CN1143837C - Sialon ceramic material for bearing ball and preparation method thereof - Google Patents

Abstract

A kind of sialon (sialon) ceramics used for bearing balls and its preparation method, adopting the formula of the present invention and specific sintering process, the obtained material is composed of 25-45wt% α-sialon phase, β phase Si ₃ N ₄ , B phase (Y ₂ SiAlO ₅ N), H phase (Y ₅ (SiO ₄ ) ₃ N), a small amount of YAG phase (3Y ₂ O ₃ ·5Al ₂ O ₃ ) and glass phase. A three-stage heat preservation sintering method is adopted, and in order to prevent pyrolysis in the second and third stages, the nitrogen pressure in the furnace is kept in the range of 0.15-0.22 and 1.0-5.0 MPa respectively. The crushing load of the prepared bearing ball is not less than 45% of the bearing steel ball of the same size, and the Vickers hardness is greater than 16.5GPa. The porosity (volume ratio) on the polished section of the ball is <0.2%.

Description

Sialon ceramic material for bearing ball and preparation method thereof

The invention relates to a sialon ceramic material and a preparation method thereof, in particular to a sialon ceramic material which can be used for bearing balls and a preparation method thereof. It belongs to the field of silicon nitride (Si ₃ N ₄ ) based materials.

Sialon is a solid solution of Si ₃ N ₄ , which was first proposed by scientists in 1978. There are two variants of α-sialon and β-sialon. Sialon material has the characteristics of high hardness and high strength, so it is a very promising wear-resistant structural ceramic material.

The molecular formula of β-sialon is Si _6-Z Al _Z O _Z N _8-Z . It is obtained by replacing Z Si-N bonds with Z Al-O bonds in β-Si ₃ N ₄ ; the molecular formula of α-sialon is Me _x Si _12-(m+n) Al _m+n O _n N _{16- n} , is obtained after n Si-N bonds are replaced by n Al-O bonds in α-Si ₃ N ₄ and m Si-N bonds are replaced by m Al-N bonds. The balance is compensated by the introduction of x metal atoms Me. When the metal atom Me is a Y element, x=m/3. Y-α/βsialon is generally synthesized by heat-treating Si ₃ N ₄ , AlN, Al ₂ O ₃ and Y ₂ O ₃ powders mixed in different proportions. Sialon dense ceramics can be obtained by sintering methods such as hot pressing (HP), pressureless (PLS), gas pressure (GPS) and high temperature isostatic pressing (HIP). Some intermediate phases appear during Sialon sintering: YAG, nitrogen-containing feldspar, nitrogen-containing silicate, 12H, 21R, and β ₆₀ are equal. The formation of these intermediate phases reduces the amount of transitional liquid phase and has a greater impact on sintering. Therefore, the intermediate heat preservation conditions during the sintering process, that is, the staged sintering conditions, should have a greater impact on the sintering process and the final density of the product. Through the design of solid solution and sintering process, the Sialon ceramic material with certain mechanical properties and microstructure can be obtained, which can meet the excellent comprehensive performance required by ceramic bearing balls.

Among structural ceramics, silicon nitride ceramics have high fracture toughness, good anti-rolling contact fatigue characteristics and spalling failure mode, and are used in the field of bearings, especially for making bearing balls and rollers. "Silicon Nitride Bearing Ball with High Fatigue Life" (CN 1143944A) invented by R.L. Yeckley introduced the preparation process of silicon nitride material for bearings, which has long rolling contact fatigue life. However, it is generally obtained by a more expensive hot isostatic pressing (HIP) sintering process, and the hardness of the material is not very high (<15.6GPa). The Sialon material prepared by "Sialon Composite Ceramics and Preparation Method" (CN1142478A) invented by Wang Peiling and others has high hardness and high temperature strength, but the grain size of the material is not mentioned. After two-stage sintering and long-term heat treatment Finally, the crystal grains in the material are easy to grow up to about 10 microns, and the material will have higher toughness, but it may not be suitable for bearing ball materials. Silicon nitride bearing materials have a peeling failure mode during application and processing, and grain pullout often occurs, and excessively large grains will reduce processing accuracy and service life.

The object of the present invention is to provide a kind of sialon material used for bearing ball and its preparation method, the sialon material used for bearing provided by it has high hardness, high crushing strength and low porosity, especially the material is microscopically composed of Composed of smaller crystal grains (95% of the crystal grains have a diameter within 0.3-3 microns), and are suitable for mass production.

