CN111659855A - Preparation method of mullite whisker reinforced ceramic shell for directional solidification of NbSi alloy generated in situ - Google Patents
Preparation method of mullite whisker reinforced ceramic shell for directional solidification of NbSi alloy generated in situ Download PDFInfo
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22C—FOUNDRY MOULDING
- B22C9/00—Moulds or cores; Moulding processes
- B22C9/22—Moulds for peculiarly-shaped castings
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22C—FOUNDRY MOULDING
- B22C1/00—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds
- B22C1/02—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds characterised by additives for special purposes, e.g. indicators, breakdown additives
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22C—FOUNDRY MOULDING
- B22C1/00—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds
- B22C1/02—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds characterised by additives for special purposes, e.g. indicators, breakdown additives
- B22C1/10—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds characterised by additives for special purposes, e.g. indicators, breakdown additives for influencing the hardening tendency of the mould material
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22C—FOUNDRY MOULDING
- B22C1/00—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds
- B22C1/16—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds characterised by the use of binding agents; Mixtures of binding agents
- B22C1/18—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds characterised by the use of binding agents; Mixtures of binding agents of inorganic agents
- B22C1/181—Cements, oxides or clays
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22C—FOUNDRY MOULDING
- B22C1/00—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds
- B22C1/16—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds characterised by the use of binding agents; Mixtures of binding agents
- B22C1/18—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds characterised by the use of binding agents; Mixtures of binding agents of inorganic agents
- B22C1/186—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds characterised by the use of binding agents; Mixtures of binding agents of inorganic agents contaming ammonium or metal silicates, silica sols
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22C—FOUNDRY MOULDING
- B22C3/00—Selection of compositions for coating the surfaces of moulds, cores, or patterns
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22C—FOUNDRY MOULDING
- B22C9/00—Moulds or cores; Moulding processes
- B22C9/02—Sand moulds or like moulds for shaped castings
- B22C9/04—Use of lost patterns
- B22C9/043—Removing the consumable pattern
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22C—FOUNDRY MOULDING
- B22C9/00—Moulds or cores; Moulding processes
- B22C9/12—Treating moulds or cores, e.g. drying, hardening
Abstract
A preparation method of a ceramic shell for directional solidification of NbSi alloy reinforced by in-situ generated mullite whiskers comprises the steps of immersing a blade wax pattern module into a surface layer coating, uniformly coating the surface layer coating on the surface of a wax pattern, and then performing sand spraying and drying on the wax pattern module to finish the preparation of the surface layer; coating and hanging a back layer coating on the blade wax pattern module, then spraying sand and drying the investment pattern module, repeating the process for a plurality of times, and completing the preparation of the back layer after the required thickness of the ceramic shell is reached; sealing the slurry by adopting the back layer coating, and drying to finish the preparation of the sealing slurry layer; and finally, dewaxing and roasting the ceramic shell at high temperature to obtain the ceramic shell. The ceramic shell of the invention can bear 1750 ℃ high temperature with little deformation and can not be opened under the condition of directional solidification ultrahigh temperature gradient.
Description
Technical Field
The invention relates to a technology in the field of investment casting, in particular to a method for preparing a mullite whisker reinforced ceramic shell for directional solidification of NbSi alloy.
Background
The niobium-silicon (NbSi) alloy has high melting point (not less than 1750 ℃) and low density (not more than 7.2 g/cm)3) The temperature bearing capacity is higher than that of nickel-based alloy by about 250 ℃, so that the material is one of the most potential candidate materials for manufacturing the high thrust-weight ratio aircraft engine blade, but the temperature (1750-.
Disclosure of Invention
Aiming at the defects in the prior art, the invention provides a method for preparing a mullite whisker in-situ generated reinforced NbSi alloy ceramic shell for directional solidification, and the ceramic shell can bear 1750 ℃ high temperature and has small deformation, and does not open under the ultrahigh temperature gradient condition of directional solidification, so the ceramic shell has wide application prospect in the preparation of new generation aeroengine turbine blades, and can be applied to the directional solidification precision casting of turbine blades of scramjet engines and high-performance gas turbines.
