Availableonlineatwww.sciencedirectcom ScienceDirect E噩≈RS ELSEVIER Joumal of the European Ceramic Society 27(2007)1389-1394 www.elsevier.comlocate/jeurceramsoc Alumina-zirconia layered ceramics fabricated by stacking water processed green ceramic tapes J. Gurauskis. A.J. Sanchez-Herencia C. Baudin nstinuto de Ceramica y Vidrio(CSIC), C/Kelsen 5. 28049 Madrid, Spain Available online 6 June 2006 Lamination of stacked green ceramic tapes is one of the most attractive methods for the production of multilayer ceramics with strong interfaces In this work the fabrication of high density layered ceramics by joining ceramic green tapes processed from aqueous ceramic slurries is described. Layers with different thickness and compositions in the alumina-zirconia system were combined to obtain layered ceramics with R-curve behaviour. The optimization of the sintering schedule to obtain crack free structures is described. The mechanical behaviour of the laminates, characterized by the indentation strength method, is discussed in terms of that of the constituent layers and the residual stresses developed. o 2006 Elsevier Ltd. all rights reserved. Keywords: Al2O3; Zroz; Joining: Sintering: Mechanical properties Introduction tapes.2. 3 The structural distribution of the layers was designed to develop different levels of compressive residual stresses in Present-day applications of engineering ceramics require the external layers. In these laminates the micro-structural and improved mechanical performance. One of the approaches to macro-structural reinforcement mechanisms would be activated achieve this is to activate rising crack growth resistance(r- The analysis of dilatometry data was done to control the forma- curve)by inducing toughening mechanisms. However, as most tion of processing defects and to evaluate the level of residual toughening mechanisms that originate R-curve behaviour are stresses related to the development of residual stresses at the microstruc In order to assess the r-curve behaviour of the obtained tural level, flaw tolerance for large crack sizes is usually associ- materials during fracture, the indentation strength method has ed with low strengths at small-filaw values As a solution to this been used Materials without R-curve behaviour display inden- tradeoff, ceramic laminates with strong interfaces and external tation load P-1/ versus strength dependence. 4, A deviation layers in compression are proposed. The processing of these lam- from that ideal P- dependence towards a plateau strength inated ceramic structures requires understanding of the nature level indicates that R-curve mechanism is operative. The effect of the residual stresses in order to obtain defect free structures of composition of laminated structures on indentation strength with operative reinforcing layers. 2-5 behaviour (using loads between 10 and 400 N) is discussed in One of the most versatile ceramic systems is AlO terms of the associated residual stresses ZrO2,3,6-9In this system, composite layers with different com- positions can be combined to give rise the desired residual stresses in the layers for macro-structural scale reinforcement. 2.Experimental In addition, the presence of Zro2 might originate reinforcing mechanisms,0, I which operate at the microstructural level Tapes for the production of the multilayer structures were cast In this work Al2O3 tapes with 5 and 40 vol. addi- Trom stable slurries of high purity a-Al2O3 and Y-TZP powders tions of t-ZrO2(Y-TZP) were employed to fabricate laminate in deionised water as dispersing media. The starting powders ceramic composites by stacking water processed green ceramic of a-Al2O3( Condea HPA 0.5, USA), with mean particle size of 0.35 um and specific surface area of 9.5m-/g, and a t-ZrO2 stabilized with 3 mol %o Y203(TZ3YS, TOSOH, Japan), with Corresponding author. Tel: +34917 355 840: fax: +34917 355 843 a mean particle size of 0. 4 um and a specific surface area of E-mail address: cbaudin @icv csices(C. Baudin) 6.7 m/g, were used. Two compositions, 95 vol. of a-Al2O3 0955-2219/S-see front matter o 2006 Elsevier Ltd. All rights reserved. doi: 10. 1016/j-jeurceramsoc. 2006.04.081
Journal of the European Ceramic Society 27 (2007) 1389–1394 Alumina–zirconia layered ceramics fabricated by stacking water processed green ceramic tapes J. Gurauskis, A.J. Sanchez-Herencia, C. Baud ´ ´ın ∗ Instituto de Cer ´amica y Vidrio (CSIC), C/Kelsen 5, 28049 Madrid, Spain Available online 6 June 2006 Abstract Lamination of stacked green ceramic tapes is one of the most attractive methods for the production of multilayer ceramics with strong interfaces. In this work the fabrication of high density layered ceramics by joining ceramic green tapes processed from aqueous ceramic slurries is described. Layers with different thickness and compositions in the alumina–zirconia system were combined to obtain layered ceramics with R-curve behaviour. The optimization of the sintering schedule to obtain crack free structures is described. The mechanical behaviour of the laminates, characterized by the indentation strength method, is discussed in terms of that of the constituent layers and the residual stresses developed. © 2006 Elsevier Ltd. All rights reserved. Keywords: Al2O3; ZrO2; Joining; Sintering; Mechanical properties 1. Introduction Present-day applications of engineering ceramics require improved mechanical performance. One of the approaches to achieve this is to activate rising crack growth resistance (Rcurve) by inducing toughening mechanisms.1 However, as most toughening mechanisms that originate R-curve behaviour are related to the development of residual stresses at the microstructural level, flaw tolerance for large crack sizes is usually associated with low strengths at small-flaw values. As a solution to this tradeoff, ceramic laminates with strong interfaces and external layers in compression are proposed. The processing of these laminated ceramic structures requires understanding of the nature of the residual stresses in order to obtain defect free structures with operative reinforcing layers.2–5 One of the most versatile ceramic systems is Al2O3– ZrO2. 