HK1101701A - Durable hard coating containing silicon nitride - Google Patents

Description

Durable hard coatings containing silicon nitride

Technical Field

The invention relates to a lubricant (slip) for producing a durable hard coating comprising silicon nitride on a substrate; a formed body comprising a base material and a hard coat layer applied thereon which is resistant to friction and scratch and is durable, thereby enabling the formed body to be transported; a method for producing the shaped body and the use of the shaped body, in particular as a melting crucible, in the field of corrosive non-ferrous metal melts, in particular in the field of solar-grade silicon (silicon) processing, and the use of the shaped body as a riser tube in aluminium smelting, in particular in low-pressure aluminium casting.

Background

Melting and recrystallization of silicon rods containing silicon particles, silicon granules or silicon wafers from graphite or silicon nitride, but predominantly SiO₂(fused silica) in a crucible. Silicon rods with the desired microstructure and purity are crystallized from the melt in a precisely defined cooling process, and these are subsequently sliced into thin wafers to form the active components of the optoelectronic component.

It is important here that the quality of the solar grade silicon is not adversely affected by the materials used in the process, for example the melting crucible, and that the silicon melt can be solidified without defects and removed from the crucible without damage. In this connection, it is important to prevent corrosive attack of the crucible material by the liquid silicon metal, since otherwise the melt would be contaminated. Furthermore, sticking, penetration and diffusion can also cause problems in demoulding of the silicon rod, so that there is a risk of the polycrystalline silicon mass breaking or cracking.

Since Si and SiO₂A chemical reaction occurs between the two to form volatile SiO, so the corrosive silicon melt reacts on the SiO₂The crucible is eroded. In addition, undesirable impurities from the crucible material enter the silicon melt in this manner.

In particular, adhesion of the material to the solidified or solidified silicon mass should be avoided anyway, since the silicon is subjected to very large thermal expansions so that very small quantities of the adhesive material generate mechanical stresses which lead to a destruction of the crystalline structure, forming unacceptable silicon material.

In aluminum smelting, in particular in low-pressure aluminum casting, risers made of iron alloys or fused silica are used. Due to the high corrosiveness of the aluminum melt at temperatures of 650-. Alumina or boron nitride coatings applied by dipping, brushing or spraying from lubricants containing organic binders are generally used here. However, the lifetime of these coatings is limited to hours or days due to the combined corrosive and mechanical attack of the hot melt and dross. Risers made of silicon nitride ceramics that are completely inert to corrosive attack by aluminum melts have also been used as alternatives to coated risers made of ferrous alloys or quartz. However, these silicon nitride risers are many times more expensive to manufacture than standard coated risers.

Prior Art

It is known from EP 963464B 1 that the melting crucible is made of quartz, graphite or ceramic and has a silicon nitride layer to avoid sticking between the melting crucible and the non-ferrous metal after the melting crucible has been brought into contact with the solidifying non-ferrous metal melt, for example a silicon melt. Here, the silicon nitride layer contains high-purity silicon nitride powder. The silicon nitride powder has a low oxygen content and has a particular composition. These powder coatings are applied directly by the user before the melting crucible is used and are made by dispersing high purity silicon nitride powder in a solvent and then applying it to the crucible by, for example, suspension spraying. The solvent used and any organic binder components must be removed by thermal aftertreatment.

It has been found that high purity silicon nitride itself is very chemically resistant to the silicon melt. However, the weight of the melt alone causes forced wetting or infiltration of the porous silicon nitride powder layer. Thus, the layer must have such a thickness that it cannot be completely penetrated and thus still functions as a release layer. However, such thick layers are in turn correspondingly soft and do not have particular friction resistance, so that when filling the crucibles, particular care has to be taken, let alone to avoid friction during long transport or when shipping the coated crucibles ready for use.

Conventional crucible coatings for the solar-grade silicon sector therefore have the disadvantage of low mechanical stability, since these coatings contain only silicon nitride powder, so that the coating is always carried out just before the crucible is filled with silicon powder, silicon granules or silicon wafers. It is therefore not possible to coat the crucible beforehand, rather than directly at the point of use. In addition, since the powder coating is soft, great care has to be taken when filling the crucible with bulk material to avoid damaging the layer. Furthermore, since the porous silicon nitride powder layer is infiltrated by molten silicon, there is an undesirable residue of agglomeration upon demolding.

