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A water-in-water (W/W) emulsion consists of droplets formed by the spontaneous liquid–liquid separation of two immiscible aqueous phases. The inherent properties of the W/W interfaces, low or ultra...

Graphene has been shown to be a promising solid lubricant to reduce friction and wear of the sliding counterparts, and currently is reported to only function below 600 °C. In this study, its potential as a lubricant above 600 °C was studied using a ball-on-disc tribo-meter and a rolling mill. Friction results suggest that a reduction up to 50% can be obtained with graphene nanoplatelets (GnP) under lubricated conditions between 600–700 °C when compared with dry tests. and this friction reduction can last more than 3 min. At 800 and 900 °C, the friction reduction is stable for 70 and 40 s, respectively, which indicates that GnP can potentially provide an effective lubrication for hot metal forming processes. Hot rolling experiments on steel strips indicate that GnP reduces the rolling force by 11%, 7.4%, and 6.9% at 795, 890, and 960 °C, respectively. These friction reductions are attributed to the easily sheared GnP between the rubbing interfaces. A temperature higher than 600 °C will lead to the gasification of the residual graphene on the strip surface, which is believed to reduce the black contamination from traditional graphite lubricant.

With the increasing demand for light weight design in industries such as aerospace and automotive, sheet metallic materials with high strength and low density have been intensively manufactured and widely applied. However, plastic deformation induced surface roughening is an undesirable and a practically unavoidable phenomenon for sheet metallic materials and becomes the subject of great attention in recent years. The evolution of surface roughening plays important roles in the strain limit of the deformation and also significantly influences the product quality and application. This paper provides a comprehensive overview of both experimental and numerical modelling contributions to understand the surface roughening behaviour of sheet metallic materials during various plastic deformation and manufacturing processes. It has been found that roughening of the free surface or necking of the interface is induced by grain scale strain heterogeneity and strongly dependent on the strain the material experienced, strain state and direction, grain size and texture, lubrication, mechanical properties of materials, rolling parameters, etc. By controlling and optimising these parameters, the surface roughening can be optimised for different materials and processing techniques to suit different applications.

In this paper, the tribological performance of two alkali glass lubricants (sodium phosphate and sodium silicate) at the nanoscale were compared in a reactive molecular dynamics simulation. The obt...

Improving the fire-resistant efficiency of silicone based polymeric coatings is important in the building industry and electrical utilities. In this study, the water-containing hybrid calcium carbonate (CaCO3)−silica (SiO2) colloidal capsule has been developed and optimized as an inorganic flame-retardant additive. This capsule exhibits excellent thermal stability up to 1000 °C with a remaining intact hollow spherical structure. When used as an inorganic filler at 15 wt %, it not only reduces the potential fire hazards by over 44% (i.e., the sumHRC reduced from 112.00 J/g K to 62.00 J/g K) but also improves the heat-barrier efficiency by over 30% (i.e., the temperature at the steady state reduced from 350 to 360 °C to below 250 °C) of the silicone-based polymeric coatings. In addition, the capsule−polymer composite coating exhibits excellent ductility which can withstand heat-induced mechanical stresses and prevent crack propagation under radiative heating conditions. The fire-resistant mechanism of the colloidal capsule is related strongly to the encapsulated water core and the reactions between SiO2 and CaCO3 at elevated temperatures. They contribute to a cooling effect on the flammable pyrolysis gases but also induce an insulative effect on the resulting char during combustion. The significant advances in this study will have a high impact on customizing the functional inorganic additives for a better design of the flame-retardant composite coating. The study is funded by the Australian Research Council (ARC) Discovery Project DP190103455 and Linkage Project LP160101871. The authors acknowledge the use of the JEOL 6490 SEM at the UOW Electron Microscopy Centre. published

High-temperature tribology, which is often involved during hot metal forming, is controlled via oxidation on a rubbing surface. However, for high chromium stainless steel (ST), where oxidation is strongly inhibited, the effect of counterface materials on tribological behavior is yet to be elucidated. In this study, the effects of counterfaces on the tribological behavior of 253MA ST and mild steel (MS) are investigated via a ball-on-disc test at 900 °C using a 20 N load. The results reveal that high-speed steel (HSS) experiences severe abrasive wear with MS and causes severe sticking problems with ST. Si3N4 and SiC present substantially stronger abrasive wear resistance than HSS with MS, and the friction coefficients are dependent on the type of ceramic. Both ceramics can facilitate the establishment of a thick tribo-oxide layer (> 3 µm) on ST to prevent sticking; however, this is accompanied by severe pull-out and fracture wear. The effects of the counterface on the mechanical properties of the tribo-oxide layer, near-surface transformation, and the responses of the tribo-oxide layer to friction and wear are discussed. This study contributes to the understanding of interfacial tribological behaviors when different types of tools are used on MS and ST.

