Hydroxyl Influence on Adsorption and Lubrication of an Ultrathin Aqueous Triblock Copolymer Lubricant.

This research aims to provide insights into the adsorption behaviors of two monomers of triblock copolymers (1,2-dimethoxyethane (1,2-DME) and 1,2-dimethoxypropane (1,2-DMP)) on a TiO2 surface in aqueous solution. A multiscale theoretical framework by means of the density functional theory (DFT), ab initio molecular dynamics (AIMD), and classical molecular dynamics (MD) simulations is established. The DFT calculation confirms that these molecules adsorb more energetically on a hydroxylated surface than pure oxide. There is a difference in adsorption behaviors between 1,2-DMP and 1,2-DME molecules due to the covalent bonding between carbons and oxygen of the hydroxylated TiO2 surface. The AIMD simulation reveals that the adsorption of both copolymers to the TiO2 surface is hindered by the presence of water with 1,2-DME exhibiting a weaker adsorption than 1,2-DMP. The presence of 1,2-DME on the TiO2 surface with water produced a smaller number of hydroxyl groups on the surface than 1,2-DMP. Moreover, the dissociative adsorption of water onto the rutile surface is the main cause for a chemical formation of terminating hydroxyl groups. The number of associated bonds is insignificant compared to the dissociated one since the dissociative adsorption is more favored than the associative one. MD simulation indicates that triblock copolymers adsorb stronger on the hydroxylated surface with a thinner adsorbed film thickness than that on the pure rutile. The presence of terminal hydroxyl groups on the rutile surface helps reducing the friction for aqueous 17R2 triblock copolymers, while it results in an increase of friction for normal copolymer L62.

First-principles molecular dynamics study on the surface chemistry and nanotribological properties of MgAl layered double hydroxides.

Layered double hydroxides (LDHs) are promising materials for lubrication. However, the underlying mechanism that leads to the low friction of the material is not well-understood. In this study, density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations have been used to study the reduced friction mechanism of MgAl-LDH. Our results indicate that the introduction of trivalent cations has a significant impact on the friction reduction of the LDH. Besides, the lateral force shows a strong correlation with the coverage of the hydroxyl group on the surface. By using AIMD simulation, we show that the water/hydroxide molecules interact with the surface through strong hydrogen bonds that confine the movement and the orientation of the intercalated molecules on the surface. Furthermore, the friction is reduced when the water thickness is increased. The reaction pathways of water with the LDH surface has been investigated using well-tempered metadynamics simulation. We found that the LDH can promote proton transfer, leading to the formation of hydroxide intermediates (OH), which then chemically adsorb on the surface. The chemical adsorption of the hydroxide intermediates can cleave the O-H bonds on the LDH surface.

Smart-Responsive Colloidal Capsules as an Emerging Tool to Design a Multifunctional Lubricant Additive.

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.

Porosity-induced mechanically robust superhydrophobicity by the sintering and silanization of hydrophilic porous diatomaceous earth.

HYPOTHESIS: Because they have self-similar low-surface-energy microstructures throughout the whole material block, fabricating superhydrophobic monoliths has been currently a promising remedy for the mechanical robustness of non-wetting properties. Noticeably, porous materials have microstructured interfaces throughout the complete volume, and silanization can make surfaces low-surface-energy. Therefore, the porous structure and surface silane-treatment can be combined to render hydrophilic inorganics into mechanically durable superhydrophobic monoliths. EXPERIMENTS: Superhydrophobic diatomaceous earth pellets were produced by thermal-sintering, followed by a silanization process with octyltriethoxysilane. The durability of superhydrophobicity was evaluated by changes in wetting properties, surface morphology, and chemistry after a systematic abrasion sliding test. FINDINGS: The intrinsic porosity of diatomite facilitated surface silanization throughout the whole sintered pellet, thus producing the water-repelling monolith. The abrasion sliding converted multimodal porosity of the volume to hierarchical roughness of the surface comprised of silanized particles, thereby attaining superhydrophobic properties of high contact angles over 150° and sliding angles below 20°. The tribological properties revealed useful information about the superhydrophobicity duration of the non-wetting monolith against friction. The result enables the application of porous structures in the fabrication of the anti-abrasion superhydrophobic materials even though they are originally hydrophilic.

Mechanisms of Pressure-Induced Structural Transformation in Confined Sodium Borate Glasses.

In this paper, density functional theory 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 the borate 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 oxygen and nonbridging and bridging 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 unchanged under the decompression. In addition, the formation of a sodium-rich layer can be observed when the systems were compressed to high pressure. Our simulation provides insight into sodium borate glass responses to extreme condition and the underlying electronic mechanisms that can account for these behaviors.

Author Correction: Chemical nature of alkaline polyphosphate boundary film at heated rubbing surfaces.

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper.

Tribological Behavior of Aqueous Copolymer Lubricant in Mixed Lubrication Regime.

