Velocity Accommodation Research Articles

Multi-scale morphological analysis of brake lining surface and load-bearing arrangement in the contact

In the case of braking, a diversity of friction mechanisms occurs at the interface, caused by the heterogeneity of the composition, properties, and thermal phenomena. After friction, the obtained surfaces of the brake lining materials are highly heterogeneous. Load-bearing plates which are also called contact plateaus are formed with a compacted third body and constitute the sites of energy dissipation and sliding velocity accommodation. Their arrangement and preferential localizations in the contact constitute a key factor for the understanding of friction-induced phenomena and braking performance. Within this framework, a morphological characterization method has been set up based on several measurement techniques to explore the macroscopic and microscopic contact situation of the pad-disc system. The results show that exploiting Abbott curve parameters combined with roughness parameters is an effective way to give a detailed and comprehensive morphological description of the brake pad friction surface. It can be seen that the contact plateaus have preferential locations which give a particular load-bearing arrangement in the pad-disc interface which can be explained by friction and wear mechanisms. Results reveal also the main encountered difficulties, especially regarding the significance of scales and parameters describing the morphology of the friction material surface and the load-bearing plates disposition.

Enhancing the tribological performance of hydroxypropyl methylcellulose composite coatings through nano-sized metal and oxide additives: A comparative study

Cellulose, due to its excellent mechanical and tribological properties and eco-friendliness, is suitable for environmentally friendly applications. However, its application is limited due to susceptibility to wear and damage under stress during usage. This study introduces nanosized aluminum, copper, alumina, and copper oxide into composite materials to tackle the challenges of wear and damage. This approach aims to improve the tribological performance of Hydroxypropyl methylcellulose (HPMC) composite coatings significantly. The additive particles enhanced the load-bearing capacity and wear properties of cellulose-based composite coatings. The velocity accommodation mode was provided by nano-metals through S3M2 (interaction of metal particle additives through a third-body mechanism), while for nano-oxides, it was S3M4 (expulsion of nanoparticles from the wear mark to the outer contact zone). Due to the higher specific stiffness and strength of oxides, nanofillers in the composite offered better load resistance than nano-metals, resulting in a smaller actual contact area during wear. Therefore, metal oxide nanofillers enhanced tribological properties more effectively than metal nanoparticles. Finally, detailed explanations of the tribological evidence and discussions on the mechanism were conducted.

Effect of film thickness and temperature on condensation and momentum accommodation at the liquid–vapor methane interphase in contact with a quartz substrate

In this paper, we used the Molecular Dynamics method to simulate the equilibrium vapor–liquid methane in contact with a solid quartz substrate and study the condensation process and momentum exchange at the atomic level. A large potential cutoff radius is used to determine the vapor and liquid densities at good accuracy. By tracking the motions of fluid molecules exchanged between the vapor and liquid phases and carrying out statistical analysis of residence time, penetration depth and especially velocity correlation, the mass and the velocity accommodation coefficients can be determined at the same time. The latter is based on the assumption that the reflected events take place over a short time, near the interface and thus atoms velocities are correlated while the condensation/evaporation events are not. The calculated coefficients are sensitive to the film thickness due to the presence of the quartz substrate and the layering effects. As temperature increases, atoms condense less and reflect more and the reflection is more diffusive due to increasing collision rate. Near the critical temperature, both the condensation and momentum accommodation coefficients vary significantly.

Microstructural evolution and tribological behavior of suspension plasma sprayed CuO as high-temperature lubricious coatings

Suspension plasma spraying (SPS) was used to produce three different CuO coatings by varying spray distance and solid loading within the suspension. The low spray distance (30 mm) and high solid loading (20 wt%) in the suspension resulted in the deposition of dense coatings as compared to other spray conditions (i.e., a distance of 40 mm and solid loading of 10 wt%). The tribological performance of the dense coating was investigated at 25 °C and 300 °C under dry sliding conditions. Scanning electron microscopy (SEM), X-ray diffraction (XRD), and Raman analysis were used to characterize the coating, and corresponding wear tracks to determine the wear mechanisms. The results showed that the friction coefficient of the CuO coating decreased from 0.7 at 25 °C to 0.46 at 300 °C. Similarly, the wear rate at 300 °C decreased by 50% compared to the wear rate at 25 °C. Ex-situ analysis showed an increased oxygen content on the worn surfaces at 300 °C and the formation of a smooth uniform tribofilm. At 25 °C, a thicker transfer film was observed on the counterface, which leads to a different velocity accommodation mode and, thus, relatively high friction.

