Friction Coefficient Resistance Research Articles

A Novel Ladder-Shaped Bridge Finned Tube for Convective Heat Transfer Enhancement

Abstract In order to improve the convective heat transfer efficiency of a shell-and-tube heat exchanger, a novel ladder-shaped bridge finned tube (LBFT) is presented. The LBFT possesses outer low helical integral fins, two layers of staggered transverse bridge, upper passage, middle passage, and bottom passage. The convective heat transfer performance of the LBFT is studied and experimental results show that the Nusselt numbers outside the tube and the overall heat transfer coefficients of the LBFT are significantly greater than those of the smooth tube. The bridges, bridge roots, and pores formed on the outer fins contribute to the larger heat transfer coefficient. Both the Nusselt number and the overall heat transfer coefficient decrease, while the friction resistance coefficient increases with outer helical fin pitch increasing and bridge width increasing. As the Reynolds number increases, the comprehensive performance evaluation criterion (PEC) decreases at first and then increases. The maximum PEC occurs at the Re number of 2300 and is up to 1.34.

Improvement of the mechanical and tribological behaviors by Ti-C interlayer for diamond-like carbon films on nitrile butadiene rubber

To improve the mechanical and tribological properties of diamond-like carbon (DLC) film on nitrile butadiene rubber, the DLC/Ti-C bilayer films are fabricated by a dual-target magnetron sputtering method. The morphological characterization results indicate that the roughness of DLC/Ti-C first decreases and then increases slightly with the increase of Ti content and surface roughness of all modified samples is lower than that of nitrile butadiene rubber (NBR). Raman spectra show that the change of Ti content in the Ti-C interlayer has no effect on the ratio of sp2 to sp3 bonds of surface DLC film. The nanoindentation results show that the Ti content significantly affects the hardness, Young’s modulus, hrex/hmax and elastic recovery (%). Notably, the DLC/Ti-C films with a Ti content of 12.32 ∼ 14.77 at. % show the lowest coefficient of friction (CoF, 0.1 ∼ 0.2) and superior wear resistance due to the increased transformation of sp2 into sp3 in the films. This study unravels that the DLC films' superior mechanical and tribologic performance could be realized by regulating the Ti content in the Ti-C interlayer.

Increased interaction between Ultra-High Molecular Weight Polyethylene fibres and epoxy matrices for advanced composite materials

Ultra-High Molecular Weight Polyethylene (UHMWPE) fibre is considered as an excellent strengthening component due to its high specific stiffness and strength, good wear resistance, low coefficient of friction and great chemical resistance. However, very weak interfacial adhesion with resin has hampered the development of UHMWPE-based composites with high performance. With the aim of improving the interface adhesion properties, the surface of the fibres has been decorated with PE crystal by a suitable chemical treatment. Adhesion property has been examined by pull-out tests, while tensile tests have been performed to evaluate the strength of the fibre. Preliminary results indicate that, compared to unmodified UHMWPE fibres, the overall mechanical properties of the composite with treated UHMWPE fibres and epoxy resin have been greatly improved. Moreover, the applied treatment does not weaken the strength of the fibre. The study results imply that the modified fibres have a great potential for use as reinforcing elements in high-performance composites.

R260 Rayının Mantar, Gövde ve Taban Kısımlarının Tribolojik Özelliklerinin Araştırılması

It is important to know tribological properties of rail and wheel for predicting the service life of both components. In railway, the tribological system is open and the harsh working condition can be seen as the contact stress between rail and wheel is extremely high. The behaviour of the tribological system varies depending on the chosen rail-wheel materials and also third body (water, humidity, lubricant, debris and combination of these elements). The coefficient of friction and wear resistance are strongly dominated by chosen material and its properties. Up to now, a number of researchers focused only on the tribological properties of the head of rail material. Recent experimental study has devoted to investigate the tribological properties of head, web and foot sections of R260 rail, as the mechanical properties of rail changes from the head to the foot of rail. Different sections of R260 rail have been evaluated in terms of microstructural, hardness, tribological and wear resistance properties. According to obtained results of the ball-on-disc wear tests, the highest wear resistance belongs the head of R260 rail as expected. The coefficients of friction of the web, head and foot of R260 have been found as 0.39, 0.35 and 0.38, respectively.

