A View of the Textron Processing Equipment
Textron Specialty Materials (TSM) is the largest producer of chemically vapor deposited boron and silicon carbide mono-filaments in the United States, though the division also has over a 30 year history of developing high-performance composite materials for demanding applications. For Textron, the DOE CFCC Program represents a very important opportunity for pursuit of commercial markets for high-temperature CFCC materials.
Before the CFCC Program began, Textron had gained a good deal of useful experience in the fabrication and testing of hot pressed and hot isostatically pressed continuous fiber-reinforced ceramic composites. Virtually all of this work was performed under outside sponsorship from military agencies. Obviously, much less attention was focused on component cost reductions than on superior technical performance.
Fabrication Process
TSM felt that a cost-effective CFCC fabrication route would be
(1) Completely net-shape or near-net-shape processing characteristics, obviating the need for expensive process tooling;
(2) ability to use low-cost, widely available starting materials and compositing methods;
(3) speed and flexibility; and
(4) tailorability, exemplified in having the means to change matrix composition or fiber selection to suit the specific technical and cost requirements of a given component application.
Textron believed these requirements could be met by choosing gas phase reaction synthesis (gaseous nitridation) as the primary route for processing CFCCs. This is a net shape technique for making nitride bonded silicon carbide (NB SiC) or nitride-bonded silicon nitride (NBSN) ceramic matrices. It begins with a high solids, water-based, dispersion of Si + SiC or pure Si commingled with ceramic fiber tows, fabrics, or filaments during the preform construction process.
After completing the nitridation process, the NB SiC or NBSN matrix contains 10 to 20% porosity, which can sometimes present a problem with regard to CFCC materials stability in certain thermo-oxidative (i.e., combustion) environments. For this reason, TSM began an effort in its Phase 1 Program to use its Rapid Densification(TM) infiltration process to infiltrate open porosity in nitride-bonded CFCC matrix with stoichiometric silicon carbide.
Although this step will not be required in all component applications, it offers end users a performance discriminator that may offer substantially enhanced materials lifetimes in certain demanding process environments.
The TSM RD(TM) process is an extremely rapid deposition/infiltration technique that utilizes an imposed thermal gradient in the substrate material to effect infiltration of open pores. The thermal gradient in the RD(TM) process is established by submersing the component to be protectively coated in a source reagent, which must be liquid at ambient temperature and pressure, although deposition actually occurs while the part is being heated from a vapor/liquid transus created at the interface between the heated part and the source reagent.
As might be expected, deposition rates are much faster than those normally encountered in gas phase processes (such as CVI or CVD); hence, considerable economic benefit could be realized by using the RDª process for protective coating of NB SiC CFCCs with a layer of adherent silicone carbide.
A View of the Textron Processing Equipment
Textron Team
Textron made a determined effort to fill all links in the CFCC food chain during its Phase 1 program by building a team that has a wide range of OEM , end-user, and materials fabrication experience. To this end, research focused on development of analytical and fabrication methods that support the long-range objective of manufacturing thin-walled CFCC tubes for the primary metals and chemical process industries. The team includes
- Textron Specialty Materials - primary fabricator of NB SiC or NBSN CFCC materials, components, and fabrication processes .
- Doehler-Jarvis Die Casting - the largest aluminum die caster in the U.S., they will serve as a host site for Textron CFCC immersion burners.
- Hauck Manufacturing - a 100-year old-company specializing in the manufacture of radiant and immersion burners for the metals processing industry. Hauck also serves as an additional CFCC burner test site.
- Williams International - an OEM of small military and industrial turbines that is investigating possibilities for use of CFCC hardware in commercial power generation markets. Textron is developing a CFCC combustor can for an industrial cogeneration unit.
- Materials Science Corporation - a materials modeling and design firm doing macromodeling and micromodeling of CFCC materials behavior.
- Stone and Webster Engineering - an architect/engineering firm examining the use of CFCC reformer and pyrolyis components in the petrochemical processing industry.
- Nova Industrial Ceramics - a materials fabricator with wide experience in reaction forming of monolithic silicon-based ceramics, Nova is assisting Textron in developing CFCC matrix materials.
- University of Dayton Research Institute - UDRI and Textron are collaborating on developing hoop tensile (burst) testing of tubular CFCC components for materials screening.
- IBIS Associates - IBIS is developing an activity-based cost model of the TSM CFCC fabrication process.
Phase 1 and 2
Because use of nitride bonding combined with automated composite preform fabrication had not been proven as a viable CFCC fabrication process before the Phase 1 effort, The principal technical objective was to demonstrate that coupon- and subcomponent-scale CFCC specimens could be made by such methods.
Textron conducted technical and economic impact assessments for use of CFCC components in process industry applications targeted by TSM and their CFCC Program team members.
Task 1 applications assessments indicated that CFCC immersion and radiant burner tubes represented viable near-term, economically attractive component applications in the metals processing industry. Catalytic reformers and reformer tubes were found to be attractive, longer-term component applications in the petrochemical processing.
Textron systematically investigated many processing techniques and CFCC materials combinations that made use of filament-, fabric-, and yarn-based reinforcements. It found that the best-performing materials system consisted of nitride-bonded silicon carbide or nitride-bonded silicon nitride reinforced with 5 to 20 volume percent of TSM SCS-6(TM) CVD SiC monofilament.
Early in Phase 2, Textron decided that the primary thrust of the program would be to fabricate a series of CFCC immersion burners that would be field tested at a host facility. After consultation with teammates Hauck Manufacturing and Doehler-Jarvis, Textron embarked on an effort to produce six CFCC immersion tubes that could be directly inserted into a small production-scale aluminum holding furnace at the Doehler casting facility in Toledo, Ohio. This would be a valuable opportunity for generating in-the-field component performance data in an actual production environment.
A View Inside the Textron Process Plant (color photo, 155.6k)
Sixth Tube Completed
Textron began the fabrication and process development activities to produce six CFCC burner tubes a year ago. The company recently completed the sixth CFCC outer burner, which was composed of SCS-6(TM) reinforced NB SiC in a ± 20 degree layup. A four axis McClean-Anderson filament winder was used to make all tubes with integral flanges and domed ends; this maximizes component durability in these critical areas.
Textron has shown that gas phase reaction synthesis, when combined with their RD(TM) protective coating scheme, can offer a cost-effective and net-shape automated processing scheme for producing a useful range of CFCC component shapes. Processing trials and simulation tests demonstrated that high-strength, thermodynamically stable CFCCs can be made with SCS-6(TM) reinforced NB SiC or NBSN.
Later in the Phase 2 Program, Textron will investigate prospects to work with other oxide and nonoxide yarn- type reinforcements so that prospective end users and OEMs have a range of technical and cost options from which to choose.
Perhaps most important, Textron has shown the ability to fabricate open- or close-end CFCC tubes that have integral flanges and domes. Facilities capable of filament winding and nitriding CFCC components up to 10 feet long exist today and are available for future program or commercial market requirements.
Comments to: mgc@ornl.gov
Revised: July 7, 1995
