Asperity Contact Research Articles

The influence of local wear and contact roughness on mixed lubrication of marine stern bearing with misaligned shaft

AbstractThe marine stern bearing provides supporting force by lubricating film to minimise the contact friction with the propeller shaft. A model considers local wear and asperity contact is proposed to investigate the mixed lubrication of bearing with misalignment. The finite difference method and over‐relaxation iteration method are employed to solve the average Reynolds equation. The lubrication behaviour includes hydrodynamic pressure, film thickness, contact force and friction coefficient were calculated. The influence of sliding speed, local wear, contact roughness, misalignment angle, elastic deformation and eccentricity ratio is discussed in detail. The critical speed from mixed lubrication to fluid lubrication is obtained by employing the Stribeck curve. Moreover, the dimensionless average hydrodynamic pressure, film thickness and the peak value of contact force are compared.

Tribological properties of hexagonal boron nitride nanoparticles as a lubricating grease additive

AbstractThe friction and anti‐wear behaviours of hexagonal boron nitride (h‐BN) nanoparticles as a lithium lubricating grease additive are being investigated under sliding conditions. The grease with 3 wt% 60 nm h‐BN particles and the grease with 1 wt% 500 nm h‐BN particles lead to 22.34% and 20.18% reductions in the wear scar diameter, respectively, although the friction coefficients slightly decrease. The boric acid H3BO3 with layer‐crystal structure produces during the friction process, and the synergistic effect of h‐BN nanoparticles and H3BO3 makes friction reduction and wear‐resistant enhancement. Furthermore, the 60 nm h‐BN particles filled in the asperity valleys of the friction surface make a rolling effect and establish a protective film, while the 500 nm h‐BN particles shield the asperity contact between friction pairs and make a polishing effect as well.

Mixed thermo-elasto-hydrodynamic lubrication and wear coupling simulation analysis for dynamical load journal bearings

In the crankshaft-connecting rod system of the diesel engine, the complicated lubrication and wear mechanisms of journal bearings are comprehensively affected by the elastic deformation effect, the asperity contact effect, the thermal effect as well as the lubricating oil viscosity-temperature effect. This paper proposes an adaptive iterative method based on a mixed thermo-elasto-hydrodynamic (TEHD) lubrication and wear coupling model to investigate the lubrication and wear behaviors of journal bearings in the mixed lubrication. The evolution laws of the oil film thickness, the hydrodynamic pressure, the asperity contact pressure, the friction power loss, and the wear distribution are discussed and compared under different running times, oil temperatures, external loads, and shaft rotational speeds. The three-dimensional surface models based on the asperity contact power loss and the maximum wear depth with the variation of the running time and the oil temperature are fitted under specific external loads and shaft rotational speeds. The simulation results demonstrate that the bearing wear process includes the initial wear stage and the stable wear stage, which has a significant influence on the changing trend and distribution of lubricating parameters. The three-dimensional surface fitting models can provide theoretical guidance for the future performance prediction of journal bearings under time-varying working conditions.

A fluid-structure coupled transient mixed lubrication model for piston ring lubrication property analysis with CMFF method

The bore's circumferential cross-section is not always a perfect circle, due to manufacturing errors, installation, and the influence of high-temperature and high-pressure gas, especially for a cylinder liner of the two-stroke marine diesel engine. This non-axisymmetric feature significantly impacts the lubrication properties of the piston ring and cylinder liner tribo-pair. To study the tribology and lubrication properties of the tribo-pair, a fluid-structure coupled transient mixed lubrication model is developed, which incorporates the piston ring curved beam model and mixed lubrication model. The key challenge lies in integrating the interface contact issue between the piston ring and bore with the structural deformation, requiring a multi-scale modeling strategy. To address this challenge, a method of coupling mesh between the finite difference method and the finite element method (CMFF) is proposed. The approach achieves the requirement for a fine mesh for the interface contact solution while maintaining the accuracy of the curved beam model for the structural response of the piston ring. Based on the model, the lubrication performance and deformation behavior of the piston ring in an elliptical bore are comprehensively analyzed. The results are compared with those of the rigid ring model to explore the overall differences. Results reveal that the current model predicts the friction is mostly caused by asperity contact between solids at the top dead center, whereas the rigid ring model predicts viscous shearing of the lubricant. The discrepancy is mainly due to the reduction of the oil film thickness between the piston ring and the cylinder liner following the consideration of ring deformation. Moreover, the minimum clearance exhibited at the piston ring end gap demonstrates the thorough consideration of the local and global lubrication performance of piston rings in the proposed model. This is significant for the resolution of local friction and wear problems of piston rings in engineering applications. Based on the work, a new framework is established for the study of the coupled performance of piston ring dynamics and tribology.

