J. Haslam et al. Journal of the European Ceramic Society 20(2000)607-618 Cracking phenomena for the tensile loading of a uni within the fibers and matrix is identical. It is possible directional CMC containing crack deflecting interfaces that the failure strain(Em) of the porous matrix can be can be related to the composite's stress/strain behavior. equal or even larger than the failure strain of the fibers During the initial loading, the behavior is linear and (Ee). Applying Hook's Law, one can show that characterized by the combined elastic modulus of the fiber and matrix weighted by the appropriate volume Er= E (1) raction of each. As loading proceeds, matrix cracking initiates without fiber failure. Matrix cracking is char- where Emf and Em f are the failure stress and elastic acterized by a decreased slope of the stress-strain curve. modulus of the matrix(m) and fibers(f). Using proper Multiple matrix cracking generally occurs prior to the ties of a low density Al2O3 matrix material and Al2O3 initiation of fiber failure. Prior to and during fiber fail- fibers(e.g. 0m 200 MPa, o 2000 MPa, Emx40 GPa ure, the cracked matrix is held together by the fibers, and Er= 400 GPa) we can see that it is reasonable to be which now supports nearly all of the applied load. Fiber able to fabricate a porous matrix with a failure strain failure, and thus CMC failure occurs at a high strain that approaches that of a strong fiber. Therefore, in (0.5 to 1.0%), indicative of a strong fiber with a low tension, a large fraction of the strength and strain to elastic modulus. For many commercial and experi- failure of strong fibers can be achieved in a ceramic mental CMCs the stress for matrix cracking lies between composite that contains a porous matrix 40 and 100 MPa whereas composite failure(fiber fail- The second role of the porous matrix is to allow fibers ure)does not occur until the stresses exceed 150 to 300 to be isolated from cracks within the matrix In porous MPa. Thus, CMCs that have been developed over the materials the crack front can be non-continuous and last 25 years to contain crack deflecting interfaces can crack extension must occur by the continued breaking not only be relatively notch insensitive, but they can of the solid phase units, i.e. fracture has to be reinitiated also exhibit higher strains to failure relative to mono- in the solid phase within the high stress field of the pro- lithic ceramics(e.g Si3N4, with a mean tensile strength pagating crack. A comparable example of this fracture of 1000 MPa, has a strain to failure of 0.3%) mode is the extension of a crack within cloth. where the Approximately 5 years ago another type of CMC was fracture of each fiber is independent of the last to fail inadvertently discovered. 3.4 Unlike the CMCs with This mode of crack extension occurs in powder com- weak ' fiber matrix interfaces, the matrix and fibers are pacts that have been heated to produce necks between bonded together in these ' new'CMCs. The second touching particles. Observing the fr acture change is that the matrix in the 'new'CMCs is purpo- these very porous materials one can see that fracture(or sely made to be porous. Despite these two major chan- crack extension,)occurred by the breaking of grain ges, both of which are not taught by mechanics of pairs at grain boundaries. A continuous crack front conventional CMCs, the new CMC with well-bonded does not exist in these porous materials fiber/matrix interfaces and porous matrixes are notch The lack of a crack front in a porous matrix means insensitive. In addition, although not as high as the that embedded fibers never see an extending crack front conventional CMCs, their failure strain is larger than as the matrix fails. Fiber fracture within a very porous conventional monolithic ceramics. 5 matrix must initiate within the fiber itself. ie. from flaws The mode of failure of these new composites is dif- either on the surface or within the fiber, and not by the ferent from the older CMCs. The stress/strain behavior propagation of a crack within the matrix. Thus, fibers in of tensile specimens is nearly linear to failure, indicating a very porous matrix can fracture in the same manner as that both the matrix and fibers fail at about the same they do when they exist as a bundle, without a matrix. failure strain. Tensile failure can occur at 200 MPa: The high failure strain of the fibers becomes the failure CMngth is also relatively notch insensitive. 5 These strain of the composite because the matrix will have a oxide matrix and can be very stable in air to tempera- As detailed elsewhere, -5 one method to produce the tures where the fibers begin to degrade. It can be fiber composites described above is to pack particles expected that the processing of the new CMCs is much around the fibers within a fiber preform by pressure fil less complex and less costly. Eliminating fiber coatings is tration and then strengthen the porous matrix. In this a significant advantage in processing and reducing cost. method, a fiber preform(3-D weave, stacked layers of The porous matrix appears to play a critical role in cloth, etc. ) is mounted on a filter within a die cavity. A achieving a notch insensitive strength and a high failure pressure is exerted to a dispersed slurry to cause the strain. One role concerns the strain to failure. When a particles to stream though the preform to become trap- composite is loaded in tension, the fibers will support ped at the filter, and to build up a consolidated layer much of the load due to their much larger elastic mod- within the fiber preform. The slurry must be formulated ulus relative to the porous matrix. Although the fiber such that the particles are repulsive with respect to must carry the major portion of the stress, the strain themselves and the fibers. The particles must also beCracking phenomena for the tensile loading