《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_Fabrication and performance of Al2O3-TiC

MSA-25534: No of Pages 9 ARTICLE IN PRESS Materials Science and Engineering A XXx(2009)xXx-XXX Contents lists available at Science Direct Materials Science and engineering A ELSEVIER journalhomepagewww.elsevier.com/locate/msea Fabrication and performance of Al2O3/W, T1)C+ Al2O3/TiC multilayered ceramic cutting tools Deng Jianxin*, Duan Zhenxing, Yun Dongling, Zhang Hui, Ai Xing, Zhao Jun School of Mechanical Engineering, Shandong University, Jinan 250061, Shandong Province, PR China ARTICLE INFO A BSTRACT Al2O3/(W, Ti)C+AlO3/TiC multilayered ceramic cutting tools with different thickness ratios were pro duced by hot pressing. The residual stresses inside the layered materials during fabrication were in revised form 8 September 2009 calculated by means of the finite element method. The mechanical properties at the outer layer of the online xxx yered materials were measured. The cutting performance of the layere compared with an unstressed reference tool. Results showed that multilayered structure in AWT+At layered ceramic materials can induce excess residual stresses during fabrication. These residual stresses Ceramic tools yered materials outer layer of the layered ceramic materials is greatly improved compared with that of the stress-free one. esidual stress These multilayered tools can minimize the flank wear and edge chipping compared with the stress-free tool. The mechanisms responsible were determined to be the formation of compressive residual stress on the outer layer of the layered tools, which led to an increase in resistance to fracture. Thickness ratios were found to have a profound effect on the residual stresses, the fracture toughness, and the cutting performance of the layered tools o 2009 Elsevier B V. All rights reserved. 1. Introduction and offer advantages with respect to friction and wear behaviors Ceramic cutting tools usually perform better in high speed In the past decade, layered ceramics that take advantage of machining and in the machining of high hardness workpiece mate- several strengthening mechanisms, such as crack deflection [7 rials as compared to high-speed steel and carbide tools. However, independence on the initial flaw[8, introduction of weakinterfaces the use of Al2O3 ceramic cutting tools, even fully densified, may 9], containment of martensitic transformation 10, or existence limited by their properties, such as their low strength and frac- of porous layers [11, have been studied extensively. Among them. ture toughness. Since about 1970, Al2 O3 ceramic cutting tools have one of the most common mechanisms is the incorporation of resid- improved remarkably. These improvements are mainly due to[ 1, 2: ual stresses arising from thermal expansion coefficients mismatch, (1)microstructures have been refined by controlling and improving so that the surface layer is under compression. Several mod manufacturing processes: (2)toughening mechanisms have been els and theoretical calculations, and experimental measurement developed, such as whisker toughening and transformation tough- methods have been proposed [8, 12-16 to determine the stress ening, thus improving the fracture toughness of ceramic tools: amount and distribution in laminated structures. These compres and(3)surfaces have been conditioned by the removal of cracks sive stresses can increase the fracture strength, damage resistance, and irregularities. Considerable improvements have been achieved fatigue properties as well as fracture toughness of layered ceram- in tool properties such as flexural strength, fracture toughness, ics (16-28]. The effectiveness of laminated structures in improving hardness, and wear resistance by incorporating one or more other the tribological properties has been reported by tarlazzi et al components into the Al2O3 base material to form ceramic com- [29, 30]. Toschi et al. [31 showed that laminated hybrid struc posite tool materials. The reinforcing component is often in the tures can improve the sliding wear resistance of alumina Portu shape of particles or whiskers. Some of these tool materials, such et al. [32 reported that laminated structure with compressive Al2O3/TiC, Al2O3/TiB2, Al2O3/ZrO2, AlO3/Ti(CN), Al2O3/(WTiC. residual stresses within the surface regions was a suitable and Al2O3/SiCw, have been used in various machining applications to obtain composite materials with superior abrasive wear resis- tance. Deng et al. [33-38 suggested that layered structures in ceramic nozzles can improve their erosion wear resistance in abra- +8653188392047 sive air-jet machining. Nicola [39 produced an alumina/zirconia E-mailaddress:jxdeng@sdu.edu.cn(DJianxin). laminated cutting tool and found that laminated structures are 0921-5093/ S-see front matter o 2009 Elsevier B V. All rights reserved. doi:10.016msea2009.09020 Please cite this article in press as: D Jianxin, et al, Mater Sci Eng. A(2009). doi: 10.1016/j. msea. 2009.09.020

