Cation self diffusion in nanocrystalline monoclinic zirconia
Final Report Abstract
Monoclinic zirconia polycrystal (MZP) is a technologically important material. For example, it was recently shown that MZP exhibits superplasticity like behavior. However, there seems to be limited work on MZP due to the difficulties in processing it to nearly full density. MZP reversibly transforms to tetragonal phase at 1170 °C and upon cooling a volume change of about 4.5% occurs, causing microcracks. This eventually leads to the fracture of the material if the densification temperature exceeds 1170 °C. To overcome this difficulty in processing, commercially obtained high purity monoclinic zirconia powder was densified using hot isostatic pressing (HIP) at 1000 °C for 1 hour, under argon pressure of 200 MPa, which had sufficient driving force to cause densification compared to densification techniques without the application of pressure. For an effective transmission of pressure to occur, MZP was encapsulated with steel and with boron nitride as protective as well as release agent. It was observed that the maximum of the average densities obtained through HIP runs was 93.4% of theoretical density, while the average final grain size was 220 nanometer. However, due to inhomogeneous density distribution, some of the HIPed regions exhibited nearly full density. Some samples turned dark gray after HIP. An extensive post-HIP analysis using X-ray Photoelectron Spectroscopy (XPS) revealed no reduction of Zr4+ and no significant diffusion of boron or nitrogen during HIP. However, carbon contamination was found, which is shown to come from the alcohol used to dissolve BN (in order to apply it as a protective coating for zirconia). In addition, a comparison of HIP with sintering in air and vacuum revealed that HIP was the most effective tool in the densification of commercial monoclinic zirconia powder, with minimum residual open porosity. Apart from this, grain boundary sliding effect on the cation diffusivity in 3 mol% yttria stabilized tetragonal zirconia (3YTZ) was studied to develop a better understanding of the rate controlling mechanism during the superplasticity. Lattice and grain boundary diffusivities of hafnium in 3YTZ , which has similar ionic radius with zirconium, was measured at 1325 °C and 1350 °C, under constant compressive stress of 20 and 50 MPa. The experimental data revealed no significant changes in the diffusivities from the diffusivities of undeformed specimens. The diffusivity data was fitted into the grain boundary sliding models and it indicated grain boundary diffusion controlled grain boundary sliding might be the rate controlling mechanism for the high temperature deformation of coarse grained 3YTZ.
