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Improvement in friction by cw Nd:YAG laser surface treatment on cast iron cylinder bore
Applied Surface Science 205 (2003) 289–296
Improvement in friction by cw Nd:YAG laser surface treatment on cast iron cylinder bore G. Duffeta,*, P. Sallamanda, A.B. Vannesb a
Laboratoire Laser et Traitements des Mate´riaux (LTm), IUT Le Creusot, 71200 Le Creusot, France Inge´nierie et Fonctionnalisation des Surfaces(IFOS), Ecole Centrale de Lyon (ECL), BP 163 69131 Ecully Cedex, France
b
Received 24 June 2002; received in revised form 30 September 2002; accepted 30 September 2002
Abstract The reduction of the oil-film thickness in the piston-assembly of automobile engines has lead to it now being necessary to add a cylinder–bore surface treatment, so that oil reserves can be created on it. However in doing this, the initial plateaued texture must be kept. With a Nd:YAG laser source treatments oil reserves holes are formed, which respect the function of the initial bore surface texture. The material, cast iron, is heterogeneous: perlitic matrix and carbon lamellae. The laser treatment will make use of this heterogeneity: as under a cw laser beam, using graphite, with its good insulating material properties, carbon lamellae, which are near the surface, are revealed. Also a high density of lamellae in a hollow against the surface are obtained. In order to improve these new surfaces, friction tests were conducted on a friction bench using samples from engine components as test pieces. In order to know bore roughness evolution, 3D surface topography measurements were made before and after as well as friction tests on surfaces without laser treatment and with laser treatment. The results indicate that these holes can improve oil lubrication time. # 2002 Elsevier Science B.V. All rights reserved. PACS: 42.62.-b; 81.65.Ya; 81.40.Pq Keywords: Cast iron; Laser treatment; Roughness; Friction
1. Introduction In the investigation of engine-oil, consumptionreduction, the lubricating mechanism between a piston ring and cylinder liner is the most important tribological system. In this mechanism, an important factor which affects the friction is the cylinder liner surface. Even modest friction reductions between the * Corresponding author. Tel.: þ33-3-85-42-43-17; fax: þ33-2-85-42-43-29. E-mail address: [email protected] (G. Duffet).
ring and the liner will lead to a significant reduction in the fuel consumption. Some liner surface processes follow three stages: boring, base honing and plateau honing. With boring and base honing processes, it is possible to generate not only surface valleys but also peaks. Thus, the goal of plateau honing is to remove the surface peaks and thereby to generate plateaus between the surface valleys. Ren Jeng [1] has investigated the tribological performance of plateaued and non-plateaued surfaces. The results show that, in the hydrodynamic lubrication regime, plateaued and non-plateaued surfaces have the
0169-4332/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 1 1 1 9 - 4
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G. Duffet et al. / Applied Surface Science 205 (2003) 289–296
same friction an that in the mixed lubrication regime, the plateaued surface has less friction, even though the plateaued surfaces and non-plateaued surfaces have the same root-mean-square of surface heights (Rq) but a very different surface height skewness parameter (Ssk). There have been a lot of studies concerned with the changes of the cylinder surface topography in the different stages of the engine life: the initial stage of engine life [2], bore polish [3], cylinder bore wear [4,5], etc. Furthermore, after a period of time the cylinder bore texture changes, even for the midstroke region, which is supposed to work in the hydrodynamic regime. The result of the current trends is to increase the pressure and the temperature of the piston assembly and then to decrease the oil film thickness [6]. Laser treatment, without any material supply, plays an important role in the surface-processing field: surface hardening, remelting and other modifications. The advantages of laser-heating are that the thermal influence zone and surface distortions are reduced to a minimum. For example, Ganeev [7] has shown the possibility of increasing the surface hardening of steels by low-power CO2 laser, which can then satisfy the requirements of various areas of engineering but with smaller speeds of processing. For our application, the laser machining speed has to be important so that this treatment can be inserted in the industrial process. For this purpose, a high-power cw Nd:YAG laser is used: the cw laser manufactured by Haas company (HL3006D type) is able to deliver up to 3 kW of laser power.
Furthermore, the laser beam is dispatched to the work piece after crossing the 600 mm diameter optical cable and the focusing system: the optical cable advantage is the simplicity of processing narrows zones, like a cylinder surface. The goal of our work is to obtain an amelioration, even modest, of the ring and liner surface friction by laser treatment of the liner surface with respect to the function of the initial plateaued texture. The bore surface had to be treated without oxidation or melting. The idea was to use the material heterogeneity, perlitic matrix and carbon lamellae, under a cw laser beam and then, to create oil reserves on the liner surface. In new engines, the oil film thickness is of the order of 1 mm or less: near top dead center (TDC) and bottom dead center (BDC) where ring velocity goes to zero and where the oil has time to ‘‘drain’’ from the walls [5]. So it is important to create oil reserves on the liner surface. Furthermore, the laser treatment is local and can be fitted to the encountered lubrication. It is not necessary to treat the whole cylinder bore surface. The characterisation criterion will be the friction coefficient evolution for a fixed oil-quantity and the reference is the plateau honing surface (without laser surface treatment).