Mechanical and tribological characterization of graphene nanoplatelets/Al2O3 reinforced epoxy hybrid composites

This study aimed to determine the effects of varying Aluminum oxide (Al2O3) and Graphene Nanoplatelets (GNP) concentrations on epoxy-based polymer composites’ mechanical and tribological characteristics. Al2O3 (1 to 5 wt.%) and GNP (0 to 1 wt.%) reinforced epoxy matrix composites were produced with sonication and stirring magnetic route. Mechanical, microstructural and tribological properties of composites were investigated via X-ray diffraction (XRD), Archimedes’ Method, Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Spectroscopy (EDX), Raman spectroscopy, hardness, impact, tensile, compressive and wear tests. The highest ultimate tensile strength and compressive strength were found at 62.83 and 137 MPa for 3 wt.% reinforced Al2O3 epoxy matrix composites, and it was observed that there was a 38% and 17% increase compared to neat epoxy, respectively. The highest impact strength was found for 3 wt.% Al2O3 and 1 wt.% GNP-reinforced composites and increased by 19% compared to neat epoxy. The best hardness was found for 3 wt.% reinforced Al2O3 and 0.25 wt.% GNP-reinforced composite, and there was a 22% increase in hardness compared to neat epoxy. Also, the highest wear resistance (1.686x10−6 mm3/Nm) and lowest Coefficient of Friction (COF, 0.13755) were determined for the same specimen.

Friction and wear properties of polyacrylonitrile‐ and pitch‐based carbon fiber‐reinforced polymer matrix composites containing silicon carbide nanoparticles

AbstractThis study investigated the friction and wear properties of high‐strength polyacrylonitrile (HS‐PAN)‐, high‐modulus polyacrylonitrile (HM‐PAN)‐, and high‐modulus pitch (HM‐pitch)‐based carbon fiber‐reinforced polymer matrix composites (CFRPs) with silicon carbide (SiC) nanoparticles. The fiber orientation of the CFRP specimens was set as 8‐ply unidirectional laminates, denoted as [0]8. Ball‐on‐disk (cyclic) wear testing was accompanied by laser, digital, and scanning electron microscope observations. For the HS‐PAN CFRP, the coefficient of friction and wear resistance were the highest among the HM‐PAN and HM‐pitch CFRPs and wear resistance was improved by the addition of SiC‐nanoparticles. Furthermore, there was no significant enhancement of wear resistance for the SiC‐nanoparticle‐filled and nanoparticle‐unfilled HM‐PAN CFRP. The wear resistance was improved by adding SiC‐nanoparticles for the HM‐pitch CFRP.

A practical empirical formula for the calculation of ship added friction-resistance due to (bio)fouling

An accurate prediction of the ship added friction-resistance due to (bio)fouling is important in the maritime sector in view of environmental sustainability. For industrial needs, the prediction method must not only be accurate but also be practical. An empirical formula is proposed in this study for the calculation of ship added friction-resistance due to (bio)fouling. An optimization method was applied, in which the objective function is defined as the least mean square error between the predicted results and data derived from Demirel et al.'s diagrams. The mathematical model to be fitted with the data involves power functions whose form resembles the functional relationship between the added friction-resistance coefficient and Reynolds number, and between the added friction-resistance coefficient and roughness ratio. The coefficients, powers and constant in the mathematical model were determined by using a nonlinear generalized reduced-gradient method. Comparisons of the formula predictions with CFD calculation results for a flat plate and the Kriso Container Ship show a small percentage difference of less than 5%. Further, the formula predictions differ less than 12% from the results obtained from methods utilizing topographical roughness parameters available in the literature.

