阅读说明：本技术 一种高精度氮化硅陶瓷微球及其制备方法和应用 (High-precision silicon nitride ceramic microsphere and preparation method and application thereof ) 是由 张伟儒 公平 徐金梦 王文雪 孙峰 董廷霞 高翔 宋健 吕沛远 于 2021-10-26 设计创作，主要内容包括：本发明提供了一种高精度氮化硅陶瓷微球及其制备方法和应用,属于陶瓷球制备技术领域。本发明解决了氮化硅陶瓷微球存在气孔、异常大晶粒、表面凹坑及雪花等缺陷,实现了氮化硅陶瓷微球的高精度制备。实施例的结果表明,本发明制备的高精度氮化硅陶瓷微球维氏硬度HV10最高达到1524kg/mm～(2),断裂韧性最高达到8.6MPa·m～(1/2),表面粗糙度0.006μm≤Ra≤0.008μm,球直径变动量0.03μm≤V-(Dw)≤0.08μm,球形误差0.03μm≤△S-(ph)≤0.08μm,各项性能指标均符合GB/T308.2-2010/ISO3290-2：2008《滚动轴承珠第2部分：氮化硅陶瓷微珠》标准要求的G3级标准要求。(The invention provides a high-precision silicon nitride ceramic microsphere and a preparation method and application thereof, belonging to the technical field of ceramic ball preparation. The invention solves the defects of air holes, abnormal large crystal grains, surface pits, snowflakes and the like of the silicon nitride ceramic microspheres and realizes the high-precision preparation of the silicon nitride ceramic microspheres. The results of the examples show that the Vickers hardness HV10 of the high-precision silicon nitride ceramic microspheres prepared by the invention can reach 1524kg/mm at most 2 The highest fracture toughness reaches 8.6 MPa.m 1/2 Ra is more than or equal to 0.006 mu m and less than or equal to 0.008 mu m in surface roughness, and V is more than or equal to 0.03 mu m in ball diameter variation Dw Less than or equal to 0.08 mu m, and the spherical error is less than or equal to 0.03 mu m ph Less than or equal to 0.08 mu m, and all performance indexes of the alloy meet GB/T308.2-2010/ISO 3290-2: 2008 "rolling bearing bead part 2: the standard requirement of G3 grade of the standard requirement of silicon nitride ceramic micro-beads.)

1. A preparation method of high-precision silicon nitride ceramic microspheres is characterized by comprising the following steps:

carrying out ball milling and mixing on the silicon nitride powder, the sintering aid and the dispersion liquid to obtain mixed feed liquid;

carrying out spray granulation on the mixed feed liquid to obtain granulation powder;

pressing and molding the granulation powder to obtain a pressed silicon nitride ceramic microsphere biscuit;

loading the pressed silicon nitride ceramic microsphere biscuit and the large-size silicon nitride ceramic biscuit ball into a crucible to sequentially perform atmosphere pressure sintering and hot isostatic pressing sintering to obtain a silicon nitride blank ball; the diameter of the large-size silicon nitride ceramic biscuit ball is 5-50 times of that of the silicon nitride ceramic microsphere biscuit; the crucible is a multilayer side wall crucible, and the multilayer side wall sequentially comprises a graphite outer wall, an intermediate wall and an inner wall from outside to inside; the extension direction of the middle wall and the extension direction of the inner wall are consistent with the extension direction of the outer wall of the crucible, and the middle wall and the inner wall are made of silicon nitride; the gap between the side walls of the adjacent layers is larger than the diameter of the silicon nitride ceramic blank ball with larger size;

and grinding the silicon nitride blank ball to obtain the high-precision silicon nitride ceramic microsphere.

2. The method according to claim 1, wherein the larger-sized silicon nitride ceramic green body balls have a diameter of 20 to 35 mm; the diameter of the pressed silicon nitride ceramic microsphere biscuit is 0.8-4 mm.

3. The method of claim 1 or 2, wherein the mass ratio of the green compact of pressed silicon nitride ceramic microspheres to the green compact of larger silicon nitride ceramic spheres is 1: (1-3).

4. The preparation method according to claim 1, wherein the temperature of the atmosphere pressure sintering is 1700-1800 ℃, the holding time is 1-3 h, and the atmosphere pressure sintering is carried out under the protection of nitrogen.

5. The preparation method of claim 1, wherein the hot isostatic pressing sintering temperature is 1800-1850 ℃, the holding time is 0.5-1 h, the hot isostatic pressing sintering is carried out under the protection of nitrogen, and the pressure of the nitrogen during the hot isostatic pressing sintering is 200-210 MPa.

6. The preparation method according to claim 1, wherein the mass of the sintering aid is 3-5% of the total mass of the silicon nitride powder and the sintering aid.

7. The production method according to claim 1 or 6, wherein the sintering aid comprises one or more of aluminum oxide, lanthanum oxide, neodymium oxide, ytterbium oxide, erbium oxide, and samarium oxide.

8. The method of claim 1, wherein the green pressed silicon nitride ceramic microspheres and the larger sized green silicon nitride ceramic spheres are loaded into cavities formed between layers and inside walls of a multi-layer sidewall crucible.