The effect of aviation anti-wear additives on tribofilm formation and micropitting propensity

This article studies the effect of anti-wear additives on micropitting and tribofilm formation. In particular, anti-wear additives used in aviation applications have been compared against a commonly used automotive anti-wear additive (zinc dialkyldithiophosphate ((ZDDP)) that is known to promote micropitting. All tests were performed under test conditions representative of the Power GearBox within Rolls-Royce's new engine, the UltraFan©. Tests using a micropitting rig found that ZDDP formed micropits the fastest, which then propagated onto the largest amount of wear showing the catastrophic effects of micropitting. Whereas other aviation anti-wear additives, such as tricresyl phosphate and additive X, formed micropits more slowly but faster than the formulation without additives containing only base oil. Focus variation microscopy was used to characterise the micropits and found that ZDDP formed smaller and shallower micropits than the other anti-wear additives, and as a result, the wear track of the ZDDP roller had the lowest roughness. The unadditised base oil, whilst it generated micropits more slowly, propagated quickly to a more severe failure mode, showing the harmful effects of having no anti-wear additive present. The profilometer results showed the counterface ring roughness for ZDDP remained the highest, confirming that ZDDP forms a tribofilm quickly. This protects asperities, which consequently promotes micropitting, as high localised asperity contact pressures are maintained. This was further confirmed by evaluating the tribofilms using a mini traction machine with spacer layer imaging to form tribofilms under representative conditions. This demonstrated that ZDDP forms a thicker tribofilm than the aviation anti-additives and at a faster rate. Overall, the study showed that the aviation anti-wear additives promote micropitting less than ZDDP but more than base oil alone. The slower action of the tribofilm formation allows the initial running-in of surfaces, which slows down the initiation of micropitting.

Multiscale Wear Simulation in Textured, Lubricated Contacts

Specific surface textures may reduce the friction and increase the lifting forces in lubricated contacts. For the detrimental operating condition of mixed friction, wear is induced by the solid contact. In this study, a methodology for wear calculation in textured, lubricated contacts is presented that considers the wear-induced surface topography evolution. Based on the Reynolds differential equation, the mass-conserving cavitation model according to Jakobsson, Floberg, and Olsson (JFO), a wear-dependent asperity contact pressure curve and the wear equation according to Archard, wear in a wedge-shaped, textured lubrication gap was calculated. The results show the wear behavior of textured lubrication gaps. Based on the wear simulations, the tribological behavior of the textured surfaces compared to smooth surfaces is discussed. It is evident that textures, which improve the tribological performance in the hydrodynamic lubrication regime, are not necessarily associated with low wear values in a lubrication condition in the mixed friction regime. The analysis of the wear-dependent parameters initially showed a ‘recovery’ of the tribological system with increasing wear until the performance decreased again after a specific reversal point. This behavior is attributed to the relative position of the surface textures in the lubrication gap.

Influence of thermal effect in piston skirt lubrication considering thermal deformation of piston and cylinder liner

With the development of high-strength marine diesel engines, piston skirt lubrication plays an increasingly important role. In this study, to ensure that the mixed lubrication can be applied, the contact of the PS corner and CL is simplified to a spring–damper system. A coupled dynamical–tribological numerical model of a piston skirt is proposed by combining mixed lubrication and the secondary motion of the piston. The model is then used to study how the thermal effect of the piston skirt-cylinder liner friction pair affects (i) the secondary motion of the piston, (ii) the contact force of the piston-skirt corner points, and (iii) the lubrication characteristics, while considering ring friction, friction moment of piston pin and three-dimensional thermal deformation of the piston and cylinder liner. The results show that the secondary motion under the thermal condition is less than that under the cold condition, and the reverse rotation of the piston under the thermal condition occurs earlier. Asperity contact and corner points contact appear under the thermal condition, and the friction mean effective pressure under the cold condition is 47.7% greater than that under the thermal condition. Overall, it is necessary to consider the thermal effect of the piston skirt-cylinder liner friction pair.