Please cite this article in press as: D. Jianxin, et al., Mater. Sci. Eng. A (2009), doi:10.1016/j.msea.2009.09.020 ARTICLE IN PRESS GModel MSA-25534; No. of Pages 9 Materials Science and Engineering A xxx (2009) xxx–xxx Contents lists available at ScienceDirect Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea Fabrication and performance of Al2O3/(W,Ti)C + Al2O3/TiC multilayered ceramic cutting tools Deng Jianxin∗, Duan Zhenxing, Yun Dongling, Zhang Hui, Ai Xing, Zhao Jun School of Mechanical Engineering, Shandong University, Jinan 250061, Shandong Province, PR China article info Article history: Received 4 August 2009 Received in revised form 8 September 2009 Accepted 8 September 2009 Available online xxx Keywords: Ceramic tools Layered materials Residual stress Al2O3 abstract Al2O3/(W,Ti)C + Al2O3/TiC multilayered ceramic cutting tools with different thickness ratios were pro￾duced by hot pressing. The residual stresses inside the layered materials during fabrication were calculated by means of the finite element method. The mechanical properties at the outer layer of the layered materials were measured. The cutting performance of the layered tools was investigated and compared with an unstressed reference tool. Results showed that multilayered structure in AWT + AT layered ceramic materials can induce excess residual stresses during fabrication. These residual stresses are compressive in the AWT outer layer and tensile in the AT internal layer. The fracture toughness at the outer layer of the layered ceramic materials is greatly improved compared with that of the stress-free one. These multilayered tools can minimize the flank wear and edge chipping compared with the stress-free tool. The mechanisms responsible were determined to be the formation of compressive residual stress on the outer layer of the layered tools, which led to an increase in resistance to fracture. Thickness ratios were found to have a profound effect on the residual stresses, the fracture toughness, and the cutting performance of the layered tools. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Ceramic cutting tools usually perform better in high speed machining and in the machining of high hardness workpiece mate￾rials as compared to high-speed steel and carbide tools. However, the use of Al2O3 ceramic cutting tools, even fully densified, may be limited by their properties, such as their low strength and frac￾ture toughness. Since about 1970, Al2O3 ceramic cutting tools have improved remarkably. These improvements aremainly due to [1,2]: (1) microstructures have been refined by controlling and improving manufacturing processes; (2) toughening mechanisms have been developed, such as whisker toughening and transformation tough￾ening, thus improving the fracture toughness of ceramic tools; and (3) surfaces have been conditioned by the removal of cracks and irregularities. Considerable improvements have been achieved in tool properties such as flexural strength, fracture toughness, hardness, and wear resistance by incorporating one or more other components into the Al2O3 base material to form ceramic com￾posite tool materials. The reinforcing component is often in the shape of particles or whiskers. Some of these tool materials, such as Al2O3/TiC, Al2O3/TiB2, Al2O3/ZrO2, Al2O3/Ti(CN), Al2O3/(WTi)C, and Al2O3/SiCw, have been used in various machining applications ∗ Corresponding author. Tel.: +86 531 88392047. E-mail address: jxdeng@sdu.edu.cn (D. Jianxin). and offer advantages with respect to friction and wear behaviors [3–6]. In the past decade, layered ceramics that take advantage of several strengthening mechanisms, such as crack deflection [7], independence on the initial flaw [8], introduction of weak interfaces [9], containment of martensitic transformation [10], or existence of porous layers [11], have been studied extensively. Among them, one of the most common mechanisms is the incorporation of resid￾ual stresses arising from thermal expansion coefficients mismatch, so that the surface layer is under compression. Several mod￾els and theoretical calculations, and experimental measurement methods have been proposed [8,12–16] to determine the stress amount and distribution in laminated structures. These compres￾sive stresses can increase the fracture strength, damage resistance, fatigue properties, as well