GB2217349A

GB2217349A - Vapour deposited self-sealing ceramic coatings

Info

Publication number: GB2217349A
Application number: GB8807395A
Authority: GB
Inventors: Allan Matthews
Original assignee: University of Hull
Current assignee: University of Hull
Priority date: 1988-03-29
Filing date: 1988-03-29
Publication date: 1989-10-25
Anticipated expiration: 2008-03-29
Also published as: GB8807395D0; GB2217349B

Abstract

A method of producing a self-sealing ceramic coating on a substrate wherein the substrate is supported in a vacuum chamber and material from a material source is deposited onto the substrate in the presence of a reactive gas to produce a ceramic coating of material, and the surface of the coating is then depleted of reactive gas by bombarding it with an inert or non-reactive gas, thereby forming a sub-stoichiometric layer in the surface of the ceramic coating. The inert or non-reactive gas may be introduced into the chamber immediately before the surface of the ceramic coating is depleted of reactive gas or it is present in the vacuum chamber with the reactive gas throughout the coating procedure. The rate of bombardment by the inert or non-reactive gas may be increased by a) interrupting the supply of reactive gas to the chamber or b) increasing the plasma intensity within the chamber. The source material may be evaporated into the chamber from a solid source, as in physical vapour deposition or may be introduced from an external source, as in chemical vapour deposition. The use of various methods of physical vapour deposition is envisaged. The substrate may be a metal, semiconductor or an insulator. The method is particularly effective in producing a multilayered structure in which there is a gradual charge from a metallic layer to a ceramic layer.

Description

DESCRIPTION "Vapour Deposited Self-Sealing Ceramic Coatings" The present invention relates to self-sealing ceramic coatings and, more particularly, to a method of producing self-sealing ceramic coatings.

It is well known that metallic surfaces in aggressive environments (oxidation, carburisation, nitridation, attack by molten metals sulphidation, etc.) at high temperatures are unstable thermodynamic systems which are susceptible to non-aqueous corrosion. In order to protect metallic surfaces from the effects of such aggressive environments it is also knojn to use ceramic barrier coatings. These are usually thermodynamically stable but are easily damaged under the action of thermodynamic stresses owing to mismatches of their thermal expansion coefficients with those of the metallic substrates and are brittle, particularly when under tension. Moreover, ceramic coatings are to a greater or lesser extent porous to aggressive species, for example, oxide ceramic coatings are porous to oxygen.

The brittleness of ceramic coatings can be reduced by using a nultilayer structure, in which there is a gradual change from metallic to ceramic character, and by decreasing the elastic modulus of the ceramic material by incorporating a porous material. However, an increase in the porosity of the coatings results in an increase in the corrosion of the substrate because the aggressive species are more easily able to defuse through the pores and attack the substrate interface.

As described by Perugini "Plasma Sprayed Self Sealing Composites, etc." Journal of Thin Solid Films, 108(1983), pages 415 to 425, self-sealing ceramic coatings provide a reduction in brittleness without a concomitant reduction in the protection against corrosion. In these self Sealing ceramic coatings a closed pore structure -is formed within a layer of the coating by typochemical oxidation reactions which result in the formation of stable ceramic oxides with a considerable increase in volume; these oxides seal the channels connecting the pores. Indeed, a positive self-sealing effect (which changes the original porosity t a closed porosity) can be obtained by in-service conversion when the reaction system is self-limiting (self stabilising).

Thus, any accidental cracks appearing in the coating ill be sealed in a similar manner and hence the coating will be self-healing.

Perugini proposes that the self-sealing coating be applied using an atmospheric plasma-spraying process and suggests that an argon plasma be used with three starting materials, namely: A. a Ni or Cu alloy melting in the range 800-1350 C, or Ni metal.; B. Cr metal; and, C. ceramic oxide, a compound or a mixture of ceramic oxides melting not lower than 1900 C.

material A forms a metallic bond with the metallic substrate. The oxide formed by material B seals the pores and any accidental cracks in the coating and at the same time ensures a metallurgical bond with material C.

Material C imparts refractory properties and chemical stability to the coating.

It has been shown that ceramic barrier coatings can be applied by electron beam physical vapour deposition (EBPVD) and more recently by plasma assisted PVD.

Typically the process involves either the direct vaporisation of a suitable ceramic, for example, yttria stabilised zirconia,or "reactive evaporation", wherein a reactive gas, such as oxygen, is introduced into the process chamber to provide or augment the supply of oxygen at the growing film surface,in which case a metal may be evaporated. A radio frequency or direct current electrical bias may be appled to the substrate to be coated, which results in the coating species arriving at the substrate with high energy, thus ensuring a dense, well adhered film.

It is an object of the present invention to provide a method of producing a self sealing ceramic coating on a substrate.

According to the present invention there is provided a method of producing a self-sealing ceramic coating on a substrate, wherein the substrate is supported in a vacuum chamber, material from a material source is deposited onto the substrate in the presence of a reactive gas to produce a ceramic coating of material, and the surface of the coating is depleted of reactive gas by bombarding the surface thereof with an inert or non-reactive gas, thereby forming a sub-stoichiometric layer in the surface of the ceramic coating.

By "non-reactive gas" it is meant any gas which will not readily react with the source material.

The inert or non-reactive gas may be introduced into the chamber immediately before the surface of the ceramic coating is depleted of reactive gas, or may be present in the vacuum chamber with the reactive gas throughout the coating procedure.

The source material may be evaporated into the chamber from a solid source, as in physical vapour deposition, or may be introduced from an external gaseous source, as in chemical vapour deposition.

