PVD bond coat

Abstract

A superalloy workpiece includes a superalloy substrate and an interface layer (IF-1) of essentially the same superalloy composition directly on a surface of the superalloy substrate. A transition layer (TL) of essentially the same superalloy and superalloy oxides or a different metal composition and different metal oxides is on the interface layer (IF-1). The oxygen content of the transition layer increases from the interface layer (IF-1) towards a barrier layer (IF-2) of super alloy oxides or of different metal oxides.

Claims

1. A superalloy workpiece comprising a superalloy substrate and an interface layer (IF-1) of essentially the same superalloy composition as the superalloy substrate directly on a surface of the superalloy substrate, followed by a transition layer (TL) of essentially the same superalloy composition as the superalloy substrate and superalloy oxides from that superalloy composition whereby oxygen content of the transition layer is increasing from interface layer (IF-1) towards a barrier layer (IF-2) of super alloy oxides wherein the interface layer (IF-1) has a crystal structure which is coherent to a crystal structure of the surface of the superalloy substrate.

2. The superalloy workpiece according to claim 1, wherein the oxygen content in the transition layer increases stepwise or gradually from the interface layer (IF-1) to the barrier layer (IF-2).

3. The superalloy workpiece according to claim 1, wherein a concentration of at least one metallic element in the transition layer increases stepwise or gradually from the interface layer (IF-1) to the barrier layer (IF-2).

4. The superalloy workpiece according to claim 1, comprising a ceramic top layer on the surface of the barrier layer (IF-2).

5. A superalloy workpiece comprising a superalloy substrate and an interface layer (IF-1) of essentially the same superalloy composition as the superalloy substrate directly on a surface of the superalloy substrate, followed by a transition layer (TL) of a different metal composition and different metal oxides from the different metal composition whereby oxygen content of the transition layer is increasing from the interface layer (IF-1) towards a barrier layer (IF-2) of the different metal oxides wherein the different metal composition differs from essentially the same superalloy composition by at least a concentration of at least one element or an addition of at least one further element, wherein the interface layer (IF-1) has a crystal structure which is coherent to a crystal structure of the surface of the superalloy (SA) substrate.

6. The superalloy workpiece according to claim 5 wherein the at least one further element has an electronegativity of equal to or smaller than 1.4.

7. The superalloy workpiece according to claim 5 wherein the at least one further element comprises a lanthanide.

8. The superalloy workpiece according to claim 7 wherein the lanthanide comprises at least one of La, Er, or Yb.

9. The superalloy workpiece according to claim 5, wherein when the different metal composition differs by the addition of at least one further element, the different metal composition differs from the superalloy composition by an addition of at least one of the following further elements: Mg, Al, Cr, Er, Y, Zr, La, Hf or Si.

10. The superalloy workpiece according to claim 9, wherein a concentration of at least one metallic element or silicon in the transition layer increases stepwise or gradually from the interface layer (IF-1) to the barrier layer (IF-2).

11. The superalloy workpiece according to claim 5, wherein the different metal oxides comprise at least one of the following oxides or a mixture thereof: aluminumoxide, aluminum-chromiumoxide, erbiumoxide, yttriumoxide, yttrium-aluminumoxide, magnesium-aluminumoxide, aluminum-siliciumoxide or hafnium-siliciumoxide.

12. The superalloy workpiece according to claim 11, wherein the different metal oxides comprise at least one of the following oxides in the respectively given crystal structure or a mixture thereof: Al.sub.2O.sub.3 comprising a corundum crystal structure, (AlCr).sub.2O.sub.3 comprising a corundum crystal structure, Er.sub.2O.sub.3 comprising a cubic crystal structure, or Y.sub.2O.sub.3 comprising a cubic crystal structure.

13. The superalloy workpiece according to claim 12, wherein more than 55% by volume of the respective crystal structure are corundum crystal structure for Al.sub.2O.sub.3 and/or (AlCr).sub.2O.sub.3.

14. The superalloy workpiece according to claim 12, wherein more than 55% by volume of the respective crystal structure are cubic crystal structure for Er.sub.2O.sub.3 and/or Y.sub.2O.sub.3.

15. The superalloy workpiece according to claim 5, wherein the different metal oxides comprise an aluminum-containing oxide and the transition layer and/or the barrier layer comprises aluminum containing-droplets.

16. The superalloy workpiece according to claim 5, wherein the different metal oxides comprise a chromium-containing oxide and the transition layer and/or the barrier layer comprises chromium containing-droplets.

