The present invention relates to so-called "MCrAlY" overlay coating systems modified to enhance resistance of superalloy gas turbine components to high temperature oxidation and corrosion attack.
The term "MCrAlY" is a shorthand way of referring to well known temperature and oxidation/corrosion resistant alloy systems comprising in general one or more of nickel, cobalt and iron as the major "M" constituent, together with chromium and aluminium in quite large amounts, plus (usually) a minor amount of yttrium or other rare earth element. In weight percentage terms, such alloys may be broadly defined as having the following compositions:
For example, one well known alloy, used as the basis of the present work, has a nominal composition in weight percent of:
As mentioned above, these alloys are frequently used as protective coatings for superalloy components exposed to high temperatures in corrosive environments. In tests, we have found that the above CoCrAlY alloy, in the form of an overlay coating applied by an argon shrouded plasma spraying technique, has given satisfactory high temperature protection and mechanical properties for nickel based superalloys such as IN738. This superalloy is available from the International Nickel company, and has the nominal composition in weight percent of:
One problem with all protective coatings of this type, particularly in highly corrosive marine environments, is gradual degradation in service due to continued mobility of elements within the coatings and from the base material at the high operating temperatures of the turbines of gas turbine engines. Elements such as Mo,W,Ti,Ta, and Cr present within a superalloy such as IN738 migrate into the coating, thus reducing the basic mechanical properties of the superalloy. Furthermore, elements at the surface of the coatings, such as Al,Cr,Co,Ni,Ti, etc., are consumed during service by oxidation and sulphidation and therefore have to be replaced continuously to afford continued protection. Such replacement occurs by means of migration to the surface from the bulk of the coating and ultimately from the superalloy base material.
It is already known to improve the ability of MCrAlY protective coatings to resist high temperature oxidation and corrosion by aluminising them to produce an aluminide layer on top of the MCrAlY coating. In this context, "high temperature" means above about 750 °C. For example, British patent GB 1457033 discloses nickel- or cobalt - based superalloy components coated as follows:
Such coatings have been further improved upon by incorporating further refractory oxidation/corrosion resistant metallic elements such as platinum and hafnium into the outer aluminium coating and then diffusion heat treating the resulting duplex coating to produce an aluminide layer containing the refractory elements. For example, patent no. US 4123594 discloses Fe,Co, and Ni based superalloy components which are coated as follows:
This patent also mentions rhenium and paladium as possible constituents of the outer portions of the coatings as additions to, or substitutions for, platinum and hafnium.
The contents of the above prior patent specifications should be consulted for further and more complete details of the compositions and processes summarised above.
It should be noted that, at least to some extent, MCrAlY coatings do act as a barrier between the base material and any outer aluminide coating to limit migration of base material elements, particularly aluminium and chromium, during service.
We prefer to apply MCrAlY coatings by the argon shrouded plasma spray process as mentioned above, proprietary to Union Carbide Coatings Service Corporation, of Indianapolis, U.S.A. After spraying, the coatings can be aluminised, but must, of course, be heat treated to inter-diffuse them with the base material.
As a result of aluminising an existing MCrAlY coating, stable aluminides of the "M" constituent(s) of the MCrAlY coating are produced in the top layer of the resulting coating, such as nickel and/or cobalt aluminides, resulting in improved hot oxidation/corrosion resistance. In service, some of the Al component in the aluminides gradually migrates to the surface of the coating, where it is converted to alumina (Al₂O₃), so producing a corrosion/oxidation resistant barrier. Nevertheless, this barrier is gradually eroded and is replaced by further oxidation of aluminium migrating from the aluminides below.
Ideally, the required extra Al to maintain the concentration of aluminides in the surface layers of the coating could be supplied from the MCrAlY inner portion of the coating. But in the case of MCrAlY alloys with aluminium concentrations near the lower end of the above-mentioned ranges (say, 4 to 20 wt.%, e.g., 12%, as in the specific example mentioned above), the Al content is insufficient for the migration mechanism to be able to continue to supply the deficient surface layers with Al for a lengthy period. Despite this, MCrAlY coatings with such relatively low aluminium contents are preferred because higher Al contents tend to make the coating brittle in service after the necessary diffusion heat treatment has been carried out.
Consequently, it is desired to improve the hot corrosion and oxidation performance of aluminised plasma-sprayed MCrAlY coatings in which the MCrAlY component of the coating has a relatively low Al content of 4 to 20wt.% in the as-applied condition.
In addition to the hot oxidation/corrosion resistance referred to above, it is also desirable to improve the ability of MCrAlY coatings to resist low temperature oxidation and corrosion, "low temperature" in the present context meaning about 550 - 750 °C. This, of course, should be accomplished without any detrimental effect on the high temperature protection afforded by the coatings.
