ovember 2006 Oxide Fiber Composites 3313 100m (a) Dc Fig 9. Damage and fracture mechanisms in a porousmatrix oxide Fig. 11. Minimal fiber pullout on the fracture surface of a mullite- ntinuous-fiber ceramic composite(a) Crack deflection and interface ased porous matrix continuous-fiber ceramic composite strengthened g in a notched bend specimen. The specimen was infiltrated with an excessive amount of a precursor-derived alumina.(Courtes with epoxy while under load and then sectioned and polished.(b, c) M.A Uncorrelated fiber failure and pullout Material consists of Nextel720 fibers in an eight-harness satin weave and a mullite-alumina matrix. matrix, coated-fiber systems. That is, matrix cracks deflect along the fiber-matrix interface and fibers subsequently fail in an un- To ensure a morphologically stable pore structure, the matri- correlated manner, leading to pullout( Fig 9). Additionally, the ces typically consist of two dissimilar phases, distinguished by degree of notch sensitivity, characterized by open hole tension their sintering kinetics. The major phase is present as a contigu- tests, is comparable in the two classes of materials(Fig. 10). In ous 3D particle network. In turn, the network is bonded by a contrast, when the matrix is sintered or densified excessively, ei- less refractory ceramic or glass binder, in the form of either ther through processing or subsequent elevated temperature(in smaller sinterable particles or the product of precursor pyrolysis. service)exposure, embrittlement ensues. This is manifested in Particles in the main network dictate the long-term stability of planar fracture surfaces with minimal fiber pullout and signi the matrix against sintering, whereas particle junctions formed antly reduced toughness(Fig. Il) y the binder control the mechanical integrity of the matrix Additionally, the junctions at the fiber surface control the inter facial toughness (2) Debonding Mechanics When properly implemented, porous-matrix CFCCs exhibit fracture characteristics similar to those of conventional dense- twofold. Firstly, the bond between the matrix and the fibers is inherently weak. That is, the interface toughness, Ti. can be no greater than that of the matrix itself; for typical porosity levels 30 Notch Insensitive: ON/oo=1 nitude lower than that of the fibers, Tf, thereby ensuring a low. o6061A toughness interface. Secondly, because energy release rates scale ith elastic moduli. the red 10 △1018Stee leads to a reduction in the driving force for matrix cracks Oxide CFCCs To achieve high toughness in CFCCs, matrix cracks must deflect into the fiber/matrix interface rather than penetrate into mullitealumina he fibers. (A second condition-that interface sliding occur with SIC CFCCs only moderate resistance-must also be satisfied. ) The condi tions that satisfy this requirement are plotted in Fig. 12(a) 06 Deflection is predicted when the toughness ratio, Ti/Tf is less than the energy release rate ratio, Ga/Gp, associated with defec- 田 Glass PMCs. tion and penetration. The latter is a function of the elastic mis- atch parameter B04 △=(Er-Em) Sensitive: ON/oo=1/ko =0. 40 where E is the plane strain modulus, and the subscripts f and m denote fiber and matrix, respectively. For porous-matrix sys- 0 tems, A takes on high values(>0.5); hence, the allowable tough ness ratio is also higl Hole Diameter, 2a( mm) Because of similarities in the matrix and fiber constituents in oxide CFCCs of present interest, the nature of bonding at the Fig 10. Open-hole tensile strength of metals, oxide, and Sic uIs-fiber ceramic composites(CFCCs), and polymer matrix composite fiber-matrix interface is similar to that between particles in the go is the unnotched tensile strength and ko is the elastic stress concen- natrix. Consequently, their toughnesses are expected to move in tration factor. All composites have two-dimensional fiber architectures tandem: that is, Ti=ol m where o is a non-dimensional par (either laminated or woven)and loads are applied parallel to one of the meter. As the packing density of matrix particles at the fiber fiber he normalized hole diameter is a/w=0.2 for all cases except surface is lower than that in the bulk, i<Im and hence o<I the oxide CFCC, wherein afw=1/3 For conservative design, o is taken to be 1To ensure a morphologically stable pore