Availableonlineatwww.sciencedirect.com 魔 。 ScienceDirect WEAR ELSEVIER Wear263(2007)872-877 www.elseviercomlocate/wea Evaluation of scuffing behavior of single-crystal zirconia ceramic materials C. Lorenzo-Martin, O.O. Ajayi, D. Singh, J.L. Routbort Argonne National Laboratory Energy Systems Division, 9700S. Cass Avenue, Argonne, IL 60439, United States Received 16 August 2006; received in revised form 13 December 2006: accepted 18 December 2006 Available online 23 May 2007 Abstract Scuffing, described as sudden catastrophic failure of lubricated sliding surfaces, is usually characterized by a sudden rapid increase in friction, temperature, and noise, and is an important failure mode on sliding surfaces. In metallic materials, scuffing results in severe plastic deformatio of surfaces in contact. This study evaluated the scuffing behavior of two variants of zirconia(ZrO2) ceramic. Using a block-on-ring contact configuration and unformulated polyalphaolefin(PAO)lubricant, step-load-increase scuffing tests were conducted with single crystals of cubic ZrO2-9.5% Y2O3 and tetragonal ZrO2-3% Y2O3 Phenomenological"scuffing", characterized by a sudden rise in friction coefficient and noise, vas observed in the cubic material. For this material, "scuffing"occurred by sudden fracture at the end of test. The tetragonal material underwent no sudden failure(scuffing). This lack of scuffing is attributed to the sequential operation of three plastic deformation mechanisms: ferroelastic domain switching, tetragonal-to-monoclinic phase transformation, and dislocation slip as the frictional stress and energy dissipation pathway Published by Elsevier B.V. Keywords: Scuffing: Zirconia; Plastic deformation; Phase transformation; Ferroelastic; Fracture 1. Introduction sible for scuffing and the development of theories to predict scuffing behavior, mostly in metallic materials. In spite of these Scuffing is a tribological failure event described as a sudden efforts, scuffing remains one of the least understood phenom- catastrophic failure of lubricated sliding surfaces. It is character- ena in the field of tribology. Because of the complexities of this ized by a sudden rise in friction, contact temperature, vibration, phenomenon, the majority of studies have focused on scuffing and noise, resulting in surface roughening through severe plas- prevention strategies, based mostly on phenomenological obser- tic flow and loss of surface integrity [1]. Because of its sudden vations [2]. Indeed, many variables influence the occurrence of nd catastrophic nature, scuffing poses a major reliability prob- scuffing: the materials in contact; the method of preparation of lem for tribological components. Many machine components the sliding surfaces; the composition and nature of the lubri- involved in sliding contact such as gears, seals, and bearings are cant; and the contact operating conditions(temperature, sliding all susceptible to scuffing failure at some point in their oper- ed, and load) ating life. Moreover, there is an ever-increasing technological Scuffing resistance depends on the material properties an demand for higher power density and higher relative sliding the surface finish. In general, heterogeneous materials and speeds in tribological systems and components, thereby rais- superfinished surfaces are more scuffing resistant [2-4]. Also, ing their susceptibility to scuffing failure. For these reasons, the chemistry and rheological properties of the lubricant affect the study of scuffing and its prevention have attracted a strong the occurrence of scuffing. Lubricants containing extreme pres interest in the field of tribology for many decades. Significant sure(EP)additives usually produce a protective surface reaction efforts have been devoted to the study of mechanisms respon- layer(the so-called boundary films), which delay or may even prevent the onset of scuffing [5]. In terms of operating contact 4 Work supported by the Department of Energy, under Contract DE-AC02. conditions, higher temperatures, sliding speeds, and loads often 06CH11357 ranslate to a higher propensity for scuffing failure [6] Corresponding author. Tel. +1 630 252 9021: fax: +1 630 2524798. In view of the increasing demands on tribological com- E- mail address. ajayi@anl. gov (O.O. Ajayi ponents, the phenomenologically based strategies for scuffing 0043-1648/S-see front matter. Published by Elsevier B.V. doi:10.1016wea2006.12054
Wear 263 (2007) 872–877 Evaluation of scuffing behavior of single-crystal zirconia ceramic materials C. Lorenzo-Martin, O.O. Ajayi ∗, D. Singh, J.L. Routbort Argonne National Laboratory, Energy Systems Division, 9700 S. Cass Avenue, Argonne, IL 60439, United States Received 16 August 2006; received in revised form 13 December 2006; accepted 18 December 2006 Available online 23 May 2007 Abstract Scuffing, described as sudden catastrophic failure of lubricated sliding surfaces, is usually characterized by a sudden rapid increase in friction, temperature, and noise, and is an important failure mode on sliding surfaces. In metallic materials, scuffing results in severe plastic deformation of surfaces in contact. This study evaluated the scuffing behavior of two variants of zirconia (ZrO2) ceramic. Using a block-on-ring contact configuration and unformulated polyalphaolefin (PAO) lubricant, step-load-increase scuffing tests were conducted with single crystals of cubic ZrO2–9.5% Y2O3 and tetragonal ZrO2–3% Y2O3. Phenomenological “scuffing”, characterized by a sudden rise in friction coefficient and noise, was observed in the cubic material. For this material, “scuffing” occurred by sudden fracture at the end of test. The tetragonal material underwent no sudden failure (scuffing). This lack of scuffing is attributed to the sequential operation of three plastic deformation mechanisms: ferroelastic domain switching, tetragonal-to-monoclinic phase transformation, and dislocation slip as the frictional stress and energy dissipation pathway. Published by Elsevier B.V. Keywords: Scuffing; Zirconia; Plastic deformation; Phase transformation; Ferroelastic; Fracture 1. Introduction Scuffing is a tribological failure event described as a sudden catastrophic failure of lubricated