Effect of two different rubbers as secondary binders on the friction and wear characteristics of non-asbestos organic (NAO) brake friction materials

Binder provides structural integrity by holding all ingredients in the composition of a brake friction material. The modified binders have played a major role in improving the frictional performance and thermal resistance of the friction material. The present research work evaluates the influence of secondary binders (Nitrile Butadiene rubber (NBR) and Styrene Butadiene rubber (SBR)) on the tribological performance of the friction material using a full-scale inertia brake dynamometer as per JASO C406 standard. Three brake pads were developed by varying the type and composition of secondary rubber binder (5%NBR, 5%SBR and 2.5%NBR + 2.5%SBR) with rest of the ingredients kept unaltered. It was found that the quantity of SBR rubber powder present as secondary binder improved dry and wet recovery. Friction coefficient (μ) exhibited better stability during the fade with the inclusion of both the rubber powders. The friction material with the inclusion of both the NBR and SBR rubber powders exhibited overall better performance than compared to the inclusion of only one secondary binder rubber in the composition. The worn-out surface of the developed friction materials and the counter discs were characterised using FESEM.     

Influence of amount and modification of resin on fade and recovery behavior of non-asbestos organic (NAO) friction materials

Phenol formaldehyde resin is one of the most important ingredients in friction materials that binds the other multiple ingredients firmly. The type and amount of resin in the friction material is very critical for structural integrity of the composite. Present work focuses on the influence of resin percentage and modification of straight phenolic resin by cashew nut shell liquid (CNSL) on fade and recovery behavior of friction composites developed in the laboratory. These composites were based on variation in amount of CNSL modified resin (10, 12.5, and 15 wt.%) keeping other ingredients same. Fade and recovery studies on these composites were done according to ECE R-90 regulation. The friction coefficient (μ) (all types viz. performance, fade, and recovery), extent of fade and recovery, increase in counterface temperature, wear and mechanical properties were significantly influenced by the variation in amount of resin. It was observed that with increase in amount of resin all types of μ decreased and extent of fade increased. Most of the mechanical properties and wear, however, improved with increase in percentage of resin. Since μ is more important for friction materials rather than wear, it was concluded that 10 wt.% CNSL resin was an optimum amount for best friction performance in severe operating conditions. The performance properties of these composites when compared with similar composites based on unmodified resin in earlier work, it was revealed that the modification of phenolic resin by CNSL deteriorated the fade and recovery performance.     

Contributions of adhesion and hysteresis to coefficient of friction between shoe and floor surfaces: effects of floor roughness and sliding speed

Abstract

Improving shoe–floor friction in order to reduce slip and fall accidents requires thorough understanding of the factors that contribute to friction. The friction between a sliding viscoelastic material (shoe) and a hard surface (floor) has two major components: adhesion and hysteresis. This study aimed to quantify the effects of floor roughness and sliding speed on adhesion and hysteresis to determine how each component contributes to the coefficient of friction. Experiments were conducted on a pin on disc tribometer using ceramic tiles with three levels of roughness, six sliding speeds, two common shoe materials and four liquid lubricants. Hysteresis was measured using a lubricant that minimised adhesion. Dry and lubricated adhesion was measured by subtracting hysteresis from the coefficient of friction. Analysis of variance regression models were used to determine the contributions of hysteresis, dry adhesion, sliding speed and fluid to lubricated coefficient of friction. Increased floor roughness led to increased hysteresis, while increased sliding speed reduced both adhesion and hysteresis. These findings are consistent with theory that states that larger asperities increase hysteretic deformation and that sliding speed affects deformation and real area of contact between a viscoelastic material and a hard surface. The model correctly predicted 83% of variation in coefficient of friction based on dry adhesion, hysteresis and fluid dependent constants. The sensitivity of hysteresis friction to shoe material and floor roughness indicates that optimising these parameters may be effective at reducing slip accidents on oily floor surfaces.     

