DE69326938T2 - ALUMINUM SILICIDE COATINGS, SURFACE COATING COMPOSITIONS, COATING METHODS AND IMPROVED COATED PRODUCTS - Google Patents

Description

Technical area

The present invention relates to aluminide coatings for aluminizing heat-resistant alloy substrates, to the slurry coating compositions for the coatings and to a method for applying such coatings, and in particular the invention relates to composite coatings containing aluminum-enriched phases and silicon-enriched phases to improve their resistance to heat corrosion and heat oxidation.

During operation, components in the turbine section of a gas turbine engine are exposed to hot combustion gases that can reach temperatures of 1200ºC (2200ºF). These components are typically made of nickel-cobalt-based superalloys specially adapted to maintain strength at these temperatures. However, when the combustion gases are exposed to oxygen at such high temperatures, these refractory materials begin to convert to their natural metal oxide form. The nickel and cobalt oxide scales that form on the surface of these alloys do not adhere firmly. During thermal cycling, they crack and chip away from the surface, exposing large areas of untreated substrate to the environment. In this way, oxidation causes surface roughening and eventually unprotected parts made of these alloys are consumed (see Fig. 1). The addition of sodium or impurities containing chlorine and sulfur to the combustion gases accelerates this deterioration. Above about 540ºC (1004ºF), Sodium and sulphur and form low-melting sulphate salts, which not only dissolve dioxide films on the nickel and cobalt, but also directly attack the substrates (Fig. 2).

State of the art

One solution to the problem caused by hot corrosion and hot oxidation, which is widespread in gas turbine engines, is to introduce aluminum into the surface of a superalloy component. This process is known as aluminizing or aluminizing. The aluminum forms a stable intermetallic compound with both nickel and cobalt. The oxide layer that this compound forms at high temperatures is no longer a metal oxide of nickel or cobalt, but instead a tough, tightly adhering protective layer of aluminum oxide Al₂O₃ (see Fig. 3).

A variety of commercially available coatings are based on this protective scheme. Sometimes aluminum is deposited from a vapor phase in a process that has become known as 'pack aluminization.' In pack aluminization, aluminum powder reacts with halide activators to create gaseous compounds that condense on the metal surface and react to produce aluminum metal. The aluminum atoms diffuse into the substrate and react to produce intermetallic aluminides. This process is described in detail in several patents, for example, U.S. Patent No. 3,257,230 (Wochtell et al.). This patent is hereby incorporated by reference.

Known MCrAlY coating alloys are also based on aluminum oxide films, which provide heat corrosion resistance. Due to the presence of chromium and yttrium in the film, the aluminum content in these coatings does not need to be The resistance to corrosion does not appear to be as high as with pack aluminizing; however, the protection is still derived from a tightly adhering crust of aluminum oxide.

'Slurry aluminizing' is another alternative method for producing an alumina-forming aluminide intermetallic coating on a superalloy. In the slurry process, an aluminum-filled slurry coating is first deposited on the hardware. When the coated part is heated in a protective atmosphere, the aluminum in the film melts and reacts with the substrate to form the desired intermetallic phases.

The demonstrable resistance of aluminide coatings to hot corrosion and hot oxidation is a consequence of the thermodynamic stability of the alumina crust that forms thereon. However, these coatings have some susceptibility to 'low temperature' hot corrosion failures at about 700 to 800ºC from the alkali metal oxides (e.g. Na2O) and the acidic oxides of refractory metals (e.g. MoO3 and W2O3).

Silicon dioxide (SiO2) is another very stable oxide. Like aluminum, silicon forms stable intermetallic compounds (silicides) with nickel and cobalt, as well as chromium and other elements typically found in refractory alloys, such as molybdenum, tungsten and titanium. This reduces the segregation of these elements into the outer surface protective oxide layer, thus improving the protective effect. In addition, unlike aluminum, silicon is incapable of forming sulfides, and silicon is resistant to sulfur diffusion. As a result, silicide coatings produced by pack or slurry processes have been used on refractory alloys to improve resistance to hot corrosion. and hot oxidation. The silicides have been found particularly useful against sulphurous attack at 'low' temperatures of 700 to 800ºC. The advantages of silicon-based coatings have been widely described, for example by F. Fitzer and J. Schlicting in the paper 'Coatings Containing Chromium, Aluminum and Silicon for High Temperature Alloys', presented at a meeting of the National Association of Corrosion Engineers held in San Diego, California, March 2-6, 1981, and published on pages 604-614 in 'High Temperature Corrosion' (ed. Robert A. Rapp). This paper is hereby incorporated by reference.

To avoid any misunderstanding, it should be mentioned that silicon is classified as a metallic element for the purposes of this description.

The advantages of aluminizing and siliconizing are combined in processes where both aluminum and silicon are simultaneously deposited on a metallic surface, usually on the surface of a superalloy. One such process, described in US-A-4,310,574 (Deadmore et al.), hereby incorporated by reference, involves depositing a silicon-filled organic slurry on a surface, after which the surface is aluminumized by conventional pack aluminizing. The aluminum carries the silicon from the slurry with it as it diffuses into the superalloy from the pack mixture. Deadmore et al. ('574) demonstrate that the resulting silicon-enriched aluminide has better resistance to oxidation at 1093°C than aluminides without silicon.

Another means of producing so-called 'silicon-modified' or 'silicon-enriched' aluminides is to apply a slurry coating containing powder ized aluminum and silicon metal to an alloy substrate containing aluminide and silicide-forming elements, followed by heating to 760ºC (1500ºF). As the aluminum and silicon in the slurry melt, they react with the substrate and preferentially diffuse. The aluminum alloys with nickel or cobalt in the substrate, while the silicon alloys with chromium or other silicide formers. The end result is a composite aluminide-silicide coating. This process is often referred to as the silicon modified slurry aluminide process and is used commercially under the brand name 'SermaLoy J' (a trademark registration of Sermatech International, Limerick, Pennsylvania, USA).

The metal handbook 'Aluminium Coating of Steels' of the American Society for Metals describes on pages 489-497 the coating of steel and superalloys with aluminide coatings by a slurry process. It also describes the coating of steel and superalloys with slurries containing 5 to 12 wt.% silicon to inhibit diffusion into the substrate.

US-A-4 500 364 describes the coating of steels and superalloys with aluminum-silicon coatings by a slurry process. It is also described here that the slurry contains an aluminum-silicon eutectic, an aluminum-silicon hypereutectic or elemental aluminum powder in combination with elemental silicon powder. This produces a coating with a total silicon content of 20 to 80 wt.%. US-A-5 120 613 describes the coating of ferritic parts with an aluminum-silicon coating by spraying an aluminum-silicon alloy containing 6 to 15 wt.% silicon.

Generally speaking, these known techniques and coating compositions aim to protect the silicon in the layer of the coating exposed to the harsh conditions described.

Alloy substrates suitable for this form of coating include nickel-based superalloys, cobalt-based superalloys and austenitic stainless steels. It has been found that the elements corresponding to the alloy-forming elements are present throughout the entire extent of the coating, but are differentially combined with aluminum and silicon components of the coating such that the silicon-enriched phases are differentially distributed throughout the thickness of the coating relative to the aluminum-enriched phases.

For use, the SermaLoy J slurry coating is supplied in a composition consisting of elemental metal powders of silicon and aluminum in an acidic water solution of inorganic salts as a binder. Approximately 15% by weight of the total metal powder content of the slurry is silicon powder. The total composition of the slurry is approximately as follows in weight percent:

Aluminium powder - 35%

Silicon powder - 6%

Water - 47%

Binder salts (dissolved in water) - 12%

A preferred way of preparing the composition is to mix the metal powder components beforehand and prepare the binder solution separately, after which the powder is mixed with the solution. Other ways of preparing the composition can be easily created.

This binder is chosen to cure into a solid matrix that holds the metal pigments in contact with the metallic surface while heating to diffusion temperature. The binder is also selected such that it volatilizes during diffusion to form residues that only loosely adhere to the surface after diffusion is complete.

In tests, the silicon-modified aluminide coating resulting from application of the slurry to superalloy articles followed by diffusion heat treatment has been shown to be extremely resistant to sulfur attack over a wide range of working temperatures. Details of some of the tests have been published by the American Society of Mechanical Engineers (ASME) in the following publication: F. N. Davis and C. E. Grinell, 'Engine Experience of Turbine Materials and Coatings' (1982). This publication is incorporated by reference. This coating has recently been applied to numerous industrial turbines and to marine turbines.

The paper reports that SermaLoy J provides good resistance to sulphidation at low and high temperatures. Because of the very satisfactory properties of the SermaLoy J coating, it is used as a standard for comparison in the tests discussed here.

The theoretical basis for the above improvement is probably the following:

Diffusion heat treating a superalloy or austenitic stainless steel substrate coated with an aluminum-silicon slurry in an inert atmosphere or in a vacuum causes certain elements from the substrate and the slurry that have a special affinity for each other to diffuse and combine with each other.

At the diffusion temperature, aluminum from the coating and nickel and/or cobalt from the superalloy or stainless steel substrate migrate into each other and combine to form nickel aluminides. Similarly, silicon in the coating has an affinity for the metallic chromium of the substrate and for molybdenum, tantalum and titanium if these metals are present, and therefore combines with one or more of these metals to form silicides.

However, silicon moves through the coating to the substrate much more slowly than aluminum, and therefore the outer parts of the coating become relatively enriched in silicon. Since chromium is present in the superalloys and austenitic stainless steels in much larger amounts than the other elements for which silicon has an affinity, the silicon largely combines with the chromium during the diffusion heat treatment and an outer coating layer is produced that is richer in chromium silicide than the rest of the coating.

For further reference in the specification, it is convenient to identify various zones in a typical SermaLoy-J coating that has been subjected to a heat treatment at 870 to 885°C. Inspection of the coating reveals a silicon-enriched surface zone where chromium silicide is specifically concentrated. This zone transitions into a 'deposition zone' that extends deeper into the coating and consists of alternating layers of silicide phases and aluminide phases. Below the deposition zone is an 'aluminide zone' where the aluminide phases predominate but where silicide precipitates are also present. At the interface with the substrate material is a diffusion zone where the coating and substrate material have diffused into each other.

