Brief introduction of high-tech structural ceramic materials and the development and application advantages of ceramic bearings

Abstract：Starting with the analysis of high-tech ceramic materials, this paper expounds the performance characteristics of ceramic bearing materials and the comparison of steel bearing materials, as well as the development trend and prospect of ceramic bearings at home and abroad at present.

Keywords: structural ceramic, ceramic bearing

As we all know, bearing is an indispensable basic component in contemporary mechanical equipment. Although its rise is only a hundred years old, with superb modern science and technology, there are more than 100,000 kinds of bearings. In recent years, with the social progress and the high-speed development of science and technology, the application environment and conditions of bearings have become more and more diversified, and the requirements for the structure, material and performance of bearings have become higher and higher. Some high-tech fields and machinery working in some special environments, such as aerospace, nuclear energy, metallurgy, chemical industry, petroleum, instruments, machinery, electronics, textiles, pharmaceuticals and other industries, need to operate at high temperature and high speed. These new requirements are far from being satisfied only by improving the structure or lubrication conditions of traditional metal bearings, so it is necessary to develop new materials and fundamentally make breakthroughs and innovations. Studies at home and abroad have found that some ceramic materials have excellent performance, can withstand the harsh working environment that metal materials and polymer materials are not competent for, and have all the important characteristics required by bearing materials. Therefore, the application of ceramic materials in bearing manufacturing industry has become a hot spot in the development and application of high-tech in the world, and has become a symbol of material technology revolution in mechanical industry.

1. Brief introduction of high-tech structural ceramic materials

High-temperature structural ceramics are called advanced ceramics in Europe and America, fine ceramics in Japan, and high-tech ceramics in China at present. Compared with traditional ceramic materials (daily-use ceramics and building ceramics), high-tech ceramic materials have an essential difference, that is, the raw materials of traditional ceramics are natural minerals such as Shi Ying, feldspar and clay, while the raw materials of high-tech ceramic materials are chemical raw materials that have been artificially synthesized or refined by ー series. High-tech ceramic materials are usually made of ultra-fine powder (sub-micron and nano-powder) after ultra-high temperature and high pressure sintering. Compared with traditional ceramic materials, its strength has been improved hundreds of times, and with the excellent high temperature resistance, wear resistance, corrosion resistance, insulation and other characteristics of ceramic materials, it has been widely used in many important fields, especially in bearing manufacturing.

At present, high-tech structural ceramics mainly include silicon nitride (Si₃N₄), oxide dislocation (ZrO₂) and alumina (Al₂O₃), etc. Its performance is compared with that of bearing steel as shown in the following table.

Material performance comparison table

From the comparison table of material properties, it can be seen that structural ceramic materials have higher hardness and strength than bearing steel. In bearing applications that require high performance, structural ceramic materials are considered to have the best mechanical and physical comprehensive characteristics. Comprehensive rolling contact peeling test and bearing test prove that dense and homogeneous ceramic materials have good rolling contact fatigue resistance (this property is very important for the requirements of bearing rolling elements). Ceramic bearings with silicon nitride ceramic balls as rolling elements can significantly improve the contact fatigue life of bearings and greatly expand the application fields of rolling bearings. However, the traditional steel rolling bearing repeatedly produces contact pressure and deformation due to the external load and rotation at the contact point between the sleeve ring and the rolling body. Due to the limitation of the material properties of ordinary steel bearings, the main form of bearing failure is fatigue spalling, so the fatigue life is short and the application range is greatly limited.

2. Development and application advantages of ceramic bearings

With the rapid development of modern machinery industry, new engineering materials and ceramics have been popularized and applied in the engineering field, and the requirements for bearing performance have become higher and higher. Since 1990s, developed countries such as the United States, Sweden and Japan have been competing to develop ceramic bearings, which can be used instead of metal bearings in special working environments such as high speed, high temperature and corrosion. In view of this situation, many research institutes at home and abroad are engaged in the research and development of ceramic bearings and hybrid ceramic bearings (that is, rolling bearings with inner and outer rings made of metal and rolling bodies made of high-tech ceramics), which have become the most promising new industry in the bearing industry with its incomparable advantages. Specific advantages are:

2.1 High speed

The weight of ceramic material is only 40% of the weight of the same steel. The characteristic of low density can realize the lightweight and high speed of the bearing, so that the ceramic bearing can inhibit the increase and slippage of the roller load caused by centrifugal force when rotating at high speed. The ceramic bearing

The rotation speed is 1.3 ~ 1.5 times that of steel bearings, and its DN value can reach 3 million. For example, angular contact ball bearings have a certain contact angle, and their rolling bodies and raceway surfaces will rotate and slide. When ceramic rolling bodies with low density are used, the rotation and sliding are not only small, but also beneficial to bearing heating and surface damage, which is also very beneficial to aerospace vehicles.