The invention is realized by the following technical scheme:
the invention relates to a preparation method of a ceramic shell for directional solidification of NbSi alloy enhanced by in-situ generation of mullite whiskers, which comprises the steps of immersing a blade wax pattern module into a surface layer coating, uniformly coating the surface layer coating on the surface of a wax pattern, and then spraying sand and drying the wax pattern module to finish the preparation of the surface layer; coating and hanging a back layer coating on the blade wax pattern module, then spraying sand and drying the investment pattern module, repeating the process for a plurality of times, and completing the preparation of the back layer after the required thickness of the ceramic shell is reached; sealing the slurry by adopting the back layer coating, and drying to finish the preparation of the sealing slurry layer; and finally, dewaxing and roasting the ceramic shell at high temperature to obtain the ceramic shell.
The high-temperature roasting refers to: and (3) dewaxing the dried ceramic shell in a high-pressure steam dewaxing kettle, roasting in the atmosphere for 2 hours, and cooling to room temperature in air, wherein the roasting temperature is preferably 1000 ℃.
The surface coating comprises: yttrium oxide sol serving as a binder, electric melting yttrium oxide ceramic powder serving as a surface layer refractory filler and auxiliary materials, wherein the proportion of the binder in the surface layer coating is 20-30 wt.%; the proportion of ceramic powder is 69.6-79.6 wt.%.
The auxiliary materials comprise a wetting agent and a defoaming agent, the weight ratio of the wetting agent to the defoaming agent is 1:1, and the total weight ratio of the wetting agent to the defoaming agent in the surface layer coating is 0.4 wt.%.
The back layer coating comprises: alkaline silica sol as binder, fused corundum powder as refractory filler, aluminium hydroxide and lignin fiber or lignocellulose as filler, aluminium fluoride (AlF) as mullite promoter3) And an auxiliary material, wherein: the proportion of binder in the backing coating is 20-30 wt.%; 46.8-76.8 wt.% of fused corundum powder and 1-10 wt.% of aluminum hydroxide; 1-10 wt.% lignin fiber; 1-3 wt.% aluminum fluoride; the adjuvant is 0.2 wt.%.
The auxiliary materials comprise defoaming agents.
The wetting agent and the defoaming agent are common additives sold in the wax mould casting industry.
The viscosity of the surface layer coating is 18-40s measured by a No. 4 Won cup.
The pH value of the yttria sol in the surface layer coating is 3.5-4.5, and the weight percentage of the yttria content is more than or equal to 15%.
The granularity of the electric melting yttrium oxide powder in the surface layer coating is 200-325 meshes, and the weight percentage of the content of yttrium oxide is more than or equal to 99 percent.
The granularity of the electric melting yttrium oxide sand in the surface layer coating is 80-120 meshes, and the weight percentage of the content of yttrium oxide is more than or equal to 99 percent.
The coating of the backing layer has a viscosity of 10-30s measured using a # 5 beaker.
The pH value of the silica sol in the back layer coating is 8-10, and the weight percentage of the silica sol is more than or equal to 30 percent.
The granularity of the fused corundum powder in the back layer coating is 200-325 meshes, and the weight percentage of the content of the alumina is more than or equal to 99 percent.
The granularity of the aluminum hydroxide in the back layer coating is 325-600 meshes, and the weight percentage of the content of the aluminum hydroxide is more than or equal to 99 percent.
The lignin fiber in the back layer coating is an organic flocculent fiber substance obtained by chemical treatment and mechanical processing of natural wood, and the length of the organic flocculent fiber substance is 100-500 mu m.
The aluminum fluoride in the back layer coating is analytically pure anhydrous aluminum fluoride.
The granularity of the sand of the fused corundum powder in the back layer coating is 46 meshes and 36 meshes, and the weight percentage of the content of the alumina is more than or equal to 99 percent.
Preferably, the top coat coating, sand pouring and back coat coating, sand pouring processes are repeated several times before high temperature firing until the desired shell thickness is achieved. Wherein: the surface layer sand spraying adopts fused yttrium oxide sand of 80-120 meshes; the granularity of the electrofused corundum adopted by the first sand spraying of the back layer is 46 meshes; the granularity of the electro-fused corundum sand adopted in the subsequent sand spraying is 24 meshes.
The blade wax pattern module is preferably cleaned by a cleaning agent, rinsed by clean water and dried by compressed air.
