Hot pressing sintering (HP) is a mechanically pressurized sintering method. This method is to place ceramic powder in a mold cavity and heat the powder to the sintering temperature while pressurizing. Since the driving force is supplemented by external pressure, densification can be achieved in a relatively short time, and a microstructure with fine and uniform grains can be obtained. Therefore, for high-temperature ceramic materials (such as Si3N4, B, C, SiC, TiB2, ZrB2) that are difficult to sinter with covalent bonds, hot pressing sintering is an effective densification technology. Hot pressing sintering can obtain ceramic products with a density close to the theoretical density at a slightly lower temperature of 100℃~200℃ below the normal pressure sintering temperature; hot pressing sintering can also improve the performance of the product, such as transparency, conductivity, mechanical properties and reliability of use.
However, hot pressing sintering can usually only produce products with a single shape, and in many cases, post-processing will greatly increase the manufacturing cost. Nevertheless, after more than 40 years of development, hot pressing sintering has evolved from simple laboratory research to widespread industrial application and has become a mature and important sintering process.
Hot pressing furnace and mold materials:
Powders or preformed blanks are usually placed in the mold, heated and pressured at the same time. Depending on the application, the working temperature can be as high as 2500C, and the working pressure is usually 10~75 MPa. In hot pressing sintering, the maximum pressure that can be applied is limited by the strength of the mold. For commonly used graphite molds, the pressure can generally reach 40 MPa.
Using special graphite molds or more expensive high-temperature metal (such as Nimonic alloy) or high-temperature ceramic (such as Al2O3, SiC) molds, the pressure can be increased to 75MPa. For mold materials such as Al2O3 and SiC, due to the limitations of preparation technology and cost, they can only be used to make small-sized molds (such as 5cm in diameter); the gap between the mold and the pressure head should be slightly larger when using it, and a coating is also required to prevent sintering or welding between the pressure head and the mold.
Another limiting factor is that trace impurities (such as SiO2) will cause severe creep in such molds. Usually 0.1% impurities will greatly reduce the use pressure and temperature, so 99% pure alumina is not suitable for hot pressing molds. Hot pressed SiC molds have been commercially used for hot pressing ferrite parts in air or other atmospheres (because the reducing atmosphere of graphite molds cannot be used to make ferrites).
Graphite is the most commonly used mold material because it is relatively cheap, easy to process, and has very good high temperature creep resistance. Graphite oxidizes slowly below 1200°C and can be placed in an oxidizing atmosphere for a short time. Above 1200°C, it must be used in an inert or reducing atmosphere. Because graphite may react with ceramic samples at high temperatures, causing erosion of the contact surface or sample sticking to the mold wall, boron nitride is usually coated on the graphite mold wall to avoid reaction and facilitate demolding of the sample after sintering.
Hot Pressing Sintering Process:
Although hot pressing sintering increases the densification driving force by applying pressure, sintering aids are also required for some difficult-to-sinter ceramic materials, especially those with strong covalent bonds and small self-expansion coefficients.
Sintering aids can provide high diffusion rate channels (such as liquid phase on grain boundaries) at sintering temperature ratios, thereby promoting densification. However, since the application of pressure increases the densification driving force, the amount of sintering aid required is less than that of normal pressure sintering.
As with the pressureless sintering method, the powder particle size and uniformity also have a significant effect on the hot pressing densification rate. The hot pressing sintering powder particle size should be submicron (<1 μm), with a narrow particle size distribution and no hard agglomerates.
The friction of the mold wall can reduce the densification rate and lead to uneven densification. To this end, the friction can be reduced by the following two ways:
① Reduce the high temperature reaction between the sample and the plate wall. Boron nitride can be coated on the contact surface of the mold;
② Try to hot press flat samples (such as discs or sheets). In fact, hot pressing sintering is most suitable for preparing flat products. The effect of applied pressure on powder particles during hot pressing sintering.
The shape change of a representative powder unit (such as three grains) is similar to that of the entire powder compact. The grains become flat in the direction of applied pressure, which is also the possible formation of hot pressing sintering. Texture (i.e., the preferred orientation of grains or the selective growth in a specific direction). Usually, the preferred orientation or selective growth direction of hot pressed grains is perpendicular to the direction of applied pressure. In order to obtain high-density samples, it is necessary to select appropriate pressure and temperature rise system. Generally, the mold is heated, and the powder in the mold cavity is gradually heated to the hot pressing temperature or below the hot pressing temperature under the action of uniaxial pressure. The actual pressurization system varies depending on different powders, and the main purpose is to fully eliminate the pores in the blank. The holding time at the hot pressing temperature varies depending on the characteristics of the powder, ranging from a few minutes to a few hours, generally 0. 5~2h. The hot pressing pressure is generally released when the predetermined density (usually complete densification) is reached, and the pressure is released at the hot pressing sintering temperature or just when cooling begins, because cracks will appear in the product during the cooling process. The hot pressing sintering temperature is 100~200C lower than the temperature of normal pressure sintering. The hot pressing sintering temperature of conventional oxide materials is shown in Table 4-5. In addition, the hot pressing sintering temperature of conventional high-temperature resistant borides, carbides and nitrides is generally 1700-1900 degrees when sintering aids are used.
