G de Portu et al. /Composites: Part B 37(2006)556-567 565 0037×MAAA·s2·s3s4 ①=492MI 0.02 0.01 回20 0.00 1.0 3.0 sliding distance x normal load(m*N as a function of sliding distance. The dashed line )and the black lines to laminated samples. The during wear process, can be present between two bodies in relative motion. The present authors have analyzed the abrasive behaviour of three laminated composites with 50 MP different thickness ratio among the layers. As shown in a 386472558644730 previous paragraph this difference leads to different values and distribution of surface stresses(Fig. 8). A comparison between abrasive wear resistance of bulk Fig. 15. Weibull plots of AA and A/AZ laminates Ref [22]. (cold isostatic pressed)Al2O3 and laminated composites in the materials whose tribological behaviour is superior to that of system Al2O3-ZrO2 has shown the positive effect of the stresses In Fig. 13, the worn volume as a function of sliding distance is reported. The dashed line refers to the monolithic 5.4. fracture strength alumina (MA)and the black lines to laminated samples Sample AA is a material obtained by lamination of only Several studies [16-23 have shown that laminated alumina layers whereas S2, S3 and S4 are hybrid laminates structures have higher bending strength than monolithic with increasing thickness ratio among AZ and A layers(as materials with chemical composition similar to that of the shown in Fig. 8). The slope of those lines is the specific wear outer layers coefficient K. If k is plotted as a function of residual stresses Pascual et al. [22] have compared the strength distribution (Fig. 14) it appears evident that for a surface residual stress of a ceramic laminate made from alumina and alumina +3Y- higher than about 30 MPa the abrasive wear resistance is TZP(A/AZ) layers with the distribution of a pure alumina increased The production of laminated structures with compressive deflections or bifurcations at the fracture surface were residual stresses at the surface thus makes it possible to obtain observed. As for the AA material, only surface defects were found as failure origins. So a similar surface defect population can be assumed to be responsible for failure of the different batches of samples. In this situation, Weibull statistic can be applied to fracture behaviour of both laminates and stresse-free material. It turned out that the difference in 0.003- ◆S3 characteristic strength and the Weibull modulu attributed to the residual compressive stress in the outer layer 日0.002 of the A/Az-laminate In Fig. 15, the Weibull plots of AA and S4◆ A/AZ laminates are compared. It can be seen that the laminated 0.00l structure exhibit higher modulus(m). The effect of a residual cold isostatic pressed alumina(MA) compressive stress at the surface leads thus to an increased strength of the specimens and a decrease in scatter, testifying a superior reliability. compressive residual stress(MPa) 6. Summary Fig. 14. Wear coefficient K as function of induced residual stresses in the mos superficial 30 um underneath the surface. As in Fig 13, dashes line shows the The motivation for studying and producing laminated value of K for the bulk alumina(MA) ceramic composites have been illustrated. It resides in theduring wear process, can be present between two bodies in relative motion. The present authors have analyzed the abrasive behaviour of three laminated composites with different thickness ratio among the layers. As shown in a previous paragraph this difference leads to different values and distribution of surface stresses (Fig. 8). A comparison between abrasive wear resistance of bulk (cold isostatic pressed) Al2O3 and laminated composites in the system Al2O3–ZrO2 has shown the positive effect of the stresses. In Fig. 13, the worn volume as a function of sliding distance is reported. The dashed line refers to the monolithic alumina (MA) and the black lines to laminated samples. Sample AA is a material obtained by lamination of only alumina layers whereas S2, S3 and S4 are hybrid laminates with increasing thickness ratio among AZ and A layers (as shown in Fig. 8). The slope of those lines is the specific wear coefficient k. If k is plotted as a function of residual stresses (Fig. 14) it appears evident that for a surface residual stress higher than about 30 MPa the abrasive wear resistance is increased. The production of laminated structures with compressive residual stresses at the surface thus makes it possible to obtain materials whose tribological behaviour is superior to that of stress-free materials. 5.4. Fracture strength Several studies [16–23] have shown that laminated structures have higher bending strength than monolithic materials with chemical composition similar to that of the outer layers. Pascual et al. [22] have compared the strength distribution of a ceramic laminate made from alumina and alumina C3Y￾TZP (A/AZ) layers with the distribution of a pure alumina laminate (AA). In the investigated A/AZ laminates no deflections or bifurcations at the fracture surface were observed. As for the AA material, only surface defects were found as failure origins. So a similar surface defect population can be assumed to be responsible for failure of the different batches of samples. In this situation, Weibull statistic can be applied to fracture behaviour of both laminates and stresse-free material. It turned out that the difference in both the characteristic strength and the Weibull modulus can be attributed to the residual compressive stress in the outer layer of the A/AZ-laminate. In Fig. 15, the Weibull plots of AA and A/AZ laminates are compared. It can be seen that the laminated structure exhibit higher modulus (m). The effect of a residual compressive stress at the surface leads thus to an increased strength of the specimens and a decrease in scatter, testifying a superior reliability. 6. Summary The motivation for studying and producing laminated ceramic composites have been illustrated. It resides in the Fig. 13. Worn volume as a function of sliding distance. The dashed line refers to the bulk alumina (MA) and the black lines to laminated samples. The slope of those lines is the specific wear coefficient k. Fig. 14. Wear coefficient k as function of induced residual stresses in the most superficial 30 mm underneath the surface. As in Fig. 13, dashes line shows the value of k for the bulk alumina (MA). Fig. 15. Weibull plots of AA and A/AZ laminates Ref. [22]. G. de Portu et al. / Composites: Part B 37 (2006) 556–567 565