Sliding wear and rolling contact fatigue behavior of M50 steel coated with atomic layer deposited lubricious oxide nanolaminates

Atomic layer deposition (ALD) was employed to deposit highly conformal and uniform nanolaminate coatings of ZnO/Al2O3/ZrO2/Al2O3 on M50 bearing steel substrates for sliding wear testing, as well as on CrN coated M50 bearing steel cylindrical rods and cups for three-ball on rod rolling contact fatigue (RCF) testing. The nanocrystalline ZnO top layer was structurally-engineered to achieve low surface energy, textured (0002)-oriented grains. To achieve this preferred orientation, amorphous Al2O3 was deposited as a seed layer, while crystalline ZrO2 acted as a high toughness/load bearing layer. The ALD nanolaminates reduced both the sliding wear rate (2 × 10-7 mm3/N·m) and enhanced the RCF resistance of M50 steel out to 6 million rolling contact fatigue cycles with no signs of spall. Based on cross-sectional transmission electron microscopy studies inside the wear surfaces, it was determined that sliding and rolling-induced plastic deformation was observed in ZnO layer where basal plane stacking faults were sheared parallel to the sliding and rolling directions. This intrafilm shear velocity accommodation mode mitigates the friction, reduces wear, and inhibits brittle fracture. Therefore, ALD ZnO/Al2O3/ZrO2/Al2O3 nanolaminates are candidate coatings that provide both sliding and RCF resistance in moving mechanical assemblies that require thin (∼10-300 nm), uniform and conformal solid lubricants.

Aluminum and alumina/MoS2/cellulose derivative composite: design and performance

Nanoparticles were added to improve the tribological performance of the biopolymer-based composite films. Aluminum and alumina were used as additives. The matrix of the composite was MoS2/hydroxypropyl methylcellulose (HPMC). The ternary additive/MoS2/HPMC hybrid composites were successfully prepared via solvent evaporation. The surface morphology, thickness, microstructure, and wear scars were analyzed using scanning electron microscopy. X-ray diffraction was used to analyze the crystal structures of the nanoparticles in the composite films. Finally, a wear test was conducted to determine the tribology behavior and was discussed using the third-body theory. Because of the high surface-area-to-volume ratio of the additives, nanoparticles were exposed and densely distributed on the composite surface. Disclosed nanoparticles caused peaks and valleys and showed more significant undulations, prompting a highly rough surface. The addition of nanoparticles enhanced the load capacity of the composite films by 155%. In the meantime, nanoparticle additives significantly reduced the coefficient of friction by 50% and improved anti-wear performance by five times. The nanoparticles in the wear scar exhibited an excellent third-body mechanism during the wear process, coordinating the velocity accommodation mode between the two rubbing surfaces and the transfer load.

Simulation of Energy Dissipation and Heat Transfers of a Braking System Using the Discrete Element Method: Role of Roughness and Granular Plateaus

Abstract The objective of this study focuses on the energy dissipation by friction on the interface of a braking system and the effects of roughness and granular plateaus on heat propagation. Faced with the difficulty of defining velocity accommodation and thermal partition between the two bodies in contact (disk and pad, for example,), the authors model the third body (friction) layer with circular particles detached from the pad. From a numerical point of view, this paper proposes a strategy of storing mechanical calculations in steady-state and using it for successive thermal processing in discrete element method (DEM) code. Thus, the heat is generated due to interparticle friction and is dissipated in the disk/pad interface by conductance. Accordingly, this coupling micro–macro model aims to determine the temperature rise of the pad/disk interface and to identify the equivalent thermal resistance. In line with that, the authors provide discussions of these parameters compared to experimental/empirical data as reported in the literature review and limitations of the model.

