In Phase I of the program, Dow Corning formed an industry team consisting of Kaiser Aerotech, Solar Turbines, Sundstrand Fluid Handling, MSNW, Techniweave, MSC, and Argonne National Laboratory. The specific objectives were:
- Assess suitability of CFCCs for the chosen industrial applications.
- Demonstrate the feasibility of fabricating 2D CFCCs at a manufacturing site.
- Investigate improvements to the existing material/process.
- Improve mechanistic understanding of composite failure mechanisms.
Applications under consideration in this program were initially divided into two areas:
1. Stationary gas turbines and heat exchanger components (Solar Turbines, Inc.)
- Heat exchanger tubes and headers
- Combustor liners
- Turbine blades, vanes, and disks
2. Chemical pump components (Sundstrand Fluid Handling)
- Containment shell for canned motor pump.
Material and Processing
The PIP process that Dow Corning uses to fabricate CFCCs is widely recognized as a versatile method to fabricate large, complex-shaped structures. In comparison with other ceramic composite fabrication processes, the PIP process offers significantly greater flexibility. By utilizing low temperature forming and molding steps typically used in organic matrix composites, the PIP approach allows one initially to use existing equipment and processing technology.
PIP Process
In the PIP process , the matrix polymer is first impregnated into the fiber architecture of choice and then cured by conventional methods. Both the initial shaping and fabrication of the composite are carried out with low-temperature processing equipment. The composite is then pyrolyzed to temperatures greater than 1000 degrees C to convert the preceramic matrix polymer to a ceramic. Subsequent impregnation and pyrolysis steps are carried out to achieve the desired final density or open porosity.
The CFCCs fabricated by Dow Corning have a versatile chemistry and can consist of various fiber, interface coatings, and matrix chemistries.
The currently available systems are divided into the following classes of materials: SylramicTM 100 series composites, A carbon-coated Nicalon fiber in an amorphous SiOC matrix for maximum use temperature of less than 450 degrees C in oxidizing environments and up to 1100 degrees C in inert environments. Sylramic(TM) 200 series composites-a proprietary coated Nicalon fiber in an amorphous SiNC matrix for use up to 1200-1250 degrees C in an oxidizing environment.
Variants within a given series are available (i.e., 101, 201 etc.) that have been tailored to meet specific application requirements by modifying matrix fillers and processing conditions.
Fabrication Methods
The ability to fabricate preforms having the desired shape and architecture is of primary importance in the CFCC industry. The containment shell for a canned motor pump and a gas turbine combustor liner requires the fabrication of thin-walled tubes. There are a number of processes that can fabricate such a shape, including
- filament winding
- hand lay-up of prepreg
- braiding
- resin transfer molding
- 3-D weaving
Each of these techniques, combined with further processing, can have its own particular merit, such as low cost, fiber orientation and architecture, and size, depending on the design and stress requirements of the particular application. The photograph (graphic, 74k), which demonstrates the versatile shape capability of the PIP process, shows a number of components fabricated by Dow Corning in other programs, including an 8-inch diameter, thin-wall tube fabricated by means of a filament-winding process.
Mechanical Properties
The composites fabricated in the Phase I program were evaluated at the High Temperature Material Testing Laboratory at Oak Ridge National Laboratory under a User Agreement. The data generated has shown that Dow Corning's CFCC demonstrates both high tensile strength and tough composite behavior at both room and elevated temperatures. Other mechanical properties measured in Phase I included low-cycle fatigue and creep rupture behavior for CFCCs, with a range of matrix compositions at both room and elevated temperatures. The table shows the typical properties for a Sylramic(TM) 201 CFCC.
Dow Corning has also recently initiated a CRADA with ORNL to investigate the mechanics of creep deformation in ceramic fibers and PIP-derived CFCCs.
Typical properties of PIP derived CFCCs (Sylramic 201). 2D 8 harness satin Nicalon in SiNC matrix, 50% fiber volume.
Applications
In the Phase 1 program, both turbine and heat-exchanger applications that utilized a CFCC were assessed for design requirements, impact, and energy benefits. Specific components evaluated included combustor liners, turbine blades, vanes and disks in the gas turbine hot section, and headers and tubes for the heat-recovery section. Based on the application assessment methodology carried out in the Phase I program, it was concluded that the gas turbine combustor liner and the first-stage gas turbine disk made from PIP-derived CFCC material would provide the greatest benefits.
The potential energy savings of using CFCCs in the turbine area was projected to be 22x10(12) BTU/year at a minimum. Other benefits include reduction in cooling air requirements, reduction in the need for strategic materials used in the gas turbines, and reduced emissions.
Containment Shell
The second industrial application evaluated in Phase I is thin-walled containment shells for canned motor pumps (graphic, 98k) used extensively in the chemical and oil refining industry. The use of CFCCs in this application will enable the fabricator of these pumps to satisfy a current market need for high-power, high-speed chemical pumps.
Sundstrand Fluid Handling is a market leader in the manufacture of centrifugal pumps and compressors for the petrochemical, chemical, and general industries. Canned motor pumps are typically used in chemical processing and petroleum refining plants. Common uses are the handling of hazardous liquids in low-pressure processes up to a maximum temperature of 450 degrees C.
These pumps integrate the pump and the motor, thereby eliminating the requirement for the shaft seal. The motor stator is sealed from the pump process fluid by the containment shell. The process fluid being pumped circulates through the motor, lubricating the bearings and removing heat from the motor stator and rotor. The primary requirements for a CFCC in this application include high hoop strength, corrosion resistance, and low electrical conductivity.
Benefits
By providing an improved resistance to corrosion and imparting damage tolerance to the liner shells making it less susceptible to failure from mechanical rubs, erosion, or fluid corrosion, CFCCs will provide an environmental benefit through a reduction in potentially hazardous fluid spills and leakages in the chemical industry.
Sealless pumps fabricated with CFCC containment shells will enable the manufacture of energy efficient, superior performance products in many fluid pumping applications. Improved market share in these applications will boost U.S. industrial productivity and competitiveness.
CFCC containment shells realize energy savings because they virtually eliminate eddy current losses. High- speed applications also allow more efficient pump-specific speeds to be obtained. Potential energy saving are projected to be 544x10(6) BTU/year.
Phase II Program
The Dow Corning team for CFCC Phase II consists of Kaiser Aerotech, Solar Turbines, Sundstrand Fluid Handling, and Synterials. In this Phase, the team will carry out the following activities:
- Material/Process Optimization and Testing
- Process/Fabrication Optimization and Component Fabrication
- Design & Testing of Representative Components
- Development of CFCC Joining Techniques
- CFCC Surface Coating Development
- Joint Venture Formed for Commercialization.
In May 1993, Dow Corning Corporation and Kaiser Aerospace and Electronics Corporation formed a joint venture partnership to pursue the commercialization of CFCCs.
This partnership, known as Kaiser Ceramic Composites, combines the manufacturing expertise of Kaiser Aerotech in composites and high-temperature materials with the Dow Corning polymer-derived ceramic technology and processing knowledge. The company's mission is to research, develop, manufacture, and sell SylramicTM composites.
Comments to: mgc@ornl.gov
Revised: July 7, 1995
