［Abstract］ Bearing ceramic ball is the key component in ceramic ball bearing． For the grinding processing of bearing ceramic ball，the grinding method of ball and grinding fluid are the key factors affecting the processing quality and efficiency of ceramic ball． Theprinciple of ball formation was briefly described，the research progress of ceramic ball grinding method was summarized，different ceramic ball grinding methods were compared and analyzed，and a variable curvature groove grinding method to meet the batch processing was proposed． The composition and role of ceramic ball grinding fluid was introduced，and the development status of bearing ceramic ball grinding fluid was reviewed． The development of ceramic ball grinding technology for bearings was presented in terms of processing accuracy，batch processing and green manufacturing．
［Keywords］ ceramic ball; ball formation principle; grinding method; bulk processing; grinding fluid
The development of ceramic ball grinding fluid
Grinding and processing technology mainly includes mechanical grinding processing technology, chemical grinding processing technology, and chemical mechanical grinding processing technology. These grinding processes require grinding fluid to assist in processing. Grinding fluid can remove material and refine the surface of the workpiece. Once the ball processing method and corresponding equipment are determined, the main factors that affect the quality and efficiency of ceramic ball processing are processing parameters and grinding fluid. Especially compared with steel ball processing, the application and development of grinding fluids are of great significance to the grinding and processing of bearing ceramic balls.
Main components and functions of grinding fluid
The grinding fluid is mainly composed of abrasives, base fluid, additives, etc. The abrasive is equivalent to a cutting tool, which can remove surface allowances. The hardness and size of the abrasive affect the processing efficiency and accuracy. The base fluid is the carrier of the abrasive and needs to suspend the abrasive sufficiently. Additives mainly assist abrasive grinding and improve the grinding environment. The abrasive in the grinding fluid acts as a cutting tool to shear and compress the ball, removing surface material. The probability of surface damage caused by abrasive grinding of the ball in the grinding fluid is lower than that caused by bonded abrasives and grinding wheels. If the abrasive cannot be evenly dispersed in the base fluid during the grinding process, the removed debris will adhere to the surface of the workpiece, resulting in a decrease in the surface quality of the workpiece, and the life of the grinding tool will also be affected. If grinding debris stays for a long time during processing, it may cause scratches on the surface of the ball. The additives in the grinding fluid can reduce the adhesion of debris, take away the debris, avoid affecting the processing quality, and ensure the processing accuracy.
The grinding fluid mainly has the following functions:
(1) Cutting. The abrasive in the grinding fluid acts as a cutting tool, shearing and compressing the ball to remove surface material. The probability of surface damage caused by abrasive grinding of the ball in the grinding fluid is lower than that caused by bonded abrasives and grinding wheels.
(2) Lubrication. When the surface material of the ball is removed to form chips, the grinding fluid can form a lubricating film between the ball and the chips, reducing friction, improving the rotation or rolling efficiency of the ball, and protecting the surface of the processed ball and improving processing efficiency.
(3) Cooling. The grinding fluid can absorb the high temperature generated by mutual squeezing between the ball and the abrasive in the processing process, reduce the temperature of the surface of the ball, the grinding disc, and the equipment, avoid surface damage to the ball due to overheating, equipment damage and other issues caused by excessive temperature.
(4) Cleaning. If grinding debris stays for a long time during processing, it may cause scratches on the surface of the ball. The additives in the grinding fluid can reduce the adhesion of debris, take away the debris, avoid affecting the processing quality, and ensure the processing accuracy.
The ceramic ball grinding fluid in this article is mainly classified based on whether there are magnetic particles and whether the abrasive reacts with silicon nitride materials, including traditional ceramic ball grinding fluids, magnetic fluid grinding fluids, and chemical-assisted grinding fluids.
Traditional Ceramic Ball Grinding Processing Grinding Fluid
The traditional ceramic grinding process mainly uses grinding fluids containing hard and brittle abrasives such as diamond, boron carbide, and silicon carbide. The hardness of the three types of abrasives is diamond, nitride boron, and silicon carbide. Diamond grinding fluid can be divided into single crystal diamond grinding fluid, polycrystalline diamond grinding fluid, and nano diamond grinding fluid according to different abrasives. Polycrystalline diamond has better wear resistance and toughness than single crystal diamond, and polycrystalline diamond grinding fluid is more widely used in the grinding of nitride silicon ceramic balls. Nano diamond not only has the hardness of diamond, but also has the characteristics of nanometer particles and ultra-large specific surface area, which greatly improves the precision of grinding. Used diamond grinding fluid to process nitride silicon ceramic balls and achieved very good results, with a ball diameter of less than 62 nm and a surface roughness of less than 1.48 nm, which meets the requirements of the national G3 level standard. Used water-based boron carbide grinding fluids of different concentrations and particle sizes to grind nitride silicon ceramic balls and determined the main form of ceramic material removal. Used silicon carbide grinding fluid to grind ceramic materials, and the surface roughness was reduced from the initial 1.49 × 10^3 nm to 2.2 × 10^2 nm, greatly improving the surface roughness.
