Influence of high velocities and high current densities on the friction and wear behavior of copper-graphite brushes

The friction and wear behavior of Morganite CM1S powder metallurgy copper-graphite brushes at sliding velocities up to 160 m s^?1 and current densities up to 870 A cm^?2 is presented. The brushes had a cross-sectional area of 1.2 cm² and the loads employed ranged from 9 to 45 N. The wear rates as a function of velocity and the voltage drop per brush as a function of sliding velocity, brush pressure and current were determined. The wear rates under a current of 600 A are of the same order as those obtained under no current conditions. The minimum difference was obtained at a sliding velocity of 100 m s^?1 (4.1 × 10^?4 cm km^?1 with current compared with 3.4 × 10^?4 cm km^?1 without current). The wear rate exhibited by the positive brush was lower than that of the negative brush at any sliding velocity.At constant current and sliding velocity the contact voltage drop decreases with increasing brush load. The voltage drop exhibited by the positive brush is always lower than that of the negative brush. The contact voltage drop varies little with sliding speed when the current and the brush load are kept constant. At constant brush load and constant sliding speed the voltage drop increases monotonically when the current is increased. It has been determined that local rotor waviness, even of small amplitude, can produce sufficient brush bouncing to cause excessive sparking which results in pronounced damage to the brushes and rotor surface.     

Friction and wear properties of two types of copper — Graphite brushes under severe sliding conditions

High speed dry friction experiments using two copper-graphite brushes against an AISI 4340 steel rotor were conducted at sliding velocities up to 230 m s⁻¹ and at current densities up to 526 A cm⁻². One brush was a commercial powder metallurgy (PM) specimen and the other was a graphite fiber-Cu/Sn matrix composite material. The composite brush was prepared by a proprietary process of liquid-metal infiltration and was run with the graphite fibers perpendicular to the rotor surface. The coefficient of friction was determined as a function of velocity, the wear rates were determined as a function of velocity and the voltage drop was determined as a function of velocity and current. The results show that the coefficient of friction exhibited by the PM brush is lower than that of the composite material at any velocity tested. The wear rates without current are much higher for the PM than for the composite brush, but they are of the same order when a current of 600 A is passed through. The voltage drop at the brush-rotor interface shows a similar variation with velocity for the two brushes, but the variation of the voltage drop as a function of current is different for the two specimens. The voltage drop increases almost linearly with increasing current for the PM brush. For the composite brush it exhibits a sharp increase up to about 50 A and then varies very little up to the maximum current of about 600 A. The damage done to the rotor surface in the case of heavy sparking is more pronounced with the PM brush than with the composite brush. It appears that the difference between the high temperature mechanical properties of the two types of brushes is responsible for their different behavior under severe sliding conditions.     

High speed tribological properties of graphite fiber/ Cu-Sn matrix composites

High speed dry friction experiments of graphite fiber/Cu-Sn matrix composites against steel were conducted at sliding velocities up to 235 m s^?1. The composite samples were prepared by the method of liquid metal infiltration. It has been determined that the friction coefficient and the wear rate depend on the amount of tin in the matrix, orientation of fibers relative to the sliding surface, the sliding velocity, the graphite grain size and the degree of liquid metal infiltration within the fibers. The increase in tin content leads to a decrease in both friction and wear due to an increase in matrix hardness. Specimens tested with the fibers oriented perpendicular to the sliding surface exhibit better frictional behavior than those with fibers parallel to the sliding surface. Both friction coefficient and wear rate reach a minimum value at a velocity between about 120 and 180 m s^?1. Large graphite grain size and poor liquid metal infiltration within the fibers have a detrimental effect on wear.     

