Coating on an article

Abstract

An article comprises a substrate comprising at least a first element, an intermediate layer on the substrate comprising a Laves phase including at least the first element and a second element different to the first element, and a protective alloy coating on the intermediate layer. The first element is preferably Nb, Ta, Mo or W, the substrate most preferably comprising at least 50 atomic percentage niobium. The second element is preferably Al, Cr or Co. The protective coating is preferably an aluminide. A method of producing the article comprises depositing a layer of the second element onto the surface of the substrate, preferably by sputtering, electroplating or chemical vapour deposition, diffusing the first element into the layer to form the Laves phase, and forming the coating, preferably by electroplating. The article may be a turbine blade.

Description

[1] The present invention relates to a coating on an article, particularly a protective coating to protect against high temperature, and a method of applying a coating to a substrate. [2] Currently a protective coating used on high performance turbine blades made of superalloy comprises either a diffusion coating of the intermetallic alloy NiAl, (Ni,Pt)Al or PtAl, or a custom alloy coating, known as MCrAlX, where M is at least one of Ni, Co, Fe and X is yttrium (Y) or another oxygen active element. In some arrangements a diffusion barrier is provided between the superalloy substrate and the high temperature protective coating . The diffusion barrier may be a precious metal, for example platinum (coverting a NiAl diffusion coating to (Pt,Ni)Al). During exposure to high temperature, aluminium in the (Pt,Ni)Al protective coating system, or within the MCrAlX alloy coating, reacts with oxygen to develop AI2O3 as a protective oxide scale. [3] In order to achieve even higher temperature performance protective coatings comprising IrAl intermetallic alloy, or ceramics (oxides and nitrides) environmental barriers have been proposed. US8,247,085 discloses a single layer, intermetallic, protective coating which includes a silicon modified Laves phase. A protective silica oxide scale is formed on this coating when exposed to a high temperature oxidising environment. [4] According to the present invention there is provided an article comprising: a substrate comprising at least a first element; an intermediate layer on the substrate comprising a Laves phase including at least the first element and a second element different to the first element; and a protective alloy coating on the intermediate layer. This has the advantage that the Laves phase acts as a diffusion barrier which can be stable at high temperature and stable with the underlying substrate. [5] In an embodiment the protective coating further comprises a third element different to the first and second elements. Thus the protective alloy layer my rely on an element different to the first and second elements. [6] In an embodiment the intermediate layer comprises the third element. In this way the third element can be used to form the Laves phase. [7] In an embodiment the third element is selected from the group comprising: Al, Si, Fe, preferably wherein the third element is Al. these elements are particularly suited to forming a protective alloy coating. In the case of Al, exposure to high temperature results in a protective oxide coating. [8] In an embodiment the protective coating comprises at least a fourth element different to the first and second elements, preferably wherein the fourth element is substantially absent from the intermediate layer. This allows free choice of the alloy composition of the protective layer so that the properties of the protective alloy layer can be tailored. [9] In an embodiment wherein the fourth element is of Ru, Rh, Pd, Os, Ir, Pt, Re. These elements are particularly suited to the task as they have high melting points and a re relatively inert. The most suitable elements for the fourth element are platinum and iridium.

[10] In an embodiment the first element is one or more selected from the group comprising: Ti, V, Cr, Zr, Mn, Co, Nb, Mo, Ru, Rh, Hf, Ta, W, Re, Os, Ir, Nd. These elements are common in the type of alloys which might benefit from the present inventive coating and which are known to form Laves phases which particularly suitable properties in as the intermediate layer. Nb, Ta, W, Mo, Ti, Hf, Zr and optionally V and Cr are particularly suited to the task, with Nb, Ta, Mo and W being the most optimal for the first element.

[11] In an embodiment the substrate comprises an alloy containing niobium, preferably wherein the substrate comprises at least 50at% niobium. Such substrates have been shown to benefit in particular from the intermediate layer and protective alloy coating. For example the substrate is comprised of FS-85, C-103, Cb521, Cb752, a RCCA, or a RHEA.

[12] In an embodiment the second element is one or more selected from the group comprising: Cr, Co, Al, Hf, Zr, V, Si, Fe, Ni, Mn. These elements are particularly suited as the second element because they form suitable Laves phases. Amongst those Al, Cr and Co are the most preferred.

[13] In an embodiment the intermediate layer comprises a C14 Laves phase. This type of Laves phase is particularly suitable to the task as it has a high melting point and good elevated temperature stability, relatively low required thickness to act as an effective diffusion barrier and can be produced in cost effective ways.

[14] In an embodiment the protective coating has a BCC (B2) aluminide structure. Such a structure is particularly suited as it can have a high temperature thermal stability and hold large amounts of aluminium to provide aluminium for formation of a protective oxide layer.