According to the different base fluids, traditional ceramic ball grinding fluids can be divided into water-based and oil-based fluids. The water-based grinding fluid has excellent heat dissipation performance and good cooling effect, while the oil-based grinding fluid has excellent lubrication effect, with less damage to the surface of the processed ball and can protect the already processed surface, avoiding secondary scratches.
Magnetic Fluid Grinding Processing Grinding Fluid
The magnetic fluid grinding processing grinding fluid generally consists of magnetic particles, non-magnetic particles, surface-active agents, and base carriers. The magnetic particles in the magnetic fluid grinding fluid are usually Fe3O4 particles. Non-magnetic abrasives (such as B4C, CeO2, Cr2O3, etc.) are dispersed in the magnetic fluid to obtain the magnetic fluid grinding fluid. The selection of abrasives is important. Used magnetic fluid grinding fluids with different abrasives to grind nitride silicon ceramics, and found that when the abrasive was B4C, the material removal rate of the processed ball surface was the highest, and when the abrasive was Cr2O3, the precision of the processed ball was the highest. Proposed the use of sodium carbonate to alleviate the oxidation of iron powder during the polishing process of ceramic balls, which improved the service life of the magnetic fluid grinding fluid. Developed an oil-based magnetic fluid grinding fluid with good dispersibility and stability, and almost no agglomeration occurred. Selected hydroxyl iron powder with a particle size of 4 μm and CeO2 abrasive with a particle size of 1 μm, and prepared a magnetic rheological polishing solution by adding glycerol, water, and rust inhibitor in a certain proportion. The ceramic ball with an initial surface roughness of 55 nm was polished for 3 hours, and the ball precision was within (1.1-2.2) × 10^2 nm, with a surface roughness of Ra = 5 nm, which meets the requirements of the Chinese G5 standard.
The magnetic particles and material removal rate in the grinding fluid continuously decrease with the grinding time, and the expensive cost of the magnetic fluid does not meet the requirements of modern environmentally friendly grinding fluids. Magnetic fluid grinding fluids are mainly divided into oil-based and water-based, and oil-based grinding fluids have good stability and anti-oxidation ability, while water-based grinding fluids have higher shear yield stress and higher grinding efficiency.
Chemical mechanical grinding liquid
Chemical mechanical grinding combines the advantages of mechanical and chemical grinding, avoiding the problems of surface damage caused by mechanical grinding and low efficiency and poor consistency of chemical polishing. The grinding liquid provides a chemical action during the chemical mechanical grinding process. The base liquid selected for different objects varies. Generally, water is selected as the base liquid for processing silicon nitride ceramic materials. Water and abrasive particles generate high temperatures during collision, and quickly undergo chemical reactions to generate reaction products with a size of 0.1 nm, which immediately participate in the grinding process, making the ball surface smoother. Water not only participates in chemical reactions but also promotes chemical mechanical polishing.
Chemical mechanical grinding processing mainly selects suitable abrasives according to the physical and chemical properties of the processed parts, with CeO2, Fe2O3, Cr2O3, etc. being used. The above abrasives can all react with silicon nitride materials under certain conditions, breaking the chemical bonds in silicon nitride and re-forming weaker chemical bonds to generate silica with lower hardness, making it easier to remove the ceramic material on the ball surface and reducing surface damage to the ball.
In addition to reacting with some abrasives, silicon nitride materials can also undergo hydrolysis reactions, with the specific formulas as follows.
The reaction of silicon nitride with cerium oxide is: Si3N4 + CeO → 2SiO2 + CeO1.72 + CeO1.83 + Ce2O3 + N2(g)
The reaction of silicon nitride with iron oxide is: Si3N4 + Fe2O → 3SiO2 + Fe2N + N2(g)
The reaction of silicon nitride with chromium oxide is: Si3N4 + Cr2O → 3SiO2 + CrN
The hydrolysis reaction of silicon nitride is: Si3N4 + H2 → SiO2 + NH3(g)
The product silica resulting from the above chemical reactions has a much lower hardness than silicon nitride and is similar in hardness to abrasive particles. Under the mechanical action of abrasive particles, the silica ceramic material on the surface of the ceramic ball can be removed, and after the removal of the silica, reactions on the new ceramic surface can increase the efficiency of ceramic ball processing.
