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Scratching of alumina in various environments
International Journal of Refractory Metals & Hard Materials 17 (1999) 111±115
Scratching of alumina in various environments L. Rapoport *, I. Lapsker, A. Rayhel Department of Science, Center for Technological Education P.O.B. 305, Holon 58102, Israel Received 9 March 1998; accepted 14 December 1998
Abstract The purpose of this work was to study the frictional and wear behavior of alumina under scratching in various environments. The scratch tests were performed at loads from 0.5 to 50 N and a sliding velocity of V 0.4 mm sÿ1 during 100 cycles in water, paran oil and in laboratory air (humidity 50%). Diamond standard spherical indenter was used as the contrabody. The acoustic emission, friction coecient, wear losses and failure development in the scratch tracks were evaluated. It was found that the friction coecient increased during some cycles and then a steady friction state with approximately constant value of the friction coecient was reached in all environments. The maximal friction coecient, (f 0.2), was in contact with paran oil, while the minimal value of f 0.09 was obtained in the wear test with water. The opposite eect was obtained in the determination of the wear rate. The width of the wear track was maximal and minimal in contact with water and paran oil, respectively. The trapping and agglomeration of wear particles was observed in the wear track under contact with air and water. Intergranular and transgranular fractures were found to be the dominant damage mechanisms in contact with paran oil. Close relation between signals of acoustic emission and friction and wear properties was exhibited under dierent environmental conditions. The bond between the trapped wear particles and the substrate was analyzed. It is supposed that the trapping of the wear particles is caused by triboelectri®cation of alumina. This eect was maximal in dry friction. Almost no trapping was observed in contact with paran oil. Ó 1999 Elsevier Science Ltd. All rights reserved. Keywords: Scratching; Alumina; Tribo®lm; Brittle fracture
1. Introduction Alumina is widely used in mechanical seals, roller bearings and cutting tools over a broad range of contact parameters [1,2]. The tribological behavior of alumina has been extensively studied [3±15]. Recently the analysis of friction and wear of ceramics was based on indentation fracture mechanics concepts [3,4]. However these concepts are generally in poor agreement with the experimental results. The other concept based on the determination of transition from one dominant wear mechanism to another was discussed in [5±11]. Wear transition from deformation-controlled to fracturecontrolled removal of alumina was revealed in dierent tests with a change in sliding speeds [5,6], loads [7±9], temperatures [10] and environments [5,6,11]. In view of controversial results in the tribological behavior of ceramics, essential attention has been given to the for-
*
Corresponding author. Tel.: +00972 3 5026601; fax: +00972 35026619; e-mail:[email protected].
mation of tribo®lms [12±15]. For alumina, the reduction of the friction coecient and wear rate was attributed to the formation of hydroxide on the contact surface due to the tribochemical reaction [12]. The agglomeration process of wear particles of alumina was observed at various water vapor pressures [15]. Scratching is usually used to study the mechanisms of abrasive wear and grinding of ceramics. The review of the literature shows that the eect of environment on the tribo®lm formation under scratching of ceramics has not been obtained yet. The main goal was to study the frictional and wear behavior of alumina under scratching in various environments. 2. Experimental procedure In order to determine the eect of original roughness on machining, single-point scratching of alumina in virgin state was used. The experiment was carried out on home-made scratch tester at loads from 0.5 to 50 N and a sliding velocity V 0.4 mm sÿ1 in water, paran oil
0263-4368/99/$ ± see front matter Ó 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 3 - 4 3 6 8 ( 9 8 ) 0 0 0 7 6 - 6
112
L. Rapoport et al. / International Journal of Refractory Metals & Hard Materials 17 (1999) 111±115
and in laboratory air (humidity 50%) during 100 cycles. Alumina a ÿ Al2 O3 with a purity of 99% and a standard spherical tip Rockwell C diamond indenter were used in this experiment. Friction force, width of the wear track, acoustic emission (AE) and the morphology of the surface of wear track were evaluated in this work. The frictional force was measured with an instrumented arm and recorded on a chart recorder. The strain gauges were bonded on the steel arm and connected to an ampli®er in a half-bridge con®guration. The signals of AE were recorded by a microprocessor-based monitoring system AET 5000. All specimens were cleaned before and after testing with hexane using an ultrasonic bath. 3. Results and discussion Figure 1 shows the dependence of the friction coecient vs. the number of cycles. Originally, the