Availableonlineatwww.sciencedirect.com DIRECT E噩≈3S SEVIER Journal of the European Ceramic Society 25(2005)847-856 www.elsevier.com/locate/jeurceramsoc Design and processing of Al2O3-Al2TiO5 layered structures Salvador Bueno Rodrigo moreno Carmen Baudin* Instituto de Ceramica y Vidrio, CS/C, Campus de Cantoblanco, 28049 Madrid, Spain Received 5 February 2004; received in revised form 22 April 2004; accepted 1 May 2004 Available online 10 July 2004 Abstract Al2O3-Al2 TiOs layered composites were manufactured by a colloidal route from aqueous Al2O and TiO2 suspensions with 50 vol % solids The mechanical behaviours of individual monolithic composite materials were combined and taken as basis for the design of the layered tructures. Residual stresses which are likely to occur due to processing and thermally introduced misfits were calculated and considered for the manufacture of the laminates Monoliths with 10, 30 and 40 vol. of second phase showed that increasing proportions of aluminium titanate decrease strength and increase the non-inear behaviour In order to obtain the desired combination of mechanical behaviours of the layers, two laminate designs with external and central layer of one composition and the alternating internal layer of the other composition were chosen taking into account chemical compatibility and development of residual stresses. In the system AAlO, external and central layers of monophase Al2O3 with high strength were combined with intermediate layers of Al2 O3 with 10 vol %of Al2TiOs. The system A10A40 was selected to combine low strength and energy absorbing intermediate layers of Al2O with 40 vol % of Al]TiOs and sufficient strength provided by external layers of Al2O3 with 10 vol %of Al2TiOs The stress-strain behaviour of the laminates was linear up to their failure stresses, with apparent strain for zero load after fracture larger than that corresponding to the monoliths of the same composition as that of the external layers. Moreover, the stress drop of the laminate samples occurred in step-like form thus suggesting the occurrence of additional energy consuming processes during fracture C 2004 Elsevier Ltd. All rights reserved Keywords: Ceramic laminates; Slip casting, Sintering; Al2O3; Al2TiOs: Laminates 1. Introduction the alumina-aluminium titanate system. These authors fab- ricated trilaminates with surface layers consisting of a ho. Improved flaw tolerance and toughness with alumina mogeneous mixture of alumina-20 vol aluminium titanate (Al2O3/aluminium titanate (Al2TiOs) composites have and a flaw tolerant inner layer of the same composition with been reported previously. -6 The toughening mechanisms heterogeneous microstructure. As opposite to laminate de acting in these composites are crack bridging and microc sign in which the high strength is due to residual compressive racking and, therefore, toughening is often associated with stresses acting in the outer layers, 9. 0 such a design would rather low strength. Both mechanisms are originated by assure also high strength for increasing temperature. The the residual stresses that develop during cooling from the limit of this approach is the difficulty to obtain co-sinterec sintering temperature due to thermal expansion mismatch layers of the same composition with large microstructural between alumina and aluminium titanate differences and therefore with significant differences in the In composite materials in which ceramic layers of differ- mechanical behaviour ent composition and, or microstructure are combined, the In a previous work, the processing conditions to achieve properties can be tailored to be superior to those of the con- crack free and completely reacted alumina-aluminium ti- stituent layers. In particular, it is possible to achieve high tanate monolithic composites were established. Uniform flaw tolerance, without sacrificing strength, by using a lami- distribution of the second phase was obtained by a strict nate design in which an R-curve material is located between control of the colloid chemistry of the mixture and grain hig igh strength layers, as demonstrated by Russo et al.in growth was controlled by using a thermal treatment at relatively low temperature. Increased sintering tempera- Corresponding author. Tel. +34 91 735 5840; fax: +34 91 735 5843. ture and aluminium titanate content led to microstructures E-mail address: cbaudin @icv csic es(C. Baudin) with larger grains that presented non-linear stress-strain 0955-2219/s-see front matter O 2004 Elsevier Ltd. All rights reserved doi: 10.1016/j jeurceramsoc 2004.05.001
