1526 Journal of the American Ceramic Society-Kriven and Lee Vol. 88. No 6 Table I. Variation of Thermal Expansion Coefficient and Flexural Strength for Mullite/Cordierite Mixture as a Function of Cordierite content Cordierite content(wt%) 100 Thermal expansion coefficient(×10-°C Flexural strength(MPa) 229+12205+30 172+18 4l1+18 116+2 108+23 For each cordierite content, seven to eight samples were tested in flexure, and the results represent the mean and amount of largest scatte Table Il. Bulk Density, Volume Fraction of a-Cristobalite, Average Grain Size, Strength, and Work of Fracture for Hot-Pressed Laminates at Various Annealing Times Annealing time at 1300C 10 Bulk density (g/cm) 2.68 2.67 2.63 2.65 Volume fraction of a-cristobalite (% 22 47 ize(um) l.2 Strength, Omax(MPa) 171+9 124+17 109±4 82+17 Work of fracture(kJ/m) 120 rElative density of each component in the laminated composite after hot pressing(cristobalite: 98%, cordierite: 98%, 60 wt% mullite/40 wt% cordierite mixture: 96%) For each annealing time, four to six samples were tested in flexure, and the results represent the mean and amount of largest scatter. The work-of-fracture was calculated samples showing cient Thermal expansion compatibility is an important factor in is interesting to note that the 12 h annealed sample appears to fabricating a stable, laminated structure with minimal thermal have exceeded the critical particle size for stress-induced trans- stresses at the interface. To design a laminated composite with formation and contained visible, thermally-induced macro- minimum thermal expansion difference between laminates and cracks(Fig. 6(c) yet retain reasonable strength, the 40 wt% cordierite content Figure presents load-deflection curves under un-notched, 4- was therefore selected for the mullite/cordierite matrix layers. point flexural testing, as a function of annealing time at 1300C. For the un-annealed bend bar. the curve showed brittle fracture. For the 10 h annealing case, the curve showed non-catastrophic (4) Fracture Behavior fracture. The step-wise load drops were characteristic of graceful To study the fracture behavior, the hot-pressed laminated com- failure. This implied that the matrix crack was debonding the posites having the same matrix to interphase thickness ratio of interphase, giving a relatively significant increase in work of 5: I were 4-point, flexural tested after various annealing times rain size, strength, and work of fracture for the laminates ob- tained at different annealing times are listed in table il. the bulk density was not changed following annealing at 1300C because the laminates appeared to have achieved their maxi lum density after the hot-pressing process. The strength de- creased when the annealing time was increased. The highest cristobalite ork of fracture of 2.38 kJ/m was observed in the laminate having a strength of 131 MPa at annealing times of 10 h. The 10 h annealed sample appeared to be optimal to maximize the volume of B-phase susceptible to shear stress-induced phase transformation. The average grain sizes in both the 10 and I h annealed samples were similar as expected, being 4.2 and 5.0 um, respectively, as summarized in Table Il. However, compa ison of their corresponding microstructures in Figs. 6(b)and(c), respectively, shows that there was a wide distribution of grain sizes and so only a trend can be observed. In general however, it 0.5m 0.25 (b) 0.15 O: Strength, Omax(MP 0.05 0.0 0.10 15 0.20.25 0.35 Fig 8. Load-deflection curves for laminates of 15 mullite/ cord matrix layers separated by cristobalite interphases under 4-point ile Fig 9. Optical micrographs of crack propagation in laminated samples testing as a function of annealing time at 13000 annealed at 1300C for(a)O h and(b)10 h after 4-point flexural testing.cient. Thermal expansion compatibility is an important factor in fabricating a stable, laminated structure with minimal thermal stresses at the interface. To design a laminated composite with minimum thermal expansion difference between laminates and yet retain reasonable strength, the 40 wt% cordierite content was therefore selected for the mullite/cordierite matrix layers. (4) Fracture Behavior To study the fracture behavior, the hot-pressed laminated com￾posites having the same matrix to interphase thickness ratio of 5:1 were 4-point, flexural tested after various annealing times. The bulk density, volume fraction of a-cristobalite, average grain size, strength, and work of fracture for the laminates ob￾tained at different annealing times are listed in Table II. The bulk density was not changed following annealing at 13001C because the laminates appeared to have achieved their maxi￾mum density after the hot-pressing process. The strength de￾creased when the annealing time was increased. The highest work of fracture of 2.38 kJ/m2 was observed in the laminate having a strength of 131 MPa at annealing times of 10 h. The 10 h annealed sample appeared to be optimal to maximize the volume of b-phase susceptible to shear stress-induced phase transformation. The average grain sizes in both the 10 and 12 h annealed samples were similar as expected, being 4.2 and 5.0 mm, respectively, as summarized in Table II. However, compar￾ison of their corresponding microstructures in Figs. 6(b) and (c), respectively, shows that there was a wide distribution of grain sizes and so only a trend can be observed. In general however, it is interesting to note that the 12 h annealed sample appears to have exceeded the critical particle size for stress-induced trans￾formation and contained visible, thermally-induced macro￾cracks (Fig. 6(c)). Figure 8 presents load-deflection curves under un-notched, 4- point flexural testing, as a function of annealing time at 13001C. For the un-annealed bend bar, the curve showed brittle fracture. For the 10 h annealing case, the curve showed non-catastrophic fracture. The step-wise load drops were characteristic of graceful failure. This implied that the matrix crack was debonding the interphase, giving a relatively significant increase in work of Table I. Variation of Thermal Expansion Coefficient and Flexural Strength for Mullite/Cordierite Mixture as a Function of Cordierite Content Cordierite content (wt%) 0 20 40 60 80 100 Thermal expansion coefficient ( 10
6 /1C) 4.5 3.8 3.1 2.6 2.0 1.4 Flexural strengthw (MPa) 229712 205730 172718 141718 116726 108723 w For each cordierite content, seven to eight samples were tested in flexure, and the results represent the mean and amount of largest scatter. Table II. Bulk Density, Volume Fraction of a-Cristobalite, Average Grain Size, Strength, and Work of Fracture for Hot-Pressed Laminates at Various Annealing Times Annealing time at 13001C 0 10 12 36 Bulk density (g/cm3 ) w 2.68 2.67 2.63 2.65 Volume fraction of a-cristobalite (%) 22 47 52 77 Average grain size (mm) 1.2 4.2 5.0 7.3 Strength,z smax (MPa) 17179 124717 10974 82717 Work of fracture (kJ/m2 ) 1.20 2.38 2.00 1.82 w Relative density of each component in the laminated composite after hot pressing (cristobalite: 98%, cordierite: 98%, 60 wt% mullite/40 wt% cordierite mixture: 96%). z For each annealing time, four to six samples were tested in flexure, and the results represent the mean and amount of largest scatter. The work-of-fracture was calculated from samples showing the maximum values. Fig. 8. Load–deflection curves for laminates of 15 mullite/cordierite matrix layers separated by cristobalite interphases under 4-point flexural testing, as a function of annealing time at 13001C. Fig. 9. Optical micrographs of crack propagation in laminated samples annealed at 13001C for (a) 0 h and (b) 10 h after 4-point flexural testing. 1526 Journal of the American Ceramic Society—Kriven and Lee Vol. 88, No. 6