vacuum hot pressing sintering furnace structure and application

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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, there are three main ways of pressure sintering, hot isostatic pressing sintering, hot pressing sintering and gas pressure sintering.

With the continuous development and marketization of new materials, the above three kinds of pressured sintering methods are more and more adopted. Due to the high cost of itself and production, hot isostatic pressing sintering and air pressure sintering equipment are not suitable for the needs of many manufacturers, especially some scientific research institutions. Therefore, it is of practical significance to develop and produce 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, due to the decrease of particle diameter, the increase of specific surface area, surface activation energy and contact area between particles, the driving force of final sintering increases and the number of large pores decreases. 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.

Vacuum hot pressing sintering furnace classification:

 

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 pressing sintering furnace: hot pressing 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.

Vacuum hot pressing sintering furnace composition of (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 system, water cooling system, control system, etc.

The hot press sintering furnace has high working temperature, high vacuum degree, large pressure force and good cooling condition. In addition to heating and heat preservation parts, other parts of the furnace body are strongly cooled, which puts forward higher requirements for structural design and manufacturing accuracy

(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 produced by the hot pressing device should be borne by the furnace body, the furnace body must have high strength and enough rigidity, and it must not deform under external force and high temperature. The center line of the fixed seat of the furnace bottom dynamic sealing device must be vertical to the upper flange of the furnace body to ensure the straightness of the lower ejector movement. The fixed seat of dynamic earth sealing device has good cooling conditions to extend the service life of the seal.

(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 barrel consists of insulation barrel, reflection barrel, insulation layer and barrel body, which are made of graphite, molybdenum sheet, graphite felt and stainless steel respectively. The reflecting barrel made of molybdenum sheet mainly resists the radiant heat generated by the heating element to avoid damaging the insulation layer and furnace body.

(4) The hot pressing device is composed 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.

Vacuum hot pressing sintering furnace application:

 

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 ℃.

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.

 

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: 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 to finally form the target material. 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.