structure, the matri￾ces typically consist of two dissimilar phases, distinguished by their sintering kinetics. The major phase is present as a contigu￾ous 3D particle network. In turn, the network is bonded by a less refractory ceramic or glass binder, in the form of either smaller sinterable particles or the product of precursor pyrolysis. Particles in the main network dictate the long-term stability of the matrix against sintering, whereas particle junctions formed by the binder control the mechanical integrity of the matrix. Additionally, the junctions at the fiber surface control the inter￾facial toughness. When properly implemented, porous-matrix CFCCs exhibit fracture characteristics similar to those of conventional dense￾matrix, coated-fiber systems. That is, matrix cracks deflect along the fiber–matrix interface and fibers subsequently fail in an un￾correlated manner, leading to pullout (Fig. 9). Additionally, the degree of notch sensitivity, characterized by open hole tension tests, is comparable in the two classes of materials (Fig. 10).9 In contrast, when the matrix is sintered or densified excessively, ei￾ther through processing or subsequent elevated temperature (in￾service) exposure, embrittlement ensues. This is manifested in planar fracture surfaces with minimal fiber pullout and signifi- cantly reduced toughness (Fig. 11). (2) Debonding Mechanics The role of matrix porosity in enabling damage tolerance is twofold. Firstly, the bond between the matrix and the fibers is inherently weak. That is, the interface toughness, Gi, can be no greater than that of the matrix itself; for typical porosity levels (B30%), the matrix toughness, Gm, is about an order of mag￾nitude lower than that of the fibers, Gf, thereby ensuring a low￾toughness interface. Secondly, because energy release rates scale with elastic moduli, the reduction in modulus due to porosity leads to a reduction in the driving force for matrix cracks. To achieve high toughness in CFCCs, matrix cracks must deflect into the fiber/matrix interface rather than penetrate into the fibers. (A second condition—that interface sliding occur with only moderate resistance—must also be satisfied.) The condi￾tions that satisfy this requirement are plotted in Fig. 12(a).37 Deflection is predicted when the toughness ratio, Gi/Gf, is less than the energy release rate ratio, Gd/Gp, associated with deflec￾tion and penetration. The latter is a function of the elastic mis￾match parameter, D  E
f E
ð Þ m E
f þ E
ð Þ m (1) where E
is the plane strain modulus, and the subscripts f and m denote fiber and matrix, respectively. For porous-matrix sys￾tems, D takes on high values (40.5); hence, the allowable tough￾ness ratio is also high. Because of similarities in the matrix and fiber constituents in oxide CFCCs of present interest, the nature of bonding at the fiber–matrix interface is similar to that between particles in the matrix. Consequently, their toughnesses are expected to move in tandem: that is, Gi 5 oGm where o is a non-dimensional par￾ameter. As the packing density of matrix particles at the fiber surface is lower than that in the bulk,38 GioGm and hence oo1. For conservative design, o is taken to be 1. Fig. 9. Damage and fracture mechanisms in a porous-matrix oxide continuous-fiber ceramic composite. (a) Crack deflection and interface debonding in a notched bend specimen. The specimen was infiltrated with epoxy while under load and then sectioned and polished. (b, c) Uncorrelated fiber failure and pullout. Material consists of Nextelt 720 fibers in an eight-harness satin weave and a mullite–alumina matrix. Fig. 10. Open-hole tensile strength of metals, oxide, and SiC continu￾ous-fiber ceramic composites (CFCCs), and polymer matrix composites. so is the unnotched tensile strength and ks is the elastic stress concen￾tration factor. All composites have two-dimensional fiber architectures (either laminated or woven) and loads are applied parallel to one of the fiber axes. The normalized hole diameter is a/w 5 0.2 for all cases except the oxide CFCC, wherein a/w 5 1/3. Fig. 11. Minimal fiber pullout on the fracture surface of a mullite￾based porous matrix continuous-fiber ceramic composite strengthened with an excessive amount of a precursor-derived alumina. (Courtesy M. A. Mattoni). November 2006 Oxide Fiber Composites 3313