Lubricants, Volume 11, Issue 11 (November 2023) – 40 articles

The leak tightness of the sealing system rotary shaft seal is based on the formation of an active back-pumping effect of the sealing ring. Here, the sealing ring pumps the fluid in the sealing gap back into the housing. However, this active sealing mechanism is disturbed by so-called “lead structures”. Lead structures include all types of directional structures on the sealing counterface which create rotation-dependent axial fluid pumping. Lead-affected sealing counterfaces can thus cause leakage or insufficient lubrication of the sealing contact. To ensure leak tightness, lead must be avoided or tolerated. This article investigates how different structural characteristics of lead affect the amount of fluid pumped by the shaft surface. For this purpose, 26 shafts are subjected to surface analyzing methods and an experimental pumping rate test. The interaction of various geometric features of the lead structures and their combined effect on the pumping capacity is modeled based on the measured data. Appropriated correlation models are discussed and relationships between shaft lead and its pumping effect are shown. The aim is to estimate shaft pumping rates based on surface measurements in future. The results contribute to the derivation of measurable tolerance values for lead and to the prevention of leakage. Full article

The total loss lubrication system that is typical of chainsaws is responsible for a massive dispersion in the agro-forestry environment of highly impactful pollutants, mostly of fossil origin, often well known as carcinogenic substances, which, in addition to presenting a risk to the environment, represent an important risk factor for human health, especially for chainsaw users. During its use, the chain lubricant is dispersed from the guide bar tip in the form of droplets and aerosol, or it is adsorbed on wood residues and sawdust. Then, it is subjected to drift, settles on the ground and vegetation, and can hit the operators, who, after prolonged exposures, can suffer both irritation of the respiratory tract and dermal absorption. Such a risk factor is often amplified by the widespread use of less-expensive, sometimes illegal alternatives, such as exhausted motor oils. To mitigate said negative effects, a process has been in progress for several years that is aimed at replacing conventional lubricants with synthetic or biobased oils with increasing biodegradability. As a contribution to this process, a study has been started on the possibility of using refined olive pomace oil (ROPO) as a base stock for the formulation of a totally biodegradable chainsaw lubricant. On purpose, to improve its properties of viscosity and adhesivity, such an oil was added with a biodegradable thickening agent, obtaining four formulations with different viscosity. After a lab test and a preliminary cutting test on firewood, the formulation with 2% of thickener resulted in being the best, and 3.0 g kg⁻¹ of tert-butylhydroquinone (TBHQ), a food-grade antioxidant, was then added to form the final formulation (F₂) to be compared, in the subsequent four test sessions, to a biodegradable commercial chain lubricant (S_B). The tests were carried out without changing the chainsaw setting, on different wood species, both in forest and, with the aim of increasing the repeatability of tests conditions and comparability of results, at a fixed point. The fluids’ performances were mainly evaluated based both on the operators’ opinions and on the measurements of the chain–bar temperatures and of saw chain wear related to a predefined number of cuts. As to the destiny of the fluid dispersed during cutting, the overall dispersion was assessed by considering the average working time, the consumption of chain lubricant, and the forest area cut down daily. Eventually, the amounts of inhalable and respirable dust particles as vectors of oil residues were quantified by means of personal air samplers worn by the operators and analyzed to determine any differences in the concentration of metallic elements. The test results evidenced chain temperatures that were 0.5, 4.9, and 12.5 °C higher with F₂ relating to S_B, respectively, in the cutting of trunks of fresh Pinus, Eucalyptus, and dry Pinus. They were accompanied by chain weight losses of 89.5% and 35% higher with F₂ relating to S_B, respectively, in cutting tests of Turkey oak and Poplar. Such a greater wear, however, apparently did not affect the saw chain’s cutting efficiency with F₂, since the operators declared that they did not notice any difference between the performances of the two fluids at the time of comparison. The effects of higher wear on the chain lifetime, any deriving risks for the operator’s safety, and the possibility to reduce the wear levels observed with F₂ will be explored in a further study, e.g., through different settings of the lubricating system of the chainsaw. The results of the analyses of the air-sampled dust residues that were evidenced with F₂ showed lower concentrations of respirable and inhalable particles and of some metallic elements (Al, Mg, and Ca) than those with S_B. This behavior probably depends on the different interaction between sawdust and the two fluids, which differ according to their chemical–physical characteristics (different viscosity, composition, and additives). However, it represents a positive factor in favor of the use of the ROPO-based lubricant, emphasized by the total biodegradability of its residues that are possibly contained in the dust inhaled by the operators. Full article

