DE102017206208A1 - Process for coating shaped bodies of alumino-silicate materials and coated shaped body of alumino-silicate materials - Google Patents

Abstract

A process is proposed for coating moldings of alumino-silicate materials which comprises, by means of a suspension comprising or consisting of silicon nitride particles, dispersants / liquefiers, binders, plasticizers and water and a pH in the range from 7 to 14 is carried out. In this case, the suspension is applied to the surface of a shaped body at least once and at least in some areas, and the shaped body is subsequently treated thermally at least once. Consequently, shaped bodies of aluminosilicate materials are provided which have a silicon nitride protective layer with a layer thickness of at least 700 μm and can be produced by the process according to the invention.

Description

A process is proposed for coating moldings of alumino-silicate materials which comprises, by means of a suspension comprising or consisting of silicon nitride particles, dispersants / liquefiers, binders, plasticizers and water and a pH in the range from 7 to 14 is carried out. In this case, the suspension is applied to the surface of a shaped body at least once and at least in some areas, and the shaped body is subsequently treated thermally at least once. Consequently, shaped bodies of aluminosilicate materials are provided which have a silicon nitride protective layer with a layer thickness of at least 700 μm and can be produced by the process according to the invention.

From the literature (http://publications.rwthaachen.de/record/444970/files/5166.pdf, retrieved on February 15, 2017) are known coating methods on alumino-silicate materials, which on the use of boron nitride (BN) or Aluminum titanate (Al ₂ TiO ₅ ) is based. Both materials are known to be melt-repellent to light metals such as aluminum or magnesium and some non-ferrous metals such as zinc and copper.

The disadvantage of such a coating is the strong deviation of the coefficients of thermal expansion (TAK) (BN, TAK = 2.2-3.2, aluminum titanate, TAK = 0.5-2), so that the coatings flake off during production and during use or have a low abrasion resistance. In addition, both materials have an extremely low strength, which also leads to a short service life in use due to poor wear resistance.

Starting from this prior art, it was the object of the present invention to provide an improved process for coating moldings of alumino-silicate materials and an improved mold made of alumino-silicate materials.

The object is achieved by the process for the at least partial application of a silicon nitride protective layer on shaped bodies of alumino-silicate materials according to claim 1 and the shaped body of sintered alumino-silicate materials according to claim 23. In the dependent claims preferred embodiments are shown.

1. a) Siliciumnitrid-Partikel,
2. b) mindestens ein organisches (pseudokationisches oder kationisches) Dispergier-/Verflüssigungsmittel, und/oder mindestens ein basisches anionisches Dispergier-/Verflüssigungsmittel
3. c) mindestens ein organisches Bindermittel
4. d) mindestens ein Plastifizierungsmittel
5. e) ad 100 Gew.-% Wasser,

wobei der pH-Wert der Suspension zwischen 7 und 14 liegt,
auf die Oberfläche des Formkörpers zumindest einmalig zumindest bereichsweise aufgetragen und der Formkörper anschließend mindestens einmal thermisch behandelt wird.The invention thus relates to a method for at least partially applying a silicon nitride protective layer on moldings of alumino-silicate materials, in which a suspension for coating moldings of alumino-silicate materials containing or consisting of

1. a) silicon nitride particles,
2. b) at least one organic (pseudo cationic or cationic) dispersing / liquefying agent, and / or at least one basic anionic dispersing / liquefying agent
3. c) at least one organic binder
4. d) at least one plasticizer
5. e) ad 100% by weight of water,

the pH of the suspension being between 7 and 14,
applied to the surface of the molding at least once at least partially applied and the molding is then thermally treated at least once.

The pH of the suspension in the range of pH 7-14 is responsible for the fact that when applying the suspension to the surface of a shaped body of alumino-silicate materials to a very good binding of the molecules of the organic cationic or pseudo cationic and / or anionic basic dispersing / liquefying agent on the "acidic" surface of the alumino-silicate material as well as on the surface of the Si ₃ N ₄ particles. Consequently, a good wetting of the alumino-silicate material surface is achieved by the Si ₃ N ₄ suspension.

Silicon nitride has exactly the same thermal expansion coefficient (TAK = 4.5 ppmK-1) as aluminosilicate materials (TAK = 4.5 ppm K-1) and is a hard material due to the nature of its high binding forces. When coated with a material consisting essentially of silicon nitride, there are no thermal stresses during manufacture and / or use and no damage to the coating and no reduction in the service life. Furthermore, the use of the hard material silicon nitride leads to a lifetime extension of the roller as a whole by reducing the abrasion effect by corresponding transport goods. Here, in particular, the increased strength of the silicon nitride, compared with other ceramic materials, such as BN or Al ₂ TiO ₅ comes into play.

