Silicon nitride is a compound of nitrogen and silicon that was discovered more than a hundred years ago. It was first synthesized in Germany and only began to be used in the 1950s. As an engineering material, it was valued in the 1960s. Silicon nitride is a synthetic material, and naturally occurring silicon nitride has not been found in nature.
As a high-temperature structural ceramic, silicon nitride ceramic has the characteristics of high strength, good thermal shock resistance, low high-temperature creep, wear resistance, excellent oxidation resistance and high chemical stability. It is one of the excellent engineering ceramics. Although silicon nitride has good properties, it also has a common feature of ceramics-brittleness. The fatal weakness of brittleness makes it impossible to guarantee the reliability in the application. Therefore, improving its toughness and improving its reliability has always been an important direction in the research of silicon nitride ceramics.
Toughening method
1. Particle toughening
Particle toughening is to add a certain size of particles with high elastic modulus, such as SiC, TiC, TiN, etc., to Si3N4 materials. Particle toughening has nothing to do with temperature and can be used as a high-temperature toughening mechanism. However, this method generally can only achieve a toughening effect of 40% to 70%, and its toughening effect is not obvious.
2. Phase change toughening
ZrO2 phase change toughening is to disperse ZrO2 particles in the Si3N4 matrix, and use the stress of the tetragonal monoclinic phase to induce the phase change to produce a volume change of about 5%, which can offset the applied stress and prevent the propagation of cracks to achieve the purpose of toughening.
3. Fiber toughening
Fiber toughening is the use of C, SiC and other long fibers to compound the toughening of Si3N4 ceramics. The main mechanism is crack deflection or bifurcation, pull-out effect and bridging effect.
4. Self-toughening
Self-toughening is to make a part of Si3N4 grains develop in situ into columnar grains with a higher aspect ratio by adjusting the material components and controlling the preparation process conditions, thereby obtaining various mechanisms similar to fiber toughening and achieving the toughening effect.
5. Layered toughening
In recent years, scholars at home and abroad have gained enlightenment from the biological world: the layered structure of shells can produce greater toughness, so new materials, namely layered composite ceramic materials, can be designed from the perspective of the macrostructure of the material.
6. Carbon fiber toughening
Carbon fiber is fired from raw fiber at high temperature, and undergoes low-temperature oxidation, medium-temperature carbonization, high-temperature graphitization and other processes. It has the advantages of high strength, high modulus, low density, high temperature resistance, low linear expansion coefficient, and high thermal conductivity. As a reinforcing and toughening material, it overcomes the shortcomings of other toughening materials.
The prerequisite for whether carbon fiber can play a reinforcing role in the silicon nitride matrix must first solve the actual effect of carbon fiber reinforcement, and ultimately depends on the degree of bonding between the carbon fiber and the silicon nitride matrix after sintering.
7. Toughening of carbon nanotubes
Theoretical calculations show that carbon nanotubes have extremely high strength and excellent toughness. Carbon nanotubes have excellent mechanical properties, their strength is about 100 times that of steel, but their density is only 1/6 of steel, and they have excellent toughness in the direction perpendicular to the tube axis of carbon nanotubes. They are considered to be the future "super fiber". ".
The main mechanism of carbon nanotube toughening silicon nitride ceramic composites is the fiber pullout mechanism.
Application of silicon nitride ceramics
1. Aerospace and military industry
Aviation manufacturing is the most concentrated area of high-tech in the manufacturing industry. It belongs to advanced manufacturing technology and is a leader in new materials, new processes and new technologies. Take the turbine engine of an airplane as an example to illustrate the application of silicon nitride in aviation manufacturing.
Ceramic silicon nitride is heat-resistant, and can still have high strength and rigidity at 1400°C (but the mechanical strength will decrease when it exceeds 1200°C), but it is relatively brittle. The use of continuous fiber reinforced reinforced ceramics can be applied to turbine parts, especially Ceramic blades, turbine outer rings and air bearings for small engines. In addition, the specific density of silicon nitride ceramics is only 41% of that of steel bearings, which can effectively reduce the weight of aircraft engines and reduce fuel consumption.
2. Mechanical engineering
Silicon nitride ceramics have low friction coefficient, self-lubricating properties, high strength, small thermal expansion coefficient, small volume change due to temperature, and effectively prevent the ball/seal ring from jamming. It can be made into bearing balls and mechanical seal rings.
Silicon nitride is strong, can be used in bearing manufacturing, can withstand harsh working environments, and has a higher working life than ordinary bearings, but the production cost is also relatively high.
Traditional valves are made of metal materials. Due to the limitation of metal materials, the corrosion damage of metal has a considerable influence on the wear resistance, reliability and service life of the valve; some metal valves used in the petroleum industry are susceptible to chemical corrosion and lose their work. ability. The excellent corrosion resistance, wear resistance and high temperature resistance of silicon nitride ceramics can be competent in this field.
3. Superfine grinding field
Silicon nitride has high hardness, second only to diamond and cubic boron nitride. Because of its very low consumption, it reduces the wear of the grinding media and the pollution of the grinding material, which is beneficial to obtain higher purity ultrafine powder.
4. High-performance machine tool cutting tools
In the modern machining process, the most effective way to improve machining efficiency is to use high-speed cutting technology. Silicon nitride tools are particularly suitable for rough and fine machining, high-speed cutting and heavy cutting of cast iron and high-temperature alloys. Its cutting durability is several to ten times higher than that of cemented carbide tools. Silicon nitride has very high wear resistance. It has better chemical stability than cemented carbide, and can be cut under high-speed conditions for a longer period of time. Compared with cemented carbide tools, the average efficiency is more than 3 times higher.