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Sliding wear transition for the CW614 brass alloy
Tribology International 39 (2006) 290–296 www.elsevier.com/locate/triboint
Sliding wear transition for the CW614 brass alloy Khaled Elleucha,*, Riadh Elleucha, Ridha Mnif a, Vincent Fridricib, Philippe Kapsab a
LASEM, Ecole Nationale d’Inge´nieurs de Sfax, B.P. W, 3018 Sfax, Tunisie LTDS, CNRS, UMR 5513, Ecole Centrale de Lyon, Ecully Cedex, France
b
Received 6 July 2004; received in revised form 24 January 2005; accepted 27 January 2005 Available online 13 March 2005
Abstract Dry sliding wear tests were performed on a CW614 brass alloy using a pin-on-ring configuration. Wear kinetics were measured within a load range of 20–80 N and sliding velocity ranging from 1 to 7 m/s. Chemical compositions, morphologies and microstructures of worn surfaces and wear debris were characterised by scanning electron microscope (SEM) and energy dispersive X-ray spectrometer (EDS). Two main wear regimes have been observed: severe wear and mild wear. The results of wear tests and metallographic investigations on worn surfaces have been summarised in a wear mechanism map. It was found that the wear transition is controlled by a critical temperature at the contact surface. q 2005 Elsevier Ltd. All rights reserved. Keywords: Brass; Wear; Transition; Thermal effect
1. Introduction Copper alloys are widely used because of some attractive properties such as high electrical and thermal conductivity. These materials are used in friction parts of machines, such as bearing liners, bushing, etc. Sliding wear is also an important consideration in material processing by rolling, extrusion, forging, etc. and it is important to understand microstructure changes at the work-tool interface and the associated heat generation. Furthermore, it is known that metal wear involves complicated phenomena such as metal transfer [1], mechanical alloy-forming process [2], abrasive and adhesive wear [3], dynamic recrystallization [4] and wear transitions [5,6]. Much work has already been done to understand the effects of microstructure and environmental conditions on wear. Goto et al. [7] have studied the influence of water vapour, oxygen and water in various environments on the friction and wear of a 60/40 brass in homogeneous contact during fretting corrosion. Zhang et al. [8] have investigated dry sliding friction and wear in vacuum and air of brass against stainless steel. The mutual transfer of surface materials has been found. The influence of alloying * Corresponding author. Tel.: C216 74 274 088; fax: C216 74 275 595. E-mail address: [email protected] (K. Elleuch).
0301-679X/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2005.01.036
with aluminium, manganese, silicone, tin and iron on the wear properties of special brass has been discussed mainly in severe wear conditions [9]. Analyses of the surface layer developed during dry sliding against steel have shown good correlations among composition, hardness and wear resistance. For (aCb) high-strength brass, an increase of a-phase from 8 to 23 vol% decreased the hardness from 281 to 250 HV [10]. It is assumed [11] that wear processes are mainly determined by mechanisms of surface film formation and destruction. For ductile materials, such as brass, plastic deformation can be important. Recently much research dealing with friction and wear of nanocrystalline [12,13] and submicrocrystalline [14] brass have confirmed that they have improved tribological properties. The aim of this paper is to underline the thermal effect on friction and wear of brass when rubbed against bearing steel. A wear mapping approach has been undertaken to present the wear regimes and the corresponding mechanism.
2. Experimental procedure 2.1. Material The material studied was a CW614 brass alloy with the following chemical composition in weight percent:
K. Elleuch et al. / Tribology International 39 (2006) 290–296
291
P (N)
80
40
20 V (m/s) 1 Fig. 1. Optical micrograph of a/b brass.
7
4
Fig. 3. Test conditions (P: normal force, V: sliding velocity).
