Si3N4-SiC Composites Research Articles

Abnormal Oxidation Behavior of Si<sub>3</sub>N<sub>4</sub>-SiC-Y<sub>2</sub>O<sub>3</sub> Composite at about 1000°C

A composite with the composition of 58 mass% Si3N4-32 mass% SiC-10 mass% Y2O3 was oxidized at 900-1400°C in dry and moist air atmospheres. The weight gain rate at 1400°C in dry air was as low as 0.5mg/cm2 for 100h, revealing that the composite is highly oxidation-resistant. The weight gain in moist air was 3-7 times larger than that in dry air at all the temperatures tested. The weight gain at 1000°C in moist air was almost the same as that at 1400°C in moist air. The mechanism of the abnormal oxidation at 1000°C is not the same but similar to that reported by Lange et al. At 1000°C, a protective SiO2 layer which retards the diffusion of oxygen into the composite was not formed and the SiC fine particles in the grain boundary region of the composite were oxidized. It was assumed that the volume expansion of about 30% accompanying the conversion of SiC into SiO2 generated cracks in the grain boundary region through which oxygen can penetrate toward the unoxidized composite. Four Si3N4-SiC composites with different compositions also showed large weight gain at 1000°C in moist air. These results indicate that special care should be taken in handling Si3N4-SiC composite in moist atmospheres at high temperatures.

Factors influencing the crystallization of ultrafine plasma-synthesized silicon nitride as a single powder and in composite SiC-Si3N4 powder

A study of various parameters affecting the crystallization process of amorphous silicon nitride produced by plasma gas-phase reaction was undertaken to determine the conditions under which whiskers are formed. This process is influenced by the ammonium chloride content of the starting powder and the presence of nitrogen in the furnace atmosphere. This last parameter is also influential on the α/β phase ratio, along with other factors like the silica content, temperature and duration of the thermal treatment. Heat treatment at 1500°C for 30 min under argon produced well-defined α-Si3N4 crystals with a hexagonal cross section, a mean length around 0.8 μm, and no sign of agglomeration. Under the same conditions, crystallization of silicon nitride in SiC-Si3N4 composite did not give crystals, but Si3N4 whiskers. Therefore silicon carbide plays a major role in their formation.

Production of SiC-Si3N4 composites by a one-step d.c. plasma process

Preparation en une seule etape d'une poudre ceramique composite SiC - Si 3 N 4 par reaction en phase gazeuse dans un reacteur a plasma.

Microstructures and mechanical properties of Si3N4-SiC composites prepared by combustion synthesis and HIP sintering.

The microstructures and mechanical properties of Si3N4-SiC composites containing 8 to 46 vol% Sic, which were prepared from combustion synthesis and HIP sintering without additives, were studied and compared with those of Si3N4 prepared from conventional methods. Transmission electron microscopic observation revealed the existence of some small SiC particles within the Si3N4 matrix grains. The fracture toughness of the Si3N4-SiC composite was independent of Si, C content, probably because the SiC grains were smaller than the critical size for effective toughening and the formation of elongated Si3N4 grains was not promoted by the Sic dispersions. But SiC additions modified the high-tempera-ture strength of the Si3N4-SiC composites and increased the microhardness at all temperatures.

Spherical-impact damage and strength degradation in Si3N4-SiC composites

Four types of SiC-whisker/silicon nitride and SiC-particle/silicon nitride composites were produced, and their mechanical properties and impact damage behaviour examined. All of the composites exhibited elastic response behaviour at spherical impact with Hertz cone crack initiation. Impact resistance behaviour, however, was different for each composite. This was due to the different mechanical properties produced by their microstructures. A SiC-platelet/silicon nitride composite displayed the highest resistance to crack initiation and propagation, which resulted in high impact resistance to strength degradation. On the other hand, SiC-particle/silicon nitride, SiC-whisker/silicon nitride, and large SiC-whisker/silicon nitride composites showed less impact resistance, even though they have higher mechanical properties such as bending strength and fracture toughness.

