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In this work, large-scale molecular dynamics (MD) computational simulations were performed in order to explore the sliding contact responses of rough surfaces with hexadecane lubricant and added nanoparticles. Simulation results revealed that the frictional state was dependent on the fluid, nanoparticle, and surface roughness. Three lubricating conditions were compared based on considerations of different amounts of fluid molecules. The lubricant was not able to separate the frictional contact surfaces if the quantity of lubricant molecules was insufficient. Particularly, there were no lubricating contributions when the amount of lubricant was too low, and the lubricant therefore only filled the pits in the surface roughness. Thus, the normal load was primarily supported by the contact between the two surfaces and nanoparticles, leading to significant surface morphology changes. In contrast, the frictional contact surfaces were able to be completely separated by the lubricant when there was a sufficient amount of fluid, and a very good lubricating effect could thus be achieved, resulting in a smaller friction force. In addition, the changes in surface morphology, contact area, and RMS are discussed in this paper, in order to reveal the dynamic frictional process. [ABSTRACT FROM AUTHOR]

The influence of surface texture in the form of pits on the wear resistance of tribo-compounds under conditions of limit friction was investigated. At the first stage of research, the mechanism of lubricant behavior between the contacting surfaces and inside the holes was modeled. The limiting condition of the rotation frequency of the sample (n>27) was established, under which a drop of lubricant “leaves” the hole in the upper position of the sample and remains in the space between the sample and the surface of the counter body, ensuring the regeneration of the boundary lubricating film on the surface of the tribo-contact when it is destroyed. When the rotation frequency of the sample is reduced (n<27), the lubricant droplet remains in the hole and does not affect the processes of the boundary lubricant film. At the second stage, experimental studies of tribo-combinations with a textured hole surface under conditions of extreme friction were carried out. It was established that the high wear resistance of the textured hole surfaces is provided by the high protective effect of the texture, as well as the high efficiency of the marginal lubricating film. It has been proven that the strengthening of the surface texture by the method of ion-plasma thermocyclic nitriding additionally increases the wear resistance by 1.7 times due to the high protective effect of surface nitrided layers and their high hardness (up to 9500 MPa). This strengthens the effect of inhibiting the occurrence of defects in the surface layers of tribo-contact, ensures a high rate of wetting of the places of actual contact of tribo-connections, and speeds up the process of regeneration of the boundary lubricating film. Research results can be used to modify the surface layer of heavily loaded parts operating under extreme operating conditions with limited supply of lubricant under various types of friction and wear. [ABSTRACT FROM AUTHOR]

In this work, large-scale molecular dynamics (MD) computational simulations were performed in order to explore the sliding contact responses of rough surfaces with hexadecane lubricant and added nanoparticles. Simulation results revealed that the frictional state was dependent on the fluid, nanoparticle, and surface roughness. Three lubricating conditions were compared based on considerations of different amounts of fluid molecules. The lubricant was not able to separate the frictional contact surfaces if the quantity of lubricant molecules was insufficient. Particularly, there were no lubricating contributions when the amount of lubricant was too low, and the lubricant therefore only filled the pits in the surface roughness. Thus, the normal load was primarily supported by the contact between the two surfaces and nanoparticles, leading to significant surface morphology changes. In contrast, the frictional contact surfaces were able to be completely separated by the lubricant when there was a sufficient amount of fluid, and a very good lubricating effect could thus be achieved, resulting in a smaller friction force. In addition, the changes in surface morphology, contact area, and RMS are discussed in this paper, in order to reveal the dynamic frictional process.