Table 1 Thermal and mechanical properties of brass CW 614 Young’s modulus, E (GPa)
Poisson’s ratio, n
Elastic limit, Re (MPa)
Hardness BRINELL, HB
Thermal conductivity, l (W/mK)
Specific heat, C (J/g 8C)
110
0.35
350
150
120
1.074
polishing and etching (using KELLER reagent). Microscope examination (Fig. 1) revealed that in the microstructure, the light coloured a phase precipitated in the dark coloured b phase matrix. The mechanical and thermal properties of CW614 brass alloy are given in Table 1. 2.2. Wear test The developed tribometer is a pin-on ring type. Contact configuration is chosen to be a cylinder on plane (Fig. 2). A continuous rotating motion is imposed on the steel ring at
(a)
3500 V=7m/s
wear depth (µm)
58.5%Cu, 2.8%Pb, 0.25%Sn, 0.11%Ni, 0.21%Fe, 0.01%Al and the balance Zn. The material was received in the form of extruded bar (120 mm!:6 mm) from which wear samples (cylinder of 6 mm in diameter and 32 mm in length) were machined. The surface preparation procedure of the wear test samples consisted of polishing the surface manually by 400, 600, 800 grit SiC abrasive papers, successively, and then polishing them with 1 and 0.05 mm alumina powder slurry using a low speed polishing machine. The counterface rings were cleaned in a methanol solution before each test. Microstructure of the investigated pin has been examined by optical microscope (OM) after grinding,
2800 2100 V=4m/s
1400 V=1m/s
700 0 0
500 1000 1500 2000 2500 3000 3500 Sliding distance(m)
wear depth (µm)
(b)
5000
V=7m/s V=4m/s
4000 3000
V=1m/s
2000 1000 0 0
500 1000 1500 2000 2500 3000 3500 Sliding distance(m)
Fig. 2. The developed pin-on-ring tribometer at Ecole Centrale de Lyon.
Fig. 4. Wear depth evolution as function of sliding distance for three sliding velocities (a): PZ20 N, (b): PZ40 N.
50
1
T
40
0.8
30
0.6
20
0.4
f
10
0.2
0 0
500
1000
1500
2000
2500
3000
0 3500
Sliding distance (m)
2
T
250
1.6
200
1.2
150 0.8
100
0.4
50
f
0 0
Temperature (˚C)
(c)
Friction coefficient
300
500
1000 1500 2000 2500 Sliding distance (m)
0 3000 3500
300
1.2 T
250
1
200
0.8
150
0.6
100
0.4 f
50
0.2
0 0
500
1000
1500
2000
2500
3. Results and discussion The instantaneous interpenetration is designated as wear depth and stored for the test duration. The global linear evolution, relative to this parameter, is noted for mild loading conditions. Fig. 4 gives examples for such variation. It shows a parabolic evolution as a function of sliding distance for severe loading conditions, such as vZ7 m/s and PZ20 N or PZ40 N. However, quasi-linear variation is demonstrated for mild loading conditions, such as vZ4 m/s and PZ20 N or PZ40 N. Furthermore, the friction coefficient and surface contact temperature for mild loading conditions (PZ20 N, VZ1 m/s) are almost steady and have only weak variations (a)
Friction coefficient
Temperature (˚C)
(b)
surface. The sample holder is made of ceramic to limit loss of thermal energy which arises from the friction of the contacting bodies. Simultaneously tangential force data are stored in real time during the imposed sliding distance (dZ3250 m).
Temperature (˚C)
Temperature (˚C)
(a)
K. Elleuch et al. / Tribology International 39 (2006) 290–296
Friction coefficient
292
0 3000
350 300 250 200
V=7m/s
150 100 50 0
V=4m/s V=1m/s 0
500 1000 1500 2000 2500 3000 3500 Sliding distance(m)
Sliding distance (m)
a constant velocity. The ring is a standard ring bearing steel (52100 steel), has a 33 mm as radius and is fixed on the engine. A constant weight is used to apply the normal load. A specific device using a rubber element is used to decrease vibrations, especially noted for severe loading conditions (high normal force and sliding velocity). Fig. 3 shows a diagram summarizing all operating conditions. Each point represents a separate test. Performing these tests gives birth to a wear track and debris creation. Wear depth was determined as a function of sliding distance at each normal load and sliding velocity condition. The resulting interpenetration of the contacting bodies is stored in real time using an inductive displacement sensor. The contact temperatures were measured using a chromel-alumel type thermocouple which is placed at the back of the brass sample. The measured value corresponds to the temperature of brass at 10 mm from the contact
350
V=7m/s
300 250 200 150
V=4m/s
100 V=1m/s
50 0 0 (c) Temperature (˚C)
Fig. 5. Friction coefficient and surface contact temperature evolution as function of sliding distance (a): PZ20 N, VZ1 m/s, (b): PZ20 N, VZ 7 m/s, (c): PZ80 N, VZ4 m/s.