1,3,6–9 In this system, composite layers with different compositions can be combined to give rise the desired residual stresses in the layers for macro-structural scale reinforcement. In addition, the presence of ZrO2 might originate reinforcing mechanisms,10,11 which operate at the microstructural level. In this work Al2O3 tapes with 5 and 40 vol.% additions of t-ZrO2 (Y-TZP) were employed to fabricate laminate ceramic composites by stacking water processed green ceramic ∗ Corresponding author. Tel.: +34 917 355 840; fax: +34 917 355 843. E-mail address: cbaudin@icv.csic.es (C. Baud´ın). tapes.12,13 The structural distribution of the layers was designed to develop different levels of compressive residual stresses in the external layers. In these laminates the micro-structural and macro-structural reinforcement mechanisms would be activated. The analysis of dilatometry data was done to control the formation of processing defects and to evaluate the level of residual stresses. In order to assess the R-curve behaviour of the obtained materials during fracture, the indentation strength method has been used. Materials without R-curve behaviour display indentation load P−1/3 versus strength dependence.14,15 A deviation from that ideal P−1/3 dependence towards a plateau strength level indicates that R-curve mechanism is operative. The effect of composition of laminated structures on indentation strength behaviour (using loads between 10 and 400 N) is discussed in terms of the associated residual stresses. 2. Experimental Tapes for the production of the multilayer structures were cast from stable slurries of high purity -Al2O3 and Y-TZP powders in deionised water as dispersing media. The starting powders of -Al2O3 (Condea HPA 0.5, USA), with mean particle size of 0.35m and specific surface area of 9.5 m2/g, and a t-ZrO2 stabilized with 3 mol.% Y2O3 (TZ3YS, TOSOH, Japan), with a mean particle size of 0.4m and a specific surface area of 6.7 m2/g, were used. Two compositions, 95 vol.% of -Al2O3 0955-2219/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2006.04.081
J. Gurauskis et al /Journal of the European Ceramic Society 27 (2007)1389-1394 Table I Basic composition and solid loading of the tapes fabricated Tapes Composition(vol%) Solid content(vol% Green density(th. %o) a-Al203 Y-TZP A-5 551±01 with 5 vol. of t-ZrO2(named A-5)and 60 vol %o of a-Al2O3 elastic properties from the flexural and torsional mode vibra with 40 vol %o of t-ZrO2(named A-40)were selected For each tions. The pieces were placed on the support points allowing composition, two slurries with different solid content(Table 1) first and second natural vibrations of the disc and tested by were prepared in order to obtain the tapes with different green impact("GrindoSonic MK5", J.W. Lemmens-Electronica N.V. density values. Slurry stabilisation was performed with 0. 8 wt. Belgium). The Youngs modulus, E, the shear modulus, G, and (referred to solid content)polyelectrolyte Dolapix CE 64 and the Poisson,s ratio, v, were calculated from the values of the 5 wt. o of binder DM 765(referred to solid content) was added frequencies and the dimensions and densities of the samples to suspension to obtain sufficient elasticity of green tapes. After Laminated green pieces were machined into bars(50mm x casting, the green ceramic tapes were dried at room temperature 7 mm x 4 mm) and the surfaces were smoothed with sandpaper for 24 h and subsequently at 60C for 48 h. The final thickness before sintering. Polished cross-sections of sintered materials of the green tapes obtained varied between A480 and 520 um. were examined by optical( Carl-Zeiss H-Pl, Germany) and by Dried and round shaped (Mgreen =60mm) tapes were sub- scanning electron(Zeiss DSM-950, Germany) microscopy for merged in water for 1 min, coated with the gluing agent(water interface union defects and for tunnel type defects in the case of suspension of 5 wt. of DM765) and stacked to obtain the symmetrical laminate structures. laminated pieces after cold uniaxial pressing of 18 MPa. Full The sintered bars were ground to obtain the test samples experimental details of the green processing, gluing agent selec- with final geometry (40 mm x 4 mm x 3 mm). After grinding ion and pressing procedure are given elsewhere. Monolithic the laminate structures had symmetrical distribution of the lay pieces were formed by piling seven tapes of the same compo- ers, giving final thickness of A420 um for every constituent sitions while laminates were formed by piling seven tapes of layer in laminate LI and in the case of laminate L2 final different compositions thicknesses of≈420and≈840μm .5 and A-40 layers Two different distributions of tapes were selected in a way respectively to obtain two different symmetrical laminated architectures Four-point bending tests were carried out on non- with external A-5 layers. In one of them (Ll) four A-5 lay- indented and indented samples using universal testing machine ers were alternated with three A-40 layers of the same thickness (Microtest, Spain) with inner and outer spans of 15 and 30 mm, 500 um). In the other(L2)three A-5 layers(N500 um)were respectively. The bending load was applied at a constant alternated with two A40 layers of double thickness(1000 um). crosshead speed of 0.05 mm/min. The surface of the bars to Binder burn out(1C/min up to 600C with a dwell time of be in tension during testing was polished successively using 9. 