Papers by Author: Shiro Shimada

Abstract: Si3N4-Si2N2O-TiN composite ceramics were in-situ fabricated by using following reactions of (1) 3TiO2 + Si3N4 → 3TiN + 3SiO2 + N2 and (2) Si3N4+ SiO2 → 2Si2N2O. The mixed powder of α-Si3N4, Al2O3, Y2O3 and TiO2 was hot-pressed at 24 MPa and 1800°-1900°C for 1-4 h in N2. Sintered composite ceramics were characterized by XRD, SEM, TEM, four-point bending test and Vickers indentation method. XRD results and TEM observation showed that TiN and amorphous SiO2 were formed at 1250°C by the reaction (1), and the Si2N2O phase formed by reaction (2) above 1800°C. Si3N4-Si2N2O-TiN composites consisted of ≥2 m sized Si2N2O grains with TiN and Si3N4 grains. Hardness and fracture strength of the composites were comparable to those of Si2N2O ceramics, with fracture toughness being improved at 5vol% TiN containing composites.

Abstract: HfO2 coatings were deposited on sintered SiC ceramics by a sol-gel process to improve the oxidation resistance of SiC. Crack-free HfO2 coatings ~200 nm thick were obtained by 8 cycles of dip-coating process using a mixed solution of Hf(OEt)4, DEA, and ethanol. XRD analysis indicated that the monoclinic HfO2 film was formed on the SiC ceramics. The un- and HfO2-coated SiC specimens were oxidized in Ar (purity: 99.9995%) containing a trace of oxygen or H2O at 1600°C for 10 h and the weight change was monitored by thermogravimetry. The weight of the un-coated sample decreased due to the active oxidation, while the weight loss of the HfO2-coated samples was smaller than that of un-coated sample.

Abstract: Mullite film was formed on ß-SiAlON (Si6-zAlzOzN8-z, z = 3) ceramics by reaction of Al2O3 films with silica formed on oxidation to improve a high temperature water vapor corrosion resistance. Sintered ß-SiAlON was fabricated by hot pressing of ß-sialon (z = 3) powder without additive at 1900°C and at 24 MPa for 4 h in N2. The Al2O3 film was deposited on polished ß-SiAlON ceramics by a dip coating process of Al2O3 precursor sols prepared from the alkoxide solution and aluminum sol. The Al2O3 coated sample was heated in Ar/O2 (95/5 vol.%) at 1300°C for 2 h, resulting in the formation of mullite by reaction with silica produced from sialon. The resulting mullite-coated sample was corroded in H2O/Ar/O2 (90/8/2 vol.%) at 1200°C for 2 ~ 100 h. The corroded samples were characterized by XRD and SEM-EDS for the evaluation of the corrosion resistance of mullite-coated sialon.

Abstract: TiO2 coating on Si3N4-based materials powders (Si3N4, α-Y-sialon, β-sialon) was accomplished by heating the powder suspension containing the precursor (1.0 vol% H2O + Ti(O-i-C3H7)4) from 15 to 40 °C. The success of homogenous coating was confirmed by TEM observation. TiO2 coated on the powder is nitrided with NH3 gas at 800 oC to TiN with uniform particle size 10–20 nm. Spark plasma sintering of composite TiN/ Si3N4-based particles at 1400 - 1600 °C yielded the composite ceramics with a relative density > 96% without sintering additives. Sintered TiN/ Si3N4-based composite ceramics containing 17.5 - 25 vol.% TiN showed a higher electrical conductivity > 103 −1 cm-1, enabling these ceramics to be suitable for electric discharge machining.

Abstract: Two series of mullite single crystal plates of thicknesses between 0.9 and 1.9 mm were exposed up to 12 h to a slowly flowing water rich gas mixture (O2/H2O = 80/20; 100 ml/min) at temperatures of 1670°C. The heat treated samples show up to about 50 μm into the mullite a partial surface decomposition. Decomposition products are sodium containing silicon-rich aluminosilicate glass and α-alumina within melt cavities. Concentration profiles through the plate cross sections show the effect of inward and outward diffusion of OH forming species and Na and Mg impurity enrichment on ppm level in the bulk.