Temperature (˚C)
(b)
500 1000 1500 2000 2500 3000 3500 Sliding distance(m)
300 250 V=4m/s
200 150 100
V=1m/s
50 0 0
500
1000 1500 2000 2500 3000 3500 Sliding distance(m)
Fig. 6. Surface contact temperature as function of sliding distance for different sliding velocities. (a): PZ20 N, (b): PZ40 N, (c): PZ80 N.
K. Elleuch et al. / Tribology International 39 (2006) 290–296
(Fig. 5(a)). However, for severe loading conditions, two types of variation are noted. Fig. 5(b) shows a continuous jump for both friction coefficient and temperature. However, Fig. 5(c) illustrates sudden and discontinuous variations. Similar results are found for aluminium alloy—steel [15,16] but there is no quantification of the instantaneous wear (only a global mass loss is measured at the end of the test). However, instantaneous surface temperature variation against sliding distance is available [15,16]. Wear tests performed at high speed are quickly blocked because of the transfer mechanism (seizure) which arrests the engine and ends the test. An illustration of temperature variation as a function of sliding distance is proposed in Fig. 6 for several normal loads (20, 40 and 80 N). Two steps are indicated. – First stage: at the beginning of the test, the temperature increases and then reaches a constant value. Hence,
293
thermal balance is well established between the contacted surfaces, – Second stage: when reaching 150 8C, the temperature shows a quick increase. A transition in heat kinetics is noticed at particular loading conditions which appear at variable sliding distance [16]. A close correlation between friction coefficient and surface contact temperature is confirmed and proves that temperature increase is controlled through surface phenomena which can be summarized by the friction coefficient. Analyses of the wear mechanisms are performed to explain the origins of these transitions. 3.1. Wear track on the copper alloy First of all, post mortem analysis of the pin surface (CW614) is proposed to show differences before and after transition responses.
Fig. 7. SEM images of wear scars on pin CW614 (a) PZ20 N, (b) PZ40 N, VZ7 m/s: formation of a granular layer.
294
K. Elleuch et al. / Tribology International 39 (2006) 290–296
Typical electron microscopic observations of the wear tracks are shown in Fig. 7(a). For mild loading conditions, the contact centre shows some scratches that are oriented in the sliding direction, whereas at the contact borders there is wedge formation. This phenomenon is emphasised for severe load conditions. In fact, the contact surface becomes more disturbed and plastic deformation is more intense. Hence, adhesion phenomena are emphasised, which leads to seizing. Probably, the increase of temperature due to friction heats the pin surface and softens the surface of the brass sample. Then the material will be plastically deformed and will be extruded towards the contact edges, with superposition of material layers [16]. Indeed, for severe loading conditions (PZ40 N and VZ7 m/s), the pin surface is characterized by the presence of small grey particles (Fig. 7(b)) which are gathered in clusters. These reveal physicochemical transformations produced at the interface as a consequent of the impact of heat flow on the rubbed brass. Formation of such layers can be the origin of the friction coefficient transition.
the temperature becomes ’sufficiently’ high (typically TZ150 8C), a granular layer with high mechanical properties forms as a third body (Fig. 7(b)) and gives rise to wear of the ring steel. 3.3. Wear debris Scanning electron microscope (SEM) observations coupled with energy dispersive X-ray spectrometer (EDS) analyses were carried out on wear debris, and these are collected during tests under different loading conditions (Fig. 9). These analyses underline two stages of evolution in wear debris morphology. – Flat and long particles are collected for mild loading conditions. EDX Analyses of the corresponding debris do not reveal any iron presence. Hence transfer of brass onto the bearing steel cylinder is mainly responsible for these wear debris at this stage. The particles may result from a delamination process [17]. – Small and equiaxed debris correspond to severe loading conditions. They contain iron which results from abrasion of the steel cylinder. This morphology transition can possibly be explained through the occurrence of microstructure transformation, following the increase of temperature and plastic deformation [16].
