Study on surface film in the wear of electrographite brushes againstcopper commutators for variable current and humidity
Z.L. Hu ∗, Z.H. Chen, J.T. XiaCollege of Materials Science and Engineering, Hunan University, 410082 Changsha, PR China
Received 12 July 2006; received in revised form 10 January 2007; accepted 18 January 2007Available online 20 February 2007
bstract
In this paper studies on the wear of brushes against copper commutators were briefly reviewed. The effects of alternate current density andumidity on the wear rate of electrographite brushes were investigated and the wear mechanism was discussed in detail. SEM (scanning electronicroscope) and EDS (energy dispersive X-ray analysis) were used to observe the morphology of contact surface of brushes and analyze the
ompositions of contact surface. The results show that the oxide layer on the contact surface becomes thick and the wear rate of brushes increases
ith current density. In 50% relative humidity (RH), an oxide layer can be formed on the contact surface, which can act as lubricants, and theear rate of brushes is small. While in 10% RH, no oxide layer is formed and the wear rate is large. Under a regular condition the fatigue wearechanism of electrographite brushes is supported in this study while in low humidity seizure mechanism is dominant.2007 Elsevier B.V. All rights reserved.
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eywords: Brush; Wear; Oxide layer; Fatigue; Humidity; Alternate current
. Introduction
Because graphitic carbon materials are of low friction, lowear rate and good conductivity, they are extensively used asaterials of brushes. The wear rate of brushes is one of theost important indicators for performances. However, the wearechanism of brushes is very complex because there are many
actors that can influence it. Many scholars have been studyingt for decades. Holm’s investigation shows the wear of brushesan be divided into three parts: �VM, mechanical wear, �VE,echanical wear caused by current and �VF, electrical wear
1]. This conclusion has been commonly accepted. Some exper-ments based on Holm’s theory have been done to investigateffects of contact voltage drop, arc, bulk temperature of brushes,oefficient of friction, normal load, velocity, current density,tc. on the wear of brushes [2–7]. Clark and Lancaster adoptedn optical and electron-optical examination to analyze the sur-
ace changes and proposed fatigue wear mechanism of carbonrushes [8]. Mckee et al. studied chemical factors in carbonrush wear [9]. They observed that current converging leads to
igh temperature and copper presented in commutators can acts catalysts for oxidation.
It is widely recognized that a surface film is composed ofuprous oxide, graphite grains and water and the film acts asubricants which can reduce friction and wear when motorsun [10]. Several studies were focused on the cuprous oxideayer [11]. Spry and Scherer determined the compositions of anxide layer by an electrochemical way and measured the thick-ess of an oxide layer both by electrochemical reduction and bylectrically puncturing method [12]. The investigation indicateshat the bulk temperature of commutators controls the thicknessnd growth rate of cuprous oxide and the polarity effect causedy current makes the film formed under the negative brush behinner than that under the positive brush.
To further investigate the wear mechanism of brushes, Bryantt al. invented an apparatus based on photoelastic theory whichan visualize the contact phenomena between real tribologicalurfaces. They studied and simulated the temperature and stressn a thermal mound [13,14], and proposed a particle ejection
echanism for brush wear [15].
Some recent research has been done to study the effects of
eometrical sliding surface of a commutator on the friction andear of brushes [16,17]. Wilk and Moson’s study demonstrated
hat the wear on the commutator sliding surface is of a wave
haracter and the commutator surface waviness has a markednfluence on the wear of commutator and brush [18].
Although much work has been done to study the wear ofrushes, the related mechanism has not been well understood.here are at least two points in related researches on the wearf brushes need more emphasis. First, most experiments wereonducted on a pin-on-disc machine in which current of pass-ng brushes was direct [19]. The results obtained may not bepplied to alternate current (AC) motors. Because in AC motor,he change of direct of current flow in commutated windingseads to dissipation of electromagnetic energy which is storedn windings mainly in the intercontact space. This phenomenon
ust influence the wear of brushes. Second, humidity is impor-ant to the wear of brushes and its influence on the graphitelectro-tribological behaviour was discussed in detail by Savagend Lancaster [20–24], but the effects of atmospheric humidityn the wear of brushes fixed in AC motors have rarely beentudied.