30 min)and sintering(2C/min up to 1550C with a dwell time 6 and 1 um diamond paste and the edges were chamfered after of 2 h)were performed in a single thermal treatment the polishing operation. The apparent densities of the green and sintered monolithic Three bending tests for each laminated structure were per pieces were determined by the Archimedes method in mercury formed on non-indented bars to determine fracture strength nd water, respectively. Relative densities were calculated as %o values of the calculated theoretical density of the studied composition For indentation-strength tests. three Vickers indentations using 3.99 g/cm'for a-Al2O3(ASTM 42-1468)and 6.10 g/cm each 2.5 mm apart, were made in the central part of the tensile for Y-TZP(ASTM83-113) Dilatometric curves of green mono- surface of the beams. Vickers cracks were made with the nor lithic samples(5 mm x 5 mm x 4 mm)were recorded in a differ- mal orientation to the major axis of the beam. The indentations ential dilatometer with alumina support using platinum protec- were performed in controlled displacement mode at 0.01 mm/s ion( Setaram, Setsys-16/17, France)and corrected for alumina up to maximum load (10-400N) with a holding time of 10s. ansion.The thermal treatment cycle applied to the samples The indentation imprints were measured within 15 min using in the dilatometer was identical to the sintering cycle described optical microscopy. One test was performed at each maximum previously. The heating up cycle was used to control sintering load level. All fractured samples were examined with an optical shrinkage and the cooling down cycle and allowed to evalu- microscope to determine whether the fracture proceeded from ate the differences between the actual cooling contractions of the indentation. The rising crack growth behaviour(R-curve the monoliths. The samples were placed in dilatometer having was characterized using post-indentation strength-indentation the constituent tapes orientated to the normal direction to the load relation 4, 6 using the best-fit curves of the function sensor. Two monolithic pieces of each composition were sin- tered without any previous machining in order to determine the of=aP-B (1)
1390 J. Gurauskis et al. / Journal of the European Ceramic Society 27 (2007) 1389–1394 Table 1 Basic composition and solid loading of the tapes fabricated Tapes Composition (vol.%) Solid content (vol.%) Green density (th.%) -Al2O3 Y-TZP A-5 95 5 50 59.1±0.1 A-5(I) 47 56.2±0.1 A-40 60 40 47 53.5±0.1 A-40(1) 50 55.1±0.1 with 5 vol.% of t-ZrO2 (named A-5) and 60 vol.% of -Al2O3 with 40 vol.% of t-ZrO2 (named A-40) were selected. For each composition, two slurries with different solid content (Table 1) were prepared in order to obtain the tapes with different green density values. Slurry stabilisation was performed with 0.8 wt.% (referred to solid content) polyelectrolyte Dolapix CE 64 and 5 wt.% of binder DM 765 (referred to solid content) was added to suspension to obtain sufficient elasticity of green tapes. After casting, the green ceramic tapes were dried at room temperature for 24 h and subsequently at 60 ◦C for 48 h. The final thickness of the green tapes obtained varied between ≈480 and 520m. Dried and round shaped (∅green = 60 mm) tapes were submerged in water for 1 min, coated with the gluing agent (water suspension of 5 wt.% of DM765) and stacked to obtain the laminated pieces after cold uniaxial pressing of 18 MPa. Full experimental details of the green processing, gluing agent selection and pressing procedure are given elsewhere.12,13 Monolithic pieces were formed by piling seven tapes of the same compositions while laminates were formed by piling seven tapes of different compositions. Two different distributions of tapes were selected in a way to obtain two different symmetrical laminated architectures with external A-5 layers. In one of them (L1) four A-5 layers were alternated with three A-40 layers of the same thickness (≈500m). In the other (L2) three A-5 layers (≈500m) were alternated with two A40 layers of double thickness (≈1000m). Binder burn out (1 ◦C/min up to 600 ◦C with a dwell time of 30 min) and sintering (2 ◦C/min up to 1550 ◦C with a dwell time of 2 h) were performed in a single thermal treatment. The apparent densities of the green and sintered monolithic pieces were determined by the Archimedes method in mercury and water, respectively. Relative densities were calculated as % of the calculated theoretical density of the studied composition, using 3.99 g/cm3 for -Al2O3 (ASTM 42-1468) and 6.10 g/cm3 for Y-TZP (ASTM 83-113). Dilatometric curves of green monolithic samples (5 mm × 5 mm × 4 mm) were recorded in a differential dilatometer with alumina support using platinum protection (Setaram, Setsys-16/17, France) and corrected for alumina expansion. The thermal treatment cycle applied to the samples in the dilatometer was identical to the sintering cycle described previously. The heating up cycle was used to control sintering shrinkage and the cooling down cycle and allowed to evaluate the differences between the actual cooling contractions of the monoliths. The samples were placed in dilatometer having the constituent tapes orientated to the normal direction to the sensor. Two monolithic pieces of each composition were sintered without any previous machining in order to determine the elastic properties from the flexural and torsional mode vibrations. The pieces were placed on the support points allowing first and second natural vibrations of the disc and tested by impact (“GrindoSonic MK5”, J.W. Lemmens-Electronica N.V., Belgium). The Young’s modulus, E, the shear modulus, G, and the Poisson’s ratio, ν, were calculated from the values of the frequencies and the dimensions and densities of the samples. Laminated green pieces were machined into bars (50 mm × 7 mm × 4 mm) and the surfaces were smoothed with sandpaper before sintering. Polished cross-sections of sintered materials were examined by optical (Carl-Zeiss H-P1, Germany) and by scanning electron (Zeiss DSM-950, Germany) microscopy for interface union defects and for tunnel type defects in the case of symmetrical laminate structures. The sintered bars were ground to obtain the test samples with final geometry (40 mm × 4 mm × 3 mm). After grinding, the laminate structures had symmetrical distribution of the layers, giving final thickness of ≈420m for every constituent layer in laminate L1 and in the case of laminate L2 final thicknesses of ≈420 and ≈840m for A-5 and A-40 layers, respectively. Four-point bending tests were carried out on nonindented and indented samples using universal testing machine (Microtest, Spain) with inner and outer spans of 15 and 30 mm, respectively. The bending load was applied at a constant crosshead speed