Spark Plasma Sintering

I. Abstract

This paper investigates the principles, theory, advantages, and applications of spark plasma sintering (SPS), a newly developed sintering method that uses high temperature plasma generated between the particles of a powdered material by electrical discharge. This discharge results from energizing the sample by a momentary pulse of large electric current. One can summarize the sequential effects of this electrical pulse in five key steps: (1) spark plasma is generated (2) the sample is “plasma heated” (3) the sample is “joule heated”, (4) the particles fuse into a compact, solid object by electrical field diffusion, and (5) the material undergoes plastic deformation to further densify the powder. In many cases, this method of sintering produces objects with superior mechanical properties to conventionally sintered objects. Furthermore, it produces highly dense objects in a shorter time period and at a lower temperature. With spark plasma sintering, ceramics can be sintered in minutes, as compared to hours and days using conventional processes. This emerging technology presents the opportunity to manufacture materials more cost-effectively.

II. Introduction

Within the umbrella of materials science and engineering lies the processing of various classes of materials, including metal alloys, composites, polymers, and ceramics. According to the Ceramic Tile Institute of America, a ceramic is an inorganic, nonmetallic solid processed via the action of heat and subsequent cooling.3 Among the wide variety of these inorganic, nonmetallic materials used in commercially viable applications, a common characteristic is high melting point, due to the nature of these materials themselves---they are usually bonded covalently, and thus their melting points are extremely high since melting point of a material is determined by the strength of its strongest bonds.

This characteristic of many ceramic materials makes them extremely difficult to process and shape using the traditional melting and cooling methods that are used to process polymers or glass. That being said, processing of ceramic materials often relies on a process called sintering. Sintering, rather than melting a material and then cooling it within a mold, creates objects from the powdered form of a solid material.9 During a typical sintering process, a powdered material is placed into a mold and then its temperature is raised to a point that is high, but below the melting point. As this happens, the powder particles fuse together and form a solid piece in the shape of the mold used.

The physics of this process may be explained by atomic diffusion. While diffusion takes place in all materials above absolute zero, its effect is much more apparent at higher temperatures. During the sintering process, as the temperature of the material rises, the atoms within the powder begin to diffuse across the boundaries of the particles.2 This causes the individual particles to fuse together in a continuous network. Conventional sintering processes include hot press sintering (HP) and hot isostatic pressing (HIP).8 These processes typically apply high heat and pressure external to the sample to achieve sintering. Sintering is highly useful because it allows one to produce commercial objects or parts in a practical manner that does not involve routinely melting ceramic materials at extremely high temperatures. Figure 1 shows the basic configuration of a spark plasma sintering unit:

[pic]

While sintering has been a key component of materials processing for decades, a newer method of sintering that improves upon the conventional form has been developed that allows materials to sinter at lower temperatures for less time by pulsing a strong electrical current through the powder, thus effectively charging the pores within the powder bed and generating a high temperature plasma, called a spark plasma. This process is known as spark plasma sintering (SPS), although research laboratories have coined other names to the process such as field assisted sintering technology (FAST), plasma assisted sintering, current sintering (PECS), and electric pulse assisted consolidation (EPAC).10

SPS systems present many benefits in comparison to conventional systems that use HP or HIP: primarily, it relies on the self-induced heating and pressure of the powder particles, rather than the application large amounts of heat and pressure externally.11 Other benefits include production of stronger and harder materials, ease of operation, accurate control of applied energy, shorter sintering time, safety, and reliability.11 All of these advantages have attracted attention from production and materials engineers.

III. Principles of Spark Plasma Sintering

By definition, all sparks consist of an ionized channel called plasma; however, the plasma channel generated by a conventional ignition is extremely low in net energy content such that it is negligible. A standard ignition spark is characterized by high voltage but low peak current, usually below 200 mA.5 In contrast, a plasma spark generated by spark plasma sintering is a high voltage ignition spark with a high peak current, usually around 20 to 30 A.5 Thus the net energy output of a plasma spark is several hundreds of times greater than a conventional spark. While a standard ignition spark is barely visible and generates almost no photonic energy, a plasma spark is characterized by liberation of high energy photons.5 The main driving forces for conventional sintering processes are (1) the joule heat generated externally by the power supply and (2) the plastic flow of materials that takes place due to the application of pressure.6 The SPS process pulses voltage and a DC current very quickly from a special pulse generator to a sample of powder; therefore, in addition to the aforementioned driving forces, SPS uses the energizing pulse of current at the initial stage of the process to generate spark plasma and then discharge electricity between powder particles.11 Figure 2 shows how the pulse current flows through particles inside the SPS sintering die. 11 [pic] Following the production of the spark plasma are four distinct steps, the first being plasma heating. In this step, the aforementioned mechanism is based on the discharge in the interfaces or pores between adjacent powder particles.6 The electrical discharge between the particles results in localized, instantaneous heating of particle surfaces up to several thousand degrees Celsius. Since the discharges emerge uniformly throughout the sample, the heat generated is uniform throughout. As this takes place, it purifies and activates the particle surfaces by vaporizing impurities concentrated on the surface. The purified surface layers of the particles then melt and fuse to each other forming necks between the particles. In the next step, joule heating takes place, during which the pulsed electrical current flows between particles through the necks that formed in the previous step.6 The electrical current generates joule heat, which leads to the next step: it enhances diffusion of the atoms or molecules in the necks. As mentioned previously, the physics behind any sintering process is explained by this atomic diffusion. Thus increasing the atomic diffusion between particles enhances the growth of the necks and the formation of the compressed solid. The localization of the joule heating and its uniform distribution throughout the sample cause a rapid temperature rise and fall. This reduces the size of the grain boundaries, promoting the formation of a compact. Figure 3 shows the action of the powder particles in the plasma heating and joule heating stages:6 [pic]

