Microwave and Millimeter wave Processing of Polymer-Derived Silicon Nitride

Chemical methods of processing ceramics have the potential to overcome many of the processing-related obstacles that have hindered widespread commercialization. The Southwest Research Institute (SwRI) has focused on the development of polymeric precursors to silicon nitride (Si{sub 3}N{sub 4}). One such precursor, perhydropolysilazane (or PHPS), has been shown to be a useful binder for Si{sub 3}N{sub 4} powder processing, a useful matrix precursor for the polymer infiltration/pyrolysis (PIP) processing of fiber-reinforced Si{sub 3}N{sub 4}, and a useful ceramic coating precursor for the repair of oxidation protection coatings on carbon-carbon composites. While conventional, thermal pyrolyses of these preceramics has been sufficient to demonstrate their potential, substantial cost savings could be realized if the polymer-to-ceramic conversion could be instigated with electromagnetic energy. The authors have investigated the use of millimeter wave heating as a means of converting PHPS into Si{sub 3}N{sub 4}, and report here the results of the efforts to produce bulk compacts, coatings, and fiber-reinforced ceramics.