Single Asperity Contact Research Articles

Development of a model for predicting the flash temperature in single asperity contact between TC4 titanium and Q235 steel

The friction hot surfaces generated by metal under low-speed friction is the main potential ignition source for explosive atmosphere, while the friction flash temperature is often much higher than the bulk temperature under the same conditions. Then the first to reach the ignition temperature of the combustible gas is the bulk temperature of the hot surface or the flash temperature of the contact of the asperities, which is worth studying. In view of this, by establishing a friction model, this paper conducts an in-depth study on the flash temperature of the asperities contact between the of TC4 titanium alloy and Q235 steel at the friction conditions of low speed and low load. According to the Hertz contact theory, the contact process of a single pair of asperities and the calculation method of the maximum flash temperature are analyzed, and the maximum flash temperature value is calculated using two assumptions. Combined with regression analysis, the mathematical model of the flash temperature with load and relative velocity is obtained. Under the same conditions, the maximum bulk temperature and flash temperature are calculated, so as to study the real effective ignition source of the gas at the friction conditions of low speed and low load.

Contribution of hardness and work hardening to the wear resistance of Pt-based metallic surfaces revealed by nanoscale reciprocating sliding

The contribution of hardness and work hardening to the wear resistance of Pt-based metallic surfaces under reciprocating sliding contact have been investigated by atomic force microscopy. The atomically flat surfaces of the Pt-BMG and Pt (111) enable the sliding friction to be a single-asperity contact. It was found that at low loads where the sliding contact was primarily elastic, the Pt(111) produced a larger adhesion than the Pt-BMG, which was ascribed to the larger mixing enthalpy between counterbodies. When the sliding contact entered into wear regime at high loads, the Pt-BMG that displayed a high hardness nearly-six times of Pt (111) was expected to have a higher wear resistance according to the Archard’s law, yet the Pt(111) actually performed better. We further explore that the work hardening mechanism associated with dislocated-mediated deformation contributes to the high wear resistance of Pt(111), as corroborated by a significant friction force decrease within scratched region on Pt(111) surface. However, such a hardening mechanism is not expected to operate in Pt-BMG due to the lack of mobile structural defects like dislocations, as confirmed by the little change in friction force between scratched and unscratched regions.

Single-asperity failure mechanism driven by morphology and multiaxial loading using molecular dynamics simulation

Single-asperity contact behavior under multiaxial loading is of considerable effect between mechanical contact surfaces. Taking α-Fe as the paradigm material, a single-asperity model under multiaxial loading is established using molecular dynamics (MD) methods. The displacement behavior of the rigid plane, normal loading and asperity radius effects on the failure mechanism are analyzed. The results show that: (i) Normal loading affects the asperity normal deformation state, leading to different failure mechanisms in tangential slip. The failure tends to occur near the contact surface under low normal loading, but inside both the asperity and substrate under high normal loading. (ii) Different asperity radii may cause the asperities to be in different initial deformation states. The failure mode could be judged by combining the normal loading and asperity radius. The results provide detailed description of the asperity failure mechanism and could be applied in the analysis and design of contact interfaces.

Direct Multimodal Nanoscale Visualization of Early Phosphorus-Based Antiwear Tribofilm Formation.

Understanding the mechanism of antiwear (AW) tribofilm formation and how to tune surface chemistry to control functionality is essential for the development of the next generation of oil lubricants. In particular, understanding and optimizing early AW tribofilm formation can increase the energy efficiency of mechanical systems. However, the mechanism for how these films form is not well understood. The majority of prior work has focused on analyzing only end-of-test surfaces long after the film has formed. In this work, we develop an in situ multimodal chemical imaging methodology to directly visualize the early formation of AW films on steel surfaces. We investigate an oil formulation containing a phosphorus-based additive commonly used to protect surfaces from wear and fatigue processes in machine elements, such as gears, bearings, and sliding contacts. Using nanoscale multimodal chemical imaging on combined platforms of atomic force microscopy (AFM) coupled directly with in situ nano-infrared (nano-IR) spectroscopy, and further combined ex situ with time-of-flight secondary ion mass spectrometry (ToF-SIMS), we demonstrate a direct correlation between changes in friction and local surface chemistry. In these experiments, the AFM probe acts as a single asperity contact to generate the tribofilm as well as a tool to analyze it in situ as it is forming. To verify our in situ measurements, we compare these results to the ex situ ToF-SIMS of macroscale block-on-ring tribometer-formed samples. The understanding gained here on how AW films form and how film properties can be modified by tuning the chemistry of the additives will facilitate developing transmission fluids to meet increasing demands for vehicle performance and efficiency.