9. The high-precision silicon nitride ceramic microspheres prepared by the preparation method of any one of claims 1 to 8, wherein the surface roughness Ra of the high-precision silicon nitride ceramic microspheres meets the following requirements: ra is more than or equal to 0.006 mu m and less than or equal to 0.008 mu m, and the variation V of the ball diameter_DwSatisfies the following conditions: v is not more than 0.03 mu m_DwNot more than 0.08 mu m, spherical error delta S_phSatisfies the following conditions: delta S is more than or equal to 0.03 mu m_ph≤0.08μm。

10. Use of the high precision silicon nitride ceramic microspheres of claim 9 in dental drill bearings.

Technical Field

The invention relates to the technical field of ceramic ball preparation, in particular to a high-precision silicon nitride ceramic microsphere and a preparation method and application thereof.

Background

In recent years, along with the improvement of living standard and living quality of people, the prevention and treatment of odontopathy are increasingly paid attention and popularized. At present, the demand for high-speed dental drill bearings matched with a dental handpiece is continuously increased in China. The high-speed dental drill ball bearing is a thin-wall ultralight series precise miniature ball bearing. The high-speed wear-resistant water-proof valve has the advantages of high rotating speed, high starting speed in working, low noise, capability of bearing certain axial and radial loads, good corrosion resistance and wear resistance, long-term working in a wet oral cavity without corrosion, and six months of clinical life under correct use conditions. The high-speed dental drill bearing is imported from foreign countries, and at present, although a few bearing enterprises are produced at home, the quality, specification and quantity of the high-speed dental drill bearing can not meet the market demand, so that the high-speed dental drill bearing has important significance for the research and development of the high-speed dental drill bearing.

Si₃N₄The ceramic ball has low density and light weight, and can reduce the centrifugal force and gyro moment of the ball obviouslyAnd the pre-tightening load of the bearing is reduced, the contact load in the bearing is reduced, and the friction torque and the friction temperature rise in the bearing are obviously reduced. In addition, the silicon nitride ceramic ball has the characteristics of high hardness, corrosion resistance, frictional wear, small thermal expansion coefficient and the like. It has low heat absorption and low cooling requirement, and can operate in environment with poor lubricating condition. Meanwhile, the ceramic ball is difficult to adhere and wear with the steel surface, so that the silicon nitride ceramic ball bearing has longer service life than an all-steel bearing under high-speed light load. Therefore, the silicon nitride ceramic ball is very suitable for being used as the material of the high-speed dental drill bearing rolling body.

When the bearing works, the vibration of the bearing can seriously affect the service life, thereby affecting the stability and reliability of working equipment. Compared with a ferrule and a retainer, the surface roughness of a bearing rolling body is a main cause of bearing vibration, and noise is generated in the bearing. The vibration of the single-grain ceramic sphere is mainly generated by the surface quality defect of the sphere and the geometric shape error of the sphere. At present, the surface quality defects of the silicon nitride ceramic microspheres for dental drills mainly comprise air holes, pits, abnormally large crystal grains and snowflakes, and the surface roughness of the ceramic spheres is seriously influenced, so that the service life is shortened.

Disclosure of Invention

The invention aims to provide a high-precision silicon nitride ceramic microsphere and a preparation method and application thereof, overcomes the defects of air holes, abnormal large grains, surface pits, snowflakes and the like of the silicon nitride ceramic microsphere, and realizes the high-precision preparation of the silicon nitride ceramic microsphere.

In order to achieve the above object, the present invention provides the following technical solutions:

the invention provides a preparation method of high-precision silicon nitride ceramic microspheres, which comprises the following steps:

carrying out ball milling and mixing on the silicon nitride powder, the sintering aid and the dispersion liquid to obtain mixed feed liquid;

carrying out spray granulation on the mixed feed liquid to obtain granulation powder;

pressing and molding the granulation powder to obtain a pressed silicon nitride ceramic microsphere biscuit;

loading the pressed silicon nitride ceramic microsphere biscuit and the large-size silicon nitride ceramic biscuit ball into a crucible to sequentially perform atmosphere pressure sintering and hot isostatic pressing sintering to obtain a silicon nitride blank ball; the diameter of the large-size silicon nitride ceramic biscuit ball is 5-50 times of that of the silicon nitride ceramic microsphere biscuit; the crucible is a multilayer side wall crucible, and the multilayer side wall sequentially comprises a graphite outer wall, an intermediate wall and an inner wall from outside to inside; the extension direction of the middle wall and the extension direction of the inner wall are consistent with the extension direction of the outer wall of the crucible, and the middle wall and the inner wall are made of silicon nitride; the gap between the side walls of the adjacent layers is larger than the diameter of the silicon nitride ceramic blank ball with larger size;

and grinding the silicon nitride blank ball to obtain the high-precision silicon nitride ceramic microsphere.

Preferably, the diameter of the large-size silicon nitride ceramic blank ball is 20-35 mm; the diameter of the pressed silicon nitride ceramic microsphere biscuit is 0.8-4 mm.

Preferably, the mass ratio of the pressed silicon nitride ceramic microsphere biscuit to the larger-size silicon nitride ceramic biscuit ball is 1: (1-3).

Preferably, the temperature of the atmosphere pressure sintering is 1700-1800 ℃, the heat preservation time is 1-3 h, and the atmosphere pressure sintering is carried out under the protection of nitrogen.