sliding surfaces. It is characterized by a sudden rise in friction, contact temperature, vibration, and noise, resulting in surface roughening through severe plastic flow and loss of surface integrity [1]. Because of its sudden and catastrophic nature, scuffing poses a major reliability problem for tribological components. Many machine components involved in sliding contact such as gears, seals, and bearings are all susceptible to scuffing failure at some point in their operating life. Moreover, there is an ever-increasing technological demand for higher power density and higher relative sliding speeds in tribological systems and components, thereby raising their susceptibility to scuffing failure. For these reasons, the study of scuffing and its prevention have attracted a strong interest in the field of tribology for many decades. Significant efforts have been devoted to the study of mechanisms respon- Work supported by the Department of Energy, under Contract DE-AC02- 06CH11357. ∗ Corresponding author. Tel.: +1 630 252 9021; fax: +1 630 252 4798. E-mail address: ajayi@anl.gov (O.O. Ajayi). sible for scuffing and the development of theories to predict scuffing behavior, mostly in metallic materials. In spite of these efforts, scuffing remains one of the least understood phenomena in the field of tribology. Because of the complexities of this phenomenon, the majority of studies have focused on scuffing prevention strategies, based mostly on phenomenological observations [2]. Indeed, many variables influence the occurrence of scuffing: the materials in contact; the method of preparation of the sliding surfaces; the composition and nature of the lubricant; and the contact operating conditions (temperature, sliding speed, and load). Scuffing resistance depends on the material properties and the surface finish. In general, heterogeneous materials and superfinished surfaces are more scuffing resistant [2–4]. Also, the chemistry and rheological properties of the lubricant affect the occurrence of scuffing. Lubricants containing extreme pressure (EP) additives usually produce a protective surface reaction layer (the so-called boundary films), which delay or may even prevent the onset of scuffing [5]. In terms of operating contact conditions, higher temperatures, sliding speeds, and loads often translate to a higher propensity for scuffing failure [6]. In view of the increasing demands on tribological components, the phenomenologically based strategies for scuffing 0043-1648/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.wear.2006.12.054
C. Lorenzo.Martin et al./ Wear 263(2007)872-877 prevention may no longer be adequate. Further, there is no satis- 2500 ctory scuffing prediction methodology, primarily because the basic mechanisms of the phenomenon are not fully understood Recently, a scuffing initiation mechanism based on adiabatic shear instability was proposed for steel material [1, 7]. When surfaces are in sliding contact, the local concentrated stresses at the asperities in the real area of contact often exceed the material yield strength, resulting in local plastic deformation primarily by dislocation motion at the asperities. With increas- ing plastic strain, the dislocation density will increase, resulting in work hardening. Concurrently, the work of plastic deforma- ion is converted into heat, leading to thermal softening If the rate of thermal softening exceeds the rate of work hardening at some asperities or locations in the sliding contact interface, the plastic deformation at that location becomes unstable, and this condition results in a sudden severe plastic deformation due to an adiabatic shear instability process. This is the initiation of scuffing. This initiation process is accompanied by a large heat generation in the severely deformed local material volume. This heat propels the propagation of the scuffing process into final catastrophic failure, if it is not quickly dissipated Because of the nature of their bonding (ionic and/or cova- Fig. 1. ZrO2-Y2O3 phase diagram [14]. lent), structural ceramics should behave differently compared to steel. Indeed, structural ceramic materials are currently being pound as illustrated by ZrO2-Y2 03 system phase diagram in used in some tribological systems to address scuffing problems Fig. 1. Through heat treatment and compositional variation,var- in lubricated components. Forexample, zirconia(ZrO2)ceramic lous microstructural configurations(and hence properties)can plungers have been successfully used in fuel injector systems produced for ZrO2 alloys. In that respect, ZrOz is similar to for heavy duty diesel engines, primarily to address scuffing fail- steel material ures in low-lubricity diesel fuels. Given the fact that scuffing In the present study, two variants of Y2O3-stabilized Zro2 at least in metals, involves severe plastic deformation, and that alloys were evaluated: the cubic Zro2-9% Y203 and tetrag ceramic materials in general do not plastically deform as eas- onal ZrO2-3% Y203 single crystals. The cubic crystals were ily as metals, it is reasonable to assume that ceramics are less obtained from Ceres Corp. and fabricated by the skull melt- susceptible to scuffing. However, some ceramic materials are ing method. Similarly, the tetragonal crystals, obtained from pable of plastic deformation, and other material mechanisms LMTRC of General Physics Institute of the Russian Academy may be involved in scuffing failure. For effective use of ceramic of Sciences(courtesy of Professor Elena Lomonova), were fabri- materials to address scuffing problems, it would be very instruc- cated by zone melting technique. Typical properties of the cubic tive to assess if scuffing can occur in this class of material and and tetragonal zrO2 crystals are given in Table I [8J Rectangu- by what mechanisms. lar"block "specimens of 12 mm x 6 mm x 2 mm were cut from This paper presents our study of the scuffing performance of both materials. The test surface oriented in the(100)plane was Y203-stabilized ZrO2 ceramic material single crystals using a polished with diamond paste to a surface finish of 0.06 pm Ra The counterface material used for the scuffing test is made so as to elucidate some of the basic material behavior and mech- of case carburized and hardened AIsI 4620 alloy steel with a anisms involved in the process, if scuffing does indeed occur in nominal composition of 0. 