1. Introduction
Spark plasma sintering (SPS) or pulsed electric current sintering (PECS) is a sintering technique utilizing uniaxial force and a pulsed (on-off) direct electrical current (DC) under low at‐mospheric pressure to perform high speed consolidation of the powder. This direct way of heating allows the application of very high heating and cooling rates, enhancing densifica tion over grain growth promoting diffusion mechanisms (see Fig. 1), allowing maintaining the intrinsic properties of nanopowders in their fully dense products.
It is regarded as a rapid sintering method in which the heating power is not only distributed over the volume of the powder compact homogeneously in a macroscopic scale, but more‐ over the heating power is dissipated exactly at the locations in the microscopic scale, where energy is required for the sintering process, namely at the contact points of the powder particles (see Fig. 2). This fact results in a favourable sintering behaviour with less grain growth and suppressed powder decomposition. Depending on the type of the powder, additional advantageous effects at the contact points are assumed by a couple of authors.
SPS systems offer many advantages over conventional systems using hot press (HP) sintering, hot isostatic pressing (HIP) or atmospheric furnaces, including ease of operation and ac‐curate control of sintering energy as well as high sintering speed, high reproducibility,safety and reliability. While similar in some aspects to HP, the SPS process is characterized by the application of the electric current through a power supply, leading to very rapid and efficient heating (see Fig. 3). The heating rate during the SPS process depends on the geometry of the container/sample ensemble, its thermal and electrical properties, and on the electric power supplier. Heating rates as high as 1000 °C/min can be achieved. As a consequence, the processing time typically takes some minutes depending on the material,dimensions of the piece, configuration, and equipment capacity.
On the contrary, in conventional HP techniques, the powder container is typically heated by radiation from the enclosing furnace through external heating elements and convection of inert gases if applicable. Therefore, the sample is heated as a consequence of the heat transfer occurring by conduction from the external surface of the container to the powders. The resulting heating rate is then typically slow and the process can last hours. In addition, a lot of heat is wasted as the whole volume of space is heated and the compact indirectly receives heat from the hot environment. On the other hand, SPS processes are characterized by the efficient use of the heat input, particularly when electrically insulating powders is used and the pulsed electric current is applied.
It should be however mentioned that in SPS processes the problem of adequate electrical conductivity of the powders and the achievement of homogenous temperature distribution is particularly acute. In this way, the electric current delivered during SPS processes can in general assume different intensity and waveform which depend upon the power supply characteristics.
In order to permit a homogeneous sintering behaviour, the temperature gradients inside the specimen should be minimized. Important parameters that are drastically determining the teperature distribution inside the sample are the sample material's electrical conductivity, the diewall thickness and the presence of graphite papers used to prevent direct contact between graphite parts and the specimen and used to guarantee electrical contacts between all parts.
The application of external electric current to assist sintering was initiated by Taylor in 1933, who incorporated the idea of resistance sintering during the hot pressing of cemented carbides [Taylor, 1933]. Later, Cramer patented a resistance sintering method to consolidate copper, brass and bronze in 1944 in a spot welding machine [Cremer, 1944]. The concept of compacting metallic materials to a relatively high density (>90% of theoretical) by an electric discharge process was originally proposed by Inoue in the 1960s [Inoue, 1965]. Inoue argued that a pulsed current was effective for densification at the initial sintering stages for low melting point metals (e.g., bismuth, cadmium, lead, tin) and at the later sintering stage for high melting metals (e.g., chromium, molybdenum, tungsten). In the United States, Lenel also used a spot welding machine for the sintering of metals [Lenel, 1955]. In addition to continuous pulses, some researchers also investigated a single discharge method, i.e., the powders were densified by a single discharge generated from a capacitor bank. In the late 1970s, Clyens et al. [Clyens et al., 1976], Raichenko et al. [Raichenko et al., 1973] and Geguzin et al. [Geguzin et al., 1975] studied the compaction of metal powders using electric discharge compaction (EDC) or electric discharge sintering (EDS). In all the methods cited, electrically conductive powders are heated by Joule heating generated by an electric current.
In 1990 Sumitomo Heavy Industries Ltd. (Japan), developed the first commercially operated plasma activated sintering (PAS) and spark plasma sintering (SPS) machines with punches and dies made from electrically conductive graphite [Yanagisawa et al., 1994]. One of the salient features of these machines was that, in addition to electrically conductive powders, high density was also achieved in insulating materials. In PAS process, a pulsed direct current is normally applied at room temperature for a short period of time followed by a constant DC applied during the remainder of the sintering process (see Fig. 4a). This procedure is often referred to in the literature as a "single pulse cycle process”. In the SPS process, a pulsed DC is applied repeatedly from the beginning to the end of the sintering cycle (see Fig. 4b). In this case the procedure is referred to as a “multiple pulse cycle process”. so used a spot welding machine for the sintering of metals [Lenel, 1955]. In addition to continuous pulses, some researchers also investigated a single discharge method, i.e., the powders were densified by a single discharge generated from a capacitor bank. In the late 1970s, Clyens et al. [Clyens et al., 1976], Raichenko et al. [Raichenko et al., 1973] and Geguzin et al. [Geguzin et al., 1975] studied the compaction of metal powders using electric discharge compaction (EDC) or electric discharge sintering (EDS). In all the methods cited, electrically conductive powders are heated by Joule heating generated by an electric current.
