A pin-on-disc investigation of novel nanoporous composite-based and conventional brake pad materials focussing on airborne wear particles

Wear particles originating from disc brakes contribute to particulate concentration in the urban atmosphere. In this work novel nanoporous composite-based and conventional brake materials were tested against cast-iron discs in a modified pin-on-disc machine. During testing airborne wear particles were measured online and collected on filters, which were analysed using SEM and EDX. The morphology of airborne wear particles containing elements such as iron, oxygen, and copper is presented. These results show that two of the nanoporous materials generated 3-7 times less airborne wear particles than the conventional materials. Both the conventional and nanoporous materials displayed a bimodal number distribution.     

A model for lubricated wear in a pin-on-disc configuration

An attempt is made to describe analytically wear in a lubricated pin-on-disc configuration. More than one wear mechanism may be operating simultaneously in a given situation but often the wear rate is controlled by a single dominant mechanism. In the model presented we consider two major modes of wear, namely adhesive wear and fatigue wear. An example demonstrating the calculation is provided.     

Wear behaviour of hydrogenated DLC in a pin-on-disc model test under lubrication with different diesel fuel types

The wear behaviour of hydrogenated diamond like-carbon (DLC) coating in DLC/steel tribological contact in a pin-on-disc model test under lubrication with two diesel fuels is presented in this work. The first diesel fuel was standard EN590 that contained ester-based antiwear additives. In contrast to EN590, the second diesel fuel, called GDK650, did not contain antiwear additives. It was experimentally observed that the antiwear additives are detrimental to the DLC. The effects of load, speed and temperature on the DLC and steel counterbody wear were investigated. Steel counterbody wear volume was found to be not affected by pressure, temperature, speed and lubricant, whereas the DLC-coating revealed correlation between the parameters and wear rate. Regarding the results of the tribological tests under both diesel lubrications, new mathematical wear laws were developed.     

A friction model for microforming

For the simulation of metal forming processes, input data relating to the tool–workpiece interface is necessary. For microforming applications, this input data becomes very much more critical and traditional methods are not realistic. This paper describes an approach that seeks to describe friction by modelling the geometric surface roughness of the tool. This finite-element-based model has been validated experimentally in terms of loads and metal forming using the ring test and actual surface measurements. It enables more accurate and also more flexible modelling of friction.     

An application of frictional criteria for determining galling thresholds in line contact tests

In this study the change in the kinetic coefficient of friction was investigated during a line contact threshold galling testing procedure. A substantial increase in the friction was measured at the onset of galling whereas the friction remained relatively constant during non-galled tests. From the results of this study, it appears that an increase in the kinetic coefficient of friction can be used as a necessary condition in the identification of galling. This condition will reduce the subjectivity in identifying galling in cases where galling is questionable. Results are presented for three types of stainless steel.     

A friction model evaluated with results from a bending-under-tension test

The friction in stamping is an important process parameter to control the flow of material in the tool. Consequently, it is also an important parameter in the design process of new stamping tools when numerical simulations of the forming operations are performed. In this work an advanced friction model is evaluated, which considers properties of surface topography, lubricant, sheet material, and process parameters such as sliding speed and pressure. The evaluation is made by comparing theoretical results with experimental ones obtained in a bending-under-tension friction test. The results show conformance in behaviour between the friction model and the experimental work. Furthermore, a model, which considers the influence of bulk plastic strains on the real area of contact, is investigated. The developed model predicts that the effective hardness of a surface is reduced by the presence of underlying plastic flow. It is found that when the strain rates are increased, the Stribeck curve becomes flatter and mixed lubrication is introduced at lower Hersey values. The friction model clearly shows the potential of improving the FE simulations of sheet metal forming operations, in comparison to the use of the classical Coulomb's friction model.     

Comparison of threshold galling results from two testing methods

In this investigation, threshold galling tests were performed on several types of stainless steel using area contact and line contact testing procedures. The area contact test method employed was the ASTM standard G98, button-on-block test. In this test method, the end of a cylindrical button is placed in contact with a flat block at a desired contact pressure. The button specimen is rotated through one revolution and both specimens are visually inspected for the presence of galling. The line contact test method used was the button-on-cylinder test where the end of a stationary button contacts a cylinder along its length. After a given load is applied, the cylinder is rotated for a fixed number of cycles and the wear surfaces are inspected for galling. A comparison of the threshold galling results from each of the test methods is discussed in this report. Various testing conditions that influence the onset of galling are discussed, including sliding distance, surface oxide layer thickness, and stress distribution as well as the impact of the interpretation of experimental data on reported galling threshold magnitudes.     

纳米SiC对半金属摩擦材料性能的影响

通过对比试验,研究纳米SiC对半金属摩擦材料性能的影响。试验表明,在半金属材料中,用纳米SiC代替普通的SiC粉体,摩阻材料的摩擦系数并没有明显的改善,而磨损率却大大地增加。借助扫描电镜(SEM)对混合粉体和摩擦表面的观察以及试验数据的分析,找到引起磨损增大的原因。     

A soft configuration model for wearless sliding friction

A phenomenological model of soft potential configurations is proposed to describe sliding friction in the wearless regime. This model accounts for the static and dynamic behavior. The physical origin of the logarithmic speed or time dependence for the friction coefficient is described. Means to control the friction coefficient by controlling the soft defect density are discussed. This revised version was published online in September 2006 with corrections to the Cover Date.     

