Protective coating

Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Coating contains embedded solid material

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Details

205170, 205176, 205181, 205191, 205194, 205195, 205227, 205228, C25D 1502

Patent

active

058338292

DESCRIPTION:

BRIEF SUMMARY
This is a national stage application of PCT/GB95/01746 filed Jul. 24, 1995.
The present invention relates to the provision of protective coatings, such as overlay coatings, on substrates. Such coatings are employed on components which are subjected to high temperature environments, particularly where corrosion and/or erosion is likely to occur. The primary but not necessarily sole application of such coatings is to parts of gas turbine engines, particularly superalloy components thereof, such as gas turbine shafts, rings, disks, combustion can ware, stator and rotor blades and guide vanes. The invention also relates to such parts, and to machinery and vehicles or fixed installations which incorporate such parts.
It has long been recognised that components of gas turbines, particularly the internal components thereof in the vicinity of and downstream of the combustor, need to exhibit high strength and corrosion resistance at high temperature.
It is known to provide such components with a load bearing structure of superalloy material, to provide sufficient high temperature strength. Typical superalloys used (examples are those known under the trade designations IN100, IN718, IN738, MAR-M002, MAR-M247, CMSX-4, PWA1480 and PWA1484) are the Ni, Co and Fe base superalloys, dependent upon the particular application requirements. Fe and Co base superalloys are often solid solution strengthened. Ni base alloys have Ni as the major constituent and often contain amounts of Cr Co Fe Mo W or Ta, and are often solid solution or precipitation strengthened. Precipitation strengthened Ni base alloys are widely used for gas turbine components and often contain Al Ti or Nb to produce a precipitated second phase in the course of a suitable heat treatment. Examples of Ni base precipitation strengthened superalloys used for gas turbine components are those known under the trade designations INCO 713, B-1900, IN 100, MAR-M 200, and MAR-M 247. Examples of Co base superalloys are MAR-M 509 and Haynes 188, and examples of Fe base superalloys are Incoloy 802 and Incoloy 903. Superalloy gas turbine components are sometimes wrought or cast and, for the more extreme operating conditions, may be directionally solidified or in the form of single crystal structures.
It has become common practice to coat superalloy components with corrosion resistant material since the superalloy itself will not normally be capable of withstanding the corrosive/oxidative in-service atmosphere.
One practice is to aluminise the superalloy. This is usually accomplished using a so-called pack aluminising process, or by physical vapour deposition. These processes involve diffusion of Al into the superalloy to form aluminides such as NiAl in the case of Ni base superalloys. In service, a surface layer of Al.sub.2 O.sub.3 forms to protect the material beneath and this tends to exfoliate due to thermal expansion and contraction. This is gradually repaired by outwardly diffusing Al and finally, when there is no longer sufficient Al to replace exfoliated material at a particular location, the superalloy component will be liable to rapid localised corrosion. Chromium and silicon either together or singly and alone or in addition to aluminium may likewise be diffused into the superalloy to form a surface layer including chromides or silicides. Although reference will be made hereafter mainly to aluminising it should be understood that such reference should be interpreted as alternatively referring mutatismutandis to chromising and/or siliconising.
A further practice is to coat the superalloy with an overlay of, for example, MCrAlY, MCrAlHf, MCrAlYHf, MCrAlYHfSi and MCrAlTaY where M is Co or Ni or Fe or a mixture thereof. The addition of Y Si or Hf helps to prevent exfoliation of Al.sub.2 O.sub.3 from the surface and thus extends the life of the component. These materials may be applied by plasma spraying; or by a co-deposition process, such as the process we describe in our patent GB-B-2 167 446. It is usual to coat a component with these materials to produce a layer

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patent: 4789441 (1988-12-01), Foster et al.
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patent: 4880614 (1989-11-01), Strangman et al.
patent: 5037513 (1991-08-01), Foster
patent: 5500252 (1996-03-01), Meelu

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