Small Friction Research Articles

Friction Mechanism of Sulfur-Containing Lubricant Additives Confined between Fe(100) Substrates.

Sulfur-containing lubricant additives can chemically react with metal surfaces under extreme conditions, such as high temperature and high pressure, forming protective films on the surfaces. However, the formation mechanisms and the friction-reducing and antiwear properties of these films remain unclear. In this study, we investigated the friction process of sulfur-containing additives confined between two iron surfaces using reactive molecular dynamics simulations. Our research revealed that in systems with a higher S/C ratio, an iron sulfide layer formed on the iron surfaces with Fe-S-Fe bridging bonds at the interface, resulting in relatively smaller friction and wear even under higher loads. However, in systems with lower S/C ratios, the presence of numerous interfacial Fe-Cn-Fe bridging bonds, caused by the hydrocarbon radicals released from the additives, led to the formation of thick amorphous shearing bands at the interface between the two substrates. In this case, the distributed sulfur atoms also exhibited some effect in reducing the shear resistance of the amorphous shearing bands due to the weak strength of S-Fe bonds compared to the strength of C-Fe bonds. These atomic-level insights help understand the antiwear characteristics of sulfur-containing lubricant additives confined between iron substrates.

Detection and Analysis of Anomalous Behavior in On-Orbit Satellites Using AI Algorithms

The increasing deployment of satellites for essential applications necessitates robust anomaly detection to maintain their operational integrity. Traditional methods, which depend on manual monitoring and predefined thresholds, often prove inadequate in the complex space environment. This paper investigates the application of Artificial Intelligence (AI) algorithms to improve the detection and analysis of anomalous behavior in on-orbit satellites. AI, especially through machine learning (ML) and deep learning (DL), provides advanced capabilities for processing extensive telemetry data and identifying intricate patterns. By leveraging historical data, AI systems can establish normal operational parameters and detect deviations indicating potential anomalies. Techniques such as supervised and unsupervised learning are employed to develop models with high predictive accuracy. Furthermore, AI facilitates root cause analysis by correlating anomalies with operational conditions or external factors, enabling effective corrective measures. The integration of AI also promotes autonomous satellite operations, which are crucial for deep-space missions. This advancement enhances satellite reliability and safety, supporting sustainable and progressive space exploration. In this research, machine learning algorithms were employed to develop the proposed anomaly detection system. The system aims to detect subtle failures in the spacecraft’s attitude dynamics system, particularly in the reaction wheel subsystem, by learning solely from the spacecraft's nominal behavioral data. The system was developed from a small satellite's attitude dynamics control system, which may exhibit bearing failures in the reaction wheels. Two types of anomaly detection systems were introduced: a two-sided learning anomaly detection system and a one-sided learning anomaly detection system. For this study, a two-sided learning anomaly detection system was developed using the Logistic Regression (LR) method. This provided a foundation for the training process using a machine learning approach. By learning from both nominal and failure behaviors of the satellite, the system was designed to detect small reaction wheel friction failures effectively.

Study on dynamic mechanism of granular flow erosion and entrainment based on DEM theory

BackgroundGranular flows are common on the Qinghai–Tibet Plateau and the Hengduan Mountains in China, and their dynamic process processes have obvious erosional and entrainment effects. On the one hand, the volume of the granular flow increases by a factor of several or ten, which significantly increases its ability to cause a catastrophe; on the other hand, the eroded loose material affects the granular flow dynamics process and changes its state of motion.MethodsIn this paper, the dynamic mechanism of granular flow erosion and entrainment is investigated by DEM simulation.Purpose The effects of different substrate materials and substrate boundary conditions on granular flow erosion and entrainment are analyzed, and the effects of material mixing caused by erosion and entrainment on the state of motion of granular flow are discussed. It was verified that the kinetic mechanisms of granular flow erosion and entrainment includes impact erosion, ploughing, and shear abrasion.ResultsAnd discovered that small matrix particle size, small matrix boundary friction, and small matrix thickness lead to stronger ploughing and shear abrasion; Large matrix fractal dimensions result in stronger ploughing and weaker shear abrasion, and the granular flow does not entrain large amounts of material to the accumulation zone. Meanwhile, the dynamics of erosion and entrainment of granular flow were investigated, and the results showed that: 1. The greater the erosion rate, the greater the velocity and kinetic energy of the granular flow, the greater the distance traveled, and the smaller the apparent friction angle (i.e., the greater the mobility). 2. The amount of small granules in a granular flow changes its fluidity, the more small granules there are, the more fluid it is. 3. The fit reveals that the substrate fractal dimension has the strongest effect on the velocity and kinetic energy of granular flow, followed by substrate thickness and substrate boundary friction.

