MAERIAL ELSEVIER Materials Science and Engineering A195( 1995)263-268 Fibrous monoliths: non-brittle fracture from powder-processed ceramIcs G. Hilmas, A Brady, U. Abdali, G. Zywicki, J. Halloran Department of Materials Science and Engineering, 2168 H.H. Dow Building, University of Michigan, Ann Arbor, MI48100-2l36,USA Abstract Fibrous monoliths are non brittle cerami ated from conventional powders using new methods that create a fibrous mesostructure to obtain an in situ composite C BN fibrous monoliths fracture like wood, with graceful failure occurring by shear delamination. Examples are Si3N4-BN monoliths, which have flexural strengths around 460 MPa between room temperature and 1000'C and retain about half of their load ng ability after fracture initiates, which apparent fracture energies up to 4800 J m-2. (SiC-alloy)-BN fibrous monoliths have flex rengths around 400 MPa and exhibit graceful failure between room temperature and 1200C Ceramic-metal fibrous monoliths also have graceful failure. Results are presented for (yttria-zirconia)-Ni fibrous monoliths, which fail by tensile cracking, with crack bridging by ductile metal ligaments. About 25 vol %o Ni imparts a fracture Keywords: Fibres; Monoliths; Fracture; Powders; Ceramics 1. Introduction extruded to form"green fibers", the precursor for the woody texture of fibrous monoliths. The Si3N4-BN Ceramics have no useful ductility, but their brittle- fibrous monoliths are fabricated from silicon nitride ness can be ameliorated so they can sustain load after polymer green fibers. The fibers are subsequently dip the onset of fracture and avod catastrophic crack coated with a shurry of boron nitride powder. The propagation, becoming effectively non-brittle Ceramic coated green fibers are layed up uniaxially, molded into fiber reinforced ceramic matrix composites were the a solid block, and then hot pressed, creating a mono- first practical examples of non-brittle ceramics, but lithic ceramic containing cells" of polycrystalline their application has been limited by high cost. Similar Si3 N4, the remnant of the green fibers, separated by behavior can be obtained with less expensive powder- thin " cell boundaries"of BN, the remnant of the processed ceramics if there are planes of weakness in coating. The cell-cell boundary mesostructure of the composite providing shear delamination in the fibrous monoliths creates the woody texture and ceramic during fracture. Clegg et al. 1 demonstrated enables graceful failure to be achieved. The SiC-BN very tough Sic-graphite laminated ceramics where the fibrous monoliths are made in a similar fashion using a graphite layers delaminate during flexural failure. high strength "BS80"SiC alloy for the cells. This Bs80 Based on the "fibrous monolith "microstructure design consists of 80 vol. B-SiC 20 vol. 3AIN: 1Al2O3 of Coblenz [2 we have previously reported a variety of which produces a two-phase alloy with the 6H and 2H non-brittle ceramics [3-7, with typical strengths polytypes [8, 9]. These boron-nitride-containing fibrous around 200 MPa. In this paper we present new results monoliths are non-brittle because of delamination of on more refined versions of these systems, with signifi- the weak BN cell boundaries, promoting shear crack- cantly improved strength and fracture energy ing and serving to deflect tensile cracks Fibrous monolithic(FM)ceramics have a wood-like A second variety of fibrous monolith employs texture, created by extrusion. A mixture of ductile metal cell boundaries. With as little as 7 vol% powder with thermoplastic polymers can be nickel, for example, alumina can exhibit graceful failureELSEVIER Materials Science and Engineering A195 (1995) 263-268 MATERIALS SCIENCE & ENGINEERING A Fibrous monoliths: non-brittle fracture from powder-processed ceramics G. Hilmas, A. Brady, U. Abdali, G. Zywicki, J. Halloran Department of Materials Science and Engineering, 2168 H.H. Dow Building, University of Michigan, Ann Arbor, MI48109-2136, USA Abstract Fibrous monoliths are non-brittle ceramics, fabricated from conventional powders using new methods that create a fibrous mesostructure to obtain an in situ composite. Ceramic-BN fibrous monoliths fracture like wood, with graceful failure occurring by shear delamination. Examples are Si3N4-BN fibrous monoliths, which have flexural strengths around 460 MPa between room temperature and 1000 °C and retain about half of their load-bearing ability after fracture initiates, which apparent fracture energies up to 4800 J m-2. (SiC-alloy)-BN fibrous monoliths have flexural strengths around 400 MPa and exhibit graceful failure between room temperature and 1200 °C. Ceramic-metal fibrous monoliths also have graceful failure. Results are presented for (yttria-zirconia)-Ni fibrous monoliths, which fail by tensile cracking, with crack bridging by ductile metal ligaments. About 25 vol.% Ni imparts a fracture energy of 2200 J m- 2, with a strength of 574 MPa. Keywords: Fibres; Monoliths; Fracture; Powders; Ceramics 1. Introduction Ceramics have no useful ductility, but their brittle￾ness can be ameliorated so they can sustain load after the onset of fracture and avod catastrophic crack propagation, becoming effectively non-brittle. Ceramic fiber reinforced ceramic matrix composites were the first practical examples of non-brittle ceramics, but their application has been limited by high cost. Similar behavior can be obtained with less expensive powder￾processed ceramics if there are planes of weakness in the composite providing shear delamination in the ceramic during fracture. Clegg et al. [1] demonstrated very tough SiC-graphite laminated ceramics where the graphite layers delaminate during fiexural failure. Based on the "fibrous monolith" microstructure design of Coblenz [2], we have previously reported a variety of non-brittle ceramics [3-7], with typical strengths around 200 MPa. In this paper we present new results on more refined versions of these systems, with signifi￾cantly improved strength and fracture energy. Fibrous monolithic (FM) ceramics have a wood-like fibrous texture, created by extrusion. A mixture of ceramic powder with thermoplastic polymers can be 0921-5093/95/$9.50 © 1995 - Elsevier Science S.A. All rights reserved SSD1 0921-5093(94)06525-X extruded to form "green fibers", the precursor for the woody texture of fibrous monoliths. The Si3N4-BN fibrous monoliths are fabricated from silicon nitride￾polymer green fibers. The fibers are subsequently dip coated with a slurry of boron nitride powder. The coated green fibers are layed up uniaxially, molded into a solid block, and then hot pressed, creating a mono￾lithic ceramic containing "cells" of polycrystalline Si3N4, the remnant of the green fibers, separated by thin "cell boundaries" of BN, the remnant of the coating. The cell-cell boundary mesostructure of fibrous monoliths creates the woody texture and enables graceful failure to be achieved. The SiC-BN fibrous monoliths are made in a similar fashion using a high strength "BS80" SiC alloy for the cells. This BS80 consists of 80 vol.% r-sic+20 vol.% 3A1N:lAI203, which produces a two-phase alloy with the 6H and 2H polytypes [8,9]. These boron-nitride-containing fibrous monoliths are non-brittle because of delamination of the weak BN cell boundaries, promoting shear crack￾ing and serving to deflect tensile cracks. A second variety of fibrous monolith employs ductile metal cell boundaries. With as little as 7 vol.% nickel, for example, alumina can exhibit graceful failure