The strength and strengthening of ceramics

Abstract

There are several mechanisms by which materials may be strengthened as listed: (i) Strain (or work) hardening in ductile ceramics, (ii) Solid-solution strengthening by pinning dislocations either by interstitial or substitutional atoms, (iii) Second-phase hardening, (iv) Transformation hardening, (v) Strengthening by grain boundaries. Strain hardening is a feature of ductile ceramics, but at high temperatures where brittle materials show plasticity, strain hardening does not necessarily occur. In brittle materials that show plasticity at elevated temperature, strain hardening depends on composition and conditions of the test. It is possible that, due to the recovery process, strain hardening will not be observed. Superplastic materials are characterized by high ductility and no strain hardening occurs. In particular in superplastic materials such as MgO, strain hardening is absent while in b-Si3N4 under compression it is not observed. However, it has often been observed that little or no hardening at all occurs. Either interstitial or substitutional atoms can pin dislocations and thus strengthen the material. Second phase particles not in solution can hinder dislocations in their motion with a consequent increase in strength in the ceramics. Ceramics such as those based on zirconia are likely to undergo phase transformation, in particular the yttria stabilized zirconia, which is associated with strengthening of the material. Clearly, grain boundaries are obstacles to dislocation motion and thus harden the ceramic.