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Abstract

This doctoral thesis describes the preparation of moderately porous Y-TZP ceramics with improved flexural strength. The main field of application is dentistry, where the elastic mismatch between the Y-TZP material and tooth substance influences the survival rate of fixed partial dentures (FPDs). By introducing porosity into the bulk samples a reasonable reduction in the elastic modulus (E) can be expected, as shown by many literature data. A major drawback in using this approach is that the flexural strength of the ceramics is also greatly reduced. The problem was tackled by approaching four major topics in materials science, i.e., colloidal processing, shaping, sintering and mechanical properties.
The first part of the work focused on the colloidal processing. The so-called, core-shell concept was adopted for the preparation of the starting materials. This concept exploits agglomeration, which results in a uniform distribution of nano-sized particles attached to the surface of the submicron-sized particles in the slurry. After the slip casting, the green pellets were biscuit-sintered at various temperatures in the ambient air in order to obtain moderately porous zirconia samples. In the second part, the densification behavior of the bimodal Y-TZP powder compacts consisting of nano/sub-micron-sized particles was studied and an explanation for their improved flexural strength when biscuit-sintered is provided. An in-situ-heating TEM analysis revealed that up to 800 °C only the nanoparticles sinter in a bimodal mixture without any densification. By increasing the temperature to 900 °C the densification of the nanoparticles begins and the partially densified nanoparticle clusters migrate into the contact area between the core particles. Consequently, the driving force for the sintering of the powder-blend compacts is reduced and this is reflected in a slower densification compared to that of the core material. At 1000 °C the sintered nanoparticle clusters begin to incorporate into the core material, resulting in a sharp increase in strength due to the increased neck area. The biscuit-sintered powder-blend compacts reached a plateau of strength at 670 MPa, which was achieved at a relative density of 70 %.