In order to study the effect of vibration on friction at contact interface, an experimental device was designed. The slider sliding at uniform speed on the vibrator was taken as the object. The ...mechanism of friction reduction caused by normal vibration and tangential vibration at interface was analyzed by means of experiment and theory. The experimental results show that normal vibration and tangential vibration often exist simultaneously. When the interface vibrates and the direction of the tangential relative velocity between the slider and the vibrator remains constant, the average sliding friction force of the slider is the same as that without vibration, and there is no friction reduction effect. When the interface vibrates and the tangential relative velocity direction changes periodically or stick-slip, there is an obvious friction reduction effect compared with that without vibration. Coulomb's friction law was used to establish a dynamic model for simulation, and the simulation results are in good ag
Structural superlubricity is a fascinating tribological phenomenon, in which the lateral interactions between two incommensurate contacting surfaces are effectively cancelled resulting in ultralow ...sliding friction. Here we report the experimental realization of robust superlubricity in microscale monocrystalline heterojunctions, which constitutes an important step towards the macroscopic scale-up of superlubricity. The results for interfaces between graphite and hexagonal boron nitride clearly demonstrate that structural superlubricity persists even when the aligned contact sustains external loads under ambient conditions. The observed frictional anisotropy in the heterojunctions is found to be orders of magnitude smaller than that measured for their homogeneous counterparts. Atomistic simulations reveal that the underlying frictional mechanisms in the two cases originate from completely different dynamical regimes. Our results are expected to be of a general nature and should be applicable to other van der Waals heterostructures.
•An improved dynamic model of ACBB considering various friction torques and ball motion state is proposed.•The previous acoustic emission model is extended to describe more degrees of freedom of ...ACBB.•Effects of speed, load, and defect sizes on the acoustic characteristics for ACBB are investigated.•Comparison results between the proposed and reported models depict the superiority of established approach.
Due to their special characteristics, angular contact ball bearings (ACBBs) are broadly applied in various mechancial systems. The working performance of rotating machinery can be determined by the internal ACBBs. Vibration and acoustic characteristics of ACCBs in various rotating machinery are becoming increasingly significant for the high-performance mechanical systems. An in-depth recognition of acoustic characteristics of ACBB can be helpful for condition monitoring of rotating machinery. This study proposes an improved dynamic model of ACBB to consider the influences of elastic hysteresis, differential sliding friction torques, and elastohydrodynamic lubrication (EHL) rolling on the ball motion state. A modified time-dependent excitation (TDE) model is developed for describing different kinds of defects in the outer ring of ACBB. The acoustic emission models given by Sharma 8 and Patil 9 are extended to describe more degrees of freedom of ACBB. Effects of speed, load, and defect sizes on the acoustic characteristics for ACBB are investigated. Comparison results between the proposed and reported models depict the superiority of established approach here. Note that the load, speed, and defect can greatly affect the acoustic characteristics of ACBB. The results improved that this model has ability to obtain the accurate acoustic characteristics of a defective ACBB.
High entropy alloys (HEAs) show promise as materials for structural applications, even at elevated temperatures. However, their wear behaviour over a wide range of temperatures has not been ...extensively studied. CoCrFeMnNi and AlxCoCrFeNi HEAs were subjected to pin-on-disc dry sliding wear at temperatures between 25 °C and 900 °C against an alumina ball, and the tribological performance benchmarked against AISI 304 and Inconel 718. A detailed characterisation of the wear tracks using electron microscopy and surface profilometry revealed a transition in wear mechanism from abrasive wear at room-temperature to oxidative and delamination wear above 600 °C. The wear performance of the HEAs, AlCoCrFeNi in particular, is substantially enhanced with increasing temperature, surpassing that of Inconel 718 at 900 °C. The enhanced wear performance of the HEAs above 600 °C is attributed to the formation of a compact oxide scale in the contact region, and relative subsurface strengthening in the form of a fine-grained recrystallised structure containing precipitation hardening phases.
•The wear behaviour of various high entropy alloys up to 900 °C is analysed.•Wear rates of high entropy alloys decreases with increasing temperature.•High wear resistance of AlCoCrFeNi due to alumina scale and σ-phase precipitation.
Abstract
Background: Polyimide is one of the organic polymer materials with the best comprehensive performance. It has outstanding mechanical properties, excellent thermal stability and excellent ...corrosion resistance, but pure polyimide has high coefficient of friction and wear rate. By combining graphene with polyimide, the mechanical properties of the composite are significantly reformatived, and the friction coefficient and wear rate can be reduced. Objective: The molecular models were developed to study the mechanical and tribological properties of graphene as a reinforced material. Methods: In this paper, the mechanical properties and friction and wear mechanism of materials are studied by molecular dynamics method from the microscopic point of view. The Young’s modulus and hardness of composites were calculated using the strain constant method. Results: Molecular dynamics simulation results expressed that the Young’s modulus and hardness of polymer composites benefited by approximately 115% and 42%, respectively, after the addition of the graphene-reinforced material. The average friction coefficient and wear rate of polymer composites fall by 35% and 48%, respectively. Through the calculation and statistics of the micro-information in the process of friction simulation, the internal mechanism of various situations is revealed in the atomic dimension. Conclusions: Graphene can adsorb on the surface of polymer chain segment, a strong polymer matrix, through van der Waals and electrostatic forces and can effectively resist external loading.