of 0.05 mm/min. The surface of the bars to be in tension during testing was polished successively using 9, 6 and 1 m diamond paste and the edges were chamfered after the polishing operation. Three bending tests for each laminated structure were performed on non-indented bars to determine fracture strength values. For indentation-strength tests, three Vickers indentations, each 2.5 mm apart, were made in the central part of the tensile surface of the beams. Vickers cracks were made with the normal orientation to the major axis of the beam. The indentations were performed in controlled displacement mode at 0.01 mm/s up to maximum load (10–400 N) with a holding time of 10 s. The indentation imprints were measured within 15 min using optical microscopy. One test was performed at each maximum load level. All fractured samples were examined with an optical microscope to determine whether the fracture proceeded from the indentation. The rising crack growth behaviour (R-curve) was characterized using post-indentation strength–indentation load relation14,16 using the best-fit curves of the function: σf = αP−β (1)
J. Gurauskis et al. Journal of the European Ceramic Society 27(2007)1389-1394 Fig. 1. Transversal cracking observed in symmetrical laminates. (a) Defect pattern in LI system.(b)Defect pattern in L2 system.(c) Detail of the defect shown in where, af is the post-indentation strength, P the indentation would limit the range of tape compositions to be used. An alter load, a and B are coefficients describing R-curve behaviour. The native approach would be to modify the colloidal processing standard deviation values from the best-fit curves were used to conditions to obtain tapes with compatible green densities. To calculate 95% confidence intervals explore this suggestion, the dilatometric curves recorded during sintering of the monoliths were analyzed(Fig 2a). Shrinkage 3. Results and discussion started at similar temperatures(1050C) for both monoliths but around 1150oC significant differences in shrinkage rate In previous works it was demonstrated that crack free and appeared leading to shrinkage differences that increased with homogeneous monolithic materials with the compositions A-5 temperature. Final shrinkages before the isothermal cycle at and A-40 starting from seven stacked tapes could be obtained 1550C were 17% and 22% for A-5 and A-40, respectively. employing the processing route described above. 2,3 More- During co-sintering of the layers in the laminated structures this over, no residual stresses were observed in those monoliths. 7 shrinkage difference would lead to the development of densi- Tapes obtained using the optimum colloidal parameters(plas- fication stresses. According to Cai et al., these densification ticity, green density, etc. for each suspension, led to different stresses cause formation of voids that work as nucleation sites een densities of 59. 1% and 53.5% for A-5 and A-40, respec- for transversal cracking, as that shown in Fig. I tively. When these tapes were combined to fabricate symmetrical llowing these observations, the colloidal processing laminates with structures LI and L2 the cross-sections showed parameters of the tapes, in particular the suspension solid load the presence of large transverse cracks though the internal A- were modified to obtain tapes with similar green densities 10 layers and extending(10-30 um)into the contiguous A-5 for both compositions that will be named A-5 and A-40 layers(Fig. la and b). These cracks had large opening displace- (Table 1). The dilatometric curves of monoliths fabricated using ments(20 um)and smooth crack surfaces with rounded grains these tapes with modified compositions(A-5 and A-40()are (Fig. Ic), suggesting that crack formation took place during shown in Fig 2b. Initiation of shrinkage took place at lower tem- sintering. As reported previously, 18, 19 differential sintering of perature for A-5() than for A-40,and differences in shrinkage the layers in ceramics is primary related to density differences. levels were maintained up to 1350C. From this temperature, This could be the case of this layered system fabricated using both compositions reached similar shrinkage levels and rates, apes with significantly different green densities. Caiet al. pro- giving final shrinkage mismatch before the high temperature posed to modify the composition of the solid components of the dwell less than N1%. These changes in the sintering kinetics tapes to reach similar green densities. However, this approach of A-5(and A-40) monoliths, suggested the compatibility of
J. Gurauskis et al. / Journal of the European Ceramic Society 27 (2007) 1389–1394 1391 Fig. 1. Transversal cracking observed in symmetrical laminates. (a) Defect pattern in L1 system. (b) Defect pattern in L2 system. (c) Detail of the defect shown in (a). where, σf is the post-indentation strength, P the indentation load, α and β are coefficients describing R-curve behaviour. The standard deviation values from the best-fit curves were used to calculate 95% confidence intervals. 3. Results and discussion In previous works it was demonstrated that crack free and homogeneous monolithic materials with the compositions A-5 and A-40 starting from seven stacked tapes could be obtained employing the processing route described above.12,13 Moreover, no residual stresses were observed in those monoliths.17 Tapes obtained using the optimum colloidal parameters (plasticity, green density, etc.) for each suspension, led to different green densities of 59.1% and 53.5% for A-5 and A-40, respectively. When these tapes were combined to fabricate symmetrical laminates with structures L1 and L2 the cross-sections showed the presence of large transverse cracks though the internal A- 40 layers and extending (≈10–30m) into the contiguous A-5 layers (Fig. 1a and b). These cracks had large opening displacements (≥20m) and smooth crack surfaces with rounded grains (Fig. 1c), suggesting that crack formation took place during sintering. As reported previously,2,18,19 differential sintering of the layers in ceramics is primary related to density