Availableonlineatwww.sciencedirect.co Science direct materials letters ELSEVIER Materials Letters 60(2006)3197-320 www.elseviercom/locate/matlet Oxidation behavior of 3D Hi-Nicalon/SiC composite Shoujun Wu", Aifei Cheng, Litong Zhang, Yongdong Xu, Qing Zhang National Key Laboratory of Thermostructure Composite Materials, Northwestern Pol al University, Xi'an Shaanxi 710072, People's Republic of China Available online 20 March 2006 Abstract Oxidation behavior of a three dimensional (3D) Hi-Nicalon/SiC composite with Cvd Sic coating was investigated in the simulated air using a thermogravimetric analysis(TGA)device. Below 1100C, the oxidation kinetics was controlled by gas diffusion through the defects in the Sic matrix and coating and resulted in the consumption of PyC interphase. The residual flexural strength did have not a remarkable fluctuation and the relationship between the residual strength to temperature and weight change to temperature of the 3D Hi-Nicalon/PyC/SiC composite indicated the same regularity. Above 1200C, the oxidation kinetics was controlled by oxygen diffusion through the SiO2 scale formed on the Sic coating and matrix. And the residual flexural strength of the composites was governed by the strength degradation of the Hi-Nicalon fiber. After oxidation, the fracture displacement in flexural tests increased with the weight loss increasing and the fracture mode showed a non-brittle pattern C 2006 Elsevier B V. All rights reserved. Keywords: Hi-Nicalon/PyC/SiC; Oxidation; Coating: Residual flexural strength 1. Introduction and the strength degradation of the fiber [8]. Thus, for Nicalon/ SiC composites, the degradation of mechanical properties after Silicon carbide fiber reinforced silicon carbide composites short periods of exposure to air was due to the oxidation and (SiC/SiC)exhibit better oxidation resistance than C/SiC, and are removal of the PyC interlayer [9]. Hi-Nicalon is oxygen-free considered as one of the most promising structural materials for fibers consisting of a mixture of Sic-nanocrystals( 5 nm in high temperature applications [1-3]. In order to improve the mean size)and free carbon [C/Si(at)ratio=1.39]. They do not mechanical properties of fiber reinforced silicon carbide matrix undergo decomposition at high temperatures since they do not composites, interphase of compliant material with low shear contain significant amount of SiCoy phase [10, 11].Hi- strength is necessary. It has also been recognized that the Nicalon/SiC composites are expected to be used above 1400C, interphase layers should be deposited parallel to the fiber and are being paid more and more attention [12]. However the surface and weakly bonded to one another, and the interphase oxidation behavior, especially the oxidation-induced mechan- should be strongly bonded to the fiber surface [2, 4]. Pyrocarbon ical properties changes of C-interphase Hi-Nicalon/SiC(Hi- (PyC)and hexagonal-BN(hex-BN) have been widely used as Nicalon/PyC/SiC) during oxidation have not been much the interphase materials [2, 4-6]. PyC is much liable to researched oxidation than hex-BN. However, hex-BN was poor bonded In order to deeply understanding the main oxidation to the Sic fiber surface and not sufficiently better to be viable at mechanisms of Hi-Nicalon/Py C/Sic materials and the effects above1300°C[4,7 of PyC interphase on the flexural strength after oxidation in air, The Nicalon fiber is consist of Sic-nanocrystals(1-2 nm in the oxidation behavior of a 3D Hi-Nicalon/Py C/SiC with a size)and free carbon embedded in an amorphous SiCO CVD Sic coating was investigated in the present paper. The matrix. At the oxidizing atmosphere, oxidation and decompo- weight change of the composite during oxidation was sition of SicxOy result in pores produced on the fiber surface monitored with thermogravimetric analysis (TGA)device And the relationship between the residual flexural strength to Corresponding author. Tel: +8629 8848 6068 828; fax: +8629 8849 4620. temperature and the weight change to temperature of the E-mailaddress:shoujun-wu(@163.com(S.Wu) composite were also researched. 