Preferably, the hot isostatic pressing sintering temperature is 1800-1850 ℃, the heat preservation time is 0.5-1 h, the hot isostatic pressing sintering is carried out under the protection of nitrogen, and the pressure of the nitrogen during the hot isostatic pressing sintering is 200-210 MPa.

Preferably, the mass of the sintering aid is 3-5% of the total mass of the silicon nitride powder and the sintering aid.

Preferably, the sintering aid comprises one or more of aluminum oxide, lanthanum oxide, neodymium oxide, ytterbium oxide, erbium oxide and samarium oxide.

Preferably, the pressed silicon nitride ceramic microsphere biscuit and the larger-size silicon nitride ceramic biscuit balls are filled into a cavity formed between layers and an inner wall of the multilayer side wall crucible.

The invention provides the high-precision silicon nitride ceramic microspheres prepared by the preparation method in the scheme, and the surface roughness Ra of the high-precision silicon nitride ceramic microspheres meets the following requirements: ra is more than or equal to 0.006 mu m and less than or equal to 0.008 mu m, and the variation VDw of the ball diameter meets the following conditions: 0.03 mu m is not less than VDw and not more than 0.08 mu m, and the spherical error delta Sph meets the following requirements: the delta Sph is more than or equal to 0.03 mu m and less than or equal to 0.08 mu m.

The invention provides application of the high-precision silicon nitride ceramic microspheres in the scheme in dental drill bearings.

The invention provides a preparation method of high-precision silicon nitride ceramic microspheres, which comprises the following steps: carrying out ball milling and mixing on the silicon nitride powder, the sintering aid and the dispersion liquid to obtain mixed feed liquid; carrying out spray granulation on the mixed feed liquid to obtain granulation powder; pressing and molding the granulation powder to obtain a pressed silicon nitride ceramic microsphere biscuit; loading the pressed silicon nitride ceramic microsphere biscuit and the large-size silicon nitride ceramic biscuit ball into a crucible to sequentially perform atmosphere pressure sintering and hot isostatic pressing sintering to obtain a silicon nitride blank ball; the diameter of the large-size silicon nitride ceramic biscuit ball is 5-50 times of that of the silicon nitride ceramic microsphere biscuit; the crucible is a multilayer side wall crucible, and the multilayer side wall sequentially comprises a graphite outer wall, an intermediate wall and an inner wall from outside to inside; the extension direction of the middle wall and the extension direction of the inner wall are consistent with the extension direction of the outer wall of the crucible, and the middle wall and the inner wall are made of silicon nitride; the gap between the side walls of the adjacent layers is larger than the diameter of the silicon nitride ceramic blank ball with larger size; and grinding the silicon nitride blank ball to obtain the high-precision silicon nitride ceramic microsphere.

The pressed silicon nitride ceramic microsphere biscuit and the silicon nitride ceramic biscuit ball with larger size are sintered together, and because the diameter of the silicon nitride ceramic biscuit ball with larger size is larger than that of the pressed silicon nitride ceramic microsphere biscuit, when the pressed silicon nitride ceramic microsphere biscuit is placed into a crucible, the pressed silicon nitride ceramic microsphere biscuit can be filled into the gap of the silicon nitride ceramic biscuit ball with large size, which is beneficial to the uniform heating of the pressed silicon nitride ceramic microsphere biscuit in the sintering process; in addition, the invention adopts the multilayer side wall crucible for sintering, and the middle wall and the inner part are made of silicon nitride, the silicon nitride can absorb partial heat, thereby preventing the defects of abnormal growth of crystal grains, white pits on the surface, snowflakes and the like caused by over-high local heating of the pressed silicon nitride ceramic microsphere biscuit.In addition, the atmosphere pressure sintering and the hot isostatic pressing sintering are sequentially carried out, so that the density of the silicon nitride ceramic microspheres can be further improved and the defects of pores and abnormal large grains are reduced compared with the single atmosphere pressure sintering or hot isostatic pressing sintering, and the Si is improved₃N₄Mechanical property, stability and surface quality of the ceramic ball.

Furthermore, by controlling the mass of the sintering aid to be 3-5% of the total mass of the silicon nitride powder and the sintering aid, the invention has the advantages that the liquid phase of nitrogen oxide formed by the reaction of the sintering aid and the substances on the surface layer of particles during sintering is less, the glass phase left between crystal boundaries after sintering is less, the purity of the ceramic ball is high, and the mechanical property of the silicon nitride ceramic microspheres is favorably improved.

The results of the examples show that the Vickers hardness HV10 of the high-precision silicon nitride ceramic microspheres prepared by the invention can reach 1524kg/mm at most²The highest fracture toughness reaches 8.6 MPa.m^1/2Ra is more than or equal to 0.006 mu m and less than or equal to 0.008 mu m in surface roughness, and V is more than or equal to 0.03 mu m in ball diameter variation_DwLess than or equal to 0.08 mu m, and the spherical error is less than or equal to 0.03 mu m_phLess than or equal to 0.08 mu m, and all performance indexes of the alloy meet GB/T308.2-2010/ISO 3290-2: 2008 "rolling bearing bead part 2: the standard requirement of G3 grade of the standard requirement of silicon nitride ceramic micro-beads.