2. Test method and tests 2.1. Friction The first interest of our friction bench (Fig. 1) is to use samples from engine components as test pieces: a
Fig. 1. Schematic diagram of our friction bench.
G. Duffet et al. / Applied Surface Science 205 (2003) 289–296
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Table 1 Friction test conditions Normal load (N)
Frequency (Hz)
Average speed (m/s)
Oil (ml)
Data acquisition rate
Test duration (h)
450
1
0.02
0.1
Data/10 s
2–8
top compression ring and a cylinder bore sample (24 mm 50 mm). The ring hardness value is 900 Hv0.3 and bore sample one 244 Hv0.3. The second one is to have the axis of the cylinder bore vertical as for in-line engines. The piston ring was constrained to an outside diameter of 83 mm, in a ring holder with a window in front of the cylinder bore sample. The cylinder bore sample was clamped and reciprocated through a stroke, s at a frequency f. This sample was loaded against the ring at a load, the normal load, FN. The operator controls include normal load, frequency and stroke. In this study, the frequency was set at 1 Hz and the stroke at 10 mm. During the tests, the average speed of the ring was 0.02 m/s, which is very low from the average engine speed. But near top dead center and bottom dead center, where contact conditions are then most severe, the ring velocity goes to zero.The normal load was set at 450 N, which corresponds to a cylinder pressure of 10 MPa. The test conditions can be found in Table 1. Lubrication was accomplished by placing 0.1 ml of SAE 15W40 engine oil above the ring-bore contact zone immediately prior to the start of the run. The tangential and normal loads were monitored then friction coefficient versus time was obtained. Per cycle, the friction coefficient was calculated as the ratio of the maximal tangential load to the mean normal load.
Fig. 2. Schematic friction coefficient evolution curve (initial friction coefficient invariability length ¼ ifci; friction coefficient evolution length ¼ fcel).
The friction coefficient reaches up different values by plateaux (Fig. 2). This evolution allowed the following indicators to be given. The initial friction coefficient invariability length (ifci). This value indicates how long the first plateau lasts. The friction coefficient evolution length (fcel). This indicator gives information about the degradation speed of the friction conditions. To determine this value, the evolution length before reaching a plateau at a fixed value, is calculated. Fig. 3 shows an experimental curve example. Some tests were several hours in duration to get the whole friction coefficient evolution. Other ones were shorter and stopped after the first increase to determine why the friction coefficient changed. For each surface treatment, seven repeat tests were conducted and five were kept for our study. A fresh ring was used for each test. It was too difficult to duplicate the whole environment of an engine, that is why we needed reference tests to be able to measure a change of the friction conditions after laser surface treatment. These reference tests were made on representative surface finishes, plateau honing, without laser surface treatment.
Fig. 3. Experimental friction coefficient evolution curve.
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2.2. Roughness Bore roughness is evaluated by a 3D surface topography measurement system and by TOPOSURF 3D program: this 3D topography is constituted by N 2D profiles on 250 mm squares. The roughness was measured on the surfaces before the test and also on those that had stopped just after the first friction coefficient increase, for each surface treatment. The material loss is then obtained from the profile values measured before and after a test: in order to not take into account the z mean value, the difference between valley height mean (SRvm) and peak height mean (SRpm) was used, even if no distortion occurred in the valleys. To know the scatter in the valley and peak height means, SRt is also calculated: SRt is the difference between the highest peak an the lowest valley. The root-mean-square of surface height distribution (Rq) is also taken into account. For the fluid film lubrication, the roughness height can be adequately described by its root-mean-square (Rq) value and the particular form of the surface roughness is not important [1]. In mixed lubrication, the total applied load is carried partly by the hydrodynamic action of the lubricant film and partly by asperity contacts. That is why peak radius, b, was manually measured on surface photographs for each treatment. From b and Rq, the real contact pressure Pr can be calculated [8]: 0:5 Rq Pr ¼ aE with b 1 1 n2 ðboreÞ 1 n2 ðringÞ þ ; ¼ E EðboreÞ EðringÞ
and EðboreÞ ¼ 126 GPa; EðringÞ ¼ 289 GPa; nðringÞ ¼ nðboreÞ ¼ 0:3 [9]. Pr take into account the lubricant film and asperity contacts.
