S. Bueno et al. / Journal of the European Ceramic Society 28(2008)1961-1971 1963 There are two main different procedures to determine 1389: 2003)and relative densities were calculated from these JiC,26-27,30 either based on the determination of the energy values and those of theoretical densities calculated taking values absorbed by the specimen, given by the area under the corre- of 3.99 gcm-3for alumina(ASTM 42-1468)and 3.70 g cm-3 sponding load-crack opening displacement curve, 30 or from for aluminium titanate(ASTM 26-0040 load-displacement curves by conducting tests on two spec Microstructural characterization on polished and thermally imens with different crack lengths. Both methods require etched(20C below the sintering temperature during 1 min) the identification of the propagating crack. In this work, a surfaces was performed by field emission gun scanning elec- graphical procedure was used- in which Jic is calculated tron microscopy(FEG-SEM, Hitachi, S-4700, Japan). The (Eq (5))from the difference between the areas under the load true average grain size was determined by the linear inter- (P)displacement(8)curves of the notched non-linear speci- cept method considering at least 200 grains for each phase and mens(Al) and an unnotched linear elastic specimen of the same g the correction factor 4.Chemical profiles across grain material(AE) for equal maximum loads(Pmax) boundaries were achieved by STEM-EDX (energy dispersed x- ray spectroscopy, coupled with scanning transmission electron X(Al-Ae) (5) microscopy, Tecnai F20-ST, The Netherlands) at 200 kV. Thin (W-a)B (W一a)B foils were prepared by mechanical polishing of a 3 mm diame ere a, W and B have the same meaning as before(Eq (1)). ter disk up to 15 um in thickness followed by Ar milling(PlPs The work of fracture is defined as the mean energy con- Gatan, USA, operating at 5 kV with a beam incidence of 6%0) sumption required forming the unit fracture surface area and Bars of 25 mm x 2 mm x 2.5 mm were diamond machined the additional process zone. It accounts an average value of from the sintered blocks for bend strength tests(three points, the whole fracture process that does not require any as 20 mm span, 0.5 mm min; Microtest, Spain). Engineering tion on the constitutive equation of the cracked body to deal stress-strain curves were calculated from the load values and with crack growth problems as discussed by Sakai et al. 8 The the displacement of the central part of the samples recorded work of fracture is obtained by dividing the work done on the during the bending tests and static Youngs modulus was deter specimen to propagate the crack, given by the area under the mined from the initial linear part of the curves. Given results load-displacement curves, by the area of the newly created for strength and static Youngs modulus are the average of five surfaces. For parallelepiped bars with straight through notches determinations and the standard deviation tested in flexure, this area is twice the area of the unnotched part Strength was also determined for specimens of a previously of the cross-section of the specimens studied A10 composite(named A10AT)fabricated from pow ders of Al2O3(90 vol %)and Al2TiO5 (10 vol %o)obtained by 3. Experimental procedure reaction of Al2O3 and TiO2 powders and sintered at 1500C, the starting Al2O3 and TiO2 powders used were the same as in Monoliths of monophase alumina (A) and alu- this work. nina+10 vol 9 aluminium titanate(AlO)composites were Single-Edge-V-Notch-Beams(SEVNB )of 4 mm x 6mm x manufactured by colloidal filtration from aqueous alumina, 50 mm were tested in a three points bending device using a span Al2O3, and titania, TiO2, suspensions using the optimum green of 40 mm and a cross-head speed of 0.005 mm min(Microtest, processing conditions previously established. 5.31 A mixture Spain). The compliance of the whole testing system(machine, of alumina ( =95 wt % )and titania (5 wt %)was used to supports, load cell and fixtures) was determined by testing a obtain the sintered composition with 10 vol. of aluminium thick(25 mm x 25 mm x 100 mm)unnotched alumina bar. The titanate, Al2TiO5. The starting materials were commercial obtained value was 1. 5x 10-m/N. The notches were initially a-Al2O3( Condea, HPAO5, USA) and TiO2-anatase(Merck, cut with a 150 um wide diamond wheel. Using this slot as a 808, Germany) powders. The powders were dispersed in guide, the remaining part of the notch was done with a razor deionised water by adding 0.5 wt %(on a dry solids basis) blade sprinkled with diamond pastes of successively 6 and 1 um of a carbonic acid-based polyelectrolyte (Dolapix CE64, Three relative notch depths, a, with approximately 0.4, 0.5 and Zschimmer-Schwarz, Germany). Suspensions were prepared to 0.6 of the thickness of the samples(w) were tested. The tip a solids loading of 50 vol. and ball milled with alumina radii of all notches were determined from optical observations and balls during 4 h. and they were always found to be below 20 um. The curves Plates of the materials with 70 mm x 70 mm x 10 mm dimen- load-displacement of the cross-head of the load frame were sions were obtained by slip casting, removed from the moulds recorded. All curves were corrected by subtracting the com- and dried in air at room temperature for at least 24 h. Sinter- pliance of the testing set up. ing of the green plates was performed in air in an electrical Additional tests were performed with unnotched specimens box furnace(Termiber, Spain) at heating and cooling rates of up to loads(=20N) well below the starting of the non-linear 2Cmin-I,with 4h, dwell at 1200C during heating and two behaviour and the obtained values of stiffness were used to different treatments at the maximum temperature: 2h, dwell at calculate JIc following the procedure described above(Eq (5)) 1450C and 3 h, dwell at 1550C. For all tests, samples were The fracture toughness parameters, i.e., critical stress inten- diamond machined from the sintered blocks sity factor, KiC, critical strain energy release rate, GIC, critical Densities of the sintered compacts were determined