[15] In an embodiment the article further comprises an oxide layer formed on the protective alloy coating, preferably wherein the oxide is an oxide of an element present in the protective layer. This is advantageous as the oxide layer is self-healing in that it regenerates should it get removed.

[16] In an embodiment the intermediate layer comprises less than lat% Si. Addition of small amounts of Si can promote Laves phase stability in few alloy systems and potentially improve on oxidation performance by promoting adhesion of oxide scale, however, over doping of Si can cause formation of NbxSiy phase during aluminising.

[17] In an embodiment the present invention provides a method of applying a protective coating to a substrate, the method comprising the steps of: providing a substrate comprising at least a first element; depositing a layer comprising a second element on a surface of the substrate, wherein the second element is different to the first element; diffusing the first element into the layer by heat treating the substrate with deposited layer to form a Laves phase including the first element and the second element; and forming a coating on the Laves phase. This method has the advantage of providing a way of producing a coating on a substrate which is a Laves phase.

[18] In an embodiment one or more of the depositing, diffusing and forming steps are performed using a non-line-of-sight technique. This has the advantage of allowing complex shaped articles to be covered reliably.

[19] In an embodiment the method further comprises introducing a third element, different to the first and second elements, into the coating. This allows the coating on the Laves phase to be an alloy which can be chosen to be a suitable protective coating.

[20] In an embodiment the introducing comprises a non-line-of-sight technique, optionally chemical vapour deposition. This has the advantage of allowing complex shaped articles to be covered reliably.

[21] In an embodiment the introducing introduces the third element into the Laves phase. In this embodiment the Laves phase may only be produced when the third element is so introduced.

[22] In an embodiment the introducing comprises aluminizing. This technique is well understood and therefore beneficial to use.

[23] In an embodiment the depositing comprises sputtering or electroplating or CVD. These methods are well understood meaning reliable manufacturing is possible. 3

[24] In an embodiment the forming comprises electroplating. This technique reliably forms coatings and is well understood.

[25] In an embodiment the forming the coating comprises forming a coating of at least a fourth element, wherein the fourth element is different to the first and second elements. The fourth element can be chosen to provide the coating on the Laves phase with suitable properties to act as a protective coating.

[26] The term “consisting of’ is used herein to indicate that 100% of the composition is being referred to and the presence of additional components is excluded so that percentages add up to 100%. Unless otherwise stated, all amounts are in at.%.

[27] The invention will be more fully described, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows the melting points of platinum group metals (PGMs) and their XA1 intermetallic compounds. Figure 2 shows as a bar chart and as a graph mass change of IrAl and Ir after oxidation for 3 h at different temperatures. Figure 3 shows a cross-sectional microstructure of an oxidized IrAl sample at 1600°C from literature. Figure 4 shows an element profile between a Pt protective coating and FS85 after 100 h annealing at 1200 °C. Figure 5 shows relative frequencies of occurrence for the most common intermetallic phase (IM) structures in all binary / temary system. Figure 6 shows a summary of reported Intermetallic (IM) phases in high entropy alloys (HEA). Figure 7 shows CALPHAD calculation of vertical section of Nb-Mn-Co ternary phase diagram at fix condition Nb=33.1at% (left) and heat map of C14 phase fraction as a function of temperature and Mn content (right). Figure 8 shows isothermal ternary phase diagram of Nb-Co-Cr system at 1000 °C. Figure 9 shows isothermal ternary phase diagram of Nb-Cr-Al system at 1450 °C. Figure 10 shows a design of a multi-layer coating system. Figure 11 shows the overall process flow design, based on coating structure. Figure 12 shows examples of sputtered coating structures using different sputtering parameters and targets. Figure 13 shows the Co-Nb-Cr phase diagram and a heat treatment design. Figure 14a shows a few examples of heat treatment trial, and optimized condition to transform the deposited layers of figure 12 to Laves phase. Figure 14b shows the microstructure of Laves phase on FS85 after heat treatment. Figure 15 shows Laves phase identified by Kikuchi pattern. Figure 16 shows electroplating Iridium on FS85 showing good quality. Figure 17 shows two Ir electroplating runs, showing repeatable deposition rate. Figure 18 shows transferring plating parameter from coupon to component. Figure 19 shows a design support frame for throat section of thruster. Figure 20 shows a homogeneous Ir layer plated on the inner surface of a throat section of thrustor. Figure 21 shows schematically an aluminizing process. Figure 22 shows a standard aluminizing process. Figure 23 shows a few examples of aluminizing trails. Figure 24 shows IrAl-BCC phase identified by XRD peaks.

[28] The present invention is described below with reference to a substrate comprised of niobium or a niobium alloy (i.e. an alloy containing niobium in an amount of at least 50 at%). However, the principles described below are applicable to other alloy systems and in particular alloys comprising at least 50 at% titanium, vanadium, chromium, zirconium, niobium, molybdenum, ruthenium, rhenium, hafnium, tantalum, tungsten, osmium, iridium, manganese, silicon, iron and cobalt.

