Lubricants

In this second part of the paper series, parameter investigations of the tribological system chain pin/bush contact, carried out on a specifically developed pin on bush plate model test technique, are presented. Both the pin material and the lubricant varied widely. In case of the pin materials, a Cr-N monolayer coating and a Cr-N-Fe-based multilayer coating were investigated. As for the lubricants used, two different performing engine oils from the field were tested as well as fresh oils, some of which were diluted with a soot surrogate (carbon black) and diesel fuel in different amounts. The results show, among other things, that friction and wear performance strongly depend on the combination of pin material and lubricant used. In this context, especially the Cr-N-Fe in combination with the used engine oils showed a high wear resistance and low friction losses compared to the Cr-N reference. In the case of fresh oils with soot, the friction losses were higher but comparable between the pin materials, and a slightly better wear performance of the Cr-N was observed due to an agglomeration effect of the soot surrogate. In general, it was found that especially soot-free oils show clear wear advantages independent of the pin material used. Thus, soot clearly has a wear-promoting component. The investigations of this study suggest that a leading mechanism that is based on a corrosive–abrasive effect in the tested system, but this is more related to the soot surrogate carbon black than engine soot. Full article

Alkylnaphthalene as base oils are widely applied in the production of high-performance lubricating oils. Here, we report the synthesis of multi-butylnaphthalenes by alkylation of naphthalene and n-butene with trifluoromethanesulfonic acid as catalyst. Trifluoromethanesulfonic acid exhibited excellent catalytic performance with naphthalene conversion, as high as 98.5%, and the multi-butylnaphthalenes selectivity of 98.8% under optimum conditions. To investigate the effects of the side-chain numbers on naphthalene on the lubrication performance, two kinds of alkylnaphthalenes were obtained by controlling catalyst dosage, denoted as MBN-1 (90.3% mono/di-butylnaphthalenes) and MBN-2 (98.2% tri/tetra/penta/hexa-butylnaphthalenes), respectively. The primary physiochemical properties of the synthetic oils were tested, and their tribological performance was evaluated. MBN-2, with more side chains on naphthalene, displayed more effective friction reduction and anti-wear properties than MBN-1 and the commercial alkyl naphthalene base oil AN5. Full article

The dispersion stability of carbon-based solid lubricants/lubricating oils remains a challenge to overcome. Recently, novel processing routes were developed to obtain 2D turbostratic graphite particles via solid-state reactions between B₄C and Cr₃C₂ (GBC) and between SiC and Fe (GSF) that present outstanding tribological properties in a dry scenario, as well as functionalized graphene (GNH). This work investigated the suspension stability of GBC and GSF particles (0.05 wt.%) dispersed in a low-viscosity polyol ester lubricating oil and their tribological performance. Ammonia-functionalized graphene (GNH) particles were also used as a reference. In order to evaluate the dispersion stability, in addition to the classical digital image technique, a much more assertive, reliable, quantitative and rarely reported in the literature technique was used, i.e., the STEP^TM (Space and Time-resolved Extinction Profiles) technology. Reciprocating sphere-on-flat tribological tests were carried out, in which before contact, 0.2 μL of pure oil and suspension (POE + 0.05 wt.% of solid lubricant) was applied on a flat surface. The results showed that the GBC particles remained remarkably stable and reduced the sphere wear rate by 61.8%. From the tribosystem point of view, the presence of GBC and GSF reduced the wear rate by 18.4% and 2.2%, respectively, with respect to the pure oil, while the GNH particles increased the wear rate by 4.2%. Furthermore, the wear rate was improved due to the highly disordered carbon tribolayer formation identified on both surfaces. Full article

Differingfrom the published documents on the effect of texture distributions of sliding tribo-pairs on their friction and wear properties, this study introduced eight patterns to reveal the influence of different distributions of pits on the tribological behavior of textured rolling element bearings with nylon cages under dry condition, namely: Outside-1/4 (OS1/4), Outside-1/2 (OS1/2), Outside-3/4 (OS3/4), Inside-1/4 (IS1/4), Inside-1/2 (IS1/2), Inside-3/4 (IS3/4), Bothside-1/3 (BS) and full (FP). A fiber laser marking system was used to prepare them on the raceways of the shaft washers of cylindrical roller thrust bearings (81107TN). A vertical universal wear test rig was used to obtain their coefficients of frictionunder an axial load of 2600 N and a rotating speed of 250 RPM, without any lubricant provided. Their wear losses and worn surfaces were characterized. The influence mechanism of different distributions on the tribological properties was also discussed. The results show that the self-lubricating performance of nylon cages can ensure the continuous operation (≥5 h) of cylindrical roller thrust bearings under dry condition. The influence of outside-distributed patterns on the friction and wear properties of bearings is significant. The friction-reducing effect and wear resistance of full textured group is improved but not the best. The friction-reducing and anti-wear behavior of OS1/2 is similar to that of FP. In this work, OS3/4 can provide the best tribological performance under self-lubricating conditions. Compared with the data of smooth bearings, its average coefficient of friction and wear loss can be reduced by 37.68% and 38.85%, respectively.This work would provide a valuable reference for the raceway design and reliability optimization of rolling element bearings. Full article