Single asperity sub-nano to nanoscale wear and tribocorrosion of wrought CoCrMo and additively manufactured CoCrMoW alloys

Wear and tribocorrosion of CoCrMo alloys have been associated with adverse local tissue reactions. The tribological and electrochemical processes that arise during single asperity contact at the scale of passive oxide films are poorly understood. An AFM-based ‘image-wear-image’ tribology method was applied to study the sub-nanoscale wear and tribocorrosion of traditional and additively manufactured CoCrMo and CoCrMoW alloys. Atomic wear and elasto-plastic driven wear were observed at wear depths near the oxide thickness. Contact stress, alloy, and environment significantly affected average nanowear depth. Surface degradation varied from grain to grain within LC CoCrMo and by intragranular cellular structures within AM CoCrMoW.

Atomic-scale friction between single-asperity contacts unveiled through in situ transmission electron microscopy

Friction and wear are detrimental to functionality and reduce the service life of products with mechanical elements. Here, we unveil the atomic-scale friction of a single tungsten asperity in real time through a high-resolution transmission electron microscopy investigation of a nanocontact in countermotion, induced through a piezo actuator. Molecular dynamics simulations provide insights into the sliding pathway of interface atoms and the dynamic strain/stress evolution at the interface. We observe a discrete stick-slip behaviour and an asynchronous process for the accumulation and dissipation of the strain energy together with the non-uniform motion of interface atoms. Our methodology allows for studying in situ atomic-friction phenomena and provides insights into friction phenomena at the atomic scale.

Shear-assisted contact aging of single-asperity nanojunctions

Contact aging, i.e., the strengthening of a tribological contact with time, is a phenomenon that is commonly attributed to an increase of the effective contact area between multiasperity surfaces based on contact mechanical processes. Only recently, atomic force microscopy experiments and simulations on single silica nanojunctions have also demonstrated logarithmic aging with contact time for single-asperity contacts and the effects were attributed to a gradual formation of chemical bonds in the contact, thereby suggesting an atomistic mechanism of ``contact aging.'' Generally, this atomic bond-formation process is considered as a shear-assisted thermally activated process, where both temperature and loading conditions are expected to influence the aging behavior. A direct link between aging and temperature has recently been substantiated by temperature-dependent aging experiments and complementary molecular dynamics simulations, but the potential influence of shear stress on single-asperity aging still remains largely elusive. To analyze the role of shear stress, we now apply a simple and semianalytical model for thermally activated bond formation, which explicitly considers the reduction of energy barriers by a shear-stress-related Eyring term. Based on this model, characteristic fingerprints of mechanical shear can be identified in our temperature-dependent contact aging experiments, including, for instance, the counterintuitive decrease of aging effects with increasing temperature. Our results thereby hint at a path to introduce this form of qualitative contact aging to the phenomenological rate and state friction, where the local aging effects might be closely linked to the shear-stress distribution within multiasperity contacts.

Contribution of Capillary Adhesion to Friction at Macroscopic Solid–Solid Interfaces

Capillary adhesion is commonly present in ambient conditions. It can be measured in single-asperity contacts through atomic force microscopy using a sharp probe that is pulled off a smooth substrate. However, for macroscopic multiasperity interfaces, the measured adhesive force is always close to zero because of the elastic energy stored into the deformation of surface roughness; this is known as the adhesion paradox. Here, we experimentally show how capillary adhesion influences friction between macroscopic $\mathrm{Si}$-on-$\mathrm{Si}$ interfaces, covered with native oxide, in two vapor environments: humid air and isopropyl alcohol (IPA) vapor. To quantify the adhesion contribution to friction, we present a boundary element method that successfully models the interplay between capillary adhesion, surface topography, and friction without adjustable parameters and show that the evolution of the surface topography during sliding dramatically increases capillary adhesion and thus friction. Replacing the water vapor with an organic (IPA) vapor, we find a lower adhesion due to the smaller surface tension.