3.2. Wear track on the bearing steel Fig. 8 shows two wear track surfaces on the bearing steel. For mild loading conditions (Fig. 8(a)) profilometric analyses performed on the corresponding wear scar show transfer phenomena without any significant abrasive wear. However, for the severe loading conditions, such as PZ80 N and VZ4 m/s, surface scratches are revealed with a darkened colour of the wear track (Fig. 8(b)). These two surface features can be explained by abrasion wear coupled with oxidation phenomenon. As soon as (a)
The following conclusions can be given from this work on a thermal effect on the sliding wear behaviour of brass (b)
6 amplitude (µm)
6 amplitude (µm)
4. Conclusions
4 2 0 -2
0
2000 4000 6000 8000 10000
contact width (mm)
4 2 0 -2
0
2000 4000
6000
8000 10000
contact width (mm)
Fig. 8. Friction tracks observed by optical microscope and profilometric analyses on ring steel. (a): PZ40 N, VZ1 m/s, (b): PZ80 N, VZ4 m/s.
K. Elleuch et al. / Tribology International 39 (2006) 290–296
295
Fig. 9. Debris modification as function of loading conditions (SEM observations). (a): PZ20 N, VZ1 m/s, (b): PZ20 N, VZ7 m/s.
CW614 rubbing against a ring bearing steel under unlubricated conditions. – Two wear responses are identified depending on loading conditions. Wear kinetics is found to be linear for low normal load and sliding velocity. However, for severe loading conditions wear kinetics has been found parabolic as a function of sliding distance. – There is a very close correlation between friction coefficient and temperature rise at the contact. Also, a good correlation is noted especially between wear and contact temperature. – The wear mechanisms in each wear regime were summarised in the wear mechanism map that also P(N)
delineates the loads and speeds at which wear transitions occur. Delamination wear may be the main mechanism observed in the mild wear regime. The transition to severe wear (plastic deformation and abrasive wear) was controlled by a critical surface temperature. A wear transition has been found when contact temperature reaches 150 8C (Fig. 10). – The wear transition is coupled to formation of a granular layer and transformation of wear debris. In fact, morphological change and chemical composition of wear debris depend on loading conditions. Especially, in severe wear, the debris contain iron.
References T= 150˚C
80 severe wear
40
mild wear
20
1
4
7
Fig. 10. Wear mechanisms maps for CW614.
V (m/s)
[1] Syed Asif SA, Biswas SK, Pramila Bai BN. Transfer and transitions in dry sliding wear of copper against steel. Scripta Met et Mater 1990;24: 1351–6. [2] Rigney DA, Chen LH, Naylor MGS, Rosenfield AR. Wear processes in sliding systems. Wear 1984;100:195–219. [3] Qi WX, Tu JP, Liu F, Yang YZ, Wang NY, Lu HM, et al. Microstructure and tribological behaviour of a peak aged Cu–Cr–Zr alloy. Mater Sci Eng 2003;A343:89–96. [4] Akagaki T, Kato K. Effects of hardness on the wear mode diagram in lubricated sliding friction of carbon steels. Wear 1990;141:1–15. [5] Alpas AT, Zhang J. Effect of SiC particulate reinforcement on the dry sliding wear of aluminium–silicon alloys (A356). Wear 1992;155: 83–104.