Lubrication by graphene from nanoscale to macroscale has attracted increasing interest in recent years. However, the graphene can only withstand load of 2 N (corresponds to Hertzian contact stress of 0.41 GPa) during sliding. In this work, a composite of graphene and sodium carbonate coating shows an improved load-carrying capability during lubricated contacts. Compared to the pure graphene deposited coating, the composite can reduce the friction from 0.6 to 0.16 and wear by one order of magnitude at 1.29 GPa, and can provide an effective lubrication even at the high load of 40 N (corresponds to maximum Hertzian contact stress of 2.06 GPa). The improved lubricating property by the composite of sodium carbonate and graphene results from the stronger adhesion of the composite on the steel surface and the sliding induced soft tribofilm formed at the rubbing interface. Interface observation indicates a high sp2 fraction of carbon at the top of the tribofilm which contributes to the easy slippery. This study provides a new approach to widen the applications of graphene to lubricated contacts under heavier loads.

In this study, the substructure formation in a multi-pass rolled aluminium single crystal {001} was investigated by the crystal plasticity finite element model, and the predations were validated by experimental observations at both macro- and micro-scale. A finite element model for multi-pass rolling was developed to follow the real experimental rolling scheme, by which the through-thickness macroscopic subdivision was successfully predicted up to a 90 % reduction. The macro-subdivision was featured by forming matrix bands through the thickness, and the deformation behaviours, in terms of slip activity, shear strain and crystal rotation, alternated between matrix bands. The development of matrix bands, stability of crystal orientations, and correlation between slip activity, shear strain and crystal rotation have been investigated. Another modelling method, Submodel, was used to exceedingly increase the mesh resolution in smaller regions of interest, and the experimentally observed microstructure, i.e., micro-subdivision was explicitly and spatially revealed. Similar predictions were obtained in Submodels with different element sizes, which proves the feasibility of this method in predicting microstructure formation. It was found that the substructure formation by varying slip activity and crystal rotation between domains is energy favourable. The procedure of substructure formation was explained based on the predictions, three types of substructure have been identified, and the substructure formation was discussed.

In this work, several phosphate-intercalated Mg-Al layered double hydroxides (LDHs) were synthesized and evaluated as solid lubricant additives in polyalphaolefin (PAO-4) by means of tribotesting on coupled GCr15/cast iron contacts. The effects of test parameters such as normal loads, additive concentrations, and substrate surface roughness were investigated, while the LDH crystal structure received considerable attention. Several types of structural disorder after anion exchange were identified based on X-ray diffraction (XRD) analysis. The unstable structures promote feasible shearing during sliding to improve friction and wear. In addition, antiwear properties correlate well with the anion charge number or the quantity of anion in the interlayer region. Overall, the tribological performance increased in the order HPO

Graphene is well-known as one of the best solid lubricants for its superlubricity and high mechanical strength. Weak adhesion leading to low interfacial compatibility is a significant challenge of ...

The microencapsulation technique has been proven as a powerful and flexible tool to design and develop a multifunctional additive for various applications. The significant characteristics of this technique center around the ability to control the release of the core active ingredients by tuning the porosity and the permeability of the shell. However, this original concept has faced a major roadblock in lubricant research since it causes a major breakage of the microcapsules (∼70%) under severe stressed-shearing conditions. The shell fragments generated from such unwanted events significantly influence the friction and wear performances of the counterpart, thus limiting the ongoing research of the microencapsulation technique in tribology. To solve such technical bottlenecks, we develop a new strategy of utilizing the microencapsulation technique which focuses on the smart responsiveness of the shell with the base lubricant and the synergy between the incorporated materials. In this study, the smart-responsive colloidal capsule has been developed based on our proposed concept that demonstrates outstanding performances in improving the lubricity of the conventional melt lubricant (by ∼70%) under hot metal working conditions. An unprecedented oxidation-reduction (by ∼93%) and the first instance of ultralow friction (0.07) at elevated temperatures (880 °C) have been initially achieved. This work opens a new avenue of customizing a multifunctional additive package by utilizing the smart colloidal capsules in lubrication science.

