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The main aim of this research was to investigate the influence of graphite as an additive to a plasma-sprayed alumina coating matrix for tribological trends. For the deposition process, pure alumina and alumina with graphite (5%, 10% and 15%) weight percentage powders were used. The graphite-modified alumina coatings were formed on steel substrates by atmospheric plasma spraying using an air-hydrogen plasma. The influence of graphite content on the elemental composition, phase structure, friction coefficient, and specific wear rate was investigated. It was obtained that the addition of graphite enhanced the γ-Al2O3 phase content in the as-sprayed coatings. The highest γ-Al2O3 phase fraction was obtained when the amount of graphite in the feedstock powders was 10 wt.%. The elemental composition results indicated that the concentration of carbon in the alumina-graphite coatings was up to six times lower in comparison to the initial feedstock, and had varied from ∼1.7 to 2.45 wt.%. The incorporation of graphite slightly reduced the friction coefficient of the composite coatings due to the formation of a self-lubricating layer under dry-sliding conditions. The specific wear rates were in the range of ∼10−6 mm3/(Nm) for the alumina-graphite composite coatings, which were similar to the alumina coating, and were an order lower compared to the wear rate of steel.

The present work is focused on comparing the tribological behaviour of graphene platelets and commercially available automotive additive package, combined with glyceryl isostearate (GIS) as a friction modifier. The investigation was performed with steel/steel and DLC/DLC contacts, at 25 °C and 100 °C, in all lubrication regimes and by considering the running-in effect. Under all the investigated conditions the graphene platelets provided the lowest friction in the ML and BL regimes. This is the result of forming graphene-based tribofilms which, unlike for the other tested lubricants, is relatively insensitive to temperature, tribochemical activation and the conditions before or after the running-in. By combined the use of graphene platelets and conventional additives some interactions in their tribological behaviour were also observed.

This work focuses on a tribological and contact-temperature investigation of unreinforced polyoxymethylene (POM) sliding against steel (DIN 100Cr6) under a wide range of loads and sliding speeds. We have investigated a wider range than is commonly studied, along with different heat-sink volumes of the steel counter-body (3-fold variation). Four empirical contact-load criteria, i.e., wear-change factor, mechanical load factor, thermal load factor, and critical temperature factor, were introduced to complement the evaluation of the tribological behaviour and the contact temperature. Moreover, these factors are not limited to current materials, but provide general methodological approach. Irrespective of the heat-sink volume, even with an 8-fold increase in the p · v value (0.6–4.8 MPa.m/s), the wear mechanism does not change significantly. This beneficial behaviour is mainly due to the synergistic interplay of the mechanical and thermal loads, which mutually suppress the domination and critical state of a single wear mechanism, either mechanical-based or thermal-based, which was understood through novel empirical factor analyses. Another key parameter for maintaining stable and beneficial tribological performance was the large thermal mass, i.e., the heat-sink volume of all the steel samples that suppressed the commonly observed high-temperature rise under severe contact conditions. Thus, the tribological performance was notably enhanced compared to previous studies under similar conditions, since the heat-sink volume was large enough and the balance between the thermal and mechanical loads was maintained.

Experimental and theoretical studies have been completed in this paper to investigate the interface effect in the lubrication performance of a step slider bearing. Oleophobic fluorine-containing diamond-like carbon (F-DLC) coatings were prepared on the step slider to provide low affinity to lubrication films. With an optical slider bearing test rig, it is revealed that the F-DLC coating presents an increase in the film thickness/load-carrying capacity and a decrease in the coefficient of friction (COF), which is quite different from the common idea that both the load-carrying capacity and the COF are reduced at the same time. Contact angles measurement on the step slider shows that the affinity to the lubricant of the inlet step surface is lower than that of the outlet land, which can be attributed to the rougher surface of the step surface by the laser ablation. Theoretical analyses show that various interfacial slip velocities on the inlet step and the outlet land can generate different change in the load-carrying capacity and the COF. When the step surface presents lower interfacial slip velocity of the film than the land surface, increase in load-carrying capacity and decrease in the COF can be theoretically found to support the experiments finds.

