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Straightforward design and long-term functionality for tribological considerations has prompted an extensive substitution of polymers for metals across various applications, from industrial machinery to medical devices. Lubrication of and by polymer gels/coatings, essential for ensuring the cost-effective operation and reliability of applications, has gained strong momentum by benefiting from the structural characteristics of natural lubrication systems (such as articular cartilage). The optimal synthetic strategy for lubricating polymer gels/coatings would be a holistic approach, wherein the lubrication mechanism in relation to the structural properties offers a pathway to design tailor-made materials. This review considers recent synthesis strategies for creating lubricating polymer gels/coatings from the molecular level (including polymer brushes, loops, microgels, and hydrogels), and assessing their frictional properties, as well as considering the underlying mechanism of their lubrication. [ABSTRACT FROM AUTHOR]

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Frictional behaviour of mixed Elasto-hydrodynamic lubrication (EHL) arises from the coupling of the lubricant fluid and asperity interaction frictions. Due to the difficulties in modelling asperity interaction friction, existing approaches often use a constant friction coefficient obtained in boundary lubrication to approach the asperity interaction friction coefficient in mixed EHL. In this study, the asperity interaction friction is, for the first time, considered to result from either the boundary film friction or solid-to-solid ploughing and adhesion friction depending on the local contact and deformation conditions. The friction of mixed EHL was predicted by the combination result of lubricants fluid, boundary film, and solid-to-solid frictions. This development provides an alternative and cost effective method to estimate friction coefficient in mixed EHL. [ABSTRACT FROM AUTHOR]

Tribological conditions in bulk metal forming processes are characterized by high contact pressures and large relative movements at high velocities between the tool and the workpiece. In addition, significant surface enlargements and elevated temperatures can occur. It is very challenging to reproduce such contact conditions on pin-on-disc tribometers. In this study, a method for simulating the contact conditions of a metal forming process on a pin-on disc tribometer is presented. Special test samples have been designed in order to achieve high contact pressures and surface enlargements with a homogeneous distribution of these quantities within the contact zone. It is shown that the test method delivers reproducible results and enables the detection of small changes of the contact conditions. Furthermore, microstructural changes of the real contact surface can also be simulated, which was verified by SEM imaging. [ABSTRACT FROM AUTHOR]

Magnetic head wear performance was studied using various thicknesses of D-4OH lubricant that covers hard disk media. The characteristic head wear behavior to over-push the dynamic flying height power differed for each lubricant thickness. The difference in the behavior was highly dependent on the media friction coefficient. The surface free energy also varied as a function of the lubricant thickness. According to the lubricant coverage analysis, we found that the head wear behavior and the friction were dependent on the lubricant coverage change as a function of the lubricant thickness. Furthermore, the molecular dynamics method was applied to investigate the lubricant molecular distribution; the coverage changed with the lubricant thickness similar to that in the experiment. Finally, a head-disk interface mechanism under an over-pushed state was discussed, and meniscus bridge formation was suggested to have occurred at touchdown power for each lubricant thickness. [ABSTRACT FROM AUTHOR]

The article presents an approach for selection of Stribeck curve for ester containing lubricating oils for low carbon steel. Topics include frictional characteristics of a mineral based lubricant having ester, correlate friction with process parameters and ester concentration, and standard friction measurement test-rig using principle 'Pin-on-disc'. It addresses selection of lubricants containing anti-wear (AW) and extreme pressure (EP) additives and modes of lubrication.

The film formation mechanism of lithium complex grease under starved condition was proposed based on the analysis of the relationship between grease reservoir and the finger-shaped lubricant along the rolling track using a laboratory built ball-on-disc test rig. Film thicknesses with rolling time at different slide/roll ratios were measured and discussed in detail. Experimental results showed that starvation occurred soon after the operation under pure rolling condition. In contrast, the contact remained fully flooded under slide-roll condition. The measurement of grease fingers proved that slide/roll ratio contributed to replenishing the contact by transferring more grease to the vicinity of the contact to form a lager lubricant reservoir. The volume of grease fingers, the inlet lubricant supply and the film thickness at different slide/roll ratios were found to be in good agreement. [ABSTRACT FROM AUTHOR]

Highlights: [•] We used GCIB to smooth sliding pins for friction tests of nanometer-thick PFPE films. [•] An ultra-smooth sliding pin with R p less than monolayer film thickness was achieved. [•] We used γ and h/σ to optimize the interplay of surface roughness and film thickness. [•] High γ induces high friction coefficient. Low h/σ leads to high friction coefficient. [•] Both γ and h/σ should be considered as design parameters for optimum design of HDDs. [ABSTRACT FROM AUTHOR]

