Search Results (1,308)

In this study, copper(II) oxide, titanium dioxide and yttrium(III) oxide nanoparticles were added to Group III-type base oil formulated with overbased calcium sulfonate. The nanosized oxides were treated with ethyl oleate surface modification. The tribological properties of the homogenized oil samples were tested on a linear oscillating tribometer. Friction was continuously monitored during the tribological tests. A surface analysis was performed on the worn samples: the amount of wear was determined using a digital optical and confocal microscope. The type of wear was examined with a scanning electron microscope, while the additives adhered to the surface were examined with energy-dispersive X-ray spectroscopy. From the results of the measurements, it can be concluded that the surface-modified nanoparticles worked well with the overbased calcium sulfonate and significantly reduced both wear and friction. In the present tribology system, the optimal concentration of all three oxide ceramic nanoadditives is 0.4 wt%. By using oxide nanoparticles, friction can be reduced by up to 15% and the wear volume by up to 77%. Overbased calcium sulfonate and oxide ceramic nanoparticles together form a lower friction anti-wear boundary layer on the worn surfaces. The results of the tests represent another step toward the applicability of these nanoparticles in commercial engine lubricants. It is advisable to further investigate the possibility of formulating nanoparticles into the oil. Full article

Calcium sulfonate complex greases (CSCG) have proven to be a sustainable alternative to lithium complex greases, which still require appropriate additives to deliver lubrication performance benefits under extreme working conditions such as heavy load, high speed, and high temperature. The anti-wear and friction reducing properties of CSCG enhanced by two-dimensional MoS₂ nanosheets (2D MoS₂) with a narrow lateral size and thickness distributions were evaluated by a four-ball tribometer. The results showed that the CSCG with 0.6 wt.% 2D MoS₂ performs best, with a 56.4% decrease in average friction coefficient (AFC), 16.5% reduction in wear scar diameter (WSD), 14.3% decrease in surface roughness, and a 59.4% reduction in average wear depth. Combining SEM-EDS images, Raman, and X-ray photoelectron spectra, it is illustrated that the physical transferred film and tribo-chemical film consisting of MoS₂, Fe₂O₃, FeSO₄, CaCO₃, CaO, and MoO₃ were generated on the worn surface, which improves the lubrication performance of CSCG considerably. Full article

In order to improve the tribological performance of PVD–MoS₂ coatings, which are frequently used as a solid lubricant for operating in challenging environments, e.g., in a vacuum, they can be modified with nitrogen. This work evaluates the tribological behavior and a possible compaction occurring during the initial tribological load in the rolling contact for pure and nitrogen-modified PVD–MoS₂ coatings in a vacuum. Short-running tests (1000 cycles) of coated steel discs paired with uncoated steel discs made from 100Cr6 (1.3505, AISI 52100) were conducted on a two-disc tribometer. The slide-to-roll ratio of 10.5% was kept constant, while the load was varied in two steps from 1.1 GPa to 1.6 GPa. Subsequently, a comparison was made between the worn and the pristine coatings by means of nanoindentation and an optical analysis of the wear track. The formation of a load-bearing solid lubrication was achieved for both MoS₂-variants. The main differences affected the material transfer and wear mechanisms. The worn coatings reached a similar wear coefficient of 4 × 10⁻⁶ mm³N⁻¹m⁻¹ and a possible compaction of the coatings was found, indicated through an increased indentation hardness (for MoS₂ 1158% and MoS₂:N 96% at a 1.1 GPa load). The assumed tribological mechanism changed with nitrogen modification, but scales with increasing load. The nitrogen-modified MoS₂ coating showed less compaction than pure MoS₂, while the frictional behavior was improved by a 17°% reduction of the coefficient of friction. Full article

Tribological testing of moving shaft/sealing pairs in complex environments is at the frontline of research. Machines working in abrasive conditions are subject to different wear effects. It is not only valid on Earth but especially valid for rovers and future robots used in Mars and Moon missions. The aim of our joint research with the European Space Agency is to study the abrasion phenomena of moving machine elements on Mars and the Moon by using artificial soil samples (“simulants”). This review details mainly the available simulant sources and recommend a selection of the most suitable ones for tribological testing. Moreover, the potential mating structural materials subjected to abrasive space applications are reviewed briefly. The tribological tests are exploring the features of the rotary shaft/seal relationship that is subject to dry friction and intense abrasion. By using the simulants, measurements are performed under laboratory conditions with both a sample test and a real shaft/seal connection. Parameters of the selection criteria were defined, and classification of the simulant sources were made. It was found that simulant particle size distribution and chemical substance content are detailed enough only for a limited type of available artificial Moon and Mars soil samples. Four simulants were identified and applied later in the tribological testing. For the shaft materials, based on a detailed case study of polymers, steel, and aluminum alloys, a high-strength aluminum alloy with a hard anodized surface and a stainless steel were selected for further abrasion tests. Full article