DE 10326769B 3 describes a durable boron nitride release layer for nonferrous die casting, in which a high-temperature-resistant nanosize binder is used as the boron nitride binder phase, and a lubricant for producing the release layer. In particular, SiO₂A suspension of a base sol gel matrix and boron nitride powder is applied to a metallic or inorganic non-metallic surface and the coating obtained in this way is dried and thermally hardened. At temperatures above 500 ℃, the binder system transforms into a glassy matrix which imparts mechanical stability to the compact ceramic coating formed. However, these boron nitride containing coatings cannot be used in the solar grade silicon field because boron nitride is a poor impurity in solar grade silicon.

DE 10326815 a1 describes a substrate having an anti-stick coating obtainable by applying to the substrate and hardening a coating composition comprising a) solid particles of a mould release agent other than boron nitride and b) a binder containing surface-modified nano-sized solid particles. The release agent particles are selected from the group consisting of graphite, graphite compounds, metal sulfides, metal selenides, and metal tellurides. These coatings are also not suitable for use with solar grade silicon because the mold release agents mentioned here, such as graphite or metal sulfides, selenides and tellurides, are also undesirable impurities in solar grade silicon.

Object of the Invention

It is therefore an object of the present invention to provide a lubricant for producing a durable coating on a substrate, which coating is particularly suitable for application in the field of solar-grade silicon processing without the disadvantages known from the prior art. Furthermore, the invention is to provide a durable, inexpensive coating for use in aluminum smelting, which in particular will increase the service life of the riser.

Summary of The Invention

The above object is achieved by a lubricant for producing a durable hard coating according to claim 1, a shaped body comprising a substrate with a durable hard coating according to claim 11, a process for producing the shaped body according to claim 15 and the use of the shaped body according to claims 16 to 18. Advantageous or particularly useful embodiments of the subject matter of the present patent application are described in the dependent claims.

The present invention therefore provides a lubricant for producing a durable hard coating on a substrate, comprising a) silicon nitride particles and b) a binder comprising nanosized solid particle precursors and/or nanosized solid particles prepared by a sol-gel process.

The invention further provides a shaped body comprising a substrate with a durable hard coating, wherein the hard coating has been produced from a lubricant according to the invention as defined above.

The invention further provides for the use of the shaped bodies according to the invention in the field of corrosive non-ferrous metal melts, in particular for the production of silicon melts in the form of melting crucibles, and for the use of the shaped bodies in the form of risers in aluminium smelting, in particular low-pressure aluminium casting.

The surprising effect exhibited by the hard silicon nitride coating according to the invention is that the presence of the rigidly bound silicon nitride particles does not hinder the demoulding of the solidified nonferrous metal melt and at the same time has the disadvantage that the shaped bodies of such hard coatings do not have a porous and loose silicon nitride powder layer structure during transport and loading.

It is surprising for the person skilled in the art that the SiO known from DE 10326769B 3 and DE 10326815A 1₂The base binder system is suitable for preparing durable hard silicon nitride coatings for applications envisioned in accordance with the present invention, as one skilled in the art would expect that the addition of an inorganic binder or nano-sized solid particles would make demolding a solidified non-ferrous metal melt more difficult and would enable the removal of the solidified non-ferrous metal meltImpurities are introduced into the solidified non-ferrous metal melt, in particular solar-grade silicon ingots, which are to be avoided anyway. Furthermore, the person skilled in the art foresees SiO₂The base binder system will likewise give rise to SiO in the molten silicon and binder system₂In a silicon melt and SiO₂This is the case in crucible systems. Corrosive attack of SiO by the silicon melt₂The material forms gaseous SiO which is volatilized, and more SiO₂Is exposed to a silicon melt. The coating will have to be continuously dissolved due to this reaction. This reaction is known, for example, from Crystal.Res.Technol.38, Nos. 7-8, 669-675 (2003).

The hard silicon nitride layer according to the invention has the following advantages, in particular:

the hard layer has absolute transport stability so that the coated shaped body ready for use, for example a melting crucible, can be transported to the final shipping user.