Compressive loading of shape memory alloys (SMA) is gaining considerable attention in recent years due to the improved fatigue life compared to tensile loading. This can be beneficial in applications such as dampers, actuators, and particularly elastocaloric cooling. SMA elements, however, tend to buckle under compressive loading and their stability can be enhanced by utilizing properly designed holders, i.e., structures that support SMA elements and prevent them from buckling. On the other hand, these supporting structures are in contact with SMA elements, which can cause wear and their premature failure, intensified by the lateral expansion of material under compression. In current literature, a majority of experiments are focused on reciprocating sliding wear of tungsten carbide or variations of bearing steel balls/discs/pins/rings on NiTi plates as well as on comparison of wear performance of NiTi with other materials. The aim of this present work is to theoretically and experimentally study tribological conditions between the tube and supporting element (bushing) and to find the most compatible material to NiTi in order to minimize wear, provide adequate structural support, and finally to enhance the overall fatigue behavior.

In this work, we evaluated the effect of the counter-body material (the same or dissimilar) and contact configuration (moving or stationary body), at similar contact tribological conditions, on the tribochemical and nanotopography characteristics of adsorbed surface films. Zinc dialkyldithiophosphate (ZDDP), the best performing anti-wear additive, was used in self-mated steel/steel and DLC/DLC contacts, which were compared with mixed steel/DLC and DLC/steel contacts in 1-h and 6-h sliding tests. The macroscale (tribometer) and nanoscale (atomic force microscopy) friction, thickness, topography, and chemical (attenuated total reflection-Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy) properties of the tribofilms were studied. The results revealed unexpectedly large differences in all the studied tribofilm parameters; this is because all the tribofilms are completely different; this includes the chemical composition, which is known to have a crucial effect on the nano- and macro-scale tribological properties. These results clearly demonstrate that the surface material, additives, and common contact operating parameters, that is, pressure, velocity, and temperature, crucially affect the ZDDP tribofilm as well as the position of the moving or stationary surface within the contact, and the material of the moving/stationary bodies.

A surface relief grating with a period of 30 µm is embossed onto the surface of magnetoactive elastomer (MAE) samples in the presence of a moderate magnetic field of about 180 mT. The grating, which is represented as a set of parallel stripes with two different amplitude reflectivity coefficients, is detected via diffraction of a laser beam in the reflection configuration. Due to the magnetic-field-induced plasticity effect, the grating persists on the MAE surface for at least 90 h if the magnetic field remains present. When the magnetic field is removed, the diffraction efficiency vanishes in a few minutes. The described effect is much more pronounced in MAE samples with larger content of iron filler (80 wt%) than in the samples with lower content of iron filler (70 wt%). A simple theoretical model is proposed to describe the observed dependence of the diffraction efficiency on the applied magnetic field. Possible applications of MAEs as magnetically reconfigurable diffractive optical elements are discussed. It is proposed that the described experimental method can be used as a convenient tool for investigations of the dynamics of magnetically induced plasticity of MAEs on the micrometer scale.

The effect of additives, their type and structure on the wetting of oil has received little attention so far and is thus poorly understood. This is despite the fact that a number of additives are present in every lubricating oil. Here, we report on the influence of the addition of some simple organic friction modifiers, namely, fatty acids, amides, alcohols and amines, on the static and dynamic (i.e., the advancing and receding contact angles, and the contact-angle hysteresis) wetting of synthetic PAO oil on steel. The results show that the dynamic wetting parameters are much more sensitive to changes in the wetting than the static contact angle, and thus more appropriate for wetting studies of oils, where the most pronounced effect was observed with the receding contact angle. The additives in the oil reduced the wetting of the steel surface significantly. The above-mentioned receding contact angles increased up to 8.3 times compared to the oil without additives. The influence of the additive's molecular structure was investigated by varying four of their fundamental properties. The results indicate that the most polar fatty acid decreases the wettability by 98% more than the least polar amine, the longest chain length of 18C atoms, by 76% more than the shortest chain of 11C atoms, the additives with one polar head group, by 74% more than with two polar groups, and the saturated additive, by 39% more than the unsaturated additive.

