process for the manufacture of cohærente bornitridholdige fibres with improved varmemodstandsdygtighed, tensile strength and flexibility from a boroxidfiberforstadium by heating in the ammoniakatmosfæ

摘要

Fibres incorporating a boron and nitrogen containing compound and having a boron content of about 28 to about 43.6% and a nitrogen content of about 8.5 to about 63%, by weight are formed in situ from a precursor fibre of maximum diameter 30m which incorporates throughout its thickness a boron and oxygen compound by reacting said precursor fibre with a nitrogen and hydrogen containing compound. Preferably fibres consisting essentially of boron nitride are produced from precursor fibres, of maximum diameter 10m and containing a boron oxide composition, by reaction with a gaseous nitrogen and hydrogen containing compound e.g. ammonia. The precursor fibre may be heated to a final temperature above 300 DEG C. at a temperature rise of at least 25 DEG per hour while simultaneously passing gaseous NH3 at a flow rate of at least about 0.025 litre/min/g. of fibre until the resulting fibre has the required composition. To increase the corrosion resistance of the boron nitride fibres (a) the NH3 flow rate may be reduced to below the final temperature, or (b) the precursor fibre may be rapidly heated to the final temperature and the flow rate then reduced or (c) oxygen and/or water vapour may be added in minor amounts to the ammonia. After nitriding the corrosion resistance may be further improved by heating the resulting fibres in an inert atmosphere to a stabilizing temperature of 1400-2500 DEG C. for a time of between 5 minutes and 2 hours, the higher the temperature the shorter the time. This step may be preceded by a step of heating the fibres at a lower temperature in an atmosphere containing compound containing oxygen or water vapour.