Duplex-phase CrAl coating for improved corrosion/oxidation protection

Abstract

Disclosed is a coating for protecting a component against high temperatures and aggressive media, which coating has at least one subregion whose main constituent is chromium. The layer additionally comprises aluminum, the chromium content at least in the subregion in which chromium is the main constituent being greater than 30% by weight and the aluminum content being greater than or equal to 5% by weight. The invention further provides a process for producing such a coating, comprising chromizing the surface to be coated and subsequently alitizing the chromium-rich layer produced during chromizing.

Claims

1. A process for producing a coating for protecting a component against high temperatures and aggressive media, the component being formed by an alloy having one or more metallic main constituents which make up the largest proportion of the alloy, wherein the process comprises chromizing a surface to be coated and subsequently aluminizing a chromium-rich layer produced during chromizing, the chromizing being carried out with a chemical chromium activity of at least 0.4, and wherein the process affords a coating that has an outer zone and an inner zone, the outer zone comprising -chromium phases in a matrix of a mixture of mixed crystals comprising essentially chromium, aluminum, and the one or more metallic main constituents of the alloy, and the inner zone comprising a mixed crystal zone comprising essentially chromium, aluminum, and the one or more metallic main constituents of the alloy, the proportion of chromium in a total coating being greater than 30% by weight and a proportion of aluminum in a total coating being at least 5% by weight, and wherein at least one of: (i) a proportion of chromium in the outer zone is from 30% by weight to 95% by weight of chromium; (ii) a proportion of chromium in the -chromium phases is at least 70% by weight; (iii) a proportion of aluminum in the outer zone is from 10% to 40% by weight of aluminum; (iv) the one or more metallic main constituents in the outer zone are present in a proportion of not higher than 40% by weight; (v) in the inner zone a proportion of chromium is not higher than 30% by weight, a proportion of aluminum is not higher than 30% by weight, and a proportion of the one or more main constituents is at least 30% by weight; (vi) a proportion of chromium in the total coating is from greater than 30% by weight to 90% by weight; (vii) a proportion of aluminum in the total coating is from 10% to 40% by weight; (viii) the outer zone of the coating makes up a proportion of at least 50% of the total coating; (ix) the coating has up to 10% by volume of pores having average diameters of from 2 m to 20 m; (x) the coating comprises from 1% to 15% by weight of oxides; (xi) the one or more metallic main constituents of the alloy are one or more of nickel, iron and cobalt; (xii) the chromizing is carried out using a Cr-rich slip containing a liquid phase.

2. The process of claim 1, wherein the chromizing is carried out using a Cr-rich slip containing a liquid phase.

3. The process of claim 2, wherein the slip is applied by injection molding.

4. The process of claim 1, wherein the chromizing is carried out at a temperature of from 1020 C. to 1180 C. for a period of from 2 to 20 hours.

5. The process of claim 1, wherein the aluminizing is carried out at a temperature of from 1050 C. to 1150 C. for a period of from 3 to 20 hours.

6. The process of claim 1, wherein the chemical aluminum activity during aluminizing is at least 0.3.

7. The process of claim 1, wherein a first aluminizing is followed by a second aluminizing at a lower chemical aluminum activity at a temperature of greater than or equal to 1050 C. for a period of from 3 to 20 hours.

8. The process of claim 1, wherein the chromizing and aluminizing are followed by a diffusion heat treatment at a temperature of greater than or equal to 1050 C. for a period of from 2 to 8 hours.

9. The process of claim 1, wherein a surface treatment by PVD, CVD, surface coating, electrochemical deposition and/or direct application of a material, in which one or more elements from the group platinum, palladium, hafnium, zirconium, yttrium and silicon are applied, is carried out before, during or after chromizing and/or aluminizing.

10. The process of claim 1, wherein a proportion of chromium in the outer zone is from 30% by weight to 95% by weight of chromium.

11. The process of claim 1, wherein a proportion of chromium in the -chromium phases is at least 70% by weight.

12. The process of claim 1, wherein a proportion of aluminum in the outer zone is from 10% to 40% by weight of aluminum.

13. The process of claim 1, wherein the one or more metallic main constituents in the outer zone are present in a proportion of not higher than 40% by weight.

14. The process of claim 1, wherein in the inner zone a proportion of chromium is not higher than 30% by weight, a proportion of aluminum is not higher than 30% by weight, and a proportion of the one or more main constituents is at least 30% by weight.

15. The process of claim 1, wherein a proportion of chromium in the total coating is from greater than 30% by weight to 90% by weight.

16. The process of claim 5, wherein a proportion of aluminum in the total coating is from 10% to 40% by weight.

17. The process of claim 4, wherein the outer zone of the coating makes up a proportion of at least 50% of the total coating.

18. The process of claim 3, wherein the coating has up to 10% by volume of pores having average diameters of from 2 m to 20 m.