Several studies have successfully demonstrated the friction-reduction and anti-wear properties of graphene on the nano- and micro-scales, but our understanding of the tribological behaviour of graphene on the macro-scale remains very limited. Accordingly, this study presents the macro-tribological behaviour of a graphene-containing oil in the lubrication of steel/steel contacts and DLC/DLC contacts. We show that graphene platelets, as additives to the base oil, can, especially in the boundary-lubrication regime, decrease the friction in the DLC/DLC contacts by up to 50%, as well as in the steel/steel contacts by up to 44%. The predominant lubrication mechanism of the graphene platelets was shown to be the formation of a protective tribofilm for both types of surfaces. The effects of the concentration of graphene (0–5 wt.%) and the lubrication regimes on the formation of the tribofilm are also investigated and explained with a schematic 2-D contact model.

The intent of this research was to determine the tribological trends of zirconia as an additive to a plasma-sprayed alumina coating matrix. For the deposition process, pure alumina and alumina-withzirconia (5%, 10%) weight percentages were employed to be coated on steel substrates using an airhydrogen plasma. The torch power was set to ~40 kW. The surface roughness for the alumina coatings were in the range from 2.6-3.2 μm and with additive percentages of zirconia, it was found to vary between 3.0-3.7 μm. XRD measurements indicated that the predominant phase in the alumina-zirconia coatings was tetragonal-ZrO2 (t-ZrO2) and with alumina, it was α-Al2O3 and γ-Al2O3. The tribological properties such as the friction coefficient and the wear-rate of the alumina-composite coatings had been inspected to evaluate its dependence on the type and concentration of the additive powders. An increase in friction-coefficient was observed with the addition of zirconia. The normalized wear rates were in the range of ~10-5 mm3/Nm for the composite coatings with certain exceptions.

This study focuses on the wear, friction and electrical characteristics of the newly developed, disc-disc contact design for sliding electrical contacts. Different types of graphite and copper, as the material combinations, different contact operating variables and, in particular, different directions of the electrical current, i.e., the disc polarity, were investigated under real contact conditions in a dedicated tribological test rig. The electrographite and polymer-bonded graphite were running in a self-mated contact, against copper and against each other. The polymer-bonded-graphite/polymer-bonded-graphite pair performed the best for almost all the studied parameters, while the mixed, dissimilar, polymer-bonded-graphite/electrographite combination had the worst performance. Moreover, all the pairs containing electrographite material exhibited unstable contact behaviour, and generally gave a worse overall performance. The wear of the self-mated graphite pairs was less than that of the graphite/copper pairs. The effect of the polarity on the graphite/graphite material combinations was negligible, while in the graphite/copper combinations the direction of the electrical current was found to significantly affect the wear of the discs. The wear of the positive graphite disc was up to 30% lower than the negative graphite disc, while the wear of the positive copper disc was up to 8 times more than that of the negative copper disc. Differences in the boundary contact film were observed in the graphite/copper contacts depending on the surface polarity, meaning that sliding contacts with the current flowing from the graphite disc to the copper disc saw less wear than the contacts where the current flows from the copper disc to the graphite disc.

The low-speed, high-torque, orbital hydraulic motor is a component that often uses mineral oil to convert hydraulic energy into the rotational motion of a shaft. The main purpose of our research was to investigate the possibility of using water, which is a much less environmentally damaging working fluid than oil. The limitations of water lubrication were overcome with an appropriate surface-engineering approach. The surface coated with diamond-like carbon (DLC) significantly reduced both the friction and the wear. The lowest coefficients of friction in this study, taking into account all the tested parameters, were obtained when the stainless-steel/DLC (SS/DLC) contact was tested in water. The friction of the SS/DLC in water was even lower than the friction of the SS/DLC or SS/SS contact in oil. Furthermore, the wear coefficient of the SS/DLC in water was comparable to the wear coefficient of the SS/SS in oil. The surface hardness, surface roughness and applied force were also varied, but they did not play such an important role as employing the DLC coating. The SS/DLC contact can thus be considered as a very promising solution for an orbital hydraulic motor from the tribological and environmental points of view.

