1680 Journal of the American Ceramic Society-Marshall et al Vol. 80. No. 7 DISPLACEMENT (um) DISPLACEMENT (um) B (a)Load-deflection response of notched beam of Y-Zr02 containing three layers of LaPO. near its midplane. (b) Flexural loading response notched beams of multilayered Ce-ZrO2/LaPO4.(Nominal stress is calculated for undamaged linear elastic beam. LaPO, sandwiched between relatively thick layers of ZrO2, the The calculated toughnesses are also by residual calculation of the crack driving force for a crack growing along stresses due to thermal expansion mismat layered com- the layer(either in the layer or at the interface)in terms of the posites. Since the thermal expansion co residual indentation stress field is not affected by the lower ZrO O2 are approximately affect the crack driving force only through its influence on the those two phase sibly small in composites containing only elastic modulus of the lapo,29 The lower agnitude of the indentation stresses, as determined by the Al2O3/Zr02 layers, the lower thermal expansion coefficient of (b) mm 200mm Ce-ZrO Fig. 6. Fracture of notched beam of multilayered composite of Ce-ZrO2 and LaPO.(a)Overall view after failure. (b)In situ optical micrograph during loading. (c)and (d) fracture surface after failure(arrows indicate cracks400 , . , . , , . . , , . . . . . . . . . , 300. \ \ \ w: \\\\/: \ \ '. ~'""'...' LaPO, sandwiched between relatively thick layers of Zro,, the calculation of the crack driving force for a crack growing along the layer (either in the layer or at the interface) in terms of the residual indentation stress field is not affected by the lower elastic modulus of the LaPO,?g The lower modulus would affect the crack driving force only through its influence on the magnitude of the indentation stresses, as determined by the inelastic deformation around the indentation. This effect must be small if the indentation is large compared with the monazite layer thickness, as in Fig. 9(c). 200 il i3 1 B 0 loo The calculated toughnesses are also affected by residual stresses due to thermal expansion mismatch in the layered com￾posites. Since the thermal expansion coefficients for LaPo, and Zro, are approximately equal (-10 X 0C-1),'3 residual stresses are negligibly small in composites containing only those two phases. However, in composites containing mixed Al,O,/ZrO, layers, the lower thermal expansion coefficient of Al,O, (-8 X "C-') leads to tensile stresses within the LaPO, layers in the direction parallel to the layers and balanc￾ing compressive stresses in the Al,O,/ZrO, (the latter stress Fig. 6. Fracture of notched beam of multilayered composite of Ce-Zro, and LaPo,. (a) Overall view after failure. (b) In siru optical micrograph during loading. (c) and (d) Fracture surface after failure (arrows indicate cracks)