Journal of the European Ceramic Society 25 (2005) 847–856 Design and processing of Al2O3–Al2TiO5 layered structures Salvador Bueno, Rodrigo Moreno, Carmen Baud´ın∗ Instituto de Cerámica y Vidrio, CSIC, Campus de Cantoblanco, 28049 Madrid, Spain Received 5 February 2004; received in revised form 22 April 2004; accepted 1 May 2004 Available online 10 July 2004 Abstract Al2O3–Al2TiO5 layered composites were manufactured by a colloidal route from aqueous Al2O3 and TiO2 suspensions with 50 vol.% solids. The mechanical behaviours of individual monolithic composite materials were combined and taken as basis for the design of the layered structures. Residual stresses which are likely to occur due to processing and thermally introduced misfits were calculated and considered for the manufacture of the laminates. Monoliths with 10, 30 and 40 vol.% of second phase showed that increasing proportions of aluminium titanate decrease strength and increase the non-linear behaviour. In order to obtain the desired combination of mechanical behaviours of the layers, two laminate designs with external and central layers of one composition and the alternating internal layer of the other composition were chosen taking into account chemical compatibility and development of residual stresses. In the system AA10, external and central layers of monophase Al2O3 with high strength were combined with intermediate layers of Al2O3 with 10 vol.% of Al2TiO5. The system A10A40 was selected to combine low strength and energy absorbing intermediate layers of Al2O3 with 40 vol.% of Al2TiO5 and sufficient strength provided by external layers of Al2O3 with 10 vol.% of Al2TiO5. The stress–strain behaviour of the laminates was linear up to their failure stresses, with apparent strain for zero load after fracture larger than that corresponding to the monoliths of the same composition as that of the external layers. Moreover, the stress drop of the laminate samples occurred in step-like form thus suggesting the occurrence of additional energy consuming processes during fracture. © 2004 Elsevier Ltd. All rights reserved. Keywords: Ceramic laminates; Slip casting; Sintering; Al2O3; Al2TiO5; Laminates 1. Introduction Improved flaw tolerance and toughness with alumina (Al2O3)–aluminium titanate (Al2TiO5) composites have been reported previously.1–6 The toughening mechanisms acting in these composites are crack bridging and microcracking and, therefore, toughening is often associated with rather low strength. Both mechanisms are originated by the residual stresses that develop during cooling from the sintering temperature due to thermal expansion mismatch between alumina and aluminium titanate. In composite materials in which ceramic layers of different composition and, or microstructure are combined, the properties can be tailored to be superior to those of the constituent layers.7 In particular, it is possible to achieve high flaw tolerance, without sacrificing strength, by using a laminate design in which an R-curve material is located between high strength layers, as demonstrated by Russo et al.8 in ∗ Corresponding author. Tel.: +34 91 735 5840; fax: +34 91 735 5843. E-mail address: cbaudin@icv.csic.es (C. Baud´ın). the alumina–aluminium titanate system. These authors fabricated trilaminates with surface layers consisting of a homogeneous mixture of alumina–20 vol.