Toughening method and application of silicon nitride ceramics

Silicon nitride is a compound of nitrogen and silicon that has been discovered more than a hundred years ago.
It was first synthesized in Germany and only began to be used in the 1950s
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As an engineering material, it was valued in the 1960s
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Silicon nitride is a synthetic material, and naturally occurring silicon nitride has not been found in nature
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? Silicon nitride ceramics, as a kind of high temperature structural ceramics, 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.

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Although silicon nitride has good properties, it also has the common property of ceramics—brittleness
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The fatal weakness of brittleness makes it impossible to guarantee the reliability in the application
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Therefore, improving its toughness and improving its reliability has always been an important research direction of silicon nitride ceramics
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? Toughening method of? 1, is the addition of toughening particles toughening particles of certain size particles in the Si3N4 material having a high elastic modulus, such as SiC, TiC, TiN and the like
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Particle toughening is independent of temperature and can be used as a high-temperature toughening mechanism
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However, this method generally can only achieve a toughening effect of 40% to 70%, and its toughening effect is not obvious
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2.
Phase change toughening ZrO2 phase change toughening is to disperse ZrO2 particles in the Si3N4 matrix, and use the stress of the tetragonal phase to monoclinic phase to induce the phase change to produce a volume change of about 5%, which can offset the applied stress and prevent cracks.
Expansion to achieve the purpose of toughening
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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
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Self-toughening Self-toughening is to make some 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.
Achieve toughening effect
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Layered toughening In recent years, scholars at home and abroad have gained inspiration from the biological world: the layered structure of shells can produce greater toughness, so new materials, namely layered composite ceramics, can be designed from the perspective of the macrostructure of the material.
Material
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Carbon fiber toughened carbon fiber is fired from raw fiber at high temperature, and has undergone low-temperature oxidation, medium-temperature carbonization, high-temperature graphitization and other processes.
It has high strength, high modulus, low density, high temperature resistance, low linear expansion coefficient, and thermal conductivity.
Higher merits
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As a reinforcing and toughening material, it overcomes the shortcomings of other toughening materials
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? 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
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Theoretical calculation of carbon nanotube toughening shows that carbon nanotubes have extremely high strength and excellent toughness
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Carbon nanotubes have excellent mechanical properties, their strength is about 100 times that of steel, but their density is only 1/6 that of steel, and they have excellent toughness in the direction perpendicular to the axis of the carbon nanotubes.
They are considered the future "super fiber".
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? The main mechanism of carbon nanotube toughening silicon nitride ceramic composites is the fiber pull-out mechanism
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Application of silicon nitride ceramics 1.
Aerospace and military industry.
Aerospace 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
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Take the airplane turbine engine as an example to illustrate the application of silicon nitride in aviation manufacturing
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? 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 These are ceramic blades, turbine outer rings and air bearings for small engines
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In addition, silicon nitride ceramics have a low specific density, which is only 41% of that of steel bearings, which can effectively reduce the weight of aircraft engines and reduce fuel consumption
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In the field of mechanical engineering, silicon nitride ceramics have low friction coefficient, self-lubricating property, high strength, small thermal expansion coefficient, small temperature change in volume, and effectively prevent the ball/seal ring from jamming.
It can be made into bearing balls and mechanical seal rings.

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? Silicon nitride is strong, can be used in bearing manufacturing, can withstand harsh working environment, working life is also higher than ordinary bearings, but the production cost is also relatively high
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? Traditional valves are made of metal materials.
Due to the limitation of metal materials, the corrosion damage of metal has a considerable impact 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 loss Work ability
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The excellent corrosion resistance, wear resistance, and high temperature resistance of silicon nitride ceramics can be competent in this field
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3.
In the ultrafine grinding field, silicon nitride has high hardness, which is second only to diamond and cubic boron nitride
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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
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High-performance machine tool cutting tools In the modern processing process, the most effective way to improve processing efficiency is to use high-speed cutting technology
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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 and lasts for a longer period of time.
Compared with cemented carbide tools, the efficiency is improved by more than 3 times on average
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