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.

ii.) Limitations: Despite the obvious advantages of FAST/SPS technology, there are also a number of process-related limitations.

• Risk of density and stress gradients in the component: If powders with a low thermal conductivity and high electrical resistance are sintered by FAST/SPS, there might arise an inhomogeneous temperature distribution in the component, which becomes more pronounced with increasing sample size. If exceeding a critical value, density and/or stress gradients in the sintered part result. There are also limitations with respect to the maximum height/diameter ratio. Similar to conventional pressing, wall friction can cause density gradients in the sintered part.
• Chemical and electrochemical interaction with the tool materials: The direct contact of the powder with the tools is advantageous with respect to heat transfer, but in the worst case it also leads to a chemical interaction with the tools or the inserted foils. When using graphite tools or foils, possible contamination of the component with carbon must be taken carefully into account. Therefore, adhering residues of the graphite foil and the reaction zone on the component surface must be removed mechanically, e.g. by grinding. In addition, it must be noted that the electrical field applied to the powder can trigger electrochemical reactions at the interfaces. A well-known effect is electrochemical blackening of oxide ceramics caused by the release of oxygen at low oxygen partial pressures. Furthermore, oxide ceramics that contain transition metals at the A or B position are sensitive to chemical expansion, which in the worst case leads to severe cracking in the component.
• Change of conductivity of semi-conductive and mixed-conductive materials: When sintering semi-conductive and mixed-conductive materials via FAST/SPS, it should be noted that the conductivity of the powder changes with increasing temperature and density during the FAST/SPS cycle. If the heating at the beginning of the sintering cycle is primarily based on heat conduction, this state might change suddenly if a current flow through the sample becomes possible when the temperature exceeds a specific value. The result is a complex sintering process that becomes difficult to control and which can impair reproducibility. This effect is used for specific kinds of field-assisted sintering, e.g. for flash sintering. For more details, see Chapter 8.
• Net-shaping: The FAST/SPS process reaches its limits when complex shapes are to be realized. In addition to the challenges of suitable tool design, increasing geometrical complexity might cause inhomogeneous temperature distribution and, in the case of conductive powders, inhomogeneous current flow over the entire sample cross-section. There are a number of approaches to manufacture complex shapes e.g. by using split tools or by sintering near net-shaped green parts in a powder bed directly via FAST/SPS. However, the possibilities for automating these technologies must be evaluated critical.
• Automation of the FAST/SPS process: At current state of development, cycle times in FAST/SPS systems cannot compete with conventional pressing and sintering, since a complete FAST/SPS cycle takes several minutes despite high heating rates and short dwell times. If insert foils are used to improve the electrical and thermal contact during tool assembly, this slows down the cycle time further. In order to operate FAST/SPS devices with higher productivity, partially automated, multiple tools (capacity up to several hundred components) are in use, which can optionally be filled with a robot system outside the device and then installed in the device via a manipulator. Another concept are FAST/SPS devices with a system of separate chambers in which the individual processing steps of tool assembly, powder filling, compression and cooling are carried out in a separate and sequential manner.

How to conduct a standard FAST/SPS cycle

In this section, the processing steps for sintering a ceramic powder via FAST/SPS are explained in detail. For conducting FAST/SPS cycles, the following three devices are available at the institute IEK-1 (Figure 3).

-> Figure 3
Figure 3: FAST/SPS devices at the institute IEK-1 a.) Laboratory device HP-D 5 b.) Hybrid sintering system H-HP-D25 SD/FL/MoSi c.) Sintering press DSP515 on industrial scale.

The HP-D 5 laboratory device is characterized by the following operating parameters:

• Type: HP-D 5
• Manufacturer: FCT Systeme GmbH, Rauenstein, Germany
• Maximum pressing force: 50 kN
• Maximum heating power: 37 kW
• Maximum sample diameter: 30 mm
• Possible sintering atmospheres: Vacuum, Ar, Ar / H2, N2, air
• Operating mode: FAST / SPS with pulsed direct current

The hybrid sintering device H-HP-D25 SD/FL/MoSi enables a number of different operating modes in addition to the conventional FAST/SPS mode through the optional application of additional heaters or operation with an external voltage source.

