Joumal J Am Ceram Soc, 80[7]1812-20(1997) High-Temperature Transverse fracture Toughness of Nicalon-Fiber-Reinforced CAS-Il Glass-Ceramic Matrix Composite Ramazan Kahama Department of Chemical Engineering, King Fahd University of Petroleum and minerals Dhahran 31261, Saudi arabia John F. Mandell and Max C Deibert Department of Chemical Engineering, Montana State University, Bozeman, Montana Cracking parallel to the fibers in off-axis plies is usually the reinforced glass-ceramics, producing the carbon-rich inter- ng process has been associated with the(transverse)frac- bond strong enough for load transfer, yet weafer-matrix) ture toughness, defined by the critical strain energy release debond readily and allow fiber bridging during crack propaga ate, Gye. The measurement of Ge provides basic informa- tion(perpendicular to the fibers )resulting in high longitudinal tion about the transverse crack resistance. In this study, the fracture toughness in the composite. tility of the double torsion Dt) test technique to deter However, reductions of strength and strain to failure (in nine Gi in a glass-ceramic matrix composite(Nicalon/ the fiber direction) have been observed when testing at tem- CAS-Im at temperatures up to 1000.C has been demor peratures as low as 400oC. 10 In contrast to room-tempera temperature (as does the bulk matrix); however, no evi. pullout was observed at high temperatures. It has been pro- dence of an interphase oxidizing effect on crack growth posed that the transition from tough(notch- insensitive)behav (parallel to the fibers)could be found. The inevitable mis. ior at room temperature to brittle(notch-sensitive) behavior at significant matrix crack interactions with the fibers re- tion effects at interfaces exposed to the environment. 61 5 bridging the crack in the DT specimens, in contrast to the and/or increased fiber-matrix bond strength caused by ported for other geometries such as double cantilever beam The oxidation reaction is not initiated until the matrix cracks and flexure specimens. pon stressing(in the fiber direction), allowing penetration of high-temperature air. 12.13 The air then infiltrates the composite and attacks the low-strength carbon-rich fiber-matrix inter brittlement of the lot ERAMIc and glass-ceramic composites offer a great range of gitudinal fracture process..o t was reported that upon the oxi- utility on a high-temperature basis and based on other en- formed between the fiber and the matrix 14-l vironmental considerations such as oxidation resistance,ero- sion, and chemical attack from acids, etc. 2 The proper Inspite of the well-demonstrated embrittlement effect of ties of these composites depend on the combination of the reinforced glass-ceramic composites, there is not much study properties of starting materials and the fabrication proce- on the effects of temperature on fracture parallel to the fibers dure. By choosing high-strength, relatively high-modulus, continuous(or relatively long) fibers and incorporating them (transverse fracture). It was the objective of this study to in into matrices, high-strength and tough composites can be cre- vestigate the resistance of Nicalon-fiber-reinforced calcium ated if the fiber-matrix interphase is weak enough to divert aluminosilicate glass-ceramic matrix composite(Nicalon/CAS and/or bridge cracks. The ultimate strength of such a com- posite is then controlled by the in situ fiber strength, and a study of the transverse fracture toughness, the effects of high temperature air environment on fracture toughness, and also composite tensile strength well beyond that associated with the stress corrosion crack growth(if present) matrix cracking can be achieved. 3 The Nicalon-fiber reinforced glass-ceramic composites have met these require. ics is important since there are few engineering applications interphase during processing, and with no deterioration in fiber where transverse stresses are not encountered, 17, 18 The resis- tance to crack growth parallel to the fibers is especially a de- Nicalon has a unique nonstoichiometric chemistryis that ing parallel to the fibers is usually the initial form of damage in glass-ceramic matrix composites. Chemical reactions can occur multidirectional composites. 9 This cracking process has beer associated with: the(transverse) fracture toughness, defined by of energy per unit plate thickness per unit crack extension. Unstable(fast) crack growth should occur when the strain en ergy release rate, Gr, reaches the critical value, GIc, for the G. Evans--contributing editor material. Measurements of the strain energy release rate for fractures verify that the critical strain energy release rate is indeed a material parameter when fracture occurs exclusively by opening, Mode L, and when plane-strain conditions prevail 995: approved July 10, 1996. at the crack tip. 22,23 It is denoted by Gre, referring to failure in Mode I and under plain-strain conditions. Because of their;;j ,;; 2 r.,. . .; ,.i . .. .._,...,. > .,.. J. Am. Cerarn. Soc., 80 I71 1812-20 (1597) High-Temperature Transverse Fracture Toughness of Nicalon-Fiber-Reinforced CAS-I I Glass-Ceramic Matrix Composite Ramazan Kahramant Department of Chemical Engineering, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia John F. Mandell and Max C. Deibert Department of Chemical Engineering, Montana State University, Bozeman, Montana 59717 Cracking parallel to the fibers in off-axis plies is usually the initial form of damage in composite laminates. This crack￾ing