Investigation of inhomogeneity in powder injection molding of nano zirconia

Abstract

Porcelain-veneered Y-TZP fixed dental prostheses often fail from fracture and wear. Zirconia with its exceptional homogenous properties contributes to alleviate the failure. Powder injection molding (PIM) has long been found to enable a much more homogeneous microstructure to components when compared to those from die pressing. However, PIM parts, however, may have a slight difference in properties for different areas due to the nature of the injection process of the feedstock. In this study, zirconia dental ceramic parts were fabricated by the powder injection molding technique that utilizes a multi-component binder system consisting of paraffin, high density polyethylene, polypropylene, and stearic acid aiming to improve the behavior in the densification of powder shaping. The nano ZrO2 powders were firstly granulated before mixing to ensure good dispersion in the feedstock. The homogeneity of sintered zirconia was characterized by determining density and hardness of ten equally thick-section zirconia and four bars. The result showed that TZ-3YS-E zirconia obtained a sintering density of 6.05g·cm−3 and a hardness of 1300 HV0.5 with a small fluctuation sintered at 1475°C. The homogeneity in the region away-from- gate was superior to the nearer-gate region based on evaluation of density, hardness, flexural strength and observation of microstructure, as well as the simulation of the mold filling of the feedstock. The average absolute deviation of hardness of slices 1 to 5 near the injection gate was 27.0 while that of slices 6 to 10 away from the injection gate was 14.0. The differences on the homogeneity and performances were regarded as the results of the drop of temperature and pressure in the final stage of mold filling of feedstock.