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|Thick Ceramic Coatings|
Ceramics have unique thermal, mechanical, chemical and electrical properties, but their high fabrication cost, brittleness and size and shape limitations as monolithic components restrict many potential applications. One way to avoid these drawbacks is to use ceramics as coatings on metallic substrates.
Thin ceramic coatings deposited by vapour deposition techniques (PVD and CVD) are widely used as wear and corrosion resistant coatings. These coatings include diamond, TiC, TiN, ZrN, Al2O3 etc. Since the deposition processes operate at atomic or molecular level the structure of the coatings can also be controlled on that level and well developed, dense structures having properties of dense monolithic materials can be produced. However, the thickness of these coatings is typically less than 10 mm, since thicker coatings suffer from high internal stresses or the same brittleness as dense monolithic ceramics.
Several applications require thicker ceramic coatings, e.g. for increased wear or corrosion resistance. One approach to avoid the above mentioned problems of thicker ceramic coatings has been the use of thermal spray technologies to produce the coatings. In thermal spraying ceramic feedstock powder is fed to high temperature plasma or flame, melted to droplets, which are accelerated to high speed and solidified on the substrate forming a lamellar structure (figure 1). Different coating microstructures and properties can be obtained depending on the spray technique, powder properties and spray parameters. Lamellar, low modulus structures allow thicker coatings having typical thicknesses in the range of 100 to 1000 mm to be produced. The larger thicknesses require further reinforcement of the coatings in demanding applications.
Figure 1: (a) Vertical cross-section of plasma sprayed alumina coating showing the lamellar structure, (b) polished surface parallel to the substrate showing the pore channels and microcracks in the lamellae
Another structure derived problem in thermal spray coatings is open porosity, which always exists in the coatings. This is a major drawback in applications requiring high resistance to aqueous corrosion. High velocity processes such as detonation gun and HVOF spraying produce coatings with lower porosity (1 – 3%), but are not widely used for spraying of ceramic coatings. Detonation guns are not readily available, have low deposition rates and require special safety measures to operate (explosion and noise considerations). HVOF systems, which burn propane or propylene with oxygen, typically produce low flame temperatures, which are not suitable for spraying high melting point ceramics, such as chromia and zirconia.
Thermally sprayed coatings can be improved by different post treatments, such as sealing or strengthening and adjusting the functional properties of the surfaces. Sealing is carried out by impregnating the coating with organic, inorganic or metallic materials. It may also be done by deposition of a dense layer on top of the coating or by remelting the coating surface by laser. Much research has been carried out in the Surface Engineering Laboratory of Tampere University of Technology with different organic sealants and with aluminium phosphate based inorganic sealants. Laser remelting has been tested for zirconia based thermal barrier coating (TBC) surfaces. In the following articles and in the attached references more detailed results of these approaches are presented. Lists of recent publications can be found at http://www.tut.fi/units/ms/pin/index.htm.
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