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Combined influence of an inert gas environment and a mechanical action on a graphite surface
WEAR ELSEVIER
Wear
181-183
(1995) 687-690
Combined influence of an inert gas environment mechanical action on a graphite surface
and a
F. Robert a, D. Paulmier a, H. Zai’di a, E. Schouller b ’ Laboratoire EWES, ENSEM-INPL, b Centre de Pkdologie Biologique-CNRS,
2 Avenue de la Fort% de Haye, 54516 Vandoeuvre I& Nancy, France 17 Rue Notre-Dame des Pauvms, 54501 Vandoeuvre I& Nancy, France
Received 10 May 1994; accepted 18 October 1994
Abstract The adsorption and reactivity properties of graphite wear fragments produced in a tribocontact depend on the gaseous environment surrounding the contact which conditions the type of wear (fatigue or abrasive wear) and the surface reactivity. The determination of the specific area of the wear fragments, using a nitrogen adsorption method (Brunauer, Emett and Teller method), permits to reach a description of the surface structure. We carried out on a pin on disc tribometer located in a vacuum chamber wear experiments on two samples of polycrystalline graphites, under different inert gaseous environments. We observed that the size of wear fragments (as well as the friction coefficient) mainly depends on the orientation of the crystallites. This mechanical phenomenon principally depends on the nature of the environment and on the normal load. Depending on these conditions, the powder can have different colorations, and its specific area shows variations from 140 mz g-’ to 285 m2 g-‘. Wear particles produced under inert environment present many active sites (edge sites) but their reactivity depends on their size and specific area. The most reactive can exhibit pyrophoric properties, when exposed to the ambient air. Keywords:
Inert gas; Graphite; Wear fragments; Surface reactivity
1. Introduction
Many scientists, such as Savage [6,7,10,11] and Lancaster [12,13], have shown the influence of the environment on the friction and wear of graphite, but only for reactive gases. In our studies concerning the friction of graphite under inert gases environment [l-4], we have demonstrated the influence of argon and helium on the tribological behaviour of a polycrystalline graphite sample. The lubricating effects of inert gases are explained by a mechanical action of atoms which, because of the pressure, are inserted between crystallites and increase their mobility. Under the frictional forces of the counterface asperities, the crystallites orient parallel to the surface and this leads to a large decrease of both the adhesive and abrasive components of the friction coefficient without any chemical passivation phenomenon. We also observed that the size of the wear fragments (as well as the friction coefficient) mainly depends on the surface morphology, i.e. on the orientation of crystallites. The wear particles have a superficial structure representative of the graphite sample surface from which they are extracted.
0043-1648/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDI 0043-1648(94)07064-4
Graphite is a lamellar and very anisotropic material, constituted by the superposition of hexagonal planes inside of which the carbon atoms are strongly bonded by covalent (r bonds and basal planes are weakly bonded by r Van der Waals forces. A sliding friction parallel to the planes is easy because of the low roughness and a weak shear strain due to a low surface energy 0.1 J m-*. At the opposite, in a perpendicular direction, friction forces are important because of the high roughness and surface energy 5 J m-* of the prismatic faces [5]. The aim of this paper is to study the influence of the environment on the wear particles properties of graphite such as Brunauer, Emett and Teller (BET) specific area, reactivity.
2. Description
of the experimental
conditions
A pin-on-disc tribometer (horizontal axis) located in a vacuum chamber, described in detail elsewhere [4], was used to carry out abrasive wear experiments on the two following polycrystalline graphite samples:
688
Graphite
F. Rob&
et al. I Wear 181483
sample/
.
Fig. 1. Schematic
description
of the abrasive
Graphitepowder
wear
(1995) 687-690
The properties of wear fragments of both EG 319 and MY3A are given in Tables 1 and 2. The BET analyses shown that the MY3A BET specific area has a microporous component (30%) whereas EG319 does not contain micropores. If the total specific areas are very close, Morganite wear powder can exhibit pyrophoric properties when quickly exposed to the air [2,4]. This property is due to the copper which is contained in the Morganite sample which activates the surface energy. The coloration of the wear fragments produced under vacuum is the same as that of the bulk material, which means that the particle size is of the order of the size of the surface asperities.
experiment.
