Effect of alumina added by mechanical mixing and co-doping on the densification mechanisms of zirconia nanoparticles at the initial stage of sintering

The comparative analysis of the mechanical activation and Al2O3 doping effect on the densification behavior of tetragonal 3Y-TZP nanopowders has been carried out. Two types of zirconia nanopowders doped by alumina have been studied. Namely, nanopowders obtained by the mechanical mixing and co-precipitation methods. The behavior of nanopowders at initial stage of sintering was examined by the constant rate of heating technique. It was found that the volume diffusion mechanism, which is realized by sintering the 3Y-TZP nanopowders undoped and doped with a small amount of Al2O3 nanopowders obtained by co-precipitation, ceases to function when the Al2O3 content increases to 2 wt% and the Al2O3 inclusions block the diffusion flux of Zr4+ cations from the volume of ZrO2 particles to the necks. Annealing of the structure defects caused by milling of the 3Y-TZP and Tosoh TZ-3Y powders during preliminary heating leads to the realization of the volume diffusion mechanism instead of the grain boundary diffusion mechanism at the initial sintering stage in milled nanopowders. Therefore, the GBD mechanism in the milled zirconia powders caused by the stressed and defective state of zirconia nanoparticles, and not by size effects. It was shown that the Al2O3 particles added to zirconia nanopowders by mechanical mixing did not change the volume diffusion mechanism at the initial stage of sintering of 3Y-TZP NPs until the Al2O3 concentration did not blocked the diffusion of cation to the necks. It was proposed to separate the effects of mechanical activation of nanopowders from the influence of doping on the diffusion mechanism at the initial stage of sintering of doped zirconia nanopowders. The simultaneous influence of both impacts (mechanical energy and chemical composition) can complicate and confuse reasoning.