3. Initial bore surface characterisation In this case, the surfaces are the ‘‘classical’’ plateau honing surfaces without laser treatment. The friction test conditions are given in Table 1. The results given in Table 2 are five test means. A typical friction coefficient evolution versus time curve is shown in Fig. 4. The before and after friction test, roughness measurements are given in Table 3. For these measurements, the tests are stopped just after the first friction coefficient increase. A wear loss of 50 nm is obtained during the friction tests: little wear is found in this first increase of the friction coefficient on plateau honing surfaces. On these surfaces (Fig. 5), the real pressure contact, Pr, was also calculated from b, peak radius, and from Rq, root-mean-square (Table 4).
Table 2 Initial bore surface friction test results
Mean Variation
Initial friction coefficient
Final friction coefficient
ifci (s)
fcel (s)
0.034 0.035–0.034
0.044 0.042–0.047
6690
3950
Initial friction coefficient invariability length: ifci; friction coefficient evolution length: fcel.
Rq2 ¼ Rq2 ðboreÞ þ Rq2 ðringÞ
Fig. 4. Optic photographs of cw laser treatment on polished cast iron surface and on bore surface.
G. Duffet et al. / Applied Surface Science 205 (2003) 289–296 Table 3 Initial bore surface roughness measurements
Before friction test After friction test
SRvm (nm)
SRpm (nm)
SRpm SRvm (nm)
SRt (nm)
295 265
280 255
575 525
800 800
Because no significant variation was measured, the same, radius peak, b was used. Then a rough estimate of the real contact pressure, 1400 MPa, was found.
4. Cw laser treated bore surface characterisation 4.1. Treatment with a cw laser source Engine cylinder material is cast iron with a perlitic matrix and carbon lamellae (composition in Table 5). Graphite can be a solid lubricant, hence it is interesting to expose on cylinder surface more carbon lamellae. Graphite is also a good insulating material. Using this property, the idea was to vaporise metal above the lamellae which are present near the surface. The thermal energy source is a cw Nd: YAG laser beam. The power has to be sufficient to vaporise metal
293
present above the lamellae without surface melting and the beam displacement velocity has to be important enough so that laser surface treatment can be inserted in the industrial process. These factors lead to the choice of the laser beam parameters (power and velocity). Hence, our parameters were the following: power 1200 W, velocity 6 m/min. The achieved surface showed a high density of carbon lamellae: treatments with the same conditions (laser power and velocity) were made on polished cast iron surfaces and on bore surfaces (Fig. 4). The lamella length was about a 100 mm and the width was just a few microns. Furthermore revealed lamellae were in a hollow against the surface (Fig. 6). A dry test on a cw laser treated surface leads to the same friction coefficient value as a dry test on surface finishes, plateau honing, without laser treatment. This indicates that little holes are present above the revealed carbon lamellae. These holes will constitute oil reserves during a lubricated test. Then in a first stage, cw treatment will improve lubrication conditions by the creations of oil reserves but not by the revelation of solid lubricants. 4.2. Friction tests The friction test conditions are given in Table 1. The results given in Table 6 are five test means.
Fig. 5. SEM photographs of cylinder bore cross-section without treatment and with cw laser treatment.
Table 4 Initial bore surface roughness results
Before friction test After friction test
E (GPa)
Rq (nm)
b (mm)
Rq/b (104)
Pr (MPa)
96 96
73.60 64.90
40 40
18.4 16.2
1445 1370
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Table 5 Cast iron composition (wt.%) C
Si
Mn
S
P
Ni
Cr
Mo
Ca
Al
3.07 1.81 0.87 0.11 0.11 0.08 0.41 0.57 1.46 0.036
Table 6 Cw surface friction test results
Mean Variation
Initial friction coefficient
Final friction coefficient
ifci (s)
fcel (s)
0.034 0.034–0.035
0.040 0.039–0.042
8725
3200
For these measurements, tests are stopped just after the first friction coefficient increase. A wear loss of 270 nm is obtained during friction tests: cw laser treated bore surface wear is five times greater than wear on plateau honing surface. On cw laser treated surfaces (Fig. 5), the real pressure contact, Pr, was also calculated from b, peak radius, and from Rq, root-mean-square (Table 8). Because no significant variation was measured, the same, radius peak, b was used.Then a rough estimate of the real contact pressure, 990 MPa was found.
5. Discussion Table 7 Cw surface roughness measurements
Before friction test After friction test
Fig. 7 shows the friction coefficient evolution versus time for the two studied surfaces.
SRvm (nm)
SRpm (nm)
SRpm SRvm (nm)
SRt (nm)
630 470
555 445
1190 920
2000 2000
A typical friction coefficient evolution versus time curve is shown in Fig. 2. The before and after friction test roughness measurements are given in Table 7.
