The mold is an indispensable element in the SPS system. It plays a vital and irreplaceable role in obtaining and maintaining the temperature and pressure of the sintered powder, giving the desired shape to the sintered body, and finally realizing various properties of the sintered body. So, what molds are used in the SPS process?
Graphite mold
Due to its special structure, graphite has special properties suitable for making SPS molds.
- High-temperature resistance. The melting point of graphite is 3850±50℃ and the boiling point is 4250℃. Even after ultra-high temperature arc burning, the weight loss is very small and the thermal expansion coefficient is also very small. The strength of graphite increases with the increase in temperature. At 2000℃, the strength of graphite doubles.
- Excellent electrical and thermal conductivity. The electrical conductivity of graphite is one hundred times higher than that of general non-metals. The thermal conductivity exceeds that of metal materials such as steel, iron, and lead.
- Chemical stability. Graphite has good chemical stability at room temperature and can resist acid, alkali, and organic solvent corrosion.
- Thermal shock resistance. When used at room temperature, graphite can withstand drastic changes in temperature without being damaged and has a low linear expansion coefficient. When the temperature changes suddenly, the volume of graphite does not change much and no cracks will occur.
- Good lubrication and wear resistance.
- Easy to process, has good mechanical processing performance, and can be made into molds with complex shapes and high precision.
As a mold material in SPS, graphite has a series of excellent properties, but it also has some disadvantages.
The strength is not high enough and it is difficult to withstand greater sintering pressure. Compared with metal and alloy molds, the life is shorter. For materials that do not want carburization during sintering, there may be some adverse effects. Therefore, it is necessary to develop a new mold with higher strength and a better reuse rate than the currently commonly used graphite mold. Improve the bearing capacity of the mold and reduce the cost of the mold.
Improvement of graphite molds used in SPS
Figure 1 is a standard graphite mold.
Traditional SPS sintering one sample at a time is inefficient. To improve preparation efficiency and save energy, a mold that can sinter multiple samples simultaneously has been developed.
Shape expansion
The mold for sintering discs is the most common and simple. But in many cases, the shape of the sample we need is not a disc. For this reason, it is necessary to develop and make special-shaped molds. Figure 3 is a graphite mold for preparing rectangular samples.
Cemented carbide molds used in SPS
To overcome the shortcomings of graphite molds, molds of various materials other than graphite have been developed.
Figure 4 is a mold made of cemented carbide.
Cemented carbide molds have the characteristics of high hardness, high strength, corrosion resistance, high-temperature resistance, and small expansion coefficient. Generally, tungsten-cobalt cemented carbide is used. The strength of cemented carbide is dozens of times that of graphite, and it can be used as a mold for SPS. It can withstand greater pressure, further improving the sintered body’s density and performance.
In addition, cemented carbide is much better than graphite in wear resistance and thermal fatigue resistance. Therefore, its service life is much longer than that of graphite molds, which reduces the cost of molds as a whole.
Precautions for using cemented carbide molds
There will be no worry about carburization when using cemented carbide molds for sintering materials that are easy to carburize and sensitive to carbon potential. To prevent the mutual diffusion of elements between the cemented carbide mold material and the sintered body during SPS sintering, a BN release agent can be applied to the inner wall of the mold. Isolate the mold material from the sintered body.
Since cemented carbide is made above the melting point of Co when using cemented carbide molds for SPS sintering, its maximum sintering temperature should not exceed or approach the melting point of Co.
In addition, since the resistivity of cemented carbide (such as WC-Co) is an order of magnitude lower than that of graphite, part of the pulse current used for sintering densification will be directly conducted to the negative electrode due to the low resistance of the mold, resulting in energy waste. Therefore, under the same size mold and the same sintering process, cemented carbide molds require more power than graphite molds. This also limits the scope of use of cemented carbide molds to a certain extent.
Ceramic molds used in SPS
Ceramic molds generally use oxides or carbon (nitride) compounds with very stable performance. The advantages of ceramic molds are high operating temperature, high strength, high hardness, moisture resistance, wear resistance, stain resistance, corrosion resistance, high-temperature resistance, easy cleaning, small deformation, good insulation, and a certain resistance to rapid cooling and heating. The disadvantages are high brittleness, poor plasticity, poor toughness, poor conductivity, and poor thermal conductivity. Conductive composite ceramic materials are required, and the thermal conductivity of conductive ceramics is only 1/6~1/7 of that of graphite. The sintering speed of SPS is approximately 100℃·min-1. When using ceramic molds, the hysteresis characteristics of the actual measured temperature must be fully considered.
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