We demonstrate the control of wettability of non-structured and microstructured magnetoactive elastomers (MAEs) by magnetic field. The synthesized composite materials have a concentration of carbonyl iron particles of 75 wt.% (≈27 vol.%) and three different stiffnesses of the elastomer matrix. A new method of fabrication of MAE coatings on plastic substrates is presented, which allows one to enhance the response of the apparent contact angle to the magnetic field by exposing the particle-enriched side of MAEs to water. A magnetic field is not applied during crosslinking. The highest variation of the contact angle from (113 ± 1)° in zero field up to (156 ± 2)° at about 400 mT is achieved in the MAE sample with the softest matrix. Several lamellar and pillared MAE structures are fabricated by laser micromachining. The lateral dimension of surface structures is about 50 µm and the depth varies between 3 µm and 60 µm. A systematic investigation of the effects of parameters of laser processing (laser power and the number of passages of the laser beam) on the wetting behavior of these structures in the absence and presence of a magnetic field is performed. In particular, strong anisotropy of the wetting behavior of lamellar structures is observed. The results are qualitatively discussed in the framework of the Wenzel and Cassie–Baxter models. Finally, directions of further research on magnetically controlled wettability of microstructured MAE surfaces are outlined. The obtained results may be useful for the development of magnetically controlled smart surfaces for droplet-based microfluidics.

The effect of three different film-forming additives mixed in PAO oil on the elasto-hydrodynamic (EHD) friction was investigated and compared to cases with the base oil only. When the boundary films were formed on the surfaces, the organic friction modifier (OFM) decreased the EHL friction by up to 8.7%, the ionic liquid (IL) decreased it by up to 6.4%, while the polymeric organic friction modifier (pOFM) increased it by up to 4.2%. In contrast, if the boundary films were not formed, the EHL friction remained the same as with the base oils only. The oils’ surface tension and viscosity were analysed at 25 °C and 100 °C as potential influencing effects, and the most important parameter for friction changes was found to be the surface energy. The mechanism behind this EHL friction reduction is, therefore, the effect of the boundary films on the poor oil-surface wetting and the consequent boundary slip. This study confirms the poor wetting arising from additive boundary-film formation as a relevant EHL friction-reduction mechanism. This observation is similar to that observed previously for surface coatings, thus suggesting that the boundary films of the surface-energy-reducing additives is a parameter with the potential to reduce the EHL friction.

Gearboxes that employ polymer gears are being used in an ever-increasing number of different applications. However, polymer-gear design suffers from a lack of understanding of their tribological behaviour under controlled - but at the same time realistic - conditions. One of the factors that is often overlooked in model tribological tests is the effect of the slide-to-roll ratio (SRR), which is different in gear meshing and conventionally used model tribological experiments with pure sliding. In this study, we analyse the wear mechanisms and wear coefficients, as well as the friction, in very common gear pairs of non-lubricated POM/steel contacts, but at different SRRs (25 to 150%) and at two controlled temperatures (50 and 80 °C). The load was also varied (8 and 45 N). The results show that tests at 50 °C in SRR-variable model tribotests can predict the SRR-dependent wear mechanisms along the meshing path (positions A-E) in real gears, while the 80 °C tests are too severe, leading to thermal overload. An about 10-times-larger wear coefficient, compared to the gear tests, was measured during our SRR-variable tests, which is a better estimation compared to the common pin-on-disc test with 2-4 orders-of-magnitude deviation. Furthermore, based on an accurate simulation of the friction behaviour in model tribotests, compared to gear tests, an empirical model based on tribological tests with a variable SRR was developed that predicts the coefficient of friction along the meshing contact line in real-scale gears with a 3% deviation. This study shows that the SRR- and temperature-controlled model tribological tests are relevant for understanding the real-scale polymer gears' tribological behaviour and mechanisms and furthermore they generate predictive, empirical results.

