Patent Application
20170044930
The present technology relates to environmental barrier coatings, and more particularly to environmental barrier coatings that offer improved resistance to dust deposits.
Higher operating temperatures for gas turbine engines are continuously being sought in order to improve their efficiency. However, as operating temperatures increase, the high temperature durability of materials of manufacture of engine components must be maintained. Significant advances in high temperature capabilities of engine components have been achieved through improved formulation and processing of iron, nickel, and cobalt-based superalloys. While superalloys have found wide use for components in gas turbine engines, and especially in the higher temperature sections, alternative lighter-weight, higher-temperature component materials have been proposed.
Ceramic matrix composites (CMCs) are a class of materials that include a reinforcing material surrounded by a ceramic matrix phase. Such materials, along with certain monolithic ceramics (i.e. ceramic materials without a reinforcing material), are currently being used in a variety of high temperature applications. These ceramic materials are lightweight compared to superalloys, yet can still provide strength and durability to components made therefrom. Therefore, such materials are currently being considered for many gas turbine components used in higher temperature sections of gas turbine engines, such as airfoils (blades and vanes), combustor liners, shrouds and other similar components, that can benefit from the lighter-weight and higher temperature capability offered by these materials.
CMC's and monolithic ceramic components can be coated with environmental barrier coatings (EBCs) to protect them from the harsh environments of high temperature engine sections. EBCs can provide a dense, hermetic seal against the corrosive gases in the hot combustion environment, which can rapidly oxidize silicon-containing CMCs and monolithic ceramics. Additionally, silicon oxide is not stable in high temperature steam, but is converted to volatile (gaseous) silicon hydroxide species. Thus, EBCs can help prevent dimensional changes in the ceramic component due to such oxidation and volatilization processes.
Aircraft engines including CMC components are being operated in parts of the world where dust ingested with compressed air can deposit on the surfaces of hot stage CMC components and degrade the life of the EBCs. Hot gas path engine components can experience loss of performance resulting from intake and deposition of environmental particulate matter, especially during idling, takeoff, and landing. Accumulation of dust particles on the surfaces of gas turbine components can result in component overheating; in addition, molten dust deposits can react chemically with EBC's to cause EBC cracking and spallation, followed by undesirable exposure of the underlying CMC's to the engine environment. Dust deposition can thus reduce EBC durability and lead to premature degradation of CMC's and decreased component life. There is a requirement for improvements in composition and properties of EBC's to improve resistance to dust deposition.
According to one example of the present technology, an environmental barrier coating system for a component of a gas turbine comprises at least one layer of rare earth disilicate and at least one layer of rare earth monosilicate. At least one of the at least one layer of rare earth disilicate or the at least one layer of rare earth monosilicate includes an alkaline earth oxide dopant.
According to another example of the present technology, a component of a gas turbine engine is coated with an environmental barrier coating system as described herein.
Examples of the present technology set forth herein will be better understood from the following description in conjunction with the accompanying figures, in which like reference numerals identify like elements, wherein:
Referring to
Referring to
The bond coat of the EBC systems disclosed herein may have a thickness of between about 10 μm and about 200 μm, for example from about 25-150 μm, or from about 50-125 μm. The rare earth disilicate-based layer may have a thickness of between about 10 μm and about 250 μm, for example from about 25-200 μm, or from about 50-150 μm. The rare earth monosilicate-based layer may have a thickness of between about 10 μm and about 100 μm, for example from about 10-75 μm, or from about 25-75 μm.
The various characteristics and embodiments of the rare earth silicate-based substantially hermetic layer as disclosed herein also relate to a thermal spray feedstock for producing a rare earth silicate-based substantially hermetic layer. For example, suitable rare earth silicates (RES) for use in the rare earth silicate-based hermetic layer produced by the thermal spray feedstock can include, without limitation, a rare earth element selected from the group consisting of ytterbium (Yb), yttrium (Y), scandium (Sc), lutetium (Lu), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), europium (Eu), gadolinium (Gd), terbium (Tb), promethium (Pm), and mixtures thereof.
Referring to
In one example, the composition of the REMS layer 350 is shown in region A, which is defined by the space encompassed by points 1-2-3, where point 1 is Y2O3, point 2 is Ca2Y8Si6O26, and point 3 is Y2SiO5. The mole % of the CaO dopant may be about 1-16%, for example about 10%. The amount of dopant added should not substantially change (i.e. increase or decrease) the coefficient of thermal expansion (CTE) of the REMS layer 350, for example the amount of dopant added should result in either a decrease or an increase in the CTE of the REMS layer 350 of less than about 10%. Larger decreases in the CTE of the REMS layer 350 are acceptable.
The composition of the REDS layer 340 is shown in region B, which is defined by the space encompassed by points 2-3-4, where point 4 is Y2Si2O7. The mole % of the CaO dopant may be about 1-16%, for example about 10%. The amount of dopant added should not substantially change the coefficient of thermal expansion (CTE) of the REDS layer 340, for example the amount of dopant added should result in a change (i.e. an increase or decrease) of the CTE of the REDS layer 340 of less than about 10%. As another example, the amount of the dopant increases the coefficient of thermal expansion of the REDS layer 340 by no more than about 25%.
In another example, the composition of the REDS layer 340 is shown in Region C, which is defined by the space encompassed by points 2, 4, and 5. The mole % of dopant may be about 5-20%, for example about 10%. Again, the amount of dopant should not change the coefficient of the REDS layer 340 by more than about 10%.
Alkaline-earth-rare-earth-silicates may be added to the above phase regions for short-term protection. The alkaline-earth-rare-earth-silicates may be a composition of the formula AE·RE·S, where AE is Be, Mg, Ca, Sr, Ba, Ra, or combinations thereof, and RE is Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, or combinations thereof, and S is a compound containing an anionic silicon compound. For example calcium yttrium silicate (CaYS), including Ca3Y2Si6O18 (point 5), Ca2Y2Si2O9 (point 6) and/or Ca3Y2Si3O12 (point 7) may be added to the above phase regions for short-term protection. With time, some of these additions are expected to change the chemistry of the coating layers. Any compositions within the composition space defined by points 2-5-7 and/or 2-6-7 may be added to the phase regions A and/or B/and/or C for short-term protection. In such case, the amount of CaO dopant can be higher, exceeding 16%, but preferably remain below 25%.
Although various examples have been depicted and described in detail herein, it will be apparent to those skilled in the relevant art that various modifications, additions, substitutions, and the like can be made and these are therefore considered to be within the scope of the claims which follow. For example, although the examples illustrated include a REMS layer and a REDS layer as shown, additional layers, including additional layers of REMS and REDS, may be provided, including layers between, below, and above the layers as shown, and dopant(s) may be provided to none, some, or all of the additional layers.