3.2. Influence of exposure time in argon environment -
Electrographite EG 319 P from the Carbone Lorraine Society. - Morganite MY3 A impregnated with copper. The values of the initial roughness (c.1.a.) are respectively R,= 2.4 and 2.1 pm. The experiments were performed under low4 Pa vacuum and in a pure argon environment. The exposure duration is the time during which the bulk sample is exposed to the inert gas before the tooling operation. The severe wear conditions are obtained by replacing the pin by a high-speed-steel tool which is pressed against the sample surface with a constant normal load FN = 4 or 20 N as shown in Fig. 1. The velocity is constant and equal to 0.03 m s-l. The wear powder quantity due to many passes on the sample surface is gathered in a receptacle placed under the contact. The powder is then gradually exposed to the atmosphere, so as to avoid the production of CO or CO, gases by autocombustion, and transferred to a BET analysis device. The BET method is based on the Langmuir (1918) theory which describes a gas adsorption on an homogenous surface, without interaction between the gas molecules. These specific areas are measured by nitrogen adsorption at 77 K after outgassing at 120 “C during about 20 h. These analyses have been carried out at the Centre de Pedologie Biologique of Nancy.
3. Experimental results and discussion 3.1. Friction under vacuum It is well known that graphite sliding under vacuum leads to a high friction coefficient ranging between 0.45 and 0.6 [l-4]. The crystallites which are originally disoriented conserve their position even under the friction forces of the counterface [5-8]. The cohesion between crystallites is strong and the roughness is important. Moreover, the edge sites present at the surface are at the origin of a strong adhesion.
The lubricating effect of inert gases has been widely
studied in our previous papers [l-3]. Starting from a graphite sample sliding under vacuum, the introduction of pure inert gas such as argon or helium can lead to a significant decrease of the friction coefficient down to values ranging between 0.1 and 0.02 (see Fig. 2). The gas atoms that insert into the surface ameliorate the mobility of superficial crystallites which orient parTable 1 Nitrogen BET transform
and reactivity
constant
of the electrograph&e
p-porous surface (mz g-r)
Non jkporous surface (m’ g-‘)
Total surface
Vacuum H.S.S. tool, 20 N, 3 cm s-’
0
143.90
143.90
1.2 X lo5 Pa Ar, exposure = 0 h H.S.S. tool, 20 N, 3 cm s-’
0
146.03
146.03
0
127.15
127.15
93.04
192.63
285.67
Production
conditions
1.2~ lo5 Pa Ar, exposure=50 H.S.S. tool, 20 N, 3 cm s-’
h
1.2 x 10’ Pa Ar, exposure = 24 h H.S.S. tool, 4 N, 3 cm s-’
Table 2 Nitrogen MY3A Production
BET
transform
and reactivity
Non p-porous surface (m* g-‘)
Total surface
40.11
92.82
132.93
h
38.10
168.18
206.28
15 h
72.10
100.09
172.19
Vacuum H.S.S. tool, 20 N, 3 cm s-’
1.5 X l@ Pa Ar, exposure= H.S.S.
of the morganite
p-porous surface (m’ g-‘)
conditions
1.5~105 Pa Ar, exposure=0 H.S.S. tool, 20 N, 3 cm s-’
constant
(m’ g-‘)
(m’ g-‘)
F. Robert et al. / Wear 181-183
allel to the surface under the action of the contacting asperities leading to a decrease of both the roughness down to 0.6 pm (c.1.a.) and the surface energy. One can observe this classical phenomenon in the case of sliding in a reactive environment (ambient air, water vapour or oxygen) that also leads to a sliding track comparable to high oriented pyrolytic graphite which is smooth and bright as shown in Fig. 3 [15-171. The main difference is that the lubricating effect of inert gases occurs without chemical passivation phenomenon. The absence of chemical passivation means that the wear particles produced in an inert gas environment conserve their reactivity due to the dangling bonds ((T bonds) comparable to those obtained under vacuum. The results obtained with EG 319 presented in Table 1 show that for a non-microporous material, the specific BET area does not vary significantly in the presence of argon; it decreases slightly when the duration of exposure increases before running under a high normal load of 20 N and a velocity of 3 cm s-l. This area diminution can be explained by the insertion of argon which decreases the cohesion between crystallites, which can, in turn, lead to the detachment of bigger fragments. If we consider the extracted particles as being spheres, we are able to evaluate their average radius. The ratio between volume-area of a sphere is r/3 (m”/m’) and the density of a perfect graphite lattice is about 2000 kg m- 3. A specific area of 127.15 m2 g-’ corresponds to 127.15 x 2 x lo6 = 2.54 x 10’ (m’/m’). Thus an average radius of r= 3/(2.54 x 10’) = 11.8 X 10e9 m is indicated. This value is of the order of the crystallite size which varies from l-100 nm [18]. Considering a micro-porous material such as MY3A, we can observe in Table 2 that the microporous area increases when the exposure time is longer. After a 15 h exposure under 1.5 x 105Pa argon, the microporous area is almost twice as high than that of powder obtained under either vacuum or just after the introduction of argon. In all likelihood, the gas atoms insert in the superficial defects, notably in the crystallite boundaries and need a certain time to diffuse into the material. The periodic mechanical action of the counterface will create a microcrack respiration phenomenon; the dilatation of structural defects occurs which effectively increases microporosity. 