E噩≈S Journal of the European Ceramic Society 22(2002)2777-2787 www.elsevier.com/locate/jeurcerar Intermediate temperature strength degradation in Sic/Sic composites G.N. Morschera.*, J.D. Cawley oHio Aerospace Institute(OAl), NASA Glenn Research Center, MS 106-5, Cleveland, OH 44135, US.A Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, OH 44106, US.A Received 20 November 2001; received in revised form 5 February 2002: accepted 23 February 2002 Abstract Woven silicon carbide fiber-reinforced, silicon carbide matrix composites are leading candidate materials for an advanced jet engine combustor liner application. Although the use temperature in the hot region for this application is expected to exceed 1200C, a potential life-limiting concern for this composite system exists at intermediate temperatures(800=+200C), where sig- nificant time-dependent strength degradation has been observed under stress-rupture loading. A number of factors control the degree of stress-rupture strength degradation. the major factor being the nature of the interphase separating the fiber and the matrix. BN interphases are superior to carbon interphases due to the slower oxidation kinetics of BN. A model for the intermediate temperature stress-rupture of SiC/BN/SiC composites is presented based on the observed mechanistic process that leads to strength degradation for the simple case of through-thickness matrix cracks. The approach taken has much in common with that used by urtin and coworkers, for two different composite systems. The predictions of the model are in good agreement with the rupture data for stress-rupture of both precracked and as-produced composites. Also, three approaches that dramatically improve the intermediate temperature stress-rupture properties are described: Si-doped BN, fiber spreading, and"outside debonding".C 2002 Elsevier Science Ltd. All rights reserved Keywords: BN interfaces: Mechanical properties; Oxidation; SiC/SiC composites 1. Introduction liner I because the"cold side ' of the combustor liner would be exposed to this temperature range, and this Non-oxide ceramic matrix composites(CMCs)such would be the portion of the combustor liner under the as SiC fiber-reinforced SiC matrix composites are envi This is sioned for use as high-temperature,> 1200C, compo- the combustor liner would have to be attached to a nents of gas turbine engines. -However, there exists an metal frame. Therefore, it is important to understand intermediate temperature (600 to 1000 C) regime and predict the time-dependent mechanical behavior at where significant, time-dependent, strength degradation intermediate temperatures for design of these compo- can occur Depending on the application, the inter- sites in components, as well as for finding insights mediate temperature properties may be critical for suc- toward improvement of the intermediate temperature cess of that component. For example, the strength properties. degradation at intermediate temperatures could be an The primary cause for intermediate temperature issue for some applications, e.g. a cooled combustor strength degradation is the oxidation of a non-oxide interphase, usually C or BN, that separates the fibers from the matrix. For C interphases, rapid interphase Corresponding author. Tel: +1-216-433-5512; fax: +1-216-433. removal 5-7 can be associated with fiber strength degra- dation in the form of oxide scale formation 8 or pre- E-mail address: gmorscher(a grc. nasa. gov (G.N. morscher) ferential oxidation of carbon enriched areas on the fiber a By intermediate temperature, we mean an elevated temperature below the supposed use temperature at which the material exhibits a urface. For BN interphases, BN oxidizes to form minima in mechanical properties, i.e. what is sometimes called a liquid B2O3(boria) that leads to the formation of a glass at the interphase region and in the 0955-2219/02/S- see front matter C 2002 Elsevier Science Ltd. All rights reserved. PII:S0955-2219(02)00Intermediate temperature strength degradation in SiC/SiC composites G.N. Morschera,*, J.D. Cawleyb a Ohio Aerospace Institute (OAI), NASA Glenn Research Center, MS 106-5, Cleveland, OH 44135, USA bDepartment of Materials Science andEngineering, Case Western Reserve University, Cleveland, OH 44106, USA Received 20 November 2001; received in revised form 5 February 2002; accepted 23 February 2002 Abstract Woven silicon carbide fiber-reinforced, silicon carbide matrix composites are leading candidate materials for an advanced jet engine combustor liner application. Although the use temperature in the hot region for this application is expected to exceed 1200
C, a potential life-limiting concern for this composite system exists at intermediate temperatures (800200
C), where sig￾nificant time-dependent strength degradation has been observed under stress-rupture loading. A number of factors control the degree of stress-rupture strength degradation, the major factor being the nature of the interphase separating the fiber and the matrix. BN interphases are superior to carbon interphases due to the slower oxidation kinetics of BN. A model for the intermediate temperature stress-rupture of SiC/BN/SiC composites is presented based on the observed mechanistic process that leads to strength degradation for the simple case of through-thickness matrix cracks. The approach taken has much in common with that used by Curtin and coworkers, for two different composite systems. The predictions of the model are in good agreement with the rupture data for stress-rupture of both precracked and as-produced composites. Also, three approaches that dramatically improve the intermediate temperature stress-rupture properties are described: Si-doped BN, fiber spreading, and ‘‘outside debonding’’. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: BN interfaces; Mechanical properties; Oxidation; SiC/SiC composites 1. Introduction Non-oxide ceramic matrix composites (CMCs) such as SiC fiber-reinforced SiC matrix composites are envi￾sioned for use as high-temperature, 51200
C, compo￾nents of gas turbine engines.13 However, there exists an intermediate temperature (600 to 1000
C)a regime where significant, time-dependent, strength degradation can occur.4 Depending on the application, the inter￾mediate temperature properties may be critical for suc￾cess of that component. For example, the strength degradation at intermediate temperatures could be an issue for some applications, e.g. a cooled combustor liner,1 because the ‘‘cold side’’ of the combustor liner would be exposed to this temperature range, and this would be the portion of the combustor liner under the highest tensile stress. This is especially the case where the combustor liner would have to be attached to a metal frame. Therefore, it is important to understand and predict the time-dependent mechanical behavior at intermediate temperatures for design of these compo￾sites in components, as well as for finding insights toward improvement of the intermediate temperature properties. The primary cause for intermediate temperature strength degradation is the oxidation of a non-oxide interphase, usually C or BN, that separates the fibers from the matrix.4 For C interphases, rapid interphase removal 57 can be associated with fiber strength degra￾dation in the form of oxide scale formation 8 or pre￾ferential oxidation of carbon enriched areas on the fiber surface.9 For BN interphases, BN oxidizes to form liquid B2O3 (boria) that leads to the formation of a borosilicate glass at the interphase region and in the 0955-2219/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0955-2219(02)00144-9 Journal of the European Ceramic Society 22 (2002) 2777–2787 www.elsevier.com/locate/jeurceramsoc * Corresponding author. Tel.: +1-216-433-5512; fax: +1-216-433- 5544. E-mail address: gmorscher@grc.nasa.gov (G.N. Morscher). a By intermediate temperature, we mean an elevated temperature below the supposed use temperature at which the material exhibits a minima in mechanical properties, i.e. what is sometimes called a ‘‘pest’’ condition.4