WO2004002655A1

WO2004002655A1 - Sealer coating for use on porous layers

Info

Publication number: WO2004002655A1
Application number: PCT/AU2003/000835
Authority: WO
Inventors: Stefan Gulizia; Mahnaz Jahedi
Original assignee: Cast Centre Pty Ltd
Priority date: 2002-07-01
Filing date: 2003-06-30
Publication date: 2004-01-08
Also published as: AUPS329102A0

Abstract

A die or mould coating comprising: a bond layer and a porous layer of ceramic material produced by co-deposition and heating of said ceramic material and a powder of an organic thermosetting polymer material; and an upper sealer coating comprising metal or ceramic filler particles dispersed in a carrier.

Description

SEALER COATING FOR USE ON POROUS LAYERS

Technical Field

. This invention relates to sealer coating compositions that are suitable for use on die coatings and in particular ceramic die coatings. Background of the Invention

The role of coatings used on substrates in contact with molten metal is to are control heat flow through the coating and prevent premature solidification of the molten metal in the die cavity. Additionally, thicker insulating coatings tend to increase the thermal mismatch between the coating and substrate increasing the stress on the coating and resulting in spalling of the coating.

Improved die coatings are disclosed in PCT/AU00/00239, the whole contents of which are incorporated by reference. These compositions have particular application on low pressure and gravity die casting. Prior to the above invention the surface of each metal mould or die component, which is to be contacted by molten metal, was usually coated by a commercial die coat. A ceramic-based coating was typically used at a thickness of from about 0.05 to 1.0mm. By referring to the coatings as "ceramic-based" the term "ceramic" was used in its art recognised sense as being inorganic, non-metallic materials processed or consolidated at higher temperature" (McGraw-Hill Encyclopaedia of Science and Technology 1994). The classes of materials generally considered to be ceramics are oxides, nitrides, borides, suicides and sulfides. Intermetallic compounds such as aluminates and beryllides are also considered as "ceramics" as are phosphides, antimonides and arsenides. The main function of the ceramic- based coating was to provide a degree of insulation which was intended to prevent premature solidification of the molten metal, and thereby enable the complete filling of the die cavity before solidification started. However, the coating also was to protect the die surfaces from erosion or corrosion by impingement or contact with the molten metal.

Previously known die coating technology involved the use of a water-based suspension of ceramic particles containing a water-based binder, most commonly sodium or potassium silicate. Coating mixtures of this type needed to be properly stored and mixed. The coating was applied to the prepared surface of a die component using a pressurised air spray gun. For this, the die component was preheated, typically from about 150° to 220°C, such that water was evaporated from the die surface, enabling the binder to polymerise or cure and bond the ceramic particles together and to the die surface. The die coatings produced with these known aqueous ceramic suspensions were highly porous. The level of porosity typically ranged from about 30 to 60%, depending on the size and shape of the ceramic particles and the proportion of binder used. High porosity gave the coating very good insulating properties. However, the strength of the coatings was limited by the strength of the binder used (about 6.9 MPa in the case of sodium silicate) and the level of porosity of the coating.

The invention of PCT/AU00/00239 provided an improved die coating for use on the surface of a mould or die component contacted by molten metal in low pressure or gravity die casting. The coating included a porous layer of ceramic material produced by co-deposition, using a thermal spraying procedure, of a powder of the ceramic material and a powder of a suitable organic polymer material and, after the co-deposition, heating of the polymer material to cause its removal. In that specification the term "ceramic material" was used very broadly but consistent with the definition in this technology field, outlined earlier. While the above invention provided good performance on a range of mould and die situations, problems arise particularly, but not exclusively in low draft angle areas of a die or mould. Drafts (or taper) in a mould facilitate part removal from the mould. The draft is in the offset angle in a direction parallel to the mould opening. It is generally best to allow for as much draft as possible for easy release from the mould. A common recommendation is to allow 1 to 3 degrees of the draft.

