2112 Fibrous Monolith Composites

Even though the fracture toughness of in situ, particulate, and whisker-reinforced ceramics is improved compared to monolithic ceramics, these ceramic matrix composites still fracture in a brittle mode. As is discussed in the next chapter, addition of continuous (long) fibers can result in nonbrittle fracture modes. However, continuous fibers are currently very expensive, and the resulting composites generally are not cost competitive for most industrial applications. The fibrous monolith was a concept introduced in 1988 by Coblenz [47], envisioned to produce a composite structure comparable to the use of continuous fibers, but starting with inexpensive powders.

The concept of a fibrous monolith can best be understood by describing the steps in fabrication and the resulting microstructure. A silicon nitride/boron nitride material is used as the example [48]. The first step is to extrude a viscous mixture of silicon nitride powder (plus sintering aids) and a polymer binder through a small orifice to form a long filament. The filament is then coated with a surface layer of boron nitride. Coated filament also has been achieved in a single coextrusion step. Strands of the BN-coated filament are stacked in the desired orientation (usually unidirectional) in a die and warm pressed at a temperature where the polymer deforms, typically 100-150°C. The filaments deform into flattened hexagonal "cells" that extend through the complete length of the sample and are separated from each other by the BN. The binder is burned off and the sample or part is hot pressed at about 1750°C to densify the silicon nitride. The BN is a nonreactive layer that prevents the cells from bonding to each other during hot pressing, so the silicon nitride retains a pseudofiber form. The material fractures similarly to wood with a high degree of crack deflection, debonding, and cell pull-out. Flexural strength is typically in the range 500-700 MPa, elastic modulus 270-280 GPa, and work of fracture 7000-10, 000 J/m2 [49].

Fibrous monoliths have been fabricated from a variety of other cell/cell boundary combinations including SiC/BN, SiC/C, ZrB2/BN, alumina/aluminum titanate, alumina/metals, and even a novel arrangement of diamond and WC-Co. [48-50]. The fibrous monolith that appears closest to industrial application is the dia-mond/WC-Co material. It has been constructed into inserts for drill bits for rock drilling and has performed very well in laboratory and field tests [50]. In the most extensive field test, a hammer bit with fibrous monolith inserts cut through 2500 ft of hard, silicified sandstone in search of a natural-gas deposit.

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