Fabrication of silicon nitride ceramics by dispersing in situ formed nano silicon carbide phase.

摘要

{dollar}rm Sisb3Nsb4{dollar} ceramics have many excellent properties, and are expected to be used in many applications of structural materials under sever conditions, like high temperature atmosphere. The grain boundary phase result in poor mechanical properties, especially at high temperatures, so therefore the control of grain boundary phase properties is the most important factor. This work investigates three kinds of grain boundary controlling methods, i.e., (1) controlling of liquid phase content and quantity, (2) second phase dispersion and (3) whisker reinforcement.; (1) The liquid phase content is controlled by surface modification treatments. The grain shape changes from an elongated to an equiaxed shape, and decreases in size with increasing surface oxygen impurity phase content, which in turn the decreases the fracture toughness and critical flow size. (2) The nano SiC particles are formed by the in-situ reaction between the coated carbon and the surface silicon oxide impurity phase during hot-pressing. The precipitated SiC particles dispersed in three different locations, i.e., in the grain boundary phase, at the {dollar}rm Sisb3Nsb4{dollar}-{dollar}rm Sisb3Nsb4{dollar} interface and inside of the {dollar}beta{dollar}-{dollar}rm Sisb3Nsb4{dollar} grain. The {dollar}rm Sisb3Nsb4{dollar}-SiC interface does not have other impurity phases and microcracks, and both lattices connect directly without other impurity phases. The grain boundary is toughened by the precipitated nano SiC particles in the grain boundary phase or at the {dollar}rm Sisb3Nsb4{dollar}-{dollar}rm Sisb3Nsb4{dollar} interface to help {dollar}rm Sisb3Nsb4{dollar} grain boundary bridging without a glassy phase. The reacted carbon content increases with increasing surface impurity oxygen phase content. (3) The whisker-matrix interface conditions are controlled by carbon coating on the SiC whisker surfaces. The fracture toughness and bending strength are increased by carbon coated SiC whisker reinforcement in {dollar}rm Sisb3Nsb4{dollar} matrix. For carbon coated SiC whisker reinforced composite, the SiC phase is formed by the in-situ reaction between the coated carbon and the surface silicon oxide impurity phase on the SiC whisker during hot-pressing. The SiC whisker surfaces change from smooth to rough by the newly formed SiC phase. The rough surfaces act as a resistance of whisker pullout.; For non-coated {dollar}rm Sisb3Nsb4{dollar} powder, powder washing method is useful and simple process for toughening of monolithic {dollar}rm Sisb3Nsb4{dollar} ceramics. The high fracture toughness can be obtained by this washing method. The nano SiC particles dispersed {dollar}rm Sisb3Nsb4{dollar} ceramics can be obtained by this carbon coating method. This carbon coating method can also be applied to {dollar}rm Sisb3Nsb4{dollar} matrix whisker reinforced composite system. Surface oxidized {dollar}rm Sisb3Nsb4{dollar} powders and SiC whiskers, i.e., inferior powders and whiskers, can be used in this method. Thus, this carbon coating method is a useful, simple and low cost process for toughening of {dollar}rm Sisb3Nsb4{dollar} ceramics. (Abstract shortened by UMI.)