29-08-2011, 10:44 AM
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ABSTRACT
Thermal barrier coatings (TBC) are highly advanced material systems usually applied to metallic surfaces, such as gas turbines or aero-engine parts, operating at elevated temperatures, as a form of Exhaust Heat Management. These coatings serve to insulate components from large and prolonged heat loads by utilizing thermally insulating materials which can sustain an appreciable temperature difference between the load bearing alloys and the coating surface. In doing so, these coatings can allow for higher operating temperatures while limiting the thermal exposure of structural components, extending part life by reducing oxidation and thermal fatigue. In conjunction with active film cooling, TBCs permit working fluid temperatures higher than the melting point of the metal airfoil in some turbine applications.
INTRODUCTION
Hundreds of different types of coatings are used to protect a variety of structural engineering materials from corrosion, wear, and erosion, and to provide lubrication and thermal insulation. Of all these, thermal barrier coatings (TBCs) have the most complex structure and must operate in the most demanding high-temperature environment of aircraft and industrial gas-turbine engines. Heat engines are based on considering various factors such as durability, performance and efficiency with the objective of minimizing the life cycle cost. Metallic coatings were introduced to sustain the 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 high velocity oxy-fuel (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
Generally, TBC is a two layer’s system which incorporates about 250 μm thickness layer of ceramic top coating applied to the outer surface of the substrate and about 150 μm thickness underlying of metallic bond coating. The metallic bond coating performs two functions:
(1) To provide oxidation resistance and
(2) To adhere the ceramic to the super alloy substrate physically and chemically.
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.
Fig. 1: Typical microstructure of TBC
By attaching an adherent layer of a low thermal conductivity material to the surface of an internally cooled gas turbine blade, a temperature drop can be induced across the thickness of the layer, Fig2. This results in a reduction in the metal temperature of the component to which it is applied. Using this approach temperature drop of up to 170oC at the metal surface have been estimated for 150μm thick yttria stabilized zirconia coatings. This temperature drop reduces the (thermally activated) oxidation rate of the bond coat applied to metal components, and so delays failure by oxidation. Modern TBC’s are required to not only limit heat transfer through the coating but to also protect engine components from oxidation and hot corrosion. No single coating composition appears able to satisfy these multifunctional requirements. As a result, a “coating system” has evolved.
ABSTRACT
Thermal barrier coatings (TBC) are highly advanced material systems usually applied to metallic surfaces, such as gas turbines or aero-engine parts, operating at elevated temperatures, as a form of Exhaust Heat Management. These coatings serve to insulate components from large and prolonged heat loads by utilizing thermally insulating materials which can sustain an appreciable temperature difference between the load bearing alloys and the coating surface. In doing so, these coatings can allow for higher operating temperatures while limiting the thermal exposure of structural components, extending part life by reducing oxidation and thermal fatigue. In conjunction with active film cooling, TBCs permit working fluid temperatures higher than the melting point of the metal airfoil in some turbine applications.
INTRODUCTION
Hundreds of different types of coatings are used to protect a variety of structural engineering materials from corrosion, wear, and erosion, and to provide lubrication and thermal insulation. Of all these, thermal barrier coatings (TBCs) have the most complex structure and must operate in the most demanding high-temperature environment of aircraft and industrial gas-turbine engines. Heat engines are based on considering various factors such as durability, performance and efficiency with the objective of minimizing the life cycle cost. Metallic coatings were introduced to sustain the 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 high velocity oxy-fuel (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
Generally, TBC is a two layer’s system which incorporates about 250 μm thickness layer of ceramic top coating applied to the outer surface of the substrate and about 150 μm thickness underlying of metallic bond coating. The metallic bond coating performs two functions:
(1) To provide oxidation resistance and
(2) To adhere the ceramic to the super alloy substrate physically and chemically.
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.
Fig. 1: Typical microstructure of TBC
By attaching an adherent layer of a low thermal conductivity material to the surface of an internally cooled gas turbine blade, a temperature drop can be induced across the thickness of the layer, Fig2. This results in a reduction in the metal temperature of the component to which it is applied. Using this approach temperature drop of up to 170oC at the metal surface have been estimated for 150μm thick yttria stabilized zirconia coatings. This temperature drop reduces the (thermally activated) oxidation rate of the bond coat applied to metal components, and so delays failure by oxidation. Modern TBC’s are required to not only limit heat transfer through the coating but to also protect engine components from oxidation and hot corrosion. No single coating composition appears able to satisfy these multifunctional requirements. As a result, a “coating system” has evolved.