A centrally cracked Brazilian disk (CCBD) specimen subjected to a pair of diametral compressive forces has been widely used to study mixed-mode I and II fractures of brittle and quasi-brittle materials. Reasons for using the CCBD are mainly due to its capability to introduce different mode mixities from pure mode I to pure mode II, the existence of closed-form solutions for fracture parameters, and the simple setup of compressive test. In addition to the diametrical concentrated force loading, the partially distributed pressure loading is also an important loading condition for CCBD specimen tests. Using the weight function method, analytical solutions of stress intensity factors and T stress considering the tangential loading friction for a CCBD specimen that is subjected to four typical partially distributed loads were derived, and effects of the boundary friction and load distribution angle on the fracture parameters were also explored. The results obtained are as follows: (1) For short cracks, geometric parameters YI, YII, and T* of pure mode I and II fractures decrease with an increase in the friction coefficient and load distribution angle. However, for long cracks, an increase in the friction coefficient causes an increase in pure mode-I YI, and an increase in the load distribution angle causes an increase in pure mode-II T*; (2) The influence of the load distribution angle on the fracture parameters is the most significant when the distributed pressure follows a constant function form, while it is the least significant for the case of quartic polynomial pressure; (3) The critical loading angle for pure mode II fractures decreases with an increase in the load distribution angle for short cracks, whereas it increases for long cracks. When the load distribution angle is fixed, an increase in friction can raise the critical loading angle for pure mode II fractures. These results have further improved the research of fracture parameters in CCBD specimens.

An understanding of the proper application of metalworking fluids (MWFs) is necessary for their implementation in efficient production processes. In addition, the knowledge of the process-related aspect of chip transport and the macroscopic cooling effect, the characteristics and properties of lubricant film formation, and the cooling conditions in the secondary shear zone on the chip surface, i.e., in the direct vicinity of the material separation, represent a combined fundamental scientific issue within production engineering. The aim is to transfer methods from the field of tribology of machine elements, which have already led to a considerable gain in knowledge in this discipline, to machining and to couple them with already established approaches to machining. In the case of roller bearings, the contact pressure is in the range as the pressure in the contact zone between the cutting insert and chip. Due to this, established methods might be transferred to the cutting process. In addition to classical pin-on-plate and pin-on-ring friction investigations, film thickness measurements were carried out and compared to machining tests. The coefficient of friction determined in the planing test rig is 0.48 for dry cutting, while it is 0.47 for wet cutting. These two values are much larger than the CoF with MWFs measured on the two tribometers. It is shown that the boundary friction of MWF especially influences the machining process. Thus, additives in MWF might have a high significance in machining. [ABSTRACT FROM AUTHOR]

The effect of 2,2,6,6-tetramethylpiperidine (TMP), 2,6-di-tert-butyl-4-methylphenol (DTBMP), para-cumylphenol (PCP), 2,2,6,6-tetramethylpiperidine-1-oxyl (TMPO), and dicumyl peroxide (DCP) on the anti-wear properties of dodecanethiol (DdSH), didodecyl disulfide (Dd2S2), and dibutyl disulfide (Bu2S2) in a solution of purified Vaseline oil (PVO) has been investigated. The anti-wear properties of 23 two-component compositions of additives from 28 possible combinations were studied. A rule was proposed: synergy is when the effectiveness of a mixture of additives is greater than the average result of the sum of anti-wear efficiencies of each component separately, and antagonism is when the effectiveness of a mixture of additives is less than the average result of the sum of anti-wear efficiencies of each component separately. The practical significance of the proposed method for studying two-component additives is that it can be used to confirm or refute the mechanism of action of additives. [ABSTRACT FROM AUTHOR]

This article describes the analysis of influence of biocarbon (BC) produced by pyrolysis from a pine nutshell on tribological properties of friction material. The research object has been presented by copper-based friction material doped with 12% tin with 10–40 vol % of carbon additive in the form of BC. The average particle size of BC is 10 μm. The tribological properties has been analyzed using an IM-58 friction machine. The tribological tests demonstrate that with an increase in the BC content the dynamic friction coefficient increases from 0.041 to 0.066. With the aim of comparison, addition of GE-1 grade powdered graphite results in an increase in the friction coefficient from 0.038 to 0.069. Simultaneously with the increase in friction coefficient the wear of friction material also increases not exceeding its ultimate value. Thus, at 10 vol % of BC addition the wear is 4.0 μm/km, at 40 vol %, 12.1 μm/km. Analysis of surface morphology of bronze demonstrates that the BC particles are worn uniformly with bronze layer, retaining the initial porous structure. The products of BC destruction with sizes of 50–500 nm have been introduced into the surface layer of bronze. The results demonstrate opportunities of biocarbon additive in friction material, which can be used as a substitution of conventionally used graphite. It should be mentioned that the promising opportunities of BC addition to powdered antifriction material are determined by initial raw material for its production. [ABSTRACT FROM AUTHOR]

