Search

In this paper, we present some initial results aimed at defining a framework for the analysis of thermodynamic systems with additional restrictions imposed on the intensive parameters. Specifically, for the case of chemical reactions, we considered the states of constant affinity that form isoffine submanifolds of the thermodynamic phase space. Wer discuss the problem of extending the previously obtained stability conditions to the considered class of systems.

An active seating system is designed to promote dynamic sitting. To achieve this, the user’s posture is permanently monitored by sensors. If the user does not change posture independently within a defined period of time, actuators exert a strain on the buttocks to create subconscious discomfort. This discomfort is intended to induce the user to change posture without consciously noticing it. In previous studies, measurements were carried out on an automated test bench based on a modified office chair. In this context, the actuating forces and displacement at which the user consciously perceives discomfort were determined, as well as the influence of different actuator speeds. The first measurements were carried out with a flat surfaced actuator of 20 mm diameter. This paper investigates the influence of the contact geometry (shape and size) of active seating systems on the human perception threshold. For this reason, the experiments are repeated with nine geometrically different contact geometries with flat, round, and pointed surfaces of 20, 40, and 60 mm diameter. In order to be able to compare results, all measurements are performed with the same group of participants. In addition, questionnaires are used to record subjective parameters during the test. Based on the results, the paper inquires whether, in addition to the expected influence of the diameter of the actuator, a significant influence of shape and speed can be observed.

Fictional contacts occur in many mechanical systems, and often affect their dynamic response, since the collisions cause a significant change the systems’ characteristics, namely in terms of velocities. This work describes and compared different formulations to handle frictional contacts in multi-rigid-body dynamics. For that, regularized and non-smooth techniques are revisited. In a simple manner, the regularized methods describe the contact forces as a continuous function of the indentation, while the non-smooth formulations use unilateral constraints to model the contact problems, which prevent the indentation from occurring. The main motivation for the performing this study came from the permanent interest in developing computational models for the dynamic modeling of contactimpact events under the framework of multibody systems methodologies. The problem of modeling and simulating contacts with friction in multibody systems includes several steps, the definition of the contact geometry; the determination of the contact points; the resolution of the contact itself; and the evaluation of the transitions between different contact regimens. The last two aspects are investigated in this work within the context of contact dynamics. In the sequel of this process, an application example is utilized to show the effectiveness of the modelling process of contact problems in multibody systems. Finally, future developments and new perspectives for further developments related to contact-impact problems are presented and discussed in this study., This work has been supported by FCT, under the national support to R&D units grant, with the reference project UIDB/04436/2020 and UIDP/04436/2020.

Dynamical similarities are non-standard symmetries found in a wide range of physical systems that identify solutions related by a change of scale. In this paper we will show through a series of examples how this symmetry extends to the space of couplings, as measured through observations of a system. This can be exploited to focus on observations that can be used distinguish between different theories, and identify those which give rise to identical physical evolutions. These can be reduced into a description which makes no reference to scale. The resultant systems can be derived from Herglotz's principle and generally exhibit friction. Here we will demonstrate this through three example systems: The Kepler problem, the N-body system and Friedmann-Lema\^itre-Robertson-Walker cosmology., Comment: 20 pages

We propose a system containing a single pyrene molecule sandwiched between two metallic electrodes. The transport properties of the single pyrene molecule with four configurations are investigated using a steady-state theoretical model. We calculate the transmission probability and the electric current for the structure (1, 8), the structure (1, 7), the structure (1, 5) and the structure (1, 4). By applying a gate voltage on the pyrene molecule, we calculate the thermoelectric properties. The thermoelectric and electron transport properties can be controlled by quantum interference, the contact geometry and the gate voltage. The asymmetric behavior and the splitting of resonances in the transmission spectrum occur due to applying a gate voltage on the pyrene molecule. As a result, the structures (1, 5) and (1, 7) have a maximum value of the figure of merit reaching to 0.8 at the Fermi level. According to the results, the structures (1, 5) and (1, 7) can be act as promising thermoelectric applications in molecular electronics.

