GB2582379A

GB2582379A - Method of coating carbon components

Info

Publication number: GB2582379A
Application number: GB1903996.5A
Authority: GB
Inventors: Philip Reid Daniel; Richard Butterworth James; Olivia Birch Erin; Howard Grundy Trevor
Original assignee: Tenmat Ltd
Current assignee: Tenmat Ltd
Priority date: 2019-03-22
Filing date: 2019-03-22
Publication date: 2020-09-23
Anticipated expiration: 2039-03-22
Also published as: GB201903996D0; WO2020193950A1; GB2582379B

Abstract

A method of coating a carbon component with silicon-silicon carbide. The method involves sequentially applying and drying onto at least a part of the carbon component a first composition comprising silicon carbide, carbon, a binder and a solvent, and a second composition comprising silicon, a binder and water. The coating compositions are then fired to provide the silicon-silicon carbide coating on the carbon component. The method may allow a more controlled and cost-effective silicon-silicon carbide coating, particularly for relatively complex carbon components such as degassing rotors. The binder may be styrene acrylate co-polymer. The solvent may be water. A carbon component comprising the silicon-silicon coating and coating compositions for use forming said coating are also provided. Disclosed are compositions for coating a substrate with silicon carbide and for silicizing silicon carbide.

Description

Method of Coating Carbon Components

Field

The present invention relates to a method of coating a carbon component, compositions for use in said method and to a carbon component comprising a silicon-silicon carbide coating. In particular the invention relates to a method of providing a silicon-silicon carbide coating onto a carbon component.

Background

Carbon components, such as furnace furniture, heating elements and electrodes used in molten metal processing formed of graphite, may have excellent thermal resistance properties but may be highly susceptible to oxidation at the high temperatures experienced by such components in use. Oxidation at such high temperatures severely reduces the useful life of the carbon components and continuously replacing these components can be costly.

In order to extend the useful life of such carbon components, a silicon carbide coating may be applied to the surface of carbon components having simple shapes. Such silicon carbide coatings provide the carbon component with improved resistance to high temperature oxidation. Subsequent reaction of such a silicon carbide coating with silicon provides a silicon-silicon carbide coating which further improves high temperature oxidation resistance of the carbon component.

Known methods for providing a silicon-silicon carbide coating onto a carbon component are limited to relatively simple shapes of carbon component. For example, US 5,418,011 discloses methods for forming a coating of silicon-silicon carbide on graphite pebbles used in nuclear power engineering. The method involves first coating the graphite pebbles with silicon carbide using an aqueous slip comprising finely divided silicon carbide, binder and finely divided carbon, drying the coating and then silicizing the coating with liquid silicon to form the silicon-silicon carbide coating.

US 3,275,471 discloses a method of forming a silicon-silicon carbide coating on a graphite nuclear reactor component. The method comprises applying by dipping or painting a slurry of silicon, silicon carbide, a suspension aid and a suspension medium to the graphite component and then firing the coating and the component to provide a relatively thin layer of silicon-silicon carbide of up to 0.005 inches (127 pm).

There remains a need for a method of applying a silicon-silicon carbide coating which is suitable for relatively complex shaped carbon components and which produces a relatively thick, reliable coating which is highly oxidation and wear resistant.

Summary of the Invention

It is one aim of the present invention, amongst others, to provide a method of coating a carbon component that addresses at least one disadvantage of the prior art, whether identified here or elsewhere, or to provide an alternative to existing coating methods. For instance it may be an aim of the present invention to provide a method of coating a carbon component with a silicon-silicon carbide layer which may provide improved reliability, oxidation resistance and wear resistance compared to known methods.

According to aspects of the present invention, there is provided a method, composition, kit and carbon component as set forth in the appended claims. Other features of the invention will be apparent from the dependent claims, and the description which follows.

According to a first aspect of the present invention, there is provided a method of coating a carbon component, the method comprising the steps of a) applying a first composition to at least a part of the carbon component, the first composition comprising silicon carbide, carbon, a binder and a solvent; b) drying the first composition on the carbon component; c) applying a second composition to the part of the carbon component comprising the first composition, the second composition comprising silicon, a binder and water; d) drying the second composition on the carbon component; and e) firing the carbon component to form a silicon-silicon carbide coating on the carbon 20 component.

The carbon component may be any component formed substantially of carbon, for example graphite or a carbon-carbon material (carbon fibre reinforced carbon). Suitably the carbon component is a graphite component. The carbon component may be intended for a high temperature application which may involve oxidation conditions. For example, the carbon component may be a furnace furniture component, a crucible, a heating element, an electrode, a radiating heat tube, a thermocouple protector or a brake disc for a motor vehicle.

The inventors have found that the method of this first aspect can reliably provide a carbon component with a silicon-silicon carbide coating having a thickness in the range 100 pm to 5 mm which may be highly resistant to high temperature oxidation and wear. Microscopic analysis of the silicon-silicon carbide coated carbon components have shown a penetration of the silicon into the carbon component resulting in a strong mechanical interface between the coating and the carbon component and therefore a mechanically reliable silicon-silicon carbide coating.

In particular, the inventors have found that applying silicon in a second composition to the carbon component comprising the first composition, the second composition also comprising a binder and water, enables relatively complex-shaped carbon components to be coated and therefore protected from oxidation and wear. Known techniques for providing silicon-silicon carbide coatings, for example techniques involving the absorption of liquid silicon into the silicon carbide coated carbon component during firing, have been found to be unsuitable for such relatively complex-shaped carbon components, for example a degassing rotor for use in molten metal processing.

The inventors have also found that the method of this first aspect may provide a silicon-silicon carbide coating to a carbon component in a more cost-effective manner than known methods of forming silicon-silicon carbide coatings due to reduced processing time compared to alternative methods such as chemical vapour deposition (CVD) and the potential to selectively coat only the regions of the component which require the properties imparted by the silicon-silicon carbide coating. Furthermore, the method of this first aspect may allow the formation of relatively thick coatings of up to 5 mm. Such coating thicknesses may provide advantages to the carbon component such as an increased operational lifetime due to an increased resistance to high temperature oxidation and/or mechanical wear.