This study presented in this paper was initiated to provide dataf the wear of brushes as function of AC density and humidity.he wear of brushes under different AC densities in both 50%H and 10% RH was investigated. The surface film was analyzedy SEM and EDS and a possible establishment of an oxide layeras presented. The wear mechanism of electrographite brushesas proposed.
. Experiments
In this paper a low power AC motor (type RB5540M220A)as adopted. This type of motors is commonly used for vacuum
leaners. Its commutator with 12 bars is made of copper withiameter at 17.0 mm. The power of motors is provided by dailyighting AC power and the frequency is 50 Hz. The test motors shown in Fig. 1. The test D104 brushes were provided byin Fang Yuan (Zhuzhou) Electrical Carbon Corporation. Therushes are made of 72% flake graphite, 8% carbon black and0% pitch. They were graphitized at 2200 ◦C. The dimension ofrushes is 5.0 mm × 6.0 mm × 12.0 mm and the main propertiesf brushes are listed in Table 1.
A humidifier was used to control humidity of a chamberhere the motor tester was placed.The normal load was provided by a constant-force spring
pplied to the brush to maintain contact between the brush andommutator. To keep the commutator velocity constant underifferent currents, an adjustable rotor wheel was installed in theC motor.
Except AC densities and humidities all tests were done underhe same conditions. The normal loads are 3 N/cm2, the temper-ture is 25 ◦C, the commutator surface sliding velocity is 10 m/snd humidity is 50% RH or 10% RH. In this study current den-
able 1ain properties of D104 brush
lectrical resistivity (��m) 12ulk density (g/cm3) 1.65hear strength (MPa) 20
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ig. 1. Schematic of the test motor: (1) commutator bar; (2) brush; (3) brusholder; (4) contact force spring; (5) bearing; (6) winding; (7) rotor wheel.
ities are limited to avoid generating electric spark (maximumurrent density is 6.0 A/cm2). So the wear of brushes caused bylectric spark can be neglected.
SEM and EDS were used to analyze the sliding surface ofrushes and the commutator was polished with a very fine Al2O3aper before each experiment. The worn surface of the testedamples was observed by SEM without cleaning so that allriginal features could be observed. The compositions of theubricating film on the brush were analyzed by EDS. The wearates of brushes under different conditions were measured untilhe sliding time reached 30 h.
. Results and discussion
.1. Effects of current on the wear of brushes
The wear of brushes is one of most important characteristics.he current in sliding contact deforms contact elements, alters
riction coefficient, intensifiers wear, and induces damage ofubbing surface [25]. In this study the wear rate in 50% RH andnder currents of 2.0, 4.0 and 6.0 A/cm2 was investigated andhe results are showed in Fig. 2.
Fig. 2 demonstrates the relationship between the wear volumef brushes and time. The slopes of the curves in the figure areig at first, but after 5 h they become low and almost constant.t shows that in initial stage the wear rate is high and after 5 ht becomes low and keeps steady. This may be of followingeasons:
First, when the test begins, the wear of brushes against com-utators is the wear of graphite against copper, so the wear of
rushes is higher. But when a layer of graphite is deposited on theommutators, it becomes the wear of graphite against graphite.lthough the graphite layer is thin and dynamic, it can decrease
he wear of brushes due to the lubricating effect of graphite.
Z.L. Hu et al. / Wear 26
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ig. 2. Wear volume loss of brushes under different current densitiesRH = 50%).
Second, the formation of an oxide layer needs some time. Innitial stage the oxide layer is not well developed on the interfaceetween brushes and commutators. It cannot provide adequaterotection to brushes from damaging by commutators, which
auses high friction and leads to a high wear rate.
Figs. 3 and 4 are SEM of worn brush surfaces siding after 1nd 5 h under a current density of 2.0 A/cm2, respectively. As
ig. 3. Morphology of the worn surface of brushes after 1 h sliding (RH = 50%,= 2.0 A/cm2).
ig. 4. Morphology of the worn surface of brushes after 5 h sliding (RH = 50%,= 2.0 A/cm2).