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[6] Mnif R, Elleuch K, Elleuch R, Fridrici V, Kapsa P., Thermal effect on wear behaviour of copper alloys. Second International Conference ICAME. 2004, Tunisia. [7] Goto H, Ashida M. Friction and wear of 60/40 brass during fretting corrosion under various environmental conditions. Tribol Int 1988;21: 183–90. [8] Zhang R, Zhu B. Mutual transfer of materials for dry sliding of brass against stainless steel. Wear 1990;140:207–22. [9] Sundberg M, Sundberg R. Metallographic aspects on wear of special brass. Wear 1987;115:151–65. [10] Mindivan H, Cimenoglu H, Kayali E. Microstructure and wear properties of brass synchroniser rings. Wear 2003;254:532–7. [11] Heilmann P, Don J, Sun TC, Rigney DA, Glaeser WA. Sliding wear and transfer. Wear 1983;91:171–90.
[12] Kong XL, Liu YB, Qiao LJ. Dry sliding tribological behaviors of nanocrystalline Cu–Zn surface layer after annealing in air. Wear 2004;256:747–53. [13] Gleiter H. Materials with ultrafine microstructures: retrospectives and perspectives. Nanostruct Mater 1992;1:1–19. [14] Sadykov FA, Barykin NP, Aslanyan IR. Wear of copper and its alloys with submicrocrystalline structure. Wear 1999;225–229: 649–55. [15] Zhang J, Alpas AT. Transition between mild and severe wear in aluminium alloys. Acta Mater 1997;45:513–28. [16] Wilson S, Alpas AT. Thermal effects on mild wear transitions in dry sliding of an aluminium alloy. Wear 1999;225–229:440–9. [17] Chen H, Alpas AT. Sliding wear map for the magnesium alloy Mg9Al-0.9 Zn (AZ91). Wear 2000;246:106–16.
Sliding wear transition for the CW614 brass alloy Khaled Elleucha,*, Riadh Elleucha, Ridha Mnif a, Vincent Fridricib, Philippe Kapsab a
LASEM, Ecole Nationale d’Inge´nieurs de Sfax, B.P. W, 3018 Sfax, Tunisie LTDS, CNRS, UMR 5513, Ecole Centrale de Lyon, Ecully Cedex, France
b
Received 6 July 2004; received in revised form 24 January 2005; accepted 27 January 2005 Available online 13 March 2005
Abstract Dry sliding wear tests were performed on a CW614 brass alloy using a pin-on-ring configuration. Wear kinetics were measured within a load range of 20–80 N and sliding velocity ranging from 1 to 7 m/s. Chemical compositions, morphologies and microstructures of worn surfaces and wear debris were characterised by scanning electron microscope (SEM) and energy dispersive X-ray spectrometer (EDS). Two main wear regimes have been observed: severe wear and mild wear. The results of wear tests and metallographic investigations on worn surfaces have been summarised in a wear mechanism map. It was found that the wear transition is controlled by a critical temperature at the contact surface. q 2005 Elsevier Ltd. All rights reserved. Keywords: Brass; Wear; Transition; Thermal effect
1. Introduction Copper alloys are widely used because of some attractive properties such as high electrical and thermal conductivity. These materials are used in friction parts of machines, such as bearing liners, bushing, etc. Sliding wear is also an important consideration in material processing by rolling, extrusion, forging, etc. and it is important to understand microstructure changes at the work-tool interface and the associated heat generation. Furthermore, it is known that metal wear involves complicated phenomena such as metal transfer [1], mechanical alloy-forming process [2], abrasive and adhesive wear [3], dynamic recrystallization [4] and wear transitions [5,6]. Much work has already been done to understand the effects of microstructure and environmental conditions on wear. Goto et al. [7] have studied the influence of water vapour, oxygen and water in various environments on the friction and wear of a 60/40 brass in homogeneous contact during fretting corrosion. Zhang et al. [8] have investigated dry sliding friction and wear in vacuum and air of brass against stainless steel. The mutual transfer of surface materials has been found. The influence of alloying * Corresponding author. Tel.: C216 74 274 088; fax: C216 74 275 595. E-mail address: [email protected] (K. Elleuch).