Production and performance of metal foil brushes

Electrical brushes which were made of silver, copper and aluminum foils of 12.5 and 25 μm thickness and were composed of 15–195 individual foils, were tested in purified argon on a polished copper rotor at a speed of 13 m s^?1. Brush pressures varied between 3.1 × 10³ and 2.8 × 10⁴ N m^?2 and current densities were up to about 700 A cm^?2 (about 4500 A in^?2).The observed dependence of the voltage drop across the brushes as a function of the current densities agreed closely with Holm 's contact theory as applied to foil brushes. The film resistivities were found to be near $\text{σ}{}_{\mathrm{F}}\text{= 10}{}^{\mathrm{?12}}\text{Ω}\text{m}{}^2$ for copper and silver and to be about $\text{3 × 10}{}^{\mathrm{?12}}\text{Ω}\text{m}{}^2$ for aluminum. The projected performance of foil brushes based on these results is very favorable and the future commercial use of foil brushes appears to be possible.The total loss, electrical and mechanical, through the brushes is independent of current density if the brush pressure is chosen to minimize the total loss. If so, the loss depends only on the brush speed, the hardness of the softer of the two materials involved (i.e. of foil and rotor or slip ring), the coefficient of friction and the film resistivity.Microscopic surface examinations of rotor and brushes show that the brush surface is smoothed through running the brush, whereas the rotor remains almost unaffected or is mildly roughened, as long as no arcing takes place. Arcing causes considerable surface roughening on both the brush and the rotor surface and debris is thus deposited on the rotor; this can score the brushes. Further experiments are required to determine the rate of brush wear.     

Friction and wear of sintered cast iron products

The wear and frictional characteristics of five types of sintered cast iron swarf powder, or the same powder decarbonized by a mechanical procedure, were investigated using a pin-and-disk-type apparatus. The contact pressure was 19.6 N cm^?2 and the sliding velocity varied from 0.1 to 4.0 m s^?1. The wear rate exhibited a maximum at a velocity of 1.0 m s^?1. A slight improvement was found in specimens which were decarbonized and forged before sintering. However, the wear rate at lower velocities of these specimens was inferior to that of a specimen containing 5% graphite. The wear mechanism was investigated by scanning electron microscopy and electron probe X-ray microanalysis, and it was found that oxidation wear had an important effect.     

Recent advances in electrical current collection

During the last few years several advanced rotating and linear electrical machine concepts have been developed which depend for their success on the efficient transfer of electrical current across sliding interfaces. The steady state operation of brushes at very high current densities (up to 7.75 MA m^?2) is the goal for advanced land or sea propulsion units. Even higher current densities (above 18 MA m^?2) and high sliding speeds (300 m s^?1) are required for the subsecond operation of inertial storage pulsed power sources. Speeds and current densities up to an order of magnitude higher, but for millisecond pulses, may be necessary for the linear electromagnetic acceleration of projectiles. Fundamental investigations and laboratory demonstrations of several forms of advanced current collectors for these applications are described in this paper. Monolithic brushes, metal fiber brushes and carbon fiber brushes are mentioned.     

Silicon segregation in sliding wear

Friction experiments were conducted on a couple consisting of an Fe-Ni pin sliding against a tool steel disk. The Fe-Ni pin contained a small amount of silicon (0.18%). In a mild vacuum environment (0.1 mmHg at 10% relative humidity) at loads below a critical value (13.0 N, at a sliding speed of 0.22 m s^?1), silicon segregation to the pin surface took place, resulting in the formation of a glassy film. Under these experimental conditions the friction coefficient and wear values were very low (0.28 and 5 × 10^?3mm³km^?1 respectively). At loads higher than the critical value (in the same mild vacuum environment) as well as under atmospheric conditions, no silicon segregation could be detected. The corresponding values of the friction coefficient and the wear rate were much higher (0.54 and about 0.4 mm³ km^?1 respectively). It is suggested that the beneficial glassy layer can build up only when the rate of diffusion of silicon to the surface is higher than the rate of material removal (wear).     

Some observations on two-body abrasive wear

Pin-on-disc-type two-body abrasion tests were carried out on five metals with seven particle sizes over a range of loads, lengths of travel and sliding speeds. The familiar results that two-body abrasive wear is proportional to load and to distance travelled were confirmed. The “size effect”, in which particles below about 100 μm produce progressively less wear, was shown to be independent of load, sliding speed and prior cold working. Increasing the sliding speed from 1 to about 100 mm s^?1 produced an increase in wear resistance of about 50% for AISI 1020 steel. An increase in velocity above 100 mm s^?1 had little effect on the wear resistance. Plots of the wear resistance against the hardness of the annealed metal showed significant deviations from the linear relationship reported in the literature. The result is influenced by both sliding velocity and particle size.     