Improved wear resistance of CBN tool enabled by hBN as a water-based lubricant additive through suppressing tribochemical reactions

Cubic boron nitride (CBN) tool is one of the most extensively used tools in modern industrial processing and manufacturing. The service life of CBN tools has extraordinary significance for machining efficiency and surface finish of workpieces. Here, we investigated the tribological behaviors of CBN/Si3N4 pairs to simulate the process of CBN tools cutting ceramics and demonstrated that the use of optimal 0.02 wt% non-toxic and eco-friendly hexagonal boron nitride (hBN) aqueous solution as a water-based cutting fluid enabled to reduce the wear rate of CBN by ∼64%, and improve the material removal of Si3N4 by ∼13% compared to pure water lubrication. Further analysis indicated that the physical adsorption of hBN nanosheets played a dominant role in the lubrication at the sliding interface where the direct contact of interfacial asperities was separated and then the tribochemical reactions of CBN with water were suppressed. The results help in the development of environmentally friendly lubricants to match the requirements of mechanical processing for difficult-to-machine materials.

Theoretical Investigations on Tribological Properties of Air Foil Thrust Bearings during Start-Up Process

During the start-up process, air foil thrust bearings (AFTBs) come into contact and are prone to localized overheating and even failure. Frequent starts and stops are the major sources of wear. In this study, a transient mixed lubrication model considering the air rarefaction effect is established. In the model, asperity contact between the top foil of the air foil thrust bearing (AFTB) and the rotor surface is considered using a deterministic approach, and the deformation of the foil structure is considered using the elastic-aerodynamic coupling. The influences of bearing compliance, external load, acceleration time and contact stiffness on the tribological properties during the start-up process are investigated. The results show that the lift-off speed increases with the increase of the bearing compliance and of the external load. Specifically, the lift-off speed of AFTBs increases from 1950 rpm to 5325 rpm when the bearing compliance increases linearly from 0.2 to 1.0. When the external load increases from 10 N to 80 N, the lift-off speed is increased by approximately 30 times, from 450 rpm to 13,575 rpm. Furthermore, a decrease in the acceleration time of the rotor generates a decrease in the lift-off time and an increase in the contact stiffness of the bearing leads to a decrease in the lift-off speed of the bearing. By exploring the effects of the bearing parameters and working conditions on the tribological performance of AFTBs during start-up, the results presented in this study can provide theoretical support for improving start-up performance and designing high-reliability and long-life AFTBs.

Superlubricity of TiN coating using glycerol with the addition of Cu nanoparticles

Superlubricity was achieved by the addition of copper nanoparticles (CuNPs) as an additive to glycerol, a sustainable lubricant, which was applied to TiN-coated steel that underwent ball-on-flat reciprocating testing at an average contact pressure of 54.6 MPa. A coefficient of friction of ∼ 0.007 and a wear rate of 2.0 × 10–8 mm3/N m were achieved on the TiN coating when lubricated with glycerol + 0.1 wt% CuNPs at a sliding speed of 120 mm/s. XPS and Raman analysis suggested that a surface tribofilm containing CuNPs and a carbon-based film derived from the degradation of glycerol were deposited onto the contact area of the tribopairs, which effectively prevented the direct asperity contact between the steel ball and TiN-coated steel, enabling ultra-low friction.

Fault Weakening During Short Seismic Slip Pulse Experiments: The Role of Pressurized Water and Implications for Induced Earthquakes in the Groningen Gas Field

