R. Bermejo et aL Composites Science and Technology 67(2007)1930-1938 stability was studied through zeta potential measurements was determined on monolithic materials from the reso- by using the laser Doppler velocimetry principle(Zetasizer nance frequency of bars tested in flexure by impact [33] NanoZS, Malvern, UK). For these measurements the sus-( Grindo Sonic MK5, J W. Lemmens-Electronica N.V., Bel pensions were diluted to a powder concentration of gium) following the guidelines provided by ASTM E 1876- 99 and ENV-843-2. Indentation tests were performed using Suspensions were slip cast in a plaster of Paris mould a Vickers indenter(Microtest, Spain)with a displacement with only one filtrating surface in order to obtain rate of 0. 1 mm/min up to reach a maximum load of 7 cm x 7 cm plates. Monoliths of Al2O3 with 5 vol% Y- 100 N and holding time of 10 s. Three indentations were TZP (labelled as ATZ) and Al,O3 with 30 vol% of Tz-0 placed in the middle of the outer layers along the longitu- (referred to as AMZ) were prepared. Sequential slip casting dinal direction of the sample with an offset separation of [27, 32] was used to fabricate laminates composed of 5 thick 2 mm to avoid any crack interaction. Indented samples layers of ATZ alternated with 4 thin layers of AMZ. The were fractured under four-point bendingthe loading axis thickness of the layers was controlled from the measure- normal to the layer plane) with inner and outer spans of ment of the wall thickness after different casting times for 15 mm and 30 mm, respectively. Tests were carried out both AtZ and AMZ individual suspensions. According under displacement control using a universal testing to filtration kinetic mechanisms, the thickness of a cast machine(Microtest, Spain)with a load cell of 5 Kn at a body is related to the square root of the casting time by cross head speed of 0.5 mm/min. The corresponding (5) oad-displacement curves were recorded by a software cou- pled to the testing set-up. The stress distribution under where e is the wall thickness, k is a casting constant and t is four-point bending on a prismatic bar formed by different With these data a first approximation was layers was calculated following the expression given by made regarding the casting time of each layer to obtain the [34] desired thicknesses in the green laminate. The laminate was EM cast, then sintered, and finally cut and polished in order to al y=rl measure the thickness of the layers by means of scanning electron microscopy (DSM-950, Zeiss, Germany). The where Ei is the Young's modulus of the corresponding measured thicknesses were used to recalculate new casting layer, M is the moment for the case of four-point bending constants for both slurry compositions. Using the recalcu- tests(M=FL, where Fis the applied load and the distance lated casting constants, laminates were cast to obtain thick between inner and outer spans), yna is the position of the layers of 630 um and intermediate thin layers of AMZ with neutral axis according to thicknesses of 126 um and 63 um, i.e. laminates with thick ness ratios of about 1/5 and 1/10 respectively. In all cases 1+t the outer ATZ layers were calculated to be thicker than J 2.∑=1E1·1·B the final desired dimensions in order to allow further grind- and eI the flexural rigidity of the multilayer calculated for g processes during preparation of beams for bending bending perpendicular to the interfaces between the layers Samples were pre-sintered at 900C for 30 min and cut as given by into bars of 5 mm x 5 mm x 50 mm before sintering. The resulting testing bars were then sintered at 1550 C for 2h El=3 2Er. B using heating and cooling rates of 5 C/min. The sintering =1 shrinkage and the phase transformation of the composites containing pure ZrO2 were determined on monolithic sam ples of 10 mm length and a cross section of 5 x 5 mm, using where ti is the corresponding layer thickness and B the a dilatometer(DI-24, Adamel Lhomargy, France)with an specimen width [35,36 alumina rod and selecting the same thermal cycle as the one used to sinter the laminates. Density was measured by 3. Results and discussion the Archimedes method using mercury for the green sam ples and water for the sintered ones. X-Ray diffraction 3. 1. Preparation and characterization of monoliths (XRD) was performed for phase identification by means of a diffractometer(D-5000, Siemens, Germany) with the As the transformation of ZrO,(TZ-0) from tetragonal Kocu radiation. After sintering, specimens were polished to monoclinic during cooling from sintering occurs sponta with diamond paste down to I um for SEM observation. neously in a sharp temperature range, the presence of inho- In order to avoid structural changes due to the zirconia mogeneities can generate local stress concentration that transformation, no thermal etching was applied to the may diminish the mechanical properties of the material samples For this reason ZrOz particles have to be well-dispersed For mechanical characterization bars of approximately inside the Al2O3 matrix. Such dispersion is possible 3.6 mm x 3. 