ABRADABLE COATING

Abstract

A method of forming an abradable coating includes forming a plasma; introducing a coating material, as a powder having particles in the range between 1 and 50 m, carried by a delivery gas into the plasma, having a sufficiently high specific enthalpy for at least partially melting some of the powder and vaporizing at least 5% by weight of the powder, to form a vapor phase cloud of vapor and particles; forming a plasma beam by maintaining a process pressure between 50 and 2000 Pa; defocussing the plasma beam by maintaining a process pressure between 50 and 2000 Pa; and forming from the vapor phase cloud an abradable coating, comprising columnar structures. Advantageously, the columnar structured abradable coating has an erosion resistance smaller than 30 s/mils, preferably in the range of 5 to 27 s/mils, more preferably in the range 10-25 s/mils, still more preferably in the range 15-20 s/mils.

Claims

1-15. (canceled)

16. A method of forming an abradable coating (40,41), comprising forming a plasma; introducing a coating material, in the form of a powder having particles in the range between 1 and 50 m, carried by a delivery gas into the plasma, the plasma having a sufficiently high specific enthalpy for at least partially melting some of the powder and vaporizing at least 5% by weight of the powder, so as to form a vapor phase cloud of vapor and particles; forming a plasma beam by maintaining a process pressure between 50 and 2000 Pa; defocussing the plasma beam including the vapor phase cloud; and forming from the vapor phase cloud onto a substrate (10,11) surface an abradable coating (40, 41), being part of an insulating layer system (20,30,40; 21,31,41), the abradable coating comprising columnar structures (49), depositing a gradient abradable layer, wherein depositing a gradient abradable layer comprises depositing a first sub-layer (40-a, 41-a) comprising a lamellar dense structure, a second sub-layer (40-b, 41-b) intermediate between the first sub-layer (40-a, 41-a) and a third sub-layer (40-c, 41-c), wherein the second sub-layer comprises a mixed phase crumbly structure, and the third sub-layer (40-c, 41-c), subsequent to depositing the second sub-layer, comprising the columnar structures (49).

17. The method according to claim 16, wherein the columnar structured abradable coating (40,41) has an erosion resistance smaller than 30 s/mils (equivalent to s/25.4 m), preferably in the range of 5 to 27 s/mils, more preferably in the range 10-25 s/mils, still more preferably in the range 15-20 s/mils.

18. The method according to claim 17, wherein the method comprises tuning the erosion resistance of the abradable coating (49,41) through controlling at least one of an amount of hydrogen plasma gas, a surface temperature of substrate (10,11), and a powder feet rate.

19. The method according to claim 18, wherein the surface temperature of the substrate (10,11) during the coating process is tuned to a value in the range 500 C. to 1100 C., preferably in the range 950 C. to 1050 C.

20. The method according to 18, wherein the amount of hydrogen plasma gas is tuned in the range of 0 NLPM to 10 NLPM.

21. The method according to claim 18, wherein the total powder feed rate is tuned in the range of 5 g/min to 60 g/min.

22. The method in accordance with claim 16, wherein the columnar structures (49) of abradable coating (40) have a feathery micro-structure.

23. The method in accordance with claim 22, wherein the columnar structures (49) of abradable coating (40) are structured such that, in operation within a turbine or engine, a top part (49-2) of the columnar structure may be chipped away by vane-tip 4, leaving a bottom part (49-1) unaffected.

24. The method according to claim 16, wherein forming the abradable coating comprises using a plasma spray physical vapor deposition (PS-PVD) system.

25. The method according to claim 16, wherein the method comprises forming the first sublayer (41-a) with a chemical composition commensurate with a chemical composition of a lower layer of the insulating layer system and forming the third sub-layer (40-c) with a different chemical composition for forming the columnar structured abradable coating.