% aluminium titanate and a flaw tolerant inner layer of the same composition with heterogeneous microstructure. As opposite to laminate design in which the high strength is due to residual compressive stresses acting in the outer layers,9,10 such a design would assure also high strength for increasing temperature. The limit of this approach is the difficulty to obtain co-sintered layers of the same composition with large microstructural differences and, therefore, with significant differences in the mechanical behaviour. In a previous work,11 the processing conditions to achieve crack free and completely reacted alumina–aluminium titanate monolithic composites were established. Uniform distribution of the second phase was obtained by a strict control of the colloid chemistry of the mixture and grain growth was controlled by using a thermal treatment at relatively low temperature. Increased sintering temperature and aluminium titanate content led to microstructures with larger grains that presented non-linear stress–strain 0955-2219/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2004.05.001
848 S. Bueno et al. /Journal of the European Ceramic Society 25 (2005)847-850 response, 2 in agreement with a simplified model6., 3 pro- jar and balls during 4h. These conditions were selected from osed for biphasic materials. Therefore a combination of a previous work layers with different aluminium titanate contents might lead Rheological characterisation was carried out using a ro- to simultaneous high strength and flaw tolerance. once the tational rheom tational rheometer(Haake, Rs50, Germany) with a double residual stresses due to the thermal expansion mismatch of cone/plate sensor system layers with different composition are controlled Solid discs with 20 mm in diameter were slip cast in In order to obtain the desired strength-flaw tolerance plaster of Paris moulds in order to determine the casting behaviour, the properties of the layer materials as well as rate of each suspension by measurement of the dry wal the green processing and sintering conditions of the lami- thickness(Mitutoyo, JDU25, Japan)after different casting nates need to be carefully adjusted. First, the composition times(1-16 min). For mechanical characterisation, plates and microstructure of the different layer materials should with 70 mm x 70 mm x 10 mm dimensions were also ob- be optimised to achieve the suitable mechanical behaviour. tained by slip casting for every composition. The cast bodies Second, compatible processing conditions, in particular, were carefully removed from the moulds and dried in air at sintering schedule, should be established to maintain the room temperature for at least 24h properties of the layers in the layered structure and impede The reaction sintering behaviour of the specimens was the failure of the laminate during fabrication due to incom- studied with a differential dilatometer(Adamel Lhomargy patible shrinkage of the layers. Last, residual stresses in the DI24, France)to 1550C. To obtain the monolithic mate- layers, originated by thermal expansion mismatch, have to rials, the dried blocks were sintered in air in an electrical be controlled to avoid fracture box furnace(Termiber, Spain) at heating and cooling rates In this work, the processing parameters to obtain flaw tol- of 2C min, with 4-h dwell at 1200C and 3-h dwell at erant and high strength laminates in the alumina-aluminium the maximum temperature, 1550C titanate system are investigated. Slip casting of aqueous alt The densities of the green and sintered compacts we mina and titania mixtures with high solids contents allows to determined by the Archimedes method using mercury and obtain composite materials with homogeneous microstruc- water, respectively. The crystalline phases present were de- tures and is a simple way to fabricate laminates constituted termined by X-ray diffraction(Siemens AG, D5000, Ger by relatively thick( 200-1000 um)layers. Accurate con- many)after grinding, and results were processed using the trol of the layer thickness can be reached by the control of ASTM Files for corundum(42-1468), anatase(21-1272), the wall thickness formation rate and the sintering shrinkage rutile(21-1276)and B-aluminium titanate(26-0040) of each slip formulation The sintered blocks were machined into bars of 25 mm x e First, the influence of the volume fraction of aluminium 2 mm x 2.5mm(referred to as small bars) for bend strength tanate on the stress-strain response of the composites was tests(three point bending, 20 mm span, 0.5 mm min-,Mi- studied,and from these results, characteristic layered struc- crotest, Spain)and