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This paper is on determination of the thermal conductivities of several hydrocarbon base oils by means of non-equilibrium molecular dynamics simulations using two different force fields. It aims to explore a simulation-based method for lubricant molecular design and analysis concerning heat transfer in electrical vehicle lubrication. Argon was analyzed as a reference for method evaluation, and the results reveal that the calculated conductivity strongly depends on the size of the computational domain. However, for hydrocarbon base oils, the dependence on computation domain size is less prominent as the domain size increases. The method of direct calculation in a sufficiently large computation domain and that of reciprocal extrapolation with data calculated in a much smaller domain are both applicable, and each has a certain value in oil conductivity calculation. The calculated conductivities show certain overpredictions when compared with experimentally measured results, and the overprediction factor is related to number of carbon atoms of the liquid molecules. The results reveal that the thermal conductivity of a single-chain hydrocarbon liquid is linearly proportional to the number of carbon atoms. While each additional branch increases thermal conductivity slightly, the presence of multiple branches reduces it from the ideal linear relationship. A set of equations was formulated to correlate hydrocarbon liquid thermal conductivity with molecular characteristics in terms of number of carbon atoms and number of branches. [ABSTRACT FROM AUTHOR]

We report here the impact of different alloying elements in steels on friction and wear behavior by performing ball-on-flat lubricated reciprocating tribotesting experiments on 52100 ball on steel flats with different compositions (52100, 1045, A2, D2, M2, and a specialty Cu-alloyed steel) heat-treated to give similar hardness and microstructure, with polyalphaolefin (PAO-4) as the lubricant. There are small variations of coefficient of friction among these alloys. The major observation is that steels containing high concentrations (≥ 10 wt%) of Cr, Mo, and V gave rise to markedly reduced wear compared with 52100 or plain carbon steels. D2 steel, which contains 11.5 wt% Cr as the major alloying element was the most wear-resistant. The wear resistance is strongly correlated with the efficiency of formation of carbon-containing oligomeric films at specimen surfaces as determined by Raman spectroscopy. This correlation holds for steels heat-treated to have higher hardness and with n-dodecane, a much less viscous lubricant compared with PAO-4. Given the strong affinity of chromium to oxygen, chromium should exist as Cr2O3 at the steel surfaces during testing. We have performed molecular dynamics simulation on Cr2O3 and demonstrated its ability to catalyze the formation of carbon-containing oligomeric films from hydrocarbon molecules, consistent with its known catalytic activity in other hydrocarbon reactions. We believe that chromium-containing alloys, such as D2, and coatings, such as CrN, derive their wear resistance in part from the efficient in situ formation of wear-protective carbon tribofilms at contacting asperities. [ABSTRACT FROM AUTHOR]

In the Raman analysis of tribofilms produced from organic precursors, the D- and G-band features are often observed, which resemble the characteristic bands of diamond-like carbon (DLC), amorphous carbon (a-C), or graphitic materials. This study reports experimental evidence that the D- and G-bands features in the Raman spectra of tribofilms could be generated by photochemical degradation of triboproducts due to the focused irradiation of laser beam during the Raman analysis, indicating that they are not unique to the genuine structure of the tribofilm produced via friction. This finding suggests that other complementary and non-destructive characterization is required to determine whether DLC, a-C, or graphitic species are produced tribochemically by frictional shear. [ABSTRACT FROM AUTHOR]

Under severe conditions, solid contacts take place even when parts are lubricated. Precise mathematical conditions are needed to describe the interior interface between fluid lubrication and solid-contact zones. In order to distinguish the conditions for this interface from conventional lubrication boundary conditions, they are named lubrication–contact interface conditions (LCICs). In this work, mathematical LCICs are derived with local flow continuity from the continuum mechanics point of view and pressure inequality across the interface. Numerical implementations are developed and tested with problems having simple geometries and configurations, and they are integrated into a new mixed/boundary elastohydrodynamic lubrication solver that uses a new method to determine solid-contact pressures. This solver is capable of capturing film thickness and pressure behaviors involving solid contacts. [ABSTRACT FROM AUTHOR]

