Impact of high pressures during compaction of zirconia nanopowder on material structure formation

The study covers the effect of compaction pressure during semi-dry pressing of zirconia nanopowder partially stabilized with yttria in a steel mold on the phase composition and microstructure of compacts and samples sintered at 1400 °C for 2 hours. An aqueous solution of polyvinyl alcohol was used as a temporary process binder. According to X-ray fluorescence analysis, the content of yttria in the powder synthesized by sol-gel technology (precipitation with aqueous ammonia solution from water-ethanol solutions of the corresponding reagents with the agar-agar additive) was 3.2 mol.%. The paper provides the results obtained when studying compacts and sintered samples by Raman spectroscopy, optical and atomic force microscopy. It was found that the increase in their density is not a monotonous process. There is a critical compaction pressure interval of Р = 400÷450 MPa where a sharp change in the material porosity, pore shape and size, microstructure and phase composition occurs. A monoclinic phase was observed in compacted samples along with tetragonal zirconia. Its content varies with a variation in Р values. The grinding of material grains is associated with the agglomerate destruction process and actively occurs in the interval of Р = 350÷550 MPa. A similar effect was observed by other researchers during zirconia nanopowder compaction who suggested that the nanopowder system response to the effect of pressure is related to the influence on the water component (in this case, the temporary process binder) and is due to the transition of one water form to another at 10–25 °С and 400–700 MPa.