First, the main raw materials used in the present invention are Si ₃ N ₄ (α phase ≥ 90wt%, average particle size 0.36 micron, oxygen content 1.8wt%), AlN (average particle size 0.93 micron, oxygen content 1.57wt%, nitrogen content 32.5 wt%), Al ₂ O ₃ (purity >99.9%) and Y ₂ O ₃ (purity >99.9%) powders. AlN content is 6-11wt%, better 8-9wt _% , _Al2O3 content is 1.5-4.5wt%, better 3-3.5wt% _{, Y2O3} content is 5-9wt%, better 7-7.9wt%, the Si ₃ N ₄ content is the amount needed to make up 100wt%.

Second, use pressureless, air pressure or hot isostatic pressing sintering, and better use air pressure sintering with high performance/cost ratio. The sintering process of sialon is a complex reaction sintering process. This invention adopts a unique three-stage heat preservation sintering method. In the first stage, 1710-1830°C, heat preservation for 0.5-2 hours; in the second stage, 1790-1910°C, heat preservation for 0.5- 2.5 hours; in the third stage, 1830-1950°C, keep warm for 0.5-5 hours. In order to control the pyrolysis of the sample, the nitrogen pressure in the second-stage sintering furnace was kept at 0.15-0.22 MPa, and the nitrogen pressure in the third-stage sintering furnace was kept at 1.0-5.0 MPa. The advantage of this sintering system is that when the temperature of the first stage is kept at a temperature of about 1750°C, a large number of fine α-sialon grains and β-phase Si ₃ N ₄ grains have been formed. The temperature difference between the stages is not more than 80°C, the most important thing is that the temperature difference between the third stage and the second stage is not more than 40°C, the temperature difference should not be too large, and the temperature of the first stage and the second stage should not be too low, respectively At 1710°C and 1790°C (see Example 4 for details). Combined with the formula provided by the present invention, the α-sialon grains do not grow abnormally even at high temperature, and the β-phase Si ₃ N ₄ grains are not long under the limitation of the steric effect of the α-sialon grains Large, during the high-temperature sintering process in the third stage, because there is no limitation of excessively large grains, the sample can continue to shrink and become dense, and the final sample has high density, less pores, and small grains. The sample was embedded in embedding powder having a composition of Si ₃ N ₄ :AlN:BN=8.2:0.8:1 (weight ratio). Both the sample and the buried powder were placed in a graphite crucible. The heating rate of the furnace is 10°C/min, and the cooling rate is 18°C/min from high temperature to 1300°C. In order to control the pyrolysis of the sample, the nitrogen pressure in the sintering furnace in the second stage is kept at 0.15-0.22 MPa, and the nitrogen pressure in the third stage is kept at 1.0-5.0 MPa.

The sialon ceramic material used for bearing balls prepared by the components and processes provided by the present invention is composed of α-phase sialon, β-phase Si ₃ N ₄ , B-phase (Y ₂ SiAlO ₅ N), H-phase (Y ₅ ( SiO ₄ ) ₃ N), and a small amount of YAG (3Y ₂ O ₃ ·5Al ₂ O ₃ ) and glass phase, and the content of α-sialon phase is in the range of 25-45 wt%. The grain size of 95% of the number is within 0.3-3 microns, and the number of grains larger than 5 microns is less than 1%. The porosity of the material is ≤0.2%, and the crystal grains are equiaxed.

The present invention has the following advantages:

(1) According to the specified formula and sintering process, a composite material of α-sialon and β-phase Si ₃ N ₄ with an α-sialon content of 25wt% to 45wt% can be obtained, and the strengths of the two phase materials are brought into play, and the hardness of the material is relatively high , the Vickers hardness is greater than 16.5GPa, and the fracture toughness and flexural strength at room temperature are high, respectively: K _IC : 5.5～6.2MPa·m ^1/2 , and 607～756MPa.

(2) In particular, the microstructure of the material is fine and equiaxed, consisting of 95% of fine grains with a grain size of 0.3-3 microns. This material is suitable for bearing materials, especially bearing ball materials. The crushing load of bearing balls made of this material is higher, which is mainly due to the equiaxed grains.

The advantages and novelties of the invention are further illustrated below by way of examples.