This study investigates the critical maximum undeformed equivalent chip thickness for ductile-brittle transition (DBhmax-e) of zirconia ceramics under different lubrication conditions. A DBhmax-e ...model is developed through geometry and kinematics analyses of ductile-mode grinding. Result shows that DBhmax-e decreases with increasing friction coefficient (μ). An experimental investigation is then conducted to validate the model and determine the effect of dry lubrication, minimum quantity lubrication (MQL), and nanoparticle jet minimum quantity lubrication (NJMQL) conditions on DBhmax-e. According to different formation mechanisms of debris, the grinding behavior of zirconia ceramics is categorized into elastic sliding friction, plastic removal, powder removal, and brittle removal. Grinding forces per unit undeformed chip thickness (Fn/h and Ft/h) are obtained. The lubrication condition affects the normal force and ultimately influences the resultant force on workpiece. In comparison with dry grinding (DBhmax-e = 0.8 μm), MQL and NJMQL grinding processes increase DBhmax-e by 0.99 and 1.79 μm respectively; this finding is similar to model result. The theoretical model is then assessed by different volume fractions of nanofluids under NJMQL condition with an average percentage error of less than 8.6%.
•Maximum undeformed equivalent chip thickness model was developed and verified.•Different critical ductile-brittle transition condition under different lubrication.•Critical equivalent chip thickness decreases with a increase in friction coefficient.•Grinding forces per unit undeformed equivalent chip thickness were obtained.•Grinding behavior is divided into four stages according to debris formation mechanism.
•The analysis model for the sliding frictional and adhesive coupling contact problem between the FGPM layered half-space and an insulating indenter was proposed by using the Maugis type of adhesion ...theory and extended Amonton’s law of friction.•The numerical solutions of the integral equations for present adhesive contact problem were given by using new algorithm.•The effect of the gradient index, friction coefficient and adhesion parameters on the surface electro-mechanical response of FGPM layered half-space was carried out systemically.•Based on our model, for given values of parameters R, w, βh and μ, when value of σ0 decreases from infinity to zero there is a continuous transition from the JKR approximation to the DMT approximation.
This paper proposes an analysis model for the sliding frictional and adhesive coupling contact problem in the plane strain state between the FGPM layered half-space and an insulating indenter. The electro-mechanical properties of FGPM layer vary exponentially along the thickness direction. By applying the Fourier integral transformation and superposition principle to the governing boundary value problem, the general solution for the sliding frictional and adhesive coupling contact problem can be derived by using the Maugis type of adhesion theory and extended Amonton’s law of friction. A Cauchy singular integral equation is further derived for present problem which is then numerically solved. The primary aim of this paper is to provide insight into the likely behavior for the effect of the gradient index, friction coefficient and adhesion parameters on the surface electro-mechanical response of FGPM layered half-space. For given values of parameters R, w, βh and μ, when value of σ0 decreases from infinity to zero there is a continuous transition from the JKR approximation to the DMT approximation. The research not only helps to further understand the frictional and adhesive damage mechanisms of MEMS devices composed of FGPM, but also provides reference basis for FGPM layer experimental analysis and intelligent structure design.
Low-speed marine diesel engines are widely used as propulsion power equipment on ocean-going merchant ships. There are three important sliding friction pairs in the engine: piston assembly-cylinder ...liner, piston rod-stuffing box, and crosshead slipper-guide. They greatly influence the mechanical efficiency and stability of the engine. However, there is no effective method to measure the friction of the three friction pairs so far, which limits the improvement of tribological performance and the reduction of fuel consumption in low-speed marine diesel engines. In the current study, an indirect measurement method is presented and the measurement of the friction force under fired conditions is realized for the first time. By comparing the experimental results of friction force with the model-based numerical results, it is found that they are in good agreement. The difference between the numerical and experimental maximum friction force is less than 5%. Both theory and experiments show that the friction force reaches the maximum after the top dead center, which is about one-tenth of the gas force. The work of this paper is expected to provide an effective tool for evaluating the tribological performance of marine diesel engines.
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This paper explores the use of sliding friction dampers (SFDs) as dissipative floor connectors to mitigate higher mode effects and earthquake‐induced absolute acceleration demands on steel ...concentrically braced frame (CBF) buildings. The dampers connect each floor of the steel CBF system to the diaphragms of the gravity framing system (GFS) and they allow for a relative in‐plane movement between the two systems. For this purpose, a design methodology is first proposed to define the activation forces in the SFDs so as to ensure damage‐free seismic performance in the steel CBF and the diaphragms of the GFS. The efficiency of the design methodology is demonstrated through nonlinear response history analyses on a low‐ and high‐ductility six‐story steel CBF building. The simulation results suggest that (a) the determined activation forces of the SFDs are effective in mitigating higher mode effects and in preventing story drift concentrations regardless if capacity design is employed for the CBF system; (b) the absolute acceleration demands are reduced by approximately 50% relative to those in the rigid diaphragm counterpart. Similar reductions are achieved in the lateral drift demands of the GFS at seismic intensities with return periods of 475 and 2475 years. The reduction in the variability of seismic response, both in terms of absolute floor acceleration demands and story drift ratios (SDRs) in the CBF system, is noteworthy. Limitations as well as suggestions for future work are discussed.
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