AbstractHigh‐velocity friction experiments on simulated fault gouges sheared at high normal stress and to low displacement are particularly relevant to induced seismicity, which is becoming an important topic in fault mechanics. Using a new, improved set‐up, which allows simulation of fault stress and fluid pressure () conditions approaching in‐situ reservoir values, we performed ring‐shear experiments on simulated fault gouges prepared from the source‐, reservoir‐, and caprock‐formations of the Groningen gas field. Pre‐sheared gouges were subjected to a rotational slip pulse reaching ∼1.0 m/s peak velocity and 13–16 cm total displacement at effective normal stresses () of 5–31 MPa and up to 5 MPa, using water or dry nitrogen as pore fluid. All water‐saturated gouges show strong dynamic weakening within a few cm of slip, with the lowest dynamic friction (0.2–0.4) measured at the highest . By contrast, the weakening was subtle in experiments using nitrogen. Our analyses focus on the high‐ experiments, which are more realistic and show a distinct dependence of constitutive parameters (e.g., slip‐weakening rate) on , in the form of empirical linear, power‐law or exponential relations. The results provide much‐needed constraints for numerical modeling of induced rupture propagation in the Groningen field. Based on temperature‐ and ‐measurements made in near‐direct contact with the active shear band, and using “post‐mortem” microstructures, we exclude previously‐proposed dynamic weakening mechanisms (e.g., flash heating or thermal pressurization) and suggest that water pressurization at heated asperity or grain contacts explains the weakening seen in our high‐ experiments.

Experimental and Numerical Study of the Mixed Lubrication Considering Boundary Film Strength.

For the influence of boundary film on the lubrication state of sliding friction pairs, a boundary film strength model was proposed that can comprehensively reflect the influences of film thickness, pressure, shear stress and temperature. The model parameters were obtained through fitting the test results. Then, a mixed lubrication model considering boundary film strength was established by coupling the boundary film strength model with the hydrodynamic lubrication model and the asperity contact model. The calculation program was developed using the Fortran language, which can effectively capture the tribological characteristics and action ratios of the fluid, boundary film and dry friction components. Simultaneously, the mixed lubrication model was applied to the journal bearing. A parametric analysis was performed to investigate the influences of different working conditions on lubrication performance. Under current operating conditions, the results show that: when the speed is above 200 r/min or the viscosity is higher than 0.09 Pa·s, the boundary film breakdown rate is almost 0 and the friction coefficient is lower than 0.02; when the roughness is reduced from 1.8 μm to 0.8 μm, the ultimate load of the journal bearing rises from 27 MPa to 36 MPa, an increase of about 33%; when the load exceeds 36 MPa or the temperature is higher than 100 °C, more than 25% of the boundary film breaks and the dry friction component accounts for more than 60% of the total friction, which leads to a sudden increase in the friction coefficient. Hence, the study of mixed lubrication considering boundary film strength provides theoretical guidance for accurately reflecting the actual lubrication state and improving the mechanical energy efficiency of friction pairs.

Nucleation of frictional slip: A yielding or a fracture process?

The onset of frictional sliding between contacting bodies under shear load is nucleated by the quasi-static growth of localized slip patches. After reaching a certain critical size, known as the nucleation length, these patches become unstable and continue growing dynamically, eventually causing the sliding of the entire interface. Two different theories have been used to compute the nucleation length of such patches depending on the dominant process driving their growth. If it is only the yielding of contact asperities (large-scale yielding), a stress criterion is applied, based on linear stability analysis, whereas if fracture dominates (small-scale yielding), an energy criterion is applied (Griffith’s criterion), based on fracture mechanics and classical nucleation theory. Both approaches contain important underlying assumptions that are well-suited to describe either one situation or the other. However, what happens in-between is not captured by any of them. In this work, we use numerical simulations to study what is the dominant underlying process driving nucleation for different conditions of heterogeneity in the frictional strength of the interface and what are the implications for nucleation dynamics and the onset of frictional sliding. We show that large frictional heterogeneities enable a transition from a yielding-driven nucleation phase to a fracture-driven one. This transition occurs only above a certain level of heterogeneity and can either be quasi-static (stable) or dynamic (unstable), depending on the correlation length of frictional strength along the interface and the difference in strength between the strongest and the weakest point (the amplitude). Unstable transitions generate localized dynamic slip events, whose magnitude increases with higher correlation length and decreases with larger amplitude. Our work sheds new light on the role of heterogeneity and fracture in the nucleation of frictional slip, bridging the gap between the two main governing theories for nucleation.