Solid Flow Regimes Within Dry Sliding Contacts

In this paper, we investigate the regimes of velocity accommodation and of load transmission within a dry sliding interface, in the presence of a solid and discontinuous interfacial layer (the so-called third body). To that end, an appropriate numerical framework called the multibody meshfree approach is used to implement a local model of such an interface, and a comprehensive dimensionless parametric study is performed in order to analyse the influence of the mechanical properties of the third body on the interfacial solid flow regimes, on the friction coefficient, and on the modes of energy dissipation. To that end, the concept of partial coefficient of friction is introduced. The numerical results demonstrate that the friction in the interface is limited by changes in the kinematics of the shear accommodation in the third-body layer and by the activation of different modes of energy dissipation (related either to surface area creation/destruction in the third-body layer or to bulk deformation of the solid matter composing it) which are uncorrelated in the parametric space of the mechanical properties of the third-body particles.

Micro morphological observation and mechanism analysis of boundary layer evolution in mixed powder lubrication

AbstractIn mixed‐powder‐lubricated interfaces, the lubrication performance is strongly affected by evolution of the boundary layer features. Therefore, the current investigation uses a laser microscopy system to observe the behaviours of boundary layer formed on the brass specimens during the process of graphite‐lubricated 0.45 C steel (HRC56) sliding upon brass alloy (HV160). The evolution process of mixed powder lubrication can be divided into 4 typical stages: running‐in, stable, deterioration, and damage. Multiple micro phenomena are observed in the boundary layers at different stages. These include extent of layer coverage, flaking, local adhesion, lamellar shedding, blisters, pits, and abrasion damage. These phenomena indicate that the boundary layer deteriorates from full coverage to a damaged powdery film, and eventually the serious wear on metal substrate. Moreover, an interface model of mixed powder lubrication is established to explain the changes in boundary layers. The model displays that the typically micro events above are related to the quantitative degree of overlapping of the asperities between opposing surfaces. At first, many particles bear the load collectively and adapt to velocity differences through inner shearing in powder layer. Later, plastic deformation and abrasion between asperities play a major role in surface velocity accommodation, and the metal substrates bear the load more directly.

The effect of contact stress on the sliding wear behaviour of Zn-Ni electrodeposited coatings

Electrodeposited zinc-nickel coatings, developed in the 1980's as a replacement for zinc coatings in the automotive industry, have recently gained interest in the aerospace industry to replace cadmium coatings. Due to different material properties of Zn-Ni and Cd, there is a need to characterize Zn-Ni for tribological applications. Sliding wear tests are performed on a reciprocating pin-on-flat tribometer using a steel counterface on two electrodeposited Zn-Ni coatings with different microstructure and surface topography. Tests were performed under 3, 7.5 and 12N normal loads at a relative humidity of 60% for 2000 cycles. Increasing the normal load increased the steady state friction coefficient and wear for both coatings. The smooth and dense coating was more sensitive to the change in normal load than the rough and porous coating, as the latter experienced less wear due to the columnar structure of the coating. In contrast, the smoother and dense coating, although has less wear at low loads, has more wear at high loads due to debonding of the coating. The coating morphology affected the extent of wear due to different wear and velocity accommodation mechanisms.

Tribological Analysis of UHMWPE Tibial Implants in Unicompartmental Knee Replacements: From Retrieved to In Vitro Studies

This study focuses on the identification of velocity accommodation mechanisms of UHMWPE tibial implants from unicompartmental knee replacements.A tribological analysis of two retrieved medial unicompartmental knee implants was performed. The first implant presented a high wear rate at 7years follow-up with the velocity accommodation mechanisms characterized by the formation of a plasticized thin and smooth 3rd body layer which was drawn until small UHMWPE particles were detached. The second implant presented good performances at 11years follow-up with a lower wear rate, with the velocity accommodation mechanisms characterized by the formation of a discontinuous 3rd body layer composed of small UHMWPE wear particles and biological molecules from synovial fluid.An important finding of the tribological analysis of the two retrieved UHMWPE tibial implants is that the 3rd body layer plays a key role in the wear rates, influencing the velocity accommodation mechanisms.An in vitro study of two identical UHMWPE tibial implants using two lubricating solutions, with different compositions and structures (a calf serum and a Gel-IN calf serum) was performed to explain the root causes of the major difference between the 3rd body layers formed in vivo.It was observed that the calf serum led to a high friction factor, as the molecules were unable to bind to the UHMWPE surface. However, the Gel-IN calf serum led to a lower friction factor, with the formation of a 3rd body layer on the UHMWPE surface due to protein and lipid adsorption.