Olive pit powder as multifunctional pigment for waterborne paint: influence of the bio-based filler on the aesthetics, durability and mechanical features of the polymer matrix

In this work, olive pit powder has been added into a polyurethane-acrylate top-coat paint, evaluating the effect of four different filler amounts on the aesthetics, durability and mechanical features of the composite coatings. The samples were characterized by electron microscopy and colorimetric analyses to assess the possible change in morphology and appearance introduced by the bio-based additive. The durability of the coatings was evaluated by salt spray chamber and UV-B exposures, which evidenced the water absorption tendency of the lignocellulose-based filler and its severe chemical-physical degradation, respectively. Otherwise, the filler based on olive pit causes an increase in the mechanical features of the polymeric matrix, such as abrasion resistance, hardness and static coefficient of friction. Although the coating containing high concentrations of olive pit powder demonstrated poor durability, this study revealed that the bio-based filler can be employed as multifunctional pigment, providing particular aesthetic features, but also improving the mechanical durability performance of polyurethane-acrylate paints.

NON-BIODEGRADATION OF POLYMERIZED POLYETHYLENE RADICALS AT SELECTED DUMPSITES IN KEFFI, NASARAWA STATE NIGERIA

Polythene and plastic bags pollutes the soil because these are non-biodegradable, and so, they cannot be degraded by the action of the microbes. Therefore, they remain in the soil and will pollute the soil. Polyethylene Terephthalate (PET) is therefore belongs to the class of polymers known as polyesters. In the natural environment, PET can be degraded by thermal oxidation, but hydrolytic cleavage and photo-oxidation initiated by UV light are more common under ambient conditions. In general, polyethylene offers excellent chemical and impact resistance, electrical properties and low coefficient of friction. It is considered a dielectric material. In addition, polyethylene are lightweight, easily processed and offer near-zero moisture absorption. In this study, selected dumps were covered in Keffi. The laboratory analysis was conducted. It was found out that there are two categories of Polyethylene with different properties, characteristics and functions. These are the High Density Polyethylene (HDPE) and Low Density Polyethylene (LDPE). The analysis revealed that T4 and T5 Isolate after drying contained more density of 8-9% degradation of high density polyethylene. While in the second tests of Low Density Polyethylene the T2, T7 and T8 have the highest coefficient of degradation. The residual plot of the dry weight polyethylene T9 has the highest weight in the model.

Cooling performance in a minichannel heat sink with different triangular pin-fins configurations

With the continuous progress of automotive new energy technology, the motor has become an important part of the power system, and the heat dissipation of insulated-gate bipolar transistors (IGBT) determines the reliability of the power system. Minichannel structure can be added to the thermal management system of new energy vehicles to improve the heat transfer capacity. Due to the growth of the boundary layer in the smooth minichannel flow channel, the cooling performance improvement was limited. Pin-fins and rib structures were used to break the boundary layer and increased the heat transfer area to enhance the heat transfer capacity. In this study, a numerical simulation model of minichannel with triangular pin-fins with different rotation angles was established and calculated using the SST k-omega method. The temperature field, velocity field, pressure, and vortex distribution under different configurations were discussed in detail. The jet area formed by the prism wall and the side wall of the minichannel would impact the wall and reduce the growth of the boundary layer. However, the stagnation area generated in the center and corner will reduce the improvement of heat transfer capacity. The thermo-hydraulic characteristics of different configurations at different Reynolds numbers (Re), such as Nusselt number (Nu), Darcy friction resistance coefficient (f), and performance evaluation criterion (PEC), were analyzed. As Re increased, the best and worst configurations changed, the best configuration changed from the 90°–120° structure to the 120°–120° structure, and the worst configuration changed from the 75°–60° to the 60°–60° structure. When the Re = 663, the influence of the front and rear rotation angle on the cooling performance was explored. When the rotation angle was closer to 60°, the cooling performance of the minichannel was better. And the closer the rotation angle was to 120°, the cooling performance was better. This has a reference effect on the design of minichannel heat sinks.

