GB2397257A - Article provided with a vibration damping coating - Google Patents

Abstract

A porous ceramic material such as spinel is impregnated with a viscoelastic material to provide a vibration damping coating for an article. The visco-elastic material such as polyurethane or polychloroethene or precursor thereof may be applied to the ceramic-containing layer as a solution or suspension. Layers of a sealing material and/or erosion resistant material such as the viscoelastic material or nickel may be applied over the ceramic-containing layer. The ceramic-containing layer may be formed by plasma spraying. A bond coat may be applied to the article before application of the ceramic-containing layer. The article may be a component of a gas turbine engine such as an air intake fan blade of a gas turbine engine.

Description

METHOD OF APPLYING A VIBRATION DAMPING COATING

AND AN ARTICLE PROVIDED WITH THE COATING

The present invention relates to the damping of vibration in articles and to vibration- damped articles. More particularly, although not exclusively, the invention relates to vibration damping of aerospace components such as gas turbine engine components.

The general use of coatings including a ceramic as vibration damping coatings for articles such as gas turbine components is well known in the art. A well known alternative is the use of visco-elastic materials as vibration damping materials in such coatings.

GB 2,346,415 (Rolls-Royce pie), the disclosure of which is incorporated herein by reference, discloses the use of a coating on a metallic body of a structural article as a means of vibration damping, the coating comprising a first layer over which a ceramic outer layer is applied. The first layer is formed of a material which has a brittle-ductile transition temperature below the operation temperature. This provides a layer which is ductile when in use thereby providing a visco-elastic interconnection between the article and the outer layer. This reduces the amplitude of shear vibrations on the article.

The first layer in this prior art is preferably a metal or alloy applied directly to the article, and the ceramic coating is preferably applied by an air plasma spray process.

US Patent No 3,758,233 (Cross et a/.), the disclosure of which is incorporated herein by reference, discloses a metal alloy aero-engine rotor blade provided with a multilayer vibration damping coating consisting of an outermost portion formed of an oxide ceramic or refractory carbide and an intermediate portion formed of a mixture of a metal alloy and the oxide ceramic material or refractory carbide. The intermediate portion can consist of two or more discrete layers, the layers having decreasing metal alloy content and increasing ceramic content towards the outermost layer portion. - 2

Such an outermost ceramic layer typically has a relatively hard, rough, surface, which can give rise to aerodynamic frictional energy loss during operation of the blade.

Furthermore, such coatings generally have rather low resistance to foreign object damage (FOD) and erosion.

US Patent No 4,405,284 (Albrecht et a/.), the disclosure of which is incorporated herein by reference, discloses a thermal turbomachine casing having a multilayer heat I insulation liner including a metallic bond coat in direct contact with the casing wall, a ceramic heat insulation layer bonded to the bond coat, and a porous, predominantly metallic, top layer bonded to the ceramic layer. The casing liner is stated to have the dual advantage of providing heat insulation to the casing while minimising wear suffered by a rotor caused by rubbing against the casing. However, there is no teaching or suggestion that the heat insulation casing liner would have any utility in preventing vibration in the casing. The term "porous" as used in this prior art refers i particularly to the presence of cavities in the metallic top layer. The prior art patent exemplifies a top layer including nickel and graphite constituents with cavities in the material.

The present invention is based on our surprising finding that, by impregnating a visco- elastic material (ie a material which exhibits internal damping of vibrations and also has elastic properties) into a porous ceramic- containing vibration damping coating for a metallic aerospace component, particularly but not exclusively a metallic aerospace component operating at substantially ambient temperature, the vibration damping performance of the coating is maintained or enhanced.

According to a first aspect of the present invention there is provided a method of applying a vibration damping coating to a surface of an article comprising the steps of forming a porous ceramic-containing layer on the article and impregnating the said porous layer with a visco-elastic material.

The impregnation of the viscoeiastic material into the porous ceramiccontaining layer is preferably carried out by contacting the porous ceramic-containing layer, after forming of the porous layer on the article, with a sufficiently mobile liquid form of the viscoelastic material for the viscoelastic material to be absorbed into the porous ceramic-containing layer. This may be achieved by soaking the ceramic containing layer with the liquid.

The absorption into the porous ceramic-containing layer may be assisted by adjusting the ambient pressure in generally conventional manner for impregnation treatments.

The liquid form of the viscoelastic material is selected to deposit the desired viscoelastic material within the porous structure of the ceramiccontaining layer after impregnation. The liquid form of the viscoelastic material may suitably be a solution or suspension of the viscoelastic material or of a precursor thereof.