The machining of Ti-6Al-4V alloys is challenging due to their high strength, poor thermal conductivity, and high chemical reactivity. When used in traditional machining, cryogenic coolants can reduce tool wear, thus extending tool life, improving surface finish, and requiring less power with reduced environmental effects. In this context, this study aimed to perform a machinability analysis of the surface roughness, power consumption, tool wear, and specific energy consumption of a Ti-6Al-4V titanium alloy and to comprehend the performance of dry and cryogenic machining in turning operations. A comprehensive analysis of tool wear and specific cutting energy (SCE) under dry and cryogenic machining was conducted. It was found that the machining time under a cryogenic environment was increased by 83% and 39% at 80 and 90 m/min compared to a cutting speed at 100 m/min. The higher cutting speed (100 m/min) in cryogenic environments produced an improved surface finish. Compared to dry machining, the cooling effect of liquid CO₂ helped dissipate heat and reduce thermal damage, improving surface finish. The findings revealed that in dry conditions, approximately 5.55%, 26.45%, and 27.61% less power was consumed than in cryogenic conditions at 80, 90, and 100 m/min cutting speeds, respectively. Based on the outcomes of the work, the application of cryogenic cooling can be considered an alternative to dry and flood cooling for improving the machinability of Ti-6Al-4V alloys. Full article

Lightweight materials with a density less than 3 g/cm³ as potential tribo-materials for tribological applications (e.g., space tribology) are always desired. Al₃BC₃ ceramic, a kind of ternary material, is one of the lightweight materials. In this study, dense Al₃BC₃ ceramic is prepared via a reactive hot-pressing process in a vacuum furnace. Its tribological properties are investigated in two unlubricated conditions (one is at elevated temperature up to 700 °C in air, and another is in a vacuum chamber of back pressures from 10⁵ Pa to 10⁻² Pa at room temperature) and lubricated conditions (i.e., water and ethanol as low-viscosity fluids). At 400 °C and lower temperatures in air, as well as in vacuum, the tribological property of Al₃BC₃ ceramic is poor due to the fracture of grains and formation of a mechanically mixed layer. The beneficial influence of adsorbed gas species on reducing friction is very limited. Due to the formation of lubricious tribo-oxide at 600 °C and 700 °C, the friction coefficient is reduced from ca. 0.9 at room temperature and 400 °C to ca. 0.4. In the presence of low-viscosity fluids, a high friction coefficient and wear but a polished surface are observed in water, while a low friction coefficient and wear occur in ethanol. A lubricious carbide-derived carbon (CDC) coating on top of Al₃BC₃ ceramic through high-temperature chlorination can be fabricated and the wear resistance of CDC can be improved by adjusting the chlorination parameters. The above results suggest that Al₃BC₃ ceramic is a potential lubricating material for some tribological applications. Full article

Due to the coupling of the damper journal with the elastic ring and oil film, the elastic ring squeeze film damper (ERSFD) shows better dynamic performance in comparison with the traditional squeeze film damper (SFD). Therefore, a novel rigid–elastic–oil coupled mathematical model was established. The elastic ring deformation, as the key point, is solved according to the planar bending theory. Then, based on the pressure governing equation of the oil film, using the central finite difference method, the oil film pressure was addressed. Meanwhile, the Simpson method was implemented to calculate the dynamic characteristic coefficients (equivalent stiffness and damping $Ce$) of ERSFD (DCCEs). Also, we analyzed the influence of journal eccentricity, oil film radius clearance, flexibility coefficient and damping hole diameter on the DCCEs, and the results were compared and verified with the existing literature. The sensitivity of each parameter to the DCCEs was analyzed by using the linear regression method. According to the results, the flexibility coefficient has the greatest effect on the DCCEs, followed by the oil film radius clearance. The eccentricity of the journal and damping hole diameter have the least impact. This work will provide a theoretical basis for reflecting on the bearing dynamic characteristics more truly and accurately. Full article

In this study, the thermo-oxidative stability and tribological behavior of bio-based lubricant samples synthesized from castor oil using isoamyl alcohol were evaluated. Initially, the compositional and physicochemical properties of the obtained samples were assessed using the ¹H NMR, FTIR and ASTM methods. Oxidative stability of the samples was evaluated using the Rancimat method at 110 °C under air flow. The final biolubricant sample (BL2), obtained after esterification, epoxidation and oxirane rings opening reactions, presented an oxidation stability time (OST) of 14.3 h. The thermal stability was also evaluated by thermogravimetry (TG) from the mass variations under inert and oxidative atmosphere. BL2 showed higher thermal stability compared to the other samples, demonstrating higher decomposition temperatures in both inert (339.04 °C) and oxidative (338.47 °C) atmospheres, for a mass loss of 50%. The tribological properties of the samples were evaluated using a four-ball tribometer configuration. The BL1 and BL2 samples exhibited lower friction coefficients than the mineral oil sample (MOS) by 21.5% and 43.1%, respectively. Regarding wear, the observed wear scar diameter (WSD) was also lower in BL1 and BL2 compared to MOS by 5.2% and 40.4%, respectively. The results of the tribological evaluation suggest that both samples (BL1 and BL2) have promising potential for applications in lubricating machines. Full article

