GB2167446A - Electrode deposited composite coating - Google Patents

Abstract

A substrate such as a single crystal gas turbine blade is given a protective coating by composite electrodeposition of a metal matrix M1 containing particles of CrAlM2, where M1 is Ni or Co or Fe and M2 is Y, Si, Ti, or a rare earth element. The preferred matrix is Ni or Co or NiCo and the preferred particles are CrAlY. The particle size is important and preferably 75% by weight are between 3.9 and 10.5???m. the coating is preferably heat treated to produce interdiffusion between the constituents of the matrix and the particles.

Description

SPECIFICATION Metallic protective coatings This invention relates to metallic protective coatings suitable for the protection of substrates intended to operate in hostile environments to provide improved resistance to one or more of oxidation, corrosion and erosion. The invention also relates two processesforthe production of such coatings. Such coatings may be used forvarious purposes but are particularlyvalu- able forthe protection of high temperature aerofoil components such as rotor blades and statorvanes of gas turbines. Such coatings are employed to enable materials for the components to be selected with greater consideration fortheir structural characteristics and lesser consideration for heir resistance to corrosion and the like.Examples of such materials are the so-called super-alloys and particularly unidirectional solidified and single crystal components.

There is described in EP-BI-0 024 802 a gas turbine blade fabricated from a nickel-based alloy which is provided by the plasma arc spraying technique with an overlay coating of CoCrAIY. This coating may suffer from microporisity and is therefore given a further coating of aluminium buy a pressure pulse chemical vapour deposition process. This two stage process has been found to give good results but is expensive and it is thought that it is not in commercial use. Two processesforthe production of overlay coatings which are used for the purpose described are plasma spraying and electron beam techniques. However, these are both expensive.

It is accordingly an object ofthe present invention to provide a coating which has good properties while being reasonable in price.

This invention is believed to reside in one ofthe features set out in the immediately following paragraphs or in one of the novel features included in the later parts ofthethis specification relating to more detailed examples orto any compatible combinations from eitheroneorfrom both ofthetwoaforementioned groups of features and it is to be emphasised that such combinations have been envisaged and are intendedto be disclosed hereby, even though not all ofthem are at present claimed.

According to one aspect ofthe present invention a coating is provided by composite electro-deposition, on a substrate, of a metal matrix M1 containing particles ofCrAlM2where M1 is Ni orCo orFe ortwo or all ofthese metals and M2 is Y, Si, Ti, a rare earth element or two or more of these elements. Preferably M1 is Ni or Co or NiCo. The electrodeposition may be electrolytic or electroless. Examples of apparatus and processes which may be used are described in UK-A-1 218179,1 329081 and 1 347184, it being preferred to use those described in UK-B-2 014189. The preferred particles are CrAIY.The particles could also contain one or more of Hf, Ta, Nb, Mn and Pt.

By electrodeposition there can be produced a coating in which particles forming one phase are dispersed in a matrix forming a second phase and such a coating has very desirable properties, and surface finish. The composite coatings may be contrasted with those produced by spraying techniques.

Thecoatingswill in practice be heat treated to improve their properties. An effect of heattreatment is to produce diffusion between the matrix and the particles so that the matrix of M1 also contains some Cr, Al and M2 while the particles also contain some M1.

For some applications, it is possible to treat the coating to a hot isostatic pressing (HIP) cycle to remove any traces of porisity in the coating. For example, where the matrix M1 is Ni, the coating and, of course, its substrate may be given a HIP cycle of 2 hoursat 1120 Cin argon at140 MPa.

It has been found that outstandingly good results are obtained by the close control ofthe particle size distribution. Thus, it has been found thatforthe best results particles in the as-deposited coating large than about 15,am should be avoided so far as is practicable and it is therefore preferred that at least 99% ofthe particles co-deposited in the coating are below 25,u An additional or alternative preference is that at least 97% ofthe co-deposited particles are below 1 5clam. A further additional or alternative preference is that at least 84% ofthe co-deposited particles are below 10.5clam. Particularly preferred distributions include: more than 95% by weight with particle sizes between 3.0 and 13.6m, more than 75% between 3.9 and 10.5,lem, and more than 65% between 5.0 and 10.5cm, these limits being applicable individually orcomulatively and separately or in combination with one or more of the immediately preceding preferencesfor the proportions below 25Em, and and 10.5,am respectively. For most conditions of co-deposition, these as-deposited limits will be achieved by applying the same limits to the size distribution ofthe particles in the bath butforthisto be the case care must be taken that adequate means are employed to ensure even distribution ofthe particlesthroughoutthe bath.

It has been mentioned above thatthe coating is preferably heat treated to obtain interdiffusion between the constituents of the matrix and the particles.