Lubrication in the contact area can be considered as a viscoelastic layer, especially in the presence of particles in it, or under conditions of low temperatures. The properties of this layer are often non-linear, in particular, they depend on local pressure. The paper presents a formulation and numerical-analytical method for solving the contact problem in the presence of viscoelastic layers, the compliance of which depends on the applied pressure and is included in the formulation as the corresponding operator. The layer is homogeneous or coated elastic half-space. For the selected type of operator, the influence of parameters, which characterize the nonlinearity of the model, on the distribution of contact pressure and the coefficient of friction due to hysteresis losses was analyzed. It is shown that for the nonlinear model, the maximum contact pressures are higher, and the friction coefficient is lower than for the linear model with constant compliance. The effect of non-linearity for a wide range of sliding velocities is considered. An analysis of principal shear and tensile-compressive stresses for a homogeneous elastic half-space and for a coating, in particular, for a coating-substrate interface, was also carried out. Full article

One of the most significant performance indicators for measuring the machinability of materials is tool wear and surface roughness. Choosing the best combination of cutting parameters can help reduce production costs, which is what the manufacturing industry is interested in. At the same time, industries are always looking for an alternative to conventional flood cooling since its use creates an environmental burden and health concerns for the operators. Therefore, vegetable oil-based minimum quantity cooling lubrication (MQCL) is considered a cutting environment. Sunflower oil is utilized as base fluid in MQCL and applied to the cutting zone through a nozzle. The turning experiments are conducted on C45 material which is widely used in various industrial applications, including numerous automotive components. Since flood cooling is widely utilized in machining C45, it is the present-day need to assess alternative cooling and lubricating approaches to avoid the adverse effects of flood cooling. The Taguchi method was used in the present work to minimize surface roughness and tool wear. L₉ orthogonal array was constructed, and experiments were performed on C45 steel using coated carbide cutting tools. The statistical approach is utilized to evaluate the effect of cutting parameters on output responses. The optimal cutting settings for cutting speed, feed, and depth of cut to minimize surface roughness are 100 m/min, 0.18 mm/rev, 0.150 mm, and 80 m/min, 0.18 mm/rev, and 0.150 mm for tool wear. According to the findings, cutting speed, feed rate, and depth of cut varied surface roughness by 1.9%, 78.3%, and 14.04%, and tool wear by around 43.8%, 37.9%, and 6.3%, respectively. The outcomes can be useful to metal-cutting industries to identify the combination of machining parameters with vegetable oil-based MQCL. Full article

Electrically as well as thermally conductive lubricants have drawn considerable attention and are an emerging research topic because they have unique advantages and advanced lubrication performance over traditional lubricants such as corrosion protection and efficient heat dissipation. For instance, some components of electric vehicles (EVs) such as bearings, seals, pads and gears require conductive lubricants to avoid premature failure and electromagnetic interference (EMI) problems due to induced shaft voltages and currents. This review provides a comprehensive overview of the recent developments in conductive lubricants. The review focuses on the important aspects to enhance the thermal and electrical conductivities as well as the tribological behavior (COF, and wear rate) of conductive solid, semisolid, and liquid lubricants. The lubricants that are electrically and thermally conductive with superior tribological performances have been identified through extensive literature review and presented in tabular form. This review summarizes the effect of various additives used to improve the conductive properties of the lubricants, such as polyalphaolefin oil, hydraulic oil, paraffin oil, and mineral oil. Furthermore, the review discusses the lubricating mechanism of conductive solid and liquid lubricants to facilitate a deeper understanding. Finally, the future perspectives and the research directions for conductive lubricants are also addressed. Full article