3.3. Influence of normal load in argon environment The results of this study concern the influence of the normal load on the energetic properties of the wear particles. In Table 1, we compare the results obtained with powder produced under high and low normal load (20 and 4 N) and one can observe that the specific area is about double. This important observation can be compared to results presented by Demidovitch [17] concerning reactive powder graphite
689
(199.5) 687490
0 r~-0
2
4
6
Fig. 2. Influence of the helium pressure on the friction coefficient of graphite-graphite couple. under a normal force FN of 4 N, and a sliding velocity of 30 mm s-‘.
Fig. 3. Microscopic sliding under inert
view of graphite gas environment
surface which has experienced under a normal force FN of 4
N and a sliding velocity of 30 mm SC’.
produced by grinding process in vacuum. Under these conditions, which approach sliding conditions, a partial orientation of superficial crystallites occurs which tend to present their basal plane parallel to the surface as shown in Fig. 3. The roughness decreases down to 0.6 pm c.1.a. The wear particles in part consist of some graphite flakes coming from oriented crystallites and of prismatic fragments. The powder coloration is then dark grey. As the normal load decreases wear undergoes a transition from dust to fatigue type, the ratio prismatic particles/graphite flakes as well as the wear rate decrease. Both become almost equal to zero below a critical normal load under which most of the superficial crystallites are oriented. Under these frictional conditions it appears that EG 319 fragments exhibit an significant microporous specific area of 93 m2 g- ’which is not the case at high normal load. Current research concerns the interpretation of this surprising result.
690
F. Roberf et al. I Wear 181-183
It is also possible to obtain reactive powder with a large specific area using the appropriate normal load. This load FN has to be low enough to allow the partial orientation of crystallites, but not sufficiently low to maintain a significant wear rate. 3.4. General diwmion Under vacuum, surface crystallites, on the friction track, are disoriented and wear is of abrasive type even under low normal load. When normal load increases, wear rate and the size of fragments increase (specific area is about 150 m2 g-l). Under inert gases environments (helium and argon), below a critical normal load, it occurs, under the action of the friction forces, an orientation of the surface crystallites parallel to the surface which leads to an important decrease of the friction coefficient (CL= 0.02-0.05) without any passivation phenomenon. The residual powder is very dark because the wear particles are very divided and their size seems to be independent of the normal load (specific area reaches 280 m2 g- ‘). The surface fatigue produces cracks and generates the detachment of graphite flakes. Above this critical load, wear occurs in abrasive conditions and the size of particles increases with the load (powder tint ranges from dark-grey to grey), and specific area decreases down to 140 mz g-l. Inert gases have also an important effect on the nanorheology of this lamellar material surface subjected to the counterface asperities interactions. The insertion of atoms into superficial defects increases the crystallite mobility.
4. Conclusion The reactivity properties of graphite wear particles are affected by the nature of the environment. The gaseous environment surrounding the tribo-contact modifies the surface properties such as surface energy
(1995) 687-690
and morphology. The presence of inert gas tends to increase the surface porosity because of the gas atoms that insert between crystallites and permit structural deformations under the frictional forces of the counterface. It also occurs a transition in wear from dust to fatigue type due to the orientation of the crystallites parallel to the surface. The use of inert gaseous environment and mechanical action can produce reactive carbons without the need for any chemical treatments [91P41.
