Field assisted sintering technology/spark plasma sintering

General principle of FAST/SPS

Field Assisted Sintering Technology/Spark Plasma Sintering (FAST/SPS) is a low-voltage, current-activated and pressure-supported sintering process that is based on Joule heating (= resistance heating) of the conductive tools (Figure 1) and is characterized by high heating rates and short cycle times. If electrically conductive powders are applied, the powder itself is also heated directly via the Joule effect. The FAST/SPS process is established in the industry and particularly promising when powders are to be sintered, which have a low sintering activity or an unfavorable particle morphology and particle size distribution for conventional pressing and sintering. Furthermore, the method is well suited for the production of composite materials, especially when phases will be combined, which have very different physical properties. Some typical application examples of FAST/SPS technology are presented in Chapter 7. Due to the great flexibility, the short cycle times and the comparatively simple and cost-effective production of tools, research and development is another important field of application for FAST/SPS technology and is widespread there.

Figure 1: Heated graphite tool in a FAST / SPS device.

In principle, a FAST/SPS device is a mechanical press, which at the same time also forms a high-energy current circuit. The FAST/SPS process is carried out in a conductive tool consisting of two punches and a die. Two additional cones establish the contact with the device's electrodes. The pressing force is also applied via these electrodes by means of a hydraulic pressure cylinder. Figure 2 shows a schematic sketch of such a device. FAST/SPS processes are usually carried out in a vacuum or protective gas to protect the tool and the powder inside the tool from oxidation. Accordingly, the pressing device is located in a water-cooled chamber, in which a moderate vacuum (0.5 - 20 mbar) or a defined protective gas atmosphere (e.g. Ar or N₂) can be set. The heating of the tool via the Joule effect requires the use of conductive materials for the punch and the die. The standard material for punch and die is graphite, alternatives to this are introduced in Chapter 4.

Figure 2: Schematic sketch of a FAST/SPS device.

The tool is heated by a continuous or pulsed direct current with moderate voltages <10 V and high currents in the range from 1 to several 10 kA (depending on the size of the device). Non-conductive powders can also be sintered in good quality via FAST/SPS. Here, the direct contact between the tool and the powder leads to rapid heat transfer and relatively even heating over the entire cross-section of the sample. The heating takes place primarily via thermal conduction. In order to reduce the heat loss through thermal radiation, the die can optionally be thermally insulated, e.g. with a graphite felt. In the standard configuration, heating rates of up to approx. 300 K/min can be achieved in a FAST/SPS device. The high heating rates are one of the main reasons for the fast cycle times, as grain boundary and volume diffusion necessary for mass transport during sintering are activated directly. Due to the thermal mass of the tools, extremely high cooling rates cannot be achieved. Typical cooling rates are around 100 - 150 K/min. It should be noted here that the use of thermal insulation materials further reduces the cooling rates. In addition to the rapid heating, the sintering kinetics are also supported by the applied load. Depending on the type of system and sample size, compaction pressure in the range of 50 - 100 MPa can be achieved with the standard material graphite. Special grades of graphite allow maximum pressures of up to 200 MPa. When using graphite tools, temperatures of up to approx. 2,200 °C can be achieved in FAST/SPS devices. Alternative tools made of highly heat-resistant metals or electrically conductive ceramics enable pressures of up to 400 MPa, but the maximum permissible sintering temperatures are usually significantly lower in these cases. In order to improve the electrical contact between the punches, the die and the sample, a flexible, ideally electrically conductive foil is usually positioned between the punches and the sample. Graphite has also proven to be the standard material for this foil. FAST/SPS systems are usually controlled by programming the temperature-time curve. The measurement of the temperature as a major process parameter is carried out by axially or radially arranged pyrometers or by thermocouples. A direct measurement of the sample temperature is usually not possible. In order to measure the temperature as close as possible to the sample, holes are often drilled in the tools, at the bottom of which the temperature is measured. Furthermore, especially with increasing size of sintered parts and complexity of shape, it is recommended to predict the temperature distribution in the parts using suitable FEM modeling.

Advantages and limitations of FAST/SPS

i.) Advantages: An essential feature of the FAST/SPS process is the rapid heating of the sample, which is based on the direct heating of the tool and the good heat transfer between the die and the powder. The superimposition of an uniaxial pressure during sintering further accelerates the sintering kinetics, so that the sintering temperature can often be significantly reduced compared to conventional sintering processes. Accordingly, the FAST/SPS process is a promising technology for •

• Powder with low sintering activity: FAST/SPS enables the compaction of powders that have a low sintering activity (e.g. borides, carbides, nitrides and their mixtures).
• • Powders with unfavorable powder properties: Powders can be sintered whose particle size distribution and morphology are unsuitable for conventional pressing and sintering.
• • Nanoscale powders: The rapid processing in the case of FAST/SPS makes it possible to preserve the structure of nanoscale powders in the component.
• • Materials with limited stability: FAST/SPS enables the sintering of materials that tend to decompose at high temperatures and long dwell times and to form undesirable secondary phases.
• • Materials containing phases out of thermodynamic equilibrium: Phases out of thermodynamic equilibrium can be maintained in the sintered part (e.g. amorphous phases). Based on this, properties can be achieved that are not feasible with other sintering methods.
• • Composite materials: FAST/SPS has great potential for sintering of composite materials that combine phases, which have large differences in their physical properties (e.g. very different melting points).
• • Energy-efficient processing: In general, the FAST/SPS process is classified as an energy-efficient process due to the reduced sintering temperature and the short cycle times, but the degree of automation and the number of components per sintering cycle must also be taken carefully into account for an objective comparison with other sintering technologies.