Process for producing fibres composed of or coated with carbides or nitrides

A process for preparing fibres consisting of or coated with a carbide or nitride substantially free of whiskers, which process comprises:

• forming a first reaction zone and filling said first reaction zone with microfine particles of an oxide or oxide precursor of silicon or boron containing at least one catalyst for generation of a gaseous sub-oxide of silicon or boron uniformly dispersed in said oxide or oxide precursor, said particles being uniformly dispersed within a matrix of carbon or a carbon precursor, wherein the molar ratio of the oxide present as a starting material or derivable from the oxide precursor to the carbon present as a starting material or derivable from the carbon precursor is equal to or slightly greater than the stoichiometrical ratio required for the generation of a gaseous sub-oxide or silicon or said boron;
• forming a second reaction zone closely adjacent to the first reaction zone, said second zone comprising a layer having a thickness of about 1 cm or less, and filling said second reaction zone with a porous mass having a density of about 1 g/cc or less formed of fibres selected from the group consisting of carbon fibres, fibres of a heat resistant non-carbonaceous material coated with carbon, fibres made of a carbon precursor and fibres made of a heat resistant non-carbonaceous material coated with a carbon precursor;
• heating said first reaction zone in a non-oxidizing atmosphere which promotes the formation of suboxide to a temperature at which said carbon precursor, if present, is converted to carbon, said oxide precursor, if present, is converted to said oxide and then said gaseous sub-oxide is formed by the reaction of the oxide and carbon in the first reaction zone and the gaseous sub-oxide diffuses to the second reaction zone;
• simultaneously heating said second reaction zone to a temperature no lower than that in the first reaction zone in a non-oxidizing atmosphere selected from nitrogen-free and nitrogen-containing atmospheres, and to a temperature at which said carbon precursor, if present, is converted to carbon, and at which the gaseous sub-oxide reacts with some or all of the carbon in the second reaction zone to form a substantially whisker-free carbide on said fibres or additionally with said nitrogen to form a nitride on said fibres;
• and
• separating the resulting fibres from any carbide or nitride whiskers that may also have been formed in the second reaction zone.