This invention relates to the coating of a surface of a component, for the purpose of providing it with desired physical and/or chemical properties, such as wear-resistance, corrosion-resistance, heat-resistance, friction, or wettability.

Certain components used in the oil and gas drilling and extraction industries are chromium plated to provide resistance to wear and corrosion caused by the drilling muds and detritus from the drilling process. Whilst oil based drilling muds are used, the chromium plating may provide adequate protection with respect to corrosion (but not necessarily to wear induced by abrasive particles) . However, water based muds are now being used for ecological reasons and the corrosion problems are exacerbated, particularly when high chloride conditions prevail. This corrosion problem is enhanced under these conditions because a hard chromium plating always exhibits cracking to a greater or lesser degree depending upon its thickness. The aqueous corrosive liquors can thus penetrate through the cracks to the base materials, whence the corrosion is initiated. The corrosion rate can be extreme in certain conditions, so as to allow only a limited drilling sequence before

component failure causes failure of other mating components. For instance, optimum drilling sequences of 200 hours may be required, whereas failure can occur in, say, 20 hours, with consequent costs of downtime for retrieving and replacing components in the system.

In the oil and gas extraction industry there are many components which are chromium plated but cannot be easily ground or surface finished. However, the "as-plated" chromium surface is quite adequate for many applications. A particular component is the rotor of a downhole motor which exhibits a "lobed" profile in cross-section, the lobes gradually "scrolling" down the length of the component in a spiral or screw fashion. This feature makes any surface finishing extremely difficult and costly. The rotor runs inside an elastomeric housing or stator which has a similar but negative lobed and scrolled surface and the "as-plated" chromium surface is compatible with the elastomer surface, until rapid corrosion occurs, when corrosive products etc. cause damage to the stator.

Accordingly, in the oil and gas extraction industry there is the problem of producing a surface coating which is corrosion resistant and compatible with an elastomeric surface.

Similar considerations apply to the rotor of a progressing cavity pump (such as the type of pump sold under the trade mark "Mono" ). This pump is similar to a

downhole motor in that it comprises a spiral or screw-shaped rotor which revolves in a mating elastomeric (rubber) housing. When the rotor is turned, its screw action drives a pumping fluid or slurry along the pump.

Extruder screws, such as are used in plastic moulding and injection processes, for example, are also required to be smooth and wear-resistant. The screw revolves in a stationary barrel, with the flights of the screw near or in contact with the barrel walls. Processed material is forced along the barrel by the screw action and, since the material may be very abrasive, wear of the screw flights may be very severe. Hard coatings can be applied to the screw flights to reduce wear, the coatings being ground to specific dimensions with a fine surface finish. However, it is difficult to prevent coating being applied to the valleys between the flights, where grinding is awkward. Very hard coatings containing tungsten carbide exhibit a rough as-coated surface finish which is very difficult to improve by grinding in between the flights.

In the steel industry it is now common practice to continuously cast billets of steel, which are then transported to the rolling mill while still hot. A rectangular water-cooled copper mould induces a solidified skin at the periphery of the "molten billet", while a semi-molten continuous billet or "strand" is

gradually drawn downwards from the mould through various guides and rolls, at a rate which matches the supply of molten steel to the mould via a tundish. The guides and rolls constrain the strand to the desired dimensions while it gradually solidifies during its journey through the casting machine to a horizontal position, where it is cut into the lengths required by the rolling mill.

Many components in the continuous casting machine are subject to severe wear due to various adhesive-wear mechanisms induced by the high temperatures and abrasive-wear mechanisms induced by the formation of oxide skins and exacerbated by entrainment of extraneous third body particles. The copper mould and guide components made of relative soft, high thermal conductivity copper alloys are particularly prone to high levels of wear which limit the continuous casting time. There is therefore a need for a smooth wear-resistant coating for the mould and/or other components of the continuous casting machine.

Another field where the present invention can be applied is that of gas turbine and internal combustion engines. Temperatures in the combustion areas of such engines can now be so high as to cause degradation of the metal surfaces. Thermal Barrier Coatings (TBCs) are now applied to such surfaces to reduce the metal temperatures. These coatings are normally ceramic coatings based on zirconia and are deliberately applied

so that they are porous and micro-cracked. The latter features help to provide thermal strain relief. The coatings exhibit a very rough surface, which can adversely affect the aerodynamics of gas flows, and also engine condensates or combustion products can penetrate the pores and/or micro-cracks of the coating. These combustion products have different expansion co-efficients to the TBC and can thus cause extensive cracking and spalling. In recent years there has been considerable research into the use of lasers to melt the surface of the TBC to produce a dense smooth surface to prevent ingress of condensates and improve aerodynamic behaviour. However, such procedures have not been entirely successful because of excessive surface cracking.

