Higher Friction Values Research Articles

Impact of blend properties on die filling during tableting

Based on characterization of a wide range of fillers and APIs, thirty divergent blends were composed and subsequently compressed on a rotary tablet press, varying paddle speed and turret speed. The tablet weight variability was determined of 20 grab samples consisting of each 20 tablets. Additionally, the bulk residence time, ejection force, pre-compression displacement, main compression force, die fill fraction and feed frame fill fraction were determined during each run. Multivariate data analysis was applied to investigate the relation between the process parameters, blend characteristics, product and process responses. Blends with metoprolol tartrate as API showed high ejection forces. This behavior could be linked to the high wall friction value of metoprolol tartrate. The main responses related to the die filling could be predicted via a PLS model based on blend characteristics. Tablet weight variability was highly correlated with the variability on pre-compression displacement and main compression force. A good predictive model for tablet weight variability was obtained taking the porosity, wall friction angle, flowability, density, compressibility and permeability into account. Additionally, turret speed and paddle speed were included in the calibration of the model. The applied approach can save resources (material, time) during early drug product development.

Performance Studies of Powder-Lubricated Journal Bearing Having Different Pocket Shapes at Cylindrical Bore Surface

It becomes impossible to use conventional fluid film journal bearings in the hot working environments (500–800 °C) due to rapid thermal degradation of lubricating oils. Under this situation, powder lubricants prove beneficial in spite of high friction values associated with them in comparison to lubricating oils. Thus, reduction of friction in powder-lubricated journal bearings is an essential task for making the operation energy efficient. Hence, the objective of this paper is to explore the reduction of coefficient of friction in a powder-lubricated journal bearing employing different pocket shapes (elliptical, parabolic, rectangular, and trapezoidal) placed on bore surface. Based on the investigations reported herein, it is found that the journal bearing having rectangular pocket yields least coefficient of friction among all the cases.

Observation of the a-C:H run-in behaviour for dry forming applications of aluminium

Amorphous hydrogenated carbon coatings (a-C:H) are well known for their exceptional tribological properties and are established as tool coatings for numerous forming applications. However, utilized in dry forming processes of aluminium a premature failure of an a-C:H coated tool often occurs due to strong adhesive wear. In this paper the run-in behaviour of a-C:H is investigated and as a possible reason for the premature tool failure evaluated. Therefore, oscillating ball-on-disc tribometer tests and strip drawing tests, for a more realistic emulation of real forming processes, will be conducted. According to these tests, the run-in period of a-C:H coatings is characterized by a high friction value and adhesion tendency and thus is decisive for the tool performance. Based on a subsequent analysis of the coating wear, the predominating wear mechanisms during the run-in period are discussed. The intrinsic nanomater-scale a-C:H roughness is identified as a crucial factor determining the tribological properties of the run-in behaviour. By reducing the coating roughness prior to the forming process, the adhesion tendency and friction value can be reduced significantly. The results demonstrate the tribological performance of pre-treated a-C:H coatings for dry sheet metal forming of aluminium EN AW-5083.

Design of Variable-friction devices for shoe-floor contact

In rehabilitation training, high-fidelity simulation environments are needed for reproducing the effects of slippery surfaces, in which potential balance failure conditions can be reproduced on demand. Motivated by these requirements, this article considers the design of variable-friction devices for use in the context of human walking on surfaces in which the coefficient of friction can be controlled dynamically. Various designs are described, aiming at rendering low-friction shoe-floor contact, associated with slippery surfaces such as ice, as well as higher-friction values more typical of surfaces such as pebbles, sand, or snow. These designs include an array of omnidirectional rolling elements, a combination of low- and high-friction coverings whose contact pressure distribution is controlled, and modulation of low-frequency vibration normal to the surface. Our experimentation investigated the static coefficient of friction attainable with each of these designs. Rolling elements were found to be the most slippery, providing a coefficient of friction as low as 0.03, but with significant drawbacks from the perspective of our design objectives. A controlled pressure distribution of low- and high-friction coverings allowed for a minimum coefficient of friction of 0.06. The effects of vibration amplitude and frequency on sliding velocity were also explored. Increases in amplitude resulted in higher velocities, but vibration frequencies greater than 25 Hz reduced sliding velocities. To meet our design objectives, a novel approach involving a friction-variation mechanism, embedded in a shoe sole, is proposed.