19. The process of claim 2, wherein the coating comprises from 1% to 15% by weight of oxides.

20. The process of claim 1, wherein the one or more metallic main constituents of the alloy are one or more of nickel, iron and cobalt.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The attached drawings show in

(2) FIG. 1 a diagram indicating the composition of the coating produced for the example of a chromium-aluminum coating on a nickel-based alloy;

(3) FIG. 2 a depiction of a coating as is present after the chromizing step;

(4) FIG. 3 a depiction of a coating as is present in the finished state;

(5) FIG. 4 a magnification of a transverse microsection of an exemplary coating layer according to the present invention; and

(6) FIG. 5 the distribution of Al and Cr along the depth direction in the coating layer shown in FIG. 4.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

(7) The particulars shown herein are by way of example and for purposes of illustrative discussion of the embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the present invention. In this regard, no attempt is made to show details of the present invention in more detail than is necessary for the fundamental understanding of the present invention, the description in combination with the drawings making apparent to those of skill in the art how the several forms of the present invention may be embodied in practice.

(8) FIG. 1 shows a ternary phase diagram in which the region of the composition to which the coating which has been applied according to the present invention to a nickel-based material is to be assigned is made clear. The hatched field shows the region of the composition which the coating according to the invention can have. Here, there is a high chromium content of more than 30% by weight of chromium, in particular in the range from 30% to 90% by weight of chromium, and a moderate aluminum content of from 10% to 35% by weight of aluminum. The proportion of the base material or of the main constituent thereof is below 30% by weight, i.e. in the present case below 30% by weight of nickel.

(9) FIG. 2 shows the formation of a chromium-rich layer after high-activity chromizing; here, an outer -chromium-nickel sublayer and a chromium-containing mixed crystal sublayer have been formed. The mixed crystal sublayer is formed by mixed crystals of chromium and the main constituent of the base material, i.e., for example, NiCr in the case of application to nickel-based alloys. The chromium-rich layer of the -chromium-nickel sublayer and the mixed crystal layer has a chromium content of greater than or equal to 40% by weight. Both in the outer layer and in the inner layer, nickel, elements of the base material and/or deliberately introduced platinum and palladium, silicon, hafnium, yttrium and/or zirconium can be present.

(10) The component bearing a correspondingly configured intermediate layer is subjected in a second step to an alitizing step in which aluminum diffuses into the intermediate layer so as to form an AlNiCr matrix in which -chromium phases are incorporated in an outer zone, as shown in FIG. 3. The -chromium phases can have a Cr content of more than 40% by weight, with the balance being essentially nickel. The outer zone having the bimodal microstructure makes up a proportion of more than 60% of the total layer thickness. The inner zone comprises only an NiAlCr mixed crystal having a composition of more than 30% by weight of nickel, less than 40% by weight of Cr and less than 30% by weight of Al. The -chromium phase has a proportion by volume in the bimodal microstructure of 10-90% and in the precipitated form is globulitic and ellipsoidal having a diameter of from 1 to 40 m. The AlCrNi phase correspondingly has a proportion by volume of 90% in the bimodal microstructure.

(11) The AlNiCr matrix of the outer zone comprises, in particular, Al.sub.xNi.sub.y, AlNi, Al.sub.3Ni.sub.2, Al.sub.3Ni and Cr.sub.2Al phases, while essentially NiAl mixed crystals having proportions of chromium are present in the NiAlCr mixed crystal zone of the inner zone.

(12) The -chromium phase of the outer zone has chromium contents of greater than or equal to 70% by weight of chromium, with essentially nickel being additionally dissolved in the -chromium phases. The total layer has a chemical composition of from 30% to 90% by weight of chromium, from 10% to 35% by weight of aluminum, up to 60% by weight of nickel, proportions of up to 25% by weight of platinum, palladium, up to 15% by weight of silicon, up to 15% by weight of hafnium, zirconium. The total layer thickness can be from 20 to 150 m.

(13) FIG. 4 shows a magnification of a transverse microsection of an exemplary coating layer according to the present invention. More specifically, FIG. 4 shows a bimodal microstructure of chromium rich alpha-phases embedded in an AlNiCr-matrix (substantially corresponding to the diagrammatic illustration of FIG. 3). The layer shown in FIG. 4 has a depth of 85 micrometer and exhibits along the depth direction a distribution of aluminum and chromium as shown in the diagram of FIG. 5 (the x-axis of FIG. 5 refers to the depth in micrometer, and the y-axis of FIG. 5 refers to the weight percentage of Al and Cr in the layer). As can be seen in the diagram of FIG. 5, between the upper surface of the layer and a depth of about 60 micrometer the content of chromium is between 60 wt-% and 78 wt-% and the content of aluminum is between 10 wt-% and 20 wt-%. Thereafter, the content of chromium significantly lowers.

(14) It is noted that the foregoing examples have been provided merely for the purpose of explanation and are in no way to be construed as limiting of the present invention. While the present invention has been described with reference to exemplary embodiments, it is understood that the words which have been used herein are words of description and illustration, rather than words of limitation. Changes may be made, within the purview of the appended claims, as presently stated and as amended, without departing from the scope and spirit of the present invention in its aspects. Although the present invention has been described herein with reference to particular means, materials and embodiments, the present invention is not intended to be limited to the particulars disclosed herein; rather, the present invention extends to all functionally equivalent structures, methods and uses, such as are within the scope of the appended claims.