With recent advancements in the petroleum industry, the need for multiphase pumps to transport multiphase products, such as the product of oil, water, and natural gas, arises along with their challenges. A helico-axial multiphase pump should be capable of converting fluid kinetic energy into pressure by avoiding gas–liquid separation and the gas-locking phenomenon, enabling smooth pump operation at higher gas volume fractions. In this paper, erosion on a helico-axial pump is evaluated at different flow conditions using sand particles mixed with water. A very important practical finding of this investigation is that the erosion rate increased when the flow conditions deviated from the design point of 3600 RPM. Operating the pump in off-design conditions not only reduced its hydraulic efficiency but also increased the erosion rates and hence reduces its life. It was observed that at 4800 RPM, the pump efficiency decreased by 11% and the erosion rate increased by 80%. This increase in erosion rate was attributed to the formation of local vortices upstream of the blade leading edge affecting the particle flow path and increasing its impact on the blade surface. Full article

The vacuum suction cup is often used as an end effector and widely used in wall-climbing operations. However, there are few vacuum suction cup designs and applications for oil-immersed substrates. Inspired by the surface morphology of the octopus sucker, bionic suction cups with different numbers, diameters, and spacings of the ring grooves were designed. Their normal adsorption force was evaluated on the untreated and polished steel plate in oil. The test results showed that ring grooves positively affected the adsorption force. The bionic suction cup with a groove number of 3, a diameter of 0.5 mm, and a spacing of 3 mm was the most excellent in the test. It achieved normal adsorption forces of 54.83 ± 0.48 N and 43.89 ± 0.69 N on the untreated and polished steel plate. Compared with the standard suction cup, it increased by 32.31% and 12.28% on the untreated and polished steel plate. The regression model between the normal adsorption force and design factors was established based on the adsorption force test results, and the influence law of the ring groove structure parameters on the adsorption force of suction cups on oil-immersed substrates was analyzed. The order of significant effects of groove design parameters on normal adsorption forces was groove diameters, spacings, and numbers. The finite element analysis (FEA) results show that the ring grooves could significantly increase the contact pressure, frictional stress, and sliding distance between the suction cup and the substrate. The ring groove structure effectively improves the adsorption force of the suction cup on the oil-immersed surface by forming a more effective seal and increasing the friction force and adsorption area. This study could provide a reference for developing the actuator of the oil-immersed or lubricated climbing machine. Full article

This study compares the film thickness, lubricant temperature, and traction curves of two groups of commonly used constitutive models for lubricants in thermo-elastohydrodynamic lubrication (TEHL) modelling. The first group consists of the Tait equation of state, the Doolittle Newtonian viscosity model, and the Carreau shear thinning model. The second group includes the Dowson equation of state, the Roelands–Houpert Newtonian viscosity model, and the Eyring shear thinning model. The simulations were conducted using a Computational Fluid Dynamic and Fluid-Structure Interaction (CFD-FSI) approach, which employs a homogeneous equilibrium model for the flow simulation along with a linear elastic solver to describe the deformation of the solid materials. The simulations were conducted under a load range of 100 kN/m to 200 kN/m and a slide-to-roll-ratio (SRR) range between 0 and 2 using Squalane lubricant. The results show up to a 10% deviation in central film thickness, a 31% deviation in coefficient of friction (CoF), and a 38% deviation in maximum lubricant temperature when using the different constitutive models. This study highlights the sensitivity of TEHL simulation results to the choice of constitutive models for lubricants and the importance of carefully selecting the appropriate models for specific applications. Full article

