J. dn. Ceram Soc..84]866-6802001 ournal Measuring Interphase Recession by Fiber Push-In Testing Charles A. Lewinsohn. Charles H. Henager Jr. and Russell H. Jones Pacific Northwest National Laboratory Richland, Washington 99352 Jeffrey I. Eldridge NASA Glenn Research Center. Cleveland Ohio 4413 A novel technique for measuring interphase recession in IL. Experimental Method ceramic-matrix composites(CMCs) due to oxidation is de- scribed. The technique involves fiber push-in testing and The materials were fabricated by chemical vapor infiltration were conducted on carbon-coated Hi-Nicalon SiC fibers in a (Cvi) of a preform of seven layers of a two-dimensional plain- CVI SiC matrix, where the carbon interphase had recessed due to oxidation. Estimates of interphase recession distances from thick pyrolitic carbon interphase. The resulting composite was also coated with an - 2 um thick Cvd SiC layer intended for oxidation analysis of fiber push-in tests are in reasonable agreement with protection. Samples for push-in testing that were -4 mm X 4 measurements made by optical microscopy. Besides measuring mm x 4 mm were cut from plates of CVI material the recession distance the fiber push-in test can be used to For push-in tests on unoxidized specimens, the CVD SiC investigate environmental effects on fiber bridging oxidation protection coating was removed from one of the faces of imen by grinding on an abrasive wheel. To ensure that the surfaces were smooth and flat, the exposed face and the face L. Introduction opposite to it were polished with diamond paste. For studying effects of oxidation, the face of the specimen that was to be tested Fa IBER-REINFORCED, non-oxide ceramic-matrix composites was polished before oxidation, after oxidation, but before push-in 3(CMCs)typically employ an interphase material between the testing, the face opposite the test face was polished until both faces and the matrix to allow fiber/matrix debonding, which were parallel. Specimens were oxidized for 1.5, 3.6, 5.4, and 7.2 ks 25, 60, 90, and 120 min)at 1073 K in air. This temperature was on-oxide CMCs occurs at elevated temperatures. In some in- tances failure is due to the growth of oxidation products at the and polished with diamond paste for optical microscopy. Micro- interfaces between the fibers and the matrix 8-2 In environments graphs of the specimens were created and the recession distance containing low oxygen concentrations where oxide growth is vas measured with digital micrometers, with a resulting accuracy limited, such as those anticipated in fusion energy systems, failure of 100 nm may be due to interphase recession. -I6 Therefore, methods for measuring interphase recession distances provide essential infor- (2) Fiber Push-In Testing mation for developing models that can predict CMC lifetimes. This Fiber p ests were performed using a desktop fiber study introduces an experimental approach employing fiber push-in apparatus described previously. iNdividual fibers were push-in testing to measure the interphase recession rate due to pushed in with a 70-included-angle conical diamond indenter with a 10 um diameter flat on the bottom. Because of the conical oxidation. This new approach is compared with the measurement shape of the indenter, push-in distances were limited to a few of recession distances by optical microscopy. micrometers. Loading was accomplished using a vertical transla- tion stage that moved the specimen at a speed of I um/s. All tests were performed in room-temperature air (25%60% relative ( Fiber Push-In Test Analysis, Recession Interface recession lengths were determined by analysis of the J. D. Cawley--contributing editor relationship between fiber end displacement and applied force. The first step was to make a compliance correction(based on tests where the force was applied to only the Sic matrix) to the measured displacement data so that the corrected displacement Based, in part, on work presented a the ziny /9, 199, aproaenceramie sce and where there is no debonding along the unoxidized interface and no represented the fiber end displacement. For the simplest case contact between fiber and matrix along the recessed length, the Contract no. be-aco6-76rlo 1830 with Pacific northwest national Laboratory, which is operated for the DOE by Battelle. Additional support was provided by the NASA HITEMP Program. of Energy by Battelle Memorial Institute under 小中 on Carbon com tokwturead by DuPont Lanxide Composites, Newark, DE. 866Measuring Interphase Recession by Fiber Push-In Testing Charles A. Lewinsohn,* Charles H. Henager Jr.,* and Russell H. Jones* Pacific Northwest National Laboratory,† Richland, Washington 99352 Jeffrey I. Eldridge* NASA Glenn Research Center, Cleveland, Ohio 44135 A novel technique for measuring interphase recession in ceramic-matrix composites (CMCs) due to oxidation is de￾scribed. The technique involves fiber push-in testing and