dynamic Youngs modulus( Grindosonic, tures with external layers of sufficient strength were de- Belgium). Nominal stress-apparent strain curves were cal- signed. Second, the green processing and sintering condi- culated from the load values and the displacement of the different layers were selected on the basis of those for the and apparent Youngs modulus was determined from the lin- monoliths, and recalculated with experimental results of sin- ear part of the curves. Reported bend strength and Youngs tered samples. Last, fracture of the laminates was charac- modulus values are the average of five measurements and terised to check whether the desired mechanical behaviour To determine the thermal expansion curves of the mono- was attained pieces of 10 mm x 5mm x 5mm were tested in a differential dilatometer(402 EP, Netzsch, Germany) using es of 5oC 2. Experimental curves the average thermal expansion coefficients between 25 and 850C were calculated. Reported values are the av- The starting materials were commercial a-AlO3( Con- erage of three measurements and errors are the standard de- dea, HPAO5, USA)and anatase-TiO2(Merck, 808, Ger- rations many)powders. Al2O3/TiO2 mixtures with relative TiO2 Two layered composites of five layers were fabricated contents of 0, 5, 15 and 20 wt were prepared to obtain by alternately casting each suspension. Casting times were VoI Sites with Al2 TiOs concentrations of fixed to reach the desired layer thickness considering the 0.10. 30 and 40 vol. after reaction sintering sting kinetics and sintering shrinkage of each composi- The single oxides and the mixtures were dispersed in tion. One laminate, A10A40, had the central and outer layers deionised water by adding 0.5 wt %(on a dry solids ba- (1200 um) made of AlO(A+T) and the two inner layers sis)of a carbonic acid based polyelectrolyte(Dolapix CE64, (300 um)of A40(A+T). In the other system, AAlO, the Zschimmer-Schwarz, Germany). Suspensions were prepare central and outer layers were made of alumina and the two to a solids loading of 50 vol. and ball milled with Al2O3 inner layers of AlO(A+T). Due to the eometry and
848 S. Bueno et al. / Journal of the European Ceramic Society 25 (2005) 847–856 response,12 in agreement with a simplified model6,13 proposed for biphasic materials. Therefore, a combination of layers with different aluminium titanate contents might lead to simultaneous high strength and flaw tolerance, once the residual stresses due to the thermal expansion mismatch of layers with different composition are controlled. In order to obtain the desired strength-flaw tolerance behaviour, the properties of the layer materials as well as the green processing and sintering conditions of the laminates need to be carefully adjusted. First, the composition and microstructure of the different layer materials should be optimised to achieve the suitable mechanical behaviour. Second, compatible processing conditions, in particular, sintering schedule, should be established to maintain the properties of the layers in the layered structure and impede the failure of the laminate during fabrication due to incompatible shrinkage of the layers. Last, residual stresses in the layers, originated by thermal expansion mismatch, have to be controlled to avoid fracture. In this work, the processing parameters to obtain flaw tolerant and high strength laminates in the alumina–aluminium titanate system are investigated. Slip casting of aqueous alumina and titania mixtures with high solids contents allows to obtain composite materials with homogeneous microstructures and is a simple way to fabricate laminates constituted by relatively thick (∼=200–1000m) layers. Accurate control of the layer thickness can be reached by the control of the wall thickness formation rate and the sintering shrinkage of each slip formulation. First, the influence of the volume fraction of aluminium titanate on the stress–strain response of the composites was studied, and from these results, characteristic layered structures with external layers of sufficient strength were designed. Second, the green processing and sintering conditions to fabricate laminates with controlled thickness of the different layers were selected on the basis of those for the monoliths, and recalculated with experimental results of sintered samples. Last, fracture of the laminates was characterised to check whether the desired mechanical behaviour was attained. 