The second part of the review of the boundary friction tribology of solid carbon coatings gives special priority to the effects of generating modified layers and the tribochemical interaction of alloying additives with liquid lubricant components used for improving antifriction and antiwear properties. The possibilities of improving the lubricity of several full-formula oils and model lubricant compositions used in friction units are considered. Recommendations on the structure and alloying of diamond-like carbon coatings developed for use in lubricated heavy-duty friction units are provided. [ABSTRACT FROM AUTHOR]

The effect of 2,2,6,6-tetramethylpiperidine (TMP), para-cumylphenol (PCP), 2,6-di-tert-butyl-4-methylphenol (DTBMP), dicumyl peroxide (DCP), 2,2,6,6-tetramethylpiperidine-1-oxyl (TMPO), dodecantiol (DdSH), dibutyl disulfide (Bu2S2), didodecyl disulfide (Dd2S2), and molecular oxygen on the wear index (Wi) of purified Vaseline oil (PVO), hexadecane (HD), polyethylsiloxane (PES-5), and a mixture of PES-5 containing 10 wt % PVO or HD was studied for the first time. The reason for the extremely high wear of steel surfaces in a solution of PES-5 containing 10 wt % of hydrocarbon lubricants, consists in the antagonism of polyorganosiloxanes with hydrocarbon lubricants and in the lack of molecular oxygen necessary for the formation of a servovite film. The prospect of studying mixtures of polyorganosiloxanes and hydrocarbons is to find a mixture that would have synergism in the anti-wear area. Studies of additives and gaseous media can also be promising. The results of the study of mixtures of polyorganosiloxanes containing about 10% by weight of hydrogen lubricants can be effectively used to reduce the wear of steel surfaces operating in an oxygen atmosphere. [ABSTRACT FROM AUTHOR]

An understanding of the proper application of metalworking fluids (MWFs) is necessary for their implementation in efficient production processes. In addition, the knowledge of the process-related aspect of chip transport and the macroscopic cooling effect, the characteristics and properties of lubricant film formation, and the cooling conditions in the secondary shear zone on the chip surface, i.e., in the direct vicinity of the material separation, represent a combined fundamental scientific issue within production engineering. The aim is to transfer methods from the field of tribology of machine elements, which have already led to a considerable gain in knowledge in this discipline, to machining and to couple them with already established approaches to machining. In the case of roller bearings, the contact pressure is in the range as the pressure in the contact zone between the cutting insert and chip. Due to this, established methods might be transferred to the cutting process. In addition to classical pin-on-plate and pin-on-ring friction investigations, film thickness measurements were carried out and compared to machining tests. The coefficient of friction determined in the planing test rig is 0.48 for dry cutting, while it is 0.47 for wet cutting. These two values are much larger than the CoF with MWFs measured on the two tribometers. It is shown that the boundary friction of MWF especially influences the machining process. Thus, additives in MWF might have a high significance in machining.