In this paper, the microstructures of nanocomposite ceramic tool materials are represented through Voronoi tessellation. A cohesive element model is established to perform the crack propagation simulation by introducing cohesive elements with fracture criteria into microstructure models. Both intergranular and transgranular cracking are considered in this work. The influences of nanoparticle size, microstructure type, nanoparticle volume content and interface fracture energy are analyzed, respectively. The simulation results show that the nanoparticles have changed the fracture pattern from intergranular mode in single-phase materials to intergranular–transgranular–mixed mode. It is mainly the nanoparticles along grain boundaries that have an impact on the fracture pattern change in nanocomposite ceramic tool materials. Microstructures with smaller nanoparticles, in which there are more nanoparticles dispersed along matrix grain boundaries, have higher fracture toughness. Microstructures with a nanoparticle volume content of 15% have the most obvious transgranular fracture phenomenon and the highest critical fracture energy release rate. A strong interface is useful for enhancing the fracture toughness of nanocomposite ceramic tool materials. Full article

Contacts and joints in structures, mechanisms, and dynamic systems often exhibit high localized interface shear at their edges, leading to edge microslip and fretting wear and fatigue. This introduces complexity, nonlinearity, and multiscale friction phenomena. This paper presents a novel approach to address this issue by introducing geometrical changes near contact edges. Two-dimensional contact models are developed and analyzed using asymptotic, closed-form, and numerical methods to study the effect of edge changes on pressure and shear traction. The results show that geometric changes near contact edges can effectively reduce contact edge shear, thereby inhibiting edge microslip and the resulting fretting wear and fatigue in contacts that occur under dynamic conditions. This approach has implications for reduced complexity in contacts and joints for improved capability in modeling, analysis, and measurement characterization. Full article

The increasing adoption of PEEK (polyetheretherketone) in many industrial applications has promoted intense research to optimize its lubrication along with the development of friction reducers (FRs), additives that help in reducing fuel consumption and, consequently, CO₂ emissions. In this study, the effect of FRs in improving the lubrication of PEEK–steel couplings was evaluated and their mechanism studied using the Mini Traction Machine (MTM) tribometer. Different types of FRs (such as Molybdenum dithiocarbamate, glycerol monooleate, amine and polymeric derivatives) and coupling combinations (steel/steel, steel/PEEK and PEEK/steel) were considered. The oil samples were evaluated as fresh and after a rubbing time considering different operative conditions (from high to low T, fixed load and type of contact motion), and a measurement of the tribofilm was acquired. The experimental campaign showed a ranking among FRs friction-reducing behavior and, in some cases, a synergistic effect was noted between the tribofilm containing the friction modifier and the PEEK surface. Comparing the top performing FRs with reference oil showed a reduction in friction of 22%, 21% and 37%, respectively, in steel–steel, PEEK–steel and steel–PEEK couplings, while in the standard steel–steel coupling, two out of four FRs did not reduce the friction. After conditioning in the presence of PEEK, all friction-modifier additives reduced the friction effectively. This demonstrates the promising performance of PEEK, its compatibility with friction-reducing additives, and its applicability to sliding machine parts in order to improve efficiency and thus reduce CO₂ emissions. Full article

The tribological behaviors of cast iron by laser surface texturing were experimentally compared with the behavior of untextured by unidirectional rotary sliding friction and wear tests under oil-lubricated initial line contact. The friction coefficient and temperature rise were analyzed with the increasing load applied by block-on-ring tests. In addition, the wear loss and wear mechanism were also investigated through the surface topographies analysis. The results showed that the tribological improvement strongly depended on the contact form. For the oil-lubricated initial line contact in this work, the textured surface showed a better frictional advantage with a lower friction coefficient and lower temperature rise. The hydrodynamic effect enhanced the load-carrying capacity of the oil film and increased the film thickness. The friction coefficients were 11~64% lower than those on the untextured one. Meanwhile, the textured surface deteriorated the wear behavior due to the coupling effect between the micro-cutting effect of the texture edges and the material deformations of the counter surface. The material loss induced by abrasive wear and fatigue wear was the dominant wear mechanism. Namely, the laser surface texturing improved the friction properties but reduced the wear resistance. Full article