Drawings

FIG. 1 is a schematic view of a crucible for sintering of the present invention;

FIG. 2 is a photograph of one of the silicon nitride microspheres prepared in comparative example 1;

FIG. 3 is a scanning electron micrograph of one of the silicon nitride microspheres prepared in comparative example 1.

Detailed Description

The invention provides a preparation method of high-precision silicon nitride ceramic microspheres, which comprises the following steps:

carrying out ball milling and mixing on the silicon nitride powder, the sintering aid and the dispersion liquid to obtain mixed feed liquid;

carrying out spray granulation on the mixed feed liquid to obtain granulation powder;

pressing and molding the granulation powder to obtain a pressed silicon nitride ceramic microsphere biscuit;

loading the pressed silicon nitride ceramic microsphere biscuit and the large-size silicon nitride ceramic biscuit ball into a crucible to sequentially perform atmosphere pressure sintering and hot isostatic pressing sintering to obtain a silicon nitride blank ball; the diameter of the large-size silicon nitride ceramic biscuit ball is 5-50 times of that of the silicon nitride ceramic microsphere biscuit; the crucible is a multilayer side wall crucible, and the multilayer side wall sequentially comprises a graphite outer wall, an intermediate wall and an inner wall from outside to inside; the extension direction of the middle wall and the extension direction of the inner wall are consistent with the extension direction of the outer wall of the crucible, and the middle wall and the inner wall are made of silicon nitride; the gap between the side walls of the adjacent layers is larger than the diameter of the silicon nitride ceramic blank ball with larger size;

and grinding the silicon nitride blank ball to obtain the high-precision silicon nitride ceramic microsphere.

In the present invention, the starting materials used are all commercially available products well known in the art, unless otherwise specified.

According to the invention, silicon nitride powder, a sintering aid and a dispersion liquid are subjected to ball milling and mixing to obtain a mixed feed liquid.

In the invention, the mass of the sintering aid is preferably 3-5% of the total mass of the silicon nitride powder and the sintering aid; the sintering aid preferably comprises one or more of aluminum oxide, lanthanum oxide, neodymium oxide, ytterbium oxide, erbium oxide and samarium oxide, and more preferably comprises 3% of aluminum oxide and 2% of lanthanum oxide; 2% of aluminum oxide + 1% of lutetium oxide; 2% of aluminum oxide + 2% of erbium oxide. The percentages in this paragraph are referred to relative to the total mass of the silicon nitride powder and the sintering aid.

In the present invention, the dispersion is preferably absolute ethanol; the ratio of the mass of the dispersion liquid to the total mass of the silicon nitride powder and the sintering aid is preferably (3-4): 1.

in the invention, the grinding balls used for ball milling and mixing are preferably silicon nitride grinding balls, and the ratio of the mass of the silicon nitride grinding balls to the total mass of the silicon nitride powder and the sintering aid is (1.5-3): 1, and more preferably 2: 1. In the invention, the time for ball milling and mixing is preferably 18-24 h. In the present invention, the ball milling and mixing is preferably performed by a roller mill. The invention has no special requirement on the rotating speed of the ball milling mixing, and the rotating speed which is well known in the field can be adopted.

After the mixed material liquid is obtained, the mixed material liquid is subjected to spray granulation to obtain granulation powder. In the present invention, the spray granulation is preferably a pressure spray granulation method. The spray granulation is preferably carried out in a spray granulation tower. The conditions for the spray granulation in the present invention are not particularly limited, and those well known in the art may be used. In the examples of the present invention, the inlet temperature of the spray granulation tower used was 190 ℃ and the pore size of the spray sheet was 0.9 mm. In the invention, the grain size of the granulated powder is preferably 50-100 micrometers. The invention improves the fluidity of the granulated powder by utilizing spray granulation, and is beneficial to subsequent compression molding.

After the granulation powder is obtained, the granulation powder is pressed and molded to obtain a pressed silicon nitride ceramic microsphere biscuit. In the present invention, the compression molding is preferably performed by using a ceramic bead molding apparatus described in chinese patent CN 202023003181.3. According to the invention, a strip-shaped die disc with a proper size is preferably selected according to the size of the pressed silicon nitride ceramic microsphere biscuit, and the rotating speed of the cylindrical roller die is preferably 5-10 r/min. In the invention, the diameter of the pressed silicon nitride ceramic microsphere biscuit is preferably 0.8-4 mm.

After obtaining a pressed silicon nitride ceramic microsphere biscuit, the invention loads the pressed silicon nitride ceramic microsphere biscuit and a larger-size silicon nitride ceramic biscuit ball into a crucible to sequentially carry out atmosphere pressure sintering and hot isostatic pressing sintering, thus obtaining a silicon nitride blank ball.

The chemical composition of the larger silicon nitride ceramic green body ball is not required by the invention, and the silicon nitride ceramic green body ball known in the field can be used. In the invention, the large-size nitrogen silicon nitride ceramic biscuit ball is preferably formed by dry pressing by an upper die and a lower die through a rotary press.