2C, 0.8Cr, 1.8Ni, and0.25Mo.The these materials. In later studies, we will evaluate the scuffing case depth was 0.5 mm with a surface hardness of 7.4GPa behavior of polycrystalline ceramic materials (61 Rc). The near surface case layer has a tempered martensite microstructure. The ring surface finish was 0.25 um Ra. 2. Experimental details 2.2. Scuffing test 2.. Materials The scuffing tests were conducted with a block-on-ring con Zirconia is perhaps the most versatile of the structural ceramic tact configuration using ZrO2 as the block and steel as the ring materials. This is in part due to its three major polymorphs of A photograph of the block-on-ring system is shown in Fig. 2 monoclinic(at low temperature), tetragonal (mid range tempera- The block is held stationary and loaded against a rotating ring ture), and cubic(at elevated temperature)structures. By alloying partially submerged in lubricant, thereby creating a fully flooded with other oxides such as MgO, Cao, Y2O3, different phases and well-lubricated contact interface. A three-axis load cell of Zro2 can be stabilized at room temperature. The stabilized allows measurement of normal, lateral, and traction forces. The phase is dependent on the concentration of the alloying com- loading and unloading is by a fast-response pneumatic system
C. Lorenzo-Martin et al. / Wear 263 (2007) 872–877 873 prevention may no longer be adequate. Further, there is no satisfactory scuffing prediction methodology, primarily because the basic mechanisms of the phenomenon are not fully understood. Recently, a scuffing initiation mechanism based on adiabatic shear instability was proposed for steel material [1,7]. When surfaces are in sliding contact, the local concentrated stresses at the asperities in the real area of contact often exceed the material yield strength, resulting in local plastic deformation primarily by dislocation motion at the asperities. With increasing plastic strain, the dislocation density will increase, resulting in work hardening. Concurrently, the work of plastic deformation is converted into heat, leading to thermal softening. If the rate of thermal softening exceeds the rate of work hardening at some asperities or locations in the sliding contact interface, the plastic deformation at that location becomes unstable, and this condition results in a sudden severe plastic deformation due to an adiabatic shear instability process. This is the initiation of scuffing. This initiation process is accompanied by a large heat generation in the severely deformed local material volume. This heat propels the propagation of the scuffing process into final catastrophic failure, if it is not quickly dissipated. Because of the nature of their bonding (ionic and/or covalent), structural ceramics should behave differently compared to steel. Indeed, structural ceramic materials are currently being used in some tribological systems to address scuffing problems in lubricated components. For example, zirconia (ZrO2) ceramic plungers have been successfully used in fuel injector systems for heavy duty diesel engines, primarily to address scuffing failures in low-lubricity diesel fuels. Given the fact that scuffing, at least in metals, involves severe plastic deformation, and that ceramic materials in general do not plastically deform as easily as metals, it is reasonable to assume that ceramics are less susceptible to scuffing. However, some ceramic materials are capable of plastic deformation, and other material mechanisms may be involved in scuffing failure. For effective use of ceramic materials to address scuffing problems, it would be very instructive to assess if scuffing can occur in this class of material and by what mechanisms. This paper presents our study of the scuffing performance of Y2O3-stabilized ZrO2 ceramic material single crystals using a common scuffing test method. Single-crystal materials are used so as to elucidate some of the basic material behavior and mechanisms involved in the process, if scuffing does indeed occur in these materials. In later studies, we will evaluate the scuffing behavior of polycrystalline ceramic materials. 2. Experimental details 2.1. Materials Zirconia is perhaps the most versatile of the structural ceramic materials. This is in part due to its three major polymorphs of monoclinic (at low temperature), tetragonal (mid range temperature), and cubic (at elevated temperature) structures. By alloying with other oxides such as MgO, CaO, Y2O3, different phases of ZrO2 can be stabilized at room temperature. The stabilized phase is dependent on the concentration of the alloying comFig. 1. ZrO2–Y2O3 phase diagram [14]. pound as illustrated by ZrO2–Y2O3 system phase diagram in Fig. 1. Through heat treatment and compositional variation, various microstructural configurations (and hence properties) can be produced for ZrO2 alloys. In that respect, ZrO2 is similar to steel material. In the present study, two variants of Y2O3-stabilized ZrO2 alloys were evaluated: the cubic ZrO2–9% Y2O3 and tetragonal ZrO2–3% Y2O3 single crystals. The cubic crystals were obtained from Ceres Corp. and fabricated by the skull melting method. Similarly, the tetragonal crystals, obtained from LMTRC of General Physics Institute of the Russian Academy of Sciences (courtesy of Professor Elena Lomonova), were fabricated by zone melting technique. Typical properties of the cubic and tetragonal ZrO2 crystals are given in Table 1 [8]. Rectangular “block” specimens of 12 mm × 6 mm × 2 mm were cut from both materials. The test surface oriented in the (1 0 0) plane was polished with diamond paste to a surface finish of 0.06 m Ra. The counterface material used for the scuffing test is made of case carburized and hardened AISI 4620 alloy steel with a nominal composition of 0.2C, 0.8Cr, 1.8Ni, and 0.25Mo. The case depth was 0.5 mm with a surface hardness of 7.4 GPa (61 Rc). The near surface case layer has a tempered martensite microstructure. The ring surface finish was 0.25 m Ra. 2.2. Scuffing test The scuffing tests were conducted with a block-on-ring contact configuration using ZrO2 as the block and steel as the ring. A photograph of the block-on-ring system is shown in Fig. 2. The block is held stationary and loaded against a rotating ring partially submerged in lubricant, thereby creating a fully flooded and well-lubricated contact interface. A three-axis load cell allows measurement of normal, lateral, and traction forces. The loading and unloading is by a fast-response pneumatic system