Example 1 405 grams of Si ₃ N ₄ powder (α phase ≥ 90wt%, average particle size 0.36 micron, oxygen content 1.8wt%), 40 grams of AlN (average particle size 0.93 micron, oxygen content 1.57wt%, nitrogen content 32.5 wt%), 38 grams of Y ₂ O ₃ (purity >99.9%) and 17 grams of Al ₂ O ₃ (purity >99.9%). Add 500 milliliters of ethanol, 1000 grams of silicon nitride pellets with a diameter of 10 mm, and 10.1 grams of PVB, and mix in a silicon nitride cylinder for 20 hours. After drying, unidirectional press to make a test strip of 5 mm × 5 mm × 50 mm, and then cold isostatic pressing under a pressure of 250 MPa to further increase the density of the green body.

The sintering is carried out in a graphite heating element furnace, and the sample is embedded in an embedded powder with a composition of Si ₃ N ₄ :AlN:BN=8.2:0.8:1 (weight ratio). Both the sample and the buried powder were placed in a graphite crucible. The heating rate of the furnace is 10°C/min, and the cooling rate is 18°C/min from high temperature to 1300°C. The three-stage heat preservation sintering method is adopted, and the sample heat preservation is in the first stage: 1770°C, 1 hour; the second stage: 1850°C, 1.5 hours; the third stage: 1880°C, 3 hours. In order to control the pyrolysis of the sample, the nitrogen pressure in the second-stage sintering furnace was kept at 0.18MPa, and the nitrogen pressure in the third-stage sintering furnace was kept at 1.2MPa.

After the sample is sintered, the surface is ground to a thickness of about 0.5mm, the density is measured by the Archimedes water immersion method, and the phase analysis is carried out by the X-ray diffraction method. After the polished samples were etched in molten NaOH, the morphology was observed by SEM.

The dense sample was ground and processed into a flexural strength test strip 3×4×35mm ³ , and the flexural strength was measured by the three-point bending method with a span of 30mm. The microhardness was measured under a load of 10kg, and the crack length was measured to calculate the fracture toughness.

The α-sialon content of the material is 39wt%, and the strength is 659MPa. The grain boundary phase is mainly B phase (Y ₂ SiAlO ₅ N), H phase (Y ₅ (SiO ₄ ) ₃ N), a small amount of YAG phase (3Y ₂ O ₃ 5Al ₂ O ₃ ) and glass phase, and its X-ray diffraction The picture is shown in Figure 1. In the figure, 1: β phase Si ₃ N ₄ , 2: α-sialon, 3: B phase, 4: YAG phase, 5: H phase.

Example 2 The same formulation and sintering process as in Example 1 were adopted, but the shape of the molded sample was a spherical sample with a diameter of 8 mm. During the second stage of heat preservation, the nitrogen pressure in the furnace is 0.16MPa, and in the third stage, the nitrogen pressure in the furnace is 3MPa. After sintering, the density of the sample is 3.3 g/mm ³ . After the ball is polished, the crushing load of the ball tested by the ball-ball pair pressure method is 9.7KN, which is 45.6% of the bearing steel ball of the same size. Cut the ball and polish the cut surface, and observe that the porosity is less than 0.2vol%. The microhardness on the cut surface is 17.6GPa, and the fracture toughness K _IC : 5.7MPa·m ^1/2 . After being corroded in molten NaOH, the morphology observed by scanning electron microscope is as shown in Figure 2. The grain size of 95% of the quantity is within 0.3-3 microns, and it is equiaxed, which helps it to be used as a bearing ball. Improves compressive strength and reduces porosity.

Sialon ceramic bearing balls prepared by the above method and general air pressure sintered silicon nitride (such as using 10wt% YAG as a sintering aid, sintered at 1900 ° C for 2 hours, then sintered at 1940 ° C for 2 hours, the highest 1.2MPa nitrogen protection atmosphere ) Ceramic bearing ball performance comparison has the advantages shown in Table 1.

Table 1 performance Example 2 General pressure sintered silicon nitride Microhardness (GPa) 17.3 13.2 Crushing strength/crushing strength of bearing steel balls of the same size >45% ≤30% >5 micron grain number <1% >10%

Example 3 The composition of the sialon raw material is 81.2wt% Si ₃ N ₄ (manufactured by Ube Shinsan Co., Ltd., UBE, E10), 8.1wt% AlN, 4.2wt% Al ₂ O ₃ and 6.5wt% Y ₂ O ₃ . Add ethanol and PVB in the same proportion as in Example 1, and use silicon nitride pellets with a diameter of 10 mm as the mixing medium for mixing in a silicon nitride barrel for 20 hours. After drying, dry pressing and isostatic pressing form the shape of the sample into a ball with a diameter of 8 mm and a test strip of 5 mm × 5 mm × 50 mm.

