Availableonlineatwww.sciencedirect.com SCIENCE E噩≈S Journal of the European Ceramic Society 24(2004)2339-234 www.elsevier.com/locate/jeurceramsoc Thermal shock resistance of fibrous monolithic Si3 N4/bn ceramics Young-Hag Koha, b,s*, Hae-Won Kima, Hyoun-Ee Kima, John W.Halloran a School of Materials Science and Engineering, Seoul National University, Seoul, 151-742, South Korea b Materials Science and Engineering Department, University of Michigan, Ann Arbor, MI48109-2136, USA Received 15 January 2003: received in revised form 25 June 2003: accepted 6 July 2003 Abstract Thermal shock resistance of fibrous monolithic Si3 N4/BN ceramic was investigated by measuring the strength retention after varying the temperature difference(AT)up to 1400C and was compared with that of monolithic Si3N4 Monolithic Si3 N4 showed catastrophic drop in flexural strength above AT of 1000C, while FM showed negligible reduction in flexural strength without critical temperature difference(AT). Two parameters, such as the resistance to crack initiation(R) and crack propagation(r). were used in order to explain the thermal shock behaviors of fibrous monolith and monolithic Si3N4. Furthermore crack interac- tions during flexural testing, such as delamination cracks and crack deflection, were characterized and were related to the work-of- racture (WoF) C) 2003 Elsevier Ltd. All rights reserved. Keywords: BN; Composites; Fibrous monoliths: Si3N4; Thermal shock 1. Introduction there are some possible methods to increase the thermal shock resistance of materials. For example, the addition Fibrous monoliths have been regarded as promising of ductile secondary phase into Al2O3 matrix increases materials for structural applications because of the the thermal shock resistance due to both reduced elastic noncatastrophic failure due to its unique modulus and increased fracture toughness. Also, flaw- architecture. 1-7 Fibrous monoliths are sintered or hot- tolerant material, such as fiber(or whisker)-reinforced pressed monolithic ceramics with a distinct fibrous tex- ceramics and laminated ceramics shows excellent ther ture consisting of strong cell and weak cell boundary mal shock resistance due to the increased resistance to that act as a easy crack path. One of the most promis- crack propagation through crack interactions with ing fibrous monoliths for high temperature applications toughening agents (fiber, whisker and weak inter is Si3N4/BN system because of its high strength and face) 6- However, so far, in spite of its importance for oxidation resistance at elevated temperature. -7 high temperature applications, no research has beer Since these composite materials are candidates as the done on thermal shock resistance of fibrous monolith high-temperature applications (e.g. in gas turbine In this paper, we have investigated thermal shock engines), it is inevitable to involve some kind of thermal resistance of fibrous monolithic Si3 N,/BN ceramics with shock loading. Most ceramics showed catastrophe temperature difference ranging from 800 to 1400C, by drops in mechanical properties, such as flexural measuring the retention of mechanical properties, such strength, elastic modulus, after thermal shock above the as flexural strength and work-of-fracture (WOF). For critical temperature(AT). -l5 This catastrophic drop the purpose of comparison, monolithic Si3 N4 was also in mechanical properties after thermal shock have lim- tested under the same conditions. ited the wide applications at high-temperatures Thermal shock resistance is dependent on several pri mary mechanical properties, such as fracture toughness, 2. Experimental fracture behavior, fracture strength, elastic modulus and coefficient of thermal expansion of material. 11, 2 Hence, 2. 1. Billet fabrication Fibrous monolithic Si3N4/BN ceramic was fabricated E-mail address: younghag(@engin. umich.edu (Y -H. Koh) using coextrusion process to produce a structure with 0955-2219S. see front matter C 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0955-2219(03)00644-7Thermal shock resistance of fibrous monolithic Si3N4/BN ceramics Young-Hag Koha,b,*, Hae-Won Kima , Hyoun-Ee Kima , John W. Halloranb a School of Materials Science and Engineering, Seoul National University, Seoul, 151-742, South Korea bMaterials Science and Engineering Department, University of Michigan, Ann Arbor, MI 48109-2136, USA Received 15 January 2003; received in revised form 25 June 2003; accepted 6 July 2003 Abstract Thermal shock resistance of fibrous monolithic Si3N4/BN ceramic was investigated by measuring the strength retention after varying the temperature difference (
T) up to 1400
C and was compared with that of monolithic Si3N4. Monolithic Si3N4 showed catastrophic drop in flexural strength above
T of 1000
C, while FM showed negligible reduction in flexural strength without critical temperature difference (
Tc). Two parameters, such as the resistance to crack initiation (R0 ) and crack propagation (R0000), were used in order to explain the thermal shock behaviors of fibrous monolith and monolithic Si3N4. Furthermore, crack interac￾tions during flexural testing, such as delamination cracks and crack deflection, were characterized and were related to the work-of￾fracture (WOF). # 2003 Elsevier Ltd. All rights reserved. Keywords: BN; Composites; Fibrous monoliths; Si3N4; Thermal shock 1. Introduction Fibrous monoliths have been regarded as promising materials for structural applications because of the noncatastrophic failure due to its unique architecture.17 Fibrous monoliths are sintered or hot￾pressed monolithic ceramics with a distinct fibrous tex￾ture consisting of strong cell and weak cell boundary that act as a easy crack path.1 One of the most promis￾ing fibrous monoliths for high temperature applications is Si3N4/BN system because of its high strength and oxidation resistance at elevated temperature.47 Since these composite materials are candidates as the high-temperature applications (e.g. in gas turbine engines), it is inevitable to involve some kind of thermal shock loading. Most ceramics showed catastrophic drops in mechanical properties, such as flexural strength, elastic modulus, after thermal shock above the critical temperature (
Tc).1115 This catastrophic drop in mechanical properties after thermal shock have lim￾ited the wide applications at high-temperatures. Thermal shock resistance is dependent on several pri￾mary mechanical properties, such as fracture toughness, fracture behavior, fracture strength, elastic modulus and coefficient of thermal expansion of material.11,12 Hence, there are some possible methods to increase the thermal shock resistance of materials. For example, the addition of ductile secondary phase into Al2O3 matrix increases the thermal shock resistance due to both reduced elastic modulus and increased fracture toughness.14 Also, flaw￾tolerant material, such as fiber (or whisker)-reinforced ceramics and laminated ceramics shows excellent ther￾mal shock resistance due to the increased resistance to crack propagation through crack interactions with toughening agents (fiber, whisker and weak inter￾face).1618 However, so far, in spite of its importance for high temperature applications, no research has been done on thermal shock resistance of fibrous monolith. In this paper, we have investigated thermal shock resistance of fibrous monolithic Si3N4/BN ceramics with temperature difference ranging from 800 to 1400
C, by measuring the retention of mechanical properties, such as flexural strength and work-of-fracture (WOF). For the purpose of comparison, monolithic Si3N4 was also tested under the same conditions. 2. Experimental 2.1. Billet fabrication Fibrous monolithic Si3N4/BN ceramic was fabricated using coextrusion process to produce a structure with 0955-2219/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0955-2219(03)00644-7 Journal of the European Ceramic Society 24 (2004) 2339–2347 www.elsevier.com/locate/jeurceramsoc * Corresponding author. E-mail address: younghag@engin.umich.edu (Y.-H. Koh).