5.1. Initial value of the friction coefficient and first plateau length The initial value is the same for the two cases: without treatment and cw treatment. The first plateau, ifci (Fig. 2), is at the same level, only the duration changes: in this hydrodynamic regime, the surface differences have no effect on the friction coefficient. The first plateau length, ifci (Fig. 2), is different for the two treatments.
Fig. 6. Optic and SEM photographs of revealed carbon lamellae in a hollow against the surface.
Table 8 Cw surface roughness results
Before friction test After friction test
E (GPa)
Rq (nm)
b (mm)
Rq/b (104)
Pr (MPa)
96 96
98.89 77.63
100 100
9.9 7.8
1050 935
G. Duffet et al. / Applied Surface Science 205 (2003) 289–296
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Fig. 7. Friction coefficient vs. time curves.
Cw treatment significantly yields larger first plateau length than no laser treatment (30%). Oil stays more time in the contact. Cw surface has more holes (revealed lamellae in a hollow against the surface) than without treatment surface. Furthermore, asperities are truncated by cw treatment: the peak radius decreases from 100 to 40 mm, then the surface asperities contact later, when the mixed lubrication mode begins. 5.2. First evolution length and level of the friction coefficient In mixed lubrication, friction is composed of a hydrodynamic component and a boundary component. The boundary component is due to asperity contacts. The laser treatment decreases the length of the friction coefficient evolution to reach a second plateau: cw treatment yields a 20% reduction. Cw treatment truncated asperities contact on a more important surface than untruncated asperities, so the evolution will be quicker: the boundary component is greater because the asperity contact load ratio increases. In some friction curves, the beginning of the second plateau shows a maximum as in Fig. 3. Firstly, we thought that this maximum was due to melted zones in relief near carbon lamellae; but this phenomena exists for treated surfaces and for untreated ones. It would be uncertainly an asperity adaptation.
On cw treatment surface, wear loss at the end of the first evolution had a value of 270 nm with a 990 MPa contact pressure. On the surface, that had not been treated, the wear loss at the end of the first evolution had a value of 50 nm with a 1400 MPa contact pressure. Then, the measured cw treatment wear was more important than on the surface without treatment wear because the contact surface was more important during this first evolution (despite a weaker contact pressure). Cw treatment yields lower friction coefficient value (0.040 instead of 0.044). Ren Jeng [1] showed that plateaued surfaces reach steady-state wear quickly but with a lower rate than non-plateaued surfaces and he concluded that plateaued surfaces would have lower wear during their operational life. Our cw treatment, and without treatment, surfaces show a behaviour difference very similar than plateaued and non-plateaued ones. The friction coefficient evolution of laser treated surfaces is quicker and the friction coefficient level is lower than non-treated ones.
6. Conclusions The bore texture function in ring-liner tribological system is well known and must be kept, but the current trend to have an oil film thickness decrease in the piston assembly leads to the creation of oil reserves on
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the bore surfaces. The Nd:YAG laser beams allow holes, on the initial bore texture, to be obtained. Cw laser treatment: Significantly yields larger hydrodynamic regime, oil stays more in the contact. In the mixed lubrication regime, the friction coefficient evolution is quicker but the reached level is lower on laser treatment surfaces than on without laser treatment and then the operational life length would be increased. It could be necessary to fit oil reserves to the encountered lubrication: for example, at top dead centre (TDC), boundary friction can lead to the polishing of the cylinder bore surface and to an increase in oil consumption. Galligan et al. [9] explained with an elastoplastic model of asperity contact that, in boundary lubrication, friction rises by polishing the contact surface. The exhaustion of the lubricant in the contact is an important parameter of this phenomena, so it is important to create deep oil reserves at TDC. A means of creating deep oil holes is by the use of a Q-switched Nd:YAG laser beam. This is part of another research in this treatment, the thermal effect is very low and melting material is ejected. But these oil reserves have to be efficient and their geometry must not lead to a friction rise.
Acknowledgements The authors are grateful for TOPOSURF 3D measurements from Prof. Zaouani, Laboratoire de Tribologie et Dynamique des Syste`mes, Ecole Centrale de Lyon 69131 Ecully.
