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.     

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 characteristics of powder metallurgy copper-graphite brushes at high sliding speeds

Friction experiments using several commercial powder metallurgy copper-graphite brushes against an AISI 4340 steel rotor were conducted at sliding velocities ranging from 20 to 235 m s^?1. The measured wear rates ranged from a minimum of 4.3 × 10^?5 cm km^?1 at a sliding velocity of 100 m s^?1 for a brush with high graphite content to a maximum of 8.4 × 10^?3 cm km^?1 at a sliding velocity of 230 m s^?1 for a brush with high metal content. The coefficients of friction ranged from a minimum of about 0.08 to a maximum of about 0.47 and were greatly affected by the presence of oxide layers at the sliding interface. Almost all the brushes exhibited some degree of edge breaking. The velocity at which edge breaking occurred was dependent on the powder grain size. Brushes with a large grain size seem to exhibit edge breaking at a lower speed than brushes with a fine grain size. High interface temperatures which occur at high sliding speeds result in melting of the lead-tin binders used in most powder copper-graphite brushes.     

Wear resistance of composites for sliding electrical contact from reprocessed bearing steel

The electrical contact characteristics are determined of model composites from bearing steel reprocessed from grinding waste. It is established that the tested sliding steel composites have a low contact electrical resistance and wear rate at current densities up to 300 A/cm². An assumption is made that efficient performance of these composites results from the superdispersed structure of the reprocessed bearing steel.     

TiSiC,TiSiC-Zr,and TiSiC-Cr Coatings—Corrosion Resistance and Tribological Performance in Saline Solution

TiSiC coatings alloyed with Zr and Cr were deposited on Si and 316 L steel substrates by a cathodic arc method in a CH₄ reactive atmosphere. The corrosion and wear behavior of the coatings in 0.9% NaCl solution was investigated. Corrosion resistance of the coatings was evaluated by electrochemical tests. The electrochemical polarization measurements were conducted at room temperature in the potential range ?1 and 1.5 V, with a 0.167 mVs^?1 scan rate. Compared to the uncoated 316 L substrates, the coated ones showed nobler characteristics, with more electropositive corrosion potentials, lower corrosion current densities, and higher polarization resistances. TiSiC-Zr exhibited the lowest corrosion current density (0.62 μAcm^?2) and the highest protection efficiency (69.5%). The tribological performance of the coatings under corrosive conditions (0.9% NaCl solution) was investigated using a ball-on-disc tribometer (6-mm-diameter sapphire ball, 5 N load, 0.15 ms^?1 sliding speed, 400 m sliding distance). The TiSiC-Cr coating demonstrated the best wear behavior, with a wear rate of 3.2 × 10^?6 mm³N^?1m^?1, followed by TiSiC and TiSiC-Zr. The morphologies and compositions of the worn surfaces were examined by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) in order to identify the wear mechanism. Corrosion, debris adhesion, and oxidation were found to be the dominant wear processes.     

An analysis of the main factors on the wear of brushes for automotive small brush-type DC motor

Some critical components in motors and generators have sliding electrical contacts. Electrical brushes are commonly used in those contact points to conduct current between the stationary and moving parts of a motor. Brushes are exposed both to mechanical and electrical loading. In this paper, studies on the wear of brushes against copper commutator were briefly reviewed. The main influential factors of brush wear are associated with both mechanical and electrical wear. Brush wear is affected by various factors including temperature, material properties, sliding speed, contact force, and interfacial and environmental conditions. The mechanical wear of brushes is proportional to the brush spring pressure and sliding speed, while the electrical wear of brushes is associated with current and contact voltage drop. To characterize the wear, a brush wear test machine was designed, and influential factors were measured such as electrical contact resistance, temperature, wear mass loss, and so on. The wear tests were processed using a small brush type automotive DC motor.     

Correlation analysis of brush temperature in brush-type DC motor for predicting motor life

Some critical components of motors and generators have sliding electrical contacts. Electrical brushes are usually used in these contact points to conduct current between the stationary part and the moving part of the motor. In this paper, studies on brush wear against copper commutators are briefly reviewed. The main influential factors in brush wear are associated with both mechanical wear and electrical wear. Brush wear is affected by various factors, including temperature, material properties, sliding speed, contact force, and interfacial as well as environmental conditions. The mechanical wear of brushes is proportional to brush spring pressure and sliding speed, and the electrical wear of brushes is associated with current and contact voltage drop. For characterization, a brush wear test machine is designed, and influential factors, such as electrical contact resistance, temperature, wear mass loss, and so on, are measured. The wear tests are processed using a small brush-type automotive DC motor. The study primarily aims to investigate the effects of the wear behavior of copper-graphite brushes on small brush-typed DC motors. The variable conditions of electrical current are obtained by changing the brush spring pressure and the sliding speed. The results are electrical contact resistance, voltage drop, brush surface temperature rise, and so on. Brush wear is greatly changed by electrical current, which indicates that high current itself not only produces more Joule heating but also causes an increase in voltage drop that will result in additional Joule heating.     

