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The recently suggested validity of the Hall-Petch relationship for transparent spinel ceramics with grain sizes down to 28 nm is discussed here regarding the equivalence of grain size and indentation size effects. The quantitative characterization of the samples investigated as transparent needs a correction of the authors' calculation of the theoretical transmittance. For fundamental physical reasons, this theoretical transmission of ceramics with a cubic crystal lattice does not exhibit an intrinsic grain size influence. © 2014 Acta Materialia Inc. Source

Klemm H.,Fraunhofer Institute for Ceramic Technologies and Systems
Journal of the American Ceramic Society | Year: 2010

In this paper, a summary of the development of high-temperature silicon nitride (T>1200°C) is provided. The high-temperature capacity of various advanced commercial silicon nitrides and materials under development was analyzed in comparison with a silicon nitride without sintering additives produced by hot isostatic pressing. Based on this model Si3N 4 composed of only crystalline Si3N4 grains and amorphous silica in the grain boundaries the influence of various sintering additive systems will be evaluated with focus on the high-temperature potential of the resulting materials. The specific design of the amorphous grain-boundary films is the key factor determining the properties at elevated temperatures. Advanced Si3N4 with Lu2O3 or Sc 2O3 as sintering additive are characterized by a superior elevated temperature resistance caused by effective crystallization of the grain-boundary phase. Nearly clean amorphous films between the Si 3N4 grains comparable to that of Si3N 4 without sintering additives were found to be the reason of this behavior. Benefit in the long-term stability of Si3N4 at elevated temperatures was observed in composites with SiC and MoSi2 caused by a modified oxidation mechanism. The insufficient corrosion stability in hot gas environments at elevated temperatures was found to be the main problem of Si3N4 for application in advanced gas turbines. Progress has been achieved in the development of potential material systems for environmental barrier coatings (EBC) for Si3N4; however, the long-term stability of the whole system EBC-base Si3N4 has to be subject of comprehensive future studies. Besides the superior high-temperature properties, the whole application process from cost-effective industrial production, reliability and failure probability, industrial handling up to specific conditions during the application have to be focused in order to bring advanced Si3N4 currently available to industrial application. © 2010 The American Ceramic Society. Source

Herrmann M.,Fraunhofer Institute for Ceramic Technologies and Systems
Journal of the American Ceramic Society | Year: 2013

Silicon nitride ceramics are used under conditions where high strength, hardness, and wear resistance are necessary. The increasing use of Si 3N4 ceramics in different environments demands an understanding of the relationships between microstructure and corrosion behavior. This study gives an overview of the behavior of silicon nitride in acids, bases, and hydrothermal conditions. It not only summarizes the literature data but also attempts to explain the mechanisms and to give some guidelines for the use of the materials in different environments. The stability of the ceramics against corrosion in acids and bases up to the boiling point is mostly controlled by the stability of the grain boundary. The stability can be predicted using the glass network theory. Materials with grain boundaries exhibiting a strong network, i.e., a high amount of SiO2 in the grain boundary, are stable in acids, but less stable under hydrothermal conditions and in basic solutions. Therefore, tailoring the grain boundaries can change the corrosion stability by several orders of magnitude. At temperatures above 200°C-250°C, the dissolution of the Si3N4 grains becomes a decisive factor determining the stability. © 2013 The American Ceramic Society. Source

Clausner A.,Fraunhofer Institute for Ceramic Technologies and Systems | Richter F.,TU Chemnitz
European Journal of Mechanics, A/Solids | Year: 2015

The aim of this paper is to investigate the possibilities of getting information on the initial yield stress Y from nano-indentation experiments with sharp indenters. Berkovich indenters have been used for the experiments and the data evaluation was performed by using two expanding cavity models (ECM) described in the literature, one for elastic-perfectly plastic, and one for power law work hardening materials. To characterize the ECMs, finite element simulations with extensive material parameter variations together with substantial experimental data are used. As a first result, the possibilities of determining Y using the introduced ECMs can be shown for the simulated materials (Fig. 3) and the presented selection of real specimens. Furthermore, the restriction of the ECMs to materials where the yield behavior obeys the von Mises yield criteria is discussed. In doing so it is shown that the indentation work, which can be determined directly from the indentation force-displacement curves, represents a useful quantity to assess the applicability of an ECM to a particular material. © 2014 Elsevier Masson SAS. Source

Schneider M.,Fraunhofer Institute for Ceramic Technologies and Systems | Kremmer K.,Fraunhofer Institute for Ceramic Technologies and Systems
Surface and Coatings Technology | Year: 2014

During the aluminum anodization the anodizing electrolyte changes over time and is well-known as bath aging. This alteration of the bath is mainly caused by the impact of Al3+ ions and ions of alloying elements as well as the consumption of protons. It is shown that the higher the Al3+ concentration the higher the pH value and the electrical resistance of the bath. Consequently, the oxide formation rate decreases, whereby the efficiency of layer formation increases. This work is focused on the effect of the ongoing bath aging on the microstructure of the anodic oxide layers formed on AA1050. It illustrates the fact that the ongoing bath aging significantly influences the morphology, the thickness and the pore structure of the oxide layers that are formed, but does not influence the barrier layer thickness. The pore diameter decreases with ongoing bath aging. As a direct consequence the hardness of the oxide layer increases and the pore diameter decreases. © 2014 Elsevier B.V. Source

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