Porous free standing pyrolytic boron nitride articles

A method of making an open-pored article of pyrolytic boron nitride (PBN) article which comprises:

• (a) forming an open-pored porous substrate in the shape desired for the PBN article,
• (b) placing said substrate in a reaction chamber and maintaining said reaction chamber at a reaction temperature of from 1700.degree. to 2000.degree. C. and at a reduced reaction pressure between about 0.2 and 0.5 Torr while introducing into said reaction chamber ammonia vapor diluted with a non-oxidizing gas and a gaseous boron halide diluted with a non-oxidizing gas,
• (c) maintaining said reaction chamber at said reaction temperature and reaction pressure while continuing to admit said diluted ammonia vapor and diluted gaseous boron halide and while venting spent diluent gas to maintain said reaction pressure until the desired amount of PBN has been deposited on and in said substrate,
• (d) allowing said reaction chamber to cool to ambient temperature while maintaining a non-oxidizing atmosphere in said chamber, and a pressure between about 0.01 to 0.1 Torr, and
• (e) removing the PBN coating from a portion of the surface of said substrate to expose said substrate and then removing said substrate to leave the desired porous article of PBN.