July 1997 Debonding in Multilayered Composites of zirconia and LaPO, 1679 beeeeboogoogoo0x sition (um) e-ZrO2/LaPO, comp e first six LapO, layers past the notch, whereas the ZrO2 layers has also been observed in LapO/Al,O in this region are fractured in only one or two places. At this in other material under shear stage of loading there was no damage in any layers beyond the glass matrix 品 sixth Lapo layer On the stepped fracture surface( Figs. 6(c)and adhesive joints (d )) cracking in the LaPO4 layers is seen primarily at or near the nterface between the LaPO4 and ce-ZrO, layers A preliminary observation suggestive of possible combined milar results were obtained for the composite consisting of toughening by ic transformation and interlaminar layers of Y-zrO2/Al,O, and LaPO4. Although on the fracture cracking in the PO, composite is shown in Fig 8 surfaces it appeared that separation of the layers occurred mainl The nomarski micrograph of Fig 8(a)shows a zone at or near the interfaces between the ZrO, and LaPO4, there was of uplifted material surrounding the main crack. The uplift is also substantial cracking within the LaPO layers( Fig. 7). Usu- due to the volume increase associated with the tetragonal-to- ally delamination began with simple deflection of the main crack. monoclinic phase transformation in the Ce-zrO, layers. The as in Fig. 7(a)but then continued by forming an array of echelon width of this zone increases as the crack extends from the notch cracks as in Figs. 7(b)and(c). This mechanism of delamination root, as observed previously in multilayered composites of Al2 O3 ind Ce-zr02 in which the layers were strongly bonded. How- ever, in this case there is also cracking in the LapOa layers within the transformation zone, as shown in Fig 8(b) all of the multilayered composites, the LaPO lay effective in confining the cracking due to adjacent vickers inden- tations, as shown in Figs. 9(a)and(b). The damage in the layers echelon cracks, similar to the damge observed in notched beams Cracks produced by vickers indentations located directly on LapO4 layers were used to obtain a measure of the interlaminar toughness, as shown in Fig. 9(c). The area of Fig. 9(c)lies within a beam of 4 mm thickness that consisted of Y-ZrO2/A1,O, with three layers of LapO, in the center. The lengths of the indentation times the length of the crack growing normal to the layer into the Y-ZrO2/Al, matrix. The corresponding fracture toughnesses calculated from 10 indentations using the analysis of Anstis l26are=8±4Jm2 for the interlaminar crack and T 80 +5J/m for the Y-ZrO2/AlO, matrix(using a value of 300 GPa for the elastic modulus). The matrix toughness is consistent with values reported in the literature(corresponding to a critical stress intensity factor of K. =5 MPa"2); while the interlami- nar toughness is similar to values reported for monazite itself. 3 For the composite with Y-zrO, matrix, the fracture energies were 1 um In=110±10J/m This toughness calculation is based on an analysis for a homog Fig. 4. Backscattered electron image from composite containi eneous isotropic material, whereas the measurements were obtained from composites containing layers of differing elastic Al, O, layer, showing presence of elongated Ce-La magnetoplumbite moduli (133 GPa for LapO4, 200 GPa for ZrO2, and 400 GP Thermally etched surface for AL2O3). In the configuration of Fig. 9, with a thin layer ofJuly 1997 Debonding in Multilayered Composites of Zirconia and LaPO, 1679 0 50 100 Position (pm) 150 Fig. 3. Electron microprobe measurements along a line traversing several layers in Ce-ZrOJLaPO, composite. Atomic proportions are normalized to four oxygen atoms. the first six LaPO, layers past the notch, whereas the ZrOz layers in this region are fractured in only one or two places. At this stage of loading there was no damage in any layers beyond the sixth LaPO, layer. On the stepped fracture surface (Figs. 6(c) and (d)), cracking in the LaPO, layers is seen primarily at or near the interface between the LaPO, and Ce-ZrO, layers. Similar results were obtained for the composite consisting of layers of Y-ZrO,/Al,O, and LaPo,. Although on the fracture surfaces it appeared that separation of the layers occurred mainly at or near the interfaces between the ZrO, and LaPO,, there was also substantial cracking within the LaPo, layers (Fig. 7). Usu￾ally delamination began with simple deflection of the main crack, as in Fig. 7(a) but then continued by forming an array of echelon cracks as in Figs. 7(b) and (c). This mechanism of delamination Fig. 4. Backscattered electron image from composite containing Ce-ZrOJAI20, and LaPO, layers, from region in center of Ce-ZrO,/ A1,0, layer, showing presence of elongated Ce-La magnetoplumbite. Thermally etched surface. for A1,0,). In the configuration of Fig. 9, with-a thin layer of has also been observed in LaP04/A1,03 composite^,'^ as well as in other material systems under shear loading, including uni￾directionally reinforced glass matrix cornp0sites,2~~~~ brittle adhesive joints between rigid and graphite-epoxy composite~.~~.~’ A preliminary observation suggestive of possible combined toughening by martensitic transformation and interlaminar cracking in the Ce-ZrO,/LaPO, composite is shown in Fig. 8. The Nomarski interference micrograph of Fig. 8(a) shows a zone of uplifted material surrounding the main crack. The uplift is due to the volume increase associated with the tetragonal-to￾monoclinic phase transformation in the Ce-Zro, layers. The width of this zone increases as the crack extends from the notch root, as observed previously in multilayered composites of A1,0, and Ce-Zr0,24 in which the layers were strongly bonded. How￾ever, in this case there is also cracking in the LaPO, layers within the transformation zone, as shown in Fig. 8(b). (B) Indentation Fracture In all of the multilayered composites, the LaPo, layers were effective in confining the cracking due to adjacent Vickers inden￾tations, as shown in Figs. 9(a) and (b). The damage in the layers at either side of the indentation consists mostly of arrays of echelon cracks, similar to the damge observed in notched beams. Cracks produced by Vickers indentations located directly on LaPo, layers were used to obtain a measure of the interlaminar toughness, as shown in Fig. 9(c). The area of Fig. 9(c) lies within a beam of 4 mm thickness that consisted of Y-ZrO,/Al,O, with three layers of LaPo, in the center. The lengths of the indentation cracks growing along the LaPo, layer in Fig. 9(c) are about 3 times the length of the crack growing normal to the layer into the Y-Zro, /A1,03 matrix. The corresponding fracture toughnesses calculated from 10 indentations using the analysis of Anstis et aLZ8 are I: = 8 ? 4 J/m2 for the interlaminar crack and r, = 80 ? 5 J/mz for the Y-ZrO,/Al,O, matrix (using a value of 300 GPa for the elastic modulus). The matrix toughness is consistent with values reported in the literature (corresponding to a critical stress intensity factor of K, = 5 MPa.m”2); while the interlami￾nar toughness is similar to values reported for monazite itself,’3 For the composite with Y-Zro, matrix, the fracture energies were I: = 6 2 3 J/m2, and r, = 110 2 10 J/m2. This toughness calculation is based on an analysis for a homog￾eneous isotropic material, whereas the measurements were obtained from composites containing layers of differing elastic moduli (133 GPa for LaPO,, 200 GPa for Zro,, and 400 GPa