Magnitude Of Friction Coefficient Research Articles

Study on the effect of the addition of bulk and exfoliated graphitic carbon nitride on the dry sliding behavior of a commercial friction material formulation through pin on disc and subscale dynamometer analysis

A novel analysis was conducted on the friction and wear properties of a commercial friction material formulation with the addition of bulk (named: gCN) and exfoliated (named: TEX6) graphitic carbon nitride in varying quantities: 4.5, 9, 13.5, and 18 wt%. The analysis was two-fold. In the first part, the samples were tested in the form of pins on a pin on disc testing equipment. The typical trend was that the friction coefficient magnitude increased with the addition of gCN. On the other hand, the friction coefficient decreased with the TEX6 addition. The pin wear reduced with the addition of gCN until 9 wt%. Beyond this amount, an increase in pin wear was observed. The TEX6 samples generally noted a decrease in pin wear with the increase in its content. An improvement in the characteristics of the secondary plateaus was seen with the increase in both gCN and TEX6 content. Through this study, the 13.5 wt% addition of both gCN and TEX6 was considered to display permissible friction and wear magnitude, and desirable extension and compaction of secondary contact plateaus. In the second part of the study, the formulation with 13.5 wt% of gCN and TEX6 was tested on a subscale dynamometer to replicate a real-life braking scenario. The friction and wear magnitudes were similar to the pin on disc study and the extension of the secondary contact plateaus was better than the virgin formulation. This study provides a complete analysis of the feasibility of the inclusion of g-C3N4 variations in automotive braking applications.

Reynolds number effect on statistics of turbulent flows over periodic hills

The wall-resolved large-eddy simulations of turbulent flows over periodic hills are carried out to study the Reynolds number effect on flow statistics. Five different Reynolds numbers ranging from 2800 to 37 000 are considered. The present simulations are validated by comparing the time-averaged flow statistics with those from the literature. The Reynolds number effect is first examined on the skin friction and pressure coefficients, the isosurfaces of p′ and Q criteria, and the vertical profiles of flow statistics. The results show that (1) at most locations the magnitude of friction coefficient decreases with the increase in Reynolds number, while the pressure coefficient varies in the opposite direction; (2) smaller turbulence structures arise at higher Reynolds numbers; and (3) the mean velocities and Reynolds stresses in general exhibit asymptotic behaviors with the increase in Reynolds number. The statistical properties of turbulence structures are further examined via the probability density function and time correlation of velocity fluctuations. At last, the dynamics in the separation bubble is investigated by examining the flow statistics and the budget equation of mean kinetic energy (MKE) on the coordinate with its origin fixed at the recirculation center, and the power spectral density of the velocity fluctuations. Similarities are in general observed for the mean velocities, Reynolds stresses, and the MKE budget in the rear part of the separation bubble. The mean convection term and turbulence convection term are observed playing a key role on the decrease in bubble size with the increase in Reynolds number.

A new design of soft texture analyzer tribometer (STAT) for in vitro oral lubrication study

The relative movement between food particles as well as between food particles and oral surface is considered as critically important in influencing food oral processing and sensory perception. Precise characterization of oral lubrication/friction has recently attracted a lot of attention among food researchers for insight information about the controlling mechanisms of oral sensation and sensory perception. This study is a continuation of our effort in trying to establish a feasible in vitro soft tribology technique for food oral processing and sensory studies. Specific purpose of this study is to further improve the reliability and feasibility of a TA-tribometer developed previously by the group. A lighter and soft probe was produced to make it more consistent with that of oral conditions and a new connecting mechanism between the probe and the load cell was introduced with the help of 3D printing technique. A set of near-Newtonian fluids (water, syrup solutions and glycerol) with different shear viscosity were used as testing samples to assess the reliability at a wide range of sliding speeds and load pressures. Friction coefficients obtained at various experimental conditions were integrated and plotted as Stribeck curves. Viscosity/load ratio was introduced to understand and predicate the lubrication behavior of test fluids. The calculated Viscosity/load ratio not only influenced friction coefficient magnitude but the establishment of tribological regimes. A boundary regime can be clearly seen at a low range of viscosity/load ratio (0.00569–0.0327). For viscosity/load ratios between 0.211 and 5.097, lubrication would be most likely in mixed regime. Once viscosity/load ratio goes beyond 6.627, a hydrodynamic regime becomes dominant. Results verify the feasibility of the new modification and the improved device suggests a promising application for assessment the lubrication behavior of various near-Newtonian fluids. Future experiments will involve the use of beverages and semi-solids, explore the application potential for in vitro oral lubrication studies.