The coatings according to the invention show, as mentioned above, a different composition of these layers and/or a different thickness of the layers.

Reference to related patent applications

The present application is a continuation-in-part of International Patent Application PCT/US93/04507, MULTIPLEX ALUMINIDE-SILICIDE COATING, Mehar C. Meelu, Alan T. Jones and Bruce G. McMordie (Meelu et al.), dated May 18, 1993. This application is based on British Patent Application No. 9 210 683.0, dated May 19, 1992. The present patent application incorporates PCT/US93/04507 by reference. A US patent application corresponds to this International Application. The present application is also a continuation-in-part of the mentioned US patent application, which is also incorporated by reference. Further comments regarding the base International Patent Application are provided below.

Summary of the invention

An important object of the invention relates to a slurry aluminide coating which is particularly useful for superalloy substrates which are frequently exposed to oxygen in gases at high temperatures during service. This coating has improved resistance to cracking during service and very satisfactory resistance to oxidation and corrosion conditions at heat is maintained.

According to the invention, it has been found that a coating composition comprising a slurry of metal powders in elemental form and containing aluminum and silicon and at least one of the following metals: chromium, titanium, tantalum, rhenium, manganese, hafnium, vanadium and niobium provides coatings which have unexpectedly improved properties have known coatings including SermaLoy J coatings.

According to the invention, it has been further discovered that a coating consisting of a slurry of a eutectic aluminum-silicon powder and an additional metallic powder in elemental form of at least one metal from the group consisting of Cr, Ti, Ta, Re, Mn, Hf, V and Nb in a curable binder provides coatings having unexpectedly better properties than known coatings, including SermaLoy J coatings.

According to the invention, slurries other than a purely eutectic slurry are discussed below.

In accordance with the invention, certain process modifications have been discovered which are particularly well suited to providing coatings of improved properties. In particular, the process described in the above-mentioned International and United States patent application has been found to be applicable to coating a selected superalloy substrate with the compositions of the invention.

Certain individual aspects of the invention are described below.

An important feature of the invention is that aluminide-silicide coatings provide overall a more uniform distribution of their components, particularly as regards the silicides throughout the entire thickness.

A further feature of the invention are coatings with reduced over-concentrations of silicon content in the surface zone of such coatings, resulting in an increased depth of the aforementioned silicon-enriched surface zone and the deposition zone and an increased dispersion of the silicide phases within the aluminide zone.

The invention is described in detail below.

The term 'superalloy' is well known in the art. In the present specification, this term is used to refer to nickel and cobalt based alloys suitable as substrates for the coatings of the invention. However, austenitic stainless steel substrates are also suitable for at least some of the coatings of the invention and are considered superalloys for the purposes of the invention.

Short description of the drawing

Coatings according to the invention and known coatings are described in conjunction with the accompanying drawing. In the drawing:

Fig. 1 what happens when a typical substrate of an unprotected superalloy surface is exposed to pure combustion gases,

Fig. 2 what happens when a typical substrate of an unprotected superalloy surface is exposed to combustion gases containing impurities containing chlorine and sulfur, which are often found in the marine atmosphere under hot corrosion and hot sulfidation conditions, respectively.

Fig. 3 a typical superalloy substrate which has been aluminized to create a diffused aluminide coating with a well-adhering protective layer of aluminum oxide (Al2O3),

Fig. 4 is a photomicrographic view of a cross-section of a normal SermaLoy J aluminide silicide coating example at 500x magnification, with the coating applied to a nickel-based superalloy substrate,

Fig. 5 to 13 are identical photomicrographic sections through other aluminide silicide coating examples whose structure, composition or heat treatment has been modified in accordance with the present invention,

Figs. 14A and 14B are graphs showing the result of the Electron Probe Micro Analysis (EPMA) for the distribution of the various protective coatings and superalloy substrate elements for the coating of Fig. 4, with the elemental amount in atomic percent in Fig. 14A and in weight percent in Fig. 14B plotted against a scale of micrometers through the sample,

Figs. 15 to 22, 31A and 31B are graphical representations of the results of EPMA tests for other selected coating examples, with elemental amounts in weight percent plotted against a micrometer scale through the samples,

Fig. 23 to 25 are graphs illustrating the result of the accelerated hot corrosion test of test pieces coated with various aluminide silicide coatings under investigation, with the weight loss in milligrams plotted against the test time in hours, and

Fig. 26 to 30 and 32 Photomicrographs of the various coating examples.

Detailed description of preferred embodiments of the invention

The invention provides several novel embodiments. One embodiment relates to a coating composition consisting of a slurry containing various metals in elemental powder form (or state) in a liquid binder which is preferably heat curable. One aspect of this embodiment is a powder consisting of metallic components, primarily aluminum and silicon and other metals to be described later. The powder of the mixture of metals is added to the binder which may have a wide pH range and is preferably an acidic binder.

The invention also provides a method of applying an aqueous composition to a superalloy substrate to produce a coating having high resistance to heat oxidation and heat corrosion and also having improved resistance to cracking.

The invention also provides improved coatings which have improved resistance to cracking under such prevailing conditions, particularly to the onset of cracking.

The invention also provides improved workpieces provided with such coatings, for example parts of gas turbines for industrial purposes, for marine purposes and for other purposes, for example rotor blades, turbine blades, housings and other parts.

The invention creates further novel embodiments.

One embodiment relates to a slurry of eutectic aluminum-silicon powder in a heat-curable liquid binder. The slurry is particularly suitable for producing coatings on superalloy substrates which are frequently exposed to oxygen in gases at high temperatures during service and which coatings normally tend to develop cracks. The slurry according to the invention helps to overcome these problems. The slurry may contain other metals in their elemental state which have additional beneficial properties as described below.

According to a further embodiment, the invention relates to coatings formed on such superalloys from slurries containing such an eutectic aluminum-silicon alloy. The coatings have an improved dispersion of the alloys of silicon and the metals of the substrate throughout the entire thickness of the coating. In addition, the coatings according to the invention have a reduced silicon content and an increased aluminum distribution in the outer zone of the coatings. The coatings according to the invention have excellent properties, in particular they are remarkably resistant to the initiation or start of microcracks in the coating and they have improved ductility.

A striking aspect of the coatings according to the invention is that the silicides are more uniformly distributed or dispersed (or redistributed) throughout the thickness of the coatings than is the case with coatings of the prior art, in the typical case of the SermaLoy J coating. In particular, the invention tends to provide less densely packed layers of aluminide and silicide phases rather than highly densely packed, interconnected layers. Under optimum conditions, the silicide layers are spaced further apart and the aluminide and silicide phases are more uniformly distributed throughout the thickness of the coatings. minid layer tends to grow and the silicide particles become more evenly distributed over it and the other layers of the coating.

According to a further embodiment of the invention, a coating is provided in which no surface layer is present, i.e. where the first outer layer, referred to as the deposition zone, contains layers of silicide phases and aluminide phases.

The invention further provides a method for producing the eutectic slurry, wherein elemental aluminum and silicon are brought together in eutectic proportions and then the eutectic alloy powder is produced. Thereafter, after a certain time, if desired, the eutectic powder is mixed with or without optional other metal powders into a liquid binder solution.

According to another embodiment, a method is provided for coating a superalloy substrate with a slurry, wherein the coatings are cured, subjected to diffusion heating, followed by heating the coated substrate and allowing it to cool, or curing the coating.

A further embodiment relates to the coated workpieces, generally to metallic workpieces, which have a superalloy component which is provided with the coating according to the invention. Parts which make particularly useful use of the coating according to the invention are parts of gas turbines.

According to one aspect of this invention, the diffusion process can be applied as often as necessary, according to the process described in the above-mentioned PCT application PCT/US93/04507. Coatings with further improved properties can be obtained.

Further embodiments of the invention emerge from the following detailed description.

In the description of the invention, the terms 'zone' and 'layer' are used interchangeably.

From the above description and the description below, it is clear that an important technical teaching of the invention is the distribution of silicide phases and aluminide phases in the coating, which distribution is different from the distribution of alloys in the coatings according to the prior art. In particular, the silicides are more evenly distributed over the coating.

Tests and experience gained in connection with the invention and in connection with parts exposed to severe ambient temperatures indicate that the outer silicide phases of a known SermaLoy J coating are the key to the improved hot corrosion resistance. These phases appear to displace some of the vulnerable aluminide phases from the surface layer. Unfortunately, these critical silicide phases become excessively concentrated in the surface zone of the coating microstructure after a typical coating and diffusion treatment is performed, particularly when the process is used in conjunction with superalloys. The silicon content of the surface zone can be as high as 40 wt.%, as opposed to 10 to 20 wt.% overall in the coating. This appears to make the outer portion of the coating susceptible to microcracking after a long period of service. Crack propagation occurs rapidly after a crack is initiated, even when the threshold for initiation is high. Although the cracks do not initially appear to be very serious because they do not propagate into the superalloy substrate, it would be highly desirable to prevent their occurrence or to prevent them from penetrating the coating. because this ultimately opens up corrosion paths to the substrate. This is of course a serious problem. The invention helps to solve this problem.

It further appears that the aluminum concentration level, at least within the nickel aluminide coating, but probably also within cobalt aluminide coatings, should be between about 20 and 30 wt.%, preferably about 25 to 30 wt.%. Nickel aluminide coatings with aluminum concentrations less than 20 to 25 wt.% tend to have insufficient oxidation resistance, while coatings with aluminum concentrations above about 30 wt.% are prone to cracking and premature failure. The reason for this is that a nickel aluminide coating with high aluminum concentrations tends to produce a multiphase aluminide structure which adversely affects the mechanical properties of the coating.

According to one aspect of the present invention, there is provided a slurry coating composition suitable for producing an aluminide silicide coating. The slurry consists of a metal powder in elemental form in a binder liquid. The metal powder component of the slurry composition comprises, in weight percent, the following ingredients in the respective ranges:

Al - 30 to 92.5%, preferably 70 to 92.5%, and more preferably 75 to 85%,

Si - 5 to 25%, preferably 10 to 25%, and more preferably 10 to 20%,

Cr - 2.5 to 20%, preferably 3 to 10%,

where the metal powder components are present in elemental form.