The wear characteristics of ultrahigh molecular weight polyethylene against a high density alumina ceramic under wet (distilled water) and dry conditions

In recent years there has been growing interest in the use of high density alumina ceramic material for the femoral ball in association with ultrahigh molecular weight polyethylene (UHMWPE) for the acetabular component in total replacement hip joints.The wear characteristics of UHMWPE pins sliding against a high density alumina ceramic disc in the presence of distilled water in a tri-pin-on-disc machine have been revealed in very long-term experiments reported in this paper. A total sliding distance in excess of 6000 km was achieved and very low mean wear coefficients of the order of 10^?8 mm³ N^?1 m^?1 were recorded.Experiments were also carried out over a shorter sliding distance under dry conditions and the average wear coefficient of 2 × 10^?7mm³N^?1m^?1 was consistent with earlier findings. In these dry tests, comet-like streaks of polyethylene were transferred to the ceramic counterface, but no such transfer was noted during the wet tests. When distilled water was added to the test chamber after a considerable period of dry sliding, the wear coefficient rapidly decreased to about 10^?8 mm³ N^?1 m^?1 and the streaky transfer film disappeared from the ceramic counterface.The possibility of hydrodynamic action between the wear face on the pins and the counterface was investigated by reversing the direction of sliding. Surface topography changes on both the pins and the discs and friction and bulk temperatures of the pins were recorded throughout the tests.It is concluded that the excellent dry wear coefficients of UHMWPE sliding on alumina ceramic counterfaces are about twenty times greater than those experienced by the same materials in the presence of distilled water. The tribological advantage of the ceramic with respect to stainless steel having a similar surface roughness has been confirmed in dry sliding involving UHMWPE, but further work is required to determine whether or not the same advantage can be achieved under wet conditions.     

Low wear metal sliding electrical contacts at high current density

Operation of a low wear (2 × 10^?5 mm³/(N-m)), low contact resistance copper sliding electrical contact was demonstrated. The wear rate of a lightly loaded copper–beryllium metal fiber sliding on a polished copper counterface was insensitive to (DC) current density values as great as 440 A/cm² (in a brush positive or anodic configuration). Low wear and relatively low friction (μ ～ 0.2 to 0.3) was achieved by operating the contact immersed in a liquid medium consisting of a hydrofluoroether with helium cover gas, inhibitingoxidationand providing cooling of the contact. Similar experiments performed in liquid mediums of ultrapure water and dilute (3%) hydrogen peroxide show an order of magnitude increase in wear rate and provide further insight on the role of electrochemically enhanced oxidation and the degraded contact resistance and tribological behavior of non-noble sliding electrical contacts in general. In contrast to high current density slidingin hydrofluoroether, an order of magnitude greater wear rate was observed for similar sliding conditionsin hydrogen peroxide or water without the aid of externally supplied electric potential. A conceptual model is proposed correlatingthe rate of brush wear to fatigue strength and electrochemically enhanced oxidation as a result of high current density transport through the contact. A mathematical expression was derived to calculate the approximate wear volume of a single fiber laterally contacting a slip-ring, based on direct measurement of the wear scar geometry.     

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.     

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.     

Tem Of Dislocations Under High Stress In Germanium And Doped Silicon

The stacking fault energies y of silicon (58 ± 6 mJ m^?2) and germanium (75 ± 10 mJ m^?2) were determined. Within the limits of accuracy γ was not found to change on doping with (13·8 mol m^?3 (8 × 10¹⁸ cm^?3) boron, and 1·17 mol m^?3 (7 × 10¹⁷ cm^?3) phosphorus). Freezing in dislocations under high shear stress reveals a different behaviour of screw dislocations: whereas these dislocations become wider in pure and p-silicon, they become narrower in n-silicon. From this we conclude the ratio of mobilities of the two 30° partials to be different in n- and p-silicon. Other observations on frozen dislocations are mentioned.     