2. Experimental The starting materials were commercial -Al2O3 (Condea, HPA05, USA) and anatase-TiO2 (Merck, 808, Germany) powders. Al2O3/TiO2 mixtures with relative TiO2 contents of 0, 5, 15 and 20 wt.% were prepared to obtain Al2O3/Al2TiO5 composites with Al2TiO5 concentrations of 0, 10, 30 and 40 vol.% after reaction sintering. The single oxides and the mixtures were dispersed in deionised water by adding 0.5 wt.% (on a dry solids basis) of a carbonic acid based polyelectrolyte (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.11 Rheological characterisation was carried out using a rotational rheometer (Haake, RS50, Germany) with a double cone/plate sensor system. Solid discs with 20 mm in diameter were slip cast in plaster of Paris moulds in order to determine the casting rate of each suspension by measurement of the dry wall thickness (Mitutoyo, JDU25, Japan) after different casting times (1–16 min). For mechanical characterisation, plates with 70 mm × 70 mm × 10 mm dimensions were also obtained by slip casting for every composition. 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 of the specimens was studied with a differential dilatometer (Adamel Lhomargy, DI24, France) to 1550 ◦C. To obtain the monolithic materials, the dried blocks were sintered in air in an electrical box furnace (Termiber, Spain) at heating and cooling rates of 2 ◦C min−1, with 4-h dwell at 1200 ◦C and 3-h dwell at the maximum temperature, 1550 ◦C. The densities of the green and sintered compacts were determined by the Archimedes method using mercury and water, respectively. The crystalline phases present were determined by X-ray diffraction (Siemens AG, D5000, Germany) after grinding, and results were processed using the ASTM Files for corundum (42-1468), anatase (21-1272), rutile (21-1276) and -aluminium titanate (26-0040). The sintered blocks were machined into bars of 25 mm × 2 mm × 2.5 mm (referred to as small bars) for bend strength tests (three point bending, 20 mm span, 0.5 mm min−1; Microtest, Spain) and dynamic Young’s modulus (Grindosonic, Belgium). Nominal stress–apparent strain curves were calculated from the load values and the displacement of the central part of the samples recorded during the bend tests, and apparent Young’s modulus was determined from the linear part of the curves. Reported bend strength and Young’s modulus values are the average of five measurements and errors are the standard deviations. To determine the thermal expansion curves of the monoliths, pieces of 10 mm × 5 mm × 5 mm were tested in a differential dilatometer (402 EP, Netzsch, Germany) using heating and cooling rates of 5 ◦C min−1. From the recorded curves the average thermal expansion coefficients between 25 and 850 ◦C were calculated. Reported values are the average of three measurements and errors are the standard deviations. Two layered composites of five layers were fabricated by alternately casting each suspension. Casting times were fixed to reach the desired layer thickness considering the casting kinetics and sintering shrinkage of each composition. One laminate, A10A40, had the central and outer layers (∼=1200m) made of A10(A+T) and the two inner layers (∼=300m) of A40(A+T). In the other system, AA10, the central and outer layers were made of alumina and the two inner layers of A10(A+T). Due to the geometry and dimen-