17. The superalloy workpiece according to claim 5, comprising a ceramic top layer on the surface of the barrier layer (IF-2).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the following the invention is described in an exemplary way with the help of experimental details and figures.

(2) FIGS. 1 to 9 show the following:

(3) FIG. 1 Layer concept and example of a bond coat;

(4) FIG. 2 XRD pattern of a virgin and an operated target;

(5) FIGS. 3A-3B Micrograph and EBSD of the SA-T surface;

(6) FIGS. 4A-4B TEM images of the SA-T surface;

(7) FIGS. 5A-5H EDX mapping

(8) FIGS. 6A-6C Bright- and dark-field micrographs, line scan;

(9) FIG. 7 XRD analog to FIG. 2 on sapphire

(10) FIGS. 8A-8C Layer stack: STEM bright-field, TKD, quality map;

(11) FIG. 9 TEM micrograph interface

(12) With the present invention a layer concept is introduced which is sketched in FIG. 1a. The approach is based on the formation of a “substrate-identical” interface layer (IF-1) to the bulk superalloy substrate (SA-S) and a subsequent transition layer (graded-layer) from IF-1 to a partially or fully oxidized coating ending in a second interface layer, here also barrier layer (IF-2). This IF-2 may be an oxygen diffusion barrier and/or a nucleation layer for a porous oxide as it is utilized in the design of a TBC. It could also be an ODS coating or a mixture of oxides which are formed during the oxidation of the superalloy vapour. The whole layer stack is synthesized in one process under vacuum conditions typical for Physical Vapour Deposition (PVD). Non-reactive and reactive arc evaporation is utilized to produce this coating design by in-situ processing.

(13) An example of a basic bond coat on a polycrystalline superalloy is shown in FIG. 1b, comprising an interface very similar or even identical to the superalloy basis, a transition layer which is graded with reference to the oxygen concentration, which means that the oxygen content increases from the interface to the barrier layer which is an oxidized superalloy according to the present example.

(14) The substrates as well as the targets were produced from powders with the chemical composition listed in Table 1, 2.sup.nd column. This composition corresponds to the specification of the superalloy PWA1483. However, the substrates as well as the targets were fabricated by spark plasma sintering at approximately 1200° C. and 30 MPa (PLANSEE Composite Materials GmbH). Therefore, it is likely that this material differs from the industrially utilized bulk material produced by melting and casting. In this regard it is important to remark that: the average grain size in structure is smaller than 50 μm and preferably smaller than 20 μm, the powder-metallurgical production preferably starts from alloyed powders instead from a mixture of elemental powders, the synthesis of the phases thereby takes place during the manufacture of the powders and not during the SPS process, such manufactured targets have no texture, i.e. they are characterized by random grain orientation (e.g. measured by EBSD), which is very different from targets manufactured by melting-metallurgy, the porosity in structures produced by means of SPS processes is adjusted to be smaller than 10% or preferably lower than 5%, SPS processes are conducted without involving the formation of liquid phases in temperature ranges of 1000 to 1350° C., preferably in temperature ranges of 1100 to 1300° C.

(15) Considering this, we will further denominate this material as superalloy substrate (SA-S), if it is utilized as substrate, and superalloy target (SA-T), if it is used as target for the evaporation. Small discs (∅ 60 mm) were produced from this material and machined to the size of (30 mm×10 mm×5 mm) for the SA-S. In identical processes, the SA-T discs (∅ 150 mm) were fabricated.

(16) Table 2 lists the main process parameters utilized in the cathodic arc evaporation using the SA-T as cathodes in the examples discussed in the following. Before deposition, the process chamber was evacuated below 0.02 Pa and standard heating and etching steps were performed to ensure a sufficient coating adhesion to the substrate. A net deposition time of 45 min was chosen for the non-reactive process (metallic vapour only) and was increased to 240 min for the reactive processes in oxygen. This is due to the reduced evaporation rate of the SA-T in pure oxygen reactive gas, resulting in coating thicknesses of 1.5 μm (reactive) and 2.2 μm (non-reactive), respectively. The cathodes were operated with DC arc currents of 140 A, either in metallic vapour only, or with a gas flow of 800 sccm oxygen (reactive processes) using an INNOVA batch-type production system of Oerlikon Surface Solutions AG. SA-S together with sapphire substrates were coated at substrate temperatures of approximately 550° C. Only one arc source was utilized for deposition. A symmetric bipolar bias voltage of 40 V with a frequency of 25 kHz and a negative pulse length of 36 μs and 4 μs positive pulse length was applied to the substrate during processing in oxygen.