26. The method according to claim 16, wherein the substrate (10,11) is a component of a gas turbine.

27. A turbine component or engine component, comprising an insulating layer system (20,30,40; 21,31,41)), wherein the insulating system comprises a gradient abradable layer with a first sub-layer (40-a, 41-a) comprising a lamellar dense structure, a second sub-layer (40-b, 41-b) intermediate between the first sub-layer (40-a, 41-a) and a third sub-layer (40-c, 41-c), with a mixed phase crumbly structure, and the third sub-layer (40-c, 41-c), with a columnar structures (49), such that an outer layer (40,41) of the insulating layer system forms an abradable coating comprising columnar structures (49).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] FIG. 1A schematically shows a turbine/engine component, in this case a blade.

[0040] FIG. 1B schematically shows a first embodiment of a coating system according to the invention.

[0041] FIG. 1C schematically shows a second embodiment of a coating system according to the invention.

[0042] FIG. 2 schematically shows a close-up of a blade tip cutting a path through an abradable columnar coating according to the invention.

[0043] FIG. 3 schematically shows a close-up of the micro-structure of a column of the abradable coating according to the invention

[0044] FIG. 4A schematically shows a first micro-structure obtainable with the PS-PVD process according to the invention

[0045] FIG. 4B schematically shows a second micro-structure obtainable with the PS-PVD process according to the invention

[0046] FIG. 4C schematically shows a third micro-structure obtainable with the PS-PVD process according to the invention

[0047] FIG. 5 schematically shows another embodiment of the abradable coating according to the invention, comprising a gradient coating.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0048] FIG. 1 schematically shows a gas turbine engine component, in this case a blade unit 1 comprising a base 2, a vane or air foil 3, and a vane-tip 4 which may be assembled in a gas turbine as either a stator blade unit or a rotor blade unit. The rotating compressor or rotor of an axial flow gas turbine consists of a plurality of such blade units attached to a shaft which is mounted in a shroud. In operation. The shaft and blades rotate inside the shroud. The inner surface of the turbine shroud 10,11 is most preferably coated with an abradable material which functions as a seal for the clearance gap between vane-tip 4 and shroud 10,11 in order to increase the efficiency of the turbine. FIGS. 1B and 1C schematically show two embodiments of the present invention in which a metallic substrate 10, respectively a ceramic matrix composite (CMC) substrate 11, of a turbine component such as the shroud is covered with an appropriate bond coat 20,21. Such a bond coat is optional. On top of the bond coat a thermal barrier coating (TBC) 30, respectively an environmental barrier coating (EBC) 31, is deposited. On top of these later barrier coatings, a columnar structured abradable coating 40,41 according to the invention has been deposited using the PS-PVD process. Advantageously, the columnar structured abradable coating 40,41 using the PS-PVD process is softer and more porous, respectively has a lower linear column density and more feathery structure of a column, relative to EB-PVD produced abradable coatings.

[0049] In order that the anisotropic micro-structure of the columnar structured abradable coating 40,41 is produced, a plasma must be produced with sufficiently high specific enthalpy so that a substantial portionamounting to at least 5% by weight, of the coating material changes into the vapor phase. The portion of the vaporized material which may not fully change into the vapor phase can amount to up to 70%. The plasma is produced in a burner with an electrical DC current and by means of a pin cathode and a ring-like anode. The power supplied to the plasma, respectively the effective power, must be determined empirically with respect to the resulting coating structure. The effective power, according to experience typically between 50% and 55% of the electrical power supplied to the plasma gun, is in the range from 40 to 80 kW.

