Mechanical properties of nanoscale ceramics

Abstract

The mechanism of deformation in nanosize ceramics occur either by dislocation motion or by grain boundary sliding depending on the size of the grains. In nanoceramics of grain size above ~100 nm the main deformation mechanism is by dislocation motion. At ultra-fine nano grain sizes below ~100 nm in the range <50 nm, the deformation mechanism is by grain boundary sliding. Dislocations cannot be accommodated conveniently in such nanosize materials and are prevented from motion and interactions. At levels in the hundreds of nanosized grains, a probable partial-dislocation mechanism may occur concurrently with other deformation mechanisms such as grain boundary sliding. For grain boundary sliding atomic mobility is essential, which results in a metal-like plasticity in nanoscale ceramics. One is interested in the behavior of nanoceramics under applied loads; therefore the various responses effecting static mechanical properties (tension-compression, hardness, etc.) time-dependent deformation (creep) and cyclic (fatigue) deformation are relevant. Making ceramics superplastic requires producing ultra-fine grains in the lower nanosize level, preferentially below 50 nm or even less. Various sophisticated techniques have been developed over the past decade or so, such that certain nanoceramics can now be produced with some measure of superplasticity. Superplastic materials may be thinned down, usually in a uniform manner, before breaking, without neck formation. The actual deformation mechanism is still under debate and may be material-dependent as well. Despite the various views on the exact mechanism responsible for the observed nano-behavior, it is clear from the experiments that nanoceramics may exhibit increased strength (hardness, for example), improved toughness, improved ductility and high resistance to fatigue. All these improved properties serve as safeguards against unexpected or premature fracture in service.