E驅≈3S Journal of the European Ceramic Society 20(2000)607-618 Processing and properties of an all-oxide composite with a porous matrix .. Haslam, K.e. berroth*ff la Materials Department, University of California at Santa Barbara, Santa Barbara, CA 93106, US.A 18 August 199 Abstract Processing and mechanical properties of an all-oxide fiber composite with a porous matrix are presented here. The processing approach for an all-oxide composite was developed to be simple and involve one sintering process. The composite uses a porous matrix instead of riser coatings to deflect cracks from the fibers. a processing method involving recently developed methods for reshaping and forming saturated high-volume fraction(> 50 vol%) particle bodies was used to form the composite Good infiltra- tion of the woven fiber tows was obtained. Sintering in a pure HCl gas atmosphere was used to produce a porous matrix without shrinkage during processing. The sintering process also produced coarsening which makes the microstructure stable against densi- fication during use and thereby prevents forming cracklike voids and retains sufficient porosity for crack deflection. Measurements of interlaminar shear strength and strength of the composite show that composite produced by this processing method is compar- able to previous all-oxide materials produced using the oxide fibers used here. The mechanical properties are rationalized in terms of the features on the fracture surfaces. Disintegration of the matrix to allow energy dissipation during fracture was apparent and correlates with the measurements of the fracture toughness of the material. Moderate notch insensitivity was demonstrated with a net section strength in the presence of a notch being 700% of the unnotched strength. c 2000 Elsevier Science Ltd. All rights reserved Keywords: Aluminosilicate fibres; Composites; Mechanical properties; Porosity: Sintering: ZrO2 matrix 1. Introduction matrix. A fiber within a good CMC is only expected to break when the applied load exceeds its strength An important property of any ceramic matrix com- In the late 1960s Phillips recognized that brittle, but posite(CMC) is that its strength should be relatively strong fibers could be isolated from one another within insensitive to the presence of notches. If the riders a brittle matrix by providing a path for cracks prova within a CMC are effective, the strength of a body with gating through the matrix to bypass the fibers. a"weak a notch(or hole)of any size or shape will be the same as interface between the matrix and fiber provides the path the unnotched strength of a body with same net (or for crack deflection, thus allowing the crack to propa- reduced) cross-section. That is, for an ideal CMC, one gate along the fiber /matrix interface instead of through drill the fiber. As described by He and Huchinson, the con reducing the failure load other than the effect of redu- dition for crack deflection depends on the ratio of the cing its cross sectional critical strain energy release rate for the interface and Since the failure strain of a strong fiber is generally fiber, and the elastic properties of the two materials. For much larger than a dense matrix, cracks generally first a few fiber/matrix combinations, the crack defecting extend within the matrix. In terms of crack extension, interface needs no special processing conditions. For notch insensitivity requires that the fibers must be iso- example. the carbon fibers in the CMCs produced by lated from the very high stress field of a crack within the Phillips et al. did not bond to the glass matrix. For most other CMCs. the fibers must be coated with either carbon or boron nitride films to achieve a crack deflecting inter- Researcher. High Performance face. Not only do fiber coatings introduce cost and proces- Ceramic Section, Swiss Federal Laboratories for Materials Testing sing complexity, but they are not stable in oxidizing and Research. EMPA. Dube environments and they can cause composite embrittlement 0955-2219/00/S- see front matter o 2000 Elsevier Science Ltd. All rights reserved PII:S0955-2219(99)00259-9Processing and properties of an all-oxide composite with a porous matrix J.J. Haslam, K.E. Berroth*, F.F. Lange Materials Department, University of California at Santa Barbara, Santa Barbara, CA 93106, USA Accepted 18 August 1999 Abstract Processing and mechanical properties of an all-oxide ®ber composite with a porous matrix are presented here. The processing approach for an all-oxide composite was developed to be simple and involve one sintering process. The composite uses a porous matrix instead of riser coatings to de¯ect cracks from the ®bers. A processing method involving recently developed methods for reshaping and forming saturated high-volume fraction (>50 vol%) particle bodies was used to form the composite. Good in®ltra￾tion of the woven ®ber tows was obtained. Sintering in a pure HCl gas atmosphere was used to produce a porous matrix without shrinkage during processing. The sintering process also produced coarsening which makes the microstructure stable against densi- ®cation during use and thereby prevents forming cracklike voids and retains sucient porosity for crack de¯ection. Measurements of interlaminar shear strength and strength of the composite show that composite produced by this processing method is compar￾able to previous all-oxide materials produced using the oxide ®bers used here. The mechanical properties are rationalized in terms of the features on the fracture surfaces. Disintegration of the matrix to allow energy dissipation during fracture was apparent and correlates with the measurements of the fracture toughness of the material. Moderate notch insensitivity was demonstrated with a net section strength in the presence of a notch being 700% of the unnotched strength. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Aluminosilicate ®bres; Composites; Mechanical properties; Porosity; Sintering; ZrO2 matrix 1. Introduction An important property of any ceramic matrix com￾posite (CMC) is that its strength should be relatively insensitive to the presence of notches. If the riders within a CMC are e€ective, the strength of a body with a notch (or hole) of any size or shape will be the same as the unnotched strength of a body with same net (or reduced) cross-section. That is, for an ideal CMC, one should be able to drill a hole without signi®cantly reducing the failure load other than the e€ect of redu￾cing its cross sectional area. Since the failure strain of a strong ®ber is generally much larger than a dense matrix, cracks generally ®rst extend within the matrix. In terms of crack extension, notch insensitivity requires that the ®bers must be iso￾lated from the very high stress ®eld of a crack within the matrix. A ®ber within a good CMC is only expected to break when the applied load exceeds its strength. In the late 1960s Phillips1 recognized that brittle, but strong ®bers could be isolated from one another within a brittle matrix by providing a path for cracks propa￾gating through the matrix to bypass the ®bers. A `weak' interface between the matrix and ®ber provides the path for crack de¯ection, thus allowing the crack to propa￾gate along the ®ber/matrix interface instead of through the ®ber. As described by He and Huchinson,2 the con￾dition for crack de¯ection depends on the ratio of the critical strain energy release rate for the interface and ®ber, and the elastic properties of the two materials. For a few ®ber/matrix combinations, the crack defecting interface needs no special processing conditions. For example. the carbon ®bers in the CMCs produced by Phillips et al. did not bond to the glass matrix. For most other CMCs, the ®bers must be coated with either carbon or boron nitride ®lms to achieve a crack de¯ecting inter￾face. Not only do ®ber coatings introduce cost and proces￾sing complexity, but they are not stable in oxidizing environments and they can cause composite embrittlement. 0955-2219/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0955-2219(99)00259-9 Journal of the European Ceramic Society 20 (2000) 607±618 * Corresponding author. Visiting Researcher, High Performance Ceramic Section, Swiss Federal Laboratories for Materials Testing and Research, EMPA, DuÈbendorf, Switzerland