J. Haslam et al. Journal of the European Ceramic Society 20(2000)607-618 mullite matrix using the 720 fibers.' The fact that the length, b'is the beam width, and't'is the beam thick- composites with the mullite/zirconia matrix have ness. As shown in Table 2, the values of Kl, are about strengths similar to those with a mullite/alumina matrix half of KIss suggesting that the notch lengths in the bend is a reasonable result if the point of view is taken that tests are not long enough to reach the steady state the tensile properties in the 0/90 orientation are domi- toughness of the material. The notch sensitivity of the nated by the fiber properties. Relative to CMCs with composite therefore will have a size dependent effect weak'interfaces, matrix cracking is not apparent until which was not examined here. This crack length effect near the fracture stress, but the porous matrix compo- on the notch sensitivity is similar to that in brittle sites appear to have a somewhat smaller strain to fail materials with ductile reinforcements ure. It must be remembered that in most CMCs multiple matrix cracking occurs at a stress much below the frac- 43. Composite microstructure ture stress. most of these materials have either carbon or bn interphases between the fiber and matrix and In general, the microstructure is consistent with the contain non-oxide fibers. Multiple matrix cracking mechanical properties. In the case of the 0/90 compo would therefore dramatically limit their application in sites, improved toughness is obtained from a micro- oxidizing conditions. Therefore the allowable design structure where the fibers can absorb energy of stress strain could be similar to current cmcs oncentrations by bridging crack surfaces and dissip The net-section stress at failure in the presence of the ing energy as the matrix disintegrates during fiber pull- notch can be calculated and compared to the strength at out as shown in Fig 9. Fig 9(c) shows that the particles failure of an un-notched sample as a measure of notch are bonding to the fibers. This supports the conclusion ensitivity. The ction strength can be approxi- that the energy absorption during fracture comes from mated by assuming that all the material above the plane disintegration of the matrix surrounding the fibers of the notch tip acts to apply load to the remaining un- Where these features are absent in the fracture surface notch portion of the beam. This can be done by simply lower toughness is observed as in the case of the subtracting the length of the notch from the height of -45 composite samples shown in Fig. 10. However, the the beam and using this as the new height. The net-sec- 80 MPa strength in the +/-45 floor orientation is tion strength of the notched beams compared to an un igher than the 50 MPa obtained by Levi et al.5 even notched beam gives a strength ratio that is an indication for the higher strength 610 fiber. Clearly, the lower fiber of the degree of notch sensitivity of the material. This is strength in the 720 fibers is not reducing the strength fo shown in Table 2 for the current material. The strength this fiber orientation. The absence of extensive disin ratio for the notched beams was a 0.7 indicating mod- tegration of the matrix in the +/-45% samples may erate notch insensitivity. This level of notch sensitivity is provide for a higher strength at a cost of a lower strain greater than that observed with the notched bend test to failure than is obtained with the 610 fibers. Poten- with the mullite/alumina matrix where the ratio is tial matrix(zirconia/mullite) combined ~0.922 with a lower fiber strength(720 fiber) may be the cause In addition, the notched beam specimen can be used for this result. It should be pointed out, however, that to determine the work of fracture. Additionally, the no testing of the mullite/alumina matrix composites work of fracture(WOF) can be related to a steady state with 720 fibers in the +/-45 fiber orientation has been oughness(Kiss)of the composite through the simplified performed. So, a direct comparison is not available. An advantage of the microstructure containing coarsened zirconia and mullite is the coarsening and strengthening KIss =(WOF*E)/2 (5) without shrinkage. It is also resistant to densification during high temperature, long term use in air. These where E is the elastic modulus of the un-notched sample. added benefits make this processing method useful in Because there is a crack length over which this fracture addition to the benefit of a simplified processing toughness develops in composite materials (R-curve method behavior), it is important to compare the steady state fracture toughness to the magnitude of the stress intensity caused by the notch at the failure load. The applied stress 5. Summary intensity at the notch tip of a beam in bending can be determined using the following equation A simple single-step processing method was intro- duced and evaluated for an all-oxide layered woven K1=2.66Ma1//(b°t2) (6) fiber composite in this paper. This composite and of its mechanical properties were also described in this where KI is the applied stress intensity factor, M is the paper. The method uses an infiltration of the all-oxide applied bending moment=Load"Span/4, a'is the notch fibers with sub-micron ceramic particles that results in amullite matrix using