CN1793017A - Mullite whisker-mullite composite coating and preparation process thereof - Google Patents

Abstract

The present invention obtains mullite whisker-mullite composite coating by in-situ synthesis, and its preparation method is: by coating the mixed slurry of silica sol, aluminum sol and mullite slurry in the vacuum of base material, or vacuum Coating silica sol, aluminum sol, mullite slurry layer by layer, or using tetraethyl orthosilicate and aluminum salt in accordance with the mullite stoichiometric ratio as raw materials, using hydrolysis-co-precipitation method to deposit oxides and then coating mullite The stone slurry is synthesized in situ by high-temperature sintering under the catalysis of metal fluoride mineralizers. The invention has the advantages of simple process, cheap and easy-to-obtain raw materials, excellent high-temperature oxidation resistance, thermal shock resistance and acid and alkali corrosion resistance, and can be widely used in the fields of light industry and chemical industry, and is especially suitable as a non-oxide high-temperature structure The ceramic anti-oxidation coating and the reinforced and toughened phase of the porous material improve the strength of the material and prolong the service life of the material.

Description

Mullite whisker-mullite composite coating and preparation method thereof

technical field

The invention relates to the field of non-metallic materials, in particular to a mullite whisker-mullite composite coating and a preparation method thereof.

Background technique

In recent years, with the wide application of high-temperature structural ceramics, especially non-oxide structural ceramics (such as sialon, silicon carbide, silicon nitride ceramics, etc.) . At present, the high-temperature anti-oxidation coatings used for non-oxide ceramics are mainly molybdenum disilicide, alumina, silicon oxide, spinel, mullite and other systems. Mullite has excellent mechanical properties, and its thermal expansion rate is similar to that of non-oxide ceramics such as silicon carbide in a wide range. Moreover, the preparation process of mullite coating is simple and inexpensive. The good stability of mullite coating in oxidizing atmosphere and acidic and alkaline conditions makes it have a good application prospect in the field of high-temperature oxidation-resistant coatings. The silicon-based non-oxide ceramics coated with mullite will actually form new silica due to the oxidation of the substrate during use, and become a composite coating of silica and mullite. There is a phase change volume effect at 723°C, and there is a small difference in thermal expansion between the mullite coating and the substrate after all, so under the impact of frequent thermal cycles, it is easy to cause cracking of the coating, making the protective effect of the mullite coating reduce. Whiskers, as an almost defect-free material with high strength and high elastic modulus, can effectively bear the phase transition stress, hinder the expansion of cracks, increase the toughness of the coating, and thus improve the thermal shock resistance of the coating. In recent years, the synthesis methods of mullite whiskers have been continuously expanded, but the preparation conditions of mullite whiskers are still relatively harsh, and it is difficult to disperse the whiskers, and it is difficult to prepare low-viscosity slurry with high whisker content. The addition of whiskers to the layer preparation will drastically increase the production cost of the coating.

At present, the methods for preparing mullite coating mainly include: slurry dipping coating method, sol-gel method, spraying slurry method, brushing method, plasma spraying method, vapor deposition method, etc. The slurry impregnation method is mainly used to coat the ceramic surface with mullite coating. However, due to the large particle size of the coating, the impregnation of the micropores in the matrix is not ideal, and the coating thickness is uneven, which is easy to cause cracking of the coating. In addition, the sintering temperature of mullite is very high, and it is difficult to obtain a dense coating. Sol-gel can prepare a uniform coating, and the heat treatment temperature of the coating is low, but the solid content of the sol is very low, and it takes more than ten times of coating to obtain a coating with a thickness of micron, and the sol is easy to crack when it is dried and sintered. Lead to coating defects; plasma spraying and brushing methods can only form coatings on the surface of the substrate. Although the vapor deposition method can obtain a coating deep in the substrate, the vapor deposition has high requirements on equipment, process parameters and working environment, and its industrial application is limited.

Contents of the invention

The technical problem to be solved by the present invention is: to provide a kind of problem existing in above coating technology, adopt the sol-gel with mullite stoichiometric ratio and the composite slurry of pre-synthesized mullite micropowder, improve the slurry Solid content and reduce the sintering temperature of the coating material. By introducing appropriate mineralizers, mullite whiskers were synthesized in situ during coating sintering, and a dense mullite whisker-mullite composite ceramic coating with toughening effect was obtained at a lower temperature.

The technical solution adopted by the present invention to solve its technical problems is as follows:

The mullite whisker-mullite composite coating provided by the present invention is obtained by an in-situ synthesis method, that is, a composite slurry is formed from aluminum sol, silica sol and mullite slurry as raw materials, and the substrate is coated under vacuum The material is coated, dried and sintered, and the mullite whisker-mullite composite coating is synthesized in situ under the catalysis of the metal fluoride mineralizer.