C. Lorenzo.Martin et al./Wear 263(2007)872-877 Table I Some properties of cubic and tetragonal zirconia single crystals Material Composition Density(kg/m) Hardness(GPa) Young modulus(GPa) Fracture toughness(MPam.) ZrO2-3Y203 13.5 attached to the load cell, enabling rapid unloading when scuffing was observed in all the tests conducted at various speeds. Before occurs final failure, the friction coefficient was nearly constant for each Tests were conducted at constant ring speeds of 500- test in the range of 0. 1-0. 15, with occasional perturbation espe 1750rpm, which translates to linear sliding speeds range of cially at load change points. The friction coefficients for tests 0.89-3. 11 m/s at the contact interface. The step-load increase conducted at higher speeds are slightly lower because of the protocol started at a load of 25 N, with an increase of 25 N every formation of higher lubricant fluid film thickness. Although the minute until scuffing occurred or the maximum load capacity of friction behavior is typical of what occurs during scuffing tests the test rig was reached, which is 1800N. The normal load, tan- of metal, the final failure occurred by fracture of the cubic ZrO gential and lateral forces, rotation speed, and number of cycles ceramic block as opposed to the typical severe plastic deforma were monitored continuously during the test. The friction coeffi- tion for metals. In all the tests with this material, the sudden rise cient was calculated as the ratio of tangential and normal forces. in friction at the end was accompanied by the block specimen Tests were terminated at scuffing occurrence, as detected by breaking into two pieces a sudden increase in the friction coefficient noise. and vibra- The frictional behavior during the test with tetragonal Zro2 ion. Multiple repeat tests were conducted for each material. blocks is shown in Fig. 4. For this material, all the tests at vari- An unformulated synthetic polyalphaolefin(PAO-4)was used ous speeds usually started with a relatively high value of friction lubricant so as to minimize the possible chemical effects of coefficient about 0. 2, but decreased gradual to a near steady the lubricant on scuffing behavior. The lubricant has a specific value in the range of 0.05-0.07. Like the cubic material, there gravity of 0.821, viscosity of 18.6 cSt at 40C, and a flash point was occasional perturbation in the friction at load changes, but Extensive post-scuffing test analyses were conducted on the (a)600 ZrO2 material. The surface damage modes were assessed by rmal Load(N) scanning electron microscopy(SEM). The samples were coated Friction Coeficient with a thin layer of carbon prior to SEM examination so as to avoid charging. X-ray diffraction was conducted in contact and y non-contact areas of the tetragonal material to determine the rystal structure of the material in and outside the contact areas 300 0.25 after tribological testing 0.2 3. Results and discussions 0.15 Fig 3 shows the frictional behavior during the test with the cubic ZrO2 material. In this material, the phenomenological 0200400600800100012001400 description of scuffing; i.e. a sudden rapid increase in friction Time(s) 025 block 0.15 005 cS|=82.13 02004006008001000120014001600 Time(s) Fig. 3. Variation of normal load and friction during scuffing test with cubic Fig. 2. Picture of block-on-ring contact. zirconia at(a) 500rpm and ( b)750rpm. CSI: contact severity index
874 C. Lorenzo-Martin et al. / Wear 263 (2007) 872–877 Table 1 Some properties of cubic and tetragonal zirconia single crystals Material Composition Density (kg/m3) Hardness (GPa) Young modulus (GPa) Fracture toughness (MPa m0.5) Cubic ZrO2–9Y2O3 5910 16 233 7 Tetragonal ZrO2–3Y2O3 6080 13.5 233 2 attached to the load cell, enabling rapid unloading when scuffing occurs. Tests were conducted at constant ring speeds of 500– 1750 rpm, which translates to linear sliding speeds range of 0.89–3.11 m/s at the contact interface. The step-load increase protocol started at a load of 25 N, with an increase of 25 N every minute until scuffing occurred or the maximum load capacity of the test rig was reached, which is 1800 N. The normal load, tangential and lateral forces, rotation speed, and number of cycles were monitored continuously during the test. The friction coeffi- cient was calculated as the ratio of tangential and normal forces. Tests were terminated at scuffing occurrence, as detected by a sudden increase in the friction coefficient, noise, and vibration. Multiple repeat tests were conducted for each material. An unformulated synthetic polyalphaolefin (PAO-4) was used as lubricant so as to minimize the possible chemical effects of the lubricant on scuffing behavior. The lubricant has a specific gravity of 0.821, viscosity of 18.6 cSt at 40 ◦C, and a flash point of 224 ◦C. Extensive post-scuffing test analyses were conducted on the ZrO2 material. The surface damage modes were assessed by scanning electron microscopy (SEM). The samples were coated with a thin layer of carbon prior to SEM examination so as to avoid charging. X-ray diffraction was conducted in contact and non-contact areas of the tetragonal material to determine the crystal structure of the material in and outside the contact areas after tribological testing. 