CN110330344A - A method of high porosity silicon nitride ceramics is prepared based on selective laser sintering - Google Patents

Abstract

The invention discloses a method for preparing high-porosity silicon nitride ceramics based on laser selective sintering, which belongs to the field of inorganic non-metallic ceramics prepared by additive manufacturing technology. The method includes the steps of: (1) preparing nano-sintering aids by mechanical mixing Mix uniform composite powder with silicon nitride polyhollow spheres, or prepare composite powders of nano-sintering aids coated silicon nitride polyhollow spheres by chemical co-precipitation method; (2) Design CAD model and slice it Save it as an STL file and import it into the SLS forming equipment; (3) Use the SLS forming equipment to perform laser selective sintering on the composite powder to prepare pre-sintered silicon nitride ceramics; (4) After high-temperature sintering, prepare the porous structure of the CAD model Silicon nitride ceramics. After the composite powder is prepared by the method of the present invention, pre-fired silicon nitride ceramics are prepared by laser selective sintering, and high-porosity silicon nitride ceramics are prepared through a post-processing process, which can form complex structures and form pores without degumming. High rate.

Description

A method for preparing high-porosity silicon nitride ceramics based on laser selective sintering

technical field

The invention belongs to the field of inorganic non-metallic ceramics prepared by additive manufacturing technology, and more specifically relates to a method for preparing high-porosity silicon nitride ceramics based on laser selective sintering.

Background technique

Silicon nitride is a non-metallic inorganic compound with strong covalent bonds. It has excellent properties such as high strength, high hardness, high elastic modulus, low thermal expansion coefficient, self-lubrication and wear resistance, and has attracted extensive attention. Among them, porous silicon nitride ceramics also have some porous characteristics such as light weight, heat resistance, high specific surface area, and selective permeability to gas or liquid, and are "structure-function" integrated ceramic materials. It is expected to be applied in the radome material of aerospace vehicles in the future.

At present, the preparation of porous silicon nitride ceramics mainly adopts foaming method, under-fired method, pore-forming agent method, laser selective sintering and other methods. However, the pore structure of the porous silicon nitride ceramics prepared by the foaming method is uncontrollable; The use of organic substances requires debinding treatment and is not conducive to sintering densification and phase transformation; although laser selective sintering can form silicon nitride ceramics with complex structures, at present, organic substances with low melting points are mostly used as binders, which require debinding treatment. , the production process is long; and the current laser selective sintering process requires debinding treatment in the preparation of ceramic samples with complex structures such as lattice structures, which easily causes problems such as deformation, cracking, and collapse of samples with complex structures, and it is difficult to effectively form pores. High porosity silicon nitride ceramics with a rate of more than 70%.

Therefore, for the selective laser sintering process, it is necessary to find a way to efficiently form complex structures of high-porosity silicon nitride ceramics without binders.

Contents of the invention

In view of the above defects or improvement needs of the prior art, the present invention provides a method for preparing high-porosity silicon nitride ceramics based on laser selective sintering, the purpose of which is to effectively form complex structures with high porosity without binders (porosity of more than 70%) silicon nitride ceramics, thus solving the problem that the existing method of laser selective sintering cannot form silicon nitride ceramics with a porosity of more than 70%, and debinding treatment is required, the production process is long, and the lattice The technical problems of deformation, cracking and collapse are easy to occur when forming porous silicon nitride ceramics with complex structures.

In order to achieve the above object, the present invention provides a method for preparing high-porosity silicon nitride ceramics based on laser selective sintering, comprising the following steps:

(1) Prepare a homogeneously mixed composite powder of nano-sintering aids and silicon nitride polyhollow spheres by mechanical mixing, or prepare a composite powder of nano-sintering aids coated with silicon nitride polyhollow spheres by chemical co-precipitation ;

(2) Design the CAD model, save the CAD model as an STL file containing the CAD model data information after slicing the CAD model, and import the STL file into the laser selection sintering forming equipment;

(3) Lay the composite powder obtained in step (1) in the powder cylinder of the laser selective sintering (SLS) forming equipment, set the SLS forming process parameters, and combine the STL file imported in step (2) to perform laser selective sintering , printed layer by layer to prepare pre-sintered silicon nitride ceramics;

(4) Sintering the pre-sintered silicon nitride ceramics obtained in step (3) at a high temperature under a protective atmosphere to prepare the high-porosity silicon nitride ceramics with the CAD model structure.

