Thermal Barrier Coatings
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01-03-2009, 01:35 PM
Thermal Barrier Coatings
Heat engines are based on considering various factors such as durability, performance and efficiency with the objective of minimizing the life cycle cost. For example, the turbine inlet temperature of a gas turbine having advanced air cooling and improved component materials is about 1500oC. Metallic coatings were introduced to sustain these high temperatures. The trend for the most efficient gas turbines is to exploit more recent advances in material and cooling technology by going to engine operating cycles which employ a large fraction of the maximum turbine inlet temperature capability for the entire operating cycle. Thermal Barrier Coatings (TBC) performs the important function of insulating components such as gas turbine and aero engine parts operating at elevated temperatures. Thermal barrier coatings (TBC) are layer systems deposited on thermally highly loaded metallic components, as for instance in gas turbines. TBC's are characterized by their low thermal conductivity, the coating bearing a large temperature gradient when exposed to heat flow. The most commonly used TBC material is Yttrium Stabilized Zirconia (YSZ), which exhibits resistance to thermal shock and thermal fatigue up to 1150oC. YSZ is generally deposited by plasma spraying and electron beam physical vapour deposition (EBPVD) processes. It can also be deposited by HVOF spraying for applications such as blade tip wear prevention, where the wear resistant properties of this material can also be used. The use of the TBC raises the process temperature and thus increases the efficiency.
Structure Of Thermal Barrier Coatings
Thermal Barrier Coating consists of two layers (duplex structure). The first layer, a metallic one, is called bond coat, whose function is to protect the basic material against oxidation and corrosion. The second layer is an oxide ceramic layer, which is glued or attached by a metallic bond coat to the super alloy. The oxide that is commonly used is Zirconia oxide (ZrO2) and Yttrium oxide (Y2O3). The metallic bond coat is an oxidation/hot corrosion resistant layer. The bond coat is empherically represented as MCrAlY alloy where
M - Metals like Ni, Co or Fe.
Y - Reactive metals like Yttrium.
CrAl - base metal.
Coatings are well established as an important underpinning technology for the manufacture of aeroengine and industrial turbines. Higher turbine combustion temperatures are desirable for increased engine efficiency and environmental reasons (reduction in pollutant emissions, particularly NOx), but place severe demands on the physical and chemical properties of the basic materials of fabrication.
In this context, MCrAlY coatings (where M = Co, Ni or Co/Ni) are widely applied to first and second stage turbine blades and nozzle guide vanes, where they may be used as corrosion resistant overlays or as bond-coats for use with thermal barrier coatings. In the first and second stage of a gas turbine, metal temperatures may exceed 850Ã‚Â°C, and two predominant corrosion mechanisms have been identified:
Accelerated high temperature oxidation (>950Ã‚Â°C) where reactions between the coating and oxidants in the gaseous phase produce oxides on the coating surface as well as internal penetration of oxides/sulphides within the coating, depending on the level of gas phase contaminants
Type I hot corrosion (850 - 950Ã‚Â°C) where corrosion occurs through reaction with salts deposited from the vapour phase (from impurities in the fuel). Molten sulphates flux the oxide scales, and non-protective scales, extensive internal suplhidation and a depletion zone of scale-forming elements characterize the microstructure.
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WHAT IS TBC???
• Layer of system deposited on metallic component subjected to high temp.
• Increases the mechanical component working life.
• Increases process efficiency.
Pratt & Whitney first introduced TBC on burner can in JT8D engine in 1963
This TBC consisted of zirconia stabilized by addition of 22%wt MgO
To avoid high temperature phase to low temperature phase transformation
In current system ceramic consists of 7wt% yttria partially stabilized zirconia(7YSZ)
Ceramic top coat
Thermally grown oxide(TGO)
Ceramic top coat
Top most layer
Provides thermal insulation
Low thermal conductivity (2.3 W/m*k) at temp of thousand *C
Made of Y2O3 stabilized with ZrO2 or YSZ
YSZ has high mp(2700*C)
Elastic modulus of 50GPa
75-150 micrometer thick
Made of Ni & Pt
Protect basic material against oxidation corrosion
(on one side)
Provide good adhesion to the thermal insulating layer
(on other side)
Thermally Grown Oxide(TGO):
TGO formed when ceramic top coat reacts with bond coat in very high temperature
Formation growth is slow, uniform & defect free
1-10 micrometer thick
Usually Ni or Co based super alloy
High temperature strength
Ductility and oxidation resistance
Bond coat is deposited by chemical vapor deposition
HVOF technology is also used for bond coat deposition
Ceramic top coat is deposited into two ways
Air plasma spray (APS) &
Electron Beam Physical vapour deposition (PVD)
Air Plasma Spray:
Ceramic powder is fed into a plasma torch and sprayed molten on the surface of metallic substrate
Droplets of molten material form splats on the metallic substrate
In APS top coat the orientation of the cracks and pores are normal to flow of heat which reduces thermal conductivity from 2.3 W/mo k to 0.8- 1.7 W/m0 k
EB PVD METHOD:
High quality castings
A material source of the coating material is vaporized with an electron beam and vapor uniformly condenses on the surface of the turbine blade