IoT-Based Heartbeat Rate-Monitoring Device Powered by Harvested Kinetic Energy.

Remote patient-monitoring systems are helpful since they can provide timely and effective healthcare facilities. Such online telemedicine is usually achieved with the help of sophisticated and advanced wearable sensor technologies. The modern type of wearable connected devices enable the monitoring of vital sign parameters such as: heart rate variability (HRV) also known as electrocardiogram (ECG), blood pressure (BLP), Respiratory rate and body temperature, blood pressure (BLP), respiratory rate, and body temperature. The ubiquitous problem of wearable devices is their power demand for signal transmission; such devices require frequent battery charging, which causes serious limitations to the continuous monitoring of vital data. To overcome this, the current study provides a primary report on collecting kinetic energy from daily human activities for monitoring vital human signs. The harvested energy is used to sustain the battery autonomy of wearable devices, which allows for a longer monitoring time of vital data. This study proposes a novel type of stress- or exercise-monitoring ECG device based on a microcontroller (PIC18F4550) and a Wi-Fi device (ESP8266), which is cost-effective and enables real-time monitoring of heart rate in the cloud during normal daily activities. In order to achieve both portability and maximum power, the harvester has a small structure and low friction. Neodymium magnets were chosen for their high magnetic strength, versatility, and compact size. Due to the non-linear magnetic force interaction of the magnets, the non-linear part of the dynamic equation has an inverse quadratic form. Electromechanical damping is considered in this study, and the quadratic non-linearity is approximated using MacLaurin expansion, which enables us to find the law of motion for general case studies using classical methods for dynamic equations and the suitable parameters for the harvester. The oscillations are enabled by applying an initial force, and there is a loss of energy due to the electromechanical damping. A typical numerical application is computed with Matlab 2015 software, and an ODE45 solver is used to verify the accuracy of the method.

Optimum Design of Coaxial Hydraulic Sealing Systems Made from Polytetrafluoroethylene and Its Compounds

Fluidic actuation systems are optimizable as to energy consumption by reducing the friction in the hydraulic cylinders. Polymeric materials with special antifriction properties and good resistance to hydraulic fluids can be deployed to enhance the performance of hydraulic cylinders. Small friction forces can also be ensured by facilitating the hydrodynamic separation of the elements of the friction tribosystem, namely the seal and sealed-off surface, respectively. The study presented in this paper analyzed the hydrodynamic separation phenomenon in hydraulic cylinders with coaxial sealing systems of the pistons. The process underlying the forming of the fluid film between the seal and its contact surface was considered and the formula for calculating film thickness was deduced. This paper presents graphs that describe the variation of the fluid film thickness versus the dimensional and material characteristics of the sealing systems. The study yielded recommendations as to the most adequate polymeric material to be used and the optimum dimensional characteristics of the seal.

