Ductile ceramics

Abstract

Not all ceramics are brittle at room temperature. There are some ceramics which are ductile at ambient temperatures. Such ceramics, for example are single crystals MgO, SrTiO 3, etc. They undergo plastic deformation and by dislocation motion slip lines are observed on the deformed specimens. In pure MgO at room temperature, dislocations are very mobile at comparatively low stresses. Changing the microstructure, possibly by alloying, the mobility of dislocations may be reduced and an increase in strength may be achieved. As usually observed, material undergoing plastic deformation tend to strain harden, a feature observed also in ductile ceramics. Of the several factors influencing the strength properties of ductile ceramics, grain size is outstanding. Fine grained ceramics are desirable. Originally brittle ceramics show elongation at high temperatures which is a usual observation. There is a transition temperature from brittle to ductile behavior which depends on the ceramics. One of the common methods to determine the brittle to transition temperature is by impact testing, and for this purpose various sophisticated machines have been developed. An extraordinary phenomenon related to ductility is superplasticity, where very high values of strains can be achieved before fracture. Superplastic ceramics are oxide (zirconia) or non-oxide ceramics. Well-known superplastic ceramics are SiC and FeC. The common feature of superplastic materials is the requirement of very fine grains, namely, in the nanosize range.