Optional components are further Ti, Ta and B, which are present in the composition, preferably in the following amounts: Ti - 0 to 10%, preferably 2 to 5%; Ta - 0 to 10%, preferably 2 to 5%, and boron in an amount of 0 to 2.5%, preferably 0.5 to 2%, and more preferably about 0.5 to 1%. Ti and Ta are preferably present together.

From the above it is clear that according to the invention the maximum aluminium content of 92.5% is sufficiently high so that a coating can be created with the mentioned minimum proportions of the other metallic elements. Similarly a minimum aluminium content of 30% is sufficient to create a coating containing the mentioned maximum proportions of the other metallic elements. Compositions with amounts of metal deviating from the upper and lower maximum values obviously do not give compositions with the desired properties. In particular it is noted that the lower the aluminium content of the slurry, the more difficult it is for the aluminium to melt into the coating and to diffuse easily. Accordingly it is expedient to keep the range of aluminium content within the mentioned limits.

The present invention includes regions of silicon content in the metal powder component of the slurry that are considerably greater than the 15 wt.% found in the SermaLoy J alloy. It is an important aspect of the invention that as a result of diffusion heat treating the coatings produced from the silicon enriched slurries, the microstructure of the coating becomes richer in silicon; thereby, more silicon becomes immediately available for combination with chromium and other silicide formers present in the substrate and coating before the surrounding material is relatively depleted of aluminum due to the rapid diffusion of aluminum. through the coating. According to the invention, the additional silicon in the slurry forms thicker silicide-rich surface zones.

The coatings of the invention prepared from slurries containing aluminum in the form of elemental metal powders below the 85 wt.% of Al in the SermaLoy J alloy, i.e., from about 65 to below about 85 wt.%, are distinguished from the SermaLoy J alloy by this smaller proportion of Al and by the presence of the other metallic elements identified herein.

The coatings according to the invention, which are prepared from a slurry containing silicon of the metal powder in elemental form less than 15% by weight of silicon in the SermaLoy J alloy, i.e. from about 5 to about 15% by weight, are distinguished from the SermaLoy J alloy by this lower proportion of silicon and by the presence of the other metallic elements identified herein.

The coatings of the invention, prepared from slurries containing aluminum and silicon in the ranges described below, are therefore distinguished by these two reduced proportions and the presence of the other metallic components.

Coatings according to the invention prepared from slurries containing Al and/or Si in amounts above 85 wt.% for Al and/or silicon above 15 wt.% differ from the SermaLoy J alloy because of the higher amounts of Al and/or Si. Likewise, these slurries were not expected to produce such improved coatings.

These different coatings have a combination of advantageous properties that are important in terms of Serma Loy J alloy were not foreseeable, despite the fact that SermaLoy J alloy was used on a large scale.

The chromium metal in the slurry coating composition of the invention is a silicide former and therefore the chromium is attracted to the silicon during the diffusion heat treatment. Since the chromium is already present throughout the coating at the beginning of diffusion, it quickly binds the adjacent silicon and produces a more uniform distribution of the silicide phases in the bulk of the coating than was the case with the SermaLoy J coating. Since no metal is present in the slurry for which the SermaLoy J alloy coating was made, the production of chromium silicide in this known alloy is based on an alloying process with chromium present in the metallic substrate. Accordingly, in the coating according to the invention the chromium silicide is formed principally from the chromium in the coating composition and not from the chromium in the substrate.

It has also been found according to the invention that by adding even very small amounts of chromium to the total metallic pigment content of the slurry, for example 3 to 5 wt.% at the expense of the aluminum content, a thicker silicide-rich surface zone and/or a deposition zone with fewer cracks can be obtained, i.e. with better ductility in the finished coatings. The addition of amounts greater than about 10 to 15 wt.% causes a further increase in ductility, but at the expense of hot corrosion resistance.

As a secondary consideration, the chromium beneficially modifies the alumina/silica surface oxide layer that is formed on the coating during service. Like the aluminum and silicon already present in known silicon-modified aluminide coatings, chromium, when oxidized, a thin but non-porous surface layer that forms a barrier against further oxidation.

Chromium has another valuable chemical property in combination with silicon, which modifies the oxide layer by forming an acidic oxide which provides good protection against high-temperature sulphidation.

Titanium and/or tantalum, which may optionally be present in the slurry of the present invention at the expense of aluminum content, are violent silicide formers and in this respect complement the effect produced by the inclusion of chromium in the slurry. Unexpectedly, it has been found that the presence of titanium and/or tantalum in the slurry in amounts of about 2 to 3 weight percent of the total metallic pigment content does not produce as pronounced an effect on the production of silicides in the deposit and aluminide zones of the finished coating as does the addition of chromium to the slurry. Instead, these metals tend to concentrate in the silicide-rich surface zone. That surface zone was found to be wider than that of the SermaLoy J coating, and apart from that, it is chemically different.

Ti and Ta tend to modify the composition of the chromium silicide because both metals will solid-solution with chromium silicide to create more complex silicides. They also have an affinity for nickel, forming intermetallic compounds with the nickel present in the superalloy or other high strength metallic substrates. Titanium promotes the formation of the protective alumina layer on the coating surface by assisting in the release of aluminum from the underlying aluminide surface. Titanium is not a very effective oxide coating alone, but can contribute to the formation of the aluminum oxide layer. On the other hand, tantalum forms a dense, stable oxide layer that provides excellent protection.

It has also been shown that surface zones containing Ti and Ta are more resistant to cracking than the narrower surface zones of the SermaLoy J coatings. Unexpectedly, however, this is only appreciably evident when both Ta and Ti are present, in that the presence of 2 wt.% Ta without Ti slightly increases the tendency for cracking in the coatings.

Titanium and tantalum may be replaced individually or in combination by other chemically equivalent transition elements of Group 4b and 5b of the Periodic Table, including rhenium, zirconium and hafnium and manganese of Group 2a as part of the total metallic powder content of the slurry.

The optional small amounts of boron present in the slurry may be added at the expense of the aluminum content in amounts up to 2.5 wt.%, preferably in the range between 0.5 and 2 wt.%, and most preferably in the range between 0.5 and about 1 wt.% of the total metal powder content. Boron in small amounts reduces the brittleness in the gamma nickel intermetallic compound Ni3Al present in the finished coating.

The embodiment of the invention, which uses a coating composition comprising a powder of a mixture of metals in elemental form, is further explained in the examples and in the figures.

A second important embodiment of the invention does not use aluminum and silicon in elemental form, but a eutectic aluminum-silicon alloy, as described below. This embodiment example of the invention uses a slurry coating to produce an aluminide-silicide coating, and the slurry consists of a metal powder in a binder liquid, and the metallic powder component of the slurry consists of at least one aluminum-silicon alloy in eutectic composition.

The eutectic composition of the aluminum-silicon alloy in stoichiometric amounts is 11.8 wt.% Si, the balance being 88.2 wt.% aluminum. However, according to the invention, the respective amounts of the two metals can deviate from the specific value, i.e. the stoichiometric value, to over about 7 to 14 wt.% without detrimentally affecting the metallic powder slurry. In fact, the respective amounts of the two metals can deviate by even higher percentages. Within logical and practical limits, it will be seen that the eutectic alloy - or a nearly eutectic alloy - can be substantially or completely replaced by Al and Si in elemental form. In such a situation, it will be seen that the embodiment of the invention described here as the 'eutectic embodiment' leads gradually and ultimately completely to the slurry of an embodiment of an 'elemental form of the metals'. In such a situation, it is found that up to an amount where Al and Si are not present in stoichiometric ratio, the amount of the metals and other metals in the slurry can be adjusted together. It is also found that a significant depression of the melting point and associated economic energy advantage - as taught in the description of the invention - occurs as a result of the use of the eutectic alloy and becomes smaller and less present when deviating from it. Therefore, it is clear that the skilled person will not use a eutectic alloy exclusively and such a feature is within the teachings of the invention. The invention can cannot be circumvented by using a eutectic alloy partially in one of the two embodiments of the invention.

It has been found according to the invention that the use of a eutectic Al-Si alloy in the slurry composition results in a deeper, more pronounced deposition zone, with the advantage of a reduced tendency to crack and improved resistance to hot corrosion.

It has been further unexpectedly found that the eutectic Si-Al alloy powder can be used to partially or completely replace the elemental Si and Al powders normally used in the SermaLoy J alloy or in the inventive composition described above which used elemental Si powders and Al powders.

In connection with the development of the present invention, which relates to the use of a eutectic alloy powder, an explanation for the improved coatings was discovered, but the inventors do not wish to maintain this. During diffusion in the heat treatment step of forming the coating, it is believed that Al and Si, when used as elemental constituents, tend to migrate toward each other and first form a eutectic alloy composition before being broken down to produce the aluminide and silicide phases in the coating. By using a eutectic alloy powder in the slurry, no chemical combination energy was consumed to produce the eutectic alloy in the coating. Instead, the Si is held closer to its original location in the coating and can combine more quickly with silicide formers, thereby increasing the depth of the coating deposition zone at the expense of the silicon-rich surface zone.

According to the invention, the amount of eutectic Al-Si alloy in the slurry can vary from about 36 wt.% to 100 wt.% of the total metal content, depending on the optional presence of additional metallic elemental powders. These can be present in the following weight percentage ranges:

Si - 0 to about 21%, preferably 5 to about 18%,

Cr - 0 to 20%, preferably 3 to about 10%,

Ta - 0 to about 10%, preferably not more than about 5%,

Ti - 0 to about 10%, preferably not more than about 5%,

B - 0 to about 2.5%, preferably not more than about 2%, and most preferably not more than about 1%.

At least a portion of the Ta and/or Ti components may be (if present) replaced by equal amounts of chemically similar transition elements as described above.