Figure 4 summarizes the formation of necks between particle contacts via the aforementioned steps: 11 [pic]

Finally, the material, now consisting of fused particles, undergoes plastic deformation to further reduce the porosity.6 The heated material softens and undergoes plastic deformation under the uniaxial force of the sintering unit. This plastic deformation combined with the amplified atomic diffusion, plasma heat, and joule heat, results in densification of the powder compact to over ninety-nine percent of its theoretical density. At lower pressure and temperature, the intergranular bonding inside the powder becomes more and more porous, whereas the porosity decreases at higher pressures and higher temperatures. IV. Advantages of Spark Plasma Sintering While traditional methods of processing require hours to reach peak temperature, spark plasma sintering achieves the same in minutes. The spark plasma sintering process is fast, uniform, and applicable to more than just ceramics—its functions can be extended to metals, composites, and new types of materials that cannot be conventionally sintered such as functionally graded materials (FGMs), fiber reinforced ceramics (FRC), and metal matrix composites (MMC).7 Moreover, spark plasma sintering systems are easy to use, and energy-efficient because unique to spark plasma sintering is the direct current generating heat within the sample itself.1 As mentioned previously, spark plasma sintering does not require a large application of heat because a pulse of large current directly causes the mold and powder to self-heat. This shows lower energy expenditure compared to conventional methods of sintering, where heat and pressure are external to the sample.10 In addition to the obvious benefits that save time, money, and energy, there are more technological advantages to using spark plasma sintering: for one, the porosity of the final compact produced is easily controlled, allowing maximum density to be achieved.6 SPS efficiently consolidates the powder into a uniform, high-quality compact because the spark plasma disperses the spark points uniformly, causing even heating. Spark plasma sintering also does not require preformation or addition of binders, and the surface purification and activation processes that take place via electrical discharge yield better results than those from conventional sintering.1,6

Moreover, a clear advantage of spark plasma sintering, as mentioned before, is that materials sintered in this manner are able to retain nanostructure. A key facet of spark plasma sintering is that it minimizes grain growth, which allows materials of nanocrystalline grain structure to be prepared rather than traditional microcrystalline structure; the grain size of the powder is preserved. Generally, ceramics tend to be less dense, chemically inert, very strong, very hard, and temperature resistant; however, nano-grain ceramics exhibit even greater strength and higher superplasticity, or toughness, when compared to the microcrystalline ceramics that are usually the outcome of conventional sintering. Therefore, the objects produced via spark plasma sintering may show improved mechanical properties compared to similar objects produced via conventional sintering. There are a few disadvantages of spark plasma sintering, however; although the process itself is cost-effective, an expensive pulsed direct current generator is required as an initial investment.6 This makes spark plasma sintering difficult for start-up companies to use, and thus the process would be more favored by established firms with a strong source of funds. In addition, only simple, symmetrical shapes may be prepared by spark plasma sintering6, making more complicated shapes only possible to manufacture by traditional methods. V. Applications Ceramic materials that can be sintered at high density in this manner include oxides, carbides, borides, and nitrides. Three experiments conducted in Japan used spark plasma sintering to process silicon nitride, silicon carbide, and a mixed composition powder of stainless steel and zirconia, stacked in layers by increasing concentration of zirconia. The last one is an example of a functionally graded material. In the case of silicon nitride, they were able to synthesize a sintered compact with 98.7% of the theoretical density and 18.3 GPa hardness.11 X-ray diffraction showed that grain growth was minimized. Figure 5 shows the analysis of the SPS sintered silicon nitride sample: [pic]