Four tool plunging paths including a one-time plunging path and three step-by-step plunging paths were designed to study the effects of the tool plunging path on the welded joint properties of pinless friction stir spot welding (PFSSW). The appearance, cross-sectional microstructure, welding temperature, microhardness, and tensile shear failure load of the PFSSW of thin copper sheets under different tool plunging paths were explored. Furthermore, the fracture modes of welded joints under different tool plunging paths were analyzed. Studies showed that path 1 (plunge total depth at one time) produced the largest range of stirring zone, but the grains in the stirring zone were larger and the width of the thermal-mechanical affected zone was smaller. Path 1 obtained the highest peak temperature during the welding process, and path 3 (plunge 1/3 total depth + plunge 2/3 total depth) gained the lowest peak temperature. The greater the initial plunging amount of the tool, the faster the temperature rise rate in the welding stage. The tensile shear failure loads for path 1, path 2 (plunge 1/2 total depth + plunge 1/2 total depth), path 3, and path 4 (plunge 2/3 total depth + plunge 1/3 total depth) were 8.65 kN, 8.15 kN, 8.25 kN, and 8.85 kN, respectively. The tensile shear failure load of path 4 was 2.3% higher than that of path 1. The fracture modes of welded joints under different tool plunging paths were all nugget pullout fractures. The fracture morphology indicated that the fracture type was ductile fracture. The step-by-step plunging path proposed in this work extends the traditional PFSSW process. The findings of this study can provide a reference for the selection and design of tool plunging paths for PFSSW. Full article

In order to improve the flowabilities and anti-friction and anti-wear properties of lubricants, the viscosity variations and tribological performances of oleylamine-modified Fe₃O₄ nanoparticles as mineral oil additives were systematically investigated via rotational parallel plate rheometer, ball–disc reciprocating tribometer, non-contact three-dimensional surface profiler, scanning electron microscope, energy dispersive X-ray spectrometer and X-ray photoelectron spectroscopy. Spherical monodisperse Fe₃O₄ nanoparticles were synthesized and dispersed into mineral oils to obtain lubricants with mass fractions of 1%, 3%, 5%, 8%, 10% and 20%, respectively. These lubricants have excellent stabilities within 12 months. Interestingly, the dynamic viscosity and kinematic viscosity of the lubricants first decrease and then increase with the increase in Fe₃O₄ content, and the lubricants’ viscosity is at a minimum when the mass concentration is 5%. The tensile curves also show that with the mass fraction increase, the lubricants’ tackiness and adhesion have the same change law, and both reach the lowest point when the mass concentration is 5%. Meanwhile, Fe₃O₄ nanoparticles can improve the tribological properties of the base oils. It is worth noting that the maximum reduction in the wear volume at 25 °C is up to 93.8% compared with base oils when the additive concentration of the Fe₃O₄ nanoparticles is 5 wt%. Full article

In the interest of analyzing the effect of the structural deformation root caused by gas pressure on the static features of aerostatic bearings, a fluid–structure interaction (FSI) model based on orifice-type aerostatic bearings is proposed that can predict the characteristics of aerostatic bearings more accurately by using the direct-coupling method (DCM). By using COMSOL Multiphysics, the governing equation matrix of the finite element model of structural deformation and gas film pressure was solved with the integral solution method, and the orifice boundary conditions were calculated with the root iteration method. At the same time, the static performance of I-shaped orifice-type aerostatic bearing with various supply pressures was analyzed theoretically and tested experimentally. The results show that in comparison with the calculation results without taking account of structural deformation, the theoretical values from the model derived in this paper considering the FSI effect are closer to the experimental values. Finally, by using the orthogonal design method, FSI simulation was carried out to analyze how the key dimension factors influence the structural stiffness of the spindle, and it is concluded that the thrust bearing’s stiffness is strongly influenced by the thickness of the thrust plate. Full article

The dispersion of nanomaterials in lubricating oil plays an important role in the lubrication and wear-resistance properties. In this work, supramolecular layered double hydroxides (LDHs) were prepared and added to lubricating oil with different dispersants. The content of key elements in the samples was measured by an oil element analyzer, and the dispersion properties of different samples were studied. The friction coefficient of the samples was measured by high-frequency linear vibration (SRV), and the morphology and composition were characterized by SEM to study the antiwear performance and action mechanism of LDH. The oxidation induction time of the samples was measured by RBOT to study the antioxygenic properties of LDH in lubricating oil. The results show that LDH can be well-dispersed in lubricating oil with the action of specific dispersants. After adding LDH, the antiwear performance of lubricating oil was improved, as a uniform and dense protective film was formed on the friction surface. Full article