Anisotropic tribology and electrification properties of sliding-mode triboelectric nanogenerator with groove textures

Sliding-mode triboelectric nanogenerator (S-TENG) is based on the coupling of triboelectrification and electrostatic induction, converting electrical energy from sliding motion. Introducing micro-textures into the sliding surface, and adjusting the angle between the texture and sliding direction (direction angle) may achieve performance anisotropy, which provides novel ideas for optimizing the tribology and electrification performance of S-TENG. To guide the performance optimization based on the anisotropy, in this paper, groove micro-textures were fabricated on the surface of S-TENG, and anisotropic tribology and electrification performance were obtained through changing the direction angle. Based on the surface analysis and after-cleaning tests, the mechanism of the anisotropy was explained. It is shown that the anisotropy of friction coefficient can be attributed to the changes of texture edge induced resistance and groove captured wear debris, while the voltage anisotropy is due to the variations of debris accumulated on the sliding interface and the resulting charge neutralization. Among the selected 0°–90° direction angles, S-TENG at angle of 90° exhibits relatively small stable friction coefficient and high open-circuit voltage, and thus it is recommended for the performance optimization. The open-circuit voltage is not directly associated with the friction coefficient, but closely related to the wear debris accumulated on the sliding interface. This study presents a simple and convenient method to optimize the performance of S-TENG, and help understand the correlation between its tribology and electrical performance.

Experimental Research on Dynamic Characteristics of a Multi-Disc Rotor System Supported by Aerostatic Bearings

Gas bearings have the advantages of small friction loss, wide applicable speed range, no pollution, etc., and have important application prospects in micro and small high-speed rotating machinery. However, due to its compressibility and low viscosity, its dynamic stability in high-speed rotating machinery is the key to constraining its development. The experimental study of shaft system dynamics is the main means to explore the mechanism of rotor behavior. On the test platform of dynamic characteristics of multi-disc rotor system supported by aerostatic bearings, experimental research on the nonlinear dynamic characteristics of a rotor system was carried out, and nonlinear vibration test and analysis methods, such as axial orbits, bifurcation diagrams, and spectral characteristics, were adopted, and vibration phenomena, including the critical rotational speed accumulating energy and low-frequency accumulating energy, were presented and the vibration characteristics of bearing fracture faults were presented. The bearing supply pressure and rubber damping pad were introduced as a method to suppress the low-frequency vibration of the aerostatic bearing rotor system, and its vibration-reduction effect was verified by experiments. The above results can provide technical support for vibration control and fault diagnosis of rotor systems supported by aerostatic bearings.

Correlations in a weakly interacting two-dimensional random flow.

We analytically examine fluctuations of vorticity excited by an external random force in two-dimensional fluid. We develop the perturbation theory enabling one to calculate nonlinear corrections to correlation functions of the flow fluctuations found in the linear approximation. We calculate the correction to the pair correlation function and the triple correlation function. It enables us to establish the criterion of validity of the perturbation theory for different ratios of viscosity and bottom friction. We find that the corrections to the second moment are anomalously weak in the cases of small bottom friction and small viscosity and relate the weakness to the energy and enstrophy balances. We demonstrate that at small bottom friction the triple correlation function is characterized by universal scaling behavior in some region of lengths. The developed perturbation method was verified and confirmed by direct numerical simulations.

Synergy of Al2O3/GO hybrid nanofluids: Molecular dynamics perspective on dispersion/adsorption evolution and its tribological validation

A synergistic Al2O3/GO hybrid nanofluid was developed, aiming to attain prominent tribological properties for application in mechanical engineering. On the support of a molecular dynamics approach, amorphous models were simulated, thus illustrating the agglomeration or the adsorption in nanofluids, as well as revealing the evolution of the laminated adsorbed structure. Further, single-component or hybrid nanofluids were characterized and their tribological validations were performed. The results showed that the modified nanofluids realized dispersion equilibrium and suspension stabilization at the molecular level, and their wettability or thermo-physical indexes were improved, which was beneficial for the tribological performance. Especially in hybrid nanofluids, the synergistic effect further enhanced these capabilities, thereby maintaining a small friction coefficient and a flat wear region.