as fracture toughness of layered ceram￾ics [16–28]. The effectiveness of laminated structures in improving the tribological properties has been reported by Tarlazzi et al. [29,30]. Toschi et al. [31] showed that laminated hybrid struc￾tures can improve the sliding wear resistance of alumina. Portu et al. [32] reported that laminated structure with compressive residual stresses within the surface regions was a suitable way to obtain composite materials with superior abrasive wear resis￾tance. Deng et al. [33–38] suggested that layered structures in ceramic nozzles can improve their erosion wear resistance in abra￾sive air-jet machining. Nicola [39] produced an alumina/zirconia laminated cutting tool and found that laminated structures are 0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2009.09.020

G Model MSA-25534: No of Pages 9 ARTICLE IN PRESS D Jianxin et aL Materials Science and Engineering A xxx(2009)xxx-XXx Table 1 AWT+ AT multilayered ceramic materials with different layer numbers and thickness ratios Code name LT-3 LT-6 AAWT.. Thickness ratio p effective in avoiding the microchipping on the flank zone. Sili he AWT+ AT multilayered ceramic tool con carbide whisker and titanium carbide particulate reinforced thickness ratios and layer numbers are ceramic matrix composites have been designed as multilayer struc layers consisting of different compo- tures and fabricated into cutting tool inserts by Maurice et al. nents alternate one after another but the external layers always 40, which demonstrate improvements in strength, toughness, consist of the same component (AWT). Thus the total and thermal shock resistance compared to the conventional non- ber of layers, N, in such a layered composite sample is odd aminated Due to lower thermal expansion coefficient of the AWT exter Al2O3/(W, Ti)c and Al2O3/TiC ceramics are widely used in indus- nal layer, compressive residual stresses will be formed at the trial applications such as cutting tools and dies[2,3, 41, 42 they external layer of the layered materials during fabrication. The both have high hardness and wear resistance. These two materials thickness ratio p among constituent layers is defined as the thick- have different thermal expansion coefficients; and different shrink- ness of external layer(Awr) divided by that of internal layer age during sintering. These differences are sufficient to induce (AT). residual stresses in the laminated structures made from these The starting powders used to fabricate these layered mate- two materials. In the present study, Al2O3/(W, Ti)C+AlO3/Tic rials were Al2O3, TiC, and (wTi)c solid-solution powders with multilayered ceramic tool materials with different thickness average grain size of 1-2 um, purity large than 99%. Their atios among constituent layers were produced by hot press- physical properties are listed in Table 2. The composition at ing in order to induce compressive residual stresses in the outer the external layer was A203/45 voL %(W, Ti)c, while the yer. The residual stresses inside these layered tool materi- nal layer was made with Al2O3/55 vol%TiC Composite were calculated by means of the finite element method of different mixture ratios were prepared by wet ball FEM). The mechanical properties at the outer layers were in alcohol with cemented carbide balls for 80h, respectively measured. The cutting performance of the multilayered tools Following drying, the composite powders with different mix were investigated and compared with an unstressed reference ture ratios were layered into the graphite mould one layer after another according to the material design results listed in Table 1. The sample was then hot-pressed at 1700C in flow ing nitrogen for 15 min temperature with an applied pressure 2. Materials and experimental procedures of 30 MPa to produce a circular ceramic disk. This disk has a thickness of 6.0 mm and diameter of 42 mm. For the purpose 2.1. Preparation of the Al2O3/(W, Ti)C+Al203/TiC multilayered of son, an unstressed reference awr ceramic with the compositions of Al2O3/45 vol%(W,Ti)C was also manufactured by Laminated hybrid structures constituted by alternate layers racture toughness measurement was performed using inden- with different compositions can be properly designed to induce tation method(IM)at the top surface of the outer layer of the a surface compressive residual stress. The basic idea is to cou- layered materials using the formula proposed by Cook and Lawn ple material layers with different thermal expansion coefficients 43-47J, and is given by (CTE)so that residual stresses arise during fabrication. Compres sive residual stresses are induced in the layers with lower CTE. The KiC=0.203x/C)-3/2 (1) materials selected in present study were Al2O3/(W, Ti)c (labeled as AWT)and Al2O3/Tic (labeled as AT). The reason for choosing these where 2a is the diagonal width of the indentation, c is the half- two materials as the constituent materials of layered ceramics can length of the surface crack, and Hv is the vickers hardness. be generally traced back to their good thermo-mechanical proper Hardness measurements were performed by placing vickers ties and their relatively ease of processing. The thermal expansion indentations on the top surface of the outer layer of the layered coefficient(CTE)of Al203/(W, Ti)C is 7. 