The coating is resistant to abrasion and thus provides protection against damage during filling of the shaped body, in particular during filling of the coated melting crucible with raw silicon or during handling of the shaped body. Furthermore, conventional silicon nitride powder coatings suffer from undesirable corrosion and sticking due to protective layer damage, which is not the case with the hard layer of the present invention.

The hard layer according to the invention is impermeable and stable at high temperatures and is not penetrated by molten non-ferrous metals, such as silicon, so that the solidified melt can be demoulded without sticking. No additional step of mechanically removing the bonding material is required and no rejects or fewer rejects occur. Furthermore, with conventional silicon nitride powder layers, material bonding occurs after demolding, since the porous silicon nitride layer is infiltrated by a molten non-ferrous metal, such as silicon. The bonded material must be mechanically removed, which firstly represents an additional processing step and secondly causes material loss.

In the case of coatings according to the prior art (EP 963464), there is the problem that, owing to the porosity of the silicon nitride powder layer, there is melt-substrate contactProperty, therefore, is derived from SiO₂Impurities of the crucible can diffuse into the solar grade silicon. As a result, impurities are introduced into the solar grade silicon, resulting in significant deterioration in quality, thereby producing off-spec material.

The hard layer of the present invention has a further advantage in that it acts as a barrier against diffusion of impurities due to its close structure, which prevents direct melt-substrate contact.

When a hard silicon nitride coating is used as a riser in aluminum smelting, significant cost advantages are obtained because the less expensive base material for the riser can be used continuously while the service life of the riser is also greatly increased. In addition, there is also a possibility of repair when the hard coating is damaged.

Disclosure of Invention

The binders used according to the invention, which contain precursors of nanosized solid particles and/or nanosized solid particles produced by a sol-gel process, are known in principle from DE 10326815 a 1. It has been found that silicon nitride particles can be bonded to the substrate surface durably and in a thermally stable manner by means of the binder. In a preferred embodiment, nanocomposites containing nanoparticles, especially in the form of sols, are used as binders. The nanocomposite or nanocomposite sol comprises a mixture of nano-sized solid particles prepared by a sol-gel process, and preferably an inorganic or organically modified inorganic polycondensate or precursor thereof. In the coating composition, the binder comprising the nanoparticles or the nanocomposite is usually present as a sol or dispersion. In the hardened layer, the matrix serves as a matrix template. Due to the pure ceramic structure of the layer, a number of requirements can be met. Due to the hardness and abrasion resistance of the layer, in addition to the high temperature stability and purity of the coating, the adhesion and mechanical stability of the layer on the substrate are also ensured.

The nano-sized solid particles are preferably metal oxide particles or are converted into nano-sized metal oxide particles after hardening by high-temperature treatmentAnd (4) preparing the system. The nano-sized solid particles are especially selected from SiO₂、TiO₂、ZrO₂、Al₂O₃、AlOOH、Y₂O₃、CeO₂、SnO₂Iron oxide and Ta₂O₅Or precursors of these nanosized solid particles which can be converted into these solid particles by a sol-gel process, SiO being particularly preferred₂Particles and/or conversion to nano-sized SiO by sol-gel process₂SiO of particles₂A particle precursor.

The preferred nanocomposites according to the invention and their preparation by the sol-gel process are known from the prior art, in particular from DE 10326815A 1. The nano-sized solid particles preferred herein are surface modified with a surface modifying agent having a molecular weight of less than 1500, in particular a surface modifying agent containing anhydride groups, amide groups, amino groups, SiOH groups, hydrolysable silane groups and/or β -dicarbonyl groups.

The nanocomposite material is preferably obtainable by a sol-gel process by reacting nanosized solid particles with one or more silanes of the general formula,

R_xSiA_(4-x) (I)

wherein the radicals A are identical or different and are hydroxyl groups or hydrolyzable radicals, the radicals R are identical or different and are non-hydrolyzable radicals, x is 0, 1, 2 or 3, at least 50% by molar amount of silane x.gtoreq.1. If only silanes of the formula (I) in which x ═ 0 are used, pure inorganic nanocomposites are obtained; otherwise, a preferred organic-inorganic nanocomposite is obtained.

