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. 2009.09.020Please 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 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