25 x 10-6K-I, and the CTE materials. The indentation load was 200N and a minimum of five of Al2O3/TiC is 8. x 10-6K-[41. These differences are sufficient indentations were tested to induce residual stresses in the laminated structures made from The residual stresses inside the layered ceramic tool these two materials materials during fabrication were calculated by means of Density Youngs modulus Thermal expansion Thermal conductivity Poisson's Particle Purity Manufacture (g/cm)(GPa) size (um) A2O3398380 g Antai Advanced Tech and carbide works WmyC956480 >99 Zhuzhou cemented carbide works Please cite this article in press as: D Jianxin, et al, Mater. Sci. Eng. A(2009). doi: 10. 1016/j. msea. 2009.09.020

Please cite this article in press as: D. Jianxin, et al., Mater. Sci. Eng. A (2009), doi:10.1016/j.msea.2009.09.020 ARTICLE IN PRESS GModel MSA-25534; No. of Pages 9 2 D. Jianxin et al. / Materials Science and Engineering A xxx (2009) xxx–xxx Table 1 AWT + AT multilayered ceramic materials with different layer numbers and thickness ratios. Code name LT-1 LT-2 LT-3 LT-4 LT-5 LT-6 LT-7 Structure Layer number N 3 3 3 357 9 Thickness ratio p 0.5 1 2 8 1 1 1 effective in avoiding the microchipping on the flank zone. Sili￾con carbide whisker and titanium carbide particulate reinforced ceramic matrix composites have been designed as multilayer struc￾tures and fabricated into cutting tool inserts by Maurice et al. [40], which demonstrate improvements in strength, toughness, and thermal shock resistance compared to the conventional non￾laminated ceramic composites. Al2O3/(W,Ti)C and Al2O3/TiC ceramics are widely used in indus￾trial applications such as cutting tools and dies [2,3,41,42], they both have high hardness and wear resistance. These two materials have different thermal expansion coefficients; and different shrink￾age during sintering. These differences are sufficient to induce residual stresses in the laminated structures made from these two materials. In the present study, Al2O3/(W,Ti)C + Al2O3/TiC multilayered ceramic tool materials with different thickness ratios among constituent layers were produced by hot press￾ing in order to induce compressive residual stresses in the outer layer. The residual stresses inside these layered tool materi￾als were calculated by means of the finite element method (FEM). The mechanical properties at the outer layers were measured. The cutting performance of the multilayered tools were investigated and compared with an unstressed reference tool. 2. Materials and experimental procedures 2.1. Preparation of the Al2O3/(W,Ti)C + Al2O3/TiC multilayered ceramic materials Laminated hybrid structures constituted by alternate layers with different compositions can be properly designed to induce a surface compressive residual stress. The basic idea is to cou￾ple material layers with different thermal expansion coefficients (CTE) so that residual stresses arise during fabrication. Compres￾sive residual stresses are induced in the layers with lower CTE. The materials selected in present study were Al2O3/(W,Ti)C (labeled as AWT) and Al2O3/TiC (labeled as AT). The reason for choosing these two materials as the constituent materials of layered ceramics can be generally traced back to their good thermo-mechanical proper￾ties and their relatively ease of processing. The thermal expansion coefficient (CTE) of Al2O3/(W,Ti)C is 7.25 × 10−6 K−1, and the CTE of Al2O3/TiC is 8.01 × 10−6 K−1 [41]. These differences are sufficient to induce residual stresses in the laminated structures made from these two materials. The architectures of the AWT + AT multilayered ceramic tool materials with different thickness ratios and layer numbers are listed in Table 1. The layers consisting of different compo￾nents alternate one after another, but the external layers always consist of the same component (AWT). Thus the total num￾ber of layers, N, in such a layered composite sample is odd. Due to lower thermal expansion coefficient of the AWT exter￾nal layer, compressive residual stresses will be formed at the external layer of the layered materials during fabrication. The thickness ratio p among constituent layers is defined as the thick￾ness of external layer (AWT) divided by that of internal layer (AT). The starting powders used to fabricate these layered mate￾rials were Al2O3, TiC, and (W,Ti)C solid-solution powders with average grain size of 1–2 m, purity large than 99%. Their physical properties are listed in Table 2. The composition at the external layer was Al2O3/45 vol.