The viscoelastic material is preferably an organic viscoelastic material, eg a viscoelastic polymer, and most preferably an elastomer or rubber such as polyurethane or polychlorothene ("neoprene"). The liquid may therefore be a solution comprising a monomer which cures to a polymer after being applied to the ceramic containing layer.

The method may also comprise the step of applying a sealing layer, which may comprise the visco-elastic material as used to impregnate the ceramiccontaining layer, or may be an alternative material such as an epoxy resin, over the ceramic-containing layer. Alternatively, or additionally, an over layer provided as an erosion resistant layer may also be applied to the coating, and this may also comprise the visco-elastic material, or may be an alternative material, for example a layer of nickel, applied by electroless nickel plating.

In the following description, the part of the article beneath the vibration damping coating will be termed the substrate.

The ceramic-containing layer is preferably formed on the substrate by a plasma spraying process. Preferably the ceramic-containing layer has an open-celled internal structure. - 4

The method may further comprise the step of applying a bond coat on the article and applying the ceramic-containing layer to the bond coat.

An over-layer may also be provided as an erosion-resistant protective layer, the ceramic-containing layer then being situated between the substrate and the over-layer.

The over-layer may suitably consist essentially of the viscoelastic material, which should not be absorbed when the surface pores of the ceramic-containing layer have been filled by the impregnating viscoelastic material.

The ceramic preferably consists essentially of a spinal, such as magnesiaalumina spinal.

According to a second aspect of the present invention, there is provided an article comprising a substrate and a vibration-damping surface coating for the substrate, the coating comprising a porous ceramic-containing layer and a visco-elastic material impregnated therein.

Preferably the visco-elastic material is provided within the pores of the porous ceramic- containing layer, preferably occupying at least about 50% of the pore volume of the porous ceramic-containing layer, and more preferably at least about 80% of the pore volume of the porous ceramic-containing layer.

The article may be a component of a gas turbine engine, and may be an air intake fan blade of a gas turbine engine.

The term "porous" used herein with reference to the ceramic-containing layer means that at least about 10% by volume, and preferably between 10% and 20% by volume of the layer is comprised of pores and voids.

The term "ceramic" used herein, as applied to a material of the coating, means that at least about 90% by weight of the material consists of a material having the physical properties normally associated with ceramics. Ceramics are chemical compounds - 5 typically composed of metal and nonmetal elements in non-zero oxidation states linked by strong ionic bonds, and are typically characterized by a high shear strength which correlates to a high hardness (generally greater than 1000 Knoop) and a high compressive strength. A ceramic material is relatively brittle in comparison with a metal.

The terms "metallic" and "metal" used herein, mean that at least about 90% by weight of the material consists of a material having the physical properties normally associated with metals. Metals usually consist of elements, or alloys, mixtures, adducts or complexes of elements, typically in the zero oxidation state and predominantly elements categorized as metals according to the Periodic Table of the Elements, and are typically characterized by a lower shear strength which correlates to a lower hardness value (less than 1000 Knoop) and a lower compressive strength. A metallic material is relatively ductile in comparison with a ceramic.

The term "visco-elastic" used herein, means a material with visco-elastic properties as is well known in the art. For example, the material will exhibit internal damping of vibrations and also has elastic properties. It will also creep under stress.

The term "surface" used herein includes surface portions.

In the vibration damping coating according to the present invention, the ceramic- containing layer is impregnated with the visco-elastic material provided in the cavities of the ceramic. This hybrid coating provides bi-modal damping ie the ceramic coating provides damping friction in tension, and the visco-elastic material provides damping friction when the coating is compressed.

Without wishing to be bound by theory, it is believed that the vibration damping effect of the coating may be enhanced in at least one of two ways in comparison with ceramic coatings with open pores: - 6 Firstly, a "constrained layer" effect may take place, in which, as the coating is strained, shear stresses may be created by the low modulus visco-elastic material being in contact with the high modulus ceramic material. This may give rise to a shear strain in the ceramic-containing component, and may result in the ceramic-containing component absorbing an unexpectedly large amount of vibrational energy from the system.

Secondly, the viscoelastic material may increase friction at sliding interfaces with the ceramic. As the ceramic layer flexes the presence of incompressible viscoelastic material gives rise to constraint which increases interface contact load, thus increasing friction.

