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 transformationC. Lorenzo-Martin et al. / Wear 263 (2007) 872–877 875 Fig. 4. Variation of friction coefficient and normal load during test with tetrag￾onal zirconia. 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 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 pro￾gressively 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 fail￾ure, 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 tribolog￾ical 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 differ￾ences 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 defor￾mation and limited cracking (Fig. 6b). This difference between the two materials occurred in spite of significantly higher sever￾ity of contact during the test with tetragonal material. Since none of the tests with the tetragonal material failed, a test was con￾ducted 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 mate￾rial 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 mech￾anisms 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-