The invention relates to the ceramic shell prepared by the method, wherein the bending strength of a wet blank is 7.93-9.26 MPa; the flexural strength of the shell after being baked for 2 hours at 1000 ℃ is 6.59-8.34MPa at high temperature (1500 ℃); the deflection of the shell after roasting for 2 hours at 1000 ℃ is 0.5-0.7mm at high temperature (1500 ℃); after being roasted for 2 hours at 1500 ℃, the residual flexural strength of the shell is reduced to room temperature and is 3.24 to 4.12 MPa; the air permeability of the shell after the shell is baked for 2 hours at 1000 ℃ is 26.98-29.44m4/N·min。
Technical effects
The invention integrally solves the technical problems of synchronously improving the high-temperature strength and the high-temperature creep resistance of the ceramic shell for directionally solidifying the NbSi alloy, and synchronously improving the air permeability and the collapsibility of the ceramic shell. Compared with the prior art, the invention has the technical effects that:
(1) the aluminum hydroxide in the back layer can be decomposed to form gamma-Al at about 300 DEG C2O3At 950 deg.c, it will react with SiO in silica sol2Chemical reaction is carried out to generate mullite crystal nucleus 3Al2O3·2SiO2The growth and development of mullite nuclei at higher temperatures during which AlF3The mullite crystal nucleus is promoted to grow along the c-axis direction under the action of a generated gas phase, so that the mullite crystal nucleus axially grows into the crystal whisker, the mullite crystal whisker formed by the in-situ reaction is tightly combined with other substances, and the high-temperature strength and the high-temperature creep resistance of the shell are obviously improved.
(2) The selected lignin fiber is an organic flocculent fiber substance obtained by chemical treatment and mechanical processing of natural renewable wood, is nontoxic, tasteless, pollution-free and radioactivity-free, and is easy to disperse in water; due to the capillary action of the lignin fiber structure, moisture in the system is rapidly transmitted to the surface and the interface of the back layer coating, so that the moisture in the coating is uniformly distributed, the skinning phenomenon is obviously reduced, the bonding strength and the surface strength of the back layer of the shell are obviously improved, the mechanism also obviously plays a role in cracking resistance due to the reduction of tension in the drying process, and meanwhile, the wet strength during shell dewaxing can be obviously improved, and shell cracking is inhibited; in the high-temperature roasting process of the shell, the lignin fiber is completely burnt out without residue, so that fine pores are formed inside the shell back layer, the residual strength of the shell is reduced, the deformability and collapsibility of the shell are improved, and the ventilation performance of the shell is improved. In addition, the lignin fiber is very cheap and easy to obtain.
Drawings
FIG. 1 is a schematic view of a ceramic shell layer, backing layer and a grout layer;
FIG. 2 is a photograph of the microstructure of lignocellulose;
FIG. 3 is a microstructure photograph of a (unfired) cross-section of a wood-cellulose containing shell backing layer;
FIG. 4 is a micrograph of a cross-section of a fired ceramic type backing layer containing mullite whiskers.
Detailed Description
Example 1
In the embodiment, the preparation of the ceramic shell for directional solidification of the niobium-silicon alloy blade is realized by the following steps:
a. preparing a shell surface layer coating: weighing 20kg of yttrium oxide sol, placing the yttrium oxide sol into a slurry preparation barrel, starting a stirrer, adding 79.6kg of electric melting yttrium oxide powder into the slurry preparation barrel in batches under the stirring condition, then slowly adding 0.2kg of wetting agent and defoaming agent respectively, and continuously stirring for 1h to obtain the surface coating. And transferring the surface coating to an L-shaped slurry dipping barrel for later use.
b. Preparation of the shell back layer coating: weighing 20kg of silica sol, placing the silica sol into a slurry preparation barrel, starting a stirrer, and stirring 76.8kg of fused corundum powder, 1kg of aluminum hydroxide, 1kg of lignin fiber and 1kg of AlF in batches3And 0.2kg of defoaming agent is added into the slurry preparation barrel, and the mixture is stirred for 2 hours, so that the back layer coating is obtained. The backing coating was transferred to an L-dip tank for future use.
c. And immersing the cleaned blade wax mould module dried by compressed air into the prepared surface layer coating, uniformly coating the coating on the wax mould, uniformly scattering the surface layer on the coating on the surface of the wax mould by using 80-120 meshes of electrofused yttrium oxide sand, and naturally drying at room temperature. And then, the module is subjected to slurry dipping of the back layer coating, the back layer is uniformly scattered on the module by using the electro-fused corundum sand, wherein the back layer sand of the first layer is 46 meshes, the back layer sand of the second layer is 24 meshes, the full drying is carried out under the ventilation condition, the slurry dipping and sand scattering process of scattering the electro-fused corundum sand of 24 meshes is repeated for 4 times, and finally, the back layer coating is applied for slurry sealing and then the drying is carried out. And (3) putting the dried ceramic shell into a high-pressure steam dewaxing kettle for dewaxing, then roasting in air at 1000 ℃ for 2h, and air-cooling to room temperature to obtain the ceramic shell for directional solidification of the niobium-silicon alloy blade.