Preferably, the nano-sintering aid includes one or more of nano-alumina, nano-yttrium oxide, nano-rare earth oxide and nano-silicon dioxide.

Preferably, the nanometer rare earth oxides include lanthanum oxide, neodymium oxide, cerium oxide and ytterbium oxide, and the nanometer sintering aid includes one or several nanometer rare earth oxides.

Preferably, the microstructure and properties of the prepared porous silicon nitride ceramics are regulated by adjusting the amount and type of the nano-sintering aid.

Preferably, in step (1), the mass fraction of the nano-sintering aid in the composite powder is 5%-30%.

Preferably, in step (1), when the mechanical mixing method is adopted, the powder mixing time is 12-24 hours.

Preferably, in step (1), the chemical co-precipitation method is: adding the silicon nitride polyhollow spheres into the salt solution intended to generate the nano-sintering aid for mixing, and after adding ammonia water to complete the reaction, carry out Suction filtration, drying, and calcination at 900-1200°C to obtain composite powder.

Preferably, the SLS forming process parameters are: laser power 3-15W, scanning speed 10-300mm/s, layer thickness 0.1-0.3mm, scanning distance 0.01-0.3mm.

Preferably, the process parameters of the high-temperature sintering are: a heating rate of 5-20° C./min, a sintering temperature of 1700-2000° C., and a holding time of 2-8 hours.

Preferably, the protective atmosphere is one or more of nitrogen, argon, and hydrogen.

Preferably, the CAD model is a honeycomb porous structure or a spatial topological structure or a complex functional structure with an internal interconnected pore structure.

Generally speaking, compared with the prior art, the above technical solutions conceived by the present invention can achieve the following beneficial effects:

1. The method provided by the present invention can prepare high-porosity silicon nitride ceramics with complex structures with a porosity of more than 70% based on the laser selective sintering process combined with the post-sintering treatment process; the method adopts a mechanical mixing method to prepare nano-sintering aid-nitrogen Silicon nitride polyhollow sphere composite powder, or prepare sintering aid coated silicon nitride polyhollow sphere composite powder by chemical co-precipitation method, prepare pre-sintered silicon nitride ceramics by laser selective sintering process, and further adopt high temperature sintering , to prepare porous silicon nitride ceramics with controllable microstructure and properties, which can realize porous silicon nitride ceramics with various structure and performance requirements; this method is innovative in material formulation and porous ceramic preparation process, and does not require bonding High-porosity silicon nitride ceramics with complex structures can be effectively formed using only a single agent; debinding treatment is not required, which simplifies the preparation process of porous ceramics, and effectively reduces the problems of deformation, cracking, and collapse when complex structures are formed.

2. The method provided by the present invention prepares pre-sintered porous silicon nitride ceramics based on the laser selective sintering process. By adjusting the amount and type of nano-sintering aids and the post-treatment process parameters, the microscopic properties of the prepared porous silicon nitride ceramics are improved. It has the characteristics of controllable macroscopic porous structure and microscopic pore structure, and can realize porous silicon nitride ceramics with various structural and performance requirements.

3. The method provided by the present invention can effectively form porous silicon nitride ceramics with complex structures such as lattices, such as honeycomb porous structures or spatial topological structures or complex functional structures (radiating fins, etc.) with internal interconnected pore structures, Effectively reduce the problems of deformation, cracking and collapse when these complex structures are formed.

4. The method provided by the present invention does not require molds, which reduces the manufacturing cost; since it does not need to carry out subsequent glue removal, environmental pollution is reduced.