[29] In particular, the present invention has been developed for use on substrates comprised of niobium (aka columbium) rich alloys such as refractory complex concentrated alloys (RCCAs), high entropy alloys (HEA) and niobium alloys, particularly FS-85, C103, C-127Y, Cb752, Cb-lZr, Cb-129, PWC-11.

[30] RCCAs typically contain three or more principle elements. Examples are (Nb,Ti,Zr), (Mo,Nb,Ti), (Nb,Ti,V), (Mo,Nb,Ti,Zr), (Mo,Nb,Ti,V), (Nb,Ti,V,Zr), HfNbTaZr, MoNbTaW, HfNbTaTiZr,, MoNbTaVW, and AlMoo AbTao HiZr. RCCAs are typically substantially single phase alloys in which the majority of the elements are held in solid solution, particularly high entropy alloy versions

[31] Nb and Nb alloys (as well as other refractory alloys, particularly Ta and Ta alloys) are well-known for their pesting oxidation at elevated temperature, where internal oxidation occurs in a catastrophic manner destroying the structural integrity. It is due to the fact that there is no intrinsic oxidation resistance from Nb alloys, where the NbzOs oxide scale with a 2.69 high Pilling-Bedworth ratio grows rapidly and then cracks, and its growth rate is few orders of magnitude higher than NiO. Therefore, it is highly desirable to provide a reliable coating system for Nb components at elevated temperatures in an oxidizing environment.

[32] Because service temperature demand for turbines is increasing, a good coating system is required to protect Nb components from oxidation degradation during service. In the present invention both coating composition, system design and process design are thoroughly considered. These two factors are deeply interrelated.

[33] The present inventors have noted the ability of aluminium based coatings successfully to protect substrates due to the formation of AhO? Ideally, a dense protective alpha- AI2O3 scale forms on the coating surface during thermal exposure, and this growth of AI2O3 oxide depletes Al from the underlayer coating. A BCC-aluminide structure is particularly well suited to a protective coating due to its high Al composition level, which serves as Al reservoir and its high temperature thermal stability. Thus in one embodiment a BCC-aluminide structure is used as the protective coating. The present inventors have also identified precious metal group metals with high melting as promising candidates, the melting points of pure platinum group metals (PGMs) metals and their corresponding XA1-BCC compounds are summarized in Figure 1. Ir and IrAl show the highest melting points in the table, except for Osmium, but Osmium oxide is toxic, so should not be used for a high temperature coating system where volatile Osmium oxide may form in service. As an alternative, a Re based protective coating has also been postulated because of the similarity between Re and the platinum group metals. Thus, the protective coating is a protective alloy coating i.e. a mixture of at least two metallic elements with a maximum of 2 at% non-metallic elements. Therefore the protective alloy coating is distinguished from an oxide layer which might be formed by oxidation of one or more components of the protective alloy coating.

[34] Iridium itself can serve as a good oxygen diffusion barrier for Rhenium based thrusters due to low oxygen permeability. However, the fast evaporation rate of pure Ir, as shown in Figure 2 (from Zhu, L. et al., High-Temperature Oxidation Behavior of a SingleLayer IrAl Intermetallic Coating. Oxidation of Metals, 91; (2019);749-766), and interdiffusion issues demand over 50-micron pure Ir thickness. However, a thick Ir layer can only be produced by electroplating at elevated temperature with good coating adherence. In terms of hot electroplating method, the infrastructure cost is high, and the electrolytes often includes toxic chemicals, such as Cyanide.

[35] Nonetheless, by formation of a continuous AI2O3 scale, IrAl, plus its attentive oxide, acts as diffusion barrier for oxygen penetration and suppresses evaporation of Ir, as shown in Figure 3 (from Zhu, L. et al., High-Temperature Oxidation Behavior of a SingleLayer IrAl Intermetallic Coating. Oxidation of Metals, 91; (2019);749-766). Therefore IrAl-BCC can be a suitable candidate for a protective coating, provided it is coupled with a suitable refractory metal diffusion barrier (the C14 Laves phase in this patent) for the following reasons: 1) high melting point and good elevated temperature stability, 2) required thickness is significantly less than pure Ir layer, 3) a cost-effective method can be used to deposit Ir from process design. Pure Ir coatings provide protection by ablation, and modifying the coating to include aluminum to form an IrAl coating results in a protective AI2O3 coating during use at elevated temperatures in an oxidating environment. IrAl coating is ‘self-healing’: AI2O3 will form in cracks to suppress oxygen ingress. IrAl is capable of operating at 1800 °C or greater, potentially even up to 2000 °C.