Microstructure, mechanical properties and friction behavior of magnetron-sputtered V-C coatings

This study describes reciprocating sliding friction experiments of three types of magnetron-sputtered vanadium carbide/amorphous carbon nanocomposite coatings, VC1−x/a-C, against alumina counterparts under unlubricated conditions. It links micro- to macroscale tribology in order to provide a better understanding of the sliding mechanisms of these nanocomposite coatings with varying constitution and microstructure (i.e. by varying the C/V concentration ratio between 1.2 and 2.9). Correspondingly, the volume fraction of the amorphous carbon phase (a-C) increases with increasing the carbon content as shown by electron microprobe studies, X-ray diffraction and Raman spectroscopy; simultaneously, the volume fraction of nanocrystalline vanadium carbide (VC1−x) decreases, and the crystallite size and crystallinity of the thin films decrease with increasing carbon content. These constitution and microstructure changes have an impact both on the mechanical and the tribological properties of the coatings. Mechanical properties characterization shows that the indentation hardness (H) and the reduced modulus (Er) of the coatings decrease with increasing the carbon concentration (i.e. H ranges between 15 and 33GPa and Er ranges between 239 and 391GPa for the low and high vanadium concentration respectively). A similar behavior was also observed with the results on the critical load of failure obtained from scratch tests, where the coatings with the highest vanadium concentration exhibits higher critical load of failure and thus, better coating adhesion. However, reciprocating micro- and macroscale tribological tests performed at a relative humidity level of 45±5% (i.e. normal load in the microtribological experiments is between 100mN and 1N and for the macroscale tribometry the normal load is 20N) reveal higher friction values and increased wear with the high vanadium content coatings. This sliding behavior is attributed to differences in the third body formation (i.e. carbon based transfer- and tribo-film) and velocity accommodation modes, which are analyzed ex situ by means of X-ray photoelectron spectroscopy (XPS), micro-Raman spectroscopy and atomic force microscopy.

Tribological characterisation of UHMWPE used in dual mobility total hip prosthesis

Total hip arthroplasty represents an effective solution for bone and joint diseases. Nevertheless, the hip prosthesis has a limited lifetime, in the average around fifteen years. Their improvement, especially their dual mobility is the objective of this study. Therefore, our strategy is focused on improving the material by comparing three types of polyethylene to determine the best one from a friction mechanism and wear rate minimization standpoint. A dual mobility hip prosthesis, containing a two-sided steel and cobalt chrome cup, was tested with a TORNIER hip joint simulator in calf serum. The rubbed surfaces were characterized using scanning electron microscopy (SEM), contact angle measurements, atomic force microscopy (AFM) and confocal fluorescence microscopy. All these multiscale characterization techniques (from nanoscale to millimeter and micro- scale) showed that the velocity accommodation mechanism is different from one type of polyethylene to another. The wear in the case of standard polyethylene was noticeable and the particles were large and scattered between the surface of polyethylene, the surface of the cup and in the calf serum. For the crosslinked polyethylene, the particles coming from the wear, were not as large, but they were spread the same way as the first case. Even though it shares the same accommodation principle on the detachment of the material with the crosslinked polyethylene the wear particles for the crosslinked vitaminized polyethylene were large and they were only found on the surface of the polyethylene.

