1550 Journal of the American Ceramic Society-Toschi et al Vol 86. No, 9 Indentation tests were performed on the polished surface of the Figure 5 shows the friction coefficient values measured in the laminated structures to measure residual stresses. When suffi stationary state on the various materials for the different experi- ciently high loads are used, the applied model relates the fracture mental conditions toughness to the indentation load and the lengths of the relative The hybrid laminated composite A/AZ was found to have the cracks emanating from the corners of the impressions through the lowest values of H under all experimental conditions, except for following equation: the test conducted at 150 N and 0.15 m/s, for which the value was closer to those of the other two materials. Conversely, the P monolithic alumina generally exhibits the highest values. For KI A/Az and MA materials, the friction coefficient could not be determined when testing at 150N and 0.05 m/s because the pins where K,s=toughness of the stress-free material, x-dimension- less constant(experimentally determined). P= indentation load, Additional relevant information acquired from the data are that and co crack length the friction coefficient of A/AZ is almost constant regardless of the It is possible to evaluate the parameter x by means of the best applied loads. except for P= 150 N and v=0. 15 m/s, where the fit of a regression of the experimental data of P and co, measured wear mechanism differed from that observed under other experi on the MA material and using the value of 3. 61 MPa'mfor K nental conditions. For laminated structures at a load of 100 N. the as measured by the Chevron notch technique. In this case we friction coefficients of the different materials are almost indepen- obtained x=0.089. This is close to the value of 0.080, which can dent of the sliding speed. Under a load of 150 N the friction be obtained from the equation x=S(E/H), as proposed in Ref. coefficients are highest at a sliding speed of 0. 15 m/s. All tested 23. considering $=0.016(suggested by the same authors)to be materials exhibited this behavior. The increase of H results in an he"calibration constant ncrease of the maximum principal tensile stress (oI max)at the When residual stress is present, Eq (1)becomes ontact surface, which is responsible for crack initiation (i.e, to he reduction in critical normal load needed for cracking) and Kr=xcv+ Yom vt (2) two sliding surfaces is determined by several factors: the height of crack length in the stressed material, r= 1.29 the asperities, their hardness, and adhesion of the contacting surfaces. It is easier for the asperities to deform plastically when geometrical factor, and Ores residual stress, whereas the other hardness is lower. Thus, the asperities'height and their effect on symbols have the same meaning as above frictin meanwhile, this Using the data for Kie(measured using the CNB method)and x the contact area between the two counter surfaces of the sliding (obtained previously), regression of the experimental data of P and c through Eq (2)allows us to calculate res, This analysis yielded materials to increase. Consequently, the fact that the material with value of-141 MPa(compres for g. Figure 4 shows the the higher hardness exhibits a lower friction coefficient suggests regression of vickers indentation data for the stress-free material hat the adhesion mechanism is dominant. When the wear is severe MA and hybrid laminate A/AZ. (see the next section). the third body produced during service plays In the laminated composite the load of 147. 15 N was the a role. The data show that when a severe wear regime is present the friction coefficient tends to increase with the severity of the higher loads was longer than the thickness of the external alumina test. This means that the increased amount of debris formed during layer. The model described above assumes half-penny-shape cracks. This is the case of the cracks observed in the section of a large contact surface for the two materials, thus leading to an increase in adhesive friction. As the wear of hybrid laminates uch laminates after indentation and fracture tests results in a lower amount of debris than other materials, the direct consequence Is a lower friction (3) Friction and Wear (B) Wear: Figure 6 shows the specific wear of the various (A) Friction: Observation of friction coefficient patterns ob- rained during te revealed that, in most cases, u started at a data are shown in a semilog plot that reduces differences if high value and reached a stationary condition after a running-in mpared with those expressed as absolute values, it is evident that period of variable duration. When a stationary value was not the wear of hybrid-laminated composite A/AZ is always lower reached (on two occasions for the MA material), the tendency was than the values measured on the other materials, except when the for the friction coefficient to decrease load is 50N and the sliding speed 10 m/s In this case the values for all the studied materials were almost the same. In the more severe wear conditions the depth of the groove produced by the pin was-170 um. that is, the exact length of the longer crack (170 um)induced on the surface by the indentation of a load of 147. 15 250 N used to calculate the residual stress. This means that the surface damage was limited to the external layer, which was -180 um thick and that the calculated compressive residual stress was the effective additional stress operating during the wear test. 141 MPa Wer tom ontical microscope and SEM observations, four main ir mechanisms were identified out of all the samples tested under the various experimental conditions Three of these mechanisms(microcracking, macrocracking, and 口AAz abrasion from the third body) appear at loads of 50 and 100N lor all three sliding speeds (0.05, 0.10, and 15 m/s). A fourth mechanism which induces considerable plastic deformation of the 50100150200250300350 material along the entire wear track, seems to predominate at a Crack length, um load of 150 N and sliding speed of 0. 15 m/s and can be synergetic with the others. and usit mM上个7四可m四m the value of x and the surface residual stress present in the A/AZ layer (s debris. its comminution, and subsequent compaction were caused