Search

Accurately assessing the stoichiometry and structural disorder of TiN ceramics is very important for understanding their properties, but is often not done because it requires complex Rietveld refinements of the corresponding X-ray diffraction (XRD) patterns. As an alternative, a simple procedure is developed here that allows such information to be extracted from the XRD patterns without Rietveld refinements. For this purpose, the stability ranges of super-stoichiometric and sub-stoichiometric TiN are first determined theoretically mathematically from the general principles of Wyckoff position occupation and electrical charge neutrality, and then constrained by theoretical thermodynamic calculations of the Gibbs potential. Subsequently, theoretical simulations of the XRD patterns of a large set of super-stoichiometric and sub-stoichiometric TiN ceramics in their stability ranges are used to reveal the existence of linear relationships between the stoichiometry and the intensity ratio of the hkl peak to the 200 peak (i.e. , I h k l / 200 (%) with hkl being 111, 220, 311, 222, 400, and 331), which is then exploited to formulate an analytical model that easily assesses the stoichiometry and structural disorder of TiN ceramics from I 111 / 200 (%). Finally, as an experimental validation and example of application, the stoichiometry of a TiN ceramic fabricated by spark plasma sintering is experimentally evaluated using both the Rietveld method and the proposed model showing marked agreement between their results. [ABSTRACT FROM AUTHOR]

Multiwalled carbon nanotubes (MWCNTs) have been widely considered for mechanical reinforcement of ceramic matrix composites. Nevertheless, the efficiency of this reinforcement strategy is under debate due to fabrication issues, such as a good homogenization or the location of the MWCNTs inside the matrix composite. Regarding this, the intragranular location of the MWCNTs has been deemed a crucial feature for optimizing the reinforcement compared to the typical intergranular placement achieved by conventional procedures. Recently, the sol-gel method has been reconsidered, as it promotes the intragranular placement of the MWCNTs. This work presents the mechanical characterization of these composites synthesized by the sol-gel method, where crack-bridging has been revealed as toughening mechanism. Finally, the conventional use of the bibliographical Young's modulus of pure alumina for the estimation of the fracture toughness is discussed, obtaining significant improvements of the fracture toughness when indentation measurements are treated by considering elastic moduli obtained by nanoindentation. • Crack-bridging as toughening phenomena revealed by SEM. • Nanoindentation hardness and elastic modulus show an increasing behavior with CNTs concentration. • Actual elastic modulus for the calculation of fracture toughness of the composites reveals significant fracture improvement. • Sintering parameters have to be tuned for fully dense composite. [ABSTRACT FROM AUTHOR]

This paper aims to give an answer to the following question: is the oxidation of graphene a critical issue for high-temperature plasticity in graphene-reinforced ceramics? To give a convincing reply, we will focus on two very different graphene-based ceramic composites: reduced graphene oxide (rGO)-reinforced alumina (α-Al2O3) and reduced graphene oxide (rGO)-reinforced yttria tetragonal zirconia (t-ZrO2). The processing of the powders has been made using a colloidal route, and after that, a spark plasma sintering process was performed in order to densify the samples. Creep tests were performed at temperatures between 1200–1250 °C in an argon atmosphere. The microstructure obtained by SEM of the sintered and tested specimens was characterized quantitatively to elucidate the deformation mechanism. Raman spectroscopy was carried out to check the integrity of the graphene. The average grain size was in the order of 1 µm and the shape factor was 0.7 for all the studied materials. The integrity of the graphene was checked before and after the creep experiments. The careful analysis of the creep tests shows that graphene oxide or its reduced version are not efficient phases for creep resistance improvement in general, contrary to what is reported elsewhere. However, the results permit the suggestion of a creep improvement in nanocomposites at a very high temperature regime due to an enhanced reactivity of oxygen between carbon and alumina interfaces. In the case of zirconia, the results give us the conclusion that the oxidation of graphene is a highly detrimental issue regarding the improvement of high-temperature plasticity. [ABSTRACT FROM AUTHOR]