Improving the performance of nonlinear isolator through triboelectric nanogenerator damper integrating energy harvesting

In order to solve the problem of integration of low-frequency vibration isolation and vibration energy harvesting, this paper proposes a novel nonlinear oscillator with triboelectric nanogenerator damper (NO-TENGD), which can realize different nonlinear characteristics of quasi-zero stiffness (QZS) and bistable by changing the length of rigid links. The mechanism of improving the isolation performance of NO-TENGD was analyzed and discussed with a dynamic model proposed, and the effects of excitation frequency, friction force, geometric parameters, and spring stiffness were studied. The damping can be increased by adjusting the pressure of TENGD, which can effectively suppress the resonance peak value of response amplitude and force transmissibility. The experimental results show that when the excitation frequency is larger, the amplitude response of NO-TENGD is smaller under frictionless condition, and the amplitude response of NO-TENGD under large friction condition is smaller than that under small friction one. NO-TENGD with QZS has a good vibration isolation effect, and the output power can light up 50 LED lights or electronic watches. Finally, NO-TENGD with QZS can achieve low frequency vibration suppression, and increasing damping can reduce the response amplitude and force transmissibility of the beam bridge in the resonance frequency range.

Confinement enhanced viscosity vs shear thinning in lubricated ice friction

The ice surface is known for presenting a very small kinetic friction coefficient, but the origin of this property remains highly controversial to date. In this work, we revisit recent computer simulations of ice sliding on atomically smooth substrates, using newly calculated bulk viscosities for the TIP4P/ice water model. The results show that spontaneously formed premelting films in static conditions exhibit an effective viscosity that is about twice the bulk viscosity. However, upon approaching sliding speeds in the order of m/s, the shear rate becomes very large, and the viscosities decrease by several orders of magnitude. This shows that premelting films can act as an efficient lubrication layer despite their small thickness and illustrates an interesting interplay between confinement enhanced viscosities and shear thinning. Our results suggest that the strongly thinned viscosities that operate under the high speed skating regime could largely reduce the amount of frictional heating.

Discrete element modeling of rock-filled gabions under successive boulder impacts

A discrete element model consisting of irregular crushable filling particles and a flexible wire mesh is established. The numerical model is validated against the static net punching test and the dynamic pendulum impact test to ensure that the large irreversible plastic deformations and the impact forces during successive impacts can be captured. The impact force reduces at small friction coefficients, which is associated with the significant particle rearrangement during the impact process. The important role of friction is further confirmed by the energy evolution, showing that friction is the dominant mechanism for energy dissipation, instead of the more intuitionistic collision or particle crushing. Besides, the increase of impact force with the number of impacts becomes more significant at a higher impact energy due to the faster rate of momentum exchange. A bounce-back behavior of the boulder is observed when the impact energy is low, which may attenuate the impact force like the reflection waves in real debris flow events where multiple boulders are present. Our results highlight the combined effects of contact properties and impact energy, which are valuable in the design of rigid barriers shielded by rock-filled gabions for hazard mitigation in engineering practice.

The Effect of Chisel Sharpening and Without Chisel Sharpening on the Surface Roughness of the Piston Waste Casting Material

This research begins with the casting process of motorcycle piston waste and then continues with the turning process on conventional lathes. The purpose of this machining process is to identify the quality of the surface roughness of the turned casting product. The surface roughness quality of the turning product to be applied must show high precision and not experience significant surface defects. To get the surface quality, there are many machining variables that need to be considered, one of which is the use of lathe chisels. The turning tool becomes an important component during the turning process to form various forms of products produced through the mediation of rotary motion and tool motion resulting in large and small friction. This friction that occurs will depend on the sharpness of the chisel itself. The problem studied is the use of sharpened and non-sharpened chisels in terms of product surface roughness during the turning process. In addition, this study will vary the variable thickness of the feed which varies 0.2 mm, 0.4 mm, 0.6 mm, 0.8 mm to be a factor in finding out the quality of turning products. While the spindle rotation of the lathe used is 240 rpm, 350 rpm, 430 rpm. From the experiments on turning the casting product, it was found that the lowest surface roughness quality was obtained with the N6 sharpened chisel of 1.13 mm while the N6 unsharpened chisel was 1.19 mm.Keywords: lathe chisel, casting, piston waste, turning, surface roughness value.