The suspension preferably has a pH of between 7.5 and 12, in particular between 8 and 11, since in this range the binding effect described above, ie. the adhesion of the suspension components to the surface of the molded article occurs more intensively.

The suspension used according to the invention has the additional advantage that a deep going infiltration of the suspension in the porous alumino-silicate material (> 700 microns infiltration depth) and thereby a large-scale chemical / physical bonding of the components of the suspension with the alumino-silicate material is made possible. This results in a high adhesion of the constituents to the surface of the alumino-silicate material, which leads to a high adhesion of the resulting protective layer after thermal treatment of the alumino-silicate material. Furthermore, the pores of the alumino-silicate material are sealed with Si ₃ N ₄ particles, whereby a smooth, non-powdery surface or protective layer is formed.

The suspension used according to the invention can penetrate into the molding body so strongly that the resulting overall layer in the alumino-silicate material can have a depth of at least 700 μm. Due to the enormous depth and a concentration shift in this area, the diffusion of particles of alumino-silicate material is inhibited by the layer. If the coated body of alumino-silicate materials is used as a crucible for holding liquid or crystalline metals, e.g. Aluminum, magnesium, zinc, etc., thus a contamination of the melt is counteracted with impurities of alumino-silicate material.

The silicon nitride particles may be contained at 10 to 40 wt .-%, preferably at 15.0 to 35.0 wt .-%, based on the total suspension.

1. a) eine mittlere Teilchengrößenverteilung d₁₀ von 0,20 bis 0,40 µm, d₅₀ von 0,50 bis 1,49 µm, bevorzugt von 0,50 bis 0,90 µm und/oder d₉₀ von 1,10 bis 3,90 µm, bevorzugt von 1,50 bis 2,50 µm auf, bestimmt nach dem Laserbeugungsverfahren gemäß ASTM B 822-10 , und/oder
2. b) eine spezifische Oberfläche von 4,67 bis 15,00 m²/g, bevorzugt von 8 bis 13 m²/g auf, bestimmt nach dem Gasadsorptionsverfahren gemäß ASTM D 3663-03.

In a preferred embodiment, the silicon nitride particles

1. a) an average particle size distribution d ₁₀ of 0.20 to 0.40 μm, d ₅₀ of 0.50 to 1.49 μm, preferably of 0.50 to 0.90 μm and / or d ₉₀ of 1.10 to 3 , 90 microns, preferably from 1.50 to 2.50 microns, determined according to the laser diffraction method according to ASTM B 822-10 , and or
2. b) a specific surface area of from 4.67 to 15.00 m ² / g, preferably from 8 to 13 m ² / g, determined by the gas adsorption process according to ASTM D 3663-03.

The liquefying agent may be selected from the group consisting of basic anionic dispersants and non-foaming, alkali-free pseudokationischen / cationic dispersing / liquefying agents in particular be selected from the group consisting of amino alcohols. A particularly preferred liquefying agent is 2-amino-2-methylpropanol.

In a further preferred embodiment, the at least one organic (cationic or pseudo-cationic) dispersing / liquefying agent and / or the at least one anionic basic dispersing / liquefying agent is from 0.01 to 10% by weight, preferably from 0.1 to 2% by weight .-%, more preferably at 0.16 to 0.3 wt .-%, based on the total suspension, in the suspension.

The at least one at least one organic binder present in the suspension preferably selected from the group consisting of water-soluble or water-dispersible hydrophilic polymers, in particular polyethylene glycols, polypropylene glycols, polybutylene glycols, polyvinyl alcohols, polyvinyl butyrals, polyvinyl pyrrolidones, polyvinyl acetates and partially or fully hydrolyzed polyvinyl acetates, in particular PVA M145000 vollhydrolysiert.

The at least one organic binder may contain from 0.01 to 20% by weight, preferably from 0.1 to 5% by weight, more preferably from 1.0 to 3.4% by weight, based on the total suspension, be included.

The plasticizer is in particular a polyhydric alcohol, in particular propane triol. The plasticizer is preferably present in an amount of from 0.1 to 6.0% by weight, more preferably from 0.5 to 3.0% by weight, based on the total suspension.

Furthermore, it is preferred that the suspension has a dynamic viscosity of 20 to 50 mPa s, determined by means of a rotational viscometer with an immersion measuring device according to FIG DIN 53019: 2008-09 , A viscosity in this range has a particularly advantageous effect on the penetration depth of the suspension into an alumino-silicate materials.