of a uni￾directional CMC containing crack de¯ecting interfaces can be related to the composite's stress/strain behavior. During the initial loading, the behavior is linear and characterized by the combined elastic modulus of the ®ber and matrix weighted by the appropriate volume fraction of each. As loading proceeds, matrix cracking initiates without ®ber failure. Matrix cracking is char￾acterized by a decreased slope of the stress±strain curve. Multiple matrix cracking generally occurs prior to the initiation of ®ber failure. Prior to and during ®ber fail￾ure, the cracked matrix is held together by the ®bers, which now supports nearly all of the applied load. Fiber failure, and thus CMC failure occurs at a high strain (0.5 to 1.0%), indicative of a strong ®ber with a low elastic modulus. For many commercial and experi￾mental CMCs the stress for matrix cracking lies between 40 and 100 MPa whereas composite failure (®ber fail￾ure) does not occur until the stresses exceed 150 to 300 MPa. Thus, CMCs that have been developed over the last 25 years to contain crack de¯ecting interfaces can not only be relatively notch insensitive, but they can also exhibit higher strains to failure relative to mono￾lithic ceramics (e.g. Si3N4, with a mean tensile strength of 1000 MPa, has a strain to failure of 0.3%). Approximately 5 years ago another type of CMC was inadvertently discovered.3,4 Unlike the CMCs with `weak' ®ber matrix interfaces, the matrix and ®bers are bonded together in these `new' CMCs. The second change is that the matrix in the `new' CMCs is purpo￾sely made to be porous. Despite these two major chan￾ges, both of which are not taught by mechanics of conventional CMCs, the new CMC with well-bonded ®ber/matrix interfaces and porous matrixes are notch insensitive. In addition, although not as high as the conventional CMCs, their failure strain is larger than conventional monolithic ceramics.5 The mode of failure of these new composites is dif￾ferent from the older CMCs.4 The stress/strain behavior of tensile specimens is nearly linear to failure, indicating that both the matrix and ®bers fail at about the same failure strain.5 Tensile failure can occur at 200 MPa; the strength is also relatively notch insensitive.5 These new CMCs can be processed with oxide ®bers in an oxide matrix and can be very stable in air to tempera￾tures where the ®bers begin to degrade. It can be expected that the processing of the new CMCs is much less complex and less costly. Eliminating ®ber coatings is a signi®cant advantage in processing and reducing cost. The porous matrix appears to play a critical role in achieving a notch insensitive strength and a high failure strain. One role concerns the strain to failure. When a composite is loaded in tension, the ®bers will support much of the load due to their much larger elastic mod￾ulus relative to the porous matrix. Although the ®ber must carry the major portion of the stress, the strain within the ®bers and matrix is identical. It is possible that the failure strain ("m) of the porous matrix can be equal or even larger than the failure strain of the ®bers ("f). Applying Hook's Law, one can show that "f ˆ "m; or f Ef ˆ m "m ; …1† where "m;f and Em,f are the failure stress and elastic modulus of the matrix (m) and ®bers (f). Using proper￾ties of a low density Al2O3 matrix material and Al2O3 ®bers (e.g. m200 MPa, f2000 MPa, Em40 GPa and Ef ˆ 400 GPa) we can see that it is reasonable to be able to fabricate a porous matrix with a failure strain that approaches that of a strong ®ber. Therefore, in tension, a large fraction of the strength and strain to failure of strong ®bers can be achieved in a ceramic composite that contains a porous matrix. The second role of the porous matrix is to allow ®bers to be isolated from cracks within the matrix. In porous materials the crack front can be non-continuous and crack extension must occur by the continued breaking of the solid phase units, i.e. fracture has to be reinitiated in the solid phase within the high stress ®eld of the pro￾pagating crack. A comparable example of this fracture mode is the extension of a crack within cloth, where the fracture of each ®ber is independent of the last to fail. This mode of crack extension occurs in powder com￾pacts that have been heated to produce necks between touching particles. Observing the fracture surface of these very porous materials one can see that fracture (or `crack extension') occurred by the breaking of grain pairs at grain boundaries.6 A continuous crack front does not exist in these porous materials. The lack of a crack front in a porous matrix means that embedded ®bers never see an extending crack front as the matrix fails. Fiber fracture within a very porous matrix must initiate within the ®ber itself. i.e. from ¯aws either on the surface or within the ®ber, and not by the propagation of a crack within the matrix. Thus, ®bers in a very porous matrix can fracture in the same manner as they do when they exist as a bundle, without a matrix. The high failure strain of the ®bers becomes the failure strain of the composite because the matrix will have a comparable strain to failure. As detailed elsewhere,3±5 one method to produce the ®ber composites described above is to pack particles around the ®bers within a ®ber preform by pressure ®l￾tration and then strengthen the porous matrix. In this method, a ®ber preform (3-D weave, stacked layers of cloth, etc.) is mounted on a ®lter within a die cavity. A pressure is exerted to a dispersed slurry to cause the particles to stream though the preform to become trap￾ped at the ®lter, and to build up a consolidated layer within the ®ber preform. The slurry must be formulated such that the particles are repulsive with respect to themselves and the ®bers. The particles must also be 608 J.J. Haslam et al. / Journal of the European Ceramic Society 20 (2000) 607±618