In low draft areas of a mould that is where the draft angle is less than 2°, particular problems can arise from the use of insulating coatings as described above.

Following solidification of molten metal in the die, molten metal solidifies into the pores of the ceramic layer. This results in not only an unsatisfactory finish on the die casting, but causes a great deal of drag and stress on the surface of the porous layer when the casting is removed. This surface drag considerably reduces the potential service life of the coating.

Summary of the Invention

The applicants have found that by applying a sealer coat having filler particles which fill and seal substantially all of the surface pores, the problem of metal solidifying in the pores is avoided and the service life of the layer is greatly improved.

Accordingly, the invention provides in one form an improved die or mould coating comprising a porous layer of ceramic material produced by co-deposition of a powder of the ceramic material and a powder of a suitable organic polymer material and, an upper sealer coating comprising metal or ceramic filler particles dispersed in a carrier.

The application of a sealer coat is particularly beneficial in low draft areas of mould cavities, although it is also useful in sealing mould or die components which do not have a low draft angle.

The porous ceramic layer produced by the co-deposition is heated to a temperature up to 550°C to thermally decompose and remove the polymer material prior to application of the upper sealer coating.

The porous ceramic layer preferably has a porosity of 5 to 70% after the polymeric material is removed.

The carrier for the metal or ceramic filler particle may be a low friction coefficient carrier and is preferably a heat resistant resin such as silicone.

The filler particle, preferably, on average have a particle 50% smaller than the pore size found on the surface of the ceramic porous layer and of suitable composition to provide wear resistance and lubricating properties to the sealer.

The size of the filler particles will depend on the size of the polymer molecules which are removed by the thermal' decomposition process but are typically in the range of 5 to 200μm . Suitable metal and ceramic filler particles are selected from the group of tungsten, borides, nitrides suicides and carbides. The sealer coating may contain volatile components such as organic solvents including toluene or xylene to allow the viscosity of the sealer coating to be modified to that suitable for application.

Further features, objects and advantages will become more apparent from the following detailed description of the invention and Examples. Detailed Description of the Invention

To enhance the adhesion of the porous ceramic layer to the substrate, a bond coat is preferably applied to the metal substrate. In one form, the bond coat is of substantially uniform thickness over all surfaces of the metal substrate, the mould or die components which define the die cavity. The applicants have further found that in many cases, the application of a bond coat to the metal die prior to application of the porous layer, provides better adhesion of the porous layer to the metal die and reduces the thermal mismatch between the die and the ceramic coating, further increasing the service life of the coating and protecting the surface of the mould or die. The bond coat also serves to enhance the adhesive strength of the coating.

The bond coat preferably is formed of a metallic, intermetallic or composite particulate materials. The bond layer is formed from a particulate material applied to the surface of the metal surface of the mould. The bond coat layer can be applied by a thermal spray process such as vacuum plasma spray (VPS) , atmospheric plasma spray (APS), combustion flame spraying and hyper velocity oxyfuel (HVOF) spray processes.

The metal in the bond coat may be in the metallic, intermetallic, oxide, clad or alloyed form consisting of any one or more of the metal components selected from the group of Mo, Ni, Al, Cr, Co, Y and W and may be in combination with yttria, alumina, zirconia, boron, carbon and have a particle size in the range of 5 to 250μm, typically 40 to 125μm. The bond coat preferably has a thickness of 5 to 300μm with a substantially uniform coating layer being provided over the surfaces to have the porous ceramic coat applied.