Deep-sea submersibles carry limited energy sources, so a high efficiency of the equipment is required to improve endurance. In the deep-sea environment, the hydraulic power source is filled with oil, which causes structural deformation of the power source and changes in the physical properties of the medium, leading to unknown changes in the efficiency characteristics of the power source. In order to explore the efficiency characteristics of the deep-sea hydraulic power source composed of a gear pump and a DC (direct current) brushless motor in a variable sea depth environment, we undertook the following. First, considering the effects of seawater pressure and temperature on the physical properties of the medium and the radial clearance deformation of the gear pump, a mathematical model for the total efficiency of the hydraulic power source was established. The results indicate that the deformation of the pump body is mainly determined by the seawater pressure and working pressure. Subsequently, by analyzing the effects of the two factors on the efficiency of the power source, respectively, when the oil temperature range is large enough, the total efficiency of the power source will increase and then decrease under six sea depths; the total efficiency of the power source decreases with the increase in the rotational speed. However, in a land environment, the trend of the efficiency characteristics of the power source is opposite to that of the remaining six deep-sea environments, both in terms of oil temperature and rotational speed. Finally, the efficiency trend of the power source with changes in sea depth under rated conditions was obtained. Under different sea depth ranges, the optimal operating oil temperatures and suitable rotational speed ranges of the power source could be obtained. This paper could provide a certain theoretical basis for the research and development of deep-sea equipment. Full article

Internal combustion engines, during their operation, subject the piston to high-temperature and high-pressure conditions, requiring it to endure intense, continuous reciprocating motion. This strenuous process leads to significant wear and tear. Among the engine’s crucial components, the piston ring plays a pivotal role but is particularly susceptible to wear. Therefore, extensive research has been devoted to investigating the wear of piston rings, a critical sealing component within internal combustion engines. To address the high cost of existing coating methods, which hinders widespread application, we propose a bionic design approach inspired by groove structures observed on earthworm bodies, aimed at enhancing the wear resistance of piston rings. Bionic piston rings featuring optimally designed groove structures inspired by the earthworm’s anatomy were designed. These rings exhibited varying groove depths (1 mm, 2 mm, and 3 mm), groove widths (0.1 mm, 0.3 mm, and 0.5 mm), and groove spacings (0.1 mm, 0.2 mm, and 0.3 mm). We conducted thermal–structural coupling analyses on both standard piston rings and these bionic counterparts. The results revealed that the maximum stress was concentrated at the first piston ring, precisely at the opposing region of the end gap. Thus, the initial piston ring endured the primary frictional losses. Moreover, a comparison of stress levels between bionic rings and the standard ring revealed that the bionic groove structure substantially reduced stress and minimized stress concentration, thus enhancing wear resistance. Groove width had the most notable influence on wear performance, followed by groove depth and groove spacing. Optimal wear resistance was achieved when the groove depth was 3 mm, groove width was 0.1 mm, and groove spacing was 0.1 mm. Subsequently, we constructed a piston ring friction test bench to validate the wear resistance of the most effective piston ring. The results indicated that the wear resistance of the bionic piston ring exceeded that of the standard piston ring by up to 19.627%. Therefore, incorporating a bionic groove structure within the piston ring can effectively reduce surface friction and enhance wear resistance. This, in turn, can enhance the operational lifespan of internal combustion engines under favorable working conditions. Full article

Laser cladding is a new technology for fabricating coatings with good properties, such as wear resistance, lubrication, and corrosion resistance. Usually, parts of 45 steel are used as a shaft under conditions of high-speed rotation or friction and wear, and they have a short service life and sometimes cause accidents. In order to avoid serious accidents, a cladding coating made from a Ni-based alloy with ceramic particles was fabricated via laser technology on a substrate of 45 steel in this research. The microstructure and properties were investigated via SEM, EDS, XRD, and a wear and friction tester. The results show that there was an obvious boundary between the cladding coating and the substrate. The main phases were γ(Fe, Ni), WC, TiC, Cr₂Ti, and Cr₂₃C₆. In the middle of cladding coating, the microstructure was composed of dendrite and cellular crystals, while the microstructure was composed of equiaxial crystals in the bonding region. Inside the cellular crystal, the main phase was γ~(Fe, Ni), which occasionally also showed the appearance of some white particles inside the cellular crystal. Compared with the cellular crystal, the boundary had less of the Fe and Ni elements and more of the Cr and W elements. The amount of C element around the dendrite crystal was more than that around the boundary of cellular crystal due to the long formation time of dendrite. The white particles around the boundary were carbides, such as WC and Cr₂₃C₆ phases. Meanwhile, the segregation of the Si element also appeared around the boundaries of the crystal. The maximum microhardness was 772.4 HV_0.5, which was about 3.9 times as much as the substrate’s microhardness. The friction coefficients of the 45 steel substrate and Ni-based alloy coating were usually around 0.3 and 0.1, respectively. The Ni-based coating had a smaller coefficient and more stable fluctuations. The wear volume of the cladding coating (0.16 mm³) was less than that of the substrate (1.1 mm³), which was about 14.5% of the wear volume of 45 steel substrate. The main reason was the existence of reinforced phases, such as γ~(Fe, Ni), Cr₂₃C₆, and Cr₂Ti. The added small WC and TiC particles also enhanced the wear resistance further. The main wear mechanism of the cladding coating was changed to be adhesive wear due to the ceramic particles, which was helpful in improving the service life of 45 steel. Full article