The inventors have also found that the method of the present invention may allow a carbon component to be effectively coated with silicon-silicon carbide in a more controlled manner than prior art methods. For example, the carbon component can be selectively coated with the first composition and the second composition in particular regions which require the beneficial chemical and physical properties which the coating provides in use. In prior art methods using liquid silicon in a second coating step, only the regions of a carbon component which contacted the base of a crucible containing the liquid silicon would be coated. In the method of the present invention, the carbon component can be selectively coated in any region, for example on opposing outer facing surfaces of a carbon component, which would not be feasible using prior art methods.

Also, the use of the first and second compositions in the method of this first aspect may allow the thickness of the silicon-silicon carbide coating to be more accurately controlled than in prior art methods. For example, it would be possible to calculate how much silicon carbide and silicon, and therefore how much of a given first and second composition, needs to be applied per unit area of the carbon component surface in order to provide a desired coating thickness for a certain application of the carbon component.

The first composition comprising silicon carbide, carbon, a binder and a solvent In some embodiments the first composition is an aqueous composition. Therefore the solvent of the first composition may be water.

In alternative embodiments the first composition is an organic composition. In such embodiments, the solvent may be an oil, suitably mineral oil.

The first composition suitably has a solids content of at least 60 wt%, suitably at least 65 wt%, suitably at least 70 wt%, suitably at least 75 wt%.

The first composition suitably has a solids content of up to 95 wt%, suitably up to 90 wt%, suitably up to 85 wt%.

The first composition suitably has a solids content of from 60 to 90 wt%, suitably from 65 to wt%, suitably from 70 to 85 wt%, suitably from 75 to 85 wt%.

The remainder of the first composition not provided by the solids content may be provided by the solvent, for example by water or mineral oil.

The first composition suitably comprises at least 50 wt% silicon carbide, suitably at least 55 wt%, suitably at least 60 wt%.

The first composition suitably comprises up to 75 wt% silicon carbide, suitably up to 70 wt%, suitably up to 65 wt%.

The first composition suitably comprises from 40 to 80 wt% silicon carbide, suitably from 50 to 75 wt%, suitably from 55 to 70 wt%, suitably from 55 to 65 wt%, suitably from 60 to 65 wt%.

Suitably the silicon carbide in the first composition has an average particle size of at least 2 pm, suitably at least 5 pm.

Suitably the silicon carbide in the first composition has an average particle size of up to 30 pm, suitably up to 20 pm, suitably up to 15 pm.

Suitably the silicon carbide in the first composition has an average particle size of from 2 to 50 pm, suitably from 5 to 20 pm, suitably from 7 to 15 pm. In some embodiments, the silicon carbide may have an average particle size of approximately 10 pm. Suitably the average particle sizes are D50 values measured by on laser diffraction particle size analysis (multisizer) or Coulter (electrical). Suitably the standard procedure of FEPA (Federation of European Producers of Abrasives) 42-2:2006 is used to determine the particle size of the silicon carbide.

Suitably the silicon carbide has the particle size 'F600' having an average size of 10 pm.

In some embodiments, the first composition comprises from 55 to 70 wt% silicon carbide having an average particle size of from 2 to 50 pm.

The first composition suitably comprises at least 4 wt% carbon, suitably at least 6 wt%, suitably at least 8 wt%.

The first composition suitably comprises up to 18 wt% carbon, suitably up to 14 wt%, suitably up to 12 wt%.

The first composition suitably comprises from 5 to 20 wt% carbon, suitably from 6 to 18 wt%, suitably from 6 to 14 wt%, suitably from 8 to 12 wt%. In some embodiments, the first composition comprises approximately 10 wt% carbon.

Suitably the carbon in the first composition has an average particle size of at least 0.01 pm, suitably at least 0.1 pm.

Suitably the carbon in the first composition has an average particle size of up to 5 pm, suitably up to 2 pm, suitably up to 1 pm.

Suitably the carbon in the first composition has an average particle size of from 0.01 to 2 pm, suitably from 0.01 to 1 pm, suitably from 0.1 to 1 pm. Suitably the average particle sizes are Dso values measured by on laser diffraction particle size analysis (multisizer) or Coulter (electrical).

Suitably the carbon in the first composition has an average particle size less than 1 pm.

Suitably the carbon in the first composition is carbon black. In some embodiments the carbon may be graphite powder. In some embodiments, the first composition comprises from 6 to 14 wt% carbon black having an average particle size less than 1 pm.

The inventors have found the ratio of the silicon carbide to carbon in the first composition to be important in providing an effective coating of silicon-silicon carbide at the end of the method of this first aspect. Suitably the ratio of silicon carbide to carbon is at least 2:1, suitably at least 3:1, suitably at least 4:1, suitably at least 5:1.

Suitably the ratio of silicon carbide to carbon is up to 20:1, suitably up to 15:1, suitably up to 10:1, suitably up to 8:1, suitably up to 7:1.

Suitably the ratio of silicon carbide to carbon in the first composition is from 2:1 to 20:1, suitably from 3:1 to 10:1, suitably from 4:1 to 8:1, suitably from 5:1 to 7:1 or from 6.0:1 to 6.5:1.

In some embodiments, the ratio of silicon carbide to carbon in the first composition is approximately 6.2:1 The binder of the first composition may be a polymeric material. For example, the binder may be selected from any one or more of acrylate polymers or copolymers, styrene-butadiene rubber (SBR), a polyethylene glycol (PEG), a poly(vinyl alcohol) (PVA), or a wax emulsion. Suitably the binder is a polar polymer, suitably comprising acrylate groups. Suitably the binder is an acrylate copolymer. Suitably the binder is a styrene acrylic copolymer.

The first composition suitably comprises at least 4 wt% binder, suitably at least 5 wt%, suitably at least 6 wt%, on a dry weight basis.

The first composition suitably comprises up to 20 wt% binder, suitably up to 15 wt%, suitably up to 12 wt%, suitably up to 10 wt%, on a dry weight basis.

