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High strain-to-failure porous alumina ceramics with improved mechanical properties
D. Doni Jayaseelan,S. Ueno,T. Ohji,S. Kanzaki 한양대학교 세라믹연구소 2004 Journal of Ceramic Processing Research Vol.5 No.1
A pulse electric current sintering technique was employed to fabricate porous alumina from a commercially available powder. Porosity was controllable between 20 to 50% depending on the sintering temperature. The fracture strength was correlated exponentially as a function of porosity. A high strength of ~250 MPa was achieved for the specimens, which had porosity of 30 vol.%. The strain-to-failure of the porous Al2O3 ceramics increased considerably and this improves the reliability of the specimens.
Comparison of water vapor corrosion mechanisms of polycrystalline and eutectic Lu2Si2O7
Shunkichi Ueno,D. Doni Jayaseelan,Tatsuki Ohji,Naoki Kondo,Shuzo Kanzaki 한양대학교 세라믹연구소 2004 Journal of Ceramic Processing Research Vol.5 No.2
To clarify the water vapor corrosion mechanism of the Lu2Si2O7 system, a high temperature water vapor corrosion test was performed on two microstructurally different specimens namely, polycrystalline and eutectic. From the results, we address the problem of the application of the Lu2Si2O7 system for environmental barrier coatings (EBC) with regard to their microstructure. In the case of the polycrystalline sample, the boundaries were easily corroded and the silicon nitride substrate underwent oxidation. In the case of the eutectic sample, no weight change was observed during the corrosion test. However, a trace of corrosion and poly-crystallization was observed on the sample surface. An easy processing technique for forming an eutectic protective layer on silicon nitride ceramics is also proposed.
Comparison of water vapor corrosion behavior of silicon nitride with various EBC layers
Shunkichi Ueno,D. Doni Jayaseelan,Tatsuki Ohji 한양대학교 세라믹연구소 2004 Journal of Ceramic Processing Research Vol.5 No.4
The water vapor corrosion resistance of silicon nitride ceramics with Lu2Si2O7, Yb2Si2O7 and ZrSiO4 environmental barrier coating (EBC) layers prepared by a reaction sintering method were examined at 1500 oC in a static state water vapor environment. The weights of these samples slightly increased during the corrosion test. The weight gain rates for Lu, Yb and zircon EBC samples were 2.218 × 10−6, 4.688 × 10−6 and 2.989 × 10−6 g/cm2·h, respectively. In the corrosion mechanism of Lu and Yb cases, the gas path through channels in the EBC layer were generated by the elimination of the boundary silica phase and then, the silicon nitride substrate was slightly oxidized. On the other hand, in the case of ZrSiO4, many cracks were introduced in the EBC layer during the corrosion test due to the difference in thermal expansion coefficient between the EBC layer and the substrate. However, the adhesion between the substrate and the EBC layer of ZrSiO4 was better than that of Lu and/or Yb EBC samples.
Shunkichi Ueno,D. Doni Jayaseelan,Tatsuki Ohji,Hua-Tay Lin 한양대학교 세라믹연구소 2005 Journal of Ceramic Processing Research Vol.6 No.1
The oxidation and water vapor corrosion behavior of sintered HfSiO4, ZrSiO4, TiSiO4 compacts and high density HfSiO4 coated silicon nitride ceramics were examined at 1500 oC in a static state water vapor environment. The bulk weights of HfSiO4 and ZrSiO4 decreased after the water vapor corrosion test with a weight loss rate of 7.080 × 10−6 and 1.333 × 10−6 g/cm2·h, respectively. The weight of TiSiO4 remained unchanged within the experimental error after the corrosion test. A trace of corrosion was observed on the surface of grains of corrosion-tested compacts. On the other hand, the weight of high density HfSiO4 layer (Environmental Barrier Coating, EBC layer) coated silicon nitride specimens increased with a weight gain rate 1.495 × 10−6 g/cm2·h, which is attributed by the oxidation of the silicon nitride substrate.