A COATING FOR METALLURGICAL SURFACES

Field of the Invention

A novel process for coating metallurgical devices which prevents their adherence to molten metals allows for repeat uses of metallurgical equipment. The invention relates to the metallic surfaces, the coating process, the coating, and metallurgical equipment.

Background of the Invention

Metallurgical equipment, such as crucibles and other vessels such as troughs, and casting equipment such as moulds, are contacted with molten metal and semi-solid slurries thereof, during processes such as die casting. The surface of such vessels are adhered to by the molten metal thereby requiring coatings to reduce adherence.

In general, the operation of vessels for production of semi-solid slurries involves filling the vessel with a molten alloy, allowing the alloy to cool to the semi-solid range, optionally stirring the slurry, then transferring the slurry to a die casting machine. Typically, the cycle time for this process is of the order of up to several minutes. After the slurry is transferred to the die casting machine, the crucible is cleaned, refurbished, and cycled back into service. Typically, the crucibles are advanced robotically or on a carousel through various slurry preparation and cleaning steps. It would be advantageous to have a semi-permanent coating which prevents adhesion so as to avoid lengthy and sometimes complicated cleaning and refurbishing steps.

Metallic troughs (as opposed to refractory lines troughs) for transfer of molten metal are used in locations where precision of adjustment is needed and require special protection from molten metal.

The use of various coatings ("washes") is known in the art. Boron nitride slurries, Woolastonite and Mica Wash are routinely used as a coating on metallurgical equipment. However, these require re-application after each use of the equipment and involve application as a slurry and then thorough drying to avoid the risk of explosions when contacted by molten metal. This lengthy cleaning and refurbishing step is not cost effective, reducing the efficiency in automated processes .

US 6,051,058 describes metal wall comprising a coating comprising of a mixture of boron nitride and a metal oxide. By contrast, the present invention defines the first intermediate metallic surface product to comprise a layer applied to the metallic surface to be an oxide, absent of boron nitride. Boron nitride, in a later intermediate product, is applied subsequently atop the underlying oxide layer.

WO 00/56481 describes a die coating for use on the surface of a mould or die component contacted by molten metal including a porous layer of ceramic material produced by co- deposition, using thermal spraying procedure, of a powder of the material and a powder of a suitable organic polymer material. By contrast, the present invention does not involve co-deposition of the metal oxide with the powdered material

but rather each is applied or deposited sequentially in a multi-layer coating.

Summary of the Invention

The present invention improves upon current coating methods of metallurgical equipment by modifying the order in which overlapping layers are applied, the manner in which the underlying metal oxide layer is applied, and, in part, on the content of each of the layers .

The present invention uses an underlying semi -permanent ceramic layer with a relatively smooth surface morphology modified by a second coating of known "washes" thereby avoiding the need for refurbishing after each use, and achieves particularly interesting repeated utility of the equipment when using a further dry overcoat.

A first aspect of the invention relates to a process for decreasing adherence between a metallic surface and a molten metal or a semi -solid metal slurry, the process comprising the step of: i) applying a coating onto the metallic surface, the coating comprising a layer of a metal oxide applied by plasma- spraying onto the metallic surface. This process provides an intermediate product suitable for commercialization and suitable for the further processing, namely ii) applying by a second application means a slurry of a powder material selected from the group consisting of boron nitride, mica or calcium silicate onto the layer of metal oxide and optionally iii) applying by a third application means a dry layer of

powdered material selected from the group consisting of boron nitride, mica or calcium silicate.

Accordingly, a further aspect of the invention is directed to a coating for use in decreasing the adhesion of a molten metal to a metallic surface, said coating comprising i) an underlying layer of a metal oxide ii) a second layer, atop of the underlying layer of metal oxide, comprising a powder material selected from the group consisting of boron nitride, mica or calcium silicate onto the layer of metal oxide .

The intermediate product, a metallic surface comprising a coating, wherein said coating is prepared by a process comprising i) applying by plasma- spraying a layer of metal oxide onto said metallic surface is a further aspect of the invention, as in its use in the preparation of a semipermanent coating for preventing adhesion of molten metals to metallic surfaces.

The invention therefore relates to metallurgical equipment comprising a metallic surface of the invention, such as a metallurgical vessel or crucible, a mould for shape casting or dry casting, or a metallurgical trough. Similarly, the invention is directed to a method of die casting or shape casting or any other method of casting using a mould, vessel, crucible, or trough of the invention.