A tribological approach to understand the behavior of oral-care silica during tooth brushing

This study focuses on the tribological behavior of oral care silica on the acquired enamel pellicle reproduced ex vivo. The work presents a simplified biomimetic model of the protein coated enamel. It highlights the complexity of toothbrushing mechanisms and helps to understand them. Toothbrushing reciprocating-motion tests were performed with a custom-made tribometer allowing an in situ visualization of glass samples (functionalized with a protein layer or not), toothbrush, and silica-based slurry. Samples were analyzed by correlating ESEM imaging and AFM in tapping mode measurements, adopting the third body approach. Third body behavior was correlated with the evolution of the friction coefficient, in order to establish the velocity accommodation mechanisms. The possible modifications/damages of the biomimetic enamel pellicle are identified and described.

Tribological Behavior of a Cold-Sprayed Cu–MoS2 Composite Coating During Dry Sliding Wear

Two cold spray coatings, one pure Cu and the other a Cu–MoS2 composite coating, were studied for their tribology performance in dry air. It was demonstrated that a small amount of MoS2 (1.8 ± 0.99 wt%) could significantly decrease coefficient of friction (CoF) from around 0.7 (Cu coating) to 0.14–0.15. MoS2 patches on the wear track exhibited a lower local CoF, and the main velocity accommodation mechanism was shearing MoS2-containing debris. Even though the coating wear rates were high in the early sliding (8.61–12.8 nm/cycle in penetration depth during the first 100 cycles), slow wear (0.12–0.22 nm/cycle) over the subsequent sliding was observed. It was also found that the presence of MoS2 helped to achieve high endurance of the first steady-state CoF. The dynamics of the process, material transfer, and phase transformation were examined using scanning electron microscopy, energy-dispersive X-ray spectroscopy, and Raman spectroscopy. The MoS2 patches developed on the wear track and the counterface served as reservoirs to replenish MoS2 in the contact and became depleted with sliding. Cross-sectional microstructure revealed by electron channeling contrast imaging technique showed a layer of sliding-induced microstructure, 3–5 µm thick for the Cu–MoS2 coating, and 10–30 µm thick for the Cu coating.

Mixed Mechanism of Lubrication by Lipid Bilayer Stacks.

Although the key role of lipid bilayer stacks in biological lubrication is generally accepted, the mechanisms underlying their extreme efficiency remain elusive. In this article, we report molecular dynamics simulations of lipid bilayer stacks undergoing load and shear. When the hydration level is reduced, the velocity accommodation mechanism changes from viscous shear in hydration water to interlayer sliding in the bilayers. This enables stacks of hydrated lipid bilayers to act as efficient boundary lubricants for various hydration conditions, structures, and mechanical loads. We also propose an estimation for the friction coefficient; thanks to the strong hydration forces between lipid bilayers, the high local viscosity is not in contradiction with low friction coefficients.

Brass fillers in friction composite materials: Tribological and brake squeal characterization for suitable effect evaluation

In this paper, brake pad performance of two organic matrix composites namely, Sample 1 (contains no brass filler) and Sample 2 (contains 1.5% brass filler), is studied based on tribological and squeal noise behavior. In the first stage, a pin-on-disc tribometer is used to evaluate the frictional behavior of the two pads. On the following stage, these pads are tested on squeal noise occurrence using a drag-type brake dynamometer. From the two type of tests, the results show that; (i) brass fillers play a dual role; firstly as reinforcing element of the brake pad providing primary contact sites, and secondly as solid lubricant by contributing to the formation of a layer of granular material providing velocity accommodation between the pad and the disc; (ii) brass fillers contribute to friction force stabilization and smooth sliding behavior; (iii) the presence of small weight quantity of brass filler strongly contributes to squeal occurrences; (iv) there is close correlation between pin-on-disc tribometer and brake dynamometer tests in terms of tribological aspect.

