This paper presents the technology of powder sintering by the spark plasma sintering method, also known as the field assisted sintering technique. The mechanisms, compared to other sintering techniques, advantages of this system, applied modifications and the history of the development of this technique are presented. Spark Plasma Sintering (SPS) uses uniaxial pressing and pulses of electric current. The direct flow of current through the sintered material allows high heating rates to be reached. This has a positive effect on material compaction and prevents the grain growth of sintered compact. The SPS mechanism is based on high-energy spark discharges. A low-voltage current pulse increases the kinetics of diffusion processes. The SPS temperature is up to 500 °C lower than the sintering temperature using conventional methods. The phenomena that occur during sintering with the Field Assisted Sintering Technology (FAST)/SPS method give great results for consolidating all types of materials, including those which are nonconductive. This method is used, among others, in relation to metals, alloys and ceramics, including advanced and ultra-high-temperature ceramics. Due to the good results and universality of this method, in recent years it has been developed and often used in research institutions, but also in industry.
The progress in advanced industries requires new, “perfect” materials. There is a demand for materials which are able to work under specific conditions such as very high temperatures or in particularly aggressive environments. This demand opens up many new opportunities and provides a challenge for scientists to obtain new durable materials. There are many methods of manufacturing and for the consolidation of materials. The powder sintering technology, however, allows the obtention of materials with specific designed microstructures and compositions with widely changing properties depending on the used technique and its parameters. Various powder sintering techniques have been used successfully to consolidate both metals and ceramic materials. Despite sintering being a mature method of material manufacturing known for thousands of years (it is known that the first ceramic products were made by humans over 25,000 years ago), the most significant progress in sintering methods started with the development of the pressure assisted technology. Then, the application of a current and electromagnetic field (microwave sintering, spark plasma sintering) turned out to be the milestone in powder sintering techniques (
The Field Assisted Sintering Technology (FAST)/Spark Plasma Sintering (SPS) technique uses a combination of uniaxial external pressure and electric current in order to rapidly sinter a wide range of powders. This manufacturing method allows higher heating rates (up to 1000 °C/min) and shorter holding times to be reached compared with conventional sintering techniques, which is undeniably one of the greatest advantages of the FAST/SPS method (
To date, many modifications of the FAST/SPS process and devices have been applied to meet the challenges of the fast-growing industry. In a recent study (
The FAST/SPS technique is a complex, multiphysical method of material manufacturing that combines several fields of science and requires a deep and detailed understanding of the mechanism and phenomena that occur during sintering. Although the existence of plasma during FAST/SPS processes is still controversial and has never been confirmed, the technique is extremely effective for almost all materials, making it extremely versatile.
The aim of this review is to summarise the principal knowledge about the history and development of the FAST/SPS technique, fundamental mechanisms responsible for powder sintering and, finally, some interesting modifications that have already been applied to the SPS method.
Sintering of ceramic and metallic powders is a phenomenon that occurs spontaneously with increasing temperature. As a result of the heating, after reaching the temperature in the range from 0.4 to 0.8 of the melting point, formation of necks, pore reduction and powder consolidation occur. The connection of powder grains is accompanied by the contraction of the entire system and the loose or weakly bound powder transition into a solid polycrystal. These changes are the result of mass transfer processes, initially moving particles against each other, and then moving the individual atoms. At lower temperatures, mass transport is a result of diffusion along the grain boundaries and surface diffusion. At the highest temperature, mass transport takes place due to volumetric diffusion, as well as evaporation and condensation (
These processes require both a high temperature and appropriate time. The value of these parameters strongly depends on the sintered material, its melting point or the type of atomic bonds. The sintering process is also determined by the pressing force, atmosphere used (e.g., argon, nitrogen, vacuum), powder particle size and the method of their initial preparation. The intensification of diffusion processes, and thus the shortening of the sintering duration, can be achieved by using devices that enable the generation of an appropriate pressing force, as well as heating due to the current pulse flowing directly through the sintered sample (
Spark plasma sintering, also called the field assisted sintering technique or Puls Electric Current Sintering (PECS), is a sintering technique using uniaxial pressing and electric current pulses. The direct current that flows through the sintered material, allows high heating rates to be reached. This has a positive effect on material compaction and prevents the sinter grains growth. The SPS sintering mechanism is based on high-energy spark discharges. A low-voltage current pulse increases the kinetics of diffusion processes.