Editorial: Tribology and Atomic Force Microscopy – Towards Single Asperity Contact

Editorial: Tribology and Atomic Force Microscopy – Towards Single Asperity Contact

The Contact Mechanics for Indentation of Single Asperity and Rough Surfaces

Abstract A finite element analysis of a rigid sphere contact with a deformable elastic–plastic plat called indentation model is studied. The numerical results are applied on the rough surfaces contact of the Greenwood-Williamson (G-W) model. A series of the relationships of the rough surfaces contact parameters are obtained. The contact parameters of the indentation model and the flattening model are compared in detail, and the reasons for their differences are analyzed. In the case of single asperity contact, for ω/ωc > 1, the indentation model reaches the initial plastic yield while the flattening model is ω/ωc=1. In ω/ωc=10, the plastic yield reaches the contact surface for the first time, and the corresponding point of the flattening model is relatively earlier in ω/ωc=6. The contact parameters of the rough surface in different plasticity indexes are compared again. On the point of ψ = 0.5, the contact parameters of the flattening model and the indentation model coincide perfectly. For 0.5 < ψ < 4, the difference between the parameters curves become larger and larger. To the point of ψ = 4, when the distance difference reaches the maximum, it begins to decrease until the two curves are close to coincide again. The dimensionless elastic–plastic contact hardness is introduced. The relation between the real contact area and the contact pressure of the indentation model can be acquired quickly. The results show that the geometric shape of deformable contact parts has an important effect on the contact parameters, especially for the extension of the plastic deformation region within a specific range of plasticity index.

Predicting the corrosion-wear response of an isolated austenite phase under anodic polarization

The wear-assisted corrosion response of a super-duplex stainless-steel from a single asperity contact was obtained using atomic force microscopy (AFM). The AFM tip applied increasing loads to an isolated surface region that was polarized to two different static potentials in the passive potential region of the alloy in a 0.6 M NaCl solution. The wear-assisted corrosion response was modeled using an interfacial film growth model to obtain consistent kinetics parameters for the austenite phase response across applied loads and polarization potentials. The average response was then fit with an Arrhenius model that suggested that the stress fields from the single asperity contact induced damage in the protective oxide but also disrupted other oxide-formation processes at the oxide-electrolyte interface, thereby slowing repassivation, and allowing increasingly high corrosion currents as the stress fields increased.

In situ nanoscale evaluation of pressure-induced changes in structural morphology of phosphonium phosphate ionic liquid at single-asperity contacts.

In this work, we perform atomic force microscopy (AFM) experiments to evaluate in situ the dependence of the structural morphology of trihexyltetradecylphosphonium bis(2-ethylhexyl) phosphate ([P6,6,6,14][DEHP]) ionic liquid (IL) on applied pressure. The experimental results obtained upon sliding a diamond-like-carbon-coated silicon AFM tip on mechanically polished steel at an applied pressure up to 5.5 ± 0.3 GPa indicate a structural transition of confined [P6,6,6,14][DEHP] molecules. This pressure-induced morphological change of [P6,6,6,14][DEHP] IL leads to the generation of a lubricious, solid-like interfacial layer, whose growth rate increases with applied pressure and temperature. The structural variation of [P6,6,6,14][DEHP] IL is proposed to derive from the well-ordered layering of the polar groups of ions separated by the apolar tails. These results not only shed new light on the structural organization of phosphonium-based ILs under elevated pressure, but also provide novel insights into the normal pressure-dependent lubrication mechanisms of ILs in general.