References F. Robert, D. Paulmier and H. ZaIdi, Tribologia, Finnish J. Tribal, 11 (4) (1992) 65-73. PI F. Robert, D. Paulmier and H. Zaidi, Proc. Eurotrib ‘93, 2 (1993) 291-296. [31 H. ZaIdi, F. Robert, D. Paulmier and H. Nety, Appl Sur$ Sci., 70/71 (1993) 103-108. [41 F. Robert, Thesis, Institut National de Lorraine, France, 1993. t51 F.P. Bowden and D. Tabor, The Friction and Lubrication of Solids, Clarendon, oxford, 1964. 161 R.H. Savage and D.L. Schaefer, J. Appl. Phys., 27 (2) (1956) 136-138. [71 R.H. Savage, Gen. Elect. Rev., 48 (1945) 13-20. [81 G.W. Rowe, Wear, 3 (1960) 45w2. [91 H. ZaIdi, 77&e, Institut National PO&technique de Lorraine, 1989. [lOI R.H. Savage, J. Appl. Phys., 19 (1) (1948) l-10. 1111 R.H. Savage, Ann. N.Y. Acad. Sci., 53 (1951) 862-869. 1121 J.K. Lancaster and J.R. Pritchard, /. Phys. D (AppL Phys.), 13
PI
(1980) 1551-1564. [131 J.K. Lancaster and J.R. Pritchard, 1. Phys. D (Appl. Phys.), 14 (1981) 747. [141 H. ZaIdi, A. Mezin, M. Nivoit and J. Lepage, J. Appl. SurfI Sci., 40 (1989) 103. [151 D. Paulmier and H. Zaidi, J. Vacuum, 41 (1990) 1314-1316. [161 H. Zaidi, D. Paulmier and J. Lepage, J. Appl. Surf. Sci., 44 (1990) 221-233. I171 G.B. Demidovitch, V.F. Kiselev, N.N. Lejnev and O.V. Nkitnia, J. Chimie. Phys., 65 (6) (1968) 1072-1078. 1181 D. Dumas, C. Parisot and A. Buscailhon, Carbones et Graphites, Les Techniques de I’Ing&ieur, A 7400, pp. 3-19.
Wear
181-183
(1995) 687-690
Combined influence of an inert gas environment mechanical action on a graphite surface
and a
F. Robert a, D. Paulmier a, H. Zai’di a, E. Schouller b ’ Laboratoire EWES, ENSEM-INPL, b Centre de Pkdologie Biologique-CNRS,
2 Avenue de la Fort% de Haye, 54516 Vandoeuvre I& Nancy, France 17 Rue Notre-Dame des Pauvms, 54501 Vandoeuvre I& Nancy, France
Received 10 May 1994; accepted 18 October 1994
Abstract The adsorption and reactivity properties of graphite wear fragments produced in a tribocontact depend on the gaseous environment surrounding the contact which conditions the type of wear (fatigue or abrasive wear) and the surface reactivity. The determination of the specific area of the wear fragments, using a nitrogen adsorption method (Brunauer, Emett and Teller method), permits to reach a description of the surface structure. We carried out on a pin on disc tribometer located in a vacuum chamber wear experiments on two samples of polycrystalline graphites, under different inert gaseous environments. We observed that the size of wear fragments (as well as the friction coefficient) mainly depends on the orientation of the crystallites. This mechanical phenomenon principally depends on the nature of the environment and on the normal load. Depending on these conditions, the powder can have different colorations, and its specific area shows variations from 140 mz g-’ to 285 m2 g-‘. Wear particles produced under inert environment present many active sites (edge sites) but their reactivity depends on their size and specific area. The most reactive can exhibit pyrophoric properties, when exposed to the ambient air. Keywords:
Inert gas; Graphite; Wear fragments; Surface reactivity
1. Introduction
Many scientists, such as Savage [6,7,10,11] and Lancaster [12,13], have shown the influence of the environment on the friction and wear of graphite, but only for reactive gases. In our studies concerning the friction of graphite under inert gases environment [l-4], we have demonstrated the influence of argon and helium on the tribological behaviour of a polycrystalline graphite sample. The lubricating effects of inert gases are explained by a mechanical action of atoms which, because of the pressure, are inserted between crystallites and increase their mobility. Under the frictional forces of the counterface asperities, the crystallites orient parallel to the surface and this leads to a large decrease of both the adhesive and abrasive components of the friction coefficient without any chemical passivation phenomenon. We also observed that the size of the wear fragments (as well as the friction coefficient) mainly depends on the surface morphology, i.e. on the orientation of crystallites. The wear particles have a superficial structure representative of the graphite sample surface from which they are extracted.