It is generally preferred that the coating consists essentially of the ceramic and visco- elastic components, with less than about 10% by weight of other components and provided that any such other components that may be present do not alter the essential characteristics of the ceramic and visco-elastic components.

In another embodiment, a predominantly metallic region ("bond layer" or "bond coat") of the coating may be disposed between the ceramic material and the substrate and in contact with the substrate. The interface between the predominantly ceramic region and the predominantly metallic region may be discrete or graded in a continuous manner. The predominantly metallic region may be the same as, or different from, the material of the substrate. In one example, the predominantly metallic region between the ceramic material and the substrate may comprise a nickel-containing alloy such as nickel aluminide or a nickel-chromium alloy.

Any porous vibration damping ceramic component may be used in the coating.

Preferably the ceramic has an open celled structure. Such materials are well known in the art, and include, for example, refractory metal oxides and carbides, including spinal and other crystalline forms thereof. - 7

The ceramic component is preferably a spinel. A spinel is a mixed metal oxide which has the general formula AB2O4, where A represents a divalent cation and B represents a trivalent cation. Examples of suitable divalent cations include Fez+, Mg2+, Cu2+ and Mn2+. Examples of suitable trivalent cations include Cr3+, Fe3+, and Al3+. The stoichiometry can vary between different spinel materials, and the two oxides need not be present in equal proportions.

The crystalline structure of a spinel is typically characterized by a cubic system, in which the metal atoms exist in tetrahedral and octahedral coordination. In a so-called normal spinel structure, each A atom is coordinated with four oxygen atoms (ie in tetrahedral coordination), and each B atom is coordinated with six oxygen atoms (ie in octahedral coordination). In a so-called inversed spinel, the tetrahedral positions are occupied by some of the B atoms, whilst the A atoms and the remainder of the B atoms are distributed throughout the octahedral positions. All crystalline forms are embraced by the term "spine!" as used herein.

Spinel materials are characteristically ceramics. They are relatively inert to acid or base attack, and relatively refractory to heat.

The preferred spinel for use in the present invention is magnesia-alumina spinel, ie A = Mg2+ and B = Al3+. The term "magnesia-alumina spinel" used herein includes materials in which MgAI204 is the predominant component, ie comprising more than about 50% by weight of the material, and in particular does not exclude impure or mixed materials which can nevertheless fairly be described as a magnesia-alumina spinel.

A particularly preferred magnesia-alumina spinel for use in the present invention is a magnesia-alumina spinel in which the alumina predominates, eg a 30:70 MgO:AI2O3 spinel.

The visco-elastic component, particularly the component impregnating the ceramic in the ceramic-containing layer, is preferably a synthetic rubber such as polychlorothene rubber or polyurethane rubber, or any other suitable rubber or other material which - 8 exhibits visco-elastic properties, that is, a material which exhibits creep under load and has sufficient elasticity in use.

The article is preferably an aerospace component such as a fan blade, integrally formed bladed disk, vane, drum, casing or shroud portion of a gas turbine engine, or any part or fitting thereof.

The article according to the present invention will typically be used at ambient temperature or thereabouts, for example an air intake fan blade or other component located at the cooler sections of a gas turbine engine.

The substrate is preferably formed of a material which is suitable to apply a plasma sprayed coating thereon. Alternatively, it may be formed of a material which is suitable to receive a bond coat on to which the air plasma sprayed coating is applied.

Accordingly, the substrate may be metallic, ie may comprise any metal or metal alloy and is suitably of relatively low density, for example less than about 7 gcm3, less than about 6 gcm3 or less than about 5 gcm3. The metallic substrate suitably has a relatively high melting point or melting range. For example, the melting point or midpoint of the melting range may suitably be above about 1 000 C, for example above about 1300 C, more preferably above about 1400 C, and most preferably above about 1 500 C.

The metallic substrate may comprise a first metal as the main component and any other suitable metal or metals as a further component or components. It will be appreciated that the metallic substrate may also comprise semi- and non-metallic components in addition to metallic components. These semi- and non-metallic components may typically be present in lower amounts than the main metallic component, for example less than about 5% by weight, less than about 3% by weight or less than about 1% by weight. - 9 -

The main component of the metallic substrate preferably comprises a transition metal or a transition metal alloy. The metallic substrate preferably comprises titanium, an alloy of titanium, steel or stainless steel. In a preferred embodiment, the metallic substrate comprises a titanium alloy substantially in the beta form.