In the case of silicon carbide, the experiment compared a sample sintered conventionally to one sintered by SPS, and found that the SPS sintered SiC had higher hardness and toughness and stronger intergranular bonding.11 The achieved density was greater than 99% of the theoretical density, compared to 93% conventionally sintered. No pores or granular growth were observed in the x-ray diffraction pattern of the SPS sample.11 Finally, for the functionally graded material, sintering at low temperature for a short period of time reduces residual stress in gradient materials.8 Thus no pores or cracking were observed, and strong intergranular bonding was achieved. This would not have been possible if the same stack of materials was sintered conventionally.8 Furthermore, there are many advanced ceramic applications that would benefit greatly from spark plasma sintering, such as protective body armor, energy harvesting thermoelectrics, fuel cells, cutting tools, thermal sensors, microelectronics, and extremely high temperature ceramic tiles useful in the Air Force and space travel.10 Spark plasma sintering presents the advantages because some of these applications are made of the newer types of materials that cannot be conventionally sintered, while the other applications require materials of extremely high strength and hardness, which can only be achieved by the nanocrystalline structure of a sample sintered by SPS. VI. Conclusion This paper discusses many aspects of spark plasma sintering, including its basic mechanisms, its advantages, and some useful applications. SPS offers a faster, easier, and more energy-efficient method of sintering ceramic and composite materials due to the effects of the pulsed electricity and self-heating. The benefits also extend to the microstructure of the objects produced. The rapid densification at lower temperature permits very limited grain growth, so the grain size of the powder particles is retained in the compact. This results in very strong and hard materials with superior mechanical properties. Spark plasma sintering is an exciting and revolutionary technology, and its presence is certain to soar in the near future as nanotechnology rises to the forefront of science and engineering pursuits. VII. References 1. Aalund, Robert. “Plasma Sintering.” Ceramic Industry 36.2 (2008): 1-5. Ceramicindustry.com. May 2008. Web. 19 Apr. 2012.

2. Chaim, R. “Densification Mechanisms in Spark Plasma Sintering of Nanocrystalline Ceramics.” Materials Science and Engineering: A 443.1-2 (2007): 25-32.

3. “CTIOA.” Ceramic Tile Institute of America. 2003. Web. 22 Apr. 2012. .

4. Grasso, Salvatore, Yoshio Sakka, and Giovanni Maizza. “Electric Current Activated/assisted Sintering: A Review of Patents 1906–2008.” Science and Technology of Advanced Materials 10.5 (2009).

5. “High Performance Ignition Systems Plasma Ignition FAQ.” Aquapulser.com. Aquapulser, 2011. Web. 22 Apr. 2012. .

6. Kopeliovich, Demitri. “Spark Plasma Sintering.” SubsTech. 10 Sept. 2006. Web. 20 Apr. 2012. .

7. Olevsky, Eugene, Evan Khaleghi, Cristina Garcia, and William Bradbury. “Fundamentals of Spark-Plasma Sintering: Applications to Net-Shaping of High Strength Temperature Resistant Components.” Materials Science Forum 654-656 (2010): 412-15. Print.

8. Omori M. Sakai, H. Okubo, A. Tokita, M. Kawahara, and M. Hirai. “Preparation of Functional Gradient Materials by Spark Plasma Sintering.” Symposium of Materials Research Society of Japan. 1994. Print. 9. "Sinter." Def. 2a. Oed.com. Oxford English Dictionary, Mar. 2012. Web. 21 Apr. 2012.

10. "Spark Plasma Sintering Emerges As New Technology." Research.psu.edu. Pennsylvania State University, 23 Aug. 2010. Web. 22 Apr. 2012.

11. Tokita, M et al. “Mechanism of Spark Plasma Sintering.” Ceramics 21.Mmc (2010): 605-608.

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Figure 1: A typical SPS system consists of a sintering machine with a vertical, uniaxial pressurizer, specially designed electrodes encompassing a water cooler, a vacuum chamber, a vacuum/air/argon-gas atmosphere controller, a special DC-pulse sintering power generator, a position measuring unit, a temperature measuring unit, an applied pressure display unit and various interlock safety units.

Figure 2: Flow of pulsed current between particle boundaries—the first step in fusing the particles into a compact.

Figure 3: The left side shows the process of electrical discharge between via the pulse of high current. The right side shows the process of joule heating which causes the necks between particles to form. This leads to compaction into a solid with high density.

Figure 4: Summary of all of the steps in spark plasma sintering, minus plastic deformation. This includes (1) spark plasma is generated (2) the sample is “plasma heated” (3) the sample is “joule heated”, (4) the particles fuse into a compact, solid object by electrical field diffusion.

Figure 5: The microstructure of the SPS sintered silicon nitride sample as well as the x-ray diffraction pattern shows that the grain growth is minimal, which explains the high strength of the resulting material.