Tsuneji Kameda

Development of high-strength reaction-sintered silicon carbide

Reaction sintering is one of the most attractive manufacturing processes of silicon carbide (SiC), because of dense structure, low processing temperature, good shape capability, low cost and high purity. However, mechanical properties of reaction-sintered SiC (RS-SiC) were typically much lower than normal sintered one. Particularly, the bending strength was approximately 300 MPa. In this study, in order to develop the high-strength RS-SiC, effect of the microstructure on the bending strength was examined. The bending strength of RS-SiC was recognized to be increased with decreasing the residual silicon (Si) size, and high-strength RS-SiC has been newly developed. The strength over 1000 MPa was obtained to control the residual Si size under 100 nm. Some other properties of developed high-strength RS-SiC were also evaluated.

EB-PVD process and thermal properties of hafnia-based thermal barrier coating

Abstract Thermal barrier coatings (TBCs) are being developed for the key technology of gas turbine and diesel engine applications. In general, 8 mass% Y2O3–ZrO2 (8YSZ) coating materials are used as the top coating of TBCs. The development of hafnia-based TBC was started in order to realize the high reliability and durability in comparison with 8YSZ, and the 7.5 mass% Y2O3–HfO2 (7.5YSH) was selected for coating material. By the investigation of electron-beam physical vapor deposition (EB-PVD) process using 7.5YSH ceramic ingot, 7.5YSH top coating with about 200 µm thickness could be formed. The microstructure of the 7.5YSH coated at coating temperature of 850 °C showed columnars of laminated thin crystals. On the other hand, the structure of the 7.5YSH coated at coating temperature of 950 °C showed solid columnars. From the result of sintering behavior obtained by heating test of 7.5YSH coating, it was recognized that the thermal durability of 7.5YSH coating was improved up to about 100 °C in comparison with 8YSZ coating. This tendency was confirmed by the experimental result of the thermal expansion characteristics of sintered 7.5YSH and 8YSZ. ©2003 Elsevier Science Ltd. All rights reserved.

Corrosion of silicon nitride ceramics in aqueous HF solutions

The leaching behaviours of hot-pressed Si3N4 ceramics containing Y2O3, Al2O3 and AIN as additives and hot isostatically pressed Si3N4 without additives were studied in 0.1 to 10 M HF aqueous solutions at 50 to 80° C. Silicon and aluminium ions were dissolved into the HF solutions, but yttrium ion did not dissolve at all and formed insoluble YF3. The dissolution of silicon and aluminium ions was controlled by the surface chemical reaction and the apparent activation energies were 70.5 to 87.6 kJ mol−1, respectively. The corrosion rate increased with increasing degree of crystallization of the grain boundary phases. The corrosion resulted in roughness of the surface and degradation of the fracture strength. Si3N4 ceramics containing an amorphous phase at the grain boundaries showed the most excellent resistance to corrosion with HF solution, and kept a fracture strength of above 400 M Pa even after leaching 40% of the silicon ions.

CORROSION OF SILICON NITRIDE CERAMICS UNDER HYDROTHERMAL CONDITIONS

Corrosion behaviour of Si3N4 ceramics containing Y2O3, Al2O3 and AIN as sintering aids was investigated under hydrothermal conditions at 200–300 ‡C and saturated vapour pressures of water for 1–10 days. Hydrothermal corrosion resulted in the dissolution of the Si3N4 matrix and the formation of a product layer consisting of the original grain-boundary phases and hydrated silica. The dissolution rate of Si3N4 ceramics decreased with decreasing crystallinity of the grain-boundary phase. The dissolution rate could be adequately described by a parabolic plot in the initial stage of the reaction. The apparent activation energies were 83.5–108 kJ mol−1, and the bending strength of the corroded samples decreased from ∼ 600 to 400 MPa in the initial stage of the reaction upto a weight loss of 0.004 g cm−2, and then was almost constant up to a weight loss of 0.012 g cm−2.

Oxidation behaviour of the sintered Si3N4-Y2O3-Al2O3 system

The oxidation behaviour of silicon nitride composed of Si3N4, Y2O3, Al2O3, AlN and TiO2 was investigated in dry and wet air at 1100–1400 °C. The oxidation rates were confirmed to obey the parabolic law. An activation energy of 255 kJ mol−1 was calculated from the Arrhenius plots of the results of oxidation in an air flow. In still air the oxidation rate was larger than that in an air flow, but the oxidation rate in flowing air was not affected by the air flow rate. α-cristobalite and Y2O3·2SiO2 were formed in oxidized surface layers. These crystal phases increased with increasing oxidation temperature. In particular, a higher content of α-cristobalite was obtained in still air oxidation. The existence of water vapour in flowing air greatly promoted the oxidation.