The engine power losses and sealing behaviour through compression rings have become a priority for engine manufactures in view of future stringent emissions standards. Historically, compression rings have great application in small road vehicles and high-performance engines. The sealing behaviour of the compression ring has a significant effect on generated friction. Therefore, the lubrication conditions in the compression ring-liner conjunction should be addressed. This study comprises ring-liner mixed hydrodynamics using a two-phase flow and a gas blow-by model. This is implemented to investigate the characteristics of a high-performance top compression ring for different engine conditions. Low, medium, and high driving speeds are examined according to world-wide harmonized light vehicles test cycle. Additionally, the impact of lubricant temperature and compression ring width on power loss and fuel consumption are investigated to display the better engine performance. [ABSTRACT FROM AUTHOR]

Highlights: Analytical solutions for the mode I and II stress intensity factors (SIFs) in a centrally cracked Brazilian disc (CCBD) considering tangential friction effects are proposed. The effects of the pressure distribution angle, tangential friction, and pressure distribution form on the mode I and II SIFs are explored. The loading conditions for pure mode I and II fractures of the CCBD specimens subjected to distributed pressures and frictions are discussed. [ABSTRACT FROM AUTHOR]

Currently known magnetic lubricant nanofluids have insufficiently good tribological characteristics when operating in the boundary friction mode. An adaptation method is proposed for the boundary friction of magnetic siloxane nanofluids by modifying their composition with chemically active antifriction, antiwear and extreme pressure additives. A number of lubricating compositions of magnetic nanofluids based on polyethylsiloxane PES-5 have been created. Conducted experimental studies have shown that the most significant improvement in the antifriction and antiwear properties of nanofluids is achieved with the introduction of Sovol and 3N2TE chlorine additives into their structure. Adding to the nanofluid the antioxidant additive DF-11, the fluorine-containing additive EO-1, the metal-plating compound MKF-18 did not lead to a significant improvement in the antifriction properties. It has been established that magnetic nanofluids with modifying additives are comparable in tribological properties with traditional non-magnetic liquid lubricants and are superior to plastic lubricants based on polyethylsiloxane. The developed compositions based on siloxane nanofluids will be in demand for lubrication of rolling and sliding bearings, gears, contact seals, which operate at low and elevated temperatures in a gas environment or in a vacuum. Most successfully, they can be used in vacuum and space technology, for magnetic frictional units, where only one refueling with a limited amount of lubricant is possible.

Piston skirt to cylinder liner conjunctions are amongst the major contributors to frictional power losses of Internal Combustion Engines (ICEs). Efforts have been made to mitigate the frictional losses of these conjunctions by incorporating different technologies such as texturing and application of novel coatings. Any potential technology needs to provide adequate wear resistance as well as frictional reduction in order to be practically applicable. In this paper, the piston skirt of a gasoline engine is deposited by three different variants of Graphene Oxide (GO) coatings deposited using an Electro-Phoretic Deposition (EPD) method. Their tribological performance is benchmarked against uncoated steel and graphite coated aluminium skirts. These coatings are experimentally characterised in terms of asperity level friction, topography and wear resistance. The conjunction and system level performance of these coatings considering both boundary and viscous friction and system dynamics are then evaluated using a multi-physics tribo-dynamic model. Results show that by incorporating an appropriate GO coating, the frictional power loss of the piston skirt to cylinder liner conjunction can be improved by up to 14% whilst maintaining the wear resistance of the coating at the level of an uncoated steel surface. [ABSTRACT FROM AUTHOR]

The paper presents results of the tribotechnical and acoustic characteristics of copper-based frictional composite materials depending on the composite structure and content of metallic (tin, iron) and carbon (coke, graphite) additives in the composition. Changes in the studied characteristics are most pronounced under dry friction conditions. The introduction of the additives leads to reduced friction components of noise to 56 dB. In the boundary friction mode, the greatest effect of introduced target additives on the acoustic characteristics does not exceed 11 dB relative to the material without additives. For a composite based on 12% tin bronze, an increase in the graphite content from 10 to 30 vol % results in a 5 dB noise reduction. [ABSTRACT FROM AUTHOR]