(17) The target surface was analyzed in a LEO 1530 scanning electron microscope (SEM). The chemical compositions of the SA-T and SA-S were measured by energy-dispersive X-ray spectroscopy (EDX) in the SEM.

(18) XRD measurements on polished slices of the polycrystalline target material were performed on a Bruker D8 Davinci diffractometer equipped with a Göbel mirror for the generation of a parallel beam and with a LynxEye 1D detector using Cu—Kα radiation. The measurements were carried out in 2θ/ω mode between 5-140°. For phase analysis, the software Diffrac.Eva V4.1 from Bruker was used in combination with the crystal open database (COD), an open-access collection of crystal structures published in the Journal of Applied Crystallography 42 (2009) 726-729.

(19) Conventional electron backscatter diffraction (EBSD) analyses were performed on the SA-T surfaces in a dual FIB FEG-SEM Lyra3 from Tescan, using a Digiview IV EDAX camera. An acceleration voltage of 20 kV and an emission current of 5 nA were used. Furthermore, Transmission-EBSD or Transmission Kikuchi Diffraction (TKD) was done on lift-out specimens of about 100 nm thickness, mounted on a holder with a pre-tilt angle of 20° to the pole piece with 3 mm working distance. Beam conditions were 30 kV and 5 nA. The chemical segregation was analysed by means of ion channeling contrast imaging which was performed using 30 kV and 1.5 pA Ga ions. The lift-out lamellae were finally analysed by transmission electron microscopy (TEM) in a JEOL JEM 2200fs equipped with an EDAX EDS system.

Analysis of Virgin Target (Cathode)

(20) The chemical composition of the SA-T manufactured by spark plasma sintering was investigated by EDX. Due to the large number of elements to be analyzed and their different sensitivity for this method, a quantitative analysis is difficult. However, the similarity in the materials allow (apart from C) a qualitative comparison. Table 1 shows the results for the as manufactured virgin surface of the manufactured target with numbers in relation to the total element composition in 3.sup.rd and difference (Δ) numbers to the powder composition in the 4.sup.th column. Except for carbon and tantalum, there is a fair agreement in composition with the original powder. The crystal structure of the virgin target surface obtained by XRD analysis was compared with the target surface after arc operation in non-reactive processes. The 2θ/ω scans are shown in FIG. 2.

(21) The XRD pattern of a virgin target (dotted line) shows several main peaks which can be indexed as fcc cubic (Fm-3m) with a=3.59 Å. The diffraction pattern which is observed for various elements from which the superalloy is composed of (Table 1) matches this cubic lattice. In addition to the individual elements, a multitude of different intermetallic compounds like Cr.sub.2Ni.sub.3, Al.sub.2.6Ni.sub.10.7Ta.sub.0.7, Ni.sub.0.9Ta.sub.0.1, Ni.sub.17W.sub.3, Co.sub.0.87W.sub.0.13, Ni.sub.3.28Ti.sub.0.72, Ni.sub.0.85W.sub.0.15 or CrNi can be indexed and may be considered as potential candidates for the observed fcc phase. Peaks with intensities below 1% are also visible in the XRD pattern of the virgin target surface. They may belong to the XRD pattern of tantalum oxide phases which form as a result of surface oxidation. Peaks of the XRD pattern revealed for the operated target (continuous line) a similar fcc cubic (Fm-3m) phase as observed for the virgin target surface. The peaks of the operated target are however slightly shifted towards higher angles indicating a decrease of the unit cell parameter a from 3.59 Å for the virgin target to 3.58 Å for the operated target. At the same time the peaks of the operated target are narrower than those of the virgin target which may be due to recrystallization processes on the target surface and consequently the formation of larger crystallites. The supposition of the presence of different intermetallic compounds from the X-ray diffraction analysis is in agreement with the results of the TEM measurements. They confirm that these superalloy materials are indeed composed of different intermetallic compounds (see below).