Analysis of the Wear-Assisted Corrosion of a Super-Duplex Stainless-Steel Using an Interface Film Growth Model

The wear-assisted corrosion response of a super-duplex stainless-steel alloy from a single asperity contact was measured using the contact mode of an atomic force microscope (AFM). The AFM tip, as illustrated in the figure below, applied increasing loads to an isolated surface region that was polarized to two different static potentials in a 0.6 M NaCl solution. The AFM tip was swept across the isolated, exposed, surface region of the alloy while the alloy response was measured by a potentiostat. The wear-assisted corrosion response was modeled using an interfacial film growth model (IFM) that was used to estimate kinetics parameters for the response of the alloy as a function of the applied loads and polarization potentials.The average response values from the IFM were then approximated using an Arrhenius expression that incorporated the peak applied stress and activation volume for the reaction as modifiers to the energy barrier for oxide film formation. This Arrhenius model suggested that the stress fields from the single asperity contact induced damage in the protective oxide and also disrupted other oxide-formation processes at the oxide-electrolyte interface. The induced damage and the disruption of the oxide-formation processes, as a function of the applied stress, slowed repassivation and allowed increasingly high corrosion currents as the stress fields increased.AcknowledgmentsThe U.S. Naval Research Laboratory sponsored this work under work unit 1E19 as part of its Basic Research Program. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the Office of Naval Research, the U.S. Navy, the Department of Defense, or the U.S. government.Figure CaptionSchematic diagram of experimental setup of the AFM tip loading the isolated region on the super-duplex stainless-steel alloy surface while the potentiostat is used to both polarize the alloy and measure its response to the load. Figure 1

Atomistic observation on diffusion-mediated friction between single-asperity contacts.

The field of nanotribology has long suffered from the inability to directly observe what takes place at a sliding interface. Although techniques based on atomic force microscopy have identified many friction phenomena at the nanoscale, many interpretative pitfalls still result from the indirect or ex situ characterization of contacting surfaces. Here we combined in situ high-resolution transmission electron microscopy and atomic force microscopy measurements to provide direct real-time observations of atomic-scale interfacial structure during frictional processes and discovered the formation of a loosely packed interfacial layer between two metallic asperities that enabled a low friction under tensile stress. This finding is corroborated by molecular dynamic simulations. The loosely packed interfacial layer became an ordered layer at equilibrium distances under compressive stress, which led to a transition from a low-friction to a dissipative high-friction motion. This work directly unveils a unique role of atomic diffusion in the friction of metallic contacts.

Nanoscopic humidity-dependent adhesion behaviors of 2D materials

Two-dimensional materials have gained significant attention in the research community due to their unique physicochemical and mechanical properties. Regarding their applications in devices irrespective of the involved length scale, interfacial adhesion plays an important role in nanoscopic single-asperity and macroscopic multi-asperity contacts. Therefore, this study aims at studying the adhesion behaviors of few-layered Ti3C2Tx, graphene oxide and WSe2 nano-sheets as a function of the adjusted relative humidity by atomic force microscopy. Reference experiments are conducted with graphite and silicon surfaces. The experimental results for graphene oxide and WSe2 nano-sheets show a monotonous increase of the adhesion force with increasing humidity. In contrast, silicon reference and bi-layered Ti3C2Tx nano-sheets show an increasing adhesion with increasing humidity before reaching a maximum value at a critical humidity followed by a pronounced decrease. Based upon the obtained adhesion and characterization data, the measured water adsorption behavior, and the existing knowledge for silicon regarding their superficial water structures, the observed experimental trends were connected with the respective surface groups of the nanomaterials and their ability to adsorb water on their surfaces.

Inelastic contact behaviors of nanosized single-asperity and multi-asperity on α-Fe surface: Molecular dynamic simulations

Contact surfaces are widely used in precision machines for transmitting force and torque, whose characteristics are crucial. The onset of plasticity during contact at microscale is different from macroscopic behavior. Taking α-Fe as the paradigm material, inelastic behaviors of nanosized single-asperity and multi-asperity contact on the α-Fe surface were investigated using molecular dynamics (MD) simulations. The results show that (i) the macroscopic single-asperity model limits to accurately describe the nano-sized contact behavior, especially the estimation of the onset plasticity, (ii) the offset distance can delay the onset plasticity, (iii) compared with the MD simulation, the Greenwood-Williamson (GW) elastic model has a higher estimation on the contact force, while the Chang-Etsion-Bogy (CEB) and other revised-CEB elastoplastic models underestimate the contact force.