Influence of Roof Shape on Snow Accumulation

The paper describes the snow accumulation issues on the buildings and structures covers. The main factors influencing the nature of the snow load formation on random shape coverings are revealed. The holding capacity dependence of incoherent snow on the coating configuration and the snow friction coefficient on the roof surface is established. The individual factors influence degree on the snow accumulation nature is revealed. An analytical dependence of the snow capacity to be retained on the coating surface from the roof outline and the snow adhesion forces on the coating material surface is proposed. Recommendations for taking into account the coating structures outline in the buildings and structures design allowing to clarify the snow accumulation features that are characteristic of this roof shape are given. The dependence in the parametric shape of the cover inclination critical angle from the magnitude of the snow friction coefficient on the roof surface is established. The obtained results allow clarifying the snow load for incoherent snow depending on the friction coefficient magnitude.

The Synergetic Effects of Surface Texturing and MoDDP Additive Applied to Ball-on-Disk Friction Subject to Both Flooded and Starved Lubrication Conditions

This paper reports a novel work on the synergetic effects of microscale surface texturing and lubricant friction modifier additive of molybdenum dialkyldithiophosphate (MoDDP) subject to both flooded and starved lubrication conditions. The experiments were performed on reciprocating ball-on-disk friction in GTL8 base oil with and without MoDDP. In the flooded lubrication condition, the test results demonstrated that the presence of the MoDDP additive contributed to lower friction coefficients and also more pronounced effect of surface textures on friction than in the case of the bare base oil. In the starved lubrication experiments, textured and texture-free surfaces in the oils with and without MoDDP additive were tested until an abrupt rising of friction coefficient was detected. The results showed that the magnitude of friction coefficient before terminating each test was the almost same for various tests, while the endurance time in different test conditions was significantly different. The textured surface exhibited longer endurance time than the texture-free surface, especially when the MoDDP additive was used. The mechanism of the synergetic effects of surface textures and MoDDP additive has been discussed based on the experimental observations in the following sections. This study provides a new idea for the application of surface texture in boundary lubrication when lubricant additive is contained in the lubricating oils.

Tribological Behavior of Oil-Lubricated Laser Textured Steel Surfaces in Conformal Flat and Non-Conformal Contacts

Changing the surface texture of sliding surfaces is an effective way to manipulate friction and wear of lubricated surfaces. Having realized its potential, we have done very extensive studies on the effects of laser surface texturing (LST, which involves the creation of an array of microdimples on a surface) on friction and wear behavior of oil-lubricated steel surfaces in the early 2000s. In this paper, we reviewed some of our research accomplishments and assessed future directions of the laser texturing field in many diverse industrial applications. Our studies specifically addressed the impact of laser texturing on friction and wear of both the flat conformal and initial non-conformal point contact configurations using a pin-on-disk test rig under fully-flooded synthetic oil lubricants with different viscosities. Electrical resistance measurement between pin and LST disks was also used to determine the operating lubrication regimes in relation to friction. In conformal contact, we confirmed that LST could significantly expand the operating conditions for hydrodynamic lubrication to significantly much higher loads and slower speeds. In particular, with LST and higher viscosity oils, the low-friction full hydrodynamic regime was shifted to the far left in the Stribeck diagram. Overall, the beneficial effects of laser surface texturing were more pronounced at higher speeds and loads and with higher viscosity oil. LST was also observed to reduce the magnitude of friction coefficients in the boundary regime. For the non-conformal contact configuration, we determined that LST would produce more abrasive wear on the rubbing counterface compared to the untreated surfaces due to a reduction in lubricant fluid film thickness, as well as the highly uneven and rough nature of the textured surfaces. However, this higher initial wear rate has led to faster generation of a conformal contact, and thus transition from the high-friction boundary to lower friction mixed lubrication regime, resulting in a rapid reduction in the friction coefficient with increased ball wear. Higher density of LST, lower oil viscosity, and hardness of counterface steel surface facilitate an increase of the initial wear, which promotes friction reduction. This phenomenon can be beneficial if the initial accelerated wear on the counterface is acceptable in intended applications. This paper summarizes our experimental investigation of the effect of LST on friction properties and lubrication regime transitions in a unidirectional sliding contact.