To study the full sound field distribution characteristics of full ceramic ball bearings, reduce the radiation noise of the bearings, and improve their service performance. In this paper, the sound field distribution characteristics of 6206 silicon nitride ceramic deep groove ball bearings are studied under different oil supplies. A mathematical model of the sound field distribution of full ceramic ball bearings under oil lubrication is established, and the validity of the model is verified by experimental data. The bearing cavity simulation model of the full ceramic ball bearing is established, and the influence of oil supply on the operation characteristics of the full ceramic ball bearing is studied. Through theoretical and experimental research, the circular distribution law of the noise signal of ceramic ball bearings under different oil supplies is revealed. It is found that there is an optimal fuel supply when the speed and load are constant. Under optimal oil supply lubrication conditions, the full ceramic ball bearing has the minimum radiation noise, and the bearing exhibits optimal lubrication state, vibration and temperature rise characteristics. The new contribution of this paper: with the increase in oil supply, the sound pressure level of radiation noise of full ceramic ball bearings decreases and then increases. The research results reveal the radiation noise mechanism of full ceramic ball bearings, which is of great significance for enriching its theory and method. Full article

The nonlinear time-domain lubrication characteristics of the hydrodynamic journal bearing system are studied in this paper. The motion equation of the hydrodynamic journal bearing system is established based on the balance of the relationship among the water film force, journal inertia force, and external load. The water film pressure distribution of the sliding bearing is calculated by the finite difference method. Firstly, the variation law of the water film pressure distribution with time under the external periodic load is calculated considering the inertial force of the journal. The influence of the initial eccentricity on the orbit of the journal center is studied. Secondly, the maximum water film pressure, the orbit of the journal center, eccentricity, water film pressure, and the minimum water film thickness of the bearing under the action of circumferential and unidirectional periodic external loads are calculated, and the effects of inertial force and rotational speed on the dynamic characteristics of the bearing are analyzed. Finally, the water film dynamic characteristics under low speed and heavy load are studied. The result shows that the pressure of the dimensionless water film caused by inertial force is reduced by 7 to 10 percent at the rotational speed between 200 r/min and 800 r/min, which means that the influence of inertia force cannot be ignored. Full article

The present work explored the use of fatty acid ‘Triangle ester’ molecules (Epoxidized Ester (EE), and Thiirane Ester (TE)) as antifriction and antiwear additives at varying levels for Group I and Group II mineral base oils using the standard ASTMD-4172B four-ball test. Relative to neat base oil, EE blends showed improved antifriction by ~61% and ~42% and antiwear properties by ~32% and ~41% in Group I and Group II base oils, respectively, while the TE blends showed friction reduction by ~65% and ~40% and wear reduction by ~93% and ~50% relative to the same neat base stock. Time evolution of the ‘Triangle ester’ molecules and their blends with mineral oil (modeled as hexadecane) w.r.t. conformational changes, adsorption energy, intermolecular energy, and effect of the applied stress were estimated theoretically using MD simulations. Further, optimized levels of these additives were explored for their effectiveness as a blending component for commercial engine oil (CEO) and could reduce the friction and wear of CEO by ~50% and ~30%, respectively. Full article

Lubricating greases enclose oil in porous structures of aggregated thickener particles. Their tendency to separate oil under static conditions is evaluated according to DIN 51817 or DIN ISO 22285 in tests of up to 168 h with the mass fraction of separated oil as result. With an analytical photo-centrifuge, separated oil mass fractions can be tracked in real time in the instrument operating temperature range from 4–60 °C. Due to the higher mechanical load compared to standard tests, the grease samples separate more and faster oil, significantly speeding up the analysis process. Fitting the measured data from both methods with the function w_O = w_O,∞ exp(−t/t_c), the parameters maximum oil separation w_O,∞ and characteristic time t_c are obtained as measures of oil separation extent and rate. Both parameters help to compare the two methods and to interpret the oil separation characteristics of greases. Using four commercial greases of NLGI classes 0–2, the analytical photo-centrifuge measuring method is presented in detail and its results are discussed in reference to those of standard DIN 51817. Full article