As one of the lightest structural metals, the application breadth of aluminum alloys is, to some extent, constrained by their relatively low wear resistance and hardness. However, laser cladding technology, with its low dilution rate, compact structure, excellent coating-to-substrate bonding, and environmental advantages, can significantly enhance the surface hardness and wear resistance of aluminum alloys, thus proving to be an effective surface modification strategy. This review focuses on the topic of surface laser cladding materials for aluminum alloys, detailing the application background, process, microstructure, hardness, wear resistance, and corrosion resistance of six types of coatings, namely Al-based, Ni-based, Fe-based, ceramic-based, amorphous glass, and high-entropy alloys. Each coating type’s characteristics are summarized, providing theoretical references for designing and selecting laser cladding coatings for aluminum alloy surfaces. Furthermore, a prediction and outlook for the future development of laser cladding on the surface of aluminum alloys is also presented. Full article

To improve the service behavior of gears, surface heat treatments such as nitriding or induction hardening can be performed. Since these processes are limited in their achievable maximum hardness or depth of hardness, a combination treatment could allow benefits from the advantages of both processes. The aim of this work was to show the correlation between the microstructure resulting from combination treatment and the performance of the surface layer using the example of wear behavior. The investigations focused on the impact of different nitrided states, in the combination treatment of the material EN31CrMoV9, on wear resistance. The wear was evaluated after running the two-disc test gravimetrically and optically. Nitrided-only specimens showed better wear resistance compared to those subjected to induction hardening after nitriding. Substantial differences in weight loss indicate that induction hardening worsens the wear behavior. The variants with the compound layer removed in the nitride-only state as well as in the induction hardened state showed a better wear behavior compared to the respective conditions with a compound layer. This was attributed to the lower surface roughness and the higher hardness due to less retained austenite after combination treatment. Full article

Gas foil thrust bearings (GFTBs) have been successfully used to support the axial load of oil-free microturbomachinery with low drag friction due to the low viscosity of gas or air used as a bearing lubricant. However, the widespread use of GFTBs in various high-power turbomachinery still needs reliable test data and an accurate predictive model. This research measures the height profile of a test GFTB to determine its actual incline geometry and estimate the drag torque of the GFTB. The measured GFTB height profile demonstrates that the incline geometry is closer to a quadratic curve than a line, which has been conventionally used to model GFTBs mathematically. The newly developed GFTB test rig is used to measure the lift-off speed, drag torque, and maximum load capacity of the test GFTB. A series of rotor speed-up tests estimate that the lift-off speeds of the GFTB increase with the increase in preloads. The maximum load capacity is determined by increasing the static load on the GFTB until a sudden sharp peak in the drag torque appears. The new GFTB model using quadratic incline geometry is in suitable agreement with the measured height profile of the GFTB incline and measured drag torque during the load capacity test. In addition, a comparison of the predicted GFTB performances reveals that the quadratic incline geometry model predicts a higher load capacity than the linear model. Full article

Bio-lubricants have demonstrated promising tribological and physical properties, suggesting their potential advantages in the lubrication of critical machinery components. This study investigates the impact of using blended individual and hybrid nanoadditives, such as graphene nanoplatelets, ZnO, and an ionic liquid (IL) of Trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl)phosphinate, on the rheological, tribological, and physical characteristics of rapeseed oil. A commercial cutting fluid (BLASER Vasco 6000) (VB 6000) is used for comparison. The results revealed a substantial improvement in viscosity index (VI) values for mixtures containing graphene nanoplatelets, reaching up to 150%, as compared to VB 6000. Regarding the tribological behavior, the friction coefficient achieved a reduction of up to 20% at room temperature (RT) and 26% at 60 °C for the hybrid containing all three nanoadditives (H3), outperforming the commercial fluid. Moreover, H3 demonstrated the most substantial reductions in wear volume (84%) and surface roughness (60%). The wettability of H3 benefited from the combined mechanisms of the applied nanoadditives; its application the contact angle decreased by 63%, revealing its outstanding spreadability. The results reveal the high potential of the H3 hybrid as a competitive and green metal working fluid that can replace hostile and toxic ones in industrial applications. Full article