The present invention provides a method of coating a surface of a component, comprising the following sequential steps:

(a) applying a first coating, consisting of one or more layers, to the surface by flame spraying;

(b) applying a ceramic slurry to the first coating;

(c) heating the ceramic slurry to form a second, ceramic coating;

(d) impregnating the coatings with a solution of at least one soluble compound capable of being converted, on heating, into an insoluble substance which bonds with the coatings; and

(e) heating the coatings to cause bonding and densification of the coatings by the said substance.

Step (d) preferably comprises impregnating the coatings with a solution of at least one chromia or phosphate forming compound capable of being converted into an oxide or phosphate on heating.

- Optionally, between steps (a) and (b), the method may include applying a ceramic material to the first coating, e. g. to form, in combination, a thermal barrier coating.

The (last) ceramic coating is preferably a so-called Monitox coating, which is achieved by applying to a substrate a slurry containing a chromium compound capable of being converted into chromium oxide (Cr_0,) at temperatures of at least 300\'C, heating the slurry to produce a porous ceramic coating, and then densifying and bonding the coating by one or more process cycles comprising impregnating the porous coating with at least one chromia or phosphate forming solution, removing excess i pregnant, and heating. This technique is more fully described (using chromia forming compounds) in GB-A-1 466 074.

The first coating may be a ceramic coating, a ceramo-metallic coating, a metallo-ceramic coating, or a metallic coating. It may be applied as a single layer

or may be built up in two or more layers.. The flame spraying process may comprise plasma spraying; this is preferred when producing a thermal barrier coating. For a wear and corrosion resistant coating the preferred flame spraying process is the high-velocity oxy-fuel (HVOF) process.

The HVOF process is a commercially available flame spraying process which by virtue of its high velocity produces very dense coatings which are highly bonded to the base material. Previous flame spray systems such as plasma spraying do not produce such high velocities and therefore the coatings tend to be more porous and are not so well bonded to the substrate. In the HVOF process, fuel and oxygen are combusted in a specially designed chamber which is connected to a water cooled tube or nozzle at the combustion chamber exit. The combustion products are accelerated down the nozzle, constituting a gun or torch. Powdered materials of closely controlled sizes are metered into the gun and are thus accelerated and heated as they pass along the hot high-velocity stream. Upon impacting with the substrate at a specific distance from the gun the particles are splat-quenched and build up upon each other and the substrate to form a highly bonded and dense coating.

Various coatings can be produced in this manner, but tungsten carbide-σobalt-chromium coatings are of

particular interest to the oil and gas industry because of their hardness, density, and corrosion resistance. However, all flame sprayed coatings to a greater or lesser degree can contain micro-cracks or defects.

Sometimes micro-cracks can be extremely small, to such an extent that they are very difficult to discern using normal metallographic techniques. However, it has been shown that corrosive liquors can penetrate the pathways to the substrate and thus cause corrosion. Attempts have been made to seal all pores, cracks and defects using materials such as organic epoxy polymers. These types of materials are composed of very large molecules and it can be very difficult to penetrate very small fissures etc. so as to seal them effectively. However, it is possible to seal such features by technically growing and bonding inorganic oxides and/or other compounds such as phosphates in these cracks. Penetration into these cracks is very much easier with these materials because of their relatively small atomic radius.

A flame-sprayed tungsten carbide surface exhibits a coarse as-sprayed surface, somewhat like a coarse emery paper. Such a surface is difficult to machine.

By using a Monitox coating it is possible to solve both the machining and sealing problems at the same time.

In the particular case of the rotors (and other components) of downhole motors or progressing cavity

pumps, a Monitox coating on top of an HVOF coating can both seal the HVOF coating, thus preventing corrosion, and produce a surface that is compatible with the mating elastomer surface of the stator. Unlike the HVOF surface, the Monitox surface is smooth (probably even smoother than a chromium plated surface). However, the ceramic surface by itself will not provide sufficient wear resistance to the action of the chemical muds which are pumped through the motor. As wear takes place, so the peaks of the underlying HVOF coating (preferably tungsten carbide type) are revealed and can then begin to support the wear (much as gravel set in road tar supports vehicular road wear). The peaks of HVOF coating do not damage the elastomer as sharp points, because the elastomer "sees " a continuous composite surface of HVOF coating and Monitox coating. As further minimal wear occurs, so further peaks or plateaus of hard HVOF coating are revealed which, because of the greater and increasing percentage of surface area comprising the harder, tougher, and therefore more durable HVOF coating, increases the wear resistance of the composite surface. Corrosion is also inhibited because the Monitox oxides and/or phosphates have also sealed any pathway or routes through the HVOF coating to the substrate.