Suitable examples of the abovementioned silanes of the formula (I) are likewise disclosed in DE 10326815A 1.

In particular, the coating of the invention consists of SiO₂The formed alcosol is prepared, and high-purity silicon nitride powder is dispersed in the alcosol. Since silicon nitride hydrolyzes in the presence of water, water-based formulations should not be used; instead, itPreferably SiO₂Forming alcohol sol. Furthermore, it is preferred to use high-purity starting chemicals (silicon nitride powder, silanes, alcohols, etc.) since in this way high-purity coatings are obtained which meet the requirements of the solar industry in particular.

In the shaped body of the present invention, the base material suitably comprises quartz, graphite, ceramics (including silicon nitride ceramics), SiO₂(fused silica) or an iron alloy. In a preferred embodiment, the shaped body is a melting crucible whose base material comprises quartz, graphite or ceramic, which is suitable for the processing of corrosive non-ferrous metal melts, in particular silicon melts.

In another embodiment, the shaped body is for aluminium smelting, the substrate of which comprises SiO₂(fused silica) or ferroalloys.

The process according to the invention for the preparation of shaped bodies comprises at least the following steps:

the lubricants of the present invention are applied to a substrate by single or repeated blade coating, dipping, flow coating, spin coating, spraying, brushing, or painting,

the applied lubricant hardens to form a durable hard coating on the substrate.

To improve adhesion, in some cases it is advantageous to treat the substrate with diluted or undiluted binder sols or their precursors or other primers before the contacting.

The solids content of the lubricant as a function of the selected coating process can be set by adding a solvent. For spraying, the solids content is generally set to 2 to 70% by weight, preferably 5 to 50% by weight, particularly preferably 10 to 30% by weight.

The final hardening can be carried out at room temperature or slightly elevated temperature by one or more drying steps, for example in a convection drying oven and/or by heating the shaped body itself. For oxidation-sensitive substrates, drying and/or subsequent hardening can be carried out in a protective atmosphere, for example in N₂Or in Ar or under reduced pressure.

In view of heat sensitivity, it is preferable to perform heat hardening by heat treatment at a temperature of 50 ℃ or higher, preferably 200 ℃ or higher, and particularly preferably 300 ℃ or higher. The hard layer may also be baked at a relatively high temperature, preferably 500 to 700 c, provided that the substrate is sufficiently stable at this temperature.

In another embodiment of the present invention, the hard layer may be a multi-layer.

In another embodiment of the present invention, a graded layer may be formed in which, for example, the kind and purity of silicon nitride particles used from the bottom (substrate side) upward (melt side) may be changed. Silicon nitride grades of different purity, different particle size or different particle morphology may be used herein within the coating structure. In addition, different binder contents may also be incorporated into the graded layer. These graded layers may also be made or arranged in multiple layers.

The shaped bodies of the invention having a durable hard coating are suitable for use in the field of corrosive non-ferrous metal melts, such as aluminum melts, glass melts, silicon melts, and the like. Shaped bodies in the form of melting crucibles are particularly suitable for preparing silicon melts, for containing liquid silicon and for crystallizing liquid silicon to form silicon ingots.

The shaped bodies in the form of risers are particularly suitable for use in aluminium smelting, particularly preferably for low pressure aluminium casting.

The following examples illustrate the invention.

Comparative example: standard suspension

This is a suspension of silicon nitride powder prepared as described in EP 963464, suspended in distilled water, without any additives.

For further processing (brushing, rolling, spraying), only the rheology of this suspension is critical. For spraying with a spray gun, the solids content is accordingly set, for example, to 60 to 70% by weight.

The suspension is applied to a clean, dust-free, dry crucible, if appropriate in multiple layers, in order to obtain a uniform layer thickness, for example 500. mu.m. After drying, the coating was sintered at about 1000 ℃ and 1100 ℃ before use as a melting crucible.

The resulting silicon nitride powder coating should be bubble-free, crack-free and also free from other defects.

Silicon nitride coatings prepared in this way have only a limited resistance to touch and are accordingly treated with care. Not only is the coating layer prevented from being damaged in the process of filling the silicon wafer, but also the filling process is required to be carried out so as to prevent the silicon wafer from sliding in the melting process, so that the prepared powder layer has no defects.

