The detailed introduction of Hot pressing sintering special ceramics

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In recent decades, many new materials with high performance can be developed and produced in ceramic sintering process.There are many kinds of new technology, and the equipment structure and operation process are very different.However, various sintering processes are nothing more than seeking for more driving force activation energy.Its function is to promote the process of material transfer or grain orientation and densification under high temperature.The driving force activation energy mainly includes the decrease of surface free energy, the decrease of chemical potential, the application of phase transition and its chemical reactivity, and the application of applied mechanical force, electric field, magnetic field and ultrasonic energy.And the most widely used is the external mechanical push – hot pressing sintering.

1.Definition, advantages and disadvantages of hot pressing sintering

1.1 Definition

Sintering is a general term for the densification process and phenomenon of powder or green billet at high temperature.To be specific, with the increase of temperature and the extension of time, solid particles bond with each other, and grains grow up. The voids (pores) and grain boundaries gradually decrease. Through the transfer of materials, the total volume shrinks, the density increases, and finally becomes a hard polycrystalline sintered body with only a certain microstructure, which is called sintering.

Solid phase sintering (solid state sintering) refers to a loose powder or by suppressing has certain shape of the powder compact is put in no more than the melting point of set temperature under certain atmosphere protection, the operation of heat preservation for a period of time.The temperature set is the sintering temperature, the atmosphere used is called the sintering atmosphere, and the insulation time used is called the sintering time.

Solid phase sintering (solid state sintering) refers to a loose powder or by suppressing has certain shape of the powder compact is put in no more than the melting point of set temperature under certain atmosphere protection, the operation of heat preservation for a period of time.The temperature set is the sintering temperature, the atmosphere used is called the sintering atmosphere, and the insulation time used is called the sintering time.

The sintering process can be divided into two categories: unpressurized sintering (sintering without external pressure) and pressurized sintering (sintering with external pressure).Pressure sintering is the process of applying high temperature and external pressure to loose powder or powder compacting.

Hot-pressing Sintering, Hot Pressing Sintering, HPS) is in the process of Sintering and exert a certain external force (according to the strength of the mould material can withstand, general pressure in the 10 ~ 40 mpa), make the material flow acceleration, rearrangement and densification.The temperature of hot pressing sintering is about 100℃~150℃ lower than that of normal pressure sintering, but the driving force of hot pressing sintering is 20~100 times larger than that of normal pressure sintering.

1.2 advantages and disadvantages of hot pressing sintering

(1) when hot pressing, because the powder is in a thermoplastic state, the deformation resistance is small, easy to plastic flow and densification, therefore, the required molding pressure is only 1/10 of the cold pressing method, can be formed large size of A12O3, BeO, BN, TiB2 and other products.

(2) the simultaneous heating and pressurization can help the mass transfer process of powder particles such as contact, diffusion and flow, reduce the sintering temperature and shorten the sintering time, thus inhibiting the growth of grains.

(3) the sintered body with theoretical density and porosity close to zero can be easily obtained by the hot-pressing method, the structure of fine grains can be easily obtained, the orientation effect of crystals can be easily realized and the composition change of sodium system containing high vapor pressure can be controlled, so the products with good mechanical and electrical properties can be easily obtained.

(4) it can produce products with complex shapes and accurate sizes.Disadvantages of hot pressing sintering: low productivity and high cost.

2. Production process types of hot press sintering
The production process of hot pressing sintering can be divided into vacuum hot pressing, atmosphere hot pressing, hot isostatic pressing, reaction hot pressing, vibration hot pressing, equilibrium hot pressing and ultra-high pressure sintering.Let’s briefly introduce some of them.

2.1 vacuum and atmosphere hot pressing sintering

For the products that are difficult to be sintered in air (such as light transmitters or non-oxides), the method of atmosphere sintering was studied in order to prevent their oxidation.That is, certain gas is put into the furnace to form the required atmosphere, and sintering is carried out in this atmosphere.The vacuum pressure is to pump the furnace chamber into vacuum.

Non-oxides such as Si3N4 and SiC, which are attractive in advanced ceramics, are easily oxidized at high temperatures and are sintered in nitrogen and inert gases.For materials that are easy to vaporize under normal pressure, they can be sintered under slightly higher pressure.

2.2 hot isostatic sintering

Hot isostatic pressing (HIP) is a sintering process in which isostatic pressure is applied to the loose powder when it is heated in a package.The pressure transfer medium of hot isostatic pressure is an inert gas.The hot isostatic pressing process is to put the powder into a high pressure vessel to make the powder bear the action of high temperature and balanced pressure, and then the powder is sintered into compact parts.FIG. 2 hot isostatic pressure of sintering furnace intensifies the pressing and sintering processes.High density and strength can be obtained by reducing sintering temperature, eliminating interstice and avoiding grain growth.Compared with the hot pressing method, the hot isostatic pressing temperature is lower and the product density is higher.