In this embodiment of the invention, chromium can be added at the expense of aluminum content for the same reasons as described above for coating compositions containing mixtures of elemental Al and Si powders. However, it has been found that when chromium is present in the slurry containing eutectic Al-Si alloy powder, the resulting structure of the finished coating contains an advantageous combination of features, in particular an increased depth of the surface zone and/or an increased depth of the deposition zone plus increased dispersion of the silicide phases, i.e. the silicides over the aluminide zone.

It has further been shown that when 2 to 3 wt.% each of tantalum and/or titanium is present in the slurries at the expense of aluminum content, the Ta and Ti components are more evenly distributed in the finished coating than is the case when used in slurries containing only elemental powders, where they tend to remain more concentrated near the surface.

Accordingly, according to the invention, the optionally added metals used in conjunction with the eutectic Al-Si alloy have additional advantages to which they do not appear to contribute as markedly when the slurry contains only elemental metal powders.

The coating composition containing an Al-Si eutectic alloy is described below.

In addition to the features that distinguish the coatings of the invention from known coatings as described, certain coatings of the invention also exhibit one or more of the following additional features.

A silicide-enriched surface layer of at least 2 µm thickness. A deposition layer having a minimum thickness of 12 µm for optimal results. A deposition layer at least four times as thick as the surface layer and preferably ten times as thick as the surface layer.

A surface layer having a maximum silicon content of less than about 25 wt.%. A deposition layer and an aluminide layer of more than about 15, but not more than 30 wt.% Si.

A coating in which the aluminum content does not vary by more than about 12, preferably not more than about 8, weight percent across the coating.

A deposit layer in which the aluminum content does not exceed about 30 wt.% and the aluminum content and the silicon content vary approximately inversely with depth.

A surface layer containing at least Ta or Ti in amounts greater than the weight percentage of the substrate material.

A coating that also contains chromium and has a thicker surface layer and increased dispersion of silicide layers throughout the coating.

A coating in which the layer (or zone) referred to here as the 'surface zone' is absent.

These features of the present invention may be present in the coating individually or in any combination of these individual features.

The binder used for the metallic components of the slurry according to the invention is a liquid, preferably an aqueous liquid, which is vaporizable by heat and curable by heat and preferably leaves an inorganic residue on the coating which is conveniently removed.

Such binders are known. They may consist of an acidic, a neutral or a basic liquid. They may be organic or inorganic, for example aqueous. The binder components of the slurries containing elemental and/or eutectic alloy powders of Si and Al may be organic (for example nitrocellulose or equivalent polymers) or they may consist of a solution from the class of inorganic chromate, phosphate, molybdate or tungstate solutions, as described in US-A-4 537 632 (particularly columns 9 and 10) and in US-A-4 606 967 and 4 863 516 (Sermatech International nal, Inc.), which publications are hereby incorporated by reference. The binder may also be selected from the class of water-soluble basic silicates which cure to form a tightly adherent glassy solid with loss of chemically bound water, as also described in the publications.

During heating to the diffusion temperature, the coating is subjected to a hardening phase. During diffusion, the metallic components of the binder are expelled from the coating. Remaining residues can be removed by brushing or similar treatment.

One aspect of the invention that is closely related to the compositional changes mentioned above relates to the duration and temperature of the diffusion heat treatment to which a coated article is subjected.

The diffusion heat treatment schedule for an article coated with a SermaLoy J alloy is selected to reduce the effect of the diffusion heat treatment on the mechanical properties of the substrate. The schedule is further selected to reduce the effect on the microstructure of the coating of the high temperatures to which the article, such as the turbine blades of an engine, is exposed during operation.

For example, an IN-738 alloy, i.e. a nickel-based superalloy, coated with a SermaLoy J slurry and aged, is normally subjected to a diffusion heat treatment at 870 to 885ºC for two hours. An X-40 alloy, i.e. a cobalt-based superalloy, coated in the same manner, is normally subjected to a diffusion heat treatment at about 870ºC for half an hour before being further heated to about 1000ºC and maintained at that high temperature for two hours.

Qualitatively, the following effects were observed, resulting from changes in temperature and time.

An extension of the duration of the diffusion heat treatment at the specified diffusion temperatures causes the coating to become more ductile and therefore less susceptible to cracking.

A significant decrease in the diffusion heat treatment temperature may be undesirable because of the risk of producing a coating microstructure that is susceptible to changes during subsequent operation in a gas turbine engine at high temperatures and the risk of inadequate diffusion of the coating elements into the base material.

In general, increasing the diffusion heat treatment temperature for a given treatment time produces a more uniform distribution of silicon in the coating and this treatment results in greater ductility, but at the expense of hot corrosion resistance. Reduced hot corrosion resistance results from excessive penetration of aluminum into the base superalloy and, as a result, the aluminum content of the coating is reduced.

However, in accordance with the invention it has been found that if diffusion is carried out in a temperature range above about 1000°C, but preferably not more than about 1120°C, the result is a thicker, more ductile coating with a less non-uniform distribution of aluminides and silicides throughout the coating and consequently improved mechanical properties are obtained, albeit with a somewhat reduced resistance to hot corrosion, which is however acceptable if the coated part is subjected to less stress. If a When such high diffusion temperature is combined with the addition of chromium to the slurry mixture, a useful combination of effects results and even better ductility and dispersion of silicides throughout the coating is achieved.

If the properties of the substrate material would be adversely affected by such high temperature diffusion, use may be inappropriate if it is not possible to restore the properties of the substrate material by further heat treatment. If the properties of the coated part are impaired to an unacceptable extent, it is appropriate to apply further heat treatment, such as precipitation heat treatment. For example, for a nickel-based superalloy IN-738 coated with a slurry mixture and subjected to diffusion heat treatment at 1200°C, the properties can be restored by heat treating at 840°C for 24 hours.

It is important to note that increasing the diffusion temperature much above 1120ºC would likely result in an even more pronounced decrease in hot corrosion resistance due to increased penetration of aluminum into the base superalloy material, resulting in an unacceptable reduction in the aluminum content in the coating.

The embodiment described above can be combined with the invention described in our pending International Patent Application PCT/US93/04507.

The following embodiments are not intended to limit the invention in any way.

EXAMPLES OF THE INVENTION AND COMPARISON WITH THE PRIOR ART Experimental strategy

Samples of various combinations of processes and chemical composition variations were evaluated against SermaLoy J alloy. A selection of such variations is shown in Table 1. The variables used and the objectives of the variations are summarized in the description following the table. TABLE 1

* Eutectic alloy composition = 88.3 wt.% Al, 11.7 wt.% Example 1 The aluminum and silicon powders used in the slurries: In some slurries, an Al/Si alloy powder of eutectic composition was used instead of Al powder and Si powder in elemental form as is common in the SermaLoy J alloy. This reduced the tendency of Al and Si to diffuse independently through the coating, thereby reducing the Si concentration in the surface zone.

Example 2 Addition of certain metallic elements to the slurry which are strong silicide formers: These were (in weight percent relative to the total metallic pigment content of the coating) Cr 5% or 10%, Ti 2.5%, Ta 2.5%. This controlled the movement of the silicon during the diffusion heat treatment and resulted in good silicide formation throughout the coating.

Example 3 Variation of the proportion of Al in the slurry: The Al content relative to the total metallic pigment content of the coating was varied in the range between 65 and 95 wt.%.

In that part of the range below the normal SermaLoy J aluminium content of 85 wt.%, a relative reduction in the Al content allowed the introduction of further metallic elements as described above under (ii), i.e. these metallic elements were introduced at the expense of the Al weight percent content.

In the part of the region above 85 wt.%, this produced an excess of Al in the slurry during the diffusion heat treatment, thereby diluting the outer normally silicon-rich surface zone.

Example 4 Change in the amount of Si in the slurry: The Si content relative to the total metallic pig The element content of the coating was varied in the range between 5 and 25 wt.%.

In the part of the range below the normal SermaLoy J silicon content of 15 wt.%, a relative reduction in silicon content allowed the addition of more Al.

In that part of the range above 15 wt.%, this produced a thicker silicon-enriched surface zone that was better able to resist crack initiation and enhanced the diffusion of Si through the coating. In this way, the deposition zone was deepened and the thermal expansion mismatch between the silicide surface zone and the aluminides and oxide skin was reduced.

Example 5 Addition of 1 wt.% boron to the slurry. This made the aluminide phases more ductile.

Example 6 Variations in temperature during diffusion heat treatment: The different temperatures used in the test were as follows: 760ºC, 870ºC, 885ºC and 1120ºC. Reducing the temperature below the normal SermaLoy J diffusion temperature of 870ºC to 885ºC reduced the diffusion rate of Al, thereby increasing the Al concentration in the surface zone of the coating. Raising the diffusion temperature above the normal temperature for the SermaLoy J alloy increased the diffusion rate of the heavier substrate material elements without significantly increasing the Al diffusion rate, thereby lowering both the Al concentration and the Si concentration in the surface zone.

Example 7 Variations in diffusion heat treatment time: Different times were investigated: 2, 3.5, 4, 5, 6.5 and 8 hours. An increase in diffusion time beyond the time normally used for the SermaLoy J alloy at The applied diffusion time of 2 hours caused complete diffusion of the coating.

Preparation of samples

In the studies related to the invention, variations in coating composition and treatment were investigated, and some of the experiments are shown in Table 1. Nickel-based superalloy samples made from IN-738 superalloy disks were coated with either a standard SermaLoy J aluminide silicide coating or with a coating having a modified composition and/or treatment. These disks were 12 mm in diameter and 5 mm thick.

The approximate composition of IN-738 alloy in weight percent is as follows:

60 Ni, 16 Cr, 8.5 Co, 3.45 Ti, 3.45 Al, 2.6 W, 1.7 Mo, 1.7 Ta, 0.8 Nb, 0.5 Fe, 0.5 Si , 0.2 Mn, 0.17 C, 0.1 Zr, 0.01 B.

As examples for the preparation of the slurries used in the studies, two slurries of different composition were investigated, as described below.

Slurry A:

95.0 ml water

31.5 g phosphoric acid

9.0 g chromic acid

7.3 g magnesium oxide

82.0 g aluminium powder (less than 5 µm diameter)

14.5 g silicon powder (-325 mesh)

Slurry B:

100.0 ml slurry A

7.0 g chromium powder (less than 5 µm diameter)

Additional slurries were prepared containing a eutectic aluminum-silicon alloy powder with and without other metallic elements, as described below.