The Mystery and Clarity of Leonardo da Vinci's Coefficient of Friction

The science of friction has been using the coefficient of friction as the main quantitative characteristic of the friction process for more than five centuries. The concept of the coefficient of friction as a characteristic of the resistance to the movement of rubbing surfaces in a hidden form was formulated by Leonardo da Vinci: "Each rubbing body exerts a friction resistance equal to one quarter of its weight, provided that the flat plane is in contact with the polished surface". Two centuries later, the coefficient of friction appeared explicitly, in the form of a formula, in the works of G. Amontons. It is clear that the coefficient of friction is a convenient friction parameter, easily determined in the experiment. However, what is the physical meaning of resistance to the movement of rubbing surfaces? Modern engineering believes that the coefficient of friction has no physical meaning. Thermodynamic analysis of the friction process is performed. The evolutionary patterns of the behavior of the friction contact are shown. A structural-energy interpretation of the logic of the coefficient of friction (resistance) is proposed: accumulation of potential energy of defects in the crystal structure of the deformable volume by friction contact. The static potential energy of the formed defects in the crystal structure of the contact is a measure of the decrease in the kinetic energy of the relative motion of the rubbing surfaces. Such a formulation of resistance to movement under friction has a deep physical and clear meaning for a scientist and engineer. The substantiation of Leonardo da Vinci's formulation of friction that the friction resistance should be equal to 0.25 is given. The interpretation of this rule is given for the case of friction in mechanisms (machines).

Comparative evaluation of frictional characteristics between nano coated and non coated orthodontic brackets and arch wire configuration-An experimental in vitro study.

To compare and assess the coefficient of frictional resistance between nano-coated orthodontic brackets and orthodontic archwires with conventional orthodontic brackets and archwires. In this experimental study, 128 samples were divided into 4 groups consisting of 32 orthodontic wires and brackets in each group. The samples were randomly allocated into GROUP A- ZNO nanoparticle coated archwires and brackets, GROUP B ZNO nanoparticle coated bracket and conventional archwire, GROUP C-ZNO nanoparticle coated archwire and conventional bracket, and GROUP D- conventional archwire and bracket after positioning them on special jigs frictional resistance was studied and evaluated. Bon - Ferroni test was used for inter group comparison and one way ANOVA was used for intr-group comparison. The lowest mean frictional resistance is seen with Group A (nanocoated archwire with nanocoated bracket) N = 0.3401 ± 0.420; and highest with Group D (conventional brackets with conventional archwires) N = 0.8413 ± 0.60. a significant difference in mean frictional resistance was observed between the groups (P ≤ 0.01). The frictional resistance for the groups was in the following order from lowest to highest: group A < B < C < D. The study showed decreased friction in ZNO nanoparticle coated archwires and brackets than conventional archwires and brackets.

Influence of different arc erosion durations on the wear properties of carbon skateboards/contact wires under low temperature

The probability of arc erosion occurring is greatly increased with the continuous rapid development of high-speed railways, especially when the contact wires are covered with ice due to a cold climate environment. In this study, the wear properties of two kinds of carbon skateboards (copper-impregnated and pure carbon skateboard) with different surface treatments by arc erosion were investigated under rigid temperature environmental conditions by a friction tester. Scanning electron microscopy and energy dispersive spectrometry were used to analyse the surface morphology and composition of the wear scar at the erosion interface, which contributed to identifying its damage characteristics and wear mechanism. The results show that the coefficient of friction of the copper-impregnated carbon skateboard varied from 0.034 to 0.1, while that of the pure carbon skateboard ranged from 0.031 to 0.088. Both were positively correlated with the arc erosion duration. The contact resistance of the contact pair increases with a gradual extension of the arc erosion duration. A comparison of the coefficient of friction and contact resistance, and wear rate shown that the wear properties of the two carbon skateboards are significantly different due to different arc erosion durations, and the evolution law of the wear mechanism during sliding wear is discussed.

