Description of nine product properties of silicon nitride materials

2020-07-08


Nine Product Properties of Silicon Nitride Materials

1、 Composition and structure

The molecular formula of silicon nitride is Si3N4, which belongs to covalent bonding compounds. Silicon nitride ceramics belong to polycrystalline materials, and their crystal structure belongs to the hexagonal crystal system. Generally, they are divided into α、β Both crystal orientations are composed of [SiN4] 4-tetrahedron β- Si3N4 has a high symmetry, a small molar volume, and is a thermodynamic stable phase at temperature, while α- Si3N4 is relatively easy to form dynamically. At high temperatures (1400 ℃ to 1800 ℃), α The phase will undergo a phase transition, becoming β Type, this phase transition is irreversible, so α The phase is favorable for sintering.

2、 Appearance

The appearance of silicon nitride obtained with different crystal phases is different, α- Si3N4 is a white or grayish white loose wool or needle like body, β- "Si3N4 is darker in color and appears as a dense granular polyhedron or short prism. Silicon nitride whiskers are transparent or translucent. The appearance of silicon nitride ceramics is gray, bluish gray, to grayish black, which varies depending on density and phase ratio, as well as other colors due to additives.". After polishing, the surface of silicon nitride ceramics has a metallic luster.

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3、 Density and Specific Gravity

The theoretical density of silicon nitride is 3100 ± 10kg/m3, which is actually measured α- The true specific gravity of Si3N4 is 3184 kg/m3, β- The true specific gravity of Si3N4 is 3187 kg/m3. The bulk density of silicon nitride ceramics varies greatly depending on the process, generally over 80% of the theoretical density, ranging from about 2200 to 3200 kg/m3. The main reason for the difference in density is the porosity. The porosity of reaction sintered silicon nitride is generally around 20%, with a density of 2200 to 2600 kg/m3, while the porosity of hot pressed silicon nitride is below 5%, with a density of 3000 to 3200 kg/m3, which is compared to other materials with similar uses, Not only is the density lower than that of all superalloys, but it is also one of the lower densities in high-temperature structural ceramics.

4、 Electrical insulation

Silicon nitride ceramics can be used as high-temperature insulating materials, and their performance indicators mainly depend on the synthesis method and purity. Free silicon not nitrided in the material, as well as alkali metals, alkaline earth metals, iron, titanium, nickel, and other impurities brought into the preparation process, can worsen the electrical properties of silicon nitride ceramics. Generally, the specific resistance of silicon nitride ceramics in dry media at room temperature is 1015 to 1016 ohms, and the dielectric constant is 9.4 to 9.5. At high temperatures, silicon nitride ceramics still maintain a high specific resistance value. With the improvement of process conditions, silicon nitride can enter the ranks of commonly used dielectrics.

5、 Thermal properties

Sintered silicon nitride with a low coefficient of thermal expansion of 2.53 × 10-6/℃, with a thermal conductivity of 18.42W/m · K, it has excellent thermal shock resistance, second only to quartz and microcrystalline glass. According to experimental reports, the reaction sintered silicon nitride sample with a density of 2500 kg/m3 has been cooled from 1200 ℃ to 20 ℃ for thousands of thermal cycles, and still does not break. Silicon nitride ceramics have good thermal stability, and can be used at high temperatures for a long time. It can be used up to 1400 ℃ in an oxidizing atmosphere and up to 1850 ℃ in a neutral or reducing atmosphere.

6、 Mechanical properties

Silicon nitride has a high mechanical strength. Generally, the bending strength of hot-pressed products is 500~700MPa, with a high strength of up to 1000~1200MPa; The flexural strength after reaction sintering is 200 MPa, with a high strength ranging from 300 to 400 MPa. Although the room temperature strength of reaction sintered products is not high, their strength does not decrease at a high temperature of 1200 to 1350 ℃. The high-temperature creep of silicon nitride is small. For example, the load of reaction sintered silicon nitride is 24 MPa at 1200 ℃, and its deformation is 0.5% after 1000 h.

7、 Friction coefficient and self-lubrication

The friction coefficient of silicon nitride ceramics is small, and under high temperature and high speed conditions, the increase in the friction coefficient is also small, which can ensure the normal operation of the mechanism. This is a prominent advantage of silicon nitride ceramics. When silicon nitride ceramics start to wear, the sliding friction coefficient reaches 1.0 to 1.5. After precise grinding, the friction coefficient greatly decreases, maintaining below 0.5. Therefore, silicon nitride ceramics are considered to have self-lubricating properties. The main reason for this self-lubrication, unlike graphite, boron nitride, and talc, lies in the lamellar structure of the material structure. Under pressure, the friction surface decomposes slightly to form a thin gas film, thereby reducing the sliding resistance between the friction surfaces and increasing the smoothness of the friction surface. In this way, the more friction, the smaller the resistance, and the particularly small wear amount. After continuous friction, most materials tend to gradually increase their friction coefficient due to surface wear or softening due to temperature rise.

8、 Machinability

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

9、 Chemical stability

Silicon nitride has excellent chemical properties and is resistant to corrosion by all inorganic acids except hydrofluoric acid and less than 25% sodium hydroxide solution. Its oxidation resistance temperature can reach 1400 ℃, and it can be used up to 1870 ℃ in a reducing atmosphere. It is not wet for metals (especially for AL liquids), especially for non-metals.

From the above physical and chemical properties of silicon nitride ceramics, it can be seen that the excellent performance of silicon nitride ceramics has special application value for the working environment of high-temperature, high-speed, and strongly corrosive media often encountered in modern technology. Its outstanding advantages are:

It has the following points:

(1) High mechanical strength, hardness close to corundum. The room temperature bending strength of hot pressed silicon nitride can be as high as 780-980MPa, some even higher, compared to alloy steel, and the strength can be maintained until 1200 ℃ without degradation.

(2) Mechanical self-lubrication, low surface friction coefficient, wear resistance, high elastic modulus, and high temperature resistance.

(3) Low thermal expansion coefficient, high thermal conductivity, and good thermal shock resistance.

(4) Low density, low specific gravity.

(5) Corrosion resistance, oxidation resistance.

(6) Good electrical insulation.

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