S. Bueno et al. /Journal of the European Ceramic Sociery 25(2005)847-856 Table 1 Viscosity of suspensions and relative density (percent of theoretical) of monolithic samples Alumina AIO(A+T) 30(A+T) A40(A+T) Viscosity(mPas, 500s-) p(green) 64.2±0.5 63.9±0.3 63.3±0.5 12 5±0.3 (sintered) 98.2±0.5 976±04 97.8±0.5 97.1士04 sions of laminated architectures, bars of 25 mm x 5.5mm were performed lished and chemically etched (HF x 3.5 mm for bend strength tests were obtained and tested 10 vol % I min) surfaces of the a 10A40 laminates under the same conditions as those described above. Bars of monolithic materials with the same dimensions than those of laminates were prepared for comparison. These are referred 3. Results to as large bars Scanning electron microscopy (SEM, Carl Zeiss, 3. 1. Monoliths DSM-950, Germany was performed on the fracture surfaces and the width of the layers in the laminates was measured The rheological properties of the studied suspensions were directly in the microscope. Additional SEM observations reported elsewhere. All the suspensions used in this study -0,04 Alumin 0,08 800 1000 00 120 1600 T°c] 0.000 -0,0001 A30(A+T) A40(A+T) -0,0005 -0.0006 1000 Fig. 1. Dynamic sintering curves of the monoliths. (a) Linear shrinkage, AL/Lo, vS. temperature.(b) Linear shrinkage rate d(Allo dT, vs temperature
S. Bueno et al. / Journal of the European Ceramic Society 25 (2005) 847–856 849 Table 1 Viscosity of suspensions and relative density (percent of theoretical) of monolithic samples Alumina A10(A+T) A30(A+T) A40(A+T) Viscosity (mPa s, 500 s−1) 34 43 44 51 ρ (green) 64.2 ± 0.5 63.9 ± 0.3 63.3 ± 0.5 62.5 ± 0.3 ρ (sintered) 98.2 ± 0.5 97.6 ± 0.4 97.8 ± 0.5 97.1 ± 0.4 sions of laminated architectures, bars of 25 mm × 5.5 mm × 3.5 mm for bend strength tests were obtained and tested under the same conditions as those described above. Bars of monolithic materials with the same dimensions than those of laminates were prepared for comparison. These are referred to as large bars. Scanning electron microscopy (SEM, Carl Zeiss, DSM-950, Germany) was performed on the fracture surfaces and the width of the layers in the laminates was measured directly in the microscope. Additional SEM observations Fig. 1. Dynamic sintering curves of the monoliths. (a) Linear shrinkage, L/L0, vs. temperature. (b) Linear shrinkage rate, d(L/L0)/dT, vs. temperature. were performed on polished and chemically etched (HF 10 vol.%, 1 min) surfaces of the A10A40 laminates. 3. Results 3.1. Monoliths The rheological properties of the studied suspensions were reported elsewhere.11 All the suspensions used in this study
S. Bueno et al. /Journal of the European Ceramic Society 25(2005)847-850 Fig.2. Scanning electron micrographs of fracture surfaces of the monoliths (prepared with heating and cooling rates of 2Cmin-l, with 4-h dwell at 200C and 3-h dwell at the maximum temperature, 1550C). Alumina grains appear with dark grey colour, aluminium titanate of an intermediate grey shade and titania, which would appear white, is not observed. Tensile surfaces are located at the lower part of the micrographs (a)AlO(A-+T);(b) A30(A+T);(c)A40(A+T. had viscosities of 40-50 mPas at a shear rate of 500s-I containing 30 and 40 vol. of aluminium titanate(Fig 2b (Table 1). The optimised colloidal processing led to high and c), green density composites (62.5% of theoretical density, Characteristic nominal stress-apparent strain relations for the small (25 mm x 2 mm x 2.5 mm) monolithic In Fig. I, dynamic sintering curves for alumina and the samples are shown in Fig 3a and the thermal expanse shrinkage levels are coincident at 1240C(Fig. la)and the Table 2. The curves corresponding to the monolith with sintering rates are coincident at 1150 C(Fig. 1b). These the lowest aluminium titanate content, AlO(A+T),were three curves exhibit a change of slope at temperatures of practically linear up to fracture and this material pproximately I380°C sented the highest strength, Youngs modulus and ther In Fig. 2, characteristic fracture surfaces of the com- mal expansion values and the lowest strains to fracture posite monoliths sintered at 1550C are observed. Alu- Increasing proportions of aluminium titanate decrease minium titanate is homogeneously distributed and mainly strength, Youngs modulus and thermal expansion and located at alumina triple points and grain boundaries, and increase the non-linear behaviour, with high strains to no titania is detected, according to XRD. In the samples acture with 10 vol. of aluminium titanate( Fig. 2a)the grain size 1g. 4 shows characteristic fracture surfaces of the of alumina( 5 um)is much larger than in the samples three monoliths at low magnification. Those of AlO(A+T)