More preferably, the alumino-silicate molded body is porous and preferably has a porosity of at least 15%, preferably 20 to 35%, particularly preferably 25 to 30% and / or after the thermal treatment has an average pore diameter of 50 to 2.0 μm, preferably 1.0 to 1.5 μm, in particular 1.00 to 1.25 μm.

During the application of the suspension to the shaped body of alumino-silicate materials, which is preferably in the green state, it is preferable to neutralize the silanol groups of the alumino-silicate material surface by molecules of the basic dispersing / liquefying agent, resulting in the formation of a monomolecular layer of organic molecules of the basic dispersant on the alumino-silicate material surface, which may also be modified leads. As a result of formation of numerous hydrogen bridges and higher affinity between already modified by basic dispersing / liquefying agent Si ₃ N ₄ particles and optionally modified alumino-silicate material surface then good adhesion of the coating produced to the alumino-silicate material surface or in the pores instead.

In the thermal treatment, preferably a firing process under oxidizing conditions, a permanent chemical adhesion between the alumino-silicate material surface and the coating is produced. During this step, Si-O-Si bonds are formed as a result of a chemical polycondensation reaction between reactive silanol groups of the alumino-silicate material and the Si ₃ N ₄ powder with elimination of water. This causes a covalent chemical bond of Si ₃ N ₄ coating with the alumino-silicate material surface, so that there is a good abrasion resistance of the coating.

The surface of the coating produced by the method according to the invention is smooth and has a low wettability to light and non-ferrous metal melts. This is particularly advantageous in contacting the crucible surface with liquid or crystalline silicon. In addition, the method has the advantage that it is easy and inexpensive to carry out.

The alumino-silicate material of the molding used in the process may be porous and preferably have a porosity of at least 8%, preferably 8 to 25%, particularly preferably from 10 to 15%. It is further preferred that the alumino-silicate material after the thermal treatment has an average pore diameter of 0.05 to 0.8 μm, preferably 0.1 to 0.4 μm, in particular 0.15 to 0.25 μm.

The suspension can be infiltrated into the molding in the process according to the invention at least 500 .mu.m, preferably 700 to 1000 .mu.m.

Particularly preferably, the shaped body of the alumino-silicate material is a green body. As a green body is understood according to the invention an unfired blank. In this case, it is firstly advantageous that higher infiltration depths of the suspension into the shaped body become possible by using a green body. Second, the coating of a green body instead of a fired molded body has the advantage that in only one step green body and coating can be fired (economic advantage). Thirdly, the joint firing process of green body and coating better closes the pores of the green body in the region of the coating and thereby firmly anchors the Si ₃ N ₄ particles within the aluminosilicate material, ie below the surface of the aluminosilicate material.

The thermal treatment can be carried out at temperatures from 1000 to 1300 ° C., preferably from 1100 to 1200 ° C. and / or over a period of from 2 to 5 h, preferably from 3 to 4 h.

The thermal treatment can also be carried out under oxidative conditions, which causes oxidation of the top Si ₃ N ₄ layer of the coating on the molding. This oxidized layer in turn protects the underlying layers from further oxidation so that the full separation function of the coating is maintained. This can ensure that a pure Si ₃ N ₄ layer remains on the inner surface of the molding.

In summary, the silicon nitride particles of the suspension are firmly and chemically covalently anchored in the alumino-silicate material surface. As a result, the suspension applied to the body of alumino-silicate material, after thermal treatment, forms a well-adherent, non-powdery and touch-resistant layer on the alumino-silicate material body.

The order of the suspension on the molding can be done by brushing, spraying, spraying, dipping the molding in the suspension and / or by electrostatic application. The suspension can also be applied with a specially equipped spraying device, which prevents contamination of the suspension with metals. The penetration of the coating into the pores can be assisted by a spray-mist-producing process in wet / wet technique, whereby the infiltration depth of the suspension into the shaped body can be further increased.

After application of the suspension and before the thermal treatment is preferably carried out a drying of the shaped body, preferably at temperatures of 20 ° C to 150 ° C and in particular at a humidity of 40 to 60%.

Furthermore, it is preferred that a closed silicon nitride protective layer on the substrate surface of at least 50 μm layer thickness is produced by the method according to the invention.

The alumino-silicate material preferably includes mullite and / or a mullitic bonded material and may, for example, have a residual glass phase and residual corundum. Mullite is a mineral from the mineral class of silicates, more specifically an island silicate with tetrahedral anions. It crystallizes in Orthorhombic crystal system with the chemical formula Al ₂ Al _{2 + 2x} Si _2-2x O _10-x with x = oxygen _vacancies per unit cell and develops mostly small, prismatic, often fibrous crystals in white, yellow, red or purple color. Also, colorless mullite crystals are known. The alumino-silicate material preferably contains 10-50% by weight, preferably 20-35% by weight of mullite.

