Renee C. Graef

The influence of preceramic binders on the microstructural development of silicon nitride

A number of polymeric precursors to silicon nitride were prepared and evaluated as binders in cold pressing/pressureless sintering operations. These polymers exhibited ceramic yields in excess of 75% by weight, and powder compacts made using them as binders displayed improved green handling properties. Compacts pyrolysed at 800 °C exhibited unusual microstructures, including the development of whiskers in situ. Based on microstructural observation, compacts sintered under pressureless conditions appeared to show enhanced densification relative to those processed without preceramic binders. Preceramic binders appeared to enhance the formation of β-Si3N4 and may enhance densification of compacts sintered under pressureless conditions.

The pyrolytic conversion of perhydropolysilazane into silicon nitride

Abstract Polymeric precursors to ceramics have the potential to enable production of ceramic articles in novel forms, with novel microstructures, and improved properties. The polymer-to-ceramic conversion process is not sufficiently understood for this approach to be widely utilised. Perhydropolysilazane converts to high purity silicon nitride (Si3N4), and has been shown to be a useful coating and matrix precursor. The conversion of PHPS to Si3N4 is being examined by Solid State, Magic Angle Spinning (MAS) NMR spectroscopy and other techniques. This paper summarises the results to date of this investigation.

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.

Infiltration/Pyrolysis Processing of Fiber-Reinforced Silicon Nitride

While its high-temperature strength, resistance to oxidation, and other properties make silicon nitride an attractive candidate for many advanced structural applications, its propensity for brittle failure has hindered its widespread adoption. One approach to avoiding brittle failure is through incorporation of continuous fiber-reinforcement; however, conventional (powderbased) methods of silicon nitride fabrication can degrade fibers and are not amenable to the production of complex shapes. The Southwest Research Institute has developed a number of polymeric precursors to silicon nitride which are available as thermosetting liquids, and we have shown that these materials can be used in combination with near net-shape manufacturing techniques to produce fiber-reinforced silicon nitride composites. Mechanical property tests conducted at room temperature suggest that these polymer-derived composites exhibit fracture behavior comparable to those produced through conventional techniques; micromechanical investigations conducted at 800°C indicate that non-brittle failure is maintained at elevated temperature.