Aqueous suspension of fumed oxides, particle size distribution and zeta potential

Polycrystalline specimens of alumina (Al2O3), magnesium aluminate spinel (MgAl2O4), magnesia (MgO), silicon nitride (Si3N4) and silicon carbide (SiC) were irradiated with various ions at temperatures between 200 and 450 K, and the microstructures were examined following irradiation using cross section transmission electron microscopy. Amorphization was not observed in any of the irradiated oxide ceramics, despite damage energy densities up to ∼ 70 keV/atom (∼ 70 displacements per atom). On the other hand, SiC readily amorphized after damage levels of ∼ 0.4 dpa at room temperature. Silicon nitride exhibited intermediate behavior; irradiation with Fe ions at room temperature produced amorphization in the implanted ion region after damage levels of ∼ 1 dpa. However, irradiated regions outside of the implanted ion region did not amorphize even after damage levels in excess of 5 dpa. The amorphous layer in the Fe-implanted region of Si3N4 did not appear if the specimen was simultaneously irradiated with 1 MeV He+ ions at room temperature. By comparison with published results, it is concluded that the implantation of certain chemical species has a pronounced effect on amorphization threshold dose of all five materials. Intense ionizing radiation inhibits amorphization in Si3N4, but does not appear to significantly influence the amorphization behavior of SiC.

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Influence of irradiation spectrum and implanted ions on the amorphization of ceramics

Polycrystalline Al2O3, magnesium aluminate spinel (MgAl2O4), MgO, Si3N4, and SiC were irradiated with various ions at 200-450 K, and microstructures were examined following irradiation using cross-section TEM. Amorphization was not observed in any of the irradiated oxide ceramics, despsite damage energy densities up to {similar_to}7 keV/atom (70 displacements per atom). On the other hand, SiC readily amorphized after damage levels of {similar_to}0.4 dpa at room temperature (RT). Si3N4 exhibited intermediate behavior; irradiation with Fe{sup 2+} ions at RT produced amorphization in the implanted ion region after damage levels of {similar_to}1 dpa. However, irradiated regions outside the implanted ion region did not amorphize even after damage levels > 5 dpa. The amorphous layer in the Fe-implanted region of Si3N4 did not appear if the specimen was simultaneoulsy irradiated with 1-MeV He{sup +} ions at RT. By comparison with published results, it is concluded that the implantation of certain chemical species has a pronounced effect on the amorphization threshold dose of all five materials. Intense ionizing radiation inhibits amorphization in Si3N4, but does not appear to significantly influence the amorphization of SiC.

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Yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs) are used to protect hot-components in aero-engines from hot gases. In this paper, the microstructure and thermo-physical and mechanical properties of plasma sprayed YSZ coatings under the condition of calcium–magnesium–alumina–silicate (CMAS) deposits were investigated. Si and Ca in the CMAS rapidly penetrated the coating at 1250 °C and accelerated sintering of the coating. At the interface between the CMAS and YSZ coating, the YSZ coating was partially dissolved in the CMAS, inducing the phase transformation from tetragonal phase to monoclinic phase. Also, the porosity of the coating was reduced from ∼25% to 5%. As a result, the thermal diffusivity at 1200 °C increased from 0.3 mm2/s to 0.7 mm2/s, suggesting a significant degradation in the thermal barrier effect. Also, the coating showed a ∼40% increase in the microhardness. The degradation mechanism of TBC induced by CMAS was discussed.

Microstructure and thermo-physical properties of yttria stabilized zirconia coatings with CMAS deposits

On Raman spectroscopy of zirconium oxide films

Three zirconia ceramics, ZrO2−4.5mol.%Y2O3, ZrO2−12mol.%Y2O3 and ZrO2−8mol.%MgO, were investigated using X-ray diffraction before and after plasma spraying. The amounts of monoclinic, tetragonal and cubic phases existing in the three zirconia ceramics were determined from the integrated intensity ratios in the {400} and {111} regions of the X-ray diffraction patterns. The tetragonal-to-monoclinic transformation in the plasma-sprayed coatings caused by indentation, fracture and/or heat treatment with a laser beam was examined using Raman spectroscopy. As a result, this spectroscopic technique is shown to be a powerful tool for the observation of this transformation.

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Characterization of Plasma-sprayed Zirconia Coatings by X-ray Diffraction and Raman Spectroscopy(Materials, Metallurgy & Weldability)

Characterization of plasma-sprayed zirconia coatings by X-ray diffraction and Raman spectroscopy

We have strengthened plasma-sprayed alumina coatings by incorporating SiC or Si3N4 whiskers. As a result, we found that the whisker-reinforced coatings were greatly improved in properties such as thermal shock resistance and adhesion. Major features of the plasma-sprayed Al2O3−5.0 wt % Si3N4, coating were investigated by means of scanning electron microscopy, secondary ion mass spectroscopy, X-ray diffraction, X-ray fluorescence spectroscopy and thermal radiation measurements.

Characterization of plasma-sprayed and whisker-reinforced alumina coatings

The aim of this study was to examine the effect of laser treatment for plasma-sprayed ceramic coatings by the use of several microanalytical techniques. We found that the coating layer was densified as a result of laser treatment. We also verified that the crystal phases in the coating layer were controlled by the laser treatment.

Shigeki Endo

Papers

Characterization of Plasma-sprayed Zirconia Coatings by X-ray Diffraction and Raman Spectroscopy(Materials, Metallurgy & Weldability)

Characterization of plasma-sprayed zirconia coatings by X-ray diffraction and Raman spectroscopy

Characterization of plasma-sprayed and whisker-reinforced alumina coatings

Laser treatment of plasma-sprayed ceramic coatings

The effect of pretreatments of metals on bond adhesion