Ola Lyckfeldt, Kent Rundgren, M. Sjöstedt
Freeze granulation (LS-2, PowderPro HB, Sweden) has been demonstrated as a favourable alternative to conventional granulation methods (spray drying, sieve granulation etc) in the production of granules for the pressing of high-performance ceramic powders. Freeze granulation/freeze drying prevents the migration of pressing aids or particle fines to the granule surface, as is the case in spray drying. This ensures granule homogeneity and an easy breakdown of granules during pressing. This, in turn, results in defect minimisation and optimal conditions for the sintering and the development of the desired material properties. In this study silicon nitride materials have been produced using freeze granulation, pressing and sintering to validate the performance. Materials with competitive properties were manufactured based on medium-cost, direct-nitrided powders (SicoNide P95, Permascand AB, Sweden), various pressing and sintering aid compositions as well as various pressing and sintering schedules. MgO vs Fe2O3 as sintering aid, PEG vs PVA as binder and higher pressure at the initial uniaxial pressing were found to promote the sintering performance.