Note (2) Equipment included under (c) above are chemical vapour deposition
furnaces designed or modified for the densification of carbon-carbon composites.
N ture nd Purpose: CVD furnaces are used to infuse carbon into a porous carbon preform of a rocket nozzle or reentry vehicle nose tip. This process, referred to as densification, fills up and virtually eliminates voids in the pre- form and thereby increases the density and strength of the treated object.
Method of Oper tion: CVD furnaces use either isothermal or thermal- gradient processes for densification. The object to be processed is placed in the appropriate chamber and lowered into the hot zone of the furnace. All gas, water, and electrical connections are made, and all process instrumen- tation is connected before the lid is lowered into the furnace and sealed. The process sequence of heating and supplying the deposition gases is au- tomated, but furnace operators follow the part development through view- ports built into the furnace walls.
Typic l Missile-Rel ted Uses: CVD furnaces are used to make lightweight carbon-carbon rocket nozzles and nose tips. Carbon-carbon pieces are light and strong, and can increase system performance.
Other Uses: CVD furnaces are used in coating optics, some medical in- struments and components (e.g., heart valves), and cutting tools; in coating and polishing precision surfaces; and in making semiconductors.
Appe r nce ( s m nuf ctured): CVD furnaces are large, double-walled, cylindrical vessels with gas-tight closures. Typical CVD furnaces are large be- cause they house an internal heat zone, electrically driven heaters, and insu- lation. Furnaces smaller than 1.5 m in height and 1 m in diameter are con- sidered laboratory scale and are barely able to process a single nose tip or rocket nozzle insert. Process production sizes are larger than 2 m in height and 2 m in diameter. These furnaces have several ports: at least one large port for power feeds, others for instrumentation, and, when temperatures are measured by optical or infrared pyrometers, one or more view-ports.
They are double-walled so that they can be water-cooled during operation. Power cables are large and may also be water-cooled. The actual retort is housed inside the furnace and is heated by a graphite induction or resistive heater to temperatures of up to 2,900°C. Two typical furnaces, without lids in place, are shown in Figure 7-9. An induction power supply with heavy, water-cooled connections between the power supply and the smaller of the two furnaces is also shown. The induction coil, retort, and silicon steel shunts, which prevent inductive coupling to the furnace wall, are shown in Figure 7-10. A typical production setup of CVD furnaces consists of several components, as shown from left to right, in Figure 7-11: an impregnation
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