The original publication contained an incorrect mention of the time duration for Fig. 12 in Appendix. The expanded friction curve is for the initial 30 s of sliding. [ABSTRACT FROM AUTHOR]

We recently reported the use of cyclopropanecarboxylic acid (CPCa) as a model additive that can readily react under the combined effect of flash heating and stress in steel tribocontacts to form tribopolymers, along with marked improvement in tribological performance. In this paper, we present results of how chemical structural modification of CPCa may impact on the formation of tribopolymers and hence friction and wear properties, both by experiments and molecular dynamics simulation. Four lubricant additives, viz., CPCa, cyclobutanecarboxylic acid (CBCa), cyclopropane-1,1-dicarboxylic acid (CPDCa), and cyclobutane-1,1-dicarboxylic acid (CBDCa) consisting of a metastable ring structure and one or two carboxyl groups dissolved in an ester base oil were studied. Friction and wear rate using these additives rank in the order of CPDCa < CBDCa < CPCa < CBCa. Raman spectroscopy analysis reveals that these additive molecules form tribopolymer films at the contact area. Molecular dynamics simulation shows that CPCa with the less stable cyclopropane ring fragments more readily than CBCa. Such fragmentation appears to be essential for subsequent tribopolymerization and formation of protective tribofilms. These simulations further demonstrate that having two carboxyl groups as in the case of CPDCa results in stronger binding of the additive molecules to the surface, thus increasing the residence time and hence facilitating mechanically or thermally induced dissociation and subsequent polymerization. The net result is that CPDCa gives the lowest friction and negligible wear under our testing conditions. [ABSTRACT FROM AUTHOR]

Certain contact problems must be treated as three-dimensional (3D) axial periodic problems, such as the contact of cylinders with rough surfaces and/or inhomogeneities in the materials, whose structural features in contact are confined in a finite domain in one direction but extended infinitely in the other direction. A novel numerical model for simulating the contact of machined cylindrical components containing inhomogeneities is developed via extending the concept of the 3D line-contact fast Fourier transform (FFT) algorithms. Due to the stochastic similarity of asperity and inhomogeneity distributions in the length direction, the cylinder is divided into N segments in the length direction while taking the roughness and inhomogeneities in one of these segments as representatives. The periodic convolution and FFT is used in the length direction, together with superposing the influence coefficients (ICs) of the N segments, while the discrete (circular) convolution and fast Fourier transformation (DC–FFT) is used in the non-periodic direction; this is named the DCS–FFT algorithm. The accuracy of the DCS–FFT algorithm is examined by the comparison of the numerical results for a degenerated cylindrical contact with the corresponding analytical solution, and its efficiency is evaluated through the comparison of its execution speed with that of two other FFT-based algorithms. The developed method is implemented to study the influence of inhomogeneities on subsurface stress distributions with/without the periodic length direction extension and superposition of inhomogeneity ICs. A criterion is provided to decide whether the DCS procedure is needed for the contact analysis of inhomogeneity-containing cylinders. [ABSTRACT FROM AUTHOR]

Mechanochemical reactions induced by external stress provide a unique approach for in situ synthesis of carbon tribofilms that can improve friction and wear performance. In this work, we studied how tribofilm formation and tribological performance might be related to the adsorption strength of three additives in polyalphaolefin (PAO4) as base oil, viz., cyclopropanecarboxylic acid (CPCa), cyclopropanemethanol (CPMA), and 1-cyclopropylethanol (CPEA) as characterized by two different surface-active groups –COOH and –OH. Tribo-testing results reveal that addition of 2.5 wt% CPCa to PAO4 gave the lowest friction coefficient and wear volume. FTIR and Raman analysis demonstrate substantial tribofilm formation only in the case when CPCa was used as the oil additive, not CPMA or CPEA, in spite of the fact that all three additives contain the same metastable cyclopropane ring. Thermogravimetric analysis and molecular dynamics simulations indicate the stronger adsorption of CPCa on the iron oxide surface compared with CPMA and CPEA. Weak adsorption of the latter molecules results in their desorption from the surface due to flash heating during tribotesting before they have the chance to participate in mechanochemical reactions required for tribofilm formation. The stronger binding of CPCa to the steel surface is a necessary condition for this type of surface mechanochemistry and appears critical to the efficient formation of carbon-containing tribofilms under our tribo-testing conditions. [ABSTRACT FROM AUTHOR]