The axle box bearing is a crucial component of high-speed electric multiple units (EMU) and is exposed to harsh working conditions, making it susceptible to subsurface-induced rolling contact fatigue (RCF) under long-term alternating stress. The objective of this paper is to develop a damage-coupled elastic–plastic constitutive model that can accurately predict the RCF life of EMU axle box bearings made from AISI 52100 bearing steel. The total damage is divided into elastic damage related to the shear stress range and plastic damage associated with plastic deformation. Material parameters are determined based on experimental data from the literature, and validation is conducted to ensure the validity of the model. Finally, the RCF behavior of the EMU axle box bearing, including crack initiation, crack propagation, and spalling, is simulated, and reasonable results are obtained. This study provides valuable insights into the RCF behavior of EMU axle box bearings and contributes to the accurate prediction of the fatigue life. Full article

Although aluminum alloys are widely used in the automotive and aerospace industries due to their excellent strength-to-weight ratio and good corrosion resistance, the poor tribological performance and low compatibility of these materials with lubricant anti-wear and anti-friction additives in conventional mineral oils are major limitations. In addition, environmental awareness has increased the need for more environmentally friendly lubricants. Ionic Liquids (ILs) have exhibited significant potential as lubricants and lubricant additives. One of the more interesting properties of ILs is that they can form physically-adsorbed or chemically-reacted layers that reduce friction and wear of the surfaces in contact. Among ILs, Protic Ionic Liquids (PILs) have received more attention recently because of their simple and economic synthesis route. Furthermore, the anions and cations of PILs can be selected to be considered environmentally benign. In this article, the tribological behavior of a family of six PILs are studied as additives to a biodegradable oil (BO), under aluminum-steel contact. Al2024 disks slid against AISI52100 steel balls under a normal load of 3 N and a frequency of 5 Hz at room temperature and using a ball-on-flat reciprocating tribometer. PILs used in this study, were synthesized using two strong acids, with short and long hydrocarbon chains, and three weak bases with different propensities to hydrogen bonds. Results show that, although adding just 1 wt.% of any PIL to BO reduced friction and wear, the alkyl chain length influenced the lubricating ability of these ordered fluids. Wear mechanisms and surface interaction are discussed on the basis of 3D profilometry, SEM-EDX and RAMAN spectroscopy. Full article

This paper evaluates the ability of some state-of-the-art Machine Learning models, namely SVM (support vector machines), DT (decision tree) and MLR (multiple linear regression), to predict pavement skid resistance. The study encompasses both regression and classification tasks. In the regression task, the aim is to predict the coefficient of friction values, while the classification task seeks to identify three classes of skid resistance: good, intermediate and bad. The dataset used in this work was gathered through an extensive test campaign that involved a fifth-wheel device to measure the coefficient of friction at different slip ratios on different road surfaces, vehicle speeds, tire tread depths and water depths. It was found that the RBF-SVM model, due to its ability to capture non-linear relationships between the features and the target for a relatively small dataset, is the most adapted tool compared with, on one side, MLR, linear SVM and DT models for the regression task and, on the other side, linear SVM and DT models for the classification task. The paper also discusses the strengths and weaknesses of the investigated models based on the underlying physical phenomena related to skid resistance. Full article

To comply with the high demand for efficient and sustainable lubrications, carbon-based tribofilms and/or nanomaterials have emerged as a potential solution that can resolve the current major shortcomings of phosphorus- and sulphur-rich tribofilms and protective coatings. Although their employment is still in the early stages of realization and research, these tribofilms receive significant interest due to their capability to continuously and in situ repair/replenish themselves during sliding, which has been an ultimate goal of all moving mechanical systems. Structurally, these tribofilms are complex and predominantly amorphous or disordered with/without graphitic domains (e.g., graphene/graphite, onion-like carbon, etc.). Chemically, the compositions of these tribofilms vary significantly with environments, conditions, and material precursors. Yet, the structural properties of carbon-based tribofilms remain largely ambiguous, which precludes a full understanding of the mechanisms underlying the formation and lubrication performance. This review will summarize the current state-of-art research about the in situ carbon-based tribofilms that have been published since the pioneering works. Particularly, this work will highlight the recent approaches to generate these tribofilms, their associated lubrication performance, current understanding of the formation mechanics, common analytical approaches for these tribofilms, and the compatibility of these tribofilms with other additives. Together, the overall outlooks will be drawn, demonstrating the knowledge gaps and proposing further investigation tactics to tackle these emerging issues. Full article