On sliding friction and wear of polyetheretherketone (PEEK) composites at elevated temperatures

The friction and wear behaviour of polyetheretherketone (PEEK) composites, incorporating different amounts of short carbon fibres with different surface treatments, was studied under dry sliding conditions against smooth steel on a pin-on-disc apparatus at different temperatures. Wear of the composites was reduced considerably in all cases, but, whatever the surface treatment, wear increased with increasing temperature for all proportions off fibres. For minimum friction coefficient there was an optimum proportion of fibre volume fraction of about 10 vol.%. The effect of the fibre surface treatment was not significant for the tribological behaviour of the PEEK composites. To predict wear performance, a wear model proposed by Friedrich and Voss seemed to work properly, and, furthermore, a friction model was developed to predict the friction behaviour of PEEK composites with short carbon fibres.     

Structural and phase binder state and behavior during friction of WC-(Fe-Mn-C) composites

The work presents results of analysis of the structure of the tribosurfaces and triboengineering characteristics of two composite materials WC-(Fe-Mn-C) with different structural and phase binder state after sliding on a steel disk within the range of velocities from 1 to 4 m/s under pressure 5 MPa. It is shown that the wear rate of the material heterophase binder structure containing fine crystalline granular austenite and a significant portion of tempered martensite in the original state exceeds at slower velocities (1 m/s) 3.5 times that of the composite with the austenite binder. As the velocity grows to 4 m/s, the wear resistance of these two materials approach and then nearly coincide. In the process of friction at all sliding velocities, the binders of both composites undergo γ → α-transformation, the extent and depth of which are governed by the acceleration.     

Tribological properties of solid lubricants (graphite, h-BN) for Cu-based P/M friction composites

Cu-based P/M friction composites containing graphite at weight fractions in the range of 0%, 2%, 5%, 8%, 10%, corresponding to the hexagonal boron nitride (h-BN) at weight fractions in the range of 10%, 8%, 5%, 2%, 0%, were fabricated by a P/M hot press method, respectively. The effects of graphite and h-BN on tribological properties of Cu-based P/M friction composites were investigated on a block-on-ring tester. Worn surfaces, microstructures and wear debris of the composites were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Results indicate that lubrication effects of graphite are superior to those of h-BN. With the increase of graphite content wear rates were decreased significantly. Added graphite with low contents of h-BN can stabilize friction and wear properties of Cu-based P/M friction composites.     

Variations in the properties of the friction zone of metallic composites under the effect of electric current and Pb-Sn melt in the contact gap

The paper deals with study of the wear rate and specific surface electric resistance of the contact of metallic composites in dry friction at a high current density. Friction induced structures are shown to appear in the surface layer of the composites. The specific electric resistance of the friction induced structures is estimated quantitatively. It is found that their specific electric resistance is similar to that of the known electrographites and carbon graphites, resulting in a high contact resistance. The electric resistance and wear rate of the friction zone are diminished by the introduction of a Pb-Sn melt. The possibility of developing of an efficient sliding electric contact of two metallic composites with a metal melt between their friction surfaces is noted.     

Processes in heat dynamics and modelling of friction and wear (dry and boundary friction)

Theoretical postulates of heat dynamics and modelling of friction and wear are formulated based on a system of equations including basic dependences of friction behaviour on load, velocity, time and temperature.     

Effect of matrix morphology on the wear and friction behavior of alumina nanoparticle/poly(ethylene) terephthalate composites

The friction and wear properties of poly(ethylene) terephthalate (PET) filled with alumina nanoparticles were studied. The nanoparticle loading was varied from 1 to 10 wt.%. The nanocomposite samples were tested in dry sliding against a steel counterface. The results show that the addition of nanoparticles can increase the wear resistance by nearly 2× over the unfilled polymer. The average coefficient of friction also decreased in many cases. The nanocomposites form a more adherent transfer film that protects the sample from the steel counterface, although the presence of an optimum filler content may be due to the development of abrasive agglomerates within the transfer films in the higher wt.% samples. This study varied both crystallinity and weight percent of filler in a PET matrix in an attempt to separate the effects of nanofillers and crystallinity on the tribology.     

Thermal diagnostics of friction in plain bearings with account for speed and mode of shaft motion

The paper discusses the method of thermal diagnostics of friction in plain bearings at low speeds of rotation and reciprocating motion of the shaft, in which case the assumptions that simplify the mathematical simulation of a thermal process are inadmissible. The results of a computational experiment on the recovery of the power of frictional heat generation by temperature data are presented.     