Slurry A corresponds to the slurry of sample number 1A1 in Table 1 and is a normal slurry composition as used for SermaLoy J with a solids content of about 60 wt.%. It results in a coating in which the aluminum constitutes 85 wt.% of the total metal powder content and the silicon content 15 wt.%. During manufacture, the second, third and fourth components combine to form a magnesium salt of chromic acid and phosphoric acid and this is the binder for the metallic components to ensure hardening of the coating. However, this is later driven out of the coating during diffusion heat treatment.

Slurry B has sufficient chromium powder added to provide a coating in which 10 wt.% of the total metal powder content is chromium. This corresponds to slurry sample No. 1B3 in Table 1.

Prior to application of the slurry, the samples were degreased in superheated steam from 1,1,1-trichloroethane and then blasted with a 90/120 mesh alumina abrasive to prepare the surface for coating. The blasting operation was carried out in a pressure chamber at a pressure of 20 psi or 138 kPa.

A thin moist coating was obtained from each slurry applied to the cleaned samples using conventional spraying techniques. After the water evaporated, the slurry coating was dried at 80ºC (175ºF) for at least 15 minutes and then cured at 350ºC (660ºF) for 30 minutes.

The curing temperatures for conventional coatings are suitable. It is possible to cure at higher temperatures, for example around 600ºC, to speed up the curing process, provided that the temperature chosen is sufficiently below the melting point of aluminium. It would also be possible to cure at lower temperatures, consistent with a reasonable curing time.

After the samples had cooled, the thickness of the coating was built up by applying and curing additional slurry coatings using the procedure outlined above. The weight (thickness) of the coating thus applied to the samples was standardized to a total of about 17 to 27 mg/cm2 of cured slurry. The lower end of the range is more appropriate for the conventional SermaLoy J coatings, while the upper end of the range is more appropriate for the heavier weight of the coatings of the invention incorporating the heavy metal chromium as an essential component.

Each of the coated samples was then subjected to a specific diffusion heat treatment schedule. For many samples, the standard SermaLoy J heat treatment was used, which involved heating to 870ºC (1600ºF) or 885ºC (1625ºF) and holding the samples at that temperature for 2 hours to induce diffusion between the coating components and the base material. In other tests, changes in temperature and cycle time were made as needed. was written and further experiments were carried out.

When the samples were cooled after heat treatment, the undiffused residues were removed by light blasting with -140 +220 glass beads at 20 psi (138 kPa) in an aspirator. In some variations of the heat treatment schedules, the samples were subjected to a cleaning treatment between the first and second heat treatments, for example in sample 1B9 in Table 1.

All diffused samples were cut, mounted and polished on standard laboratory equipment. Cutting was performed using conventional elastic cutting wheels in conjunction with a coolant, as would be done in normal SermaLoy J production coating test pieces testing.

Table 1 above shows different samples, their composition and heat treatment.

MICROSCOPIC AND ELECTRONIC PROBE EXAMINATION OF THE SAMPLES

First, the diffused coatings were subjected to optical or electronic microscopy and electronic probe microanalysis (EPMA).

All diffused samples were examined visually at relatively low magnification (x40) with binoculars and then microscopically. Notes were recorded for each sample regarding condition, color, microstructure, etc. In particular, the cut samples were checked for microcracks resulting from the cutting process, which provided an indication of how brittle the coating was and how the response to Bean Some of the samples were X-rayed to detect cracks and were subjected to EPMA testing to determine where the individual elements were located in the coating and what their concentration levels were.

Typical photomicrographs of the coating sections of these examples are shown in Figures 4 to 13, 26 to 30 and 32 and examples of EPMA are shown in Figures 14 to 22 and 31, corresponding to the microstructures of Figures 14 to 22. The origins of the EPMA graphics (i.e., the baselines for the beginning of each electron probe scan) were located just beyond the outer surface of the coating.

Different zones of the photomicrograph sections are indicated by the following letter codes:

A - Sample storage compound (e.g. 'Bakelite' (trademark))

B - Nickel plating - some plating surfaces were plated with nickel during sample preparation.

C - Silicon-enriched surface zone where chromium silicide and other silicides are particularly concentrated. This zone is patchy or absent in some samples.

D - Depositional zone consisting of alternating successive layers of silicide phases and aluminide phases - this zone is not always clearly distinguishable from other zones.

E - Aluminide zone, predominantly consisting of aluminide phases, but with greater or lesser amounts of silicide precipitates.

X - Precipitation zone. In samples exposed to high diffusion temperatures, an aluminide zone with predominantly silicide precipitates of refractory metals, but with a lower aluminum concentration than in samples treated at normal diffusion temperatures.

Y - Nickel enriched zone. In samples subjected to a high diffusion temperature, an aluminide zone with a high nickel content.

F - An interface diffusion zone. At the interface between the coating and the substrate material, where the grain structure has been visibly aged by the diffusion process.

G - Substrate material - in all samples. This was IN 738 superalloy material.

Figure 4 shows a conventional SermaLoy J coating with a microstructure deposited on an IN-738 substrate and subjected to the normal diffusion heat treatment as mentioned above.

As can be seen from Fig. 4, the coating is about 50 µm thick overall and shows the above zones C, D, E and F. The silicon-enriched surface zone C is only about 5 µm thick. The deposition zone D, where aluminide phases and silicide phases are mixed together, is about 15 µm thick, but the layers are somewhat interrupted. The aluminide zone E shows silicide precipitates. The interface diffusion zone F is about 5 µm thick. The latter zone does not count as part of the thickness of the coating for the purposes of the invention, since this zone is by its nature part of both the coating and the substrate material G. The zones X and Y are not present, since in the In this example, a diffusion temperature of only 870ºC was used.

This sample was subjected to an EPMA process with the result shown in Figures 14A and 14B, confirming that the coating is a composition of nickel aluminide and silicides, primarily chromium silicides. During diffusion of the SermaLoy J alloy, the affinities of Al to Ni and Si to Cr are the predominant features. The measured concentration of silicon in the surface zone is up to 38 atomic % (34 wt.%) silicon at locations within the first 10 µm of the coating surface. Cracks were evident in the sample of Figure 4 when subjected to visual inspection and X-ray inspection.

In SermaLoy J alloy, there is a high level of silicon segregation in the surface zone, which explains the superior protection over normal aluminides. However, this silicon concentration causes rapid crack propagation after cracking has started, although the threshold for crack initiation is relatively high.

Nickel aluminides with higher aluminum concentration lead to the creation of multiphase structures which adversely affect the mechanical properties of the coating. It was found that an acceptable aluminum level within the coating should be between 25 and 30 wt.%, while Fig. 14 shows Al levels consistently well above 40 atomic % (28.6 wt.%) and peaking at over 50 atomic % (35.7 wt.%).

As a result of the microscopic observations and the EPMA observations, the following conclusions were drawn regarding the effectiveness of the composition and the process changes described above.

SUMMARY OF OBSERVATIONS AND CONCLUSIONS

For convenient reference of the relative thicknesses of the zones appearing in Figures 4 through 13, reference should be made to Table 2 below. For any given sample number, the total coating thickness in µm is given, as is the approximate percent increase in total coating thickness for each sample relative to the standard SermaLoy J sample, and the thickness in µm of each Zone C, D, E, X, Y, and F is given when present. In parentheses next to each absolute zone thickness figure, the thickness is given as a percentage of the total coating thickness. However, no percentage values are given for Zone F since this zone does not count as part of the total coating thickness. TABLE 2

Comment:

1. Al/Si eutectic powder: seeks to reduce C, thickness absolute D, seeks to increase D%

2. Increasing Cr leads to thicker coatings (but not as pronounced as 1.) above) and: seeks to decrease C, thickness absolute D, seeks to increase D%

3. Increased diffusion temperature leads to much thicker coatings

Effects of additional metals (i) Addition of silicide formers to the slurry

Figures 10 to 13 show samples where the metal content of the coating that was applied and cured consisted of elemental powders modified by the addition of varying amounts of chromium ranging from 3 to 13 wt.%. The diffusion heat treatment was carried out at 885ºC for two hours.

From Table 2 it can be seen that with higher chromium addition rates, the thickness of zone C of the samples is reduced, while the thickness of zone D increases.

It is notable in Figures 10 and 12 that the high chromium content of 10 and 13.4 wt.% in samples 4E and 5E, respectively, has greatly reduced the surface zone elevation and introduced good silicide deposits and precipitation over the remainder of the coating thickness. In Figure 10, the deposit also appears to extend over zone E, although the deposit is not predominant in the lower half of the coating depth. Figure 11 (here the chromium content is only 6.7 wt.%) shows a slightly more pronounced surface zone C, with the remainder of the coating equally divided between zones D and E. Zone E shows particularly good silicide formation.

Sample 9E (Fig. 13) has only 3.3 wt.% chromium and an increased silicon content compared to sample J, and this results in a more pronounced silicon-rich surface zone C, but a thinner deposition zone E.

Fig. 5 shows the coated sample 1A6, where the metal content of the coating as applied and cured consists of elemental powders modified by the addition of 10 wt.% silicon and 5 wt.% chromium on Ko sten of the aluminum content. The diffusion heat treatment was the same as normally used for the SermaLoy J alloy. The surface layer zone C is not appreciably thicker than that of sample J, with zone D being very thin and not very pronounced. However, the coating is 40% thicker than that of sample J, and with reference to the EPMA result in Fig. 18, it is noteworthy that the weight percents of Si, Al, Cr and Ni are much more constant throughout the depth of the coating than in sample J. In particular, it is noted that the Al content is continuous over 20 to 24 wt.% and the maximum measured silicon concentration in the upper zone of the coating is 10 wt.%. The conclusion to be drawn from this is that the addition of chromium assists in the distribution of Si throughout the coating, but that the additional Si has helped to form the thicker zone C.

No cracks were observed in sample 1A6, which can be attributed to good ductility.