While diffusion of more than one of the constituents contributes to the excellent properties ofthe coatings of the invention, it is through that diffusion of Al from the particles into the matrix is particularly valuable since the availability of Al at the surface ofthe coating will lead to the formation of aluminium oxides at the surfacewhen the coated component is in use and aluminium oxides are known to provide excellent erosion and corrosion resistance in hostile environments such as those encountered by the blades and vanes of gas turbines. Preferably, the temperatures and durations of the heattreatmentareselectedto produce a minimum of 3weightpercent(wlo) of Al in the matrix phase ofthecoating and high Awl values may be beneficial up to values approaching the maximum amount of Al which is soluble in the matrix phase (thoughtto beabout4.2w/oforAl in Co).

To achieve adequate diffusion, it is probablethatthe lowest effective temperature will be 700"C but to achieve rapid diffusion high temperatures are prefer red, one suitable range being between 1 050 C and 11 50 C, that between 11000 and 11 400C being particularly preferred, the necessary duration being between one half and two and one half hours. Very good results have been achieved with one to two hours at 11 000C in vacuum, butlongertimes may be beneficial in some cases and in particular if lowertemperatures are used.

The proportions ofthe constituents ofthe particles may varywithin wide limits but preferably the proportions of Cr and Al range between 80/20 and 50/50 while the content of M2 in the particles will be substantially less than the othertwo constituents, preferably less than 2.5 w/o, preferably less than 2.0 w/o. Particularly preferred proportions are Cr- 37.3 w/o Al 01.7 w/O Y. Although CoCrAIY coatings are suitable for many application, for example for aero engines, those of NiCoCrAIY and CoNiCrAIY may be preferred for these and other applications.For marine and industrial power plants coatings containing high percentages ofchromium (e.g. up to 40 weight per cent) and with silicon (e.g. up to 10 weight per cent), platinum (e.g. upto 10 weight per cent) and hafnium (e.g. up to 5 weight percent) may be required.

It will now be appreciated that the preferred features ofthe invention may be characterised in various ways.

Thus according to one aspect (and realising that the invention has many broader aspects), the invention may be seen to reside in a substrate carrying an overlay coating of M1CrAIM2 (where M1 is Co, Ni or NiCo and M2 is Y, Si, Ti or two or all ofthese but preferably Y) applied by electrodeposition of a matrix of M1 and particles of CLAIM2 having in the as deposited state more than 97% by weight ofthe particles below 15 m,the coating having been heated treated to produce a minimum of3 weight percent of Al in the matrix.

The invention may be carried into practice in various ways butthree Examples will now be given.

Rods of Ma rM002 were coated bythetechniques and apparatus described in UK-A-2 014189.

In Example A, the bath contained a CoNi plating solution and the particles were of CrAIY containing 60 parts by weight Crto 40 parts Al and 1.7 w/o Ywith a size distribution, both in the bath and in the asdeposited coating, as given in column A ofthe following Table.

TABLE

COLTINe A B C SIZE BAND Weight Per Cent 118.4 54.9 0 0 0 54.9 33.7 0.2 0 0 33.7 23.7 2.9 0.3 0.1 23.7 17.7 5.9 1.3 0.7 17.7 13.6 14.8 4.3 2.9 13.6 10.5 20.2 17.7 11.7 10.5 8.2 28.1 38.1 30.5 8.2 6.4 12.4 18.3 21.1 6.4. 5.0 7.8 12.3 18.0 5.0 3.9 7.5 8.2 9.8 3.9 3.0 0 2 0.1 5.1 3.0 2.4 0 0 0.1 2.4 1 9 0 After plating, the rod with the composite overlay coating thereon was given a heattreatmentofone hourat 1100 C invacuum. Amicrograph (at an original magnification of X400) ofthe coating after heat treatment is shown in Figure 1. The coating comprises a matrix of CoNi into which amounts of Cr, Al and Y have diffused and particles into which amounts of Co and Ni have diffused. The matrix has approximately 20% Cr and approximately 4% Al. The coating has good properties.

In Example B,the bath contained a Co plating solution and the particles were of CrAIY containing 50 parts by weight Cur to 50 parts by weight of Al and 1.7 w/o Ywith a size distribution, both in the bath and in the as-deposited coating, as given in Column B in the above Table. The rod was heat treated as for Example Aand a micrograph ofthe coating after heat treatment is shown in Figure 2. The coating has good properties. In a test, the coating was found to last in excess of 600 hours compared with a currently used aluminidecoating which lasted only 200 hours.

In Example C, the bath contained a Co plating solution and the particles were of CrAIY containing 60 parts by weight Crto 40 Parts Al and 1.7 w/o Ywith a size distribution, both in the bath and in the asdeposited coating, as given in Column C ofthe above Table. The rod was heattreated as for Examples A and B and a micrograph ofthe coating is shown in Figure 3. It has been found that whilethe properties of the coatings of Examples A and B are good, those of Example Care outstanding. The coating isvery dense, adherent and smooth with, as can clearly be seen from Figure 3, a fine microstructure with an even distribution of particles and no zones of weakness.