[36] For elevated temperature applications, interdiffusion between the protective coating and substrate is often a key degradation mechanism for long term service. This implies that a pure Pt group metal cannot be used. A coating like Iridium, in the platinum group metals PGM family, is a good example of a coating with fast diffusion between coating and substrate. Diffusion couple experiments are used to test diffusion behaviour between pure Pt and FS-85 at 1200 °C for 100 h, with the results shown in the element profile measured by EDS in Figure 4. As can be seen, Nb diffuses from substrate and penetrates through the 100-micron thick Pt layer forming NbPt2 Once Nb reaches the Pt surface, pesting oxidation of Nb causes blistering of coating layer which damages to the integrity of the entire system. Similar issue also occurs to Ir and FS-85 system.

[37] Therefore, the present inventors have determined that a diffusion barrier is required to prevent premature failure of the coating system due to interdiffusion between coating and substrate especially at elevated temperatures, as diffusion is promoted by increasing temperature. Therefore, the present inventors have sought to manufacture a suitable diffusion barrier which can be used in conjunction with a platinum group metal / Al protective coating. Although the invention is described with reference to aluminium and PGM / Re containing protective coatings, the same principles can be applied to protective coatings containing other elements, particularly those containing silicon and / or iron, for example in the form of silicide coatings, such as R512E which is comprised of about 60 wt.% silicon, 20 wt.% chromium and 20 wt.% iron.

[38] Because multi-alloying elements are included in the combination of the substrate and the protective coating, the present inventors considered that a stable phase with good compatibility with the principle elements (in the example Nb and Ir or Pt (depending on the protective layer) might work well in the diffusion barrier. To select a stable and functional phase as a diffusion barrier, a review of the phase structures in multiple binary / temary systems was performed. The relative frequencies of occurrence for most common intermetallic (IM) phases are listed in Figure 5.

[39] The present inventors had the idea to design a diffusion barrier based on the most common phase, i.e., a phase with higher compatibility to complicated alloying systems. The A2B Laves phase was selected as the most formed phase in all binary / temary alloy systems. Among Laves phases, three crystal structures appear, normally cubic Cl5, hexagonal C14 and hexagonal C36. Laves phase as a TCP phase has a high packing density, chemical stability and within its stoichiometry the sublattice site can be engineered with compatible elements. Additionally, Laves phase is also the most frequent phase in high entropy alloy design as shown in Figure 6, where all reported IM phases in HEA are counted. This indicates that a broad design space can be expected for Laves phase.

[40] The Laves phase was also considered from the point of view of appropriate elements from the substrate and a suitable coating composition system, high temperature stability, and phase compatibility to the coating and substrate. For example, the most promising elements are listed in Table 1, including their preferential sublattice site in the A2B Laves structure, based on the chemistry of FS-85 (Nb28TalOWlZr) and IrAl.

[41] It can be seen that the Laves phase of a diffusion barrier can include a first element from the substrate (e.g. Nb or Ta for FS85 by way of example) and a second element which is not necessarily in the substrate or in the protective coating (e.g. Cr). The Laves phase may include more than one element from the substrate, for example in the case of FS85, any or all of Nb, Ta and W could be present in the Laves phase. The protective coating has a third element, which is optionally also in the Laves phase (e.g. Al). The presence of the third element, in the protective coating and in the Laves phase, may result in a better bond between the protective coating and the Laves phase in the case that a coherent bonded structure is achieved. A fourth element is present in the protective phase which may not be present in the Laves phase (e.g. Ir or Pt or another PGM), to permit the formation of a high temperature intermetallic alloy that forms a protective alumina oxide scale under high temperature oxidising environment, in one embodiment a protective alpha-alumina surface oxide scale. Table 1 Candidate elements in sublattice site of A2B structure of laves phase. B site A site Nb Cr Ta Co W Ai Mo Ti Hf Zr Mn

[42] The inventors have investigated potential systems implementing the idea of using a Laves phase as a diffusion barrier between a substrate and a protective coating.

[43] The (Mn,X)2Nb system illustrates the invention with reference to the ternary system Nb-Mn-Co as shown in Figure 7. The global minimum melting point corresponding to 29.3 at% of Mn (balanced by Co) at 1269 °C. It is a good candidate system for target temperature below melting point because a single-phase region for Laves C14 extends over a wide range of Mn concentrations.

[44] The (Al,Cr,Co)2(Nb,Ta,Mo,W) system is a suitable system. With each sublattice site defined, A site elements of the A2B laves phase are from coating system (the second and third elements) and B site elements (the first element) from substrate. Ideally, for the A site, both Aluminum and Chromium are beneficial for coating oxidation performance.

[45] From a process aspect, Aluminum can be introduced by the aluminizing process, Cobalt from electroplating and Chromium from either CVD or the electroplating process.

