9 major product properties of silicon nitride materials
1. Composition and structure
The molecular formula of silicon nitride is Si3N4, which is a compound that is covalently bonded. Silicon nitride ceramics are polycrystalline materials. The crystal structure is hexagonal. It is generally divided into two crystal orientations: α and β, both of which are composed of [SiN4]4-tetrahedral The structure of β-Si3N4 is relatively high, and the molar volume is relatively small. It is a thermodynamically stable phase at temperature, while α-Si3N4 is easy to generate kinetics. At high temperature (1400℃～1800℃), α phase will meet The phase change occurs and becomes β type. This phase change is irreversible, so the α phase is good for sintering.
The appearance of silicon nitride obtained from different crystal phases is different. α-Si3N4 is white or off-white loose wool or needle-like body, while β-Si3N4 is darker in color and presents dense granular polyhedrons or short prisms. Silicon nitride crystals It must be transparent or translucent. The appearance of silicon nitride ceramics is gray, blue-gray to gray-black, depending on the density and contrast, and other colors due to additives. After polishing, the surface of silicon nitride ceramic has metallic luster.
3. Density and specific gravity
The theoretical density of silicon nitride is 3100±10kg/m3, the actual specific gravity of α-Si3N4 is 3184kg/m3, and the true specific gravity of β-Si3N4 is 3187kg/m3. The bulk density of silicon nitride ceramics varies greatly due to the process. Generally, it is more than 80% of the theoretical density, about 2200～3200kg/m3. The porosity is the main reason for the difference in density. The reaction sintered silicon nitride The porosity is generally about 20%, and the density is 2200-2600kg/m3, while 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, and it is also a kind of lower density in high-temperature structural ceramics.
Fourth, electrical insulation
Silicon nitride ceramics can be used as high-temperature insulating materials. The quality of its performance indicators mainly depends on the synthesis method and purity. The free silicon in the material that has not been nitrided, the alkali metals, alkaline earth metals, iron, titanium, Magazines such as nickel can deteriorate the electrical properties of silicon nitride ceramics. In general, the specific resistance of silicon nitride ceramics in a dry medium at room temperature is 1015-1016 ohms, and the dielectric constant is 9.4-9.5. At high temperatures, silicon nitride ceramics still maintain a relatively high specific resistance value. With the improvement of conditions, 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. Therefore, 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 2500kg/m3 has been cooled from 1200°C to 20°C for thousands of thermal cycles without breaking. The silicon nitride ceramic has good thermal stability and can be used at high temperatures for a long time. It can be used up to 1400°C in an oxidizing atmosphere, and up to 1850°C in a neutral or reducing atmosphere.
Six, mechanical properties
Silicon nitride has high mechanical strength. Generally, the flexural strength of hot-pressed products is 500-700 MPa, and the high can reach 1000-1200 MPa; the flexural strength after reaction sintering is 200 MPa, and the high can be 300-400 MPa. Although the room temperature strength of the reaction sintered product is not high, its strength still does not decrease at a high temperature of 1200～1350℃. The high temperature creep of silicon nitride is small. For example, reaction sintered silicon nitride has a load of 24MPa at 1200℃, and its deformation becomes 0.5% after 1000h.
7. Friction coefficient and self-lubricity
The friction coefficient of silicon nitride ceramics is small. Under the conditions of high temperature and high speed, the increase of friction coefficient is also small, so the normal operation of the mechanism can be ensured. This is its outstanding advantage. Silicon nitride ceramics begin to rub against sliding The coefficient reaches 1.0 to 1.5. After precise 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., in the scaly layered structure of the material organization. Under the action of pressure, the friction surface decomposes to form a thin air 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 smaller the amount of wear. After most materials are constantly rubbed, the friction coefficient tends to increase gradually due to surface wear or softening due to temperature rise.
Silicon nitride ceramics can be machined to achieve the required shape, precision, 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 sodium hydroxide solution below 25%. Its oxidation-resistant temperature can reach 1400°C, and it can be used up to 1870°C in a reducing atmosphere. It is non-wetting to metals (especially AL liquid), especially non-metals.
From the above physical and chemical properties of silicon nitride ceramics, it can be seen that the excellent properties of silicon nitride ceramics have special use value for the working environment of high temperature, high speed, and strong corrosive media frequently encountered in modern technology. Its outstanding advantages are:
Has the following points:
(1) High mechanical strength, 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, which can be compared with alloy steel, and the strength can be maintained up to 1200°C without dropping.
(2) The machine is self-lubricating, with low surface friction coefficient, wear resistance, large elastic modulus, and 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 and oxidation resistance.
(6) Good electrical insulation.