0301-679X/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2005.01.036
with aluminium, manganese, silicone, tin and iron on the wear properties of special brass has been discussed mainly in severe wear conditions [9]. Analyses of the surface layer developed during dry sliding against steel have shown good correlations among composition, hardness and wear resistance. For (aCb) high-strength brass, an increase of a-phase from 8 to 23 vol% decreased the hardness from 281 to 250 HV [10]. It is assumed [11] that wear processes are mainly determined by mechanisms of surface film formation and destruction. For ductile materials, such as brass, plastic deformation can be important. Recently much research dealing with friction and wear of nanocrystalline [12,13] and submicrocrystalline [14] brass have confirmed that they have improved tribological properties. The aim of this paper is to underline the thermal effect on friction and wear of brass when rubbed against bearing steel. A wear mapping approach has been undertaken to present the wear regimes and the corresponding mechanism.
2. Experimental procedure 2.1. Material The material studied was a CW614 brass alloy with the following chemical composition in weight percent:
K. Elleuch et al. / Tribology International 39 (2006) 290–296
291
P (N)
80
40
20 V (m/s) 1 Fig. 1. Optical micrograph of a/b brass.
7
4
Fig. 3. Test conditions (P: normal force, V: sliding velocity).
Table 1 Thermal and mechanical properties of brass CW 614 Young’s modulus, E (GPa)
Poisson’s ratio, n
Elastic limit, Re (MPa)
Hardness BRINELL, HB
Thermal conductivity, l (W/mK)
Specific heat, C (J/g 8C)
110
0.35
350
150
120
1.074
polishing and etching (using KELLER reagent). Microscope examination (Fig. 1) revealed that in the microstructure, the light coloured a phase precipitated in the dark coloured b phase matrix. The mechanical and thermal properties of CW614 brass alloy are given in Table 1. 2.2. Wear test The developed tribometer is a pin-on ring type. Contact configuration is chosen to be a cylinder on plane (Fig. 2). A continuous rotating motion is imposed on the steel ring at
(a)
3500 V=7m/s
wear depth (µm)
58.5%Cu, 2.8%Pb, 0.25%Sn, 0.11%Ni, 0.21%Fe, 0.01%Al and the balance Zn. The material was received in the form of extruded bar (120 mm!:6 mm) from which wear samples (cylinder of 6 mm in diameter and 32 mm in length) were machined. The surface preparation procedure of the wear test samples consisted of polishing the surface manually by 400, 600, 800 grit SiC abrasive papers, successively, and then polishing them with 1 and 0.05 mm alumina powder slurry using a low speed polishing machine. The counterface rings were cleaned in a methanol solution before each test. Microstructure of the investigated pin has been examined by optical microscope (OM) after grinding,
2800 2100 V=4m/s
1400 V=1m/s
700 0 0
500 1000 1500 2000 2500 3000 3500 Sliding distance(m)
wear depth (µm)
(b)
5000
V=7m/s V=4m/s
4000 3000
V=1m/s
2000 1000 0 0
500 1000 1500 2000 2500 3000 3500 Sliding distance(m)
Fig. 2. The developed pin-on-ring tribometer at Ecole Centrale de Lyon.
Fig. 4. Wear depth evolution as function of sliding distance for three sliding velocities (a): PZ20 N, (b): PZ40 N.
50
1
T
40
0.8
30
0.6
20
0.4
f
10
0.2
0 0
500
1000
1500
2000
2500
3000
0 3500
Sliding distance (m)
2
T
250
1.6
200
1.2
150 0.8
100
0.4
50
f
0 0
Temperature (˚C)
(c)
Friction coefficient
300
500
1000 1500 2000 2500 Sliding distance (m)
0 3000 3500
300
1.2 T
250
1
200
0.8
150
0.6
100
0.4 f
50
0.2
0 0
500
1000
1500
2000
2500
3. Results and discussion The instantaneous interpenetration is designated as wear depth and stored for the test duration. The global linear evolution, relative to this parameter, is noted for mild loading conditions. Fig. 4 gives examples for such variation. It shows a parabolic evolution as a function of sliding distance for severe loading conditions, such as vZ7 m/s and PZ20 N or PZ40 N. However, quasi-linear variation is demonstrated for mild loading conditions, such as vZ4 m/s and PZ20 N or PZ40 N. Furthermore, the friction coefficient and surface contact temperature for mild loading conditions (PZ20 N, VZ1 m/s) are almost steady and have only weak variations (a)
Friction coefficient
Temperature (˚C)
(b)
surface. The sample holder is made of ceramic to limit loss of thermal energy which arises from the friction of the contacting bodies. Simultaneously tangential force data are stored in real time during the imposed sliding distance (dZ3250 m).