References [1] Y. Ren Jeng, Impact of plateaued surfaces on tribological performance, Tribol. Perform. 39 (1996) 354–361. [2] P. Pawlus, Change of cylinder surface topography in the initial stage of engine life, Wear 209 (1997) 69–83. [3] A.K. Gondal, F.A. Davis, T.S. Eyre, Bore polish, Mater. Sci. Technol. 14 (1998) 67–72. [4] B.-G. Rose´ n, R. Ohlsson, T.R. Thomas, Wear of cylinder bore micro topography, Wear 198 (1996) 271–279. [5] E.P. Becker, K.C. Ludema, A qualitative empirical model of cylinder bore wear, Wear 225 (1999) 387–404. [6] M. Priest, C.M. Taylor, Automobile engine tribologyapproaching the surface, Wear 241 (2000) 193–203. [7] R.A. Ganeev, Low-power laser hardening of steels, J. Mater. Process. Technol. 121 (2002) 414–419. [8] J.M. Georges, Frottement, usure et lubrification, CNRS editions Eyrolles, 2000. [9] J. Galligan, A.A. Torrance, G. Liraut, A scuffing test for piston ring/bore combinations: Part I. Stearic acid lubrication, Wear 236 (1999) 199–209.
Improvement in friction by cw Nd:YAG laser surface treatment on cast iron cylinder bore G. Duffeta,*, P. Sallamanda, A.B. Vannesb a
Laboratoire Laser et Traitements des Mate´riaux (LTm), IUT Le Creusot, 71200 Le Creusot, France Inge´nierie et Fonctionnalisation des Surfaces(IFOS), Ecole Centrale de Lyon (ECL), BP 163 69131 Ecully Cedex, France
b
Received 24 June 2002; received in revised form 30 September 2002; accepted 30 September 2002
Abstract The reduction of the oil-film thickness in the piston-assembly of automobile engines has lead to it now being necessary to add a cylinder–bore surface treatment, so that oil reserves can be created on it. However in doing this, the initial plateaued texture must be kept. With a Nd:YAG laser source treatments oil reserves holes are formed, which respect the function of the initial bore surface texture. The material, cast iron, is heterogeneous: perlitic matrix and carbon lamellae. The laser treatment will make use of this heterogeneity: as under a cw laser beam, using graphite, with its good insulating material properties, carbon lamellae, which are near the surface, are revealed. Also a high density of lamellae in a hollow against the surface are obtained. In order to improve these new surfaces, friction tests were conducted on a friction bench using samples from engine components as test pieces. In order to know bore roughness evolution, 3D surface topography measurements were made before and after as well as friction tests on surfaces without laser treatment and with laser treatment. The results indicate that these holes can improve oil lubrication time. # 2002 Elsevier Science B.V. All rights reserved. PACS: 42.62.-b; 81.65.Ya; 81.40.Pq Keywords: Cast iron; Laser treatment; Roughness; Friction
1. Introduction In the investigation of engine-oil, consumptionreduction, the lubricating mechanism between a piston ring and cylinder liner is the most important tribological system. In this mechanism, an important factor which affects the friction is the cylinder liner surface. Even modest friction reductions between the * Corresponding author. Tel.: þ33-3-85-42-43-17; fax: þ33-2-85-42-43-29. E-mail address: [email protected] (G. Duffet).
ring and the liner will lead to a significant reduction in the fuel consumption. Some liner surface processes follow three stages: boring, base honing and plateau honing. With boring and base honing processes, it is possible to generate not only surface valleys but also peaks. Thus, the goal of plateau honing is to remove the surface peaks and thereby to generate plateaus between the surface valleys. Ren Jeng [1] has investigated the tribological performance of plateaued and non-plateaued surfaces. The results show that, in the hydrodynamic lubrication regime, plateaued and non-plateaued surfaces have the
0169-4332/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 1 1 1 9 - 4
290
G. Duffet et al. / Applied Surface Science 205 (2003) 289–296
same friction an that in the mixed lubrication regime, the plateaued surface has less friction, even though the plateaued surfaces and non-plateaued surfaces have the same root-mean-square of surface heights (Rq) but a very different surface height skewness parameter (Ssk). There have been a lot of studies concerned with the changes of the cylinder surface topography in the different stages of the engine life: the initial stage of engine life [2], bore polish [3], cylinder bore wear [4,5], etc. Furthermore, after a period of time the cylinder bore texture changes, even for the midstroke region, which is supposed to work in the hydrodynamic regime. The result of the current trends is to increase the pressure and the temperature of the piston assembly and then to decrease the oil film thickness [6]. Laser treatment, without any material supply, plays an important role in the surface-processing field: surface hardening, remelting and other modifications. The advantages of laser-heating are that the thermal influence zone and surface distortions are reduced to a minimum. For example, Ganeev [7] has shown the possibility of increasing the surface hardening of steels by low-power CO2 laser, which can then satisfy the requirements of various areas of engineering but with smaller speeds of processing. For our application, the laser machining speed has to be important so that this treatment can be inserted in the industrial process. For this purpose, a high-power cw Nd:YAG laser is used: the cw laser manufactured by Haas company (HL3006D type) is able to deliver up to 3 kW of laser power.
