Ceramics-Beyond Your Coffee Cup
Humans have been living with ceramics for 25,000 years. We’ve been using them for cups, pipes, pottery and many other handy everyday objects. Ceramics are known for their incredible strength and superior heat resistance.
Humans have been living with ceramics for 25,000 years. We’ve been using them for cups, pipes, pottery and many other handy everyday objects. Ceramics are known for their incredible strength and superior heat resistance. But the light, strong, and heat resistant material has one fatal flaw, it is brittle. Ceramics they fracture easily under mechan- ical or thermo-mechanical loads because of cracks initiated by small defects or scratches. To overcome this deficiency, Ceramic Matrix Composites (CMCs) have been developed.
Ceramic Matrix Composite (CMC) materials are made of coated ceramic fibers surrounded by a ceramic matrix. They are tough, lightweight and capable of withstanding temperatures 300–400 degrees F hotter than metal alloys can endure. A ceramic matrix composite is diGerent than almost all other composites because the matrix is ceramic and the fiber is ceramic. Typically, combining two brittle materials yields a brittle material. But altering the bond between fiber and matrix enhances the toughness of the composite. Cracks don’t propagate into the fibers from the matrix around them. The fibers hold the material together and carry the load while slowly pulling from the matrix.
Initially, to increase the crack resistance or fracture toughness, monocrystalline whiskers or platelets were embedded into the matrix. However, the improvement was limited, and the products have found application only in some ceramic cutting tools.
The use of long multi-strand fibers has drastically increased the crack resistance, elongation and thermal shock resistance. The reinforce- ments used in CMC to enhance the fracture toughness of the combined material system while still taking advantage of the inherent high strength and Young’s modulus of the ceramic matrix. The most common reinforcement embodiment is a continuous-length ceramic fiber, with an elastic modulus that is typically somewhat higher than the matrix. The functional role of this fiber is to increase the CMC stress for progress of micro-cracks through the matrix, thereby increas- ing the energy expended during crack propagation; and then when thru-thickness cracks begin to form across the CMC at a higher stress, to bridge these cracks without fracturing, thereby providing the CMC with a high ultimate tensile strength. In this way, ceramic fiber reinforcements not only increase the composite structure’s initial resistance to crack propagation, but also allow the CMC to avoid abrupt brittle failure that is characteristic of monolithic ceramics. This behavior is distinct from the behavior of ceramic fibers in Polymer Matrix Composites (PMC) and Metal Matrix Composites (MMCs), where the fibers typically fracture prior to the matrix due to the higher failure strain capabilities of these matrices.
Ceramic fibers are made by super-heating chemical like silica until they are molten, and then spinning into a fiber form. 3M developed this technology and has developed a family of fibers known as Nextel.
Nextel is an oxide ceramic fiber, mainly based on alumina Al2O3 and Silica SiO2. This chemistry is favorable because of high stability of alumina at high temperatures. 3M produces Nextel in several grades: 312, 440, 550, 610, and 720. These fibers can be readily converted into textile structures of various fabrics, braids and tapes.
Nextel ceramic fibers 312 and 440 are aluminoborosilicate based and primarily used in heat and flame shielding applications in the aircraft and aerospace industries. Flexible and durable Nextel ceramic textiles remain flexible even after being exposed to high temperatures and harsh conditions over long periods of time, meaning fewer repairs and replacements. Primary applications are heat shields, curtains, linings, insulation, blankets and seals.
Initially used to protect NASA Space Shuttles against the heat of reentry, today Nextel 312/440 textiles and fibers are used for both high temperature and load-bearing applications in outer space. For example, blankets sewn from Nextel textiles protect the Delta II rocket engine from the plume of the solid boosters, and whipple shields made with Nextel fabrics defend the International Space Station and satellites against impact by micrometeorites and space debris.
With Nextel’s success in space flight, it only seems natural that the fabric be used in the aviation industry. The high temperature insula- tion performance makes it perfect for lining and protection surfaces through airplane compartments. It oGers lightweight, flexible and durable alternatives to metal shields for protecting aircraft engine struts and composite fan cowls.
Nextel ceramic fibers 610 (alumina) and 720 (aluminosilica) are used in ceramic, polymer and metal matrix composites. Nextel ceramic fiber 610 is noted for its outstanding single filament tensile properties.
Nextel ceramic fiber 720 finds applications in ceramic matrix compos- ites because of its high creep resistance. Applications using these fibers have been successfully demonstrated in both military and commercial jet engine applications.
CMCs have now allowed engineers to build jet engines that could take planes farther and burn less fuel. That’s because the material has two hugely winning attributes for aviation: it’s one-third the weight of metal and it doesn’t need to be air-cooled, which allows designers to build lighter and more efficient engines. There are many components were CMCs could replace metal alloys in turbine engines of aircraft and power plants. The result is the engines could operate more efficiently at higher temperatures, combusting fuel more completely and emitting fewer pollutants.
GE Aviation began developing Ox-Ox in the late 1980s as part of its CMC research eGorts. Ox-Ox was introduced on F414 exhaust seals in 2011 to improve durability. The Passport engine will be the first non-military engine to use Oxide-Oxide (Ox-Ox) CMCs. For the Passport engine, the Ox-Ox CMC material will be used on three parts: exhaust mixer, center body and core cowls. The lightweight material is resistant to high temperatures found in the exhaust area. These advantages will enhance the engine's durability and lower fuel consumption.
Both Fabric Development Inc (FDI) and Textile Products Inc (TPI) reinforce the ceramics market. FDI is currently supporting the aircraft insulation activities. TPI is a leader is weaving fabrics for CMCs and is sole source for the Passport engine program.