850 S. Bueno et al. / Journal of the European Ceramic Society 25 (2005) 847–856 Fig. 2. Scanning electron micrographs of fracture surfaces of the monoliths (prepared with heating and cooling rates of 2 ◦C min−1, with 4-h dwell at 1200 ◦C and 3-h dwell at the maximum temperature, 1550 ◦C). Alumina grains appear with dark grey colour, aluminium titanate of an intermediate grey shade and titania, which would appear white, is not observed. Tensile surfaces are located at the lower part of the micrographs. (a) A10(A+T); (b) A30(A+T); (c) A40(A+T). had viscosities of ≈40–50 mPa s at a shear rate of 500 s−1 (Table 1). The optimised colloidal processing led to high green density composites (>62.5% of theoretical density, Table 1). In Fig. 1, dynamic sintering curves for alumina and the three studied composites are plotted. For the composites, the shrinkage levels are coincident at 1240 ◦C (Fig. 1a) and the sintering rates are coincident at 1150 ◦C (Fig. 1b). These three curves exhibit a change of slope at temperatures of approximately 1380 ◦C. In Fig. 2, characteristic fracture surfaces of the composite monoliths sintered at 1550 ◦C are observed. Aluminium titanate is homogeneously distributed and mainly located at alumina triple points and grain boundaries, and no titania is detected, according to XRD. In the samples with 10 vol.% of aluminium titanate (Fig. 2a) the grain size of alumina (≈5m) is much larger than in the samples containing 30 and 40 vol.% of aluminium titanate (Fig. 2b and c), Characteristic nominal stress–apparent strain relations for the small (25 mm × 2 mm × 2.5 mm) monolithic samples are shown in Fig. 3a and the thermal expansion coefficient and mechanical properties are summarised in Table 2. The curves corresponding to the monolith with the lowest aluminium titanate content, A10(A+T), were practically linear up to fracture and this material presented the highest strength, Young’s modulus and thermal expansion values and the lowest strains to fracture. Increasing proportions of aluminium titanate decrease strength, Young’s modulus and thermal expansion and increase the non-linear behaviour, with high strains to fracture. Fig. 4 shows characteristic fracture surfaces of the three monoliths at low magnification. Those of A10(A+T)
S. Bueno et al. /Journal of the European Ceramic Sociery 25(2005)847-856 250 A10(A+T) composition with central and external layers of AlO(A+T) composition. In the system AA1O, external and central lay ers of monophase alumina were combined with intermedi ate layers of AlO(A+T) composition. As discussed below, thin intermediate layers(300 um) and thick external and central layers(1200 um) were selected Fig. 5 shows the casting kinetics for the alumina and the A30(A+T three composite slips. In all cases, the well-known propor A40(A+T) tionality between wall thickness()and the square root of time(o) is found. 4 Casting time shortens with titania con- tent for the same wall thickness Fig 6 shows characteristic fracture surfaces of the layered 0.05 0, 20 materials where the different layers are easily differentiated Strain [% because the crack path changes at the interlayers. Neverthe- less, the size of the layers indicated in Table 3, which was measured directly in the SEM on fracture surfaces, has to be taken as approximate In Fig. 7, the nominal stress-apparent strain relationships of the laminates are compared to those for large monolithic samples(25 mm x 5.5mm x 3.5 mm)with the same com- positions as those of the corresponding external layers. In all cases, the behaviour was practically linear up to fracture The slope of the linear portion was lower for the layered materials. The stress drop from the failure point occurred steeply for the monoliths, in which apparent deformation he maximum load was coincident with that for zero load fter fracture. In the laminates, a step-like way was followed and apparent strain for zero load was larger than that corre- 0.00 ,10 0.15 sponding to failure load Fig. 3. Characteristic nominal stress-apparent strain curves for monolithic Strength values for the laminates were 272+32 MPa and 47+ 20 MPa, for AA10 and A10A40, respectively small(25 mm x 2 mm x 2.5 mm) samples. (a)Curves corresponding to the materials indicated.