Furthermore, the laser beam is dispatched to the work piece after crossing the 600 mm diameter optical cable and the focusing system: the optical cable advantage is the simplicity of processing narrows zones, like a cylinder surface. The goal of our work is to obtain an amelioration, even modest, of the ring and liner surface friction by laser treatment of the liner surface with respect to the function of the initial plateaued texture. The bore surface had to be treated without oxidation or melting. The idea was to use the material heterogeneity, perlitic matrix and carbon lamellae, under a cw laser beam and then, to create oil reserves on the liner surface. In new engines, the oil film thickness is of the order of 1 mm or less: near top dead center (TDC) and bottom dead center (BDC) where ring velocity goes to zero and where the oil has time to ‘‘drain’’ from the walls [5]. So it is important to create oil reserves on the liner surface. Furthermore, the laser treatment is local and can be fitted to the encountered lubrication. It is not necessary to treat the whole cylinder bore surface. The characterisation criterion will be the friction coefficient evolution for a fixed oil-quantity and the reference is the plateau honing surface (without laser surface treatment).
2. Test method and tests 2.1. Friction The first interest of our friction bench (Fig. 1) is to use samples from engine components as test pieces: a
Fig. 1. Schematic diagram of our friction bench.
G. Duffet et al. / Applied Surface Science 205 (2003) 289–296
291
Table 1 Friction test conditions Normal load (N)
Frequency (Hz)
Average speed (m/s)
Oil (ml)
Data acquisition rate
Test duration (h)
450
1
0.02
0.1
Data/10 s
2–8
top compression ring and a cylinder bore sample (24 mm 50 mm). The ring hardness value is 900 Hv0.3 and bore sample one 244 Hv0.3. The second one is to have the axis of the cylinder bore vertical as for in-line engines. The piston ring was constrained to an outside diameter of 83 mm, in a ring holder with a window in front of the cylinder bore sample. The cylinder bore sample was clamped and reciprocated through a stroke, s at a frequency f. This sample was loaded against the ring at a load, the normal load, FN. The operator controls include normal load, frequency and stroke. In this study, the frequency was set at 1 Hz and the stroke at 10 mm. During the tests, the average speed of the ring was 0.02 m/s, which is very low from the average engine speed. But near top dead center and bottom dead center, where contact conditions are then most severe, the ring velocity goes to zero.The normal load was set at 450 N, which corresponds to a cylinder pressure of 10 MPa. The test conditions can be found in Table 1. Lubrication was accomplished by placing 0.1 ml of SAE 15W40 engine oil above the ring-bore contact zone immediately prior to the start of the run. The tangential and normal loads were monitored then friction coefficient versus time was obtained. Per cycle, the friction coefficient was calculated as the ratio of the maximal tangential load to the mean normal load.
Fig. 2. Schematic friction coefficient evolution curve (initial friction coefficient invariability length ¼ ifci; friction coefficient evolution length ¼ fcel).
The friction coefficient reaches up different values by plateaux (Fig. 2). This evolution allowed the following indicators to be given. The initial friction coefficient invariability length (ifci). This value indicates how long the first plateau lasts. The friction coefficient evolution length (fcel). This indicator gives information about the degradation speed of the friction conditions. To determine this value, the evolution length before reaching a plateau at a fixed value, is calculated. Fig. 3 shows an experimental curve example. Some tests were several hours in duration to get the whole friction coefficient evolution. Other ones were shorter and stopped after the first increase to determine why the friction coefficient changed. For each surface treatment, seven repeat tests were conducted and five were kept for our study. A fresh ring was used for each test. It was too difficult to duplicate the whole environment of an engine, that is why we needed reference tests to be able to measure a change of the friction conditions after laser surface treatment. These reference tests were made on representative surface finishes, plateau honing, without laser surface treatment.
Fig. 3. Experimental friction coefficient evolution curve.
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G. Duffet et al. / Applied Surface Science 205 (2003) 289–296
2.2. Roughness Bore roughness is evaluated by a 3D surface topography measurement system and by TOPOSURF 3D program: this 3D topography is constituted by N 2D profiles on 250 mm squares. The roughness was measured on the surfaces before the test and also on those that had stopped just after the first friction coefficient increase, for each surface treatment. The material loss is then obtained from the profile values measured before and after a test: in order to not take into account the z mean value, the difference between valley height mean (SRvm) and peak height mean (SRpm) was used, even if no distortion occurred in the valleys. To know the scatter in the valley and peak height means, SRt is also calculated: SRt is the difference between the highest peak an the lowest valley. The root-mean-square of surface height distribution (Rq) is also taken into account. For the fluid film lubrication, the roughness height can be adequately described by its root-mean-square (Rq) value and the particular form of the surface roughness is not important [1]. In mixed lubrication, the total applied load is carried partly by the hydrodynamic action of the lubricant film and partly by asperity contacts. That is why peak radius, b, was manually measured on surface photographs for each treatment. From b and Rq, the real contact pressure Pr can be calculated [8]: 0:5 Rq Pr ¼ aE with b 1 1 n2 ðboreÞ 1 n2 ðringÞ þ ; ¼ E EðboreÞ EðringÞ
and EðboreÞ ¼ 126 GPa; EðringÞ ¼ 289 GPa; nðringÞ ¼ nðboreÞ ¼ 0:3 [9]. Pr take into account the lubricant film and asperity contacts.
