I. Field of the Invention
This invention relates to a process for producing fibres composed of, or coated with, carbides or nitrides of such elements as e.g. silicon and boron.
II. Description of the Prior Art
Continuous and short fibres made of SiC and similar carbides and nitrides are very useful for reinforcing a variety of materials such as metals and ceramics in order to improve the toughness or high temperature performance of such materials. The usefulness of the fibres arises not only from their inherent hardness and tensile strength, but also from the fact that the fibres generally are stable in oxidizing atmospheres and at high temperatures and tend not to react with the ceramic or metal matrix.
From these points of view, fibres of this kind are superior to carbon fibres often used for the same purposes. For example, when carbon fibres are used to reinforce aluminum metal, enormous pressures must be used to cause the molten aluminum to impregnate the fibres because carbon is not wetted by the molten aluminum. Moreover, the carbon reacts with the aluminum to form Al.sub.4 C.sub.3 at the interface and the resulting composite is of low strength. Attempts have been made in the past to overcome these problems by coating carbon fibres with materials such as metals (e.g. Ni) or carbides or borides, etc. in order to make the fibres wettable and to make their surfaces inert. The coating methods attempted have included plasma spraying, chemical vapour deposition, sputtering, etc. but there has been limited success due to poor penetration of the coating materials into fibre bundles, high costs and unacceptable losses of coating material etc.
Despite the superiority of fibres made of carbides and the like as mentioned above, the conventional fibres of this kind have a number of problems For example, SiC fibres are often manufactured by degrading a polycarbosilane type of precursor, but the high temperature stability of such fibres is low because they undergo a crystallization change at temperatures of about 1300.degree.-1400.degree. C. and thus tend to fall apart at these temperatures. It is also difficult to manufacture long or continuous fibres of carbides and the like, and although it has been reported that continuous SiC fibres can be made by heating a precursor fibre containing SiO.sub.2 or Si particles distributed in a carbon precursor such as pitch, attempts to duplicate such a process have not been successful. SiC particles are indeed formed, but they do not sinter together even at temperatures exceeding 2000.degree. C. and thus they do not retain the original fibre shape.
It has been known that suboxides such as SiO can be formed by reacting SiO.sub.2 and C or SiO.sub.2 and Si etc. and reacted with a graphite surface to form a coating. For example JP 58104078-A, JP 540090216-A etc. refer to the reaction of SiO(g) with graphite to form a coating on the surface. The SiO(g) is produced in a separate reactor and transported to the graphite surface. This is feasible at temperatures above 1800.degree. C. as at lower temperatures SiO can re-convert to SiO.sub.2 and Si (or, in presence of CO, to SiO.sub.2 and C). Coating of a carbon fibre mass by this method results in two problems:
a) due to improper infiltration, the coating formed is non uniform; and
b) as the reaction is conducted at high temperatures, the coatings are very granular and highly porous.
Accordingly, there is a need for an improved process for producing fibres made of, or coated with, carbides or nitrides, which process overcomes some or all of the disadvantages mentioned above.