The rapid development of molecular dynamics (MD) simulations, as well as classical and reactive atomic potentials, has enabled tribologists to gain new insights into lubrication performance at the fundamental level. However, the impact of adopted potentials on the rheological properties and tribological performance of hydrocarbons has not been researched adequately. This extensive study analyzed the effects of surface structure, applied load, and force field (FF) on the thin film lubrication of hexadecane. The lubricant film became more solid-like as the applied load increased. In particular, with increasing applied load, there was an increase in the velocity slip, shear viscosity, and friction. The degree of ordering structure also changed with the applied load but rather insignificantly. It was also significantly dependent on the surface structure. The chosen FFs significantly influenced the lubrication performance, rheological properties, and molecular structure. The adaptive intermolecular reactive empirical bond order (AIREBO) potential resulted in more significant liquid-like behaviors, and the smallest velocity slip, degree of ordering structure, and shear stress were compared using the optimized potential for liquid simulations of united atoms (OPLS-UAs), condensed-phase optimized molecular potential for atomic simulation studies (COMPASS), and ReaxFF. Generally, classical potentials, such as OPLS-UA and COMPASS, exhibit more solid-like behavior than reactive potentials do. Furthermore, owing to the solid-like behavior, the lubricant temperatures obtained from OPLS-UA and COMPASS were much lower than those obtained from AIREBO and ReaxFF. The increase in shear stress, as well as the decrease in velocity slip with an increase in the surface potential parameter ζ, remained conserved for all chosen FFs, thus indicating that the proposed surface potential parameter ζ for the COMPASS FF can be verified for a wide range of atomic models.

A comparative evaluation of the friction and wear behaviors of 40CrNiMoA steel and Inconel 718 alloy sliding against Si3N4 counterparts was conducted over a large temperature range from room temperature (RT) to 800 °C. The temperature-dependent tribological properties associated with the resulting chemical mitigation and structural adaptation of the solid sliding surface were clarified by surface/interface characterizations. The results revealed desirable performance in reducing friction and wear at elevated temperatures, which was associated with the resulting oxide composite film’s adaptive lubricating capability, whereas severe abrasive wear occurred at room/ambient temperatures. The oxidative-abrasive differentials for the two alloys were further discussed by considering the combined effect of temperature and stressed-shearing conditions.

This paper introduces a three-dimensional (3D) simulation to model the elasto-plastic deformation of ductile materials by discrete element method (DEM) using cohesive plastic beam approach. The Euler–Bernoulli beam theory is applied to calculate force and torque reactions of the cohesive beam bond that can account for plastic strain. In order to simulate the elasto-plastic behavior of materials, a coupled model of plasticity and damage is proposed for the cohesive beam bond. The von Mises yield criterion, a modified perfectly plastic law and an exponential damage law are implemented to simulate behaviors of ductile materials such as steel and aluminum in tensile tests. It is then further applied to investigate the ductile fracture modes such as localized shear fracture and necking phenomenon with cup-and-cone shape. Finally, the proposed DEM model using cohesive plastic beam is applied to visualize the compression test of cylindrical steel sample and wrinkle deformation of steel tube under bucking due to axial compression. This paper is the first attempt to model ductile materials by DEM with cohesive beam bond in tension, compression and buckling. All of the numerical benchmarks demonstrate the power of DEM using cohesive beam bond to model ductile materials with fracture, large deformation and elasto-plastic behavior.

The present study investigated oxidation reactivity and hot lubricity of a sodium silicate melt at different Na2O/SiO2 ratios under elevated temperature stimulation. Static oxidation prevention was achieved at 920 °C when the Na2O/SiO2 ratio reached 1:3 (trisilicate) and 1:2 (disilicate), but it started to deteriorate in the case of 1:1 (metasilicate). At a high concentration of sodium (metasilicate), a severe corrosion reaction between the melt and oxide took place that resulted in a composite coating on the steel substrate. This high-temperature reaction accelerated the formation of ionic charges from the steel base and promoted oxidation. However, friction and wear reduction is proportional to an increase in the sodium oxide fraction. Metasilicate (1:1) exhibited excellent lubricity under the hot frictional test at 920 °C compared to other lubricants. It was due to the formation of the sodium-saturated surfaces and an amorphous silica layer, which was associated with the high-temperature reactivity of sodium toward the oxide surface. In addition, the NaFeO2-Fe2O3 composite film, as the reaction product of individual sodium charge and oxide, plays a significant role in maintaining the tribofilm stability for metasilicate, which was not present for disilicate. This study advances the understanding of how sodium-containing compounds perform oxidation prevention and generate lubricity at hot rubbed surfaces.