0043-1648/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDI 0043-1648(94)07064-4
Graphite is a lamellar and very anisotropic material, constituted by the superposition of hexagonal planes inside of which the carbon atoms are strongly bonded by covalent (r bonds and basal planes are weakly bonded by r Van der Waals forces. A sliding friction parallel to the planes is easy because of the low roughness and a weak shear strain due to a low surface energy 0.1 J m-*. At the opposite, in a perpendicular direction, friction forces are important because of the high roughness and surface energy 5 J m-* of the prismatic faces [5]. The aim of this paper is to study the influence of the environment on the wear particles properties of graphite such as Brunauer, Emett and Teller (BET) specific area, reactivity.
2. Description
of the experimental
conditions
A pin-on-disc tribometer (horizontal axis) located in a vacuum chamber, described in detail elsewhere [4], was used to carry out abrasive wear experiments on the two following polycrystalline graphite samples:
688
Graphite
F. Rob&
et al. I Wear 181483
sample/
.
Fig. 1. Schematic
description
of the abrasive
Graphitepowder
wear
(1995) 687-690
The properties of wear fragments of both EG 319 and MY3A are given in Tables 1 and 2. The BET analyses shown that the MY3A BET specific area has a microporous component (30%) whereas EG319 does not contain micropores. If the total specific areas are very close, Morganite wear powder can exhibit pyrophoric properties when quickly exposed to the air [2,4]. This property is due to the copper which is contained in the Morganite sample which activates the surface energy. The coloration of the wear fragments produced under vacuum is the same as that of the bulk material, which means that the particle size is of the order of the size of the surface asperities.
experiment.
3.2. Influence of exposure time in argon environment -
Electrographite EG 319 P from the Carbone Lorraine Society. - Morganite MY3 A impregnated with copper. The values of the initial roughness (c.1.a.) are respectively R,= 2.4 and 2.1 pm. The experiments were performed under low4 Pa vacuum and in a pure argon environment. The exposure duration is the time during which the bulk sample is exposed to the inert gas before the tooling operation. The severe wear conditions are obtained by replacing the pin by a high-speed-steel tool which is pressed against the sample surface with a constant normal load FN = 4 or 20 N as shown in Fig. 1. The velocity is constant and equal to 0.03 m s-l. The wear powder quantity due to many passes on the sample surface is gathered in a receptacle placed under the contact. The powder is then gradually exposed to the atmosphere, so as to avoid the production of CO or CO, gases by autocombustion, and transferred to a BET analysis device. The BET method is based on the Langmuir (1918) theory which describes a gas adsorption on an homogenous surface, without interaction between the gas molecules. These specific areas are measured by nitrogen adsorption at 77 K after outgassing at 120 “C during about 20 h. These analyses have been carried out at the Centre de Pedologie Biologique of Nancy.
3. Experimental results and discussion 3.1. Friction under vacuum It is well known that graphite sliding under vacuum leads to a high friction coefficient ranging between 0.45 and 0.6 [l-4]. The crystallites which are originally disoriented conserve their position even under the friction forces of the counterface [5-8]. The cohesion between crystallites is strong and the roughness is important. Moreover, the edge sites present at the surface are at the origin of a strong adhesion.