In a study reported in [58], silicon nitride ceramic balls were subjected to grinding experiments using chromium oxide and iron oxide grinding liquids, and it was found that the surface roughness of the balls ground with an iron oxide grinding liquid was lower than that of balls ground with a chromium oxide grinding liquid. This is because the hardness of chromium oxide abrasive particles is higher than that of silica, and they can cause damage to the surface of the ceramic ball under mechanical action. In the study reported in [59], chemical mechanical grinding was applied to ceramic material processing, with Fe2O3 used as a soft abrasive for chemical mechanical grinding of Si3N4. The ceramic material was removed by the chemical and mechanical action between silicon nitride ceramic material and iron oxide grinding liquid, and the material removal rate was up to 1.6 μm/h. A smooth and undamaged surface with a roughness of Ra = 20 nm was obtained. In [60], a water-based CeO2 grinding liquid was developed, and chemical mechanical grinding and polishing of silicon nitride ceramic balls were performed, resulting in a surface roughness of Ra = 4 nm. In the study reported in [61], the properties of silicon nitride materials were used to investigate the effect of HF aqueous solution on ceramic materials. It was found that HF aqueous solution can destroy the original network structure of silicon nitride ceramics and significantly reduce their strength.
Chemical mechanical grinding liquid originated from chemical corrosive liquids but has been improved with a basic liquid that has weaker corrosive properties and is less harmful to the environment. For ceramic and other non-metallic materials, the basic liquid is typically alkaline, with weak corrosive properties and fine material removal from the surface of the ball, resulting in high precision processing. Water is commonly used as the base carrier liquid for chemical mechanical grinding liquids. Water can promote chemical reactions and, under certain conditions, can react with silicon nitride materials to generate SiO2. Silicon nitride reacts with oxygen dissolved in water to generate amorphous Si-O compounds. The cooling effect of water-based grinding liquids is excellent and can absorb the high temperatures generated by the mutual squeezing of the ball and abrasive particles during the grinding process, reducing the temperature of the ball surface, grinding disc, and equipment and avoiding problems such as surface damage and equipment failure due to high temperature. The cleaning of the grinding waste liquid is relatively easy, does not pollute the environment, and has low cost, meeting the requirements of environmentally-friendly grinding liquids. If oil is used as the base carrier liquid for chemical mechanical grinding liquids, a layer of oil film will form between the abrasive particles and the ball being ground, blocking the progress of chemical reactions.
Conclusion and Outlook
Currently, the focus of ceramic ball processing research is to ensure the quality and improve the efficiency of processing, but to reduce processing costs, it is also necessary to increase the batch processing quantity. The main difficulty of the research is what kind of processing method to use, and the development and design of corresponding ceramic ball grinding special equipment, which can not only ensure the quality of ball processing and improve the efficiency of processing, but also realize large-scale processing of ball.
For the grinding and processing of bearings, the single-line curved groove processing method has high accuracy and efficiency, but the batch is limited. The multi-line curved groove processing method proposed in this article can solve the problem of batch processing well. Through innovative design of corresponding processing equipment, precision and efficient large-scale processing of ceramic ball can be realized.
Chemical mechanical grinding fluid has many advantages, participating in processing and promoting chemical reactions, improving the efficiency of grinding and processing. With the occurrence of chemical reactions, environmental protection issues need to be taken seriously, and additives should also meet the requirements of green production. Therefore, the development of grinding fluids should be an environmentally friendly type of grinding fluids that comply with green manufacturing.
In summary, how to balance high efficiency, high quality, and industrialized batch processing of these technologies, adopt advanced ceramic ball grinding methods and processing equipment, and apply efficient and green grinding fluids to grinding and processing ceramic balls is the direction of development of bearing ceramic ball grinding and processing technology:
(1) Based on the principle of grinding ceramic balls into spheres, while meeting the requirements of bearing ceramic ball processing quality and high yield rate, propose and develop new grinding and processing methods and equipment suitable for large-scale batch production of ceramic balls.
(2) Based on the material removal principle of the grinding fluid on ceramic balls, combined with the process requirements, propose a special series of grinding fluids suitable for efficient and green grinding and processing of ceramic balls, and achieve high-speed and green grinding and processing of ceramic balls.
(3) For the new ceramic ball processing method and equipment, optimize the ball grinding process parameters, select the optimal efficiency processing parameters while ensuring the processing quality, and ultimately achieve efficient and high-quality processing of bearing ceramic balls.