E≈S Journal of the European Ceramic Society 20(2000)2627-2636 Room and elevated temperature tensile properties of single tow Hi-Nicalon, carbon interphase, CVI SiC matrix minicomposites Martinez-Fernandez aGn. Morscher epartamento de Fisica de la Materia Condensada, Universidad de Sevilla, Spain bNASA Glenn Research Center, Ohio Aerospace Institute, MS 106-5, Cleveland, OH 44135, USA Received 20 January 2000: received in revised form 13 April 2000; accepted 29 April 2000 Abstract Single tow Hi-NicalonM,C interphase, CVI SiC matrix minicomposites were tested in tension at room temperature, 700, 950, and 1200C in air. Monotonic loading with modal acoustic emission monitoring was performed at room temperature in order to determine the dependence of matrix cracking on applied load. Modal acoustic emission was shown to correlate directly with the number of matrix cracks formed. Elevated temperature constant load stress-rupture and low-cycle fatigue experiments were per formed on precracked specimens. The elevated temperature rupture behavior was dependent on the precrack stress, the lower pre crack stress resulting in longer rupture life for a given stress. It was found that the rupture lives of C-interphase Hi-NicalonTM minicomposites were superior to C-interphase Ceramic Grade Nicalon M minicomposites and inferior to those of BN-interphase Hi-NicalonTM minicomposites. C 2000 Elsevier Science Ltd. All rights reserved Keywords: C-interphase; Composites: Mechanical properties; SiC-SiC; SiC fibres 1. Introduction However, this strength degradation at temperature, in air. and at stress is far more severe than could be behavi poor intermediate temperature tensile-rupture accounted for from these two mechanisms. For exam Carbon (NIC), Tokyo, Japan] reinforced CVI Sic NIC/SiC composite, the fully loaded gage length of the matrix composites with carbon interphases has been fibers could be approximated by demonstrated over temperatures ranging from 425 to 1000oC. 1-7 The time to failure for all these studies cor 1=aR/2/r (2) responds to a stress exponent, n, of approximately 1/4, 4 where time to failure, L, is directly proportional to com- where R is the fiber radius, f is the volume fraction of posite stress, o, to the power n: load-bearing fibers, and t is the interfacial shear strength. For the case where o equals 240 MPa, f t aoh () equals 0.16, t equals 10 MPa, and assuming a Weibull modulus m for individual fiber failure equal to 5, the This corresponds to a rupture strength degradation of maximum decrease in strength due to an increase in over 70% for rupture times less than 10 h. Two gage length(12 mm for the hot zone in the Ref 2 study) mechanisms have been put forward for the reduction in would be 45%. If more than one crack were in the hot strength of the NIC/SiC system with C interphases: the zone of the furnace the degradation in rupture strength increase in effective gage length from carbon volatiliza- due to this mechanism would be less. In addition, if the tion'and the flaw size increase due to oxide scale oxide scale was related to the flaw size, one would growthon the surface of the fibers. The latter mechan- expect an increase in flaw size(oxide scale thickness)of ism would predict a stress exponent of 1 /4 assuming approximately one order of magnitude if the rupture parabolic oxide growth at intermediate temperatures. strength was reduced from 2000 to 500 MPa(a 75% decrease)over 100 h at 700 C, assuming a Kic of 2.9 For the Ref. 2 study, SiO2 scales were not detectable on the fiber surfaces at these low temperatures, even though 0955-2219/00/S. see front matter C 2000 Elsevier Science Ltd. All rights reserved PII:S0955-2219(00)00138-2Room and elevated temperature tensile properties of single tow Hi-Nicalon, carbon interphase, CVI SiC matrix minicomposites J. MartõÂnez-FernaÂndez a , G.N. Morscher b,* a Departamento de FõÂsica de la Materia Condensada, Universidad de Sevilla, Spain bNASA Glenn Research Center, Ohio Aerospace Institute, MS 106-5, Cleveland, OH 44135, USA Received 20 January 2000; received in revised form 13 April 2000; accepted 29 April 2000 Abstract Single tow Hi-NicalonTM, C interphase, CVI SiC matrix minicomposites were tested in tension at room temperature, 700, 950, and 1200C in air. Monotonic loading with modal acoustic emission monitoring was performed at room temperature in order to determine the dependence of matrix cracking on applied load. Modal acoustic emission was shown to correlate directly with the number of matrix cracks formed. Elevated temperature constant load stress-rupture and low-cycle fatigue experiments were per￾formed on precracked specimens. The elevated temperature rupture behavior was dependent on the precrack stress, the lower pre￾crack stress resulting in longer rupture life for a given stress. It was found that the rupture lives of C-interphase Hi-NicalonTM minicomposites were superior to C-interphase Ceramic Grade NicalonTM minicomposites and inferior to those of BN-interphase Hi-NicalonTM minicomposites. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: C-interphase; Composites; Mechanical properties; SiC±SiC; SiC ®bres 1. Introduction The poor intermediate temperature tensile-rupture behavior in air of ceramic grade NicalonTM [Nippon Carbon (NIC), Tokyo, Japan] reinforced CVI SiC matrix composites with carbon interphases has been demonstrated over temperatures ranging from 425 to 1000C.1ÿ7 The time to failure for all these studies cor￾responds to a stress exponent, n, of approximately 1/4,4 where time to failure, t, is directly proportional to com￾posite stress, , to the power n: t  ÿn …1† This corresponds to a rupture strength degradation of over 70% for rupture times less than 10 h. Two mechanisms have been put forward for the reduction in strength of the NIC/SiC system with C interphases: the increase in e€ective gage length from carbon volatiliza￾tion3 and the ¯aw size increase due to oxide scale growth4 on the surface of the ®bers. The latter mechan￾ism would predict a stress exponent of 1/4 assuming parabolic oxide growth at intermediate temperatures. However, this strength degradation at temperature, in air, and at stress is far more severe than could be accounted for from these two mechanisms. For exam￾ple, if only one crack existed in the gage section of a NIC/SiC composite, the fully loaded gage length of the ®bers could be approximated by: 1 ˆ R=2f …2† where R is the ®ber radius, f is the volume fraction of load-bearing ®bers, and  is the interfacial shear strength.8 For the case where  equals 240 MPa, f equals 0.16,  equals 10 MPa, and assuming a Weibull modulus m for individual ®ber failure equal to 5, the maximum decrease in strength due to an increase in gage length (12 mm for the hot zone in the Ref. 2 study) would be 45%. If more than one crack were in the hot zone of the furnace the degradation in rupture strength due to this mechanism would be less. In addition, if the oxide scale was related to the ¯aw size, one would expect an increase in ¯aw size (oxide scale thickness) of approximately one order of magnitude if the rupture strength was reduced from 2000 to 500 MPa (a 75% decrease) over 100 h at 700C, assuming a KIC of 2.9 For the Ref. 2 study, SiO2 scales were not detectable on the ®ber surfaces at these low temperatures, even though 0955-2219/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0955-2219(00)00138-2 Journal of the European Ceramic Society 20 (2000) 2627±2636 * Corresponding author. E-mail address: gmorscher@grc.nasa.gov (G.N. Morscher)