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High-Tech Sintering

June 1, 2012
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Spark plasma sintering is an ever-growing class of versatile techniques for sintering particulate materials.

Spark plasma sintering (SPS) is a rapid sintering technique that is assisted by pressure and electric field. The scientific and industrial importance of SPS technologies is widely recognized.

SPS’ established advantages include low power consumption, elimination of the need for sintering aids, control of the thermal gradient for functional-graded materials (FGMs), selective control of the density in specified regions of the sample, single-step sintering/bonding, particle surface cleaning, high heating rate, and near-net-shape capability. The short sintering time is particularly suitable for preserving the initial powder grain size or nanostructure, preserving the amorphous structure of sintered materials (i.e., metallic glasses), improving bonding strength between particles, and controlling phase reactions or decomposition in the case of composites.

Historical Background

SPS technology was pioneered in the UK by A.G. Bloxam, whose first patent on pure direct current (DC) resistance sintering (RS) was published in 1906. Thereafter, Duval d’Adrian in 1922 and G.F. Taylor in 1932 developed the first resistive sintering processes combining capacitor bank, transformers, and special switching devices. This originated so-called electric discharge compaction (EDC).

As shown in Figure 1, the progress of SPS technology has been rather discontinuous. Its main driving force was initially motivated by the industrial needs of the time. SPS’ industrialization and commercialization evolved in three stages: an exploration stage (1900-1960), a development stage (1960-1990) and an exploitation stage (1990-2008). The exploration stage was mainly driven by the strong need for improved refractory products, cutting tools, abrasives and oil-free bearings. Early machines were limited by electric power, mechanical pressure and process control. The discouraging results in early work were attributed to both the lack of fundamental understanding of basic principles and poor devices (e.g., energy sources, capacitor switches and controlling units).

The development stage was strongly influenced by Inoue’s patents, which contributed to the development of SPS. He pioneered the pulsed electric current sintering (PECS) method and reconsidered the method of pressure application and the current waveforms. The patent published by Inoue in 1966 added a solid backbone to existing SPS technology. It introduced unprecedented technological innovations with new basic sintering principles using different electric current waveforms (i.e., low-frequency AC, high-frequency unidirectional AC and/or pulsed DC).

These methods were combined in one sintering process called electric discharge sintering (EDS), also known as spark sintering (SS). Spark sintering employs a unidirectional pulsed DC or a unidirectional AC eventually superimposed on DC. This process inspired the development of subsequent pulsed electric current sintering (PECS) methods, including plasma-assisted sintering (PAS), spark plasma sintering (SPS) and plasma pressure compaction (P2C).

The most recent Inoue patents have been focused on the design of more efficient current sources, as well as on the introduction of auxiliary/peripheral devices to increase the reproducibility and controllability of SPS processes. However, Inoue’s patents have not been put into wide use, owing perhaps to the high cost of equipment, lack of repeatability or low sintering efficiency.

Very few successful industrial applications were reported before 1989. The exploitation stage arrived with the worldwide availability of SPS systems. The development of fully automated SPS systems decreased production costs and increased productivity. Today, SPS is considered a mature and viable manufacturing technology. The near-net-shape capability, versatility, and the recognized technological and economical benefits of SPS have stimulated the industry sector. The principles of early patents on SPS are still used in modern apparatuses.

SPS vs. Conventional Hot Pressing

SPS basically exploits the same punch/die system concept used in the more familiar hot pressing (HP) process. A powder or green compact is placed in the die and subsequently pressed between two counter-sliding punches. Mechanical loading is normally uniaxial. It is well-documented that in the HP process, the powder densifies due to a combination of thermal and pressure effects.

SPS and HP differ significantly in the heating mode, however. In HP, an array of heating elements indirectly heats the punch/powder/die assembly by radiation and eventually by convection and/or conduction. Conversely, in SPS, the punches carry electric current, which produces direct Joule heating of the powder.

As the supplied current density can be very large, the heating rate in the powder can approach 103°C min-1. This heating rate is much higher than the 80°C min-1 that is possible for the HP process. Thus, the sintering time can be lowered and the production rate increased with SPS.

