EFFECT OF HEAT-TREATMENT ON THE ELEMENTAL DISTRIBUTION OF SI,C,N,O FIBERS

Nicalon(R) and hydridopolysilazane (HPZ) polymer-derived ceramic fibers have been heat treated in various environments at 1200-1700-degrees-C, in attempts to understand their deterioration processes and to modify their surface layer composition, which is critical for their use in ceramic and metal matrix composites. The elemental distribution at and below the surface layer has been determined, before and after treatments, by a scanning Auger microprobe (SAM). Annealing in nitrogen resulted in its penetration deep into Nicalon fibers to replace carbon and oxygen. Carbon associated with Si could be easily replaced by N, while 'free' carbon was more stable and required higher temperature (1700-degrees-C) and N2 pressure (5 MPa) to be removed. Treatment in argon caused Si3N4 decomposition, as well as C and SiC removal through reaction with Si or SiO2. Only Si + O remained in the outer layer of HPZ fibers, and only a porous Si + C material remained in the outer layer of carbon-enriched HPZ fibers. Treatment of C-HPZ fibers in wet air at 1200-degrees-C resulted in the oxidation of the Si3N4, SiC and C components, leaving a surface Si + O layer.