Si3N4 powder is the main raw material for the preparation of silicon nitride balls. Selecting the appropriate treatment method to obtain the powder with regular shape and uniform particle size distribution is the basis for the stable implementation of Si3N4 ceramic ball forming, sintering, processing and other processes.

According to the different atomization methods, the spray granulation methods of Si3N4 powder mainly include centrifugal spray granulation, pressure spray granulation and two-fluid spray granulation. Pressure spray granulation The slurry with uniform Si3N4 powder is sprayed into the granulation tower under high pressure for atomization, and the droplets are quickly dried into spherical powder by hot air flow, which can prevent the agglomeration and sedimentation of various components in the slurry. By controlling the volatilization rate of the solvent on the surface of the particles, the regular particle morphology can be obtained, and the spray granulation powder with uniform particle size distribution, good fluidity and suitable loose density can be packed. Thus, the performance of the powder filling mold is improved, and the density and uniformity of the blank are increased. Therefore, the pressure spray granulation method was selected to study the effect of the loose density of spray granulation powder on the properties of Si3N4 ceramic balls.

 

Test Material

Si3N4 powder (when the cumulative volume fraction in the particle size distribution is 50%, the corresponding particle size D50=1.5μm, α-Si3N4 content is 93%, the purity is 99.9%), Y2O3 powder (D50=1.8μm, the purity is 99.9%), Al2O3 powder (D50=2.2μm, the purity is 99.95%), etc.

 

Sample preparation

According to Si3N4∶Y2O3∶Al2O3= 92%∶4%∶4%(mass ratio), the mixture was added to the ball mill, anhydrous ethanol was used as the solvent, Si3N4 ball was used as the grinding medium for mixing and dispersing, mixing time was 24h, the mass ratio of Si3N4 ball and mixed powder was 3:1. After uniform mixing, the mass fraction of solid phase of the slurry is 55%, and the viscosity is 4000MPa·s. By controlling the inlet temperature of the spray drying tower and the diameter of the spray plate, the granulated powder with different loose density was obtained. The Si3N4 spray granulation powder was pressed into a ceramic pellet with a diameter of 8.731mm by a dry press, and then the atmosphere pressure sintering was carried out at 1850℃, the heating rate was 3℃ /min, the holding time was 1.5h, and the nitrogen pressure was 9MPa. The properties were tested after preparation.

 

Result

The screening fraction data and loose density of the granulated powder prepared by different spray granulation processes with the same batch slurry are shown in Table 1. The density of ceramic pellet blank, sintering density, bending strength, crushing load, fracture toughness and Vickers hardness of the ceramic pellet pressed by spray granulation powder with different loose density are shown in Table 2.

 

The mechanical properties of Si3N4 ceramic ball pressed by 5# granulated powder are the best. Too high or too low loose density will affect the pressing performance of the powder and the density of Si3N4 ceramic ball blanks, thus affecting the mechanical properties of Si3N4 ceramic balls. The mechanism is that the loose density directly affects the porosity of the spray granulation powder after pressing, and the gas is difficult to discharge, resulting in the long migration distance of particles and substances during the sintering process, which is not conducive to sintering densification.

The particle morphology of the 5# Si3N4 spray granulation powder is solid and spherical (Figure 1).

The microstructure of Si3N4 ceramic ball GPS by 1# ~ 9# spray granulation powder is shown in Figure 2. With the increase of loose density of granulation powder, the number of pores inside Si3N4 ceramic ball after GPS first increases and then decreases, and the density of Si3N4 ceramic ball first increases and then decreases.

SEM was used to observe the microstructure and grain fracture morphology of the crushed samples of Si3N4 ceramic balls with the best mechanical properties and poor mechanical properties. As shown in Figure 3, the density of Si3N4 ceramic balls first increased and then decreased with the increase of loose density of granulated powder. Too high or too low loose density would lead to uneven grain growth and internal pores.

 

Conclusion

With Si3N4 powder as raw material and Y2O3 and Al2O3 as sintering additives, the influence of loose density of spray granulation powder on densification and mechanical properties of ceramic ball sintering was analyzed. The following conclusions were drawn:

1) The density of Si3N4 ceramic ball blank increases first and then decreases with the increase of the bulk density of granulated powder. When the loose packing density is 0.89g /cm3, the Si3N4 ceramic ball has the highest densification degree and the best mechanical properties.