Predicting the Mine Friction Coefficient Using the GSCV-RF Hybrid Approach

The safety and reliability of a ventilation system relies on an accurate friction resistance coefficient (α), but obtaining α requires a great deal of tedious measurement work in order to determine the result, and many erroneous data are obtained. Therefore, it is vital that α be obtained quickly and accurately for the ventilation system design. In this study, a passive and active support indicator system was constructed for the prediction of α. An RF model, GSCV-RF model and BP model were constructed using the RF algorithm, GSCV algorithm and BP neural network, respectively, for α prediction. In the GSCV-RF and BP models, 160 samples complied with the prediction indicator system and were used to construct a prediction dataset and, this dataset was divided into a training set and a test set. The prediction results were based on the quantitative evaluation models of MAE, RMSE and R2. The results show that, among the three models, the GSCV-RF model’s prediction result for α was the best, the RF model performed well and the BP model performed worst. In the prediction for all the datasets obtained by GSCV-RF model, all the values of MAE and RMSE were less than 0.5, the values of R2 were more than 0.85 and the value of R2 of the passive and active support test sets were 0.8845 and 0.9294, respectively. This proved that the GSCV-RF model can offer a more accurate α and aid in the reasonable design and the safe operation of a ventilation system.

Preparation and properties of CoCrMo-Ni@Al2O3 composite by selective laser melting

In this study, nickel coated alumina (Ni@Al 2 O 3 ) particles were prepared via electroless plating, and ball milled with CoCrMo alloy powder in the proportion of 0 wt% to 2 wt%. Then the Ni@Al 2 O 3 reinforced CoCrMo alloy were manufactured by selective laser melting (SLM) process. The microstructure, mechanical property and wear resistance performance of the CoCrMo alloy were characterized. By increasing the content of Ni@Al 2 O 3 particles from 0 wt% to 1.0 wt%, the CoCrMo alloy kept flawless with the highest hardness of 435 HV 1 and best abrasion performance, which could be attributed to the pinning dislocation slip effect of Al 2 O 3 phase and the metallurgical bonding of Ni phase with CoCrMo alloy during the SLM process. When the content of Ni@Al 2 O 3 increased to 2 wt%, the inconsistent fluidity between Ni@Al 2 O 3 and CoCrMo powders has led to the uneven distribution of mixed particles during the powder spreading process, which resulted in the lowest compressive strength and wear resistance. • In the field of joint replacement surgery, the SLM-processed CoCrMo alloy implants are gaining more popularity for their excellent mechanical properties. In order to promote the wear resistance and prolong the service life of implants, small amounts of ceramic particles can be added into the alloy matrix. • In this work, we have fabricated the uniformly core-shell structure of Ni coated Al 2 O 3 particle by electroless plating, and the CoCrMo composites with different amounts of Ni@Al 2 O 3 particles were prepared by SLM technology. As the Ni phase acted as a bridge during the melting process, the surface of CoCrMo-Ni@Al 2 O 3 composites were smooth without obvious cracks. Moreover, the CoCrMo composites demonstrated better wear resistance and stable coefficient of friction for the introduction of Al 2 O 3 .

Flow fluctuations and flow friction characteristics of lead-bismuth eutectic in a vertical tube under rolling conditions

The effects of rolling motion on the flow characteristics of lead-bismuth eutectic in a vertical circular tube are investigated experimentally. The results show that the pressure drop and flow rate of the forced LBE circulation fluctuate under rolling conditions. The effect of rolling motion on the flow characteristics decreases with the increase of the corresponding static Reynolds number. The rolling motion may increase the time-averaged frictional resistance coefficient of the LBE when flow rate is low. The critical value of the impact Reynolds number is about 125,000. As a result, the time-averaged flow rate decreases. The analysis shows that it is mainly the transverse additional force caused by the rolling motion increases the frictional resistance of the LBE. An empirical correlation is established to predict the frictional resistance coefficient under rolling conditions. This study may be useful in the design and safety analysis of LBE as a working fluid under moving conditions.