Tracing single asperity wear in relation to macroscale friction during running-in

The running-in wear of a multi-asperity silicon carbide sphere-on-silicon flat interface is investigated at the micro- and nanoscale in relation to the friction behaviour of an unlubricated macroscale tribological system sliding in a unidirectional mode. Experiments and contact simulations indicated that the macroscale friction behaviour during running-in was governed by the wear behaviour of roughness asperities on the sphere and their influence on the interfacial contact pressure. Specific ploughing tracks on the flat corresponded to individual asperities on the sphere which, when worn-off, led to lower, more stable friction behaviour and mild wear at an atomic attrition-like rate. It was also found that single asperity contact simulations are unable to reliably predict multi-asperity friction and wear behaviour for this system.

Green's function method for dynamic contact calculations.

Resolving atomic scale details while capturing long-range elastic deformation is the principal difficulty when solving contact mechanics problems with computer simulations. Fully atomistic simulations must consider large blocks of atoms to support long-wavelength deformation modes, meaning that most atoms are far removed from the region of interest. Building on earlier methods that used elastic surface Green's functions to compute static substrate deformation, we present a numerically efficient dynamic Green's function technique to treat realistic, time-evolving, elastic solids. Our method solves substrate dynamics in reciprocal space and utilizes precomputed Green's functions that exactly reproduce elastic interactions without retaining the atomic degrees of freedom in the bulk. We invoke physical insights to determine the necessary number of explicit substrate layers required to capture the attenuation of subsurface waves as a function of surface wave vector. We observe that truncating substrate dynamics at depths that fall as a power of wave vector allows us to accurately model wave propagation without implementing arbitrary damping. The framework we have developed substantially accelerates molecular dynamics simulations of large elastic substrates. We apply the method to single asperity contact, impact, and sliding friction problems and present our preliminary findings.

Study of wear particles formation at single asperity contact: An experimental and numerical approach

Wear particles generated due to the relative sliding of the roll and the sheet metal in the roll bite are one of the main factors that contaminate the surface of a cold rolled steel sheet. The details of the wear mechanisms in the contact of individual roll-sheet asperities define the total amount of wear particles generated. In this paper, a micro-mechanical experimental approach is coupled with material point method (MPM) scratch simulations to study the friction and wear behavior of a single roll asperity sliding through a sheet metal. Micro-tribology experiments in the form of single asperity scratch testing showed that ploughing is the dominant wear mechanism in lubricated conditions, while wedge forming was the main wear mechanism in the absence of lubricant. The beneficial influence of chrome plating the rolls on wear particles formation is found to stem from its interaction with the lubricant as the same influence was not observed in non-lubricated test conditions. MPM single asperity simulations revealed that almost all the frictional resistance arises from deforming the substrate in the case of an interfacial shear strength corresponding to the lubricated contact. In contrast, MPM showed that, in unlubricated sliding, the frictional resistance is primarily due to shearing of the adhesion junction and tearing the deforming body. Furthermore, using the degree of wear of the scratch experiments as a benchmark, a critical plastic strain needed to produce wear particles was found to be between 3 to 4 for the investigated interstitial-free steel.

Effect of H2O2 Solution’s pH on the Human Enamel Micro and Nanowear

The effect of 30% H2O2 solutions with different pH (2, 4 and 6) on the wear of human dental enamel was studied. Additionally, the whitening efficiency and changes in hardness and morphology were evaluated. The enamel tribological response was accessed through two different approaches: ball-on-plate and atomic force microscopy (AFM), the later following a tribological model based on a single asperity contact. The results showed differences in the specific wear rate and wear mechanisms between the two used approaches. Regardless of the tribological approach used, the wear resistance of enamel was lower for pH = 2. It was observed that pH = 6 leads to a safe and efficient whitening treatment.