2084 Journal of the American Ceramic Society Levi et al. Vol 8l. No 8 N720 Ir N610 Composites Three puint Band Tests 1200"C Treatments f=40%0/° Tensile Strain (%o) (a) Displacement (mm 200"C 2 h Treatments Tensile Strain (% Fig. 11. Effect of thermal exposure on the ambient stress-strain Fig. 12. (a) Short-beam shear behavior of all-oxide CFCCs in the havior for all-oxide composites in the 0 /90 orientation:(a)effect /90 orientation. The 2 h/1200C specimens represent the as different times at 1200oC and(b)effect of two different temperatures ssed condition. The 50 h/1200C for 2 h treatments minal interlaminar shear stress is calculated using the equation T= 3P/4BD, where P, B, and D are the load, thickness, and indicating that the matrix is stable and continues t Its pth of the specimen, respectively. (b) Micrograph shows delamina- function. Increasing the treatment temperature to 1300C has a tion cracks after testing in an N720 composite more pronounced effect, stiffening the stress-strain respons but reducing the tensile strength and failure strain( Fig. 11(b)) The reduction in strength is comparable to that observed for lamination occurs mainly through the matrix regions between N610 dry bundles subjected to analogous heat treatments. the fiber layers(Fig. 12(b). Evidently, the cracks propagate However. there is also a noticeable reduction in the extent of stably, in some instances allowing the load to increase beyond pull-out, suggesting some changes in the matrix or in the de- that of the first nonlinearity. The peak stresses are typically in gree of bonding at the fiber-matrix interface the range of 8-10 MPa Fiber failure is minimal, and the speci- The above results suggest that the sintering of the matrix to mens retain load-bearing capacity at displacements >l mm the fibers causes little degradation to the reinforcement beyond Even for displacements of this order, the delamination cracks that expected from changes in its microstructure. The implica- close down upon removal of the load, and a substantial fraction tion is that the degradation of composite properties after exp of the displacement is recovered sure to elevated temperatures could be reduced by using a fiber Similar behavior is obtained in the n720 material. The onset with enhanced microstructural stability, such as N720 Similar of nonlinearity occurs at somewhat lower stresses, but the ul- studies on composites manufactured with this fiber are cur- timate shear strengths are comparable(-8 MPa). There is no rently underway significant change in the interlaminar response as the heat treat- ment time is increased from 2 to 50 h at 1200oC, again dem- VI. Interlaminar Properties onstrating the stability of the matrix microstructure The interlaminar response has been probed by performing comparison with other CFCCs) are attributable to the high mm x 3 mm) in the 0o/90 orientation. Typical curves that erty involves additional cycles of precursor impregnation and relate the nominal interlaminar shear stress to the load point pyrolysis, yielding a higher matrix density. However, excessive displacement are presented in Fig. 12(a). In the N610 material densification may have deleterious effects on the crack deflec- the first nonlinearity occurs at stresses of T =8 MPa. The tion characteristics of the matrix, leading to a degradation in subsequent response is somewhat erratic, with occasional the tensile properties along the fiber direction. An alternate abrupt load drops corresponding to delamination events. De reinforcing fibers Inindicating that the matrix is stable and continues to perform its function. Increasing the treatment temperature to 1300°C has a more pronounced effect, stiffening the stress–strain response but reducing the tensile strength and failure strain (Fig. 11(b)). The reduction in strength is comparable to that observed for N610 dry bundles subjected to analogous heat treatments.24 However, there is also a noticeable reduction in the extent of pull-out, suggesting some changes in the matrix or in the de￾gree of bonding at the fiber–matrix interface. The above results suggest that the sintering of the matrix to the fibers causes little degradation to the reinforcement beyond that expected from changes in its microstructure. The implica￾tion is that the degradation of composite properties after expo￾sure to elevated temperatures could be reduced by using a fiber with enhanced microstructural stability, such as N720. Similar studies on composites manufactured with this fiber are cur￾rently underway. VI. Interlaminar Properties The interlaminar response has been probed by performing three-point flexure tests on short-beam specimens (45 mm × 6 mm × 3 mm) in the 0°/90° orientation. Typical curves that relate the nominal interlaminar shear stress to the load point displacement are presented in Fig. 12(a). In the N610 material, the first nonlinearity occurs at stresses of t ≈ 8 MPa. The subsequent response is somewhat erratic, with occasional abrupt load drops corresponding to delamination events. De￾lamination occurs mainly through the matrix regions between the fiber layers (Fig. 12(b)). Evidently, the cracks propagate stably, in some instances allowing the load to increase beyond that of the first nonlinearity. The peak stresses are typically in the range of 8–10 MPa. Fiber failure is minimal, and the speci￾mens retain load-bearing capacity at displacements >1 mm. Even for displacements of this order, the delamination cracks close down upon removal of the load, and a substantial fraction of the displacement is recovered. Similar behavior is obtained in the N720 material. The onset of nonlinearity occurs at somewhat lower stresses, but the ul￾timate shear strengths are comparable (∼8 MPa). There is no significant change in the interlaminar response as the heat treat￾ment time is increased from 2 to 50 h at 1200°C, again dem￾onstrating the stability of the matrix microstructure. The relatively low levels of interlaminar shear strength (in comparison with other CFCCs) are attributable to the high matrix porosity. One potential strategy for improving this prop￾erty involves additional cycles of precursor impregnation and pyrolysis, yielding a higher matrix density. However, excessive densification may have deleterious effects on the crack deflec￾tion characteristics of the matrix, leading to a degradation in the tensile properties along the fiber direction. An alternate solution is to introduce through-thickness reinforcing fibers. In Fig. 11. Effect of thermal exposure on the ambient stress–strain be￾havior for all-oxide composites in the 0°/90° orientation: (a) effect of different times at 1200°C and (b) effect of two different temperatures for 2 h treatments. Fig. 12. (a) Short-beam shear behavior of all-oxide CFCCs in the 0°/90° orientation. The 2 h/1200°C specimens represent the as￾processed condition. The 50 h/1200°C specimen of N720 composite shows no evident effect of extended heat treatment on the interlaminar properties. Nominal interlaminar shear stress is calculated using the equation t 4 3P/4BD, where P, B, and D are the load, thickness, and depth of the specimen, respectively. (b) Micrograph shows delamina￾tion cracks after testing in an N720 composite. 2084 Journal of the American Ceramic Society—Levi et al. Vol. 81, No. 8