Multi-scale study of the polishing behaviour of quartz and feldspar on road surfacing aggregate

ABSTRACTIn this paper, the exact mechanisms as to how petrographic properties of aggregate influence the polishing behaviour quantitatively were identified. Since the aggregate is composed of the different rock-forming minerals, the analyses conducted in this research project focused not only on the aggregate but also on crystals/minerals. The hardest and most abundant rock-forming minerals found on earth – quartz and feldspar – were investigated with polishing tests with the Aachen Polishing Machine on granite aggregates from four sources which exhibit different mineralogical properties. Changes of the aggregate texture and minerals were studied based on texture measurements and skid resistance measurements obtained by means of the Pendulum Test and the Wehner/Schulze device. The influences of mineral composition and crystal size on the changes of the micro-texture and skid resistance of the aggregates were determined by comparing the four granites. The polishing and wearing behaviour of quartz and various feldspars (albite, microcline and plagioclase) were characterised quantitatively by means of a spectral analysis and abrasion analysis. The results show that the micro-texture of exposed minerals is greatly smoothened on wavelengths from 100 to 500 μm. Fine-grained quartz or feldspar crystals tend to be rougher than medium of large-sized crystals. Quartz exhibits more favourable properties than feldspar between 100 and 500 μm; this is especially true for fine quartz crystals. The correlation that more content of coarse-grained quartz leads to high friction value cannot be proved in this research.

Mathematical Modeling and Simulation of an Occlusion Device in a Blood Vessel.

An occlusion device is placed in an abnormal opening of the heart or its surrounding vessels to regain normal blood flow. There are various occlusion devices available for treatment of various congenital heart defects like PDA, ASD, etc. However, they have limitations like residual shunting, erosion of tissue, displacement and breakage of device, thrombus formation and sudden death. To improve efficiency and to reduce failure of occlusion devices, it is important to simulate blood flow through defect before and after placement of device. It is also important to evaluate stresses and forces exerted by blood flow on device and by the device on the vessel wall. Contact friction between device and vessel wall plays a crucial role in anchoring the device. The objective is to develop a framework to determine conditions to restrict dislocation of device in terms of contact friction. Typical occlusion devices are porous initially and later due to thrombogenesis, their porosity reduces until eventually it acts as a natural permanent plug. Thus, a porous sponge is a good model for an occlusion device. The mathematical model developed here is for differential pressure causing incipient movement of device, and minimum value of contact friction for restricting movement of the device for two shapes, cylindrical and conical, in uncompressed as well as pre compressed forms. The model for differential pressure is fitted by conducting physical experiment with sponge. Mathematically, porosity is modeled using viscous resistance and inertial resistance which are calculated by experiment and simulation with ANSYS. We perform computer experiments (simulations) on a cylindrical device in a cylindrical vessel and on a conical device in a tapered vessel to determine the differential pressure across the device and hence contact friction with varied porosity under boundary conditions as in body. The contact friction required to retain device is lesser in case of conical device compared to cylindrical device. As compression of device increases, friction require to retain it decreases. Hence, lesser porosity results in larger differential pressure and lesser compression which will eventually need higher friction values to retain the device.