Temperature (˚C)
Temperature (˚C)
(a)
K. Elleuch et al. / Tribology International 39 (2006) 290–296
Friction coefficient
292
0 3000
350 300 250 200
V=7m/s
150 100 50 0
V=4m/s V=1m/s 0
500 1000 1500 2000 2500 3000 3500 Sliding distance(m)
Sliding distance (m)
a constant velocity. The ring is a standard ring bearing steel (52100 steel), has a 33 mm as radius and is fixed on the engine. A constant weight is used to apply the normal load. A specific device using a rubber element is used to decrease vibrations, especially noted for severe loading conditions (high normal force and sliding velocity). Fig. 3 shows a diagram summarizing all operating conditions. Each point represents a separate test. Performing these tests gives birth to a wear track and debris creation. Wear depth was determined as a function of sliding distance at each normal load and sliding velocity condition. The resulting interpenetration of the contacting bodies is stored in real time using an inductive displacement sensor. The contact temperatures were measured using a chromel-alumel type thermocouple which is placed at the back of the brass sample. The measured value corresponds to the temperature of brass at 10 mm from the contact
350
V=7m/s
300 250 200 150
V=4m/s
100 V=1m/s
50 0 0 (c) Temperature (˚C)
Fig. 5. Friction coefficient and surface contact temperature evolution as function of sliding distance (a): PZ20 N, VZ1 m/s, (b): PZ20 N, VZ 7 m/s, (c): PZ80 N, VZ4 m/s.
Temperature (˚C)
(b)
500 1000 1500 2000 2500 3000 3500 Sliding distance(m)
300 250 V=4m/s
200 150 100
V=1m/s
50 0 0
500
1000 1500 2000 2500 3000 3500 Sliding distance(m)
Fig. 6. Surface contact temperature as function of sliding distance for different sliding velocities. (a): PZ20 N, (b): PZ40 N, (c): PZ80 N.
K. Elleuch et al. / Tribology International 39 (2006) 290–296
(Fig. 5(a)). However, for severe loading conditions, two types of variation are noted. Fig. 5(b) shows a continuous jump for both friction coefficient and temperature. However, Fig. 5(c) illustrates sudden and discontinuous variations. Similar results are found for aluminium alloy—steel [15,16] but there is no quantification of the instantaneous wear (only a global mass loss is measured at the end of the test). However, instantaneous surface temperature variation against sliding distance is available [15,16]. Wear tests performed at high speed are quickly blocked because of the transfer mechanism (seizure) which arrests the engine and ends the test. An illustration of temperature variation as a function of sliding distance is proposed in Fig. 6 for several normal loads (20, 40 and 80 N). Two steps are indicated. – First stage: at the beginning of the test, the temperature increases and then reaches a constant value. Hence,
293
thermal balance is well established between the contacted surfaces, – Second stage: when reaching 150 8C, the temperature shows a quick increase. A transition in heat kinetics is noticed at particular loading conditions which appear at variable sliding distance [16]. A close correlation between friction coefficient and surface contact temperature is confirmed and proves that temperature increase is controlled through surface phenomena which can be summarized by the friction coefficient. Analyses of the wear mechanisms are performed to explain the origins of these transitions. 3.1. Wear track on the copper alloy First of all, post mortem analysis of the pin surface (CW614) is proposed to show differences before and after transition responses.
Fig. 7. SEM images of wear scars on pin CW614 (a) PZ20 N, (b) PZ40 N, VZ7 m/s: formation of a granular layer.