In a preferred embodiment, the molded article is selected from the group consisting of transport rollers for treatment furnaces, crucibles, furnace lining and charging station.

According to a further preferred variant, the method is carried out in such a way that the at least partial application is carried out in several stages, wherein first a base coat and then at least one further coat is applied to the mold. The individual layers or the cover layer can be thermally treated after the respective application or finally thermally treated together in a so-called co-firing.

In this case, it is possible for the base coating to be applied to the shaped body as a suspension or solution, similar to the silicon nitride coating, and for the silicon nitride-containing suspension to be applied wet on wet to the base coat. Finally, the coating, which in this case then comprises the base coating and the actual silicon nitride coating, can be produced by simultaneous at least one thermal treatment.

Alternatively, it is also possible first to apply the base coat (e.g., by applying a suitable suspension from which the base coat can be made) and to first treat it thermally. The silicon nitride coating can then be applied to this thermally treated base coat in a further step in the manner described above.

In particular, suspensions of inorganic materials from which ceramic materials can be produced upon thermal treatment are suitable as the material for applying the basecoat.

When applying a base coat, it is particularly advantageous that the composition and the grains of the base coat can be adjusted so that it is on the one hand chemically bondable to the alumino-silicate material, on the other hand closes the base porosity of alumino-silicate material and in addition to a leveling of Unevenness on the surface of the alumino-silicate material leads. Preferably, the material of the base coat is adjusted in terms of its thermal expansion coefficient on the alumino-silicate material and the final silicon nitride layer.

Particularly preferably, the material of the base coating is selected from the group consisting of silica, silica sol or a mixture of silica and silica sol; Alumino sol, boehmite or a mixture of alumino sol and boehmite.

In all previously presented embodiments of the method according to the invention, the thermal treatment can be carried out as a single-stage or multi-stage tempering process.

The suspension used according to the invention may additionally comprise at least one inorganic dispersing agent and / or binder, preferably an SiO ₂ -containing dispersing and / or binding agent, in particular illite, silicic acid, water glass, silica gel or sol, organosilicon compounds, pyogenic ciselic acid Derivatives, as well as mixtures and combinations thereof.

The invention further provides a shaped body of sintered alumino-silicate material, in particular a roll or crucible, comprising a silicon nitride protective layer formed at least in regions on the surface of the shaped body with a layer thickness of at least 50 μm, which can be produced by the method according to the invention.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited non-patent literature

• ASTM B 822-10 [0013]
• DIN 53019: 2008-09 [0019]

Claims (26)