The first composition suitably comprises from 5 to 20 wt% binder, suitably from 5 to 15 wt%, suitably from 6 to 12 wt%, suitably from 8 to 10 wt%, on a dry weight basis. In some embodiments, the first composition comprises from 8 to 10 wt% of a styrene acrylic copolymer, on a dry weight basis. The binder may be in the form of a emulsion, for example a 50 wt% solids emulsion, suitably in water.

The first composition may comprise a thickening agent. The thickening agent may be additionally or alternatively described as a flocculent. A suitable compound for performing these functions may be a polymeric compound. A suitable compound may be selected from a cellulose ether, a polyacrylate a starch or a gum. Suitably the first composition comprises cellulose, suitably as a thickening agent. A suitable cellulose thickening agent is CulminalTM MHPC 20000 S supplied by Ashland Specialty Chemical which is a high substitution level, non-ionic cellulose ether. The inventors have found that the use of such a thickening agent provides the first composition as a stable homogeneous solution which is advantageous for providing a consistent coating on a carbon component.

Suitably the first composition comprises at least 0.01 wt% of a thickening agent, suitably at least 0.05 wt%, suitably at least 0.1 wt%.

Suitably the first composition comprises up to 2 wt% of a thickening agent, suitably up to 1.0 wt%, suitably up to 0.5 wt%, suitably up to 0.2 wt%.

Suitably the first composition comprises from 0.01 to 1.0 wt% of a thickening agent, suitably from 0.05 to 0.5 wt%, suitably from 0.1 to 0.2 wt%. In some embodiments, the first composition comprises from 0.05 to 0.5 wt% of cellulose.

In some embodiments, the first composition comprises from 55 to 70 wt% silicon carbide, from 6 to 14 wt% carbon black, from 5 to 8 wt% of a styrene acrylic copolymer (dry weight), from 0.05 to 0.5 wt% of cellulose and water (to balance).

In some embodiments, the first composition comprises from 55 to 70 wt% silicon carbide, from 6 to 14 wt% carbon black, from 5 to 8 wt% of a styrene acrylic copolymer (dry weight), from 0.05 to 0.5 wt% of cellulose and mineral oil (to balance).

In some embodiments, the first composition comprises from 55 to 70 wt% silicon carbide having an average particle size of from 5 to 20 pm, from 6 to 14 wt% carbon having an average particle size less than 1 pm, from 5 to 8 wt% of a styrene acrylic copolymer (dry weight), from 0.05 to 0.5 wt% of cellulose and water (to balance).

In some embodiments, the first composition comprises from 55 to 70 wt% silicon carbide having an average particle size of from 5 to 20 pm, from 6 to 14 wt% carbon having an average particle size less than 1 pm, from 5 to 8 wt% of a styrene acrylic copolymer (dry weight), from 0.05 to 0.5 wt% of cellulose and mineral oil (to balance).

The first composition may be formed by combining the stated materials and carrying out a blending step for a period of time. The inventors have found that the first composition forms a homogeneous solution after combining and blending the silicon carbide, carbon, binder, solvent and optionally the thickening agent. The blending may be carried out using a ball mill or a high shear mixer. The blending step may be carried out for between 1 minute and 1 hour, suitably between 5 and 30 minutes.

The first composition is suitably a liquid with a viscosity of approximately 50,000 absolute centipoise, suitably as measured by a capillary viscometer.

The first composition is suitably a liquid suitable for application to a component by spraying or brushing. Suitably step a) of the method of this first aspect is carried out by spraying, brushing or dipping. Suitably step a) is carried out by spraying the first composition onto at least a part of the carbon component.

The method of this first aspect involves step b) of drying the first composition on the carbon component. Step b) may be carried out by exposing the carbon component comprising the first composition to a temperature of at least 60 °C, suitably at least 80 °C, suitably for a period of time of at least 6 hours.

In embodiments wherein the first composition comprises silicon carbide, carbon, a binder and a water as a solvent, step b) may involve exposing the carbon component comprising the first composition to a temperature of at least 60 °C, suitably at least 70 °C, suitably at least 80 °C.

Suitably in such embodiments, step b) involves exposing the carbon component comprising the first composition to a temperature of from 80 to 130 °C, suitably from 100 to 120 °C, suitably approximately 110 °C. Suitably step b) is carried out using an air oven. Suitably this heating in step b) is carried out for a time sufficient to remove the water from the first composition to leave a substantially dry coating comprising silicon carbide, carbon, the binder and optionally the thickening agent, suitably for between 8 and 12 hours.

In embodiments wherein the first composition comprises silicon carbide, carbon, a binder and a mineral oil as a solvent, step b) may involve exposing the carbon component comprising the first composition to a temperature of at least 160 °C, suitably at least 200 °C, suitably at least 240 °C, suitably approximately 275 °C. Suitably in such embodiments, step b) involves exposing the carbon component comprising the first composition to a temperature of from 260 to 290 °C. Suitably step b) is carried out using an air oven. Suitably this heating in step b) is carried out for a time sufficient to remove the mineral oil from the first composition to leave a substantially dry coating comprising silicon carbide, carbon, the binder and optionally the thickening agent, suitably between 24 and 36 hours.

After step b), the carbon component may be considered to comprise a first coating layer comprising silicon carbide on a main body of the carbon component.

The second composition comprising silicon, a binder and water Step c) of the method of this first aspect involves applying a second composition to the part of the carbon component comprising the first composition, the second composition comprising silicon, a binder and water. Therefore the second composition is applied onto the first composition, in effect forming an outer layer of the carbon component, in the part wherein the first and second compositions are applied.

The second composition suitably has a solids content of at least 55 wt%, suitably at least wt%, suitably at least 65 wt%.

The second composition suitably has a solids content of up to 90 wt%, suitably up to 85 wt%, suitably up to 80 wt%, suitably up to 75 wt%.

The second composition suitably has a solids content of from 55 to 90 wt%, suitably from 60 to 85 wt%, suitably from 60 to 80 wt%, suitably from 65 to 75 wt%.

The remainder of the second composition not provided by the solids content may be provided by the water.