Industrial Production

The versatility of SPS is illustrated in Figures 2 and 3, which show the wide range of potential application fields and associated advanced materials. Materials processed through SPS often exhibit improved physical and mechanical properties compared with those obtained by conventional methods. Noteworthy features resulting from SPS include superplastic behavior of ultrafine ceramics, increased permittivity of ferroelectric materials, improved magnetic properties of magnetic materials, enhanced product-scale bonding, augmented thermoelectric properties, superior mechanical properties and reduced impurities segregation at grain boundaries.

Structural Materials

The large number of patents in the field of cutting tools (35) is driven by the longer operation life of SPSed tools compared with their counterparts produced by conventional sintering methods. Typically, SPS cutting tool materials comprise ultrafine/nanostructured cemented carbides, diamond, cubic boron nitride and ultra-hard materials. The composite materials (26 patents) obtained by SPS technology permit highly controllable bonding conditions with no interphase pollution, even in the case of dissimilar materials.

As mentioned previously, in comparison with conventional HP, SPS makes it possible to achieve fully dense ultrafine/nanostructured materials without using sintering aids, as patented in the case of high-performance engineering ceramics. Due to the highly metastable processing conditions, SPS is particularly suitable for the production of components to be used as oil-free bearing materials, with dispersed solid lubricant sliding cemented carbide having excellent self-lubricity.

SPS technology also demonstrates a particular suitability for the reduction of surface oxides in the case of aluminum and magnesium alloys. SPS preforms exhibit high superplasticity, which permits easy net-shaping of the products.

Abrasives represent a significant application field of SPS technology. In particular, diamond/cubic boron materials have been patented, and again the rapid SPS processing permits metastability and avoids the degradation of hard particles in the ceramic matrix while maintaining excellent bonding. It is well-known that refractory materials are difficult to densify due to their refractoriness. SPS technology has been successfully applied to a wide range of ultra-high-temperature ceramics (e.g., ZrB2) for aerospace applications.

Functional Materials

Considering the 71 patents published on magnetic materials, it is clear that SPS technology brings effective improvement in magnetic properties. Rapid heating and cooling conditions can result in both an improved corrosion resistance and magnetic properties for materials ranging from soft ferrites to rare earth sintered magnets.

Thermoelectric materials (60 patents) represent a significant application field of SPS technology. The electric field imposed during the sintering results in slightly higher electrical conductivity without affecting the Seebeck coefficient. On the other side, nanostructuring significantly lowers thermal conductivity by enhancing phonon scattering. Thus, SPS results in an augmented figure of merit compared to materials manufactured by conventional sintering techniques.

Electronic materials processed by SPS involve a wide range of applications including semiconductors and piezoelectric, ferroelectric and heat-dissipative elements for heat sinks. Carbon nanotube (CNT) ceramic composites feature excellent thermomechanical properties. Rapid processing using SPS has been demonstrated to avoid thermal degradation during the sintering process. The ceramic and metallic composites of CNTs have promising mechanical and multifunctional properties.

SPS in EU

Figure 4 shows the distribution of the SPS systems in the EU in 2008 and 2011. According to the 2008 data, over 34 SPS systems existed in the EU at that time: 10 in Germany, seven in France, and three in both Italy and Spain. The number of systems commercialized in the EU has grown exponentially between 2008-2011—from 34 to 58. The rapid growth in the number of patents in the last two decades has resulted from the worldwide spread of the technology in both the scientific community and the industrial sector. c

For more information, email the lead author at m.j.reece@qmul.ac.uk.

For Further Reading:

  1. Grasso, S.; Sakka, Y.; Maizza, G., Science and Technology of Advanced Materials, 10 (2009) 053001.
  2. Orrú, R.; Licheri, R.; Locci, A.M.; Cincotti, A.; and Cao, G., Mater. Sci. Eng., 2009, R 63 127.
  3. Munir, Z.A.; Anselmi-Tamburini, U.; and Oyanagi, M., J. Mater. Sci., 2006, 41 763.

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