2.2 High rigidity

The elastic modulus of silicon nitride is much higher than that of metal, which is 1.5 times that of metal. Therefore, the elastic deformation after stress is small, and the rigidity of relative load is high, which can increase the rigidity by about 15 ~ 20%, thus reducing the vibration of machine tools. It has gained good application value in high-precision systems, such as the spindle of ultra-precision machine tools and high-precision aerospace bearings.

2.3 Long life

Because ceramic materials have less dislocation, low mobility and high hardness, which is generally more than one time higher than that of metal, it can reduce wear loss and make ceramic bearings have good wear resistance. In addition, the mechanical strength of silicon nitride ceramics is not low, and its tensile strength and bending strength are comparable to those of metals. However, the compressive strength is extremely high, which is about 5 ~ 7 times of that of metal materials. Especially at high temperature, it can still maintain high strength and hardness, and the strength remains basically unchanged even at 12000t t. In the case of foreign substances mixed in, ceramic balls rarely peel off and lose effectiveness. Therefore, ceramic bearings usually have a longer service life, which is generally 3 ~ 5 times higher than that of steel bearings.

2.4 Low fever

Because the friction coefficient of silicon nitride ceramic material is small, which is about 30% of that of standard bearing steel, compared with metal materials, the thermal conductivity of silicon nitride ceramic material is poor, so the heat generated by ceramic bearing during working is small, which can prolong the life of grease.

2.5 Low thermal expansion

The thermal expansion coefficient of silicon nitride is about 20% of that of bearing steel, so the dimensional change of ceramic bearings with temperature change is small, thus avoiding excessive heat accumulation and fatigue spalling failure, which is beneficial to use in the environment with large temperature change.

2.6 Corrosion resistance

Because of the inactive chemical characteristics of ceramic materials, ceramic bearings have certain corrosion resistance. Therefore, ceramic bearings can be used in all applications where steel bearings fail in advance due to lack of chemical resistance, such as chemical machinery and equipment, machinery used in food, marine and other departments and applications in atomic energy equipment.

2.7 Nonmagnetic

In a strong magnetic environment, when steel bearings are used, the micro-powder worn from the bearings themselves is adsorbed between the rolling elements and the raceway surface, which becomes the main reason for the early spalling and damage of the bearings and the increase of noise. Because ceramic bearings are completely nonmagnetic and have normal load-carrying capacity, they can be used in occasions where completely nonmagnetic bearings are needed.

2.8 Insulativity

The ceramic material has high constant resistivity, so it can be used as a good insulating material to prevent the bearing from being damaged by electric arc.

Since the first set of ceramic bearings was successfully developed in the United States in 1972, countries all over the world have been competing to develop a new generation of ceramic bearings with higher performance. From the application results, the most prominent effect is that the service life and the ultimate rotational speed of the bearings have been greatly increased, and basic parts have been provided for the development of high-speed and ultra-high-speed and high-precision machine tools. Besides, the application in high temperature, corrosion, insulation, vacuum and other industries has achieved good results. For example, the dn value of ceramic bearings in high-speed spindle parts has reached 2.12 ~ 2.5 million; In the high temperature field, the high temperature resistance of ceramic bearings has reached 800て; It can work stably for a long time in strong acid, alkali, salt and other highly corrosive media. At the 200I International Machine Tool Exhibition and International Bearing Exhibition, Wazhou Group, a state-level enterprise, developed and researched ceramic bearings, and our ceramic bearings received great response when they first appeared. Among them, that most eye-catching product are:

① Self-lubricating corrosion-resistant bearing

The self-lubricating and corrosion-resistant bearing developed by ceramic materials is a technical leap in the history of rolling bearings. The bearing can work normally in vacuum, various gas liquids or even strongly corrosive gas liquids without any lubrication measures. In this way, in many occasions, such as spaceships, satellites, nuclear reactors, submarines, etc., some parts need to work for a long period without lubrication, and ceramics are the most ideal bearing parts. Because silicon nitride itself is a solid lubricating material, the bearings using silicon nitride as rolling elements can work without lubrication at all. For example, ceramic bearings specially made for Tsinghua University high-temperature gas-cooled reactor require no lubrication in any form when working in nitrogen, and ordinary steel bearings cannot be used. However, ceramic bearings can solve this problem satisfactorily, and the actual application situation is very ideal. In addition, the inner and outer rings are made of ceramic made of stainless steel.

Porcelain bearings can work in corrosive gas and liquid, which is impossible for ordinary steel rolling bearings. This greatly broadens the application range of rolling bearings. Just as the technical progress of replacing sliding bearings with rolling bearings has greatly promoted the development of machinery manufacturing industry, the popularization and application of corrosion-resistant rolling bearings will have a wide and far-reaching impact on machinery manufacturing industry.

