Thermal insulation layer
Thermal barrier coatings ( English thermal barrier coating, TBC ) are in the industry used to the material temperature to decrease in high temperature applications. One example are new and retrofitted gas turbines , in which the coating is applied in the range of a few tenths of a millimeter to millimeters on the blades and on other components in the so-called hot air path of turbines such as heat shields and combustion chamber segments.
For example, high temperatures are necessary when operating turbines in order to increase the efficiency or the degree of efficiency of the turbine. This contrasts with the strength of the material at high temperatures. For example, turbine blade materials can be used at exhaust gas temperatures above 1400 ° C, because the temperature of the material can be reduced to below 1100 ° C through a thermal insulation layer, at which the material still has a sufficiently high strength.
properties
The composition of the coating has both positive and negative properties. On the one hand, together with the “cooling air system” of the turbines, it is a very effective protection against the extreme temperatures reached; on the other hand, it demands absolute form accuracy from the manufacturing process and a high degree of sensitivity when handling the components during assembly / disassembly, because the Coating has a ceramic structure that is porous and brittle . Depending on the processing method, this coating is ivory or gray.
processing
TBC occurs primarily in industrial gas turbine systems whose turbine inlet temperature (also referred to as T 4 in the literature ) is higher than the melting point of the blading material used. TBC are coating ceramics, which are often mixed with aluminum and zirconium oxides.
The coating processes and their accuracy are of great importance. Thermal insulation layers are particularly necessary for turbine blades. The vanes and rotor blades of a turbine, which are very smooth nowadays (grain-bound single crystals made of nickel superalloys) are roughened with TBC before coating. The roughening takes place by means of a further layer, the so-called bond coat , the roughness of which is comparable to a grinding sheet (20 grain) and contains significant alloying elements such as nickel, cobalt, aluminum, molybdenum or yttrium. The thickness of the bond coat is approximately 0.1 to 0.2 mm.
The TBC is now applied to the adhesive layer. Today, this is done using three common methods: Vacuum Plasma Spraying (VPS), Atmospheric Plasma Spraying (APS) or Electron Beam Physical Vapor Deposition (EB-PVD). In all variants, the TBC material is liquefied at high temperatures and sprayed onto the surface. The thickness of the layer is 0.3 to 0.5 mm after it has often been sprayed on several times and, thanks to its ceramic elements, has very low thermal conductivities of λ TBC = 0.7 ... 0.8 W / (m · K).
Current research is working towards so-called Low Conductivity TBCs, whose thermal conductivity should be below 0.4 W / (m · K).