%(W,Ti)C, while the inter￾nal layer was made with Al2O3/55 vol.%TiC. Composite powders of different mixture ratios were prepared by wet ball milling in alcohol with cemented carbide balls for 80 h, respectively. Following drying, the composite powders with different mix￾ture ratios were layered into the graphite mould one layer after another according to the material design results listed in Table 1. The sample was then hot-pressed at 1700 ◦C in flow￾ing nitrogen for 15 min temperature with an applied pressure of 30 MPa to produce a circular ceramic disk. This disk has a thickness of 6.0 mm and diameter of 42 mm. For the purpose of comparison, an unstressed reference AWT ceramic with the compositions of Al2O3/45 vol.%(W,Ti)C was also manufactured by hot-pressing. Fracture toughness measurement was performed using inden￾tation method (IM) at the top surface of the outer layer of the layered materials using the formula proposed by Cook and Lawn [43–47], and is given by: KIC = 0.203 × c a −3/2 · √a · HV (1) where 2a is the diagonal width of the indentation, c is the half￾length of the surface crack, and HV is the Vickers hardness. Hardness measurements were performed by placing Vickers indentations on the top surface of the outer layer of the layered materials. The indentation load was 200 N and a minimum of five indentations were tested. The residual stresses inside the layered ceramic tool materials during fabrication were calculated by means of Table 2 Physical properties of Al2O3, TiC and (W,Ti)C. Starting powder Density (g/cm3) Young’s modulus (GPa) Thermal expansion (10−6 K−1) Thermal conductivity W/(m K) Poisson’s ratio Particle size (m) Purity (%) Manufacture Al2O3 3.98 380 8.0 30.2 0.27 1–2 >99 Beijing Antai Advanced Tech. and Mater. Co., Ltd TiC 4.93 500 7.4 24.3 0.20 1–2 >99 Zhuzhou cemented carbide works (W,Ti)C 9.56 480 8.5 21.4 0.25 1–2 >99 Zhuzhou cemented carbide works

MSA-25534: No of Pages 9 ARTICLE IN PRESS D Jianxin et aL. Materials Science and Engineering A xxx(2009)xxx-xXX AWT AWT AWT Al AWT AWT AWT AWT AWT AWT Fig 1. SEM micrographs of the cross-section surface of AWT+ AT multilayered ceramic materials( layer number N= 3)with thickness ratio p of (a)AWT stress-free material, b)p=0.5.(c)p=1.(d)p=2.(e)p=6,and(p=8. the finite element method (FEM). The microstructure of 3. Results and discussion the layered materials was investigated by scanning electron mIcroscopy. 3.1. Microstructural characterization of the AWT+ AT multilayered ceramic materials 2. 2. Cutting tests The SEM micrographs of the cross-section surface of the AWT+ AT multilayered ceramic materials with different thickness Cutting tests were carried out on a CA6140 lathe. the ratios(p) and number of layers(n) are shown in Figs. 1 and 2, ools used were the AWT+ AT multilayered ceramic tools respectively, where the bright layers correspond to AWT and the AWT stress-free tool having the following geometry: rake dark ones to AT composition. The layered architectures can be yo=-15, clearance angle ao=5, inclination angle As clearly seen, the Awt and at layers are all compact without voids cutting edge angle Kr=45 and are reasonably uniform and the interfaces are straight and The workpiece material used was nodular cast iron(QT420-10) well-distinguishable with a hardness of HB 190-210 in the form of round bar. No cutting Closer examination at higher magnification on the interface fluid was used in the machining processes. All tests were carrie structure of these multilayered ceramic materials is illustrated in out with the following parameters: depth of cut ap=0.5 mm, feed Fig 3. No cracks or delaminations can be detected at the interfaces rate=0. 1 mm/rev, cutting speed v=108 m/min Tool flank wear was measured using a 20x optional microscope 3.2. Residual stresses in AWT+ AT multilayered ceramic materials system linked via transducers to a digital read out The worn rake nd flank regions on the tools were examined using scanning elec he magnitude of residual stresses is proportional to the Cte tron microscopy(HITACH S-570). mismatch between constituent layers, and also depends on the AWT AWT AWT AWT AWT 2 Fig. 2. SEM micrographs of the cross-section surface of AWT+ AT multilayered ceramic materials( thickness ratio, p=l)with layer numbers of (a)N=3, (b)N=5, and (c)N=7. Please cite this article in press as: D Jianxin, et al, Mater Sci Eng. A(2009). doi: 10. 1016/j. msea. 2009.09.020

