Corrosion Science 50(2008)3132-3138 Contents lists available at Science Direct Corrosion science ELSEVIER journalhomepagewww.elsevier.com/locate/corsci Thermal and mechanical stabilities of Hi-Nicalon SiC fiber under annealing and creep in various oxygen partial pressures - Sha,, T Hinoki, A Kohyama Graduate School of energy Science, Kyoto University, Gokasho, Uji Kyoto 611-0011, Japan b Institute of Advanced Energy, Kyoto University. Gokasho, Uji, Kyoto 611-0011, Japan ARTICLE INF O A BSTRACT Article history Thermal and mechanical stabilities were investigated on Hi-Nicalon SiC fibers under annealing and creep Accepable 18May2008 13 August 2008 in various oxygen partial pressures by mass change, mechanical properties as well as microstructural online 19 August 2008 features. In the case of mechanical stability, the tensile strength of fiber is strongly dependent on the oxy- en partial pressure of testing environment, but a weak dependence of BSr creep resistance on oxygen artial pressure is appeared. The analyses of surface morphology and mass change indicated the thermal Keywords: A Ceramic tability of fiber under annealing in different environments was different. At different exposure temper B. X-ray diffraction C Oxidatio by the stress applied through BSR test. This means the thermal stability of fibers is related to not only t <posed environments, but also the mechanical state of fibers. G 2008 Elsevier Ltd. All rights reserved. 1 Introduction pressures. In such case, the Sic materials would be oxidized in pas Ceramic-matrix composites(CMCs)are been considering as the degradation of Sic materials in oxidizing environments strongly potential structural materials for advanced energy generation sys- depends on the oxidation mechanism. Jacobson[6] has generalized tems and propulsion systems. As we know, the monolithic ceramic the oxidation degradation mechanism of Sic materials in varied naterials are very brittle in the fracture behaviour. with the rein environments, but it is still insufficient because of the complexity forcement of fibers, the fracture toughness and impact capability of of service environments. The key question concerns the oxidation ceramic materials could be improved significantly. Thus, the load- kinetics: passive and active oxidation. This topic has given rise to bearing SiC fibers as reinforcements in CMCs are backbone, and much controversy for Sic materials, because the temperature acceptable performance of high temperature CMCs strongly de- boundaries for the oxidation kinetics are quite dependent not only pends upon judicious selection and incorporation of ceramic fiber on the materials themselves(purity and crystallinity) but also on reinforcement with the proper chemical, physical and mechanical the specific service environment(exposure temperature, oxygen properties. Low fiber strength and thermal stability could result partial pressure and mechanical state ) Furthermore, rarely is one in low fracture toughness and accelerate sub-critical crack mechanism operative in performance degradation of SiC materials. gation in CMCs In the past few decade years, extensive efforts have In practice, several mechanisms operate simultaneously een made to develop the SiC fibers in order to satisfy the require- SiC fibers as the reinforcements of structural materials for higl ments from the high temperature technologies. Especially, in re- temperature technologies, the most desired critical properties are cent years, the Sic fibers with high thermal stability, which were excellent high temperature mechanical and thermal stabilities. cured by electron-beam irradiation [1], are considering as the Thus, in an specific service environment, the environmental dura- promising reinforcement in CMCs. However, the mechanical and bility and the response of reinforced fibers to service environment thermal stabilities of SiC fibers as reinforcements in CMCs are very are major concern and they should be revealed for the reliability sensitive to their purity, crystallinity and service environments[2- evaluation of CMCs. In our previous works [7, 8] the fracture fea- 4 including thermal and loading history. ures for this fiber have been characterized, but no attempts were For high temperature applications, the CMCs are often sub- made to correlate the environment with microstructure and high jected to oxidative environments with different oxygen partial temperature mechanical properties. For practie dication of MCs, it is requested to accumulate Corresponding author. Tel: +81 774 38 3463: fax: +81 774 38 3467 veal the degradation mechanism of Sic fibers with a consideration E-mailaddress:shajianjun720@yahoo.com(.].Sha). of the mechanical and thermal stabilities 0010-938X/s front matter 2008 Elsevier Ltd. all rights reserved. do:101016/ J- corsi2008.08003Thermal and mechanical stabilities of Hi-Nicalon SiC fiber under annealing and creep in various oxygen partial pressures J.J. Sha a,*, T. Hinoki b , A. Kohyama b aGraduate