An example of a bond coat powder was a Metco 480-NS grade fully alloyed spheroidal, gas atomised Nickel 95% Aluminium 5% for which the data sheet indicated a particle size range of not more than 90μm and not less than 45μm. The polymer and ceramic particles are then co-deposited onto the bond coat and the co-deposited layer is heated to a temperature up to 550°C to thermally decompose and remove the polymer material prior to application of the upper sealer coating. The ceramic powders which are used in providing the porous coating may be a processed powder conventionally used in the production of ceramic articles. Thus, the powder may be selected from at least one metal compound such as oxides, nitrides, carbides and borides. Suitable examples include carbon, alumina, titania, silica, stabilized zirconia, silicon nitride, boron nitride, silicon carbide, tungsten carbide, titanium borides and zirconium boride. However, the ceramic powder may be of a suitable mineral origin such as clay minerals, hard rock ore and heavy mineral sands such as those of ilmenite, rutile and/or zircon. One particularly suitable powder is that obtained from scoria or pumice, since powder particles of these materials are internally porous and have the added benefit of being of angular form.

A wide range of plastics and like materials can be used to provide the organic polymer powder. Important requirements for selection of these are availability in a suitable powder form and an ability to withstand sufficiently the temperatures to which they are exposed during thermal deposition. A further requirement is an ability to be combusted or decomposed at practical temperatures and in practical reaction times. In large part, the materials comprise thermoplastics, such as polystyrene, styrene-acrylonitrile, polymethacrylates polyesters, polyamides, polyamide-imides and PTFE.

The respective powders, that is the ceramic powder and the polymer powder, preferably are of a relatively narrow size spectrum. In general, they preferably are of particle sizes not more than about 60 μm and not less than about

1μm in the case of the ceramic and not less than about 5 μm in the case of the polymer material.

In practice the upper sealer coating is applied to the porous layer after the polymer has been removed from that layer. The porous layer typically then has a porosity of 5 to 70%, and the surface of this layer is characterised by a plurality of pores. The sealer coating may be applied with a paint brush, air spray gun or other application means. To achieve suitable viscosity for application a number of volatile diluents may be used. It will be appreciated that relatively volatile diluents are preferred so that the sealer coating effectively penetrates the porous layer and virtually produces a smooth surface which seals the surface from the penetrating molten metal. It is preferred that the carrier is heat resistant so that it is not substantially damaged by the molten metal and so that it can also be applied when the die is hot. The carriers may include crosslinking agents, such as silicone or may be themselves crosslinkable.

Preferably the film thickness of the upper sealer coating is in the range 5 to 100 μm and more preferably approximately 10 μm.

The invention will be further described by reference to the following non limiting example.

Example 1

This Example describes the formation of a mould coating composition on a metal surface of a die having low draft regions.

Miller Thermal SG 100 Plasma Spray Torch thermal spray unit. The bond coat powder was a Metco 480-NS grade fully alloyed spheroidal, gas atomised

Nickel 95% Aluminium 5% for which the data sheet indicated a particle size range of not more than 90μm and not less than 45μm. The process settings used were as follows: -

Voltage: 33

Current: 650

Plasma Gases: Argon at 50 psi & Helium at 50 psi

Powder Feed Rate: 11.5 RPM at 35 psi Spray Distance: 100mm

Then the ceramic powder and polymer powder were mixed and subjected to a thermal spraying to form a co-deposited coating on a ladle used for transferring molten metal to a die cavity defining the surface of a low pressure metal die cast component. The ceramic powder was Metco 210 (NS/NS-1/NS-1- G) grade zirconia stabilised by 24% magnesium oxide for which the data sheet indicated a particle size range of not more than 90μm and not less than 11μm, a melting point of 2140°C and a density of 4.2gcm^"3. The polymer powder was of polymer supplied by Sulzermetco which had been ground to -150 + 45 μm (-100 +325). The powder mixture of MgO(24%) ZrO₂/polystyrene contained 15 % volume percent (3wt%) of polymer.

