Properties and Benefits of Silicon Nitride Coatings

       performance



Density and Specific Gravity

The theoretical density of silicon nitride is 3100±10kg/m3, the actual specific gravity of α-Si3N4 is 3184 kg/m3, and the true specific gravity of β-Si3N4 is 3187 kg/m3. The bulk density of silicon nitride ceramics varies greatly due to the process, generally more than 80% of the theoretical density, about 2200 ~ 3200 kg/m3, the porosity is the main reason for the difference in density, reaction sintering silicon nitride The porosity of silicon nitride is generally about 20%, and the density is 2200-2600 kg/m3, while the porosity of hot-pressed silicon nitride is below 5%, and the density is 3000-3200 kg/m3. Compared with other materials with similar uses, not only the density It is lower than all superalloys, and it is also one of the lower density among high temperature structural ceramics.



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 is not nitrided, the alkali metals, alkaline earth metals, iron, titanium, Nickel and other magazines can deteriorate the electrical properties of silicon nitride ceramics. Generally, the specific resistance of silicon nitride ceramics at room temperature and in dry medium is 1015~1016 ohms, and the dielectric constant is 9.4~9.5. At high temperature, silicon nitride ceramics still maintain a high specific resistance value. With the improvement of conditions, silicon nitride can enter the ranks of commonly used dielectrics.



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-ceramic , There are experimental reports that the reaction sintered silicon nitride sample with a density of 2500 kg/m3 is cooled from 1200 ℃ to 20 ℃ for thousands of thermal cycles, and still does not crack. Silicon nitride ceramics have good thermal stability and can be used in high temperature. long-term use. It can be used up to 1400°C in an oxidizing atmosphere and up to 1850°C in a neutral or reducing atmosphere.



mechanical properties

Silicon nitride has high mechanical strength. Generally, the flexural strength of hot-pressed products is 500-700MPa, and the highest can reach 1000-1200MPa; the flexural strength after reaction sintering is 200MPa, and the highest can be 300-400MPa. Although the room temperature strength of the reaction sintered product is not high, its strength does not decrease at the high temperature of 1200-1350℃. The high temperature creep of silicon nitride is small. 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 under the condition of high temperature and high speed, the friction coefficient is also small, so it can ensure the normal operation of the mechanism, which is one of its outstanding advantages. Silicon nitride ceramics start sliding friction when grinding The coefficient reaches 1.0 to 1.5. After precision running-in, the friction coefficient is greatly reduced and kept below 0.5, so silicon nitride ceramics are considered to be self-lubricating materials. The main reason for this self-lubricating property, different from graphite, boron nitride, talc, etc., is the scaly layered structure of the material organization. Under the action of pressure, a small amount of friction surface is decomposed 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 smaller the amount of wear. However, after continuous friction of most materials, the friction coefficient tends to gradually increase 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 is resistant to corrosion by 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 a reducing atmosphere, and it is not wet 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 performance of silicon nitride ceramics has special use value for the working environment of high temperature, high speed and strong corrosive medium often encountered in modern technology.



advantage



(1) High mechanical strength and hardness close to corundum. The room temperature bending 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 ℃ without decreasing.



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



(3) The thermal expansion coefficient is small, the thermal conductivity coefficient 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.