The installation quality of a propulsion shaft system directly affects the lubrication statuses of the bearings. The quality of the shaft system installation not only affects the progress of ship construction, but also the safety, stability, and reliability of the shaft system. This article takes sliding bearings in ship shafting as the research object and establishes a hydrodynamic lubrication model of sliding bearings while considering installation errors to address the issue of installation errors of ship stern bearings. The finite difference method and super-relaxation iteration method are used to solve the problem, and the influences of bearing installation errors on bearing lubrication characteristic parameters are explored. An installation error of the stern bearing can lead to an increase in the film pressure at both ends of the stern bearing in the axial direction, leading to a decrease in the lubrication status of the bearing. Poor lubrication and wear faults of the stern bearing are prone to occur at both ends of the stern bearing. As the installation error of the stern bearing increases, the minimum film thickness of the stern bearing decreases and the maximum film pressure increases, and as the installation error increases, the sensitivity of the aft stern bearing to the vertical installation error is greater than that of the lateral installation error, and the sensitivity of the fore stern bearing to the lateral installation error is greater than that of the vertical installation error. The sensitivity of the lateral and vertical film forces at both ends of the aft stern bearing and the fore stern bearing is greater than that of the middle part; the installation error of bearings has a significant impact on the lubrication characteristics of bearings. Full article

In this study, the potential of Fe₃Al coating material as an environmentally friendly alternative to coatings containing critical elements for brake discs was investigated. A buffer layer of Cr–Mo steel (Ferro 55) that was about 500 µm thick was applied on a gray cast iron disc to enhance the coating quality and prevent the formation of hot cracks during solidification. The microstructural analysis of the cross-section of the coating showed that the buffer layer diffused into the Fe₃Al coating, forming a combination of Fe₃Al, Fe, and Fe₃AlC_0.5 phases. The tribological properties of the Fe₃Al-coated disc were evaluated using pin-on-disc tests against two different copper-free friction materials extracted from commercial brake pads. The wear results show a coefficient of friction comparable to that of an uncoated disc (≈0.55), but with a reduction in particulate matter (PM) emissions, which decreased from 600 to 476 #/cm³. The last issue is an interesting aspect that is gaining increasing importance in view of the upcoming international standards. Full article

This work investigates the influence of altered engine oil on the tribological performance, focusing in particular on wear and interconnected tribofilm formation. For this purpose, Zinc dialkyldithiophosphate (ZDDP) additivated engine oils of different degradation levels, produced in an artificial oil alteration process, were used in tribometer tests with a nitride steel piston ring against a grey cast iron cylinder liner model contact. Parameters were chosen to simulate the boundary and mixed lubrication regime typical for the top dead centre conditions of an internal combustion engine of a passenger car. Wear of the cylinder liner specimens was continuously monitored during the tribometer tests by the radio-isotope concentration (RIC) method, and tribofilms were posteriorly investigated by X-ray photoelectron spectroscopy (XPS). The results clearly show that the steady-state wear rates for experiments with altered lubricants were significantly lower than for the experiments with fresh lubricants. XPS analysis on the formed tribofilms revealed a decrease in sulphide and an increase in sulphate states for altered oils evaluated at 120 °C oil temperature, correlating with a decrease in steady-state wear rate. This finding emphasizes the role of sulphate species in the tribofilm formation process and its anti-wear capabilities, in contrast to the sulphide species and the (poly-)phosphate species, as outlined in most of the ZDDP literature. Moreover, the RIC signal that represents the amount of wear in the engine oil showed a decrease over time for specific altered lubricants and test conditions. These “negative” trends in the wear signal are remarkable and have been identified as an incorporation of wear particles from the lubricant into the tribofilm. This finding is supported by XPS results that detected an iron-oxide layer with a remarkably similar quantity within the tribofilm on the surface. Based on these findings, an assessment of the minimum film formation rate and particle incorporation rate was achieved, which is an important basis for adequate tribofilm formation and wear models. Full article

Aiming to solve the problem of knot-tripping failure caused by severe wear between the spherical roller and planar cam of the knotter, this paper first establishes a calculation model of the spatial cam contour surface. The knot-tripping mechanism in the knotter is designed as a line-contact curved-surface cam mechanism, with the cutter arm swinging in accordance with sinusoidal acceleration. The design significantly reduces the contact stress between the cam and the roller, compared to the original knot-tripping mechanism. Additionally, it eliminates the impact between the spherical roller and the planar cam. Based on the Archard model, the calculation model for cam-roller wear in the knot-tripping mechanism has been derived and utilized for wear calculation. The wear test results of the knot-tripping mechanism with an aluminum cam show that the curved cam has a wear amount that is 43%, 56%, 46%, and 37% lower than that of the planar cam after tying the knot 200 times, 600 times, 1300 times, and 2000 times, respectively. Under the condition that the twine tension is set to 120 N, and the rotation speed of the fluted disc is 60 rpm, the deviations between the calculated value and the measured value of the wear amount of the curved cam are 9.48%, 6.01%, 7.27%, and 9.95%, respectively. This validates the accuracy of the spatial cam wear model and the correctness of the curved cam design. Full article

