Availableonlineatwww.sciencedirect.com SCIENCE E噩≈S ournal of the European Ceramic Society 24(2004)699-703 www.elsevier.com/locate/jeurceramsoc Mechanical properties of fibrous monolithic Si3 N4/BN ceramics with different cell boundary thicknesses Young- Hag Koh*, Hae-Won Kim, Hyoun-Ee Kim chool of Materials Science and Engineering, Seoul National University, Seoul, 151-742, South Korea Received 3 February 2003: received in revised form 12 March 2003; accepted 15 March 2003 Fibrous Si3 N/BN monoliths with various cell boundary thickness were fabricated by hot-pressing. The extruded Si3 N4 polymer was dip-coated with the BN-containing slurry with a different BN concentration. a cell boundary thickness increase up to 42 um by adjusting the bn concentration from 0 to 25 wt. in the slurry, and this cell boundary thickness was found to play an important ole in determining the fracture behavior of the fibrous monoliths On increasing cell boundary thickness, the flexural strength decreased, but the apparent work-of-fracture and toughness increased remarkably due to the extensive crack interactions with weak C 2003 Elsevier Ltd. All rights reserved Keywords: BN; Composites: Si3N4; Strength; Toughness; Work-of-fracture 1. Introduction Research on fibrous Si3 N4/BN monoliths has been focused on finding the mechanism of crack deflection Silicon nitride(Si N4) is regarded as one of the most and delamination+ 12,22-24 and on the optimization of promising materials for high-temperature structural their mechanical properties through processing applications due to its excellent thermomechanical improvements. 2,25 Crack interactions are strongly properties, such as high strength, hardness, and resis- dependent on the characteristics of both the cell and the tance to creep and oxidation at elevated temperatures. cell boundaries, especially their relative elastic moduli, however, its wider utilization has been limited mainly strengths, toughnesses, and thermal expansion coeffi- ecause of its catastrophic fracture behavior. Many cients. Also, the cell boundary thickness is expected to efforts have been made to prevent the catastrophic play a key role in the fracture behavior of fibrous fracture pattern of ceramics, including fiber reinfor- monoliths cement, 3 lamination, 4-10 and the production of fibrous Therefore, in the present research, we investigated the ceramics 1-2I Among those composites, fibrous mono- effect of cell boundary thickness on mechanical proper lithic ceramics consist of a primary pl l1) ties, including strength, apparent work-of-fracture, and and a tailored phase(BN cell boundary). 2 Non-catas- fracture toughness of fibrous monolithic Si3 N4/BN trophic failures are frequently observed in fibrous composites. The thickness of the cell boundaries was monolithic materials because of crack interactions with controlled by changing the bn concentration in the e weak cell boundaries 12-21 In other words. cracks slurry used for the dip-coating process propagate through the cell boundaries, which results in a high fracture energy requirement, as is the case of crack deflection or crack delamination 2. Experimental procedures A high purity a-Si3 N4 powder (E-10, Ube Industries, 4 Corresponding author. Now with University of Michigan. Tokyo, Japan)was mixed with 5 wt. yttria(Grade F, H. C. Starck GmbH Co., Berlin, Germany) and 2 0955-2219/03/S. see front matter C 2003 Elsevier Ltd. All rights reserved. doi:10.1016S0955-221903)00266-8Mechanical properties of fibrous monolithic Si3N4/BN ceramics with different cell boundary thicknesses Young-Hag Koh*, Hae-Won Kim, Hyoun-Ee Kim School of Materials Science and Engineering, Seoul National University, Seoul, 151-742, South Korea Received 3 February 2003; received in revised form 12 March 2003; accepted 15 March 2003 Abstract Fibrous Si3N4/BN monoliths with various cell boundary thickness were fabricated by hot-pressing. The extruded Si3N4 polymer was dip-coated with the BN-containing slurry with a different BN concentration. A cell boundary thickness increase up to 42 mm by adjusting the BN concentration from 0 to 25 wt.% in the slurry, and this cell boundary thickness was found to play an important role in determining the fracture behavior of the fibrous monoliths. On increasing cell boundary thickness, the flexural strength decreased, but the apparent work-of-fracture and toughness increased remarkably due to the extensive crack interactions with weak cell boundaries. This is related to the change in stored energy before fracture initiation and in interfacial fracture resistance. # 2003 Elsevier Ltd. All rights reserved. Keywords: BN; Composites; Si3N4; Strength; Toughness; Work-of-fracture 1. Introduction Silicon nitride (Si3N4) is regarded as one of the most promising materials for high-temperature structural applications due to its excellent thermomechanical properties, such as high strength, hardness, and resis￾tance to creep and oxidation at elevated temperatures. However, its wider utilization has been limited mainly because of its catastrophic fracture behavior. Many efforts have been made to prevent the catastrophic fracture pattern of ceramics,1,2 including fiber reinfor￾cement,3 lamination,4
10 and the production of fibrous ceramics.11
21 Among those composites, fibrous mono￾lithic ceramics consist of a primary phase (Si3N4 cell) and a tailored phase (BN cell boundary).12 Non-catas￾trophic failures are frequently observed in fibrous monolithic materials because of crack interactions with the weak cell boundaries.12
21 In other words, cracks propagate through the cell boundaries, which results in a high fracture energy requirement, as is the case of crack deflection or crack delamination. Research on fibrous Si3N4/BN monoliths has been focused on finding the mechanism of crack deflection and delamination4,12,22
24 and on the optimization of their mechanical properties through processing improvements.12,25 Crack interactions are strongly dependent on the characteristics of both the cell and the cell boundaries, especially their relative elastic moduli, strengths, toughnesses, and thermal expansion coeffi- cients.12 Also, the cell boundary thickness is expected to play a key role in the fracture behavior of fibrous monoliths. Therefore, in the present research, we investigated the effect of cell boundary thickness on mechanical proper￾ties, including strength, apparent work-of-fracture, and fracture toughness of fibrous monolithic Si3N4/BN composites. The thickness of the cell boundaries was controlled by changing the BN concentration in the slurry used for the dip-coating process. 2. Experimental procedures A high purity a-Si3N4 powder (E-10, Ube Industries, Tokyo, Japan) was mixed with 5 wt.% yttria (Grade F, H. C. Starck GmbH & Co., Berlin, Germany) and 2 0955-2219/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0955-2219(03)00266-8 Journal of the European Ceramic Society 24 (2004) 699–703 www.elsevier.com/locate/jeurceramsoc * Corresponding author. Now with University of Michigan. E-mail address: younghag@engin.umich.edu (Y.-H. Koh)