In the embodiment, the green compact rupture strength of the ceramic shell for directional solidification of the niobium-silicon alloy blade prepared by the method is 7.93 MPa; the flexural strength of the shell after being baked for 2 hours at 1000 ℃ at high temperature (1500 ℃) is 6.59 MPa; the deflection of the shell after roasting for 2 hours at 1000 ℃ is 0.7mm at high temperature (1500 ℃); after being roasted for 2 hours at 1500 ℃, the residual flexural strength of the shell is reduced to room temperature and is 3.24 MPa; the air permeability of the shell after the shell is baked for 2 hours at 1000 ℃ is 26.98m4/N·min。
Example 2
In the embodiment, the preparation of the ceramic shell for directional solidification of the niobium-silicon alloy blade is realized by the following steps:
a. preparing a shell surface layer coating: weighing 25kg of yttrium oxide sol, placing the yttrium oxide sol into a slurry preparation barrel, starting a stirrer, adding 74.6kg of electric melting yttrium oxide powder into the slurry preparation barrel in batches under the stirring condition, then slowly adding 0.2kg of wetting agent and defoaming agent respectively, and continuously stirring for 1h to obtain the surface coating. And transferring the surface coating to an L-shaped slurry dipping barrel for later use.
b. Preparation of the shell back layer coating: weighing 30kg of silica sol, placing the silica sol into a slurry preparation barrel, starting a stirrer, and stirring 57.8kg of fused corundum powder, 5kg of aluminum hydroxide, 5kg of lignin fiber and 2kg of AlF in batches3And 0.2kg of defoaming agent is added into the slurry preparation barrel, and the mixture is stirred for 2 hours, so that the back layer coating is obtained. The backing coating was transferred to an L-dip tank for future use.
c. And immersing the cleaned blade wax mould module dried by compressed air into the prepared surface layer coating, uniformly coating the coating on the wax mould, uniformly scattering the surface layer on the coating on the surface of the wax mould by using 80-120 meshes of electrofused yttrium oxide sand, and naturally drying at room temperature. And then, the module is subjected to slurry dipping of the back layer coating, the back layer is uniformly scattered on the module by using the electro-fused corundum sand, wherein the back layer sand of the first layer is 46 meshes, the back layer sand of the second layer is 24 meshes, the full drying is carried out under the ventilation condition, the slurry dipping and sand scattering process of scattering the electro-fused corundum sand of 24 meshes is repeated for 4 times, and finally, the back layer coating is applied for slurry sealing and then the drying is carried out. And (3) putting the dried ceramic shell into a high-pressure steam dewaxing kettle for dewaxing, then roasting in air at 1000 ℃ for 2h, and air-cooling to room temperature to obtain the ceramic shell for directional solidification of the niobium-silicon alloy blade.
In the embodiment, the green compact rupture strength of the ceramic shell for directional solidification of the niobium-silicon alloy blade prepared by the method is 9.26 MPa; the flexural strength of the shell after being baked for 2 hours at 1000 ℃ at high temperature (1500 ℃) is 8.34 MPa; the deflection of the shell after roasting for 2 hours at 1000 ℃ is 0.5mm at high temperature (1500 ℃); after being roasted for 2 hours at 1500 ℃, the residual flexural strength of the shell is reduced to room temperature and is 3.98 MPa; the air permeability of the shell after the shell is baked for 2 hours at 1000 ℃ is 28.56m4/N·min。
Example 3
In the embodiment, the preparation of the ceramic shell for directional solidification of the niobium-silicon alloy blade is realized by the following steps:
a. preparing a shell surface layer coating: weighing 30kg of yttrium oxide sol, placing the yttrium oxide sol into a slurry preparation barrel, starting a stirrer, adding 69.6kg of electric melting yttrium oxide powder into the slurry preparation barrel in batches under the stirring condition, then slowly adding 0.2kg of wetting agent and defoaming agent respectively, and continuously stirring for 1h to obtain the surface coating. And transferring the surface coating to an L-shaped slurry dipping barrel for later use.