M. A. Akyol and R. Sar? [On semi-slant ??-Riemannian submersions, Mediterr. J. Math. 14(6) (2017) 234.] defined semi-slant ??-Riemannian submersions from Sasakian manifolds onto Riemannian manifolds. As a generalization of the above notion and natural generalization of anti-invariant ??-Riemannian submersions, semi-invariant ??-Riemannian submersions and slant submersions, we study hemi-slant ??-Riemannian submersions from Sasakian manifolds onto Riemannian manifolds. We obtain the geometry of foliations, give some examples and find necessary and sufficient condition for the base manifold to be a locally product manifold. Moreover, we obtain some curvature relations from Sasakian space forms between the total space, the base space and the fibres.

Liouville domains have become central objects in symplectic and contact geometry. However, the auxiliary data they involve --- namely, Liouville forms --- and the non-compactness of their completions generate some inconvenience. The notion of ideal Liouville domains is designed to suppress these awkward aspects and to let symplectic structures play the leading role. Ideal Liouville domains are compact manifolds with boundary whose interior carries a symplectic form satisfying some tameness condition along the boundary. Their definition and their basic properties are presented in the first part of these notes, while the second part discusses their relevance in contact geometry., Comment: 22 pages

In this paper, we discuss about almost complex structures and complex structures on Riemannian manifolds, symplectic manifolds and contact manifolds. We have also shown a special comparison between complex symplectic geometry and complex contact geometry. Also, the existence of a complex submanifold of n-dimensional complex manifold which intersects a real submanifold GANIT J. Bangladesh Math. Soc.Vol. 39 (2019) 119-126

This paper studies the instantaneous second-order total freedom of two smooth objects initially in contact at a single point. The analytical determination of the set of physically allowed second-order motions is formulated in the Euclidean space using the screw theoretic concepts of twist and twist-derivative. Mathematical expression that characterizes the nature of second-order motion for an arbitrary contact geometry is derived in terms of twist coordinates, twist-derivative coordinates and the principle normal curvatures of the two surfaces at the point of contact. It is shown that the characteristic of this expression is equivalent to that of the derivative of reciprocal product of the instantaneous twist and contact normal line. The efficacy of the theory developed is demonstrated through two illustrative examples.

Studies on the eutectic reaction of core materials are important for more accurate evaluation of progression of a reactor severe accident since such reactions provide significant input to the followed in- and ex-vessel phenomena. In this study, motivated to obtain some knowledge and database for enhanced understanding on this reaction, a series of simulated experiments was performed using the Pb-Sn couple due to their lower eutectic point. The experiments were conducted in a self-designed experimental system, which mainly consists of a sample holder and a visible resistance furnace. To acquire relatively comprehensive understanding, different experimental parameters such as the reaction temperature (463–483 K), contact pressure (0.43–1.15 MPa) and the contact geometry (square, upright and inverted isosceles trapezoids) were varied. Based on the experimental observation and quantitative data obtained, influence of experimental conditions on the reaction rate as well as the chemical composition and specific heat capacity of reaction product was analyzed. It is found that the reaction temperature and contact pressure have noticeable impact on the reaction rate, while the role of contact geometry seems to be insignificant. As for the composition of reaction product, despite an evident effect of reaction temperature and contact geometry, non-remarkable impact of the contact pressure is found. Although the composition of reaction product is varied at different conditions, the change in macroscopic specific heat capacity of the reaction product is confirmed to be limited over present scope of experimental parameters. The obtained results in this work to some degree have confirmed the good applicability of our experimental system developed, thus providing us with promoted confidence for future investigations on eutectic interaction using actual reactor materials. Knowledge and fundamental data from our work might be also utilized for the future development and verifications of eutectic-reaction models in reactor severe accident analysis codes.

A two-parametric geometry optimization problem for a symmetrical punch with compound curvature in frictionless contact with and an elastic half-plane is considered. An almost constant pressure distribution is sought for by means of least-square method. Another sub-optimal solution, which equalizes the three local maximum peaks (one at the center, and two at either side of the contact interval), is constructed using an asymptotic modeling approach and compared with the exact solution known in the literature.

This transaction proposes a symmetric bistaggered dual-gate organic thin-film transistor (TFT) to minimize the difference between the top and bottom channel current characteristics and demonstrates it through experiment and 2-D simulation. In dual-gate organic TFTs, asymmetric top and bottom channel currents are mainly a result of their asymmetric contact geometry. To address this geometric limitation, the contact electrodes were sandwiched between the two organic semiconductor layers to be placed in a staggered position to both top and bottom gates. Two-way current crowding at the bistaggered contact electrodes leads to high-charge injection efficiency in both channels.