[0050] The process pressure of the PS-PVD method for producing the abradable coatings according to the invention has a value between 50 and 2000 Pa, preferably between 100 and 800 Pa. Powder is injected into the plasma from 1 or more (such as 2, 3, or 4) injectors using a delivery gas. The process gas for the production of the plasma is a mixture of inert gases, in particular a mixture of argon Ar and helium He, with the volume ratio of Ar to He advantageously lying in the range from 2:1 to 1:4. The total gas flow is in the range from 30 to 150 NLPM (Normal Litres Per Minute). The total powder feed rate lies between 5 and 60 g/min, preferably between 10 and 40 g/min. The plasma has a sufficiently high specific enthalpy for at least partially melting some of the powder and vaporizing at least 5% by weight of the powder, so as to form a vapor phase cloud of vapor and particles. A plasma beam is formed by maintaining a process pressure between 50 and 2000 Pa and defocused, including the vapor phase cloud of vapor and particles in the defocusing plasma. The substrate is preferably moved with rotating or pivoting movements relative to this cloud during the material application. Typically, the substrate 10,11 surface temperature during the coating process is in the range of 500 C. and 1100 C. and is heated using the plasma jet. Alternatively, however, the surface temperature may also be controlled using other heat sources, such as another plasma gun, induction, or quartz lamps. The spray distance from the plasma gun to the substrate typically is around 900 mm. Using the PS-PVD process, the abradable coating is built up by growth of the columnar structure. The total coating thickness has values between 20 m and 2000 m, preferably values between 200 m and 1000 m.

[0051] An oxide ceramic material, or a material which includes oxide ceramic components, is suitable for the manufacture of a columnar structured abradable coating 40,41 using the method in accordance with the invention, with the oxide ceramic material being in particular a zirconium oxide, in particular a zirconium oxide which is fully or partly stabilized with yttrium, cerium or other rare earths. The material used as the stabilizer is added to the zirconium oxide as an alloy in the form of an oxide of the rare earths, for example yttrium Y, cerium or scandium, withfor the example of Ythe oxide forming a portion of 5 to 20% by weight, such as 8%.

[0052] In order that the powder beam is reshaped by the defocusing plasma into a vapor phase cloud of vapor and particles from which a coating results with the desired micro-structure, the powdery starting material must have a very fine primary grain (preferably in the range 1-3 m) which may (loosely) agglomerate to larger powder particles. The size distribution of the powder particles is typically determined by means of a laser scattering method. The size distribution of the powder particles lies to a substantial portion in the range between 1 m and 50 m, preferably between 3 m and 25 m. Various methods can be used to manufacture the powder particles: for example, spray drying or a combination of melting and subsequent breaking and/or grinding of the solidified melt.

[0053] In case of a metallic turbine component substrate 10, comprising for instance a Ni or Co base alloy, optional bond coating 20 may comprise an NiAl alloy or an NiCr alloy. TBC 30, for instance made using Zirconium oxide stabilized with yttrium Y (such as ZrO.sub.2-8% Y.sub.2O.sub.3) as the coating material, typically has a coating thickness ranging between 10 m and 300 m, preferably between 25 m and 150 m. TBC 30 in particular comprises a metal aluminide, or an MCrAlY alloy, with M standing for one of the metals Fe, Co or Ni or of a ceramic oxide material. It preferably has an either dense, columnar, directional or unidirectional structure.

[0054] In case of a CMC turbine component substrate 11, comprising carbon fiber reinforced silicon carbide composites (C/SiC) and silicon carbide fiber reinforced silicon carbide composites (SiC/SiC), the optional bond coat 21 may comprise a Si-based metal. EBC 31, for instance made of mullites (Al.sub.2O.sub.3 SiO.sub.2) with different proportion of Al.sub.2O.sub.3 and SiO.sub.2, or silicates materials such as Yb.sub.2O.sub.3, Yb.sub.2Si.sub.2O.sub.7, Yb.sub.2SiO.sub.5 and/or a combination of both mullites and silicates, typically has a coating thickness ranging between 10 m and 300 m, preferably between 25 m and 150 m.

[0055] The part layers of the complete coating system are preferably all applied in a single work cycle without interruption using the PS-PVD processes. After the application, the coating system may be heat treated as a whole, if necessary.