In this work, a numerical model simulating friction hysteresis for lubricated rough and textured surfaces in contact is presented. Friction hysteresis occurs in sliding contacts that are subjected to a non-constant (e.g., sinusoidal) motion. It refers to the phenomenon where the observed friction force during acceleration differs from that during deceleration. Besides the dynamics of the sliding system, a classic mixed friction model is adopted, in which the transient Reynolds equation for the description of the thin lubricant film is combined with a statistical Greenwood–Williamson model for the description of rough surface asperity contacts. The model enables the prediction of the friction hysteresis for predefined contact descriptions (i.e., surface profile and roughness, lubricant, etc.) and allows the study of the physics and parametric influences of dynamically sliding contacts. In this paper, it is shown that (i) friction hysteresis is captured by classic transient models for mixed lubrication; (ii) system parameters, such as roughness, applied load, viscosity and velocity, including the offset, amplitude and motion reversal, influence the shape and area of friction hysteresis; and (iii) the selection of the aforementioned parameters may minimize friction hysteresis. Full article

Electropolishing at high current densities without agitation of the electrolyte results in a pitting phenomenon that produces dimple-like surface features. Although pitting is unfavorable in the electropolishing process, its effect on surface modification, such as surface texturing, has not been thoroughly investigated. Surface topography and chemical composition analyses of electropolished steel revealed surface pits and an oxide surface layer, indicating the presence of surface texture and coating. The resulting surface is characterized by negative skewness and high kurtosis values. The tribological behavior of the electropolished steel-bronze pair is investigated by evaluating coefficients of friction and bronze wear using sliding tests conducted in mixed and boundary lubrication regimes. The results are compared to those of the ground steel-bronze pair. In the mixed and upper range of the boundary lubrication regime, coefficients of friction reduction up to 30% and shorter running-in phases are observed for electropolished steel (electropolished steel μ_avg = 0.019 vs. ground steel μ_avg = 0.028). In contrast, the coefficient of friction increased in the lower range of boundary lubrication regime by 50% (electropolished steel μ_avg = 0.098 vs. ground steel μ_avg = 0.065). Electropolishing, as a cost- and time-effective method applicable to complex geometries, presents an alternative method for achieving surface modifications aimed at friction reduction and improved tribological behavior for non-conformal contacts in the boundary and mixed lubrication regimes. Full article

A water hydraulic axial piston pump has become the preferred power component of environmentally friendly water hydraulic transmission systems, due to its advantages of a compact structure, high power density, and so on. The poor friction and wear performance in the water medium, especially under extreme conditions of high speed and high pressure, limit the engineering application of the water hydraulic axial piston pump. In this review, the research progress for key friction pair materials (such as special corrosion-resistant alloys, engineering plastics, and engineering ceramics) for water hydraulic axial piston pumps is, firstly, summarized. Secondly, inspired by nature, the processing methods, lubrication drag-reduction mechanism, and tribological properties of the biomimetic surface textures are discussed. The effects of the surface texture shape, equivalent diameter, depth, and arrangement on the pump’s tribological properties are reviewed in detail. Finally, the application status of, and problems with, surface texture technology in water hydraulic axial piston pumps are summarized. It is suggested that future studies should focus on the multi-field coupling lubrication anti-friction mechanism of the multi-type composite texture under extreme conditions and mixed lubrication; and the anti-wear performance of the texture coupled with a coating modification, to further promote the surface texture in the field of lubrication antifriction engineering applications. Full article

Bacterial cellulose (BC) is a biodegradable, non-toxic, natural substance that can be obtained by culturing bacteria. It can be approached in various ways from physical, chemical, and biological points. BC nanoparticles have been applied as lubricating additives to improve the load capacity, anti-wear, and friction. The microcellular foaming process was created using a technology based on the saturation of the polymer by supercritical CO₂ and rapid decompression. An increase in saturation pressure leads to an increase in the molecular potential energy of CO₂, which can be more easily compressed into the cellulose matrix. Moreover, the high crystallinity and water content combination contribute to thermal stability. Specimen membranes produced by Komagataeibacter xylinus prepared with a thickness of 2 mm were saturated in supercritical condition, 10 MPa of CO₂ for 4 h, and foamed at a temperature of 120 °C in a hot press. After the foaming process, we used dry ice to cool the BC. Before foaming, the friction coefficient continuously increased with the increase in cycles, and after foaming, a stable friction coefficient of 0.3 or less was secured despite the increase in the cycle. The microcellular foaming process significantly reduced and made BC’s coefficient of friction stable. Full article