G de Portu et al. /Composites: Part B 37(2006)556-567 563 500um length along the eross section/m Fig 8. Stress profiles and micrograph of the cross section in multilayered specimen with different thickness ratio among the layers(S2, S3 and $4). Each profile refers to the specimen showed on its right. In order to clarify the graph, only one half of the stress profile is plotted. wear resistance) or to the whole structure (strength and indentation tests and compared the response of the laminates to toughness) the response of a stress-free alumina. They have observed that the laminated composites present a better resistance to damage 5.1. Surface damage resistance than pure alumina, both under constant and cyclic loading. These results are summarized in Figs. 9 and 10. From these figures, it As already mentioned considerable effort have been done in can be seen also that, in both cases, A/LAZ presents better results characterizing the mechanical properties of the laminat than A/AZ. This can be correlated to the existence of the highe composites [16-23]. All these studies have shown that these multilayer ceramic composites present better fracture proper laminated materials(see Fig 8) ties than their monolithic counterparts and that, in some cases they mitigate the brittleness of the ceramics by presenting a 52. Wear resistance graceful failure'behaviour Most of the studies, however, deal with crack propagation The presence of residual stresses at the surface of laminated through layers, usually in bending tests, but there is not much composites can improve the wear resistance both under sliding research done about the behaviour of these materials under and abrasive conditions repetitive and/or sliding surface loads, which in some cases It has been verified [34 that under sliding conditions the may be closer to real-life applications. This is particularly true friction coefficients, for the pairings of monolithic alumina in cases such as biomedical implants or bearings, wher with laminated composite A/AZ, were always lower than those ceramics are used because of their biocompatibility and good for stress free materials(Fig. ID) tribological behaviour. At the same time the data for specific wear have shown Jimenez-Pique et al. [23] have investigated the contact fatigue (Fig. 12)how the presence of compressive stresses plays an esponse of symmetrical laminated ceramics of Al, and important role in improving the wear resistance of the alumina Al2O3-ZrO2 with compressive residual stresses at the surface. when the applied loads and sliding speeds induce a wear For that purpose they have carried out cyclic and static Hertzian mechanism that is mainly ascribable to micro- and macro- cracking. When the wear mechanism is associated with the △A2AZ 700△ ▲△A2AZ A/AZ 乙 300 0o100010001000001000000 Number of cycles Fig. 9. Applied indenter load against time for a stress corros ing tests Fig 10. Applied indentation load against number of c the cyclic loading for the three materials. Empty points indicate no apparent n the fatigue tests for MA, A/AZ surface of the sample, while closed points indicate the existence of a well- apparent damage on the surface of the sample, while developed ring crack Ref. [231 existence of a well-developed ring crack Ref. [23]-wear resistance) or to the whole structure (strength and toughness). 5.1. Surface damage resistance As already mentioned considerable effort have been done in characterizing the mechanical properties of the laminated composites [16–23]. All these studies have shown that these multilayer ceramic composites present better fracture proper￾ties than their monolithic counterparts and that, in some cases, they mitigate the brittleness of the ceramics by presenting a ‘graceful failure’ behaviour. Most of the studies, however, deal with crack propagation through layers, usually in bending tests, but there is not much research done about the behaviour of these materials under repetitive and/or sliding surface loads, which in some cases may be closer to real-life applications. This is particularly true in cases such as biomedical implants or bearings, where ceramics are used because of their biocompatibility and good tribological behaviour. Jime´nez-Pique´ et al. [23] have investigated the contact fatigue response of symmetrical laminated ceramics of Al2O3 and Al2O3–ZrO2 with compressive residual stresses at the surface. For that purpose they have carried out cyclic and static Hertzian indentation tests and compared the response of the laminates to the response of a stress-free alumina. They have observed that the laminated composites present a better resistance to damage than pure alumina, both under constant and cyclic loading. These results are summarized in Figs. 9 and 10. From these figures, it can be seen also that, in both cases, A/2AZ presents better results than A/AZ. This can be correlated to the existence of the higher compressive residual stresses at the surface in the A/2AZ laminated materials (see Fig. 8). 5.2. Wear resistance The presence of residual stresses at the surface of laminated composites can improve the wear resistance both under sliding and abrasive conditions. It has been verified [34] that under sliding conditions the friction coefficients, for the pairings of monolithic alumina with laminated composite A/AZ, were always lower than those for stress free materials (Fig. 11). At the same time the data for specific wear have shown (Fig. 12) how the presence of compressive stresses plays an important role in improving the wear resistance of the alumina when the applied loads and sliding speeds induce a wear mechanism that is mainly ascribable to micro- and macro￾cracking. When the wear mechanism is associated with the Fig. 8. Stress profiles and micrograph of the cross section in multilayered specimen with different thickness ratio among the layers (S2, S3 and S4). Each profile refers to the specimen showed on its right. In order to clarify the graph, only one half of the stress profile is plotted. Fig. 9. Applied indenter load against time for a stress corrosion cracking tests for the three materials. Empty points indicate no apparent damage on the surface of the sample, while closed points indicate the existence of a well￾developed ring crack Ref. [23]. Fig. 10. Applied indentation load against number of cycles of the cyclic loading fatigue tests for MA, A/AZ and A/2AZ materials. Empty points indicate no apparent damage on the surface of the sample, while closed points indicate the existence of a well-developed ring crack Ref. [23]. G. de Portu et al. / Composites: Part B 37 (2006) 556–567 563