[0056] In the plasma spraying process of the invention an additional heat source, such as another plasma gun, a quartz lamp, or induction source, can also be used in order to carry out the deposition of the coating material within a predetermined temperature range. The temperature of the substrate 10, 11 is pre-set in the range between 500 C. and 1100 C., preferably in the temperature range 950 C. to 1050 C. An infrared lamp or plasma jet can, for example, be used as an auxiliary heat source. In this arrangement a supply of heat from the heat source and the temperature in the substrate which is to be coated can be controlled or regulated independently of the already named process parameters. The temperature control can be carried out with usual measuring methods (using infrared sensors, thermal sensors, etc.).

[0057] The method in accordance with the invention can be used to coat components exposed to high process temperatures with a columnar structured abradable coating. Such components are, for example, components of a stationary gas turbine or of an airplane power plant: namely turbine blades, in particular guide blades or runner blades, or even components which can be exposed to hot gas such as a heat shield and shroud.

[0058] FIG. 2 schematically shows a close-up of the top layers of a coating system, with a TBC 30, respectively an EBC 31 covered with a columnar structured abradable coating 40,41. Also shown is an air foil or vane 3 with a vane-tip 4 of a turbine blade 1 creating a cutting path through the abradable coating 40,41 under operation condition of the turbine. As can be seen, vane 3 creates a well-defined cutting path through the columnar structured abradable coating 40,41. Advantageously, the columnar structured abradable coating has such a low erosion resistance and such a spacing between the individual columns 49 that vane-tip 4 wears of individual columns 49 under the tip without effecting neighbouring columns 49. The columnar structured abradable coating according to the invention has an erosion resistance <30 s/mils, preferably in the range of 5 to 27 s/mils, more preferably in the range 10 to 25 s/mils, even more preferably in the range between 15 and 20 s/mils. Erosion resistances in this range essentially result in that the wall of the cutting path is defined by a single columnar structure 49. The erosion resistance of the columnar structured coating can be tuned by controlling the density of the columnar structures. Lower densities can be realized by reducing and/or removing the amount of Hydrogen plasma gas in the process gas, reducing the surface temperature during the coating process, and increasing the powder feed rate of the coating material. Thus, in an embodiment, the method according to the invention comprises tuning an erosion resistance of the abradable coating through controlling at least one of the amount of hydrogen plasma gas, the surface temperature of substrate 10,11, and the powder feet rate.

[0059] The thermal conductivity of the columnar structured abradable coating 40 is similar to a TBC 30, and may be substantially lower in case of very porous coatings, i.e. coatings 40 with a low density of columnar structures 49.

[0060] FIG. 3 shows schematically a close-up of the microstructure of a columnar structure 49. As can been seen, the columnar structures 49 have a feathery and loose structure when produced with the PS-PVD process. These feathery structures help reduce the erosion resistance in comparison to a dense crystal growth of needles as is known from EB-PVD. Furthermore, the feathery structure allows vane-tip 4 to create a cutting path by consecutively chipping off individual feathers or feather parts from columnar structure 49 as vane-tip 4 expands under the operating temperature conditions of the turbine. Advantageously, the low or soft erosion resistance of the abradable coating according to the invention allows for a top part 49-2 of the columnar structure to be chipped of by vane-tip 4, while bottom part 49-1 is unaffected and still adheres to the lower layers of the coating system.

[0061] FIG. 4 shows schematically different microstructures of abradable coating 40,41 on top of TBC 30, respectively EBC 31. These can be obtained using the PS-PVD process according to the invention by controlling the coating temperature and the plasma gas mixture. The working pressure and power level of the PS-PVD process are in the same range as described above in conjunction with FIG. 1.

[0062] In FIG. 4A a relative dense columnar structure is produced using a plasma mixture of Ar, He, and H.sub.2. Typically, the Ar/He ratio ranges from 2:1 to 1:4, and preferably is 1:2, while the flow rate ranges from 30 to 150 NLPM. The H2 gas flow may range from 1 to 16 NLPM, preferably from 1 to 10 NLPM. As a typical example: the gas flow rate for the PS-PVD process is 30 NLPM Ar, 65 NLPM He, and 10 NLPM H.sub.2. The substrate temperature during the coating process is in the range 700 C. to 1100 C., preferably between 950 C. and 1000 C. The width and linear density of PS-PVD produced columns under these operating conditions is in the range of 10-50 m with approximately 4 columns/100 m (i.e. an intercolumn space 0 to 5 m). Thermal conductivity of such a columnar structured abradable coating is in the range 1.0-2.5 W/m.Math.K.

