Meeting the Contact-Mechanics Challenge Müser, Martin H.; Dapp, Wolf B.; Bugnicourt, Romain ...
Tribology letters,
12/2017, Letnik:
65, Številka:
4
Journal Article
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This paper summarizes the submissions to a recently announced contact-mechanics modeling challenge. The task was to solve a typical, albeit mathematically fully defined problem on the adhesion ...between nominally flat surfaces. The surface topography of the rough, rigid substrate, the elastic properties of the indenter, as well as the short-range adhesion between indenter and substrate, were specified so that diverse quantities of interest, e.g., the distribution of interfacial stresses at a given load or the mean gap as a function of load, could be computed and compared to a reference solution. Many different solution strategies were pursued, ranging from traditional asperity-based models via Persson theory and brute-force computational approaches, to real-laboratory experiments and all-atom molecular dynamics simulations of a model, in which the original assignment was scaled down to the atomistic scale. While each submission contained satisfying answers for at least a subset of the posed questions, efficiency, versatility, and accuracy differed between methods, the more precise methods being, in general, computationally more complex. The aim of this paper is to provide both theorists and experimentalists with benchmarks to decide which method is the most appropriate for a particular application and to gauge the errors associated with each one.
Significance Granular materials are used for a wide variety of technological applications in nearly every industry and also serve as an instructive prototype for far-from-equilibrium behavior. ...Although they have received considerable attention, there are few general guidelines describing how they collectively organize and yield under shear, especially in physically relevant frictional grains. Using simulations, we explore the response of granular packings to slow periodic shear, systematically varying both the friction and shear. We discover a previously unidentified state in which the otherwise chaotic motion gives way to a precisely repetitive limit cycle for each individual grain. Understanding how grains self-organize into stable, periodic limit cycle configurations in this unlikely system foretells new approaches to understand yielding and memory formation in disordered materials.
The disordered static structure and chaotic dynamics of frictional granular matter has occupied scientists for centuries, yet there are few organizational principles or guiding rules for this highly hysteretic, dissipative material. We show that cyclic shear of a granular material leads to dynamic self-organization into several phases with different spatial and temporal order. Using numerical simulations, we present a phase diagram in strain–friction space that shows chaotic dispersion, crystal formation, vortex patterns, and most unusually a disordered phase in which each particle precisely retraces its unique path. However, the system is not reversible. Rather, the trajectory of each particle, and the entire frictional, many–degrees-of-freedom system, organizes itself into a limit cycle absorbing state. Of particular note is that fact that the cyclic states are spatially disordered, whereas the ordered states are chaotic.
Press hardening tools are expected to operate satisfactorily for thousands of forming cycles before being replaced. An approach to enhance the durability of tools in operation, and improve the ...process economy, is to refurbish them by depositing new material on the tool through welding, or hardfacing, when its surface is adversely damaged. The welding process changes the microstructure of the original tool steel and the deposited material also has a different microstructure, as well as different chemical composition to facilitate the welding process. Dissimilar friction behaviour between the original tool steel and the welded material can lead to unstable friction forces during forming. This work therefore focuses on understanding the tribological behaviour of different hardfacing materials, deposited through TIG welding, and their interaction with Al-Si coated boron steel at high temperatures. The tribological behaviour is evaluated using a hot-strip drawing tribometer capable of simulating the sliding conditions in the press hardening process. The effect of temperature on the microstructure and on the friction and wear behaviour of the weld material is also studied by conducting tests at room temperature and at 500 °C. In general, friction and the governing wear mechanisms are similar for the evaluated hardfacing materials. Friction stability was observed to be closely related to the temperature of the workpiece material. Temperature of the tool significantly affects the formation and structure of material transfer layers onto tool steels.
•Investigation of hardfacings as surface treatment for improving wear resistance of press hardening dies.•Tool temperature and workpiece temperature are considered.•Friction instabilities increase with workpiece temperature.•High tool steel temperature enables long sliding distances.•Wear mechanisms on tool and workpiece are described in detail.
