Diffusion barrier layers

Details Diffusion barrier layers Diffusion barrier layers

1995-10-25

1998-04-21

Wyszomierski, George

Stock material or miscellaneous articles

All metal or with adjacent metals

Composite; i.e., plural, adjacent, spatially distinct metal...

428670, 428680, 148525, 148527, 4273837, B32B 1501

Patent

active

057416044

DESCRIPTION:

BRIEF SUMMARY
This application is a 35 U.S.C. .sctn. 371 national phase application of PCT/GB94/00301, filed Feb. 15, 1994.
The present invention relates to protective coatings for metallic articles, and in particular to stable diffusion barrier coatings which can be applied to inhibit the deterioration of substrate materials as a result of interaction with external stimuli.


BACKGROUND OF THE INVENTION

The utility of many materials is limited by their tolerance of operating conditions since, in hostile environments such as extended exposure to high temperatures and temperature cycling of the type found in gas turbine engines, environmental degradation can be very severe.
Over the years, technology has developed a variety of protective e coatings to extend the operating lifetime and/or the maximum permitted working temperature of many materials. However, the coatings of choice for particular applications, for example coatings having good oxidation or corrosion resistance, are not necessarily compatible with the substrate material to which they are applied. In many cases there are unfavourable interactions between the material of the substrate and the coating composition, with the result that the physical and mechanical properties of the substrate are compromised. Deterioration of metallic materials is accelerated at elevated operating temperatures.
Formation of intermetallic protection layers of high temperature components is known in JP 570155364, where a PtAl.sub.2 discontinuous intermetallic phase of approximately 35-50 .mu.m is formed by a diffusion pack aluminising process at a temperature of 1150.degree. C. High temperature materials are defined as those materials capable of operating at temperatures of 500.degree. C. or greater. This protection layer does not afford uniform protection for the substrate material.
There is therefore a need for a uniform coating technique for high temperature components which will allow the best possible coatings for a given purpose to be applied to a particular substrate regardless of the interactions which might otherwise occur between the two.


BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic illustration of the sequence of steps necessary to carry out the invention.


DESCRIPTION OF THE INVENTION

According to the present invention, this is achieved by applying a stable continuous uniform diffusion barrier as an intermediate coating between the protective coating and the surface of the high temperature substrate material. The diffusion barrier serves to inhibit the breakdown of the protective coating system by minimising coating/substrate interactions, such that efficacy of the protective coating is maintained even though the composition of the coating may be altered by loss through surface oxidation/corrosion. The diffusion barrier also helps to preserve the physical and mechanical properties of the substrate, by limiting unfavourable interactions.
In particular, the inventive technique relies on the in situ formation of a continuous stable diffusion barrier by means of sequential layering and subsequent reaction treatment of suitable metallic species to produce a diffusion barrier of intermetallic form. This inventive concept extends also to diffusion barriers consisting of multi-intermetallic layers each limiting the diffusion of a specific element (or elements), and is not necessarily limited to the formation of a single intermetallic diffusion barrier layer of homogeneous structure. By selection of appropriate intermetallic species, interdiffusion of the protective coating through the barrier can be minimised.
The invention is a method of producing a continuous stable intermetallic diffusion barrier on a metallic high temperature article, the method comprising the steps of: article; article to a depth sufficient to provide a predetermined molar ratio of the first and second metals, and combine to form an intermetallic species.
The term metallic is used to define substrates made of metal, intermetallic or alloy materials.
For certain applications, it may be advanta

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