The co-deposition of the powder mixture was performed using a Miller Thermal SG 100 Plasma Spray Torch and a Miller Thermal powder feeder, under the following settings:

Voltage: 34 Current: 750

Plasma Gases: Argon at 50 psi & Helium at 50 psi

Powder Feed Rate: 2.88 (rpm) at 35 psi

Spray Distance: 100 mm

Following co-deposition of the blended powders, the deposited coating was heated to 450°C for one hour to cause the polymer to decompose. As well as plasma application other thermal application techniques may also be used. A commercial sealer coating composition, Metcoseal SA (Sulzer Metco consisting of aluminium flakes and silicone binder, was applied by paintbrush to the low draft regions to form a coating approximately 25 μm thick. After evaporation of the sealant a sealer coat was formed. A number of castings were produced from molten aluminium in the mould and it was observed that in the low draft region of the mould, the mould coating composition was largely undamaged. This allowed the mould to be used for many more moulding operations than if the sealer coating was not used.

Claims

1. A die or mould coating comprising: a bond layer and a porous layer of ceramic material produced by co- deposition and heating of said ceramic material and a powder of an organic thermosetting polymer material; and an upper sealer coating comprising metal or ceramic filler particles dispersed in a carrier.

2. The die or mould coating of claim 1 , wherein the porous layer has a porosity of 5 to 70%.

3. The die or mould coating of claim 1 , wherein the porous layer has a thickness in the range of 50μm to 1 μm.

4. The die or mould coating of claim 1 , wherein ceramic material in the porous layer is at least one compound selected from the group of carbon, alumina, titania, silica, stabilized zirconia, silicon nitride, boron nitride, silicon carbide, tungsten carbide, titanium borides and zirconium boride.

5. The die or mould coating of claim 1 , wherein the organic polymer material is at least one compound selected from the group of polyester, polystyrene, styrene- acrylonitrile, polymethacrylates polyesters, poiyamides, polyamide-imides and PTFE.

6. The die or mould coating of claim 1 , wherein the metal or ceramic filler particles of the upper sealer coating is ceramic particles selected from the groups consisting of borides, nitrides, suicides and carbides or any metal particles including tungsten and aluminium.

7. The die or mould coating of claim 1 , wherein the carrier for the metal or ceramic filler particles of the upper sealer coating is a heat resistant resin selected from the group of silicone resins.

8. The coating of claim 1 , wherein the bond layer is applied in a substantially uniform thickness over the metal surfaces of the mould or die components which define a die cavity.

9. The coating of claim 1 , wherein the thickness of the sealer coating is in the range of 5 to 10Oμm.

10. The coating of claim 1 , wherein the thickness of the sealer coating is in the range of 5 to 10μm.

11. The coating of claim 6, wherein metal or ceramic filler particles, on average have a particle which is 50% of the average size of the pores.

12. The coating of claim 6, wherein the size of the metal or ceramic filler particles is in the range of 5 to 200μm.

13. A method of forming a coating on a die or mould component comprising the steps of: applying a bond layer to the die or mould component; co-depositing a layer of a powder of ceramic material and a powder of an organic polymer material on to the coated die or mould component; heating the co-deposited layer to bind the ceramic material and thermally decompose the organic polymer material to form a porous ceramic layer; and applying an upper sealer coating comprising metal or ceramic filler particles dispersed in a carrier.

14. The method of claim 13, wherein the co-deposited layer is heated to a temperature up to 550°C to thermally decompose and remove the polymer material.

15. The method of claim 1 , wherein the bond layer is applied in a substantially uniform thickness over the metal surfaces of the mould or die components which define a die cavity.

16. The method of claim 1 , wherein the thickness of the sealer coating is in the range of 5 to 100μm.

17. The method of claim 1 , wherein the thickness of the sealer coating is in the range of 5 to 10μm.

18. The method of claim 6, wherein metal or ceramic filler particles, on average have a particle which is 50% of the average size of the pores.

19. The method of claim 6, wherein the size of the metal or ceramic filler particles is in the range of 5 to 200μm.

20. The method of claim 13, wherein the coating is applied to low draft areas of a die or mould.