3. Results and discussions Fig. 3 shows the frictional behavior during the test with the cubic ZrO2 material. In this material, the phenomenological description of scuffing; i.e. a sudden rapid increase in friction Fig. 2. Picture of block-on-ring contact. was observed in all the tests conducted at various speeds. Before final failure, the friction coefficient was nearly constant for each test in the range of 0.1–0.15, with occasional perturbation especially at load change points. The friction coefficients for tests conducted at higher speeds are slightly lower because of the formation of higher lubricant fluid film thickness. Although the friction behavior is typical of what occurs during scuffing tests of metal, the final failure occurred by fracture of the cubic ZrO2 ceramic block as opposed to the typical severe plastic deformation for metals. In all the tests with this material, the sudden rise in friction at the end was accompanied by the block specimen breaking into two pieces. The frictional behavior during the test with tetragonal ZrO2 blocks is shown in Fig. 4. For this material, all the tests at various speeds usually started with a relatively high value of friction coefficient about 0.2, but decreased gradual to a near steady value in the range of 0.05–0.07. Like the cubic material, there was occasional perturbation in the friction at load changes, but Fig. 3. Variation of normal load and friction during scuffing test with cubic zirconia at (a) 500 rpm and (b) 750 rpm. CSI: contact severity index
C: Lorenzo.Martin et al. /Wear 263(2007)872-877 2000 02004006008001000120014001600 Fig 4. Variation of friction coefficient and normal load during test with tetrag unlike the cubic material. no irreversible sudden rise in fric tion occurred at the end of the test signifying the occurrence of scuffing. Indeed, phenomenological scuffing did not occur in all the tests with tetragonal ZrO2 material up to the maximum peed and load range of our test rig. Because of the high severity of contact in tests with tetragonal ZrO2 material, considerable heat was generated as indicated by the excessive smoking of the lubricant. Although we did not measure the contact temp the lubricant reservoir temperature was certainly in excess of the lubricant flash temperature of 224C. The contact temperature is certainly much higher than that. Clearly, the tribological behavior differs significantly between tetragonal and cubic ZrO2 single crystals under the pro- gressively increasing contact severity of the step-load scuffing ANL EMC150k118mm×150SE(M)3/1005 test. The scuffing resistance of sliding surfaces can be assessed by the contact severity index(CSI) parameter, which is defined the product of the friction coefficient, the sliding speed, and the normal load at the point of scuffing, i.e. CSI=uSL Fig. 5. SEM micrograph of contact area in(a)cubic ZrO2 and(b) tetragonal This parameter is a measure of the frictional energy required ZrO,, both showing material transfer from the steel ring. to cause scuffing at a sliding contact interface. The average CSI in the tests with cubic material is about 82 at the point of fail- failure in order to examine the nature of damage in the mate- ure, compared to 280 for tetragonal material without failure, as rial prior to final failure. There was no evidence of macro-level indicated in Figs. 3 and 4. The different scuffing behavior of the plastic deformation, only localized damage by brittle fractures two materials reflects differences in the operating mechanisms was observed in the contact area(Fig. 7a). These areas of at the contact interface during the scuffing test, in spite of some localized surface damage may be connected to the formation of ring material transfer, as indicated in Fig. 7b for the cubic One and perhaps the only similarity between the tribolog- material. ical performance of the cubic and tetragonal ZrO2 materials The observed plastic deformation of the tetragonal material is the occurrence of some transfer of ring steel material into reflects the possible operation of several deformation mech- the ceramic surface, as shown in Fig. 5. However, the differ- anisms during the progressively increasing contact severity ences between the two materials are much more profound. As scuffing test. At the initial stages of relatively low contact stress, shown in Fig 6a, the cubic material exhibited extensive cracking, ferroelastic domain switching by reorientation of tetragonal generally oriented perpendicular to the sliding direction. There domains is responsible for plastic deformation [10]. A crystal was evidence of little or no plastic deformation. The tetragonal is ferroelastic if it has two or more stable orientation states that material,on the other hand, underwent significant plastic defor- can be readily changed from one to another when subjected to mation and limited cracking(Fig. 6b). This difference between mechanical stress. Tetragonal Zro2-3% Y2O3 single crystal, has the two materials occurred in spite of significantly higher sever- been shown to readily deform plastically by ferroelastic domain ty of contact during the test with tetragonal material. Since none switching [10, 11]. At high contact stress, plastic deformation of of the tests with the tetragonal material failed, a test was con- the tetragonal crystal can also occur by a phase transformation ducted with the cubic material and terminated before the final process, from tetragonal to monoclinic. This transformation
C. Lorenzo-Martin et al. / Wear 263 (2007) 872–877 875 Fig. 4. Variation of friction coefficient and normal load during test with tetragonal zirconia. unlike the cubic material, no irreversible sudden rise in friction occurred at the end of the test signifying the occurrence of scuffing. Indeed, phenomenological scuffing did not occur in all the tests with tetragonal ZrO2 material up to the maximum speed and load range of our test rig. Because of the high severity of contact in tests with tetragonal ZrO2 material, considerable heat was generated as indicated by the excessive smoking of the lubricant. Although we did not measure the contact temperature, the lubricant reservoir temperature was certainly in excess of the lubricant flash temperature of 224 ◦C. The contact temperature is certainly much higher than that. Clearly, the tribological behavior differs significantly between tetragonal and cubic ZrO2 single crystals under the progressively increasing contact severity of the step-load scuffing test. The scuffing resistance of sliding surfaces can be assessed by the contact severity index (CSI) parameter, which is defined as the product of the friction coefficient, the sliding speed, and the