Cutting Process Tribometer Experiments for Evaluation of Friction Coefficient Close to a CFRP Machining Operation

Finishing operations of Carbon Fibre Reinforced Polymers (CFRP), such as drilling or milling are known to generate heavy tool wear. Consequently, ultra-hard cutting materials like diamond-coated carbide tools or PCD-tools are utilised. To further improve the lifetime of these tools, it is recommended to adjust the tool geometry, process parameters and coating to the relevant application. Usually, the process optimisations base on knowledge from fundamental experiments in similar materials or rarely on tool wear modelling. To improve suitable force and tool wear models, the friction coefficients between tool and workpiece under realistic conditions are required.To address the lack of friction coefficient at the tool/workpiece interface, a cutting process tribometer (CP-T) is utilised. This tribometer enables for friction measurements between diamond-coated cemented carbide pins and freshly generated CFRP surfaces downstream of a orthogonal cutting process. The tribological conditions in the measurement setup (e.g. velocity, pressure and presence of loose particles) are very similar to those occurring at the tool/workpiece interface during the cutting process. It is shown that the magnitude of friction coefficient as well as the influence of the fibre orientation is much lower compared to results of pin-on-disk tribometer (POD-T) experiments. This measurement data is of high relevance for machining force and wear modelling as well as cutting process optimisation.

On the residual stress modeling of shot-peened AISI 4340 steel: finite element and response surface methods

Shot peening is a well-known process in applying the residual stress on the surface of industrial parts. The induced residual stress improves fatigue life. In this study, the effects of shot peening parameters such as shot diameter, shot speed, friction coefficient, and the number of impacts on the applied residual stress will be evaluated. To assess these parameters effect, firstly the shot peening process has been simulated by finite element method. Then, effects of the process parameters on the residual stress have been evaluated by response surface method as a statistical approach. Finally, a strong model is presented to predict the maximum residual stress induced by shot peening process in AISI 4340 steel. Also, the optimum parameters for the maximum residual stress are achieved. The results indicate that effect of shot diameter on the induced residual stress is increased by increasing the shot speed. Also, enhancing the friction coefficient magnitude always cannot lead to increase in the residual stress.

Controlling the Quality of Two-Way Euler/Lagrange Numerical Modeling of Bubbling and Spouted Fluidized Beds Dynamics

We numerically simulate three-dimensional fluidized beds of monodisperse spheres using a two-way Euler/Lagrange method. Particles trajectories are tracked in a Lagrangian way, and particles collisions are computed by a soft sphere model. The fluid conservation equations are written in a classical Eulerian fashion and are locally averaged on cells 1 order of magnitude larger than particles. We detail the equations of the model and their numerical implementation. We study the influence of numerical parameters and simulation domain size on computed results in a biperiodic fluidized bed. We then validate the model against theoretical results for a bubbling fluidized bed and against experimental data for a single- and multi-nozzle spouted bed. Finally, we investigate the influence of the Coulomb friction coefficient magnitude on a single-nozzle spouted bed dynamics in order to emphasize the importance of tangential friction in such processes.

Modeling of equal channel forward extrusion force using response surface approach

Equal channel forward extrusion is a new severe plastic deformation method that has been developed in recent years. This study investigates the effect of significant parameters on the equal channel forward extrusion pressing force. First, the process was modeled by finite element method and has been validated using experimental results. Next, response surface method and analysis of variance were applied to investigate the influences of equal channel forward extrusion parameters such as friction coefficient magnitude, length-to-width ratio and main deformation zone height on the pressing force. Finally, a new formula is presented for prediction of equal channel forward extrusion pressing force using statistical modeling.

Investigation of Efficiency of Use of High-Temperature Greases in Steel Rolling - Part 1

It has been shown the efficiency of use of high-temperature greases “Natol-H-1(2)” in steel rolling in the result of research of friction coefficients (steel-to-steel) for various rolling pressures on application of greases “Natol”. Medium-temperature putted greases Natol-M-1(2) and pumpable consistent greases “Natol-Р-1(2, 3)” are not suitable for steel rolling. The influence of various components and their percentages in greases on magnitude of friction coefficients was found.

Local buckling behavior of core plate in all-steel buckling restrained braces

This study presents the finite element analysis results of the proposed all-steel buckling restrained brace (BRB) models. The objective of the analysis is to conduct a parametric study to investigate the effect of interface detail of BRBs on local buckling behavior of the core plate. Moreover, the effects of magnitude of friction coefficient (¦I) between the core and the buckling restraining mechanism (BRM) contact, the size of gap between the core and the BRM, and the unbonding material thickness on local buckling behavior of core plate in BRBs are investigated through the finite element analysis method. Based on the results, the interface configuration, the magnitude of friction coefficient at the interface, and the size of gap could significantly affect the local buckling behavior of core plate in all-steel BRBs. However, the thickness change in unbonding material was not found to considerably affect the overall response of BRBs. Finally, an appropriate interface detail in BRBs was suggested for design purposes based on the finite element analyses results.