Tribological Properties of Hard Carbon Films on Zirconia Ceramics

In this study, the authors investigated the tribological properties of hard diamondlike carbon (DLC) films on magnesia-partially stabilized zirconia (MgO-PSZ) substrates over a wide range of bads, speeds, temperatures, and counterface materials. The films were 2 μm thick and produced by ion-beam deposition at room temperature. Tribological tests were conducted on a ball-on-disk machine with MgO-PSZ balls, in open air of 30 to 50% relative humidity under contact loads of 1 to 50 N, at sliding velocities of 0.1 to 6 m/s, and at temperatures of 400°C. Al₂O₃ and Si₃N₄ balls were also rubbed against the DLC-coaled MgO-PSZ disks, primarily to assess their friction and wear performance and to compare it with that of MgO-PSZ balls. A series of long-duration lifetime tests was run at speeds of 1, 2, and 6 m/s under a 5 N load to assess the durability of these DLC films. Results showed that the friction coefficients of MgO-PSZ balls sliding against MgO-PSZ disks were 0.5-0.8, and the average specific wear rates of MgO-PSZ balls ranged from 1 × 10^?5 to 5 × 10^?4 mm³/N·m, depending on sliding velocity, contact load, and ambient temperature. The friction coefficients of MgO-PSZ balls sliding against the DLC-coaled MgO-PSZ disks ranged from 0.03 to 0.1. The average specific wear rates of MgO-PSZ, balls were reduced by three to four orders of magnitude when rubbed against the DLC-coaled disks. These DLC films could last 1.5 to 4 million cycles, depending on sliding velocity. Scanning electron microscopy and micro-laser Raman spectroscopy were used to elucidate the microstructural and chemical nature of the DLC films and worn surfaces.     

An equation for the abrasive wear of elastomeric O-ring materials

An empirical equation was obtained via dimensional analysis, which relates the abrasive wear volume to the friction force, specimen load, sliding distance and specimen breaking strength for O-ring materials. Wear experiments on O-rings molded from four nitrile compounds and one polyurethane material were conducted on a special pin-disc-type testing machine. Specimens cut from a size 330 O-ring were held against a roughened rotating steel wear cylinder with a load which varied from 5 to 15 lbf. Both the specimen and the wear cylinder were immersed in an abrasive mud of the type used for oil well drilling. The sliding velocity was held constant at 11 in s^?1. The wear resistance of the polyurethane was two times better than the best nitrile compound.     

Device for the control and measurement of brush forces to an accuracy of a few grams-force while monitoring brush resistance and brush wear at currents up to 500 A or more

A brush holder is described which permits the electrical performance of a brush to be tested under currents of up to at least 500 A and speeds of up to at least 70 m s^?1 and under brush forces that can be applied and maintained within close limits. Simultaneously, the brush wear can be monitored continuously.     

Friction and wear behaviour of aluminium alloy coconut shell char particulate composites

The friction and wear characteristics of Al-11.8%Si alloys containing 10–25 vol.% (3–8 wt.%) dispersions of coconut shell char particles (average size, 125 μm) were evaluated under dry conditions with a pin-on-disc machine. At the lower sliding speed of 0.56 m s^?1, the wear rates and friction coefficients of the composites decreased with increasing volume per cent of dispersed char particles in the aluminium alloy matrix. Scanning electron microscopy observations have revealed the presence of adhering shell char fragments on the worn-out surface of the composites and the average roughness R_a for the worn-out surface of the composite (Al-11.8%Si-8%char) was much less (1.9 μm) than for the worn-out surface of the matrix (3.2 μm). At the higher sliding speed of 5.38 m s^?1, the wear rates increased with increasing volume per cent of dispersed char particles in the matrix and the R_a value for the composite (Al-11.8%Si-8%char) was higher (5.2 μm) than for the matrix (4.6 μm). The worn-out surface of the composites did not show the presence of adhering shell char fragments. The reduction in wear rates and friction coefficients of composites at the lower sliding speed of 0.56 m s^?1 with respect to the matrix alloy wear was attributed to the presence of adhered fragmented bits of shell char on the wearing composite surface.     