process has been associated with the (transverse) frac￾ture toughness, defined by the critical strain energy release rate, GI,. The measurement of GI, provides basic inf'orma￾tion about the transverse crack resistance. In this study, the utility of the double torsion (DT) test technique to deter￾mine GI, in a glass-ceramic matrix composite (Nicalod CAS-11) at temperatures up to 1OOO"C has been demon￾strated. GI, did decrease moderately with increasing temperature (as does the bulk matrix); however, no evi￾dence of an interphase oxidizing effect on crack growth (parallel to the fibers) could be found. The inevitable mis￾alignment of fibers in the material was not very efficient at bridging the crack in the DT specimens, in contrast to the significant matrix crack interactions with the fibers re￾ported for other geometries such as double cantilever beam and flexure specimens. I. Introduction ERAMIC and glass-ceramic composites offer a great range of C utility on a high-temperature basis and based on other en￾vironmental considerations such as oxidation resistance, ero￾sion, and chemical attack from acids, etc.lV2 The proper￾ties of these composites depend on the combination of the properties of starting materials and the fabrication proce￾dure.' By choosing high-strength, relatively high-modulus, continuous (or relatively long) fibers and incorporating them into matrices, high-strength and tough composites can be cre￾ated if the fiber-matrix interphase is weak enough to divert and/or bridge cracks. The ultimate strength of such a com￾posite is then controlled by the in situ fiber strength, and a composite tensile strength well beyond that associated with matrix cracking can be a~hieved.~ The Nicalon-fiber￾reinforced glass-ceramic composites have met these require￾ments through the development of a carbon-rich fiber-matrix interphase during processing, and with no deterioration in fiber strength. Nicalon has a unique nonstoichiometric chemisw5 that makes it particularly suited to the development of high-strength glass-ceramic matrix composites. Chemical reactions can occur between the fiber and matrix during processing of Nicalon￾A. G. Evans-contributing editor Manuscript No. 192457. Received July 17. 1995; approved July 10. 1996. 'Author to whom correspondence should be addressed. reinforced glass-ceramics, producing the carbon-rich inter￾phase regi~n.~."~ The carbon-rich layer forms a (fiber-matrix) bond strong enough for load transfer, yet weak enough to debond readily and allow fiber bridging during crack propaga￾tion (perpendicular to the fibers) resulting in high longitudinal fracture toughness in the c0mposite.6*~ However, reductions of strength and strain to failure (in the fiber direction) have been observed when testing at tem￾peratures as low as 400°C.L0 In contrast to room-tempera￾ture fractures, a nearIy planar fracture surface with little fiber pullout was observed at high temperatures. It has been pro￾posed that the transition from tough (notch-insensitive) behav￾ior at room temperature to brittle (notch-sensitive) behavior at elevated temperatures is likely due to fiber strength degradation and/or increased fiber-matrix bond strength caused by oxida￾tion effects at interfaces exposed to the The oxidation reaction is not initiated until the matrix cracks upon stressing (in the fiber direction), allowing penetration of high-temperature air.12J3 The air then infiltrates the composite and attacks the low-strength carbon-rich fiber-matrix inter￾phase in such a way as to cause an embrittlement of the lon￾gitudinal fracture process.'S6 It was reported that upon the oxi￾dation of the interphase carbon layer a stronger silica bond is formed between the fiber and the matix.'"I6 Inspite of the well-demonstrated embrittlement effect of temperature on the longitudinal behavior of the Nicalon￾reinforced glass-ceramic composites, there is not much study on the effects of temperature on fracture parallel to the fibers (transverse fracture). It was the objective of this study to in￾vestigate the resistance of Nicalon-fiber-reinforced calcium alurninosilicate glass-ceramic matrix composite (NicalodCAS￾II) to crack growth parallel to the fibers. This included the study of the transverse fracture toughness, the effects of high￾temperature air environment on fracture toughness, and also the stress corrosion crack growth (if present). Studying the transverse properties of fiber-reinforced ceram￾ics is important since there are few engineering applications where transverse stresses are not en~ountered.'~.'~ The resis￾tance to crack growth parallel to the fibers is especially a de￾termining factor for the useful design stress range since crack￾ing parallel to the fibers is usually the initial form of damage in multidirectional composites.'9 This cracking process has been associated with the (transverse) fracture toughness, defined by the critical strain energy release rate, GIc.20*21 G,, has the units of energy per unit plate thickness per unit crack extension. Unstable (fast) crack growth should occur when the strain en￾ergy release rate, GI, reaches the critical value, G,,. for the material. Measurements of the strain energy release rate for fractures verify that the critical strain energy release rate is indeed a material parameter when fracture occurs exclusively by opening, Mode I, and when plane-strain conditions prevail at the crack tip.22,23 It is denoted by Gn, refemng to failure in Mode I and under plain-strain conditions. Because of their ' 1812