Example 1: hard silicon nitride coating

For silicon nitride suspension according to one embodiment of the invention, a 60 wt.% dispersion of silicon nitride powder in absolute ethanol was prepared. For this purpose, the powder can be initially charged and continuously introduced into the dispersion medium, or the powder can be stirred into the initially charged ethanol.

An equal amount of base stock (ino)^®sil S-38, Inomat GmbH) was stirred into an ethanol dispersion of silicon nitride to make a sprayable suspension containing 30 wt.% silicon nitride.

The suspension is applied in layers by spraying in a "wet-on-wet" manner, giving a layer thickness of up to about 40 μm. After "air drying" at room temperature, the coating was dried in a drying oven and then sintered at 500 ℃ for 30 minutes.

The now coated crucible can be used in a melting process. The obtained defect-free Si ingot can be released from the mold without any problem.

Example 2: hard silicon nitride coating

For silicon nitride suspension according to another embodiment of the invention, a 60 wt.% dispersion of silicon nitride powder in anhydrous ethanol was prepared. For this purpose, the powder can be initially charged and continuously introduced into the dispersion medium, or the powder can be stirred into the initially charged ethanol.

The base material (ino) is mixed with the silicon nitride in a ratio of 2: 1^®sil S-38, Inomat GmbH) was stirred into an ethanol dispersion of silicon nitride to prepare a suspension containing 40% by weight of silicon nitride.

The high viscosity suspension is applied by dipping, pouring, or brushing/rolling, in a layer thickness of up to about 100 μm.

After "air drying" at room temperature, the coating was dried in a drying oven and then sintered at 500 ℃ for 30 minutes.

The now coated crucible can be used in a melting process. The obtained defect-free Si ingot can be released from the mold without any problem.

Example 3: hard silicon nitride coating

For silicon nitride suspensions used for coating solar crucibles according to another embodiment of the invention, suspensions containing 30% by weight of silicon nitride powder were produced directly in a liquid ethanol base. For this purpose, silicon nitride powder is continuously introduced into the binder (ino) by stirring^®sil S-38, Inomat GmbH). To homogenize the mixture, it is processed on a roll mill for several hours.

The 30% strength by weight non-agglomerating silicon nitride suspension obtained in this way is applied in layers by spraying in a "wet-on-wet" manner, giving a layer thickness of up to about 40 μm. After "air drying" at room temperature, the coating was dried in a drying oven and subsequently sintered at 500 ℃ for 30 minutes.

The now coated crucible can be used in a melting process. The obtained defect-free Si ingot can be released from the mold without any problem.

Example 4: hard silicon nitride coating

For silicon nitride suspensions used for coating solar crucibles according to another embodiment of the invention, suspensions containing 60% by weight of silicon nitride powder were produced directly in a liquid ethanol base. For this purpose, silicon nitrideThe powder is continuously introduced into the base (ino) by stirring^®sil S-38, Inomat GmbH). Alternatively, the binder can also be introduced into the initially charged silicon nitride powder a little at a time. To homogenize the mixture, it is processed on a roll mill for several hours.

The non-agglomerating silicon nitride suspensions obtained in this way at a concentration of 60% by weight were applied in multiple layers by brushing and rolling, the layer thickness being up to about 100. mu.m. After "air drying" at room temperature, the coating was dried in a drying oven and then sintered at 500 ℃ for 30 minutes.

The now coated crucible can be used in a melting process. The obtained defect-free Si ingot can be released from the mold without any problem.

The embodiment of the silicon nitride coating of the invention described in the examples differs from the reference coating of the prior art by its lower layer thickness. Despite the low layer thickness, functional, i.e. defect-free (bubble-free, crack-free), release layers can always be produced. Due to the presence of the binder, these layers have significantly higher bond strength and scratch resistance than standard silicon nitride powder coatings. Despite the thin coating, the layer is not damaged when the silicon wafer is loaded or/and melted, thus avoiding contact between the melt and the crucible, which contact, when solidified, leads to sticking and thus to spalling and cracking.

