Properties and technology of silicon nitride

1、 Product nature

Composition and structure

The molecular formula of silicon nitride is Si3N4, Silicon nitride ceramics are polycrystalline materials. The crystal structure of silicon nitride ceramics belongs to hexagonal crystal system. They are generally divided into two crystal orientations, i.e. α and β. They are all composed of [SiN4] 4-tetrahedron, of which β - Si3N4 has high symmetry, small molar volume, and is a thermodynamic stable phase at temperature, while α - Si3N4 is a thermodynamic stable phase- Si3N4 is easy to form in kinetics. At high temperature (1400 ℃ ~ 1800 ℃), α phase will change into β phase, which is irreversible, so α phase is favorable for sintering.

appearance

The appearance of silicon nitride obtained from different crystal phases is different, α - Si3N4 is white or gray white loose wool like or needle like, β- Si3N4 is a dense granular polyhedron or short prism with dark color. Silicon nitride whiskers are transparent or translucent. The appearance of silicon nitride ceramics is gray, blue gray to gray black. It varies with density and phase ratio, and it also has other colors due to additives. After polishing, the surface of silicon nitride ceramic has metallic luster.

Density and specific gravity

The theoretical density of silicon nitride is 3100 ± 10kg / m3, the real specific gravity of α - Si3N4 is 3184 kg / m3, and that of β - Si3N4 is 3187 kg / m3. The bulk density of silicon nitride ceramics changes greatly due to the process, generally more than 80% of the theoretical density, about 2200-3200 kg / m3, and the porosity is the main reason for the different density. The porosity of reaction sintered silicon nitride is generally about 20%, the density is 2200-2600 kg / m3, while the porosity of hot pressed silicon nitride is less than 5%, and the density is 3000-3200 Kg / m3, compared with other materials of similar use, not only the density is lower than all superalloys, but also the density is lower in high-temperature structural ceramics.

Electrical insulation

Silicon nitride ceramics can be used as high-temperature insulating materials. The performance index of silicon nitride ceramics mainly depends on the synthesis method and purity. The free silicon which is not nitrided in the materials can deteriorate the electrical properties of silicon nitride ceramics, such as alkali metal, alkaline earth metal, iron, titanium, nickel and other magazines brought in during the preparation. In general, the specific resistance of silicon nitride ceramics is 1015-1016 ohm in dry medium at room temperature, and the dielectric constant is 9.4-9.5. At high temperature, the specific resistance of silicon nitride ceramics is still high. With the improvement of process conditions, silicon nitride can enter the ranks of common dielectric.

thermal properties 

Sintered silicon nitride has a low thermal expansion coefficient of 2.53 × 10 - 6 / ℃ and a thermal conductivity of 18.42 w / m · K, so it has excellent thermal shock resistance, second only to quartz and glass ceramics, Some experimental reports show that the reaction sintered silicon nitride samples with a density of 2500 kg / m3 have been cooled from 1200 ℃ to 20 ℃ for thousands of thermal cycles, and still do not crack. The silicon nitride ceramics have good thermal stability and can be used in high temperature for a long time. It can be used up to 1400 ℃ in oxidizing atmosphere and 1850 ℃ in neutral or reducing atmosphere.

Mechanical properties

Silicon nitride has high mechanical strength, the bending strength of general hot pressed products is 500-700mpa, and the high one can reach 1000-1200mpa; the bending strength after reaction sintering is 200MPa, and the high one can reach 300-400mpa. Although the room temperature strength of the reaction sintered products is not high, its strength still does not decrease at 1200-1350 ℃. For example, the load of reaction sintered silicon nitride is 24MPa at 1200 ℃, and its deformation is 0.5% after 1000h.

Friction coefficient and self lubrication

The friction coefficient of silicon nitride ceramics is small, and the increase range of the friction coefficient is small under the condition of high temperature and high speed, so it can ensure the normal operation of the mechanism. This is an outstanding advantage of silicon nitride ceramics. The sliding friction coefficient of silicon nitride ceramics reaches 1.0 to 1.5 at the beginning of grinding. After the precise running in, the friction coefficient is greatly reduced and kept below 0.5. Therefore, silicon nitride ceramics are considered to have Self lubricating material. Different from graphite, boron nitride, talc and so on, the main reason for this self-lubricating property is the scale layered structure of the material structure. Under the action of pressure, the friction surface decomposes slightly to form a thin gas film, thus reducing the sliding resistance between the friction surfaces and increasing the smoothness of the friction surfaces. In this way, the more friction, the less resistance, and the smaller wear amount. However, most materials, after continuous friction, tend to increase friction coefficient due to surface wear or softening due to temperature rise.

Machinability

Silicon nitride ceramics can be machined to achieve the required shape, precision and surface finish.

chemical stability

Silicon nitride has excellent chemical properties and can resist the corrosion of all inorganic acids except hydrofluoric acid and sodium hydroxide solution below 25%. Its oxidation resistance temperature can reach 1400 ℃, and it can be used up to 1870 ℃ in reducing atmosphere. It does not wet metal (especially al liquid) especially non-metal.

