Kazushi Kishi

Oxidation behavior of Mo≤5Si3C≤1 and its composites

The oxidation behavior of Mo≤5Si3C≤1 and its composites was studied in air over the temperature range of 500°C–1600°C. Experiments revealed poor oxidation resistance of monolithic Mo≤5Si3C≤1 at high temperature. The oxidation was quite rapid at 1200°C and above, resulting in complete oxidation of specimens in a short time. The addition of 2.0 wt% boron was found to produce a Mo≤5Si3C≤1 composite with three other phases of MoB, MoSi2, and SiC, and showed remarkable improvement in oxidation resistance. The mechanism for the improvement was attributed to the viscous sintering of the scale to close the pores formed during the initial oxidation period. Oxidation tests were also conducted on SiC-Mo≤5Si3C≤1 composite at 800°C, 1300°C and 1600°C for more than 100 hours. The oxidation resistance of the composite was found to be very good. The results demonstrate that, though oxidation resistance of monolithic Mo≤5Si3C≤1 is far insufficient for high-temperature applications, boron-modification and/or composites with SiC are viable methods to improve oxidation resistance to a practically acceptable level.

Infra-Red Absorption Spectroscopic Study of β-Sialons in the System Si3N4–Al2O3·AlN

β-sialons (Si6-zAlzOzN8-z) have been formed in the system Si3N4–Al2O3AlN using hot-pressing at 1800°C under a pressure of 200 kg/cm2 in 1 atm. N2. In the spectral ranges 1050–850 cm-1 and 600–300 cm-1, certain infra-red bands decreased linearly in their frequencies as z increased. The bands in the former range are assigned to the stretching vibration of Si–N and the bands in the latter range to the bending vibration of Si–N–Si and N–Si–N groups in the β-Si3N4 three-dimensional network. When z increases, Si and N in the β-Si3N4 network are largely replaced by Al and O, respectively. This results in an increasing degree of structural disorder of the network and a decrease in the force constants for those vibrations. The decrease in the frequency can be explained by this decrease in the force constants.

Influence of microstructure on strength and fracture toughness of?-Sialon

Abstractβ-Sialon (Si6−zAlzOzN8−z) withz = 0.5 was fabricated by hot-pressing of a spray dried mixture of α−Si3N4 and aluminium-isopropoxide solution. Phase composition, flexural strength and microstructure of a sintered body were investigated. Phases identified by XRD were β-Sialon and a small amount of O′-Sialon. The flexural strength (three-point bending) was about 1500 MN m−2. This value, about three times higher than that of β-Sialon fabricated from α-Si3N4 and α-Al2O3 powder, was mainly due to the homogeneous microstructure without large defects such like clusters of large grains. β-Sialon was heat treated at 2000 °C for 2 h in 4 M Pa N2 to develop elongated β-Sialon grains with high aspect ratio. Microstructure, flexural strength and fracture toughness (KIc) of it were investigated. Both strength andKIc were lower than those of hot-pressed sample, even though an elongated microstructure was achieved. This fact showed that the toughening of β-Sialon with elongated grains could not be achieved without grain boundary phase which resulted in a crack deflection.

Room temperature strength of β-sialon (z = 0.5) fabricated using fine grain size alumina powder

Abstract The room temperature strength of β-sialon with z=0.5 fabricated using a mixture of fine grain size Al2O3 and Si3N4 powders was measured, and the effect of a small, amount of Al(Oi-Pr)3 addition as a dispersant on its strength was also investigated. This β-sialon showed an average strength of 1.00 GPa. It was much higher than that of β-sia1on from traditionally produced Si3N4 and Al2O3 powders as a result of decreasing the size of defects responsible for fracture origin. Mixing of Si3N4 and Al2O3 powders in a Al(Oi–Pr)3 solution brought further increase in strength to 1.25 GPa because of the smaller size and reduced number of defects. However, the strength of β-sialon is still controlled by the defects with the size of less than 10 μm.

Oxidation behaviour of Β-sialon fabricated from α-Si3N4 and aluminium iso-propoxide

AbstractΒ-sialon with z=0.5 was fabricated by hot pressing of a spray-dried mixture of α-Si3N4 and aluminium iso-propoxide solution. The oxidation behaviour of this Β-sialon was investigated, comparing it with commercial Β-sialon containing Y2O3 as a sintering aid. Oxidation tests were carried out at 1200 and 1400‡C for 25 to 200 h in air. The oxide layer of aluminium isopropoxide-derived Β-sialon was thin, dense, smooth and homogeneous without bubbles and cracks. The strength after oxidation at 1400‡C for 200 h was about 800 MN m−2, almost the same value as before oxidation. The oxide layer of Y2O3-doped Β-sialon was thick and inhomogeneous, containing many bubbles, cracks and grown needle-like crystallites (Y2Si2O7). The strength after oxidation at 1200‡C for 200 h fell to 1/2(440 MN m−2) because of pit formation in the oxide layer, and at 1400‡C for 200 h fell to 1/4(200 MN m−2) because of severe swelling and flaking of the oxide layer. The high oxidation resistance of aluminium iso-propoxide derived Β-sialon was mainly due to its homogeneous microstructure and freedom from foreign constituents such as Y2O3.

Mechanical and electrical properties of multilayer composites of silicon carbide

Multilayer composite disks of SiC-TiC-Ni system were fabricated by a powder sintering technique. The composition was graded stepwise between SiC and TiC, and between TiC and Ni. Cracking in the ceramic part was prevented by increase of the number of stacked layers. It was found by the Vickers indentation technique that large tensile stresses existed on the surface of SiC layer and at the disk edge. The reduction of residual stress by compositional gradient was confirmed by an analytical calculation. Furthermore, the electrical property of SiC-TiC gradient material was evaluated. The electrical conductivity measured in the direction from SiC to TiC revealed the ohmic behavior, indicating that metallic TiC is useful as electrode of semiconductive SiC.

Some properties of β-Sialon and β-Sialon-SiC composite annealed under high N2 gas pressure

Bending strength, hardness, thermal expansion coefficient, fracture toughness and weight gain (1300°C, 100h in air) of the β-sialon and β-sialon-50wt% SiC composite hot pressed at 1850°C and annealed at 1900° and 2000°C were measured.(1) The strength and hardness of β-sialon and β-sialon-50wt% SiC composite decrease with increase in annealing temperature.(2) Fracture toughness of β-sialon and β-sialon-50wt% SiC composite is about 3.0MNm-3/2 and 4.3MNm-3/2 respectively. They are independent of annealing temperature.(3) Thermal expansion coefficient of β-sialon as hot-pressed and annealed at 1900° and 2000°C is 2.4-2.5×10-6°C-1 That of β-sialon-50wt% SiC composite as hot-pressed and annealed at 1900°C is 3.0×10-6°C-1 and annealed at 2000°C is 3.5×10-6°C-1.(4) Weight gain of both β-sialon and β-sialon-50wt% SiC oxidized at 1300°C for 100h is nearly zero.