by J-integral, JiC, and work of fracture, ywoF, were calculated the Archimedes's method in water(European Standard en from the curves obtained during the sEvnb tests for the threeS. Bueno et al. / Journal of the European Ceramic Society 28 (2008) 1961–1971 1963 There are two main different procedures to determine JIC, 26–27,30 either based on the determination of the energy absorbed by the specimen, given by the area under the corre￾sponding load–crack opening displacement curve,30 or from load–displacement curves by conducting tests on two spec￾imens with different crack lengths.26 Both methods require the identification of the propagating crack. In this work, a graphical procedure was used27 in which JIC is calculated (Eq. (5)) from the difference between the areas under the load (P)–displacement (δ) curves of the notched non-linear speci￾mens (AI) and an unnotched linear elastic specimen of the same material (AE) for equal maximum loads (Pmax): JIC= 2 (W − a)B × δmax 0 Pdδ= 2 (W − a)B × (AI − AE) (5) where a, W and B have the same meaning as before (Eq. (1)). The work of fracture is defined as the mean energy con￾sumption required forming the unit fracture surface area and the additional process zone. It accounts an average value of the whole fracture process that does not require any assump￾tion on the constitutive equation of the cracked body to deal with crack growth problems as discussed by Sakai et al.18 The work of fracture is obtained by dividing the work done on the specimen to propagate the crack, given by the area under the load–displacement curves, by the area of the newly created surfaces. For parallelepiped bars with straight through notches tested in flexure, this area is twice the area of the unnotched part of the cross-section of the specimens. 3. Experimental procedure Monoliths of monophase alumina (A) and alu￾mina + 10 vol.% aluminium titanate (A10) composites were manufactured by colloidal filtration from aqueous alumina, Al2O3, and titania, TiO2, suspensions using the optimum green processing conditions previously established.15,31 A mixture of alumina (∼=95 wt.%) and titania (∼=5 wt.%) was used to obtain the sintered composition with 10 vol.% of aluminium titanate, Al2TiO5. The starting materials were commercial α-Al2O3 (Condea, HPA05, USA) and TiO2-anatase (Merck, 808, Germany) powders. The powders 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 alumina jar and balls during 4 h. Plates of the materials with 70 mm × 70 mm × 10 mm dimen￾sions were obtained by slip casting, removed from the moulds and dried in air at room temperature for at least 24 h. Sinter￾ing of the green plates was performed 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 during heating and two different treatments at the maximum temperature: 2 h, dwell at 1450 ◦C and 3 h, dwell at 1550 ◦C. For all tests, samples were diamond machined from the sintered blocks. Densities of the sintered compacts were determined by the Archimedes’s 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 cm−3 for alumina (ASTM 42-1468) and 3.70 g cm−3 for aluminium titanate (ASTM 26-0040). Microstructural characterization on polished and thermally etched (20 ◦C below the sintering temperature during 1 min) surfaces was performed by field emission gun scanning elec￾tron microscopy (FEG-SEM, Hitachi, S-4700, Japan). The true average grain size was determined by the linear inter￾cept method considering at least 200 grains for each phase and using the correction factor 4/π. 32 Chemical profiles across grain boundaries were achieved by STEM–EDX (energy dispersed X￾ray spectroscopy, coupled with scanning transmission electron microscopy, Tecnai F20-ST, The Netherlands) at 200 kV. Thin foils were prepared by mechanical polishing of a 3 mm diame￾ter disk up to 15m in thickness followed by Ar+ milling (PIPS Gatan, USA, operating at 5 kV with a beam incidence of 6%). Bars of 25 mm × 2 mm × 2.5 mm were diamond machined from the sintered blocks for bend strength tests (three points, 20 mm span, 0.5 mm min−1; Microtest, Spain). Engineering stress–strain curves were calculated from the load values and the displacement of the central part of the samples recorded during the bending tests and static Young’s modulus was deter￾mined from the initial linear part of the curves. Given results for strength and static Young’s modulus are the average of five determinations and the standard deviation. Strength was also determined for specimens of a previously studied A10 composite8 (named A10AT) fabricated from pow￾ders of Al2O3 (90 vol.%) and Al2TiO5 (10 vol.%) obtained by reaction of Al2O3 and TiO2 powders33 and sintered at 1500 ◦C; the starting Al2O3 and TiO2 powders used were the same as in this work. Single-Edge-V-Notch-Beams (SEVNB) of 4 mm × 6 mm × 50 mm were tested in a three points bending device using a span of 40 mm and a cross-head speed of 0.005 mm min−1 (Microtest, Spain). The compliance of the whole testing system (machine, supports, load cell and fixtures) was determined by testing a thick (25 mm × 25 mm × 100 mm) unnotched alumina bar. The obtained value was 1.5 × 10−7 m/N. The notches were initially cut with a 150 m 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 m. Three relative notch depths, α, with approximately 0.4, 0.5 and 0.6 of the thickness of the samples (W) were tested. The tip radii of all notches were determined from optical observations and they were always found to be below 20 m. The curves load–displacement of the cross-head of the load frame were recorded. All curves were corrected by subtracting the com￾pliance of the testing set up. Additional tests were performed with unnotched specimens up to loads (∼=20 N) well below the starting of the non-linear behaviour and the obtained values of stiffness were used to calculate JIC following the procedure described above (Eq. (5)). The fracture toughness parameters, i.e., critical stress inten￾sity factor, KIC, critical strain energy release rate, GIC, critical J-integral, JIC, and work of fracture, γWOF, were calculated from the curves obtained during the SEVNB tests for the three