Synergistic effect of graphene and β-Si3N4 whisker enables Si3N4 ceramic composites to obtain ultra-low friction coefficient

This work aimed to further enhance the tribological properties of Si3N4-based ceramics by introducing β-Si3N4 whisker (β-Si3N4w) into multilayer graphene (MLG)/Si3N4 ceramics to inhibit the reduction in the hardness of ceramic matrix induced by MLG. Therefore, the Si3N4 ceramics containing 1 wt% MLG and 1–5 wt% β-Si3N4w were prepared. The tribological properties of specimens at room temperature were evaluated using a ball-disk rotating tribo-tester. The coefficient of friction (COF) and wear resistance of MLG/β-Si3N4w/Si3N4 ceramic composites (MwSNCC) were analyzed. The existence of MLG and β-Si3N4w enabled the dry sliding COF of MwSNCC to notably reduce to 0.04–0.07, which was 89.43–95.40% lower than that of monolithic Si3N4 ceramics, and the wear resistance was enhanced by 33–62.40%.

Waste rock segregation during disposal: Calibration and upscaling of discrete element simulations

Large volumes of waste rock are produced during mining operations and are often disposed of in large piles on the surface. Segregation caused by particle sizes and deposition methods usually leads to heterogenous internal structures, increasing the risk for hydrogeotechnical and geochemical instabilities. Disposal optimization could improve the geotechnical stability of piles but are difficult to test at field scale. Discrete element methods are therefore used to investigate waste rock flow behaviour. The main challenge is, however, their calibration and extrapolation to field scale. A new calibration process was proposed and tested in this study based on laboratory repose angle tests and field natural repose angles. Calibrated models were then used to reproduce waste rock segregation, which was compared with laboratory and field measurements. Results indicated that numerical models calibrated using repose angle tests were able to reproduce waste rock segregation, and friction and rolling resistance coefficients were scale dependent. • A new method was proposed to calibrate discrete element models for waste rock disposal simulations. • The calibration process was validated at laboratory scale and then extrapolated to field scale. • Repose angle tests and segregation tests were designed for model calibration and validation at laboratory scale. • Input parameters such as friction and rolling resistance coefficients are scale dependent for segregation simulations.

Experimental Research on the Flow and Heat Transfer Characteristics of Subcritical and Supercritical Water in the Vertical Upward Smooth and Rifled Tubes

Experiments were conducted to investigate the heat transfer and flow characteristics of the vertical upward smooth and rifled tubes from subcritical to supercritical pressure. The distributions of wall temperature and heat transfer coefficient (HTC) were obtained, and the HTC correlations and friction resistance coefficient correlations were fitted with experimental data. In addition, the influences of heat flux and type of tube on heat transfer performance were analyzed. The research shows that heat flux has different influences on the heat transfer characteristics under different pressures. The increase in heat flux improves the heat transfer characteristics in the nucleate boiling region, yet it leads to the advance in heat transfer deterioration. However, for supercritical water, the increase in heat flux reduces the heat transfer ability. In addition, using the rifled tube not only improves the heat transfer performance, but also inhibits the occurrence of heat transfer deterioration. The fitted correlations have great predictive ability for the heat transfer coefficient and friction resistance coefficient, and the average relative fitting errors are limited to 20%.

Effects of Streptococcus mutans and their metabolites on the wear behavior of dental restorative materials

The wear behavior of dental restorative materials is highly related to the biolubricating medium in the oral environment. Bacteria, along with their metabolic products, are essential substances in the oral cavity and have not been studied as a potential factor affecting lubrication performance during mastication. In this study, the effects of the Streptococcus mutans bacterial cells and their metabolites were investigated on the wear behavior of resin composites, polymer-infiltrated ceramic networks and zirconium-lithium silicate glass-ceramics. A reciprocating friction test and quantitative analysis of the wear morphology were utilized to determine the coefficient of friction (COF) and wear resistance of the test materials. The results showed that the bacterial metabolite medium significantly reduces the COF and wear rate of the three restorative materials and provide better protection against superficial abrasion. When tested under lactic acid medium, a key acid production in bacterial metabolites, similar wear reduction results were observed in the three materials, which confirmed that lactic acid should be accountable for the excellent lubricating property of bacterial metabolites. Furthermore, the resin composite with lower wettability exhibited a more significant wear reduction than the other two materials when lubricating with a bacterial metabolite medium. These findings provide novel insights into the biological basis of lubrication mechanisms in the oral cavity under high-loading and low-velocity conditions.