2.3 reaction hot pressing sintering

this is a chemical reaction process that may occur in the powder at high temperature.It is a kind of hot pressing sintering technology which can be used according to the situation.That is to say, in the sintering mass transfer process, in addition to the use of surface free energy and mechanical forces to promote, plus a chemical reaction energy as a driving force or activation energy.In order to reduce the sintering temperature, the sintering difficulty is reduced to obtain dense ceramics.

3. Densification process of hot pressing sintering

the densification process of hot pressing sintering can be divided into three parts: the early stage of hot pressing, the middle stage of hot pressing and the later stage of hot pressing.

Ø hot-pressing pressure after mainly refers to the heat in the early years of the initial dozen to a few minutes, the relative density from 5 ~ 60% up to 90%.Compared with normal sintering, this period is characterized by a rapid increase in density and the disappearance of most pores at this time.In the body of the billet, the following changes mainly occurred: particle rearrangement under pressure, local fragmentation or plastic mass transfer caused by grain boundary slip, and filling of large aggregate clearance.The higher the temperature, the higher the pressure and the faster the density increases.However, with the increase of the density, the contact surface of the particle is significantly increased, and the force distributed on the unit surface is greatly reduced, so the particle boundary slippage is not easy. Instead, a large number of squeezed particle boundaries appear, and the densification speed slows down.

Ø hot-pressing mid – although at this time does not reject a small amount of interface slip and powder particle rearrangement, but large area, long LiJie sliding is not possible, the main mass transfer mechanism should be pressure under the action of space point spread, and accompanied with the porosity in the LiJie disappear.At the early stage of extrusion, the pressure difference between the particle boundaries is large, so the space point concentration and diffusion velocity are also large.So the density is not increasing too slowly.However, by the late stage of extrusion, the boundary pressure of particles everywhere was approaching equilibrium, the mass transfer of this creep type was no longer obvious, and the densification rate was greatly reduced.

In this period, the effect of external pressure is not obvious, and the main mass transfer forces and mass transfer mechanisms are similar to those in normal sintering period. The particle boundary movement under the impetus of interfacial pressure difference and the further elimination of pores along the particle boundary.Because the surface curvature difference as the driving force is independent of the applied pressure, the particle boundary velocity is basically independent of the applied pressure.Only under the action of external pressure, the grains stick more closely together and the grain boundary is more dense, which is more conducive to the particle jumping over the grain boundary and recrystallization.

4. hot-pressing sintering equipment

The structure of the hot-pressing sintering equipment is divided according to the heating and pressing methods, the atmosphere adopted and other factors.Electric heating is usually used in the hot pressing process.The most common methods are: direct heating to die pressing or burning material through electricity;The die is put in the electric furnace to heat the die;Direct induction heating of the conductive die;A non-conductive die is placed in a conductive tube for induction heating.

Graphite is the most widely used die material for hot pressing equipment.The price of graphite is not too expensive, easy to machining, in a large temperature range with a low density, low resistance, good thermal stability and has enough mechanical strength, and can form a protective atmosphere.The compressive strength of graphite used in the actual die is 35-45mpa.High strength graphite, can be used under pressure up to 70MPa.The limitation of graphite die is its low mechanical strength (it cannot work under high pressure) and its ability to reduce certain materials, especially oxides.Graphite can also react with transition metals and nitrides and silicides from transition metals.

Besides graphite die, metal die is most widely used, especially copper alloy die.Metal dies are used to make polycrystalline optical materials such as magnesium fluoride, magnesium oxide, and lead selenide.Oxide and ceramic dies are rarely used because of their poor thermal stability, difficulty in processing, and not always compatible with the pressed material.

5. Scope of application of hot pressing sintering
Compared with normal pressure sintering, the sintering temperature is much lower, and the porosity in the sintered body is low and the density is high.Because sintering at a lower temperature inhibits the growth of grains, the resulting sintered bodies have finer grains and higher mechanical strength.Hot pressing sintering is widely used in the preparation of materials that are difficult to densify under normal conditions of no pressure and the preparation of nano-ceramics.In the modern material industry, there are two kinds of industries that use the powder raw material to sinter into shape, one is the powder metallurgy industry, the other is the special ceramic industry.There are two main sintering processes used, one is cold pressing and then sintering, and the other is hot pressing sintering.The results show that the product has no oxidation, low porosity, less impurities, and higher alloying degree, so as to improve the comprehensive performance of the product.

Hot pressing sintering is a technological progress, which should be widely needed in the market. Its application fields include: (1) tools: diamond and cubic boron nitride products;Cemented carbide products;Cermet, powder high speed steel products.(2) electrical: soft magnetic, hard magnetic, high temperature magnetic materials;Ferrite, electric contact material, metal electrothermal material, electric vacuum material.(3) special materials: powder superalloy, oxide dispersion strengthening material, carbon (boron, nitrogen) dispersion strengthening material, fiber strengthening material, high purity heat resistant metal (tantalum, niobium, molybdenum, tungsten, beryllium) and alloy, composite metal, etc.(4) mechanical parts: widely used in automobile, aircraft, ship, agricultural machinery, office machinery, hydraulic parts, machine tools, home appliances and other fields.Especially wear-resistant and fragile key parts.

Copyright: SIMUWU Vacuum Furnace

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