The jet engine is the most complex element of an aircraft and one of the most complex human-made products ever developed, housing thousands of individual components — and ultimately determining fuel efficiency for aircraft.
Traditionally, certain materials have been used specifically for different parts of the jet engine as seen in the image below.
However, considering that the melting point of current superalloys is around 1,850°C, the challenge becomes finding materials that will withstand hotter temperatures; and as such, the search for new materials has come to the fore. The advent of lean-burn engines, with temperature potentials as high as 2,100°C, has helped drive demand for these new materials. To achieve higher thrust, higher operating temperatures must be realized and for higher efficiency, engines must be made significantly lighter without loss of thrust. In either case, new families of materials need to be developed that have higher melting points and greater intrinsic strength.
A saga of different categories of materials for high-temperature applications:
For high-temperature applications, exclusively four categories of materials can be considered, which are shown in the figure below:
Composite materials embody the emerging moiety of the high temperature aerospace material sector. They bestow certain advantages over their counterparts including reduced weight, increased fuel efficiency in conjunction with the ease of handling, shaping and repairing aspects- these factors play a significant role in designing the commercial aircraft. Advanced composites made from high-performance metal matrix composites (MMCs), ceramic matrix composites (CMCs), and fiber-reinforced polymers (FRPs) are used in high-performance aerospace systems to provide additional functional benefits such as temperature resistance, radar absorption, and flutter suppression.
Metal and ceramic matrix composites
Metal and ceramic matrix composites are also excellent materials for aerospace structural parts. Erosion and deformation concerns presented by refractory metals and/or other materials used in high temperature applications can annihilate the efficient functioning of the aircraft structures in consideration. The ceramic matrix composites (CMCs) are intended to obliterate these concerns owing to their appropriate functional properties that mitigate poor structural performance, thermal shock, thermal conductivity, and oxidation.
CMCs can work at a much higher temperature (difference ~500°F) than nickel superalloys with the added advantage of lowering of weight (their weight is 33% of nickel superalloys that were utilized); this accounts for the enhanced progressive use of CMCs in military and commercial jet engines.
Good impact resistance and stability at high operating temperatures make the silicon carbide (SiC)/SiC ceramic matrix composite system a desirable option for jet engines. Niobium-silicide-based composites show good oxidation resistance, reasonable fracture toughness, good resistance to pesting (intermediate-temperature pulverization), good high-temperature strength, and good impact resistance, good fatigue resistance, and they can be cast reasonably well.
The advantageous propositions offered by CMC parts in the jet engine market segment can be considered as the driving force for the improvement of CMC parts in variations of the best selling aircraft programs, for example, the B737 Max and A320neo. With the incorporation of five CMC applications in GE9X engine for upcoming variation B777x, this interest can be expected to thrive more.
Carbon carbon composites
Carbon carbon composites are prodigiously terrific materials for aerospace structures and have been used hitherto in the space industry for re-entry heat shields; rocket nozzles; parts of gas turbine engines such as flaps, vanes, seals, liners, and tail cones. The extraordinary characteristics that make them suitable for aerospace applications include their ability to retain mechanical and physical properties at very high temperatures along with a high degree of toughness and inertness, apart from being lightweight.
Fiber-reinforced composites have been tremendously utilized in military and civil aircrafts, spacecrafts, and satellites. Progress has been made by developing different types of advanced fibrous architectures (two-dimensional and three-dimensional woven, knitted, and braided) which are used as preforms of composite materials.
Sandwich composites made of advanced 3D sandwich fibrous architectures with integrated face and core solve the delamination problems. These can be preferably utilized in the future for aerospace parts.
High shear and torsional strength and stiffness, high transverse strength and modulus, damage tolerance and fatigue life, notch insensitivity, high fracture toughness, and the possibility to develop complex and near-net-shape composites are some important advantages of braided composites. The rocket nozzle, fan blade containment case, aircraft propellers, and stator vanes are some aerospace structural parts constructed using braided composites.
New kids on the block for jet engines?
Auxetic composites are another category of composite materials that, in addition to having the negative Poisson’s ratio, also have high shear modulus, synclastic curvature, high fracture toughness, enhanced crack growth resistance, high damping resistance, and high energy absorption capability. These properties conform to some of the most important criteria while making a decision for selecting a particular material.