Description of drawings

Fig. 1 is the flow chart of the method for preparing high-porosity silicon nitride ceramics based on laser selective sintering provided by the present invention;

Fig. 2 is an example of an octahedron space topology CAD model provided by the present invention;

Fig. 3 is the example of the honeycomb porous structure CAD model that the interior provided by the present invention has interconnected pore structure;

Figure 4(a) is a low magnification view of the microscopic morphology of the pre-sintered silicon nitride ceramic prepared in Example 1 of the present invention;

Figure 4(b) is a high-magnification view of the microscopic morphology of the pre-sintered silicon nitride ceramic prepared in Example 1 of the present invention;

Figure 5(a) is a low-magnification view of the microscopic morphology of the porous silicon nitride ceramic prepared in Example 1 of the present invention;

Fig. 5(b) is a high-magnification view of the microscopic morphology of the porous silicon nitride ceramic prepared in Example 1 of the present invention.

Detailed ways

In order to make the object, technical solution and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described here are only used to explain the present invention, not to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not constitute a conflict with each other.

The invention provides a method for preparing high-porosity silicon nitride ceramics based on laser selective sintering, specifically using nano-sintering aids as binders and sintering aids, based on SLS forming technology combined with slice data information of CAD models, nearly Pre-sintered porous silicon nitride ceramics are prepared by net shaping, and the porous silicon nitride ceramics are formed after sintering. As shown in Figure 1, the steps include:

(1) Prepare a homogeneously mixed composite powder of nano-sintering aids and silicon nitride polyhollow spheres by mechanical mixing, or prepare a composite powder of nano-sintering aids coated with silicon nitride polyhollow spheres by chemical co-precipitation .

Silicon nitride poly hollow spheres, the English full name is Si ₃ N ₄ poly-hollow microspheres, see the paper (Preparation of porous Si ₃ N ₄ ceramics via tailoring solid loading of Si ₃ N ₄ slurry and Si ₃ N ₄ poly-hollow microsphere content[ J]. Journal of Advanced Ceramics, 2015, 4(4): 260-266.).

The present invention can control the microstructure and performance of the porous ceramic green body by adjusting the type and amount of nano-sintering aids; since the sintering aids will affect the densification and phase transition in the sintering process of silicon nitride ceramics, such as sintering The addition of a large amount of additives may increase the phase transition and densification rate, increasing the density of silicon nitride ceramics, but the corresponding high-temperature mechanical properties may decrease. Therefore, the microstructure and performance of silicon nitride ceramic parts can be regulated by adjusting the amount and type of nano-sintering aids. Among them, the sintering aid is preferably one or more of nano-alumina, nano-yttrium oxide, nano-rare earth oxides (lanthanum oxide, neodymium oxide, cerium oxide, ytterbium oxide, etc.) and nano-silica.

Specifically, the composite powder for SLS forming is prepared by a mechanical mixing method, the amount of the nano-sintering additive accounts for 5% to 30% of the total mass of the composite powder, and the powder mixing time is 12 to 24 hours.

The composite powder for SLS forming is prepared by the chemical co-precipitation coating method. The specific method is: the salt solution corresponding to the nano-sintering aid based on the powder mass of 5% to 30% and the corresponding mass fraction of 70 to 95% of silicon nitride The ceramic powders are mixed, and after adding ammonia water to complete the reaction, suction filtration, drying, and calcination at 900-1200° C. are performed to obtain composite powder-coated silicon nitride ceramic powders.

(2) Optimally design the CAD (Computer Aided Design, Computer Aided Design) model of porous silicon nitride ceramics with a complex structure according to the requirements, slice the CAD model and save it as an STL file containing the CAD model data information, and import it into laser selective sintering forming equipment. The applicable CAD model of the present invention does not limit the shape, structure and size, and can also be optimized into a honeycomb porous structure with a connected pore structure inside or a spatial topological structure such as an octahedron or a dodecahedron or a functional device with a complex structure according to application requirements. Such as insulation sheet and so on. Figure 2 shows an example of an application-oriented optimized octahedron spatial topology CAD model, and Figure 3 shows an example of an application-oriented optimized honeycomb porous structure CAD model with interconnected pore structures inside.