等离子弧淬火扫描螺距对摩擦磨损的影响

通过研究等离子弧淬火扫描螺距对表面摩擦磨损的影响，从摩擦学角度寻求等离子弧淬火的最佳范围，为今后淬火参数控制提供理论依据。研究表明：等离子弧淬火使被扫描表面产生了5μm左右的波度，促使相互运动表面产生油楔作用，使边界润滑向混合润滑状态转变，使摩擦系数降低，耐磨性提高；在扫描螺距为2．7时的摩擦磨损综合性能要好一些，而扫描螺距过大，并口扫描螺距小于淬火宽度时，都会使淬火表面质量下降，从而导致表面减摩性与耐磨性能降低，因此应当给予充分的考虑和合理的设计。     

A new tribometer for friction and wear studies of dental materials and hard tooth tissues

This paper presents a new tribometer developed for a study of the tribological behaviours of dental materials and hard tooth tissues. The device simulates oral kinematic conditions and the loading produced during masticatory process. The tribometer is similar to the existing devices regarding the kinematic features, i.e. it produces an adjustable oscillating movement. However, the device machine is equipped with a unique pneumatic system of loading controlled via computer by special software called TOOTHY. The programmable system allows easy adjusting of the loading parameters such as the magnitude of normal force and its amplitude or pattern of cyclic loading. In this way different combinations of loads can be applied thus making the investigation of different wear situations possible. The device has two full bridge strain gauges for the measurement of loading and friction forces, by which the coefficient of friction is determined.The use of the tribometer is illustrated by a comparative study of tribological behaviour of human enamel subjected to two- and three-body friction, and to two different loading patterns as well. The obtained results are discussed.     

A phenomenological friction model of tripod constant velocity (CV) joints

Constant velocity (CV) joints have been favored for automotive applications, compared to universal joints, due to their superiority of constant velocity torque transfer and plunging capability. High speed and sport utility vehicles with large joint articulation angles, demand lower plunging friction inside their CV joints to meet noise and vibration requirements, thus requiring a more thorough understanding of their internal friction characteristics. In this paper, a phenomenological CV joint friction model was developed to model the friction behavior of tripod CV joints by using an instrumented CV joint friction apparatus with tripod-type joint assemblies. Experiments were conduced under different operating conditions of oscillatory speeds, CV joint articulation angles, lubrication, and torque. The experimental data and physical parameters were used to develop a physics-based phenomenological CV joint dynamic friction model. It was found that the proposed friction model captures the experimental data well, and the model was used to predict the external generated axial force, which is the main source of force that causes vehicle vibration problems.     

Modelling of friction and wear for designing cryogenic valves

From a tribological point of view, the selection of materials for seals for valves of cryogenic rocket engines is a critical issue for the designer. Due to the lack of comprehensive information on this topic, data have been obtained in the framework of research programmes.In the first step, two polymers potentially usable for sealing cryogenic fluids were identified. They were submitted to general test conditions to gain some fundamental understanding of their tribological behaviour when immersed in a cryogenic fluid. This fundamental understanding proved to be helpful for an efficient technical approach.This approach with the test conditions as close as possible to those met in real valves has been used in the second step. In order to obtain the best information with the minimum number of tests, the statistical method of Doehlert has been adopted. The use of such a method led to the construction of surface response for modelling friction and wear. These surfaces and their associated equation are directly and easily usable by the designer. This way of treating technical tribological problem is illustrated by an example.     

3D Analysis of cold rolling using a constitutive model for interface friction

Full three-dimensional numerical analyses are carried out for the cold rolling of plates of finite width, to study the effect of the width spread during rolling. The contact and friction between roll and plate is modeled in terms of an interface constitutive model that accounts for the friction forces in the rolling direction as well as those in the transverse direction that give resistance to the width spread. At low normal pressures Coulomb friction is represented while at high normal pressure a yield stress limitation of the maximum tangential stress is incorporated, and slip as well as no slip is accounted for. Finite strain elasto-plasticity is applied for the plate material, using mostly isotropic hardening or in a few cases kinematic hardening to represent the effect of a rounded vertex on the yield surface. In addition, for a given plate thickness and degree of reduction the effect of different values of the roll radius and the effect of different values of the plate width are analysed.     

Tribological performance of raw and formulated RBD palm kernel with ZDDP additive

This study compares tribological performance of refined, bleached and deodorized (RBD) palm kernel (PK) as an alternative lubricant. An analysis was made for chemically modified RBD PK with zinc dialkyl dithiophosphate (ZDDP) additive to determine its tribological performance using modified pin-on-disc tribotester. Commercial mineral oil (SAE 40) was used as the benchmark in this study. The conditions for this experiment are sliding speed at 1.5 m/s, a normal load at 9.81 N, weight percentage of ZDDP for 0, 3 and 5%, lubricant quantity of 2.5 ml and test duration of 60 min. The findings revealed that RBD PK oil exhibits better anti-friction and anti-wear performance compared to commercial mineral oil (SAE 40). Besides, coefficient of friction is less dependent on ZDDP concentration, but anti-wear ability is dependent on the ZDDP additive concentration. ZDDP additive acts as a good anti-wear and antioxidant additive in RBD palm kernel.     

