R. A. Lowden and M. A. Karnitz, Oak Ridge National Laboratory, Oak Ridge, Tennessee
United States industry has a critical need for materials that are lighter, stronger, more corrosion resistant, and capable of performing at elevated temperatures for applications in energy and related technologies. Continuous fiber ceramic composites offer the potential to meet many of these demands in a variety of industrial areas. The opportunities for CFCC materials are extremely diverse and cut across many industrial sectors. Significant energy savings and reductions in emissions are projected for the utilization of these advanced materials in only a few critical applications such as gas turbine power generators, high pressure heat exchangers, radiant burners, and aluminum processing.
Because of the widespread benefits of CFCCs, the United States Department of Energy Office of Industrial Technologies has supported work in this area since 1987. Recently, a more comprehensive strategy and plan was conceived, and a program initiated to enhance or develop the primary processing methods for the reliable and cost effective fabrication of CFCC components for industrial applications. The Continuous Fiber Ceramic Composites Program began in 1991 and has been executed mainly by industry using existing facilities and expertise. The CFCC Program has been underway for two years, and the industrial teams are completing the first phase of the effort, which emphasized applications assessment and process feasibility. The Program is about to enter into Phase II, in which efforts will focus on process engineering, and the fabrication of representative components or prototypes.
As the Program enters this and future phases, the cost and availability of constituents materials and precursors become key issues. The industrial teams are responsible for process development, and thus matrix precursors are their concern. However, due to the level of funding, and emphasis being placed on processing, the CFCC Program is not supporting research and development activities for reinforcements. Significant programs in this area have been and continue to be funded by other Government agencies (NASA and ARPA). It was decided that the Program would start with existing fibers and track the developments within other organizations. The cost, availability, and performance of reinforcements are of significant importance for the success of the CFCC Program.
Cost competitiveness and product reliability are the overwhelming concerns of the private sector when considering adoption of CFCCs for industrial applications. Within the context of these considerations, the specific technical needs for CFCC development and commercialization were assessed in a study that involved a comprehensive literature search and discussions with over 120 organizations. In order of importance as expressed by the industrial and R&D communities, fiber development, was the top priority. More environmentally stable and corrosion resistant fibers, with higher temperature capability (> 1000° C) at greatly reduced cost (< $50/lb) are desired for the manufacturing of cost effective ceramic composite components for industrial applications.
It has been shown that as processing becomes more efficient and costs are reduced, the contribution of fibers to the final value of a component increases substantially. In a recent report by Stinton and Holzl entitled, "Commercialization Assessment of CVI Technology," completed as part of the Air Force Processing Science of Chemical Vapor Infiltration Program, it was shown that as furnace time and machining costs decrease, the cost of the reinforcement has a more significant impact on the cost of the final part. The study examined costs based on reactor type and size, precursors, reinforcement, preform preparation, fixturing, furnace/processing time, and machining. Nicalon fiber reinforced silicon carbide was used as an example. In the selected composites, the reinforcement is responsible for < 10% of the cost of thin-walled tubes or plates, while processing and machining contribute 80%. A SiC matrix composite plate, 200 x 200 x 3 mm, for testing and evaluation, costs $4000, with the Nicalon fabric being 5% of the value. However, it was determined that as the size and complexity of the part increased, and processing time decreased with improved reactor efficiency, the reinforcement could account for > 25% of the value of a component. A composite disk of similar construction, 250 mm in diameter and 20 mm thick, requires almost $1500 of fabric to fabricate the preform. If processed employing the more rapid forced CVI technique, densification can be accomplished in 36 h, and the cost of a part has been estimated to be $3000. In this case, reinforcement is 50% of the value of a finished piece. This demonstrates undeniably that the cost of reinforcements will have a major impact on the price of components.
In order to more effectively coordinate CFCC efforts with the numerous other ceramic composite programs, and to avoid duplication, a separate task has been established to assess ceramic fiber and reinforcement costs, availability, and manufacturing. Interests are in price, based on quantity and demand; manufacturing capabilities (capacity); and properties and specifications of currently available fibers or other reinforcements. In addition, projections for future expansion and/or cost reduction, with increased demand, are very beneficial in this evaluation. Information on "developmental" fibers is also being collected and considered, if available and appropriate.
