Thermal barrier coating

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. [F.Yu and T.D.Bennett (2005). J.Appl. Phys.97,013520 (2005)] . 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 flame temperatures higher than the melting point of the metal airfoil in some turbine applications.

Anatomy

Thermal barrier coatings consist of four layers. They are the metal substrate, metallic bond coat, thermally grown oxide, and ceramic topcoat. The metal substrate and metallic bond coat are metal layers and the thermally grown oxide and topcoat are ceramic layers. The metal substrate is typically a high temperature nickel or cobalt alloy that is either in single crystal or polycrystalline form. Typically 5-12 other elements are added to the alloy. [Padture, N.P., M. Gell, and E.H. Jordan, Thermal Barrier Coatings for Gas-Turbine Engine Applications. Science, 2002. 296: p. 280-284.] The metallic bond coat is an alloy typically with the composition of NiCoCrAlY. The bond coat creates a bond between the ceramic coat and substrate. This layer is responsible for forming the thermally grown oxide, which is the third layer, when the TBC is subjected to high temperatures. The thermally grown oxide consists of alumina, and this layer protects the substrate from thermal oxidation and corrosion by serving as an oxygen diffusion barrier. Finally, the last coat is the ceramic topcoat. It is composed of yttria-stabilized zirconia (YSZ) which is desirable for having very low conductivity while remaining stable at nominal operating temperatures typically seen in applications. This layer creates the largest thermal gradient of the TBC and keeps the lower layers at a lower temperature than the surface.

Processing

In industry, thermal barrier coatings are produced in a number of ways:
* Electrostatic Spray Assisted Vapour Deposition ESAVD
* Electron Beam Physical Vapor Deposition: EBPVD
* Air Plasma Spray
* Direct Vapor Deposition

Additionally, the development of advanced coatings and processing methods is a field of active research. One such example is the Solution precursor plasma spray process which has been used to create TBCs with some of the lowest reported thermal conductivities while not sacrificing thermal cyclic durability.

References

* [http://www.fz-juelich.de/iwv/iwv1/index.php?index=63 Institute for Materials and Processes in Energy Systems (IWV)] V.kurup, A.prasad, D.parab, R.thosar of Plasma-Sprayed Thermal Barrier Coatings Under Mechanical Stress, J. Thermal Spray. 13 (2004) 390.

Safety in Metal Spraying

Metal spraying need not be a dangerous process, if the equipment is treated with care, and correct spraying practices are followed. As with any industrial process, there are a number of hazards, of which the operator should be aware, and against which specific precautions should be taken.

Ideally, equipment should be operated automatically, in enclosures specially designed to extract fumes, reduce noise levels, and present direct viewing of the spraying head. Such techniques will also produce coatings that are more consistent. There are occasions when the type of components being treated, or their low production levels, require manual equipment operation. Under these conditions, a number of hazards, peculiar to thermal spraying, are experienced, in addition to those commonly encountered in production or processing industries.

Noise

Metal spraying equipment uses compressed gases, which create noise. Sound levels vary with the type of spraying equipment, the material being sprayed, and the operating parameters. Typical sound pressure levels taken 1 meter behind the arc.

Light

Combustion spraying equipment produces an intense flame, which may have a peak temperature more than 3,100°C, and is very bright. Electric arc spraying produces ultra-violet light, which may damage delicate body tissues. Spray booths, and enclosures, should be fitted with ultra-violet absorbent dark glass. Where this is not possible, operators, and others in the vicinity, should wear protective goggles containing BS grade 6 green glass. Opaque screens should be placed around spraying areas. The nozzle of an arc pistol should never be viewed directly, unless it is certain that no power is available to the equipment.

Dust and fumes

The atomization of molten materials, produces a certain amount of dust and fumes. Proper extraction facilities are vital, not only for personal safety, but to minimize entrapment of re-frozen particles in the sprayed coatings. The use of breathing masks, fitted with suitable filters, is strongly recommended, where equipment cannot be isolated. Certain materials offer specific known hazards.

#Finely divided metal particles are potentially pyrophoric and none should be allowed to accumulate.
#Certain materials e.g. aluminum, zinc and other base metals may react with water to evolve hydrogen. This is potentially explosive and special precautions are necessary in fume extraction equipment.
#Fumes of certain materials, notably zinc and copper alloys are unpleasant to smell, and, in certain individuals, may cause a fever-type reaction. This may occur some time after spraying and usually subsides rapidly. If it does not, medical advice must be sought.

Heat

Combustion spraying guns use oxygen and fuel gases. The fuel gases are potentially explosive. In particular, acetylene may only be used under approved conditions. Oxygen, while not explosive, will sustain combustion, and many materials will spontaneously ignite, if excessive oxygen levels are present. Care must be taken to avoid leakage, and to isolate oxygen and fuel gas supplies, when not in use.

Electricity

Electric arc guns operate at low voltages (below 45 DC), but at relatively high currents. They may be safely hand-held. The power supply units are connected to 440 volt AC sources, and must be treated with caution.

Compressed Air

The air supply, to spray guns, is at high pressure. It should not be directed towards people. The motor air supply is lubricated, and should never be fitted to a breathing apparatus. Any breathing equipment, used with the thermal spraying process, must be supplied with air of breathing quality.

Other Links

* [http://www.msm.cam.ac.uk/mmc Gordon Laboratory, University of Cambridge]
* [http://www.aflame.com/ A-Flame Metal Spraying Equipment] located in Cincinnati, OH USA T 513-831-4284 E-Mail aflame@fuse.net

Shiladitya Paul "et al.", "Effects of Impurity Content on the Sintering Chatacteristics of Plasma-Sprayed Zirconia", J. Therm. Spray Technol., 16 (2007) 798

Shiladitya Paul "et al.", "The Effects of Heat Treatment on Pore Architecture and Associated Property Changes in Plasma Sprayed TBC's", ITSC 2007 Proceedings, Beijing, China, May 14-17, 2007, p. 411,416.


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