The preparation method of in-situ synthesis mullite whisker-mullite composite coating provided by the invention, its steps are as follows:

(1) Prepare mullite slurry: aluminum raw material and quartz raw material are mixed according to the requirement of mullite stoichiometric ratio according to the mass ratio of alumina and silicon dioxide, and mixed with 30-50% water of the total mass of raw materials, Ball mill for 10 to 24 hours, then add metal fluoride mineralizer with 4-10% of the total mass of raw materials, ball mill for 1 to 2 hours, form a block green body, and keep the green body at 1400-1550°C for 1 hour to synthesize mullite , and then through pulverization and ball milling to obtain a mullite slurry with an average particle size of 0.5-1.5 μm; or, directly use industrial fused mullite as a raw material, add a metal fluoride mineralizer with a total mass of 4-10% of the raw material, and then Obtained by crushing and ball milling.

(2) Coating coating: the substrate material to be coated is coated with a composite slurry of silica sol, aluminum sol and mullite slurry in a vacuum with a stoichiometric ratio consistent with the composition of mullite, or layer by layer in vacuum Sequential coating of silica sol, aluminum sol, mullite slurry, or vacuum sequential coating of silica sol, aluminum sol and metal fluoride supersaturated solution; As a raw material, the oxide is deposited by hydrolysis-co-precipitation method, and then coated with mullite slurry; the coated material is dried at 60-90°C.

(3) Sintering the dried coating material at 1100-1500° C. for 1-3 hours to obtain an in-situ synthesized mullite whisker-mullite composite coating.

Compared with the prior art, the present invention has the following remarkable effects:

One. Adopting the method of sol-gel and pre-synthesized mullite micropowder composite slurry with mullite stoichiometric ratio to prepare slurry with high solid content, which is conducive to obtaining a moderate thickness coating without dry cracks; Simple sol-coated coatings are thin, dry and easy to crack.

Shown by Fig. 2, Fig. 3: mullite whisker-mullite composite coating thickness distribution of the present invention is even, and in the mullite coating, in situ grows the mullite whisker with certain length-to-diameter ratio in a considerable amount, and The concentration of whisker is relatively high, and the coating has no cracking phenomenon after 40 thermal cycle shock resistance tests (room temperature to 1200 °C).

Figures 4 and 5 show that the thickness of the traditional single mullite coating is very uneven, and after 40 thermal cycle shock (room temperature to 1200°C) tests, obvious cracks appear along the valleys of the coating.

Second, the sintering temperature of the mullite whisker-mullite composite coating is reduced by using highly active aluminum or silicon raw materials, which is beneficial to the densification of the sintering of the coating (see Figure 2).

Third. Under the action of mineralizer, the mullite whisker-mullite composite coating was synthesized in situ with highly active raw materials, so that the coating has excellent high temperature oxidation resistance, thermal shock resistance and acid resistance Alkali corrosion performance (see Figure 6); improve the shortcomings of the traditional single mullite coating in these aspects (see Figure 4 and Figure 5).

Figure 6 shows the comparison curve of oxidation weight gain between the mullite whisker-mullite composite coating of the present invention and the traditional single mullite coating. This curve is obtained after the material coated with mullite whisker-mullite composite coating is subjected to 40 thermal cycle shock resistance (room temperature to 1200° C.) experiments.

The above-mentioned heat cycle shock resistance test, the method is: put the sample in an electric furnace with a constant temperature of 1200 ° C for 15 minutes, take out the sample with a crucible and cool it to room temperature in the air, and then place it in an electric furnace at 1200 ° C for heat preservation. This is a cycle. Repeat the above operation 40 times. It can be seen from Fig. 6 that the oxidation gain of the composite coating of the present invention is only half of that of the traditional single coating.

Its four. process is simple, and raw material source is extensive, process is simple, and production cost is low.

Five. Wide range of applications: It can be widely used in light industry, chemical industry and aerospace fields, especially suitable for anti-oxidation of high-temperature non-oxide structural ceramics, satellite attitude-adjusting rocket throat lining materials, C-C composite materials, etc. that need to withstand repeated thermal shocks , thermal shock-resistant coatings and porous materials such as filter materials, thermal insulation materials and catalyst carriers, the reinforcement and toughening phase can improve the performance of the material, broaden the scope of use of the material and prolong the service life of the material.

Description of drawings

Figure 1: X-ray diffraction (XRD) spectrum of mullite in mullite slurry.