The objective of the work was to carry out a comprehensive study to investigate the tribological response of the oil-in-water (o/w) emulsion systems between polydimethylsiloxane disc and steel ball ...surface using a ball-on-disc tribology assembly (3 point-contact). Simultaneously, oral and finger tactile sensory measurements were recorded to predict ‘smoothness’, in order to establish a reliable correlation between instrumental characterization and human perception. We hypothesized that the lubrication behavior of emulsions (constant oil droplet size) with varying oil mass fractions stabilized by different emulsifiers influences smoothness perception. Different emulsifiers studied were whey protein isolate (pH 6.7 & pH 3.5), modified starch and lysozyme while oil mass fractions of 30, 20, 10, 5 and 1 w% were examined. In-vitro studies with simulated saliva were also performed on the tribometer. All the emulsions showed typical Stribeck curve patterns with clear boundary and the mixed regime, while, the hydrodynamic regime was observed for high oil mass fractions. Friction coefficient (CoF) for all the emulsions was significantly different at most of the oil mass fractions (P < 0.05) and was compared with the smoothness scores (sensory analysis) along with discussion on plausible physical mechanisms for perception. Correlation and regression analyses provide empirical evidence that the o/w emulsions can be differentiated based on tribology studies and tactile perception. Smoothness is significantly correlated with the friction force on the tongue especially at sliding speeds between 0.1 and 30 mm/s constituting the mixed regime and hydrodynamic regime. Also, oral tactile smoothness scores correlated highly with CoF values than finger tactile.
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•Smoothness perception is related to friction between the tongue and the palate.•The o/w emulsions can be differentiated for smoothness perception based on oil mass fraction and type of emulsifier.•COF & smoothness scores show significant correlations between 0.1 and 30 mm/sec constituting mixed and hydrodynamic regimes.•Oral tactile measurements correlated strongly with friction coefficients than finger tactile measurements.•Tongue surface is highly sensitive to the vibro-tactile sensation compared to the innervation of the finger tips.
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•316L stainless steel samples were coated with the plasma spray coating method.•The wear behaviors of coated samples at room and high temperatures were studied.•Yttria stabilized ...zirconia (YSZ) coating has increased the wear resistance of 316L.
Yttria Stabilized Zirconia (YSZ) coatings by plasma spray coating were performed to investigate the behavior of 316L Stainless Steel (SS) under high temperatures. The structural and mechanical properties of YSZ films were examined by successful scanning electron microscopy and Vickers microhardness. A tribotester determined the friction and wear properties at 500 and 1000 °C temperature conditions. The results showed that the surface hardness values of the YSZ-coated samples were significantly improved due to obtained ceramic layer compared with untreated 316L samples. Although the abrasive wear mechanism was observed in both the coated and uncoated samples, it was observed that higher temperatures produced smoother wear tracks. Also, increased wear test temperatures resulted in micro cracks and structural degradations on the surface of untreated stainless steel samples.
The potential of lower friction and desired wear for nickel-based composite coating system at high temperatures was explored, where multiple oxides were incorporated into nickel matrix during the ...atmospheric plasma spraying process. The phase composition and microstructure of wear track were characterized by XRD, Raman, SEM/EDS, and FIB/STEM. The further addition of MoO3 allows for an effective reduction of friction coefficient at 800 °C. The surficial and interfacial analysis well confirmed that a protective tribo-layer presenting on the contacting surface, in which binary oxides (A-TiO2, R–TiO2, ZnO, NiO), molybdates and titanates (ZnMoO4, NiMoO4, and Zn2TiO4) were identifiable. The association of friction and wear properties with interface chemistry was discussed for the nickel-based coatings with the addition of multiple oxides (TiO2–ZnO–MoO3).
•Ni-based coatings with TiO2–ZnO–MoO3 multiple oxides were prepared.•Composite coatings demonstrated the improved lubricating capability at 800 °C.•Molybdates and titanates were formed during the spraying process and sliding test.•The tribo-layers after wear test at 800 °C were analyzed.