In general, examination of other samples not shown showed that the addition of chromium to the slurry resulted in a thicker deposition zone D at the expense of the silicon-enriched surface zone C. This also increased the dispersion of the silicides throughout the zone E. Additional silicon could be added to obtain the formation of a thicker silicon-enriched surface zone C if desired.

However, when chromium was added to the slurries that had a eutectic Al-Si alloy, the effect was not as pronounced.

The results when tantalum and titanium were added to the slurry showed a different effect on the coating, as can be seen from Figs. 15 to 16 and 19.

When applied to an IN 738 substrate, a SermaLoy J coating, the concentrations of Ta and Ti in the silicon-rich surface zone C are about 1 wt.% and 1 wt.%, respectively (see Fig. 14A). However, Sample 1A2 (Fig. 15B) and Sample 1A5 (Fig. 17B) show Ta concentrations up to 3%, while Samples 1A4 and 1A7 show Ti concentrations up to 5% and 10%, respectively. The EPMA record here shows that the additional amounts of Ta and Ti were incorporated into the finished coating, but instead of forming silicides within the body of the coating, Ta and Ti were concentrated in zone C, at least when used in conjunction with elemental Al and Si powders.

Tantalum without titanium was found to favorably affect the coatings with respect to cracking, probably due to the mismatch in atomic size between Ta and Si, which caused stress in the microstructure of the surface zone where Ta was concentrated. However, when Ti and Ta were used together, the silicide-rich zones became wider than those of the normal SermaLoy J alloy and this tended to make them less susceptible to cracking.

The chemical differences created in Zone C by Ta and Ti relative to SermaLoy J alloy are due to their ability to form silicides and to solid-solution with chromium silicide to form complex silicides and their affinity for nickel from the substrate to form intermetallics such as Ni3Ti. In addition, both resulted in the creation of a protective alumina skin on the surface of the coating during service as this assisted in the release of aluminum from the aluminide phase. Although TiO2 does not provide effective protection on its own, it can help to improve the cohesion of the alumina skin. Tantalum also forms a dense stable oxide during service and this oxide provides good protection.

When slurries containing a eutectic Al-Si alloy and Ta and Ti were examined, the Ta and Ti components were found to be more evenly distributed throughout the coating than when elemental Si and Al powders were used. This is clear from comparing the distribution of Ti and Ta (especially Ti) in the outermost layers of coating 1A15 (a composition containing eutectic powders with Ti and Ta) as shown in Fig. 21B, compared with the distribution of those elements in the standard SermaLoy J alloy (Fig. 14B). This observation confirms the effect mentioned in paragraph (ii) concerning the more uniform distribution of silicides throughout the coating when eutectic Al-Si powders are used.

(ii) Use of a eutectic Al-Si alloy powder in the slurry

Fig. 6 is an electron scanning micrograph of coating sample 1A9, where the metal content of the coating that was applied and cured consisted entirely of powdered aluminum-silicon alloy of eutectic composition. The diffusion heat treatment was the same as that normally used for SermaLoy J alloy. Fig. 6 should be viewed in conjunction with Fig. 20, which shows the result of the corresponding EPMA test.

The coating is about 85 µm deep, which is 35 µm (70%) deeper than that of Fig. 4. Zone F is flat and fine-grained, as in Fig. 4, and this leads to good mechanical properties at the interface.

In contrast to Fig. 4, zone C in Fig. 6 is very thin and patchy and was not considered as part of the total coating thickness, but zone D comprises about the outer 40% of the coating thickness, namely about 35 µm. In addition, the deposition in zone D is not as interrupted as in the same zone according to Fig. 4.

Zone E in Fig. 6 is about 20 µm thicker than the same zone in Fig. 4, but it occupies about the same proportion of the total coating thickness.

From Fig. 20 it can be seen that the silicon in sample 1A9 is more uniformly distributed throughout the coating than in sample J, with the measured concentration now exceeding about 8%.

A microscopic examination revealed no cracks in the coating.

Sample 9U (see Fig. 7A) was treated at a slightly higher diffusion temperature than samples J and 1A9, but still normal for SermaLoy J coating production. It contained approximately 69 wt.% of eutectic Al-Si powder with elemental additions of aluminum and silicon and about 3 wt.% chromium.

The total coating thickness is only 20% greater than in Fig. 4, with zone C being slightly more pronounced than in Fig. 6, but more patchy than in Fig. 4. Although zone D is thicker for sample 9U than for sample J, this difference is not pronounced. It is clear that a portion of the silicide content spread as layers over zone D in Fig. 6 (coating 1A9) has segregated into the surface zone C of Fig. 7A (coating 9U, eutectic powder + Cr powder).

By adding Ti and Ta powder to the Al-Si eutectic alloy powder and chromium powder in 1A15 slurry, an aluminide with a more homogeneous microstructure is produced than any other eutectic alloy slurry that does not contain these components. None of the deposits of the silicon-rich phases evident in coating 1A9 (zone D in Fig. 6) is visible in the outer zone of the 1A15 aluminide (Fig. 7B). These silicides are instead uniformly distributed within the nickel aluminides in that zone. Although the microprobe analysis in Figs. 21A and 21B showed that there was an equal proportion of silicon in 1A15 (up to about 6%) as in the other coatings, Fig. 21A teaches that this silicon was quite uniformly distributed throughout this outer zone. This is not the case with the structure of coating 1A9 (Fig. 20). In the coating produced by diffusion of the eutectic Al-Si alloy without the addition of Cr, Ti or Ta, the silicon concentrations in the outer layers are more variable.

Furthermore, it can be seen from Fig. 21 that Cr, Ti and Ta are distributed quite uniformly throughout the coating. When Cr was added to the eutectic slurry without Ti and Ta (coating 9U, Fig. 7A), the chromium silicides remained visibly concentrated in the outer zones C and D.

This evidence showed that the Ti and Ta metal added to the slurries are contained in the aluminide produced when these slurries are diffused. These additions distribute Si and Cr further throughout the coating than would be possible without these additions, and there is mixing and enhancing the homogenizing effect of the eutectic Al-Si alloy powder.

General conclusions drawn from the examination of the other samples are consistent with the above considerations. It was found that according to the invention, those slurries containing the Al-Si alloy in eutectic composition gave thicker coatings than slurries containing elemental Al and Si powders. It was apparent that some of the silicides, particularly near the surface, at normal SermaLoy J alloy were more evenly distributed throughout the thickness of the coating.

(iii) Addition of Al or Si to the slurry

Examination of the samples showed that the addition of more silicon to the slurry produced the silicide-enriched outer zones which were thicker, but were found to have a lower Si content than in a normal SermaLoy J alloy. This is illustrated in the case of coating sample 1A6 in Figures 5 and 18 as described above.

(iv) Addition of boron to the slurry

Difficulties were expected in determining the effect that boron has on the coatings due to the limitations of the EPMA technique used. One effect of including boron as an optional component of the coatings according to the invention is that it is likely to make at least the aluminide phases of the coating more ductile. Since this is a means of lowering the melting point, smaller amounts are preferably used to provide beneficial effects on the coating in a high temperature environment.

(v) Change in diffusion heat treatment temperature

For nickel-based alloys, the silicon-aluminium slurries must diffuse above about 854ºC, and preferably between 870 and 885ºC, to obtain an aluminium-enriched NiAl phase without producing a brittle stoichiometric NiAl3 phase. This critical temperature was confirmed in this review when the diffusion temperature was increased to 760ºC with the aim of was lowered to reduce the diffusion rate of Al and consequently an increase in Al concentration (ie reduction in Si concentration) in the outer zone of these coatings. In this case, thinner coatings were produced and their Al levels were too high. It was not surprising when it was found that cracking was more severe in samples diffused at 760ºC than those diffused at 870-885ºC.

Diffusion at temperatures above 1000ºC produced coatings that were thicker and more ductile than those produced at 870ºC, while achieving even better dispersion of the silicon-rich phases in the coating.

When diffusion at 1000ºC and above was combined with the addition of chromium into the slurry, a useful combination of effects was obtained and these samples were particularly resistant to cracking.

Diffusion at 870ºC to 885ºC (normal diffusion temperatures) causes what can be described as a 'highly active aluminizing process', with inward diffusion of aluminum being the main reaction.

At 1120ºC, the outward diffusion of heavy elements, particularly nickel in a nickel-based material, becomes more pronounced and this results in a lower activation aluminization process. As nickel moves into the coating, other elements that are normally in solid solution with nickel remain in the substrate to precipitate columnar carbides in the diffusion interzone. Some samples, for example 1B9 (Fig. 8) and 1B18 (Figs. 9 and 22), were therefore first exposed to normal diffusion temperatures for two hours and then exposed to elevated diffusion temperatures for a further two hours, to obtain the high/low aluminide activity characteristics.

Examination of the samples showed that when the diffusion temperature was increased to 1120ºC, an interface diffusion zone F containing columnar carbides was always produced. Compared with sample J and some others, lower Al concentrations were achieved in the body of the coating and a lower Si content was found in the outer zone. This can be seen from Figs. 9 and 22.

Sample 1B9 in Fig. 8 shows the result of high-temperature diffusion of a slurry whose metal powder content is a 100% eutectic Al-Si alloy.

Most notable is the greatly increased total coating thickness, 130% greater than that of sample J, resulting from the above-mentioned thickening effect of the eutectic alloy and the increased diffusion temperature. In contrast to sample 1A9, which consists of a eutectic alloy powder but diffused at lower temperatures, a distinct 5 µm thick silicon-enriched surface zone C results, presumably as a result of the greater inward diffusion of aluminum. However, a thick deposition zone was produced as in sample 1A9, and good precipitation formation over the underlying zones is probably evident.

Figures 9 and 22 also show features resulting from the combination of two different coating modification changes, namely the use of an additional amount of a violent silicide former in the slurry mixture (+10 wt.% Cr) and an increased diffusion temperature of 1120°C. The microstructure shows a patchy zone C and a thicker deposit zone D, despite the fact that the total thickness of the coating is slightly less than that of sample 1B9 in Figure 8. Chromium appears to have the effect of enhancing silicide formation within zones D, X and Y at the expense of zone C.

