EnglishViews: 2455 Author: Site Editor Publish Time: 2020-06-29 Origin: Site
From the perspective of the seals used in the industry at present, no matter spring seal, gland packing, pump seal, elastomer bellows seal or non contacting mechanical seal, most of them have the defects of poor sealing performance, easy to be corroded and short working life. This is because most of these seals are made of metal materials. When they are used for fluid sealing, the chemicals in the fluid medium will wash and erode the seals, resulting in seal failure. Most mechanical seal leakage is caused by the end face of the seal. Most mechanical seal materials are hard alloy, stainless steel, titanium alloy, ceramics, etc. Due to poor corrosion resistance, metal materials can not be used in corrosive, high-speed and high-pressure situations. Compared with metal materials, ceramic materials have the advantages of high melting point, corrosion resistance, oxidation resistance, high hardness, high temperature resistance, high strength, etc., which can effectively overcome many defects of metal seal, and become more and more popular in the seal industry. In recent years, silicon carbide materials have developed rapidly. Silicon carbide seal is widely used in mechanical, automotive, petroleum, chemical and other fields.
Silicon carbide is usually made by sintering. For the silicon carbide used for sealing, three sintering methods are adopted: normal pressure sintering, reaction sintering and hot pressing sintering. The three methods have their own characteristics, so the properties of sintered silicon carbide products are also different.
The mechanical properties of silicon carbide sealing materials are mainly described from the following parameters: density, hardness, bending strength, elastic modulus, CTE and thermal conductivity. The specific performance parameters are shown in the table below.
Mechanical Properties | Normal Pressure Sintering | Reaction Sintering | Hot Pressing Sintering |
Density/g·cm-3 | 3.14~3.18 | 3.2 | 3.1 |
Hardness/kg·mm-2 | 2500 | 2100 | 1700 |
Bending strength/Mpa | 410 | 640 | 380 |
Elastic modulus/GPa | 410 | 450 | 350 |
CTE/K-1 | 4.7×10-6 | 4.8×10-6 | 4.5×10-6 |
Thermal conductivity/(W/m·K),20℃ | 110 | 130 | 140 |
The corrosion resistance of silicon carbide ceramics prepared by three sintering methods in some common strong acids and bases is shown in the table below. The values in the table are the mass reduction value of silicon carbide in solution (mg / cm2·a).
Test Solution | Temperature/℃ | Normal Pressure Sintering | Reaction Sintering | Hot Pressing Sintering |
98% Sulfuric acid | 100 | 1.8 | 87.0 | 55.0 |
50% Sodium hydroxide | 100 | 2.5 | 75.0 | ＞1000 |
53% Hydrofluoric acid | 25 | ＜0.2 | — | 7.9 |
85% Phosphoric acid | 100 | ＜0.2 | ＜0.2 | 8.8 |
70% Nitric acid | 100 | ＜0.2 | ＜0.2 | 0.5 |
45% Potassium hydroxide | 100 | ＜0.2 | 3.0 | ＞1000 |
25% Hydrochloric acid | 70 | ＜0.2 | 14.0 | 0.9 |
3. As a sealing material, silicon carbide has its unique tribological properties, such as good thermal conductivity, self lubrication, high hardness, etc., so it can meet the sealing requirements of many industries. Self lubrication is mainly due to the micro graphite or carbon particles in silicon carbide. When the sealing surface rotates, these micro particles will quickly form a layer of graphite film on the end face, playing the role of solid lubricant. So silicon carbide material has good wear resistance.
