Improvement of fracture toughness in hot isostatically pressed mixtures of ultrafine and coarse-grained SiC ceramics

摘要

Recently, much work has been performed on nano-sized structural ceramics [1-3]. These materials exhibit several interesting features including ex-treme hardness and superplasticity. Due to the high number of grain boundary atoms, plastic deforma-tion at low temperature was expected, but experi-mental results (e.g. on TiO2) are controversial [4,5]. Probably also the presence of pores in the nano-crystalline material influences the mechanical prop-erties. For non-oxide ceramics a ductilization effect has not yet been reported. This might be due to the high activation energies of diffusion in covalent-bond non-oxide ceramics. A significantly faster diffusion mechanism in nanocrystalline compared to coarse-grained materials cannot be expected be-cause of similar grain boundary structures in both types of materials [6]. An estimation of the potential for plastic deformation can be made using a Coble creep mechanism [4]. An upper limit of creep rate can be evaluated if the lowest activation energy for grain boundary diffusion given in [7] (3.14 eV) is taken and the grain boundary diffusion coefficient extrapolated down to room temperature. Then a normalized creep rate of l(T36%/GPa/h results for a grain size of 10 nm. Plastic deformation of SiC at room temperature is therefore (at least for a Coble creep mechanism) very unlikely even for nanocrys-talline materials.