Structural and mechanical properties of (Cr, Ni) N single and gradient layer coatings deposited on mild steel by magnetron sputtering

Ternary single and gradient layer (Cr, Ni) N thin films were deposited on the mild steel substrate by unbalanced magnetron sputtering technique in order to evaluate mechanical properties for machine tools and automotive applications. Microstructure, chemical composition, surface morphology and phase analysis were carried out using field emission scanning electron microscopy, energy dispersive X-ray spectroscopy, atomic force microscopy and X-ray diffraction, respectively. Both single and gradient layer of (Cr, Ni) N coatings show a significant increment in mechanical properties such as hardness, adhesion strength and surface roughness along with the reduction of friction coefficient. Mechanical tests revealed that the hardness of the gradient layer increased up to 3.1 times due to the formation of Cr2N and Ni phase whereas single layer showed the least friction. Single layer CrNiN layer exhibited 27.2% less surface roughness (Ra) in comparison with gradient layer. High values of surface roughness, hardness, thickness and friction could be correlated with high film-to-substrate adhesion (Lc2) for the gradient layer.

Lateral coefficient of friction for characterizing winter road conditions

Real Time Traction Tool (RT3)-Curve was used in this study to evaluate the effect of ice and snow on tire–road lateral friction coefficient, herein referred to as the Halliday Friction Number (HFN). The field experiments for the study were performed in winter 2012–2013 on the University of Alberta’s test road facility in Edmonton, Alberta. Each run was repeated at three target speeds under varied road conditions, bare dry, dry with ice patches, ice, and three levels of snow accumulation. No considerable correlation was found between vehicle speed and the friction measurements for bare dry, ice- and snow-covered conditions. Expectedly, the bare dry asphalt concrete surface had the highest HFN, the presence of ice reduced the dry surface friction by 55%. The accumulation of snow on the dry surface reduced the HFN further than ice, by 69, 75, and 81% for light, moderate, and heavy snow, respectively. A falling trend was observed for friction as more snow accumulated on the ground. Analysis of the effect of number of truck passes over ice at −3.5 and −5 °C showed that ice can become more slippery after each pass of traffic. A similar analysis for snow revealed that more passes over moderate snow will compact the fresh snow into a slippery surface. For light snow, even at low temperatures (<−10 °C), passes of traffic will melt the snow through frictional heat and result in higher friction values.

Nb-B-C thin films for electrical contact applications deposited by magnetron sputtering

The high wear resistance, high chemical inertness, and high electrical conductivity of magnetron-sputtered transition metal diborides make them a candidate material for sliding electrical contacts. However, their high hardness makes it difficult to penetrate surface oxides, resulting in a high electrical contact resistance. In this study, the authors have investigated how the contact resistance can be improved by the formation of softer Nb-B-C films. The Nb-B-C films were deposited by magnetron sputtering and shown to exhibit a nanocomposite microstructure consisting of nanocrystalline NbB2−x grains with a solid solution of C separated by an amorphous BCx phase. The formation of the BCx phase reduces the hardness from 41 GPa for the NbB2−x film to 19 GPa at 36 at. % C. As a consequence the contact resistance is drastically reduced and the lowest contact resistance of 35 mΩ (contact force 5 N) is achieved for a film containing 30 at. % C. However, crack formation and subsequent delamination and fragmentation is observed for the C-containing Nb-B-C films in tribology tests resulting in high friction values for these films.

Double nut ball screw with improved operating characteristics

The performance and availability of production machines is strongly affected by the operating characteristics of the installed ball screws. In this context, the value of preloading is an important property greatly influencing the quality of the feed motion as well as the expected service life. Dynamic applications often require ball screws with high preloading values. As a result, such systems typically suffer from high friction values with the involved wear and heat generation. This paper presents a novel design principle for ball screws, allowing for a considerable reduction of the preloading and hence an overall improvement of the operating characteristics.

