138 S Bueno, C Baudin/Composites: 2009)137-143 for deflection to be efficient very high values of the porosity are Al203 jar and balls during 4 h. These conditions were selected from needed [12]. Main drawback of the second approach is that crack a previous work [29] deflection limits the wear resistance of the materials Solid discs with 20 mm in diameter were slip cast in plaster of From all ceramics, alumina(Al2O3)presents the highest thermal Paris moulds in order to determine the casting rate of each suspen- stability together with high hardness sustained up to temperatures sion by determination of the dry wall thickness(Mitutoyo, JDU25 over 1200C. therefore is the natural ceramic for wear. Japan)after different casting times(1-16 min). For Young s modu- n this work, we present an alumina-based layered ceramic- lus and strength, plates with 70 x 70 x 6 mm dimensions were ceramic laminate designed on the basis of a combination of both obtained by slip casting of the a10 and A30 slips. The cast bodies discussed approaches. It is constituted by relatively stiff and brittle were carefully removed from the moulds and dried in air at room external layers and microcracked internal layers to produce multi- temperature for at least 24 h ple crack deflection at the microstructural scale, thus, limiting The reaction sintering behaviour was studied with a differential delamination lengths in order not to loose structural integrity. Spe- dilatometer(Adamel Lhomargy, DI24, France)to 1550C using cial care was given to adjust the processing variables that permit- small(5 x 5 x 5 mm)green samples of the monoliths with alt ed the fabrication of the designed laminated by sequential slip mina detector and the sintering schedule was selected from tl casting and sintering. tained results. To obtain the monolithic composites, the dried As internal material an alumina -aluminium titanate(Al2TiO5) plates were sintered in air in an electrical box furnace(termiber composite containing 30 vol% of aluiminium titanate previously Spain)at heating and cooling rates of 2C min, with 4 h dwell studied was chosen [15. In this material, extensive deflection and at 1200C and 2 h dwell at the maximum temperature, 1450C branching of the main crack along the pre-existing microcracks oc. Densities of the sintered compacts were determined by the archi- curred during fracture. An alumina -aluminium titanate composite medes method in water(European Standard En 1389: 2003)and with relatively high strength (10 voL% aluminium titanate, relative densities were calculated from these values and those of or= 261+6 MPa, 3-points bending, samples 2x 2.5 x 30 mm, theoretical densities calculated taking values of 3.99 g cm- for span 20 mm [16) was chosen to constitute the external layers. alumina(a-Al2O3, ASTM 42-1468)and 3.70 g cm- for aluminium External composite layers instead of pure alumina ones were chosen titanate(B-Al2TiOs, ASTM 26-0040) at the expense of strength(for monophase alumina or=456+ Additional sintering experiments(samples 12 x 5 mm) 29 MPa, 3-points bending, samples 2 x 2.5 x 30 mm, span 20 mm were performed in a differential dilatometer (Setaram, Setsys 16/ [16) in order to assure compatible sintering between the layers. 18, France)with alumina detector reproducing the same thermal On the one hand it is well-known that titania accelerates the initial treatment schedule as that used to obtain the final materials in or sintering of alumina[17-19] and, on the other, the formation of alu- der to determine strain during cooling. minium titanate is expansive(Ava 11%, calculated using density The sintered blocks of the monoliths were machined into bars of values of 3.99 g cm-for a-Al203 ASTM File 42-1468, 3.70 g cm-3 50 x 3 x 4 mm for dynamic Youngs modulus determinations. for B-Al, TiOs, ASTM File 26-0040, and 3. 89 g cm- for TiO2-anatasa, from the resonance frequency of the bars in flexure(Grindosonic, ASTM Files 21-1272)thus, an arrest of the shrinkage rate occurs at J.W. Lemmens, Belgium) and bend strength tests( 4-points bend- the temperature of aluminium titanate formation (1390.C) ing, 40-20 mm span, 0.5 mm min": Microtest, Spain) Reported Youngs modulus and bend strength values are the average of five The laminate design was done taking into account the total measurements and errors are strain of monolithic materials of both compositions when cooling A symmetric laminated structure of five layers was fabricated from the sintering temperature and their Youngs modulus in order by casting each suspension alternately Casting times were fixed to limit tensile residual stresses in the external layers to reach the desired layer thickness considering the casting kinet The work of fracture has been chosen as mechanical parameter ics and sintering shrinkage of each composition. The laminate had to establish the relationships between the mechanical behaviour of the central (1200 um)and outer layers(e2100 um )made of A10 the laminate and that of the constituent layers. The advantage of and the two inner layers(e300 um)of A30 this energy parameter is that it does not require any assumpt Microstructure of polished cross sections was characterised in a about the constitutive equation of the body with the crack to dis- field emission gun scanning electron microscopy(FEG-SEM, Hit- cuss its propagation [22]. Thus, it can be used to describe behav- achi, S-4700, Japan) Thermally etched (1440C-1min) specimens iours which separate from linearity and it is an additive were analysed. Additional observations were performed on chem- parameter that makes it possible to quantify the different contri- ically etched(hF 10 vol%-1 min) specimens in order to assure that butions to energy dissipation during fracture [23-27 the thermal etching did not produce further microcracking in the The apparent toughness as proposed by Clegg et al. 9. 