differences. This could be the case of this layered system fabricated using tapes with significantly different green densities. Cai et al.18 proposed to modify the composition of the solid components of the tapes to reach similar green densities. However, this approach would limit the range of tape compositions to be used. An alternative approach would be to modify the colloidal processing conditions to obtain tapes with compatible green densities. To explore this suggestion, the dilatometric curves recorded during sintering of the monoliths were analyzed (Fig. 2a). Shrinkage started at similar temperatures (≈1050 ◦C) for both monoliths but around 1150 ◦C significant differences in shrinkage rate appeared leading to shrinkage differences that increased with temperature. Final shrinkages before the isothermal cycle at 1550 ◦C were 17% and 22% for A-5 and A-40, respectively. During co-sintering of the layers in the laminated structures this shrinkage difference would lead to the development of densi- fication stresses. According to Cai et al.,18 these densification stresses cause formation of voids that work as nucleation sites for transversal cracking, as that shown in Fig. 1. Following these observations, the colloidal processing parameters of the tapes, in particular the suspension solid load, were modified to obtain tapes with similar green densities for both compositions that will be named A-5(I) and A-40(I) (Table 1). The dilatometric curves of monoliths fabricated using these tapes with modified compositions (A-5(I) and A-40(I)) are shown in Fig. 2b. Initiation of shrinkage took place at lower temperature for A-5(I) than for A-40(I), and differences in shrinkage levels were maintained up to 1350 ◦C. From this temperature, both compositions reached similar shrinkage levels and rates, giving final shrinkage mismatch before the high temperature dwell less than ≈1%. These changes in the sintering kinetics of A-5(I) and A-40(I) monoliths, suggested the compatibility of
1392 J. Gurauskis et al /Journal of the European Ceramic Society 27 (2007)1389-1394 0.0 00 2.0×104 60×104 上8.0x10 (a) Temperature('C) Fig. 4. SEM micrograph, showing interface zone between A-5) and A-40 2.0x10 tapes. Good and uniform union between layers is observed. .10 ng levels and rates in A-5( and A-40() before acceleration occurred(Fig. 2b). Furthermore, the A-40Q)monolith reached 6.0x10s the high temperature dwell (1550C)almost dense(total shrink -0.20 age during dwell time <1%, Fig. 6a), while the A-5()mon lith experienced significant shrinkage (3%0, Fig 6a) during the isothermal treatment; thus, the layers with composition A-40 800900100011001200130014001500 would be subjected to tensile densification stresses during the Temperature(C) final sintering period of the laminates. Nevertheless, none of these features of the sintering behaviour led to cracking of the Fig. 2. Dilatometer and sintering kinetics curves for monolithic samples. (a) laminates constituted lese layers for which the tempera Samples with different green densities. (b)Samples with adjusted green densi ture ranges for acceleration towards maximum sintering rates were coincident(Fig. 2b). On the contrary, the mentioned perature ranges for monoliths A-5 and A-40 were significantly tapes with similar green densities for co-sintering in laminated different(1200-1400C and 1350-1450C for A-5 and A-40, structures. Cross section observation of the sintered laminates respectively, Fig. 2a) and transverse cracking took place in the fabricated from monoliths with the modified green densities(A- laminates constituted by these tapes(Fig. 1) 50)and A-40()showed defect free structures( Fig 3)with good All specimens for testing were fabricated using the optimized interface union(Fig. 4) between the constituent tapes tapes A-50 and A-40()(Table 1). The elastic properties of These results demonstrate that elimination of densification monolithic samples are given in Table 2. stresses in the temperature range in which acceleration towards Fig 5 plots the indentation strength-indentation load relation naximum sintering rates of the layers occurs is the most impor- ships for laminated specimens of systems LI and L2 Strength tant for these laminated structures. During the initial stage of values were significantly higher(22%) for L2 than for LI densification, there were significant differences between sinter- through the whole range of indentation loads which shows that 40 56g Fig 3. SEM micrographs at the polished cross-section showing the structure of symmetrical laminates. (a) Laminate LI.(b)Laminate L2
1392 J. Gurauskis et al. / Journal of the European Ceramic Society 27 (2007) 1389–1394 Fig. 2. Dilatometer and sintering kinetics curves for monolithic samples. (a) Samples with different green densities. (b) Samples with adjusted green densities. tapes with similar green densities for co-sintering in laminated structures. Cross section observation of the sintered laminates fabricated from monoliths with the modified green densities (A- 5(I) and A-40(I)) showed defect free structures (Fig. 3) with good interface union (Fig. 4) between the constituent tapes. These results demonstrate that elimination of densifications stresses in the temperature range in which acceleration towards maximum sintering rates of the layers occurs is the most important for these laminated structures. During the initial stage of densification, there were significant differences between sinterFig. 