Fig. 2: A 2000 times scanning electron microscope (SEM) image of the mullite whisker-mullite composite coating prepared by the present invention after 40 thermal cycle shock tests.

Fig. 3: 10,000 times scanning electron microscope (SEM) image of the mullite whisker-mullite composite coating prepared by the present invention after 40 thermal cycle shock tests.

Figure 4: 5000X SEM image of a traditional single mullite coating after 40 thermal cycle shock tests.

Figure 5: 10,000X side SEM image of a traditional single mullite coating after 40 thermal cycle shock tests.

Figure 6: Comparison curve of oxidation weight gain between the mullite whisker-mullite composite coating of the present invention and the traditional single mullite coating.

In the above-mentioned Fig. 2, Fig. 3, Fig. 4 and Fig. 5, the temperature of the thermal cycle shock test is from room temperature to 1200°C.

Detailed ways

The invention relates to a mullite whisker-mullite composite coating and a preparation method thereof.

The present invention will be further described below in conjunction with the embodiments and accompanying drawings.

1. Mullite whisker-mullite composite coating

Mullite whisker-mullite composite coating is obtained by in-situ synthesis method, that is, aluminum sol, silica sol and mullite slurry are used as raw materials to form a composite slurry, and the base material is coated under vacuum, and after drying Sintering, under the catalysis of the metal fluoride mineralizer, the mullite whisker-mullite composite coating is synthesized in situ, and the thickness of the coating is 1-5 μm.

From the spectrum shown in Figure 1, it can be seen that in the mullite whisker-mullite composite coating, the mullite phase is the main crystal phase and contains a small amount of alumina and quartz.

2. Method for in-situ synthesis of mullite whisker-mullite composite coating

This method is: by coating silica sol, aluminum sol, mullite slurry layer by layer in the vacuum of the base material, or adopting the mixed slurry of vacuum coating silica sol, aluminum sol and mullite slurry, or in accordance with mullite Orthosilicate ethyl silicate and aluminum salt with stone stoichiometric ratio are used as raw materials to deposit oxide by hydrolysis-co-precipitation method and then coat mullite slurry. Under the catalysis of metal fluoride mineralizer, mollite is synthesized in situ by high temperature sintering. Mullite whisker-mullite composite coating.

(1) The method adopts three steps of preparation of mullite slurry, coating coating and coating sintering, as follows:

1. Preparation of mullite slurry: mix aluminum raw materials and quartz raw materials according to the mass ratio of alumina and silicon dioxide in accordance with the requirements of mullite stoichiometric ratio, and mix with water with 30-50% of the total mass of raw materials, and ball mill After 10 to 24 hours, add a metal fluoride mineralizer with 4-10% of the total mass of raw materials, ball mill for 1 to 2 hours, and form a block green body, and keep the green body at 1400-1550°C for 1 hour to synthesize mullite. Then pulverize and ball mill to obtain mullite slurry with an average particle size of 1 to 2 μm; or, directly use industrial fused mullite raw materials, add a metal fluoride mineralizer of 4-10% of the total mass of raw materials, and then pulverize and obtained by ball milling.

The aluminum raw material mentioned above can be selected from aluminum oxide, aluminum hydroxide, aluminum sol, aluminum ammonium sulfate, aluminum salt, clay, kaolin, or obtained from one of the aluminum-containing raw materials, or a mixture thereof. Quartz raw materials can choose white carbon black, silica sol, etc. Mineralizers are metal fluorides, such as: AlF ₃ , CaF ₂ , BaF ₂ and so on.

2. Coating coating: the substrate material to be coated is coated with a composite slurry of silica sol, aluminum sol and mullite slurry whose stoichiometric ratio conforms to the composition of mullite in vacuum, or layer by layer in vacuum Coating silica sol, aluminum sol, mullite slurry, or vacuum coating silica sol, aluminum sol and metal fluoride supersaturated solution in sequence; The raw material is to deposit oxide by hydrolysis-co-precipitation method, and then coat mullite slurry; the coated material is dried at 60-90°C.

3. Coating sintering: sintering the dried coating material at 1100-1500° C. for 1-3 hours to obtain an in-situ synthesized mullite whisker-mullite composite coating. The XRD spectrum of the synthesized mullite is shown in Figure 1.

Based on the fact that silicon-based non-oxide ceramics will form new silicon dioxide after oxidation, the formulation of the coating of the present invention can be designed to be composed of aluminum-rich mullite.

