We have thus far dealt with the resistance to crack propagation at opposite extremes of material representation, continuum solid and atomic lattice. It is now appropriate to investigate the problem at an intermediate level, that of the microstructure. By ‘microstructure’ we mean the compositional configuration of discrete structural ‘defects’: voids, inclusions, secondphase particles (volume defects); secondary crack surfaces, grain boundaries, stacking faults, twin or phase boundaries (surface defects); dislocations (line defects). It is principally at this intermediate level that significant improvements in the mechanical properties of traditional brittle polycrystalline ceramics (cf. table 3.1) may be realised. By tailoring the microstructure it is possible to introduce an interactive defect structure that acts as an effective restraint on crack propagation and thus enhances the material toughness.

In this chapter we examine some of these ‘toughening’ interactions. We identify two classes of restraint. The first involves purely geometrical processes, deflections along or across weak interfaces, etc. The responsible microstructural elements may be regarded as ‘transitory obstacles’, in the sense that their impeding influence lasts only for the duration of crackfront intersection. Because of their ephemeral nature such interactions are relatively ineffective as sources of toughening, accounting at very most for increases of a factor of four in crack-resistance energy R or, equivalently, a factor of two in toughness T.

The second class of restraint comprises shielding processes. The critical interactions occur away from the tip, within a ‘frontal zone’ ahead or at a ‘bridged interface’ behind.