• Type: H-HP-D25 SD/FL/MoSi
• Manufacturer: FCT Systeme GmbH, Rauenstein, Germany
• Maximum pressing force: 250 kN
• Maximum sample diameter: 100 mm (FAST/SPS mode), 20 mm (Flash sintering mode)
• Maximum heating power (FAST / SPS): 60 kW
• Maximum heating power (Induction): 80 kW
• Maximum heating power (MoSi2): 10 kW
• Maximum heating power (Hybrid heating): 100 kW
• External voltage source: AC/DC, 1000 V, max. 25 A
• Possible sintering atmospheres: Vacuum, Ar, Ar / H2, N2, air
• Operation modes: i.) FAST / SPS with pulsed direct current ii.) Hybrid FAST/SPS with induction coil or MoSi2 furnace as additional heater iii.) Hot press with MoSi2 heater iv.) Flash SPS with defined heating power v.) Flash sintering with an external voltage source and MoSi2 heater.

The DSP515 sintering press is a device, which can be used for industrial production and enables the scaling up of promising results from experiments on lab-scale.

• Type: DSP515
• Manufacturer: Dr. Fritsch Sondermaschinen GmbH, Fellbach, Germany
• Maxiumum pressing force: 555 kN
• Electrode size: 180 x 180 mm2
• Maximum sample size: approx. 160 x 160 mm2
• Maximum heating power: 170 kW
• Possible sintering atmospheres: Vacuum, Ar, Ar / H2, N2
• Operating modes: i.) FAST/SPS with constant direct current ii.) Flash SPS with defined heating power.
• Debinding unit: The system is also equipped with a cooling trap for debinding of tape-cast parts.

Below, the steps of a standard FAST/SPS cycle using a graphite tool with a sample diameter of 20 mm are described. An additional graphite foil (thickness approx. 0.35 mm) is inserted into the graphite tool. The thickness of the foil must be taken into account when designing the tool. The inserted foil has two main functions. On the one hand, it improves the electrical and thermal contact between the punch and the die as well as between the powder, the face of the punch and the die. On the other hand, it avoids chemical interactions between powder and tool, so that the tool life is increased. The FAST/SPS cycle shown here was carried out in the HP-D 5 laboratory device.

i.) Sample preparation: Figure 4 shows the parts of a lab-scale graphite tool (inner diameter 20 mm) as well as the graphite insert foils (thickness 0.35 mm), which are sized with respect to the tool dimensions. In a first step, the graphite foils are positioned on the inner wall of the die and on the faces of the punches. It is important that the foils has a tight contact to the tool components and that there are no kinks or gaps after positioning. For health protection, the graphite tool should be filled in a laboratory hood. Filling can be carried out on a balance. This increases the accuracy of the powder quantity weighed in.

-> Figure 4
Figure 4: Preparation of the graphite tool and the insert foil for the FAST/SPS cycle. For accurate sample preparation, the powder should be directly balanced in the tool. Furthermore, it is recommended to handle the powder in a hood.

After filling in the tool, the powder is covered with an additional graphite foil and pre-pressed in a manual laboratory press with 50 MPa (16 KN for diameter 20 mm tool) (Figure 5). In order to avoid sticking of powder particles on the face the upper punch, the upper punch is removed again and powder residues are manually removed. Afterwards, the upper punch is placed again in the tool.

-> Figure 5
Figure 5: a.) Filling the mold and positioning the graphite foil b.) Pre-pressing of the FAST/SPS sample in a manual hand press c.) Removal of the upper punch after the pressing process and removal of adhering powder particles.

ii.) Mounting of the tool and programming the FAST/SPS device: At the laboratory system HP-D 5, the programming of the FAST/SPS parameters can be done via a process controller type Stange SE607 (Figure 6). The main operating parameters temperature, pressing force, pulse length of the current and moving speed of the lower electrode is programmed in individual program segments. During the FAST/SPS cycle, all process data are shown on the controller display as a numerical values and as a time-resolved graph. In parallel, all data are saved via a data logger integrated in the system. By default, the data are transferred to an external PC for data management and further evaluation. Individual parameter sets of the users are also stored on the PC and can be easily transferred to the controller if required.

-> Figure 6
Figure 6: Display of the controller of the HP-D 5 device giving a compact overview of all process-relevant data.

Figure 7 shows the graphite tool mounted into the HP-D 5 devices as well as the CuBe electrodes. Optionally, the graphite tool can be thermally isolated with a jacket made of a graphite felt (not shown here). A graphite cord can be used to fix the graphite felt on the tool.