② High and low temperature resistant bearing

Because the mechanical properties of metal materials will decrease greatly when the temperature rises or falls, and lubrication is easy to fail at high and low temperatures, the bearing life is generally short at high and low temperatures. The high and low temperature resistant bearing developed by using the high and low temperature resistant characteristics of ceramic materials has achieved a major technical breakthrough in this field. At present, the high-temperature bearing can withstand "350て" and the low-temperature bearing can withstand 230 ー. With the development of high and low temperature resistant ceramic bearing technology, it will be widely used in many extremely important fields of national defense and civil use, such as aircraft engines, rocket engines, marine diesel engines, steel mills, generators, motors, superconductors, etc.

Generally speaking, ceramic rolling bearing is the newest bearing industry with the highest technical content, which needs to be developed and utilized in a wider field, and its potential application field is very huge.

3. The development status and prospect of ceramic bearings

The research of ceramic bearings in China started late, but it is still in the experimental research stage. The main reasons that affect the wide application of ceramic materials in ball bearings are its difficult machinability and high manufacturing cost. There are two main problems to be solved in the research and development of high-speed and high-precision ceramic ball bearings in the whole bearing industry: one is the research on the blank making of ceramic balls, the other is the research on the processing technology of high-precision ceramics, and there is little research on new materials such as SiC, ZrO₂, ZTA, Sailon and so on.

Secondly, the production of ceramic bearings is the industrial combination of high-tech ceramics and bearing manufacturing technology, both of which have a high degree of specialization. If the industrial combination of ceramics and bearings is not solved, it will be an empty talk to produce ceramic bearings that meet the market demand in large quantities and at low cost. Nowadays, domestic enterprises that can produce bearings can't manufacture ceramic materials, and those that can produce ceramic materials can't manufacture bearings. There is a serious disconnect between majors and disciplines in practical application. Recently, researchers of Guangdong Morgan Metallurgical Industry Co., Ltd. in our city joined forces with universities to start with industrialization and carry out a series of development and research on corresponding equipment, technology and tooling. Great progress has been made in the research of ceramic bearing balls, and the processing speed has been more than doubled compared with the existing domestic technology, and all technical indexes have reached or exceeded the G5 standard requirements. It can promote the industrialization and commercialization of ceramic bearings in China. However, in order to do a good job in the industrial production from research and development to application of ceramic bearings, further research and exploration are needed in the following aspects: First, research the rolling contact performance of ceramic bearings under the conditions of wider application range and worse lubrication conditions; The second is to study the structural coordination design of ceramic bearing related parts, as well as the reliability and economy of processing; Third, the nondestructive testing method and damage prediction technology of related parts of ceramic bearings; Fourthly, the inspection standards of ceramic bearings are formulated.

Since the Eighth Five-Year Plan, the research and development of ceramic ball bearings has been listed as a key scientific and technological project. The development of ceramic bearings is in the ascendant, and the practical application of nano-materials and composite materials has brought greater vitality and vitality to the development of ceramic bearings. Ceramic bearings are two typical high-yield products, that is, high-tech content and high added value. They are internationally recognized as the mainstream products of new bearings and will occupy 40 ~ 60% of the bearing market. Recently, the market size is about 1 billion, which is mainly used in high-speed machine tools, high-speed motors and low-temperature equipment. At present, the development and production can gradually replace all application areas of precision and medium-speed full bearings. The price of precision ceramic bearings is 1.5 ~ 1.8 times that of similar steel bearings, and the service life can be extended by at least 3 times. According to the current production situation in China, it is far from meeting the market demand, so the market prospect is very promising. Although the price is relatively high, under the same conditions, the comprehensive economic benefits are very significant. In practical application, the service life of ceramic bearings is 5~10 times that of heat-resistant steel bearings and 5~15 times that of stainless steel bearings. Moreover, the price of products of the same specification is obviously lower than that of foreign countries. It is reported that, taking the outer diameter of 32mm as an example, the ceramic ball bearings in the United States cost 80 USD, while the domestic ceramic bearings cost only 12 USD in 110 yuan. There is a strong international market competition among products of the same quality. It can be seen that high-performance ceramic bearing technology has great economic benefits and extensive social benefits. Its application prospect is very broad, even as someone predicted: ceramic bearings will completely replace steel bearings in the future.

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Ceramic bearings are comprised of ceramic rolling elements (balls) enclosed in a ferrous (typically steel) inner and outer race. Since these bearings are constructed from many materials, they often operate in hybrid capacities. We also build ceramic bearings (i.e., bearings with ceramic races), but they are not as common because they are less durable. We construct the rolling elements in ceramic bearings using several ceramic materials, including • Silicon nitride (Si3N4) • Aluminum oxide (Al2O3) • Zirconium oxide (ZrO2) • Silicon carbide (SiC) Applications that involve high speeds and high loads often involve ceramic bearings. These bearings offer several benefits over conventional steel bearings, including enhanced acceleration capability, increased stiffness, and reduced friction. Hybrid ceramic bearings also offer the benefit of not allowing electrical currents to run through the bearing, which allows for prolonged use.