Oxidation Behavior of Hot-Pressed Si<sub>3</sub>N<sub>4</sub>-SiC Composites

The Si3N4-SiC composites which is composed of Si3N4 matrix doped with 2wt% Al2O3 and 5wt% Y2O3, and 50wt% SiC powder having a different mean particle sizes, were fabricated by hot-pressing. Their oxidation behavior was investigated and compared with the commercial Si3N4 sintered material. An oxidation test was carried out in an electric furnace at 1300°C for 100 hours. The measurement of bending strength, weight gain, surface roughness and microscopic observation of sintered materials were carried out after oxidation. Weight gain of the Si3N4-SiC composite was lower than that of the commercial Si3N4 sintered material. The crystalline phases of sintered materials after oxidation test were β-Si3N4, α-SiO2, Y2O3⋅2SiO2 and SiC. The bending strength of Si3N4-SiC composites slightly increased the value after oxidation.

In Situ Polysilane‐Derived Silicon Carbide Particulates Dispersed in Silicon Nitride Composite

A dense (97% of theoretical density) Si3N4—SiC composite containing 10 wt%β‐SiC was prepared by introducing a SiC phase by the pyrolysis of a polymeric SiC precursor. The composite material was produced by mixing an alkyl/aryl‐substituted polysilane with Si3N4 powder and, by subsequently forming green compacts, pyrolyzing the polymeric species, and finally sintering the sample. Synthesis and characterization of the polymeric compound was described. Its transformation reactions to SiC and the characterization of the ceramic residue were also studied. High ceramic yields were obtained by curing the as‐synthesized polysilane at 500°C in an Ar atmosphere. The heat treatment had no effect on the good solubility of the polymeric precursor in organic solvents. This was important for processes such as infiltration, sealing, and coating and for the mixing of the polymer with powders for the preparation of homogeneous composite ceramics. The dense microstructure of the pyrolyzed and sintered Si3N4 powder–polysilane mixture exhibited reduced grain growth of the Si3N4 particles and a very homogeneous distribution of the in situ‐formed β‐SiC phase.

Infrared and Raman spectroscopic studies of Si3N4-SiC composites

Infrared and Raman spectroscopic studies of Si3N4-SiC composites

Microstructural Changes of Amorphous Si-N-C Fine Powders at High Temperatures

Microstructural changes of amorphous Si-N-C fine powders were studied by X-ray diffraction, transmission electron microscopy, IR spectroscopy and chemical analysis. The samples examined had varying contents of Si, N and C (C5: Si-35.8N-4.8C, C8: Si-32.8N-8.1C, C17: Si-29.0N-17.3C, C20: Si-24.2N-19.7C), and were prepared by pyrolysis of hexamethyldisilazane ((Me3Si)2NH, Me=CH3). These samples were heat-treated in an argon atmosphere at 1743 and 1803K for periods of up to 64.8ks. X-ray diffraction showed that the crystallized phase consisted mainly of α-Si3N4 and β-SiC. In the C5 and C8 powders, the size of crystallized β-SiC was 20-100nm under all heat treatment conditions, and β-SiC in C17 and C20 powders grew 50-1500nm and 20-200nm in size respectively, after heat treatment at 1803K for 64.8ks. The morphology of α-Si3N4 in C5 and C8 powders was rod-shaped, and the size of the α-Si3N4 was larger than that of β-SiC. On the other hand, the fine-grained and the thin-plated α-Si3N4 were observed in all of the samples heat-treated for 64.8ks at 1803K. We believe that the formation of fine-grained and thin-plated α-Si3N4 relate to the development of mechanical properties of Si3N4-SiC composites.

High-temperature catalyst supports based on nonmetallic refractory compounds

A study was made of conditions of preparation of supports based on MgO-Si3N4 and Si3N4-SiC composites for catalysts intended for high-temperature processes. It is shown that synthesized supports of these compositions surpass GIAP-3 industrial catalyst in thermal fatigue resistance and mechanical strength.