Please cite this article in press as: D. Jianxin, et al., Mater. Sci. Eng. A (2009), doi:10.1016/j.msea.2009.09.020 ARTICLE IN PRESS GModel MSA-25534; No. of Pages 9 D. Jianxin et al. / Materials Science and Engineering A xxx (2009) xxx–xxx 3 Fig. 1. SEM micrographs of the cross-section surface of AWT + AT multilayered ceramic materials (layer number N = 3) with thickness ratio p of (a) AWT stress-free material, (b) p = 0.5, (c) p = 1, (d) p = 2, (e) p = 6, and (f) p = 8. the finite element method (FEM). The microstructure of the layered materials was investigated by scanning electron microscopy. 2.2. Cutting tests Cutting tests were carried out on a CA6140 lathe. The cutting tools used were the AWT + AT multilayered ceramic tools and the AWT stress-free tool having the following geometry: rake angle o = −15◦, clearance angle ˛o = 5◦, inclination angle s = −5◦, side cutting edge angle Kr = 45◦. The workpiece material used was nodular cast iron (QT420-10) with a hardness of HB 190–210 in the form of round bar. No cutting fluid was used in the machining processes. All tests were carried out with the following parameters: depth of cut ap = 0.5 mm, feed rate f = 0.1 mm/rev, cutting speed v = 108 m/min. Tool flank wear was measured using a 20× optional microscope system linked via transducers to a digital read out. The worn rake and flank regions on the tools were examined using scanning elec￾tron microscopy (HITACH S-570). 3. Results and discussion 3.1. Microstructural characterization of the AWT + AT multilayered ceramic materials The SEM micrographs of the cross-section surface of the AWT + AT multilayered ceramic materials with different thickness ratios (p) and number of layers (N) are shown in Figs. 1 and 2, respectively, where the bright layers correspond to AWT and the dark ones to AT composition. The layered architectures can be clearly seen, the AWT and AT layers are all compact without voids, and are reasonably uniform and the interfaces are straight and well-distinguishable. Closer examination at higher magnification on the interface structure of these multilayered ceramic materials is illustrated in Fig. 3. No cracks or delaminations can be detected at the interfaces. 3.2. Residual stresses in AWT + AT multilayered ceramic materials The magnitude of residual stresses is proportional to the CTE mismatch between constituent layers, and also depends on the Fig. 2. SEM micrographs of the cross-section surface of AWT + AT multilayered ceramic materials (thickness ratio, p = 1) with layer numbers of (a) N = 3, (b) N = 5, and (c) N = 7.

G Model MSA-25534:N a orages ARTICLE IN PRESS D Jianxin et aL Materials Science and Engineering A xxx(2009)xxx-XXx (a) (b) Interface Interface AWT 5 mm nterface Fig 3. SEM micrographs of the interface structure of AWT+ AT multilayered ceramic materials. Table 3 Material properties of AwT and AT [411- Compositions(vol%) Density (g/cm) Thermal expansion(10-5K-) Elastic modulus(GPa) Poissons ratio AWT Al2O3/45%(W, Ti)c 650 7250 448 Al,O3/ 55%TiC 4.73 417 0223 geometry of the layered structure, in particular on thickness ratios [8, 12-15]. The overall residual stress field is rather complex an thus difficult to predict by theoretical calculations: while finite ele ment methods give more accurate estimation of residual stresses d the character of their distribution [17, 48, 49]. Therefore, three- dimensional finite element method(FEM)is used as a means of umerically evaluating the residual stresses and their distribution inside the AWT+AT multilayered ceramic materials during fab rication. In view of the symmetry, three-quarter of a cylindrical model, whose geometry and size were taken from actual hot pressed samples, was represented. Fig 4 shows the FEM gridding 42 model for calculation and the coordinates for the stress analysis. An axisymmetric calculation was preferred and steady state boundary conditions were invoked Table 3 lists the material properties of the AWt and aT materials[41 The model is cooled from sinter g temperature 1700C to room temperature 20 C with a uniform temperature field. The results of the residual stresses in LT-4 and LT-7 multilayered ceramic materials with thickness ratio of 8 and 1 during fabrica- tion are shown in Fig. 5. It is indicated that excess residual stresses were formed inside the layered tool materials. Compressive resid- Fig 4. Finite element method gridding model for calculation and coordinates for ual stresses were introduced to AWT external layer, and tensile the stress analysis. Please cite this article in press as: D Jianxin, et al, Mater. Sci. Eng. A(2009). doi: 10. 1016/j. msea. 2009.09.020