b. Preparation of the shell back layer coating: weighing 40kg of silica sol, placing the silica sol into a slurry preparation barrel, starting a stirrer, and stirring 36.8kg of fused corundum powder, 10kg of aluminum hydroxide, 10kg of lignin fiber and 3kg of AlF in batches3And 0.2kg of defoaming agent is added into the slurry preparation barrel, and the mixture is stirred for 2 hours, so that the back layer coating is obtained. The backing coating was transferred to an L-dip tank for future use.
c. And immersing the cleaned blade wax mould module dried by compressed air into the prepared surface layer coating, uniformly coating the coating on the wax mould, uniformly scattering the surface layer on the coating on the surface of the wax mould by using 80-120 meshes of electrofused yttrium oxide sand, and naturally drying at room temperature. And then, the module is subjected to slurry dipping of the back layer coating, then the back layer is uniformly scattered on the module by using electro-fused corundum sand, wherein the first layer of back layer sand is 46 meshes, the second layer of back layer sand is 24 meshes, the full drying is carried out under the ventilation condition, the process is repeated for 4 times, and finally, the back layer coating is applied for sealing slurry, and then the drying is carried out. And (3) putting the dried ceramic shell into a high-pressure steam dewaxing kettle for dewaxing, then roasting in air at 1000 ℃ for 2h, and air-cooling to room temperature to obtain the ceramic shell for directional solidification of the niobium-silicon alloy blade.
In the embodiment, the green compact rupture strength of the ceramic shell for directional solidification of the niobium-silicon alloy blade prepared by the method is 8.43 MPa; the flexural strength of the shell after being baked for 2 hours at 1000 ℃ at high temperature (1500 ℃) is 7.32 MPa; the deflection of the shell after roasting for 2 hours at 1000 ℃ is 0.6mm at high temperature (1500 ℃); after being roasted for 2 hours at 1500 ℃, the residual flexural strength of the shell is reduced to room temperature and is 3.24 MPa; the air permeability of the shell after the shell is baked for 2 hours at 1000 ℃ is 29.44m4/N·min。
According to the invention, the lignin fiber is applied to the preparation of the ceramic shell for the first time so as to improve the wet strength, air permeability and collapsibility of the shell; the mullite whisker generated in situ is used as a reinforcing phase for the first time to improve the high-temperature strength and the high-temperature creep resistance of the shell.
Compared with the prior art, the bending strength of the wet blank prepared by the method is 7.93-9.26 MPa; the flexural strength of the shell after being baked for 2 hours at 1000 ℃ is 6.59-8.34MPa at high temperature (1500 ℃); the deflection of the shell after roasting for 2 hours at 1000 ℃ is 0.5-0.7mm at high temperature (1500 ℃); after being roasted for 2 hours at 1500 ℃, the residual flexural strength of the shell is reduced to room temperature and is 3.24 to 4.12 MPa; the air permeability of the shell after the shell is baked for 2 hours at 1000 ℃ is 26.98-29.44m4min/N. Compared with the prior art, the wet blank rupture strength of the traditional fused corundum ceramic shell is 4.78 MPa; the flexural strength of the shell after being baked for 2 hours at 1000 ℃ at high temperature (1500 ℃) is 5.15 MPa; the deflection of the shell after roasting for 2 hours at 1000 ℃ is 1.83mm at high temperature (1500 ℃); after being roasted for 2 hours at 1500 ℃, the residual flexural strength of the shell is reduced to room temperature and is 4.78 MPa; the air permeability of the shell after the shell is baked for 2 hours at 1000 ℃ is 11.24m4/N·min。
The foregoing embodiments may be modified in many different ways by those skilled in the art without departing from the spirit and scope of the invention, which is defined by the appended claims and all changes that come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.
Claims (9)
1. A preparation method of a ceramic shell for directional solidification of NbSi alloy reinforced by in-situ generated mullite whiskers is characterized in that a blade wax pattern module is immersed into a surface layer coating to uniformly coat the surface layer coating on the surface of a wax pattern, and then the wax pattern module is subjected to sand spraying and drying to complete the preparation of the surface layer; coating and hanging a back layer coating on the blade wax pattern module, then spraying sand and drying the investment pattern module, repeating the process for a plurality of times, and completing the preparation of the back layer after the required thickness of the ceramic shell is reached; sealing the slurry by adopting the back layer coating, and drying to finish the preparation of the sealing slurry layer; finally, dewaxing and roasting the ceramic shell at high temperature to obtain the ceramic shell;
the surface coating comprises: yttrium oxide sol as a binder, electric melting yttrium oxide ceramic powder as a refractory filler of a surface layer and auxiliary materials;
the back layer coating comprises: alkaline silica sol as binder, fused corundum powder as refractory filler, aluminium hydroxide and lignin fiber as filler, AlF as mullite promoter3And an auxiliary material.