The reactive gas may be introduced into the chamber from an external source, or may be present in the source material, as, for example, where the source material comprises a metal oxide, in which case the reactive gas is oxygen.

The surface of the coating is depleted of reactive gas by increasing the rate of bombardment thereof by the inert or non-reactive gas. This can be achieved either by interrupting the supply of reactive gas to the chamber, or be increasing the plasma intensity within the chamber.

Of course, where the reactive gas is introduced into the chamber from an external source it is a relatively simple matter to close off the supply of reactive gas.

Alternatively, where the reactive gas is present in the source material itself the evaporation or introduction, of source material into the chamber can be temporarily halted, or the plasma within the chamber can be increased so that the rate of bombardment of the surface of the coating by inert or non-reactive gas is greater than the rate at which reactive gas is laid down in the coating.

As will be readily understood any desired number of self-sealing layers can be produced within the coating by periodically carrying out the method of the present invention as the coating is laid down. It will be beneficial if the self sealing layer is between ceramic layers as this will provide a constraint against expansion during conversion of the metal, e.g. by oxidation, carborisation etc..

Where the source material comprises an oxide, bombardment with hydrogen has been found effective in depleting the surface of oxygen. Alternatively, an inert gas such as argon may be used.

In the present invention the following source material build up sequence is given, by way of example. Firstly a deposit of stabilised zirconium oxide, typically several microns thick, is produced by direct evaporation in a glow discharge. This deposit can if desired, be laid over a bond coating, such as a MCrAlY alloy or an aluminide layer. Following deposition of the zirconium oxide layer, the evaporation of source material is stopped and hydrogen and/or argon is introduced into the chamber. The resulting glow discharge will bombard the zircomum oxide coating, to deplete the oxygen and leave zirconium, plus a lesser amount of the stabilising phase, such as yttrium.The process can then be repeated to lay down a further layer of ceramic, which is bombarded to deplete the coating surface of oxygen, and repeated again and again until the required coating thickness is obtained. This may be in excess of 100 microns thickness for a typical thermal barrier application.

The method of the present invention is particularly effective in producing a multilayered structure in which there is a gradual change from a metallic layer to a ceramic layer, as described by Perugini. In this respect, the duration of the bombardment stage can be varied and a timer can be provided to this effect which enables the process to proceed under automatic control. In addition the method of the present invention can be modified to deposit any preferred metallic phase instead of the nonevaporative stage.

Using the method of the present invention metals, semiconductors and insulators can be coated. Different coating materials can be evaporated using, for example, resistance heated boats, an arc source, a sputter source or an electron beam source. The method of the present invention is amenable to use in an ionisation assisted physical vapour deposition mode, using either an RF or DC plasma.

Claims

1. A method of producing a self-sealing ceramic coating on a substrate, wherein the substrate is supported in a vacuum chamber, material from a material source is deposited onto the substrate in the presence of a reactive gas to produce a ceramic coating of material, and the surface of the coating is depleted of reactive gas by bombarding the surface thereof with a further gas, thereby forming a sub-stoichiometric layer in the surface of the ceramic coating.

2. A method according to claim 1, wherein the further gas is introduced into the chamber immediately before the surface of the ceramic coating is depleted of reactive gas.

3. A method according to claim 1, wherein the further gas is present in the vacuum chamber with the reactive gas throughout the coating procedure.

4. A method according to any preceding claim, wherein the further gas comprises an inert or nonreactive gas.

5. A method according to claim 4, wherein the further gas comprises hydrogen.

6. A method according to claim 4 or claim 5, wherein the further gas comprises argon.

7. A method according to any preceding claim, wherein the source material is evaporated into the chamber from a solid source, as in physical vapour deposition.

8. A method according to any preceeding Claim, wherein a plurality of sub-stoichiometric layers can be produced within a coating by periodically carrying out the method of the present invention as the coating is laid down.

8. A method according to any one of claims 1 to 6, wherein the source material is introduced from an external gaseous source, as in chemical vapour deposition.

9. A method according to any preceding claim, wherein the rate of bombardment by the further gas is increased by interrupting the supply of reactive gas to the chamber.

10. A method according to any one of claims 1 to 8, wherein the rate of bombardment by the further gas is increased by increasing the plasma intensity within the chamber.

11. A method according to any preceding claim, wherein a plurality of sub-stoichiometric layers can be produced within a coating by periodically carrying out the method of the present invention as the coating is laid down.

Amendments to the claims have been filed as follows

1. A method of producing a self-sealing ceramic coating on a substrate, wherein the substrate is supported in a vacuum chamber, material from a material source is deposited onto the substrate in the presence of a reactive gas to produce a ceramic coating of material, and the surface of the coating.is depleted of reactive gas by bombarding the surface thereof with an inert or nonreactive gas, thereby forming a sub-stoichiometric layer in the surface of the ceramic coating.

2. A method according to Claim 1, wherein the inert or non-reactive gas is introduced into the chamber immediately before the surface of the ceramic coating is depleted of reactive gas.

3. A method according to Claim 1, wherein the inert or non-reactive gas is present in the vacuum chamber with the reactive gas throughout the coating procedure.

4. A method according to any preceeding Claim, wherein the source material is evaporated into the chamber from a solid source, as in physical vapour deposition.

5. A method according to any one of Claims 1 to 3, wherein the source material is introduced from an external gaseous source, as in chemical vapour deposition.

6. A method according to any preceeding Claim, wherein the rate of bombardment by the inert or nonreactive gas is increased byinterruptingthe supply of reactive gas to the chamber.

7. A method according to any one of Claims 1 to 5, wherein the rate of bombardment by the inert or nonreactive gas is increased by increasing the plasma intensity within the chamber.