normal load at the point of scuffing, i.e. CSI =SL [9]. This parameter is a measure of the frictional energy required to cause scuffing at a sliding contact interface. The average CSI in the tests with cubic material is about 82 at the point of failure, compared to 280 for tetragonal material without failure, as indicated in Figs. 3 and 4. The different scuffing behavior of the two materials reflects differences in the operating mechanisms at the contact interface during the scuffing test, in spite of some similarities. One and perhaps the only similarity between the tribological performance of the cubic and tetragonal ZrO2 materials is the occurrence of some transfer of ring steel material into the ceramic surface, as shown in Fig. 5. However, the differences between the two materials are much more profound. As shown in Fig. 6a, the cubic material exhibited extensive cracking, generally oriented perpendicular to the sliding direction. There was evidence of little or no plastic deformation. The tetragonal material, on the other hand, underwent significant plastic deformation and limited cracking (Fig. 6b). This difference between the two materials occurred in spite of significantly higher severity of contact during the test with tetragonal material. Since none of the tests with the tetragonal material failed, a test was conducted with the cubic material and terminated before the final Fig. 5. SEM micrograph of contact area in (a) cubic ZrO2 and (b) tetragonal ZrO2, both showing material transfer from the steel ring. failure in order to examine the nature of damage in the material prior to final failure. There was no evidence of macro-level plastic deformation, only localized damage by brittle fractures was observed in the contact area (Fig. 7a). These areas of localized surface damage may be connected to the formation of ring material transfer, as indicated in Fig. 7b for the cubic material. The observed plastic deformation of the tetragonal material reflects the possible operation of several deformation mechanisms during the progressively increasing contact severity scuffing test. At the initial stages of relatively low contact stress, ferroelastic domain switching by reorientation of tetragonal domains is responsible for plastic deformation [10]. A crystal is ferroelastic if it has two or more stable orientation states that can be readily changed from one to another when subjected to mechanical stress. Tetragonal ZrO2–3% Y2O3 single crystal, has been shown to readily deform plastically by ferroelastic domain switching [10,11]. At high contact stress, plastic deformation of the tetragonal crystal can also occur by a phase transformation process, from tetragonal to monoclinic. This transformation-
876 C. Lorenzo.Martin et al./Wear 263(2007)872-877 ANL_EMC 150KV 117mm x500 SE(M)3/10/05 100um 100kv123mmx4.00k7310616:44 Fig. 6. SEM micrograph of the edge of contact area on(a)cubic ZrO2 showing Fig. 7. SEM micrograph of cubic ZrO2 showing(a)localized surface damage b extensive cracking and (b) tetragonal ZrO2 showing mainly plastic deformation. brittle fracture in earlier stages of scuffing test and (b) transfer of steel material onto areas of localized damage at conclusion of test. induced plasticity is stress induced and is accompanied by a volume increase which imposes compressive stresses, thereby suppressing cracking and fracture. This is indeed the principle for the transformation toughening of zro or zro contain- ing structural ceramic materials and composites. In the present study, X-ray diffraction from the wear track and the non-contact areas of the tetragonal material did show the presence of mon aclinic phase in the wear track(Fig. 8). Thus, the plasticity observed in the tetragonal ZrO material during scuffing test 3 10 is partially due to the tetragonal-to-monoclinic phase transfor- a ation in the contact area. Plastic deformation can also occur in s 2 tetragonal ZrO2 by slip through dislocation mobility, especiall at elevated temperatures. As indicated earlier, the contact tem- perature with tetragonal material under the high contact severity achieved towards the end of the test is high. It is thus possi ble that some deformation by dislocation motion was activated in the latter stage of the scuffing test with the tetragonal Zro material. The deformation by this mechanism obviously was not Angle(2 Tetha) Damage by cracking and fracture was limited in the tetragonal ZrOz material showing the formation of monoclinncps vear areas in tetragonal to the level of shear instability, hence no catastrophic failure. Fig 8. x-ray diffraction spectra of non-contact and
876 C. Lorenzo-Martin et al. / Wear 263 (2007) 872–877 Fig. 6. SEM micrograph of the edge of contact area on (a) cubic ZrO2 showing extensive cracking and (b) tetragonal ZrO2 showing mainly plastic deformation. induced plasticity is stress induced and is accompanied by a volume increase which imposes compressive stresses, thereby suppressing cracking and fracture. This is indeed the principle for the transformation toughening of ZrO2 or ZrO2 containing structural ceramic materials and composites. In the present study, X-ray diffraction from the wear track and the non-contact areas of the tetragonal material did show the presence of monoclinic phase in the wear track (Fig. 8). Thus, the plasticity observed in the tetragonal ZrO2 material during scuffing test is partially due to the tetragonal-to-monoclinic phase transformation in the contact area. Plastic deformation can also occur in tetragonal ZrO2 by slip through dislocation mobility, especially at elevated temperatures. As indicated earlier, the contact temperature with tetragonal material under the high contact severity achieved towards the end of the test is high. It is thus possible that some deformation by dislocation motion was activated in the latter stage of the scuffing test with the tetragonal ZrO2 material. The deformation by this mechanism obviously was not to the level of shear instability, hence no catastrophic failure. Damage by cracking and fracture was limited in the tetragonal Fig. 7. SEM micrograph of cubic ZrO2 showing (a) localized surface damage by brittle fracture in earlier stages of scuffing test and (b) transfer of steel material onto areas of localized damage at conclusion of test. Fig. 8. X-ray diffraction spectra of non-contact and wear areas in tetragonal ZrO2 material showing the formation of monoclinic phase after testing