friction coecient increased during some cycles and then the approximate constant value of the friction coecient was reached at all environments. At steady friction, the maximal friction coecient, (f 0.2), was in contact with paran oil, while the minimal value of f 0.09 was obtained with water. The increase in friction coecient at constant load was found to be associated with a growth of contact surface due to severe fracture of the rough virgin surface and the further agglomeration and compaction of the wear particles, Fig. 2. This phenomenon might possibly be explained on the basis of the Bowden±Tabor model of friction [16]. The friction force is expressed as F AS Fp , where A is the area of contact, S the shear strength, and Fp the plowing term. We did not observe plowing in the wear track of the
alumina. So the plowing term can be ignored. Thus the increase in friction coecient might possibly be associated with an increase of the contact surface area. It was found that after one-side pass, the tribo®lms were formed, Fig. 2(a). With subsequent cycling, tribo®lms grew due to agglomeration of fresh wear particles, Fig. 2(b),(c). Then, the tribo®lms delaminated, Fig. 2(d). Thus the steady friction state was associated with a brittle fracture of the alumina, subsequent agglomeration, compaction and delamination of tribo®lms. The morphology of the wear track in contact with water was similar to that observed in dry friction. The dierence was in the thickness of the tribo®lm and in the quantity of wear particles adhered to the wear surface. The lesser the quantity of the wear particles, the thinner the tribo®lms agglomerated to the surface. Relatively smooth and thin tribo®lm in contact with water was apparently responsible for the low friction coecient. The tribological behavior of the alumina in contact with paran oil was essentially dierent on other environments. The trapping of the wear particles occurred only at ®rst and second cycles, Fig. 3(a). Almost no agglomeration and compaction of the wear particles was observed with a subsequent cycling. Intergranular and transgranular fracture of alumina grains was obtained, Fig. 3(b),(d). This state was preserved in the steady friction. Thus brittle fracture of rough virgin surface of the alumina may be the main cause of the high friction coecient in contact with paran oil. It was found that the dependence of the width of wear tracks on sliding distance was opposite to the dependence of friction coecient, Fig. 4. The width of the wear track was maximal in contact with water, while the
Fig. 1. Dependence of the friction coecient on number of cycles.
L. Rapoport et al. / International Journal of Refractory Metals & Hard Materials 17 (1999) 111±115
113
Fig. 2. Formation and destruction of surface ®lms in dry friction of alumina.
minimal value was obtained in the wear test with paran oil. It is believed that paran oil protects the surface of alumina on straight contact with a diamond indenter. Preservation of lubricant restricts the process of crack growth and thus decreases the wear rate. From the other side, it has been shown that the hostile environment leads to suciently high crack growth due to the transportation of hostile elements to the tip of the crack [17]. Thereafter one of the main causes of crack growth in contact with water may be the penetration of hostile environment to the tips of cracks under scratching. The curves of wear appeared to be closed to the dependencies of the AE signals, Fig. 5. Severe brittle fracture accompanied high AE level under all environmental conditions during some ®rst cycles. Decreasing the AE after some cycles of friction was associated with the stage of compaction of wear particles leading to formation of relatively smooth surface. At steady friction state, the de®nite relation between the trapped and fractured islands was preserved and the level of the AE was rested a constant. In other words, the trapped and
compacted wear particles were delaminated after some cycles and then redestroyed and trapped again. It is known that the formation of wear particles accompanied by brittle fracture leads to high level of AE [18]. More `noised' the wear tracks the higher the wear rate. Actually, the highest level of AE was observed in contact with water when severe brittle fracture occurred. The low level of AE con®rmed the low crack growth and wear rate in contact with paran oil. It is interesting to note that the bond between the trapped wear particles and the substrate obtained at ®rst cycle of sliding was so strong that a blow by compressed air and water ¯ushing did not principally change the bond observed. The strong bond was apparently caused by electrostatic cohesion resulting from triboelectri®cation. This eect decreased in water leading to the formation of thin tribo®lms. The presence of water increases electric conductivity among wear particles. Reducing a surface charging and electrostatic force results in the diculty of wear particle agglomeration [15]. Low electric conductivity of the alumina led to the formation of thick tribo®lms at dry friction.