Effect of Hardness Ratio on the Wear Performance and Subsurface Evolution of Ni3Al Matrix Composites

A favorable hardness ratio (H_disk/H_pin = H_d/H_p) could lead to a transition to mild wear during sliding contact. To determine a more appropriate H_d/H_p value for the sliding wear, the dry sliding pin-on-disk wear tests of Ni₃Al matrix composites (NMCs) with multilayer graphene (MLG) are undertaken at H_d/H_p values of 0.99, 0.83, 0.42, and 0.35 at sliding speeds of 0.1, 0.3, 0.5, and 0.7 m/s. It is found that the tribological properties of NMCs are strongly affected by the various hardness ratios. At 0.1 m/s, the friction coefficient decreases with a decrease in H_d/H_p value. The low friction coefficient is 0.14 and the wear rate is 0.9 × 10^?5 mm³ N^?1m^?1 under the ceramic counterpart with H_d/H_p of 0.35. At 0.7 m/s, the tribological properties show the opposite trend with a decrease in H_d/H_p. At an H_d/H_p of 0.35, the smooth compact layer on the worn surface could decrease the friction at 0.1 m/s, and the improved hardness in the subsurface by strain hardening would play an important role in the improvement of wear resistance. Under the metal counterpart with H_d/H_p of 0.99, plastic deformation only occurs on the contact surface and the MLG could suppress further shear deformation in the subsurface, leading to a low wear rate (2.4 × 10^?5 mm³ N^?1m^?1) and friction coefficient (0.15) at 0.7 m/s.     

High Wear Resistance of Magnetron Sputtered Cr<Subscript>80</Subscript>Si<Subscript>20</Subscript>N Nanocomposite Coatings: Almost Independent of Hardness

Hardness has been popularly considered as an essential factor defining the wear resistance of hard coatings. Here, we report magnetron sputtered Cr₈₀Si₂₀N nanocomposite coatings, of widely varied packing densities, that exhibited identical specific wear rates, while the hardness changed over a wide range (from ~12 to ~36 GPa). All the Cr₈₀Si₂₀N coatings were free of extended and uninterrupted columnar boundaries, and retained low specific wear rates in the ball-on-plate sliding tests against Al₂O₃ counterpart with a normal load of 5 N (less than 3.0?×?10^?16 m³/N m under ambient condition and less than 2.0?×?10^?15 m³/N m under 3.5 wt% NaCl solution, respectively). Post examination reveals extensive interruption or termination of cracks in the wear tracks of the under-dense coatings, indicative of extrinsic toughening mechanisms by effective relief of local contact stress. Our results suggest that a critical role of toughening rather than hardening, played in enhancing the wear resistance of hard coatings, and thus would pave a way to develop highly wear-resistant coatings with a low hardness.     

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.     

Effect of electrical current on the tribological behavior of the Cu-WS2-G composites in air and vacuum

As the traditional graphite-based composites cannot meet the requirement of rapid developing modern industry, novel sliding electrical contact materials with high self-lubricating performance in multiple environments are eagerly required. Herein a copper-based composite with WS2 and graphite as solid lubricant are fabricated by powder metallurgy hot-pressed method. The friction and wear behaviors of the composites with and without current are investigated under the condition with sliding velocity of 10 m/s and normal load of 2.5N/cm 2 in both air and vacuum. Morphologies of the worn surfaces are observed by optical microscope and compositions of the lubricating films are analyzed by XPS. Surface profile curves and roughness of the worn surfaces are obtained by 2205 surface profiler. The results of wear tests show that the friction coefficient and wear volume loss of the composites with current are greater than that without current in both air and vacuum due to the adverse effects of electrical current which damaged the lubricating film partially and roughed the worn surfaces. XPS results demonstrate that the lubricating film formed in air is composed of oxides of Cu, WS2 , elemental S and graphite, while the lubricating film formed in vacuum is composed of Cu, WS2 and graphite. Because of the synergetic lubricating action of oxides of Cu, WS2 and graphite, the composites show low friction coefficient and wear volume loss in air condition. Owing to the fact that graphite loses its lubricity which makes WS2 become the only lubricant, severe adhesive and abrasive wear occur and result in a high value of wear rate in vacuum condition. The formation of the lubricating film on the contact interface between the brush and ring is one of the factors which can greatly affect the wear performance of the brushes. The low contact voltage drop of the composites in vacuum condition is attributed to the high content of Cu in the surface film. This study fabricated a kind of new sliding electrical contact self-lubricating composite with dual-lubricant which can work well in both air and vacuum environments and provides a comprehensive analysis on the lubrication mechanisms of the composite.     

