Papers by Author: L.K.L. Falk

Abstract: This paper is focussed on the development of microstructure during liquid phase sintering and post-densification crystallisation heat treatment of ceramic materials based on the α- and β-Si3N4 structures. Grain shape and size distributions, assessed by quantitative microscopy in combination with stereological methods, and fine scale microstructures, investigated by electron diffraction and high resolution imaging and microanalysis in the transmission electron microscope, are discussed in relation to the fabrication process and the overall composition of the ceramic material.

Abstract: This paper is focussed on the development of microstructure during crystallisation heat treatment of B-phase parent glasses with composition (e/o) 35R:45Si:20Al:83O:17N, where R = Er, Yb, Y or a mixture of Y and Yb. Extensive high resolution analytical transmission electron microscopy has shown that the lenticular B-phase crystals take up a substantial range of composition. The element R is always clearly anti-correlated with the Si, and a larger R3+ cation radius moves the composition range to lower R contents. It is suggested that a locally increased density in the bi-dimensional network of randomly oriented (Si,Al)(O,N)4 tetrahedra is associated with an increased density of vacancies in the R3+ cation lattice.

Abstract: The intergranular microstructure in a liquid phase sintered silicon nitride based ceramic may be viewed as an oxynitride glass-ceramic. This work is concerned with the incorporation of yttrium B-phase, which is a five-component phase, into the intergranular regions of silicon nitride ceramics. The silicon nitride materials were fabricated with the addition of a powdered B-phase parent glass with composition (e/o) 35Y:45Si:20Al:83O:17N, or the addition of a mixture of Y2O3, SiO2 and Al2O3 with cation composition (e/o) 35Y:45Si:20Al. The starting powder mixtures contained 10 wt% of sintering additives. Sintering for 2 h at 1800°C was followed by a two-step post-densification heat treatment in order to promote nucleation and growth of yttrium B-phase. Detailed imaging and elemental analysis of the intergranular regions was carried out by EDX in a FEGTEM.

Abstract: Five-component B-phase may be readily formed through the nucleation and crystallisation heat treatment of nitrogen-rich parent glasses with composition (e/o) 35R:45Si:20Al:83O:17N. This paper is focussed on the B-phase structure where R stands for ytterbium, erbium or yttrium. Fine probe EDX analysis in the TEM has shown that the lenticular B-phase crystals take up a substantial range of composition and that the element R is always clearly anti-correlated with silicon. A larger R3+ cation radius moves the B-phase composition range to lower R contents, and as a consequence of the anti-correlation with silicon, the silicon solid solution range goes to higher values. The EDX results lend support to a B-phase structure consisting of two-dimensional network of randomly linked (Si,Al)(O,N)4 tetrahedra between layers of R3+ cations. It is suggested that, in addition to the random substitution of silicon by aluminium in the (Si,Al)(O,N)4 tetrahedra, a locally increased density in the bi-dimensional network of randomly oriented tetrahedra is associated with an increased density of vacancies in the R3+ cation lattice.

Abstract: The tensile creep behaviour of a mullite-SiC nanocomposite containing 5 vol% of SiC particles deformed under stresses from 4 to 50 MPa at 1400 °C has been studied. After grain-size effects had been accounted for, the creep-rate of the nanocomposite was found to be approximately 30× less than that of the monolithic mullite. It is suggested that this reduction is caused not by a threshold stress but by the extra work required to drive diffusion in the low diffusivity SiC particles so that they can move with the grain boundaries during creep. A model is presented which predicts the rate of creep under these conditions and gives reasonable agreement with the experiments at low stresses.