Method of applying an abrasive and protective armor overlay and tool

Abstract

The invention relates to a method to produce a metallic coating (7, 7',7'',7''') with secondary particles (19), especially abrasive ceramic particles (19), wherein the secondary particles (19) are injected from outside into the spray (10) of a HVOF gun (13) using a separate supply (16).

Description

[0001] Method of applying an abrasive and protective armor overlay and tool

[0002] The invention relates to a method of applying an abrasive and protective armor overlay especially to tips of rotating components of gas turbine engine and a corresponding tool.

[0003] In gas turbine engines clearance control is extremely important to improve the engine efficiency and reduce harmful emissions .

[0004] One method of clearance control involves application of abrasive protective overlays on blade tips which cut into a coating of a stationary counter surface in the case of a rub event, which are quite known:

[0005] D. B. Allen US 2005 / 0129522 Al;

[0006] D. B. Allen WO 2014 / 074370 A2 ;

[0007] Burton A. Kushner, Anthony J. Rotolico, John E. Nerz, Law- erence A. Saia, US 5,059,095;

[0008] J.D. Shell and H.J. Farr US 5, 952,110;

[0009] J. R. Faughnan et al. US 2016 / 0024942 Al;

[0010] X-T Luo, and C-J. Li, "Thermal Stability of Microstructure and Hardness of Cold-Sprayed cBN / NiCrAl Nanocomposite Coating" JTST, Vol. 21, pp . 578-585 (2012) . cBN in a superalloy or MCrAlY matrix is the most frequently selected option.

[0011] There are several methods to apply cBN on the blade tips including electroplating, brazing, and laser welding.

[0012] For power turbines where commissioning is not as well controlled as in aviation engines or where prolonged high temperature operation is inevitable, the electroplated or brazed cBN particles do not form a suitable overlay.

[0013] This is because the coating thickness for electroplated and brazed versions are limited to monolayers of abrasive and several tens of microns of the matrix. Welded overlays on the other hand, might produce overlays with multiple layers of abrasive and the matrix thickness in millimeter range.

[0014] Therefore, a more common method for powder turbines is via laser powder welding method: US 8, 647,073 B2 Hoebel et al or US 10,259,720 B2, Balbach et al...

[0015] In laser powder welding (LPW, sometimes also referred to as laser cladding and laser metal forming) a laser beam creates a molten pool at the substrate surface. The abrasive and the matrix material powders are transported via a carrier gas and injected into the molten pool around the laser beam. The abrasive and matrix material are premixed in the hopper for a single injection in the desired ratio, or the ratio can be achieved by controlling the mass feed rates of the two components in case of dual injection. LPW can be used to economically and repeatably build thick protective and abrasive overlays on blade tips with complex shapes.

[0016] However, LPW process like any welding process tends to generate heat effected zone in the base-alloy during deposition of the abrasive layer. This can lead to cracking of the tip especially if the blade is not made of a single crystal unlike in US '073 B2 Hoebel et al. Additionally the heat input to the cBN particles in the state-of-the-art LPW process can lead to loss of cBN particles due to high temperature oxidation or decomposition or can reduce the size of the cBN particles that end up in the weld overlay. The reduced amount and / or the smaller size of cBN particles adversely affect the cutting behavior of the blades.

[0017] The problem is not fully resolved yet.

[0018] With respect to brazing and electroplating, the coating thickness is rather limited to mono-grains and some tens of micrometers, which might offer too low wear resistance during contact of rotor and stator parts.

[0019] In contrast if welding is considered, the layer thickness might be high enough. However, the high heat of the process causes cracks within the base alloy, which is tried to be solved to some extend by applying buffer layers. This results only in a partial solution for the cracking issue. Furthermore, the issue of a loss of particles of cBN or size reduction is during the welding process has not yet solved.

[0020] It is therefore the aim of the invention to overcome these problems .

[0021] The problem is solved by a method according to claim 1 and a tool according claim 8.

[0022] In the dependent claims further advantages are listed which can be arbitrarily combined with each other to yield further advantages .

[0023] It shows figure 1 a schematic view of the tool used for the method, figure 2-4 examples of coatings produced with this method.

[0024] The figures and the description are only examples of the invention .

[0025] This invention proposes the use of high velocity oxy fuel (HVOF) thermal spray process technology to simultaneously spray the matrix material and the abrasive material in a dual injection (non-premixed powder form) set up.

[0026] Figure 1 shows an exemplary tool 1 comprising an HVOF spray gun 13 using powder which creates a spray 10 with a matrix material .

[0027] The matrix material is deposited on a substrate 4 which yields in a coating 7.

[0028] Abrasive particles 19 are bypassed the spray gun 13 using a separate and distinct supply 16 which transports using a carrier gas the abrasive particles 19.

[0029] The supply 16 is preferably fixed to the spray gun 13. The end of the supply 16 has a length from the end of the spray gun and is used at a certain distance 22 to the substrate 4.

[0030] But the length of the supply 16 and by this the distance to a surface or substrate 4 can also be adjusted.

[0031] The key technical features by which the invention solves the problem are the use of HVOF technology for spraying and the special setup for the injection of the abrasive particles separate from the matrix and away from the flame.

[0032] Advantages of using HVOF compared to LPW process:

[0033] - Unlike the LPW process, the HVOF process causes minimal heat input to the substrate 4 made of a base-alloy.

[0034] - Unlike the LPW process, HVOF process creates a peening effect on the substrate 4 surface. The above features can reduce the tip cracking tendency.

[0035] - The process HVOF is capable of generating extremely dense and oxidation resistant coatings / overlays 7.

