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Silicon carbide materials have developed rapidly in the field of materials due to their stable chemical properties, high thermal conductivity, small thermal expansion coefficient, small density, good wear resistance, high hardness, high mechanical strength, chemical corrosion resistance and other characteristics.
Silicon carbide ceramics began in the 1960s. Before that, silicon carbide was mainly used as mechanical grinding materials and refractory materials. However, with the development of advanced ceramics, people are not satisfied with the preparation of traditional silicon carbide ceramics. In recent years, various kinds of silicon carbide ceramic based multiphase ceramics have emerged one after another, improving all aspects of the performance of single-phase materials, making silicon carbide ceramics more widely used. Now it is widely used in ceramic ball bearings, valves, semiconductor materials, measuring instruments, aerospace and other fields.
1、 Properties of silicon carbide
The chemical stability of silicon carbide is closely related to its oxidation characteristics. Silicon carbide itself is easy to be oxidized, but after it is oxidized, a layer of silicon dioxide film is formed, and the oxidation process is gradually hindered.
In air, silicon carbide begins to oxidize at 800 ℃, but slowly; With the increase of temperature, the oxidation speed is accelerated rapidly. The oxidation rate of silicon carbide in oxygen is 1.6 times faster than that in air; The rate of oxidation slows down with time.
Pure silicon carbide is colorless and transparent crystal. Industrial silicon carbide can be colorless, light yellow, light green, dark green, light blue, dark blue and black. The degree of transparency decreases in turn when the color deepens. The abrasive industry divides silicon carbide into black silicon carbide and green silicon carbide according to color. Among them, colorless to dark green is classified as green silicon carbide, and light blue to black is classified as black silicon carbide. The mechanical properties of black and green silicon carbide are slightly different. Green silicon carbide is relatively brittle, and the abrasive tools made of green silicon carbide are rich in self sharpening; Black silicon carbide is tough, so the two kinds of silicon carbide have different uses.
2、 Preparation of silicon carbide ceramics
1. Preparation of ultrafine silicon carbide
In recent years, the preparation methods of superfine silicon carbide powder developed in the field of high-tech mainly fall into three categories: solid phase method, liquid phase method and gas phase method.
A. The solid phase method mainly includes the following:
Carbothermal reduction method, direct reaction method of Si and C (including high-temperature self propagating synthesis method and mechanical alloying method).
B Liquid phase method mainly includes the following:
Sol gel method, polymer thermal decomposition method and solvothermal method.
C Gas phase method mainly includes the following:
Vapor Reactive Deposition (CVD), Plasma, Laser Induced Vapor
2. Preparation of silicon carbide ceramics
Silicon carbide is a typical covalent bond bound stable compound. Due to its low diffusion coefficient, it is difficult to achieve densification by conventional sintering methods. It must be achieved by adding some sintering additives to reduce surface energy or increase external pressure.
a. Reaction bonded silicon carbide ceramics
This method is based on α- SiC and C are used as raw materials, and an appropriate amount of binder is added for molding and drying, and then put into the embedded material containing Si. When the green body is heated in the furnace for more than 1400 ℃, the Si around the green body melts, or penetrates into the capillary of the green body in liquid or gas state, and reacts with the C in the green body to generate SiC. The generated SiC gradually fills the pores in the green body, and α- SiC is connected to achieve the densification of products and strength.
b. Hot pressed silicon carbide ceramics
Hot pressing sintering means that a certain pressure is applied during the sintering process. The existence of the pressure increases the atomic diffusion rate and the sintering driving force, thus speeding up the sintering process. However, under high pressure, there will be directionally growing grains perpendicular to the pressure direction in the sintered body. In order to avoid this phenomenon, hot isostatic pressing sintering can be used.
c. Pressureless sintered silicon carbide ceramics
The pressureless sintering is considered to be the most promising sintering method for SiC sintering. SiC components with complex shape and large size can be prepared by pressureless sintering. According to different sintering mechanisms, pressureless sintering can be divided into solid phase sintering and liquid phase sintering.
3、 Application of silicon carbide ceramics
a. Silicon carbide ceramic ball
Precision ball is an important element in roundness instrument, gyroscope, bearing and precision measurement, and it is often used as the benchmark of precision measurement. At present, the precision ball is mainly made of metal materials, but there are some problems such as poor wear resistance and large deformation due to temperature. Ceramic ball has been widely used in precision bearings and other parts because of its light weight, high hardness, wear resistance, high temperature resistance, corrosion resistance, small thermal expansion coefficient and other advantages.
b. Silicon carbide abrasive tools
The hardness of silicon carbide is very high, and it can be prepared into various grinding wheels, sandpaper and abrasives, which are mainly used in the machining industry. The Mohs hardness of silicon carbide is 9.2~9.6, second only to diamond and boron carbide, and it is a commonly used abrasive.
c. Silicon carbide matrix composites
Due to the inherent brittleness of ceramic materials, its application in aerospace and other high-tech fields is limited, so much attention is paid to the toughening of silicon carbide materials; The use of silicon carbide braid can not only improve the toughness of ceramic materials, but also improve the strength and modulus of materials in varying degrees. Silicon carbide matrix composites have been widely used in high temperature thermal structure of aerospace field due to their high toughness, high strength and excellent oxidation resistance.