Application of Collins’ upper bound theorem to upsetting processes

The standard formulation of the upper bound theorem is incompatible with the Coulomb friction law. However, this law is widely used in metal forming processes analysis. An alternative formulation based on non-kinematically admissible velocity fields can evaluate the load required to deform the material. The present paper adopts this formulation for evaluating the compression force in solid and hollow cylinder compression processes. The trial velocity fields are continuous. The field for the solid cylinder involves no parameter for minimization, and the field for the hollow cylinder involves one parameter. Parametric analyses of the solutions are provided. The compression forces found are compared to available results at small friction coefficients. The solution is also in agreement with physical expectations. In particular, the solution for the hollow cylinder compression predicts that the radial velocity is everywhere positive in the friction coefficient is small enough. For larger friction coefficients, the radius where the radial velocity vanishes propagates from the inner radius as the friction coefficient increases. The final result can be used in conjunction with standard procedures for evaluating the friction coefficient.

Investigation on the effect and growth mechanism of two-stage MAO coating

As a more popular surface treatment method, micro-arc oxidation technology can well solve the problems of insufficient strength and poor corrosion resistance of aluminum alloy surface. In this paper, a composite coating was obtained by using a two-stage micro-arc oxidation method different from the conventional micro-arc oxidation treatment, which solved the problem of incompatibility between the two types of electrolytes and obtained a coating with superior effect. The microscopic morphology and elemental composition were characterized by using Energy Dispersive Spectrometer (EDS)/X-ray Diffraction (XRD)/Scanning Electron Microscopy (SEM). The coating thickness was tested using a coating thickness gauge, its tribological properties were tested by friction experiments, and its corrosion resistance was tested by electrochemical corrosion tests. The results show that the coating has the advantages of large thickness, dense, good anti-wear performance, small friction coefficient and excellent corrosion resistance, which enhanced the performance of MAO coating and was an important guide for further work.

Particle-photon radiative interactions and thermalization.

We analyze the statistical properties of radiative transitions for a molecular system possessing discrete, equally spaced, energy levels, interacting with thermal radiation at constant temperature. A radiative fluctuation-dissipation theorem is derived and the particle velocity distribution analyzed. It is shown analytically that, neglecting molecular collisions, the velocity distribution function cannot be Gaussian, as the equilibrium value for the kurtosis κ is different from κ=3. A Maxwellian velocity distribution can be recovered in the limit of small radiative friction.

Epoxidized Malaysian Elaeis guineensis palm kernel oil trimethylolpropane polyol ester as green renewable biolubricants

Green renewable biolubricants base stock based on polyol triester of epoxidized Malaysian Elaeis guineensis palm kernel oil and trimethylolpropane was produced and optimized by D-optimal design of response surface methodology. The in-situ generated performic acid catalyst was used to epoxidize the palm kernel oil polyol triester (PKO-TMPE). The epoxidation process was improved and the subsequent epoxidized palm kernel oil-polyol triester (EPKO-TMPE) was assessed for its lubrication possessions. The optimal condition for the epoxidation process was attained at the unsaturation, hydrogen peroxide and formic acid molar ratios of 1.00: 7.67: 5.00, temperature of 51.85 °C with 1.62 h reaction time, respectively. High epoxide yield of 92.5% and 100.0% relative conversion oxirane with low iodine value of 0.40 g/100 g I2 of EPKO-TMPE was produced at the optimal condition. The resultant epoxidized polyol ester displayed respectable lubrication properties with high flash point at 335 °C and oxidative stability temperature at 278 °C, decent viscosity index of 187 and pour point at −19 °C. Tribological assessments exhibited that EPKO-TMPE has revealed small coefficient friction of 0.16 at 40 °C and 0.18 at 100 °C in the hydrodynamic region and equivalent with some marketable lubricants. The rheological investigations showed EPKO-TMPE can be classified as ISO VG 46 lubricant grade with decent lubrication possessions of Newtonian fluids. This renewable green industrial biolubricant is likely to be applicable for turbine, compressor fluids and hydraulic oil applications.