C: Lorenzo.Martin et al. /Wear 263(2007)872-877 material because of the operation of plasticity-based frictional 3. This resilience of the tetragonal material is the result of stress dissipation mechanisms instead the sequential operation of several frictional stress dis- Results of the present study provided some valuable insight sipation mechanisms of plastic deformation. Ferroelastic into the role of composition and microstructure of materials domain switching occurred at low loads then tetragonal-to- on their scuffing behavior Two variants of zro, material with monoclinic phase transformation, and finally deformation by different amounts of Y2O3 alloying composition(3% for tetrag slip at elevated temperatures towards the end of the test onal and 9% for cubic), showed significantly different behavior 4. Based on the results of the present study, materials with multi- during a scuffing test. This behavior reflects differences in the ple sequential dissipation mechanisms are expected to exhibit crystal structure and properties of the two variants. For the cubic higher scuffing resistance. ZrO2-9% Y203 material, all the scuffing tests were terminated a result of catastrophic failure by fracture. Although limited Acknowledgments plastic deformation of cubic Zro2 by slip is possible, it can only occur at elevated temperatures [12]. In the present study, the This work was supported by the Office of Freedom Car and failure in the material is dominated by brittle fracture. At the Vehicle Technologies of the U.S. Department of Energy under early stages of the scuffing test, tensile stresses at local asperity contract DE-ACO2-06CHI1357. We thank Professor Elena contact are high enough to cause localized cracking. The rela- Lomonova of the laser Materials and Technologies Research tive ease of forming cracks in cubic material occurs because of Center of General Physics Institute(LMTRC), Moscow for sup- its much lower fracture toughness compared to the tetragonal plying the tetragonal zirconia material used in this study material:1.8-1.9 and 6.0-7.0 MPam.for cubic and tetragonal material, respectively [8, 13]. With increasing contact seventy References via load increase, the stress is high enough to cause a final frac ture of the cubic ZrO2 block and the consequent sudden rise in [11 O.O. Ajayi, J.G. Hersberger, J. Zhang, H. Yoon, G.R. Fenske, Microstruc friction coefficient and"scuffing tural evolution during scuffing of hardened 4340 steel-implication for In general, this study demonstrated a materials approach to scuffing mechanism, Tribol. Int. 38(3)(2005)277-28 [2 K.C. Ludema, A review of scuffing and running-in of lubricated sur- enhance the scuffing resistance of sliding surfaces. materials faces, with asperities and oxides in perspective, Wear 100(1984)315- with multiple, preferably sequential, stress dissipation mecha- nisms(such as tetragonal zirconia) are likely to exhibit high [3] Y-Z. Lee, B.J. Kim, The influence of the boundary lubricating conditions scuffing resistance compared to materials with primarily a single of three different fluids on the plastic fatigue related mechanism of we dissipation mechanism. Indeed, this principle can be general and scuffing, Wear 232(1999)116-121. [4]K. Kim, K.C. Ludema, A correlation between low cycle fatigue properties ized to tribological contact interfaces. Sliding contact interface and scuffing properties of 4340 steel, J. Tribol. 117(1995)617-621 with multiple sequential frictional dissipation mechanisms are [5] E.F. Escobar-Jaramillo, The additive EP-condition and the critical scuffing expected to be less susceptible to sudden catastrophic failure limit for rolling-sliding. Tribol. Trans. 118(1996)125-130 such as scuffin [6] A. Dyson, Scuffing, Treatise Mater. Sci. Technol. 13(1979)175-216 [7] J. Hershberger, O.O. Ajayi, J. Zhang, H. Yoon, G.R. Fenske, Evidence of scuffing initiation by adiabatic shear instability, Wear 258(2005) 4. Conclusions [8]R.P. Ingel, D. Lewis, B.A. Bender, R.W. Rice, Physical, microstructural The scuffing performance of two variants of single-crystal and thermomechanical properties of ZrO2 single crystals, in: N. Claussen, ZrO, materials; the tetragonal ZrO, with 3%Y2O3 and the M. Ruhle, A.H. Heuer(Eds ) Advances in Ceramics, vol. 12, American ZrO2 with 9%Y2O3 was evaluated with a block-on-ring contact (9) M.E. Alzoubi, O.O. Ajayi, J.B. Woodford, A. Erdemir, G.R. Fenske, Scuff- configuration. Using a step loading protocol, both materials were ing performance of amorphous carbon coating during dry-sliding contact, tested against case carburized and hardened AISI 4620 steel Tribol. Trans.44(2001)591-596 lubricated with unformulated PAO-4 synthetic oil. The following [10] D. Baither, M. Bartsch, B. Baufeld, A Tikhonovsky, A Foitzik, M Ruhle, onclusions are drawn from the present study U Messerschmidt, Ferroelastic and plastic deformation of t-zirconia single crystals, J Am Ceram Soc. 84(2001)1755-1762. [11] F.R. Chien, FJ. Ubic, V. Prakash, A H. Heuer, Stress-induced marten 1. The cubic material showed a sudden catastrophic failure sitic transformation and ferroelastic deformation adjacent microhardness haracteristic of scuffing The mechanism of failure was, indents in tetragonal zirconia single crystals, Acta Mater. 46(1998) however, by fracture instead of severe plastic deformation typical for metallic materials. In all cases the terminal point [12]U Messerschmidt, D. Baither, B. Baufeld, M. Bartsch, Plastic deformation of failure for cubic material was the breaking of the zro zirconia single crystals, Mater. Sci. Eng. A 233(1997)61-74 block sample in two [13] D. Michel, L Mazerolles, M. Perez, Y. Jorba, Fracture of metastable tetrag. onal zirconia crystals, J Mater. Sci. 18(1983)2618-2628 2. In the tetragonal material, there was no sudden catastrophic [14] V. Srikanth, E.C. Subbarao, Acoustic emission study of phase relations failure in spite of much higher contact severity during testin in low-Y2O3 portion of ZrOz-Y2O3 system, J Mater. Sci. 29(199 with the material 3363-3371
