October 2005 Tailored Residual Stresses in Ceramic laminates engineered laminate(AM) 450 wM 15 monolith(AMO 1 mm 100 Fig. 7. Failure stress data as function of the indentation load for the engineered laminates(AM)and monolithic samples(AMO). Each point Fig 9. Optical micrographs showing the typical fracture surface of ar corresponds to the average of five samples. Straight lines correspond te linear fitting curves in the log-log diagram. indicate the surface faw where fracture initiated the black arrows indi- cate the arrested crack in the engineered laminate while, as expected, the strength of monolithic laminates is relat- that the material shows also some warning before final failur d to the indentation load as described by This phenomenon is quite unusual in brittle materials. Figure 9 shows the typical fracture surface of an engineered or pl The markings on the fracture surface show that the surface e (11) AM laminate compared with that of a ceramic monolith(AM where k, in the specific case, calculated by linear fitting (in the extended at first along the surface, being arrested by the engi- log-log diagram), is equal to 0.28, corresponding closely to the peered residual stress profile in the in-depth propagation; then, theoretical value(N 0.3 after this stable growth stage only, the through-thickness notch To further emphasize this point becomes unstable and leads to the sample failure. Conversely one should note also that the strength of indented AM laminates corresponds closely to the design value( a 400 MPa). From the growth of surface cracks in the ceramic monolith is typically Fig. 7, it appears surprising that the standard deviation of n as the critical load is reached figure 9 shows strength for indented alumina samples is smaller than for AM also the perfect adhesion of the various layers both in the whole- composites; such a result can be probably related in part to the alumina and in the engineered laminate indicating a satisfacto- limited number of samples(five)tested at each indentation load rily implemented thermo-compression and sintering process. In although further investigations are certainly needed addition, detailed observations did not reveal any delamination The presence of stable crack growth phenomena was ob- effect. By comparing Figs 8 and 9 it should be noted that orig- served in the engineered AM laminates. Samples subjected to inal surface flaws, natural or produced by indentation, propa- bending loads exhibited in fact multiple cracking before final gate along the tensile surface and arrest at around the same failure. i.e. several cracks are formed on the tensile surface of depth. Any of the resulting through-thickness cracks can then he sample above a specific threshold stress( a 200 MPa). Such lead to ultimate failure, this specific event being therefore racks are arrested or undergo stable growth and only one lead onger related to the initial faw size to the failure of the body. Figure 8 shows clearly that surface stable cracks derive either from indentation faws or surface pores; interestingly, the critical fracture does not necessarily V. Conclusions start from the larger indentation flaws. Such behavior indicates This study shows that it is possible to produce highly reliable ceramic materials whose"" strength can be a priori de fined and controlled through an innovative design procedi arrested cracks 1 mm The material can be also designed to support bending loads more efficiently than homogeneous materials, since the material is improved only where required, i.e., near the surface In the present work ceramic laminates composed of alumina/ mullite composite layers have been designed to possess strength equal to A 400 MPa and to promote the stable growth of cracks as deep as 180 um. The material has been then produced by stacking and sintering together laminae obtained by tape asting. The engineered composite showed strength is equal to 458+32 MPa thus confirming the validity of the experimental approach adopted. The stable growth of surface cracks, deriving either from natural defects and indentation flaws has also been observe indentation precrack The laminated bodies here presented are therefore natural andidates for structural applications, particularly when high mechanical reliability and damage tolerance in severe conditions re required, as in the case of load-bearing components in the Fig 8. Optical micrograph showing the arrested cracks produced upo automotive and aircraft industries, biomedical prostheses, chem- ending on indented alt ical plant linings and safety systems even when more complex d=100N shapes like plates, shells or tubes are requiredwhile, as expected, the strength of monolithic laminates is relat￾ed to the indentation load as described by: sf Pk ¼ constant (11) where k, in the specific case, calculated by linear fitting (in the log-log diagram), is equal to 0.28, corresponding closely to the theoretical value ( 0.33).1,17 To further emphasize this point one should note also that the strength of indented AM laminates corresponds closely to the design value ( 400 MPa). From Fig. 7, it appears surprising that the standard deviation of strength for indented alumina samples is smaller than for AM composites; such a result can be probably related in part to the limited number of samples (five) tested at each indentation load although further investigations are certainly needed. The presence of stable crack growth phenomena was ob￾served in the engineered AM laminates. Samples subjected to bending loads exhibited in fact multiple cracking before final failure, i.e., several cracks are formed on the tensile surface of the sample above a specific threshold stress ( 200 MPa). Such cracks are arrested or undergo stable growth and only one leads to the failure of the body. Figure 8 shows clearly that surface stable cracks derive either from indentation flaws or surface pores; interestingly, the critical fracture does not necessarily start from the larger indentation flaws. Such behavior indicates that the material shows also some warning before final failure. This phenomenon is quite unusual in brittle materials. Figure 9 shows the typical fracture surface of an engineered AM laminate compared with that of a ceramic monolith (AM0). The markings on the fracture surface show that the surface flaw, extended at first along the surface, being arrested by the engi￾neered residual stress profile in the in-depth propagation; then, after this stable growth stage only, the through-thickness notch becomes unstable and leads to the sample failure. Conversely, the growth of surface cracks in the ceramic monolith is typically unstable as soon as the critical load is reached. Figure 9 shows also the perfect adhesion of the various layers both in the whole￾alumina and in the engineered laminate indicating a satisfacto￾rily implemented thermo-compression and sintering process. In addition, detailed observations did not reveal any delamination effect. By comparing Figs 8 and 9 it should be noted that orig￾inal surface flaws, natural or produced by indentation, propa￾gate along the tensile surface and arrest at around the same depth. Any of the resulting through-thickness cracks can then lead to ultimate failure, this specific event being therefore no longer related to the initial flaw size. V. Conclusions This study shows that it is possible to produce highly reliable ceramic materials whose ‘‘constant’’ strength can be a priori de- fined and controlled through an innovative design procedure. The material can be also designed to support bending loads more efficiently than homogeneous materials, since the material is improved only where required, i.e., near the surface. In the present work ceramic laminates composed of alumina/ mullite composite layers have been designed to possess strength equal to 400 MPa and to promote the stable growth of cracks as deep as 180 mm. The material has been then produced by stacking and sintering together laminae obtained by tape casting. The engineered composite showed strength is equal to 458732 MPa thus confirming the validity of the experimental approach adopted. The stable growth of surface cracks, deriving either from natural defects and indentation flaws, has also been observed. The laminated bodies here presented are therefore natural candidates for structural applications, particularly when high mechanical reliability and damage tolerance in severe conditions are required, as in the case of load-bearing components in the automotive and aircraft industries, biomedical prostheses, chem￾ical plant linings and safety systems even when more complex shapes like plates, shells or tubes are required. 0 150 300 450 600 10 100 failure stress, σf (MPa) indentation load (N) engineered laminate (AM) monolith (AM0) Fig. 7. Failure stress data as function of the indentation load for the engineered laminates (AM) and monolithic samples (AM0). Each point corresponds to the average of five samples. Straight lines correspond to linear fitting curves in the log–log diagram. Fig. 8. Optical micrograph showing the arrested cracks produced upon bending on indented alumina monolithic specimen (indentation load 5 100 N). Fig. 9. Optical micrographs showing the typical fracture surface of an engineered laminate (a) and of a ceramic monolith (b). The white arrows indicate the surface flaw where fracture initiated; the black arrows indi￾cate the arrested crack in the engineered laminate. October 2005 Tailored Residual Stresses in Ceramic Laminates 2831