An investigation is made into the lubrication capabilities of solid-lubricated liquid carbon dioxide (LCO2) in comparison to flood lubrication, straight LCO2 and oil-lubricated LCO2 (MQL). The coefficient of friction is determined via tribological experiments, similar to machining, using an open tribometer which features an uncoated carbide insert sliding against a workpiece. Tribological experiments reveal superior performance of solid-lubricated LCO2. The milling experiments as well indicate that solid-lubricated LCO2 significantly reduces wear. The machined-surface topography is examined using high-magnification SEM, which shows no presence of adhered solid particles on the workpiece surface, providing a completely dry machining process.

Neat (NPP) and recycled (RPP) polypropylene matrix materials were used to prepare wood–polymer composites with untreated wood fibers up to 40 wt.%. Long-term creep properties obtained through the time-temperature superposition showed superior creep resistance of composites with NPP matrix. In part, this is attributed to their higher crystallinity and better interfacial adhesion caused by the formation of a transcrystalline layer. This difference resulted in up to 25% creep compliance reduction of composites with NPP matrix compared to composites with recycled (RPP) polypropylene matrix, which does not form a transcrystalline layer between the fibers and polymer matrix. Despite the overall inferior creep performance of composites with RPP matrix, from the 20 wt.% on, the creep compliance is comparable and even surpasses the creep performance of unfilled NPP matrix and can be a promising way to promote sustainability.

Fundamental knowledge of frictional phenomena in cutting processes is elemental for their further development. Both pin-on-disc and open tribometer are able to replicate the sliding velocities and contact pressures during cutting to some extent; however, open tribometer can simulate continuous sliding against freshly regenerated surface. In this paper, the influence of kinematics on the friction coefficient measurement has been analyzed, considering exactly determined contact and lubrication conditions. The suitability of both methods for simulating the friction in cutting processes is discussed, and an applicability area is proposed for both methods, leading to a better modelling of cutting processes.

This paper reports on the performance and tribological mechanisms in a newly developed design for an electrical sliding contact in terms of sliding velocity, normal force and electrical current during 24-h experiments. Copper discs were run against hard-carbon graphite, electrographite and polymer-bonded graphite. The average wear rates of the copper discs against the three graphite materials were reduced to 10−8 and 10−7 mm3/Nm, and the graphite discs to 10−6 mm3/Nm, which is for 1 and 1–2 orders of magnitude less, than copper slip-rings and carbon brushes in conventional systems, respectively. The coefficient of friction was 0.2–0.4, depending on the conditions, which is comparable to the values of conventional electrical sliding contact. Tribofilms are generated in almost all contacts, and crucially determine the contact performance. The polymer-bonded-graphite/Cu formed the most compliant tribofilms that unify and decrease the local contact pressures and temperatures, and have the largest surface coverage that reduces the wear and contact resistance, which in turn reduces the contact instabilities and so the friction.

Al2O3 and Al2O3-graphite composite coatings were prepared on stainless steel by atmospheric plasma spraying. The influence of spraying distance and graphite addition on the coating microstructure, phase composition and tribological properties were investigated. The elemental composition results indicated that the increase of spraying distance slightly increased the graphite concentration in composites. The X-ray diffraction results of Al2O3 coatings revealed the presence of α-Al2O3, γ-Al2O3 and β-Al2O3 phases. The friction coefficient of Al2O3 coatings varied in the range of 0.74–0.75, whereas with the addition of graphite into the alumina powders, the friction coefficient of the coatings reduced to 0.34–0.38. It was found that the wear resistance of the Al2O3-graphite composite coating was superior to that of the Al2O3 coating when the spraying distance was 60 mm.

