Jet engines, gas turbines, and other systems rely on advanced materials that resist deterioration and retain strength when exposed to extreme heat. The internal parts and components in such equipment experience temperatures up to or exceeding 2,500°F during operation. Without specially engineered materials that resist high temperatures, the performance and reliability of critical systems would suffer.
What Properties Are Needed?
For a material to adequately withstand high service temperatures, it requires an exceptionally high melting point, as well as excellent resistance to creep and other forms of deformation over time. The material must resist oxidation, corrosion, erosion, and other chemical breakdown processes exacerbated by heat. It should retain most of its room temperature mechanical properties when heated. High temperature durability often requires materials that perform well after repeated heating and cooling cycles without cracking or losing strength. Weight is also a key consideration, as lighter components place less demand on other hardware.
Refractory Metals Offer High Melting Points
Certain metals classified as refractory metals, including tungsten, molybdenum, tantalum, and niobium, have exceptionally high melting points ranging from 4,150°F to 4,730°F. These exceed the melting points of more common engineering metals like steel, titanium, and aluminum alloys. Refractory metal alloys offer enhanced strength beyond their pure forms at the cost of some lost melting point. But as temperatures approach the melting threshold, most metals become significantly weaker according to the power law creep equation. Refractory metals in air environments also suffer accelerated oxidation at higher temperatures. Protective coatings are often necessary for oxidation resistance.
Ceramics Withstand Oxidation but Are Brittle
Many advanced ceramic materials boast hot hardness, refractory properties on a par with metals while resisting oxidation, corrosion, and erosion far better. Ceramics such as silicon carbide, silicon nitride and aluminum oxide maintain strength at temperatures exceeding 2,000°F. Their melting points also similarly exceed 4,000°F. Compositions resist breakdown via oxidation much better than metals. But monolithic ceramics have low tensile strength and fracture toughness properties at room temperature that worsen relative to metals as temperatures increase.
Ceramic Matrix Composites Combine Properties
Ceramic matrix composites (CMCs) offer the temperature resistance of ceramics while incorporating reinforcing fibers or particulates to eliminate brittle failure. The fibers, often silicon carbide, carbon, or oxide ceramics, strengthen and toughen the matrix material. According to the experts at Axiom Materials, CMCs act more gracelessly when overloaded. For example, silicon carbide CMC turbine parts offer 60% the density of nickel alloys, operate over 100°F hotter, and resist oxidation.
Thermal Barrier Coatings Insulate Components
Thermal barrier coatings (TBCs) applied to alloy hardware add insulation against heat flowing into components. A multilayer TBC system typically comprises a metallic alloy bond coat applied to the substrate. A thick ceramic top coating with exceptionally low thermal conductivity insulates the surface. An interfacial thermally grown oxide (TGO) layer composed of aluminum oxide forms between layers. The total coating creates a gradient, lowering the temperature experienced by the alloy component.
Continuing Advancements
While existing technology has yielded capable high temperature materials, ongoing innovations target even lighter weight options with longer part life under demanding service regimes. Materials and coatings with nano-engineered compositions and microstructures offer paths to further push property limits. And additive manufacturing methods uniquely construct some components with advanced heat-resistant alloys. Helping critical systems withstand intense operating environments for longer service remains pivotal toward improved efficiency and reduced maintenance.
Conclusion
The development of high-temperature resistant materials has enabled tremendous advances in systems like jet engines, gas turbines, and other high-performance equipment. Though already impressively capable, researchers continue to enhance material technologies for withstanding intense heat to empower further performance gains in critical equipment.