Substantial quantities of Al (as well as some Cr and Y) have diffused out ofthe particles into the Co matrix and Co as diffused into the particles so the coating consists of a 2-phase alloy, the matrix phase containing cobalt chromium aluminium and yttrium and the particles contain the same constituents but in different proportions. Upon oxidation at 1 000 Cthis alloy developed an oxide on the surface which was rich in aluminium oxide and which was particularly resistant to erosion and corrosion. In a test on a burner rig test at 11 00 C, the coating was found to last over600 hours compared to coating of similar overall composition produced by argon shrouded plasma spraying which only survived 400 hours.

Itwill now be appreciated that the invention provides overlay coatings having extremely beneficialproperties. Itshould notedthatthecoatings employ fine particles which are evenly distributed in an equiaxed matrix, thus producing a very high quality surface finish requiring little or no additional work; in general, the coating will be given a heat treatment but no otherfinshing treatment. This may be contrasted with sprayed and physical vapour deposition processes which put down coatings tending to have structures which produce coarse finishes requiring very Iengthlyfinishing. These coatings usually require peening to eliminate comprehensive stresses in the surface and to encourage recrystallisation on heat treatment. Plasma sprayed deposits normally also require a polishing operation because ofthe rough surface produced by this process.

Claims (23)

1. Acoating produced by composite elec trodeposition, on a substrate, of a metal matrix M1 containing particles of CrAIM2,where M1 is Ni or Co orFe ortwo or all ofthese metals and M2 is Y, Si, Ti, a rare earth element or two or more ofthese elements.

2. A coating as claimed in Claim 1 in which M2 is Y.

3. Acoating as claimed in Claim 1 or Claim 2 in which the particles also contain one or more of Hf, Ta, Nb, Mn and Pt.

4. A method of producing a coating on a substrate which comprises electrodepositing a metal matrix M1 from a bath containing particles of CrAIM2 so asto codepositthe particles with the matrix, M1 being Ni or Co or Fe or two or all of these metals and M2 being Y, Si, Ti, a rare earth element ortwo or more ofthese elements.

5. A method as claimed in Claim 4 in which M2 is Y.

6. A method as claimed in Claim 4 or Claim 5in which the particles also contain one or more of Hf, Ta, Nb, Mn and Pt.

7. A method as claimed in any of Claims 4to 6 in which at least 99% by weight of the particles in the as-deposited coating are below 25 m.

8. A method as claimed in any of Claims 4to 7 in which at least 97% by weight ofthe particles in the as-deposited coating are below 15 m.

9. A method as claimed in any of Claims 4to 8 in which at least 84% by weight ofthe particles in the as-deposited coating are below 10.5,am.

10. A method as claimed in any of Claims 4to 9 in which at least 95% by weight of the particles are between 3.0 and 13.6 m.

11. A method as claimed in any of Claims 4to 10 in which at least 75% by weight ofthe particles are between 3.9 and 10.5 m.

12. Amethodasclaimed inanyofClaims4toll in which at least 65% by weight ofthe particles are between 5.0 and 10.5,am.

13. A method as claimed in any of Claims 4to 12 which includes heattreating the as-deposited coating to obtain interdiffusion between the constituents of the matrix and the particles.

14. A method as claimed in Claim 13 in which the heattreatment produces a coating having a minimum of weight percent of Al in the matrix phase.

15. A method as claimed in Claim 13 or Claim 14 in which the heat treatment is effected at tempera turesbetween 1050 C and 1150"C.

16. A method as claimed in Claim 13 or Claim 14 in which theheattreatmentis affected attemperatures between 1 Q90 C and 11 40eC.

17. A method as claimed in Claim 13 or Claim 14 in whichtheh.eattrnatment is effected at 1100 Cin vacuum for between one and two hours.

18. A method as claimed in any of Claims 4to 17 in which the proportion of Cr and Al in the as-deposited particles is between 80/20 and 50/50.

19. A method as claimed in any of Claims 4to 18 in which the proportion of M2 in the as-deposited particles is less than 2.5 weight percent.

20. A method as claimed in anyof Claims 4to 18 in which the proportion of M2 in the as-deposited particles is less than 2.0 weight percent.

21. A method as claimed in any of Claims 4to 18 in which the as-deposited particles consist of 37.3 weightpercentAl, 1.7 weight percentYandthe balance Cr.

22. A substrate carrying an overlay coating of M1CrAIM2 (where M1 is Co, Ni or NiCo and M2 is Y, Si, Ti ortwo or all of Y, Si and Ti) applied by electro-codeposition of a matrix of M1 and particles of CrAIM2 having in the as-deposited state more than 97% by weight of the particles below 15 m, the coating having been heat treated to produce a minimum of weight percent of Al in the matrix.

23. A method of producing a coating on a substrate substantially as described herein with reference to the foregoing Example A or Example B or Example C.