There are several names for this method in the literature. In addition to SPS, it is also called Plasma Assisted Sintering (PAS) and FAST (Field Assisted Sintering Technology). Moreover, the Pulse Plasma Sintering (PPS) method is common, which is characterised by similar phenomena as in the above-mentioned methods, but as it was reported (
Currently, the SPS mechanism is explained by discharges arising in the spaces between the sintered powder particles, causing local temperature increases, powder surface melting and neck formation. After creating the neck, resistance heating occurs as a result of the current flow through a conductor with a small cross-section (
The SPS temperature is up to 500 °C lower than the sintering temperature used in conventional sintering methods. Evaporation, melting and sintering between the powder particles takes place in a few to several dozen minutes, including the heating time and holding at the sintering temperature (
The SPS technique is characterised by direct heating of the sintered material as a result of the current flow; thus, it has a high heating rate compared to traditional HP (hot pressing) and Hot Isostatic Pressing (HIP) processes. This is especially so when compared to the heating rates obtained during free sintering, or even microwave sintering (
It should be mentioned that in SPS processes, powder conductivity is highly important, as well as achieving a homogeneous current density and temperature distribution. In materials characterised by high electrical resistance, the current flows through the graphite set, which acts as a heating element located around the sintered material. The density of current flowing through the graphite elements and the sintered material, depending on its electrical conductivity, is shown in
The development of sintering methods assisted with electric currents began in the 1930s. Initially, resistance sintering was utilised in the hot-pressing process. Then, spot welding equipment was introduced. The exploitation of an external electric current to assist sintering was initiated in 1933 by Taylor, who patented a device and a method combining resistance sintering and hot pressing of sintered carbides (
The SPS method was developed in Japan. In 1962, Dr. Kiyoshi Inoue of Japax Inc. elaborated the Spark Sintering (SS) method, which was then extended by introducing current pulses at the initial stage of heating. The second generation of powder consolidation equipment was developed in 1986 by Inoue-Japax Research Inc. and was called PAS (plasma assisted/activated sintering). Currently, the SPS method is recognised as the third generation (after SS and PAS) of such methods, initiated by Sumitomo Coal Mining Co., Ltd. (Japan) in 1989 (
SPS devices can be used to consolidate conductive materials as well as materials with high electrical resistances. The development of both methods and devices for using impulse current enabled the consolidation of a wide range of metallic, ceramic and composite materials, also at an industrial scale. Currently, SPS devices are used not only in material sintering, but also in the processes of solid phase material diffusion bonding or surface layer modification. In the years 2000–2009, there was a tendency to use the SPS method in industrial production at small and medium-series scales. Process automation and the creation of industrial lines, which include, among others, devices with impulse heating systems, constitute the fourth generation of the SPS system. SPS devices adapted to other technologies, sometimes far away from the sintering process, which resulted in numerous research and industrial works being carried out in recent years; these can be thought of as the fifth generation of SPS technology. A chart showing the increase in the number of publications on SPS since 1994 is presented in
The powder consolidation methods of the second and third generations (PAS and SPS) differ mainly in the duration of the stage in which the pulse current is used to heat the material. In the PAS method, heating with an impulse current is carried out at the beginning of the process, and in the subsequent stages of sintering it is realised by applying a direct current (
The Łukasiewicz—Krakow Institute of Technology possesses an SPS machine manufactured by an FCT System GmbH, model HP D5. This device is characterised by a maximum sintering temperature of 2400 °C, pulse duration in the range of 1 to 255 ms and an additional option to regulate the time interval between the pulses as well as between the group of pulses. The pressing force can achieve values in the range of 3–50 kN and the maximum force is defined by the graphite element’s strength. Additionally, the SPS machine is equipped with a hermetic chamber that enables the performance of the process with vacuum conditions (50 Pa) or to provide a protective atmosphere (argon, nitrogen). Moreover, the control system offers heating rate regulation ranging from 5 to 400 °C/min.
The HP D5 device can be used for the sintering of various metallic, ceramic and composite materials—in particular, the so-called advanced ceramics, including ceramic composites reinforced with diamond and cubic boron nitride (cBN) (
The sintering behaviour and mechanical properties of Al2O3-based matrix composites reinforced with up to 30 vol.% of diamond and cBN were investigated. Additionally, pure Al2O3 powder was treated separately under identical SPS conditions for the purpose of comparison. The sintering behaviour of the composites is different than that of the Al2O3 matrix sintered separately. The Young’s modulus of Al2O3 increases with temperature up to a specific value and does not change thereafter. The composites attain maximum values of density and Young’s modulus around the sintering temperature of 1300–1400 °C. This difference can be explained by the cubic to hexagonal phase transitions. During SPS, two opposing processes occur: densification of the material (increase in density and Young’s modulus), and diamond/cBN → graphite/hexagonal boron nitride (hBN) transformation at higher temperatures (reduction in density and Young’s modulus). The best composites reinforced with diamond have high relative densities (99%), and moderate Young’s modulus (74% of theoretical value); nevertheless, the hardness and fracture toughness of these sinters are too low in relation to the values assumed. Notably, good improvements in fracture toughness, of about 1.5 MPa∙m1/2 on average over that of Al2O3, are observed in the case of composites sintered with cBN. Crack deflection on cBN grains is identified as the main toughening mechanism of the materials investigated (
Despite the SPS technology being a well-known method that ensures great results both in process parameters (sintering time reduction and lowering sintering temperature) and material properties, some improvements are still being developed. One of the most promising routes for further SPS evolution is a sintering method called Flash SPS (FSPS), which is a combination of the standard SPS method and flash sintering.