Composites mainly reinforced with nanoclay, carbon nanotubes (CNTs) and metal nanoparticles are increasingly used in the aerospace sector due to their usage resulting in better interface and reinforcement efficiency, while imparting new features like electrical and thermal conductivity, electromagnetic shielding, self-sensing ability, gas barrier properties, and fire retardancy.
Hybrid multiscale composites
Hybrid multiscale composites are yet another important category of nanocomposites to be considered for advanced applications in aerospace. These category of nanocomposites incorporate nanomaterials like nanoclay, carbon fibres, carbon nanotubes and metal nanoparticles into conventional composites.
Continuous monitoring of strain is highly important to improve safety of aerospace structures, thereby reducing maintenance by detection of damages at early stages. For this purpose, self-sensing composites have a significant role to play. Examples include mostly composites such as continuous carbon fiber/polymer matrix composites, short carbon fiber composites, hybrid carbon fiber/carbon particle composites, carbon fiber/glass fiber composites, and glass fiber/carbon nanotube composites.
Resistance to crack propagation is one of the major advantages offered by composite materials as compared to their metal counterparts. But, there are some composites which can perform automatic repairing of damage using healing agents. Self-healing composites use healing agents within microcapsules, hollow fibers, or vascular networks in the structure of composites. The initiation or growth of cracks will break the containers of the healing agents. Consequently, these healing agents will be released and cracks get repaired without any external intervention. In fact, self-sensing and self-healing composites can be utilized simultaneously to detect the damage and heal the damage in the advanced composite structures.
Intermetallics have garnered considerable interest for jet engine applications, especially ƴ-TiAl, NiAl, and the platinum-group metal (PGM) compounds. TiAl is not preferably used in commercial jet engines because of its low room temperature ductility (1%-2%), low fracture toughness, high stress sensitivity of fatigue life, apart from having a modest melting point of 1,500°C. On the other hand, alloy parts based on this NiAl intermetallic material have been successfully tested. These tests took into consideration its favorable properties for jet engine applications, such as a high melting point (1,650°C), good thermal conductivity, low density, and intrinsic oxidation resistance.
PGM-based intermetallic alloys fall into two classes:
- Isomorphous with Ni3Al (e.g., Pt3Al)
- Isomorphous with NiAl (e.g., RuAl).
In both cases, the advantages of PGM-based intermetallics over Ni-based superalloys are a significantly higher melting point (1,500ᵒC for Pt3Al and 2,100ᵒC for RuAl) and inherent oxidation resistance, albeit with some increase in density. The Inconel (nickel-chromium-iron) alloys are frequently used in turbine engines because of their ability to maintain their strength and corrosion resistance under extremely high-temperature conditions. GE used IN-738 as a first-stage blade material from 1971 until 1984, when it was replaced by GTD-111. It is now used as a second-stage material. It was specifically designed for land-based turbines rather than aircraft gas turbines. Nimonic 105 was used on the Rolls-Royce Spey.
Ceramics are well known for being stable at much higher temperatures; but because they are inherently brittle, they possess an enormous impediment in their use in withstanding the rigors of assembly, unless meticulous research for innovative materials and system architecture brings forth novel solutions. Ceramics and ceramic-metal mixes are used in combustion chambers because of their high heat resistance. They have very high melting points and don’t require cooling systems, so they make for lighter, less complicated engine parts. The downside is that they tend to fracture under stress, so engineers seek to create new ceramic composites that incorporate other materials to improve properties.
Refractory metals are generally not considered good prospects for aerospace applications due to the fact that none of them satisfactorily meets the criterion of being oxidation resistant, and almost all of them, with the exception of chromium, are significantly denser than the existing Ni-based alloys. Combustion chambers are generally made up of superalloys with refractory metals such as tungsten, molybdenum, niobium, and tantalum.