-> Figure 7
Figure 7: View on the processing chamber of the HP-D 5 device with mounted graphite tool. When installing the tool and the two cones, it should be noted that graphite is a brittle and shock-sensitive material and therefore breaks very easily. For this reason, sudden contact of all graphite parts with the other system components (stamp, chamber walls) must be avoided.

After the graphite tool has been inserted and fixed by lifting the lower punch, the container door is closed and the FAST/SPS cycle can be started.

iii.) Choice and regulation of the sintering atmosphere: A FAST/SPS cycle is usually carried out in vacuum or under protective gas atmosphere. Accordingly, the process chamber is evacuated by a vacuum pump at the beginning of the cycle. In the laboratory device HP-D 5, a vacuum of approx. 0.5 mbar is achieved; in larger systems, this value can also be higher (e.g. 20 mbar for the DSP515). The quality of the vacuum depends on the sealing of the process chamber and the power of the vacuum pump. If aiming on a high vacuum, cost of the FAST/SPS device significantly increases. After evacuation, the chamber can optionally be flooded with gases such as N₂, Ar or Ar/H₂ (static atmosphere control) or a continuous gas flow (dynamic atmosphere control). Figure 8 shows the valves for feeding the individual gases from the institute's gas store into the system. The evacuation of the process chamber and the optional supply of protective gases to the process chamber is regulated by the system control and takes place automated. The pressure in the process chamber is continuously recorded in the HP-D 5 device by two sensors with different measuring ranges (coarse sensor 1.0 - 1300 mbar, fine sensor 0.01 - 13 mbar), which are positioned at the gas outlet. For operation of the system under relative pressure (reduced pressure in relation to the ambient pressure), the system is also equipped with a relative pressure sensor, which enables to regulate the relative pressure in the range of 5 - 50 mbar. Static or dynamic operation is also possible in the relative pressure mode. A safety valve is used to protect the processing chamber against overpressure. The valve opens at an overpressure of 100 mbar above the ambient pressure. The pressure curve during the FAST/SPS cycle is recorded continuously. An analysis of the pressure curve can be helpful if components outgas from the sample or if the sample decomposes under the FAST/SPS conditions.

-> Figure 8
Figure 8: Valves for flooding the processing chamber with the protective gases Ar, N₂, Ar/H₂.

When sintering oxide ceramics via FAST/SPS, it should be noted that the sintering conditions have a reducing effect, i.e. there is a release oxygen atoms from the lattice, which increases with decreasing oxygen partial pressure. The effect is aggravated in the case of direct contact with the graphite tools or the graphite foils. The release of oxygen is visually noticeable in the darkening of the ceramic. This effect is superimposed by the uptake of carbon. If the release of oxygen exceeds a critical value, the sample can undergo chemical expansion, which can be up to several percent (as e.g. the case for gadolinium-doped cerium oxide or barium-strontium-cobalt oxides). Usually, this effect leads to severe cracking of the sample. Furthermore, the crystal lattice can become unstable if the oxygen release progresses further. In worst case, reduction of components down to the metallic phase might appear.

In principle, it is possible to operate a FAST/SPS device with technical air. Such operation must be done with special care, since most materials used for the tools are unstable in air and start to strongly oxidize if the temperature exceeds a critical value (usually around 400°C). It should also be noted that the powder in the tool is encapsulated in such a way that the access of oxygen to the powder is prevented to a large extent.

iv.) Performing of the FAST/SPS cycle: After the system has been prepared, the selected program is started and runs automatically. Figure 9 shows the graphite die during the heating process. As already mentioned, the operation of FAST/SPS devices is usually temperature-controlled. This means that the controller defines the temperature and the system regulates the current and voltage in such a way that the temperature profile is adhered to the specification as precisely as possible. In order to keep the deviations from the programmed temperature curve as low as possible, the system is equipped with a PID control (PID = Proportional-Integral-Derivative). This control unit iteratively adapt the optimal current and voltage curves for heating the tool to the respective electrical resistance of the tool.

-> Figure 9
Figure 9: View on the heated graphite tool (punch diameter 20 mm) in the laboratory device HP-D 5. When operating a FAST/SPS system, it should be noted that the high currents in the system cause strong electromagnetic fields, which can damage pacemakers and others sensitive electronic devices.