In the case where the metallic substrate is a titanium alloy, it will comprise titanium as the main component and preferably one or more subsidiary components selected from the group consisting of aluminium, beryllium, bismuth, chromium, cobalt, gallium, hafnium, iron, manganese, molybdenum, niobium, nickel, oxygen, rhenium, tantalum, tin, tungsten, vanadium and zirconium. This alloy may also suitably comprise one or more semi- or non-metallic elements selected from the group consisting of boron, carbon, silicon, phosphorus, arsenic, selenium, antimony and tellurium. These elements may serve to increase the oxidation, creep or burning resistance of the metallic substrate.

Titanium may be present in such a titanium alloy in an amount greater than about 40% by weight, for example greater than about 50% by weight, greater than about 60% by weight or greater than about 70% by weight and in some embodiments may be present in an amount greater than about 80% by weight.

The amount in which the subsidiary component or components are present is determined by the use to which the metallic substrate will be put, as will be well understood by those skilled in this art. For example, the metallic substrate may be a ternary alloy comprising titanium, vanadium and chromium. Certain compositions of this type are especially preferred for certain applications wherein the titanium is present substantially in the beta form under most temperature conditions ie has less than about 3wt% alpha phase titanium, preferably less than about 2wt% alpha phase titanium.

Such beta titanium alloys are based on ternary compositions of titaniumvanadium- chromium which occur in the titanium-vanadium-chromium phase diagram bounded by the points Ti-22V-1 3Cr, Ti-22V-36Cr, and Ti40V-1 3Cr. These compositions are known to have useful mechanical properties such as high creep strength and a lack of combustibility at temperatures of up to at least about 650 C. In such compositions, the - 10 titanium is preferably present in an amount greater than about 40% by weight, for example greater than about 50% by weight. The chromium is preferably present in an amount greater than about 10% by weight, for example greater than about 15% by weight or greater than about 25% by weight. This concentration of chromium is necessary to provide the required nonburning characteristics of the alloy at these high temperatures. Vanadium may be present in an amount greater than about 20% by weight, for example greater than 25% by weight or greater than about 30% by weight.

A specific alloy of this type has a composition comprising about 50wt% titanium, about 35wt% vanadium and about 15wt% chromium.

In other applications, the elements of the alloy composition will be significantly different.

For example, the metallic substrate may comprise titanium and other metals or semi- metals selected from the group consisting of aluminium, chromium, copper, iron, molybdenum, niobium, silicon, carbon, tin, vanadium and zirconium. In such alloys, aluminium is preferably present in an amount less than 1 Owt%, for example less than 8 wt%; chromium is preferably present in an amount less than 10wt%, for example less than 8wt%; copper is preferably present in an amount less than 5wt%, for example less than 3wt%; iron is preferably present in an amount less than 5wt%, for example less than 3wt%; molybdenum is preferably present in an amount less than 10wwt%, for example less than 8wt%; niobium is preferably present in an amount less than 6wt%, for example less than 4wt%; silicon is preferably present in an amount less than 2wt%, for example less than 1wt%; carbon is preferably present in an amount less than 1wt%, for example less than 0. 5wt%; tin is preferably present in an amount less than 16wt%, for example less than 12wt%; vanadium is preferably present in an amount less than 1 5wt%, for example less than 1 Owt%; and zirconium is preferably present in an amount less than 8wt%, for example less than 6wt%. A specific example of such an alloy is Ti- 6AI-4V.

The coating may be applied to the entire surface of the component, or to portions of the component such as those regions which encounter the largest vibrational forces. - 11

The substrate may initially be prepared for coating in a conventional manner, eg peening, decreasing and other surface treatments.

The coating may be applied by any convenient method for depositing porous ceramics and visco-elastic materials to substrates.

Such deposition methods will be well known to those skilled in the art. Examples for applying the ceramic-containing layer include: plasma spraying (eg air plasma spraying), physical vapour deposition, chemical vapour deposition, high velocity oxyfuel deposition, sol-gel deposition.

The preferred deposition technique for depositing the ceramic layer is air plasma spraying. In essence, a powder is entrained in a very high temperature plasma flame, where it is rapidly heated to a molten or softened state and accelerated to a high velocity. The hot material passes through a nozzle and impacts on the substrate surface, where it rapidly cools, forming the coating. It is preferred that a so-called "cold plasma spraying" process is used, whereby the temperature of the material impacting the substrate is maintained sufficiently low to avoid heat damage to the substrate.