Fretting wear behavior of Zn–Ni alloy coatings

Cadmium coatings are used in the aerospace industry primarily as a corrosion resistant plating, but also in applications where tribological properties are also important. Due to the carcinogenic nature of Cd, many coatings have been proposed as replacements, with Zn–Ni being the leading candidate. In this study, we examine two Zn–Ni coatings with differences in their surface roughness and their content of through thickness defects, including pores and cracks. The morphological differences between the coatings had a noticeable effect on their fretting wear behavior.A customized tribometer, with a reciprocating rounded pin (AISI 440C steel) on flat (Zn–Ni) geometry, was used to perform fretting wear tests. The two morphologically different Zn–Ni coatings were tested at room temperature using ±70, 100 and 150μm displacements and 133N and 447N constant normal loads. The surface of the wear scar was analyzed using scanning electron microscopy coupled with energy dispersive X-ray spectroscopy for changes in morphology and chemistry. Wear scar depth was measured from surface profiles obtained using confocal microscopy.Hysteresis fretting loops of the tests showed that for both coatings, at ±70μm displacement remains in no slip condition, at ±100μm in the mixed slip condition, and at ±150μm displacement remains in gross slip condition. Although the coatings had similar stick–slip behavior, the smoother coating has a slower progression of wear from the no slip to gross slip conditions than the rougher coatings. Also, differences in the wear scar morphologies are attributed to the differences in the coating morphologies, which resulted in different wear and velocity accommodation mechanisms.

High frequency reciprocating sliding wear behavior and mechanisms of quaternary metal oxide coatings

Wear resistant coatings based on quaternary metal oxides were studied with comparisons made to more traditional ternary metal oxides. While much is known about ternary metal oxide wear behavior and mechanisms from room to higher temperatures, little is known about quaternary oxides; for instance, the role of the fourth element in determining the coating crystalline state and defect structure and how they control tribological properties. To this end, the system (ZnTiZr)xOy was deposited by atomic layer deposition (ALD) and compared to previously studied wear resistant nanocrystalline ALD ZnTiO3 coatings. X-ray diffraction determined that both as-deposited and ex situ 550°C annealed ternary and quaternary coatings exhibit ZnTiO3 phase or solid solution Zn(Ti,Zr)O3 phase only, respectively. However, the coatings have completely different growth structures where the ternary ZnTiO3 coatings exhibit textured (104) nanocolumnar grains and the quaternary Zn(Ti,Zr)O3 coatings are predominantly amorphous with some nanocrystalline grains. High frequency reciprocating sliding on these coatings revealed that the growth structure did not influence the wear rate (~1×10−6mm3/Nm). However, the crystal structure-dependent deformation mechanisms were different as revealed by FIB–SEM and HRTEM analyses inside worn surfaces. It was determined that both coatings show surface deformation due to ductile layering/smearing with no evidence of brittle fracture. Wear reduction of the ZnTiO3 coating was due to nanoscale sliding-induced plastic deformation when (104) stacking faults were sheared along the sliding direction resulting in an intrafilm shear velocity accommodation mode. Conversely, wear reduction in the quaternary Zn(Ti,Zr)O3 coating was a result of a tribofilm/mechanically mixed layer, composed of amorphous transferred silica from the sliding Si3N4 counterface and refined nanocrystalline Zn(Ti,Zr)O3 grains from the coating. Both wear surfaces contained solid cylindrical rolls or roll-ups often found in ceramic–ceramic reciprocating sliding. New insights revealed that the rolls originate in the tribofilm, with similar composition and structure, and evolve to the surface with continued sliding.

Experimental and computational studies on stacking faults in zinc titanate

Zinc titanate (ZnTiO3) thin films grown by atomic layer deposition with ilmenite structure have recently been identified as an excellent solid lubricant, where low interfacial shear and friction are achieved due to intrafilm shear velocity accommodation in sliding contacts. In this Letter, high resolution transmission electron microscopy with electron diffraction revealed that extensive stacking faults are present on ZnTiO3 textured (104) planes. These growth stacking faults serve as a pathway for dislocations to glide parallel to the sliding direction and hence achieve low interfacial shear/friction. Generalized stacking fault energy plots also known as γ-surfaces were computed for the (104) surface of ZnTiO3 using energy minimization method with classical effective partial charge potential and verified by using density functional theory first principles calculations for stacking fault energies along certain directions. These two are in qualitative agreement but classical simulations generally overestimate the energies. In addition, the lowest energy path was determined to be along the [451¯] direction and the most favorable glide system is {104} ⟨451¯⟩ that is responsible for the experimentally observed sliding-induced ductility.