Calf serum is defined as a test fluid for in vitro knee wear simulation studies in the ISO standard. However, protein degradation typically occurs during in vitro wear simulation. The current study should indicate whether increased loads change the rheological properties of the test fluid and may, therefore, lead to favorable tribological behavior and reduced wear. Three different load levels were simulated in a displacement-controlled knee wear simulation study. The gravimetric wear rates were determined, pressure measurements were performed, and the dynamic viscosity of the test fluids were analyzed after the simulation of 0.5 × 10⁶ cycles. The lowest load level led to the lowest wear rate, and the lowest contact pressure and contact area, compared to the medium and high-load level. Although, the high-load level led to the highest contact pressure and contact area, the wear rates were comparable to the medium-load level. The rheological measurements revealed the highest dynamic viscosity for the high-load level and no differences could be found between the medium and low loading condition. To perform realistic wear simulation studies, the reproduction of the in vivo interrelationships between the shear forces and wear are necessary. Full article

In aerospace, aviation, nuclear power, and other high-tech fields, some essential moving parts must operate under high vacuum, high load, intense radiation, and other conditions. Under such extreme conditions, only solid lubricating materials can meet the lubrication requirements. Traditional material modification methods have problems such as high energy consumption, severe pollution, and narrow scope of application. Plasma modification technology can overcome these shortcomings. This paper focuses on several commonly used plasma preparation techniques for solid lubricating coatings, including plasma chemical heat treatment, physical vapor deposition, plasma immersion ion implantation and deposition, plasma spraying, and plasma electrolytic oxidation. Subsequently, the material systems of metal-based solid lubrication coatings are reviewed: soft metals, oxides, sulfides, nitrides, and carbon-based materials. Finally, found that the development of new solid lubricants, the improvement of existing preparation technology, and the development of new processes are the key development directions in the future. Full article

Press fits are a simple and effective method for assembling a shaft into a hub for different applications in the mechanical engineering field. This method consists of forcing to pass a shaft into a hub via axial insertion. As a result of the difference in the diameters of both components of the shaft and hub, a radial interference is generated, causing a contact pressure at the interface shaft–hub. Contact pressure and the friction coefficient are key factors influencing the maximum transmitted torque. So, in this study, different scenarios for the assembly of a press fit were simulated using finite elements (FE) in order to reveal the influence of this key parameter on the manufacturing-induced stresses in the hub. This way, different friction conditions were considered in terms of the friction coefficient from the frictionless case to a case of high dry friction. In addition, different hub geometries were analyzed including conventional hubs and chamfer hubs with optimal geometry that allows lowering the localized stress concentrations at the hub edges. This way, a more realistic estimation of the final stress state of a press fit is obtained. According to the obtained results, the friction coefficient is revealed as a key parameter in the resulting stress field, causing a non-uniform distribution of stress that can affect the mechanical performance of the press-fit assembly. Full article

ADC12 aluminum alloy has been widely used in the aerospace, ship, and automotive fields because of its high specific strength, excellent die-casting performance, and wear resistance. Adhesion wear is the main wear mechanism of high-speed milling ADC12 aluminum alloy. The most important factor affecting adhesion wear is the tool–chip interface friction, which is directly manifested in the tool–chip interface temperature. Therefore, the temperature variation during the milling of aluminum alloy is analyzed using a temperature field model and infrared temperature measurement technology. Then, the tool wear morphology and the tool wear land width are observed using a scanning electron microscope. Finally, the tool wear mechanism considering the tool–chip interface temperature is discussed. The tool–chip interface temperature is related to the friction angle, tool–chip contact length, and friction force at the rake face, which increases first and then decreases as the cutting speed and feed rate increase. During the formation of the adhesive layer, the tool–chip interface temperature increases, the change rate of the cutting force and the tool wear rate increase, and adhesion, oxidation, and abrasive and delamination wear are generated on the tool surface. With the increase in temperature, the tool wear rate increases, the molten adhesive layer on the tool surface is accompanied by crack propagation, and adhesion wear, oxidation wear, and abrasive wear occur on the tool surface. Full article