2. The method for preparing the ceramic shell for directional solidification of the NbSi alloy reinforced by in-situ generated mullite whiskers as claimed in claim 1, wherein the high-temperature roasting is as follows: and (3) putting the dried ceramic shell into a high-pressure steam dewaxing kettle for dewaxing, then roasting for 2 hours in the atmosphere, and cooling to room temperature in the air to obtain the ceramic shell.
3. The method for preparing the ceramic shell for directional solidification of the NbSi alloy reinforced by in-situ generated mullite whiskers as claimed in claim 1, wherein the proportion of the binder in the surface coating is 20-30 wt.%; the proportion of ceramic powder is 69.6-79.6 wt.%.
4. The method for preparing the ceramic shell for directional solidification of the NbSi alloy reinforced by in-situ generated mullite whiskers, as claimed in claim 3, wherein the auxiliary materials in the surface coating comprise a wetting agent and an antifoaming agent in a weight ratio of 1:1, and the total weight ratio of the auxiliary materials in the surface coating is 0.4 wt.%.
5. The method of claim 1, wherein the back layer coating comprises 20-30 wt.% of a binder; 46.8-76.8 wt.% of fused corundum powder and 1-10 wt.% of aluminum hydroxide; 1-10 wt.% lignin fiber; 1-3 wt.% of a mullite promoter; the adjuvant is 0.2 wt.%.
6. The method of claim 5, wherein the back layer coating comprises an anti-foaming agent as an auxiliary material.
7. The method for preparing the ceramic shell for directional solidification of the NbSi alloy reinforced by in-situ generated mullite whiskers according to claim 1 or 3, wherein the pH value of yttria sol in the surface coating is 3.5-4.5, and the content of yttria is more than or equal to 15% by weight;
the granularity of the electric melting yttrium oxide powder in the surface layer coating is 200-325 meshes, and the weight percentage of the content of yttrium oxide is more than or equal to 99 percent;
the granularity of the electric melting yttrium oxide sand in the surface layer coating is 80-120 meshes, and the weight percentage of the content of yttrium oxide is more than or equal to 99 percent;
the pH value of silica sol in the back layer coating is 8-10, and the weight percentage of the content of silica is more than or equal to 30 percent;
the granularity of the fused corundum powder in the back layer coating is 200-325 meshes, and the weight percentage of the content of the alumina is more than or equal to 99 percent;
the granularity of the aluminum hydroxide in the back layer coating is 325-600 meshes, and the weight percentage of the content of the aluminum hydroxide is more than or equal to 99 percent;
the lignin fiber in the back layer coating is an organic flocculent fiber substance obtained by chemical treatment and mechanical processing of natural wood, and the length of the organic flocculent fiber substance is 100-500 mu m;
the aluminum fluoride in the back layer coating is analytically pure anhydrous aluminum fluoride;
the granularity of the sand of the fused corundum powder in the back layer coating is 46 meshes and 36 meshes, and the weight percentage of the content of the alumina is more than or equal to 99 percent.
8. The method for preparing the mullite whisker reinforced NbSi alloy directional solidification ceramic shell as claimed in claim 1, wherein the steps of coating and sand pouring of the surface layer and coating and sand pouring of the back layer are repeated for several times before high-temperature roasting until the required shell thickness is reached, wherein: the granularity of the electrofused corundum adopted by the first sand spraying of the back layer is 46 meshes; the granularity of the electro-fused corundum sand adopted in the subsequent sand spraying is 36 meshes.
9. A ceramic shell mould prepared by the method of any one of claims 1 to 8, wherein the green compact has a flexural strength of 7.93 to 9.26 MPa; the flexural strength of the shell after being roasted for 2 hours at 1000 ℃ is 6.59-8.34MPa at high temperature; the deflection of the shell after roasting for 2 hours at 1000 ℃ is 0.5-0.7mm at high temperature; after being roasted for 2 hours at 1500 ℃, the residual flexural strength of the shell is reduced to room temperature and is 3.24 to 4.12 MPa; the air permeability of the shell after the shell is baked for 2 hours at 1000 ℃ is 26.98-29.44m4/N·min。
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