Extrusion-based additive manufacturing of yttria-partially-stabilized zirconia ceramics

Because of their outstanding mechanical properties, chemical stability, and biocompatibility, 3-mol % yttria-stabilised tetragonal zirconia polycrystals (3Y-TZP), known as zirconia ceramics in dentistry, are an important choice for various types of prosthesis. In addition to extensive use for crown and bridge construction, considerable interest has been generated for applications in implant dentistry, including full-contour zirconia crowns as supra-constructions, zirconia abutments, and novel zirconia implants. However, their use among dentist and researchers is controversial, especially compared with the well-established implants made of titanium alloys. As a latecomer, the merits and limitations of 3Y-TZP are awaiting careful investigation. Design, manufacturing, and clinical operation guidelines are urgently needed. The aim of this review was to address the present status of the application of zirconia ceramics related to implant dentistry by analysing the published data from both in vitro and ...

https://www.tandfonline.com/doi/pdf/10.1080/17436753.2016.1264537

Zirconia ceramics in metal-free implant dentistry

Purpose

The clinical failures of zirconia dental restorations are often caused by extrinsic artifacts introduced by processing. The aim of this study was to investigate the micro-defects and residual stresses generated during the multistep process of zirconia dental restorations.



Materials and Methods

Thermal spray granulated 3Y-TZP powders were dry pressed by two tools exhibiting distinctly different Young's moduli, cold isostatic pressed (CIP-ed), and pressure-less fully sintered. The green bodies pressed by a stiff tool were treated with different procedures: direct milling (green milling) followed by fully sintering; half-sintering and milling (raw milling) with or without fully sintering; and fully sintering followed by grinding. The fully sintered 3Y-TZP crowns were clinically adjusted using both a diamond bur and SiC bur, respectively. Phase composition and microstructure of the pressed, milled, and ground surfaces were studied by XRD and SEM.



Results

Tetragonal phase was the main phase of all detected 3Y-TZP specimens. Excessive residual stresses introduced by raw milling and grinding were confirmed by a strained T (111) peak, monoclinic phase, and obviously changed I(002)t/I(200)t ratio. The residual stresses would form a compressive stress layer, while it was too shallow to inhibit crack propagation even for ground specimens. Large voids with high-coordination numbers were the common packing micro-defects. Once formed, they were barely healed by CIP-ing and sintering. A stiff pressing tool was confirmed to be useful for reducing the surface packing voids. Milling removed the surface voids, but was no help for the interior ones. Raw milling introduced more serious chippings, most originating from the existing packing voids, than green milling due to its brittle failure and was less recommended for production. Grinding dense 3Y-TZP caused surface grain refinement and much more severe micro-defects, especially when clinical adjustment was applied by diamond bur compared to SiC bur.



Conclusions

Micro-defects and residual stresses are introduced and accumulated through the entire production chain and determine the final microstructure of zirconia dental restorations. Several procedural improvements are offered and expected to reduce processing micro-defects.

Effect of multistep processing technique on the formation of micro-defects and residual stresses in zirconia dental restorations.

Flexural strength and translucency characterization of aesthetic monolithic zirconia and relevance to clinical indications: A systematic review.

Quantitative phase analysis in tetragonal-rich tetragonal/monoclinic two phase zirconia by Raman spectroscopy

The fractography of a new grade of zirconia ceramics, known as self-glazed zirconia, was investigated. The as-sintered intact top surface was made with superior smoothness that mimicked the optical appearances of the natural teeth enamel. The beneath surface opposite to this was made hierarchically rough with microscopic pits of the size up to 60 μm together with grain-level roughness of about 2 μm. The three-point bending test of the samples made with the hierarchically rough surface being tensile one demonstrated an average bending strength of 1120 ± 70 MPa and a Weibull modulus of as high as 18 ascribed to the improved structural homogeneity. Surface topography was found the main origins of crack initiation leading to fracture. The observed unusually predominant transgranular fracture mode of submicron-sized grains disclosed a possible toughening mechanism of disassembling of mesocrystalline grains that differs significantly from the commonly quoted phase transformation toughening of this category of ceramics.

Fractography of self-glazed zirconia with improved reliability

A suspension of zirconia powder (TZ3YSE) with a solids loading of 50 vol% was prepared by ball milling. Binders were added and some of the suspension was diluted to 40, 30 and 20 vol% before freeze granulation was performed. A spray dried material (TZ3YSEB) was used as a reference. The pore size distribution of the different granules was evaluated and from the microstructure it was shown that inhomogeneities were present in both the freeze granulated as well as in the spray dried granules. In addition, the density, microstructure as well as the strength of sintered materials prepared from the granules were studied. The results showed that a high green density or sintered density was not sufficient in order to achieve a high strength material. It was further shown that the strength was significantly influenced by the granule density and not by the inhomogeneities found in the granules.

Effects of granule density on strength and granule related defects in zirconia

Crack-free zirconia ceramics were consolidated via sintering by intense thermal radiation (SITR) approach at 1600–1700°C for 3–5 min. The resulted ceramic bulks can achieve a relative densi...

https://www.tandfonline.com/doi/pdf/10.1080/17436753.2018.1449580

Erik Adolfsson

Papers

Fractography of self-glazed zirconia with improved reliability

Effects of granule density on strength and granule related defects in zirconia

Dense and strong ZrO2 ceramics fully densified in <15 min