analysis of the load–displacement curves. Fiber push-in tests were conducted on carbon-coated Hi-Nicalon SiC fibers in a CVI SiC matrix, where the carbon interphase had recessed due to oxidation. Estimates of interphase recession distances from analysis of fiber push-in tests are in reasonable agreement with measurements made by optical microscopy. Besides measuring the recession distance, the fiber push-in test can be used to investigate environmental effects on fiber bridging. I. Introduction FIBER-REINFORCED, non-oxide ceramic-matrix composites (CMCs) typically employ an interphase material between the fiber and the matrix to allow fiber/matrix debonding, which provides higher toughness and greater reliability relative to unre￾inforced ceramics.1–7 It is well known that rapid failure of many non-oxide CMCs occurs at elevated temperatures. In some in￾stances failure is due to the growth of oxidation products at the interfaces between the fibers and the matrix.8–12 In environments containing low oxygen concentrations where oxide growth is limited, such as those anticipated in fusion energy systems, failure may be due to interphase recession.13–16 Therefore, methods for measuring interphase recession distances provide essential infor￾mation for developing models that can predict CMC lifetimes. This study introduces an experimental approach employing fiber push-in testing to measure the interphase recession rate due to oxidation. This new approach is compared with the measurement of recession distances by optical microscopy. II. Experimental Method (1) Materials The materials were fabricated‡ by chemical vapor infiltration (CVI) of a preform of seven layers of a two-dimensional plain￾weave (0/90°) cloth of Hi-Nicalon®§ fibers coated with a 1 mm thick pyrolitic carbon interphase. The resulting composite was also coated with an ;2 mm thick CVD SiC layer intended for oxidation protection. Samples for push-in testing that were ;4 mm 3 4 mm 3 4 mm were cut from plates of CVI material. For push-in tests on unoxidized specimens, the CVD SiC oxidation protection coating was removed from one of the faces of each specimen by grinding on an abrasive wheel. To ensure that the surfaces were smooth and flat, the exposed face and the face opposite to it were polished with diamond paste. For studying effects of oxidation, the face of the specimen that was to be tested was polished before oxidation; after oxidation, but before push-in testing, the face opposite the test face was polished until both faces were parallel. Specimens were oxidized for 1.5, 3.6, 5.4, and 7.2 ks (25, 60, 90, and 120 min) at 1073 K in air. This temperature was chosen so as to minimize microstructural changes in the fibers. Some specimens were cut with a diamond saw, mounted in resin, and polished with diamond paste for optical microscopy. Micro￾graphs of the specimens were created and the recession distance was measured with digital micrometers, with a resulting accuracy of 100 nm. (2) Fiber Push-In Testing Fiber push-in tests were performed using a desktop fiber push-in apparatus described previously.17 Individual fibers were pushed in with a 70°-included-angle conical diamond indenter with a 10 mm diameter flat on the bottom. Because of the conical shape of the indenter, push-in distances were limited to a few micrometers. Loading was accomplished using a vertical transla￾tion stage that moved the specimen at a speed of 1 mm/s. All tests were performed in room-temperature air (25%–60% relative humidity). (3) Fiber Push-In Test Analysis, Recession Length Determination Interface recession lengths were determined by analysis of the relationship between fiber end displacement and applied force. The first step was to make a compliance correction (based on tests where the force was applied to only the SiC matrix) to the measured displacement data so that the corrected displacement represented the fiber end displacement. For the simplest case where there is no debonding along the unoxidized interface and no contact between fiber and matrix along the recessed length, the J. D. Cawley—contributing editor Manuscript No. 189152. Received August 19, 1999; approved January 2, 2001. Based, in part, on work presented at the 22nd Annual Cocoa Beach Conference and Exposition of the Engineering Ceramics Division of the American Ceramic Society, January 1998. Supported, in part, by Basic Energy Sciences under U.S. Department of Energy (DOE) Contract No. DE-AC06-76RLO 1830 with Pacific Northwest National Laboratory, which is operated for the DOE by Battelle. Additional support was provided by the NASA HITEMP Program. *Member, American Ceramic Society. † Operated for the U.S. Department of Energy by Battelle Memorial Institute under Contract No. DE-AC06-76RLO 1830. ‡ Composites were manufactured by DuPont Lanxide Composites, Newark, DE. § Nippon Carbon Co., Tokyo, Japan. 866 journal J. Am. Ceram. Soc., 84 [4] 866–68 (2001)