GB2182347A - An overlay coating alloy - Google Patents

Abstract

The alloy has a base of nickel and/or cobalt or iron and comprises 20-40 Cr; 6-12 Al; 0.5-12 Si (weight percent). A specific alloy for marine turbine use is 35 Cr-8 Al-8 Si balance Ni (weight percent). The coating is deposited by sputter ion plating, or alternatively by plasma spraying or electro-deposition.

Description

SPECIFICATION Overlay coating of superalloys This invention relates to overlay coating of the socalled superalloys for enhancement of their resistance to surface degradation by oxidation corrosion and hot erosion. The superalloys are an internationally recognised group of materials each developed for high performance applications involving relatively high stresses and temperatures generally above 540"C where resistance to oxidation and corrosion is required, and having a composition based on nickel, cobolt, iron or nickel plus iron, with chromium as an alloying ingredient.

Early nickel based superalloys forturbine blade applications included a high percentage of chromium (eg 20 weight percent) and this confired a good resistance to both oxidation and corrosion attack, by formation of a surface chromium oxide scale. More recent superalloys, devised for improved engine performance through operation at higher temperatures, have reduced chromium content (as low as 5 weight percent) and lower resistance to surface degradation. These alloys are usually given increased resistanceto surface degradation bythe provision of a protective coating.

There is a wide range of materials and processes used in the coating ofsuperalloycomponentsforgas turbine applications. The properties required of a suitable coating material embrace the following: a. high resistance to oxidation- and/orsulphidation corrosion; b. sufficient ductility to enable the coatig to withstand changes in the dimensions ofthe coated com ponent without cracking thereof; c. compatabilitywith the alloy of the component in terms of constitution and coefficient of thermal expansion; and d. ease of application.

Anti-degradation coatings for superalloys fall into two largely distinct categories. The first of these is the diffusion coating wherein the degradation resistance is generated by interdiffusion of the coating material and substrate alloy. The second category is that of the overlay coatings. In these coatings the degradation resistance is an inherent property of the coating and the degree of interdiffusion between coating and substrate alloy is much reduced.

Diffusion coatings produced by the so-called packaluminising or pack-cementation chemical vapour deposition (cvd) processes have been widely used as have, to a lesser extent, coatings produced by the broadlysimilarchromising and siliconising processes. The pack-aluminising processes form aluminides of nickel, cobalt or iron depending upon the composition of the substrate alloy. Aluminide coatings have very good oxidation resistance attempera- tures up to 11 00'C.Chromised coatings have good resistance to sulphidation corrosion attemperatures uptoapproximately800Cbutdo nothavesignifi- cant thermal stability in contact with oxygen bearing atmospheres at temperatures above approximately 850 C. Silicon enriched coatings produced by siliconising also have a restricted temperature capability.The interdiffision between coating elements and substrate which is necessary to convertthe diffu- sion coating elementtothestablealuminide chromide or siliconide form can detract from the mechanical properties of the substrate component, in particular by reducing the load-bearing crosssectional area which reduction can be very significant in the case ofthin-walled components (such as turbine blades with internal cooling passages) orat leading and trailing edge regions. In castings having wall thicknesses ofthe order of 1 mm some 30"C i n creep rupture properties can be lost from this cause.

Aluminide coatings produced by cvd processes tend to be susceptibleto sulphidation corrosion attack which is undesirable in gas turbine engines employed in marine environments where sea salt accelerated corrosion can be severe, the mechanisms of corrosion by contaminated gas stream being numerous and complicated.

Overlay coatings may be deposited by physical vapour deposition (pvd) methods. Although these coatings do require some limited inter-diffusion between coating and substrate to facilitate good bonding they do not rely on diffusion with substrate el ements for the formation of the coating itself and loss of mechanical properties of the substrate component is, therefore, minimal. Overlay coatings are also more ductile than the aluminide (cvd) coatings at temperatures below a pproxi m ately 8000C.

Overlay coatings utilising the MCrAIYsystem (where M is a Fe, Co, or Ni base) are well known.