This study provides the results of analyzing the influence of carbon-containing powder additives in the form of powders of fine GK-1 pencil graphite, GE-1 coarse graphite, and foundry coke fractions smaller than 63 and 100–200 μm on the setting and transfer of materials using mineral and synthetic oils. It is established that foundry coke powder allows for improving the resistance of material during seizure. Thus, an increase in the seizure load from 1.4 to 1.7 MPa for mineral oil and from 1.0 to 1.4 MPa for synthetic oil is registered at a sliding speed of 2 m/s. The use of finely dispersed pencil graphite as part of the friction material during friction in synthetic oil increases the speed during seizure; thus, an increase in speed up to 4 m/s is registered at a load of 1 MPa, while an increase in speed for pure 12% bronze is not higher than 2 m/s. [ABSTRACT FROM AUTHOR]

Debris‐flow runout is a fascinating process to understand due to its implications for downstream alluvial fans. Based on the propagation‐deposition behaviors of the Dongyuege (DYG) debris flow, in Yunnan, the effect of biofilms on channel surfaces on debris‐flow runout is investigated in laboratory flumes with two different internal surfaces: surfaces are lined with granite slabs (Model I) and gravel (Model II), respectively. Our results show that biofilms can significantly reduce frictional resistance to flows. They increase flow velocities, slow down the deceleration of the snouts, prolong runout distances, and subsequently extend the areas covered with resulting deposits, thus greatly assisting the propagation of experimental debris flows. Slippery biofilms consisting mainly of diatoms and their extracellular mucus (ECM) reduce the contact friction between the flume‐beds and the overlying fluids, and greatly promote the propagation of tested flows. Well‐developed biofilms are found on the underwater channel surfaces of the DYG Creek. Acting as lubricating layers, they likely played a key role in the DYG debris‐flow runout. Most of the debris transported during the DYG event was deposited on overbanks, and the sediment that caused the disaster was transported to the populated fan region through the stream‐bed clad in the thick biofilms. Owing to their impacts on the development and width of the temporary debris dam breach, the stream‐bed covered with biofilms became a direct contributor to the debris‐flow hazard. Because of the ubiquitous presence of biofilms on mountain stream‐bed surfaces, the development of perennial streamflows can be viewed as an indicator of gully susceptibility to debris flows threatening creek fans. The underwater areas of pre‐event channel cross‐sections should be regarded as slip or low‐friction boundaries, and the parts above stream‐levels can be viewed as no‐slip boundaries. © 2019 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

This paper presents the impact of coating topography in piston ring-liner conjunction under mixed regime of lubrication using low viscosity oils. The study provides a time efficient analytical model including mixed-hydrodynamics regime of lubrication under different contact conditions. The method modified the expressions of the contact load and area of Greenwood-Tripp model in order to capture the real asperities interaction into contact. The model represents the tribological behavior of a thin top ring at Top Dead Centre, where boundary and mixed conditions are predominant. Electroplated CrN and PVD TiN coated rings were studied to predict the ring friction. The results are compared with an uncoated steel ring. The CrN coating shows slighter coefficient of friction, due to the coating morphology and roughness parameters. The TiN coating presents thicker lubricant films and higher coefficient of friction because the surface topography is quite rough with high peaks. This can be explained because of the major contribution of the roughness parameter and asperity slope in the boundary friction prediction.

This article presents an investigation of the dynamical contact between two atomically flat surfaces separated by an ultrathin lubricant film. Using a thermodynamic approach we describe the second-order phase transition between two structural states of the lubricant which leads to the stick–slip mode of boundary friction. An analytical description and numerical simulation with radial distributions of the order parameter, stress and strain were performed to investigate the spatial inhomogeneity. It is shown that in the case when the driving device is connected to the upper part of the friction block through an elastic spring, the frequency of the melting/solidification phase transitions increases with time.