[46] The advantage of this system is that full range solubility of Al, Cr and Co in A site at elevated temperature, as shown in Figure 8, meaning the phase composition can be quite flexible and good stability of phase structure against composition change from degradation during service is achieved. Because both Al and Cr are beneficial for oxidation performance of the coating system, such a design can act as Al reservoir to prolong coating service life. Cr on the other hand can enhance high temperature phase stability. The ternary phase diagram for the Nb-Cr-Al phase diagram (Figure 9) shows good stability of Al in the NbCr2 phase which is beneficial to allow for aluminization. The isopleth diagram for NbCr2 with aluminum additions shows that by increasing Al content, C14 phase can be stabilized at full temperature range although C14 melting point also decreases with higher Al level as trade-off.

[47] To summarize the coating system structure, the coating system, as shown in Figure 10, includes: • Top: Protecting coating layer: preferably a IrAl coating which provides oxidation resistance by forming an oxide layer such as AI2O3 scale (not shown) • Intermediate Laves layer: preferably (Al,Cr,Co)2Nb Laves as a diffusion barrier layer to suppress interdiffusion between the protective coating and the substrate • Substrate, which may be a refractory alloy substrate, for instance comprising Nb and / or Ta

[48] Therefore, an article created using this system comprises a substrate comprising at least a first element (e.g. Nb, Ta), an intermediate layer on the substrate which is comprised of a Laves phase which includes at least the first element and a second element (e.g. Cr) which is different to the first element. A protective coating is then positioned on the intermediate layer. The intermediate coating may comprise at least a third element (e.g. Al) which is different to the first and second elements. The third element is also present in the protective coating and may be diffused from the environmental protective coating. A fourth element is present in the protective coating (e.g. Ir or a different Pt group metal or Re).

[49] In the example of Figure 10, the first element is niobium and is present on the B site of the Laves phase of the intermediate layer. The second element is chromium (present on the A site of the Laves phase) and the third element is aluminium. In this system, a fourth element is present in the protective coating, namely iridium. This third element may also be present in the Laves phase in an A site, possibly diffused from the environmental protective coating. For example, in the Figure 9 example, the intermediate layer could be comprised of (Cr,Al)2Nb so that the second element is chromium with the third element being aluminium (with the Cr present to stabilize the C14 Laves phase).

[50] It is predictable that other elements could comprise the first, second, third and fourth elements. This is because elements in the same group of the periodic table behave similarly and as shown in Figure 5 Laves phase of the form A2B is particularly common in binary and ternary systems. Additionally, the first, second, third and fourth elements may be more than one type of element. This is particularly the case for the first and second elements and the third element, as described above, with a high melting point, such as one or more platinum group elements (Ru, Rh, Pd, Os, Ir, Pt) or Re. Of those, iridium is most preferred for the reasons given above, however platinum is cheaper than iridium and may therefore still be of interest as the third element in the protective coating.

[51] The other element in the protective coating is preferably aluminum. However, the present invention is not limited to that and additionally other elements may be present or other elements may be present as an alternative / addition to aluminum. As explained above, the fourth element may for example be silicon and / or iron such as used in the R512E coating.

[52] The first element is preferably niobium. However, any other refractory type elements may additionally or alternatively be present. For example, the first element may be one or more selected from Ti, V, Cr, Zr, Nb, Mo, Co, Ru, Rh, Hf, Ta, W, Re, Os, Ir. Of these Nb, Ta, W, Mo, Ti, Hf, Zr and optionally V and Cr are the most preferred, with niobium, tantalum, molybdenum, hafnium and tungsten being the very most preferred as these are common elements in high-temperature refractory alloys.

[53] The second group of elements includes chromium as in the above example and this is the most preferred alloying element. However, as can be seen from Figure 8, cobalt is also a potential second element in substitution or in addition to chromium. Figure 7 shows that manganese may also be suitable in substitution to Cr and Co or in addition to one of those elements or both of them. Elements from the same group of the periodic table include aluminum, vanadium, silicon, iron, nickel, manganese, hafnium and zirconium, and these may also be suitable as the second element in addition or in substitution. Of these, chromium, cobalt (and manganese) are the most preferable, as is aluminum in the case that the third element is not aluminum. A disadvantage of silicon is that it can cause formation of NbxSiy phase during aluminising (see below) and so in an embodiment silicon in the intermediate phase is limited to 1.0at% or less.

[54] Although the present invention can be put into practice using any Laves phase, preferably the Laves phase is a C14 Laves phase because this phase has been shown to be compatible with aluminum and have stability at high temperature. Cl5 Laves phase has lower high temperature stability and is not compatible with aluminum. C36 Laves phase on the other hand is less common. C14 Laves phase has been found to occur in at least the following binary systems Cr-Nb, Co-Ta, Fe-Nb, Fe-Ti, Cr-Ti, Co-Ti and Zr-Al. C14 Laves phase can form in ternary systems that do not form a binary Laves phase. Such C14 Laves phases could also be a base for a stable diffusion barrier and are included in the present invention.

[55] As shown in Figure 11, a long process chain is designed to produce the right coating structure. The process design strategy and process optimization are explained in this section in detail.

