Impact of High Pressures during the Compaction of Zirconia Nanopowder on Material Structure Formation

Abstract

The influence of compaction pressure during semidry compaction in a steel mold of zirconia powder partially stabilized by yttria on the phase composition and microstructure of formed compacts, as well as samples sintered at 1400°C for 2 h, is investigated. An aqueous solution of polyvinyl alcohol is selected as a temporary manufacturing binder. The yttria content in the powder synthesized according to the sol–gel technology (the deposition by an aqueous-ammonia solution from aqueous ethanol solutions of corresponding reagents with the agar-agar additive) is 3.2 mol % according to the X-ray fluorescent analysis data. The results of studying the compacts and sintered samples by Raman spectroscopy, optical microscopy, and atomic force microscopy are presented. It is established that an increase in their density is not monotonic. There is the critical range of compaction pressures P = 400–450 MPa, in which the porosity, pore shape and sizes, microstructure, and phase composition of the material vary abruptly. A monoclinic phase, the content of which varies upon varying P, is fixed in compacted samples along with tetragonal zirconia. Material grain grinding is associated with the destruction of agglomerates and actively occurs in range P = 350–550 MPa. A similar effect when studying the compaction process of nanopowders of zirconia is also noted by other researchers, who assumed that the response of the nanopowder system on the pressure effect is associated with the influence on the aqueous component (the temporary manufacturing binder in this case) and is caused by the transition of one form of water into another one at 10–25°C and a pressure of 400–700 MPa.