4. SEM micrograph, showing interface zone between A-5(I) and A-40(I) tapes. Good and uniform union between layers is observed. ing levels and rates in A-5(I) and A-40(I) before acceleration occurred (Fig. 2b). Furthermore, the A-40(I) monolith reached the high temperature dwell (1550 ◦C) almost dense (total shrinkage during dwell time <1%, Fig. 6a), while the A-5(I) monolith experienced significant shrinkage (3%, Fig. 6a) during the isothermal treatment; thus, the layers with composition A-40(I) would be subjected to tensile densification stresses during the final sintering period of the laminates. Nevertheless, none of these features of the sintering behaviour led to cracking of the laminates constituted by these layers for which the temperature ranges for acceleration towards maximum sintering rates were coincident (Fig. 2b). On the contrary, the mentioned temperature ranges for monoliths A-5 and A-40 were significantly different (≈1200–1400 ◦C and 1350–1450 ◦C for A-5 and A-40, respectively, Fig. 2a) and transverse cracking took place in the laminates constituted by these tapes (Fig. 1). All specimens for testing were fabricated using the optimized tapes A-5(I) and A-40(I) (Table 1). The elastic properties of monolithic samples are given in Table 2. Fig. 5 plots the indentation strength-indentation load relationships for laminated specimens of systems L1 and L2. Strength values were significantly higher (≈22%) for L2 than for L1 through the whole range of indentation loads, which shows that Fig. 3. SEM micrographs at the polished cross-section showing the structure of symmetrical laminates. (a) Laminate L1. (b) Laminate L2
J. Gurauskis et al /Journal of the European Ceramic Society 27 (2007)1389-1394 1393 rial properties determined from monolithic pieces Youngs modulus(GPa) Shear modulus(GPa) 403=002 4.79=001 309±2 0.26+001 the laminate structure increased the strength values. In a prev In order to determine AE, the cooling part of the sinte ous work, the distribution of residual stresses across the section ing cycle from 1200C to room temperature was analyzed of LI specimens was determined by the piezo-spectroscopic This temperature range was chosen because no accommoda technique. Compressive stresses were found to develop in the tion of deformation mismatch by diffusional processes occurs external alumina layers, smaller at the surface and increas- in alumina materials at temperatures lower than 1200C.In ing with depth up to maximum values close to the interface Fig. 6b the analyzed dilatometric curves are plotted. Equal between these layers and the contiguous A-40 layers. Maximum levels of deformation for both compositions at 1200C have stress values determined at the interior of the specimens were been assumed because the differences in shrinkage betw of the same order as those calculated using the simplified model the layers in the laminates would be accommodated by S190 MPa) of a symmetric plate constituted by alternate lay- diffusional processes at high temperatures. From this fig ers of the same thickness having a uniform biaxial distribution ure, AE=EA-5-EA-40=0.9 x 10, which gives compressive of stresses across each layer. Using this approach, the arising stresses in the A-5 layers of -187 and -268 MPa for esidual stresses within A-5 and A-40 composition layers are LI and L2 structure, respectively. The higher strength val- ues found for L2 as compared to LI can be explained AEEA +(EA-snA-shA-5/EA-4n A-40h A-40) nA-40 hA-40 -0.20 where As is the strain mismatch between the layers, 1A-5A-40 Shrinkage during dwell time and hA-5 A-40 are the number and the thickness of the layers respectively, and EA.5.A-40 is the reduced Young's modulu -023 2004006008001000120014001600 Slope 0.09 2.6 300 -2,0x10 lope 0.33 023 200 8 .0x10 Alumina 95% Conf, band 80×106 Log Indentation Load (N) Fig. 5. Logarithm of observed post-indentation strength versus the logarithm of Temperature(°C) indentation load for the laminate systems studied. The solid lines are linear fit values(Eg.(1)and the dashed lines are 95% confidence limit. The correspond- Fig. 6. Dilatometer curves corresponding to A-5) and A-40 composition ing fracture strength data versus indentation load scales represented too. As a samples with similar densities.(a) Data of isothermal cycle(dwell time).(b) term of comparison data for fine-grained alumina 4 are also represented Data of cooling down cycle
J. Gurauskis et al. / Journal of the European Ceramic Society 27 (2007) 1389–1394 1393 Table 2 Material properties determined from monolithic pieces Monolithic material ρsintered Young’s modulus (GPa) Shear modulus (GPa) Poisson coefficient g/cm3 th.% A-5(I) 4.03±0.02 98.7±0.1 389±4 155±2 0.25±0.01 A-40(I) 4.79±0.01 99.2±0.1 309±2 121±2 0.26±0.01 the laminate structure increased the strength values. In a previous work, the distribution of residual stresses across the section of L1 specimens was determined by the piezo-spectroscopic technique.9 Compressive stresses were found to develop in the external alumina layers, smaller at the surface and increasing with depth up to maximum values close to the interface between these layers and the contiguous A-40 layers. Maximum stress values determined at the interior of the specimens were of the same order as those calculated using the simplified model (≈190 MPa) of a symmetric plate constituted by alternate layers of the same thickness having a uniform biaxial distribution of stresses across each layer.20 Using this approach, the arising residual stresses within A-5 and A-40 composition layers are given by: σA-5 = − εE A-5 1 + (E A-5nA-5hA-5/E A-40nA-40hA-40) (2) σA-40 = −σA-5 nA-5 nA-40 hA-5 hA-40 (3) where ε is the strain mismatch between the layers, nA-5, A-40 and hA-5, A-40 are the number and the thickness of the layers, respectively, and E A-5, A-40 is the reduced Young’s modulus: E i = Ei 1 − νi (4) Fig. 5. Logarithm of observed post-indentation strength versus the logarithm of indentation load for the laminate systems studied. The solid lines are linear fit values (Eq. (1)) and the dashed lines are 95% confidence limit. The corresponding fracture strength data versus indentation load scales represented too. As a term of comparison data for fine-grained alumina14 are also represented. In order to determine ε, the cooling part of the sintering cycle from 1200 ◦C to room temperature was analyzed. This temperature range was chosen because no accommodation of deformation mismatch by diffusional processes occurs in alumina materials at temperatures lower than 1200 ◦C.2 In Fig. 6b the analyzed dilatometric curves are plotted. Equal levels of deformation for both compositions at 1200 ◦C have been assumed because the differences in shrinkage between the layers in the laminates would be accommodated by diffusional processes at high temperatures. From this figure, ε = εA-5 − εA-40 = 0.9 × 10−6, which gives compressive stresses in the A-5(I) layers of −187 and −268 MPa for L1 and L2 structure, respectively. The higher strength values found for L2 as compared to L1 can be explained Fig. 6. Dilatometer curves corresponding to A-5(I) and A-40(I) composition samples with similar densities. (a) Data of isothermal cycle (dwell time). (b) Data of cooling down cycle