(2) Specific examples

Example 1:

(1) Preparation of mullite slurry: ultrafine industrial α-Al ₂ O ₃ powder (AL ₂ O ₃ ≥99.6%, d ₅₀ ≤1.0 μm) and industrial quartz powder (SiO ₂ ≥99.1%, d ₅₀ ≤ 1.0μm) as raw materials, add 5% alumina sol (pH: 6.7～7.3, _{Al2O3 concentration} : 10±0.5%) and silica sol (pH: 6.5～8.0, _SiO2 concentration: 30±1% ) to make the ratio of alumina and silicon dioxide conform to the mullite stoichiometric ratio (the mass ratio of Al ₂ O ₃ : SiO ₂ is 3: 1), add 6% BaF ₂ (chemically pure), and adjust the pH value to 7.0～ 8.0, according to the ratio of material: ball: water of 1:1:0.5, ball mill for 10-24 hours, cast grout to form a green body, and keep the green body at 1400-1550°C for 1-3 hours to synthesize mullite. The synthesized mullite is crushed to less than 2 mm, and placed in a planetary ball mill for 2 hours according to the ratio of material: ball: water 1: 1.5: 0.4, to make a slurry with a particle size d ₅₀ ≤ 1.5 μm.

(2) Coating: Coat recrystallized silicon carbide with SiO ₂ sol (pH: 6.5-8.0, SiO ₂ content: 30±1%) in vacuum (vacuum degree: 1-2×10 ⁴ Pa). Dry at 70-80°C; apply Al ₂ O ₃ sol in vacuum (1-2×10 ⁴ Pa) and dry slowly at 70-80°C. Vacuum-coat mullite, silicon, and aluminum sol (pH 7.0 ～8.0) mixed slurry (the relative mass content of mullite is 35±3%), and dry at 70～80°C. The above-mentioned two-step process of coating is repeated 3 times to ensure the thickness of the coating. 1200～1400°C air Medium sintering for 1 to 2 hours for densification.

(3) Coating sealing layer: grind the pre-prepared high-quartz borosilicate glass with a material: water: ball ratio of 1:1:0.4 in a planetary ball mill for 1 to 2 hours (feeding particle size ≤ 4mm) To form a glass slurry, take the glass slurry (d ₅₀ ≤ 1.0 μm), and dry the sample prepared in vacuum coating (vacuum degree: 1-2×10 ⁴ Pa) at 70-80°C.

(4) Coating sintering: heat the sealed and coated ceramic material in air at 1300-1500°C for one hour to melt the glass layer and seal the coating, densify the coating, and synthesize mullite in situ Whisker-mullite composite coating.

Example 2:

(1) Preparation of mullite slurry: use kaolin as the main raw material (SiO ₂ : 52±0.5%, Al ₂ O ₃ : 45±0.5%, d ₅₀ ≤2.0 μm), add Al ₂ O ₃ and SiO ₂ (particle size and purity are the same as in Example 1), adjust the ratio of the two to make it meet the mullite ratio (Al ₂ O ₃ : SiO ₂ mass ratio is 3: 1), add 8% BaF ₂ as a mineralizer . The process of ball milling, molding, sintering and preparing mullite slurry is the same as in Example 1.

(2) Coating coating: vacuum-coat recrystallized silicon carbide with SiO ₂ sol (pH: 6.5～8.0, SiO ₂ content: 10±1%) and Al ₂ O ₃ sol (same as Example 1), repeat 3 Second-rate. Vacuum-coat mullite slurry and dry at 70-80°C. The two-step coating process can be repeated 3 times depending on the specific situation to ensure the thickness of the coating. The coated material is sintered in air at 1100-1400° C. (same as Example 1).

In this example, the steps of coating the sealing layer and densifying the coating are the same as in Example 1.

Example 3:

(1) Preparation of mullite slurry: industrial fused mullite and its mass 5% BaF ₂ , 5% AlF ₃ are ball milled in a planetary ball mill to prepare mullite slurry (mullite particles d ₅₀ =1.5 μm).

(2) Coating: SiO ₂ sol was vacuum-coated on the silicon nitride bonded silicon carbide material with a porosity of 11%, and dried (same as Example 2). Al ₂ O ₃ sol was vacuum coated and dried (same as in Example 1). Vacuum coating mullite slurry, drying (same as embodiment 2), ensures the compactness of coating. Sintering in air at 1100-1400°C (same as Example 1).

In this example, the steps of coating the sealing layer and densifying the coating are the same as in Example 1.

Example 4:

The recrystallized silicon carbide was oxidized and pretreated at 1200-1300°C for 24 hours, then vacuum-coated with Al ₂ O ₃ sol (same as Example 1), and slowly dried at 70-80°C. Vacuum coating of 8% AlF ₃ supersaturated solution, the solution should be stirred quickly before coating the AlF ₃ supersaturated solution. Dry at 60-80°C and repeat the operation 5 times. Then, it is sintered in air at 1000-1400°C to make it densified, and the mullite whisker-mullite composite coating is formed in situ.