Recently, there has been an increasing trend in researches focusing on improving the performance of the biomedical implants. The clinicians used metallic implants to treat bone imperfections and ...fractures. The commonly used metals (Stainless steel, Ti-alloys and Co-alloy) failed to prove long-term durability and did not build sufficient bond with human bone. Since the invention of bioactive materials, which can generate chemical bond with bones, the researchers proposed combining the superior mechanical properties of metals and bioactivity of bioactive materials. This can be achieved by cladding bioactive material on metallic substrate. Different techniques, like thermal spraying, electron magnetron sputtering, laser cladding, etc., were proposed to successfully deposit bioactive materials on metallic substrates. In this article, we will discuss the potential of available metallic alloys and bioactive materials in biomedical implants including different techniques used in depositing bioactive materials on metallic implants.
•Review of mechanical properties & biocompatibility of most popular metallic alloys.•Review the potential of bulk metallic glass alloys in biomedical implants.•Review of bioceramics, especially hydroxyapatite and Bioglass, properties.•Review on different coating techniques of metals with bioactive layers.
Solid lubricants like metal sulfides and fluorides are important ingredients in stabilizing the friction in brake pads. It is essential to consider the oxidation temperature and the compatibility of ...the metal sulfides. This study emphasises the importance of incorporating a variety of solid lubricants based on oxidation temperature regimes. Low-temperature solid lubricants such as molybdenum disulfide, iron sulfide, and bismuth sulfide, as well as high-temperature solid lubricants such as tin(II)sulfide, tin(IV)sulfide, and calcium fluoride were used in the preparation of brake pads. The combination of low and high-temperature solid lubricant mix was tested for tribological performance in brake pads. The results show the necessity of using both low and high-temperature solid lubricants to enhance the tribological performance of brake pads.
•Diverse solid lubricants in brake friction composites.•High and low-temperature solid lubricants for enhanced brake performance.•The combination of solid lubricants cut brake costs by reducing the use of costlier one.
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•Zirconium phosphate (ZrP) as an additive in titanium complex grease.•The tribological properties of zirconium phosphate and graphite are compared.•Zirconium phosphate has excellent ...anti-wear properties.•The lubrication mechanism is explored.
Zirconium phosphate (ZrP) was selected as an additive in titanium complex grease. Firstly, the tribological properties of ZrP were studied at different concentrations in titanium complex grease. The best tribological performance of the grease was identified with 2 wt% ZrP, which showed COF decreased by 10.8% and WSD decreased by 19.7% compared to the base grease. Subsequently, The ZrP demonstrated excellent anti-wear properties than graphite at low concentrations. Finally, XPS analysis was conducted to investigate the titanium complex grease's lubricating behavior and the additives' mechanism in the grease. The results indicate that the additives' friction reduction and anti-wear properties were attributed to the physical adsorption of solid accumulation, filling and chemical adsorption of new friction film.
Nanocomposite films that consist of niobium nitride with silver nanoinclusions were created using unbalanced magnetron sputtering to investigate their potential as adaptive, friction reducing ...coatings. The coatings were tribotested against a Si3N4 counterface in the 25 to 1000°C temperature range. The coatings displayed coefficients of friction in the 0.15 to 0.30 range at T>700°C. Post-wear testing structural and chemical characterization revealed that, in the low to mid-temperature range, silver migrated to the surface to reduce friction. At higher temperatures, oxygen, silver and the transition metal reacted to form lubricious binary metal oxide phases (silver niobate) in addition to pure silver. In situ Raman spectroscopy measurements were taken during heating and wear testing at 750°C to identify the evolution of phases in the coatings surface and in the wear track. The analysis of the in situ Raman spectroscopy data revealed the various stages of formation of these binary metal oxides. The coatings were subsequently doped with MoS2 to investigate the effect of the introduction of a low temperature lubricant. The addition of MoS2 did not appreciably reduce the room temperature coefficient of friction, likely due to the miscibility of this compound with the transition metal nitride. However, the coefficient of friction was significantly reduced at high temperatures because of the synergistic lubricious effect of silver niobates and molybdates.
► Deposition of NbN/Ag and NbN/Ag/MoS2 nanocomposite coatings by unbalanced magnetron sputtering. ► Wear testing of NbN/Ag and NbN/Ag/MoS2 coatings from 25°C to 1000°C. ► Enhanced tribological properties compared to monolithic NbN coatings.
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