Three Applications of Vacuum Hot Pressing Sintering Furnace

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1. Advanced ceramic hot pressing sintering (B4C sintering)

Densification and 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%). As a result, the elimination of pores, grain boundary and volume diffusion in boron carbide need to take place sufficiently above 2200°C.

Fig 1. The lattice and structural fragments of boron carbide

Generally speaking, ordinary boron carbide powder can only reach a relative density of 80%-87% when sintered at 2250-2300°C under normal pressure. The mechanism is volume diffusion when the temperature is close to the melting point of boron carbide. Sintering under such high temperature conditions will rapidly coarsen and grow crystal grains, which is not conducive to the elimination of pores, and a large number of residual pores will be generated to affect the compactness of the material.
Boron carbide is a typical stable compound bonded by covalent bonds. With its low diffusion coefficient, it is difficult to achieve densification by conventional sintering methods. It is necessary to add some sintering aids to reduce surface energy or increase Surface area, and the use of special processes to obtain dense silicon carbide ceramics.

Hot press sintering is currently the most widely used rapid sintering method for manufacturing boron carbide and its composite engineering devices. Hot press sintering with additives can strongly promote the densification rate, and can obtain materials close to the physical and chemical density, and significantly improve product performance. The additives currently used are similar to those used for atmospheric sintering. Liquid phase sintering occupies an important position in the sintering of silicon carbide and its composite materials.

Experiments show that the relative density of boron carbide ceramics reaches 91.6%, the Young’s modulus at room temperature is 292.5GPa, the Poisson’s ratio at room temperature is 0.16, and the temperature is proportional to the expansion coefficient at 0~1000℃. The thermal conductivity is reduced.

2. Hot Pressing and Sintering of the Target Material

Generally, the fusion casting method cannot achieve the preparation of refractory metal sputtering targets. For two or more metals with large differences in melting point and density, it is generally difficult to obtain an alloy target with uniform composition by using ordinary fusion casting methods. For inorganic non-metallic targets and composite targets, the molten casting method is even more powerless, and the powder metallurgy method is the best way to solve the technical problems of preparing the above-mentioned target materials. At the same time, the powder metallurgy process also has the advantages of easily obtaining a uniform fine-grain structure, saving raw materials, and high production efficiency. It has become the main preparation method and research hotspot of magnetron sputtering targets.

Powder metallurgy method: melting alloy raw materials with a certain ratio, pouring them into ingots and then pulverizing them, then isostatically pressing the pulverized powders, and then sintering them at high temperatures to finally form targets. Commonly used powder metallurgy processes include cold pressing, vacuum hot pressing and hot isostatic pressing, etc., while vacuum hot pressing sintering can prepare large-size, high-density flat targets, and is the most widely used.

3. Metal/Ceramic Diffusion Welding Connection

Due to the fundamentally different chemical bond structures of ceramic materials and metal materials, and the special physical and chemical properties of ceramics themselves, there are many problems in connection with metals or ceramics themselves. It is mainly reflected in the following two problems. One: Ceramic materials are mainly composed of ionic bonds and covalent bonds, while metal materials are mainly composed of metal bonds, and the two are almost non-wetting. Therefore, we need to consider the wetting of ceramic and metal materials. The second problem is that the linear expansion coefficients of the two are generally different. When heat sealing or mechanical connection is used, there will be a large residual stress at the joint between ceramic and metal, which weakens the mechanical properties of the joint or even damages the joint. Cracking, so it is necessary to consider the problem of thermal stress mitigation at the junction.

Solid-phase diffusion bonding is the most studied high-temperature ceramic/metal connection method. Its working principle is: two materials of the same or dissimilar species are in close contact at high temperature and a certain pressure, the surface is plastically deformed, and mutual diffusion between atoms is realized. A good joint is formed.

The advantages of this process are stable quality at the joints, high connection strength, large cross-section joints can be welded, multiple joints can be welded at a time, high efficiency, an intermediate layer can be added, and no surface metallization of ceramic materials is required. However, the process of this method is complicated and can meet the application requirements under high temperature and corrosion resistance conditions. There are high requirements on the state of the connection surface and the connection equipment.

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