A Curve-fit Traction Coefficient Relation of Mixed EHL Line Contact with Hydraulic Fluid and Steel Surfaces

Accurate modeling of hydraulic gear machine meshing torque loss, considering possible asperity contact, is required for better prediction of its hydromechanical efficiency. A model for predicting the mixed thermal EHL line contact traction coefficient is implemented in this study, considering experimental density and viscosity properties. Simulation results of traction coefficient are validated with experiments at two operating conditions. The defining components of the traction coefficient, the asperity load ratio and the hydrodynamic component are obtained at wide range of inputs. Finally, explicit curve-fit formulations of the asperity load ratio and the hydrodynamic component of traction coefficient are created in terms of input non-dimensional variables, which can be used for modeling the torque loss in a simple yet accurate way.

Shear-induced distortion of clay minerals aids in dynamic weakening of shallow faults during earthquakes

Coseismic faulting may activate complicated physicochemical processes in the fault zone through both mechanical deformation and frictional heating. While plenty of work emphasizes the great importance of thermal effects on dynamic fault weakening, less attention has been paid to the mechanochemical effects. We study the effects of mechanochemical changes of chlorite on its dynamic weakening process via low- and high-velocity friction experiments, simultaneous thermal analysis, X-ray diffraction, microstructural observation, and numerical modeling in this work. With higher degrees of deformation, the experimentally sheared chlorite samples generally show more pronounced reductions in the onset temperature of thermal decomposition (Td0; from 561°C to 189°C in the extreme case) and more striking increases in the reaction rate. The exponential-decay formula can well describe the relation between Td0 and the mechanical work (integral of shear stress with respect to displacement) in the shear deformation. Moreover, the presence of pore water and the variation in chlorite content could significantly affect the mechanochemical changes of the sheared chlorite-bearing samples. Taking the markedly lowered Td0 as the characteristic temperature leading to thermal softening of asperity contacts, numerical modeling shows that flash weakening of chlorite-rich samples can be significantly promoted, dwarfing the thermochemical pressurization that is also enhanced by mechanochemical effects though. This is largely supported by high-velocity experiments (∼m/s) performed on the pre-deformed chlorite samples, which show both lower peak friction and fracture energy, and earlier activation of dynamic weakening. Clearly, shear deformation plays a key role in aiding thermally-activated mechanisms in weakening faults and facilitating rupture propagation during earthquakes.

Wear simulation in lubricated contacts considering wear-dependent surface topography changes

Along with friction, wear in lubricated contacts is one of the most essential design criteria. The use of low-viscosity lubricants, as well as changing operating conditions, may lead to the wear-causing operating condition of mixed friction. For wear assessment, in the last decades numerical simulations have been confirmed as a powerful tool. The level of detail of the simulation model has a significant influence on the simulation results. The local wear heights for a hydrodynamic journal bearing under constant operating conditions are calculated numerically. A variant study is used to determine the influence of elastic deformation, cavitation model and the consideration of the wear-dependent surface topography. The surface topography, which is subject to change especially during running-in, is taken into account in the form of wear-dependent asperity contact pressure curves, respectively wear-dependent flow factors. To validate the simulation results, tests were carried out on a rotational tribometer. The results show that the consideration of the wear-dependent surface topography has a significant influence on the wear development and thus the trend evaluated from the experiment can be better assessed.

Frictional properties of MoS2 on a multi-level rough wall under starved lubrication.

Owing to nano-MoS2's excellent anti-friction and anti-wear properties, nano-MoS2, which can act as a nano-additive in lubricating oil or solid lubricants, is believed to have great potential in the lubrication of power machinery and moving parts of a spacecraft. The molecular dynamics method was used to construct a rough surface and a multi-level asperity structure to simulate starved lubrication before oil film breakdown, and the lubrication mechanism of MoS2 as a nano-additive or directly coated on the textured surface could reduce the friction coefficient and wear was explained from the atomic perspective. Simulations showed that the multilayer MoS2 played a role of load-bearing at light load or low velocity, and slipped into the grooves to repair the surface under heavy load or high velocity. Even if local asperity contact occurs, MoS2 nanoparticles could accelerate the detachment of the initial asperity contact to prevent large-scale adhesion. The MoS2 nanoparticles transformed the pure liquid oil film into a liquid-solid composite oil film, which was more suitable for lubrication under heavy load and high velocity because it increased the contact area, protected the friction surface and prevented asperity contact. The proposed lubrication mechanism contributes to understanding the frictional properties of layered nanomaterials under extreme conditions and provides a reference for further application of MoS2 materials in the field of lubrication.