Please cite this article in press as: D. Jianxin, et al., Mater. Sci. Eng. A (2009), doi:10.1016/j.msea.2009.09.020 ARTICLE IN PRESS GModel MSA-25534; No. of Pages 9 4 D. Jianxin et al. / Materials Science and Engineering A xxx (2009) xxx–xxx Fig. 3. SEM micrographs of the interface structure of AWT + AT multilayered ceramic materials. Table 3 Material properties of AWT and AT [41]. Code name Compositions (vol.%) Density (g/cm3) Thermal expansion (10−6 K−1) Elastic modulus (GPa) Poisson’s ratio AWT Al2O3/45%(W,Ti)C 6.50 7.250 448 0.232 AT Al2O3/55%TiC 4.73 8.014 417 0.223 geometry of the layered structure, in particular on thickness ratios [8,12–15]. The overall residual stress field is rather complex and thus difficult to predict by theoretical calculations; while finite ele￾ment methods give more accurate estimation of residual stresses and the character of their distribution [17,48,49]. Therefore, three￾dimensional finite element method (FEM) is used as a means of numerically evaluating the residual stresses and their distribution inside the AWT + AT multilayered ceramic materials during fab￾rication. In view of the symmetry, three-quarter of a cylindrical model, whose geometry and size were taken from actual hot￾pressed samples, was represented. Fig. 4 shows the FEM gridding model for calculation and the coordinates for the stress analysis. An axisymmetric calculation was preferred and steady state boundary conditions were invoked. Table 3 lists the material properties of the AWT and AT materials [41]. The model is cooled from sinter￾ing temperature 1700 ◦C to room temperature 20 ◦C with a uniform temperature field. The results of the residual stresses in LT-4 and LT-7 multilayered ceramic materials with thickness ratio of 8 and 1 during fabrica￾tion are shown in Fig. 5. It is indicated that excess residual stresses were formed inside the layered tool materials. Compressive resid￾ual stresses were introduced to AWT external layer, and tensile Fig. 4. Finite element method gridding model for calculation and coordinates for the stress analysis

MSA-25534: No of Pages 9 ARTICLE IN PRESS D Jianxin et aL. Materials Science and Engineering A xxx(2009)xxx-xXX 180-136-92-48-5039126170214 Fig. 5. Residual stress in AWT+ AT three-layered materials with thickness ratio of (a) p=8 and (b)p=l residual stresses were formed in AT internal layer. The maximum pendicular to the interface produced by a vickers impression on compressive stress in AWT external layer was -180 MPa for LT-7, AT internal layers, are longer than those parallel to the interface and-287 MPa for LT-4: while the maximum tensile stress in At (Fig. &c), indicating the presence of tensile stresses in AT layer par- internal layer was 214 MPa for LT-7, and 171 MPa for LT-4. allel to the interface Fig 6 shows the residual stresses along the radial direction in Closer examination e crack tip(fig. 8d) shows that the the outer surface of AwT external layer of the multilayered mate- indention crack initiated in AWT layer can propagate across the rials with different thickness ratios. It is evident that the residual interface to the at layer; while the crack developed in At layer was stresses in the outer surface of AwT external layer are all com- arrested at the interface, and cannot extend through the interface pressive whatever the thickness ratios. They kept almost constant to AWT layer(Fig. Se). This phenomenon also indicates that there at the central area of the sample, and decreased gradually at the are compressive stresses in AwT external layer and tensile stresses edge of the sample. The higher the thickness ratios, the higher the in AT internal layer, and the outer compressive layers have been compressive stresses in AWT external layer. The residual stresses in AT internal layer were tensile(fig. 5). and decreased with the increasing of thickness ratios(Fig. 7). The 3.3. Mechanical properties at the outer layer of the AW+AT LT-1 with thickness ratio of 0.5 showed 276 MPa tensile stresses at multilayered ceramic materials its internal layer, and the Lt-4 with thickness ratio of 8 had only The results of fracture toughness and hardness at the outer An initial indication of the presence of compressive stresses in layer of the Aw+ AT multilayered materials with different thick- AWT external layer and tensile stress in AT internal layer can be ness ratios among constituent layers are presented in Table 4. It is een in Fig 8. It is evident that, in the case of AwT outer layer indicated