Secondary air systems of civil aircraft engines require labyrinth seals with a minimum gap clearance for optimal functioning. In the case of high speed contact during the engine running-in period, an abradable material is deposited on the stationary part of the seal to limit the damage of the rotating shaft, which is made of a titanium alloy. Such situations are potentially critical for the seal; hence, the present study aims to observe the material behaviour during these contact conditions and to establish the tribological circuit of a third body through the interface. A high-speed contact test rig was developed to recreate contact conditions occurring in an aircraft engine. Two contact configurations occurring in different locations of the engine, with different materials and surface areas were explored. Thermal and mechanical instrumentation were used in each test. The influence of the contact geometry and the test conditions show that material flows through the contact determine the life cycle of the contact (by establishing a balance between the source, internal and material flow) and allows for the control of the thermomechanical constraints in a high-speed contact.

Lubrication is often employed in fretting contacts to reduce wear and stresses associated with high friction. Owing to the very small displacements associated with fretting, penetration of lubricating oils into the contact may not be effective. The efficacy of the penetration of the lubricant into the contact is very difficult to observe experimentally, and accordingly, this paper presents a numerical simulation of a lubricated fretting contact using a Coupled–Eulerian–Lagrangian (CEL) finite element method. Meso-scale CEL finite element models are developed to simulate the cylinder-on-flat arrangement used experimentally at the University of Nottingham in which the roughness of contact surfaces is characterized as fractal geometry by the Weierstrass-Mandelbrot (W-M) function. The fluid–solid and solid–solid contact in the lubricated fretting contact are simulated, and from these, wear and friction coefficients are determined. The effects of contact geometry on lubricated fretting contacts and lubricant on fretting wear are modelled and compared with experimental observations. Results indicate that oil lubrication reduces fretting wear and friction effectively in the less-conforming contacts but has little effect in the more-conforming contacts.

This paper focuses on the systematic type synthesis of parallel robot manipulators by using new structural formulas based on the screw theory. New structural formulas as a total number of screw in kinematic pairs , number of screws with variable pitch , total number of screws that represent the contact geometry of lower and higher joint elements (t), mobility equation for robot manipulators (M), dimension of the closed loop , motion of end effector of parallel manipulator , number degree of freedom of kinematic pairs , refers to find the kinematic structure of robot manipulators realizing a specified motion requirement. Twenty kinematic pairs with structural parameters are introduced. History of six structural formulas using for structural synthesis of parallel robot manipulators from space and different subspaces are presented as a table with equations, authors, years and some commentaries. The structural synthesis approach is based on the elementary notions of screw theory. Using the proposed of structural formulas approach, families of platform manipulators are constructed from a set of structural units. This paper is appropriate for engineers with interest in robotics, rovers, space docking parallel manipulators and screw theory.

We present a fast and feature-complete differentiable physics engine, Nimble (nimblephysics.org), that supports Lagrangian dynamics and hard contact constraints for articulated rigid body simulation. Our differentiable physics engine offers a complete set of features that are typically only available in non-differentiable physics simulators commonly used by robotics applications. We solve contact constraints precisely using linear complementarity problems (LCPs). We present efficient and novel analytical gradients through the LCP formulation of inelastic contact that exploit the sparsity of the LCP solution. We support complex contact geometry, and gradients approximating continuous-time elastic collision. We also introduce a novel method to compute complementarity-aware gradients that help downstream optimization tasks avoid stalling in saddle points. We show that an implementation of this combination in an existing physics engine (DART) is capable of a 87x single-core speedup over finite-differencing in computing analytical Jacobians for a single timestep, while preserving all the expressiveness of original DART.