In the invention, the diameter of the large-size silicon nitride ceramic green ball is 5-50 times of the diameter of the pressed silicon nitride ceramic microsphere green body, and further preferably, the diameter of the large-size silicon nitride ceramic green ball is 20-35 mm, and the diameter of the pressed silicon nitride ceramic microsphere green body is 0.8-4 mm. In the invention, the mass ratio of the pressed silicon nitride ceramic microsphere biscuit to the larger-size silicon nitride ceramic biscuit ball is preferably 1: (1-3), more preferably 1: (1.5-2.5). The invention carries out atmosphere pressure sintering and hot isostatic pressing sintering on the pressed silicon nitride ceramic microsphere biscuit and the silicon nitride ceramic biscuit ball with larger size in sequence, because the diameter of the silicon nitride ceramic biscuit ball with larger size is larger than that of the pressed silicon nitride ceramic microsphere biscuit, when the ceramic ball is arranged in a crucible, the pressed silicon nitride ceramic microsphere biscuit can be filled in the gap of the silicon nitride ceramic biscuit ball with larger size, which is beneficial to the uniform heating of the pressed silicon nitride ceramic microsphere biscuit in the sintering process and prevents the defects of abnormal growth of crystal grains, white pits, snowflakes and the like on the surface caused by over-high local heating.

In the invention, the crucible is a multilayer side wall crucible, as shown in fig. 1, the multilayer side wall comprises a graphite outer wall, an intermediate wall and an inner wall from outside to inside in sequence; the extension direction of the middle wall and the extension direction of the inner wall are consistent with the extension direction of the outer wall of the crucible, and the middle wall and the inner wall are made of silicon nitride; the gap between the side walls of the adjacent layers is larger than the diameter of the silicon nitride ceramic blank ball. In the invention, the multi-layer side wall crucible is of an integrated structure, and the graphite outer wall, the intermediate wall and the inner wall share the same crucible bottom.

In the invention, the thickness of the inner wall is preferably 2-4 mm, and the inner diameter of a concentric circle formed by the inner wall is preferably 90-100 mm; the thickness of the middle wall is preferably 3-5 mm, and the inner diameter of a concentric circle formed by the middle wall is preferably 180-200 mm; the thickness of the graphite outer wall is preferably 8-10 mm, and the inner diameter is preferably 300-320 mm. In the invention, the bottom thickness of the multilayer side wall crucible is preferably 6-8 mm. In the invention, the heights of the side walls of the multi-layer side wall crucible are preferably the same, and the heights are preferably 15-17 mm.

In the present invention, it is preferable that the green compact of the pressed silicon nitride ceramic microspheres and the green compact of the larger-sized silicon nitride ceramic are put into cavities formed between layers and inner walls of the multi-layer side-wall crucible during sintering. According to the invention, the multilayer side wall crucible is adopted to sequentially carry out atmosphere pressure sintering and hot isostatic pressing sintering, the middle wall and the inner wall are made of silicon nitride, and the silicon nitride can absorb part of heat, so that the defects of abnormal growth of crystal grains, white pits on the surface, snowflakes and the like caused by over-high local heating of the pressed silicon nitride ceramic microsphere biscuit are avoided.

In the invention, the temperature of the atmosphere pressure sintering is preferably 1700-1800 ℃, and more preferably 1720-1780 ℃; the heat preservation time is preferably 1-3 h, and more preferably 2 h; the atmospheric pressure sintering is preferably carried out under nitrogen protection. According to the invention, the temperature is preferably raised from room temperature to the atmospheric pressure sintering temperature, and the heating rate is preferably 5-20 ℃/min. The invention utilizes atmosphere pressure sintering to obtain Si without open pores₃N₄A ceramic.

After the atmospheric pressure sintering is completed, the invention preferably cools to room temperature and then performs hot isostatic pressing sintering. In the invention, the hot isostatic pressing sintering temperature is preferably 1800-1850 ℃, the heat preservation time is 0.5-1 h, the hot isostatic pressing sintering is carried out under the protection of nitrogen, and the pressure of the nitrogen during the hot isostatic pressing sintering is 200-210 MPa. In the present invention, the rate of temperature increase to the hot isostatic pressing sintering temperature is preferably 5 to 20 ℃/min. The hot isostatic pressing sintering is carried out after the atmosphere pressure sintering, so that the density of the silicon nitride ceramic microspheres can be further improved, and the defect of abnormal large grains is reduced, thereby improving the Si content₃N₄The compactness, the mechanical property, the stability and the surface quality of the ceramic ball.

In the invention, the multi-layer side wall crucible is adopted in the atmospheric pressure sintering and the hot isostatic pressing sintering.

After the hot isostatic pressing sintering is finished, the silicon nitride ceramic green ball is removed to obtain the silicon nitride green ball.

After the silicon nitride blank ball is obtained, the silicon nitride blank ball is ground to obtain the high-precision silicon nitride ceramic microsphere. In the present invention, the grinding process preferably includes performing rough grinding, fine grinding, finish grinding, lapping, and ultra lapping in this order. In the present invention, the grinding process is preferably performed by using a batch processing apparatus for silicon nitride ceramic micro beads disclosed in chinese patent CN 202021836565.0. In the invention, the upper and lower grinding plates of the grinding device are cast iron plates, and the specification is preferably phi 660mm multiplied by phi 420 mm.

