Showing papers by "Tanguy Rouxel published in 1993"

Crystallization and properties of a Si-Y-Al-O-N glass ceramic

Abstract: Glasses have been prepared in the Si-Y-Al-O-N system by melting mixtures of silica, alumina, yttria, and silicon nitride. One particular glass containing 17 equiv% N has been investigated to observe the crystallization at temperatures in the range 1,050--1,300 C. The major crystalline phase observed is Y[sub 2]Si[sub 2]O[sub 7], existing as the [alpha] form below 1,200 C and the [beta] form above this temperature. Properties of the glass-ceramic, including thermal expansion coefficient, hardness, electrical resistivity, and creep have been assessed.

Intragranular crack deflection and crystallographic slip in Si3N4/SiC nano-composites

Abstract: Crack deflection is a potential mechanism for toughening essentially brittle materials. Deflection generally occurs along grain boundaries and is enhanced by particles with high aspect ratios. Si 3 N 4 /SiC ceramic/ceramic composites contain elongated β Si 3 N 4 grains as a major phase and provide a good illustration for this mechanism. As a result, such materials demonstrate a relatively good fracture toughness. It is shown in this study, using transmission electron microscopy (TEM), that transgranular cracks, which develop at low temperatures and/or high deformation rates (present case), may also be deflected by intragranular SiC inclusions. Cracks use the 100 planes and propagate along the 〈100〉 directions of the silicon nitride hexagonal lattice. Further, after large plastic deformations at high temperature, slip defects are observed in silicon nitride grains, with many step-like deviations (‘cross’ slip), and the slip system is found to be [001] {100}.

Improvement of creep resistance of sintered silicon nitride by hot isostatic exudation of intergranular glass

Abstract: Postsintering treatment of pressureless-sintered silicon nitride at 1,250 C (i.e., above the glass transition temperature of the amorphous phase) by hot isostatic pressing was performed to diminish the quantity of residual intergranular amorphous phase that results from densification aids. The samples were first embedded in a powder (SiC or Al[sub 2]O[sub 3]), which worked as a diffusion barrier and a pressure transmitter; then, both sample and surrounding powder were encapsulated in evacuated tubes of borosilicate glass or stainless steel. Energy dispersive X-ray spectrometry proved that, during the treatment, a noticeable amount of intergranular vitreous phase exuded out of the sample into powder pores. As a consequence, the creep resistance was enhanced by a factor of 3. When the HIP treatment was associated with a subsequent crystallization treatment, a further improvement in creep resistance was obtained.