[0063] In FIG. 4B a lower density columnar structure is produced by applying a gas mixture of Ar and He. In other words, the H.sub.2 gas flow has been removed from the mixture. Remaining operation conditions are the same as in FIG. 4A. The width and linear density of PS-PVD produced columns under these operating conditions is in the range of 5-15 m with approximately 7 columns/100 m (i.e. an intercolumn space >5 m). Thermal conductivity of such a columnar structured abradable coating is in the range 0.8-1.5 W/m.Math.K.

[0064] In FIG. 4C a crumbly structure is obtained, essentially a mixed phase of the columnar structure and the lamellar dense layer, by reducing the substrate temperature during the deposition process to a temperature in the range 500 C. to 700 C. The remaining operating process conditions are similar as those for FIGS. 4A and 4B. The powder feed rate is a further parameter influencing the mixed phase composition. An increase in the feed rate reduces the number of particles in the vapor phase, thus allowing the tuning of the mixed phase coating.

[0065] FIG. 5 schematically shows a coating system comprising a gradient abradable coating. The turbine component may have a metallic substrate 10, respectively a CMC substrate 11. Optionally, an appropriate bond coat 20, respectively 21 is applied to the substrate. Subsequently, a TBC layer 30, respectively an EBC layer 31 has been deposited using the PS-PVD process. And on top a gradient abradable coating 40, respectively 41, has been deposited using the PS-PVD process. A first sub-layer 40-a/41-a of gradient coating 40,41 comprises a lamellar dense layer, an optional second sub-layer 40-b/41-b of gradient coating 40, 41 comprises a mixed phase layer, and a third sub-layer at the top comprises a columnar structured abradable layer 40-c/41-c. Advantageously, the gradient ensures an excellent bonding of the abradable coating 40,41 to the underlying TBC 30, respectively EBC 31 layer. Especially in the latter case, in view of the differences in the chemical composition of the EBC and abradable coating layer, the gradient ensures adherence as the chemical composition of the coating material in the three sub-layers may be tuned from one that is commensurate with the EBC to one that is optimal for functioning as a seal to the clearing gap.

[0066] Operating parameters for the first sub-layer typically are: work pressure 50 Pa to 80000 Pa, preferably 100 Pa to 1000 Pa; effective power of the plasma jet 40 kW to 80 kW; Total gas flow, comprising Ar and optionally He and/or H.sub.2, in the range of 30 NLPM to 150 NLPM; with, in case the gas flow comprises a Ar/He mixture, an Ar:He ratio in the range 10:1 to 1:1, typically 4:1, and 0<H.sub.2<20 NLPM; a total powder feed rate in the range of 5-120 g/min, preferably 20-80 g/min, ideally between 40 and 80 g/min; substrate temperature in the range of 500 C. to 1100 C.

[0067] Operating parameters of the third sub-layer typically are as described for the embodiment in FIG. 1.

[0068] Thus, in an embodiment the method comprises depositing a gradient abradable coating by controlling at least one of the substrate temperature, the powder feed rate, and the gas flow mixture. In one example, a first sub-layer 40-a was produced with a 80/40/10 NLPM Ar/He/H2 gas flow mixture, a 240 g/min feed rate, a 1.5 mbar work pressure, and a substrate temperature 900 C., while the third sub-layer 40-c was produced with a 30/60/0 NLPM Ar/He/H2 gas flow mixture, a 233 10 g/min feed rate, a 1.5 mbar work pressure, an a substrate temperature 1000 C. The working parameters for the second sub-layer 40-b were intermediate to the aforementioned parameter sets.