Purpose of thermal barrier coatings of turbine blades.
At elevated temperatures, the durability of turbine turbine engine blades is largely determined by the ability to protect them with heat-resistant coatings. Air cooling supplied to the internal cavity ensures normal operation at metal temperatures up to 1000 - 1200 ° С, depending on the type of superalloy. With a subsequent increase in gas temperature, the use of blades without coatings becomes impossible, since a further increase in the temperature of the heat flux supplied to them can lead to a deterioration in their cooling and, as a consequence, an increase in the surface temperature above the critical one. This adversely affects the resource of the working blades. Replacing damaged blades in the course of repair is a rather costly and time-consuming procedure. An effective way to protect the internal and flow surfaces of the blades from corrosion damage and high-temperature oxidation is the application of modern heat-protective coatings (HRC) by air-plasma spraying, designed to reduce heat influx to the blade material.
Materials for plasma spraying of thermal barrier coatings.
The process of protection consists in the formation of various oxides, mainly on the basis of aluminum - this is Al2O3. Alloys with this metal are characterized by high protective qualities and relative cheapness. Among the actively used sublayers are aluminides NiAl or Ni2Al3, MeCrAlY.
The main types of coatings and their features:
- metal - in the MeCrAlY alloy, based on the type of material of the part, as well as the operating conditions, nickel, cobalt, or their combinations can be used (iron can also be used in the blades of steel);
- chrome-designed to prevent high-temperature corrosion, but its use only affects the effect on the substrate and has an impact on the formation of a coating layer with it;
- aluminum - mass content does not exceed 10–12%. Oxidation can be prevented by increasing the aluminum content, but this can also cause a decrease in the plasticity of the coating;
- Yttrium can significantly increase the adhesion of the oxide layer. It is also capable of forming rods connecting the oxide layer and the coating. But the main function of using yttrium is the ability to form compounds with sulfur, which do not allow the division of the oxide layer;
- silicon - significantly increases the resistance to natural oxidation, but also leads to a decrease in the melting temperature of the coating of the blades. It was experimentally established that the allowable silicon content should not exceed 2.5% by weight;
- additions of rhenium and talan also contribute to increasing the resistance of the blades to cyclic or isothermal oxidation;
- deposition of platinum underlayer - 5–10 microns thick. Although its cost is high, it often pays off, since after using it, the duration of using blades increases up to 3 times. Moreover, platinum not only forms a diffusion barrier under titanium, but also increases the diffusion of aluminum.
The use of heat-protective coatings allows to achieve:
- reduction of the cooling of the blade (up to 36%), while maintaining heat resistance;
- a significant increase in the heat resistance of the blades while maintaining a constant mode of operation (i.e. this allows the blades to maintain lower temperatures at a constant temperature at the turbine inlet).
Ceramic heat-protective coatings are used, consisting of zirconium dioxide and yttria, which is used to stabilize the qualities of zirconium dioxide and reduce the thermal conductivity of ceramics.
Technology of plasma spraying of turbine blades
JSC "Plakart" produces heat-shielding coatings for elements of the hot turbine path at our production site in Moscow by means of high-speed and plasma spraying.
Layers are created in this way:
- the first layer is formed on the blade or on the binder coating by applying to the high-speed jet dense, pre-sintered particles of the applied heat-resistant material. When forming the coating, the particles of the first layer, or at least most of them, completely or almost completely melt before hitting the substrate or the binder coating, forming a durable non-porous layer;
- the second layer, by spraying onto the first layer of particles of heat-shielding material with a microstructure different from the type of structure of the first powder. Particles upon receipt of the second layer are characterized by a porous, more open structure, compared with the elements of the first layer.
Production Plakart certified for the application of repair coatings on the elements of the hot tract leading foreign and Russian manufacturers of turbines: GE, ODK, Power Machines. The quality system is certified according to ISO 9001.