Densification of ?active? zirconia powders during hot pressing

Abstract

We studied the influence of temperature and heat process time for zirconium nitrate on the phase composition, dispersion of grains and crystalline lattice defects in the resulting zirconia, and also on its capacity for subsequent compaction during hot pressing. This capacity is connected with the formation, during a short heating period up to 400°C, of metastable tetragonal and cubic modifications having a highly defective crystalline lattice. We also studied the densification during hot pressing of active zirconia obtained by decomposing zirconium nitrate. During decomposition of zirconium nitrate at 400°C with a short soak we noted the formation of zirconium dioxide which was actively compacted at 1100°C and a pressure of 200 kg/cm2. We studied the influence of additions of CaO and the methods of adding it on the densification of active zirconia. An increase in the content of CaO leads to a reduction in the capacity of ZrO2 for compaction during hot pressing. It is necessary to introduce the additive to the nitrate before it decomposes. During hot pressing of active zirconia containing additions of CaO and Y2O3 at low temperatures (1100–1300°C) cubic solid solutions are not formed owing to the short process time, and the zirconia in the specimens consists of monoclinic form. Subsequent heating of the pressed specimens forms cubic solid solutions. It was found that the introduction of 10–20% additive of zirconia obtained by heat processing the nitrate to industrial ultrapure zirconium dioxide yields dense specimens with a reduction in the optimum pressing temperature of 200°C.