The lubricating effect of inert gases has been widely
studied in our previous papers [l-3]. Starting from a graphite sample sliding under vacuum, the introduction of pure inert gas such as argon or helium can lead to a significant decrease of the friction coefficient down to values ranging between 0.1 and 0.02 (see Fig. 2). The gas atoms that insert into the surface ameliorate the mobility of superficial crystallites which orient parTable 1 Nitrogen BET transform
and reactivity
constant
of the electrograph&e
p-porous surface (mz g-r)
Non jkporous surface (m’ g-‘)
Total surface
Vacuum H.S.S. tool, 20 N, 3 cm s-’
0
143.90
143.90
1.2 X lo5 Pa Ar, exposure = 0 h H.S.S. tool, 20 N, 3 cm s-’
0
146.03
146.03
0
127.15
127.15
93.04
192.63
285.67
Production
conditions
1.2~ lo5 Pa Ar, exposure=50 H.S.S. tool, 20 N, 3 cm s-’
h
1.2 x 10’ Pa Ar, exposure = 24 h H.S.S. tool, 4 N, 3 cm s-’
Table 2 Nitrogen MY3A Production
BET
transform
and reactivity
Non p-porous surface (m* g-‘)
Total surface
40.11
92.82
132.93
h
38.10
168.18
206.28
15 h
72.10
100.09
172.19
Vacuum H.S.S. tool, 20 N, 3 cm s-’
1.5 X l@ Pa Ar, exposure= H.S.S.
of the morganite
p-porous surface (m’ g-‘)
conditions
1.5~105 Pa Ar, exposure=0 H.S.S. tool, 20 N, 3 cm s-’
constant
(m’ g-‘)
(m’ g-‘)
F. Robert et al. / Wear 181-183
allel to the surface under the action of the contacting asperities leading to a decrease of both the roughness down to 0.6 pm (c.1.a.) and the surface energy. One can observe this classical phenomenon in the case of sliding in a reactive environment (ambient air, water vapour or oxygen) that also leads to a sliding track comparable to high oriented pyrolytic graphite which is smooth and bright as shown in Fig. 3 [15-171. The main difference is that the lubricating effect of inert gases occurs without chemical passivation phenomenon. The absence of chemical passivation means that the wear particles produced in an inert gas environment conserve their reactivity due to the dangling bonds ((T bonds) comparable to those obtained under vacuum. The results obtained with EG 319 presented in Table 1 show that for a non-microporous material, the specific BET area does not vary significantly in the presence of argon; it decreases slightly when the duration of exposure increases before running under a high normal load of 20 N and a velocity of 3 cm s-l. This area diminution can be explained by the insertion of argon which decreases the cohesion between crystallites, which can, in turn, lead to the detachment of bigger fragments. If we consider the extracted particles as being spheres, we are able to evaluate their average radius. The ratio between volume-area of a sphere is r/3 (m”/m’) and the density of a perfect graphite lattice is about 2000 kg m- 3. A specific area of 127.15 m2 g-’ corresponds to 127.15 x 2 x lo6 = 2.54 x 10’ (m’/m’). Thus an average radius of r= 3/(2.54 x 10’) = 11.8 X 10e9 m is indicated. This value is of the order of the crystallite size which varies from l-100 nm [18]. Considering a micro-porous material such as MY3A, we can observe in Table 2 that the microporous area increases when the exposure time is longer. After a 15 h exposure under 1.5 x 105Pa argon, the microporous area is almost twice as high than that of powder obtained under either vacuum or just after the introduction of argon. In all likelihood, the gas atoms insert in the superficial defects, notably in the crystallite boundaries and need a certain time to diffuse into the material. The periodic mechanical action of the counterface will create a microcrack respiration phenomenon; the dilatation of structural defects occurs which effectively increases microporosity. 3.3. Influence of normal load in argon environment The results of this study concern the influence of the normal load on the energetic properties of the wear particles. In Table 1, we compare the results obtained with powder produced under high and low normal load (20 and 4 N) and one can observe that the specific area is about double. This important observation can be compared to results presented by Demidovitch [17] concerning reactive powder graphite
689
(199.5) 687490
0 r~-0
2
4
6
Fig. 2. Influence of the helium pressure on the friction coefficient of graphite-graphite couple. under a normal force FN of 4 N, and a sliding velocity of 30 mm s-‘.
Fig. 3. Microscopic sliding under inert
view of graphite gas environment
surface which has experienced under a normal force FN of 4
N and a sliding velocity of 30 mm SC’.
produced by grinding process in vacuum. Under these conditions, which approach sliding conditions, a partial orientation of superficial crystallites occurs which tend to present their basal plane parallel to the surface as shown in Fig. 3. The roughness decreases down to 0.6 pm c.1.a. The wear particles in part consist of some graphite flakes coming from oriented crystallites and of prismatic fragments. The powder coloration is then dark grey. As the normal load decreases wear undergoes a transition from dust to fatigue type, the ratio prismatic particles/graphite flakes as well as the wear rate decrease. Both become almost equal to zero below a critical normal load under which most of the superficial crystallites are oriented. Under these frictional conditions it appears that EG 319 fragments exhibit an significant microporous specific area of 93 m2 g- ’which is not the case at high normal load. Current research concerns the interpretation of this surprising result.