[56] Step 1 is providing the substrate which includes the first element. As described above, the coating has been optimized for use on additively manufactured substrates, although this is not essential, and the coating is suitable for being applied to substrates formed in other ways such as casting, forging, machining, cold forming etc.. Preferably, the substrate is cleaned, for example using an organic solvent, ]ethyl alcohol or IPA.

[57] The next step 2 is to deposit a layer of the second element on the surface of the substrate. As described above, the second element is different to the first element which is present in the substrate. Although the first and second elements are different, in an embodiment the substrate also can include the second element, in addition to the first element.

[58] Because the invention is optimized for use with additively manufactured articles with a complex three dimensional shape, preferably the layer is deposited using a non-line-of-site technique. One such technique is multi-target sputtering (PVD and ionplating), a second is CVD (chemical vapor deposition) and a third is electroplating.

[59] It is preferred not to directly deposit the Laves phase but instead to use a two-step process whereby the Laves phase is formed in-situ, whereby first key elements are introduced onto the surface of the substrate (e.g. the second element and optionally the third element) before being transformed into a continuous Laves phase using a well-controlled heat treatment step, during which the first element diffuses from the substrate into the second (and third if present) element.

[60] In an embodiment a multi-target Magnetron sputtering system is used in the depositing step 2 to deposit the second element on the substrate surface and possibly the third element. There are two main advantages of the system to make a thin layer for diffusion barrier phase design: 1) free choice of elements: changing sputtering targets and sputtering multiple targets simultaneously allows in situ alloying; 2) good control of composition / element ratio by adjusting sputtering parameters of each target. Calibration is performed on each target to understand sputtering rate distribution, with tooling factors measured to define suitable sputtering distribution for each target positioning. After some trials, optimized sputtering recipes can be defined to achieve the target composition of deposited coating. A few examples are shown in figure 12 where, in this figure, the element is followed by the power and where more than one element is mentioned cosputtering occurred. As can be seen, homogeneous layers were achieved by the design of sputtering parameters, which enabled phase transformation from as-sputtered amorphous structure to ordered Laves phase with proper heat treatment design, explained next. In an embodiment the sputtering process may sputter more than one element. For example, a mixture of chromium and aluminum (or tantalum) could be sputtered.

[61] The third step 3 is a heat treatment step to develop the deposited layer to form into the Laves phase.

[62] A heat treatment study was conducted to develop a suitable heat treatment procedure to form and stabilize the Laves phase. To assist the design of heat treatment profile, the phase diagram is reviewed. An example can be illustrated by Nb-Cr-Co ternary system, as shown in Figure 13. After a few heat treatment trials as illustrated in figure 14a (which are the same tests as shown in figure 12), a dense Laves phase layer was made, as shown in the right hand side of Figure 14a. A 2 hour vacuum heat treatment of a chromium deposited layer on FS-85 produced the desired phase structure, as confirmed by XRD and EBSD as shown in figure 15.

[63] Figure 14b shows a case where trivalent Chromium electroplating was conducted to deposit a thicker Cr layer than in the examples for figure 14a. A heat treatment condition was identified to transform Cr to Laves phase. The heat treatment condition is 1400 C for 2 h.

[64] In the fourth step 4, a coating on the Laves phase is formed. For example, the coating can comprise the third element. Preferably the deposition is done by a non-line-of-site technique. In an embodiment, the technique used is electroplating. In the preferred embodiment, the third element is iridium and an iridium electroplating process will now be described.

[65] A parameter study on plating was conducted to find a suitable plating window, and factors have been optimized to achieve the best coating quality. The factors are listed below: • Selection of Ir salt for electroplating • Plating temperature window • Current density • Anode design • Concentration of Ir in electrolyte • Optimized pre-plating cleaning procedure • Agitation of electrolyte

[66] A standard recipe is defined for electroplating of Iridium on Nb and Nb alloys, details see below: Sample cleaning 1. Clean sample in ultrasonic IPA for 10 min 2. Grit blast both surfaces of the sample (220 alumina grit, 20 psi). 3. Brush sample to remove alumina grit. 4. Clean sample in ultrasonic IPA for 10 min. Prepare the solution 5. The electrolyte solution is composed of hydrogen hexachloroiridate hydrate diluted in deionized water (H2IrC16 xH20). A mass of 2.2 g of salt in 300 ml of deionized water should provide a concentration of 2.93 g of Ir / 1. [Before each plating session, the concentration of Ir in the electrolyte was measured. For the measurement of Ir in the electrolyte, a micropipette to take 3 - 4 drops of solution was used, and three measurements of Ir concentration was taken, using the XRF. An average of these three measurements was calculated.

[67] It was found that if the Ir concentration (from the average of the three measurements) is between 2.9g / l of Ir - 3.2 g / 1 of Ir, no further adjustment is needed. If the measurement falls below this range, more salt was added to the solution until this range is achieved. If the measurement falls above this range, the solution was diluted with deionized water until the measurement falls within this range.

