R Bermejo et al. Composites Science and Technology 67(2007)1930-1938 1933 electrical charge on the surface of the powders, leading to a AtZ compacts recorded during a complete sintering cycle good stability of the final slurry. Fig. I shows the zeta In this figure a close-up view of the transformation zone is potential vs. deflocculant content for both Al_O3 and also plotted. It can be observed that the ATZ compact ZrO, powders in water. The TZ-0 material presents a neg- starts to sinter at 1000C and shrinks 13% whereas the ative zeta potential for all the measurement range, whereas AMZ starts to sinter around 1100C and shrinks 14% the sign of the surface charge of Al2O3 changes for deflocc- These small differences in the starting temperature for sin- ulant contents of 0.5-0.6%, thus indicating that 0.6% is the tering and the final shrinkage can be explained due to the lower content of deflocculant required to avoid heterocoag- higher packing density of the atz green bodies. At ulation. It can also be observed that over a deflocculant 1330C both samples reach the same shrinkage rate. The content of 0. 8 wt% the absolute value of zeta potential is most interesting differences between those curves are higher than 30 mV for both powders, which is accepted observed in the cooling ramp. While AMz sample rapidly as a limit value that indicates a high stabilization of casting expands at 730C due to the martensitic phase transfor Following the above ideas, suspensions of ATZ and mation of the zirconia particles from tetragonal to mono- AMZ with 36.5 vol%/(approximately 70 wt%)solids dis- clinic, the AtZ sample maintains linear shrinkage during persed with 0. 8 wt% polyelectrolyte were prepared and slip cooling. The magnitude of the expansion due to phase cast in order to obtain the kinetic equations used to calcu- transformation in the AMZ samples is of 0. 16%. XRD late the casting time for the laminates. Kinetic constants analysis indicates that most of the zirconia present in the for ATZ and AMZ slurries, determined as the slope of AMZ samples has transformed into the monoclinic phase the wall thickness- casting time curve, were measured as and only a 5-10% remaining tetragonal zirconia was 0.00303 and 0.00216 mm/s, respectively. Green densities detected in this sample. Density measurements of A of cast monolithic samples were 69%th for ATZ andand AMZ compacts are listed in Table l, along with the 62%th for AMZ compacts. data obtained from the characterization of the sintered Monolithic samples were used to separately study the monolithic samples [36]. ntering behaviour of the layers forming the laminates Fig. 2 shows the dilatometric curves for both AMZ and 3. 2. Sequential slip casting of laminates with residual stresses On the basis of the kinetic constants calculated from the monolithic sample characterization, a multilayer piece with o Zrt predicted layer thicknesses of 630 um for ATZ and 63 um for AMZ was cast. Fig. 3 plots the variation of wall thick wt%D-3021 ness as a function of casting time for both AMz and atZ suspensions, as well as the step-like procedure used to cast 1.0 the multilayers. In order to form the first ATZ layer of a certain thickness, the casting time was fixed following the ATZ casting curve(ATZ equation in Fig. 3). After that, the suspension left was poured out of the mould and the new slip to form the AMz layer poured on top of the Fig.I.Variation of zeta potential with the dispersant content for the already formed ATZ layer. The thickness of this new powders used to prepare the slurries. AMZ layer corresponds to a fixed casting time given by the aMz casting curve(AMZ equation in Fig. 3). This procedure was repeated to alternatively form the rest of the layers. Hence, for casting a new layer with a thickness eAtz, the atZ slurry was maintained in the mould for a casting time tATz, dictated by the AtZ casting curve, as shown in the referred figure. Likewise, attempting to form an AMz layer of thickness eAMz, the AMz slurry was drain cast for a time tamz, following the AMz casting curve(Fig 3). After casting the whole multilayer, the green plate was dried in air inside the mould for two days until it shrank enough to free from the mould walls. Aft 02004006008001000120014001600 ing the test-piece laminate, the thickness of the resulting layers was assessed and the kinetic constants then refined yielding new values of 0.00433 and 0.01233 for ATZ and Fig. 2. Dilatometric curves from monolithic samples of ATz and AMz AMz layers, respectively. Considering those recalculated materials. A close-up of the 900-600C interval during cooling is also constants and the sintering shrinkage of each compact plotted to show the tetragonal to monoclinic zirconia phase Fig. 2), two sets of laminates composed of 5 thick ATZ transfe layers of 550 um and 4 thin AMZ layers of 110 um andelectrical charge on the surface