(22) A micrograph of the SA-T surface obtained from SEM with backscattered electrons using 20 kV beam voltage is displayed in FIG. 3a. The contrast in the backscattered image is mainly due to grain orientation. This was verified by a corresponding EBSD crystal orientation map of the investigated surface which is shown in a black and white (bw) version with FIG. 3b. The EBSD analysis indicates 88% high angle and 12% low angle grain boundaries and 7% Σ3 twin (60°@(111)) boundaries with an average grain size of (5.9±3.1) μm. The white spots in the observed backscattered image of FIG. 3a were identified in the TEM as precipitates rich in titanium and tantalum. An enlarged section with different grains is shown in the bright-field and dark-field scanning transmission electron microscopy images in FIGS. 4a and b, respectively. An EDX mapping of this detail is given in FIG. 5. This mapping indicates that Cr (sub-FIG. 5b), Co (FIG. 5c) and Mo (FIG. 5g) are segregating together, also within the grains. The same holds for Ni (FIG. 5a), Al (FIG. 5h), Ti (FIG. 5e) and Ta (FIG. 5d). In addition, the mapping suggests that the precipitates consist mainly of Ta and Ti.

(23) As mentioned earlier, the XRD pattern obtained from the surface of as manufactured and operated targets can be indexed with fcc phases for which different intermetallic compounds may be potential candidates (FIG. 2). This assumption is supported by STEM investigations, where chemical segregation was observed within and between the grains. FIG. 6 shows as examples bright-field (6a) and dark-field (6b) micrographs for the transitions across two grain boundaries. The arrow in FIG. 6a indicates the position for which the EDX line scan shown in FIG. 6c was performed. The qualitative distribution of only the predominant elements is plotted and it changes significantly between the two investigated grains. Segregation of Ni/Al and Co/Cr is observable, which is in good agreement with the mapping shown in FIG. 5. This was the case for many similar line scans, which indicate the presence of more than one fcc phase with very similar lattice constants.

(24) The analysis of the target indicates that the spark plasma sintering process produces a target material with polycrystalline structure of nearly random grain orientation. In addition, the analysis proves the presence of different intermetallic phases with similar lattice constants and the existence of precipitates in the produced material.

Analysis of Operated Targets

(25) In a next step, the as manufactured targets were utilized as cathodes and evaporated by arc. The evaporation was performed under the conditions mentioned in Table 2. In the non-reactive process, no additional gases were utilized during evaporation. This approach relinquishes of the possible reduced incorporation of droplets in the deposited coatings due to multiple scattering with gas atoms, however, it allows to maintain the higher degree of ionization and the higher kinetic energy of the metallic vapour supporting coating condensation at higher energy. The reactive process was performed in oxygen only. The value of oxygen flow was chosen to ensure an oxygen to evaporated metal atom ratio of about 4 to 5 to produce the IF-2 (oxidized super alloy layer) which should result in a nearly full oxidation of the coating. The chemical compositions of the targets after non-reactive process A and reactive process B were measured by EDX and are given in Table 1 together with the difference (Δ) to the original powder composition (5.sup.th to 8.sup.th column). The analysis of the target surface indicates a slight reduction in Al and Cr from non-reactive to reactive process, but no drastic change in the composition for the other target elements. The XRD pattern of the target surface after arc operation in non-reactive mode is given in FIG. 2 (continuous line). Compared with the virgin target (dotted line), peaks of the operated target are narrower and shifted towards higher angles. They can as well be assigned to a fcc cubic phase (Fm-3m). The average unit cell of the operated target is slightly smaller, and the lattice parameter decreases from 3.584 Å (before operation) to 3.568 Å (after operation) and the reduced full width at half maximum (FWHM) indicates recrystallization processes on the target surface.

Coating Synthesis

(26) Coatings were synthesized with the parameters of process A given in Table 2 by non-reactive processing to investigate if the chemical composition of the target can also be maintained in the coating. The composition obtained by EDX is displayed in Table 3, in both cases coating A has the composition of the interface layer (IF-1). Except for C, for which EDX is not sensitive and accurate enough, the analysis indicates only for Al concentration and, to some extent, for Ti concentration a reduction in the coating. Initial XRD analysis of the coatings on the SA-S substrate was performed. As coating and SA-S have very similar lattice constants, the observed Bragg reflections could not be assigned unambiguously to the coating. Therefore, the measurements have been repeated for coatings on sapphire substrates (FIG. 7).

(27) The first of the two observed phases, denoted as M-1 with a=3.60 Å, (black lines, left side of the peaks), is nearly identical with the phase of the uncoated SA-S (a=3.59 Å) (FIG. 7). Reflections of the second phase M-2 (grey lines, right side of the peaks), are shifted towards higher 2θ angles (a=3.56 Å). This indicates that the nucleation behaviour on the sapphire substrate is slightly different. The lattice constant of the phase M-2 has been determined to be approximately 3.56 Å. The TEM investigations of the target (and substrate) material already indicated more than one intermetallic phase and EDX mapping showed that there are at least two groups of elements in addition to precipitates which are segregating together. It is likely that these two groups condensate at different temperatures which results in this phase separation.

