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. 2009.09.020Please 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