for calculating the correlation of a signal with a Gaussian function is described. The network behaves as a Gaussian filter and has two outputs: the first approximates the noisy signal and the second represents the filtered signal. The filtered output provides improvement by a factor of ten in the signal-to-noise ratio. A higher order Gaussian filter was synthesized by combining several Hermite functions together.

Atomistic simulation of tensile deformation behavior of ∑5 tilt grain boundaries in copper bicrystal.

Experiments on polycrystalline metallic samples have indicated that Grain boundary (GB) structure can affect many material properties related to fracture and plasticity. In this study, atomistic simulations are employed to investigate the structures and mechanical behavior of both symmetric and asymmetric ∑5[0 0 1] tilt GBs of copper bicrystal. First, the equilibrium GB structures are generated by molecular statics simulation at 0K. The results show that the ∑5 asymmetric GBs with different inclination angles (φ) are composed of only two structural units corresponding to the two ∑5 symmetric GBs. Molecular dynamics simulations are then conducted to investigate the mechanical response and the underlying deformation mechanisms of bicrystal models with different ∑5 GBs under tension. Tensile deformation is applied under both 'free' and 'constrained' boundary conditions. Simulation results revealed different mechanical properties of the symmetric and asymmetric GBs and indicated that stress state can play an important role in the deformation mechanisms of nanocrystalline materials.

Asymmetric cryorolling for fabrication of nanostructural aluminum sheets.

Nanostructural Al 1050 sheets were produced using a novel method of asymmetric cryorolling under ratios of upper and down rolling velocities (RUDV) of 1.1, 1.2, 1.3, and 1.4. Sheets were rolled to about 0.17 mm from 1.5 mm. Both the strength and ductility of Al 1050 sheets increase with RUDVs. Tensile strength of Al sheets with the RUDV 1.4 is larger 22.3% of that for RUDV 1.1, which is 196 MPa. The TEM observations show the grain size is 360 nm when the RUDV is 1.1, and 211 nm for RUDV 1.4.