Flash sintering itself (without SPS), as a novel method based on applying an electric field to a conventionally heated sample, allows the reduction in the time and temperature required to sinter ceramic materials. The sample, initially nonconductive, is heated by radiation in a conventional furnace. At the same time, an electric field is applied through the electrodes (usually highly conductive metal), forcing a current flow in the ceramic body. During the so-called “flash event”, rapid densification takes place as well as a sudden sample resistance drop and thermal runaway of internally generated Joule heat. It is possible to obtain almost a fully dense body while flash sintering in only a few seconds. Moreover, this process enables one to achieve metastable phases and produce ceramics with nanometric grains (
To date, some configurations have already been developed for carrying out FSPS processes. Die isolated from punches or die-less configurations (
Although flash sintering and Flash SPS are considered as innovative methods leading to extremely desirable process parameters and material properties, a more detailed understanding of the “flash event” phenomenon is still required.
In summary, the SPS equipment allows the sintering of a wide variety of materials and operations at lower temperatures and pressures compared to the free sinter or conventional HP (hot pressing) process. It affects the reduction in grain growth by shortening the sintering process time. In addition, it enables materials to be shaped without prepressing, isostatic densification and drying. Moreover, it allows the visualisation of changes in the sintering process parameters (temperature, time, heating and cooling rates, pressure force) and the visualisation of changes in the resistance of the sintered material. This technique is used to produce and shape a wide range of materials: metallic, composites and ceramics, including superhard materials. Furthermore, spark plasma sintering is a modern method that enables the simultaneous performance of syntheses with the features of self-developing high-temperature synthesis (SHS) and the densification of the produced materials. The phenomena accompanying the SPS processes favours the formation of new compounds and also contributes to the reduction in the sintering temperature required to obtain a material with a high relative density.
Conceptualisation, J.L.-Ł. and P.P.; Methodology, J.L.-Ł. and P.P.; Validation, P.K.; Formal Analysis, P.K.; Investigation, M.P.; Resources, J.L.-Ł., P.P. and P.K.; Data Curation, J.L.-Ł., P.P. and P.K.; Writing—Original Draft Preparation, J.L.-Ł., P.P. and K.B.; Writing—Review and Editing, J.L.-Ł. and P.K.; Visualisation, P.P.; Supervision, P.P. and P.K.; Project Administration, P.P.; Funding Acquisition, P.P.
This work was carried out in the Łukasiewicz Research Network—Krakow Institute of Technology, under the following project supported by the Ministry of Science and Higher Education of Poland (subvention): “Preparation of ceramic composites with the strengthening phase in the form of diamond nanostructures using HPHT method” (Project No. DS 4402/1031). The results presented were also partially achieved within the DURACER project, entitled “Durable ceramic composites with super-hard particles for wear-resistant cutting tools”, cofounded by the Polish National Centre for Research and Development within the framework of the M-ERA.NET programme (Agreement No. M-ERA.NET2/2017/1/2018).
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
Model of mass transport during sintering in the two-grain system (
Diagram of the current flow during Spark Plasma Sintering (SPS) method
Comparison of the heating rate and duration of the sintering process of ceramics with the Al2O3 matrix sintered by SPS, microwave and free sintering (
Scheme of hot pressing (HP) (
Difference (ΔT) between set temperature (TA) and the measured temperature (TI), and the time (t *) needed to equalise the temperature in the processes: (
Current flow during SPS heating, depending on the electrical conductivity of the sintered material: (
The results of modelling the temperature distribution on: graphite elements with different geometries and electrical properties (
Number of publications with terms SPS or spark plasma sintering in the title, according to the Web of Science database (as of 25 November 2020) (
Differences in the manner of heating materials in the Plasma Assisted Sintering (PAS) (
Impulse heating during sintering by SPS method with the use of devices from FCT GmbH (
SPS device scheme
SPS sintering device, type HP D5 by FCT (
The influence of the pyrometer location on the temperature difference between the sample and the place of measurement. Test conditions: TiN material, relative density 97%, sample dimensions ϕ50 × 10 mm and sample temperature 1500 °C [FCT GmbH advertising materials]
Example of Field Assisted Sintering Technology (FAST)/SPS sintering curves
Example of sample shrinkage during FAST/SPS process
Flash SPS configurations: (