Development of a galling resistance test method with a uniform stress distribution

In this study, a new galling test method is presented along with results from a pilot testing program. The pilot program was performed on type 303 stainless steel. The new testing configuration consists of two concentrically aligned hollow cylindrical specimens loaded along their longitudinal axes. The specimens are held in alignment using a custom made alignment pin. The pin maintains concentricity between the mated surfaces while enabling the contact interface to self-align. The resulting contact surface is in the shape of an annulus and has a uniform stress distribution. The new testing configuration eliminates the stress concentration that exists in the ASTM G98 Standard Test Method for Galling Resistance of Materials. The results of this study show that the galling wear phenomena do not transition from non-galled to galled at a single value. Galling is shown to be stochastic in nature.     

Effects of silicon carbide particle sizes on friction-wear properties of friction composites designed for car brake lining applications

Apart from the service conditions during the braking (e.g. applied pressure, velocity of rotor) the friction-wear properties of friction composites used as a car brake lining are directly influenced by their composition. Among the components used for a car brake lining, the chemical and structural nature of the abrasives, jointly with the morphology and size of the particles, influence the friction parameters and stability of the composite. In the present paper the effect of silicon carbide abrasive of various particle sizes (40, 10, and 3 μm) on the friction-wear properties of friction composites based on potassium titanate ceramics is summarized. The composites with an increasing amount of the abrasive in composition (3.4, 5.6, 9.0, and 14.6 vol%) for each SiC size were prepared. The highest values of friction coefficient as well as the lowest fade for the composites containing the finest SiC fraction (median value 3 μm) were obtained. Contrary to the friction coefficient, the values of specific wear rate decrease with increasing SiC particle size. Transport of the iron particles, originated from the cast iron rotor, increases with SiC particle size and decreases with the testing temperature.     

A fully coupled thermo-mechanical model of friction stir welding

This paper presents a new developed fully coupled thermo-mechanical model of the friction stir welding process. Results indicate that the rotation of the shoulder can accelerate the material flow behavior near the top surface. The material deformation and the temperature field can have relations with the microstructural evolution. The texture of the appearance of the friction stir welds can correlate well with the equivalent plastic strain distributions on the top surface. The temperature field in the friction stir welding process is approximately symmetric to the welding line. The material flows in different thicknesses are different. The shoulder can have a significant effect on material behaviors on the top surface, but this effect is greatly weakened when the material gets closer to the bottom surface of the welding plate.     

Test results of friction factor for round-hole roughness surfaces in closely spaced channel flow of water

For examining friction-factor characteristics of round-hole pattern surfaces which are usually applied on damper seals, flat plate test apparatus is designed and fabricated. The measurement method of leakage and pressure distribution along round-hole pattern specimen with different hole area is described and a method for determining the Fanning friction factor is discussed. Results show that the round-hole pattern surfaces provide a much larger friction factor than smooth surface, and the friction factor vs. clearance behavior yields that the friction factor generally decreases as the clearance increases unlike the results of Nava’s flat plate test. As the hole depth is decreased, the friction factor is increased, and maximum friction factor is obtained for 50% of hole area. Since the present experimental friction factor results show coincident characteristics with Moody’s friction factor model, empirical friction factors for round-hole pattern surfaces are obtained by using the Moody’s formula based on curve-fit of the experimental data. Results of Villasmil’s 2D CFD simulation support the present experimental test result.     

Wear and friction behavior of sand cast brake rotor made of A359-20 vol% SiC particle composites sliding against automobile friction material

The effect of load range of 30-100 N and speed range of 3-12 m/s on the wear and friction behavior of sand cast brake rotor made of A359-20 vol% SiC particle composites sliding against automobile friction material was investigated. Dry sliding frictional and wear behavior were investigated in a pin-on-disc type apparatus. Automobile friction material was used as pin, while the A359-20 vol% SiC particle composites formed the rotating disc. For comparison, the wear and friction behavior of commercially used cast iron brake rotor were studied. The results showed that the wear rate of the composite disc decreased with increasing the applied load from 30 to 50 N and increased with increasing the load from 50 to100 N. However, the wear rate of the composite disc decreased with increasing the sliding speed at all levels of load applied in the present work. For all sliding speeds, the friction coefficient of the composite disc decreased with applied load. The worn surfaces as well as wear debris were studied using scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analyzer and X-ray diffraction (XRD) technique. At load of 50 N and speed range of 3-12 m/s, the worn surface of the composite disc showed a dark adherent layer, which mostly consisted of constituents of the friction material. This layer acted as a protective coating and lubricant, resulting in an improvement in the wear resistance of the composite.