Experimental Investigation on Friction Coefficient of Composite Materials Sliding Against SS 201 and SS 301 Counterfaces

In this research, friction coefficients of composite materials such as gear fiber reinforced plastic (gear fiber) and glass fiber reinforced plastic (glass fiber) are investigated and compared. In the experiments, gear fiber and glass fiber slide against different austenitic stainless steels such as stainless steel 201 (SS 201) and stainless steel 301 (SS 301). Experiments are carried out at low loads 2, 4 and 6N, low sliding velocities 0.2, 0.4 and 0.6 m/s and relative humidity 70%. The obtained results reveal that in general, friction coefficient of gear fiber and glass fiber increases with the increase in normal load and sliding velocity. Results show that friction coefficient of glass fiber-SS 201 pair is the highest and gear fiber-SS 301 pair is the lowest within the observed range of normal load and sliding velocity. On the other hand, it is found that friction coefficient of glass fiber-SS 301 pair is slightly higher than that of gear fiber-SS 201 pair. During the running-in process, friction coefficient of gear fiber and glass fiber steadily increases with the increase in rubbing time and after certain duration of rubbing, it remains constant regardless of the counterface material. The obtained results reveal that for the observed range, the influence of normal load on the frictional properties of gear fibe and glass fiber is greater than that of sliding velocity. At identical operating conditions, the magnitudes of friction coefficient of gear fiber and glass fiber are different depending on normal load, sliding velocity and counterface material.

Friction mechanism in diamond-like carbon film sliding against various counterbodies

In the present study, the tribological properties of hydrogenated diamond-like carbon (DLC) coating were investigated using various sliding counterbodies such as steel, Al2O3 and SiC balls. A friction mechanism based on surface chemistry and mechanical properties of sliding interfaces is proposed. During initial sliding passes, the high friction coefficient is governed by the surface roughness of the sliding interfaces. However, for longer sliding cycles, the sliding interfaces get smoothened and magnitude of friction coefficient is reduced. Under these experimental conditions, appropriate description of friction involves formation of carbonaceous transferlayer on ball sliding surface have been investigated.

온도변화에 따른 주석 도금한 전기 커넥터의 미동마멸 부식 거동

In this study, we conduct fretting corrosion tests on tin-plated brass coupons to investigate the effect of temperature on fretting corrosion for various span amplitudes. We prepare a coupled fretting corrosion specimens using a tin-plated brass coupon with a thickness of <TEX>$10{\mu}m$</TEX>. One specimen is a flat coupon and the other specimen is a coupon with a protuberance in 1 mm radius, which is produced using 2 mm diameter steel ball. We conduct fretting corrosion tests at <TEX>$25^{\circ}C$</TEX>, <TEX>$50^{\circ}C$</TEX>, <TEX>$75^{\circ}C$</TEX>, <TEX>$100^{\circ}C$</TEX> by rubbing the coupled coupons together at the contact between the flat and protuberance coupons. We measure electric resistance of the contact during the fretting corrosion test period. There is increase in resistance with fretting cycles. It is found that rate of increase in electric resistance becomes faster with increase in testing temperature. Magnitude of friction coefficient increases with fretting span amplitudes. And, change in friction coefficient becomes desensitized to the increment in span amplitude. Assuming that failure cycle is the cycle with an electric resistance of <TEX>$0.01{\Omega}$</TEX>, we find that failure lifetime (<TEX>$N_f$</TEX>) decreases with increase in testing temperature. Furthermore, based on the assumption that the damage rate of the connector is inversely related to the failure cycle, we calculate the activation energy for fretting damage to be 13.6 kJ/mole by using the Arrhenius equation. We propose a method to predict failure cycle at different temperatures for span amplitudes below <TEX>$30{\mu}m$</TEX>. Friction coefficients generally increase with increase in span amplitude and decrease in testing temperature.

Beyond surface selection: The impact of different methodologies on tribological measurements

Emerging links between food friction and sensory texture have prompted an increase in food tribology studies. Soft tribological surfaces (e.g. elastomers) are used in these studies to mimic conditions in the oral cavity. However, measurement protocols and tribological surfaces vary among food tribological studies. Because the majority of these studies use empirical measurements, it is difficult to quantitatively compare their results. Although the effects of surface hydrophobicity, roughness, and modulus on friction coefficient measurements have been examined, there has been little study of variability due to differences in measurement parameters. Therefore, this study examined the effect of different measurement parameters on friction coefficient magnitude and variation. Tribological data were affected by multiple measurement parameters. A measurement protocol that yielded repeatable measurements with low variation was proposed based on these results. It is suggested that a standard protocol for food tribological measurements be adopted to enable proper data comparison among studies.

