332 Composites

Textron Systems Corporation makes tubular CFCCs products by gas-phase reaction, combining elements of ceramic slip casting, filament winding, and gas-phase nitridation bonding. The result is either a nitride-bonded silicon carbide or nitride-bonded silicon nitride ceramic depending upon the materials and process.

Textron tubular products are formed by drawing silicon carbide monofilaments (or yarn) through an aqueous-based slurry of silicon carbide containing a binder (polymer), silicon powder, and silicon carbide particulates. The coated filaments are wound onto a drum or segmented mandrel at ±30° and dried. After the binder is removed, the silicon powder is converted to silicon nitride by nitriding/heating in a gas, typically containing nitrogen, ammonia, and hydrogen. This converts the silicon to silicon nitride, creating a matrix of silicon nitride to bond the silicon carbide powders and fibers together into a strong composite.

The green silicon carbide preform is densified by submerging it in a liquid silicon carbide precursor and heating by induction to a temperature above the decomposition temperature of the precursor. The extreme heat transfer environment imposed on the preform causes a steep thermal gradient to develop through

FIGURE 3.15 Cylindrical shapes are usually formed by winding ceramic fibers onto mandrel and then impregnating with ceramic matrix.

the thickness. Densification occurs by deposition of silicon carbide into open porosity of the green form, beginning at the inside of the composite and moving to the external surface through control of the thermal gradient. The final cylinder is completely net shape.

For most tubes, Textron uses its large SCS-6 silicon carbide fiber. For small cylinders, such as a 1- to 4-in.-diameter combustor liner of a small missile turbine engine, a smaller diameter fiber, such as its SCS-9 is required to bend around the small radius.

McDermott makes tubes by winding onto mandrels and infiltrating with aluminum oxide precursor sol (Fig. 3.15). The sol is converted into aluminum oxide by heating. It also uses an alternate process of laying-up cloth onto round mandrels, hardening, and removing from the mandrel.

Honeywell Advanced Composites Corporation, Newark, Delaware, makes CFCCs of silicon carbide and oxide fibers, silicon carbide and aluminum oxide matrices in various combinations depending upon the desired CFCC properties. It manufactures CFCCs with three processes: chemical vapor infiltration (CVI), melt infiltration, and directed oxidation.

At the start of each process, tooling holds the shape of a preform of fiber made of silicon carbide or carbon or metal oxide. The CVI process deposits a carbon or boron nitride (from boron chloride, ammonia, and hydrogen) coating onto the fiber. The coating is selected based upon the operating environment of the finished product. Once the coating is applied, the preform is rigid and free-standing in its desired geometry. The preform is infiltrated with methyltrichlorosilane and hydrogen vapors that react to form a silicon carbide matrix between the coated fibers. This is an isobaric, isothermal infiltration process conducted near 1000°C (1652°F) under reduced pressure. This process results in parts of various shapes and sizes including flat plates, cylinders, and more complex parts. Tables 3.8 and 3.9 list typical properties of Honeywell's Enhanced CFCC containing silicon carbide fiber in a plain weave and five harness satin fabric, respectively. Note that properties are retained at high temperatures.

Honeywell's DIMOX directed metal oxidation (DMO) process involves the growth of an oxide matrix through a preform of silicon carbide or oxide fibers.

TABLE 3.8 Properties of Honeywell Enhanced CFCC with Plain Weave Nicalon™ Fibers

Nominal Value at Temperature

TABLE 3.8 Properties of Honeywell Enhanced CFCC with Plain Weave Nicalon™ Fibers

Property

Units

23°F

2012°F

Density

lb/in.3

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