690
F. Roberf et al. I Wear 181-183
It is also possible to obtain reactive powder with a large specific area using the appropriate normal load. This load FN has to be low enough to allow the partial orientation of crystallites, but not sufficiently low to maintain a significant wear rate. 3.4. General diwmion Under vacuum, surface crystallites, on the friction track, are disoriented and wear is of abrasive type even under low normal load. When normal load increases, wear rate and the size of fragments increase (specific area is about 150 m2 g-l). Under inert gases environments (helium and argon), below a critical normal load, it occurs, under the action of the friction forces, an orientation of the surface crystallites parallel to the surface which leads to an important decrease of the friction coefficient (CL= 0.02-0.05) without any passivation phenomenon. The residual powder is very dark because the wear particles are very divided and their size seems to be independent of the normal load (specific area reaches 280 m2 g- ‘). The surface fatigue produces cracks and generates the detachment of graphite flakes. Above this critical load, wear occurs in abrasive conditions and the size of particles increases with the load (powder tint ranges from dark-grey to grey), and specific area decreases down to 140 mz g-l. Inert gases have also an important effect on the nanorheology of this lamellar material surface subjected to the counterface asperities interactions. The insertion of atoms into superficial defects increases the crystallite mobility.
4. Conclusion The reactivity properties of graphite wear particles are affected by the nature of the environment. The gaseous environment surrounding the tribo-contact modifies the surface properties such as surface energy
(1995) 687-690
and morphology. The presence of inert gas tends to increase the surface porosity because of the gas atoms that insert between crystallites and permit structural deformations under the frictional forces of the counterface. It also occurs a transition in wear from dust to fatigue type due to the orientation of the crystallites parallel to the surface. The use of inert gaseous environment and mechanical action can produce reactive carbons without the need for any chemical treatments [91P41.
References F. Robert, D. Paulmier and H. ZaIdi, Tribologia, Finnish J. Tribal, 11 (4) (1992) 65-73. PI F. Robert, D. Paulmier and H. Zaidi, Proc. Eurotrib ‘93, 2 (1993) 291-296. [31 H. ZaIdi, F. Robert, D. Paulmier and H. Nety, Appl Sur$ Sci., 70/71 (1993) 103-108. [41 F. Robert, Thesis, Institut National de Lorraine, France, 1993. t51 F.P. Bowden and D. Tabor, The Friction and Lubrication of Solids, Clarendon, oxford, 1964. 161 R.H. Savage and D.L. Schaefer, J. Appl. Phys., 27 (2) (1956) 136-138. [71 R.H. Savage, Gen. Elect. Rev., 48 (1945) 13-20. [81 G.W. Rowe, Wear, 3 (1960) 45w2. [91 H. ZaIdi, 77&e, Institut National PO&technique de Lorraine, 1989. [lOI R.H. Savage, J. Appl. Phys., 19 (1) (1948) l-10. 1111 R.H. Savage, Ann. N.Y. Acad. Sci., 53 (1951) 862-869. 1121 J.K. Lancaster and J.R. Pritchard, /. Phys. D (AppL Phys.), 13
PI
(1980) 1551-1564. [131 J.K. Lancaster and J.R. Pritchard, 1. Phys. D (Appl. Phys.), 14 (1981) 747. [141 H. ZaIdi, A. Mezin, M. Nivoit and J. Lepage, J. Appl. SurfI Sci., 40 (1989) 103. [151 D. Paulmier and H. Zaidi, J. Vacuum, 41 (1990) 1314-1316. [161 H. Zaidi, D. Paulmier and J. Lepage, J. Appl. Surf. Sci., 44 (1990) 221-233. I171 G.B. Demidovitch, V.F. Kiselev, N.N. Lejnev and O.V. Nkitnia, J. Chimie. Phys., 65 (6) (1968) 1072-1078. 1181 D. Dumas, C. Parisot and A. Buscailhon, Carbones et Graphites, Les Techniques de I’Ing&ieur, A 7400, pp. 3-19.