The high diffusion temperature has, as explained above, a modified zone F relative to sample J. Migration of the heavier (heat-resistant) elements into the IN 738 base material, i.e. Mo, W, Ti, Cr, Ta and Nb, was increased, together with the carbon already contained in the alloy. These heat-resistant elements are violent carbide formers and, as a result, their movement through the coating is arrested by carbide formation at the interface. This is advantageous because their oxides have a detrimental effect on the protective properties of the surface skin of the coating which forms during service, so they should be kept away from the surface as far as possible.

A problem with performing a high temperature diffusion heat treatment at 1120ºC is that it has an adverse effect on the mechanical properties of IN 738 alloy and other superalloys because the strengthening grain boundary precipitates produced by normal precipitation heat treatment fall back into solid solution. However, the correct structure can be restored by subjecting the coated alloy to a further precipitation heat treatment. This is 24 hours at 840ºC for IN 738 alloy.

(vi) Change in the duration of the diffusion heat treatment

Increasing the time at the normal diffusion temperature from 870ºC to 885ºC resulted in a reduction in cracking in the samples. Increasing the diffusion heat treatment time also produced some thicker coatings, but the effect was not as pronounced as it was at the increased temperature.

Overall, little change in the coating structure was observed, demonstrating that the times used in Serma-Loy J production, i.e. 2 hours, are sufficient to produce a properly diffused coating.

Corrosion tests

Accelerated heat corrosion testing was carried out on specimens of pins made of IN 738. These pins had a diameter of 6.5 mm and were 65 mm long, and were coated with various designs as described above and manufactured by different processes.

Prior to application of the coating slurry, the samples were degreased in a hot vapor of 1,1,1-trichloroethane and then blasted with alumina grit (90/120 mesh) at 20 psi or 138 kPa.

A thin wet coating of each slurry was applied to the irradiated samples using conventional spray guns. After the water evaporated, the sprayed slurry was dried at 80ºC (175ºF) for at least 15 minutes and then cured at 350ºC (660ºF) for 30 minutes.

After the samples were cooled, the coating thickness was built up by applying additional slurry coatings according to the procedure described with respect to the disk samples.

Each of these coated samples was then subjected to a diffusion heat treatment during which a diffusion sion at 885ºC for two hours. After cooling the samples following their heat treatment, non-diffused residues were removed by light irradiation with -140 +220 glass bead grit at 20 psi (138 kPa) in an exhaust blower.

In the hot corrosion tests, the pins were heated for 3 minutes in an open flame fed with propane, reaching a steady temperature of 950ºC (1740ºF) within 60 seconds. Then, the hot pins were quenched to room temperature within 60 seconds in a finely sprayed aqueous solution containing 1 wt% Na2SO4 and 10 wt% NaCl. After three minutes of spraying, the complete heating and quenching cycle was repeated, and this continued for a total of 140 hours. Approximately every seven to eight hours, the test was interrupted and the pins were removed, cleaned ultrasonically in deionized water, weighed, and visually inspected. All defective pins were removed and replaced with clean pins to keep the mass constant.

Three different tests were conducted, using four or five different plating samples in each test, each with different plating samples on three pins, thus 12 to 15 pins were used in each test. Three of these pins in each test were coated with normal SermaLoy-J alloy.

The results of this test are illustrated in Figures 23 (15 pins), 24 (12 pins) and 25 (15 pins) and these figures show graphs indicating the weight loss in milligrams versus the test time in hours. On these graphs the solid lines refer to different types of coating samples and they only give an average weight value for each pin and therefore do not refer to individual pin results, which are discussed below. The graph lines for pins coated with normal SermaLoy J alloy are labeled 'J'.

Two of the coating samples in Fig. 23 are worth special consideration.

Three pins were tested for coating sample 1A9, which had the coating composition shown in Table 1. During approximately the first 40 hours of the test, the 1A9 samples lost significantly less weight at a slower rate than the other samples, including the SermaLoy J specimens. After 40 to 50 hours, two of the 1A9 samples began to deteriorate rapidly, although one, labeled 1A9' on the graph, showed better continued performance than all of the other samples until the test was completed at 60 hours.

The other notable coating sample in Fig. 23 is number 1A6, and the test again includes three pins that outlasted the SermaLoy J alloy coated pins. As can be seen from Table 1, this coating sample, described in connection with Figs. 5 and 18, consisted of elemental silicon and aluminum powders with 5 wt.% chromium and 10 wt.% silicon added at the expense of the usual aluminum content of a normal SermaLoy J alloy. The better performance is probably a consequence of the presence of a thick silicon-rich surface layer C resulting from the additional silicon and the addition of chromium as a vigorous silicide former.

Samples 1B3 and 1B9, which were subjected to high temperature diffusion heat treatment (Table 1), did not show optimal behavior. It is assumed that the The decrease in hot corrosion resistance is a consequence of the depletion of the aluminum content in the coatings, which has migrated into the substrate material during the high temperature treatment.

Nevertheless, in service where hot corrosion is not a serious problem, the thicker, more ductile coating and the better dispersion of the silicides therein, as indicated in the examples, can be considered an advantageous arrangement of the high temperature treatment, especially in combination with additions of silicide formers to the slurry.

Referring now to Fig. 24, the behavior of samples 7E, 4E and 9E is noteworthy. Although on average the 4E and 7E pins showed weight loss at a greater rate than the SermaLoy J alloy after approximately 75 hours, their prior weight loss was less than the SermaLoy J samples. In addition, one of the 7E pins, designated 7E' on the graph, consistently showed a much lower weight loss rate after 75 hours than the SermaLoy J samples, and the test was completed with a good thickness of coating remaining, while the three SermaLoy J samples were destroyed.

It is noteworthy that after 75 hours, one of the 4E pins showed weight loss at a noticeably lower rate than the SermaLoy J pins until approximately 120 hours were reached. This pin is indicated as 4E' on the graph.

The fact that most 4E and 7E pins did not show as favorable results as 4E and 7E may be a result of the additional heavy chromium powder in the coating slurry, making it difficult to ensure a uniform coating across the pin surfaces while preparing.

With respect to coating sample 9E, it is important to note that during the earlier stages of the test the pins appeared to exhibit less favorable behavior than the others, but their behavior was more uniform at the end of the test and they were not subjected to rapid weight loss of the remaining coating that characterizes the SermaLoy J coating samples. There was still some coating thickness available on all of the 9E pins at the end of the test.

Referring now to Fig. 25, the test results for three coating samples 9U (Table 1) are compared with three SermaLoy J samples and also with three samples of each of the three other coatings, labeled 1U, 2U and 6U, not shown in Table 1. All of these samples contained eutectic Al-Si alloy powder in the coating slurry.

Coating 1U served as a kind of negative standard to test the assumptions of the experimental strategy. The slurry contained, in proportion to the metal pigment content, chromium by weight with Si and Al contents of only 5 wt.% and 75 wt.%, respectively, with part of the aluminum content and silicon content being elemental powders and part consisting of a eutectic alloy powder. The aluminide produced by this slurry appeared to be very ductile, but did not exhibit the microstructure build-up that has been shown to be necessary for good hot corrosion resistance. The pins coated with this composition failed quickly (see Fig. 25).

Coating 2U contained no chromium, with the silicon content and aluminum content being 25 wt% and 75 wt%, respectively. As before, part of the aluminum content and silicon content consisted of elemental powders and part consisted of a eutectic alloy powder.

Coating 6U also contained no chromium, and the silicon content and aluminum content were 15 wt% and 85 wt%, respectively. Therefore, in terms of total Al and Si content, it is the same as a standard SermaLoy J slurry, but again, part of the aluminum content and silicon content consisted of elemental powders, and part consisted of a eutectic alloy powder.

All of the 2U and 6U pins failed long before the failure of the SermaLoy J samples, but at 140 hours the three 9U pins still had coatings while the three SermaLoy J pins had failed, as indicated by the intersection point X of the J line and the 9U line on the plot.

The final overall ranking of the best coating samples in terms of weight loss during the corrosion tests is as follows, starting with the lowest weight loss: 9E, J, 9U, 4E and 7E.

A pin coated with coating sample 9E provides a coating with improved performance. When a coating of example 9U is applied to a gas turbine engine rotor blade, a similarly improved coating is obtained. Similarly, a coating on the airfoil working section according to sample 1A6 provides a coating with a thicker surface layer with reduced silicon content and improved performance.

Other turbine parts such as vanes, disks, walls of combustion chambers, afterburners, thrust reversers, shafts and accessories therefor such as fasteners, bolts, etc., benefit considerably from the coatings according to the invention.

A fastener which is tested with the coating sample 9U coated provides a coating with improved performance.

The coatings of the invention are not limited to gas turbine engines, but include coatings for any coated superalloy parts subjected to working conditions normally encountered in reciprocating machinery, spacecraft (aerodynamic, heated surfaces), steam power turbines, and the like.

The tests reported above were carried out on samples consisting of a nickel-based IN 738 superalloy. Cobalt-based superalloys and austenitic stainless steel superalloys give comparably improved coatings when coated in accordance with the invention.

Hot corrosion resistance combined with resistance to crack initiation in the coating of a superalloy is a serious problem facing the industry. The problem becomes increasingly critical as the conditions of use become more demanding. The present invention contributes to solving this problem in this field.