1394 J. Gurauskis et al /Journal of the European Ceramic Society 27 (2007)1389-1394 by the difference in residual stresses (80 MPa) which is References of the same order as differences between strength values (100 MPa) though the whole range of indentation loads 1. Russo, C J, Harmer, M P, Chan, H M. and Miller, G A Design of laminated ceramic composite for improved strength and toughness. J. Am Unlike strength levels, similar B values(Eq (1), within the 2. Cerm.Soc.1992,75(12),3396-3400. 2. Hillman, C, Suo, Z. G and Lange, F. F, Cracking of laminates subjected 95%o confidence limits, were found for both systems. These val- to biaxial tensile stresses. J Am. Ceram. Soc., 1996. 79(8). 2127-2133 ues were much lower than the corresponding for fine grained 3. Sanchez-Herencia, A. J, Gurauskis, J. and Baudin, C, Processing of alumina, for which the coefficient B value is 0.33(indenta- AlzO3/Y-TZP laminates from water-based cast tapes. Compos. B, in press tion load P- versus strength dependence), which reveals available online April 4. 2006]. similar R-curve behaviour during fracture of both laminated 4. Green. D. J. Cai, P. Z. and Messing. G. L, Residual stresses in structures, in spite of the level of residual stresses. Taking into alumina-zirconia laminates. J. Eur Ceram. Soc., 1999, 19(13-14). 2511-2517 account the actual distribution of compressive residual stresses 5.Rao, MP, Sanchez-Herencia, AJ,Beltz,GE.McMeeking,RMand through the external alumina layer, the origin of the observed Lange, F. F. Laminar ceramics that exhibit a threshold strength. Science, R-curve behaviour can be envisaged as follows For small inden 1999,286(5437),102-105 (33 um) are located at the zone of the specimens with O G tation loads(e.g 10 N)the cracks associated to the indentation 6. Sanchez-Herencia, A J, James, L and Lange, F. F, Bifurcation in alumina plates produced by a phase transformation in central, alumina/zirconia thin layers. J. Eur. Ceram Soc., 2000, 20(9), 1297-1300 residual stresses. As the indentation load increases, the larger 7. Chartier, T and Rouxel, T, Tape-cast alumina-zirconia laminates: process- cracks associated are subjected to increasing residual stresses, ing and mechanical properties. J. Eur. Ceram. Soc., 1997, 17(2-3), 299- which leads to increasing apparent toughness. For the maximum load used(400N), the crack(320 um) has to overcome 8. Cai, P. Z, Green, D. J. and Messing, G. L, Mechanical character tion of AlO3/ZrO hybrid laminates. J. Eur. Ceram Soc., 1998, 18 high residual stresses found close to the interface(420 um) 2025-2034. the characteristics of the process depend om do This process occurs in a similar way for both syster 9. de Portu. G. Gurauskis J, Micele. L, Sanchez-Herencia, A J, Baudin, C. and Pezzotti, G, PieZo-spectroscopic characterization of alumina-zirconia of the outer alumina layer and not on the level of layered composites. J Mater. Sci, in press [available online April 10, 2006] 10. Claussen, N, Stress-induced transformation of tetragonal ZrO2 particles in stresses ceramic matrices. J. Am. Ceram. Soc 1978. 61(1-2), 85-86 I1. Szutkowska, M, Fracture resistance behavior of alumina-zirconia compos- 4. Conclusions ites. J. Mater Proc. Technol. 2004. 153-54. 868-874 12. Gurauskis, J, Sanchez-Herencia, A.J. and Baudin, C, Joining green cerar Defect free laminated ceramics with strong interfaces and tapes made from water-based slurries by applying low pressures at ambient temperature. J. Eur Ceram. Soc., 2005, 25(15), 3403-341 high level of residual stresses can be fabricated from green pieces 13. Gurauskis, J, Sanchez-Herencia, A J and Baudin, C, Al,O,/Y-TZP and obtained by stacking water processed tape cast tapes. The match- Y-TZP materials fabricated by stacking layers obtained by aqueous tape ing of density of the constituent tapes is fundamental to allow asting. J. Eur Ceran. Soc. 2006. 26(8), 1489-1496 co-sintering of the different layers 14. Chantikul. P. Anstis. G. R. Lawn. B. R. and Marshall. D. B. A critical- The strength levels obtained with a particular laminated struc- Strength method. J. Am. Ceram. Soc., 1981, 64(9), 539-543. ture are determined by the actual values of the residual stresses. 15. Chantikul, P, Bennison, S J and Lawn, B. R Role of grain-size in the The relationship between the crack size and its resistance to rength and r-curve properties of alumina. J. Am. Ceram. Soc., 1990. 73(8). grow is determined by the distribution of these stresses through 2419-2427 the external layer 16. Krause, R. F, Rising fracture-toughness from the bending strength of indented alumina beams. J. Am. Ceram. Soc., 1988. 71(5).338- Acknowledgments 17. Ruiz-Hervias, J. Bruno, G, Gurauskis, J, Sanchez-Herencia, A. J. and Baudin, C, Neutron diffraction investigation for possible anisotropy within This work was supported by the projects CICYT MAT 2003 monolithic Al2O3/Y-TZP composites fabricated by stacking together cast tapes. Scr Mate,2006,54(6,1133-1137 00836 and CAM GR MATo7072004(Spain) 18. Cai, P. Z, Green, D J and Messing. G. L, Constrained densification of alu- Work supported in part by the European Community mina/zirconia hybrid laminates. 1. Experimental observations of processing Human Potential Programme under contract HPRN-CT-2002 defects. J. Am. Ceram Soc., 1997, 80(8), 1929-1939 00203 SICMAC]. 19. Cai, P. Z, Green, D. J. and Messing. G. L, Constrained densification of Jonas Gurauskis acknowledges the financial support pro- alumina/zirconia hybrid laminates. 2. Viscoelastic stress computation. Anm ceran.Soc.,1997,80(8),1940-1948. vided through the European Community's Human Potential 20. Chartier, T, Merle, D and Besson, J. L, Laminar ceramic composites Programme under contract HPRN-CT-2002-00203 [ SICMAC] Eur Ceran.Soc,1995,15(2),101-107
