PROCESSING NANOCRYSTALLINE CERAMICS FOR APPLICATIONS IN SUPERPLASTICITY

The production of nanocrystalline ceramics for subsequent use in super plastic forming operations requires that the ceramics be made in large quantities, with high densities, and under stringent grain growth con trol. To make large amounts of nanocrystalline starting powders, two p opular wet chemical techniques (precipitation from salt solutions and alkoxide hydrolysis) can be used and are described in this paper. Unfo rtunately, pressureless sintering of these powders does not typically lead to the high densities and ultrafine grain sizes desired in the fi nal product. Sintering data suggest that pore shrinkage occurs only wh en grains reach a critical size with respect to the pore size; thus, i f the ceramic contains large pores, densification can require signific ant grain growth. Separation of large pores from grain boundaries may also occur and lead to incomplete densification, even at extremely lar ge grain sizes. In all cases the pressureless sintering behavior of th e nanocrystalline ceramics appears to adhere to well established theor ies used to explain the sintering of conventional, larger-grained cera mics. During both pressureless sintering and sinter-forging experiment s, the grain size of a nanocrystalline ceramic is identical to the ave rage spacing between open pores in the sample. Pressureless sintering results in the closure of these pinning pores by about 90% density and thus leads to a substantial grain growth at densities greater than 90 %. Sinter-forging, however, often allows one to maintain a stable popu lation of small open pores (for pinning purposes) throughout sintering , while preferentially eliminating the large pores which detract from the sample density. The deformation regime in which sinter-forging is performed has a decided effect on whether large pores or small pores a re eliminated preferentially and, consequently, on whether a high dens ity and fine grain size combination is achieved or not.