3. Initial bore surface characterisation In this case, the surfaces are the ‘‘classical’’ plateau honing surfaces without laser treatment. The friction test conditions are given in Table 1. The results given in Table 2 are five test means. A typical friction coefficient evolution versus time curve is shown in Fig. 4. The before and after friction test, roughness measurements are given in Table 3. For these measurements, the tests are stopped just after the first friction coefficient increase. A wear loss of 50 nm is obtained during the friction tests: little wear is found in this first increase of the friction coefficient on plateau honing surfaces. On these surfaces (Fig. 5), the real pressure contact, Pr, was also calculated from b, peak radius, and from Rq, root-mean-square (Table 4).
Table 2 Initial bore surface friction test results
Mean Variation
Initial friction coefficient
Final friction coefficient
ifci (s)
fcel (s)
0.034 0.035–0.034
0.044 0.042–0.047
6690
3950
Initial friction coefficient invariability length: ifci; friction coefficient evolution length: fcel.
Rq2 ¼ Rq2 ðboreÞ þ Rq2 ðringÞ
Fig. 4. Optic photographs of cw laser treatment on polished cast iron surface and on bore surface.
G. Duffet et al. / Applied Surface Science 205 (2003) 289–296 Table 3 Initial bore surface roughness measurements
Before friction test After friction test
SRvm (nm)
SRpm (nm)
SRpm SRvm (nm)
SRt (nm)
295 265
280 255
575 525
800 800
Because no significant variation was measured, the same, radius peak, b was used. Then a rough estimate of the real contact pressure, 1400 MPa, was found.
4. Cw laser treated bore surface characterisation 4.1. Treatment with a cw laser source Engine cylinder material is cast iron with a perlitic matrix and carbon lamellae (composition in Table 5). Graphite can be a solid lubricant, hence it is interesting to expose on cylinder surface more carbon lamellae. Graphite is also a good insulating material. Using this property, the idea was to vaporise metal above the lamellae which are present near the surface. The thermal energy source is a cw Nd: YAG laser beam. The power has to be sufficient to vaporise metal
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present above the lamellae without surface melting and the beam displacement velocity has to be important enough so that laser surface treatment can be inserted in the industrial process. These factors lead to the choice of the laser beam parameters (power and velocity). Hence, our parameters were the following: power 1200 W, velocity 6 m/min. The achieved surface showed a high density of carbon lamellae: treatments with the same conditions (laser power and velocity) were made on polished cast iron surfaces and on bore surfaces (Fig. 4). The lamella length was about a 100 mm and the width was just a few microns. Furthermore revealed lamellae were in a hollow against the surface (Fig. 6). A dry test on a cw laser treated surface leads to the same friction coefficient value as a dry test on surface finishes, plateau honing, without laser treatment. This indicates that little holes are present above the revealed carbon lamellae. These holes will constitute oil reserves during a lubricated test. Then in a first stage, cw treatment will improve lubrication conditions by the creations of oil reserves but not by the revelation of solid lubricants. 4.2. Friction tests The friction test conditions are given in Table 1. The results given in Table 6 are five test means.
Fig. 5. SEM photographs of cylinder bore cross-section without treatment and with cw laser treatment.
Table 4 Initial bore surface roughness results
Before friction test After friction test
E (GPa)
Rq (nm)
b (mm)
Rq/b (104)
Pr (MPa)
96 96
73.60 64.90
40 40
18.4 16.2
1445 1370
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Table 5 Cast iron composition (wt.%) C
Si
Mn
S
P
Ni
Cr
Mo
Ca
Al
3.07 1.81 0.87 0.11 0.11 0.08 0.41 0.57 1.46 0.036
Table 6 Cw surface friction test results
Mean Variation
Initial friction coefficient
Final friction coefficient
ifci (s)
fcel (s)
0.034 0.034–0.035
0.040 0.039–0.042
8725
3200
For these measurements, tests are stopped just after the first friction coefficient increase. A wear loss of 270 nm is obtained during friction tests: cw laser treated bore surface wear is five times greater than wear on plateau honing surface. On cw laser treated surfaces (Fig. 5), the real pressure contact, Pr, was also calculated from b, peak radius, and from Rq, root-mean-square (Table 8). Because no significant variation was measured, the same, radius peak, b was used.Then a rough estimate of the real contact pressure, 990 MPa was found.