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emonstrated by these two figures, sliding after 1 h, the surfacelm on brush is not well developed. It is thin and not continu-us. But after 5 h it is almost continuous and uniform and almostovers the whole surface of brushes. It may be because after 5 hliding the rate of formation surface film is equal to that of dam-ging of surface film and it is nearly in a dynamic equilibriumnd steady state. Above analysis is supported by our observa-ion for the worn surfaces of commutator and brush which areovered by a uniform and stable film after 5 h sliding.
From Fig. 2 it is obvious that with increasing current density,he wear rate increases rapidly. In a steady state, the wear ratesnder current densities of 2.0, 4.0 and 6.0 A/cm2 are about 0.3,.4 and 0.6 mm3/h, respectively.
According to Holm’s theory on electrical contact [1], therere three types of contact areas exist at a carbon-copper junctionn the experiment, which are, respectively, ordinary “a” spots,ajor “a” spots and “b” spots. An ordinary “a” spot is in physical
ontact with solid copper and a major “a” spot is one where solidarbon is in physical contact with pools of molten copper. A “b”pot is where carbon is in physical contact with solid copperxide. Although ordinary and major “a” spots only comprise aery small part of the whole apparent area, they conduct nearlyll the current in a sliding system. The contact resistance Rcuring the electrical wear includes the resistance of the filmf and constriction resistance Rs arising from the geometricalffect of current lines from specimen bulk to spot. The contactesistance can be expressed by:
c = Rf + Rs = ρ1 + ρ2
4α+ σ
πα2 , (1)
here α is the contact spot radius, ρ1 and ρ2 are the electri-al resistivity of brush and commutator, respectively. σ is theesistance of unit area of the film.
Fig. 5(a)–(c) are the morphologies of brushes after 5 h slidingnder current densities of 2.0, 4.0 and 6.0 A/cm2, respectively.t is observed that the surface films on brushes change a lot withncreasing current densities and Table 2 shows its compositionsnder different current densities. The data in Table 2 are pre-ented by atom number percentage. When the current densityncreases from 2.0 to 6.0 A/cm2, copper content increases from.5 to 4.5%. This is because with increasing current density,ore Joule heat is produced, and more copper is oxidized. Thus
t leads to a thicker oxide layer. Fig. 5(a) shows a loose oxideayer while Fig. 5(c) shows a compact and thicker layer formed.
With the increase of current density, the cuprous oxide layerecomes thicker and Rc increases, also. So more Joule heatakes temperature of “a” spots adjacent higher, which can be
igher than the oxide temperature of carbon matrix of brushes. It
able 2urface element analysis of brushes under different currents (RH = 50%)
ample set Under differentcurrents (A/cm2)
Surface element analysis (%)
Else elements Cu
2.0 97.5 2.54.0 96.4 3.66.0 95.5 4.5
14 Z.L. Hu et al. / Wear 26
Fig. 5. SEM of the worn brush surfaces under different current densities(RH = 50%). (a) 2.0 A/cm2 (b) 4.0 A/cm2 (c) 6.0 A/cm2.
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ill lead to more graphite grains to be oxidized. Thermo gravi-etric analysis of graphite grains of brushes shows that the
raphite grains begin to be oxidized at 510 ◦C. Because ordi-ary and major “a” spots only comprise a very small part of thehole apparent area and high temperature at “a” spots adjacent
s transient, the rate of oxidizing wear of brushes is small.On the other hand, high temperature caused by Joule heating
nhibits the ability of the film to remain tightly bound to the baseaterial. With increasing current density, more Joule heating is
roduced, and major “a” spots and even “b” spots are punc-ured. Thus, surface film is damaged and its lubrication actions reduced. From Fig. 5(c) it is clear that the oxide layer at somereas was punctured. The related wear mechanism of brushesill be analyzed in Section 3.3.Based on the above analysis, a conclusion can be drawn that
higher current density leads to a higher wear rate of brushesnd makes an oxide layer thicker, and a thicker oxide layer willncrease the contact resistance and the wear of brushes, evenhough the oxide layer can act as lubricants.