Porous carbon fiber-reinforced Al-Si alloy matrix composites and carbon fiber felt-reinforced Al-Si alloy matrix composites with carbon content of 10 wt.% were prepared by die casting. The dry tribological properties of these two composites and Al-Si alloy were studied using a ball-on-disc rotational tribometer in the rotational speed range of 300 r/min to 1000 r/min, and the wear mechanisms were analyzed in combination with the wear morphology. The results show that the friction coefficient and wear rate of these two composites are lower than the Al-Si alloy at different speeds. With the increase in rotational speed, the friction coefficient of the two composites and Al-Si alloy first increases and then decreases, and the wear rate gradually increases. The wear mechanisms of the two composites and Al-Si alloy change from abrasive wear and adhesive wear to delamination wear, but the node speed of the change in the wear mechanism of the composites to delamination wear is higher, and the wear degree is relatively slight. In addition, the comprehensive tribological properties of carbon fiber felt-reinforced Al-Si alloy matrix composites are better than the porous carbon fiber-reinforced Al-Si alloy matrix composites. Full article

The corrosion inhibition properties of three spiro-isoxazoline derivatives, namely 3,4-diphenyl-1,7-dioxa-2-azaspiro[4.4]non-2-en-6-one (DDA), 3-phenyl-4-(p-tolyl)-1,7-dioxa-2-azaspiro[4.4]non-2-en-6-one (PDA) and 4-(4-methoxyphenyl)-3-phenyl-1,7-dioxa-2-azaspiro[4.4]non-2-en-6-one (MDA) on carbon steel in 1.0 mol/L HCl acid medium were experimentally and computationally investigated. The experimental results showed that the inhibitory efficiency reached remarkable values of 76.26, 80.31, and 82.91%, respectively, for DDA, PDA and MDA at a maximum concentration of 10⁻³ mol/L. The potentiodynamic polarization curves (PPCs) showed that investigated compounds had a mixed type character, controlling both anodic and cathodic corrosion reactions. In addition, electrochemical impedance spectroscopy (EIS) indicated that the addition of increasing concentration of tested compounds to HCl solutions led to a significant increase in the polarization resistance of the carbon steel, which was accompanied with a simultaneous decrease in the double layer capacitance. On the other hand, the morphological study of the metal surface by scanning electron microscope (SEM) and energy dispersive X-ray (EDX) confirmed the effective protection of the carbon steel by the inhibitors against corrosion through the formation of a protective film on its surface. The adsorption characteristics of investigated compounds on carbon steel were assessed at microscopic level using Density Functional Based Tight Binding (DFTB) simulation, which revealed the formation of covalent bonds between inhibitors’ atoms and Fe atoms. Furthermore, additional insights into the compounds’ reactivity and adsorption configurations on steel surface were obtained from global reactivity descriptors and Monte Carlo simulation. The present work’s outcomes are interesting for further design and performance evaluation of effective organic corrosion inhibitors for acid environments. Full article

In the contact of rough surfaces, most contact patches are at the scale of micrometers, and thus, their contact deformation can be dominated by the size-dependent plasticity. In this paper, we propose a new strategy to analyze the role of strain gradient plasticity in the contact response between a realistic rough surface and a rigid plane, which modifies the incremental contact model based on the mechanism-based gradient plasticity (MSGP) theory. For several different rough surfaces with their topography measured experimentally, the relations between applied load and real contact area are derived in a simple but effective way. It is found that strain gradient plasticity significantly increases the level of mean contact pressure. The hardening effect caused by strain gradient plasticity weakens somewhat as the contact area increases. Compared with previous methods, the present model might be more efficient and of wider application. Full article

Based on the basic mechanism and bionics principle that texture affects the dynamic pressure effect of lubricating medium, a V-shaped texture that converges along the sliding direction is designed. Through numerical simulation, the optimal geometric parameters and distribution of the V-shaped and textures are obtained. A textured surface with various texture features is prepared using a nanosecond ultraviolet laser with bearing steel as substrate. Tribological experiments with friction and wear tester are performed to investigate the effect of characteristic parameters and distribution of surface texture on the lubrication performance and the lubrication properties are compared and analyzed with that of circular texture. Hence, this investigation provides a research direction to improve the lubrication performance between frictional pairs under fluid lubrication condition to reduce the frictional wear of mechanical systems. The results show that under the conditions of optimal parameters, due to the effect of convergence and extrusion on the flow of lubrication medium, the V-shape texture is better than the circular texture in improving the lubrication performance. The optimal characteristic parameters of V-shape texture are: 60° for the angle between the two wings, 0.53 for shape parameter, 25.9% for area ratio, 13 μm depth, 60% texture area coverage ratio, and the inlet of flow field of the texture distribution position. Full article