The plasma spraying procedure is typically performed using a conventional plasma spraying apparatus comprising an anode (eg of copper) and a cathode (eg of tungsten), both of which are cooled (eg by water). Plasma gas (eg argon, nitrogen, hydrogen or helium) flows around the cathode and through the anode. The anode is formed into a constricting nozzle, through which the plasma stream and powder particles are ejected.

The plasma is initiated by a high voltage discharge, which causes localised ionization and a conductive path for a DC (direct current) electric arc to form between the cathode and the anode. The resistance heating from the arc causes the gas to reach extreme temperatures, dissociate and ionise to form a plasma. The plasma then exits the anode nozzle as a free or neutral plasma flame (ie plasma which does not carry any electric current). - 12

The plasma spraying apparatus is normally located between about 25 and about 150mm from the metallic substrate surface.

If a bond layer is applied to the substrate it may also be applied using a plasma spraying process, and the ceramic and bond layer components or their precursors are typically supplied as separate powders for the plasma spraying deposition process, the rate of supply and the nature of the supplied materials being chosen according to the deposition procedure being employed, and the desired composition and structure of the coating.

The technique of plasma spraying mixtures of ceramic and metal powders onto a metallic substrate is described, for example, in US Patent No 4, 481,237, the disclosure of which is incorporated herein by reference.

The method of applying the ceramic-containing layer to the substrate may comprise an initial step of applying a metallic base layer or bond coat on the substrate. The ceramic is subsequently built up on the base layer, eg by way of a gradual change from above about 90% w/w, more particularly about 100% w/w, base layer metal to above about 90% w/w, more particularly about 100% w/w of the ceramic, and the ceramic layer continues at about 100% w/w for a predetermined depth.

Alternatively, however, the base layer can be dispensed with, and the initial step could be applying the ceramic to the substrate, forming a discrete, non-diffuse substrate- ceramic layer interface.

After the step of applying the ceramic, the visco-elastic material is impregnated into the ceramic containing layer. The visco-elastic material or a precursor thereof is typically dissolved, thinned or suspended in a solvent or liquid carrier prior contact with the ceramic-containing layer. If the visco-elastic material is a polymer, this may be dissolved or suspended in a suitable solvent such as petroleum spirit. The liquid is preferably applied to the ceramic by soaking the ceramic with the liquid for a suitable length of time, eg for approximately 15 minutes. The article may then be placed in a - 13 vacuum chamber (at a pressure of less than 1 00mBar), and this is then vented to a higher pressure so as to aid the penetration of the visco-elastic material into the pores and cavities of the ceramic.

Alternatively, the ceramic is subjected to an elevated pressure after being treated with the solution, of preferably between 2 x 105 (2 Bar) and 3 x 105 Pa (3 Bar) so as to aid penetration of the solution into the pores and cavities.

Certain viscoelastic materials, eg synthetic rubbers, are suitably applied in the form of solutions or dispersions in a solvent or carrier liquid.

The liquid form of the viscoelastic material can be allowed to dry after the impregnation into the pores of the ceramic containing layer, whereby the viscoelastic material is deposited within the pores and the solvent or carrier evaporates to the atmosphere.

Other viscoelastic materials, eg a viscoelastic resin, or a viscoelastic polymer, can be formed in situ within the pores of the ceramic-containing layer as a result of a curing or polymerization reaction taking place within the pores between precursor materials (eg A and B resin components or monomers) which are deposited within the pores, from liquid forms of the said precursor materials which are applied simultaneously or sequentially to the porous ceramic-containing layer and absorbed into the internal porous structure of the ceramic-containing layer.

The treated coating is then dried or cured as appropriate, preferably at a temperature up to approximately 1 50 C. This allows the solvent to evaporate and the monomers to link to create the desired polymer.

Preferably a sealing layer is then applied to the ceramic-containing layer once the visco-elastic material has cured completely or substantially. This may be an epoxy resin or more preferably a visco- elastic material, and may be the same material as used to impregnate the ceramic. This sealing layer may be applied by dipping the ceramic impregnated with the visco-elastic material into the sealing layer material in liquid form.

Alternatively the sealing layer material may be brushed or aerosol sprayed on to the - 14 ceramic/visco-elastic material. The article may then be placed in a vacuum chamber, or pressurized, as described above.