Friction analysis of thermally sprayed coatings finished by ball burnishing and grinding

Thermally sprayed coatings offer a promising approach as efficient method to increase the wear-resistance of sheet metal forming tools. However, the roughness of thermally sprayed surfaces is quite high. The use of these coatings for deep drawing tools results in poor sheet surface qualities and low drawing ratios. Because it is suspected that high friction is the reason for the low drawability, hard metal coatings (WC–12Co), deposited by high velocity oxygen fuel flame-spraying, were machined by grinding and ball burnishing to improve their friction behavior and the accuracy of the tool shape. The investigation was conducted by plane strip drawing tests. Strips of high strength steel were mated with these novel and effective coatings at different normal contact pressures and drawing velocities. Uncoated friction elements made of C60 steel were considered as reference during the analysis. The results revealed that coated but unmachined friction elements showed high friction values, which led to scratch marks on the sheet surface after drawing. Applying the finishing processes, the friction coefficient could be reduced significantly. Additionally, deep drawing tests were carried out to determine the drawing ratio for coated, unmachined as well as for processed, coated dies. Thermally sprayed and ball burnished as well as thermally sprayed and ground coatings are feasible for deep drawing. Due to the post treatment, the drawing ratio β = 1.8 was increased to 2.0. This is consistent to the results of the friction tests.

Surface roughness of three types of modern plastic bracket slot floors and frictional resistance

To quantitatively analyze the surface roughness of the slot floors of three types of modern plastic brackets and to measure static frictional force during sliding mechanics in vitro. Control groups comprised stainless steel brackets and monocrystalline ceramic brackets. Test groups comprised three types of 0.022-in slot, Roth prescription, plastic, maxillary right central incisor brackets. Test groups included glass fiber-reinforced polycarbonate, filler-reinforced polycarbonate, and hybrid polymer with inserted metal slot brackets. The static frictional resistance caused by sliding movements with an archwire (stainless steel) in vitro was quantitatively analyzed. Both scanning electron microscope and three-dimensional optical surface profiling were used. Scanning electron microscope and three-dimensional optical surface profiler revealed that all as-received brackets had irregular slot floor surfaces, and both irregularity and roughness increased after the archwire sliding test. The ceramic brackets in the control group showed significantly lower surface roughness values and higher frictional values during the archwire sliding test compared with the other brackets. The glass or filler-reinforced plastic brackets exhibited significantly higher static frictional values than the metallic slot type brackets (P < .001). The hybrid polymer with inserted metal slot brackets showed relatively lower surface roughness and frictional values compared with the stainless steel control bracket. Glass or filler-reinforced plastic brackets showed higher frictional resistance than metallic slot-type brackets. A plastic bracket with inserted metal slot may be the best choice among plastic brackets for low frictional resistance and to avoid damage from sliding movements of the archwire.

A Survey of Soil-Reinforcement Interface Friction

The most important aspect of reinforced earth is the sliding shear resistance between the soil and reinforcements expressed in terms of a sliding friction angle or a coefficient of friction. A number of different procedures have been followed in order to quantify this physical property and as a result there is a wide range of reported values. In the present paper, the friction coefficient values of smooth and rough brass strips as determined by modified direct shear tests and by pull out and wall rotation tests in a model box are given and interpreted. For smooth strips, it is found that direct shear and pull out tests give comparable results and the friction coefficients are not influenced by the density. For rough strips, however, pull out tests may give much higher friction values than the direct shear tests as density increases. In wall rotation tests which are an attempt to represent the friction mobilization mechanism in reinforced earth, the friction coefficient values appear to be limited by the values obtained from direct shear tests for both smooth and rough strips, irrespective of density

Surface interactions in lubrication of titanium, aluminium, and titanium–aluminium alloys with the ionic liquid [C2mim]Tf2N under increasing temperature