[68] The deposition rates are approximately 1 pm / h for Ir concentrations in the range of 2.7 g of Ir / 1 - 4 g of Ir / 1 (at a current density of 0.4 A / dm2, at a temperature between 79 - 82 °C, and a pH in the range 2 - 2.2.

[69] The pH of the electrolyte solution for the plating was 2 - 2.2 (this value does not seem to change after 80 hours of plating, so does not need to be adjusted). A magnetic stirrer was placed in the beaker, and the agitation of the electrolyte was on level 1.5.

[70] The temperature of the solution was 79 - 82 °C.

[71] Once the sample has been cleaned in IP A, the mass of the sample was measured with a microbalance. A crocodile clip was attached to the sample and a black electrical heat resistant tape was used to mask an area not to be exposed to the electrolyte. A new crocodile clip (and banana plug adapter) was used at the start of each plating session.

[72] An alternative and preferred option is to spot weld a platinum wire to the top side of the sample (cathode). This is to avoid any corrosion of the crocodile clip (or banana plug holding the clip), which negatively affects the plating rate. The other end of the platinum wire will be attached to a crocodile clip (ideally outside of the beaker), which is then connected to the negative terminal of a power supply.

[73] A platinum wire was tied to an iridium oxide coated anode and the anode was submerged into the electrolyte as much as possible, without touching the magnetic stirrer or the bottom (or sides) of the beaker. The other end of the anode was attached to the crocodile clip (ideally outside of the beaker), which was then connected to the positive terminal of a power supply.

[74] The distance between anode and cathode during plating of flat coupons was kept at 4 cm, and ideally, the sample and the anode should be facing each other and positioned as vertical as possible into the solution.

[75] The current density for optimized plating rates has been determined to be between 0.35 A / dm2 - 0.45 A / dm2. Therefore, the plating of flat coupons has been set to 0.4 A / dm2.

[76] After 1 hour of plating, the mass of the sample was measured. Based on the weight change, the amount of iridium which was consumed was calculated and added to the corresponding amount of Ir salt in the solution. 15

[77] By using the standard electroplating procedure, an 8 micron Ir coating was successfully plated on Nb with good adhesion as shown in Figure 16 (a protective Ni layer was deposited before preparing the sample so as to protect the iridium coating during sample preparation) and a repeatable 1 micron / h Iridium deposition rate can be achieved as shown in Figure 17.

[78] The above describes typical parameters for Ir plating of a flat coupon surface. Now it is shown how those principles can be transferred to a situation where a complex 3-d component needs to be plated with reference to figure 18. A few modifications were designed for plating a rocket thrustor component, including designing a support frame (Figure 19) and anode design. A narrow throat section of a thrustor was used as the most representative structure for parameter transfer study. Figure 20 shows that it is possible successfully to coat the inner area of throat section.

[79] The final step 5 is to introduce the fourth element into the coating to form the protective coating. In the present example, the fourth element is aluminum. Preferably, the aluminum is introduced using a non-line-of-site technique. A suitable process has been found to comprise CVD aluminizing using the over-chip technique. Over chip means a process where the sample is placed above a source without being in contact (i.e. a CVD process) at 1000°C for one hour.

[80] The goal is to produce IrAl-BCC phase coating to provide oxidation resistance. In order to introduce Al into the coating system, CVD aluminizing process is chosen as illustrated in Figure 21. The aluminizing process was assessed at different process conditions, including pack aluminizing (where the sample is immersed in a powder mixture containing the source (aluminum) and optionally an inert filler powder) and overchip aluminizing, as shown in Figure 22. Modeling using Thermo-Calc and Factsage database were used to assist the optimization of CVD chemistry and process parameters optimization.

[81] Activity of Al source, activator and process temperature were optimized based on modeling results. CVD runs were conducted on different combinations including FS85, Ir coated FS85, Cr coupons to find the best parameters (Figure 23). Based on microstructure characterization results, after CVD, over-chip aluminising at 1000 °C proved a good solution as the IrAl-BCC phase was identified after CVD by XRD (Figure 24), whilst no low temperature intermetallic phases formed. This shows that the Al has penetrated through the Iridium and into the Laves phase (so that the third element (aluminium) is present in the protective alloy coating and in the intermediate layer Laves phase).

[82] In an alternative to sputtering the second element in Step 2 electroplating is possible, though hexavalent Cr plating is banned or strictly controlled in many EU countries and UK because hexavalent Cr can be Carcinogenic. Trivalent Cr plating is possible, and some commercial Cr plating suppliers are available in UK.