STL is the standard file type used by most rapid prototyping systems. STL uses triangular meshes to represent 3D CAD models.

(3) Select the SLS forming process parameters, according to the STL file data information imported in step (2), perform laser selective sintering, and print layer by layer to prepare pre-sintered porous silicon nitride ceramics corresponding to the CAD structure.

Specifically, the SLS forming process parameters are preferably as follows: laser power of 3-15W, scanning speed of 10-300mm/s, layer thickness of 0.1-0.3mm, and scanning distance of 0.01-0.3mm.

(4) After the pre-sintered porous silicon nitride ceramic prepared in the step (3) is sintered at high temperature, a porous silicon nitride ceramic part with a special pore structure designed by the CAD model is prepared.

Specifically, the high-temperature sintering process parameters are preferably: heating rate 5-20°C/min, sintering temperature 1700-2000°C, holding time 2-8h, protective atmosphere of one or more of nitrogen, argon, and hydrogen .

In the present invention, silicon nitride polyhollow spheres are used as raw materials and pore-forming agents, and are mechanically mixed or chemically co-precipitated with nano-sintering aids to prepare composite powders, and pre-fired silicon nitride ceramics are prepared by direct laser selective sintering. And through the post-processing process to prepare silicon nitride ceramics, it has the characteristics of no need to deglue, complex structure can be formed, high porosity of formed parts, high dimensional accuracy and so on.

The method for preparing high-porosity silicon nitride ceramics based on laser selective sintering provided by the present invention will be further described in detail below with reference to the accompanying drawings and examples.

Example 1:

(1) In this example, silicon nitride polyhollow spheres and nano-sintering aids are chemically co-precipitated and coated to prepare a composite powder of silicon nitride polyhollow spheres coated with nano-sintering aids; specifically,

Mix 93g silicon nitride polyhollow spheres with a salt solution made up of 15g aluminum nitrate and 17g yttrium nitrate, add ammonia water until the reaction is complete, perform suction filtration, dry, and calcinate at 1000°C to obtain a uniformly mixed composite powder Body, that is, coated silicon nitride ceramic powder, wherein the nano-sintering aid accounts for 7% of the mass fraction of the composite powder;

(2) Design an application-oriented and optimized octahedral spatial topology CAD model, and convert the CAD model slice into an STL file, and import the STL file into the SLS forming equipment;

(3) On the SLS forming equipment, select the following SLS forming process parameters to form porous silicon nitride ceramic green bodies, that is, pre-sintered silicon nitride ceramics: laser power 9W, scanning speed 50mm/s, layer thickness 0.15mm , the scanning distance is 0.15 mm; the microscopic topography of the pre-sintered silicon nitride ceramic prepared in this embodiment is shown in Figure 4(a) and Figure 4(b).

(4) Sinter the porous silicon nitride ceramic green body obtained in step (3) at a high temperature in a nitrogen atmosphere. % porous Si ₃ N ₄ ceramic parts.

The microscopic morphology of the porous Si ₃ N ₄ ceramics prepared in this example with a porosity of 75% is shown in Fig. 5(a) and Fig. 5(b). Example 2:

(1) Mechanically mixing 70g of silicon nitride polyhollow spheres and 30g of nano-lanthanum oxide for 24 hours to obtain a composite powder, wherein the amount of nano-sintering aid is 30% of the total composite powder mass;

(2) Establish an application-oriented optimized honeycomb porous structure CAD model with interconnected pore structure inside, and convert the CAD model slice into an STL file, and import the STL file into the SLS forming equipment;

(3) On the SLS forming equipment, select the following SLS forming process parameters to form porous silicon nitride ceramic green bodies, that is, pre-sintered silicon nitride ceramics: laser power 12W, scanning speed 100mm/s, layer thickness 0.3mm , the scanning distance is 0.20mm;

(4) Sinter the porous silicon nitride ceramic green body obtained in step (3) at high temperature in an argon atmosphere. 73% porous Si ₃ N ₄ ceramic parts.

