The Cutting Edge of Aerospace Metallurgy - CMCs (Ceramic Matrix Composites)

By Frank Rovella
Over the recent holiday, while gorging on 10 times the caloric intake of Djibouti, the discussion at the table turned, as it always does in our house, to advanced metallurgy. An unnamed aerospace engineer was giving me an overview of new developments in the use of Ceramic Matrix Composites (CMCs) in jet engines. Unfortunately, the discussion was limited due to the proprietary nature of the materials that his company was working with. If you’ve watched enough X-Files episodes, I’m sure you know that the truth is out there. After a little research I found that all the major jet engine/gas turbine players such as Rolls Royce, GE, Pratt & Whitney, and Siemens, all have something in development, and are understandably pretty tight-lipped about it. When I think of CMCs, I think of ceramic brake disks like the type used in F1 racing and supercars. While these applications are impressive, it’s pretty tame compared to the high pressure, high heat environment of a jet engine. What’s more, if you know something about single crystal superalloys, which is the current standard for high-pressure turbine blades, you’ll know that creating a superior material sounds like alien technology. As with any composite, the goal is to combine the favorable characteristics of dissimilar materials to create something unique. In the case of CMCs, the goal is to create a material that withstands tremendous forces at extremely high temperatures for prolonged periods of time. Although superalloys can and have been filling this need, the Aerospace industry is under ever-increasing pressure to heighten engine efficiency, power, and durability. This means higher heat, higher temperatures, and higher speeds. According to an article in the MIT Technology Review, GE and Pratt & Whitney are the furthest along in the development of a new generation of jet engines that utilizes CMC technology.  These Silicon carbide-based composites can handle temperatures to 1200°C /2200°F,  require less cooling, and are 1/3 the weight of superalloys. This is a Godsend to Aerospace designers trying to meet the demands rising fuel costs. The first iteration of the new GE engine is expected to use 15% less fuel, with reduced emission and no power loss. Pratt & Whitney has a similar product in the pipeline, also boasting 15% fuel reduction, which if history is any indication, will more than likely outperform GE’s entry. The cutting edge nature of CMCs is based in cutting edge manufacturing technology. GE has recently invested 195 million dollars in a new plant in North Carolina, solely to produce CMC turbine blades and rotors. This facility will work in conjunction with GE’s ceramics lab in Delaware, which will supply it with CMC sheets. The sheets are composed of ceramics coated carbon fiber that is bonded with a polymer material, and placed in molds and formed in an autoclave. The formed shapes are then placed in a furnace to burn off the polymer material. This creates a solid lightweight part known as a “hollow shell of fibers”, which is then subject to further thermal processing that permeates the carbon fibers with silicon, and does so without altering the geometry. Since ceramics are often used for cutting metal, cutting something as hard as CMCs is a big part of the new process, one that no one involved is willing to talk about… yet. As one would expect, there’s a lot more to CMCs, and as the technology grows expect to see these super materials in more and more applications.