1394 J. Gurauskis et al. / Journal of the European Ceramic Society 27 (2007) 1389–1394 by the difference in residual stresses (≈80 MPa) which is of the same order as differences between strength values (≈100 MPa) though the whole range of indentation loads (Fig. 5). Unlike strength levels, similar β values (Eq. (1)), within the 95% confidence limits, were found for both systems. These values were much lower than the corresponding for fine grained alumina, for which the coefficient β value is 0.33 (indentation load P−1/3 versus strength dependence), which reveals similar R-curve behaviour during fracture of both laminated structures, in spite of the level of residual stresses. Taking into account the actual distribution of compressive residual stresses through the external alumina layer, the origin of the observed R-curve behaviour can be envisaged as follows. For small indentation loads (e.g.: 10 N) the cracks associated to the indentations (≈33m) are located at the zone of the specimens with low residual stresses. As the indentation load increases, the larger cracks associated are subjected to increasing residual stresses, which leads to increasing apparent toughness. For the maximum load used (400 N), the crack (≈320m) has to overcome the high residual stresses found close to the interface (≈420m). This process occurs in a similar way for both systems, because the characteristics of the process depend on the thickness of the outer alumina layer and not on the level of residual stresses. 4. Conclusions Defect free laminated ceramics with strong interfaces and high level of residual stresses can be fabricated from green pieces obtained by stacking water processed tape cast tapes. The matching of density of the constituent tapes is fundamental to allow co-sintering of the different layers. The strength levels obtained with a particular laminated structure are determined by the actual values of the residual stresses. The relationship between the crack size and its resistance to grow is determined by the distribution of these stresses through the external layer. Acknowledgments This work was supported by the projects CICYT MAT 2003- 00836 and CAM GR MAT07072004 (Spain). Work supported in part by the European Community’s Human Potential Programme under contract HPRN-CT-2002- 00203 [SICMAC]. Jonas Gurauskis acknowledges the financial support provided through the European Community’s Human Potential Programme under contract HPRN-CT-2002-00203 [SICMAC]. References 1. Russo, C. J., Harmer, M. P., Chan, H. M. and Miller, G. A., Design of laminated ceramic composite for improved strength and toughness. J. Am. Ceram. Soc., 1992, 75(12), 3396–3400. 2. Hillman, C., Suo, Z. G. and Lange, F. F., Cracking of laminates subjected to biaxial tensile stresses. J. Am. Ceram. Soc., 1996, 79(8), 2127–2133. 3. Sanchez-Herencia, A. J., Gurauskis, J. and Baud ´ ´ın, C., Processing of Al2O3/Y-TZP laminates from water-based cast tapes. Compos. B, in press [available online April 4, 2006]. 4. Green, D. J., Cai, P. Z. and Messing, G. L., Residual stresses in alumina–zirconia laminates. J. Eur. Ceram. Soc., 1999, 19(13–14), 2511–2517. 5. Rao, M. P., Sanchez-Herencia, A. J., Beltz, G. E., McMeeking, R. M. and Lange, F. F., Laminar ceramics that exhibit a threshold strength. Science, 1999, 286(5437), 102–105. 6. Sanchez-Herencia, A. J., James, L. and Lange, F. F., Bifurcation in alumina plates produced by a phase transformation in central, alumina/zirconia thin layers. J. Eur. Ceram. Soc., 2000, 20(9), 1297–1300. 7. Chartier, T. and Rouxel, T., Tape-cast alumina–zirconia laminates: processing and mechanical properties. J. Eur. Ceram. Soc., 1997, 17(2–3), 299– 308. 8. Cai, P. Z., Green, D. J. and Messing, G. L., Mechanical characterization of Al2O3/ZrO2 hybrid laminates. J. Eur. Ceram. Soc., 1998, 18(14), 2025–2034. 9. de Portu, G., Gurauskis, J., Micele, L., Sanchez-Herencia, A. J., Baudin, C. and Pezzotti, G., Piezo-spectroscopic characterization of alumina–zirconia layered composites. J. Mater. Sci., in press [available online April 10, 2006]. 10. Claussen, N., Stress-induced transformation of tetragonal ZrO2 particles in ceramic matrices. J. Am. Ceram. Soc., 1978, 61(1–2), 85–86. 11. Szutkowska, M., Fracture resistance behavior of alumina–zirconia composites. J. Mater. Proc. Technol., 2004, 153–54, 868–874. 12. Gurauskis, J., Sanchez-Herencia, A. J. and Baudin, C., Joining green ceramic tapes made from water-based slurries by applying low pressures at ambient temperature. J. Eur. Ceram. Soc., 2005, 25(15), 3403–3411. 13. Gurauskis, J., Sanchez-Herencia, A. J. and Baudin, C., Al ´ 2O3/Y-TZP and Y-TZP materials fabricated by stacking layers obtained by aqueous tape casting. J. Eur. Ceram. Soc., 2006, 26(8), 1489–1496. 14. Chantikul, P., Anstis, G. R., Lawn, B. R. and Marshall, D. B., A criticalevaluation of indentation techniques for measuring fracture-toughness. 2. Strength method. J. Am. Ceram. Soc., 1981, 64(9), 539–543. 15. Chantikul, P., Bennison, S. J. and Lawn, B. R., Role of grain-size in the strength and R-curve properties of alumina. J. Am. Ceram. Soc., 1990, 73(8), 2419–2427. 16. Krause, R. F., Rising fracture-toughness from the bending strength of indented alumina beams. J. Am. Ceram. Soc., 1988, 71(5), 338– 343. 17. Ruiz-Herv´ıas, J., Bruno, G., Gurauskis, J., Sanchez-Herencia, A. J. and ´ Baud´ın, C., Neutron diffraction investigation for possible anisotropy within monolithic Al2O3/Y-TZP composites fabricated by stacking together cast tapes. Scr. Mater., 2006, 54(6), 1133–1137. 18. Cai, P. Z., Green, D. J. and Messing, G. L., Constrained densification of alumina/zirconia hybrid laminates. 1. Experimental observations of processing defects. J. Am. Ceram. Soc., 1997, 80(8), 1929–1939. 19. Cai, P. Z., Green, D. J. and Messing, G. L., Constrained densification of alumina/zirconia hybrid laminates. 2. Viscoelastic stress computation. J. Am. Ceram. Soc., 1997, 80(8), 1940–1948. 20. Chartier, T., Merle, D. and Besson, J. L., Laminar ceramic composites. J. Eur. Ceram. Soc., 1995, 15(2), 101–107.