C. Lorenzo-Martin et al. / Wear 263 (2007) 872–877 877 material because of the operation of plasticity-based frictional stress dissipation mechanisms instead. Results of the present study provided some valuable insight into the role of composition and microstructure of materials on their scuffing behavior. Two variants of ZrO2 material with different amounts of Y2O3 alloying composition (3% for tetragonal and 9% for cubic), showed significantly different behavior during a scuffing test. This behavior reflects differences in the crystal structure and properties of the two variants. For the cubic ZrO2–9% Y2O3 material, all the scuffing tests were terminated as a result of catastrophic failure by fracture. Although limited plastic deformation of cubic ZrO2 by slip is possible, it can only occur at elevated temperatures [12]. In the present study, the failure in the material is dominated by brittle fracture. At the early stages of the scuffing test, tensile stresses at local asperity contact are high enough to cause localized cracking. The relative ease of forming cracks in cubic material occurs because of its much lower fracture toughness compared to the tetragonal material: 1.8–1.9 and 6.0–7.0 MPa m0.5 for cubic and tetragonal material, respectively [8,13]. With increasing contact seventy via load increase, the stress is high enough to cause a final fracture of the cubic ZrO2 block and the consequent sudden rise in friction coefficient and “scuffing”. In general, this study demonstrated a materials approach to enhance the scuffing resistance of sliding surfaces. Materials with multiple, preferably sequential, stress dissipation mechanisms (such as tetragonal zirconia) are likely to exhibit high scuffing resistance compared to materials with primarily a single dissipation mechanism. Indeed, this principle can be generalized to tribological contact interfaces. Sliding contact interface with multiple sequential frictional dissipation mechanisms are expected to be less susceptible to sudden catastrophic failure, such as scuffing. 4. Conclusions The scuffing performance of two variants of single-crystal ZrO2 materials; the tetragonal ZrO2 with 3% Y2O3 and the cubic ZrO2 with 9% Y2O3 was evaluated with a block-on-ring contact configuration. Using a step loading protocol, both materials were tested against case carburized and hardened AISI 4620 steel, lubricated with unformulated PAO-4 synthetic oil. The following conclusions are drawn from the present study: 1. The cubic material showed a sudden catastrophic failure characteristic of scuffing. The mechanism of failure was, however, by fracture instead of severe plastic deformation typical for metallic materials. In all cases the terminal point of failure for cubic material was the breaking of the ZrO2 block sample in two. 2. In the tetragonal material, there was no sudden catastrophic failure in spite of much higher contact severity during testing with the material. 3. This resilience of the tetragonal material is the result of the sequential operation of several frictional stress dissipation mechanisms of plastic deformation. Ferroelastic domain switching occurred at low loads, then tetragonal-tomonoclinic phase transformation, and finally deformation by slip at elevated temperatures towards the end of the test. 4. Based on the results of the present study, materials with multiple sequential dissipation mechanisms are expected to exhibit higher scuffing resistance. Acknowledgments This work was supported by the Office of FreedomCar and Vehicle Technologies of the U.S. Department of Energy under contract DE-AC02-06CH11357. We thank Professor Elena Lomonova of the Laser Materials and Technologies Research Center of General Physics Institute (LMTRC), Moscow for supplying the tetragonal zirconia material used in this study. References [1] O.O. Ajayi, J.G. Hersberger, J. Zhang, H. Yoon, G.R. Fenske, Microstructural evolution during scuffing of hardened 4340 steel—implication for scuffing mechanism, Tribol. Int. 38 (3) (2005) 277–282. [2] K.C. Ludema, A review of scuffing and running-in of lubricated surfaces, with asperities and oxides in perspective, Wear 100 (1984) 315– 331. [3] Y.-Z. Lee, B.-J. Kim, The influence of the boundary lubricating conditions of three different fluids on the plastic fatigue related mechanism of wear and scuffing, Wear 232 (1999) 116–121. [4] K. Kim, K.C. Ludema, A correlation between low cycle fatigue properties and scuffing properties of 4340 steel, J. Tribol. 117 (1995) 617–621. [5] E.F. Escobar-Jaramillo, The additive EP-condition and the critical scuffing limit for rolling-sliding, Tribol. Trans. 118 (1996) 125–130. [6] A. Dyson, Scuffing, Treatise Mater. Sci. Technol. 13 (1979) 175–216. [7] J. Hershberger, O.O. Ajayi, J. Zhang, H. Yoon, G.R. Fenske, Evidence of scuffing initiation by adiabatic shear instability, Wear 258 (2005) 1471–1478. [8] R.P. Ingel, D. Lewis, B.A. Bender, R.W. Rice, Physical, microstructural, and thermomechanical properties of ZrO2 single crystals, in: N. Claussen, M. Ruhle, A.H. Heuer (Eds.), Advances in Ceramics, vol. 12, American Ceramic Society, Columbus, OH, 1984, pp. 408–414. [9] M.F. Alzoubi, O.O. Ajayi, J.B. Woodford, A. Erdemir, G.R. Fenske, Scuffing performance of amorphous carbon coating during dry-sliding contact, Tribol. Trans. 44 (2001) 591–596. [10] D. Baither, M. Bartsch, B. Baufeld, A. Tikhonovsky, A. Foitzik, M. Ruhle, U. Messerschmidt, Ferroelastic and plastic deformation of t-zirconia single crystals, J. Am. Ceram. Soc. 84 (2001) 1755–1762. [11] F.R. Chien, F.J. Ubic, V. Prakash, A.H. Heuer, Stress-induced martensitic transformation and ferroelastic deformation adjacent microhardness indents in tetragonal zirconia single crystals, Acta Mater. 46 (1998) 2151–2171. [12] U. Messerschmidt, D. Baither, B. Baufeld, M. Bartsch, Plastic deformation of zirconia single crystals, Mater. Sci. Eng. A 233 (1997) 61–74. [13] D. Michel, L. Mazerolles, M. Perez, Y. Jorba, Fracture of metastable tetragonal zirconia crystals, J. Mater. Sci. 18 (1983) 2618–2628. [14] V. Srikanth, E.C. Subbarao, Acoustic emission study of phase relations in low-Y2O3 portion of ZrO2–Y2O3 system, J. Mater. Sci. 29 (1994) 3363–3371