The behaviour of commercial purity aluminium processed by high pressure torsion is studied for a wide range of strains. The microstructure and texture are used to characterise the HPT deformation. Two different stages of microstructure evolution, i.e. grain fragmentation and grain elongation, are first found before attainment of steady state during HPT deformation. The grain fragmentation is not a linear function of shear strain in HPT. Significant grain refinement mainly occurs in the equivalent strain ranges of ∼0 to 1 and ∼11 to 32. Preferred simple shear orientations appear and vanish periodically due to the intrinsic instability of the shear direction in HPT. The lattice rotation of ultra-fine grain material is related to the grain size in the steady state. The strong ideal components have larger grain size than weak ideal components and non-ideal components. But the sensitivity of fragmentation to grain orientation tends to disappear as the steady state is attained.

Lubrication is vital in enabling the key machinery contact components to operate durably and reliably at extremely high temperatures for space aviation and metal forming. In this work, the expanded graphite in sodium metasilicate solution as high temperature lubricant was evaluated using a ball-on-disc tribometer. The results suggest that the use of this mixture can remarkably reduce the friction and wear of steel counterparts by up to 85% and 60% at 850 °C, respectively, compared with unlubricated condition. These improvements result from the formed tribo-layer containing amorphous carbon and sodium carbonate melt, as well as the two different ordered crystal layers on the rubbing interface induced by high temperature, pressure and shear. The expanded graphite in sodium metasilicate solution studied here for the first time will extend its application for high temperature lubrication due to its excellent performance.

We have developed a reactive force field (ReaxFF), which is able to reproduce accurately the physical and chemical properties of a comprehensive Fe/Na/P/O system. This ReaxFF was trained systematic...

This work mainly considered the effect of different TiO2 additions and of sintering temperatures on the structural change, densification and mechanical properties of ZrO2–TiO2 ceramic composites obtained by cold compaction and subsequent sintering. The results demonstrated that the structural transformation happens from pristine monoclinic zirconia into tetragonal zirconia, amount of cubic phase in as-obtained ZrO2–TiO2 specimens could be distinguished as well. The increasing concentration of TiO2 addition facilitated lower the sintering temperature and densification of ZrO2 matrix. The grain growth and bulk density of ZrO2–TiO2 ceramic composites varied with the sintering temperatures and dopant concentrations. Full evaluation of the role of TiO2 addition and sintering temperature on the mechanical properties of ZrO2–TiO2 samples was carried out in terms of Vickers hardness, flexural strength and fracture toughness. In particular, the ZrO2 matrix with a value of 5 wt % TiO2 generated the desired flexural strength and fracture toughness at the sintering temperature of 1400 °C.

Alkaline borate–silicate composites provide desirable protection for the machinery moving contacts in high-temperature coating and lubricant applications, but their behavior depends closely on the ...

In this paper, Density Functional Theory (DFT) simulations were conducted to investigate the structural adaptation of sodium borates xNa2O·(100-x)B2O3 (x = 25, 33, 50, and 60 mol %) during the compression/decompression between 0 and 10 GPa. The sodium borates are confined between two Fe2O3 substrates and undergo the compression by reducing the gap between the two surfaces. The results reveal that the borates response to the load through a two-stage transformation: rearrangement at low pressure and polymerization at high pressure. The pressure required to initiate the polymerization depends directly on the portion of fourfold-coordinated ([4]B) boron in the sodium borates. We found that the polymerization occurs through three different mechanisms to form BO4 tetrahedra with surface’s oxygen, non-bonding, and bonding oxygen. The electronic structure was analyzed to understand the nature of these mechanisms. The conversions from BO3 to BO4 are mostly irreversible as a large number of newly formed BO4 remain ...

The influence of solute H on the interaction between screw dislocations and vicinal twin boundaries in Ni bicrystals is investigated using molecular dynamics (MD) simulations. Several interaction mechanisms such as dislocation transmission, nucleation and reflection are reported for different glide planes and grain boundary (GB) structures. The presence of H tends to transform these interactions into ones involving dislocation absorption. This disorders the atomic structure of GBs and establishes a local stress state, which promotes the ultimate failure of GBs due to the formation of vacancies. These findings will deepen our understanding on the experimentally-observed H embrittlement in metallic materials.