Investigation of floor slipperiness is critical in studying the risk of slips and falls. This research conducted friction measurements and employee survey on floor slipperiness in a student cafeteria in a university in China in order to study the correlation between the two types of floor slipperiness measure. It was found that the fryer area in kitchen 1 and the meat defrost sink in kitchen 2 had significantly the lowest coefficient of friction (COF) values among all areas in the same kitchen. The results showed that the levels of friction on the floors in different areas in each kitchen were significantly different. Employee perceptions of floor slipperiness among areas were also significantly different. The level of friction and employee perception of floor slipperiness was highly correlated (ρ = 0.87). This implies a good agreement between the two measures. A linear regression model was established to describe the relationship between the perceived floor slipperiness and the measured COF. The model was significant atp < 0.0001 with anR2of 0.59. [ABSTRACT FROM AUTHOR]

Advances in internal combustion engines make it necessary to study in depth their contribution to engine efficiency, and thus to friction mechanical losses, especially those generated in the piston-cylinder liner assembly. In this paper, the design of a novel test rig is presented, based on the floating liner principle, with the key objective of having the capability of developing multiple parameters tests in a relatively quick and easy way. The final design integrates a single-cylinder internal combustion engine, a mechanical solution that allows the liner to float, three piezoelectric force sensors and a novel design of lateral support based on ball transfer units. Reproducibility and repeatability of tests were assessed demonstrating the capability of the test rig to develop reliable friction measurements. Some parameters tests were developed to evaluate the sensitivity of the friction force measurements to variations in the working conditions, oil temperature and engine speed. A theoretical model for the piston compression ring is presented for both fully flooded and starved lubrication conditions, including asperity interactions and lubricant rheology. Comparisons of these approaches with experimental results showed that, for the working conditions of the test rig, lubrication of the compression ring is better estimated under fully flooded lubrication condition, as the starved lubrication analysis results in over-estimated friction force values at higher engine speed regimes. • Design and validation of a floating liner for measurement of piston friction force. • Experimental compression ring friction as function of speed and oil temperature. • Piston compression ring model for fully flooded and starved lubrication conditions. • Comparison of experimental and theoretical results for the piston compression ring. • Piston compression ring working mainly under fully flooded lubrication conditions. [ABSTRACT FROM AUTHOR]

One commonly used lubricant in rolling bearings is grease, which consists of base oil, thickener and small amounts of additives. Commercial greases are mostly produced from petrochemical base oil and thickener. Recently, the development of base oils from renewable resources have been significantly focused on in the lubricant industry. However, to produce an entirely bio-based grease, the thickener must also be produced from renewable materials. Therefore, this work presents the design and evaluation of three different bio-based polymer thickener systems. Tribological tests are performed to characterize lubrication properties of developed bio-based greases. The effect of thickener type on film thickness and friction behavior of the produced bio-based greases is evaluated on a ball-on-disc tribometer. Moreover, the results are compared to a commercial petrochemical grease chosen as benchmark. [ABSTRACT FROM AUTHOR]

In this study, a novel apparatus was designed and constructed to perform micro-sliding friction experiments while simultaneously observing the motion of micro-sized spheres using a visual inspection technique. The apparatus comprises a precision elevation stage that is used to elevate a flat mica disk, with the microspheres on top of it, to bring it into contact with a stationary surface and apply low loads on the contact. During micro-sliding experiments, it was found that the velocity of the center of the microsphere was half the velocity of the mica disk; in addition, friction force measurements revealed a very low coefficient of friction (about 0.03), indicating the rolling motion of the microspheres. The main outcome of our study was the verification of the hypothesis that spherical particles can be used to avoid direct contact among flat surfaces and can also introduce rolling motion within the system. The pure rolling motion of the microspheres sandwiched between a stationary and a moving flat surface supports the idea of adding rigid spherical particles to oil lubricants to further reduce friction and wear in the system by performing as micro and nanoscale ball bearings. [ABSTRACT FROM AUTHOR]

The subject of this paper is the solution to specific problems in the measurement of the coefficient of the sliding friction of material pairs using a variable incidence tribometer. The aim of this work was to solve the questions of the measurement of the static coefficient of friction using a variable incidence tribometer from the perspective of metrology. In particular, we intended to research the expression of the credibility of this measurement and the possibility of increasing the credibility of the measurement. Another problem that needs to be solved is the method of carrying out the measurement, which has an impact on its achieved uncertainty. Sliding friction is a phenomenon that depends on many material properties such as contact area roughness, moisture, lubricants, temperature, and relative motion velocity. If environmental conditions are defined, the main input parameter is the relative motion velocity. For this input quantity, friction force becomes a symmetrical problem, and it is only necessary to explore this phenomenon for positive values of velocities. Symmetry in this area simplifies the research of this science topic. [ABSTRACT FROM AUTHOR]