Effect of SiC particles on the friction and wear behavior of Ti3Si(Al)C2-based composites

The non-lubricated, sliding friction and wear behavior of Ti₃Si(Al)C₂ and SiC-reinforced Ti₃Si(Al)C₂ composites against AISI 52100 bearing steel ball were investigated using a ball-on-flat, reciprocating tribometer at room temperature. The contact load was varied from 5 to 20 N. For monolithic Ti₃Si(Al)C₂, high friction coefficients between 0.61 and 0.90 and wear rates between 1.79 × 10⁻³ and 2.68 × 10⁻³ mm³ (N m)⁻¹ were measured. With increasing SiC content in the composites, both the friction coefficients and the wear rates were significantly decreased. The friction coefficients reduced to a value between 0.38 and 0.50, and the wear rates to between 2.64 × 10⁻⁴ and 1.93 × 10⁻⁵ mm³ (N m)⁻¹ when the SiC content ranged from 10 to 30 vol.%. The enhanced wear resistance of Ti₃Si(Al)C₂ is mainly attributed to the facts that the hard SiC particles inhibit the plastic deformation and fracture of the soft matrix, the oxide debris lubricate the counterpair, and the wear mode converts from adhesive wear to abrasive wear during dry sliding.     

Effects of information loss in texture details due to the PIFS encoding on load and friction in hydrodynamic bearings

Textured surfaces can significantly improve the performance of hydrodynamic bearings. However, there is no generally accepted method for their accurate and automated 3D characterization. A promising solution to this problem is partition iterated function system (PIFS) model, which encapsulates information about 3D topography of textured surfaces. However, some loss in surface details can occur. Therefore, before PIFS could be used, effects of this information loss on load and friction need to be investigated. In this study, this issue was addressed using a textured hydrodynamic pad bearing. The results obtained showed that PIFS models might become useful in characterization of textured surfaces.     

Effects of sliding velocity and normal load on friction and wear characteristics of multi-layered diamond-like carbon (DLC) coating prepared by reactive sputtering

Friction and wear tests were performed to investigate effects of sliding velocity and normal load on tribological characteristics of a multi-layered diamond-like carbon (DLC) coating for machine elements. The DLC coatings which consist of sequentially deposited gradient Cr/CrN, W-doped DLC (a-C:H:W) and DLC (a-C:H) layers were formed on carburized SCM 415 Cr–Mo steel disks using a reactive sputtering system. The tests against AISI 52100 steel balls were performed under various sliding velocities (0.0625, 0.125, 0.25, 0.5, 1 and 2 m/s) and normal loads (6.1, 20.7 and 49.0 N) in ambient air (relative humidity=26±2%, temperature=18±2 °C). Each test was conducted for 20 km sliding distance without lubricating oil. The results show that friction coefficients decrease with the increase in sliding velocity and normal load. Wear rates of both surfaces decrease with the increase in normal load. The increase in sliding velocity leads initially to the increase in wear rates up to the maximum value. Then, they decrease, as the sliding velocity increases above specific value that corresponds to the maximum wear rate. Through surface observation and analysis, it is confirmed that formation of transfer layers and graphitized degree of wear surfaces of DLC coatings mainly affect its tribological characteristics.     

Comparison of different methods to calibrate torsional spring constant and photodetector for atomic force microscopy friction measurements in air and liquid

A number of atomic force microscopy cantilevers have been exhaustively calibrated by a number of techniques to obtain both normal and frictional force constants to evaluate the relative accuracy of the different methods. These were of either direct or indirect character-the latter relies on cantilever resonant frequencies. The so-called Sader [Rev. Sci. Instrum. 70, 3967 (1999)] and Cleveland [Rev. Sci. Instrum. 64, 403 (1993)] techniques are compared for the normal force constant calibration and while agreement was good, a systematic difference was observed. For the torsional force constants, all the techniques displayed a certain scatter but the agreement was highly encouraging. By far the simplest technique is that of Sader, and it is suggested in view of this validation that this method should be generally adopted. The issue of the photodetector calibration is also addressed since this is necessary to obtain the cantilever twist from which the torsional force is calculated. Here a technique of obtaining the torsional photodetector sensitivity by combining the direct and indirect methods is proposed. Direct calibration measurements were conducted in liquid as well as air, and a conversion factor was obtained showing that quantitative friction measurements in liquid are equally feasible provided the correct calibration is performed.     