Lubricant has been widely applied to reduce wear and friction between the contact surfaces when they are in relative motion. In the current study, a nonequilibrium molecular dynamics (NEMD) simulation was specifically established to conduct a comprehensive investigation on the dynamic contact between two iron surfaces in a boundary friction system considering the mixed C4-alkane and nanoparticles as lubricant. The main research objective was to explore the effects of fluid and nanoparticles addition on the surface contact and friction force. It was found that nanoparticles acted like ball bearings between the contact surfaces, leading to a change of sliding friction mode to rolling friction mode. Under normal loads, plastic deformation occurred at the top surface because nanoparticles were mainly supporting the normal load. By increasing the number of C4-alkane molecules between two contact surfaces, the contact condition has been changed from partial to full lubrication. In addition, an attractive force from the solid–liquid LJ interaction between C4-alkane and surfaces was observed at the early stage of sliding, due to the large space formed by wall surfaces and nanoparticles. The findings in this paper would be beneficial for understanding the frictional behavior of a simple lubricant with or without nanoparticles addition in a small confinement.

In this study, the crystal plasticity finite element method model was used to study the deformation in a single crystal aluminium processed by accumulative roll-bonding (ARB). The predicted...

The study revealed a facile method to hydrophobize enamel coatings with a thermally stable hydrophobicity by using halogenic acid etching and perfluorooctyl triethoxysilane surface modification. The results showed that the modification was successfully achieved as evidenced by the presence of the organic modifier from the measurements by FTIR and XPS. SEM/EDS and 3D optical profilometer showed that hydrofluoric/hydrochloric acid exposed the distinct inner microstructures of glass-ceramic/glassy enamels, with corresponding structures of multi particles/island morphology in micro and sub-micro scales. The combination of these unique structures and fluoroalkyl silane modifier improved the contact angle of the enamel surfaces up to 134°, compared to 110° of the non-etched samples. The resulted hydrophobic film was thermally stable up to 400 °C and able to withstand a temperature of 200 °C in a period of 15 h. During a 60 min water condensation testing, the obtained hydrophobic enamel surfaces demonstrated an intriguing dropwise water condensation which is advantageous to the surface heat transfer. During mechanical sliding, the nano scaled multi-spike surface of crystalized enamel was more vulnerable than the micro island structured surface of glassy enamel. The former experienced a sharp decrease in hydrophobicity due to both the loss of modifier and the collapse of fragile nanostructure, and the latter still remained hydrophobic thanks to a mechanically stable micro structure. The study showed that the combination of the facile etching process and widely-used fluoroalkyl silanization could be utilized to introduce hydrophobicity on a large scale to improve the surface function of the conventional enamel coating for various heat transfer applications.

The segregation of solute H has a profound effect on mechanical properties of grain boundaries (GBs). In this work, systematic molecular dynamics (MD) simulations have been performed to elucidate the influence of H environment on dislocation nucleation process and fracture response of different Ni GBs. The results show that the effect of H on dislocation nucleation depends strongly on GB structures. The presence of H decreases the yield stress required to nucleate dislocations from GBs with C and D structural units (SUs), while increases the yield stress of GBs with E SUs. This difference is attributed to different deformation mechanisms related to the dislocation nucleation from various GBs with H. By analysing the decohesion results, it is found that H has stronger embrittling effect on GBs with E SUs than C and D SUs, originating from the fact that H significantly elongates the Ni-Ni bonds around the open E SUs while slightly expands the compact C and D SUs. These findings present fundamental understanding on H embrittlement mechanism and provide mechanistic insights for engineering microstructure against H embrittlement in metallic materials.

By using large-scale molecular dynamics (MD) simulations, the influence of solute hydrogen (H) segregation into several typical [ 1 1 ¯ 0 ] symmetric tilt grain boundaries (STGBs) on the shear response and coupled grain boundary (GB) migration of bicrystals in α-Fe is systematically examined. Depending on our geometric model of coupling for [ 1 1 ¯ 0 ] STGBs in body-centred cubic (BCC) metals, two different coupling branches (〈100〉 and 〈111〉) are predicted and further validated by the MD results. Our atomistic simulations show that solute H atoms impede coupled GB motion irrespective of the GB structure, which mainly stems from the fact that H considerably modifies the local atomic structures of the GBs. At high H concentrations, the response of GBs to shear deformation changes from GB coupling to pure GB sliding. In addition, it is found that H can facilitate the vacancy generation via enhancing the interaction of GB dislocations within the framework of the H-enhanced localized plasticity (HELP) mechanism. Although H-vacancy clusters are formed by solute H combining with nucleated vacancies, they cannot grow larger owing to the migration of GBs with extensive dislocation plasticity. This directly prevents the occurrence of H embrittlement (HE). These findings deepen our overall understanding of the role of solute H in GB-mediated plasticity process (GB migration) of metallic materials and provide a possible path to designing new materials with high resistance to HE.