In the invention, during the coarse grinding, the pressure applied between the upper grinding plate and the lower grinding plate is preferably (0.8-1) × 10KN, the main shaft rotating speed is preferably 100-120 r/min, and the machining allowance of the coarse grinding is preferably 250-350 μm.

In the invention, during fine grinding, the pressure applied between the upper grinding plate and the lower grinding plate is preferably (0.6-0.9) × 10KN, the main shaft rotating speed is preferably 80-100 r/min, and the machining allowance of the fine grinding is preferably more than or equal to 150 mu m and less than 250 mu m.

In the invention, during the fine grinding, the pressure applied between the upper grinding plate and the lower grinding plate is preferably (0.4-0.7) multiplied by 10KN, the rotation speed of the main shaft is preferably 70-80 r/min, and the machining allowance of the fine grinding is preferably more than or equal to 50 mu m and less than 100 mu m.

In the present invention, the pressure applied between the upper and lower polishing plates is preferably (0.3 to 0.6) × 10KN, the spindle rotation speed is preferably 60 to 70r/min, and the finishing allowance is preferably equal to or more than 30 μm and less than 50 μm.

In the present invention, in the ultra-lapping, the pressure applied between the upper and lower polishing plates is preferably (0.1 to 0.3) × 10KN, the spindle rotation speed is preferably 50 to 60r/min, and the machining allowance of the ultra-lapping is 0, that is, the target size is reached.

The invention provides the high-precision silicon nitride ceramic microspheres prepared by the preparation method in the scheme, and the surface roughness Ra of the high-precision silicon nitride ceramic microspheres meets the following requirements: ra is more than or equal to 0.006 mu m and less than or equal to 0.008 mu m, and the variation VDw of the ball diameter meets the following conditions: 0.03 mu m is not less than VDw and not more than 0.08 mu m, and the spherical error delta Sph meets the following requirements: the delta Sph is more than or equal to 0.03 mu m and less than or equal to 0.08 mu m. In the invention, the Vickers hardness HV10 of the high-precision silicon nitride ceramic microspheres reaches up to 1480kg/mm²The maximum fracture toughness reaches 8 MPa.m^{1 /2}All performance indexes of the alloy all meet GB/T308.2-2010/ISO 3290-2: 2008 "rolling bearing bead part 2: the standard requirement of G3 grade of the standard requirement of silicon nitride ceramic micro-beads. In the invention, the size of the high-precision silicon nitride ceramic microspheres is preferably 0.4-1 mm.

The invention provides application of the high-precision silicon nitride ceramic microspheres in the scheme in dental drill bearings.

The high-precision silicon nitride ceramic microspheres provided by the invention and the preparation method and application thereof are described in detail below with reference to examples, but the invention is not to be construed as being limited by the scope of the invention.

The specifications of the multi-wall crucible used in the following examples are:

the thickness of the inner wall is 3mm, and the inner diameter of a concentric circle formed by the inner wall is 100 mm; the thickness of the middle wall is 4mm, and the inner diameter of a concentric circle formed by the middle wall is 180 mm; the thickness of the graphite outer wall is 10mm, and the inner diameter is 320 mm. The bottom thickness of the multi-layer side wall crucible is 6mm, and the height of each side wall of the multi-layer side wall crucible is the same and is 15 mm.

Example 1

(1) Taking aluminum oxide with the content of 2 wt%, erbium oxide with the content of 2 wt% and silicon nitride powder with the content of 96 wt% as powder, taking absolute ethyl alcohol (the weight is 2 times of the total weight of the powder) as dispersion liquid, putting the powder, the absolute ethyl alcohol and silicon nitride grinding balls (the weight is 2 times of the total weight of the powder) into a roller mill for ball milling and mixing for 24 hours to obtain mixed liquid;

(2) granulating the mixed feed liquid by a spray granulation tower in a pressure spray granulation mode to obtain granulated powder, wherein the inlet temperature of the spray granulation tower is 190 ℃, and the pore diameter of a spray piece is 0.9mm, so that spherical particles with the particle size of 50-100 microns are obtained;

(3) pressing and molding the sprayed powder by using the ceramic microsphere molding device in CN202023003181.3, wherein the size of a pressed silicon nitride ceramic microsphere biscuit is 2.0-2.1 mm;

(4) placing the pressed silicon nitride ceramic microsphere biscuit and a silicon nitride ceramic biscuit ball with the diameter phi of 20mm into a multi-layer side wall crucible according to the mass ratio of 1:2, placing the crucible into an atmosphere pressure sintering furnace, wherein the sintering temperature is 1790 ℃, the heating rate is 8 ℃/min, the highest temperature heat preservation time is 1.5h, and the sintering process adopts nitrogen gas under normal pressure protection; hot isostatic pressing sintering temperature is 1830 ℃, heating rate is 10 ℃/min, the highest temperature heat preservation time is 1 hour, and nitrogen pressure is 200MPa in the sintering process, so as to obtain silicon nitride blank balls;