Example 3:

(1) Mechanically mixing 95g of silicon nitride polyhollow spheres and 5g of nano-silicon dioxide for 12 hours to obtain a composite powder, wherein the amount of nano-sintering aid is 5% of the total composite powder mass;

(2) Establish an application-optimized CAD model of a honeycomb porous structure with a connected pore structure inside, and convert the CAD model slice into an STL file, and import the STL file into the SLS forming equipment;

(3) On the SLS forming equipment, select the following SLS forming process parameters to form porous silicon nitride ceramic green bodies, that is, pre-sintered silicon nitride ceramics: laser power 3W, scanning speed 10mm/s, layer thickness 0.1mm , the scanning distance is 0.3mm;

(4) Sinter the porous silicon nitride ceramic green body obtained in step (3) at a high temperature in an argon atmosphere. The high temperature sintering process parameter is 10°C/min, the temperature is raised to 2000°C, and the temperature is kept for 4 hours. Finally, a porosity 80% porous Si ₃ N ₄ ceramic parts.

Example 4:

(1) Mix 93g silicon nitride polyhollow spheres, 64g aluminum nitrate, and 72g yttrium nitrate into a solution, add ammonia water until the reaction is complete, perform suction filtration, dry, and calcine at 1200°C to obtain a composite powder, That is, the composite powder of silicon nitride polyhollow spheres coated with nano-sintering aids, the amount of nano-sintering aids after coating accounts for 30% of the mass of the composite powder;

(2) Establish an application-oriented and optimized octahedral spatial topology CAD model, and convert the CAD model slice into an STL file, and import the STL file into the SLS forming equipment;

(3) On the SLS forming equipment, select the following SLS forming process parameters to form a porous silicon nitride ceramic green body: laser power 15W, scanning speed 300mm/s, layer thickness 0.20mm, scanning distance 0.01mm;

(4) The porous silicon nitride ceramic green body obtained in step (3) is subjected to high-temperature sintering in a hydrogen/argon mixed atmosphere, the high-temperature sintering process parameter is 15°C/min, the temperature is raised to 1800°C, and the temperature is kept for 6 hours, and finally prepared _Porous Si3N4 ceramic parts with 70% porosity _.

Example 5:

(1) Mechanically mix 90g of silicon nitride polyhollow spheres, 5g of nano-ytterbium oxide and 5g of nano-yttrium oxide for 18 hours to obtain a composite powder;

(2) Establish an application-oriented and optimized octahedral spatial topology CAD model, and convert the CAD model slice into an STL file, and import the STL file into the SLS forming equipment;

(3) On the SLS forming equipment, select the following SLS forming process parameters to form porous silicon nitride ceramic green bodies, that is, pre-sintered silicon nitride ceramics: laser power 5W, scanning speed 30mm/s, layer thickness 0.10mm , the scanning distance is 0.05mm;

(4) Carry out high-temperature sintering of the porous silicon nitride ceramic green body obtained in step (3) in a nitrogen atmosphere. % porous Si ₃ N ₄ ceramic parts.

Embodiment 6:

(1) Mix 93g of silicon nitride polyhollow spheres, 11g of aluminum nitrate, and 12g of yttrium nitrate into a solution, add ammonia water until the reaction is complete, perform suction filtration, dry, and calcinate at 900°C to obtain a composite powder, That is, for the coated silicon nitride ceramic powder, the amount of sintering aid added after coating is 5% of the total composite powder mass;

(2) Establish an application-oriented and optimized octahedron spatial topology CAD model, and convert the CAD model slice into an STL file, and import the STL file into the SLS forming equipment;

(3) On the SLS forming equipment, select the following SLS forming process parameters to form a porous silicon nitride ceramic green body: laser power 8W, scanning speed 200mm/s, layer thickness 0.30mm, scanning distance 0.10mm;

(4) Carry out high-temperature sintering of the porous silicon nitride ceramic green body obtained in step (3) in a nitrogen atmosphere. % porous Si ₃ N ₄ ceramic parts.

