ADM guidance-Ceramics: guidance to the use of fractography in failure analysis of brittle materials.

OBJECTIVES: To provide background information and guidance as to how to use fractography accurately, a powerful tool for failure analysis of dental ceramic structures. METHODS: An extended palette of qualitative and quantitative fractography is provided, both for in vivo and in vitro fracture surface analyses. As visual support, this guidance document will provide micrographs of typical critical ceramic processing flaws, differentiating between pre- versus post sintering cracks, grinding damage related failures and occlusal contact wear origins and of failures due to surface degradation. RESULTS: The documentation emphasizes good labeling of crack features, precise indication of the direction of crack propagation (dcp), identification of the fracture origin, the use of fractographic photomontage of critical flaws or flaw labeling on strength data graphics. A compilation of recommendations for specific applications of fractography in Dentistry is also provided. SIGNIFICANCE: This guidance document will contribute to a more accurate use of fractography and help researchers to better identify, describe and understand the causes of failure, for both clinical and laboratory-scale situations. If adequately performed at a large scale, fractography will assist in optimizing the methods of processing and designing of restorative materials and components. Clinical failures may be better understood and consequently reduced by sending out the correct message regarding the fracture origin in clinical trials.

Thermal gradients and residual stresses in veneered Y-TZP frameworks.

OBJECTIVES: The occurrence of "chipping" of all-ceramic restorations with Y-TZP frameworks has resulted in various designs and cooling procedures recommended for reducing such behavior. In this paper the temperature gradients during fast and slow cooling for conventional and anatomical designs are compared as well as an optical procedure to directly compare the influence of cooling rate on residual stress. METHODS: This investigation quantifies the temperature gradients between the inner and outer surfaces of crowns measured with thermocouples during two different cooling methods with uniform and anatomical frameworks. In the first method the crown was removed from the furnace after commencement of cooling whereas for the second method the crown was cooled to the glass transition temperature (600°C) before removal. Direct observation of the residual stresses was made with an optical polarimeter and thin slices of veneered copings. RESULTS: This study observed that slow cooling decreases the temperature differences but still differences of up to 88°C were observed. For the fast cooled crown, temperature differences of more than 100°C for the uniform and 140°C for the anatomical framework at temperatures above the glass transition temperature were recorded. Optical polarimeter observations indicated much lower stresses within the porcelain layer upon cooling by removing the crown below the glass transition temperature. CONCLUSION: Slow cooling during the final veneering of dental restorations with zirconia frameworks reduces the temperature gradients and residual stresses within the porcelain layer, which represent one possible cause for chipping. An anatomical designed framework did not show the same reduction extent.

XRD2 micro-diffraction analysis of the interface between Y-TZP and veneering porcelain: role of application methods.

OBJECTIVES: The metastability of the tetragonal crystal structure of yttria partial stabilized zirconia polycrystalline (Y-TZP) ceramics is a basis of concern for dental restorations. Reactions between the porcelain and the Y-TZP framework may result in a reduction of the stability of the zirconia and interface bonding caused by a transformation from tetragonal to monoclinic crystalline structure during veneering. METHODS: XRD(2) micro-diffraction measurements were carried out on tapered veneered cross-sections of the interface area to generate locally resolved information of the phase content in this region. To get a high intensity X-ray beam for short measurement times a focussing polycapillary with a spot size of app. 50 microm was used to evaluate the porcelain zirconia interface. RESULTS: Under almost all conditions the phase composition of zirconia grains at the interface revealed both the monoclinic and tetragonal structure. These observations indicate that destabilization of the tetragonal phase of zirconia occurs at the interface during veneering with porcelain. SIGNIFICANCE: These results and their relevance to the long-term stability of the interface adhesion between zirconia and veneering porcelain as well as the tetragonal to monoclinic crystal transformations at the interface are discussed.

SEM observations of porcelain Y-TZP interface.

OBJECTIVES: The metastability of the tetragonal phase of yttria tetragonal zirconia polycrystalline (Y-TZP) ceramics is a cause for concern in dental crown and bridge applications. One specific problematic area is the nature of the interface between the veneering porcelain and the Y-TZP framework and whether the associated preparation procedures and reactions result in a reduction of the stability of the zirconia. METHODS: To investigate this aspect, high-resolution SEM observations were made of polished and etched (HF content gel) cross-sections of the interface area. Dry and moist veneering porcelain powders were built up on the zirconia base. RESULTS: In some instances the zirconia grains at the interface appear to show multiplicity of subgrain faceting whereas in other instances they do not. The latter indicate destabilisation of the tetragonal phase occurs and in addition that the porcelain veneering material wets and some dissolution of the Y-TZP occurs. SIGNIFICANCE: These results and their relevance to the long-term stability of the interface adhesion to the veneering porcelain as well as possible tetragonal to monoclinic crystal transformations at the interface are discussed.