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Fig. 3. Formation and destruction of surface ®lms in contact with paran oil.
Fig. 4. The eect of environment on wear of the alumina.
L. Rapoport et al. / International Journal of Refractory Metals & Hard Materials 17 (1999) 111±115
115
Fig. 5. The eect of environment on acoustic emission under friction of the alumina.
4. Conclusions The trapping and agglomeration of wear particles was observed on the surface of the alumina under singlepoint scratching in air and water. Intergranular and transgranular fractures were dominant damage mechanisms in contact with paran oil. The friction coecient increased with a trapping and compaction of the wear particles and then an approximately constant value of friction coecient was reached. Minimal and maximal friction coecients were observed in water and paran oil, respectively. The opposite eect was obtained in the analysis of the wear rate. The width of the wear track was maximal and minimal in contact with water and paran oil, respectively. The wear particles that are agglomerated and compacted do not preserve the surface of the alumina on severe damage. The trapping of the wear particles is apparently caused by triboelectri®cation of the alumina. This eect was maximal in dry friction. The trapping is practically absent in contact with paran oil.
[5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
References [1] Assanabe S. Application of ceramics for tribological componenets. Tribol Int 1987;120:354±60. [2] Fisher G. Refractory used ± practicality of high technology ceramics. Ceram Bull 1987;66:1103±08. [3] Lawn B, Wilshaw R. Review Indentation fracture: principles and applications. J Mater Sci 1975;10:1049±81. [4] Evans AG, Marshall DB, Rigney DA. editor. Wear mechanisms of ceramics. In: Rigney DA editor. Fundamentals of friction and
[16] [17] [18]
wear of materials. ASM, Metals Park OH: ASM Mater. Sci. Semin. 1981:439±52. Wang YS, Hsu MS, Munro RG. Ceramics wear maps: alumina. STLE Lubr Eng 1991;47:63±9. Deckman DE, Jahanmir S, Hsu SM. Wear mechanisms of alphaalumina lubricated with a paran oil. Wear 1991;149:155±61. Cho SJ, Hockey BJ, Lawn BR, Bennison SJ. Grain size and Rcurve eects in the abrasive wear of alumina. J Am Ceram Soc 1989;72:1249±52. Jahanmir S, Dong X. Mechanism of mild to severe wear transition in alpha-alumina. J Tribol 1992;114:403±11. Hsu SM, Wang SY, Munro RG. Quantitative wear maps as a visualization of wear mechanism transitions in ceramic materials. Wear 1989;134:1±8. Dong X, Jahanmir S, Hsu SM. Tribological characteristics of aalumina at elevated temperatures. J Am Ceram Soc 1991;74:1036±44. Gates RS, Hsu MS, Klause EE. Ceramic tribology: methodology and mechanisms of alumina wear. Tribol Trans 1989;32:357±66. Sasaki S., The eect of the surrounding atmosphere on friction and wear of alumina, zirconia, silicon carbide, and silicon nitride. Wear 1989;134:185±200. Komvopoulos K, Li H. The eect of tribo®lm formation and humidity on the friction and wear properties of ceramic materials. Trans. ASME, J Tribol. 1992;114:131±40. Jeng M-C, Yan L-Y. Environmental eects on wear behavior of alumina. Wear 1993;161:111±19. Xu J, Kato K. The eect of water vapor on the agglomeration of wear particles of ceramics. Wear 1997;202:165±71. Bowden FP, Tabor D. The friction and lubrication of solids. Part 1, Clarendon Press, Oxford, 1964. Rapoport L, Salganik RL, Gotlib VA, A model of brittledominated wear due to delayed fracture (vertical crack growth). Wear 1995;188:66±174. Rapoport L, Petrov Yu, Vainberg V, Voronina I. Study of the dynamics of metal friction process by the acoustic emission method. `Trenie Iznos Mash' in Russian; `Friction and Wear in Soviet Union' in USA , 1981;2:305±09.