A survey of the U. S. manufacturers and distributors of ceramic fibers and reinforcements was conducted to gather pertinent information. Requests for the aforementioned information were sent to the organizations, and 100% of those queried responded. The information is summarized in Tables 1 and 2. The information is broken down by company, and reinforcement type, ie. fibers and filaments.
The polymer-derived Si-C-O fibers, Nicalon and Tyranno, are being used in a significant portion of the composites currently under consideration for use in U. S. aerospace, energy, and industrial applications. These reinforcements are imported, and although production quantities for these material has been increasing, costs have risen due to changes in the yen/dollar exchange rate (300 yen/$ to 110 yen/$). Dow Corning, the U. S. distributor of the Nicalon fibers, stated that sales volumes need to double or triple costs would occur for Tyranno fibers, imported by Textron Specialty Materials, and it was speculated that scale-up of the Tyranno manufacturing facilities could be achieved in < 2 years. At present, there is no lower-cost, domestic replacement for these fibers. A number of equivalent reinforcements have been, and continue to be, in development by domestic organizations, but none has yet to be competitive in availability or cost.
The properties and thermochemical stability of the polymer-derived fibers have seen great improvements through changes in processing and composition, e.g. reduction in oxygen content. Processing techniques such as electron beam irradiation curing have produced small quantities of high quality fibers, however, the costs of these advancements are excessive at this time. Promising results for Si-C fibers have also been reported by Dow Corning and the University of Florida, but it will be some time before these materials will see production, if at all.
Stoichiometric SiC reinforcements are under development at Carborundum, ie. the vapor-liquid-solid (VLS) single crystal fibrils, and the polycrystalline SiC fibers formed by sintering powder. The fibrils have exhibited some of the highest strengths measured for SiC reinforcements. Both SiC materials are extremely creep resistant. The VLS process is in the developmental phase with small quantities of fibril available for evaluation. Carborundum anticipates having sintered fiber in continuous tow from a prepilot fiber line in the second quarter of 1994.
A number of alumina-based fibers are readily available from domestic and foreign sources. These fibers are used as reinforcements in non-structural ceramic matrix composite components such as radiant burners, furnace liners, and hot gas filters, however, they have not been considered for use in composites for high temperature structural applications. Many of the Nextel fibers are available < $200/lb, and the Altex fiber is offered at $125/lb for individual users who require quantities > 2,000 lb/year. Oxides are the lowest cost, available fibers and fabrics, but concerns regarding performance, stability, and life hinder their use in structural ceramic composite parts. It is important to note that the Nextel 610 alumina fibers are currently $1200/lb, but when in full production, cost will be reduced to < $200/lb. Although production quantities are proprietary, 3M appears to have set a goal of < $200/lb for their commercial fibers.
After many years of development, MER SiC fibers and fabric are available and have been distributed in small quantities. Carbon fibers are converted to a carbon-rich SiC material using a proprietary gas-phase process. It appears MER is preparing for scale-up of the process and projected costs are less than other Si-C fibers ($300/lb). Due to the limited availability of these materials, they have not seen the intense evaluation as have other reinforcements.
A low-cost method to produce SiC multifilament tows using carbon fibers is also being investigated at Georgia Tech Research Institute. Preparation of SiC fiber tow employing a continuous CVD process has been demonstrated. An economic analysis indicates that the fiber tow could be produced for $50/lb. The fiber is in development with no information regarding stability or properties available.
A variety of monofilaments are available, however, most are produced in small quantities or are unavailable for general consumption. Although these reinforcements may not be ideal for all ceramic composite applications, they do offer some advantages in strength, stiffness, and composition as compared to small diameter fibers. The most interesting development with regard to the filaments is the commitment to scale-up and manufacturing. In all cases, the manufacturers of monofilaments are prepared to enhance, or are currently increasing production capabilities. Substantial reductions in the cost of filaments are projected with scale-up. A decrease of 80% in the price of SCS-6 monofilaments is suggested for a 400% increase in demand, and thus production. Saphikon believes the cost of the single crystal alumina filaments will be reduced from $22,000/lb to < $1000/lb when pilot scale production begins.
