Mullite ceramic whisker composite article exhibiting high-temperature strength

DETAILED DESCRIPTION As previously noted, strong, high-temperature materials have great potential for use in high performance applications such as gas turbine engines, cutting tools, and extrusion dies.
Mullite is an attractive matrix material for use in the fabrication of components for such applications, and more particularly for whisker-mullite composites, because of its relatively low thermal expansion, high creep resistance, relatively high melting point and low density.
It has proven difficult in the past to sinter mullite to full density since the crystals formed during sintering tend to be of acicular habit.
It is known that the addition of zirconia to mullite tends to prevent the formation of glassy grain boundary phases and also inhibits the formation of acicular grains.
Also, it is known that dense mullite-zirconia ceramics can be prepared by adding sintering aids such as MgO, CaO or TiO.
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2 to batches containing zircon and alumina.
However, the refractoriness of the resulting mullite matrix is generally reduced by such sintering aids due to the formation of a glassy grain boundary phase upon sintering.
It is also possible to obtain higher density mullite products by controlling the solid state reaction occurring between alumina and zircon through control of the firing schedule.
Again, however, this can require a two-stage treatment which may be less economical than a direct method for producing mullite products.
Also, since zircon is a natural mineral the impurities in it can vary significantly from location to location, making it difficult to manufacture consistent products.
In the present invention, tough mullite ceramics comprising zirconia are prepared using finely divided silica and zirconia, preferably derived from dissociated zircon, as batch raw materials.
As noted, the advantage of using batch materials such as dissociated zircon is that the particle size of the zirconia is smaller, and also that the reaction during sintering can occur among silica, zirconia and alumina rather than between zircon and alumina