Claims (66)

1. An aluminide-silicide coating on a heat resistant superalloy substrate (G), which coating has a highly dispersed distribution of silicide phases throughout the thickness of the coating, the coating having a resistance to the development of cracks and wherein the coating also has a resistance to oxidation and hot corrosion conditions, the coating having a layer (D) having a continuity of silicide phases and aluminide phases and a predominant aluminide layer (E) beneath and adjacent to the layer (D), and wherein the coating is formed from a slurry containing aluminum, silicon and a powder of a silicide-forming metal in an amount effective to promote the generation and dispersion in the coating of silicides corresponding to the metal. 2. An aluminide silicide coating according to claim 1, further comprising a silicon-enriched surface layer (C) above and adjacent to the continuous layer (D), which surface layer (C) has an increased silicon content compared to the continuous layer (D). 3. Aluminide-silicide coating according to claim 1, in which the layer (D) containing a continuum of silicide phases and aluminide phases is a layer consisting of interlayered layers of aluminum and silicide phases. 4. Aluminide silicide coating according to claim 2, in which the layer of interlayered layers (D) is at least 12 µm thick. 5. An aluminide-silicide coating according to claim 1, wherein the layer (D) consisting of a continuum of silicide phases and aluminide phases is a deposition layer consisting of a substantially continuous interlayered layer of aluminide phases and silicide phases and is at least four times as thick as the surface layer. 6. An aluminide silicide coating according to claim 5, wherein the deposition layer (D) is at least ten times as thick as the surface layer. 7. An aluminide-silicide coating according to claim 1, wherein the aluminide phase contains silicide precipitates substantially uniformly dispersed therein. 8. The aluminide-silicide coating of claim 1, wherein the aluminum in the layer (D) consisting of a continuum of silicide phases and aluminide phases and in the aluminide layer (E) is in the range of about 15 wt.% to about 30 wt.%. 9. The aluminide silicide coating of claim 8, wherein the aluminum is in the range between about 20 wt. % and about 30 wt. %. 10. The aluminide silicide coating of claim 8, wherein the aluminum content does not change by more than about 12 wt.% throughout the coating. 11. The aluminide silicide coating of claim 10, wherein the aluminum content does not vary by more than about 8 wt.% throughout the coating. 12. Aluminide-silicide coating according to claim 8, in which in the layer (D) and in the aluminide layer (E) the aluminum and the silicon change in depth approximately inversely proportional to one another. 13. An aluminide silicide coating according to claim 12, wherein the aluminum content in the coating does not exceed about 30 wt.%. 14. An aluminide silicide coating according to claim 12, wherein the surface layer (C) contains at least one of the elements Ta or Ti in solid solution with chromium silicide. 15. An aluminide silicide coating according to claim 14, in which the proportion of Ta or Ti in the surface layer (C) exceeds the weight percentage of the substrate material. 16. An aluminide silicide coating according to claim 15, wherein Ti or Ta is uniformly dispersed throughout the thickness of the coating. 17. An aluminide silicide coating according to claim 12, which contains chromium. 18. An aluminide silicide coating according to claim 17, wherein the chromium does not exceed about 10 wt.%. 19. An aluminide silicide coating according to claim 1, wherein the silicide former metal contains at least one of the following elements: chromium, tantalum and titanium. 20. An aluminide silicide coating according to claim 19, wherein the slurry further contains chromium and at least tantalum powder and/or titanium powder. 21. An aluminide silicide coating according to claim 1, wherein the silicide former metal contains at least one of the elements of rhenium, hafnium, manganese and elements of Group 4b and 5b from the Periodic Table of Elements. 22. Aluminide silicide coating according to claim 2, in which the surface layer (C) contains a maximum of about 25 wt. % silicon. 23. An aluminide silicide coating according to claim 22, wherein the aluminum in the aluminide layer (E) is in the range of about 15 wt.% and 25 wt.% and does not change by more than about 12 wt.% throughout the coating. 24. The aluminide silicide coating of claim 19, further comprising both Ti and Ta in the surface layer. 25. A slurry for coating a heat resistant superalloy substrate having resistance to crack development while maintaining good oxidation resistance and heat corrosion resistance, said slurry comprising an eutectic aluminum-silicon alloy powder and an additional metallic powder in elemental form selected from at least one metal of the group comprising Cr, Ti, Ta, Re, Mn, Hf, V and Nb, said metallic powder being contained in a heat curable liquid binder. 26. The slurry of claim 25, wherein the proportion of eutectic alloy in the slurry is in the range of about 36 to about 100 weight percent of the total metal content of the slurry. 27. The slurry of claim 25 additionally containing Si metal powder in elemental form in the range of about 5 to about 21 wt.%. 28. A slurry according to claim 27, wherein the chromium content does not exceed about 10% by weight. 29. A slurry according to claim 28, wherein the Ti content does not exceed about 5 wt%. 30. The slurry of claim 29, wherein Ta does not exceed about 5 wt%. 31. A slurry according to claim 30, which additionally contains B metal powder in elemental form not exceeding about 2 wt.%. 32. A slurry according to claim 31, which contains Si and Cr. 33. A slurry according to claim 32, which contains Ta and Ti. 34. A slurry according to any one of claims 25 to 33, wherein the heat-curable binder is an aqueous acidic binder. 35. The slurry of claim 34 wherein the aqueous acidic binder is selected from the class of inorganic chromate, phosphate, molybdate and tungstate. 36. A slurry according to claim 25, which additionally contains metal powder of Si or B in elemental form, wherein the heat-curable binder is an aqueous acidic binder. 37. The slurry of claim 25 additionally containing metal powder of silicon in the range of about 5 to about 18%. 38. A slurry for an aluminide silicide coating on a heat resistant superalloy substrate having resistance to oxidation and hot corrosion, the slurry containing metal powders in elemental form consisting of aluminum and silicon and at least one of the following metals: chromium, titanium, tantalum, rhenium, manganese, hafnium, vanadium and niobium. 39. A slurry according to claim 38, which contains at least chromium or additional metal powders of boron and at least one metal of titanium or tantalum. 40. The slurry of claim 38, wherein the aluminum is present in the range between about 30 wt.% and about 92.5 wt.%, silicon is present in the range between about 10 and about 25 wt.%, and chromium is present in the range of about 2.5 to 10 wt.%. 41. The slurry of claim 39, which additionally contains metal powder of boron in the range of between 0.5 to about 2.5 wt.%. 42. The slurry of claim 39, wherein the titanium and tantalum content ranges from 0.5 to about 10 wt.%. 43. The slurry of claim 38 wherein the aluminum content is in the range of less than about 85 to about 65 weight percent. 44. The slurry of claim 38 wherein the aluminum content is in the range of greater than 85 to about 95 weight percent. 45. The slurry of claim 39 containing both titanium and tantalum. 46. A slurry according to claim 38, wherein the heat-curable binder is an aqueous inorganic acidic binder. 47. The slurry of claim 38, wherein the metal is Cr. 48. A process for producing an aluminide-silicide coating from a slurry of an aluminum-silicon eutectic or from a slurry of metal powders in elemental form in a heat-curable liquid binder, the slurry containing a powder of a silicide-forming metal in an amount effective to promote the formation and dispersion of silicides corresponding to the metal in the coating, wherein a coating of the slurry is applied to a superalloy substrate (G) and the applied coating is subjected to diffusion heating, thereby forming a deposition layer (D) containing alternating substantially continuous interlayers of aluminide phases and silicide phases, and wherein a diffusion interface layer (E) is present between the coating and the substrate. 49. The method of claim 48, wherein the diffusion heating temperature is in the range of about 950°C to about 1120°C. 50. The method of claim 48, wherein the diffusion temperature is in the range of about 850°C to about 1120°C. 51. A process according to claim 48, wherein the diffusion temperature is below 870°C or above 885°C. 52. A method according to claim 48, wherein the curing of the coating occurs during heating. 53. A method according to claim 48, wherein a surface layer (C) is produced on the deposition layer (D). 54. Turbine part (G) made of a superalloy, coated with an aluminide-silicide coating which Coating containing highly dispersed silicide phases throughout its thickness and the coated portion having resistance to cracking, resistance to oxidation and resistance to hot corrosion, and wherein the coating comprises a deposit layer (D) consisting of substantially continuous interlayers of aluminide phases and silicide phases and a predominant aluminide layer (E) beneath and adjacent to layer (D) which predominantly contains silicide precipitates, the coating being formed from a slurry containing aluminum, silicon and a powder of silicide-forming metal in an amount effective to promote the formation and dispersion of silicides corresponding to the metal in the coating. 55. A superalloy coated turbine part according to claim 54, wherein the coating includes an outer layer (C) over and adjacent to the deposit layer (D). 56. A superalloy coated turbine part according to claim 55, wherein the outer layer is substantially continuous. 57. A superalloy coated turbine part according to claim 56, wherein the silicide concentration in the coating does not exceed 8% at any one location. 58. A diffusion heat treated aluminide-silicide coating structure for a refractory alloy substrate (G) containing mainly proportions of at least one of Ni and Co, the coating having a plurality of depthwise facing zones including a distinct deposit layer (D) consisting of intermediate layers of aluminide phases and silicide phases, wherein a distinct aluminide layer (E) having distinct silicide precipitates are disposed between the deposition layer and a diffusion interface layer (F) of the coating on the substrate (G), the coating being made from a slurry containing aluminum, silicon and a powder of a silicide-forming metal in an amount sufficient to promote the formation and dispersion in the coating of the silicides corresponding to the metal. 59. The coating structure of claim 58, wherein the silicide-forming metal contains at least one of the elements of chromium, tantalum, titanium, rhenium, zirconium, hafnium, manganese, and elements of Group 4b and 5b of the Periodic Table. 60. Coating structure according to claim 58, wherein a silicon-enriched surface layer (C) is provided having a maximum silicon content of less than 25 wt.% on the deposition layer (D). 61. A coating structure according to claim 60, wherein the surface layer (C) is at least 2 µm thick and the deposition layer (D) has a thickness at least four times as great as the surface layer (C), the deposition layer (D) being at least 12 µm thick. 62. Coating structure according to claim 61, wherein the deposition layer (D) is at least ten times as thick as the silicon-enriched surface layer (C). 63. Coating structure according to one of claims 58 to 62, wherein the aluminum content in the deposition layer (D) and the aluminide layer (E) is considerably higher than 15 wt.% and not greater than 30 wt.%. 64. The coating of claim 63, wherein the aluminum content of the coating does not change by more than about 12 wt.%. 65. Coating according to one of claims 58 to 64, in which the aluminum content in the deposition layer (D) and the aluminide layer (E) is not more than 30 wt.% and the aluminum content and silicon content therein change in depth approximately inversely proportional to one another. 66. A coating according to any one of claims 58 to 65, wherein the substrate contains proportions of at least one element of Ta and Ti and the surface layer (C) contains at least one element of Ta and Ti in amounts greater than the weight percentage of the substrate material (G).