(b) Characteristic ratio of specific elastic energy at fracture(scratched area) to the whole specific energy expended during 4. Discussion the test for A40(A+T)materials. 4I. Monoliths samples were flat whereas those of the A30(A+T)and A40(A+T)samples were highly tortuous The objective of this work was the development of laminates on the basis of the properties of the monolithic 3. 2. laminate composite materials that would constitute the different lay ers. Therefore, the selection of the sintering schedule for Two laminates with five layers were fabricated. The sys- the monoliths was performed to ensure the possibility of tem A10A40 combines intermediate layers of A40(A+T) laminate fabrication. Moreover, complete reaction between Table 2 Thermal expansion coefficient and mechanical properties of monolithic materials Alumina AlO(A+T) A30(A+T) A40(A+T) a2ss0°c×10-6(°C-1) 8.8±0.2 8.3±0.3 4.7±0.2 4.1±0.1 388±5 333±9 146±6 107±3 376士6 60±8 43±1 0.99±0.01 0.76±003 0.72±003 Bending strength(MPa) Small samples(25×2×2.5) 230±1 76±4 61土 Large samples(25×5.5×3.5 304±36 189±6 62士2 43±1
S. Bueno et al. / Journal of the European Ceramic Society 25 (2005) 847–856 851 Fig. 3. Characteristic nominal stress–apparent strain curves for monolithic small (25 mm × 2 mm × 2.5 mm) samples. (a) Curves corresponding to the materials indicated. (b) Characteristic ratio of specific elastic energy at fracture (scratched area) to the whole specific energy expended during the test for A40(A+T) materials. samples were flat whereas those of the A30(A+T) and A40(A+T) samples were highly tortuous. 3.2. Laminates Two laminates with five layers were fabricated. The system A10A40 combines intermediate layers of A40(A+T) Table 2 Thermal expansion coefficient and mechanical properties of monolithic materials Alumina A10(A+T) A30(A+T) A40(A+T) α25–850 ◦C × 10−6 ( ◦C−1) 8.8 ± 0.2 8.3 ± 0.3 4.7 ± 0.2 4.1 ± 0.1 Edynamic (GPa) 388 ± 5 333 ± 9 146 ± 6 107 ± 3 Estatic (GPa) 376 ± 6 202 ± 10 60 ± 8 43 ± 1 Brittleness parameter – 0.99 ± 0.01 0.76 ± 0.03 0.72 ± 0.03 Bending strength (MPa) Small samples (25 × 2 × 2.5) – 230 ± 1 76 ± 4 61 ± 1 Large samples (25 × 5.5 × 3.5) 304 ± 36 189 ± 6 62 ± 2 43 ± 1 composition with central and external layers of A10(A+T) composition. In the system AA10, external and central layers of monophase alumina were combined with intermediate layers of A10(A+T) composition. As discussed below, thin intermediate layers (≈300m) and thick external and central layers (≈1200m) were selected. Fig. 5 shows the casting kinetics for the alumina and the three composite slips. In all cases, the well-known proportionality between wall thickness (l) and the square root of time (t) is found.14 Casting time shortens with titania content for the same wall thickness. Fig. 6 shows characteristic fracture surfaces of the layered materials where the different layers are easily differentiated because the crack path changes at the interlayers. Nevertheless, the size of the layers indicated in Table 3, which was measured directly in the SEM on fracture surfaces, has to be taken as approximate. In Fig. 7, the nominal stress–apparent strain relationships of the laminates are compared to those for large monolithic samples (25 mm × 5.5 mm × 3.5 mm) with the same compositions as those of the corresponding external layers. In all cases, the behaviour was practically linear up to fracture. The slope of the linear portion was lower for the layered materials. The stress drop from the failure point occurred steeply for the monoliths, in which apparent deformation at the maximum load was coincident with that for zero load after fracture. In the laminates, a step-like way was followed and apparent strain for zero load was larger than that corresponding to failure load. Strength values for the laminates were 272 ± 32 MPa and 147 ± 20 MPa, for AA10 and A10A40, respectively. 4. Discussion 4.1. Monoliths The objective of this work was the development of laminates on the basis of the properties of the monolithic composite materials that would constitute the different layers. Therefore, the selection of the sintering schedule for the monoliths was performed to ensure the possibility of laminate fabrication. Moreover, complete reaction between