H. Tomaszewski et al 120 MPa and then sintered at 1600C. The larger The tests of controlled crack growth were per- shrinkage of the Y-ZrO2 during sintering caused formed using Zwick machine of 1446 type. The that in some layered composites the mixed compo- notched beams were loaded in three-point bending sition of 50 vol% AlO3 and Y-ZrO2 was used with I um/min loading speed and 40 mm bearing instead of a pure AlO3 to minimize this mismatch distance. The crack was initiated and slowly grown ( Table 1) Micrographs of composite samples with step by step in a controlled way by permanent both types of barrier layers were shown in Fig. 1. loading and remowing of the load. This procedure The samples after sintering were cut and ground to results in less than 100 um increase of crack length the dimensions of 45x4x 1.5 mm and one surface by one step. The path of the crack during fracture perpendicular to the layers was polished. The sharp of layered composite was registered by SEM using notch in the center of the beams was prepared with of OPTON DSM 950 microscope. All experiments two diamond saws: 0.200 and 0-025 mm(Fig. 2) were done at room temperature in normal air environments The spatial distribution of residual stresses Table 1. Shrinkage of materials used for Y-ZrO2/Al2O3 com- within the alumina and a mixture of alumina and posite preparation in a sintering temperature zirconia layer of the composites was measured Material used Shrinkage after sintering using the piezospectroscopic technique. The inl600°C(%) method is based on the photostimulated fluores- Y-ZrO, cence from trace Cr+3 ions in alumina Al,O The frequency shift Av of the two lines in the R Mixture of 50 vol% Al,O3 and Y-Zro doublet is a measure of the elastic strain within the volume of material excited by the laser, following where: Ili are the piezospectroscopic coefficients 100 Fm and oi are the stress components The made using an optical microscope with an attached spectrometer(DILOR X4800) An argon ion laser ing at wavelength of 514.5 nm was used the excitation source. In each experiment a region of 5968 interest in the sample was first selected using the microscope then the laser beam was focused to a spot BShoNse on that feature. This way the alumina or a mixture layer of composite were scanned by 5 to 10 um steps (a The intensities of the stimulated R, and R fluores- cence lines were typically collected by scanning the 时的 25k 188pm M35 Fig. 1. Microstructure of layered composite with Y-TZP natrix and barrier layers(darker regions) consisted of (a) Fig. 2. An example of notched beam used in controlled crack alumina and(b) a mixture of alumina and zirconia propagation tests120 MPa and then sintered at 1600C. The larger shrinkage of the Y±ZrO2 during sintering caused that in some layered composites the mixed compo￾sition of 50 vol% Al2O3 and Y±ZrO2 was used instead of a pure Al2O3 to minimize this mismatch (Table 1). Micrographs of composite samples with both types of barrier layers were shown in Fig. 1. The samples after sintering were cut and ground to the dimensions of 4541.5 mm and one surface perpendicular to the layers was polished. The sharp notch in the center of the beams was prepared with two diamond saws: 0.200 and 0.025 mm (Fig. 2). The tests of controlled crack growth were per￾formed using Zwick machine of 1446 type. The notched beams were loaded in three-point bending with 1m/min loading speed and 40 mm bearing distance. The crack was initiated and slowly grown step by step in a controlled way by permanent loading and remowing of the load. This procedure results in less than 100m increase of crack length by one step. The path of the crack during fracture of layered composite was registered by SEM using of OPTON DSM 950 microscope. All experiments were done at room temperature in normal air environments. The spatial distribution of residual stresses within the alumina and a mixture of alumina and zirconia layer of the composites was measured using the piezospectroscopic technique. The method is based on the photostimulated ¯uores￾cence from trace Cr+3 ions in alumina. The frequency shift
 of the two lines in the R￾doublet is a measure of the elastic strain within the volume of material excited by the laser, following tensorial relation:10
 ˆ ijij where: ij are the piezospectroscopic coecients and ij are the stress components. The piezospectroscopic measurements were made using an optical microscope with an attached spectrometer (DILOR X4800). An argon ion laser operating at wavelength of 514.5 nm was used as the excitation source. In each experiment a region of interest in the sample was ®rst selected using the microscope then the laser beam was focused to a spot on that feature. This way the alumina or a mixture layer of composite were scanned by 5 to 10m steps. The intensities of the stimulated R1 and R2 ¯uores￾cence lines were typically collected by scanning the Fig. 1. Microstructure of layered composite with Y±TZP matrix and barrier layers (darker regions) consisted of (a) alumina and (b) a mixture of alumina and zirconia. Fig. 2. An example of notched beam used in controlled crack propagation tests. Table 1. Shrinkage of materials used for Y-ZrO2/Al2O3 com￾posite preparation in a sintering temperature Material used Shrinkage after sintering in 1600C (%) Y±ZrO2 19.04 Al2O3 16.27 Mixture of 50 vol% Al2O3 and Y±ZrO2 18.47 256 H. Tomaszewski et al.