.2. Effects of humidity on the wear of brush
Humidity is very important to the wear of brushes [26,27].he wear volume loss of brushes in 50% RH and 10% RH (undercurrent density of 4.0 A/cm2) as a function of wear time is
hown in Fig. 6. It shows that the wear rate of brushes in 10%H is about two times higher than that in 50% RH.
According to the theories of Savage and Lancaster [20–24],n high humidity the water vapor molecules tend to cover thexposed graphite surface by forming transient monolayers. Thisay, the free surface energy is lowered, which reduces cohe-
ion and adhesion between contact areas and makes friction andear decrease. In low humidity, graphite can not obtain its sat-rated surface and the adsorption water film only covers a partf graphite surface; the free surface energy is high; the contactolecules between graphite and copper may seize; and it makes
he friction and the wear of brushes increase largely, even when
dusting wear happens.
Fig. 7 is the morphology of worn brushes in 10% RH. Its obvious that there are dust and fragmentary grains on theurfaces of brushes for absent of water.
ig. 6. Wear volume loss of brushes under different humidities (I = 4.0 A/cm2).
Z.L. Hu et al. / Wear 26
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bmiiof the difference of thermal property of raw materials, there arealways a lot of micro cracks and pores in the matrix of electro-graghite brushes. Fig. 8 is SEM of the unworn surface of brushes.
ig. 7. Morphology of the worn brush surface (RH = 10%, I = 4.0 A/cm2).
Table 3 was EDS of the worn brush surfaces in 50% RH and0% RH under a current density of 4.0 A/cm2. It shows thathere is no copper element on the surface brush in 10% RH andemonstrates that there is no or very little cuprous oxide formedn the interface in 10% RH. This is because that water vapouracilitates the formation of cuprous oxide protective layer andater might act as a catalyst to form cuprous oxide. Reactionsn the interface may be expressed as following formula:
2O = H+ + OH−
2 + 4H+ + Cu = Cu2O + 2H2O
Water can be dissociated into H+ and OH− ion under cur-ent, and standard electrode potential of (ϕo
O2/H+ ) is 1.229 V,
hich increases oxidizing ability of O2 and can oxidize Cu intou2O. Under the condition of absence of water or little quantityf water, oxidizing ability of O2 is not strong enough to oxi-ize Cu into Cu2O, which leads to a high rate of brushes wear.igs. 7 and 5(b) are SEM of worn surfaces of brushes under
he same condition except different humidities. Fig. 7 showshat in 10% RH there are only graphite grains on the surface ofrushes while it can be obviously seen an oxide layer on surfacef brushes from Fig. 5(b). No oxide layer is formed at interfacen low humidity, the lubricating action is reduced and the wearf brushes increases.
In a word, when humidity is low, water film can not cover thehole exposed graphite surfaces and the oxide layer can not be
ormed on the contact surfaces, so the wear rate of brushes isery high.
able 3urface element analysis of brushes in 50% RH and 10% RH (I = 4.0 A/cm2)
ample set Humidity Surface element analysis (%)
Else elements Cu
50% RH 96.4 3.610% RH 100 0
4 (2008) 11–17 15
.3. Analysis of the surface film and the wear mechanism oflectrographite brushes
According to previous research [12], copper in the surfacelm was in form of Cu2O instead of CuO. It is thought thatu is easier oxidized into Cu2+ other than Cu+ for ϕo
Cu2+/Cu <oCu+/Cu (ϕo
Cu2+/Cu = 0.33 V, ϕoCu/Cu = 0.52 V) based on the
heory on oxidizing reaction. So in ambient air when Cu is heatedill to more than 150 ◦C, it is oxidized into CuO instead of Cu2O.
Although many researchers investigated the surface film dueo its importance, few explored its establishment and the condi-ion of its formation. Here a possible establishment is presented.
When motors run, the major and ordinary “a” spots in theontact areas produce heat because of friction and Joule heating.his makes major and ordinary “a” spots and adjacent area a high
emperature at which Cu can be oxidized into CuO and Cu2O.ut the thermal stability of Cu2O is better than that of CuO
at ambient air Cu2O is decomposed at 1800 ◦C while CuO at100 ◦C), so at almost same time all CuO and a part of Cu2Ore decomposed into Cu and O2. The contact time of major andrdinary “a” spots is transient due to rotation, and Cu2O in thesepots cools rapidly to the bulk temperature of brushes. So Cu2Os formed and no CuO is found in the surface film.
