For the sintering of metal, ceramic and some refractory metal intermetallic compound powder, two sintering methods are generally adopted, namely pressureless sintering and pressured sintering. At present, the commonly used pressure sintering mainly adopts the following three ways: hot isostatic pressing sintering, hot pressing sintering and gas pressure sintering. With the continuous development of a large number of new materials and the demand of industrial mass production, the above three kinds of pressured sintering are widely used in the preparation of new materials. Due to its special structure, the production cost of hot isostatic pressing sintering and gas pressure sintering equipment is relatively expensive, which is not the optimal choice for many manufacturers. Therefore, it is of practical significance to develop a low-cost vacuum hot pressing sintering furnace. The present research and production practice have proved that the densification process of hot pressed materials includes plastic flow, viscous flow, diffusion and creep. When plastic and viscous flow become the dominant densification mechanism, the powder can be densified rapidly and the microstructure can be controlled. The advantage of hot pressing is that the equipment investment is small. Although the pressure is one order of magnitude lower than that of isostatic pressing, the longitudinal heat flow is limited because the pressure bearing material in the hot pressing machine can be refined, the uniformity of the temperature field of the workpiece is improved, and the energy consumption is greatly reduced. Large diameter materials can be prepared, and the hot pressing machine can be controlled by IT technology, so as to effectively control the densification process and quality of materials.
Hot pressing sintering is a process of densification of products by using heat energy and mechanical energy. The characteristic of this process is that the sintering temperature can be 200 ~ 400 ℃ lower than that of normal pressure sintering according to the size of the applied pressure, and the added energy makes the densification speed of the products faster, so the fully dense and fine grain products can be completed in a lower temperature and a shorter time; while the vacuum hot pressing sintering can be further improved because of the high vacuum degree maintained in the hot pressing process It can effectively reduce the sintering temperature of the product and effectively remove the gas in the micro pores, so as to further promote the densification process of hot pressed materials. As we all know, the reduction of sintering temperature, for micro powder products, can effectively prevent the growth of grain, for the stability of the final product quality is of great significance. Especially for the near nanometer cemented carbide, the decrease of particle diameter, the increase of specific surface area, surface activation energy and contact area between particles lead to the increase of the driving force of final sintering and the decrease of the generation and number of pores. Now it is found that the sintering driving force of nano powder products is dozens or even hundreds of times that of ordinary products. Restraining the growth of grain in sintering process is the most critical process to obtain nanocrystalline products, and vacuum hot pressing sintering process can achieve this well, which has been proved in the actual production of structural ceramics and ITO targets.
1、 Classification of vacuum hot pressing sintering furnace
The vacuum hot pressing sintering furnace can be classified as follows according to the service environment:
1). Atmospheric hot press sintering furnace: mainly oxygen hot press sintering, hot press sintering in atmospheric environment;
2). Atmosphere hot pressing sintering furnace: hot pressing sintering through atmosphere protection or chemical reaction;
3). Vacuum hot press sintering furnace: hot press sintering is carried out in a certain high vacuum environment. The sintered materials are anaerobic or react with oxygen, so they must be sintered in a vacuum environment.
According to the temperature, vacuum hot pressing sintering furnace can be classified as follows:
1). Room temperature ~ 800 ℃: for the furnace whose temperature is below 800 ℃, the heater often uses iron chromium aluminum, nickel chromium wire as heating elements, and the insulation material often uses high temperature aluminum silicate insulation felt.
2). Normal temperature ~ 1600 ℃: metal molybdenum, silicon molybdenum rod, silicon carbon rod and graphite rod are often used as heating elements when the temperature is 1000 ℃ ~ 1600 ℃. Composite carbon felt, mullite felt and graphite felt are often used as insulation materials.
3). Room temperature ~ 2400 ℃: when the temperature is 1600 ℃ ~ 2400 ℃, graphite tube and tungsten are often used as heating elements or induction heating method, and graphite felt is commonly used as insulation material.
2、 Composition of vacuum hot pressing sintering furnace (resistance heating)
Vacuum hot pressing furnace is mainly composed of furnace body, furnace door, heating and heat preservation and temperature measurement system, pressure system, vacuum atmosphere system, water cooling system, control system, safety protection system, etc.