Brief Description of the Figure

Figure 1 depicts a cross-section view of one aspect of the invention of a metal with three- layered coating.

Detailed Description of the Invention

A first aspect of the invention relates, in part, to applying a coating to a metal surface so as to obtain a specific surface topography to minimize surface tensions in order to minimize the need for refurbishing of surfaces in between uses in metallurgy, as discussed above. Although the formation of a ceramic layer by means of the use of a metal oxide on a metal surface is known, the present inventors have found the plasma spray drying of the metal oxide provides a rounded plasma topography (see Figure 1) . The metal oxide is substantially permanent and has a relatively smooth surface morphology. The relatively smooth surface morphology is achieved by plasma spraying. The rounded dimples of metal oxide over the metallic surface is achieved preferably by plasma spray drying. This application means has been found to be favorable over typical coating means which provides a rough surface which favors surface adhesion.

The thickness of the metal oxide layer is preferably less than about 300 microns, and more preferably less than about 200 microns in thickness.

The coating of plasma sprayed metal oxide surface on a metallic surface is stable on the metallic surface and the metallic objects using this coating can be sold as such. The present inventors have found that the metallic surface having a surface morphology achieved by plasma spraying can serve as an intermediate product for use in the preparation of a

metallurgical device with reduced adhesion to molten metals in that it can be treated with a non-permanent agent selected from boron nitride, mica, talc, graphite, calcium silicate, sodium silicate or calcium aluminum silicate in slurry form, so as to form a second layer to the metallic surface. This differs from the conventional technology in that, in the present invention, the non-permanent agents and metal oxides are not co-deposited so as to form a composite first layer, but rather sequentially added so as to form a multi- layer coating .

The inventors have found that an excessively rough metal oxide surface is undesirable. An excessively rough metal oxide coating does not retain sufficient non-permanent agent to be effective. The metal oxide coating preferably has an average roughness (Ra) of less than 10 microns, and more preferably less than 8 microns .

The roughness of the metal oxide surface is measured, for example, using a Mahr Perthometer (Model PGK120) and Marsurf XR20 analysis software. Accordingly, the first aspect of the invention relates to the substantially permanent intermediate product - the metallic surface comprising the plasma sprayed metal oxide layer. This aspect of the invention relates to a metallic surface comprising a coating, wherein said coating is prepared by a process comprising i) applying by plasma- spraying a layer of metal oxide onto said metallic surface. Alternatively stated, this aspect of the invention relates to a metallic surface comprising a coating, wherein said coating is prepared by a process comprising i) applying a relatively smooth surface layer of metal oxide onto said metallic

surface. Applying the metal oxide directly to the metallic surface by plasma- spraying so as to provide a surface morphology or topography that is smooth or uniform. The uniform surface achieved by the plasma- spraying allows for the coated surface to be suitable to the further aspects of the invention, namely the coatings of the invention comprising further layers. The plasma- sprayed metal oxide coating provides an intermediate for the adhesion resistant coatings of the invention. The coatings of the invention are resistant to the adhesion, chemical attack, and erosive attack of molten aluminum alloys or a semi -solid slurry of aluminum alloy.

The metal oxide applied by plasma- spraying is suitably selected from the group consisting of alumina, niobia, zirconia, spinel, and mixtures of thereof or other metallic oxides. In further contrast to conventional methods, the layer of metal oxide on said metallic surface is substantially free of boron nitride, mica, sodium silicate, or calcium silicate or other water-based binders. The spinel may be MgAl ₂ O ₄ , MgCr ₂ O ₄ , FeAl ₂ O ₄ or FeCr ₂ O ₄ .

The oxide layer is preferably less than 300 microns, and although thicker layers may be used, there is no technical advantage to thicker layers.

The metal oxide is deposited or otherwise applied to a metallic surface, wherein metallic surface is a metal selected from the group consisting of stainless steel, mild steel, cast iron, aluminum, aluminum alloys, copper, copper alloys, titanium and alloys, tungsten and alloys, tantalum and alloys, molybdenum and alloys, niobium and alloys.

The metallic surface having a layer of metal oxide applied thereto cannot itself, according to the present invention, allow for reduced adherence to a molten alloy - the metallic surface having a layer of metal oxide applied thereto by plasma spraying serves as a distributable intermediate product for use in the preparation of a further aspect of the invention wherein the coating is prepared by a process further comprising ii) applying by a second application means a slurry of a powder material selected from the group consisting of boron nitride, a mica or calcium silicate onto said layer of metal oxide .