Suitably the silicon of the second composition is in the form of a powder having an average particle size of at least 20 pm, suitably greater than 20 pm, suitably of at least 100 pm, suitably at least 200 pm. The inventors have found that using silicon powder having an average particle size of approximately 500 pm is particularly effective in the method of this first aspect. If silicon powder having an average particle size of less than 20 pm is used then the surface area to volume ratio may be too high, which may result in insufficient silicizing due to surface impurities, such as silica and losses of silicon from the carbon component in the form of silicon vapour.

The second composition suitably comprises at least 40 wt% silicon, suitably at least 50 wt%, suitably at least 60 wt%.

The second composition suitably comprises up to 90 wt% silicon, suitably up to 80 wt%, suitably up to 70 wt%.

The second composition suitably comprises from 40 to 90 wt% silicon, suitably from 50 to 75 wt%, suitably from 55 to 70 wt%, suitably from 60 to 65 wt%. In some embodiments, the second composition comprises from 55 to 70 wt% silicon in the form of a powder having an average particle size of at least 20 pm, suitably of at least 200 pm.

Suitably the second composition is applied to the carbon component in an amount which provides a weight of silicon which is 2 to 10 times greater than the weight of silicon carbide provided by the first composition, suitably 2 to 5 times greater, suitably approximately 3 times greater.

The binder of the second composition may be a polymeric material. For example, the binder may be selected from any one or more of acrylate polymers or copolymers, styrene-butadiene rubber (SBR), a polyethylene glycol (PEG), a poly(vinyl alcohol) (PVA), or a wax emulsion. Suitably the binder is a polar polymer, suitably comprising acrylate groups. Suitably the binder is an acrylate copolymer. Suitably the binder is a styrene acrylic copolymer. The binder of the second composition may be the same as the binder used in the first composition.

The second composition suitably comprises at least 2 wt% binder, suitably at least 3 wt%, suitably at least 4 wt%, on a dry weight basis.

The second composition suitably comprises up to 20 wt% binder, suitably up to 10 wt%, suitably up to 8 wt%, suitably up to 7 wt%, on a dry weight basis.

The second composition suitably comprises from 2 to 20 wt% binder, suitably from 2 to wt%, suitably from 3 to 10 wt%, suitably from 4 to 8 wt%, suitably from 5 to 8 wt%. In some embodiments, the second composition comprises from 3 to 8 wt% of a styrene acrylic copolymer, on a dry weight basis. The binder may be in the form of a solution or an emulsion, for example a 50 wt% solids emulsion, suitably in water.

The second composition may comprise a thickening agent. The thickening agent may be additionally or alternatively described as a flocculent. A suitable compound for performing these functions may be a polymeric compound and may be as described above in relation to the thickening agent used in the first composition.

Suitably the second composition comprises at least 0.01 wt% of a thickening agent, suitably at least 0.05 wt%, suitably at least 0.1 wt%.

Suitably the second composition comprises up to 5.0 wt% of a thickening agent, suitably up to 3.0 wt%, suitably up to 1.0 wt%.

Suitably the second composition comprises from 0.01 to 5.0 wt% of a thickening agent, suitably from 0.05 to 5.0 wt%, suitably from 0.1 to 2.0 wt%. In some embodiments, the second composition comprises from 0.05 to 1.0 wt% of cellulose.

In some embodiments, the second composition comprises from 55 to 70 wt% of silicon, from 3 to 10 wt% of a styrene acrylic copolymer, from 0.05 to 5.0 wt% of cellulose and water (to balance).

In some embodiments, the second composition comprises from 55 to 70 wt°/0 of silicon having an average particle size of at least than 20 pm, from 3 to 10 wt% of a styrene acrylic copolymer, from 0.05 to 5.0 wt% of cellulose and water (to balance).

Suitably the second composition has a viscosity which allows it to be applied by brushing. The second composition may be paste-like. The second composition may have a greater viscosity than the first composition. This greater viscosity suitably enables a thicker coating to be applied to facilitate effective silicon delivery to the coating and the carbon component to ensure sufficient silicon is available to the carbon component during firing in order to fully silicize the silicon carbide coating.

Step c) may be carried out by brushing the second composition onto the part of the carbon component comprising the first composition. In some embodiments step c) is carried out by spraying the second composition onto the part of the carbon component comprising the first composition.

Step d) involves drying the second composition on the carbon component. Step d) may involve exposing the carbon component comprising the second composition to a temperature of at least 70 °C, suitably at least 80 °C, suitably at least 90 °C. Suitably in such embodiments, step d) involves exposing the carbon component comprising the second composition to a temperature of from 80 to 170 °C, suitably from 90 to 130 °C, suitably approximately 110 °C. Suitably step d) is carried out using an air oven. Suitably this heating in step d) is carried out for a time sufficient to remove the water from the second composition to leave a substantially dry coating comprising silicon, the binder and optionally a thickening agent, suitably for between 8 and 12 hours.

In some embodiments, the drying step d) involves exposing the carbon component comprising the second composition to a temperature of at least 200 °C, suitably at least 220 °C, suitably for at least 6 hours, suitably for at least 8 hours. Suitably step d) is carried out using an air oven. Suitably the temperature of step d) is up to 300 °C, suitably up to 275 °C.

After step d), the carbon component may be considered to comprise a second coating layer comprising silicon on top of the first coating layer comprising silicon carbide which is in turn on top of the main body of the carbon component.

After step d), the carbon component may be considered to be in a "green state" ready to be fired to produce the final carbon component comprising a silicon-silicon carbide coating.

Step e) of the method of this first aspect involves firing the carbon component to form a silicon-silicon carbide coating on the carbon component. The firing of step e) suitably involves a multi-stage heating process. Step e) may comprise a first stage of exposing the carbon component to a temperature which removes the binder from the coating (the binder having been provided in the first and second compositions). Suitably this first stage involves exposing the carbon component to a temperature of from 400 to 600 °C, suitably from 450 to 575 °C, suitably from 500 to 550 °C. Suitably this first stage involves exposing the carbon component to said temperatures for at least 10 minutes, suitably at least 15 minutes, suitably at least minutes, suitably at least 30 minutes. Suitably this first stage involves exposing the carbon component to said temperatures for up to 120 minutes, suitably up to 60 minutes, suitably up to 40 minutes. Suitably this first stage involves exposing the carbon component to a temperature of from 400 to 600 °C for a period of from 15 to 120 minutes, suitably approximately 30 minutes.