These coatings are designed to produce an alumina or alternatively a chromia scale as may be required by the intended engine environment. The yttrium promotes adherence of the corrosion resistant alumina or chromia scale. The base alloy, ie the M in the MCrAIYformula, is selected from the group of candidates on the basis of compatability with the base material in the substrate alloy. Whilst mixtures of eg Ni and Co have been utilised as the base in gredients in McrAlY overlay coatings for Co-based superalloys it is usual to use a coating base ingredientwhich is the same as that of the substrate alloy.In alloys ofthis system it is usually considered that an individual alloy having recognisable properties may be adequately defined by specification of the individual ranges forM, Cr, Al and Ywith Fe, Ni and Co being mutually interchangeable as the M ingredient. The use ofthe alloy as a coating will require appropriate matching ofthe base ofthe coating alloy to the base of the substrate alloy as will of course be apparenttothoseskilled intheart.

Alloys suitable for use as overlay coatings on gas turbine component materials such as nickel-based superalloys can be produced having very good resistanceto sulphidation corrosion.

One such range of alloys is described by Higginbotam petal in U K Patent Specification No. 1,426,438 and is primarily intended to combat sulphidation corrosion in the temperature range 700-900'C while still retaining adequate oxidation resistance at elevated temperatures. The alloy comprises a chromium con tent of 12 to 20 wt%, chromium being the principal element employed to enhance resistanceto sulphid- ation corrosion and oxidation. The specification states that work on simple alloys indicated a practical upperlimit of 20wt% chromium. However,thisstate- ment is apparently based on the false premise that the mechanisms ofsulphidation corrosion between 700 and 900"C are all comparably similar.This is not the case as research has since shown. To reduce fuel consumption many marine gas turbine installations in ships are run at reduced power levels with the re sultthattheir operating temperatures often fall to below750 C. It was previously thought that higher temperature sulphidation corrosion processes were caused by molten sodium sulphate, Na2SO4, condensed on the blade surface and,therefore, capable of severe attack only attemperatures near to or above the melting point of Na2SO4, ie 884'C.This view persisted when gasturbineswere operated at relatively high power levels and temperatures of operation were nearthe melting point of 884"C. However, when operating temperatures fell to 750'C and below as a consequence of reduced power running, instances have been experienced of very severe corrosion of blades and nozzle guide vanes, corrosion rates sometimes being in excess ofthose experienced atthe higher operating temperatures. It was then realised that corrosion could not be due to the action of Na2SO4 alone.Extensive research has shown the cause of lowtemperature sulphidation corrosion to be due to traces ofsulphurtrioxide, SO3, in the engine gas stream reacting with cobalt and nickel ox ides present in the protective oxide film on the com ponent surface to form cobalt and nickel sulphates.

These cobalt and nickel salts react with Na2SO4, to form mixed sulphates having a melting point below 650 C thus enhancing the corrosion rate. Above about 750"C the sulphurtrioxide becomes less stable underturbine conditions, hence, the corrosion rate diminishes. At temperatures above 850the pred ominantreaction occurring is that of the formation of oxides of aluminium, nickel, chromium etc, even in marine atmospheres. Theformation of these oxides hamperthe reactions occurring during sulphidation corrosion and thus retards the rate of corrosion.

UK Patent Specification 1,426,438 describes only tests carried out at 870"C and 1050"C where oxidation reactions and not corrosion reactions predominate.

The statementthatthe practical upper limit of chromium content is 20% does not, therefore, take into account the sulphidation corrosion mechanisms occurring below approximately 750"C. This is a part- icularly important requirement for marine gas turb ine engines.

One prior art document disclosing overlay coat ingswith a higher chromium contentistheApp licants' European patent 0,025,263. This patent con cerns overlay coatings utilising a MCrTiAI system (where M is Ni or Co). For this system Si and Hfare disclosed as optional ingredients. Whilstthe App licant's previous MCrTiAl compositions were inten ded to provide overlay coatings having resistance to sulphidation corrosion to a level in advance of the art at least in a marine engine environment it has now been demonstrated in the Applicant's current experi ments that a new overlay coating composition can provide yet superior performance.

Accordingly one aspect of the present invention is an overlay coating alloy for a superalloy component, consisting essentiallyofthefollowing ingredients in the ranges stated below: chromium 20 to 40 percent aluminium 6to 12 percent silicon 0.5 to 12 percent balance M, where M is a single metal orcombination of metals selected from the group consisting of nickel,cobaltand iron.

All compositions given in this specification and the appendent claims are in percentage by weight unless specifically stated to be otherwise.

A preferred overlay coating alloy consists essentially of: chromium 30 to 40 percent aluminium 6to 12 percent silicon 0.5to 10 percent balance M where M is as defined above.