A healthy natural synovial joint is very important for painless active movement of the natural musculoskeletal system. The right functioning of natural synovial joints ensures well lubricated contact surfaces with a very low friction coefficient and wear of cartilage tissue. The present paper deals with a new method for visualization of lubricating film with simultaneous measurements of the friction coefficient. This can contribute to better understanding of lubricating film formation in a natural synovial joint. A newly developed device, a reciprocating tribometer, is used to allow for simultaneous measurement of friction forces with contact visualization by fluorescence microscopy. The software allowing for snaps processing and subsequent evaluation of fluorescence records is developed. The evaluation software and the follow-up evaluation procedure are also described. The experiments with cartilage samples and model synovial fluid are carried out, and the new software is applied to provide their evaluation. The primary results explaining a connection between lubrication and friction are presented. The results show a more significant impact of albumin proteins on the lubrication process, whereas its clusters create a more stable lubrication layer. A decreasing trend of protein cluster count, which corresponds to a decrease in the thickness of the lubrication film, is found in all experiments. The results highlight a deeper connection between the cartilage friction and the lubrication film formation, which allows for better understanding of the cartilage lubrication mechanism. [ABSTRACT FROM AUTHOR]

In this work, friction loss investigations and comparisons of three different four-cylinder engines for passenger car applications are presented, using a recently developed combined approach. By merging extensive experimental with reliable and predictive journal bearing simulation results, a sub-assembly-resolved friction loss analysis of the piston group, crankshaft journal bearings and valve train is conducted for all three engines. The engines have been chosen individually based on their specific power output and crank train geometry. The measurement program covers a wide range of corresponding engine operation points (identical speed, load and thermal boundary conditions). In addition, the investigations are carried out for different engine media supply temperatures ranging from 70 ∘ C to 110 ∘ C for a comprehensive consideration of the friction losses at reduced lubricant viscosity. For reasons of comparability, all investigations conducted in this work have been carried out using the same modern SAE 5W30 lubricant. This is done to exclude influences from different lubricant properties which may have significant effects on the tribological behaviour of the engines' sub-assemblies. While the diesel engine showed a friction reduction potential over the entire engine operation range when increasing the engine media supply temperatures, the gasoline engines showed a different behaviour. For the gasoline engines, disadvantages arise especially at low engine speeds. By using the developed combined approach, it was possible to assign mixed lubrication regimes at the valve train systems and at the piston groups. [ABSTRACT FROM AUTHOR]

This work presents the application of a combined approach to investigate the friction losses in a modern four-cylinder passenger-car diesel engine. The approach connects the results from engine friction measurements using the indication method and the results from journal-bearing simulations. The utilization of the method enables a subassembly-resolved friction loss analysis that yields the losses of the piston group, crankshaft journal bearings, and valve train (including the timing drive and crankshaft seals). The engine and engine subassembly friction losses are investigated over the full speed and load range, covering more than 120 engine operation points at different engine media supply temperatures ranging from 70 to 110 ∘ C. The subsequently decreasing lubricant viscosity due to higher engine media supply temperatures allow for the identification of friction reduction potentials as well as possible risks due to an onset of mixed lubrication. Furthermore, additional strip-tests have been conducted to determine the friction losses of the crankshaft radial lip seals, the timing drive, and the crankshaft journal bearings, thus enabling a verification of the calculated journal-bearing friction losses with measurement results. For the investigated diesel engine, a friction reduction potential of up to 21% could be determined when increasing the engine media supply temperature from 70 to 110 ∘ C, at engine speeds higher than n = 1500 rpm and part load operating conditions. At low engine speeds and high load operations, the friction loss reduction potential is considerably decreased and below 8%, indicating mixed lubrication regimes at the piston group and valve train. [ABSTRACT FROM AUTHOR]

The tribological system plays a critical part in designing robust and efficient cold forging operations. The appropriate selection of lubrication allows to forge defect-free workpieces with high dimensional precision and desired surface finish while ensuring that no defects, such as cracks or seams, occur. Additionally, friction and wear are highly affected by the choice of tribological system, which in turn influence the cost-effectiveness of the forging operation by preventing premature tool failure. Next to the employed tool coating and work piece material, the lubrication system and work piece surface topography are the main factors influencing the aforementioned constraints when designing efficient forging operations. In order to choose the appropriate tribological system before implementing it within an industrial forging operation, tribometers are used to characterize the performance of the tribological system. In this paper, the necessity to account for not only the tribological loads when designing these tribometer tests as is typical for existing methodologies, but also for process and lubricant specific properties will be highlighted. With the help of the tribometer sliding compression test, it will be shown that using liquid lubricants necessitates the need to account for the escape of lubricant, while this is not true for solid lubricants. The escape of lubricant from the contact zone is governed by lubricant properties as well as the contact kinematics and may lead to significantly different results regarding friction and wear. In order to account for this escape, the tribometer test must be specifically designed to reproduce the contact kinematics of the investigated industrial forging operation. [ABSTRACT FROM AUTHOR]

An introduction is presented in which the editor discusses various reports within the issue on topics including physical joint, friction measurements and lubrication.