An erosion resistant over-layer may be applied to the coating as well as, or instead of, the sealing material. This may also be the visco-elastic material, or may be any suitable erosion resistant material such as Nickel, preferably applied by electroless nickel plating. Electroless nickel plating is the plating of nickel deposits, which may; contain phosphorus and boron, onto catalytic metallic or catalysed non-metallic substrates by a reducing chemical reaction. t The present invention enables articles to be provided with a vibration damping coating, such that the coated articles have improved damping characteristics, in comparison with articles provided with known vibration damping coatings, while providing a; damping coating of similar mass and thickness to conventional ceramic vibration damping coating layers.

The invention has been tested using polyurethane as a viscoelastic material and plasma-sprayed magnesia alumina spinal as a porous ceramic material, and was found to provide an additional 50% more vibration damping capacity, in comparison with the vibration damping capacity of an unimpregnated plasma-sprayed magnesia alumina spinal coating.

This invention is considered likely to be of particular utility in relation to aerospace i components which in use encounter significant vibration, for example component parts of gas turbine engines at the air intake end of the engine.

The present invention has been broadly described without limitation. Variations and modifications as will be readily apparent to those skilled in this art are intended to be covered by the present application and resulting patents. ; - 15

Claims (31)

1. A method of applying a vibration damping coating to a surface of an article comprising the steps of forming a porous ceramic-containing layer on the article and impregnating the said porous layer with a viscoelastic material.

2. A method as claimed in claim 1, wherein the second material is applied to the ceramic-containing layer as a liquid.

3. A method as claimed in claim 2, wherein the liquid is a solution or suspension of the viscoelastic material, or a precursor thereof.

4. A method as claimed in claim 3, wherein the liquid is a solution comprising a monomer which cures to a polymer after being applied to the ceramic containing layer.

5. A method as claimed in any one of the preceding claims, wherein the viscoelastic material is a rubber.

6. A method as claimed in any one of the preceding claims, wherein the viscoelastic material is polyurethane.

7. A method as claimed in claim 5, wherein the polymer is polychloroethene.

8. A method as claimed in any one of claims 2 to 7, wherein the ceramiccontaining layer is soaked with the liquid.

9. A method as claimed in any one of claims 4 to 8, further comprising the step of curing the viscoelastic material, or precursor thereof. - 16

10. A method as claimed in any one of the preceding claims, further comprising the step of applying one or more additional layers of a sealing material and/or erosion resistant material over the ceramic- containing layer.

1 1. A method as claimed in claim 10, wherein the sealing material and/or the erosion resistant material is the visco-elastic material.

12. A method as claimed in claim 10, where the sealing material and/or the erosion resistant material is nickel.

13. A method as claimed in claim 12, wherein the nickel is applied by electroless nickel plating.

14. A method as claimed in any one of the preceding claims, further comprising the step of applying a bond layer to the surface of the article before applying the ceramic-containing layer to the bond coat.

15. A method as claimed in any one of the preceding claims, wherein the ceramic- containing layer is formed on the substrate by a plasma spraying process.

16. A method of applying a vibration damping coating to a surface of an article as claimed in claim 1 and substantially as described herein.

17. An article comprising a substrate and a vibration-damping surface coating for the substrate, the coating comprising a porous ceramic-containing layer and a visco- elastic material impregnated therein.

18. An article as claimed in claim 17, wherein the visco-elastic material is provided within the pores of at least 50% of the porous ceramiccontaining layer.

19. An article as claimed in claim 18, wherein the visco-elastic material is provided in the pores of at least 80% of the porous ceramic-containing layer. - 17

20. An article as claimed in any one of claims 17 to 19, wherein the visco-elastic material is a polymer.

21. An article as claimed in claim 20, wherein the visco-elastic material is a rubber.

22. An article as claimed in claim 21, wherein the visco-elastic material is polyurethane.

23. An article as claimed in any one of claims 17 to 22, further comprising an over layer, the ceramic-containing layer being situated between the substrate and the over layer.

24. An article as claimed in claim 23, wherein the over layer consists essentially of the visco-elastic material.

25. An article as claimed in claim 23, wherein the over layer consists essentially of nickel.

26. An article as claimed in any one of claims 17 to 25, wherein the ceramic consists essentially of a spinel.

27. An article as claimed in claim 26, wherein the spinal is magnesiaalumina spinet.

28. An article as claimed in any one of claims 17 to 27, which is a component of a gas turbine engine.

29. An article as claimed claim 28, which is an air intake fan blade of a gas turbine engine.

30. A coating substantially as described herein. - 18

31. An article comprising a coating as claimed in claim 30.