9 product properties of silicon nitride
1. Composition and structure
The molecular formula of silicon nitride is Si3N4, which is a covalently bonded compound. Silicon nitride ceramics are polycrystalline materials, and the crystal structure belongs to the hexagonal crystal system. They are generally divided into two crystal directions, α and β, both of which are composed of [SiN4]4- The structure of the body, where β-Si3N4 has a high symmetry and a small molar volume, is a thermodynamically stable phase at temperature, while α-Si3N4 is easier to form dynamically. At high temperatures (1400°C to 1800°C), the α phase meets A phase change occurs and becomes a β type. This phase change is irreversible, so the α phase is beneficial to sintering.
The appearance of silicon nitride is different in different crystal phases, α-Si3N4 is white or off-white loose wool or needle-like body, β-Si3N4 is darker in color, showing dense granular polyhedron or short prism, silicon nitride crystal It must be transparent or translucent. The appearance of silicon nitride ceramics is gray, blue gray to gray black, which varies with density and comparative examples, and other colors due to additives. The silicon nitride ceramic surface has a metallic luster after being polished.
3. Density and specific gravity
The theoretical density of silicon nitride is 3100±10kg/m3, the actual measured specific gravity of α-Si3N4 is 3184kg/m3, and the true specific gravity of β-Si3N4 is 3187kg/m3. The volume density of silicon nitride ceramics varies greatly due to the process, generally more than 80% of the theoretical density, about 2200 ~ 3200kg/m3, the level of porosity is the main reason for the different density, reaction sintered silicon nitride The porosity is generally around 20%, the density is 2200-2600kg/m3, and the porosity of hot-pressed silicon nitride is below 5%, and the density is 3000-3200kg/m3. Compared with other materials with similar uses, not only the density is lower than all High-temperature alloy, but also a kind of low density in high-temperature structural ceramics.
4. Electrical insulation
Silicon nitride ceramics can be used as high-temperature insulating materials, and the performance index depends on the synthesis method and purity. The free silicon that is not nitrided in the material, the alkali metals, alkaline earth metals, iron, titanium, etc. brought in during the preparation Magazines such as nickel can deteriorate 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. As conditions improve, silicon nitride can enter the ranks of commonly used dielectrics.
5. Thermal properties
Sintered silicon nitride has a low thermal expansion coefficient of 2.53×10-6/℃ and a thermal conductivity of 18.42W/m·K, so it has excellent thermal shock resistance, second only to quartz and glass-ceramics. The experimental report shows that the reaction sintered silicon nitride sample with a density of 2500 kg/m3 is cooled from 1200°C to 20°C for thousands of thermal cycles, and it still does not crack. The silicon nitride ceramic has good thermal stability and can be used for a long time at high temperatures. It can be used up to 1400°C in oxidizing atmosphere and 1850°C in neutral or reducing atmosphere.
6. Mechanical properties
Silicon nitride has high mechanical strength. The flexural strength of general hot-pressed products is 500-700MPa, and the high can reach 1000-1200MPa; the flexural strength after reaction sintering is 200MPa, and the high can be 300-400MPa. Although the room-temperature strength of the reaction sintered product is not high, its strength still does not decrease under the high temperature of 1200～1350℃. The high temperature creep of silicon nitride is small. For example, the reaction sintered silicon nitride has a load of 24 MPa at 1200°C and its shape changes to 0.5% after 1000 hours.
7. Friction coefficient and self-lubricity
The friction coefficient of silicon nitride ceramics is small, and under high temperature and high speed conditions, the increase in friction coefficient is also small, so it can ensure the normal operation of the mechanism. This is an outstanding advantage of it. The silicon nitride ceramics begin to rub against the friction during grinding The coefficient reaches 1.0 to 1.5. After precision running-in, the friction coefficient is greatly reduced and remains below 0.5, so silicon nitride ceramics are considered to be self-lubricating materials. The main reason for this self-lubricity is different from graphite, boron nitride, talc, etc. lies in the scale structure of the material structure. Under the action of pressure, the friction surface decomposes to form a thin gas film, so that the sliding resistance between the friction surfaces is reduced, and the smoothness of the friction surface is increased. In this way, the more friction, the smaller the resistance, and the amount of wear is also particularly small. After most friction, the friction coefficient tends to increase gradually due to surface wear or temperature softening.
Silicon nitride ceramics can be machined to achieve the required shape and accuracy, surface finish.
9. Chemical stability
Silicon nitride has excellent chemical properties and can withstand corrosion of all inorganic acids except hydrofluoric acid and sodium hydroxide solution below 25%. Its oxidation resistance temperature can reach 1400 ℃, can be used up to 1870 ℃ in a reducing atmosphere, it is not wet to metals (especially AL liquid), especially non-metals.
It can be known from the physical and chemical properties of the above silicon nitride ceramics that the excellent performance of silicon nitride ceramics has a special use value for the working environment of high temperature, high speed and strong corrosive media often encountered by modern technology. Its outstanding advantages are:
has the following points:
(1) High mechanical strength and hardness close to corundum. The room temperature flexural strength of hot-pressed silicon nitride can be as high as 780-980MPa, and some are even higher. It can be compared with alloy steel, and the strength can be maintained until 1200 ℃ without decreasing.
(2) Mechanical self-lubrication, small surface friction coefficient, wear resistance, large elastic modulus, high temperature resistance.
(3) The coefficient of thermal expansion is small, the coefficient of thermal conductivity is large, and the thermal shock resistance is good.
(4) Low density and small specific gravity.
(5) Corrosion resistance and oxidation resistance.
(6) Good electrical insulation.