CompositesWorld
Issue link: https://cw.epubxp.com/i/546021
CompositesWorld.com 39 NEWS N E W S N S N E W S E N W S W Aeroengine CMCs supplier GKN Aerospace (Redditch, UK), "CFRP products are moving mainly from the front of the engine towards the back and also from the outside, moving inward." With much higher temperature resistance, CMCs are moving in the opposite direction, i.e., from the rear — the engine "hot zone" — towards the front and from the inside towards the outside (see Fig. 1, p. 38). "Tere will always be a mix of materials," says Runnemalm, observing that integration of CFRP, CMCs and metals will be a challenge, including joining techniques, how to deal with the stress distribution, assembly and disassembly requirements and managing the transition between heat zones in the engine. Te main types of CMCs include silicon carbide (SiC) fbers reinforcing a SiC matrix (SiC/SiC), carbon/carbon (C/C), C/SiC and Ox/Ox where the oxide is typically alumina (see Table 1, this page). SiC/SiC components used in oxidation-causing environ- ments must be protected, using environmental barrier coatings (EBCs), and even the SiC fbers must be coated to prevent attack from oxygen molecules difusing through the porous matrix. Because they do not require a carbon coating on the fbers or EBCs, Ox/Ox composites ofer lower cost. However, they lag in thermomechanical properties vs. SiC/SiC. CMCs take fight GE Aviation (Cincinnati, OH, US) expects a tenfold increase in the use of CMCs in its engines over the next decade (see "Learn More"). One reason is that, unlike metals in the hot zone, CMCs don't need to be air-cooled, freeing up fow to boost the engine's propulsion and efciency. GE's frst CMC parts to enter service will be the static frst stage HP compressor shroud in the LEAP engine. Also called HP turbine shrouds, and reportedly some of the hottest parts in a turbofan, 18 of these direct airfow to ensure turbine blade efciency. Tey are manufactured using SiC/SiC at GE's CMC-dedicated Asheville, NC, US, factory, which also will make the inner and outer combus- tion liners and stage 1 and 2 nozzles for the GE9X engine. Tese parts not only reduce cooling air requirements but also improve durability. Finished parts from Asheville will proceed to Advanced Ceramic Coatings (ACC, Hickory, NC), which operates from the US facility of surface treatment specialist Turbocoating (Rubbiano di Solignano, Italy). ACC will apply GE's proprietary EBCs to protect SiC/SiC components from surface recession (erosion) and expects to deliver its frst coated parts later this year. Although CMCs were frst targeted to static applications, the real revolution, say proponents, will come with dropping weight and cooling in rotating parts. In February 2015, GE successfully tested CMC rotating parts in an F-14 military jet engine. Similar to the fan disk reduction enabled by CFRP fan blades, these CMC LP turbine blades allow smaller, lighter metal turbine disks (see Fig. 2, below) and bearings and other parts can be downsized, multiplying weight savings by as much as a factor of three. According to GE Aviation's CMC design section leader Jonathan Blank, replacing nickel alloys with CMCs inside the engine is a huge step and sets the stage for revolutionary jet engine design changes. GE is also exploring use of CMCs in helicopter engines and in gas turbines and compressors for electrical power plants. Property Unit SiC/SiC C/SiC C/C Ox/Ox Fiber content vol.-% 40–60 10–70 40–60 30–50 Porosity vol.-% 10–15 1–20 8–23 10–40 Density g/cm 3 2.3–2.9 1.8–2.8 1.4–1.7 2.1–2.8 Tensile strength MPa 150–360 80–540 14–1100 70–280 Bending strength MPa 280–550 80–700 120–1200 80–630 Strain-to-failure % 0.1–0.7 0.5–1.1 0.1–0.8 0.12–0.4 Young's modulus GPa 70–270 30–150 10–480 50–210 Fracture toughness MPa·m1/2 25–32 25–30 5,7–,3 58–69 Thermal conductivity W/m·K 6–20 10–130 10–70 1–4 Coefcient of thermal expansion ppm/K 2.8–5.2 0–7 0.6–8.4 2–7.5 Maximum service temperature °C 1100–1600 1350–2100 2000–2100 1000–1100 Table 1 Ceramic matrix composite properties Material properties of typical CMCs at ambient temperature, where the range spans minimum and maximum values of each property in diferent directions or for diferent CMC quali- ties (Ox/Ox covers CMC with alumina fbers and alumina or alumino-silicate matrix). Source | Friedrich Raether, Fraunhofer Center for High Temperature Materials and Design HTL (Bayreuth, Germany), www.htl.fraunhofer.de FIG. 2 CMC fan blades A turbine rotor undergoing testing with lightweight CMC blades that enable smaller and lighter metal disks (shiny steel part in the center), reducing overall engine weight and boosting fuel efciency. Yellow blades are covered with an environmental barrier coating. Source | GE Aviation