Ce mémoire se présente comme une promenade dans le domaine de la topologie symplectique et de la géométrie de contact, présentant dans leur contexte les résultats que j’ai obtenus depuis la fin de ma thèse.Le fil conducteur de mes recherches est la question suivante: Dans le cadre d’une variété symplectique à bord de type contact, quelle information l’intérieur possède t-il sur le bord et réciproquement, quelle information le bord possède t-il sur l’intérieur?Ce mémoire est divisé en deux parties correspondant aux deux volets de la question ci-dessus.La première partie est consacrée à l’étude du nombre minimal d’orbites périodiques du champ de Reeb d’abord sur des hypersurfaces étoilées dans R^{2n} ensuite sur des variétés plus générales. Un des outils principaux est l’homologie symplectique S^1-équivariante positive; elle est construite à partir d’orbites périodiques de champs de vecteurs hamiltoniens sur une variété symplectique dont le bord est la variété de contact considérée.La deuxième partie est consacrée aux plongements symplectiques d’une variété symplectique dans une autre et plus précisément à leur obstructions (capacités symplectiques). Nous présentons une nouvelle construction d’une suite de capacités symplectiques ainsi que quelques applications et calculs. La conjecture forte de Viterbo énonce que toutes les capacités normalisées coïncident sur les domaines convexes de R^{2n}. Nous en donnons une démonstration en dimension 4 dans le cadre des domaines toriques monotones (que nous introduisons). Nous définissons une nouvelle notion d’équivalence de plongements symplectiques et donnons des exemples de plongements non-équivalents.Le dernier chapitre présente certaines perspectives envisagées pour mes recherches futures.

Organic molecular devices are promising candidates for thermoelectric applications. By using density functional theory combined with non-equilibrium Green’s function method, we have investigated the thermoelectric properties of a single-molecule junction, which is constructed by 2,7-dialkyl [1] benzothieno [3,2-b] [1] benzothiophene derivatives (Cn-BTBT) connected with zigzag graphene nanoribbon electrodes. It is found that the phonon thermal conductance decreases with the increase of alkyl edge branches due to the local resonance. And the thermoelectric performance can be enhanced significantly by substituting bithiophene or biphosphole bridge anchor groups. A good thermoelectric figure of merit about 0.56 can be obtained near Fermi level. Moreover, we also found that the molecular device can be changed from p-type into n-type by changing the contact points, resulting in the transformation of Seebeck coefficient from positive to negative.

A first-order asymptotic analysis of the Griffith energy balance in the Johnson–Kendall–Roberts model of adhesive contact under non-symmetric perturbation of the contact geometry is presented. The pull-off force is evaluated in explicit form. A particular case of adhesive contact between a relatively stiff sphere and an elastic half-space is considered under the assumption that the sphere geometry is changed by the application of an arbitrary lateral normal surface loading. The effect of the sphere Poisson’s ratio on controlling the adhesive pull-off force is considered.This article is part of a discussion meeting issue ‘A cracking approach to inventing new tough materials: fracture stranger than friction’.

Although concentrated photovoltaic (CPV) systems can achieve high energy conversion efficiencies due to their use of concentrating optics and high-efficient multi-junction solar cells, power losses arising from the front surface reflection and metallization shading typically contribute a significant fraction to the total power loss for these devices. There is an opportunity to reduce these losses by optimizing the geometry of the metal contacts, however this requires an understanding of the optical flux and angular distribution of the light delivered to the surface of the cell. This study aimed to determine the influence of secondary optical elements (SOEs) in CPV systems on the optical flux and angular distribution of the light. The optical flux and angular distribution for refractive SOEs with straight and tapered sides were compared using ray-tracing simulations. It is shown that both SOEs can homogenize the flux distribution at the receiver. However, the ray angle range incident on the solar cell receiver remains the same for the homogenizer with straight edges and largely increased for the homogenizer with tapered sides. This has important implications for solar cell metal contact geometry design.

Vision-based tactile sensors have the potential to provide important contact geometry to localize the objective with visual occlusion. However, it is challenging to measure high-resolution 3D contact geometry for a compact robot finger, to simultaneously meet optical and mechanical constraints. In this work, we present the GelSight Wedge sensor, which is optimized to have a compact shape for robot fingers, while achieving high-resolution 3D reconstruction. We evaluate the 3D reconstruction under different lighting configurations, and extend the method from 3 lights to 1 or 2 lights. We demonstrate the flexibility of the design by shrinking the sensor to the size of a human finger for fine manipulation tasks. We also show the effectiveness and potential of the reconstructed 3D geometry for pose tracking in the 3D space., Comment: ICRA 2021