28 for sintered specimens. laminates with weak interfaces has been used to compare the Single-Edge-V-Notch-Beams(SEvNB) of 4 x 6 x 50 mm,ma- mechanical performance of the proposed laminate with that of chined from the laminated sintered blocks, were tested in a other structural ceramics 3-points bending device using a span of 40 mm and a cross-head speed of 0.005 mm min(Microtest, Spain). The notches were ini tially cut with a 150 um wide diamond wheel. Using this slot as a guide, the remaining part of the notch was done with a razor blade sprinkled with diamond pastes of successively 6 and 1 um Speci The starting materials were commercial Alumina(a-Al2O3, Con- mens with relative notch depths of about 0.8 of the thickness of dea, HPAO5, USA)and titania(anatase-TiO2, Merck, 808, Germany) the first external layer, corresponding to 0. 26 of the specimen owders. Al 2O3/TiO2 mixtures with relative TiO2 contents of 5 and thickness( W), were tested. The tip radii of all notches were deter 15 wt% were prepared to obtain alumina/aluminium titanate com- mined from optical observations and they were always found to be posites with second phase contents of 10 and 30 vol% after reac- below 20 um. The curves load-displacement of the cross-head of tion sintering, named A10 and A30, respectively he load frame were recorded and corrected by subtracting the The mixtures were dispersed in deionised water by adding compliance of the testing set up(machine, supports, load cell 0.5 wt%(on a dry solids basis) of a carbonic acid based polyelectro- and fixtures, 1.5 x 10-m/N)determined by testing a thick lyte(Dolapix CE64, Zschimmer-Schwarz, Germany). Suspensions (25 x 25 x 100 mm )unnotched alumina bar. Three specimens were prepared to a solids loading of 50 vol% and ball milled with were tested and the curves were found to be practically coincident.for deflection to be efficient very high values of the porosity are needed [12]. Main drawback of the second approach is that crack deflection limits the wear resistance of the materials. From all ceramics, alumina (Al2O3) presents the highest thermal stability together with high hardness sustained up to temperatures over 1200 C, therefore, is the natural ceramic for wear. In this work, we present an alumina-based layered ceramic￾ceramic laminate designed on the basis of a combination of both discussed approaches. It is constituted by relatively stiff and brittle external layers and microcracked internal layers to produce multi￾ple crack deflection at the microstructural scale, thus, limiting delamination lengths in order not to loose structural integrity. Spe￾cial care was given to adjust the processing variables that permit￾ted the fabrication of the designed laminated by sequential slip casting and sintering. As internal material an alumina–aluminium titanate (Al2TiO5) composite containing 30 vol.% of aluiminium titanate previously studied was chosen [15]. In this material, extensive deflection and branching of the main crack along the pre-existing microcracks oc￾curred during fracture. An alumina–aluminium titanate composite with relatively high strength (10 vol.% aluminium titanate, rf = 261 ± 6 MPa, 3-points bending, samples 2  2.5  30 mm3 , span 20 mm [16]) was chosen to constitute the external layers. External composite layers instead of pure alumina ones were chosen at the expense of strength (for monophase alumina rf = 456 ± 29 MPa, 3-points bending, samples 2  2.5  30 mm3 , span 20 mm [16]) in order to assure compatible sintering between the layers. On the one hand, it is well-known that titania accelerates the initial sintering of alumina [17–19] and, on the other, the formation of alu￾minium titanate is expansive (DV 11%, calculated using density values of 3.99 g cm3 for a-Al2O3, ASTM File 42-1468, 3.70 g cm3 for b-Al2TiO5, ASTM File 26-0040, and 3.89 g cm3 for TiO2-anatasa, ASTM Files 21-1272) thus, an arrest of the shrinkage rate occurs at the temperature of aluminium titanate formation (1390 C) [20,21]. The laminate design was done taking into account the total strain of monolithic materials of both compositions when cooling from the sintering temperature and their Young’s modulus in order to limit tensile residual stresses in the external layers. The work of fracture has been chosen as mechanical parameter to establish the relationships between the mechanical behaviour of the laminate and that of the constituent layers. The advantage of this energy parameter is that it does not require any assumptions about the constitutive equation of the body with the crack to dis￾cuss its propagation [22]. Thus, it can be used to describe behav￾iours which separate from linearity and it is an additive parameter that makes it possible to quantify the different contri￾butions to energy dissipation during fracture [23–27]. The apparent toughness as proposed by Clegg et al. [9,28] for laminates with weak interfaces has been used to compare the mechanical performance of the proposed laminate with that of other structural ceramics. 