The chemical environment and mechanical environment in which the turbine works are very harsh, and the huge stress generated by the rotation speed of 6000 to 30,000 revolutions per minute requires the turbine material to have excellent mechanical properties. In the more than 70 years since the appearance of gas turbine engines, advances in material science and the application of cooling measures have significantly increased the temperature resistance of high-temperature components of the engine.
At present, titanium alloys are mostly used in the manufacture of cold-end parts, while a large number of nickel-based alloys are used in the manufacture of hot-end parts, with the addition of rare rhenium, hafnium and other refractory metals, so that it has excellent temperature resistance and stiffness strength. The advancement of materials science makes it possible for future engines to use light-weight heat-resistant materials, including intermetallic compounds and metal matrix composite materials (composites composed of metal or alloy as a matrix and reinforcements such as fibers, whiskers, and particles).
The turbine blades of the current large turbofan engines are made of single crystal growth, which has reliable mechanical properties, excellent fatigue strength and creep resistance (plastic deformation under high temperature and high strain environments). The equiaxed crystal structure has good mechanical properties in all directions; the oriented crystal turbine blade has improved mechanical properties in the longitudinal direction; the single crystal turbine blade has superior mechanical properties in the longitudinal direction and improved heat resistance.
Ceramic materials are characterized by hardness, oxidation resistance and high temperature resistance. In the future, if their impact resistance can achieve technological breakthroughs, it will be very suitable for making high temperature turbine blades.
Turbine blade surface coating technology
In order to improve the high temperature creep strength and high temperature oxidation corrosion resistance required by the blade, one of the methods is to borrow surface coating technology to provide protection. There are two types of coatings commonly used at present: diffusion coatings and cladding coatings. The typical composition of diffusion coating is protective Al2O3, Cr2O or SiO2 (the numbers are all subscripts). The typical composition of the cladding coating is MCrAlY, which can be prepared by physical vapor deposition or plasma spraying.
Data show that the oxidation resistance of the vapor-deposited NiCoCrAlY composite coating is 1.5 times that of the monoaluminide coating, and the vacuum plasma sprayed NiCoCrAlY+ (Si, Hf) coating is 5 times that of it. Hf and Si can improve the adhesion of alumina and delay the thickening of oxide scale.
Ceramic thermal barrier coatings are used as heat insulation layers for turbine air-cooled cooling blades. For example, zirconia coatings can reduce the surface temperature of superalloys by 100 to 200 degrees Celsius. If applied to third-generation single crystal alloys and combined with advanced cooling technology, it can Cooling 350 degrees Celsius, which is expected to greatly increase the temperature in front of the engine’s turbine.
Vapor-deposited ceramic coating (EB-PVD) produced by electron beam evaporation can provide columnar ceramic structure. It is an order of magnitude longer than plasma sprayed coatings in terms of exfoliation life. For example, the PWA142 blade adopts the PWA73 coating of the physical vapor deposition method to improve the corrosion resistance and oxidation resistance of the blade, and will not have a harmful effect on the level of tensile, long-lasting creep and high-cycle fatigue performance.