One example of an overlay coating in accordance with the current invention was prepared to the formula 35Cr-8AI-8Si balance Ni. This alloy was deposited upon superalloy components having the nominal composition: Ni(bal)-8.5 Co-l 6.0 Cr-1.75 Mo-2.6W- 1.75 Ta-0.9 Nb-3.4 Al-3.4 Ti-0.1 7 C-0.01 B-0.1 Zrto a thickness of 75 to 125 Fm by a sputter ion plating process.These coated components were subjected to comparative trial in the Applicant's marine engine experiments consisting of several thousand hours of accumulated engine running, against commercially available coating such as a platinum-aluminium diffusion coating, MCrAIY overlay coating and the Applicant's own prior art NiCrAITi overlay coatings and werefoundto be superiorto these other coatings. It is presently considered that a composition defined as follows: chromium 30to40 aluminium 6to12 silicon 6to10 balance M where M is as defined above; represents a best optimisation of the invention for marine turbine usage.The relatively high silicon content is particularly important in this environment because it enhances the hot corrosion resistance of the coating relative to those compositions that are silicon-free.

This appears to be primarily dueto the highly prot- ective nature of alumina films containing a proportion of silica. Coating compositions containing a proportion of silicon appears to have a better structural integrity, especially under marine gas turbine operating conditions, than those that are without silicon.

It is considered that the coating alloy of the present invention may also be optimised for application to a high-temperature high-performance aero engine environment. In such applications the predominant degradation mechanism is likely to to be oxidation rather than corrosion caused by sea salt residues.

Moreover the increase in maximum temperature introduces yet further complications. For these appliations a high silicon content is undesirable because silicon reduces the melting point of the coating and a high silicon content would foster undesirable interdiffusion between coating and coated component which would impairthe mechanical properties ofthe substrate. A reduction in silicon content would also improve the ductilitytemperature characteristic of the coating. However retention of at least 0.5 percent of silicon is considered necessary even in the above mentioned environment in orderto foster a neces sary degree of oxide scale adhesion.

For these high-temperature high-performance aero engine environments the aluminium should be towards the upper end of the range of the invention in orderto provide maximal long term high temperature oxidation resistance. Furthermorethe chromiumcontentshould belowerthanthatrequi- red for a marine engine environment in orderto avoid loss of coating through evaporation of chromium oxide. Accordingly it is presently considered that a coating alloy to the following essential composition represents a best optimisation for an aero turbine.

chromium 25to30 aluminium 10to12 silicon 0.5to 6 balance M where M is as defined above.

It is considered that cobalt and nickel may be exchanged for one another or combined in any proportion within the overall limit on the level of'M'without detriment to the coating alloy. Iron should preferably be used as the base of the coating alloy when the coating isto be applied to an iron-base substrate.

The coating alloy may also include minor additions of reactive elements such as yttrium, or rare earth elements, or hafnium or precious metals such as platinum or rhodium for their known benefits.

The presently preferred method of depositing the overlay coating alloyofthe invention is bysputter ion plating. This process iswell knownintheartso no description is given here. Whilstthe properties of the present coating alloy are to some extent dependent upon the adequacy of the method of deposition in regard to its ability to provide a coating of con sistentthickness and composition together with a good microstructure it is considered that other ion plating processes and allied processes, or plasma spraying, or electro deposition may be suitable as an alternative.

Asecond aspectofthe invention described herein is a superalloy component coated with a coating alloy according to any of the coating alloy composition of the first aspect of the invention. Preferably the coated component is one coated by sputter ion plating.

Claims (7)

1. An overlay coating alloy for superalloy com ponents, consisting essentially of the following in the proportions specified by weight: chromium 20 to 40 percent aluminium 6to 12 percent silicon 0.5to 12 percent balance M where M is a single metal or combination of metals selected from the group consisting of nickel, cobalt and iron.

2. An overlay coating alloy as claim in Claim 1 wherein the chromium content is 30 to 40 percent by weight.

3. An overlay coating alloy as claimed in Claim 2 wherein the aluminium content is 6to 12 percent by weight and the silicon content is 6 to 10 percent by weight.

4. An overlay coating alloy as claimed in Claim 1 wherein the aluminium content is 10 to 12 percent by weight and the silicon content is 0.5 to 6 percent by weight.

5. An overlay coating alloy as claimed in Claim 4 wherein the chromium content is 25to 30 percent by weight.

6. An superalloy component having an overlay coating comprising an overlay coating alloy as claimed in any one ofthe preceding claims.

7. A superalloy component as claimed in Claim 6 wherein the overlay coating is one deposited by sputterion plating.