E驅≈3S al of the euro Plastic deformation of silicon nitride/boron nitride fibrous monoliths A.R. de arellano-Lopeza,*,S. Lopez-Pomberoa, A Dominguez-Rodriguez a J. L. Routbort b d. singh b K.c. goretta Departamento de Fisica de la Materia Condensada, Universidad de sevilla, PO Box 1065, 41080 Seville, spain Energy Technology Division, Argonne National Laboratory, Argonne, IL 60349-4838, USA Received 27 November 1999: received in revised form 31 May 2000: accepted 1l June 2000 High-temperature compressive creep of unidirectional Si3 N4/BN fibrous monoliths has been investigated at 1300-1500oC in an inert tmosphere. The results were then compared to those for deformation of the Si3 N4 and bn base materials. Plasticity of the fibrous monoliths was limited to very low stresses when the Si3N4 cells were oriented perpendicular to the stress axis because the bn cell steady-state deformation controlled by deformation of the Si3 N4 cells was achieved. C 2001 Elsevier Science Ltd. All rights reserved 9 boundaries failed, followed by failure of the Si3 N4 cells. In the fibrous monolith in which cells were oriented parallel to the stress ax Keywords: Creep; BN; Fibrous monoliths; Mechanical properties; Si3N4 1. Introduction creep.,0 Fracture testing of Si3 N4/BN fibrous monoliths in air at temperatures up to 1400C has indicated that Ceramic fibrous monoliths generally consist of strong shear failures become dominant at 1100C and above ceramic cells that are surrounded by a weaker cell and that creep of the Si3 n4 phase becomes significant at boundary(see schematic diagram in Fig. la). These 21400 C. The creep rates have not been quantified monoliths are produced from ceramic powders by con We report here on compressive creep of Si3 N4/BN ventional fabrication techniques, such as extrusion 1.2 fibrous monoliths that were tested in inert atmosphe They exhibit graceful failure; in flexure, they splinter. -5 For comparison, Si3 N4 and BN specimens with compo- In many applications, fibrous monoliths may offer a sitions similar to those of the components of the fibrous low-cost alternative to conventional continuouS-fiber monolith phase were also tested ceramic composites. Several compositions of ceramics and cermets have een processed successfully in rous mon 2. Experimental details The most thoroughly investigated fibrous monolith consists of Si3 N4 cells and a continuous bn cell 2. 1. Specimen fabrication boundary. 3- Through appropriate selection of initial powders, and extrusion and hot-pressing parameters The fibrous monoliths were produced by Advanced strong, very tough Si3 N4/bn products have been ramics Research of Tucson, AZ. They were fabri produced. The high toughness is due primarily to cated from A325-Hm-diameter Si3 N4/BN coextruded crack deflection along the weaker bn phase green filaments that were produced by melt coextrusion Fracture of Si3 N4/BN fibrous monoliths has been of a blend of N52 vol. ceramic powder mixture in an studied extensively at both room and high tempe- ethylene-based copolymer binder. The coextruded ratures 3-I However, very little effort has focused on filaments contained nominally 85 vol. core Si3N4 material(E-10, Ube Industries, Tokyo) and 15 vol% Corresponding author. Tel. +34-95-4552891; ext 96; fax: +34- n cladding(HCP Grade, Advanced Ceramics Cor- poration, Cleveland ) The Si3N4 was of a sinterable E-mail address: ramirez(@ cica. es(A R. de Arellano-Lopez) composition, 92 wt. commercial Si3 N4 powder, 6 0955-2219/01/S. see front matter C 2001 Elsevier Science Ltd. All rights reserved PII:S0955-2219(00)00175Plastic deformation of silicon nitride/boron nitride ®brous monoliths A.R. de Arellano-LoÂpez a,*, S. LoÂpez-Pombero a , A. DomõÂnguez-RodrõÂguez a , J.L. Routbort b, D. Singh b, K.C. Goretta b a Departamento de Fisica de la Materia Condensada, Universidad de Sevilla, PO Box 1065, 41080 Seville, Spain bEnergy Technology Division, Argonne National Laboratory, Argonne, IL 60349-4838, USA Received 27 November 1999; received in revised form 31 May 2000; accepted 11 June 2000 Abstract High-temperature compressive creep of unidirectional Si3N4/BN ®brous monoliths has been investigated at 1300±1500C in an inert atmosphere. The results were then compared to those for deformation of the Si3N4 and BN base materials. Plasticity of the ®brous monoliths was limited to very low stresses when the Si3N4 cells were oriented perpendicular to the stress axis because the BN cell boundaries failed, followed by failure of the Si3N4 cells. In the ®brous monolith in which cells were oriented parallel to the stress axis, steady-state deformation controlled by deformation of the Si3N4 cells was achieved. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Creep; BN; Fibrous monoliths; Mechanical properties; Si3N4 1. Introduction Ceramic ®brous monoliths generally consist of strong ceramic cells that are surrounded by a weaker cell boundary (see schematic diagram in Fig. 1a). These monoliths are produced from ceramic powders by con￾ventional fabrication techniques, such as extrusion.1,2 They exhibit graceful failure; in ¯exure, they splinter.1ÿ5 In many applications, ®brous monoliths may o€er a low-cost alternative to conventional continuous-®ber ceramic composites. Several compositions of ceramics and cermets have been processed successfully in ®brous monolithic form.4 The most thoroughly investigated ®brous monolith consists of Si3N4 cells and a continuous BN cell boundary.3ÿ5 Through appropriate selection of initial powders, and extrusion and hot-pressing parameters, strong, very tough Si3N4/BN products have been produced.2ÿ7 The high toughness is due primarily to crack de¯ection along the weaker BN phase. Fracture of Si3N4/BN ®brous monoliths has been studied extensively at both room and high tempe￾ratures.3ÿ11 However, very little e€ort has focused on creep.9,10 Fracture testing of Si3N4/BN ®brous monoliths in air at temperatures up to 1400C has indicated that shear failures become dominant at 1100C and above and that creep of the Si3N4 phase becomes signi®cant at 1400C.11 The creep rates have not been quanti®ed. We report here on compressive creep of Si3N4/BN ®brous monoliths that were tested in inert atmosphere. For comparison, Si3N4 and BN specimens with compo￾sitions similar to those of the components of the ®brous monolith phase were also tested. 2. Experimental details 2.1. Specimen fabrication The ®brous monoliths were produced by Advanced Ceramics Research of Tucson, AZ. They were fabri￾cated from 325-mm-diameter Si3N4/BN coextruded green ®laments5 that were produced by melt coextrusion of a blend of 52 vol.% ceramic powder mixture in an ethylene-based copolymer binder.11 The coextruded ®laments contained nominally 85 vol.% core Si3N4 material (E-10, Ube Industries, Tokyo) and 15 vol.% BN cladding (HCP Grade, Advanced Ceramics Cor￾poration, Cleveland). The Si3N4 was of a sinterable composition, 92 wt.% commercial Si3N4 powder, 6 0955-2219/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0955-2219(00)00175-8 Journal of the European Ceramic Society 21 (2001) 245±250 www.elsevier.com/locate/jeurceramsoc * Corresponding author. Tel.: +34-95-4552891; ext. 96; fax: +34- 95-4612097. E-mail address: ramirez@cica.es (A.R. de Arellano-LoÂpez)