One known way of improving low temperature oxidation/ corrosion resistance of any coating system is to increase the concentration of chromium, at least near the surface layer of the coating. Generally speaking, it has been found that low temperature oxidation/ corrosion resistance improves as Cr concentration levels in the coating increase to about 40%, but that further increases above 40% are deleterious. The process of increasing the Cr content of an existing coating is called chromising, and is usually accomplished by known vapour or pack chromising methods, such as those marketed by Chromalloy Corporation.
Chromising improves low temperature oxidation/ corrosion resistance of coating systems because Cr oxidises to form chromia, Cr₂O₃, which forms a protective oxide film at the coating surface. Unfortunately, chromising is ineffective to improve high temperature oxidation/corrosion resistance of coating systems. This is because Cr₂O₃ dissociates into Cr and gaseous CrO₃ at temperatures above about 750°C.
Accordingly, the present invention includes a process for providing an M-based superalloy base material, where M is at least one of iron, cobalt and nickel, with a graded multiplex protective coating system containing aluminides in and near its surface, the process comprising the steps of:
Preferably, the platinum layer has a thickness of 5 to 15µm when applied, and the subsequent diffusing heat treatment is conveniently that normally utilised to restore the properties of the base material after the aluminising step. In the case of alloy IN738, this is one hour diffusion treatment at 1120°C, gas fan quench, and age 24 hours at 845°C.
Given the above heat treatment time, the thickness of the platinum layer as deposited is very important to the final structure of the above coating. Utilising a deposited platinum layer having a thickness of 10 to 15µm gives a structure comprising an MCrAlY coating having a single surface layer containing platinum modified aluminide. This is achieved because during heat treatment the platinum and the underlying aluminides inter-diffuse completely.
Alternatively, to obtain a structure comprising an MCrAlY coating having a double surface layer in the form of a top layer containing platinum modified aluminide and a sub-surface layer containing aluminides substantially without platinum modification, the deposited platinum layer should have a thickness of 5 to 10µm. This can be achieved because during heat treatment the platinum and the underlying aluminides do not inter-diffuse completely.
The present invention also includes an alternative process for providing an M-based superalloy base material, where M is at least one of iron, cobalt and nickel, with a graded multiplex protective coating system containing aluminides with enhanced aluminium content in and near its surface, the process comprising the steps of:
Preferably, the platinum layer has a thickness of 5 to 10µm when applied.
The time of aluminising is very important to the final structure of the above coating. Utilising a deposited platinum layer of the above thickness, to obtain a structure comprising an MCrAlY coating having a single surface layer containing platinum modified aluminide, the platinised coating should be aluminised only for a relatively short time of about 4 to 6 hours at the aluminising temperature. This will allow the aluminium to diffuse into the platinised coating only about as far as the platinum has already diffused. Alternatively, to obtain a structure comprising an MCrAlY coating having a double surface layer in the form of a top layer containing platinum modified aluminide and a sub-surface layer containing aluminides substantially without platinum modification, the platinised coating should be aluminised for a relatively long time of about 6 to 12 hours at the aluminising temperature. This will allow the aluminium to diffuse into the platinised coating further than the platinum has already diffused.
The chromising step in the above processes, if performed, is performed essentially to obtain a large gradient of concentration of Cr in and near the surface of the MCrAlY coating. This increases its low temperature sulphidation resistance.
An increase in the chromium content of various superalloy coating systems increases their low temperature sulphidation resistance, though an upper limit of about 40wt.% Cr content is usually adhered to because too high a Cr content causes excessive brittleness. This low temperature sulphidation resistance is achieved because Cr in the coating oxidises to chromia (Cr₂O₃), which is a relatively inert at temperatures below about 750°C. However, above 750°C, chromia dissociates, with the escape from the surface of the coating of CrO₃ gas and consequent disruption of the oxide surface scale, leaving the underlying layer open to further corrosive attack. Hence, Cr enrichment of MCrAlY coatings is conventionally understood to be ineffective to increase sulphidation resistance at temperatures above about 750°C.
However, in our invention, we have found that by enriching the Cr content of the MCrAlY coating, preferably up to its maximum advisable limit of about 40wt.% utilising the chromising step, and thereafter aluminising, optionally platinising (alternatively, and preferably, this optional platinising step can be done before aluminising, which has the effect of increasing the concentration of Al in the resulting layer of platinum modified aluminide), and finally heat treating, we produce a synergistic effect in which the presence of high concentrations of aluminides along with the high concentrations of chromium in the surface layer(s) of the MCrAlY coating changes the reaction kinetics of the chromia at the surface of the coating at temperatures above about 750°C, such that dissociation of chromia encourages the formation of further alumina, which is resistant to corrosive attack at these temperatures.