Wear behavior of polymeric compositions in dry reciprocating sliding

The development of Stirling engines as high efficiency motors for automobiles is critically dependent on the ability of sliding seals to operate for long periods (up to 3500 h) without excessive leakage or wear. Piston rings must run completely dry while sealing against hydrogen leakage under a pressure drop of 5–20 MPa. Main seals, cooled by oil, must seal against similar pressures without admitting any oil which could contaminate the high temperature sections of the engine.The results of an initial search for materials with lower wear rates than the polytetrafluoroethylene (PTFE) polymer compositions currently used in test engines are described in this paper. On the basis of a literature review, the results of engine development tests and experience with solid lubricants, several commercially available formulations were selected for a screening test program. In addition, some custom formulations were prepared by commercial vendors.A reciprocating screening tester, operating at 1200 cycles min^?1 with a stroke of 40 mm, was used to determine material wear rates at a PV factor below the first PV limit of the reference PTFE material. Materials with wear rates in the range of 0.13 × 10^?6–0.16 × 10^?6 mm³ N^?1 m^?1 (the reference material wear rate was 0.40 × 10^?6 mm³ N^?1 were found.Friction coefficients were also measured since friction power loss in the seals represents a significant portion of the total mechanical engine losses. Compositions which significantly reduced the friction coefficient while retaining a low wear rate were found.     

Surface chemistry-controlled tribological behavior of silicon and diamond

SEM tribometric experiments were performed with Si(100) vs. Si(100) interfaces in mode-rate vacuum to 850°C. The results are compared with similar tests previously completed with fine-cauliflowered PCD (PCD_fcf) mated against itself, and polished C(100)-textured polycrystalline diamond (PCD_C(100)) sliding against Si(100). All data agree with a hypothesis connecting the thermal desorption of adsorbates and wear with the generation of dangling bonds on the sliding surfaces. Linking of the counterfaces by the free radicals appears to be the main cause of high adhesion and friction. The high friction can be drastically reduced by dissociative chemisorption of certain passivating gaseous species condensing at sufficiently low surface temperatures. Strong circumstantial evidence continues to mount for the incremental reduction in high temperature friction being caused by surface reconstruction. Deconstruction of the sliding surfaces and the reemergence of high friction eventually occurs on discontinued heating, until the adsorbates chemisorb on the cooled surfaces. There, the friction drops to a level determined by the characteristic shear strength of the interfaces and the wear-induced increase in the real area of contact. The maximum friction measured at high temperatures in vacuum, indicative of the most intensive interaction of dangling bonds at the interface, scaled only approximately with the 1.8 times strength of the C-C versus the Si-Si bonds. The 1.6 experimental ratio is lower than the theoretical, reflecting the broad distribution of dangling bond energies (densities of surface trap states) for PCD and even for polished Si(100). The wear rate of Si(100) sliding against itself is about four-orders-of-magnitude higher (~ 2 × 10^-12 m³/(Nm)) than that of unpolished PCD_fcf vs. itself (4 × 10^-16 m³/(Nm)) or rough and unpolished PCD_C(100) wearing its polished version (8.5 × 10^-16 m³/(Nm)).     

Behaviour of fibre brushes under transient atmospheric conditions

Voltage drop and friction measurements were made on fibre brushes working under transient atmospheric conditions. These transient conditions correspond to the introduction of ball-bearing greases or of water into the chamber containing the experimental set-up under a dry nitrogen atmosphere.It is shown that vapours from a particular grease have a contaminating effect on the sliding contacts. When the same grease is heated up to 100 °C and when the disc on which the brushes are rubbing is gold plated, very low frequency oscillations in the cathodic voltage drop are observed. This phenomenon is interpreted by assuming that the surface film resistance and the surface temperature are dependent on each other. In addition, a short and simplified linear stability study of sliding contacts is given.It is shown that the behaviour of brushes in an atmosphere having a variable relative humidity strongly depends on the surface material (i.e. gold or copper) of the disc. The major conclusion emerging from the results presented is that, under certain circumstances, water vapour may play the role of an adsorption inhibitor for organic molecules whereas, in other cases, it may act as a lubricant.     

A monitor for heavy ion flux density control based on detection of recoil protons during irradiation of polymer films

An monitor for control of accelerated particle flux density used in irradiation of polymer films with heavy ions with a low track density (10³?5 × 10⁷ cm^?2) is based on detection of recoil protons. It has been designed to control irradiation of a film with a width of up to 650 mm and is composed of five PIN diodes with a hydrogenous target placed in front of each diode. The sensitivity of the monitor with a polyethylene target to the density of a xenon ion beam is described by a linear function; its value is (4.0±0.5) × 10^?3 pulses/(ion/cm²). The instrument is capable of monitoring ion flux densities of <10⁶ cm^?2, for which all other methods are ineffective. It can be used to determine the track density in a finished product, bypassing the stage of a check with an electron microscope. The signals from the monitor can also be used to introduce a feedback in a system of automatic beam tuning in order to improve the film irradiation quality. The radiation hardness of silicon detectors is ≈ 10¹² protons/cm². At an average detector load of ≈ 10⁴ protons/cm², the average service life of the monitor is 10⁸ s of continuous operation.