Increase in hardness for flash sintered ceramics

A study of the hardness values of flash sintered multiphase ceramics was carried out to determine the effect of flash sintering on mechanical property. A three-phase ceramic of equal volume percent of Al2O3, MgAl2O4, and 8YSZ was compared to single phase Al2O3, MgAl2O4, and 8YSZ. Samples were flash sintered with an isothermal furnace temperature of 1450°C, a field of 680 V/cm, and a current limit set to 50mA/mm2. Control samples were made by conventional sintering and two-step sinter forging. Vickers hardness tests were conducted to evaluate hardness as a function of process parameters. Initial results revel an increase of hardness for flash sintered samples compared to conventional sintered and two-step sinter-forged samples. The two-step sinter-forged samples and flash sintered three-phase samples had similar grain sizes and density, and the increase in hardness is hypothesized to be a result of increase point defects resulting from flash sintering. Please click Additional Files below to see the full abstract

On the CMAS Problem in Thermal Barrier Coatings%253A Benchmarking Thermochemical Resistance of Oxides Alternative to YSZ Through a Microscopic Standpoint

This study focuses on experimental modelling of the failure of Thermal Barrier Coatings (TBCs) due to attack of CMAS (Calcia-Magnesia-Alumina-Silicate), which is often found in harsh environments, via glassy phase infiltration. Volcanic ash and dust, sand particles, and fly ash, which contain CMAS, are imminent threats impeding predictable lifetimes of TBCs. Such incurrence directly affects the geometry and clinging to bond coat, and intrinsic material properties such as thermal conductivity and crystal structure of TBC are modified after exposure to CMAS, which ultimately results in delamination, spallation and failure of the coating material. The scope of this work is to survey the reactivity of CMAS with various oxide systems, and evaluate possible oxide systems that can be replaced and%252For used with Yttria-stabilized Zirconia (YSZ) by investigating the penetration depth and reactivity after sintering with CMAS. A cost-effective method to observe the reaction of candidate oxides with CMAS is suggested and administired%253B understanding the main mechanism that causes the failure of top coat in the wake of CMAS infiltration, and seeking solutions for the problem is performed by taking advantage of Scanning Electron Microscopy (SEM). Recently suggested ceramic oxide systems that form in pyrochlore structure, some perovskite structures in various compositions, monazite, mullite and YSZ are studied. The possible outcome consequent upon CMAS infiltration are concluded and course for designing novel material systems that are expected to withstand the CMAS attacks better than the state-of-the-art 4mole%25 YSZ is defined. 5%25 mole Yb-doped SrZrO3(5Yb-SZ) and favored pyrochlores such as Gd2r2O7 and GdYbZr2O7 are found to be better mitigating CMAS attacks

Electrical Conductivity of Mullite Ceramics

The electrical conductivity of a lab-produced homogeneous mullite ceramic sintered at 1625 degrees C for 10h with low porosity was measured by impedance spectroscopy in the 0.01Hz to 1MHz frequency range at temperatures between 300 degrees C and 1400 degrees C in air. The electrical conductivity of the mullite ceramic is low at 300 degrees C (approximate to 0.5x10-9Scm-1), typical for a ceramic insulator. Up to approximate to 800 degrees C, the conductivity only slightly increases (approximate to 0.5x10-6Scm-1 at 800 degrees C) corresponding to a relatively low activation energy (0.68eV) of the process. Above approximate to 800 degrees C, the temperature-dependent increase in the electrical conductivity is higher (approximate to 10-5Scm-1 at 1400 degrees C), which goes along with a higher activation energy (1.14eV). The electrical conductivity of the mullite ceramic and its temperature-dependence are compared with prior studies. The conductivity of polycrystalline mullite is found to lie in-between those of the strong insulator -alumina and the excellent ion conductor Y-doped zirconia. The electrical conductivity of the mullite ceramic in the low-temperature field (< approximate to 800 degrees C) is approximately one order of magnitude higher than that of the mullite single crystals. This difference is essentially attributed to electronic grain-boundary conductivity in the polycrystalline ceramic material. The electronic grain-boundary conductivity may be triggered by defects at grain boundaries. At high temperatures, above approximate to 800 degrees C, and up to 1400 degrees C gradually increasing ionic oxygen conductivity dominates