Although a number of experiments have been attempted to investigate the lubrication of aqueous copolymer lubricant, which is applied widely in metalworking operations, a comprehensive theoretical investigation at atomistic level is still lacking. This study addresses the influence of loading pressure and copolymer concentration on the structural properties and tribological performance of aqueous copolymer solution of poly(propylene oxide)-poly(ethylene oxide)-poly(propylene oxide) (PPO-PEO-PPO) at mixed lubrication using a molecular dynamic (MD) simulation. An effective interfacial potential, which has been derived from density functional theory (DFT) calculations, was employed for the interactions between the fluid's molecules and iron surface. The simulation results have indicated that the triblock copolymer is physisorption on iron surface. Under confinement by iron surfaces, the copolymer molecules form lamellar structure in aqueous solution and behave differently from its bulk state. The lubrication performance of aqueous copolymer lubricant increases with concentration, but the friction reduction is insignificant at high loading pressure. Additionally, the plastic deformation of asperity is dependent on both copolymer concentration and loading pressure, and the wear behavior shows a linear dependence of friction force on the number of transferred atoms between contacting asperities.

Chemical nature of alkaline polyphosphate boundary film at heated rubbing surfaces.

Alkaline polyphosphate has been demonstrated to be able to reduce significant wear and friction of sliding interfaces under heavy loads (>1 GPa) and elevated temperature (800 °C and above) conditions, e.g. hot metal manufacturing. The chemical composition and fine structure of polyphosphate lubricating film is not well understood as well as the role of alkaline elements within the reaction film at hot rubbing surface. This work makes use of the coupling surface analytical techniques on the alkaline polyphosphate tribofilm, XANES, TOF-SIMS and FIB/TEM. The data show the composition in gradient distribution and trilaminar structure of tribofilm: a shorter chain phosphate overlying a long chain polyphosphate that adheres onto oxide steel base through a short chain phosphate. The chemical hardness model well explains the anti-abrasive mechanism of alkaline polyphosphate at elevated temperatures and also predicts a depolymerisation and simultaneous cross-linking of the polyphosphate glass. The role of alkaline elements in the lubrication mechanism is especially explained. This work firstly serves as a basis for a detailed study of alkaline polyphosphate tribofilm at temperature over 600 °C.

A new insight into ductile fracture of ultrafine-grained Al-Mg alloys.

It is well known that when coarse-grained metals undergo severe plastic deformation to be transformed into nano-grained metals, their ductility is reduced. However, there are no ductile fracture criteria developed based on grain refinement. In this paper, we propose a new relationship between ductile fracture and grain refinement during deformation, considering factors besides void nucleation and growth. Ultrafine-grained Al-Mg alloy sheets were fabricated using different rolling techniques at room and cryogenic temperatures. It is proposed for the first time that features of the microstructure near the fracture surface can be used to explain the ductile fracture post necking directly. We found that as grains are refined to a nano size which approaches the theoretical minimum achievable value, the material becomes brittle at the shear band zone. This may explain the tendency for ductile fracture in metals under plastic deformation.

Asymmetric kernel regression.

Kernel regression is one model that has been applied to explain or design radial-basis neural networks. Practical application of the kernel regression method has shown that bias errors caused by the boundaries of the data can seriously effect the accuracy of this type of regression. This paper investigates the correction of boundary error by substituting an asymmetric kernel function for the symmetric kernel function at data points close to the boundary. The asymmetric kernel function allows a much closer approach to the boundary to be achieved without adversely effecting the noise-filtering properties of the kernel regression.

A deformation mechanism of hard metal surrounded by soft metal during roll forming.

It is interesting to imagine what would happen when a mixture of soft-boiled eggs and stones is deformed together. A foil made of pure Ti is stronger than that made of Cu. When a composite Cu/Ti foil deforms, the harder Ti will penetrate into the softer Cu in the convex shapes according to previously reported results. In this paper, we describe the fabrication of multilayer Cu/Ti foils by the roll bonding technique and report our observations. The experimental results lead us to propose a new deformation mechanism for a hard metal surrounded by a soft metal during rolling of a laminated foil, particularly when the thickness of hard metal foil (Ti, 25 µm) is much less than that of the soft metal foil (Cu, 300 µm). Transmission Electron Microscope (TEM) imaging results show that the hard metal penetrates into the soft metal in the form of concave protrusions. Finite element simulations of the rolling process of a Cu/Ti/Cu composite foil are described. Finally, we focus on an analysis of the deformation mechanism of Ti foils and its effects on grain refinement, and propose a grain refinement mechanism from the inside to the outside of the laminates during rolling.

Fabrication of ultra-thin nanostructured bimetallic foils by Accumulative Roll Bonding and Asymmetric Rolling.

This paper reports a new technique that combines the features of Accumulative Roll Bonding (ARB) and Asymmetric Rolling (AR). This technique has been developed to enable production of ultra-thin bimetallic foils. Initially, 1.5 mm thick AA1050 and AA6061 foils were roll-bonded using ARB at 200°C, with 50% reduction. The resulting 1.5 mm bimetallic foil was subsequently thinned to 0.04 mm through four AR passes at room temperature. The speed ratio between the upper and lower AR rolls was 1:1.3. The tensile strength of the bimetallic foil was seen to increase with reduction in thickness. The ductility of the foil was seen to reduce upon decreasing the foil thickness from 1.5 mm to 0.14 mm, but increase upon further reduction in thickness from 0.14 mm to 0.04 mm. The grain size was about 140 nm for the AA6061 layer and 235 nm for the AA1050 layer, after the third AR pass.