The hot pressing sintering furnace has the advantages of high working temperature, high vacuum degree, high combined pressure and good cooling conditions. Except for the heating body and the heat preservation part, the other parts of the furnace can be cooled quickly, which has high safety and energy consumption. To meet the above characteristics, there are high requirements for structural design and manufacturing accuracy. Let's understand its structural characteristics
1) The furnace body is made of stainless steel with double-layer structure, which is divided into inner and outer barrel bodies. The furnace bottom head is processed by spinning process. Because the force generated by the hot pressing device is borne by the furnace body, the furnace body must have high strength and enough stiffness, and it will not deform under external force and high temperature. The dynamic sealing device of the furnace bottom is fixed on the center line of the furnace body and must be perpendicular to the flange plate to ensure that the lower ejector rod moves along the straight line. The dynamic sealing device has a good cooling system to extend the service life of the seals.
2) The furnace door is composed of furnace head and flange, lifting oil cylinder and upper pressure head. The furnace door is a double-layer structure. In order to bear the pressure transmitted by the lower ejector, it must have enough strength and stiffness. The lower end of the upper ram is in the high temperature zone when it works, so it must be forced cooled.
3) The insulation system consists of insulation layer, reflecting layer and insulation layer, which are made of graphite, molybdenum sheet, graphite felt and stainless steel respectively. The reflective layer made of molybdenum sheet mainly resists the radiant heat generated by the heating element, so as not to damage the insulation layer and furnace body.
4) The pressure system consists of hydraulic station, oil cylinder, lifting oil cylinder, lower ejector and controller. The hydraulic station has two pressure outputs, one is to drive the furnace cover up and down, the other is to drive the hot press cylinder. There are two ways to control the hot press cylinder, manual or automatic. During automatic operation, the pressure can be automatically maintained according to the set pressure, so that the product is always in a constant pressure state. The speed adjustment range of the oil cylinder is 50 ~ 300 mm / min. The hot press oil cylinder must have high stability and can't shake when it works under load. The movement quality of the hot press cylinder is one of the important conditions to prepare high quality products.
3、 Application of vacuum hot pressing sintering furnace
1). Hot pressing sintering of advanced ceramics (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 ℃.
Lattice and structural fragment of boron carbide
Generally speaking, the ordinary boron carbide powder can only reach 80% to 87% relative density when sintered at 2250-2300 ℃. The mechanism is the volume diffusion when the temperature is close to the melting point of boron carbide. The grain will be coarsened and grown rapidly under such high temperature, which is not conducive to the elimination of pores, and a large number of residual pores will be produced to affect the densification of the material.
Boron carbide is a typical stable compound which is combined by covalent bond. With its low diffusion coefficient, it is difficult to densify it by conventional sintering method. It is necessary to add some sintering aids to reduce surface energy or increase surface area, and to obtain compact silicon carbide ceramics by special processing.
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). Hot pressing sintering of target
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 powerless, and the powder metallurgy method is the solution to prepare the above target The best way to solve the 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: smelting alloy raw materials with certain proportion of components, casting into ingots, then crushing, forming the powder by isostatic pressure, sintering at high temperature, and finally forming target. The common PM processes include cold pressing, vacuum hot pressing and hot isostatic pressure, etc. However, vacuum hot pressing sintering can be used to prepare large-scale and high density plane target materials, which is 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, which are almost non wetting. Therefore, the wettability of ceramic and metal materials needs to be considered. Second, the coefficient of linear expansion of the two materials is generally quite different. When heat sealing or mechanical connection is adopted, the joint between ceramic and metal will be closed There will be a large residual stress, which will weaken the mechanical properties of the joint and even cause the joint to be damaged and cracked. Therefore, it is necessary to consider the problem of thermal stress relief at the junction.
Solid phase diffusion connection is a kind of ceramic / metal connection method which has been studied more and more recently. Its working principle is: two same or different materials are closely contacted at high temperature and certain pressure, plastic deformation occurs on the surface, and atoms can diffuse with each other to form a good joint.
The advantages of this process are that the joint is of stable quality, high connection strength, large cross-section joint, multiple joints can be welded at a time, with high efficiency, and the intermediate layer can be increased, and the surface metallization of ceramic materials is not required. But the process of this method is complex, which can meet the application requirements of high temperature and corrosion resistance. High requirements for connection surface state and connection equipment.
4、 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. At present, with the development of social progress, science and technology, hot pressing sintering gradually tends to the direction of digital model and artificial intelligence automation.
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