Friction Coefficient and Wear Rate of Different Materials Sliding Against Stainless Steel

This paper examines the relation between friction/wear and different types of steel materials under different normal loads and sliding velocities and to explore the possibility of adding controlled normal load and sliding velocity to a mechanical process. In order to do so, a pin on disc apparatus is designed and fabricated. Experiments are carried out when different types of disc materials such as stainless steel 304 (SS 304), stainless steel 316 (SS 316) and mild steel slide against stainless steel 304 (SS 304) pin. Variations of friction coefficient with the duration of rubbing at different normal loads and sliding velocities are investigated. Results show that friction coefficient varies with duration of rubbing, normal load and sliding velocity. In general, friction coefficient increases for a certain duration of rubbing and after that it remains constant for the rest of the experimental time. The obtained results reveal that friction coefficient decreases with the increase in normal load for all the tested materials. It is also found that friction coefficient increases with the increase in sliding velocity for all the materials investigated. Moreover, wear rate increases with the increase in normal load and sliding velocity. At identical operating condition, the magnitudes of friction coefficient and wear rate are different for different materials depending on sliding velocity and normal load.

Friction Coefficient of Polymer and Composite Materials Sliding against Stainless Steel

An endeavor has been made to study and compare the friction coefficient of different polymer and composite materials. Experiments were carried out when stainless steel 304 (SS 304) pin slides on different types of composite and polymer materials such as cloth reinforced ebonite (commercially known as gear fiber), glass fiber reinforced plastic (glass fiber), nylon and polytetrafluoroethylene (PTFE). Experiments were conducted at normal load 5, 7.5, 10 N, sliding velocity 0.5, 0.75, 1 m/s and relative humidity 70%. Variations of friction coefficient with the duration of rubbing at different normal loads and sliding velocities were investigated. Results show that friction coefficient varies with duration of rubbing, normal load and sliding velocity. In general, friction coefficient increases with the increase in normal load and sliding velocity for all the tested materials except nylon. At identical operating conditions, the magnitudes of friction coefficient are different for different polymer and composite materials.

Effect of Load and Sliding Velocity on Friction Coefficient of Aluminum Sliding Against Different Pin Materials

<b> </b>In the present research, friction coefficients of aluminum sliding against different pin materials are investigated and compared. In order to do so, a pin on disc apparatus is designed and fabricated. Experiments are carried out when different types of pin such as aluminum, copper and brass slide on aluminum disc. Experiments are conducted at normal load 5, 7.5 and 10 N, sliding velocity 0.5, 0.75 and 1 m/s and relative humidity 70%. Variations of friction coefficient with the duration of rubbing at different normal load and sliding velocity are investigated. Results show that friction coefficient varies with duration of rubbing, normal load and sliding velocity. In general, friction coefficient increases for a certain duration of rubbing and after that it remains constant for the rest of the experimental time. Moreover, the obtained results reveal that friction coefficient increases with the increase in normal load and sliding velocity for all the tested pairs. The magnitudes of friction coefficient are different for different material pairs depending on normal load and sliding velocity.

Frictional behaviour of engineering surfaces in overall lubrication regimes of point contacts

The aim of the present paper is to study experimentally and numerically the frictional behaviour of engineering surfaces within all lubrication regions of point contacts. For this reason, a numerical solution proposed elsewhere by the current authors, which can predict friction under the different lubrication modes of elastohydrodynamic, mixed, and boundary lubrications, is introduced. Based on a deterministic model of mixed lubrication, the solution was combined with the variation of the lubricating films’ physical state during the transition of lubrication modes. Results show that roughness amplitude has a great effect on the transition of friction regimes. In addition, it is also observed that variation of the friction coefficient has nearly the same trend as the true contact area ratio in the mixed lubrication state. Meanwhile, it is concluded that transverse roughness has better film-forming capacity than longitudinal roughness and thus leads to a lower magnitude of friction coefficient if the operating conditions are the same. Analysis of the mechanism of friction behaviour suggests that the true contact area ratio determines the friction behaviour of engineering surfaces in mixed lubrication. In smooth contacts, the comparison of experiment tests and simulation results suggests that friction variation results from gradual change of the liquid lubricant to solid-like matter with diminishing film thickness.