Further study on oxide layer will be of important significanceo improve technology, modify formula of brushes and make theear mechanism of brushes understood better.Clark, Lancaster and White’s studies proved that the wear
echanism of electrographite brushes is fatigue mechanism8,19]. The data in this study supported their statements.
Electrographite brushes are composed of grains of carbonlack, coke, natural or artificial graphite and pitch binder. Theiranufacturing process includes baking in which the temperature
s often over 1000 ◦C and graphitizing in which the temperatures over 2200 ◦C. When material of brushes is heated, because
Fig. 8. Morphology of the unworn surface of brushes.
16 Z.L. Hu et al. / Wear 26
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ig. 9. SEM of the worn surface of brushes under a current of 6.0 A/cmRH = 50%).
rom it one can see that there are a lot of micro cracks and poresn matrix of brushes.
Electrographite brushes are brittle materials, so fracture muste concerned on the mechanism of wear. Cracks in matrix oflectrographite brushes are initial sources of wear. Under slid-ng conditions, mechanical stress within the contact surface ofrushes is produced for two reasons: first, friction and Joule heatngender big temperature gradient between the contact surfacend its adjacent area; second, the drastic temperature change onhe same spot in the contact surface is created because of tran-ient contact. Such factors generate great mechanical stress thatnitiates new cracks and propagates existing cracks within theontact surface. When the cracks propagate to detach materialrom the surface by external force such as friction, wear andear particles are produced.Fig. 9 is the morphology of worn surface of brushes under a
urrent density of 6.0 A/cm2 and in 50% RH. It clearly showshat there are a lot of micro cracks, propagated cracks and weararticles on the surface of brushes.
Some micro plough and scratch on the surfaces of brushes cane seen in Fig. 9, which indicates that mild abrasive wear exists.lthough in this study the alternate current is limited not to
ause spark, and the surfaces of commutators are approximatelymooth, but it is still possible that there may be little blistersn the surfaces of commutators which leads to ploughing andcratching on the surface of brushes.
Moreover, Joule heat makes contact spots high temperaturet which the grain of brushes can be oxidized. This is analyzedn Section 3.1.
Above analysis is applied to a regular condition in which anxide layer is formed. When in a low humidity, an oxide layeran not be formed on the contact surface and graphite grains
re not well oriented and uncovered by water vapour. Directontact between brushes and commutators leads a high frictionnd contact molecules at interface may seize. Wear mecha-ism of brushes is primarily seizure mechanism [20]. From
4 (2008) 11–17
ig. 7 one can see many fragmentary grains are produced onhe contact surface, which is a result of shattering of seizure
aterials.In general, in a sliding system under a regular condition,
his research supports the fatigue wear mechanism of electro-raphite brushes. The wear process may include: initiation oficrocracks, propagation of microcracks caused by mechanical
tress because of thermal expansion and detaching of microc-acks to form wear particles. Then new microcracks is initiatednd propagated to be detached to form new wear particles. Andhis process repeats until the end of operating life of brushes.t the same time abrasive and oxidizing wear exist, but they are
ess significant compared with fatigue wear.While in low humidity, no oxide layer is formed and the wear
echanism is primarily seizure mechanism.
. Conclusions
. With increasing the current density, an oxide layer on the con-tact surface of electrographite brushes becomes thicker andthe wear rate of brushes increases. A thicker oxide layer willincrease the contact resistance and make the wear of brushesincrease although the oxide layer can act as lubricants. So anoptimum thick of oxide layer is desired.
. In 10% RH cuprous oxide layer can not be formed on the con-tact surface and the wear rate of brushes is high. In 50% RH,cuprous oxide layer is formed and the wear rate of brushesis low.
. The data in this study support the fatigue wear mechanismin 50% RH, although oxidizing and abrasive wear may exist,they are less significant compared with fatigue wear. In 10%RH, seizure mechanism may dominate the wear of electro-graphite brushes.
. Surface film is of key factor to the wear of brushes and furtherstudy on it will be of theoretical guide to improve technologyand modify formula of brushes.
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[
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