October 2003 Porous Oxide Matrix Composite Reinforced with Oxide Fibers 1737 Matrix porosity 38. 2%, 1200.C/100 h vDN6101200°c Matrix porosity 38.2%, 1200'C/2h QDN6101200° cooH Matrix porosity 43.2%, 1200.C/100h EEEx三 Matrix porosity 43. 2%, 1200.C/2 h mml Variation in maximun stress with matrix porosity in essed condition and after Is sed spec shear behavior for all-oxide composites with two filled symbols, after agin C(100h) ty levels. An increase in shear strength is seen after 00 h compared with as-processed specimens(sintered that high matrix strength can be obtained by strengthening the SEM observations showed no significant changes in the matrix structure after heat treatment at 1200C for 100 h. However, aging (3) In-Plane Flexure Testing at 1300C/(100 h)promoted an evolution of the flaws caused by matrix densification (Fig. 4(b). Others have reported matri notched composite specimens as proces e n-plane mec erties for un aging. The densification with associated matrix damage at 1200C for longer strength of the N720 specimens was >170 d the n610 periods(1000 h) in similar materials pecimens was >280 MPa in as-processed condition, which is (2) Interlaminar Shear Strength consistent with data presented by Levi, Zok, and co-workers" for the previously described(see Section m(I) method of pro- Typical stress/displacement respons for the inter- cessing porous mullite/alumina matrix composites. Figure 7 shows laminar shear tests are shown in Fig. 5 n initial linear representative stress-strain curves for the rtion, a slight reduction in stiffness rved after the Figure 8 shows fracture surfaces of a N720 composite. The 0 maximum load was obtained, followed by a number of load drops. fiber tows break in a random fashion over a wide range of axial In general the observed behavior is similar to the phenomenon of locations producing fiber brushes. The locations of fracture within equential 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 strength. 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 shear holes that contained fibers that fracture within the matrix. In strength increases with decreasing porosity. Heat treatment at composites with porous matrixes, no such holes can be observed 1200.C for 100 h also seems to increase the interlaminar shear and the matrix rather disintegrates into smaller pieces, some of strength. Although no matrix densification could be observed after which still are bonded to the fibers. 23, 24,28 The amount of fiber this heat treatment(see Section Ill(I), the results imply that the ull-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 material(38 vol%)was tion stress measured for the least porous d wnlolattoni et al2for redistribution of the precursor(used to strengthen the matrix)in edges of the test bar.24 29 This behavior has been explained by 11-12 MPa, which is close to that measure a composite with a more dense matrix (34 vol% and -12 MPa). the absence of a gelling step in the manufacturing process The composite used by Mattoni et al had a matrix with a Thermal stability of the composites was studied by aging mullite/alumina composition of 80/20. This observation suggests specimens in air for 100 h at 1200 and 1300@C and then tested at Table Il. Properties of In-Plane Unnotched Composite Three-Point Bend Tests Flexure strength Elastic modulus Panel Fibe Heat treatment (MPa) Failure modet N720 200°C/2h 163 ABABABCDCDCD N720 200°C/100h) 179 N720 l89 N720 300°C100h) N610 200°C(2h N610 200°C(2h N610 TTTTTTTMMTTT CN6101300°C/(100h) l86 N6101300°C(100h) l81 Failure mode: T, tensile, M, mixed mode(e. g, buckling, delamination)SEM observations showed no significant changes in the matrix structure after heat treatment at 1200°C for 100 h. However, aging at 1300°C/(100 h) promoted an evolution of the flaws caused by matrix densification (Fig. 4(b)). Others have reported matrix densification with associated matrix damage at 1200°C for longer periods (1000 h) in similar materials.31 (2) Interlaminar Shear Strength Typical stress/displacement responses obtained for the inter￾laminar shear tests are shown in Fig. 5. After an initial linear portion, 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 sequential delamination failure.42 The failure mode was delami￾nation in all tests, and the maximum interlaminar shear stresses calculated according to Eq. (3) are a measure of the matrix strength. Interlaminar shear strength as a function of matrix porosity is shown in Fig. 6. These data suggest that the shear strength increases with decreasing porosity. Heat treatment at 1200°C for 100 h also seems to increase the interlaminar shear strength. Although no matrix densification could be observed after this heat treatment (see Section III(1)), the results imply that the mullite/alumina matrix network has strengthened. The delamina￾tion stress measured for the least porous material (38 vol%) was 11–12 MPa, which is close to that measured by Mattoni et al.29 for a composite with a more dense matrix (
34 vol% and
12 MPa). The composite used by Mattoni et al. had a matrix with a mullite/alumina composition of 80/20. This observation suggests that high matrix strength can be obtained by strengthening the network without decreasing porosity. (3) In-Plane Flexure Testing Table II reports the in-plane mechanical properties for un￾notched composite specimens as processed and after aging. The strength of the N720 specimens was 170 MPa, and the N610 specimens was 280 MPa in as-processed condition, which is consistent with data presented by Levi, Zok, and co-workers28–30 for the previously described (see Section III(1)) method of pro￾cessing porous mullite/alumina matrix composites. Figure 7 shows representative stress–strain curves for the composites. Figure 8 shows fracture surfaces of a N720 composite. The 0° fiber tows break in a random fashion over a wide range of axial locations producing fiber brushes. The locations of fracture within individual tows also show a distribution (Fig. 8(a)). These obser￾vations show that the porous matrix is an efficient crack deflector both within and between fiber tows. In more conventional CMCs with crack-deflecting matrix/fiber interfaces, one can observe holes that contained fibers that fracture within the matrix.17 In composites with porous matrixes, no such holes can be observed and the matrix rather disintegrates into smaller pieces, some of which still are bonded to the fibers.23,24,28 The amount of fiber pull-out appears to be relatively uniform over the fracture surface, whereas others have observed more coplanar fracture near the edges of the test bar.24,29 This behavior has been explained by redistribution of the precursor (used to strengthen the matrix) in the absence of a gelling step in the manufacturing process. Thermal stability of the composites was studied by aging specimens in air for 100 h at 1200° and 1300°C and then tested at Fig. 5. Short-beam shear behavior for all-oxide composites with two different matrix porosity levels. An increase in shear strength is seen after aging at 1200°C for 100 h compared with as-processed specimens (sintered at 1200°C for 2 h). Fig. 6. Variation in maximum shear stress with matrix porosity in as-processed condition and after aging: open symbols, as-processed spec￾imens; filled symbols, after aging at 1200°C/(100 h). Table II. Properties of In-Plane Unnotched Composite Three-Point Bend Tests Panel Fiber Heat treatment Flexure strength (MPa) Elastic modulus (GPa) Failure mode† A N720 1200°C/(2 h) 163 62 T B N720 1200°C/(2 h) 177 60 T A N720 1200°C/(100 h) 179 65 T B N720 1200°C/(100 h) 189 64 T A N720 1300°C/(100 h) 129 70 T B N720 1300°C/(100 h) 139 71 T C N610 1200°C/(2 h) 289 85 T D N610 1200°C/(2 h) 226 85 M C N610 1200°C/(100 h) 251 92 M D N610 1200°C/(100 h) 276 91 T C N610 1300°C/(100 h) 186 95 T D N610 1300°C/(100 h) 181 96 T † Failure mode: T, tensile; M, mixed mode (e.g., buckling, delamination). October 2003 Porous Oxide Matrix Composite Reinforced with Oxide Fibers 1737