School of Energy Science, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan b Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan article info Article history: Received 18 May 2008 Accepted 13 August 2008 Available online 19 August 2008 Keywords: A. Ceramic B. SEM B. X-ray diffraction C. Oxidation abstract Thermal and mechanical stabilities were investigated on Hi-Nicalon SiC fibers under annealing and creep in various oxygen partial pressures by mass change, mechanical properties as well as microstructural features. In the case of mechanical stability, the tensile strength of fiber is strongly dependent on the oxy￾gen partial pressure of testing environment, but a weak dependence of BSR creep resistance on oxygen partial pressure is appeared. The analyses of surface morphology and mass change indicated the thermal stability of fiber under annealing in different environments was different. At different exposure temper￾atures and oxygen partial pressure levels, the different oxidation regimes are responsible for the strength degradation and microstructure change of fibers. Furthermore, the microstructure change is also affected by the stress applied through BSR test. This means the thermal stability of fibers is related to not only the exposed environments, but also the mechanical state of fibers. 2008 Elsevier Ltd. All rights reserved. 1. Introduction Ceramic–matrix composites (CMCs) are been considering as the potential structural materials for advanced energy generation sys￾tems and propulsion systems. As we know, the monolithic ceramic materials are very brittle in the fracture behaviour. With the rein￾forcement of fibers, the fracture toughness and impact capability of ceramic materials could be improved significantly. Thus, the load￾bearing SiC fibers as reinforcements in CMCs are backbone, and acceptable performance of high temperature CMCs strongly de￾pends upon judicious selection and incorporation of ceramic fiber reinforcement with the proper chemical, physical and mechanical properties. Low fiber strength and thermal stability could result in low fracture toughness and accelerate sub-critical crack propa￾gation in CMCs. In the past few decade years, extensive efforts have been made to develop the SiC fibers in order to satisfy the require￾ments from the high temperature technologies. Especially, in re￾cent years, the SiC fibers with high thermal stability, which were cured by electron-beam irradiation [1], are considering as the promising reinforcement in CMCs. However, the mechanical and thermal stabilities of SiC fibers as reinforcements in CMCs are very sensitive to their purity, crystallinity and service environments [2– 4] including thermal and loading history. For high temperature applications, the CMCs are often sub￾jected to oxidative environments with different oxygen partial pressures. In such case, the SiC materials would be oxidized in pas￾sive/active oxidation regime [5]. As we know, the performance degradation of SiC materials in oxidizing environments strongly depends on the oxidation mechanism. Jacobson [6] has generalized the oxidation degradation mechanism of SiC materials in varied environments, but it is still insufficient because of the complexity of service environments. The key question concerns the oxidation kinetics: passive and active oxidation. This topic has given rise to much controversy for SiC materials, because the temperature boundaries for the oxidation kinetics are quite dependent not only on the materials themselves (purity and crystallinity), but also on the specific service environment (exposure temperature, oxygen partial pressure and mechanical state). Furthermore, rarely is one mechanism operative in performance degradation of SiC materials. In practice, several mechanisms operate simultaneously. SiC fibers as the reinforcements of structural materials for high temperature technologies, the most desired critical properties are excellent high temperature mechanical and thermal stabilities. Thus, in an specific service environment, the environmental dura￾bility and the response of reinforced fibers to service environment are major concern and they should be revealed for the reliability evaluation of CMCs. In our previous works [7,8], the fracture fea￾tures for this fiber have been characterized, but no attempts were made to correlate the environment with microstructure and high temperature mechanical properties. For practical application of CMCs, it is requested to accumulate experimental data and to re￾veal the degradation mechanism of SiC fibers with a consideration of the mechanical and thermal stabilities. 0010-938X/$ - see front matter 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2008.08.003 * Corresponding author. Tel.: +81 774 38 3463; fax: +81 774 38 3467. E-mail address: shajianjun720@yahoo.com (J.J. Sha). Corrosion Science 50 (2008) 3132–3138 Contents lists available at ScienceDirect Corrosion Science journal homepage: www.elsevier.com/locate/corsci