2. Experimental The starting materials were commercial Alumina (a-Al2O3, Con￾dea, HPA05, USA) and titania (anatase-TiO2, Merck, 808, Germany) powders. Al2O3/TiO2 mixtures with relative TiO2 contents of 5 and 15 wt.% were prepared to obtain alumina/aluminium titanate com￾posites with second phase contents of 10 and 30 vol.% after reac￾tion sintering, named A10 and A30, respectively. The mixtures were dispersed in deionised water by adding 0.5 wt.% (on a dry solids basis) of a carbonic acid based polyelectro￾lyte (Dolapix CE64, Zschimmer-Schwarz, Germany). Suspensions were prepared to a solids loading of 50 vol.% and ball milled with Al2O3 jar and balls during 4 h. These conditions were selected from a previous work [29]. Solid discs with 20 mm in diameter were slip cast in plaster of Paris moulds in order to determine the casting rate of each suspen￾sion by determination of the dry wall thickness (Mitutoyo, JDU25, Japan) after different casting times (1–16 min). For Young’s modu￾lus and strength, plates with 70  70  6 mm3 dimensions were obtained by slip casting of the A10 and A30 slips. The cast bodies were carefully removed from the moulds and dried in air at room temperature for at least 24 h. The reaction sintering behaviour was studied with a differential dilatometer (Adamel Lhomargy, DI24, France) to 1550 C using small (5  5  5 mm3 ) green samples of the monoliths with alu￾mina detector and the sintering schedule was selected from the ob￾tained results. To obtain the monolithic composites, the dried plates were sintered in air in an electrical box furnace (Termiber, Spain) at heating and cooling rates of 2 C min1 , with 4 h dwell at 1200 C and 2 h dwell at the maximum temperature, 1450 C. Densities of the sintered compacts were determined by the Archi￾medes method in water (European Standard EN 1389:2003) and relative densities were calculated from these values and those of theoretical densities calculated taking values of 3.99 g cm3 for alumina (a-Al2O3, ASTM 42-1468) and 3.70 g cm3 for aluminium titanate (b -Al2TiO5, ASTM 26-0040). Additional sintering experiments (samples 12  5  5 mm3 ) were performed in a differential dilatometer (Setaram, Setsys 16/ 18, France) with alumina detector reproducing the same thermal treatment schedule as that used to obtain the final materials in or￾der to determine strain during cooling. The sintered blocks of the monoliths were machined into bars of 50  3  4 mm3 for dynamic Young’s modulus determinations, from the resonance frequency of the bars in flexure (Grindosonic, J.W. Lemmens, Belgium) and bend strength tests (4-points bend￾ing, 40–20 mm span, 0.5 mm min1 ; Microtest, Spain). Reported Young’s modulus and bend strength values are the average of five measurements and errors are the standard deviations. A symmetric laminated structure of five layers was fabricated by casting each suspension alternately. Casting times were fixed to reach the desired layer thickness considering the casting kinet￾ics and sintering shrinkage of each composition. The laminate had the central (1200 lm) and outer layers (ffi2100 lm) made of A10 and the two inner layers (ffi300 lm) of A30. Microstructure of polished cross sections was characterised in a field emission gun scanning electron microscopy (FEG-SEM, Hit￾achi, S-4700, Japan). Thermally etched (1440 C–1min) specimens were analysed. Additional observations were performed on chem￾ically etched (HF 10 vol.%–1 min) specimens in order to assure that the thermal etching did not produce further microcracking in the sintered specimens. Single-Edge-V-Notch-Beams (SEVNB) of 4  6  50 mm3 , ma￾chined from the laminated sintered blocks, were tested in a 3-points bending device using a span of 40 mm and a cross-head speed of 0.005 mm min1 (Microtest, Spain). The notches were ini￾tially cut with a 150 lm wide diamond wheel. Using this slot as a guide, the remaining part of the notch was done with a razor blade sprinkled with diamond pastes of successively 6 and 1 lm. Speci￾mens with relative notch depths of about 0.8 of the thickness of the first external layer, corresponding to 0.26 of the specimen thickness (W), were tested. The tip radii of all notches were deter￾mined from optical observations and they were always found to be below 20 lm. The curves load–displacement of the cross-head of the load frame were recorded and corrected by subtracting the compliance of the testing set up (machine, supports, load cell and fixtures, 1.5  107 m/N) determined by testing a thick (25  25  100 mm3 ) unnotched alumina bar. Three specimens were tested and the curves were found to be practically coincident. 138 S. Bueno, C. Baudín / Composites: Part A 40 (2009) 137–143