The silicon nitride coating of the invention described is distinguished by the following features: the solids content, the ratio of silicon nitride to binder and the coating thickness and the corresponding concentration of the suspension (which determines the technique used for applying the suspension) and a defect-free layer can be obtained: the higher the ratio of silicon nitride to binder, the thicker the coating; the lower the ratio of silicon nitride to binder, the harder and the higher the scratch resistance.

Depending on the respective melting/solar grade silicon production process, the optimal coating system may be so selected (matching the suspension preparation, the coating process and the respective melting process).

Claims (18)

1. A lubricant for producing a durable hard coating on a substrate comprising:

a) silicon nitride particles and

b) a binder comprising nanosized solid particle precursors and/or nanosized solid particles prepared by a sol-gel process.

2. A lubricant according to claim 1, wherein the binder comprises a nanocomposite comprising nanosized solid particles in the form of an organically modified inorganic polycondensate or a precursor thereof.

3. A lubricant according to claim 2, wherein the organically modified inorganic polycondensate or precursor thereof is an organically modified inorganic polysiloxane or precursor thereof.

4. A lubricant according to at least one of the preceding claims, wherein the nano-sized solid particles are metal oxide particles.

5. A lubricant according to at least one of the preceding claims, wherein the nanosized solid particles are selected from SiO₂、TiO₂、ZrO₂、Al₂O₃、AlOOH、Y₂O₃、CeO₂、SnO₂Iron oxide and Ta₂O₅Or these nano-sized solid particle precursors that can be converted to these solid particles by a sol-gel process.

6. A lubricant according to at least one of the preceding claims, wherein SiO₂Particles and/or conversion to nano-sized SiO by sol-gel process₂SiO of particles₂The particle precursors are present as nano-sized solid particles.

7. A lubricant according to at least one of the preceding claims, wherein said nanosized solid particles have been surface modified with a surface modifier having a molecular weight of less than 1500.

8. A lubricant according to at least one of the preceding claims, wherein said nanosized solid particles have been modified by a surface modifier comprising anhydride groups, amide groups, amino groups, SiOH groups, hydrolysable silane groups and/or β -dicarbonyl groups.

9. A lubricant according to at least one of the preceding claims 2 to 7, wherein the nanocomposite is obtainable by a sol-gel process by reacting nanosized solid particles with one or more silanes of the general formula (I),

R_xSiA_(4-x) (I)

wherein the radicals A are identical or different and are hydroxyl groups or hydrolyzable radicals, the radicals R are identical or different and are non-hydrolyzable radicals, x is 0, 1, 2 or 3, at least 50% by molar amount of silane x.gtoreq.1.

10. A lubricant according to claim 1, wherein the binder is obtainable by a sol-gel process by reacting one or more silanes of the general formula (I),

R_xSiA_(4-x) (I)

wherein the radicals A are identical or different and are hydroxyl groups or hydrolyzable radicals, the radicals R are identical or different and are non-hydrolyzable radicals, x is 0, 1, 2 or 3, at least 50% by molar amount of silane x.gtoreq.1.

11. Shaped body comprising a substrate with a durable hard coating, wherein the hard coating has been prepared from a lubricant according to at least one of claims 1 to 10.

12. The shaped body according to claim 11, wherein the substrate comprises quartz, graphite, ceramic, SiO₂(fused silica) or an iron alloy.

13. The shaped body as claimed in claim 11 and/or 12, wherein the shaped body is a melting crucible for corrosive nonferrous metal melts whose base material comprises quartz, graphite or ceramic.

14. Shaped body according to claim 11 and/or 12, wherein the shaped body is a substrate comprising SiO₂(fused silica) or ferroalloys riser tubes for aluminum smelting.

15. Process for the preparation of a shaped body according to at least one of the preceding claims 11 to 14, comprising the steps of:

applying the lubricant to the substrate by single or repeated blade coating, dipping, flow coating, spin coating, spraying, brushing, or painting;

the applied lubricant hardens to form a durable hard coating on the substrate.

16. Use of a shaped body according to at least one of the preceding claims 11 to 14 in the field of corrosive nonferrous metal melts.

17. Use of a melting crucible according to claim 13 for preparing a silicon melt, for containing liquid silicon and/or for preparing a silicon block for crystallization of liquid silicon.

18. Use of the riser according to claim 14 in aluminium smelting, especially in low pressure aluminium casting.