Step e) may comprise a second stage of exposing the carbon component to a temperature above the melting point of silicon, suitably a temperature of at least 1,400 °C, suitably at least 1,450 °C, suitably at least 1,500 °C. Suitably this second stage involves exposing the carbon component to a temperature at which the silicon provided by the second composition melts, penetrates the first coating layer and reacts with the carbon and silicon carbide in the first coating layer to produce silicon-silicon carbide (i.e. silicizing the first coating layer). The silicon suitably also penetrates the main body of the carbon component underneath the first coating layer. Suitably this second stage involves exposing the carbon component to a temperature of from 1,400 to 1,800 °C, suitably from 1,400 to 1,700 °C, suitably from 1,450 to 1,650 °C, suitably from 1,500 to 1,600 °C, suitably approximately 1,550 °C. Suitably this second stage involves exposing the carbon component to said temperatures for at least 30 minutes, suitably for at least 60 minutes, suitably for at least 80 minutes. Suitably this first stage involves exposing the carbon component to said temperatures for up to 180 minutes, suitably for up to 120 minutes, suitably for up to 100 minutes. Suitably this second stage involves exposing the carbon component to a temperature of from 1,400 to 1,800 °C for a period of from 10 to minutes, suitably approximately 90 minutes.

Suitably step e) is carried out under vacuum, suitably at a pressure of 1 mbar or less. Suitably step e) is carried out in a vacuum furnace. Step e) may be carried out in a vacuum furnace under an inert atmosphere, for example nitrogen.

Step e) may involve providing additional silicon to the vessel in which the carbon component is fired. Such additional silicon may ensure that sufficient silicon is available to the carbon component during firing in order to fully silicize the silicon carbide coating.

After step e), the carbon component may be considered to be in a finished or "fired" state ready for usage in the intended application. After step e), the carbon component comprises a silicon-silicon carbide coating. The coating on the carbon component after step e) may be considered to be a reaction-bonded silicon-silicon carbide coating.

The method of this first aspect is carried out on at least a part of the carbon component and therefore provides a silicon-silicon carbide coating onto at least a part of the carbon component. In some embodiments, only a part or parts of the carbon component will require coating with the silicon-silicon carbide coating. For example a motor vehicle brake disc may only require the silicon-silicon carbide coating on the braking surface. The parts of the carbon component which are not intended to be coated in the method may be masked off using suitable techniques.

The method of this first aspect may be carried out on the entire surface of the carbon component. In some embodiments, the entire surface of the carbon component will require coating with the silicon-silicon carbide coating to allow the carbon component to perform its desired function. For example, furnace furniture may require coating on all surfaces in order to perform its desired function whilst resisting oxidation.

After step e), the silicon-silicon carbide coated carbon component may be inspected for coating quality and coverage. The silicon-silicon carbide coated carbon component may be shot blasted to test the integrity of the silicon-silicon carbide coating.

The inventors have found that the method of this first aspect can provide a silicon-silicon carbide coated carbon component which is resistant to oxidation at temperatures of at least 1,000 °C and is also highly resistant to wear. The silicon-silicon carbide coated carbon component may also have high resistance to thermal shock.

The silicon-silicon carbide coating produced by the method of this first aspect is suitably non-porous, suitably substantially 100% non-porous, suitably 100% non-porous. Any porosity present in the coating may adversely affect the oxidation resistance and/or mechanical integrity of the coating.

The method of this first aspect may involve weighing the carbon component before step a) and after step b) to determine the weight of the first coating layer. Then the amount of second composition to use in step c) can be determined in order to provide the desired ratio of silicon carbide (in the first coating layer) to silicon (in the second coating layer), on a weight by weight basis. The carbon component can then be weighed after step d) to check that the desired weight of silicon has been applied in the second coating layer.

Suitably the method steps are carried out in the order step a) followed by step b) followed by step c) followed by step d) followed by step e).

Steps a) and b) may repeated before step c) is carried out. Repeating steps a) and b) may be carried out to provide a thicker first coating layer than would be possible by carrying out steps a) and b) only once. For example, steps a) and b) may be repeated multiple times to provide a silicon-silicon carbide coating, after steps e), of up to 5 mm.

The carbon component As discussed above, the carbon component may be any component formed substantially of carbon, for example graphite or a carbon-carbon material (carbon fibre reinforced carbon).

The carbon component is suitably a relatively high density carbon material. Suitably the carbon component has a density of at least 1.3 g/cc, suitably at least 1.4 g/cc, suitably at least 1.5 g/cc, suitably at least 1.6 g/cc. Suitably the carbon component has a relatively low porosity, suitably a porosity of up to 40 %, suitably up to 30 %, suitably up to 20 %, suitably up to 15 %, suitably up to 10 %. Suitably the porosity of carbon component is measured by the method of ASTM C20.

Suitably the carbon component is a graphite component, suitably a high density isostatically pressed graphite, for example a graphite with a density of approximately 1.8 g/cc. Suitably the graphite has a porosity of 10 % or less.

According to a second aspect of the present invention, there is provided a composition for coating a substrate with silicon carbide, the composition comprising silicon carbide, carbon, a styrene acrylate copolymer and water.

The composition of this second aspect may have any of the suitable features and advantages of the first composition of the first aspect.

The substrate on which the composition of this second aspect is used may be a carbon component as described in relation to the first aspect.

Suitably the composition of this second aspect comprises from 55 to 70 wt% silicon carbide having an average particle size of from 5 to 20 pm, from 6 to 14 wt% carbon having an average particle size less than 1 pm, from 5 to 8 wt% of a styrene acrylic copolymer (dry weight), from 0.05 to 0.5 wt% of cellulose and water (to balance).

In some embodiments, the composition of this second aspect comprises from 55 to 70 wt% silicon carbide having an average particle size of from 5 to 20 pm, from 6 to 14 wt% carbon having an average particle size less than 1 pm, from 5 to 8 wt% of a styrene acrylic copolymer (dry weight), from 0.05 to 0.5 wt% of cellulose and mineral oil (to balance).