Abstract—The article describes the effect of tin and graphite content in the composition of a copper-based friction material (FM) on the structure, tribological properties, and formation of the contact surface. It is shown that an increase in tin from 6 to 12 wt % leads to an increase in the friction coefficient and a decrease in the running-in time for wear resistance. When the tin content in the friction material is 6 wt %, during friction, plastic flow of the contact surface material occurs with the formation of a "third" layer. The type and content of graphite also significantly affect the tribotechnical properties of the FM: an increase in graphite content of more than 20 vol % increases the friction coefficient; when GK-1 pencil grade graphite is used, the friction coefficient increases to a lesser extent than for GE-1 elemental grade graphite, and the wear resistance increases by 2.5–3.0 times. [ABSTRACT FROM AUTHOR]

Accurate simulation performs a crucial role in the design and development of new modern internal combustion engines. In the case of piston rings, simulations are used to effectively predict generated friction and power loss of proposed designs. These are consequences of viscous shear of a thin lubricant film, likewise boundary friction caused by direct interaction of piston rings with the cylinder liner/bore surface. The most commonly used model for determining boundary friction is that of Greenwood and Tripp. The model requires the pressure coefficient of boundary shear strength of asperities from the softer of the contacting surfaces as an input. This parameter needs to be measured. The paper describes the process of measurement using an Atomic Force Microscope (AFM), both for a dry surface and that wetted by the presence of a lubricant layer. For realistic results, the investigated specimen is a used, tested engine cylinder liner where boundary active lubricant additives are bonded to its surface as well as combustion products. This approach is as opposed to the previously reported works using new flat surfaces with base oil or partially formulated lubricants and has not been previously reported in the literature. The results show that for used cylinder liners, the measured boundary shear strength of asperities varies according to location along the stroke. Results are reported for the top dead centre, mid-stroke and bottom dead centre locations. The measurements are subsequently used with two-dimensional Reynolds solution for a top compression ring-liner contact, where it is found that accurate localised predictions of generated friction and power loss can be made instead of the usual average value approach reported in the literature. [ABSTRACT FROM AUTHOR]

Abstract For future engines to become more efficient, with enhanced fuel economy and increased power output, accurate prediction of new designs is required over the full lifetime of an engine. The analysis uses of a local pressure coefficient of boundary shear strength of asperities value, taking into account the localised effects of surface texture, coating and surface deposition. XPS spectra analysis was also carried out to identify the surface depositions as a result of combustion, not previously taken into account during piston ring pack simulation. It was found that piston varnish on the liner corresponded to higher values of the pressure coefficient of boundary shear strength of asperities, showing the importance of using real system components run under representative operating conditions. Highlights • The paper presents measurements of ς from a used real cylinder liner surface. • XPS is used to identify chemical composition of liner surface regions. • Depositions and tribo-films on the liner surface are characterised. • Results show that boundary friction is dependent on the by-products of combustion over time. • Simulation shows that results for friction can vary by 30% at peak combustion pressure. [ABSTRACT FROM AUTHOR]

• A microscopic dynamic force balance is proposed for assessing the bentonite homogenization process with initial technological voids. • The model utilized exhibits strong robustness in predicting the dry density evolution and the extent of heterogeneous distribution. • High friction factor at the bentonite and steel interface allows for the maintenance of large stress gradients and the associated density gradients within the material. During the construction of the nuclear waste geological repository, the inevitable creation of technological gaps will lead to the redistribution of bentonite material along the wetting path upon water intake, thus impacting its localized hydraulic and mechanical properties. It is therefore of the utmost importance to be able to understand and predict the dry density inhomogeneous distribution throughout the buffer zone. This work proposes a microscopic dynamic force balance framework, incorporating the influence of van der Waals force, diffuse double layer force, and boundary friction force, to explore the underlying mechanism of the bentonite homogenization process upon hydration. The microscopic assumption postulates that bentonite materials are an assembly of numerous parallel thin sheets with other dimensions substantially exceeding the thickness, establishing an association between the aforementioned force and the material's volume fraction. Particular attention is directed toward exploring the mechanism of boundary friction's effects on the inhomogeneous distribution of dry density with hydration time. The application of the framework to examine the impact of engineered voids with 10% and 20% of the total volume yields a satisfactory reproduction in terms of the spatial and temporal evolution of dry density, validating the robust prediction capacity of the established model. The investigation indicates that compacted blocks in proximity to technological voids experience a rapid reduction in dry density upon hydration, leading to the homogenization process characterized by elevated values in the initial void-affected region and diminished values in the innermost area. Whereas full homogenization is yet to be achieved within the observation period of up to 2160 h, and the greater the variability of the dry density within the specimens containing larger initial voids. Further analysis reveals that as the hydration process advances, the friction force within the portion featuring the initial void exhibits a gradual increase, while emergence gradually decreases in the inner swelling zone. Notably, the high friction factor at the bentonite and steel interface allows for the maintenance of large stress gradients and the associated density gradients. [ABSTRACT FROM AUTHOR]