the 720 ®bers.5 The fact that the composites with the mullite/zirconia matrix have strengths similar to those with a mullite/alumina matrix is a reasonable result if the point of view is taken that the tensile properties in the 0/90 orientation are domi￾nated by the ®ber properties. Relative to CMCs with `weak' interfaces, matrix cracking is not apparent until near the fracture stress, but the porous matrix compo￾sites appear to have a somewhat smaller strain to fail￾ure. It must be remembered that in most CMCs multiple matrix cracking occurs at a stress much below the frac￾ture stress. Most of these materials have either carbon or BN interphases between the ®ber and matrix and contain non-oxide ®bers. Multiple matrix cracking would therefore dramatically limit their application in oxidizing conditions. Therefore the allowable design strain could be similar to current CMCs. The net-section stress at failure in the presence of the notch can be calculated and compared to the strength at failure of an un-notched sample as a measure of notch sensitivity. The net-section strength can be approxi￾mated by assuming that all the material above the plane of the notch tip acts to apply load to the remaining un￾notch portion of the beam. This can be done by simply subtracting the length of the notch from the height of the beam and using this as the new height. The net-sec￾tion strength of the notched beams compared to an un￾notched beam gives a strength ratio that is an indication of the degree of notch sensitivity of the material. This is shown in Table 2 for the current material. The strength ratio for the notched beams was  0.7 indicating mod￾erate notch insensitivity. This level of notch sensitivity is greater than that observed with the notched bend test with the mullite/alumina matrix where the ratio is 0.9.22 In addition, the notched beam specimen can be used to determine the work of fracture. Additionally, the work of fracture (WOF) can be related to a steady state toughness (KIss) of the composite through the simpli®ed equation: KIss ˆ …WOF E† 1=2 …5† where E is the elastic modulus of the un-notched sample. Because there is a crack length over which this fracture toughness develops in composite materials (R-curve behavior), it is important to compare the steady state fracture toughness to the magnitude of the stress intensity caused by the notch at the failure load. The applied stress intensity at the notch tip of a beam in bending can be determined using the following equation: KI ˆ 2:66Ma1=2 =…bt 2 †; …6† where KI is the applied stress intensity factor, M is the applied bending moment=Load Span/4, `a' is the notch length, `b' is the beam width, and `t' is the beam thick￾ness. As shown in Table 2, the values of KI, are about half of KIss suggesting that the notch lengths in the bend tests are not long enough to reach the steady state toughness of the material. The notch sensitivity of the composite therefore will have a size dependent e€ect which was not examined here. This crack length e€ect on the notch sensitivity is similar to that in brittle materials with ductile reinforcements.29 4.3. Composite microstructure In general, the microstructure is consistent with the mechanical properties. In the case of the 0/90 compo￾sites, improved toughness is obtained from a micro￾structure where the ®bers can absorb energy of stress concentrations by bridging crack surfaces and dissipat￾ing energy as the matrix disintegrates during ®ber pull￾out as shown in Fig. 9. Fig. 9(c) shows that the particles are bonding to the ®bers. This supports the conclusion that the energy absorption during fracture comes from disintegration of the matrix surrounding the ®bers. Where these features are absent in the fracture surface lower toughness is observed as in the case of the +/ ÿ45 composite samples shown in Fig. 10. However, the 80 MPa strength in the +/ÿ45 ¯oor orientation is higher than the 5O MPa obtained by Levi et al.5 even for the higher strength 610 ®ber. Clearly, the lower ®ber strength in the 720 ®bers is not reducing the strength for this ®ber orientation. The absence of extensive disin￾tegration of the matrix in the +/ÿ45 samples may provide for a higher strength at a cost of a lower strain to failure than is obtained with the 610 ®bers. Poten￾tially, a stronger matrix (zirconia/mullite) combined with a lower ®ber strength (720 ®ber) may be the cause for this result. It should be pointed out, however, that no testing of the mullite/alumina matrix composites with 720 ®bers in the +/ÿ45 ®ber orientation has been performed. So, a direct comparison is not available. An advantage of the microstructure containing coarsened zirconia and mullite is the coarsening and strengthening without shrinkage. It is also resistant to densi®cation during high temperature, long term use in air. These added bene®ts make this processing method useful in addition to the bene®t of a simpli®ed processing method. 5. Summary A simple single-step processing method was intro￾duced and evaluated for an all-oxide layered woven ®ber composite in this paper. This composite and some of its mechanical properties were also described in this paper. The method uses an in®ltration of the all-oxide ®bers with sub-micron ceramic particles that results in a 616 J.J. Haslam et al. / Journal of the European Ceramic Society 20 (2000) 607±618