2) When the bulk density of spray granulation powder is 0.89g /cm3, the formed Si3N4 ceramic ball has the smallest pores, uniform grain size, and mainly adopts the mode of transgranular fracture.

Silicon nitride (Si3N4) balls show great potential in the field of high-speed, high-precision, long-life bearings due to their excellent properties, such as lightweight, self-lubricating, high insulation, non-magnetic, high elastic modulus and excellent corrosion resistance. This paper aims to discuss the optimization of the spray granulation process of Si3N4 powder, especially the pressure spray granulation method, in order to improve the density, mechanical properties and microstructure of Si3N4 balls, so as to meet the urgent demand for high-performance bearing materials in aerospace, automotive and wind power generation fields.

 

With the rapid development of industrial technology, the requirements for material properties are increasingly stringent. As a new type of high performance bearing material, Si3N4 ceramic ball has become a research hotspot to further improve its performance. Among them, the blank density, as a key factor affecting the final performance of ceramic balls, is directly related to the number of micro-pores and fatigue life of ceramic balls after sintering. Therefore, optimizing the spray granulation process of Si3N4 powder to achieve regular powder morphology, uniform particle size distribution and increased bulk density is an effective way to improve the performance of ceramic balls.

 

Performance Advantages and Application Fields of Si3N4 Balls

With its unique physical and chemical properties, Si3N4 ceramic balls show significant advantages in many fields: low mass reduces the overall load of the bearing system and improves the operating efficiency; Self-lubricating features reduce lubrication requirements and reduce maintenance costs; High insulation and non-magnetic suitable for special environments; High elastic modulus and corrosion resistance ensure long-term stable operation under extreme conditions. These advantages make Si3N4 ceramic balls widely used in high-speed, high-precision and long-life bearing systems such as machine tool spindles, electric spindles, automotive engines, aerospace engines and wind turbines.

 

Effect of Spray Granulation Process on Properties of Si3N4 Ball

 

The Importance of Blank Density

The density of blank is a key parameter in the preparation of ceramic ball, which directly affects the density and properties of ceramic ball after sintering. Increasing the density of the blank is helpful to reduce the formation of pores in the sintering process, so as to improve the mechanical properties and fatigue life of ceramic balls.

 

The Relationship Between the Morphology of Spray-pelleted Powder and the Bulk Density

Spray granulation is one of the important processes for preparing high quality ceramic powders. Through pressure spray granulation, Si3N4 slurry is sprayed into the granulation tower under high pressure, and the droplets are quickly dried into spherical powder in the hot air flow. In this process, controlling the evaporation rate of the solvent on the particle surface is the key to obtain the powder with regular morphology and uniform particle size distribution. The regular spherical powder is beneficial to increase the loose density of the powder, and then improve the density and uniformity of the blank.

 

Optimization Strategy

1. Optimize the slurry formula: adjust the proportion of each component in the slurry and the use of dispersant to reduce agglomeration and settling stratification and improve the uniformity and stability of the slurry.

 

2. Precise control of spray parameters, such as spray pressure, nozzle design, hot air flow temperature, etc., to optimize the particle size distribution and drying rate of droplets, to ensure the regularity of powder morphology and the improvement of loose density.

Post-treatment optimization: The powder after spray granulation is screened and graded, and unqualified particles are removed to further improve the overall quality of the powder.

 

3. Experiment and result analysis

Through a series of experiments, we found that the raw density of Si3N4 ceramic balls prepared by optimized pressure spray granulation technology was significantly increased, and the sintered ceramic balls showed lower microporosity, higher hardness and better fatigue resistance. The microstructure analysis shows that the grain structure of the ceramic ball is more dense and the interface bonding strength is enhanced.

 

 

Conclusion and Prospect

In this paper, through the optimization of Si3N4 spray granulation process, especially the in-depth study of pressure spray granulation method, the density, mechanical properties and microstructure of Si3N4 ceramic balls have been successfully improved, which provides theoretical basis and technical support for the preparation of high-performance ceramic balls. In the future, with the continuous progress of material science and preparation technology, Si3N4 ceramic balls will show their unique advantages in more high-end fields and promote the sustainable development of related industries. At the same time, further exploration of new spray granulation technology and process parameter optimization will be an important direction to improve the performance of ceramic balls. Optimizing the Si3N4 spray granulation process to improve the performance of ceramic balls: Performance advantages, application areas and process exploration