Mechanical spectroscopy of the high-temperature brittle-to-ductile transition in ceramics and cermets

Abstract

In ceramics such as alumina, zirconia or silicon nitride, the high-temperature “brittle-to-ductile” transition is associated with grain boundary sliding often lubricated by an intergranular glassy film. This dissipative mechanism gives rise to a mechanical loss peak, which turns into an exponential background at higher temperature. When the amount of intergranular glassy phase is higher, the exponential background is globally higher and so is the creep rate. When the mechanical loss peak is due to amorphous pockets surrounded by a rigid skeleton as in silicon nitride, it may account for the toughness improvement by energy dissipation in these glassy pockets, the surrounding skeleton being responsible for creep resistance. Good examples of two-phase materials, in which one phase is responsible for mechanical strength, the other for toughness, are the cermets or the cemented carbides. For instance, in WC–Co, a relaxation peak has been found in the Co binder phase which can be correlated with an increase in toughness, the high-temperature exponential background accounting for the creep behavior of the material.