Application of vacuum hot pressing sintering furnace

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1. Advanced ceramic hot pressing sintering (B4C sintering)
The densification sintering of pure boron carbide is extremely difficult because its covalent bond reaches 93.94%, which is much higher than that of silicon carbide (88%) and silicon nitride (70%). Therefore, the elimination of pores, grain boundary and volume diffusion need to take place above 2200 ℃.

Generally speaking, the relative density of ordinary boron carbide powder can only reach 80% – 87% when sintered at 2250-2300 ℃ under normal pressure. The mechanism is volume diffusion when the temperature is close to the melting point of boron carbide. When sintered at such a high temperature, the grains will coarsen and grow rapidly, which is not conducive to the elimination of pores, and a large number of residual pores will be produced, which will affect the compactness of the material.

Boron carbide is a kind of typical stable compound by covalent bond. Because of its low diffusion coefficient, it is difficult to achieve densification by conventional sintering method. It is necessary to add some sintering additives to reduce the surface energy or increase the surface area, and adopt special process to obtain dense silicon carbide ceramics.

Hot pressing sintering is the most widely used rapid sintering method for manufacturing boron carbide and its composite engineering devices. Hot pressing sintering with additives can strongly promote the densification rate, and can obtain materials close to the physical and chemical density, which can significantly improve the product performance. The additives used at present are similar to those used in atmospheric sintering. Liquid phase sintering plays an important role in the sintering of SiC and its composites.

The results show that the relative density, Young’s modulus and Poisson’s ratio of boron carbide ceramics are 91.6%, 292.5gpa and 0.16 respectively after hot pressing sintering at 2150 ℃ for 10 min. the temperature is proportional to the coefficient of linear expansion and the coefficient of thermal conductivity is decreased from 0 ℃ to 1000 ℃.

2. Target hot pressing sintering
Generally, the melt casting method can not achieve the preparation of refractory metal sputtering target. For two or more kinds of metals with large difference in melting point and density, it is difficult to obtain the alloy target with uniform composition by using the ordinary melt casting method. For inorganic non-metallic target and composite target, the melt casting method is even more powerless, Powder metallurgy is the best way to solve the above technical problems. At the same time, powder metallurgy technology has the advantages of easy to obtain uniform fine grain structure, saving raw materials and high production efficiency, which has become the main preparation method and research hotspot of magnetron sputtering target.

Powder metallurgy method; The alloy raw materials with a certain composition ratio are melted, poured into ingots, and then crushed. The crushed powder is shaped by isostatic pressing, and then sintered at high temperature, and finally the target material is formed. The commonly used powder metallurgy processes include cold pressing, vacuum hot pressing and hot isostatic pressing, and vacuum hot pressing sintering can produce large-size, high-density planar targets, which are most widely used.

3. Metal / ceramic diffusion bonding
Because the chemical bond structure of ceramic materials and metal materials is fundamentally different, coupled with the special physical and chemical properties of ceramic itself, there are many problems in the connection between ceramic materials and metal or ceramic itself. It is mainly reflected in the following two problems: first, ceramic materials are mainly composed of ionic bonds and covalent bonds, while metal materials are mainly composed of metal bonds, and they are hardly wetted. Therefore, the wettability of ceramic and metal materials should be considered. Second, the coefficient of linear expansion of the two materials is generally quite different. When heat sealing or mechanical connection is used, the wettability of ceramic and metal materials should be considered, There will be large residual stress at the joint of ceramic and metal, which will weaken the mechanical properties of the joint and even cause the joint to be damaged and cracked. Therefore, the thermal stress relief at the joint should be considered.

Solid phase diffusion bonding is a kind of high temperature resistant ceramic / metal bonding method which is widely studied at present. Its working principle is: two same or different materials are in close contact under high temperature and certain pressure, plastic deformation occurs on the surface, atoms realize mutual diffusion and form a good joint.

The advantages of this process are that the quality of joints is stable, the connection strength is high, the joints with larger cross-section can be welded, multiple joints can be welded at one time, the efficiency is high, the intermediate layer can be increased, and the surface metallization of ceramic materials is not required. However, the process is complex and can meet the application requirements of high temperature and corrosion resistance. It has high requirements for the connection surface state and connection equipment.

The development trend of hot pressing sintering technology
Hot pressing sintering has been attracting great attention, but its progress in the industry is not significant. Only a few special products have been successfully achieved, such as dense boron carbide for nuclear industry, magnesium fluoride window for military industry, and special tungsten carbide, cutting tools and special target materials. The main reason for limiting the application of hot pressing sintering is high cost. It is usually necessary to fix a set of devices with pressure and heating system to burn a sample, and the geometry of the sample is limited to the cylinder shape. However, these reasons also promote the isostatic sintering. However, there are still many technical problems to be solved, especially the isostatic pressure mold, and the initial investment is large. With the development of science and technology, hot pressing sintering is becoming more and more popular in the direction of digital model artificial intelligence automation.

Learn More : Vacuum Hot Pressing Sintering Furnace