The coating slurry may be an aqueous slurry of boron nitride, mica, talc, graphite, calcium silicate, sodium silicate or calcium aluminum silicate. Alternatively, the slurry of boron nitride, mica, talc, graphite, calcium silicate, or sodium silicate or calcium aluminum silicate may comprise a liquid selected from the group consisting of water, alcohol, ketones, aldehydes, aliphatic or aromatic hydrocarbons The liquid should preferably chosen from the group of liquids that wets the powder, and should readily evaporate on mild heating. The slurry may be deposited or otherwise applied to the metal oxide layer of the metallic surface by a second application means selected from the group consisting of spraying, painting, spreading with a squeegee or sponge, rolling, pouring or dipping. In a highly suitable embodiment of the invention, the applying by a second application means of a slurry of a powder material selected from the group consisting of boron nitride, a mica or calcium silicate etc onto said layer of metal oxide is followed by a

drying step. The drying step is intended to reduce the content of moisture, such as to remove moisture or substantially dry the layer. The drying step comprises exposing the metallic surface to a temperature from 100 to 1000 ⁰ C, typically from 200 to 900 ⁰ C, suitably from 300 to 800 ⁰ C, such as from 400 to 700 ⁰ C, such as about 500 C to 650 ⁰ C, such as about 60O ⁰ C adequate to remove all carrier solvent.

The drying step preferably further involves the removal of any excess material or any oxide derivative of the powder material which may have formed upon contact with the metal oxide or from the drying process. The removal of any excess material or any oxide derivative of the powder material which may have formed is suitably performed by a physical process such as brushing, gentle abrasion or air cleaning.

Powder from the slurry fills the interstices between the plasma sprayed oxide particles so that the total thickness of the combined materials is substantially the same as the plasma sprayed oxide and slurry sufficient to do this is used.

Drying should be done long enough to reduce moisture levels to about 1% residual water or less. This takes, for example, about 1 to 2 hours at 600 C

The inventors have found that the metallic surfaces of the invention comprise a metal oxide layer and a further layer of dried powder material selected from boron nitride, mica, talc, graphite, calcium silicate, or sodium silicate or calcium aluminum silicate allow for at least 20 uses of the

metallic surface before metal adhesion occurs. This compares favorably with existing methodologies.

Surprisingly, the inventors have found that applying by a third application means a layer iii) of dry powdered material (before every 1 to 5 uses of the metallic surface) , wherein the layer iii) is selected from the group consisting of boron nitride, mica, talc, graphite, calcium silicate, or sodium silicate or calcium aluminum silicate results in a metallic surface resistant to adhesion for over 100-200 repeat uses before the slurry layer is replaced. Suitably, the third application means is electrostatic spray application.

The 3 -layer coating of the invention provides a dramatic increase in the number of repeat uses compared to conventional methodologies while using components readily available to the user, thereby not increasing costs to the user. The number of repeat uses avoids lengthy and complicated cleaning and refurbishing steps, thereby increasing the productivity of the user .

A typical 3 -layer coating of the present invention is illustrated schematically in Figure 1. The coating is applied to a metallic substrate 1 and comprises first the plasma sprayed layer 2 which is typically less than 300 microns thick with an average roughness Ra of less than 10 microns. On top and filling in the interstices as well as smoothening the asperity of the surface is the slurry applied layer 3. Interstices are defined as spaces in the surface. Asperity is defined as an unevenness or roughness of a surface. The thickness of the combined layers 2 and 3 is approximately the

same as layer 2 alone, since the slurry fills in the interstices in layer 2 and any excess is generally brushed off after drying. A final layer 4 is a thin layer, applied dry, for example by electrostatic spraying.

The number of repeat uses for the metallic surface will depend to a certain extent on the utility of the metallic surface be it within a metallurgical vessel, the inner walls of a mould for shape casting, or of a mould for die casting, or for use in metallurgical trough. The temperature of use will affect this as well.

A further aspect of the invention is directed to a coating for use in decreasing the adhesion of a molten metal to a metallic surface, said coating comprising i) an underlying layer of a metal oxide; ii) a second layer, atop of the underlying layer of metal oxide, comprising a powder material selected from the group consisting of boron nitride, mica, talc, graphite, calcium silicate, or sodium silicate or calcium aluminum silicate onto the layer of metal oxide. The coating comprising the underlying and second layers prevent adhesion of molten metals or their cooled counterparts, the resulting semi-solid slurries of metal to a metallic surface. These coatings allow for at least 20 uses without the need to refurbish the layers.