Can every physical system simulate any Turing machine? This is a classical problem that is intimately connected with the undecidability of certain physical phenomena. Concerning fluid flows, Moore [C. Moore, Nonlinearity 4, 199 (1991)] asked if hydrodynamics is capable of performing computations. More recently, Tao launched a program based on the Turing completeness of the Euler equations to address the blow-up problem in the Navier¿Stokes equations. In this direction, the undecidability of some physical systems has been studied in recent years, from the quantum gap problem to quantum-field theories. To the best of our knowledge, the existence of undecidable particle paths of three-dimensional fluid flows has remained an elusive open problem since Moore¿s works in the early 1990s. In this article, we construct a Turing complete stationary Euler flow on a Riemannian S3 and speculate on its implications concerning Tao¿s approach to the blow-up problem in the Navier¿Stokes equations., Robert Cardona was supported by the Spanish Ministry of Economy and Competitiveness, through the María de Maeztu Program for Units of Excellence in R&D (MDM-2014-0445) via an FPI grant. R.C. and E.M. are partially supported by Grants MTM2015-69135-P/FEDER, the Spanish Ministry of Science and Innovation PID2019-103849GB-I00/AEI/10.13039/501100011033, and Agència de Gestió d’Ajuts Universitaris i de Recerca Grant 2017SGR932. E.M. is supported by the Catalan Institution for Research and Advanced Studies via an ICREA Academia Prize 2016. D.P.-S. is supported by MICINN Grant MTM PID2019-106715GB-C21 and MCIU Grant Europa Excelencia EUR2019-103821. F.P. is supported by MICINN/FEDER Grants MTM2016-79400-P and PID2019-108936GB-C21. This work was partially supported by ICMAT–Severo Ochoa Grant CEX2019-000904-S.

Liu et al. recently published a study on friction in heavily loaded lubricated contacts. They compare measurements obtained from tests carried out on two tribometers, at constant entrainment speed, temperature and contact pressure. They observe deviations they attribute to the influence of scale and contact geometry effects. We show, by means of experiments conducted under the same conditions and with the same lubricant, that three alternative effects, somehow dependent on each other, can explain their results: very different elastohydrodynamic conditions, the pervasive presence of significant thermal effects and the occurrence of a mixed lubrication regime in some tests. This discussion provides an opportunity to reconsider how to conduct friction tests with in mind to focus on the lubricant response only. Proposals are presented in terms of operating conditions and lubricant selection, with the subsequent objective to better understand the mechanisms behind friction in highly loaded EHD contacts.

A cryogenic bolometer with Neutron Transmutation Doped Ge as mK thermometer is being developed to study neutrinoless double beta decay (NDBD) in 124Sn. The performance and sensitivity of the sensor strongly depends on the irradiation as well as fabrication (etching, annealing, contact deposition) process. The role of contact fabrication on the sensor performance is presented here. The performace of NTD Ge sensors with wrap-around contact and face-type contacts is compared at T < 100 mK. It is observed that NTD Ge sensors with face-type contacts show Mott behaviour upto lower temperature ~35 mK as compared to that for sensors with wrap-around contacts. Further, detailed noise measurements on several indigeously developed NTD Ge sensors have been carried out in temperature range of 20-70 mK to understand its impact on bolometer resolution and have been described in following section.

In this paper, we are concerned with completely integrable Hamiltonian systems and generalized action–angle coordinates in the setting of contact geometry. We investigate the deformations of the Sasaki–Einstein structures, keeping the Reeb vector field fixed, but changing the contact form. We examine the modifications of the action–angle coordinates by the Sasaki–Ricci flow. We then pass to the particular cases of the contact structures of the five-dimensional Sasaki–Einstein manifolds T1,1 and Yp,q.

We used density functional theory (DFT) calculations to investigate the physical origin of the mechano-chemical response of materials interfaces. Our results show that the mechano-chemical response can be decomposed into the contribution from the interface itself (deformation of interfacial bonds) and a contribution from the underlying solid. The relative contributions depend on the stiffness of these regions and the contact geometry, which affects the stress distribution within the bulk region. We demonstrate that, contrary to what is commonly assumed, the contribution to the activation volume from the elastic deformation of the surrounding bulk is significant and, in some case, may be dominant. We also show that the activation volume and the mechanochemical response of interfaces should be finite due to the effects on the stiffness and stress distribution within the near-surface bulk region. Our results indicate that the large range of activation volumes measured in the previous experiments even for the same material system might originate from the different degrees of contributions probed from the bulk vs. interface.

