New Concepts in the Design of Tough Ceramics

Abstract

The brittle nature of ceramics hinders their applications as structural components. Two alternatives are adopted to solve the problems [1,2]. One alternative refines the processing details to reduce the flaw size in ceramics. The strength is consequently enhanced though the toughness remains low. The other approach targets the toughness improvement through the addition of toughening reinforcement. Ceramic or metallic reinforcement is incorporated into a ceramic matrix. The reinforcement interacts with the pre-existing and/or service-induced cracks to slow down their propagation. The toughness of the brittle matrix is thus improved through such interactions. Though the strength may be sacrificed slightly by adopting this approach, the reinforcement also acts as a stress concentration site. However, the latter approach is attractive, as the reliability of ceramics during the subsequent usage can be improved significantly. The development of tough ceramic-matrix composites has been thoroughly studied since 1982 [1]. Many ceramic or metallic materials, such as zirconia [3], SiC whisker [2], nickel [4] and silver [5] have been used as toughening reinforcements. The presence of these toughening agents enhances the toughness of ceramics through the generation of various toughening mechanisms. These toughening mechanisms may be active either in a process zone around the major crack or in bridging crack surfaces [6]. The propagation of cracks is hampered due to the effect of these mechanisms, resulting in an increase in the toughness of the ceramics. The presence of a single toughening agent frequently induces more than one toughening mechanism. For example, the addition of zirconia particles can induce transformation toughening, microcrack toughening and crack deflection mechanisms [7]. The metallic inclusions can either bridge the crack surfaces through their plastic deformation [4,5] or deflect the crack [5]. These mechanisms operate simultaneously to a different extent within the brittle matrix. However, the contribution to toughness from each mechanism has yet to be quantified. Several theoretical or experimental attempts have been made recently to differentiate between individual contributions [8,9]. These studies assumed that all the individual contributions to total toughness enhancement could be added together.