The sintering process is the same as in Example 1, and the material contains 29 wt% of α-phase sialon. The breaking strength of the material is 692 MPa, the Vickers hardness is 17.3 GPa, and the porosity is less than 0.2 vol% when the ball polishing section is observed under a 100 times optical microscope.

Example 4 The composition of the sialon raw material is the same as in Example 1 and Example 2, and the same molding process is used to shape it into a spherical shape or a test bar. The buried powder, furnace and heating rate for sintering are also the same as in Example 1 and Example 2, and the sintering system Change to: Sample insulation in the first stage: 1680°C, 1 hour; second stage: 1780°C, 1.5 hours; third stage: 1900°C, 3 hours. In order to control the pyrolysis of the sample, the nitrogen pressure in the second-stage sintering furnace was kept at 0.18MPa, and the nitrogen pressure in the third-stage sintering furnace was kept at 1.2MPa. Due to the large temperature difference in each heat preservation stage and the low temperature in the first two stages, the grains grew abnormally in the third stage, and a large number of grains larger than 3 microns appeared in the material microstructure, as shown in Figure 3. The support effect produced by the oversized crystal grains affects the final densification, and the porosity of the ball-polished cut surface is observed under a 100-fold optical microscope, and the porosity is >0.5vol%. This shows that the formulation of the present invention must be combined with the sintering process of the present invention to achieve the desired performance.

The sialon ceramics of the present invention can be used for (but not limited to) ceramic bearing balls or rollers, and can also be used for other wear-resistant parts, such as ceramic cutting tools, ceramic sandblasting nozzles and the like.

Claims (5)

1. A sintered sialon ceramic material for bearings, mainly composed of α-sialon phase and β-Si ₃ N ₄ phase, characterized in that: (1) It consists of 25~45wt% α-sialon phase, β-Si ₃ N ₄ phase, B phase Y ₂ SiAlO ₅ N, H phase Y ₅ (SiO ₄ ) ₃ N, a small amount of YAG phase 3Y ₂ O ₃ · 5Al ₂ O ₃ and glass phase composition; the content of AlN is 6-11wt%, the content of Al ₂ O ₃ is 1.5-4.5wt%, the content of Y ₂ O ₃ is 5-9wt%, and the content of Si ₃ N ₄ is 100wt%. the required amount; (2) The microstructure of the material is fine and equiaxed, 95% of the grains are within 0.3-3 microns, and the grains larger than 5 microns are less than 1%. 2. The sintered sialon ceramic material for bearing balls according to claim 1, characterized in that the crushing load is not less than 45% of that of bearing steel balls of the same size. 3. The method for preparing sintered sialon ceramic materials for bearing balls according to claim 1, characterized in that: (1) The main raw materials used are Si ₃ N ₄ , AlN, Al ₂ O ₃ and Y ₂ O ₃ powder, according to AlN content 6-11wt%, Al ₂ O ₃ content 1.5-4.5wt%, Y ₂ O ₃ content 5-9wt%, the content of Si ₃ N ₄ is the amount and proportion required for 100wt%; (2) Adopt three-stage heat preservation and sintering method, in the first stage 1710～1830℃, heat preservation 0.5～2 hours; in the second stage 1790～1910℃, 0.5～2.5 hours; in the third stage 1830～1950℃, 0.5～5 hours; the temperature difference between the second stage and the first stage is not more than 80°C, the temperature difference between the third stage and the second stage is not more than 40°C; and the nitrogen pressure in the second stage sintering furnace is kept at 0.15-0.22Mpa, The nitrogen pressure in the stage sintering furnace is kept at 1.0-5.0Mpa; (3) The samples were sintered by embedding in embedding powder with the composition Si ₃ N ₄ :AlN:BN=8.2:0.8:1 (weight ratio). 4. The method for preparing sintered sialon ceramic materials for bearing balls according to claim ₃ , characterized in that: the content of AlN is 8-9wt%, the content of _Al2O3 is 3-3.5wt%, and the content of _{Y2O 3} content is 7-7.9 wt%, and the Si ₃ N ₄ content is the amount needed to make up 100 wt%. 5. The preparation method of the sintered sialon ceramic material used for bearing balls according to claim 3 or 4, characterized in that: the sample and the embedded powder are all placed in a graphite crucible, the heating rate is 10°C/min, and the cooling rate From high temperature to 1300°C is 18°C/min.

Images