Laser quenching is usually performed on a localized area of workpiece surfaces through a certain thermal cycle. As a result, the treated area possesses improved mechanical properties. The quality of the treatment results depends on the selection of laser process parameters, which relies on the knowledge of the temperature field caused by laser heating. This paper reports a model for the evaluation of the laser-induced thermal field. This model assumes a laser beam of uniform intensity and considers the effect of the base temperature on the total temperature. Surface temperatures were computed and compared for different scanning velocities and laser spot sizes. The analyses indicate that the temperature in later-heated areas is much higher than that in earlier-heated areas. [ABSTRACT FROM AUTHOR]

Damping systems using granular particles as the damping medium are promising for application in extreme temperature conditions. In particle-based damping systems, mechanical energy is dissipated through inelastic collisions and friction between particles. In this work, we use particle dynamics simulations to investigate and compare the damping mechanisms of a piston-type thrust damper and a box-type oscillation damper. The mechanisms of energy transfer and energy dissipation are investigated. The roles of friction and inelastic collisions, as well as the wall effects in energy dissipation, are examined. The simulation results provide better understanding of the particle damping mechanisms, which may help in the design of next generation particle damping devices. [ABSTRACT FROM AUTHOR]

The contact of nominally flat surfaces can be treated as a bilateral periodic contact problem considering the stochastic surface similarity to the asperity distribution in a representative region. This similarity treatment method can be extended to material inhomogeneities. A novel numerical model for simulating the elastoplastic contact between nominally flat surfaces of materials containing inhomogeneities or coatings is developed via extending the concept of the discrete convolution and FFT (DC–FFT) algorithm with double superpositions of influence coefficients, which is named the DCSS–FFT algorithm. Several cases are analyzed with this new algorithm to examine its convenience, efficiency, and accuracy in dealing with complicated nominally flat–flat contact problems. The effects of surface roughness and material inhomogeneity are explored, and the mechanisms of contact surface failure are discussed. [ABSTRACT FROM AUTHOR]

Diamond-like carbon (DLC) is a class of amorphous carbon materials used as wear-resistant coatings in tribo-components. The chemical inertness of DLC surfaces, while important in many applications, makes them incompatible with additives in commercial lubricants. Specifically, DLC surfaces do not permit strong adsorption of friction modifier molecules. This study aims at improving the adsorption of these friction modifier molecules on DLC surfaces through an acid treatment. Water contact angle measurements show that such a treatment results in improved hydrophilicity of the DLC. XPS analysis demonstrates 50% increase in the uptake of ArmeenT, an organic friction modifier, on DLC after the acid treatment. This increased ArmeenT adsorption is accompanied by marked decrease in friction in micro-scale friction experiments. [ABSTRACT FROM AUTHOR]

The protein kinase D family of serine/threonine kinases, particularly PKD1, has been implicated in the regulation of a complex array of fundamental biological processes. However, its function and mechanism underlying PKD1-mediated the bone development and osteoblast differentiation are not fully understood. Here we demonstrate that loss of PKD1 function led to impaired bone development and osteoblast differentiation through STAT3 and p38 MAPK signaling using in vitro and in vivo bone-specific conditional PKD1-knockout (PKD1-KO) mice models. These mice developed markedly craniofacial dysplasia, scapula dysplasia, long bone length shortage and body weight decrease compared with wild-type littermates. Moreover, deletion of PKD1 in vivo reduced trabecular development and activity of osteoblast development, confirmed by Micro-CT and histological staining as well as expression of osteoblastic marker (OPN, Runx2 and OSX). Mechanistically, loss of PKD1 mediated the downregulation of osteoblast markers and impaired osteoblast differentiation through STAT3 and p38 MAPK signaling pathways. Taken together, these results demonstrated that PKD1 contributes to the osteoblast differentiation and bone development via elevation of osteoblast markers through activation of STAT3 and p38 MAPK signaling pathways. [ABSTRACT FROM AUTHOR]