Fretting wear is a common phenomenon that happens between contact parts when there is an oscillatory relative movement. To investigate wear characteristics history in the fretting process, the finite element method (FEM) is commonly applied to simulate the fretting by considering the wear in the model. In most literature publications, the wear coefficient is considered as a constant, which is not a real case based on the experimental results. To consider the variation of wear coefficient, a double-linear model is applied in this paper, and the tribologically transformed structure (TTS) phase is considered in the study of the wear coefficient variation model. By using these models for variable wear coefficient for both flat and cylinder, the difference of wear characteristics, plastic strain, and stress between variable wear coefficient model (VWCM) and constant wear coefficient model (CWCM) are analyzed. The results show that the variable wear coefficient has no significant effect on the wear characteristic at the end of the process in the gross sliding regime. However, in the partial slip regime, the effect of variable wear coefficient on wear characteristics is significant. Due to the difference in contact geometry in the fretting process between VWCM and CWCM, the tangential and shear stress and equivalent plastic strain also show differences during the fretting process.

International audience; The case of a frictionless contact between a spherical body and a flat metallic glass is studied using a mesoscopic description of plasticity combined with a semi-analytical description of the elastic deformation in a contact geometry (code ISAAC). Plasticity is described by irreversible strain rearrangements in the maximum deviatoric strain direction, above some random strain threshold. In the absence of adhesion or friction, the plastic deformation is initiated below the surface. To represent the singularities due to adhesion, initial rearrangements are forced at the boundary of the contact. Then, the structural disorder is introduced in two different levels: either in the local strain thresholds for plasticity or in the residual plastic strains. It is shown that the spatial organization of plastic rearrangements is not universal, but it is very dependent on the choice of disorder and external loading conditions. Spatial curved shear bands may appear below the contact but only for a very specific set of parameters, especially those characterizing the random thresholds compared to externally induced strain gradients.

We study a long-recognised but under-appreciated symmetry called "dynamical similarity" and illustrate its relevance to many important conceptual problems in fundamental physics. Dynamical similarities are general transformations of a system where the unit of Hamilton's principal function is rescaled, and therefore represent a kind of dynamical scaling symmetry with formal properties that differ from many standard symmetries. To study this symmetry, we develop a general framework for symmetries that distinguishes the observable and surplus structures of a theory by using the minimal freely specifiable initial data for the theory that is necessary to achieve empirical adequacy. This framework is then applied to well-studied examples including Galilean invariance and the symmetries of the Kepler problem. We find that our framework gives a precise dynamical criterion for identifying the observables of those systems, and that those observables agree with epistemic expectations. We then apply our framework to dynamical similarity. First we give a general definition of dynamical similarity. Then we show, with the help of some previous results, how the dynamics of our observables leads to singularity resolution and the emergence of an arrow of time in cosmology., 1 figure. Version accepted to Synthese. Substantive improvements and clarifications

Soft optical tactile sensors enable robots to manipulate deformable objects by capturing important features such as high-resolution contact geometry and estimations of object compliance. This work presents a variable stiffness soft tactile end-effector called StRETcH, a Soft to Resistive Elastic Tactile Hand, that is easily manufactured and integrated with a robotic arm. An elastic membrane is suspended between two robotic fingers, and a depth sensor capturing the deformations of the elastic membrane enables sub-millimeter accurate estimates of contact geometries. The parallel-jaw gripper varies the stiffness of the membrane by uni-axially stretching it, which controllably modulates StRETcH's effective modulus from approximately 4kPa to 9kPa. This work uses StRETcH to reconstruct the contact geometry of rigid and deformable objects, estimate the stiffness of four balloons filled with different substances, and manipulate dough into a desired shape., Comment: 8 pages, 8 figures, To be published in Proc. 2020 IEEE International Conference on Robotics and Automation (ICRA), expanded figures, added clarification for stiffness estimation (Sec. VI-B, paragraph 4)

In the presented work, tribological interactions of tetrahedral amorphous carbon (ta-C) coatings with variations of sp3 content have been systematically studied in the boundary lubrication regime with emphasis on fatty acid-based lubricants. Manifold phenomena regarding wear and friction have been observed that exceed the conventional understanding of lubricant-surface interaction. Distinct superlubricity behavior of ta-C was found when lubricated with glycerol or oleic acid. It was shown that both the acid function and the double bond in the lubricant molecules are essential features to achieve superlubricity. Superlubricity is not confined to self-mated ta-C surfaces, but also happens in contact to steel. The contact situation has important influence on tribological behavior. Wear rates change by 1–2 orders of magnitude by altering the contact geometry and loading conditions of an oleic-acid-lubricated ta-C/steel pairing. This ta-C wear phenomenon is attributed to tribochemical wear.