According to a third aspect of the present invention, there is provided a composition for silicizing a substrate coated with silicon carbide, the composition comprising silicon, a styrene acrylate copolymer and water.

The composition of this third aspect may have any of the suitable features and advantages of the second composition of the first aspect.

The substrate on which the composition of this third aspect is used may be a carbon component as described in relation to the first aspect.

Suitably the composition of this third aspect comprises from 55 to 70 wt% of silicon having an average particle size of at least than 20 pm, from 3 to 10 wt% of a styrene acrylic copolymer, from 0.05 to 5.0 wt% of cellulose and water (to balance).

According to a fourth aspect of the present invention, there is provided a kit for providing a silicon-silicon carbide coating on a substrate, the kit comprising a first composition and a second composition; wherein the first composition comprises silicon carbide, carbon, a binder and a solvent; and wherein the second composition comprises silicon, a binder and water.

The first and second compositions may have any of the suitable features and advantages of the first and second compositions of the first aspect respectively.

Suitably, the kit of this fourth aspect comprises a first composition according to the second aspect and a second composition according to the third aspect.

The kit of this fourth aspect comprises two separate compositions -the first composition and the second composition. Suitably the first composition and the second composition are separately packaged. The first composition and the second composition may be provided in separate containers, suitably sealed containers. Suitably the first composition and the second composition are stable on storage under ambient conditions and may be considered "shelf stable". Suitably the kit does not require any special storage or handling conditions in addition to being stored under ambient conditions. Suitably the kit can be safely transported without special handling conditions being required.

The first and second compositions provided in the kit are intended to used sequentially in a method according to the first aspect to provide a silicon-silicon carbide coating onto a carbon component.

According to a fifth aspect of the present invention, there is provided a carbon component comprising a silicon-silicon carbide coating and an intermediate zone underneath the silicon-silicon carbide coating, on at least a part of the carbon component; wherein the silicon-silicon carbide coating has a thickness of from 0.5 mm to 5 mm; wherein the intermediate zone comprises carbon and silicon carbide and has a thickness of at least 0.5 mm.

Suitably the intermediate zone is directly underneath the silicon-silicon carbide coating and extends into the bulk of the carbon component by at least 0.5 mm, suitably at least 0.8 mm, suitably at least 1 mm. The intermediate zone is where silicon has penetrated into the bulk of the carbon component during formation of the silicon-silicon carbide coating and has reacted to form silicon carbide within the carbon component.

Suitably the intermediate zone comprises from 5 to 25 wt% silicon carbide, suitably 10 to wt% silicon carbide.

The inventors have found that the intermediate zone may provide improved mechanical properties to the carbon component, for example the silicon-silicon carbide coating may have an improved bonding to the carbon component as a result of the intermediate zone.

The carbon component and the silicon-silicon carbide coating may have any of the suitable features and advantages described in relation to the first aspect.

The carbon component comprises the silicon-silicon carbide coating and the associated intermediate zone on at least a part of the carbon component. Suitably the carbon component comprises the silicon-silicon carbide coating and the associated intermediate zone in parts of the carbon component which require the beneficial chemical and physical properties which the coating provides in use. In some embodiments, the carbon component may comprise the silicon-silicon carbide coating and the associated intermediate zone on opposing outer facing surfaces, for example on an inside and outside surface of a degassing rotor head for use in molten metal purification.

The silicon-silicon carbide coating on the carbon component of this fifth aspect may be formed by a method according to the first aspect. Suitably the intermediate zone is also formed during the method of the first aspect.

In some embodiments, the carbon component of this fifth aspect may be a component which is exposed to high temperature oxidising conditions in use. For example, a furnace, kiln furniture, a thermocouple, a heating element sleeve or a component used in molten metal handling.

In some embodiments, the carbon component of this fifth aspect may be a component which is exposed to high mechanical wear in use. For example, a high temperature pipe liner or a cyclone or hydrocyclone wear part.

In some embodiments, the carbon component of this fifth aspect may be a component which requires high friction properties in use. For example, a mechanical seal face or counterface.

In some embodiments, the carbon component of this fifth aspect may be a component which requires high thermal resistance in use. For example, a weld former or a burner nozzle. Graphite is typically used as weld former backing plate due to its high thermal resistance and because it is not readily wetted by molten steel. However, such graphite weld former backing plates suffer from fragility. A weld former backing plate according to this fifth aspect may have improved strength compared to known weld former backing plates.

In some embodiments, the carbon component of this fifth aspect may be a component which requires high thermal conductivity in use. For example, a thermocouple sleeve or heating element or a high temperature heat exchanger. Such components may benefit from the high thermal conductivity of the silicon-silicon carbide coating of the carbon component of this fifth aspect.

In some embodiments, the carbon component of this fifth aspect may be a component which requires a high strength to weight ratio in use. For example, turbine mechanical seal faces, ballistic armour, furnace or kiln beams or ceramic pistons. The speed at which a turbine can run may be limited by the weight and strength of the seal face material, which is typically silicon carbide. Due to the advantageous silicon-silicon carbide coating described herein, the carbon component of this fifth aspect may have a strength to weight ratio which may allow a reduction in weight of this component which in turn may allow the turbine to run faster and therefore more efficiently. Likewise, the other types of components mentioned above could benefit from the advantageous strength to weight ratio of the carbon component of this fifth aspect.

In some embodiments, the carbon component of this fifth aspect may be a component which requires high corrosion/chemical resistance in use. For example, furnace furniture used in a corrosive atmosphere, such as when liquid silicon is present.

Brief Description Of The Drawings

For a better understanding of the invention, and to show how example embodiments may be carried into effect, reference will now be made to the accompanying drawings in which: Figure 1 is a perspective view of a degassing rotor coated with silicon-silicon carbide according to the first aspect of the present invention.

Figure 2 shows a microscope image (x4 magnification) of a cross-section of a carbon component coated with silicon-silicon carbide according to the first aspect of the present invention.

Figures 3 and 4 show a microscope image (x40 magnification) of a cross-section of a carbon component coated with silicon-silicon carbide according to the first aspect of the present invention.