Examples of various prior art coatings and coatings according to the invention will now be described with reference to the accompanying figures, in which:
In tests of the effectiveness of various modifications to the CoCrAlY overlay coating system described above, the overlay coating was first applied to IN738 alloy bars by the previously mentioned argon shrouded plasma spray technique. Extra elements were then introduced into the outer layer of the coating using various methods as described below:
In addition to the above six processes involving modification of an initial MCrAlY coating, two further coating systems were tried:
The major coating processes referred to above are characterised as follows.
The vapour aluminising process took place in an argon-filled retort vessel, with the parts to be aluminised being suspended in close proximity over packs of aluminising powder which liberate aluminium halide gas when heated. Aluminium from the gas is deposited onto the parts and diffuses into it due to the elevated temperature. A commercial example of this technique is the RT69 (trade name) process available from Chromalloy U.K. Limited, of Bramble Way, Clover Nook Industrial Estate, Somercoates, Derby DE55 4RH, England, or its parent company, Chromalloy Research and Technology, Blaisdell, Orangeburg, NY 10962, U.S.A.
Platinum plating was accomplished by an electroplating process, again available from Chromalloy at the sites mentioned above under the trade name RT22.
Vapour chromising is performed in a similar way to vapour aluminising and is also available from Chromalloy under the trade name CN70.
The samples processed as outlined above were subjected to simulated isothermal sulphidation tests. The coated test pieces were placed in a Nimonic boat and covered with 50/50 mixture of Na₂SiO₃ and MoS₂ paste. The samples were then tested at 850°C in a muffle furnace for times of up to 3550 hours. At intervals a sample of each coating system type was removed and carefully prepared for microscopic examination. The depth of attack was recorded for each test piece. Results of this test are given in Figure 4, which is a graph of depth of penetration of the coating in µm (microns) against time of the test in hours.
Below is a short commentary on the microscopic examinations of the tested samples for each type of coating mentioned in paragraphs 1 to 8 above.
As can be seen from Figure 4, overall best performance in the sulphidation tests was given by coating (5), which had been chromised, aluminised and platinised with the 8µm (micron) platinum plate diffused layer.
Next best performers were coatings (4), (6), (3) and (2), which were very close together in apparent sulphidation performance. Coating (4) had been processed in the same way as coating (5), except for the application of the thicker 16µm (micron) platinum plate. Coating (6) had been chromised and aluminised, but not platinised. Coatings (3) and (2) had been aluminised and platinised, but not chromised.
The trend shown here is that sulphidation performance is increased by the chromising step, and increased even further by platinising, provided that aluminium concentrations in the platinum modified aluminide layer can be kept as high as possible. Lower concentrations of aluminium in such cases are associated with application of thicker platinum plate for diffusion.
Figure 5 shows a typical microstructure of coating (5) at x200 magnification before sulphidation testing, with the various layers indicated. Figure 6 shows the result of an electron probe microanalysis of this coating, in which the concentration in weight percentage terms of various elements in the coating is graphically plotted against the depth of the coating in µm (microns). Here it can be seen that the Cr content of the coating is up to about 40wt.% in the chromised layer, up to about 20 wt.% in the platinum aluminide layer and about 25wt.% retained in the body of the MCrAlY coating. On the other hand, the highest aluminium content is only about 20 wt.% and is less than this near the surface of the coating in the platinum modified layer. It should be noted that the results of the electron probe microanalysis as plotted are subject to variation due to experimental error and to local variations in element concentrations in the small scale microstructure.
From the work as detailed above it was evident that with samples (4) and (5) the Al concentration in the outer zones was lower than is generally accepted to be optimum in aluminide coatings. Consequently, further work was undertaken with test pieces in which the process activity and sequence of operation were altered to increase the Al concentration level to 25 - 30%. Alterations were essentially to apply the platinum plate directly onto the chromised MCrAlY overlay coating and platinise by diffusion heat treating at 1050°C for 2 hours before the aluminising step. Depending upon the depth of the platinum plate as applied and the time for which the platinised coating was subsequently aluminised (thicker platinum modified layers require longer aluminising times), it was found that the structures illustrated in Figures 5 and 6 could be reproduced, but with increased Al concentrations of 25-30% in the aluminide layers. This increased Al concentration enabled achievement of the required synergistic effect of extra alumina scale production at the surface of the coating at high temperatures consequent upon dissociation of chromia, thereby enhancing high temperature sulphidation resistance.
Although of maximum advantage when utilised in conjunction with the chromising step, the method of platinising an MCrAlY coating before aluminising it, in order to gain higher Al concentrations in the platinum modified aluminide surface layer of the finished coating, will also plainly be advantageous when utilised without the prior chromising step.
Although the above description has concentrated on the use of a CoCrAlY overlay coating on an IN738 nickel based superalloy substrate, the invention is plainly more generally applicable to MCrAlY-type coatings on a variety of superalloy substrates.