Folding, a common defect in plastic forming, is prone to occur and very complicated especially in the forming of multi-cavity parts by multi-direction loading, which is the most representative and difficult plastic forming process. Physical simulation experiment (PSE) with Plasticine provides an intuitive method for study of metal flow and folding defect. However, the reasonable PSE and evaluation of it are lack of understanding. In this work, taking a typical multi-cavity part, 316LN tee valve, as object, physical simulation experiment and evaluation for folding defect were investigated by considering the Plasticine composition, layered thickness, and material modeling, along with boundary friction. The results show that the oil-based clay Plasticine with layered thickness of 2 mm could accurately reflect material flow characteristics due to its good performance. Constitutive modeling of Plasticine based on the similar strain hardening index and proportional load, combined with similar friction conditions at room temperature could well macroscopically characterize the metal flow and folding defect evolution in actual forming process. The reliability and applicability of the physical simulation method proposed are further validated by comparative analysis of the material flow law and folding defect with finite element simulations under four representative loading paths. [ABSTRACT FROM AUTHOR]

Abstract: The effect of temperature on the friction coefficient under boundary lubrication by cholesteric liquid crystal nanomaterials, as well as by their solutions in industrial oil, is studied. It is established that the minimum friction coefficient for the investigated substances is reached in the mesomorphic state. In the isotropic state, an insignificant monotonic increase in the friction coefficient with increasing temperature is observed. The friction coefficient also decreases with the increasing molecular weight of the lubricant. Under friction in solutions, an abrupt increase in the friction coefficient occurs when a certain critical temperature is reached, which could be caused by the temperature destruction of the adsorbed layer. [ABSTRACT FROM AUTHOR]

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The behavior of a liquid argon layer compressed between absolutely solid diamond surfaces with a periodic atomic relief has been studied by the method of molecular dynamics. The film consists of one or two layers of molecules for which the model of absolutely elastic spheres has been used. The equilibrium and dynamic properties of the film dependently on the load and shear force applied to surfaces and the number of argon layers have been investigated. The performed modeling allows us to compare the behavior of systems with smooth and rough plates. The results show that most of the properties of ultrathin argon films confined between surfaces of both types are similar. [ABSTRACT FROM AUTHOR]

Here we consider melting of an ultrathin lubricant layer between two atomically smooth solid surfaces taking into account the stress dependence of the lubricant shear modulus and its decrease with increasing stress (strain). In the adiabatic approximation with the stress relaxation time far longer than strain and temperature relaxation times, a Langevin equation is written and its respective Fokker-Planck equation is derived using the Stratonovich calculus. Phase diagrams for the steady case are presented illustrating the effect of the system parameters on the lubricant behavior. A joint numerical and analytical analysis demonstrates a very close match between probability distributions at different parameters. It is shown that in a limited stress range, a self-similar mode of dry friction is established showing up in self-similar behavior of stress time series. [ABSTRACT FROM AUTHOR]