0167-577X/S-see front matter o 2006 Elsevier B V. All rights reserved. doi:10.1016/ malet200602.072Oxidation behavior of 3D Hi–Nicalon/SiC composite Shoujun Wu ⁎, Laifei Cheng, Litong Zhang, Yongdong Xu, Qing Zhang National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical University, Xi'an Shaanxi 710072, People's Republic of China Received 18 September 2005; accepted 22 February 2006 Available online 20 March 2006 Abstract Oxidation behavior of a three dimensional (3D) Hi–Nicalon/SiC composite with CVD SiC coating was investigated in the simulated air using a thermogravimetric analysis (TGA) device. Below 1100 °C, the oxidation kinetics was controlled by gas diffusion through the defects in the SiC matrix and coating and resulted in the consumption of PyC interphase. The residual flexural strength did have not a remarkable fluctuation and the relationship between the residual strength to temperature and weight change to temperature of the 3D Hi–Nicalon/PyC/SiC composite indicated the same regularity. Above 1200 °C, the oxidation kinetics was controlled by oxygen diffusion through the SiO2 scale formed on the SiC coating and matrix. And the residual flexural strength of the composites was governed by the strength degradation of the Hi–Nicalon fiber. After oxidation, the fracture displacement in flexural tests increased with the weight loss increasing and the fracture mode showed a non-brittle pattern. © 2006 Elsevier B.V. All rights reserved. Keywords: Hi–Nicalon/PyC/SiC; Oxidation; Coating; Residual flexural strength 1. Introduction Silicon carbide fiber reinforced silicon carbide composites (SiC/SiC) exhibit better oxidation resistance than C/SiC, and are considered as one of the most promising structural materials for high temperature applications [1–3]. In order to improve the mechanical properties of fiber reinforced silicon carbide matrix composites, interphase of compliant material with low shear strength is necessary. It has also been recognized that the interphase layers should be deposited parallel to the fiber surface and weakly bonded to one another, and the interphase should be strongly bonded to the fiber surface [2,4]. Pyrocarbon (PyC) and hexagonal–BN (hex–BN) have been widely used as the interphase materials [2,4–6]. PyC is much liable to oxidation than hex–BN. However, hex–BN was poor bonded to the SiC fiber surface and not sufficiently better to be viable at above 1300 °C [4,7]. The Nicalon fiber is consist of SiC–nanocrystals (1∼2 nm in size) and free carbon embedded in an amorphous SiCxOy matrix. At the oxidizing atmosphere, oxidation and decompo￾sition of SiCxOy result in pores produced on the fiber surface and the strength degradation of the fiber [8]. Thus, for Nicalon/ SiC composites, the degradation of mechanical properties after short periods of exposure to air was due to the oxidation and removal of the PyC interlayer [9]. Hi–Nicalon is oxygen–free fibers consisting of a mixture of SiC–nanocrystals (≈5 nm in mean size) and free carbon [C/Si (at) ratio = 1.39]. They do not undergo decomposition at high temperatures since they do not contain significant amount of SiCxOy phase [10,11]. Hi– Nicalon/SiC composites are expected to be used above 1400 °C, and are being paid more and more attention [12]. However the oxidation behavior, especially the oxidation–induced mechan￾ical properties changes of C–interphase Hi–Nicalon/SiC (Hi– Nicalon/PyC/SiC) during oxidation have not been much researched. In order to deeply understanding the main oxidation mechanisms of Hi–Nicalon/PyC/SiC materials and the effects of PyC interphase on the flexural strength after oxidation in air, the oxidation behavior of a 3D Hi–Nicalon/PyC/SiC with a CVD SiC coating was investigated in the present paper. The weight change of the composite during oxidation was monitored with thermogravimetric analysis (TGA) device. And the relationship between the residual flexural strength to temperature and the weight change to temperature of the composite were also researched. Materials Letters 60 (2006) 3197–3201 www.elsevier.com/locate/matlet ⁎ Corresponding author. Tel.: +86 29 8848 6068 828; fax: +86 29 8849 4620. E-mail address: shoujun_wu@163.com (S. Wu). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.02.072