Takeaways from recent developments in industry and academia:
The increasing production rate of composite-rich fighter aircraft such as the F-35 is likely to give an impetus to the demand for high-temperature composites in the industry. In addition to using composites for extremely high-tech carbon fiber fan blades, jet engine manufacturers are also incorporating CMC parts that can withstand extremely high temperatures in the hot sections. The use of CMC parts enables weight reduction for engines and allows them to run at much higher temperatures, improving performance and efficiency. These ceramic matrix composites have taken the market in an aggressive manner. For example, while 50% of the Boeing 787 Dreamliner’s weight is made up of composites, Airbus, with its new A350 XWB, surpasses this with 53% composite content.
To achieve an excellent strength-to-weight ratio while pushing the service temperatures into the range from 600°F to 1,000°F in newer jet engines and fifth-generation fighter aircrafts has created an upsurging demand for composite materials. Lockheed Martin’s F-35, which has incorporated high-temperature composites, is among the best-selling fifth-generation aircraft. About 35% of the aircraft is made of composites, approximately 50% of which are high-temperature composites.
A few of the pivotal companies operating in the global CMC segment are Applied Thin Films Inc., 3M Company, General Electric Company, Ube Industries Ltd., and CoorsTek Inc. Here’s an overview of the most notable developments in the space:
- Okayama-based manufacturer Nakashima Propeller has developed a special carbon fiber reinforced polymer (CFRP) that allows the propellers to be 40%-50% lighter than conventional products, apart from boosting the fuel efficiency of a vessel by 5%-6%.
- Toray Industries has fabricated its own high-performance CFRP for fan blades of aircraft engines.
- Rolls-Royce, which is one of the biggest aircraft engine manufacturers, has been working on SiC CMCs that can endure high temperatures of 1,900ᵒC. These are also two-thirds lighter than traditional nickel-based alloys. Commercially, SiC fibers are developed by a technology owned solely by Nippon Carbon and Ube Industries.
- General Electric — the largest jet engine maker — has collaborated with Nippon Carbon to manufacture its CMC material for jet engines, which is projected to improve fuel efficiency by 2%. GE Aviation’s two-plant manufacturing site in Asheville, North Carolina, is among the first operations to mass produce CMC components for commercial jet engines. According to GE, the GE9X engine will be the most fuel-efficient jet engine the company has ever produced on a per-pounds-of-thrust basis. The GE9X was developed on the foundations of the GE90 which was developed back in the early 1990s, and powers the Boeing 777. The GE9X is designed to achieve an overall pressure ratio of 60:1 and bypass ratio of approximately 10:1, and has CMC material in the combustor and turbine.
- Professor Jim Williams of Ohio State University focused on introducing several new materials and processes into the jet engine business at Boeing and Rockwell as well as GE. Very recently, Professor Kyosuke Yoshimi of Tohoku University’s Graduate School of Engineering and his colleagues have identified a metal that may surpass even nickel superalloys for aerospace applications.
- Titanium carbide–reinforced, molybdenum-silicon-boron–based alloy is a promising new material whose high-temperature strength was identified under constant forces in the temperature range of 1,400°C to 1,600°C that may be suited for applications including aircraft jet engines and gas turbines for electric power generation.
Opportunities galore – Can the constraints provide a solution?
Design of aerospace structures using advanced composites is a cross-functional activity with a strong interplay between materials, mechanics, and the manufacturing process. It is important to realize that the use of composites requires an integrated approach between the development of new materials and innovation in design and manufacturing to ensure functionality. Considerations of ductility and toughness cannot be undervalued. The CMCs market in aircraft engines is projected to witness an impressive double-digit growth rate over the next five years, driven by the development of CMC applications in the best-selling aircrafts or their variants, owing to intrinsic advantages such as temperature resistance up to 260 °C higher than nickel alloys at just one-third the weight. There should be continuous replacement of nickel alloys with CMCs in both low-pressure and high-pressure engine zones during the forecast period. Furthermore, increasing aircraft deliveries and demand for fuel-efficient aircraft will further propel the demand for CMC in aircraft engines. Hence, for the time being, ceramic matrix composites are making a strong contribution to what some consider a disruptive advancement in aircraft engines — a game-changing transition to an integrated propulsion system that incorporates advanced materials and technology. It remains to be seen how multidisciplinary optimization can provide for efficient design and fabrication of such high-temperature materials for jet engines.
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