Sliding electrical contacts are used in devices for the conduction of electrical power or the electrical signals between stationary and moving parts. The most widely used electrical-sliding system is still the conventional slip ring and brush assembly. In the last decade, the lifetime of ring-brush contacts was extended through the use of better contact-material combinations and design improvements. However, the extremely rapid electrification of many more components has put severe demands on this extended lifetime. This paper presents a novel, patented design for two sliding discs that allows the use of advantageous graphite/graphite contacts. In the literature there is a serious lack of understanding the performance of sliding electrical contacts with graphite/graphite pairs, which depends on the key contact parameters, as well as the influence and potential of the different available graphite materials. The normal load, the contact velocity, the electric current, and the three self-mated graphite contacts, i.e., hard carbon, electrographite and polymer-bonded graphite, were investigated. The results indicate that the contact conditions influence the performance, which is also very dependent on the graphite materials. The polymer-bonded graphite showed the best results in this study.

The multi-asperity contacts between two nominally flat surfaces were studied experimentally with submicron resolution and compared to the most common elastic-plastic statistical models to provide information that is seriously lacking today. The contact pressure, the real contact area and the asperity deformation were investigated for realistic steel specimens with different roughness in the full engineering loading range up to the material's macro yield stress. The experimental results showed that the roughness strongly affects the real contact area and the deformation behaviour of the asperities. Experimental results and the statistical models shows good agreement for the smoothest surface under all loads, while at loads above ≈0.3∙Y the models overestimate the real contact area for 2–6 times.

Modern trends in machining include sustainable manufacturing with the focus on reducing the consumption of conventional cutting fluids. These alternative cooling-lubrication principles include Minimum Quantity Lubrication (MQL), cryogenic machining and dry machining. In the absence of abundant quantities of cutting fluid, adequate lubrication is needed, especially when difficult-to-machine materials are considered. High friction during machining leads to heat generation, which adversely affects the tool life, workpiece surface quality and energy consumption. Consequently, lower cutting speeds are employed. Solid lubricants such as molybdenum disulphide or graphite provide excellent lubrication in severe conditions, where contact pressures and temperatures exceed the operational limits for conventional oils. These solid lubricants can be added to cutting fluids, e.g. in oil for MQL, to enhance their thermal and tribological properties. Their unique structure can provide lower friction coefficient, thus reducing the generated heat and prolonging the tool life. Presented in the paper is a performance evaluation of solid lubricant (MoS 2 , average particle size 2 μm) from the tribological viewpoint. An open tribometer has been developed to estimate the friction coefficient between uncoated carbide tool and AISI 1045 workpiece at machining-like conditions which include high contact pressures and sliding speeds. To show the potential of solid lubricant, a comparison has been made with dry, flooding, MQL, cryogenic and combination of MQL and cryogenic conditions. Molybdenum disulphide showed excellent lubrication performance under all considered sliding speeds, outperforming the conventional lubricants, opening the possibility of further research in the area of solid lubricant assisted machining.

Polymer gears are used in many devices due to their advantages, among other, dry-running possibilities. As part of their design, the life-time predictions for fatigue and wear are among the most critical and necessary for any application. In a wear life-time prediction, the wear coefficient is the key parameter. The VDI 2736 guideline considers the wear coefficients obtained from pin-on-disc tests. Due to the obvious possible differences in wear coefficients from pin-on-disc tests and actual gear tests, these data can vary considerably. Moreover, the methodology for determining wear coefficients in a real-scale gear test has several possible variations that can lead to significant differences. This study evaluates several methods of determining the wear coefficient, i.e., pin-on-disc, as well as seven methodologies based on real-scale gear tests. Two different polyacetal (POM) materials (Delrin 500P and Hostaform C9091), typical for polymer gears, were used for the pin-on-disc analyses and a re-evaluation from the literature. The Delrin 500P was additionally used in real-scale gear tests to provide a direct comparison of the wear coefficients with the selected methods. The overall conclusion is that the wear coefficient of polymers used in gear-design calculations should be obtained from real-scale gear tests. This can be concluded from empirical wear coefficient results, as well as from surface wear mechanisms analyses.