The underlying layer has a relatively smooth surface topography of metal oxide and is applied by plasma- spraying a layer of metal oxide onto said metallic surface. The underlying layer is substantially free of boron nitride, mica, sodium silicate, or calcium silicate.

The second layer is typically applied to the underlying layer by a applying a slurry. The second layer suitably comprises boron nitride, mica, talc, graphite, calcium silicate, or sodium silicate or calcium aluminum silicate Typically, the second layer has been dried to reduce the moisture content, and is preferably substantially dry.

Preferably, the coating further comprises iii) an overcoat layer selected from the group consisting of powdered boron nitride, mica, talc, graphite, calcium silicate, or sodium silicate or calcium aluminum silicate.

A key aspect of the invention relates to the manner in which the metallic surface is treated or the manner in which the coating is prepared, namely the process by which each are made. The invention is further directed to a process for decreasing the adherence between a metallic surface and a molten metal or a semi-solid metal slurry comprising the steps of i) applying by plasma- spraying a layer of metal oxide onto said metallic surface. As stated this method of applying the metal oxide layer provides a stable intermediate product which can be used in the preparation of the metallic surface of the invention. The applying of the metal oxide is such that a relatively smooth surface layer of metal oxide is formed onto said metallic surface. This smooth topography allows opposing objectives to be accomplished: adequate trapping or retaining of the further powder material of the second layer while minimizing sharp edges at the surface interface with the molten metal, said sharp edges promoting the adherence between a metallic surface and a molten metal or a semi-solid metal slurry.

The actual decrease in adherence requires the process to further comprise the step of ii) applying by a second application means a slurry of a powder material selected from the group consisting of boron nitride, mica or calcium silicate onto the layer of metal oxide. Typically, the second application means is plasma spraying. The applying of a slurry of a powder material is usually followed by a drying step, said drying step typically comprising subjecting the metallic surface to a temperature from 100 to 1000 ⁰ C. This substantially removes excess moisture from the slurry. The inventors have found that the presence of excess moisture results molten metal explosions when utilized. Preferably, the drying step comprises subjecting the metallic surface to a temperature from 100 to 1000 ⁰ C and physical removal of any oxide derivative of the powder material .

To reduce the amount of moisture, other solvents can be used in combination with water to form the slurry. However, this increases potential costs. Suitably, the slurry is of boron nitride as an aqueous slurry, but can be a mixed solvent system such as with water and an alcohol or other liquid miscible with water, or a solvent comprising one or more miscible components selected from water, alcohol, ketones, aldehydes, aliphatic or aromatic hydrocarbons. Optimal results in terms of increased number of possible repeat uses of the metallic object is achieved when the process further comprises a step iii) of applying by a third application means a layer of powdered material selected from the group consisting of boron nitride, mica, talc, graphite, calcium silicate, or sodium silicate or calcium aluminum silicate. Suitably, the

third application means is electrospray application. This provides an adhesion resistant coating, wherein the coating is resistant to the adhesion of molten metal or a semi-solid metal slurry, most notably wherein the coating is resistant to the adhesion of molten aluminum alloys or a semi-solid slurry of aluminum alloy.

EXAMPLES

Example 1

A vessel with an interior of stainless steel and a cylindrical interior of 25 cm to more than 35 cm in length and 8 to 17 cm in diameter is subjected to the method of the invention .

i) An underlying coating of alumina was applied by plasma spraying using commercial plasma spraying process and material supplied by Weir Canada.

ii) An aqueous slurry of boron nitride was then sprayed onto the oxide surface and dried for 2 hours at about 600 ⁰ C to remove moisture, then brushed to remove excess material and any boron oxide that may have formed.

iii) The surface is briefly sprayed with a dry boron nitride powder by means of electrostatic spray before use.

When the oxide layer and only the aqueous slurry of boron nitride spray was used, the coating had up to about 30 uses before metal adhesion rendered the vessel unusable and the aqueous slurry had to be re -applied.

Use of the slurry of boron nitride on the oxide surface followed by a brief spray each time of dry boron nitride powder resulted in more than about 100 to 200 uses.

The oxide layer on the stainless steel surface itself did not need replacing even after 2000 to 3000 uses.