High-strength silicon nitride ceramics obtained by grain-boundary crystallization

A monolithic type of toughened silicon nitride ceramics has been developed from Si3N4-Y2O3 systems. However, because of the existence of a second phase, the fracture strength decreases at elevated temperatures. To improve the high-temperature strength of silicon nitride, some additional components were investigated. It was found that the addition of hafnia to the Si3N4-Y2O3-AIN system gave a greater high-temperature strength based on the promotion of grain-boundary phase crystallization: namely, 126 kg mm−2 in 3-point bend strength at 1300 °C for the hot-pressed specimen, and 90 kg mm−2 at 1300 °C for the pressureless sintered specimen. The role of the hafnia in crystallization is not yet clear, and is being characterized by electron microscopy and microanalysis.

Effect of structure of interfacial coating layer on mechanical properties of continuous fiber reinforced reaction sintered silicon carbide matrix composite

A dense silicon carbide matrix composite reinforced by Hi-Nicalon fibers CVD coated with boron nitride and silicon carbide was fabricated by slurry impregnation and subsequent reaction sintering with molten silicon. The effect of the structure and the thickness of the silicon carbide layer of the fiber coating on the mechanical properties of the composite were investigated. That is, three types of silicon carbide layers, namely a dense structure with a thickness of 0.15 μm and two porous structures with a thickness of 0.15 μm and 0.48 μm, respectively, were investigated. As a result, excellent strength property of ceramic matrix composite (CMC) was obtained in the case of the dense silicon carbide (SiC) layer. The thickness effect of the SiC layer on the strength was smaller than that of the structure.

Development of Continuous Fiber Reinforced Reaction Sintered Silicon Carbide Matrix Composite for Gas Turbine Hot Parts Application

A dense silicon carbide matrix composite reinforced by Hi-Nicalon fibers coated with boron nitride was fabricated by slurry impregnation and subsequent reaction sintering with molten silicon. The effect of the fiber coating structure and the infiltrating metal composition on the mechanical properties of the composite was investigated. The fabrication process for the combustor liner and the shroud segment for a 15 MW gas turbine was developed. The small combustor liner and the shroud segment with some machined notches were evaluated in a combustion test at 1773 K and atmospheric pressure. Excellent durability for gas turbine hot parts was recognized.Copyright

Effect of TiO2 addition on oxidation behavior of sintered bodies composed of Si3N4-Y2O3-Al2O3-AlN

The effect of TiO2 content on the oxidation of sintered bodies from the conventional Si3N4-Y2O3-Al2O3-AlN system was investigated. Sintered specimens composed of Si3N4, Y2O3, Al2O3, and AlN, with a ratio of 100 : 5 : 3 : 3 wt% and containing TiO2 in the range of 0 to 5 wt% to Si3N4, were fabricated at 1775 °C for 4 h at 0.5 MPa of N2. Oxidation at 1200 to 1400 °C for a maximum of 100 h was performed in atmospheres of dry and wet air flows. The relation between weight gain and oxidation time was confirmed to obey the parabolic law. The activation energies decreased with TiO2 content. In the phases present in the specimens oxidized at 1300 °C for 100 h in dry air, Y3Al5O12 and TiN, which had existed before oxidation, disappeared. Alpha-cristobalite and Y2O3·2TiO2 (Y2T) appeared in their place and increased with increasing TiO2 content. In those oxidized at 1400 °C, α -cristobalite was dominant and very small amounts of Y2O3·2SiO2 and Y2T were contained. There was a tendency for more α -cristobalite to form in oxidation in wet air than in dry air. Therefore, moisture was confirmed to affect the crystallization of SiO2 formed during oxidation. Judging from the lower activation energy, the crystallization, and the pores formation, we concluded that the addition of TiO2 decreases oxidation resistance.

Influence of Temperature on Fatigue and Creep Strength of Ceramic Matrix Composites.

For studying the mechanical properties of composite material at high temperature, fatigue and creep tests were carried out for two types of ceramics matrix composites, that is, the reaction bonded and the precursor SiC/SiC composites. And then analysis of data regression was conducted for evaluating damage accumulation. As a result, following conclusions were obtained. 1) At room temperature, fatigue strength of precursor CMC is above twice higher than that of reaction bonded CMC, but at 1 300°C, fatigue strength of both composites is reversed in the range of longer life. 2) Fatigue and creep strength of reaction bonded CMC correspond each other if it is explained that fractures are caused by the crack extension process of time dependence except for flexure strength. 3) In precursor CMC, fatigue and creep strength for a short term coincide with flexure strength, which can be also explained by the crack extension process of time dependence. But high temperature strength for a long term rapidly decline because of internal oxidation.