Scratching of alumina in various environments L. Rapoport *, I. Lapsker, A. Rayhel Department of Science, Center for Technological Education P.O.B. 305, Holon 58102, Israel Received 9 March 1998; accepted 14 December 1998
Abstract The purpose of this work was to study the frictional and wear behavior of alumina under scratching in various environments. The scratch tests were performed at loads from 0.5 to 50 N and a sliding velocity of V 0.4 mm sÿ1 during 100 cycles in water, paran oil and in laboratory air (humidity 50%). Diamond standard spherical indenter was used as the contrabody. The acoustic emission, friction coecient, wear losses and failure development in the scratch tracks were evaluated. It was found that the friction coecient increased during some cycles and then a steady friction state with approximately constant value of the friction coecient was reached in all environments. The maximal friction coecient, (f 0.2), was in contact with paran oil, while the minimal value of f 0.09 was obtained in the wear test with water. The opposite eect was obtained in the determination of the wear rate. The width of the wear track was maximal and minimal in contact with water and paran oil, respectively. The trapping and agglomeration of wear particles was observed in the wear track under contact with air and water. Intergranular and transgranular fractures were found to be the dominant damage mechanisms in contact with paran oil. Close relation between signals of acoustic emission and friction and wear properties was exhibited under dierent environmental conditions. The bond between the trapped wear particles and the substrate was analyzed. It is supposed that the trapping of the wear particles is caused by triboelectri®cation of alumina. This eect was maximal in dry friction. Almost no trapping was observed in contact with paran oil. Ó 1999 Elsevier Science Ltd. All rights reserved. Keywords: Scratching; Alumina; Tribo®lm; Brittle fracture
1. Introduction Alumina is widely used in mechanical seals, roller bearings and cutting tools over a broad range of contact parameters [1,2]. The tribological behavior of alumina has been extensively studied [3±15]. Recently the analysis of friction and wear of ceramics was based on indentation fracture mechanics concepts [3,4]. However these concepts are generally in poor agreement with the experimental results. The other concept based on the determination of transition from one dominant wear mechanism to another was discussed in [5±11]. Wear transition from deformation-controlled to fracturecontrolled removal of alumina was revealed in dierent tests with a change in sliding speeds [5,6], loads [7±9], temperatures [10] and environments [5,6,11]. In view of controversial results in the tribological behavior of ceramics, essential attention has been given to the for-
*
Corresponding author. Tel.: +00972 3 5026601; fax: +00972 35026619; e-mail:[email protected].
mation of tribo®lms [12±15]. For alumina, the reduction of the friction coecient and wear rate was attributed to the formation of hydroxide on the contact surface due to the tribochemical reaction [12]. The agglomeration process of wear particles of alumina was observed at various water vapor pressures [15]. Scratching is usually used to study the mechanisms of abrasive wear and grinding of ceramics. The review of the literature shows that the eect of environment on the tribo®lm formation under scratching of ceramics has not been obtained yet. The main goal was to study the frictional and wear behavior of alumina under scratching in various environments. 2. Experimental procedure In order to determine the eect of original roughness on machining, single-point scratching of alumina in virgin state was used. The experiment was carried out on home-made scratch tester at loads from 0.5 to 50 N and a sliding velocity V 0.4 mm sÿ1 in water, paran oil
0263-4368/99/$ ± see front matter Ó 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 3 - 4 3 6 8 ( 9 8 ) 0 0 0 7 6 - 6
112
L. Rapoport et al. / International Journal of Refractory Metals & Hard Materials 17 (1999) 111±115
and in laboratory air (humidity 50%) during 100 cycles. Alumina a ÿ Al2 O3 with a purity of 99% and a standard spherical tip Rockwell C diamond indenter were used in this experiment. Friction force, width of the wear track, acoustic emission (AE) and the morphology of the surface of wear track were evaluated in this work. The frictional force was measured with an instrumented arm and recorded on a chart recorder. The strain gauges were bonded on the steel arm and connected to an ampli®er in a half-bridge con®guration. The signals of AE were recorded by a microprocessor-based monitoring system AET 5000. All specimens were cleaned before and after testing with hexane using an ultrasonic bath. 3. Results and discussion Figure 1 shows the dependence of the friction coecient vs. the number of cycles. Originally, the friction coecient increased during some cycles and then the approximate constant value of the friction coecient was reached at all environments. At steady friction, the maximal friction coecient, (f 0.2), was in contact with paran oil, while the minimal value of f 0.09 was obtained with water. The increase in friction coecient at constant load was found to be associated with a growth of contact surface due to severe fracture of the rough virgin surface and the further agglomeration and compaction of the wear particles, Fig. 2. This phenomenon might possibly be explained on the basis of the Bowden±Tabor model of friction [16]. The friction force is expressed as F AS Fp , where A is the area of contact, S the shear strength, and Fp the plowing term. We did not observe plowing in the wear track of the
