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Ceramic matrix composites (CMCs) are becoming widely used in aerospace applications (especially in rocket motors and turbine engine hot exhaust areas) as a method to reduce weight while increasing performance and efficiency. Continuous-fiber CMCs typically consist of a silicon carbide (SiC) ceramic matrix and SiC ceramic fibers, which together create a SiC/SiC composite. These composites can withstand extreme temperatures, making them ideal for propulsion system components.
CMCs are replacing the nickel, chromium and titanium alloys historically used in high-temperature applications, resulting in reduced structure weight (as much as 50% for certain applications). Although significant weight savings and performance benefits can be achieved through the introduction of CMCs into spacecraft and aerospace components, some drawbacks need to be considered and addressed to fully realize the potential of this family of materials.
The machining and grinding of ceramic matrix composites, particularly SiC CMCs, are difficult and costly processes. CMC machining costs can represent up to 75% of the total production cost for a particular component.
Traditional PracticesSilicon carbide CMCs are abrasive in nature, causing traditional cutting tools to suffer from short operating lives. The geometry and construction material of traditional cutting tools have been major factors in their inability to increase material removal rates and thereby decrease manufacturing costs. New tool materials and/or manufacturing methods must be developed for the machining of CMCs to reduce manufacturing costs and make them a more viable material option for spacecraft and aircraft components.
A variety of materials and coatings, including polycrystalline diamond (PCD), have been applied to the machining of CMCs. It has been demonstrated that a small increase in material removal rate is possible, though not enough to drastically reduce manufacturing costs. Different machining coolants have also been used in place of traditional glycol-based coolants to dissipate the large amount of heat generated during the machining of these materials. Coolants like alcohol have reduced the heat at the tool/substrate interface and produced a high-precision surface on the final part, but this does not solve the problem of high manufacturing costs.
Grinding techniques have also been employed in the fabrication of CMC components, and they typically produce good surface quality. However, low material removal efficiencies and high grinding wheel wear result in high machining and recurring tooling costs. Also, standard grinding techniques are not applicable to surfaces with complex geometries.
Laser-Assisted MachiningUntil recently, no practical method to economically machine ceramics and CMCs has existed with the high precision required for many components. Laser-assisted machining (LAM) was developed in an effort to reduce the high manufacturing costs typically associated with ceramics while maintaining the base structure of the material. In laser-assisted machining, a laser is used as an intense heat source to change the deformation behavior of the ceramic substrate from brittle to ductile just prior to material removal with a cutting tool, without melting or sublimation.
To maintain the integrity of the material, the power output of the laser must be carefully controlled so as to heat only the substrate to the depth of cut of the machining process. Cutting tools, such as PCD-coated tools, are typically used in conjunction with the LAM process. This helps to extend the tool life far beyond that of standard machining practices in ceramics.
When compared to traditional machining techniques for ceramics, LAM has been shown to achieve material removal rates nearly 20 times that of ultrasonic machining and over 100 times that of grinding. Typically, ceramics are machined using a series of expensive grinding machines, which can be replaced by a single (and potentially lower-cost) LAM system.
Developing a New SystemAurora Flight Sciences (AFS) and Kansas State University (KSU) have teamed up to develop multi-axis LAM capability for SiC CMC materials.* The program is sponsored through the West Virginia High Technology Consortium (WVHTC) Foundation’s Innovative Research initiative, which seeks to stimulate technological innovation in the private sector and strengthen the role of small businesses and universities in conducting research that supports NASA’s latest critical technology needs.
The technology to be developed during this program will enable the high-speed precision machining of CMCs at a significantly reduced cost, making production of current and new CMC structures more cost-effective and viable. The overall goals of this program are to demonstrate the technical feasibility of using LAM for the fabrication of CMC components, perform limited micro-structure analysis and physical property testing on LAM-processed articles, and demonstrate the cost/time savings that can be achieved with the developed technology. The CMC material used during this program was SiC fiber in a SiC matrix.
In order to determine the LAM operational parameters for the CMC material, predictive numerical models were developed using a finite element analysis (FEA) model approach. The heat transfer and thermal stress distribution in the material was modeled and verified using actual experimental data collected during the fabrication of a test article. The FEA models were used to predict the laser power, beam diameter and preheating time, as well as the tool path temperature.