(5) carrying out coarse grinding, fine grinding and ultra-fine grinding on the silicon nitride blank balls, wherein the upper grinding plate and the lower grinding plate are cast iron plates made of cast iron and have the specification of phi 660mm multiplied by phi 420mm, the pressure applied between the upper grinding plate and the lower grinding plate in the coarse grinding process is 1 multiplied by 10KN, and the rotating speed of a main shaft is 110 r/min; in the fine grinding procedure, the pressure applied between the upper grinding plate and the lower grinding plate is 0.9 multiplied by 10KN, and the rotating speed of the main shaft is 90 r/min; the pressure applied between the upper grinding plate and the lower grinding plate in the fine grinding procedure is 0.7 multiplied by 10KN, and the rotating speed of the main shaft is 80 r/min; in the lapping procedure, the pressure applied between the upper grinding plate and the lower grinding plate is 0.6 multiplied by 10KN, and the rotating speed of the main shaft is 70 r/min; in the ultra-fine grinding procedure, the pressure applied between the upper grinding plate and the lower grinding plate is 0.3 multiplied by 10KN, the rotating speed of the main shaft is 60r/min, and the finished product of the silicon nitride ceramic microsphere with the diameter of 1mm is obtained.

Example 2

(1) Taking 3 wt% of alumina, 2 wt% of lanthanum oxide and 95 wt% of silicon nitride as powder, taking absolute ethyl alcohol (the weight is 2 times of the total weight of the powder) as dispersion liquid, putting the powder, the dispersion liquid and silicon nitride grinding balls (the weight is 2 times of the total weight of the powder) into a roller mill for ball milling and mixing for 24 hours to obtain mixed feed liquid;

(2) granulating the mixed material liquid by a spray granulation tower in a pressure spray granulation mode to obtain granulated powder, wherein the inlet temperature of the spray granulation tower is 190 ℃, the aperture of a spray piece is 0.9mm, and spherical particles with the particle size of 50-100 microns are obtained;

(3) pressing and molding the sprayed powder by using the ceramic microsphere molding device in CN202023003181.3, wherein the size of a pressed silicon nitride ceramic microsphere biscuit is 1.3-1.4 mm;

(4) putting the pressed silicon nitride ceramic microsphere biscuit and a silicon nitride ceramic biscuit ball with the diameter phi of 20mm into a graphite crucible according to the mass ratio of 1:2.5, putting the graphite crucible into an atmosphere pressure sintering furnace, wherein the sintering temperature is 1760 ℃, the heating rate is 9 ℃/min, the maximum temperature holding time is 2h, and the sintering process adopts nitrogen gas under normal pressure protection; hot isostatic pressing sintering temperature is 1800 ℃, heating rate is 8 ℃/min, the highest temperature heat preservation time is 1 hour, the total sintering time is 10 hours, and the nitrogen pressure is 200MPa in the sintering process, so that silicon nitride blank balls are obtained;

(5) carrying out coarse grinding, fine grinding and ultra-fine grinding on the silicon nitride blank balls, wherein the upper grinding plate and the lower grinding plate are cast iron plates made of cast iron and have the specification of phi 660mm multiplied by phi 420mm, the pressure applied between the upper grinding plate and the lower grinding plate in the coarse grinding process is 0.8 multiplied by 10KN, and the rotating speed of a main shaft is 110 r/min; in the fine grinding procedure, the pressure applied between the upper grinding plate and the lower grinding plate is 0.7 multiplied by 10KN, and the rotating speed of the main shaft is 90 r/min; the pressure applied between the upper grinding plate and the lower grinding plate in the fine grinding procedure is 0.6 multiplied by 10KN, and the rotating speed of the main shaft is 70 r/min; in the lapping procedure, the pressure applied between the upper grinding plate and the lower grinding plate is 0.5 multiplied by 10KN, and the rotating speed of the main shaft is 60 r/min; the pressure applied between the upper grinding plate and the lower grinding plate in the super-fine grinding procedure is 0.2 multiplied by 10KN, the rotating speed of a main shaft is 60r/min, and a finished product of silicon nitride ceramic microspheres with phi of 0.8mm is obtained.

Example 3

(1) Taking aluminum oxide with the content of 2 wt%, lutetium oxide with the content of 1 wt% and silicon nitride powder with the content of 97 wt% as powder, taking absolute ethyl alcohol (the weight is 2 times of the total weight of the powder) as dispersion liquid, putting the powder, the absolute ethyl alcohol and silicon nitride grinding balls (the weight is 2 times of the total weight of the powder) into a roller mill for ball milling and mixing for 24 hours to obtain mixed liquid;

(2) granulating the mixed feed liquid by a spray granulation tower in a pressure spray granulation mode to obtain granulated powder, wherein the inlet temperature of the spray granulation tower is 190 ℃, the pore diameter of a spray piece is 0.9mm, and spherical particles with the particle size of 50-100 microns are obtained;

(3) pressing and molding the sprayed powder by using the ceramic microsphere molding device in CN202023003181.3, wherein the size of a pressed silicon nitride ceramic microsphere biscuit is 0.8-0.9 mm;

(4) putting the pressed silicon nitride ceramic microsphere biscuit and a silicon nitride ceramic biscuit ball with the diameter phi of 20mm into a graphite crucible according to the mass ratio of 1:2.5, putting the graphite crucible into an atmosphere pressure sintering furnace, wherein the sintering temperature is 1750 ℃, the heating rate is 10 ℃/min, the maximum temperature holding time is 2h, and the sintering process adopts nitrogen gas under normal pressure protection; hot isostatic pressing sintering temperature is 1750 ℃, heating rate is 12 ℃/min, the highest temperature heat preservation time is 0.5 hour, the total sintering time is 9.5 hours, and the nitrogen pressure in the sintering process is 200MPa, so that silicon nitride blank balls are obtained;

