June 2005 Transformation Weakening of Interphas 1527 (a) Cristobalite 6难编 mcloone/cordierite mulmte/cordierite ∮)21 100ul 100u Fig. 11. Scanning electron micrograph of indentation crack pattern (b) hot-pressed laminate annealed for 10 h at 1300.C. Residual stresses at interfaces due to thermal expansion mis- u match can have an effect on fracture behavior. In this laminated composite, however, the thermal expansion coefficient of the matrix, which had a 40 wt% cordierite content, was quite close to that of the B-cristobalite interphase. Therefore, the effect of residual stress on fracture behavior was considered to be To examine the interaction between crack propagation and the laminated microstructure. vickers indentation cracks were introduced and the sEM micrograph is shown in Fig. 11. In- dent-induced cracks, in the laminated hot-pre ssed nd annealed layer into the adjacent matrix layer. 100um IV. Conclusi Fig 10. Scanning electron micrographs of hot-pressed laminates ealed at 1300.C for(a) 10 h and(b)36 h A pronounced, Vickers in- An oxide-laminated composite consisting of a mullite/cordierite duced macrocrack was observed to have propagated in the cristobalite rix separated by nterphase in the laminate annealed for 36 h terphase has successfully been engineered. Matrix-crack deflec- tion by shear stress-induced phase transformation was observed at a critical doped cristobalite grain size of 4-5 um, resulting in fracture. The optical micrographs confirming crack deflection a comparatively high work of fracture. The best mechanical the laminated samples are seen in Figs. 9(a) and(b). In ehavior was exhibited by an interphase annealed for 10 h at omparison with the un-annealed laminate, the crack was 1300.C. The macrocracks, which were caused by spontaneous visibly deflected along the interphase, especially in the central thermally-induced phase transformation during the cooling shear region of the cristobalite layer in the composite anneale process, provided an easier propagation path for crack deflec- for 10 h tion without requiring significant crack energy for propagation The thermally-induced cracks in the hot-pressed laminated Conventional ceramic pr cristobalite layer before conducting flexural strength tests are and hot pressing were used, but the concept of transformation shown in Fig. 10. In the bulk sample annealed for 10 h, ne macrocracks were observed as seen in Fig. 6(b). Similarly in the graceful failure has essentially been demonstrated. In continu- laminate annealed for 10 h. no macrocracks were observed ous fiber reinforced CMCs. fibrous monoliths. or laminated (Fig. 10(a)), although some transverse, intergranular micro- composites, significant toughening can be achieved by shear-in- cracking appears to be present in the cristobalite layers. These duced, transformation weakening, causing debonding of inter may be specimen preparation artifacts due to shear-induced ses. As such, this mechanism has the potential for developing ransformation resulting from grinding and polishing of samples fully dense composites, which initially have both high strength for SEM examination and hence release of matrix constraint. In s well as potentially high toughness comparison, however, the 36 h annealed sample, having an av- erage grain size of x7.3 um clearly contained macrocracks propagating within the cristobalite layer(Fig. 10(b)). The 36 h nnealed laminated sample had a lower strength and work of fracture than did the laminate annealed at 10 h as seen in fig. 8 hich is supported by the U.S. Department of Energy under Grant DEFG02-91 This observation is consistent with the hypothesis that the 10 h ER45439, at the University of Illinois at Urbana-Champaign nnealed sample, being close to the critical particle size for stress-induced transformation, was able to absorb more fracture energy than could the 36 h annealed sample, which was over aged and hence required little or no fracture energy to nucleate the transformation ACEm图2B pment of High Toughness Ceramics,fracture. The optical micrographs confirming crack deflection in the laminated samples are seen in Figs. 9(a) and (b). In comparison with the un-annealed laminate, the crack was visibly deflected along the interphase, especially in the central shear region of the cristobalite layer in the composite annealed for 10 h. The thermally-induced cracks in the hot-pressed laminated cristobalite layer before conducting flexural strength tests are shown in Fig. 10. In the bulk sample annealed for 10 h, no macrocracks were observed as seen in Fig. 6(b). Similarly in the laminate annealed for 10 h, no macrocracks were observed (Fig. 10(a)), although some transverse, intergranular micro￾cracking appears to be present in the cristobalite layers. These may be specimen preparation artifacts due to shear-induced transformation resulting from grinding and polishing of samples for SEM examination, and hence release of matrix constraint. In comparison, however, the 36 h annealed sample, having an av￾erage grain size of B7.3 mm clearly contained macrocracks propagating within the cristobalite layer (Fig. 10(b)). The 36 h annealed laminated sample had a lower strength and work of fracture than did the laminate annealed at 10 h, as seen in Fig. 8. This observation is consistent with the hypothesis that the 10 h annealed sample, being close to the critical particle size for stress-induced transformation, was able to absorb more fracture energy than could the 36 h annealed sample, which was over￾aged and hence required little or no fracture energy to nucleate the transformation. Residual stresses at interfaces due to thermal expansion mis￾match can have an effect on fracture behavior. In this laminated composite, however, the thermal expansion coefficient of the matrix, which had a 40 wt% cordierite content, was quite close to that of the b-cristobalite interphase. Therefore, the effect of residual stress on fracture behavior was considered to be negligible. To examine the interaction between crack propagation and the laminated microstructure, Vickers indentation cracks were introduced and the SEM micrograph is shown in Fig. 11. In￾dent-induced cracks, in the laminated hot-pressed and annealed for 10 h, displayed a propagation path through the mullite/cor￾dierite layer. However, the crack did not cross the cristobalite layer into the adjacent matrix layer. IV. Conclusions An oxide-laminated composite consisting of a mullite/cordierite matrix separated by a transformation-weakened cristobalite in￾terphase has successfully been engineered. Matrix-crack deflec￾tion by shear stress-induced phase transformation was observed at a critical doped cristobalite grain size of 4–5 mm, resulting in a comparatively high work of fracture. The best mechanical behavior was exhibited by an interphase annealed for 10 h at 13001C. The macrocracks, which were caused by spontaneous thermally-induced phase transformation during the cooling process, provided an easier propagation path for crack deflec￾tion without requiring significant crack energy for propagation. Conventional ceramic processing techniques of tape casting and hot pressing were used, but the concept of transformation weakening and debonding of interphases leading to overall graceful failure has essentially been demonstrated. In continu￾ous fiber reinforced CMCs, fibrous monoliths, or laminated composites, significant toughening can be achieved by shear-in￾duced, transformation weakening, causing debonding of inter￾phases. As such, this mechanism has the potential for developing fully dense composites, which initially have both high strength as well as potentially high toughness. Acknowledgment Use is acknowledged of some of the facilities maintained in the Center for Microanalysis of Materials, of the Frederick Seitz Materials Research Laboratory, which is supported by the U.S. Department of Energy under Grant DEFG02-91- ER45439, at the University of Illinois at Urbana-Champaign. References 1 A. G. Evans, ‘‘Perspectives on the Development of High Toughness Ceramics,’’ J. Am. Ceram. Soc., 73 [2] 187–206 (1990). Fig. 10. Scanning electron micrographs of hot-pressed laminates an￾nealed at 13001C for (a) 10 h and (b) 36 h. A pronounced, Vickers in￾duced macrocrack was observed to have propagated in the cristobalite interphase in the laminate annealed for 36 h. Fig. 11. Scanning electron micrograph of indentation crack pattern in hot-pressed laminate annealed for 10 h at 13001C. June 2005 Transformation Weakening of Interphases 1527