The team also developed a distinct element method (DEM) model to simulate the material removal process using LAM techniques. The development of the models assists with the future development of LAM technology by providing a means to rapidly predict operational parameters in a production environment.
A 3 x 5 in. key characteristic test article for fabrication using LAM techniques was developed to simulate features typically found in space propulsion and turbine engine components. Key features included leading and trailing edge geometries, 0.093-0.248 in. diameter holes and a 3-axis slot (see Figure 1). The test article design represents features currently found in propulsion structures and features that are not possible to install using current techniques, such as 3-axis slot milling and counterbores.
The holes, with their associated counterbores, represent the range of hole sizes of interest to the program. The article also features a full radius edge, as well as a radius and chamfer combination edge, to represent the leading and trailing edges and throat geometries found in propulsion structures. The modeled article relies on 0.090-in. material that is face-milled to a final thickness of 0.085 in. by removing 0.0025 in. per side.
*While retaining ownership of all key intellectual property, Aurora is open to collaborations and licensing options for its new technology.
Tooling SelectionCutting tool materials were selected to support operational sequences and to function properly with laser heating requirements. The operational sequences for fabrication of the article were determined in an effort to minimize the equipment and setup time/costs associated with fabrication. In any manufacturing operation, non-recurring costs like setup, equipment and tooling are a major factor in selecting the fabrication method, and often result in costs higher than the performance of the machining operation. Working to reduce non-recurring costs was a key aspect during all steps of the program to reduce fabrication costs associated with laser-assisted machining technology.
The selected cutting tools included a custom-designed polycrystalline cubic boron nitride (PCBN) insert cutter for face milling and diamond-coated fluted cutters. The use of an insert cutter instead of a traditional fluted cutter reduces non-recurring tooling costs associated with manufacture by enabling the replacement of just the insert and not the entire tool. With coated fluted cutting tools, once the cutting surfaces wear and the cutting edge is diminished, regrinding the surface is impossible without removing the coating.
The selection of a diamond application method for the fluted tools was a critical aspect to the final tool selection. There are many commercially available PCD tool coatings that utilize various application methods. The most common method of application is to resin bond or “glue” the diamond particles to the outside of the cutting tool. Another method is to grow the crystals on the tool surface. Both of these methods produce acceptable coatings for standard applications, but can only withstand temperatures of up to approximately 600°F.
Due to the high tool path temperature on the CMC surface as a result of laser heating, both the resin bond and PCD growth application methods would fail during use. A novel metal bond coating method was developed that is capable of withstanding temperatures in excess of 2400°F.
Because of the thermal conductivity of the material, the area to be machined must be relatively small in order to facilitate laser heating of the CMC substrate during hole installation. When using helix drills, the tool must remove the entire area of the hole, which in a LAM application would require that the entire area to be removed be heated. This technique would be applicable to small hole sizes (<0.015 in.) but is not possible with larger hole sizes.
An alternate method is to step up the drill sizes to the final desired hole size, but this would significantly increase manufacturing costs. Therefore, a hole installation technique was developed that only requires a small area be heated in order to produce the final hole size in a single operation, thereby reducing operational run time.
Testing and ResultsTo determine an initial assessment of the developed LAM operational parameters and machining methodology, trials in homogenous silicon nitride (Si3N4) were completed using a 3-axis LAM system currently in place at Kansas State University (see Figure 2). Figure 3 illustrates the face milling, groove and chamfer installations that have been completed on the Si3N4 ceramic.
The results of this trial have indicated the need for the development of an improved heating method for the fabrication of large components. The program team is currently revising the heating strategy for LAM prior to the fabrication of the test article, and this should be completed in early 2008.
For more information, contact: Aurora Flight Sciences, 9950 Wakeman Dr., Manassas, VA 20110; (703) 396-6304; e-mail firstname.lastname@example.org; or visit www.aurora.aero.com. Kansas State University, IMSE Department, 212 Durland Hall, Manhattan, KS 66506; (785) 532-3731; e-mail email@example.com; or visit www.ksu.edu.