[0036] Advantages of using modified injection of abrasive particles 19 compared to the standard injection in HVOF process (and LPW process)

[0037] - Abrasive particles 19 are less exposed to the high temperature oxidizing atmosphere prior to getting deposited and therefore less loss of cBN particles and less reduction of abrasive particle like cBN particle size

[0038] - Typically for standard HVOF process, the powder particle 19 size is small, which helps to accelerate the particles. Fine abrasive particles are not desirable for cutting applications such as that intended for the gas turbine blades. On the contrary, in the set up current disclosure, appropriately selected injector will allow spraying of the desirable coarse particles 19 as large as 250pm.

[0039] - The abrasive particle feed rate can be independently controlled and adjusted during deposition of the full thickness of the coating. This would allow the freedom to change the microstructure of the composite coating through its thickness. Figures 2 to 4 show schematic examples of the different through-thickness microstructure variations possible to achieve with the disclosed tool 1 using one powder feed stock for the spray gun 13.

[0040] Figure 2 shows a staggered structure 7' , wherein the particles 19 are not in area direct on or near the substrate 4.

[0041] Figure 3 shows a graded structure 7' ' , wherein the abrasive particles 19 have a higher density towards the outer surface 25 of the coating 7' ' .

[0042] In both examples of Fig. 2, 3 the abrasive particles 19 can also protrude the outer surface 28 like shown in the coating 7' ' ' in figure 4.

[0043] The disclosed method can be applied both liquid-fuel and gas-fuel version of the HVOF spray guns 13.

[0044] Furthermore, although this disclosure uses cBN particles as an example of the abrasive material, it is in no way intended to be limited to this material and can be applied and any suitable abrasive and matrix material combinations. Possible abrasive materials include but are not limited to SiC, TaC, VC, B4C Si3N4, AIN, A12O3, ZrO2, zirconia-toughened alumina (ZTA) etc...

[0045] HVOF has been used in studies to spray premixed Inconel 718 / cBN feedstock injected either central in gas powered HVOF or using the injection ports of liquid fueled HVOF systems [K. Shivalingaiah, „HVOF sprayed Inconel 718 / cubic boron nitride composite coatings: ''microstructure, microhardness and slurry erosive behaviour" in Mater. Res. Express 6 (2019) 1265i8] . In that study, the feedstock material used rather small cBN grain sizes of below 10pm, which were usually alloyed with the matrix powder using the ball milling process. Consequently, the abrasive grains were not large and do not protrude outside the sprayed coating by process and due to their small sizes.

[0046] Luo and Li used the cold-spray method to fabricate samples of cBN / NiCrAl nanocomposite coating for their study [X-T Luo, and C-J. Li, "Thermal Stability of Microstructure and Hardness of Cold-Sprayed cBN / NiCrAl Nanocomposite Coating" JTST, Vol. 21, pp . 578-585 (2012) ] . However, nano structural composites are not suitable for high temperature applications and fine abrasive grains do not provide optimal cutting behavior. Also, cold spraying is not suitable for superalloys which do not possess sufficient ductility.

[0047] The current invention suggests the use of larger particles 19 of 50pm - 250pm, which are fed separately from the matrix and are injected outside the spray gun 13 (Fig. 1) . This procedure has advantages over standard HVOF spraying: The cBN particles are larger in size to provide a more effective abrasion (cutting) process. In addition, the degeneration of the abrasives 19 due to heat is considerably less. Another advantage is the tailoring of the coating structure itself (Fig. 2 -4) .

[0048] The adhesion (buffer) layer can be sprayed using the matrix material only, which can be followed by a graded structure or even a cBN layer on top of the matrix depending on the demixing of both materials in the powder jet. Overall, the coating by liquid fueled HVOF provides compressive stress which is advantageous for suppressing cracking tendency of the base alloy. The low heat input and compressive stresses in our disclosure are beneficial compared to LPW process, particularly for the components made of polycrystalline base-alloys .

Claims

Method to produce a metallic coating (7, 7' , 7' ' , 7' ' ' ) with secondary particles (19) , especially abrasive ceramic particles (19) , wherein the secondary particles (19) are injected from outside into the spray (10) of a HVOF gun (13) using a separate supply (16) . Method according to claim 1, wherein the secondary particles (19) are applied at distance (22) of 50% to 75% of a distance of spray gun (13) to substrate ( 4 ) . Method according to one of the claims 1 or 2, wherein cBN, SiC, TaC, VC, B4C Si3N4, AIN, A12O3, ZrO2, zirconia-toughened alumina (ZTA) or mixtures of them are used for the secondary particles (19) . Method according to any of the claims 1, 2 or 3, wherein the secondary particles (19) have a particle size between 50pm - 250pm. Method according to any of the claims 1, 2, 3 or 4, wherein a buffer layer is applied to the substrate (4) without secondary particles (19) . Method according to any of the claims 1, 2, 3, 4 or 5, wherein a graded structure of abrasive particles (19) are generated. Method according to any of the claims 1, 2, 3, 4, 5 or6, wherein the particles (19) protrude an outer surface (28) of the coating (7, 7 ' , 7 ' ' , 7 ' ' ' ) , especially by increasing the supply of abrasive particles (19) by the supply (16) .

8. Tool (1) for performing a method according to any of the claims 1, 2, 3, 4, 5, 6 or 7, which comprises a HVOF spray gun (13) a separate supply (16) for secondary particles (19) outside the area of the spray (10) , which (16) is especially attached to the spray gun (13) .

9. Tool according to claim 8, wherein the length of the supply (16) can be adjusted.

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