Detailed Description Of The Example Embodiments

Figure 1 shows a degassing rotor (100) comprising a rotor head (110) and a threaded opening (120) for receiving a shaft (not shown). The rotor head (110) is generally cone shaped with an open base (111) and notches (112). Such a degassing rotor is typically used in the purification of mainly non-ferrous metals and alloys, for example for aluminium and its alloys. In use, the rotor head (110) is immersed in a vat of molten metal to be purified and a suitable gas is injected through the shaft and out of the base (111) of the rotor head (110). The shaft is driven to rotate the rotor head (110) to create a stream of bubbles from the gas feed which sweeps dissolved hydrogen and sometimes other impurities to the surface of the molten metal where they are easily removed. The degassing rotor experiences very high temperatures in use and has a relatively complex shape which does not lend itself to be coated by placing in a liquid, for example liquid silicon. The degassing rotor (100) has been coated with a silicon-silicon carbide coating, using a method of the present invention, on all surfaces apart from the threaded opening (120) and the associated socket (121) where the degassing rotor is coupled to a shaft. The two-stage coating of the present invention using the first and the second coating compositions facilitates the efficient formation of an effective silicon-silicon carbide coating on the degassing rotor (100), despite the degassing rotor having a relatively complex shape. The degassing rotor (100) had a significantly improved service life than degassing

rotors of the prior art.

Coating method A silicon-silicon carbide coating was provided onto a carbon component, a graphite crucible, by the following method. A crucible was formed by machining from a block of graphite (Grade G330 supplied by Tokai). The properties of this graphite are shown below in Table 1.

Table 1 -Typical properties of G330 Graphite Density 1.79 g cm-3 Hardness Shore 56 Coefficient of thermal expansion S 1 4.8 x 10- °C-Thermal conductivity 104 W mk-1 Pore size 2.2 pm Porosity 15 Grain Size 13 pm A first composition comprising silicon carbide, carbon, a styrene acrylic binder, cellulose and water was prepared by mixing the materials listed below in Table 2 and blending for 5-30 minutes using a high shear mixer.

Table 2 -First composition Ingredient Wet Weight (%) Water 13.98 Styrene Acrylic (50 wt% solids) 13.98 Carbon Black 9.99 Silicon Carbide 61.91 Methyl Cellulose 0.14 The materials listed above were obtained from the suppliers noted below in Table 2a. The grades and average particle sizes of these components is also noted in Table 2a.

Table 2a

Component Manufacturer Grade Particle Size Carbon Black Cancarb Thermax N990 280 nm Silicon Carbide Sika Abri F600 9.3 pm Methyl Cellulose Ashland Cuminal MHPC 255-330 pm 20000P Styrene Acrylic OrganikKimya Orgal PST 50 A N/A The weight of the uncoated crucible was measured and noted. The first composition was then applied to the entire inner and outer surfaces of the crucible using a spray gun to provide an even thickness of coating over the crucible. The first composition was then dried at 80 °C in an air oven for 8-12 hours to provide the crucible comprising a first coating layer of silicon carbide.

The weight of the crucible comprising the silicon carbide coating was then measured and compared to the weight of the uncoated crucible to give the weight of coating applied.

A second composition comprising silicon, a styrene acrylic binder, cellulose and water was prepared by mixing the materials listed below in Table 3.

Table 3 -Second composition Ingredient Wet Weight (%) Water 24.68 Styrene Acrylic (50 % solids) 12.50 Silicon 62.51 Methyl Cellulose 0.31 The materials listed above were obtained from the suppliers noted below in Table 3a. The grades and average particle sizes of these components is also noted in Table 3a.

Table 3a

Component Manufacturer Grade Particle Size Silicon Elkem ASA Silicon Silgrain HQ 0.2-0.8 mm Methyl Cellulose Ashland Cuminal MHPC 255-330 pm 20000P Styrene Acrylic OrganikKimya Orgal PST 50 A N/A The amounts of silicon carbide and silicon in the first and second compositions respectively and the weight of the first composition applied to the crucible were used to calculate the amount of the second composition required to provide 4 times the amount of silicon compared to silicon carbide (on a weight by weight basis). This amount of the second composition was applied to the entire inner and outer surfaces of the crucible, on top of the first composition, using a brush or spatula. The second composition was then dried at 150 °C and then at 225 °C in an air oven for at least 8 hours to provide a "green state" crucible comprising a silicon coating on top of the silicon carbide coating.

The weight of the crucible was then measured to check the desired amount of silicon-containing second composition had been applied and dried on to the crucible. The crucible was then fired in a furnace with approximately 200 g of additional silicon to produce the desired reaction bonded silicon-silicon carbide coating on the crucible. The heating sequence used was as follows: a. ramp to 525 °C at 2.3 °C a minute, dwell time 30 minutes; b. ramp to 1350 °C at 4.0 °C a minute, dwell time 15 minutes; c. ramp to 1550 °C at 0.4 °C a minute, dwell time 90 minutes; d. furnace cooled.

The silicon-silicon carbide coated crucible was then inspected and cleaned with a shot blaster to determine the integrity of the silicon-silicon carbide coating.

The silicon-silicon carbide coated crucible exhibited excellent high temperature oxidation resistance. After exposure to oxidising conditions at 1,000 °C for 17 hours, the crucible exhibited virtually no mass loss. By comparison, an uncoated graphite crucible was completely consumed by oxidation under these conditions.

A comparison of graphite samples prepared as above using two different grades of silicon in the second composition (Elkem and Pilamec) showed that superior high temperature oxidation resistance was provided by the Elkem Silgrain with the larger average particle size. An uncoated graphite sample was also tested as a control.