The majority of the fibers of interest to the CFCC Program participants are > $300/lb. A heat exchanger tube, 3 m in length with a diameter of 100 mm and a wall thickness of 5 mm, would require 11.5 lb, or > $4,000 of fiber assuming 45 vol % reinforcement (typical fiber contents for a filament wound structure). This cost could be reduced accordingly with fiber cost decreases, e.g. the same component would use $1500 - $2000 of Nextel 440 or Altex. Extrapolating this to the manufacture of large quantities of components, it is evident that the cost savings associated with cheaper fibers could be substantial.
The industrial teams currently participating in the CFCC Program were surveyed to determined which reinforcements were of interest, approximate application temperatures, and key issues concerning reinforcements in individual projects. The results are given in Table 3 . With the exception of one or two efforts, high temperature performance was the principal concern. High-temperature strength and stability are of primary importance. Fundamentally, the users desire a fiber that is much like Nicalonreg., an easily handled, small diameter fiber, but with improved high-temperature properties and stability. Cost was secondary, lower cost is desirable but not imperative. Many users believe there are applications in which currently available reinforcements can be utilized; however, long-term stability was questionable.
The upper limit of use temperature appears to be 1400° C (2500° F). A common application drives this requirement, an uncooled combustion chamber for a gas turbine (Solar Turbine, Inc.). Internal surface temperatures approaching 1400° C are predicted. In this case, the users are willing to sacrifice cost for performance; however, even in this component, fiber costs similar to the current price for Nicalonreg. ($360/lb.) are desired. In the lower temperature applications, cost becomes a critical issue. An order of magnitude reduction in cost would be of great benefit in finding a broader range of new applications for CFCCs. In many of these cases, the composites would be used as direct replacements for metal parts, thus cost is the major issue.
This analysis has only addressed the reinforcement and not preform preparation, application of an interface coating, or other related procedures. Any additional handling or processing steps would add cost to the final component. For example, the fabrication of preforms will be dependent upon application. Multidirectional weaves would obviously be more expensive than fabric lay-ups or filament winding. Interlayer deposition will most likely be accomplished as an initial stage of the densification. Coatings would be applied to the final preform due to the difficulties in handling and weaving of coated reinforcements.
In conclusion, the industrial participants in the CFCC Program are moving closer to production of representative components and prototypes. Emphasis has been placed upon applications, and process feasibility. Great progress has been made, however, there are concerns regarding the cost, performance, and availability of reinforcements. Preliminary surveys of the users determined a need for improvement in stability and performance, and a significant reduction in cost. Meeting these requirements is a considerable challenge but is prerequisite to the successful adoption of CFCCs in industrial applications.
Nicalon, Nippon Carbon, Tokyo, Japan
Tyranno, Ube Industries, Japan
Nextel, 3M Corporation, St. Paul, MN
Altex, Sumitomo, Japan
Almax, Mitsui Mining Company, Ltd., Tokyo, Japan
The issues and concerns regarding reinforcements for CFCCs to be used in industrial applications were presented to the IHPTET Fiber Consortium at the Interagency Working Group Meeting held in Cocoa Beach, Florida on January 10, 1994. The recommendations were based on discussions between the participants in the Department of Energy Office of Industrial Technology CFCC Program. The suggestions are summarized following the format established by the Fiber Consortium, i.e. Fiber Property Goals for CMC (CFCC) Applications.
1) Diameter:
2) Room-Temperature Strength:
3) Room-Temperature Elastic Modulus:
4) Creep:
5) Stress Rupture:
6) CTE:
7) Oxidation resistant (in-composite performance)
8) Textile-Grade Multifilament Fiber:
9) Cost:
* Same as Fiber Consortium
In addition to physical property and cost targets for fibers, the following programmatic suggestions were also given:
|| Fiber Report || CFCC Program ||
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Last Updated, September 9, 1995
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