The tribological behaviour and surface interactions of Al2011/AISI 52100 steel at 25°C, 100°C, and 200°C, using the room-temperature ionic liquid (IL) 1-ethyl-3-methylimidazolium bis(trifluoromethanesulphonyl)imide ([C2mim][Tf2N]) as lubricant have been studied and compared with that of Ti6Al4V and Ti Grade 3. The best tribological performance at room temperature is that of Ti6Al4V, while Ti Grade 3 shows the lowest friction coefficients and wear rates at high temperature. Al2011 shows constant friction coefficients with no running-in periods. The tribochemical interactions between [C2mim][Tf2N] and Al2011 produce a tribolayer on the steel ball containing sulphur from the IL anion combined with Cu from the Al2011 alloy, while wear debris are mainly composed of aluminium oxide and fluoride. Al2011 wear debris morphology evolves from powder at 25°C, to adhesion flat flakes at 100°C, to abrasion chips at 200°C. The titanium alloys show high running-in friction values, with running-in distances that increase with increasing temperature. Titanium Grade 3 shows an adhesive wear mechanism, with material transference to the steel ball and formation of a tribolayer on the wear track as a result of the tribochemical interactions with the IL anion. At 200°C, Ti6Al4V shows a more abrasive wear mechanism than Ti Grade 3, due to severe corrosion of the steel ball.

Friction properties of clay compounds

Friction is one of the most important factors in energy consumption and in the lifetime of machinery used to prepare and produce heavy-clay products such as brick and roof tiles. Production of traditional heavy-clay products often demands additives to change the properties of the clay processed or those of the products. The effects of different additions of organic and non-organic additives on the friction coefficients of clay compounds were investigated. As opening agents sand and ground brick dust, while as pore forming additives sawdust and ground sunflower seed shell were mixed to the clay in 0, 3 and 5 wt.%. The water content value was set to 20% and 24% of the compounds. Investigation was performed by a special tribometer up to the plastic deformation of a sample. The sand and brick dust lowered the coefficient of friction to different extents. Raising the amount of sand from 3% to 5% caused some 30% decrease in friction while the effect of brick dust was not so significant. Sunflower seed shell lowered friction at 24% moisture content while increasing it at 20% moisture content. Addition of saw dust resulted in higher friction value as compared to pure clay. The results revealed how additives behaved in the clay matrix.

Microtribology friction measurements on microtextured silicon

This paper describes the results of microtribology sliding experiments that were carried out on a range of micropatterned silicon surfaces. The objective of this work was to identify critical pattern features that control friction and fluid interaction on surfaces under a broad range of contact conditions and develop models to explain these relationships. These scratch tests were carried out in air, and with deionised water. Tests were carried out under a range of test loads and speeds, from low to high. It was found that in the dry tests, the measured friction was higher on the patterned surfaces than on the non-patterned surface. For the liquid experiments, there was a reduction in friction in some cases, dependent on the pattern and conditions used, indicating that the liquid acted as a lubricant. The greatest reduction in friction was obtained with a herringbone pattern aligned perpendicular to the direction of travel of the probe. However, for higher load conditions, high friction coefficients were measured for many of the patterns. The surfaces of the tested silicon samples were examined with optical microscopy and in high resolution scanning electron microscopy so that the interactions between the probe and the sample could be determined. It was found that the high friction values that had been observed in many experiments were correlated with considerable damage to the structure of the patterned silicon surface.

Ionic Liquids as Lubricants of Titanium–Steel Contact. Part 2: Friction, Wear and Surface Interactions at High Temperature

The tribological behaviour and surface interactions of titanium sliding against AISI 52100 steel have been studied at 200 and 300 °C in the presence of two commercial imidazolium room temperature ionic liquid (ILs): 1-octyl-3-methylimidazolium tetrafluoroborate (L108) and 1-hexyl-3-methylimidazolium hexafluorophosphate (LP106). L108 presents the higher thermal stability but gives higher friction coefficients and wear rates than LP106, with long running-in periods and high friction values, both at 200 and 300 °C. Friction and wear rates for LP106 are lower and decrease as the temperature increases from 25 to 200 °C. At 200 °C, LP106 shows a constant friction coefficient, without running-in, produces a mild wear on titanium and no surface damage on steel. LP106 fails at 300 °C, close to its degradation temperature, due to tribochemical decomposition through partial dissociation of the hexafluorophosphate anion, with formation of a phosphorus-rich layer on the steel ball, while the titanium wear track surface is heterogeneous, showing regions with the presence of fluoride and others with the presence of phosphate. When the steel ball is substituted for a ruby sphere under the same conditions at 300 °C, a low friction coefficient and mild wear is observed, due to the higher stability of the LP106 lubricant at the ruby–titanium interface. The friction coefficients, wear mechanisms and surface interactions have been studied by means of friction-distance records, SEM, EDX and XPS.

