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 index874 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, tan￾gential 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 vibra￾tion. 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 espe￾cially 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 deforma￾tion 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 vari￾ous 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