alumina. So the plowing term can be ignored. Thus the increase in friction coecient might possibly be associated with an increase of the contact surface area. It was found that after one-side pass, the tribo®lms were formed, Fig. 2(a). With subsequent cycling, tribo®lms grew due to agglomeration of fresh wear particles, Fig. 2(b),(c). Then, the tribo®lms delaminated, Fig. 2(d). Thus the steady friction state was associated with a brittle fracture of the alumina, subsequent agglomeration, compaction and delamination of tribo®lms. The morphology of the wear track in contact with water was similar to that observed in dry friction. The dierence was in the thickness of the tribo®lm and in the quantity of wear particles adhered to the wear surface. The lesser the quantity of the wear particles, the thinner the tribo®lms agglomerated to the surface. Relatively smooth and thin tribo®lm in contact with water was apparently responsible for the low friction coecient. The tribological behavior of the alumina in contact with paran oil was essentially dierent on other environments. The trapping of the wear particles occurred only at ®rst and second cycles, Fig. 3(a). Almost no agglomeration and compaction of the wear particles was observed with a subsequent cycling. Intergranular and transgranular fracture of alumina grains was obtained, Fig. 3(b),(d). This state was preserved in the steady friction. Thus brittle fracture of rough virgin surface of the alumina may be the main cause of the high friction coecient in contact with paran oil. It was found that the dependence of the width of wear tracks on sliding distance was opposite to the dependence of friction coecient, Fig. 4. The width of the wear track was maximal in contact with water, while the
Fig. 1. Dependence of the friction coecient on number of cycles.
L. Rapoport et al. / International Journal of Refractory Metals & Hard Materials 17 (1999) 111±115
113
Fig. 2. Formation and destruction of surface ®lms in dry friction of alumina.
minimal value was obtained in the wear test with paran oil. It is believed that paran oil protects the surface of alumina on straight contact with a diamond indenter. Preservation of lubricant restricts the process of crack growth and thus decreases the wear rate. From the other side, it has been shown that the hostile environment leads to suciently high crack growth due to the transportation of hostile elements to the tip of the crack [17]. Thereafter one of the main causes of crack growth in contact with water may be the penetration of hostile environment to the tips of cracks under scratching. The curves of wear appeared to be closed to the dependencies of the AE signals, Fig. 5. Severe brittle fracture accompanied high AE level under all environmental conditions during some ®rst cycles. Decreasing the AE after some cycles of friction was associated with the stage of compaction of wear particles leading to formation of relatively smooth surface. At steady friction state, the de®nite relation between the trapped and fractured islands was preserved and the level of the AE was rested a constant. In other words, the trapped and
compacted wear particles were delaminated after some cycles and then redestroyed and trapped again. It is known that the formation of wear particles accompanied by brittle fracture leads to high level of AE [18]. More `noised' the wear tracks the higher the wear rate. Actually, the highest level of AE was observed in contact with water when severe brittle fracture occurred. The low level of AE con®rmed the low crack growth and wear rate in contact with paran oil. It is interesting to note that the bond between the trapped wear particles and the substrate obtained at ®rst cycle of sliding was so strong that a blow by compressed air and water ¯ushing did not principally change the bond observed. The strong bond was apparently caused by electrostatic cohesion resulting from triboelectri®cation. This eect decreased in water leading to the formation of thin tribo®lms. The presence of water increases electric conductivity among wear particles. Reducing a surface charging and electrostatic force results in the diculty of wear particle agglomeration [15]. Low electric conductivity of the alumina led to the formation of thick tribo®lms at dry friction.