The results of tests of a Zr-Al-B boundary friction detonation coating in a wide range of changes under friction conditions have been presented. A comparative analysis of the obtained characteristics of friction and wear in order to evaluate the tribotechnical properties of amorphous-crystalline coatings has been carried out. These results have been compared with parallel tests of coatings based on tungsten carbide, samples of hardened steel and bronze, and the bearing of the sliding layer. The formation of solid solutions has been established and the introduction of oxygen in zirconium that corresponds to the formation on the friction surfaces of the secondary structures of the first type, a characteristic feature of which is their surface localization, ultra-dispersed structure, and ability to minimize disruption and shield unacceptable adhesion phenomena. The use of the Auger electron microscope confirmed that oxygen completely replaces the sulfur in the surface structures. [ABSTRACT FROM AUTHOR]

It is necessary to develop an analytical solution in order to combine predictions with measured tribological parameters and fundamentally understand the mechanism of lubrication in a typical region of engine cycle, using tribometric studies. This paper deals with the development of such a representative approach. An analytical, rather than a numerical approach is expounded, as it is shown to suffice for the purpose of precise time-efficient predictions, which conform well to the measurements. The effect of surface topography, material and operating conditions are ascertained for the representative case of top compression ring-cylinder liner contact at the top dead centre reversal in transition from the compression to power stroke. Stainless steel uncoated surface used as press fit cylinder liners for niche original equipment manufacturer applications are compared with those furnished with a Nickel-Silicon Carbide wear-resistant coating of choice in high performance motorsport. © 2017 The Authors Lubrication Science Published by John Wiley & Sons Ltd. [ABSTRACT FROM AUTHOR]

The tribology of lubricated poly(ether ether ketone) (PEEK)--steel tribosystems was investigated in the static and boundary friction regime. Pentaerythrite ester and trimellitic acid ester were used as lubricants. The lubricants differed in their molecular structure but showed only minor differences in their rheological and cohesive energetic properties. In order to investigate the effect of the lubricants on static and dynamic friction, macroscopic oscillatory tribometry experiments and gliding experiments were carried out. The surface and interfacial energies of PEEK, lubricant, and steel, which can be used to determine e.g., the spreading tendency of the lubricant, were employed to explain the tribological characteristics of the system. The gliding experiments exhibited a velocity dependence of the coefficient of friction which indicates a "rate-and-state" type of friction. Trimellitic acid ester resulted in the lowest static and dynamic friction values due to its high tendency to spread. A different static friction behavior was found for pentaerythrite ester, which indicates a possible lubricant-induced mobilization of the polymer surface chains. This finding was supported by an increased interaction energy measured by contact angle measurements. The macroscopic oscillatory tribometry was shown to be a unique and precise method to investigate the transition from static to dynamic friction and to quantify macroscopic adhesive friction. Both types of experiments used a polished steel surface as a frictional partner, which favors the adhesive component of friction. However, the results for the lubricated contacts of polished steel surfaces in the boundary lubrication regime can give some insight into effects of the polymer transfer to rough steel surfaces in the mixed lubrication regime. [ABSTRACT FROM AUTHOR]

Different aspects of the tribological performance of black oxidized surfaces in rolling–sliding boundary-lubricated or mixed lubricated contacts are overviewed in the present work. It covers the topics of antimicropitting resistance, antismearing effects, reduction in boundary friction, repelling tribochemical attacks of aggressive lubricants or additives, running-in behavior, and the effect of black oxide coating on bearing life. The results of the component laboratory experiments, full bearing tests, and theoretical modeling, performed within the frame of the present work, shed light upon the benefits and limitations of using black oxide coating for bearing applications. [ABSTRACT FROM AUTHOR]

Wear assessment of critical components subjected to relative sliding is a key factor for the development of advanced materials and surface treatments in automotive industry. Simulation of wear process of the engine components is considered as a good alternative for experimental testing which is costly and time-consuming, but it requires a reliable experimental data for model fine-tuning. Therefore, friction and wear of cylinder liner against a piston ring were experimentally studied in simulated laboratory tests. The parameters which were controlled in these tests included oil type, lubrication starvation, surface finishing and surface coatings. The obtained experimental data were fed into a specific simulation model (AVL Excite-Power Unit). Comparison of experimental and simulated results yielded the error below 5%. [ABSTRACT FROM AUTHOR]