Preceramic Polymer Derived Si₃N₄ Composites with Continuous Fiber Reinforcement

Southwest Research Institute (SwRI) has developed a number of polymeric precursors to silicon nitride that are low-viscosity, thermosetting resins. These preceramic polymers are effective matrix precursors in infiltration/pyrolysis composite processing for the fabrication of fiber-reinforced Si₃N₄ composites.

Background

Excellent high temperature strength retention and oxidation resistance give ceramics the potential to replace metals in structural applications such as airframes and engines. The high strength to density ration of ceramic components help decrease a system's total weight, which increases operating efficiency.

Traditional ceramic fabrication involves the consolidation of micron sized ceramic particles through treatment at high temperature and pressure. Because the particles are hard and abrasive, reinforcing fibers often break during traditional fabrication methods causing the resulting composite to exhibit mechanical properties inferior to that of the unreinforced matrix. Traditional powder consolidation techniques are also not amenable to the low-cost production of complex shapes.

What are Preceramic Polymers?

Preceramic polymers are composed of a chain of main-group inorganic elements with organic appendages. When heated to sufficient temperature, the organic appendages are shed to leave an amorphous network of inorganic elements. Subsequent heat treatment transforms this material into crystalline ceramic.

A particularly useful family of polymeric precursors to silicon nitride has been developed at SwRI. These materials exist as waxes or low-viscosity thermosetting liquids. They exhibit very high ceramic yields (>85% by weight) and produce ceramic of exceptional purity (>95% Si3N4).

The liquid members of this family enable the fabrication of fiber-reinforced ceramics by methods commonly associated with polymer composites.

Composite Processing

To form a composite, the reinforcement cloth is prepregged with preceramic polymer and then stacked. Conventional vacuum bag technology facilitates composite cure at temperatures to 500°F with applied overpressures of 50-200 psig.

Polymer-to-ceramic conversion occurs at temperatures above 1450°F under flowing nitrogen. Subsequent reinfiltrations raise the composite density to the desired level.

Typical density history of a silicon nitride matrix composite during processing. Due to the inherent density change (from 1 to 3 g/ml) multiple reinfiltrations are required to achieve maximum density.

Other Applications

Ceramic Coatings

Polymer-based ceramic coating techniques may be used for field repair of damaged oxidation protection coatings and for factory floor post-assembly sealing of fastener sites in coated carbon-carbon (C/C).

Oxidative weight loss under simulated mission conditions of damaged-and-repaired, protectively coated composites. SwRI-repaired composites are virtually indistinguishable from undamaged composites.

Non-fugitive Binders

Preceramic binders, used in place of fugitive binders during Si₃N₄ powder processing, can maximize green body strength and affect void formation and microstructural development during densification.

Microstructure of silicon nitride, with 25% by weight preceramic polymer binder, after pyrolysis to 1475°F.

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