(5) carrying out coarse grinding, fine grinding and ultra-fine grinding on the silicon nitride blank balls, wherein the upper grinding plate and the lower grinding plate are cast iron plates made of cast iron and have the specification of phi 660mm multiplied by phi 420mm, the pressure applied between the upper grinding plate and the lower grinding plate in the coarse grinding process is 0.8 multiplied by 10KN, and the rotating speed of a main shaft is 100 r/min; in the fine grinding procedure, the pressure applied between the upper grinding plate and the lower grinding plate is 0.6 multiplied by 10KN, and the rotating speed of the main shaft is 80 r/min; the pressure applied between the upper grinding plate and the lower grinding plate in the fine grinding procedure is 0.4 multiplied by 10KN, and the rotating speed of the main shaft is 70 r/min; in the lapping procedure, the pressure applied between the upper grinding plate and the lower grinding plate is 0.3 multiplied by 10KN, and the rotating speed of the main shaft is 60 r/min; the pressure applied between the upper grinding plate and the lower grinding plate in the super-fine grinding procedure is 0.1 multiplied by 10KN, the rotating speed of a main shaft is 50r/min, and a finished product of silicon nitride ceramic microspheres with phi of 0.4mm is obtained.

Comparative example 1

The only difference from example 1 is the use of a common single layer crucible.

Comparative example 2

The only difference from example 1 is that no silicon nitride ceramic green body balls with a diameter of phi 20mm were used in the sintering.

Comparative example 3

The only difference from example 1 is that the hot isostatic pressing sintering is omitted.

Comparative example 4

The only difference from example 1 is that the atmospheric pressure sintering is omitted.

And (3) carrying out mechanical property test and precision test on the finished products of the embodiments 1-3 and the comparative examples 1-4, wherein the mechanical property test refers to ASTM F2094-2018a, and the precision test refers to GB/T308.2-2010/ISO 3290-2: 2008 "rolling bearing bead part 2: silicon nitride ceramic beads, the test results are shown in table 1.

TABLE 1 mechanical Properties and accuracies of silicon nitride ceramic balls prepared in examples 1 to 3 and comparative examples 1 to 4

Note: the abnormal large grains can only be observed by a scanning electron microscope, and the number of balls in each batch is large, so the ratio cannot be counted, and the abnormal large grains are not listed in table 1.

As can be seen from Table 1, the compactness of the silicon nitride ceramic microspheres prepared in examples 1, 2 and 3 reaches more than 99.7%, the material properties such as Vickers hardness, fracture toughness and three-point bending strength reach the ASTM F2094-2018a I grade material standard, the precision grade reaches the GB/T308.2G 3 grade standard, and the proportion of white pit and snowflake defects is lower than 0.5%. The compactness of the silicon nitride ceramic microspheres prepared in the comparative example 1 reaches 99.53 percent, the hardness only reaches the ASTM F2094-2018a II-grade material standard, the fracture toughness and the three-point bending strength reach the I-grade material standard, the precision grade reaches the GB/T308.2G 10-grade standard, and the white dot proportion and the snowflake proportion are respectively 32 percent and 18 percent; the compactness of the silicon nitride ceramic microspheres prepared in the comparative example 2 reaches 99.41 percent, the hardness only reaches the ASTM F2094-2018a II-grade material standard, the fracture toughness and the three-point bending strength reach the I-grade material standard, the precision grade reaches the GB/T308.2G 10-grade standard, and the white dot proportion and the snowflake proportion are respectively 35 percent and 21 percent; the compactness of the silicon nitride ceramic microspheres prepared in the comparative example 3 only reaches 95.11 percent, the hardness is lower than the ASTM F2094-2018a grade III material standard, the fracture toughness reaches the II grade material standard, the three-point bending strength reaches the grade III material standard, the precision grade reaches the GB/T308.2G 20 grade standard, and the white dot proportion and the snowflake proportion are respectively 26 percent and 13 percent; comparative example 4 the density of the prepared silicon nitride ceramic microspheres reaches 96.32%, the hardness and fracture toughness reaches the standard of ASTM F2094-2018a II-grade material, the three-point bending strength reaches the standard of III-grade material, the precision grade reaches the standard of GB/T308.2G 20, and the white dot proportion and the snowflake proportion are respectively 30% and 10%. The invention solves the defects of air holes, abnormal large grains, surface pits, snowflakes and the like of the silicon nitride ceramic microspheres by adopting a sintering mode of combining atmosphere pressure sintering and hot isostatic pressing sintering, adopting larger-size silicon nitride ceramic green balls and adopting a multilayer side wall crucible for sintering, and realizing the high-precision preparation of the silicon nitride ceramic microspheres.

Fig. 2 is a photograph of one of the silicon nitride microspheres prepared in comparative example 1, and it can be seen from fig. 2 that white pits appear on the surface of the silicon nitride microspheres. FIG. 3 is a scanning electron micrograph of one of the silicon nitride microspheres prepared in comparative example 1, and it can be seen that abnormally large grains appear.

The foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.