The Monitox coating is applied in an aqueous slurry form containing various oxides and chemicals. This layer is then dried and heated (up to 500 ^* C) and at this

stage is "soft", in that it is a collection of hard particles which are not very well bonded to each other or the substrate. Thus at this stage the coating can be easily "machined". The coating is hardened and densified by a cyclic process of impregnation in chemicals and further heat treatment. Approximately 4-15 impregnation/heat treatment cycles are usually required to produce the optimum coating.

Another and alternative method of providing a composite HVOF/Monitox coating to the above components is as follows.

After applying the Monitox coating and after, say, 2 or 3 cycles of impregnation/heat treatment, the Monitox coating can be easily "scraped" so as to reveal the harder HVOF surface underneath. Thus a composite surface can be exposed which constitutes a specific ratio of HVOF/ceramic surface. This composite surface is then processed by the necessary and remaining impregnation/heat treatment cycles to bring the Monitox ceramic surface up to its optimum condition. By this means, the initial "bedding-in" process that is

« described above is eliminated and therefore closer tolerances between the stator and rotor can be maintained if necessary.

The method described in the preceding paragraph is particularly applicable to components of continuous casting machines which components (e. g. of the mould or

the guides) are to be in frictional contact with the solidified steel. A thin composite coating improves the operating lifetime of such components. A smooth surface finish is important to allow even and uniform sliding of the solidified and oxidised surface of the steel strand. It is important to maintain only a thin coating in order to minimise any reduction in thermal conductivity (particularly in the mould). The HVOF coating resists abrasive wear and the ceramic coating resists adhesive wear and provides a compatible sliding surface for the oxidised steel strand.

The HVOF/Monitox composite coating is also applicable to the screw of an extruder.

This composite surfacing technique can also be employed elsewhere and in other industries. For example the Monitox coating exhibits different characteristics to flame-sprayed coatings such as friction, wettability, etc. It is therefore possible to create a surface condition which exhibits specific characteristics due to the combination of both a flame-sprayed coating and a Monitox ceramic coating.

By using the Monitox process on a TBC in the same way as described above, it is possible to bond Monitox ceramic onto a TBC to produce a dense surface layer which is very much smoother than the original T. B. C. Such a TBC/Monitox combined coating is suitable for components of internal combustion engines and gas

turbine engines.

Example 1: process for coating a downhole motor rotor.

(I) Degrease the rotor.

•(2) Mask where required.

(3) Grit-blast.

(4) Apply tungsten carbide-cobalt-chromium coating (or other wear and corrosion resistant coating) by HVOF spraying.

(5) Clean and dress as required.

(6) Prefire at approximately 400 - 500"C.

(7) Mask where required.

(8) Grit-blast.

(9) Re-mask.

(10) Apply Monitox slurry.

(II) Fire at approximately 400 - 500 "C.

(12) Cool.

(13) Remove excess ceramic.

(14) Impregnate the coatings with oxide or phosphate forming compounds.

(15) Fire at approximately 400 - 500 ^* C.

(16) Repeat operations (14) and (15) for 2 - 20 cycles.

(17) Finish, where necessary, by non-dimensional or precision machining, to achieve the required surface quality.

The same sequence of steps can be applied to a pump rotor, an extruder screw, or a component of a continuous casting machine.

Example 2: process for applying thermal barrier coatings to engine components.

(1) Degrease the component.

(2) Mask where required.

(3) Grit-blast.

(4) Apply bonding coating (typically Co-Ni-Cr-Al-Y) by plasma spraying.

(5) Applying ceramic coating (typically zirconia partially stabilised with yttria).

(6) Clean and dress as required.

(7) Prefire at approximately 400 - 500 ^* C.

(8) Mask where required.

(9) Apply Monitox slurry.

(10) Fire at approximately 400 - 500\'C.

(11) Cool.

(12) Remove excess ceramic.

(13) Impregnate the coatings with oxide or phosphate forming compounds.

(14) Fire at approximately 400 - 500"C.

(15) Repeat operations (13) and (14) for 2 - 20 cycles.

(16) Finish, where necessary, by non-dimensional or precision machining, to achieve the required surface quality.