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Protective coatings for gas turbine airfoils—State of the art and science
Thin Solid Films, 53 (1978) 223-224 © ElsevierSequoiaS.A., Lausanne--Printedin the Netherlands
223
PROTECTIVE COATINGS FOR GAS TURBINE AIRFOILS---STATE OF THE ART AND SCIENCE* G. W. GOWARD Pratt and Whitney Aircraft, East Hartford, Conn. 06108 (U.S.A.)
For almost all contemporary applications of gas turbine engines, protective coatings must be used for hot section airfoils to achieve maximum cycle efficiency with economically acceptable durability. Depending on the particular application (aircraft propulsion, ship propulsion, electrical power generation) the degradation of available coatings can involve, singly or in various combinations, oxidation, oxidation accelerated by contaminants (hot corrosion), particle erosion and thermal stress effects such as surface deformation and fatigue cracking. All protective coatings in current use are metallic alloys optimized in composition to form oxide barriers, with lowest possible growth rates, between the various corrodents and the underlying coatings. Usually the oxide barrier is alumina, although isolated reports indicate that chromia or perhaps silica may be more satisfactory for certain applications. In the virtual absence of extraneous airfoil deposits, coating degradation proceeds principally by repeated damage of the alumina scale by thermal Cycling which leads to depletion of aluminum in the coating alloy to a level at which less protective oxides form. The rate of this degradation can be increased by dilution of the coating and by incorporation of base alloy elements inimical to the formation of alumina by interdiffusion of the base alloy and the coating. The degradation rate can be decreased by improvement of the oxide adherence by the addition of oxygenactive elements such as yttrium or stable oxide particles such as alumina to the coating alloys. Additions of noble metals such as platinum also improve the adherence of the oxide barrier, in addition to inhibiting coating-alloy interdiffusion. In most applications of the gas turbine, greater or lesser amounts of contaminants contained in both fuel and air result in the deposition of various compounds on the surfaces of the hot section airfoils. Many of these compounds can significantly increase the rate of conversion of coatings and base alloys to oxides. One useful model of this degradation process views molten sulfate salts, principally sodium sulfate, as the major corroding medium with the rate of corrosion being affected by various additions to or modifications of this salt. For example, transfer of sulfur from the sodium sulfate to the coating alloy to form sulfides renders the salt more basic in terms of oxide ion concentration and the basic molten salt can then dissolve potentially protective oxide barriers. Incorporation of sulfur in the *Abstract of a paper presented at the International Conferenceon Metallurgical Coatings, San Francisco,California,U.S.A.,April3-7, 1978.
224
AUTHORS' ABSTRACTS
underlying coating alloy can negatively affect oxidation resistance. Alternatively, incorporation of acidic species, such as sulfur trioxide and vanadium pentoxide, renders the salt acidic and it can again dissolve protective oxide films and allow penetration of sulfur into the coating alloy. Depending on the specific environment, various combinations of chromium and/or aluminum in cobalt- and nickel-based coating alloys can provide practically useful protection against hot corrosion. Recent research indicates that erosion, caused by abrasive particles entrained in high velocity gas streams similar to those in turbines, can accelerate both oxidative and hot corrosive degradation of coatings and base alloys. Few if any coatings have been optimized to resist this particular mode of degradation. In turbine applications where frequent power changes are required, thermally induced stresses can cause fatigue cracking of coatings and the cracks can propagate into the load-bearing base alloys. Coating compositions can be adjusted to provide more resistance to such cracking but usually by sacrificing oxidation and/or hot corrosion properties. Thus various coating compositions can be designed to resist most of the identified degradation processes but all too often such coatings still degrade at economically unacceptable rates. It must be concluded that much more research on degradation mechanisms is required to provide more sophisticated guidance to the development of coating compositions more resistant to the wide variety of turbine environments. So-called "thermal barrier" insulative coatings based on relatively thick films of stabilized zirconia have shown some promise in improving turbine airfoil durability. The thermal stress or spalling resistance of these coatings on gas turbine airfoils has not been fully established and, in terms of current understanding of hot corrosion mechanisms, neither has the corrosion resistance. Extensive evaluation programs are needed to determine the full potential for airfoil protection. Early results indicate that further development will be required to achieve extensive practical application of thermal barrier coatings. The need for coatings with improved oxidation and hot corrosion resistance has necessitated the development of manufacturing methods capable of synthesizing coatings with complex and closely controlled compositions. Electron beam physical vapor deposition has been developed to a practical state for application of the first generation of such coatings, typified by the various M-Cr-A1-Y compositions. Recent investigations of sputtering and plasma spraying indicate that these processes show considerable promise of application for overlay coatings, particularly where more complex compositions are required.