Flow and Conjugate Heat Transfer of Swirl Chamber With Micro-Ribs in Turbine Vane Leading Edge

Abstract Swirl cooling can provide effective protection for the turbine vane leading edge (LE). In this paper, a swirl cooling model for improving the turbine vane heat transfer is established. The model includes the high-temperature mainstream region, LE region, and swirl cooling region. The conjugate heat transfer (CHT) method is used to examine the influence of wall structures on swirl cooling. Then, the best surface structure in the studied range is selected to further analyze the impact of the coolant inlet mass flow. The results show that the circumferential micro-rib structure has a more excellent performance in both fluid flow and cooling performance. The hindering effect of the micro-ribs can effectively avoid the development of axial cross-flow, thus enhancing the heat transfer with a small friction loss increment and providing a lower surface temperature and more uniform temperature distribution. When the inlet mass flowrate improves, the thermal performance factor increases and the LE temperature decreases gradually. Under the same pumping power condition, the circumferential micro-ribs structure has higher heat transfer efficiency. This investigation can provide a new design for further improving the thermal performance of swirl cooling for turbine vanes.

2次均質化法を用いた多結晶材料の圧縮変形における寸法依存性と加工条件の関係評価

In a hot steel rolling process, the rolling force and the strain distribution in a rolled material is strongly affected by the friction coefficient between the work roll and the rolled material and the rolling shape factor which is the ratio of the contact length between the work roll and the material to the material thickness. In this respect, many numerical attempts have been made to reveal their relationship. These simulations often suppose the homogeneity of the material because of simplicity. However, the recent researchers have suggested that the heterogeneity in microstructure can cause size effect on deformation behavior of polycrystalline material. In order to clarify the size effect in the rolling process, this study investigates the size effect on the deformation force and the strain distribution in the plane strain compression which is simple and similar to the flat rolling in effects of the friction coefficient and the shape factor, using the second-order homogenization-based finite element (2nd HMFE) simulations. Simulations are performed by two-dimensional 2nd HMFE with different friction coefficients, shape factors, work hardening parameters and size ratios between micro and macrostructures, which are related to strain heterogeneity in a deformed material. As a result, in case of the small friction coefficient, relatively uniform deformation tends to occur in the material, which results in small effects of the size ratio on deformation behavior. In contrast, in case of the large friction coefficient, size effects on the compression force and the strain distribution appears to be large when the shape factor of a deformed material approaches 1. It is considered to be because high strain gradient tends to reduce the plastic work in the microstructure and a deformation band in which the strain gradient is higher is formed in the material in such a case.

Effects of prestress implementation on self-stress state in large-scale tensegrity structure

Tensegrity structures are spatial, reticulated, and lightweight structures that are composed of struts and cables. Stability is provided by a self-stress state between tensioned and compressed members obtained by prestress. Although the form finding of tensegrity structures has attracted significant attention, few studies have focused on the structural concept implementation in large-scale systems and prestress implementation in particular. This work investigates the effects of the prestress implementation on the self-stress state in a large-scale tensegrity structure with interconnected struts. The suspended, 6.2-m-tall structure consists of stainless steel cables and carbon-fiber composite tubes. Different uniform and non-uniform prestress and support condition scenarios were applied and the effects on the self-stress state were investigated experimentally by measuring strains in the carbon tubes, stresses in cables, and reaction forces. Numerical modeling was performed using the dynamic relaxation method, whose predicted stress values were compared to experimental data. The results revealed that the cable-by-cable prestress implementation led to lower effective prestress levels than nominally predicted due to a continuous stress redistribution. Non-uniformly applied prestress resulted in a uniform stress state due to the same continuous stress redistribution. Small joint restrictions in rotational angles and small joint friction hindered geometry and stress distribution from changing when the support conditions changed. The importance of extending the analysis of tensegrity structures beyond form finding and sizing is thus demonstrated; prestress implementation and joint design also need attention.