2. The basic SPS configuration and process
The basic configuration of a typical SPS system is shown in Figure 5. The system consists of a SPS sintering machine with vertical single-axis pressurization and built-in water-cooled special energizing mechanism, a water-cooled vacuum chamber, atmosphere controls, vacuum exhaust unit, special sintering DC pulse generator and a SPS controller. The powder materials are stacked between the die and punch on the sintering stage in the chamber and held between the electrodes. Under pressure and pulse energized, the temperature quickly rises to 1000~2500 °C above the ambient temperature, resulting in the production of a high quality sintered compact in only a few minutes.
3. Principles and mechanism of the SPS process
The SPS process is based on the electrical spark discharge phenomenon: a high energy, low voltage spark pulse current momentarily generates spark plasma at high localized temperatures, from several to ten thousand ℃ between the particles resulting in optimum thermal and electrolytic diffusion. SPS sintering temperatures range from low to over 2000 ℃ which are 200 to 500 ℃ lower than with conventional sintering. Vaporization, melting and sintering are completed in short periods of approximately 5 to 20 minutes, including temperature rise and holding times. Several explanations have been proposed for the effect of SPS:
3.1. Plasma generation
It was originally claimed by Inoue and the SPS process inventors that the pulses generated sparks and even plasma discharges between the particle contacts, which were the reason that the processes were named, spark plasma sintering and plasma activated sintering [Inoue, 1965, Yanagisawa et al., 1994]. They claimed that ionization at the particle contact due to spark discharg‐es developed “impulsive pressures” that facilitated diffusion of the atoms at contacts. Groza [Groza et al, 1999] suggested that a pulsed current had a cleaning effect on the particle surfaces based on the observation of a grain boundary without oxidation formed between particles.
Whether plasma is generated or not has not yet been confirmed directly by experiments. Therefore, there is no conclusive evidence for the effect of a plasma generation in SPS. The occurrence of a plasma discharge is still debated, but it seems to be widely accepted that occasional electric discharges may take place on a microscopic level [Groza et al., 2000].
3.2. Electroplastic effect
Metal powders have been observed to exhibit lower yield strength under an electric fieldRaichenko et al. [Raichenko et al., 1973] and Conrad [Conrad, 2002] independently studied electroplastic phenomena.
3.2. Electroplastic effect
Metal powders have been observed to exhibit lower yield strength under an electric fieldRaichenko et al. [Raichenko et al., 1973] and Conrad [Conrad, 2002] independently studied electroplastic phenomena.
3.3. Joule heating
Joule heating due to the passage of electric current through particles assists in the welding of the particles under mechanical pressure. The intense joule heating effect at the particle conducting surface can often result in reaching the boiling point and therefore leads to localized vaporization or cleaning of powder surfaces [Tiwari et al., 2009]. Such phenomenon ensures favourable path for current flow.
3.4. Pulsed current
The ON-OFF DC pulse energizing method generates: (1) spark plasma, (2) spark impact pressure, (3) Joule heating, and (4) an electrical field diffusion effect.
In the SPS process, the powder particle surfaces are more easily purified and activated than in conventional electrical sintering processes and material transfers at both the micro and macro levels are promoted, so a high-quality sintered compact is obtained at a lower temperature and in a shorter time than with conventional processes. Figure 6 illustrates how pulse current flows through powder particles inside the SPS sintering die.
The SPS process is an electrical sintering technique which applies an ON-OFF DC pulse voltage and current from a special pulse generator to a powder of particles (see fig. 7), and in addition to the factors promoting sintering described above, also effectively discharges between particles of powder occurring at the initial stage of the pulse energizing for sintering.
High temperature sputtering phenomenon generated by spark plasma and spark impact pressure eliminates adsorptive gas and impurities existing on the surface of the powder particles. The action of the electrical field causes high-speed diffusion due to the high-speed migration of ions.
3.5. Mechanical pressure
When a spark discharge appears in a gap or at the contact point between the particles of a material, a local high temperature-state (discharge column) of several to ten thousands of degrees centigrade is generated momentarily. This causes evaporation and melting on the surface of powder particles in the SPS process, and "necks" are formed around the area of contact between particles. Figure 8 shows the basic mechanism of neck formation by spark plasma
Figure 9a shows the behavior in the initial stage of neck formation due to sparks in the plasma. The heat is transferred immediately from the center of the spark discharge column to the sphere surface and diffused so that intergranular bonding portion is quickly cooled. As seen in Figure 9b which show several necks, the pulse energizing method causes spark discharges one after another between particles. Even with a single particle, the number of positions where necks are formed between adjacent particles increases as the discharges are repeated. Figure 9c shows the condition of an SPS sintered grain boundary which is plastic deformed after the sintering has progressed further.
Spark Plasma Sintering