[83] Mixed precious metal aluminide coatings are also possible. For example, a (Pd, Ru)Al coating can also provide oxidation resistance. PdAl melting point is 1645 °C at 50:50, however, modification of composition is required to stabilize the high temperature phase of the system to prevent degradation. Addition of Ru to (Pd, Ru)Al is a good design as (1) RuAl shows high melting point above 1920 °C, (2) good solubility between Pd and Ru in BCC sublattice, (3) Ru is added to avoid phase transformation from B2 to 3’, as cracking of PdAl has been reported causing by the phase transformation. Therefore, (Pd,Ru)Al is one example of mixed precious metal aluminide coatings that are candidates for high temperature coating design.

[84] Alternatively, a high TRL coating developed by HITEMCO, namely R512E coating, is a suitable protective coating to provide protection to aNb base alloy. R512E coating can provide a reliable life up to 1500 °C for few hours by forming an adherent mixed-oxide layer at coating surface. In terms of the process, R512E coating can be applied by a slurry process and then heat treatment to fuse the coating and substrate and promotes various layer formation, which is theoretically compatible with the complex component geometry.

Claims

1. An article comprising:a substrate comprising at least a first element;an intermediate layer on the substrate comprising a Laves phase including at least the first element and a second element different to the first element; anda protective alloy coating on the intermediate layer.

2. The article of claim 1, wherein the protective coating further comprises a third element different to the first and second elements.

3. The article of claim 2, wherein the intermediate layer comprises the third element.

4. The article of claim 1 or 2, wherein the third element is selected from the groupcomprising: Al, Si, Fe, preferably wherein the third element is Al.

5. The article of any preceding claim, wherein the protective coating comprises at least a fourth element different to the first and second elements, preferably wherein the fourth element is substantially absent from the intermediate layer.

6. The article of claim 5, wherein the fourth element is of Ru, Rh, Pd, Os, Ir, Pt, Re.

7. The article of claim 5, wherein the third element is platinum or iridium.

8. The article of any preceding claim, wherein the first element is one or more selectedfrom the group comprising: Ti, V, Cr, Zr, Mn, Co, Nb, Mo, Ru, Rh, Hf, Ta, W, Re, Os, Ir, Nd.

9. The article of any of claims 1 to 7, wherein the first element is one or more selected from the group comprising: Nb, Ta, W, Mo, Ti, Hf, Zr and optionally V and Cr.

10. The article of any of claims 1 to 7, wherein the first element is one or more selected from the group comprising: Nb, Ta, Mo, W.

11. The article of any of the preceding claims, wherein the substrate comprises an alloy containing niobium, preferably wherein the substrate comprises at least 50 at% niobium.

12. The article of any of the preceding claims, wherein the substrate is comprised of FS-85, C103, Cb521, Cb752, aRCCA, or a RHEA.

13. The article of any of the preceding claims, wherein the second element is one or more selected from the group comprising: Cr, Co, Al, Hf, Zr, V, Si, Fe, Ni, Mn.

14. The article of any preceding claim, wherein the second element is one or more selected from the group comprising: Al, Cr and Co.

15. The article of any preceding claim, wherein the intermediate layer comprises a C14 Laves phase.

16. The article of any preceding claim, wherein the substrate is an additively manufactured substrate.

17. The article of any preceding claim, wherein the protective coating has a BCC (B2) aluminide structure.

18. The article of any of the preceding claims, further comprising an oxide layer formed on the protective alloy coating, preferably wherein the oxide is an oxide of an element present in the protective layer.

19. The article of any preceding claim, wherein the intermediate layer comprises less than lat% Si.

20. A method of applying a protective coating to a substrate, the method comprising the steps of:providing a substrate comprising at least a first element;depositing a layer comprising a second element on a surface of the substrate, wherein the second element is different to the first element;diffusing the first element into the layer by heat treating the substrate with deposited layer to form a Laves phase including the first element and the second element; andforming a coating on the Laves phase.

21. The method of claim 20, wherein one or more of the depositing, diffusing and forming steps are performed using a non-line-of-sight technique.

22. The method of claim 20 or 21, further comprising:introducing a third element, different to the first and second elements, into the coating.

23. The method of claim 22, wherein the introducing comprises a non-line-of-sight technique, optionally chemical vapour deposition.

24. The method of claim 22 or 23, wherein the introducing introduces the third element into the Laves phase.

25. The method of any of claims 22 to 24, wherein the introducing comprises aluminizing.

26. The method of any of claims 20 to 25, wherein the depositing comprises sputtering.

27. The method of any of claims 20 to 25, wherein the depositing compriseselectroplating.

28. The method of any of claims 20 to 25, wherein the depositing comprises CVD.

29. The method of any of claims 20 to 28, wherein the forming comprises electroplating.

30. The method of any of claims 20 to 29, wherein forming the coating comprisesforming a coating of at least a fourth element, wherein the fourth element is different to the first and second elements.

31. The method of any of claims 20 to 30, wherein the method is for manufacturing the article of any of claims 1 to 19.

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