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L. Rapoport et al. / International Journal of Refractory Metals & Hard Materials 17 (1999) 111±115
Fig. 3. Formation and destruction of surface ®lms in contact with paran oil.
Fig. 4. The eect of environment on wear of the alumina.
L. Rapoport et al. / International Journal of Refractory Metals & Hard Materials 17 (1999) 111±115
115
Fig. 5. The eect of environment on acoustic emission under friction of the alumina.
4. Conclusions The trapping and agglomeration of wear particles was observed on the surface of the alumina under singlepoint scratching in air and water. Intergranular and transgranular fractures were dominant damage mechanisms in contact with paran oil. The friction coecient increased with a trapping and compaction of the wear particles and then an approximately constant value of friction coecient was reached. Minimal and maximal friction coecients were observed in water and paran oil, respectively. The opposite eect was obtained in the analysis of the wear rate. The width of the wear track was maximal and minimal in contact with water and paran oil, respectively. The wear particles that are agglomerated and compacted do not preserve the surface of the alumina on severe damage. The trapping of the wear particles is apparently caused by triboelectri®cation of the alumina. This eect was maximal in dry friction. The trapping is practically absent in contact with paran oil.
[5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
References [1] Assanabe S. Application of ceramics for tribological componenets. Tribol Int 1987;120:354±60. [2] Fisher G. Refractory used ± practicality of high technology ceramics. Ceram Bull 1987;66:1103±08. [3] Lawn B, Wilshaw R. Review Indentation fracture: principles and applications. J Mater Sci 1975;10:1049±81. [4] Evans AG, Marshall DB, Rigney DA. editor. Wear mechanisms of ceramics. In: Rigney DA editor. Fundamentals of friction and
[16] [17] [18]
wear of materials. ASM, Metals Park OH: ASM Mater. Sci. Semin. 1981:439±52. Wang YS, Hsu MS, Munro RG. Ceramics wear maps: alumina. STLE Lubr Eng 1991;47:63±9. Deckman DE, Jahanmir S, Hsu SM. Wear mechanisms of alphaalumina lubricated with a paran oil. Wear 1991;149:155±61. Cho SJ, Hockey BJ, Lawn BR, Bennison SJ. Grain size and Rcurve eects in the abrasive wear of alumina. J Am Ceram Soc 1989;72:1249±52. Jahanmir S, Dong X. Mechanism of mild to severe wear transition in alpha-alumina. J Tribol 1992;114:403±11. Hsu SM, Wang SY, Munro RG. Quantitative wear maps as a visualization of wear mechanism transitions in ceramic materials. Wear 1989;134:1±8. Dong X, Jahanmir S, Hsu SM. Tribological characteristics of aalumina at elevated temperatures. J Am Ceram Soc 1991;74:1036±44. Gates RS, Hsu MS, Klause EE. Ceramic tribology: methodology and mechanisms of alumina wear. Tribol Trans 1989;32:357±66. Sasaki S., The eect of the surrounding atmosphere on friction and wear of alumina, zirconia, silicon carbide, and silicon nitride. Wear 1989;134:185±200. Komvopoulos K, Li H. The eect of tribo®lm formation and humidity on the friction and wear properties of ceramic materials. Trans. ASME, J Tribol. 1992;114:131±40. Jeng M-C, Yan L-Y. Environmental eects on wear behavior of alumina. Wear 1993;161:111±19. Xu J, Kato K. The eect of water vapor on the agglomeration of wear particles of ceramics. Wear 1997;202:165±71. Bowden FP, Tabor D. The friction and lubrication of solids. Part 1, Clarendon Press, Oxford, 1964. Rapoport L, Salganik RL, Gotlib VA, A model of brittledominated wear due to delayed fracture (vertical crack growth). Wear 1995;188:66±174. Rapoport L, Petrov Yu, Vainberg V, Voronina I. Study of the dynamics of metal friction process by the acoustic emission method. `Trenie Iznos Mash' in Russian; `Friction and Wear in Soviet Union' in USA , 1981;2:305±09.