Heat checking in the contact zone of a bearing seal (a two-dimensional model of a single moving asperity)

When two bodies are in sliding contact under heavy loads, local high temperature may occur as a result of excessive frictional heating near the contacting surfaces. Because of a combination of thermal heating and the mechanical load, the material may crack in the neighborhood of the contact zone. This phenomenon is called heat checking. It commonly occurs in mechanical seals and brakes. In recent years there has been increased emphasis on finding a solution. In this paper a simple model is proposed to determine those parameters which can be optimized to control the heat checking of the materials.Because of the size difference between the contact area and the seal, the mathematical model is represented by a half-space subjected to a fast-moving load which is distributed over a small area. The load comes from a combination of an arbitrarily distributed heat source and mechanical loads of pressure and friction. The general solutions are expressed in the form of integral equations with determined Green's functions. Numerical results for fracture criteria are then obtained using a computer program.For the current problem, a nominal pressure of 365 MPa (53000 Ibf in^?2) and a corresponding friction of 183 MPa (26500 Ibf in^?2) are used. The induced heat source is 1.5 × 10⁷ in Ibf in^?2 s^?1 (1.70 × 10⁶Jm^?2s^?1). Such a load results in fracture where the crack is first initiated immediately beneath the surface at the trailing edge of the moving load.     

The effect of flat-on-ring sample alignment on sliding friction break-in curves for aluminum bronze on 52100 steel

The effects of test sample fixturing on the interpretation of frictional break-in behavior are described for dry sliding flat-on-ring tests of CDA 688 bronze on 52100 steel. It is demonstrated that for otherwise similar test conditions (i.e. 10 N load, 20 cm s^?1 velocity, 1 μm polished block surfaces and flowing argon gas environment) tilt of the fixed flat block can affect the break-in duration for friction and for wear because of the rate at which a balance of steady state sliding surface contact conditions is achieved.     

A new concept for producing ultrafine-grained metallic structures via an intermediate strain rate: Experiments and modeling

A novel experimental methodology to produce ultrafine-grained metallic microstructures, which is applied on aluminum is proposed in this work. In fact, the ultrafine-grained aluminum polycrystal is made from commercial purity powder by a combination of hot isostatic pressing (HIP) and dynamic severe plastic deformation (DSPD). After the first step, the bulk consolidated material showed a random texture and homogeneous microstructure of equiaxed grains with an average size of 2 μm. The material is then subsequently impacted, using a falling weight at a maximum impact velocity of 9.2 m/sec. The resulting material shows a microstructure having an average grain size of about 500 nm with a strong gradient of fiber-like crystallographic texture perpendicular to the impact direction. The mechanical properties of the impacted material are then characterized under compression tests at room temperature under a strain rate of 10^?4 s^?1. The effect of the change of the deformation path on the mechanical response parallel and perpendicular to the impact direction is also investigated. These results are discussed in relation with microstructure. Further, a new extension of a micromechanical approach developed by Abdul-Latif et al., [2] is proposed to predict the grain size effect on the enhancement of the mechanical strength of polycrystals. Within the framework of small strain hypothesis, the elastic anisotropy of the grain and grain rotation are neglected for the sake of simplicity. The local inelastic deformation heterogeneity is determined through the slip theory. It is assumed that the yield strength increases linearly with decreasing grain size as in Hall–Petch relationship. It is obviously recognized that the model with its new extension describes fairly well the effect of the grain size on the strain–stress behavior of the sub-micrometer aluminum.     

Effect of Compressive Deformation on Wear Property of Extruded ZK60 Magnesium Alloy

The present study investigates the dry sliding wear behavior of precompressed ZK60 alloy using a ball-on-disc wear test machine under normal loads of 20 and 40N at a sliding speed of 0.5m s^?1 for a constant sliding distance of 1,500m. Microstructures and worn surfaces were characterized by optical microscopy and scanning electron microscopy. Microhardness measurements were taken to evaluate mechanical properties. Results showed that both the microhardness and wear resistance of uncompressived alloy can be improved with precompression. This is attributed to the grain refinement via subdivision of twin lamellae generated by compressive deformation. Furthermore, the worn surface studies showed a mixed type of wear mechanisms: oxidative and delamination wear took place under higher applied loads.     