(28) In additional experiments, a complete stack of layers was investigated according to process B. After initial pre-treatment of the SA-S as described above, the IF-1 was formed by arc evaporation in non-reactive mode and without additional interfaces at the SA-S with a thickness of about 500 nm. In subsequent steps, 800 sccm oxygen was fed to the arc evaporation process and a short transition from the non-reactive to the reactive mode was performed. Together with the double rotation of the substrate, this results in a multilayer structure and finally in the nucleation of an oxide coating of about 1.5 μm. A STEM bright-field image of the complete layer stack is shown in FIG. 8a. The interface between the substrate and the interface layer IF-1 is indicated by a dashed line in FIGS. 8b and c. The interface has been investigated in greater detail by TKD in FIG. 8c and a corresponding image quality map in FIG. 8b, here in black and white. Orientation mapping indicated epitaxial growth at grains in the region of IF-1 followed by the nucleation of many and very small grains with arbitrary orientation and finally the growth of larger grains nucleating at the finer grains of this transition region and forming the oxidized region of the layer stack. A high resolution (HR)-TEM micrograph of an enlarged region of the interface is given in FIG. 9. The micrograph demonstrates that the lattice planes of the ST-A and the coating are parallel with the same distance between the planes confirming once more the epitaxial growth of the coating on the substrate.

(29) Thereby it is shown in detail the possibility to create a complete layer stack for a bond coat by cathodic arc evaporation in an in-situ process sequence, i.e. without interruption of vacuum. It was demonstrated that targets from powders nearly identical in chemical composition with a superalloy substrate can be fabricated and utilized as cathodes in arc evaporation. The targets can be operated in non-reactive and reactive deposition processes. The investigation of the target surface after processing with and without oxygen reactive gas, revealed only little influence on chemical composition and crystal structure. Coatings synthesized in non-reactive deposition mode are also similar in chemical compositions and crystal structure with respect to the targets. The approach to create a complete layer stack for the bond coat in one process, allows a design principle of grading profiles either by the controlled addition of the reactive oxygen gas or by the operation of additional targets with the same or different elemental compositions. In addition, epitaxial growth could be observed at the grains of the polycrystalline substrate at the substrate interface. The addition of oxygen to the running arc evaporation process results in a fine-grained transition region and finally a nucleation of larger crystallites in the fully oxidized region of the layer stack. The presented approach has the potential to realize epitaxial growth at arbitrary superalloy materials and to perform gradients to coatings with different chemical composition and functionality.

(30) TABLE-US-00001 TABLE 1 Powder Com- Target Composition posi- (Δ % refers to powder composition) Ele- tion as produced Process A Process B ment [wt. %] [wt. %] [Δ %] [wt. %] [Δ %] [wt. %] [Δ %] C 0.07 0.9 >10.sup.3  0.5 614 0.4 471 Al 3.6 3.8    5.6 3.1 −13.9 1.6 −55.6 Ta 5 8.2   64.0 5.2 4.0 4.6 −8.0 W 3.8 4.5   18.4 3.6 −5.3 4.6 21.1 Mo 1.9 2    5.3 1.2 −36.8 1.6 −15.8 Ti 4.1 3.8  −7.3 2.9 −29.3 2.2 −46.3 Cr 12.2 11.2  −8.2 14.2 16.4 11.8 −3.3 Co 9 8.5  −5.6 8.9 −1.1 9.6 6.7 Ni 60.33 57.1  −5.4 60.3 0.0 63.6 5.4

(31) TABLE-US-00002 TABLE 2 Arc Oxygen Deposition Substrate Process Current Flow Time Bias Coating [A] [sccm] [min] [V] Interface A 140 0 45 −40 none B 140 800 240 −40 1. Coating A (500 nm) 2. Transition in oxygen from 0 to 800 sccm within 200 nm

(32) TABLE-US-00003 TABLE 3 Powder Composition Composition Coating A EDX Element [wt. %] [wt. %] [Δ %] C 0.07 0.5 614.3 Al 3.6 1.2 −66.7 Ta 5 6.4 28.0 W 3.8 4.2 10.5 Mo 1.9 1.5 −21.1 Ti 4.1 2.8 −31.7 Cr 12.2 14 14.8 Co 9 9.3 3.3 Ni 60.33 60.1 −0.4