294
K. Elleuch et al. / Tribology International 39 (2006) 290–296
Typical electron microscopic observations of the wear tracks are shown in Fig. 7(a). For mild loading conditions, the contact centre shows some scratches that are oriented in the sliding direction, whereas at the contact borders there is wedge formation. This phenomenon is emphasised for severe load conditions. In fact, the contact surface becomes more disturbed and plastic deformation is more intense. Hence, adhesion phenomena are emphasised, which leads to seizing. Probably, the increase of temperature due to friction heats the pin surface and softens the surface of the brass sample. Then the material will be plastically deformed and will be extruded towards the contact edges, with superposition of material layers [16]. Indeed, for severe loading conditions (PZ40 N and VZ7 m/s), the pin surface is characterized by the presence of small grey particles (Fig. 7(b)) which are gathered in clusters. These reveal physicochemical transformations produced at the interface as a consequent of the impact of heat flow on the rubbed brass. Formation of such layers can be the origin of the friction coefficient transition.
the temperature becomes ’sufficiently’ high (typically TZ150 8C), a granular layer with high mechanical properties forms as a third body (Fig. 7(b)) and gives rise to wear of the ring steel. 3.3. Wear debris Scanning electron microscope (SEM) observations coupled with energy dispersive X-ray spectrometer (EDS) analyses were carried out on wear debris, and these are collected during tests under different loading conditions (Fig. 9). These analyses underline two stages of evolution in wear debris morphology. – Flat and long particles are collected for mild loading conditions. EDX Analyses of the corresponding debris do not reveal any iron presence. Hence transfer of brass onto the bearing steel cylinder is mainly responsible for these wear debris at this stage. The particles may result from a delamination process [17]. – Small and equiaxed debris correspond to severe loading conditions. They contain iron which results from abrasion of the steel cylinder. This morphology transition can possibly be explained through the occurrence of microstructure transformation, following the increase of temperature and plastic deformation [16].
3.2. Wear track on the bearing steel Fig. 8 shows two wear track surfaces on the bearing steel. For mild loading conditions (Fig. 8(a)) profilometric analyses performed on the corresponding wear scar show transfer phenomena without any significant abrasive wear. However, for the severe loading conditions, such as PZ80 N and VZ4 m/s, surface scratches are revealed with a darkened colour of the wear track (Fig. 8(b)). These two surface features can be explained by abrasion wear coupled with oxidation phenomenon. As soon as (a)
The following conclusions can be given from this work on a thermal effect on the sliding wear behaviour of brass (b)
6 amplitude (µm)
6 amplitude (µm)
4. Conclusions
4 2 0 -2
0
2000 4000 6000 8000 10000
contact width (mm)
4 2 0 -2
0
2000 4000
6000
8000 10000
contact width (mm)
Fig. 8. Friction tracks observed by optical microscope and profilometric analyses on ring steel. (a): PZ40 N, VZ1 m/s, (b): PZ80 N, VZ4 m/s.
K. Elleuch et al. / Tribology International 39 (2006) 290–296
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Fig. 9. Debris modification as function of loading conditions (SEM observations). (a): PZ20 N, VZ1 m/s, (b): PZ20 N, VZ7 m/s.
CW614 rubbing against a ring bearing steel under unlubricated conditions. – Two wear responses are identified depending on loading conditions. Wear kinetics is found to be linear for low normal load and sliding velocity. However, for severe loading conditions wear kinetics has been found parabolic as a function of sliding distance. – There is a very close correlation between friction coefficient and temperature rise at the contact. Also, a good correlation is noted especially between wear and contact temperature. – The wear mechanisms in each wear regime were summarised in the wear mechanism map that also P(N)
delineates the loads and speeds at which wear transitions occur. Delamination wear may be the main mechanism observed in the mild wear regime. The transition to severe wear (plastic deformation and abrasive wear) was controlled by a critical surface temperature. A wear transition has been found when contact temperature reaches 150 8C (Fig. 10). – The wear transition is coupled to formation of a granular layer and transformation of wear debris. In fact, morphological change and chemical composition of wear debris depend on loading conditions. Especially, in severe wear, the debris contain iron.
References T= 150˚C
80 severe wear
40
mild wear
20
1
4
7
Fig. 10. Wear mechanisms maps for CW614.
V (m/s)
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