Thermal Barrier Coatings

Thermal Barrier Coatings

Definition

Thermal barrier coatings (TBC) are layer systems deposited on thermally highly loaded metallic components, as for instance in gas turbines. The accompanying figure shows a stator blade of a stationary gas turbine, furnished with a plasma sprayed thermal barrier coating of YSZ (Siemens Power Generation). The cooling of the components causes a pronounced reduction of the metal temperature, which leads to a prolongation of the mechanical component's lifetime. Alternatively, the use of thermal barrier coatings allows to raise the process temperature, obtaining thus an increased efficiency.

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 850°C, and two predominant corrosion mechanisms have been identified:

Accelerated high temperature oxidation (>950°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 - 950°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.

Thermal barrier coatings are highly advanced material systems applied to metallic surfaces, such as gas turbine or aero-engine parts, operating at elevated temperatures. These coatings serve to insulate metallic 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 fact, in conjunction with active film cooling, TBCs permit working fluid temperatures higher than the melting point of the metal airfoil in some turbine applications.

Background

Thermal Barrier Caotings are typically ceramic composites based on zirconia, alumina, and titanium.. The high hardnes, wear resisrance and good chemical stability of TBCs make them very desirable in cutting tool applications. TBCs provide good resistance against the corrosive, high temperature environment of aircraft engines as well. Wear resistance can increase between 200 to 500% with the addition of a TBC to a tool. Chemical vapor deposition (CVD), plasma vapor deposition (PVD), and electron beam physical vapor deposition (EBPVD) are the primary deposition methods for these coatings.


The adhesion quality of the TBC to the substrate is considered to be one of the limiting factors for use of these materials. Previous studies using pull-off methods to determine the adhesion do not sufficiently describe the mechanism of failure. In addition, a large difference in the thermal expansion coefficient between the interface and the substrate is a potential cause for spalling of the coating. This research will attempt to characterize the mechanical properties of the interface, in particular the interface fracture resistance and progressive debonding.

Cutting Tool Applications

The addition of a TBC is credited for increasing cutting speeds of tools and for providing deeper cuts. In particular, TBCs provide excellent wear resistance which is necessary in the harsh tool environment. Wear mechanisms of cutting tools, include crater, attrition, flank, and abrasive wear. It has been shown that TBCs can limit the crater and attrition wear processes.

Multi-layer coatings increase performance as the combination exhibits the best qualities of each coating. These coatings are also known to produce finer grain sizes and minimize chopping. The research will demonstrate the reliability of the combinations already in service.