Pressing of granulated ceramic powders is the most frequently used method to form ceramic implants in orthopaedics. In general, the granulated ceramic powders are prepared by spray drying of ceramic suspensions containing submicrometer and nanometer ceramic particles as well as organic additives. The spray drying can result in too hard granules that crush partially during pressing or too soft granules that crush premature. Therefore, investigating the compressibility of the granules is an essential step during development of new ceramic materials for orthopaedic use.
Alumina-zirconia composite granules differing in zirconia content and granules morphology were compacted with a universal testing machine. The load was applied to the upper punch of a pressing die in a number of subsequent cycles in which the load was increased to a given value, held for 10 s and decreased. In each subsequent cycle, the maximal load was gradually increased to cover a pressure range from 0.16 to 200 MPa. The density of the compacts and their relaxation were deduced from the cross head position of the mechanical testing machine and plotted as a function of the pressure. The strength of the granules, the interval of plastic deformation of the granules and the minimal pressure required for defect-free pressing the granules were deduced from the compaction curves.
In addition, compacts were prepared at different pressures and investigated with scanning electron microscopy and microcomputer tomography. The microstructure of the obtained compacts correlated well with the data deduced from the compaction curves.
The results showed that the compaction of granulated powders with a mechanical testing machine in multiple steps can be used to investigate the compressibility of spray dried granules. Further, it is an easy to use and fast method, which is of great importance for developing new ceramic materials.