Effect of temperature on the frictional behaviour of an aluminium alloy sliding against steel during ball-on-disc tests

Ball-on-disc tests were carried out at elevated temperatures to determine the friction behaviour of the 7475 aluminium alloy against steel, resembling the material mating during hot extrusion. Friction coefficient was found to increase markedly with rising temperature, suggesting that the commonly accepted assumption of an unvaried friction coefficient in the FE simulation of the aluminium extrusion process involving temperature evolution is incorrect. A theoretical analysis showed that the high friction values measured during high-temperature ball-on-disc tests could be partly attributed to the increased plowing friction caused by the severe deformation of the aluminium alloy. The effect of temperature on the stress and strain distributions at the ball/disc interface was revealed by means of three-dimensional finite-element (3D FE) simulation. Increased deformation in the aluminium disc around the contact point with rising temperature was confirmed. The alterations of the strain state and stress state around the contact point were considered responsible for the generation of wear debris, the oscillations of friction coefficient along the wear track and the increase of friction coefficient with rising temperature.

Tribological, physicochemical and thermal study of the abrupt friction transition during carbon/carbon composite friction

Due to their thermo-mechanical performances maintained at high temperature, C/C composites demonstrate their advantages in high energy aeronautical braking. During friction a typical tribological behavior was observed. At first, an abrupt transition of friction coefficient takes place systematically from a weak value (∼0.15) to a high value (∼0.35). Before this transition, an extremely weak wear rate is associated to a weak friction coefficient value and no gas exchanges occur in the contact, this regime being called non-reactive. From the abrupt transition, a very high wear rate and strong gas exchanges associated to the high friction value occur (reactive regime). In this paper, for a better understanding of the mechanisms governing the abrupt transition during C/C composites tribological, mechanical, thermal and physico-chemical data are analyzed. Friction tests are performed on a 3D C/C composite consisting of PAN-based fibers and CVI pyrocarbon matrix in using a pin-on-disc tribometer equipped with a mass spectrometer allowing the in situ gas exchange analysis (CO 2 production, O 2 consumption) in the contact. To follow the disc surface temperature evolution before, during and after the transition, a thermal infrared camera is used. After friction, worn surfaces and interfaces are characterized by optical microscopy and scanning electron microscopy. In a final analysis, a mechanism is proposed to explain the abrupt transition.

CuO, ZrO 2 and ZnO nanoparticles as antiwear additive in oil lubricants

This work presents and discusses the antiwear behaviour of nanoparticle suspensions in a polyalphaolefin (PAO 6). CuO, ZnO and ZrO 2 nanoparticles were separately dispersed at 0.5%, 1.0% and 2.0%wt. in PAO 6 using an ultrasonic probe for 2 min. AW properties were obtained using a TE53SLIM tribometer with a block-on-ring configuration. Tests were made under a load of 165 N, sliding speed of 2 m/s and a total distance of 3.066 m. Wear surfaces were analysed by scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) after wear tests. The study led to the following conclusions: all nanoparticle suspensions exhibited reductions in friction and wear compared to the base oil; the suspensions with 0.5% of ZnO and ZrO 2 had the best general tribological behaviour, exhibiting high friction and wear reduction values even at low deposition levels on the wear surface; CuO suspensions showed the highest friction coefficient and lowest wear per nanoparticle content of 2%; and the antiwear mechanism of nanoparticulate additive was produced by tribo-sintering.