"Abradable coatings are characterized by a friable structure of carefully selected materials. These coatings are difficult to engineer because they must be at the same time readily abradable and mechanically stable to withstand the harsh operating conditions of a gas turbine. There is a demand from the aerospace and energy industries for the production of turbines that operate at higher temperatures, i.e. temperatures higher than about 1100.degree. C. Operation at higher temperatures translates into higher efficiency, higher economy and less pollution. As a consequence, it is desirable that the abradable coatings also follow this trend, i.e., they are able to operate at higher temperatures.
"In order to achieve this goal, two main types of high temperature abradable coatings are currently in use. The first one is based on the combination of a high temperature alloy (CoNiCrAlY), a self-lubricating material (BN) and a polymer (polyester). The metallic alloy provides the oxidation resistance and mechanical integrity at high temperatures. The BN lowers the friction coefficient of the coating and the polyester produces high amounts of porosity (producing a friable structure) after it is burned out of the coating.
"The second type of high temperature abradable currently in use is based on a ceramic material (ZrO.sub.2-6-8 wt % Y.sub.2O.sub.3), BN and polyester. The ceramic material provides the mechanical and chemical integrity at high temperatures. Like the metallic abradable, the BN also lowers the friction coefficient and the polyester also creates a network of porosity in the coating microstructure (after being burned out), therefore making a friable ceramic material.
"Despite the success of the current approaches, there are still problems to be solved. For example, when spraying a composite material with very different physical properties, such as CoNiCrAlY and polyester or ZrO.sub.2-7 wt % Y.sub.2O.sub.3 and polyester, it is very difficult to have consistency in the spraying process, therefore these types of coatings may exhibit homogeneity problems. Further, after coating deposition, the polymer must be burned out of the coating to create porosity. This process takes hours and raises the cost of the process in terms of time and money.
"Thermal Barrier Coatings (TBCs)
"TBCs are deposited on the surface of metal parts that are routinely subjected to thermal shock (e.g., turbine blades and combustion chambers of aircraft and land based gas turbines, etc.) to decrease heat transfer between e.g. hot gases arising from the combustion of fuel (e.g., kerosene) and the metallic parts. TBCs are normally made of two layers of coatings. The first layer is generally a metallic bond coat (BC), which is deposited directly (via thermal spray) on the metallic surface of the blades and combustion chambers. The BC layer (coating) is usually made of CoNiCrAlY alloys and the typical BC thickness varies from 100 to 200 .mu.m. The main function of the BC is to protect the metallic parts of the turbine against high temperature oxidation and to serve as a support coating or anchor coating for the second layer. The second layer (also known as top coat) deposited (via thermal spray) on the BC layer, is a ceramic coating usually based on zirconia (ZrO.sub.2). The typical thickness of the ceramic top coat varies from 250 to 500 .mu.m. The main function of the ceramic top coat, due to its inherent mechanical integrity, stability, low thermal diffusivity/conductivity and chemical resistance up to high temperatures, is to protect the metallic parts of the turbine against the high temperature environment of the combustion of fuel in the turbine engine. With the use of TBCs it is possible to increase the compressor and combustion chamber efficiencies (by burning fuel at higher temperatures) and decrease fuel consumption. Today, most of the aviation and land based gas turbines make use of TBCs.
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