2 mm x 40 mm were prepared. Elastic modulus through the addition of a deflocculant that develops anstability was studied through zeta potential measurements by using the laser Doppler velocimetry principle (Zetasizer NanoZS, Malvern, UK). For these measurements the sus￾pensions were diluted to a powder concentration of 100 mg/l. Suspensions were slip cast in a plaster of Paris mould with only one filtrating surface in order to obtain 7 cm · 7 cm plates. Monoliths of Al2O3 with 5 vol% Y￾TZP (labelled as ATZ) and Al2O3 with 30 vol% of TZ-0 (referred to as AMZ) were prepared. Sequential slip casting [27,32] was used to fabricate laminates composed of 5 thick layers of ATZ alternated with 4 thin layers of AMZ. The thickness of the layers was controlled from the measure￾ment of the wall thickness after different casting times for both ATZ and AMZ individual suspensions. According to filtration kinetic mechanisms, the thickness of a cast body is related to the square root of the casting time by e ¼ kt1=2 ð5Þ where e is the wall thickness, k is a casting constant and t is the casting time. With these data a first approximation was made regarding the casting time of each layer to obtain the desired thicknesses in the green laminate. The laminate was cast, then sintered, and finally cut and polished in order to measure the thickness of the layers by means of scanning electron microscopy (DSM-950, Zeiss, Germany). The measured thicknesses were used to recalculate new casting constants for both slurry compositions. Using the recalcu￾lated casting constants, laminates were cast to obtain thick layers of 630 lm and intermediate thin layers of AMZ with thicknesses of 126 lm and 63 lm, i.e. laminates with thick￾ness ratios of about 1/5 and 1/10 respectively. In all cases the outer ATZ layers were calculated to be thicker than the final desired dimensions in order to allow further grind￾ing processes during preparation of beams for bending tests. Samples were pre-sintered at 900 C for 30 min and cut into bars of 5 mm · 5 mm · 50 mm before sintering. The resulting testing bars were then sintered at 1550 C for 2 h using heating and cooling rates of 5 C/min. The sintering shrinkage and the phase transformation of the composites containing pure ZrO2 were determined on monolithic sam￾ples of 10 mm length and a cross section of 5 · 5 mm, using a dilatometer (DI-24, Adamel Lhomargy, France) with an alumina rod and selecting the same thermal cycle as the one used to sinter the laminates. Density was measured by the Archimedes method using mercury for the green sam￾ples and water for the sintered ones. X-Ray diffraction (XRD) was performed for phase identification by means of a diffractometer (D-5000, Siemens, Germany) with the KaCu radiation. After sintering, specimens were polished with diamond paste down to 1 lm for SEM observation. In order to avoid structural changes due to the zirconia transformation, no thermal etching was applied to the samples. For mechanical characterization bars of approximately 3.6 mm · 3.2 mm · 40 mm were prepared. Elastic modulus was determined on monolithic materials from the reso￾nance frequency of bars tested in flexure by impact [33] (GrindoSonic MK5, J.W. Lemmens-Electronica N.V., Bel￾gium) following the guidelines provided by ASTM E 1876- 99 and ENV-843-2. Indentation tests were performed using a Vickers indenter (Microtest, Spain) with a displacement rate of 0.1 mm/min up to reach a maximum load of 100 N and holding time of 10 s. Three indentations were placed in the middle of the outer layers along the longitu￾dinal direction of the sample with an offset separation of 2 mm to avoid any crack interaction. Indented samples were fractured under four-point bending (the loading axis normal to the layer plane) with inner and outer spans of 15 mm and 30 mm, respectively. Tests were carried out under displacement control using a universal testing machine (Microtest, Spain) with a load cell of 5 KN at a cross head speed of 0.5 mm/min. The corresponding load–displacement curves were recorded by a software cou￾pled to the testing set-up. The stress distribution under four-point bending on a prismatic bar formed by different layers was calculated following the expression given by [34]: ri;y ¼ EiM EI ðy ynaÞ ð6Þ where Ei is the Young’s modulus of the corresponding layer, M is the moment for the case of four-point bending tests (M = Fl, where F is the applied load and l the distance between inner and outer spans), yna is the position of the neutral axis according to yna ¼ Pn i¼1Ei  ti  B  2  Pi1 j¼1tj þ ti 2  Pn i¼1Ei  ti  B ð7Þ and EI the flexural rigidity of the multilayer calculated for bending perpendicular to the interfaces between the layers as given by EI ¼ 1 3  Xn i¼1 Ei  B  Xi j¼1 tj yna !3 þ yna Xi1 j¼1 tj !3 0 @ 1 A ð8Þ where ti is the corresponding layer thickness and B the specimen width [35,36]. 3. Results and discussion 3.1. Preparation and characterization of monoliths As the transformation of ZrO2 (TZ-0) from tetragonal to monoclinic during cooling from sintering occurs sponta￾neously in a sharp temperature range, the presence of inho￾mogeneities can generate local stress concentration that may diminish the mechanical properties of the material. For this reason ZrO2 particles have to be well-dispersed inside the Al2O3 matrix. Such dispersion is possible through the addition of a deflocculant that develops an 1932 R. Bermejo et al. / Composites Science and Technology 67 (2007) 1930–1938