Table 4 -Mass Loss Due to Oxidation at 1000 °C over 17 hours Sample -silicon grade Weight -Si particle size Before (g) After (g) Change (g) Change (%) 1 layer -Elkem --0.5 67.7237 67.7173 -0.0064 -0.009 mm 1 layer -Pilamec -15 65.9248 65.2145 -0.7103 -1.077 Pm Graphite -none 55.4435 0.0000 -55.4435 -100.000 Table 5 -Mass Loss Due to Oxidation at 800 °C over 2.5 hours Sample -silicon grade -Si Weight particle size Before (g) After (g) Change (g) Change (%) 1 layer -Elkem --0.5 mm 66.4759 66.4731 -0.0028 -0.004 1 layer -Pilamec -15 pm 65.2195 65.1373 -0.0822 -0.126 Graphite -none 53.7653 32.2767 -21.4886 -39.967 Figures 2, 3 and 4 show microscope images a cross-section of a carbon component (200) made of graphite coated with silicon-silicon carbide according to the method described above. Figure 2 is a x4 magnification image which shows the carbon component (200) comprises a base graphite material (210) having an outer coating of silicon-silicon carbide (220) and an intermediate zone (230) comprising graphite which has reacted with excess silicon to produce silicon carbide within the graphite, the intermediate zone (230) being arranged between the base graphite material (210) and the silicon-silicon carbide coating (220).

Figure 3 shows a x40 magnification image of the silicon-silicon carbide coating (220) adjoining the intermediate zone (230) at bondline (221). Silicon carbide "islands" (232) are clearly discernible in the intermediate zone (230).

Figure 4 shows a x40 magnification image of the intermediate zone (230) adjoining the base graphite material (210) at boundary (231). Again, silicon carbide "islands" (232) are clearly discernible in the intermediate zone (230), but not in the base graphite material (210).

This microstructure may provide the carbon component with the advantageous chemical and physical properties discussed above.

In summary, the present invention provides a method of coating a carbon component with silicon-silicon carbide. The method involves sequentially applying and drying onto at least a part of the carbon component a first composition comprising silicon carbide; carbon, a binder and a solvent, and a second composition comprising silicon, a binder and water. The coating compositions are then fired to provide the silicon-silicon carbide coating on the carbon component. The method may allow a more controlled and cost effective silicon-silicon carbide coating, particularly for relatively complex carbon components such as degassing rotors. A carbon component comprising the silicon-silicon coating and coating compositions for use forming said coating are also provided.

Although a few preferred embodiments have been shown and described, it will be appreciated by those skilled in the art that various changes and modifications might be made without departing from the scope of the invention, as defined in the appended claims.

Throughout this specification, the term "comprising" or "comprises" means including the component(s) specified but not to the exclusion of the presence of other components. The term "consisting essentially of or "consists essentially of means including the components specified but excluding other components except for materials present as impurities, unavoidable materials present as a result of processes used to provide the components, and components added for a purpose other than achieving the technical effect of the invention. Typically, when referring to compositions, a composition consisting essentially of a set of components will comprise less than 5% by weight, typically less than 3% by weight, more typically less than 1% by weight of non-specified components.

The term "consisting of or "consists of means including the components specified but excluding addition of other components.

Whenever appropriate, depending upon the context, the use of the term "comprises" or "comprising" may also be taken to encompass or include the meaning "consists essentially of or "consisting essentially of', and may also be taken to include the meaning "consists of or "consisting of'.

For the avoidance of doubt, wherein amounts of components in a composition are described in wt%, this means the weight percentage of the specified component in relation to the whole composition referred to. For example, "wherein the first composition comprises from 50 to 75 wt% silicon carbide" means that 50 to 75 wt% of the first composition is provided by silicon carbide.

The optional features set out herein may be used either individually or in combination with each other where appropriate and particularly in the combinations as set out in the accompanying claims. The optional features for each aspect or exemplary embodiment of the invention as set out herein are also to be read as applicable to any other aspect or exemplary embodiments of the invention, where appropriate. In other words, the skilled person reading this specification should consider the optional features for each exemplary embodiment of the invention as interchangeable and combinable between different exemplary embodiments.

Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any accompanying claims, and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

Claims 1. A method of coating a carbon component, the method comprising the steps of: a) applying a first composition to at least a part of the carbon component, the first composition comprising silicon carbide, carbon, a binder and a solvent; b) drying the first composition on the component; c) applying a second composition to the part of the carbon component comprising the first composition, the second composition comprising silicon, a binder and water; d) drying the second composition on the carbon component; and e) firing the carbon component to form a silicon-silicon carbide coating on the carbon 10 component.
2. The method according to claim 1, wherein the solvent of the first composition is water.
3. The method according to claim 1 or claim 2, wherein the binder of the first composition is a styrene acrylic copolymer.
4. The method according to any preceding claim, wherein the silicon carbide and the carbon are present in the first composition in a weight ratio of from 4:1 to 8:1.
5. The method according to any preceding claim, wherein the silicon carbide has an average particle size of from 5 to 20 pm.
6. The method according to any preceding claim, wherein the carbon has an average particle size less than 1 pm.
7. The method according to any preceding claim, wherein the first composition comprises a thickening agent.
8. The method according to any preceding claim, wherein the silicon of the second composition is in the form of a powder having an average particle of at least 20 pm.
9. The method according to any preceding claim, wherein the second composition is applied to the carbon component in an amount which provides an amount of the silicon which is 2 to 5 times greater than the weight of silicon carbide provided by the first composition.
10. The method according to any preceding claim, wherein step c) is carried out by spraying the second composition onto the part of the carbon component comprising the first composition.
11. The method according to any preceding claim, wherein steps a) and b) are repeated before step c) is carried out.
12. A composition for coating a substrate with silicon carbide, the composition comprising silicon carbide, carbon, a styrene acrylate copolymer and water.
13. A composition for silicizing a substrate coated with silicon carbide, the composition comprising silicon, a styrene acrylate copolymer and water.
14. A kit for providing a silicon-silicon carbide coating on a substrate, the kit comprising a first composition and a second composition; wherein the first composition comprises silicon carbide, carbon, a binder and a solvent; and wherein the second composition comprises silicon, a binder and water.
15. A carbon component comprising a silicon-silicon carbide coating and an intermediate zone underneath the silicon-silicon carbide coating, on at least a part of the carbon component; wherein the silicon-silicon carbide coating has a thickness of from 0.5 mm to 5 mm; wherein the intermediate zone comprises carbon and silicon carbide and has a thickness of at least 0.5 mm.