5. Discussion Table 7 Cw surface roughness measurements
Before friction test After friction test
Fig. 7 shows the friction coefficient evolution versus time for the two studied surfaces.
SRvm (nm)
SRpm (nm)
SRpm SRvm (nm)
SRt (nm)
630 470
555 445
1190 920
2000 2000
A typical friction coefficient evolution versus time curve is shown in Fig. 2. The before and after friction test roughness measurements are given in Table 7.
5.1. Initial value of the friction coefficient and first plateau length The initial value is the same for the two cases: without treatment and cw treatment. The first plateau, ifci (Fig. 2), is at the same level, only the duration changes: in this hydrodynamic regime, the surface differences have no effect on the friction coefficient. The first plateau length, ifci (Fig. 2), is different for the two treatments.
Fig. 6. Optic and SEM photographs of revealed carbon lamellae in a hollow against the surface.
Table 8 Cw surface roughness results
Before friction test After friction test
E (GPa)
Rq (nm)
b (mm)
Rq/b (104)
Pr (MPa)
96 96
98.89 77.63
100 100
9.9 7.8
1050 935
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Fig. 7. Friction coefficient vs. time curves.
Cw treatment significantly yields larger first plateau length than no laser treatment (30%). Oil stays more time in the contact. Cw surface has more holes (revealed lamellae in a hollow against the surface) than without treatment surface. Furthermore, asperities are truncated by cw treatment: the peak radius decreases from 100 to 40 mm, then the surface asperities contact later, when the mixed lubrication mode begins. 5.2. First evolution length and level of the friction coefficient In mixed lubrication, friction is composed of a hydrodynamic component and a boundary component. The boundary component is due to asperity contacts. The laser treatment decreases the length of the friction coefficient evolution to reach a second plateau: cw treatment yields a 20% reduction. Cw treatment truncated asperities contact on a more important surface than untruncated asperities, so the evolution will be quicker: the boundary component is greater because the asperity contact load ratio increases. In some friction curves, the beginning of the second plateau shows a maximum as in Fig. 3. Firstly, we thought that this maximum was due to melted zones in relief near carbon lamellae; but this phenomena exists for treated surfaces and for untreated ones. It would be uncertainly an asperity adaptation.
On cw treatment surface, wear loss at the end of the first evolution had a value of 270 nm with a 990 MPa contact pressure. On the surface, that had not been treated, the wear loss at the end of the first evolution had a value of 50 nm with a 1400 MPa contact pressure. Then, the measured cw treatment wear was more important than on the surface without treatment wear because the contact surface was more important during this first evolution (despite a weaker contact pressure). Cw treatment yields lower friction coefficient value (0.040 instead of 0.044). Ren Jeng [1] showed that plateaued surfaces reach steady-state wear quickly but with a lower rate than non-plateaued surfaces and he concluded that plateaued surfaces would have lower wear during their operational life. Our cw treatment, and without treatment, surfaces show a behaviour difference very similar than plateaued and non-plateaued ones. The friction coefficient evolution of laser treated surfaces is quicker and the friction coefficient level is lower than non-treated ones.
6. Conclusions The bore texture function in ring-liner tribological system is well known and must be kept, but the current trend to have an oil film thickness decrease in the piston assembly leads to the creation of oil reserves on
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the bore surfaces. The Nd:YAG laser beams allow holes, on the initial bore texture, to be obtained. Cw laser treatment: Significantly yields larger hydrodynamic regime, oil stays more in the contact. In the mixed lubrication regime, the friction coefficient evolution is quicker but the reached level is lower on laser treatment surfaces than on without laser treatment and then the operational life length would be increased. It could be necessary to fit oil reserves to the encountered lubrication: for example, at top dead centre (TDC), boundary friction can lead to the polishing of the cylinder bore surface and to an increase in oil consumption. Galligan et al. [9] explained with an elastoplastic model of asperity contact that, in boundary lubrication, friction rises by polishing the contact surface. The exhaustion of the lubricant in the contact is an important parameter of this phenomena, so it is important to create deep oil reserves at TDC. A means of creating deep oil holes is by the use of a Q-switched Nd:YAG laser beam. This is part of another research in this treatment, the thermal effect is very low and melting material is ejected. But these oil reserves have to be efficient and their geometry must not lead to a friction rise.
Acknowledgements The authors are grateful for TOPOSURF 3D measurements from Prof. Zaouani, Laboratoire de Tribologie et Dynamique des Syste`mes, Ecole Centrale de Lyon 69131 Ecully.
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