223
PROTECTIVE COATINGS FOR GAS TURBINE AIRFOILS---STATE OF THE ART AND SCIENCE* G. W. GOWARD Pratt and Whitney Aircraft, East Hartford, Conn. 06108 (U.S.A.)
For almost all contemporary applications of gas turbine engines, protective coatings must be used for hot section airfoils to achieve maximum cycle efficiency with economically acceptable durability. Depending on the particular application (aircraft propulsion, ship propulsion, electrical power generation) the degradation of available coatings can involve, singly or in various combinations, oxidation, oxidation accelerated by contaminants (hot corrosion), particle erosion and thermal stress effects such as surface deformation and fatigue cracking. All protective coatings in current use are metallic alloys optimized in composition to form oxide barriers, with lowest possible growth rates, between the various corrodents and the underlying coatings. Usually the oxide barrier is alumina, although isolated reports indicate that chromia or perhaps silica may be more satisfactory for certain applications. In the virtual absence of extraneous airfoil deposits, coating degradation proceeds principally by repeated damage of the alumina scale by thermal Cycling which leads to depletion of aluminum in the coating alloy to a level at which less protective oxides form. The rate of this degradation can be increased by dilution of the coating and by incorporation of base alloy elements inimical to the formation of alumina by interdiffusion of the base alloy and the coating. The degradation rate can be decreased by improvement of the oxide adherence by the addition of oxygenactive elements such as yttrium or stable oxide particles such as alumina to the coating alloys. Additions of noble metals such as platinum also improve the adherence of the oxide barrier, in addition to inhibiting coating-alloy interdiffusion. In most applications of the gas turbine, greater or lesser amounts of contaminants contained in both fuel and air result in the deposition of various compounds on the surfaces of the hot section airfoils. Many of these compounds can significantly increase the rate of conversion of coatings and base alloys to oxides. One useful model of this degradation process views molten sulfate salts, principally sodium sulfate, as the major corroding medium with the rate of corrosion being affected by various additions to or modifications of this salt. For example, transfer of sulfur from the sodium sulfate to the coating alloy to form sulfides renders the salt more basic in terms of oxide ion concentration and the basic molten salt can then dissolve potentially protective oxide barriers. Incorporation of sulfur in the *Abstract of a paper presented at the International Conferenceon Metallurgical Coatings, San Francisco,California,U.S.A.,April3-7, 1978.
224
AUTHORS' ABSTRACTS
underlying coating alloy can negatively affect oxidation resistance. Alternatively, incorporation of acidic species, such as sulfur trioxide and vanadium pentoxide, renders the salt acidic and it can again dissolve protective oxide films and allow penetration of sulfur into the coating alloy. Depending on the specific environment, various combinations of chromium and/or aluminum in cobalt- and nickel-based coating alloys can provide practically useful protection against hot corrosion. Recent research indicates that erosion, caused by abrasive particles entrained in high velocity gas streams similar to those in turbines, can accelerate both oxidative and hot corrosive degradation of coatings and base alloys. Few if any coatings have been optimized to resist this particular mode of degradation. In turbine applications where frequent power changes are required, thermally induced stresses can cause fatigue cracking of coatings and the cracks can propagate into the load-bearing base alloys. Coating compositions can be adjusted to provide more resistance to such cracking but usually by sacrificing oxidation and/or hot corrosion properties. Thus various coating compositions can be designed to resist most of the identified degradation processes but all too often such coatings still degrade at economically unacceptable rates. It must be concluded that much more research on degradation mechanisms is required to provide more sophisticated guidance to the development of coating compositions more resistant to the wide variety of turbine environments. So-called "thermal barrier" insulative coatings based on relatively thick films of stabilized zirconia have shown some promise in improving turbine airfoil durability. The thermal stress or spalling resistance of these coatings on gas turbine airfoils has not been fully established and, in terms of current understanding of hot corrosion mechanisms, neither has the corrosion resistance. Extensive evaluation programs are needed to determine the full potential for airfoil protection. Early results indicate that further development will be required to achieve extensive practical application of thermal barrier coatings. The need for coatings with improved oxidation and hot corrosion resistance has necessitated the development of manufacturing methods capable of synthesizing coatings with complex and closely controlled compositions. Electron beam physical vapor deposition has been developed to a practical state for application of the first generation of such coatings, typified by the various M-Cr-A1-Y compositions. Recent investigations of sputtering and plasma spraying indicate that these processes show considerable promise of application for overlay coatings, particularly where more complex compositions are required.