Method for continuous detection of different friction and wear regimes and application to the behaviour of ceramics at high temperature

This paper describes tests carried out on a tribometer functioning at high temperature and equipped to record continuously the various test parameters, namely, the normal applied load, the tangential friction force, the differential movement of the contact surfaces, and the temperature of the sample. Dry friction tests, conducted on two aluminas of different purities, showed different degradation regimes at both 20 and 800°C. Thus, three successive periods of functioning were identified and correlated with transitions in the mechanism of degradation of the surfaces or in the mechanism of interfacial adaptation in the contact. The first period corresponds basically to the adaptation mechanisms of the interfaces with no wear material generated. The two subsequent periods demonstrate the degradation mechanisms of the surfaces leading to the production of more or less wear material and resulting in different states of equilibrium between production, recirculation, and loss of wear debris through the contact.     

Tribological behaviour of ZnAl alloys (ZA27) compared with bronze when used as a bearing material with high load and at very low speed

In this paper the tribological behaviour of ZnAl alloy (ZA27) as a bearing material is analysed using experimentally determined Stribeck's curves. Comparisons involving friction coefficient measurements and surface damage observations are reported to illustrate the behaviour of this new kind of bearing material under critical running conditions.Numerous experimental data were therefore compiled and lead to the conclusion that ZnAl alloy is indeed a good substitute material for SAE 660 bronze: a high load (100 bar) at a slow rotation (0.2 rev min^?1) could even be supported by a neutral oil without seizure. Although the boundary behaviour of the two materials is similar at 10 and 100 bar, the ZA27 alloy clearly shows superior behaviour at 50 bar.     

Experimental investigation and simulation of heat flux into metallic surfaces due to single discharges in micro-electrochemical arc machining (micro-ECAM)

In this work, single discharges of electrochemical arc machining are examined. The heat-affected zone is analyzed, and a model is set up to simulate the heat transfer into the workpiece. As an input parameter of the simulation, the temperature of the electrochemical arc machining process was determined to be 3,500 K by means of emission spectroscopy. The simulation shows that the diameter of the heat-affected zone is less dependent on discharge duration and heat transfer due to heat flux than on the arc spot diameter. As a result of the investigation, it became clear that varying diameters of the heat-affected zone have to evolve from different diameters of the plasma channel’s arc spot. Understanding the heat distribution into the workpiece in electrochemical arc machining with micro-machining parameters allows the further development of a micro-drilling process for electrically conductive materials based on electrochemical arc machining.     

Re(de)construction-induced friction signatures of polished polycrystalline diamond films in vacuum and hydrogen

Pin-on-flat SEM tribometry was performed with polished, mostly C(100)-textured and acid-cleaned polycrystalline CVD diamond films heated to 950°C then cooled to room temperature. Testing in~1.33×10^-3 Pa = 1× 10^-5 Torr vacuum was followed by similar experimentation in 13 to 40 Pa (0.1to 0.3 Torr) partial pressures of 99.999%-pure H₂. In vacuum, all tests showed the characteristic step function-with-trough coefficient of friction (COF) signatures previously hypothesized as footprints of wear- and thermal desorption-induced generation, re(de)construction and passivation of the danglingσ bonds on the interacting surfaces. In hydrogen, all wear tracks exhibited stepfunction-like COF curves caused by adsorbate de(re)sorption on heating and cooling. A distinct re(de)construction COF trough obtained at the highest temperatures could be duplicated during repeated sliding in the same track on a large number (but not all) of the wear paths. The repeatable, incremental reduction in COF at the onset of heating and its substantial reduction on final cooling are attributed to tribocatalytically enhanced dissociative chemisorption of molecular hydrogen. The wear rates of the polished diamondon the pin tip, as controlled by the progressively reduced unit stresses caused by the enlargement of the wear scar, are between3.9×10^-16 and 2.6×10^-16m³/(N m) in P_{H 2}, in good agreement with previous data. This revised version was published online in June 2006 with corrections to the Cover Date.