Views: 0 Author: Site Editor Publish Time: 2023-02-02 Origin: 粉体圈儿
A large number of experiments have proved that "ball" is the weakest part of precision bearings working under high speed environment, and about 60%-70% of high-speed bearing failures are caused by steel ball fatigue of different degrees. In order to improve the performance of high-speed bearings and prolong their fatigue life, the application of structural ceramics to manufacture ball or other bearing parts at home and abroad can significantly improve the performance and life of "high-speed bearings".
There are a wide variety of ceramics, but in bearing applications where high performance is required, silicon nitride is considered to have the best combined mechanical and physical properties. The important reason is that other ceramics are damaged in a catastrophic way, while silicon nitride ceramics are damaged in a way similar to the failure of bearing steel, that is, local spalling. Therefore, as a rolling bearing material, from the point of view of rolling fatigue life and reliability, only silicon nitride can be competent! The following will be a simple analysis of the advantages of silicon nitride bearing materials and their manufacturing.
①Smaller than density, small centrifugal force - more conducive to high-speed operation
The density of silicon nitride ceramic material is about 3.2×103kg/m3, while the density of bearing steel is about 7.8×103kg/m3. The density of silicon nitride ceramics is only about 40% of the density of bearing steel. When the rolling body uses silicon nitride ceramics, the bearing can effectively inhibit the increase of the rolling body load caused by centrifugal force at high speed rotation. Therefore, the use of low density silicon nitride ball is more conducive to the high speed development of bearings.
②Heat resistance - can adapt to higher temperature working conditions
General steel bearing use temperature more than 120℃, hardness will reduce, rolling life will also decline. However, silicon nitride completely maintains its original hardness in the range of 400℃, and only at about 800℃, its hardness and strength began to decline. Therefore, for bearings used in high temperature environments, silicon nitride material is very suitable. For example: aircraft jet engines, gas turbines, nuclear reactor systems, X-ray tube tungsten disk, as well as rockets, spacecraft.
③ Small linear expansion coefficient - can be used in the field of environmental temperature change
The linear expansion coefficient of silicon nitride is about 1/4 of that of bearing steel, so the size change of silicon nitride bearing material with temperature change is small. Therefore, the bearing made of silicon nitride ceramic material can effectively prevent the bearing material from being "locked" due to the size change caused by the temperature change. In order to ensure the stable operation of equipment, reduce the loss caused by equipment failure.
④Excellent self-lubrication performance
Silicon nitride ceramic material itself has anti-friction, anti-wear, lubrication functions, under poor lubrication conditions, such as lean oil lubrication, no oil dry friction, showing superior anti-friction self-lubrication performance, has a good emergency state, can effectively avoid equipment sudden failure caused by the loss.
Hot-pressed silicon nitride ball 'gets smooth' after 10 years of work
⑤ Stable chemical properties - corrosion resistance
Silicon nitride has good chemical resistance to most acids and bases. Only hydrofluoric acid or hydrochloric acid and nitric acid mixture can corrode silicon nitride. Therefore, the bearing material prepared by silicon nitride can be in corrosive acid, alkali, salt and other solutions for a long time, compared with steel bearings, its average life will be 4-25 times higher than stainless steel bearings. Can be used in chemical machinery equipment, food, Marine, sewage treatment and other departments used in the machine, reduce the trouble caused by corrosion.
⑥ No magnetism, insulation
In the strong magnetic environment, the use of steel bearings, from the bearing itself wear down the powder is adsorbed on the rolling body and rolling surface, which will become the main reason for early spalling damage and noise increase of bearings. The use of bearing steel for traction motor of railway vehicles will cause electrical corrosion and reduce the life of traction motor bearing. Using insulated ceramic silicon nitride bearing material can effectively improve the effect of electric corrosion on bearing.
Silicon nitride ceramic ball is an important part of silicon nitride ceramic bearing. The manufacturing process of silicon nitride ceramic ball can be divided into four stages. The first stage is the preparation of si3N4 raw material, that is, the Si3N4 powder and sintering additives are uniformly mixed in a certain proportion. The second stage is the molding of silicon nitride ceramic ball, commonly used extrusion molding, injection molding, dry pressing molding and grouting molding and other molding methods. The third stage is si3N4 ceramic ball densification sintering, sintering methods include atmospheric sintering, hot pressing sintering, pressure sintering and hot isostatic sintering. The fourth stage is post-treatment grinding processing, namely rough grinding, fine grinding, rough grinding, fine polishing and polishing.
① Preparation of high-end silicon nitride powder
The main synthesis methods of silicon nitride powder are silicon nitride method and chemical synthesis method. In China, the silicon nitride method is used. Compared with the powder prepared by chemical synthesis method, the powder prepared by the latter has high purity, good sphericity, high sintering activity and low influence on the stability of silicon raw material, which is the preferred raw material for the preparation of high-precision silicon nitride ball bearing.
②performance deficiency
Compared with bearing steel, its low toughness, high hardness and moderate bending strength are still the fatal weaknesses of silicon nitride ceramic materials sensitive to damage and defects. High reliability of silicon nitride ceramic bearing requires complete non-destructive testing. The difference of mechanical properties of ceramic bearing results in obvious changes in the stiffness of ceramic bearing, which will lead to obvious changes in the supporting stiffness, dynamic response and additional dynamic load of the rotor system.
③The restriction of precision grinding technology
In order to manufacture ultra-precision high-end silicon nitride ball bearings, the first task is to manufacture ultra-precision silicon nitride ceramic balls. The basic points to ensure ultra-precision mass production of silicon nitride ball are:
● the surface of silicon nitride ball to achieve equal probability grinding, that is, to ensure that every particle on the ball has the same grinding probability;
● Grinding efficiency with automatic size selectivity, that is, the large ball or long axis direction automatic priority grinding;
● Grinding technology is easy to achieve large-scale industrialization and low cost production. However, all kinds of grinding techniques reported publicly in China cannot meet the above three requirements at the same time.
⑤Rolling contact fatigue of silicon nitride ball bearing
A large number of studies have shown that the RCF (rolling contact fatigue) performance of Si3N4 ball depends on its production technology level, and there are many main factors affecting its performance, which can be roughly summarized as pore, surface micro-crack (annular/linear shape), surface strength, residual stress, surface roughness and other internal causes, and lubrication conditions, load, speed and other external causes. The influence of the above factors should be fully considered when preparing silicon nitride ball bearings.
Surface microstructure of silicon nitride ceramic bearing