A method for at least partially applying a silicon nitride protective layer on moldings of alumino-silicate materials, in which a suspension containing or consisting of a) silicon nitride particles, b) at least one organic cationic or pseudo-cationic dispersing / liquefying agent, and / or at least one anionic basic dispersing / liquefying agent, c) at least one organic binder, d) at least one plasticizer, e) ad 100% by weight of water, the pH of the suspension being between 7 and 14, applied to the surface of the molding at least once at least partially applied and the molding is then thermally treated at least once. Method according to Claim 1 , characterized in that the pH of the suspension is between 7.5 and 12, preferably between 8 and 11. Method according to one of the preceding claims, characterized in that the silicon nitride particles to 10 to 40 wt .-%, preferably from 15.0 to 35.0 wt .-%, based on the total suspension are included. Method according to one of the preceding claims, characterized in that the silicon nitride particles a) has an average particle size distribution d ₁₀ of 0.20 to 0.40 microns, d ₅₀ from 0.50 to 1.49 microns, preferably from 0.50 to 0.90 μm and / or d ₉₀ from 1.10 to 3.90 μm, preferably from 1.50 to 2.50 μm, determined according to the laser diffraction method according to ASTM B 822-10, and / or b) has a specific surface area from 4.67 to 15.00 m ² / g, preferably from 8 to 13 m ² / g, determined by the gas adsorption process according to ASTM D 3663-03. Method according to one of the preceding claims, characterized in that the at least one organic cationic or pseudokationische dispersing / liquefying agent is selected from the group consisting of amino alcohols, preferably 2-amino-2-methylpropanol, basic anionic dispersants / liquefiers and mixtures or combinations hereof. Method according to one of the preceding claims, characterized in that the at least one organic (cationic or pseudokationisches) dispersing / liquefying agent and / or the at least one anionic basic dispersing / liquefying agent to 0.01 to 10 wt .-%, preferably to 0 , 1 to 2 wt .-%, particularly preferably from 0.16 to 0.3 wt .-%, based on the total suspension is included. Method according to one of the preceding claims, characterized in that the at least one organic binder is selected from the group consisting of water-soluble or water-dispersible hydrophilic polymers, in particular polyethylene glycols, polypropylene glycols, polybutylene glycols, polyvinyl alcohols, polyvinyl butyrals, polyvinyl pyrrolidones, polyvinyl acetates and partially or vollverseifte Polyvinyl acetates, especially PVA M145000 fully hydrolyzed. Method according to one of the preceding claims, characterized in that the at least one organic binder to 0.01 to 20 wt .-%, preferably 0.1 to 5 wt .-%, particularly preferably from 1.0 to 3.4 wt .-%, based on the total suspension is included. Method according to one of the preceding claims, characterized in that the at least one plasticizer is selected from the group consisting of polyhydric alcohols, in particular propanetriol and / or preferably in an amount of 0.1 to 6.0 wt .-%, more preferably of 0.5 to 3.0 wt .-%, based on the total suspension is included. Method according to one of the preceding claims, characterized in that it has a dynamic viscosity of 20 to 50 mPa s, determined by means of a rotational viscometer with an immersion measuring device according to DIN 53019: 2008-09. Method according to one of the preceding claims, characterized in that the alumino-silicate molded body is porous and preferably has a porosity of at least 15%, preferably 20 to 35%, particularly preferably from 25 to 30% and / or after the thermal treatment a middle Pore diameter of 0.50 to 2.0 .mu.m, preferably 1.0 to 1.5 .mu.m, in particular 1.00 to 1.25 .mu.m. Method according to the preceding claim, characterized in that the suspension is infiltrated into the shaped body at least 500 μm, preferably 700 to 1000 μm. Method according to one of the preceding claims, characterized in that the shaped body of alumino-silicate material is a green body. Method according to one of the preceding claims, characterized in that the thermal treatment at temperatures of 1000 to 1300 ° C, preferably from 1100 to 1200 ° C and / or over a period of 2 to 5 hours, preferably from 3 to 4 hours performed becomes. Method according to one of the preceding claims, characterized in that the application of the suspension by means of brushing, spraying, spraying, dipping the shaped body in the suspension and / or by electrostatic application. Method according to one of the preceding claims, characterized in that after application of the suspension and before the thermal treatment, a drying of the shaped body, preferably at temperatures of 20 ° C to 150 ° C and in particular at a humidity of 40 to 60%. Method according to one of the preceding claims, characterized in that a closed silicon nitride protective layer on the substrate surface of at least 20 microns layer thickness is generated. Method according to one of the preceding claims, characterized in that the alumino-silicate material mullite and / or mullitisch bound materials and amorphous residual glass phase and Restkorund contains.selected is selected from the group consisting of mullite and / or mullitisch bound materials. Method according to one of the preceding claims, characterized in that the shaped body is selected from the group consisting of transport rollers for treatment furnaces, crucibles, furnace lining and Charginggestelle. Method according to one of the preceding claims, characterized in that after molding of the alumino-silicate material is subjected directly after the thermal treatment of a mechanical surface treatment. Method according to the preceding claim, characterized in that the mechanical surface treatment is carried out grinding and / or polishing with hard-machining tools, in particular diamond-coated grinding wheels and / or with or without grinding aids, in particular grinding pastes. Method according to one of the preceding claims, characterized in that the at least partially applying is carried out in several stages, wherein first a base coat and then at least one further cover layer is applied to the shaped body. A method according to the preceding claim, characterized in that the base coating of silica, silica sol or a mixture of silica and silica sol, alumino sol, boehmite or a mixture of alumino sol and boehmite is formed. Method according to one of the preceding claims, characterized in that thermal treatment is carried out as a single-stage or multi-stage annealing process. Method according to one of the preceding claims, characterized in that the suspension additionally comprises at least one inorganic dispersant and / or binder, preferably a SiO ₂ -containing dispersing and / or binder, in particular illite, silica, water glass, silica gel or sol , Si-organic compounds, pyogenic silicic acid derivatives, as well as mixtures and combinations thereof. Shaped body of sintered alumino-silicate material, in particular a roll, e.g. a transport roller for treatment furnaces or a crucible, comprising a silicon nitride protective layer formed at least in regions on the surface of the molded body with a layer thickness of at least 50 μm, preparable by a method according to one of the preceding claims.