Tatsuo Morimoto

Multilayer coating for carbon-carbon composites

Abstract Because of the excellent mechanical properties at elevated temperature, carbon-fiber reinforced carbon (carbon-carbon) composites have received much attention for high-temperature structural applications. One serious drawback against actual use of carbon-carbon composites is their poor oxidation resistance in high-temperature oxidizing atmospheres. In many attempts to develop an effective oxidation-protection coating, silicon carbide coating has shown the best performance for short periods of up to 1900 K. For longer periods and higher temperature applications, a challenging coating system should be developed. There exist certain difficulties in establishing a new coating system, such as coefficients of thermal expansion (CTE) mismatch between coating layer and carbon-carbon composites, and chemical stability of the interface. This paper describes a basic idea for a new multilayer coating system, the candidate materials for each coating layer, and the characteristics of the new coating system.

Improvement of the corrosion resistance of alumina–chromia ceramic materials in molten slag

Abstract In the previous study, we found out that the corrosion of ceramic materials in molten slag depended strongly on the viscosity of the slag and the basicity gap between the ceramics and the slag. In this study, we propose experimental equation to predict the corrosion rate of a ceramic material in a molten slag as functions of the viscosity and the basicity gap on the basis of the corrosion tests. Cr2O3–Al2O3 ceramic materials were used for the tests in the SiO2–CaO–B2O3 based model molten slag. The obtained experimental equation is as follows: logΔ D=0.18 ∗ log Δ B−0.55 ∗ log η+C where, ΔD is the depth of corrosion (μm), ΔB the basicity gap between the slag and specimen, η the viscosity of slag (Poise). C is the constant that depends on the conditions of the corrosion. The value is 2.7 for the testing temperature of 1573 K, holding time of 3600 s and rotating speed of specimen of 5.2 cm s−1 within this corrosion test.

Synthesis of β-Si3N4 particles from α-Si3N4 particles

Abstract This report describes an investigation of the synthesis of β-Si 3 N 4 particles from α-Si 3 N 4 particles. The β fraction of Si 3 N 4 particles was found to depend on temperature, heating time, and the type of crucibles in which the Si 3 N 4 particles were heated. When Si 3 N 4 particles were heated in a crucible made of carbon, most α-Si 3 N 4 particles converted to β-Si 3 N 4 after heating at 2000°C for 90 min in an atmosphere of N 2 of 9 kgf/cm 2 . The morphology of the resulting β-Si 3 N 4 particles appeared as a whisker shape. When Si 3 N 4 particles were heated in a crucible made of boron nitride, most α-Si 3 N 4 particles converted to β-Si 3 N 4 after heating at 2000°C for 480min in an atmosphere of N 2 of 9kgf/cm 2 . The resulting morphology was equiaxed. It is suspected that the transformation occurs via the gas phase and is affected by the partial pressure of oxygen in the atmosphere.

Mass Spectrometric Study of Oxidation of SiC in Low-Pressure Oxygen

Passive-to-active oxidation transitions in chemical vapor deposited (CVD) SiC under low total pressures (0.8-440 Pa) in the temperature range 1250-1700°C have been investigated. A mass spectrometer sampling system equipped with a xenon image furnace was used to detect the transition points. As the temperature increased under fixed oxygen pressure, the ion current at m/e 28, which corresponds to the partial pressure of CO(g) on the surface of CVD-SiC samples, indicated two distinct changes in the measured partial pressure of CO(g). One can be assigned to a transition from the passive oxidation to bubble formation and the other can be assigned to a transition from bubble formation to active oxidation. An explanation is proposed based on available kinetic and thermodynamic data for these.

Influence of impurities in Al2O3 ceramics on hot corrosion resistance against molten salt

Abstract This report describes an investigation of the influence of impurities of Al2O3 on the hot corrosion resistance against V2O5–Na2SO4 molten salt. Thickness of damage zone in Al2O3 ceramics whose purity is high depends on the holding time linearly. On the other hand, thickness of damage zone in Al2O3 whose purity is relative low depends on square root of holding time. In the impurities, SiO2 affected the corrosion rate of Al2O3 especially. Grain boundaries are suffered from hot corrosion. SiO2 would influence the diffusion rate of corrosive elements through grain boundaries and the corrosion rate.

An additional carbon layer over the surface of C/C composite substrate with the aid of polyvinyl chloride

Abstract The novel bonding of an additional carbon layer over a C/C composite substrate surface was studied. Furan resin was selected for the additional carbon layer precursor. Molding was performed using a hot press apparatus. Polyvinyl chloride resin (PVC) was used as the bonding agent. The bonding mechanisms were studied with thermal gravity analysis, thermal gravity-mass spectroscopy, and high-temperature electron paramagnetic resonance spectroscopy. No peel-off was observed at the boundary between the carbon layer derived from furan resin/graphite powder and the C/C composite after loading by an external stress. The spins in thin carbon derived from PVC are recombined and are erased by the particular sites of graphite surface as one of roles of enhancement of adhesion.