It is easy for those skilled in the art to understand that the above descriptions are only preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements and improvements made within the spirit and principles of the present invention, All should be included within the protection scope of the present invention.

Claims (10)

1. a kind of method for preparing high porosity silicon nitride ceramics based on selective laser sintering, which is characterized in that including walking as follows It is rapid:

(1) the uniformly mixed composite granule of sintering of nano-material auxiliary agent and the poly- hollow sphere of silicon nitride is prepared using mechanical mixing, or Person prepares the composite granule that sintering of nano-material auxiliary agent coats the poly- hollow sphere of silicon nitride by chemical coprecipitation；

(2) Design CAD model saves as the STL text comprising CAD model data information after carrying out slicing treatment to the CAD model Part imports the stl file in selective laser sintering former；

(3) composite granule that step (1) obtains is layed in selective laser sintering former powder cylinder, setting SLS shapes work Skill parameter carries out selective laser sintering, successively prints in conjunction with the stl file imported in step (2), prepares pre-sintering nitridation Silicon ceramics；

(4) the pre-sintering silicon nitride ceramics obtained to step (3) carries out high temperature sintering under protective atmosphere, prepares the CAD The high porosity silicon nitride ceramics of model structure.

2. a kind of method that high porosity silicon nitride ceramics is prepared based on selective laser sintering according to claim 1, It is characterized in that, the sintering of nano-material auxiliary agent includes nano aluminium oxide, nano yttrium oxide, oxide nano rare earth and nanometer titanium dioxide One or more of silicon.

3. a kind of method that high porosity silicon nitride ceramics is prepared based on selective laser sintering according to claim 2, It is characterized in that, the oxide nano rare earth includes lanthana, neodymia, cerium oxide and ytterbium oxide, the sintering of nano-material auxiliary agent Including one such or several oxide nano rare earths.

4. a kind of method that high porosity silicon nitride ceramics is prepared based on selective laser sintering according to claim 2 or 3, It is characterized in that, by adjusting the additional amount and type of the sintering of nano-material auxiliary agent, to the high porosity silicon nitride being prepared The microstructure and properties of ceramics are regulated and controled.

5. a kind of method that high porosity silicon nitride ceramics is prepared based on selective laser sintering according to claim 1, It is characterized in that, in step (1), the mass fraction of the sintering of nano-material auxiliary agent is 5%~30%.

6. a kind of method that high porosity silicon nitride ceramics is prepared based on selective laser sintering according to claim 5, Be characterized in that, in step (1), when using mechanical mixing, mixing time be 12~for 24 hours.

7. a kind of method that high porosity silicon nitride ceramics is prepared based on selective laser sintering according to claim 5, It is characterized in that, in step (1), the chemical coprecipitation are as follows: by the poly- hollow sphere of the silicon nitride, the quasi- generation nanometer is added Mixed in the salting liquid of sintering aid, be added ammonium hydroxide fully reacting after, filtered, dried, and 900~1200 DEG C into Row calcining obtains composite granule.

8. -7 any a kind of side for preparing high porosity silicon nitride ceramics based on selective laser sintering according to claim 1 Method, which is characterized in that the SLS forming parameters are as follows: 3~15W of laser power, 10~300mm/s of scanning speed are layered layer Thickness 0.1~0.3mm, 0.01~0.3mm of sweep span.

9. -7 any a kind of side for preparing high porosity silicon nitride ceramics based on selective laser sintering according to claim 1 Method, which is characterized in that the technological parameter of the high temperature sintering are as follows: 5~20 DEG C/min of heating rate, sintering temperature be 1700~ 2000 DEG C, soaking time is 2~8h.

10. -7 any described a kind of preparing high porosity silicon nitride ceramics based on selective laser sintering according to claim 1 Method, which is characterized in that the protective atmosphere is one or more of nitrogen, argon gas, hydrogen.