that the outer layer of the layered materials with high (Fig 8a), the impression produced by a Vickers indenter causes the thickness ratio shows high fracture toughness and hardness. By formation of cracks parallel to the interface which are longer than comparison with the stress-free tool (AWT), the fracture tough- the perpendicular ones, suggesting the presence of compressive ness at the outer layer of the layered materials is much more stresses in AWT outer layer. At the same time, the cracks per- improved, and rose from 4.9MPam'2 for AWT stress-free mate rial to 10.4 MPam/2 for LT-4 layered material, representing a um increase of 5.5 MPam/2. While the hardness rose from 240 lLI Radial position r(mm) Thickness ratio p Fig. sidual stress along the radial direction external layer of the Fig. 7. Effect of thickness ratio p on the residual tensile stress in the At internal AwT ree-layered materials with different thickness ratio p. layer of the AWT+ AT three-layered ceramic materials. Please cite this article in press as: D Jianxin, et al, Mater Sci Eng. A(2009). doi: 10.1016/j. msea. 2009.09.020

Please cite this article in press as: D. Jianxin, et al., Mater. Sci. Eng. A (2009), doi:10.1016/j.msea.2009.09.020 ARTICLE IN PRESS GModel MSA-25534; No. of Pages 9 D. Jianxin et al. / Materials Science and Engineering A xxx (2009) xxx–xxx 5 Fig. 5. Residual stress in AWT + AT three-layered materials with thickness ratio of (a) p = 8 and (b) p = 1. residual stresses were formed in AT internal layer. The maximum compressive stress in AWT external layer was −180 MPa for LT-7, and −287 MPa for LT-4; while the maximum tensile stress in AT internal layer was 214 MPa for LT-7, and 171 MPa for LT-4. Fig. 6 shows the residual stresses along the radial direction in the outer surface of AWT external layer of the multilayered mate￾rials with different thickness ratios. It is evident that the residual stresses in the outer surface of AWT external layer are all com￾pressive whatever the thickness ratios. They kept almost constant at the central area of the sample, and decreased gradually at the edge of the sample. The higher the thickness ratios, the higher the compressive stresses in AWT external layer. The residual stresses in AT internal layer were tensile (Fig. 5), and decreased with the increasing of thickness ratios (Fig. 7). The LT-1 with thickness ratio of 0.5 showed 276 MPa tensile stresses at its internal layer, and the LT-4 with thickness ratio of 8 had only 71 MPa. An initial indication of the presence of compressive stresses in AWT external layer and tensile stress in AT internal layer can be seen in Fig. 8. It is evident that, in the case of AWT outer layer (Fig. 8a), the impression produced by a Vickers indenter causes the formation of cracks parallel to the interface which are longer than the perpendicular ones, suggesting the presence of compressive stresses in AWT outer layer. At the same time, the cracks per￾Fig. 6. Residual stress along the radial direction in the AWT external layer of the AWT + AT three-layered materials with different thickness ratio p. pendicular to the interface, produced by a Vickers impression on AT internal layers, are longer than those parallel to the interface (Fig. 8c), indicating the presence of tensile stresses in AT layer par￾allel to the interface. Closer examination on the crack tip (Fig. 8d) shows that the indention crack initiated in AWT layer can propagate across the interface to the AT layer; while the crack developed in AT layer was arrested at the interface, and cannot extend through the interface to AWT layer (Fig. 8e). This phenomenon also indicates that there are compressive stresses in AWT external layer and tensile stresses in AT internal layer, and the outer compressive layers have been proved to be able to stop cracks. 3.3. Mechanical properties at the outer layer of the AW + AT multilayered ceramic materials The results of fracture toughness and hardness at the outer layer of the AW + AT multilayered materials with different thick￾ness ratios among constituent layers are presented in Table 4. It is indicated that the outer layer of the layered materials with high thickness ratio shows high fracture toughness and hardness. By comparison with the stress-free tool (AWT), the fracture tough￾ness at the outer layer of the layered materials is much more improved, and rose from 4.9 MPa m1/2 for AWT stress-free mate￾rial to 10.4 MPa m1/2 for LT-4 layered material, representing a maximum increase of 5.5 MPa m1/2. While the hardness rose from Fig. 7. Effect of thickness ratio p on the residual tensile stress in the AT internal layer of the AWT + AT three-layered ceramic materials