08-05-2012, 12:10 PM
thermal barrier coating
TBC-Final.doc (Size: 2.77 MB / Downloads: 81)
. INTRODUCTION
The world-wide energy saving and reduction of global warming effect gas (CO2) are required very strongly. In order to comply with the requirements, the effort to develop the high efficiency gas turbine has been made and therefore, the improvements of gas turbine blade material, cooling technology and thermal barrier coating (TBC) on the blades have been carried out continuously.
The thermal efficiency of gas turbine engines is largely determined by the turbine entry temperature (TET): a higher TET results in higher efficiency. Consequently, the development of ever more efficient gas turbines has been paced by developments in high temperature – high strength materials, protective and heat resistant coatings, and component cooling, as shown in figure 1.1.
Alloy development
Alloys were developed with improved strength capability at higher temperatures. The sequence of material systems followed a path from heat resisting steels and wrought Ni-Cr alloys in the nineteen-fifties to conventionally cast Ni-based super alloys in the nineteen-sixties and -seventies to the directionally solidified and single crystal alloys in the nineteen-eighties and -nineties. However, the strength improvements were realized at the expense of less resistance to oxidation and hot corrosion. Therefore protective metallic surface coatings were developed. The earliest coatings were diffusion coatings to increase the aluminium content of the component surface. More recently, overlay MCrAlY coatings (where M = Ni, Co or NiCo) were developed.
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. In general, the bond coating materials are MCrAlYs (M:Ni and/or Co) and top coating materials are partially stabilized zirconia (PSZ). The thermal conductivity of PSZ coating is about 2 W/mK and lower than that of supper alloys.
By attaching an adherent layer of a low thermal conductivity material to the surface of a internally cooled gas turbine blade, a temperature drop can be induced across the thickness of the layer, Fig. 1.2. 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. It also retards the onset of thermally induced failure mechanisms (i.e. thermal fatigue) that contribute to component durability and life. It is important to note that coatings of this type are currently used only for component life extension at current operating temperatures. They are not used to increase the operating temperature of the engine. However, the development of a “prime reliant” TBC system, for which the probability of failure is sufficiently low, would allow these coatings to be used to increase the engine operating temperature and lead to significant improvements in engine performance.
TBC Techniques
Mainly there are two techniques to apply TBC;
-Air Plasma spraying (APS)
-EB-PVD process
PLASMA SPRAYING OF TBC’s
Coating structure and quality are determined mainly by the temperature, velocity and size (distribution) of the incident particles. The particles should be molten before impact and have velocities sufficient to enable spreading out and flowing into the irregularities of the previously deposited material.
The particle temperature depends on the plasma temperature, the heat transfer of the plasma to the particles, the heat conduction in the particles and the dwell time of the particles in the plasma. The particle velocity depends on the plasma velocity and viscosity, the size, geometry and density of the particles, and the dwell time of the particles in the plasma. The particle size (distribution) is determined by the powder stock material. The particle size affects the microstructure and also the particles’ velocities and temperatures. Moreover, a wider variation in the particle size distribution causes larger variations in particle velocity and temperature.