October 2003 Porous Oxide Matrix Composite Reinforced with Oxide Fibers 1737 15 Matrix porosity 38. 2%, 1200"C/100 o"D◆ 的的布∽ DN101200°cPn rix porosity 38. 2%, 1200C/ h 9DN6101200c100h Matrix porosity 43. 2%, 1200 C/100h Matrix porosity 43.2%, 1200C/2h 060810 Matrⅸ porosity(% on in maximum shear stress with matrix porosity in lition and after aging: open symbols, as-processed spec- Fig, 5. shear behavior for all-oxide composites with two bols, after aging at 1200 C/( 100 h) different osity levels, An increase in shear strength is seen after that high matrix strength can be obtained by strengthening the network without decreasing porosity EM observations showed no signif in the ma structure after heat treatment at 1200C for 100 h. However, aging (3) In-Plane Flexure Testing noted an evolution of f ws caused by matrix densification (Fig. 4(b). Others reported matrix Table II reports the in-plane mechanical properties for un- densification with associated matrix damage at 1200 C for longer notched composite specimens as processed and after aging. The periods(1000 h) in similar materials. strength of the N720 specimens was >170 MPa, and the N61o specimens was >280 MPa in as-processed condition, which is consistent with data presented by Levi, Zok, and co-workers" (2) Interlaminar Shear Strength for the previously described(see Section Ill( I) method of pro- Typical stress/displacement responses obtained for the inter- cessing porous mullite/alumina matrix composites. Figure 7 shows laminar shear tests are shown in Fig. 5. After an initial linear representative stress-strain curves for the composites ortion, a slight reduction in stiffness was observed after the maximum load was obtained, followed by a number of load drops In general the observed behavior is similar to the phenomenon of locations producing fiber brushes. The locations of fracture within sequential delamination failure. The failure mode was delani- individual tows also show a distribution(Fig. 8(a). These obser- nation in all tests. and the maximum interlaminar shear stresses vations show that the porous matrix is an efficient crack deflector calculated according to Eq. (3)are a measure of the matrix both within and between fiber tows. In more conventional CMCs rength. Interlaminar shear strength as a function of matrix with crack-deflecting matrix/fiber interfaces, one can observe porosity is shown in Fig. 6. These data suggest that the shea holes that contained fibers that fracture within the matrix. 7In rength increases with decreasing porosity. Heat treatment at composites with porous matrixes, no such holes can be observed strength. Although no matrix densification could be observed after which still are bonded to the fibers. 2. 24.2 The amount of fiber this heat treatment (see Section Ill(1), the results imply that the pull-out appears to be relatively uniform over the fracture surface, mullite/alumina matrix network has strengthened. The delamina- whereas others have observed more coplanar fracture near the tion stress measured for the least porous material (38 vol%) was edges of the test bar. This behavior has been explained by redistribution of the precursor(used to strengthen the matrix) in a composite with a more dense matrix(34 vol% and-12 MPa). the absence of a gelling step in the manufacturing process mullite/alumina composition of 80/20. This observation suggests specimens in air for 100 h at 1200 and 1300 C and then tested ag The composite used by Mattoni et al. had a matrix with a Table IL. Properties of In-Plane Unnotched Composite Three-Point Bend Tests Flexure strengt Elastic modulus Fiber Heat treatment Failure mode' N720 1200°C(2h) T N720 200°C/(2h) N720 1200°C/(100h) 1200°C/(100h) N720 1300C/(100h N720 300C/(100h 200°C/2h 85 N610 1200°C2h N 1200°c(100h) N6101200°C(100h N610 1300°C(100h) N60 300°C/100h 18 Failure mode; T, tensile: M, mixed mode (e. g, buckling, delamination)