Peter Greil

Oxygen distribution in silicon nitride powders

The oxygen content of ten different silicon nitride powders was determined by bulk chemical analysis and surface-sensitive X-ray photoemission spectroscopy (XPS). In silicon nitride powders prepared from silicon and silica by nitridation and carbothermal reduction in a nitrogen atmosphere, respectively, only a minor part of the total oxygen content of 0.9 to 2.5 wt% was found in a surface layer of less than 1 nm thick, whereas an appreciable amount can be attributed to oxygen dissolved in the bulk. Powders made by silicon diimide decomposition, however, are characterized by a higher oxygen concentration at the particle surface relative to the bulk, which may be further reduced by chemical treatment. The surface layer composition corresponds to an intermediate state between silica and silicon oxynitride.

Formation of silicon oxinitride from Si3N4 and SiO2 in the presence of Al2O3

Abstract The formation of Si 2 N 2 O from SiO 2 and α-Si 3 N 4 powder mixtures with addition of 3 mol % Al 2 O 3 was investigated. An eutectic at 98 mole % SiO 2 and 2 mole % Si 3 N 4 was formed at 1863 K which is decreased by the Al 2 O 3 , thus enabling the formation of Si 2 N 2 O via the liquid phase. Microstructural analysis by TEM showed a strong orientation between Si 2 N 2 O and α-Si 2 N 4 crystals. Oxidation resistance and bending strength were also measured.

Sintering and HIPping of silicon nitride-silicon carbide composite materials☆

Abstract Si 3 N 4 composite materials containing up to 60 vol.% of dispersed β-SiC particles were sintered with Y 2 O 3 and Al 2 O 3 at 1850°C and 0·1 MPa N 2 . Fractional density decreased from 0·97 to 0·91 when the SiC content increased from 0 to 60 vol.%. Simultaneously a retardation of grain growth and reduced pore size was found. Subsequent HIPping at 2000°C with N 2 -pressure of 100 MPa resulted in almost complete elimination of presintered closed pores and a final density of 0·99 up to 20 vol.% of SiC. During HIPping Si 3 N 4 is formed by reaction of SiC or C with the SiO 2 intergranular glass in the presence of high N 2 -pressure. While fracture toughness shows no significant influence of SiC content, a reduction of critical defect size with increasing SiC content results in a distinct increase of fracture strength particularly after HIPping.

Evaluation of the microstructure of ß-SiAlON solid solution materials containing different amounts of amorphous grain boundary phase

In order to test the possibility of quantitative microstructural anlaysis in Si3N4 ceramics, β solid solution materials containing different amounts of an amorphous phase were examined by transmission electron microscopy. The results show the amount and distribution of the amorphous phase and the influence of this phase on the grain morphology. This shows that within certain limits, set by the transmission electron microscopy, such a characterization is possible.

Rheology of aqueous silicon nitride suspensions

Abstract Highly concentrated silicon nitride slips with solid-volume concentrations φ of up to 0·475 were prepared from preoxidized, submicron powders in aqueous solutions at pH>8. Viscosity measurements reveal a pseudo-plastic flow behaviour at φ >0·35, which could not be described by existing flow equations. On the basis of the elastic-floc model for highly concentrated suspensions, a modified flow equation was derived, which is only determined by the initial floc-volume ratio and the particle-packing density. The calculated viscosity fits very well with the experimental values from low to high particle-volume concentrations φ from 0·2 to 0·475.

Sintering and strength of silicon nitride-silicon carbide composites

Abstract The sintering, microstructure, fracture toughness and strength of silicon carbide-dispersed silicon nitride composite systems were investigated. The composite powders could be sintered with aluminium oxide and yttrium oxide additives at normal pressure. Nearly complete densification was achieved by hipping presintered compacts. The silicon carbide particle dispersion retarded both grain and pore growth during sintering. The strength was found to increase with silicon carbide content, whereas fracture toughness remained constant. Thus strengthening of the composite is attributed to the decrease in flaw size.

Sintering of Si3N4 with liquid in the system Ce2O3AINSiO2

Abstract Liquid phase sintering of Si3N4 with melts from the system Ce2O3AINSiO2 has been studied. The glass forming region in this system and the reaction products formed during sintering at 1750–1800°C were analysed. Sintering of Si3N4 with two melt compositions selected from outside the glass forming region yields fully dense Si3N4. Post sintering treatment at 1300°C resulted in devitrification with consequent improvement of high temperature mechanical properties. The mechanical properties of Si3N4 sintered with liquids in the system Ce2O3AINSiO2 were found to be inferior to those of liquids selected from Y2O3AINSiO2, but superior to those selected from the system MgOAINSiO2.