of the powders, leading to a good stability of the final slurry. Fig. 1 shows the zeta potential vs. deflocculant content for both Al2O3 and ZrO2 powders in water. The TZ-0 material presents a neg￾ative zeta potential for all the measurement range, whereas the sign of the surface charge of Al2O3 changes for deflocc￾ulant contents of 0.5–0.6%, thus indicating that 0.6% is the lower content of deflocculant required to avoid heterocoag￾ulation. It can also be observed that over a deflocculant content of 0.8 wt% the absolute value of zeta potential is higher than 30 mV for both powders, which is accepted as a limit value that indicates a high stabilization of casting slips. Following the above ideas, suspensions of ATZ and AMZ with 36.5 vol% (approximately 70 wt%) solids dis￾persed with 0.8 wt% polyelectrolyte were prepared and slip cast in order to obtain the kinetic equations used to calcu￾late the casting time for the laminates. Kinetic constants for ATZ and AMZ slurries, determined as the slope of the wall thickness – casting time curve, were measured as 0.00303 and 0.00216 mm2 /s, respectively. Green densities of cast monolithic samples were 69%th for ATZ and 62%th for AMZ compacts. Monolithic samples were used to separately study the sintering behaviour of the layers forming the laminates. Fig. 2 shows the dilatometric curves for both AMZ and ATZ compacts recorded during a complete sintering cycle. In this figure a close-up view of the transformation zone is also plotted. It can be observed that the ATZ compact starts to sinter at 1000 C and shrinks 13% whereas the AMZ starts to sinter around 1100 C and shrinks 14%. These small differences in the starting temperature for sin￾tering and the final shrinkage can be explained due to the higher packing density of the ATZ green bodies. At 1330 C both samples reach the same shrinkage rate. The most interesting differences between those curves are observed in the cooling ramp. While AMZ sample rapidly expands at 730 C due to the martensitic phase transfor￾mation of the zirconia particles from tetragonal to mono￾clinic, the ATZ sample maintains linear shrinkage during cooling. The magnitude of the expansion due to phase transformation in the AMZ samples is of 0.16%. XRD analysis indicates that most of the zirconia present in the AMZ samples has transformed into the monoclinic phase, and only a 5–10% remaining tetragonal zirconia was detected in this sample. Density measurements of ATZ and AMZ compacts are listed in Table 1, along with the data obtained from the characterization of the sintered monolithic samples [36]. 3.2. Sequential slip casting of laminates with residual stresses On the basis of the kinetic constants calculated from the monolithic sample characterization, a multilayer piece with predicted layer thicknesses of 630 lm for ATZ and 63 lm for AMZ was cast. Fig. 3 plots the variation of wall thick￾ness as a function of casting time for both AMZ and ATZ suspensions, as well as the step-like procedure used to cast the multilayers. In order to form the first ATZ layer of a certain thickness, the casting time was fixed following the ATZ casting curve (ATZ equation in Fig. 3). After that, the suspension left was poured out of the mould and the new slip to form the AMZ layer poured on top of the already formed ATZ layer. The thickness of this new AMZ layer corresponds to a fixed casting time given by the AMZ casting curve (AMZ equation in Fig. 3). This procedure was repeated to alternatively form the rest of the layers. Hence, for casting a new layer with a thickness eATZ, the ATZ slurry was maintained in the mould for a casting time tATZ, dictated by the ATZ casting curve, as shown in the referred figure. Likewise, attempting to form an AMZ layer of thickness eAMZ, the AMZ slurry was drain cast for a time tAMZ, following the AMZ casting curve (Fig. 3). After casting the whole multilayer, the green plate was dried in air inside the mould for two days until it shrank enough to free from the mould walls. After sinter￾ing the test-piece laminate, the thickness of the resulting layers was assessed and the kinetic constants then refined, yielding new values of 0.00433 and 0.01233 for ATZ and AMZ layers, respectively. Considering those recalculated constants and the sintering shrinkage of each compact (Fig. 2), two sets of laminates composed of 5 thick ATZ layers of 550 lm and 4 thin AMZ layers of 110 lm and Fig. 1. Variation of zeta potential with the dispersant content for the powders used to prepare the slurries. Fig. 2. Dilatometric curves from monolithic samples of ATZ and AMZ recorded using the thermal cycle employed to sinter the multilayered materials. A close-up of the 900–600 C interval during cooling is also plotted to show the tetragonal to monoclinic zirconia phase transformation. R. Bermejo et al. / Composites Science and Technology 67 (2007) 1930–1938 1933