From the above physical and chemical properties of silicon nitride ceramics, we can see that the excellent properties of silicon nitride ceramics have special use value for the working environment of high temperature, high speed and strong corrosive medium that modern technology often encounters. Its outstanding advantages are:

It has the following advantages:

(1) The mechanical strength is high and the hardness is close to that of corundum. The bending strength of hot pressed silicon nitride at room temperature can be as high as 780-980mpa, some even higher, which can be compared with alloy steel, and the strength can be maintained up to 1200 ℃.

(2) Mechanical self lubrication, small surface friction coefficient, wear resistance, large modulus of elasticity, high temperature resistance.

(3) The thermal expansion coefficient is small, the thermal conductivity is large, and the thermal shock resistance is good.

(4) Low density and small specific gravity.

(5) Corrosion resistance, oxidation resistance.

(6) Good electrical insulation.

2、 Product use

(1) In the metallurgical industry, we make crucible, muffle furnace, burner, heating fixture, mold, aluminum conduit, thermowell, aluminum electrolytic cell lining and other thermal equipment components.

(2) High speed turning tools, bearings, supports for heat treatment of metal parts, rotor engine blades, guide vanes and turbine blades of gas turbines are manufactured in the mechanical manufacturing industry.

(3) In the chemical industry, it is used as ball valve, pump body, sealing ring, filter, heat exchanger component and fixed catalyst carrier, combustion boat and evaporation dish.

(4) In semiconductor, aviation, atomic energy and other industries, it is used to manufacture switch circuit substrate, film capacitor, electrical insulator for high temperature or temperature change, radar wire cover, missile tail nozzle, support and isolation parts in atomic reactor, and carrier of nuclear fission material.

(5) It can be used as artificial joint in pharmaceutical industry.

Technology Overview

The technological process of preparing silicon nitride ceramic products is generally composed of raw material treatment, powder synthesis, powder treatment, forming, green body treatment, sintering and ceramic body treatment.

The preparation process of silicon nitride ceramics is mainly classified according to different methods and orders of synthesis, molding and sintering.

Reactive sintering (RS)

Reaction sintered silicon nitride is formed by the mixture of Si powder or Si powder and Si3N4 Powder, nitrogen is applied at about 1200 ℃ for pre nitriding, and then the required parts are machined, and finally the final nitriding sintering is carried out at about 1400 ℃. In this process, there is no need to add sintering aids, so the material strength will not be significantly reduced at high temperature. At the same time, reaction sintered silicon nitride has the characteristics of non shrinkage and can be used to prepare the parts with complex shapes. However, due to the low density of 70% ～ 90% and the existence of a large number of pores, the mechanical properties of the products are greatly affected.

Normal pressure sintering (PLS)

The normal pressure sintering silicon nitride is made by mixing high-purity, ultra-fine, high α phase silicon nitride powder with a small amount of sintering aids, and preparing by forming, sintering and other processes. In the sintering process, α phase dissolves into liquid phase, then precipitates on β - Si3N4 crystal nucleus and becomes β - Si3N4, which is conducive to the sintering process. During sintering, nitrogen must be introduced to inhibit the high temperature decomposition of Si3N4. The ceramics with complex shape and excellent properties can be obtained by sintering under normal pressure. The disadvantage of the ceramics is that the sintering shrinkage is large, generally 16% ～ 26%, which is easy to crack and deform the products.

Re sintering (PS)

The reaction sintered Si3N4 sinter blank was placed in silicon nitride powder in the presence of sintering aids, and then re sintered at high temperature to obtain dense Si3N4 products. The sintering aid can be introduced into the ball milling of silicon powder, or it can be impregnated after reaction sintering. Because the reaction sintering process can be pre processed and the shrinkage is only 6% ～ 10% in the re sintering process, the parts with complex shape and excellent performance can be prepared.

Hot press sintering (HP)

Silicon nitride powder and sintering aid were placed in graphite mould and sintered under high temperature. As the external pressure increases the driving force of sintering, the α - β transformation and densification speed are accelerated. High strength silicon nitride ceramics with a density of more than 95% can be obtained by hot pressing, which have high properties and short manufacturing cycle. However, this method can only produce products with simple shape, and the processing cost of parts with complex shape is high. Because of unidirectional pressure, there is a preferred orientation in the structure, so the performance is different in the parallel and vertical direction with the hot pressing surface.

Gas pressure sintering (GPS)

Air pressure sintering is to sinter Si3N4 compacts in 5-12mpa nitrogen at 1800-2100 ℃. Due to the high nitrogen pressure, the decomposition temperature of Si3N4 is increased, which is conducive to the selection of sintering aids that can form high refractoriness intergranular phase to improve the high temperature performance of the material.

Hip method

The mixture powder of silicon nitride and sintering aid is encapsulated in a metal or glass sheath, and then sintered under high temperature by high pressure gas after vacuumizing. The commonly used pressure is 200MPa, and the temperature is 2000 ℃. The theoretical density of silicon nitride can be achieved, but its process is complex and its cost is high.

In recent years, some other sintering and densification processes have been developed, such as ultra-high pressure sintering, chemical vapor deposition, explosive forming, etc.