Fast Alternative to CVD Densification for Carbon-Carbon Composites
So-called carbon-carbon materials are made of fibers and a carbon matrix and can withstand temperatures to the order of 3,500°C. In the seventies, CEA's DAM (Division of Military Applications) was already tackling these materials, with a focus on nuclear missile re-entry bodies for the naval forces. This is when a new and faster manufacturing process was devised for these materials. Although a patent was filed for the process in 1982, fast densification was not developed for many years until Textron, a US company, bought the license in 1988 and undertook to industrialize the process. In 1996, SEP (a division of the Snecma group) learned about the process. The company is currently developing the process jointly with the DAM. Their goal is to engineer an industrial prototype by 2001.
The story of the process started at the CEA research center in Grenoble. In the early seventies, a scientist began to look into carbon deposits, under a research program on fast neutron reactors. Unlike what was being done in standard processes where a decomposed gas was the starting point for generating carbon, the scientist decided to start with a liquid. The main advantage of this process was that it produced a much higher carbon concentration. However, after positive findings his work stopped because the research program was closed down.
The Division of Military Applications began looking into processes to engineer the critical carbon-carbon materials for the manufacture of re-entry bodies, especially nosecones that are subjected to the highest stress levels during the atmosphere re-entry phase. "There were two types of processes, the standard process using a gas that we tested extensively at the Bruyères-le-Châtel center." "The other process worked with pitch," Michel Houdayer explains. The DAM team learned of this process and decided to take up its study. Fairly simple research proved that it was an extremely fast process producing a carbon similar to the one from standard processes (i.e., using a gas). "We observed that densification speed was 50 to 100 times faster than for the standard process," says Michel Houdayer, Head of the Materials Department at the DAM.
From F16 Fighter Planes to Racing Cars
However, the process was not retained for re-entry bodies. The program was immediately stopped and a patent was filed. Industrialists Aérospatiale and SEP expressed no interest in the process, preferring the chemical vapor infiltration process where methane in a 1,100°C kiln cracks, trapping the carbon in its structure. Consequently, the liquid-vapor densification process called Kalamazoo was shelved for several years until Textron, a US company, found the patent. The Americans wanted to see if the process could be used for the faster, cost-saving manufacture of plane brakes, which had been made with the standard process until then.
In September 1989, CEA-trained, Textron technicians engineered a prototype to check the application of the process to 3D composites. Textron continued industrialization of the process and adjusted it to the simultaneous manufacture of several parts to further improve cost-savings. The company has recently gotten approval of its brakes for US F-16 fighter planes and has just built a new plant for this purpose. Textron-made brakes have already been installed on some racing cars and in 1997, one of them came in third at Indianapolis.
Building an Industrial Prototype
Following Textron's overseas advertising of these applications, SEP (a division of theSnecma group) decided to contact the CEA. The company and research organization quickly signed an agreement on the acquisition of a process license and on research development to validate the technical feasibility of fast densification on samples in a lab-size setup. Once positive findings were found on the lab installation, scientists at the Carbon and Composite Laboratory (Department of Materials) at the CEA Ripault Center decided to increase installation scale and, jointly with Snecma, designed a bigger installation. Michel Houdayer puts in a nutshell, "The goal is to gradually engineer an industrial prototype that can prove the effective cost-saving aspect of the process."
It took two years to design the new facility that is almost on an industrial scale. This "gas factory" will serve to test different operational options likely to improve the standard process. The installation first used water to validate the proper working order of its various organs. Recently, the second stage has replaced the water with a liquid hydrocarbon. In 2001, the findings are expected to give the green light for the building of an industrial prototype.
A Process with a Strong Industrial Potential
Fast densification can only process a few parts at the same time. It remains to be seen whether the process is economically competitive with the currently used standard process that, albeit long, can process many parts at once. The stakes are high for Snecma. The company is looking beyond current applications (in civil and military aeronautics and for racing cars) toward target markets such as heavy vehicles, the railways and top-of-the-line cars. However, process potential does not stop there. As part of a contract with the US Air Force, the CEA has proven that the process is also viable for engineering other ceramic composites.
Contact:
Mr Eric Charrier
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