Effect of Zero-Sap Antiwear/Ep Lubricant Additives on Nano-Mechanical Properties of Tribofilms Formed on AISI 52100 Steel Grade

Numerous nations have recently passed stricter environmental legislation due to excessive release of harmful emissions like sulfur (S), phosphorus (P), and zinc (Zn) in conventional lubricating oil additives used in contemporary internal combustion engines. Use of borate ester lubricant additives with no sulfated ash, phosphorus, or sulfur effect (zero-SAPS) is one way to reduce this effect. The resistance of tribofilms from oils containing borate esters on AISI 52100 steel grade to plastic deformation is not entirely understood, yet. This study compares Zinc Dialkyl Dithiophosphate (ZDDP) to the tribological performance and Nano-mechanical behavior of tribofilms made from each of the antiwear/extreme pressure borate esters in Polyalphaolefin (PAO6) base oil. According to tribological test findings on a reciprocating steel pin-on-steel plate sliding rig, the boundary films formed by the borate esters reduced friction and wear in a manner similar to that of ZDDP. On the wear scar, the Nano-indentation technique was used to characterize the Nano-mechanical properties of the tribofilms. The ratios of reduced elastic modulus to hardness (E’/H) for the tribofilms of both borate esters are not only higher than the steel substrate but also comparable to ZDDP. This shows that boron-containing tribofilms offered ZDDP-like tribological performance and also have the ability to release absorbed frictional energy when moving across elastic asperity contacts. Consequently, using borate additives in lubricating oils in IC engines is a method of reducing exhaust emission modern vehicles without adversely affecting tribological performance.Key Words:Anti-wear Additives, Borate Esters, Plasticity Index.

Temperature Effect on Load Distribution, Friction, and Wear of a Grease-Lubricated Spherical Roller Bearing (SRB)

This article investigates the effect of temperature on the lubrication and wear behavior of a grease-lubricated spherical roller bearing (SRB) assuming mixed-elastohydrodynamic lubrication (mixed-EHL) where the contact load is shared between the lubricant film and contact asperities. Temperature effects on internal load distribution and elastic modulus and their subsequent effect on the wear are also analyzed. The effect of temperature is investigated, first on the film thickness and load sharing, and then on the friction and wear. If the temperature rises from ambient to 110 °C, the increase in the contact patch dimensions, due to the decrease of elastic modulus, is only approximately 1% and the decrease in mean contact pressure is approximately 2%. Moreover, the load at decreases by approximately 4%, and the load zone increases by approximately 5%. As the temperature rises from 50 to 110 °C, the asperity portion of the contact load at position is increased by 350%. At 50 °C, only 20% of the total traction is contributed by the contact asperity, which is 80% at 110 °C. For the temperature rise from 50 to 110 °C, the wear volume loss increases by 238% and nonlinearly, due to viscosity reduction and its effect on the reduction of film thickness. The isolated change of internal load distribution and elastic modulus due to temperature rise causes a relatively insignificant change in wear. Hence, the predominant effect of temperature rise on wear is due to a decrease in lubricant film thickness causing the asperity component of the contact load to increase.

Quasi-Static Sliding Wear Analysis of 3D Rough Surface Considering Changes in the Point of Contact

This study analyzed adhesive wear in periodic sliding motion using a quasi-static deterministic wear model that considered changes in the point of contact of asperities as well as changes in the surface and statistical parameters. The contact pressure was calculated using the semi-analytical method (SAM) based on the periodic properties of the rough surface, and the wear was analyzed by obtaining the wear depth for each node using the Archard wear model. We took into account that the sliding distances of the upper moving surface and the lower stationary surface are different according to the actual size of the two objects. We compared the results of the quasi-static wear analysis with the truncation model and the deterministic fixed model, which did not consider the change in the asperity contact point. In the truncation model, an error in the estimation of the radius occurred in the process of fitting the tip of asperities with a sphere. As the asperities became flatter by wear, this error accumulated, which revealed a difference in the deterministic wear analysis results. As a result of the wear analysis on the periodic surface, the RMS roughness of the positively skewed surface decreased the fastest, the skewness increased in the negative direction, and the kurtosis initially decreased and then increased. In addition, wear scars occurred due to the difference in wear depth between the lower stationary surface and the upper moving surface.