Minimum quantity lubrication (MQL) is a potential technology for reducing the consumption of cutting fluids in machining processes. However, there is a need for further improvement in its lubrication and cooling properties. Nanofluid MQL (NMQL) and ultrasonic vibration-assisted machining are both effective methods of enhancing MQL. To achieve an optimal result, this work presents a new method of combining nanofluid MQL with ultrasonic vibration assistance in a turning process. Comparative experimental studies were conducted for two types of turning processes of aluminum alloy 6061, including conventional turning (CT) and ultrasonic vibration-assisted turning (UVAT). For each turning process, five types of lubricating methods were applied, including dry, MQL, nanofluid MQL with graphene nanosheets (GN-MQL), nanofluid MQL with diamond nanoparticles (DN-MQL), and nanofluid MQL with a diamond/graphene hybrid (GN+DN-MQL). A specific cutting energy and areal surface roughness were adopted to evaluate the machinability. The results show that the new method can further improve the machining performance by reducing the specific cutting energy and areal surface roughness, compared with the NMQL turning process and UVAT process. The diamond nanoparticles are easy to embed on the workpiece surface under the UVAT process, which can increase the specific cutting energy and Sa as compared to the MQL method. The graphene nanosheets can produce the interlayer shear effect and be squeezed into the workpiece, thus reducing the specific cutting energy. The results provide a new way for the development of eco-friendly machining. Full article

This study aims to evaluate the tribological properties of two protic ionic liquids (PILs) under different tribological conditions as a sustainable alternative for mineral oil-based neat lubricants. The synthesis of PILs in this study uses a relatively simple and less expensive method. The Fourier transform infrared (FTIR) spectroscopy results help validate the synthesised PILs’ formation. Further, their physicochemical and tribological properties were investigated. The PILs as neat lubricants were tested on a ball-on-plate reciprocating tribometer using bearing steel–bearing steel and bearing steel–aluminium alloy friction pairs at 30 °C and 80 °C. The results show that the investigated PILs significantly reduced the coefficient of friction and wear. The dodecylamine-based PILs performed better in friction and wear reduction than the other investigated lubricants. The formation of the adsorption layer on the friction pairs was assumed to be the dominant friction and wear reduction mechanism. Full article

In a previous paper, methods that have been used to quantify grease mechanical degradation were compared, finding that crossover stress is a practical method for estimating the cone penetration value of a grease using a small sample. This paper covers techniques that have not generally been applied to modeling grease degradation and indicates their usefulness in characterizing the state of a grease. Three methods are examined, each using a different flow profile: rotation, oscillation, and normal force/extension. It is found that crossover stress is likely still the best choice for estimating cone penetration, and a fast, practical method is introduced here. In addition, a procedure for evaluating pull-off force is provided that describes some of the stretching behavior experienced by grease in a rolling contact; this method can also be used as an estimate of cone penetration. Finally, the applications of a “start-up yield” measurement are covered, providing details about the significance of wall slip as well as an independent way of estimating cone penetration. Full article

Tribotronics represents the modulation of friction via an external electric potential, a field with promising ramifications for intelligent devices, precision manufacturing, and biomedical applications. A profound elucidation of mechanisms that allow for potential-controlled friction is foundational to further research in this tribotronic domain. This article provides a comprehensive review of the research progress in electro-controlled friction over the past few decades, approached from the perspective of the boundary lubrication film at the friction interface, a direct influencer of electro-controlled friction performance. The mechanisms of potential-controlled friction are categorized into three distinct classifications, contingent on the formation mode of the boundary lubrication film: potential-induced interfacial redox reactions, interfacial physical adsorption, and interfacial phase structure transformations. Furthermore, an outlook on the application prospects of electro-controlled friction is provided. Finally, several research directions worth exploring in the field of electro-controlled friction are proposed. The authors hope that this article will further promote the application of electro-controlled friction technology in engineering and provide intellectual inspiration for related researchers. Full article

Aqueous solutions of water and ethylene glycol (EG) are prevalently employed in braking, heat transfer, and lubrication systems. However, the precise mechanism through which water content affects the lubricative attributes of EG solutions remains elusive. This research systematically examines the tribological characteristics of EG solutions at varying concentrations using a ceramic–TiAlN friction-pair system. As the concentration of EG increases, the sequential transformation of the associated molecular complex structure in the lubricating medium can be described as follows: [H₂O]_m·EG → [H₂O]_m·[EG]_n → H₂O·[EG]_n. Among them, the stoichiometric coefficients “m” and “n” are the simplest mole ratio of H₂O and EG in the molecular complex structure, respectively. The most favorable EG concentration was determined to be 50 wt.%. At this concentration, a flexible molecular complex adsorption structure ([H₂O]_m·[EG]_n) with a significant bearing capacity (due to intense hydrogen bonding) forms on the surface of the friction pair, which results in a reduction in the running-in duration and facilitates the achievement of superlubricity, and the coefficient of friction (COF) is about 0.0047. Solutions containing 50 wt.% EG enhance the load-bearing ability and hydrophilicity of the lubricating medium. Moreover, they minimize the roughness of the worn region and curtail the adhesive forces and shear stress at the frictional interface, enabling the realization of superlubricity. Consequently, this research offers valuable insights into the optimal water-to-EG ratio, revealing the mechanism of a superlubricity system that possesses exceptional tribological attributes and holds significant potential for practical applications. Full article