Embodiment 5: replace pre-oxidation treatment in embodiment 4 with vacuum coating silica sol (same as embodiment 2), in-situ generation mullite whisker-mullite composite coating (same as embodiment 4)

Embodiment 6:

(1) The preparation of mullite slurry is the same as in Example 1.

(2) Coating: prepare 1.2 mol/L ethanol solution of tetraethyl orthosilicate, adjust the pH value to 2-3 with dilute nitric acid, stir at room temperature for 2-3 hours, let stand for 24 hours, and then mix with 0.7 mol/L aluminum nitrate aqueous solution mixed with acetic acid as dispersant. Slowly add ammonia water until the pH value is 3-7.5 to make the solution precipitate. After the precipitation occurs, quickly add ammonia water until the pH value reaches 9-9.5 to prepare a co-precipitation mixture. Vacuum coating (vacuum degree of 1-2×10 ⁴ Pa) coprecipitation mixture, after drying, vacuum (vacuum degree of 1-2×10 ⁴ Pa) coating 5% supersaturated BaF ₂ solution, drying at 70-80°C , repeat the operation 3 times. Vacuum coating mixed mullite, aluminum sol, silica sol mixed slurry, sintering in air at 1200～1400 ℃ (same as embodiment 1).

In this example, the steps of coating the sealing layer and densifying the coating are the same as in Example 1.

Embodiment 7:

The silicon carbide porous filter material was used instead of silicon carbide and silicon nitride, and then, according to Example 3, a mullite whisker-mullite composite coating was prepared.

Claims (7)

1. A composite coating, characterized in that mullite whisker-mullite composite coating, which is obtained by in-situ synthesis method, that is: form composite material with aluminum sol, silica sol and mullite slurry as raw materials The slurry is coated on the base material under vacuum, dried and sintered, and the mullite whisker-mullite composite coating is synthesized in situ under the catalysis of the metal fluoride mineralizer. 2. The composite coating according to claim 1, characterized in that the thickness of the mullite whisker-mullite composite coating is 1-5 μm. 3. A preparation method for a composite coating, comprising preparing mullite slurry, coating coating and coating sintering, characterized in that it is a preparation method for in-situ synthesis of mullite whiskers-mullite composite coating , the steps are as follows: (1) Prepare mullite slurry: aluminum raw material and quartz raw material are mixed according to the requirement of mullite stoichiometric ratio according to the mass ratio of alumina and silicon dioxide, and mixed with 30-50% water of the total mass of raw materials, Ball mill for 10 to 24 hours, then add metal fluoride mineralizer with 4-10% of the total mass of raw materials, ball mill for 1 to 2 hours, form a block green body, and keep the green body at 1400-1550°C for 1 hour to synthesize mullite , and then through pulverization and ball milling to obtain a mullite slurry with an average particle size of 0.5-1.5 μm; or, directly use industrial fused mullite as a raw material, add a metal fluoride mineralizer with a total mass of 4-10% of the raw material, and then Obtained by crushing and ball milling, (2) Coating coating: the substrate material to be coated is coated with a composite slurry of silica sol, aluminum sol and mullite slurry in a vacuum with a stoichiometric ratio consistent with the composition of mullite, or layer by layer in vacuum Sequential coating of silica sol, aluminum sol, mullite slurry, or vacuum sequential coating of silica sol, aluminum sol and metal fluoride supersaturated solution; As a raw material, the oxide is deposited by hydrolysis-co-precipitation method, and then coated with mullite slurry; the coated material is dried at 60-90°C, (3) Sintering the dried coating material at 1100-1500° C. for 1-3 hours to obtain an in-situ synthesized mullite whisker-mullite composite coating. 4. The preparation method of the composite coating according to claim 3, characterized in that: in-situ synthesis of mullite whiskers in the coating. 5. the preparation method of composite coating according to claim 3 is characterized in that: aluminum raw material is aluminum oxide, aluminum hydroxide, aluminum sol, aluminum ammonium sulfate, aluminum salt, clay, kaolin, or is made of aluminum-containing raw material one of them, or a mixture thereof. 6 . The preparation method of the composite coating according to claim 3 , wherein the metal fluoride mineralizer is AlF ₃ , CaF ₂ , BaF ₂ , or a mixture thereof. 7. The preparation method of the composite coating according to claim 3, characterized in that: the quartz raw material is white carbon black or silica sol, quartz powder.

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