The role of residual stresses in layered composites of y-ZrO2 and A120 261 4 Conclu The aim of this work was to determine the residual stress effect on character of crack ation 5e ya layered ceramic composites prepared by sequential centrifuging of powder suspensions During tests of controlled crack growth a distinct crack deflection in alumina layers was observed. As it occured the value of crack deflection angle was proportional to the layer thickness. In the case of layer thicknesses 16973 below 10 um the crack was found to be undeflected In barrier layers made of an oxide mixture crack deflection did not happen independently on layer thickness. This observations have been explained by measurements of residual stress distribution in bar. rier layers. The magnitude of compressive stress in alumina layer on the layer boundary was indepen dent on layer thickness. However the layer thickness 1898 affected the gradient of stresses. The compressive stresses were found to decrease from the boundary of layer to the centre of alumina layer and here reached the minimum. The correlation between the value of crack deflection angle and the magnitude of stress gradient was observed. In the case of layer with thicknesses less than 10 um, where crack did not deflect, the compressive stress gradient reached 16981 very small value. In the barrier layers made of an oxide mixture, higher compressive stresses were found. however the distribution and local character of these stresses resulted in the lack of crack deflec- (b) tion independently on layer thickness Elongation of crack way caused by crack deflec 12. The crack path during fracture of Y-ZrO,/Al, O, tion in alumina layer seemed to be responsible for posite with barrier layers made of alumina and zirconia observed enhancement in toughness of composites studied in a function of layer thickness Acknowledgements This work was supported by Polish Committee for Scientific Research under grant No. 7S202 04607 References 1. Clarke, F.J. P, Residual strain and fracture stress-grain size relationships in brittle solids. Acta Metall, 1964 Position across the layer, um 2. Evans, A. G, Microfracture from thermal expansion ani- sotropy. Acta Metall., 1978, 26( 3. Clarke. D. R. Microfracture from anisotropic shape changes etal.,1980,28(3) 4. Tvegaard. v. and Hutchinson, J. w. Microcraking in ceramics induced by thermal expansion or elastic aniso- tropy. J. Amer. Ceram Soc., 1988, 71(3), 157-16 5. Ortiz. M. and Molinari, A. Microstructural residua the layer, um tresses in ceramic materials. J. Mech. Phvs. Solids, 1988 4) Fig. 13. Frequency shift of the Ri line and compressive stres 6. Cook, R. F. Fairbanks. C. J. Lawn in 45 um thick barrier layer made of an oxide mixture of Y.-W, Crack resistance by interfacial bi Y-ZrO2/Al,O3 composite as a function of position across the etermining strength characteristics. J. lay 2(3),4 Conclusions The aim of this work was to determine the residual stress e€ect on character of crack propagation in layered ceramic composites prepared by sequential centrifuging of powder suspensions. During tests of controlled crack growth a distinct crack de¯ection in alumina layers was observed. As it occured the value of crack de¯ection angle was proportional to the layer thickness. In the case of layer thicknesses below 10m the crack was found to be unde¯ected. In barrier layers made of an oxide mixture crack de¯ection did not happen independently on layer thickness. This observations have been explained by measurements of residual stress distribution in bar￾rier layers. The magnitude of compressive stress in alumina layer on the layer boundary was indepen￾dent on layer thickness. However the layer thickness a€ected the gradient of stresses. The compressive stresses were found to decrease from the boundary of layer to the centre of alumina layer and here reached the minimum. The correlation between the value of crack de¯ection angle and the magnitude of stress gradient was observed. In the case of layer with thicknesses less than 10m, where crack did not de¯ect, the compressive stress gradient reached very small value. In the barrier layers made of an oxide mixture, higher compressive stresses were found. However the distribution and local character of these stresses resulted in the lack of crack de¯ec￾tion independently on layer thickness. Elongation of crack way caused by crack de¯ec￾tion in alumina layer seemed to be responsible for observed enhancement in toughness of composites studied in a function of layer thickness. Acknowledgements This work was supported by Polish Committee for Scienti®c Research under grant No. 7S202 04607. References 1. Clarke, F. J. P., Residual strain and fracture stress±grain size relationships in brittle solids. Acta Metall., 1964, 12(2), 139±143. 2. Evans, A. G., Microfracture from thermal expansion ani￾sotropy. Acta Metall., 1978, 26(5), 1845±1853. 3. Clarke, D. R., Microfracture in brittle solids resulting from anisotropic shape changes. Acta Metall., 1980, 28(3), 913±924. 4. Tvegaard, V. and Hutchinson, J. W., Microcraking in ceramics induced by thermal expansion or elastic aniso￾tropy. J. Amer. Ceram. Soc., 1988, 71(3), 157±166. 5. Ortiz, M. and Molinari, A., Microstructural residual stresses in ceramic materials. J. Mech. Phys. Solids, 1988, 36(4), 385±400. 6. Cook, R. F., Fairbanks, C. J., Lawn, B. R. and Mai, Y.-W., Crack resistance by interfacial bridging: its role in determining strength characteristics. J. Mater. Res., 1987, 2(3), 345±356. Fig. 13. Frequency shift of the R1 line and compressive stres￾ses in 45m thick barrier layer made of an oxide mixture of Y±ZrO2/Al2O3 composite as a function of position across the layer. Fig. 12. The crack path during fracture of Y±ZrO2/Al2O3 composite with barrier layers made of alumina and zirconia mixture. The role of residual stresses in layered composites of Y±ZrO2 and Al2O3 261