Sliding wear characteristics of gas boronized steel

The wear characteristics and the mechanism of sliding wear of boronized steel under unlubricated conditions were studied. Characteristic wear curves of FeB and Fe₂B boride layers formed on SAE 1045 steel were similar in form. The maximum wear rates were obtained under a sliding velocity of 0.30 m s^?1 for FeB specimens and 0.50 m s^?1 for Fe₂B specimens. Under such conditions both mechanical wear caused by scratching and oxidative wear occurred. Under conditions of mild wear the wear loss was caused mainly by oxidative wear. Under conditions of heavy wear destruction of the sliding surface was caused by thermal stress. The wear debris was composed principally of iron oxides (α Fe₂O₃, Fe₃O₄) formed by oxidative wear, α iron and borides (FeB, Fe₂B) produced by mechanical wear and B₂O₃ produced by the preferential oxidation of boron in the boride layer.     

The Influence of Lubrication on Head and Disk Wear

Magnetic disks are usually lubricated with fluorocarbon-type lubricants to reduce head and disk wear during the start/stop process of the disk rotation. In this paper, the influence of disk lubrication on the tribological characteristics of the head/disk interface is investigated by pin-on-disk wear tests and the head/disk friction tests.

The anti-wear performance of a lubricant is very high. For example, a lubricant coating of 8.4 × 10^?5 mg/cm² exhibits 1/20 of the ferrite pin wear rate of an unlubricated disk. For a lubricated disk, ferrite pin wear decreases at increased sliding velocities as high as 10 m/s, while pin wear increases rapidly with increased velocity for an unlubricated disk. The lubricant used here performs well in suppressing the wear increase caused by increased load. Regarding friction characteristics, however, an excessive amount of lubricant induces severe head/disk sticking, causing head crash. With respect to head/disk sticking, the upper-limit of the amount of lubricant is 8.4 × 10^?5 mg/cm².     

High temperature tribology of ceramics

The friction and wear behaviour of self-mated couples of MgO---ZrO₂, Al₂O₃ and two types of SiSiC were studied under dry sliding conditions in a special pin-on-disc high temperature tribometer. The temperature was varied between 25 and 1000°C, and the sliding speed from 0.03 m s⁻¹ to 3 m s⁻¹. The morphology of the worn surfaces was studied by means of SEM, and their phase distribution by X-ray diffraction and TEM analyses. The results show that the wear coefficients of all couples mostly increase with increasing temperature and sliding velocity. The wear of MgO---ZrO₂ is influenced by tribo-induced phase transformations while α-Al₂O₃ retains its original structure for all test conditions. For SiSiC delamination and fatigue of the interface Si/ß-SiC predominate. At higher temperatures and sliding velocities tribo-oxidation is effective. The friction coefficients lie between 0.5 and 1.0 under steady-state conditions but for short test durations lower values can occur. The couple SiSiC/SiSiC has low friction coefficients at low sliding velocities and temperatures, even if the steady-state region is reached.     

The effect of sliding time and speed on the wear of composite materials

The friction and wear properties of an Al 201 alloy and a unidirectionally oriented graphite fiber-aluminum matrix composite (T50-Al 201) were investigated. The experiments were conducted on a pin-on-disc type friction machine. The diameter of the pin was 0.22 cm and the load 4.46 N. The sliding velocity varied between 0.17 and 0.43 m s^?1. The disc counterface was of commercially pure iron. It has been found that the friction coefficient μ and the wear rate W_L of the composite material decrease as the sliding time is increased until a steady state value is reached. The steady state wear rate is proportional to the reciprocal of the sliding speed in accord with a recently proposed model. Scanning electron microscopy and Auger electron spectroscopy observations indicate that the high initial values of μ and W_L are due to a high degree of matrix adhesion to the counterface accompanied by fiber breaking and transfer. The low steady state values of μ and W_L are due to the formation of a film that impedes adhesion and confers some degree of self-lubrication. It is suggested that the observed variation of W_L with sliding speed is related to changes in the degree of subsurface damage as the velocity is varied.