The spark plasma sintering process offers significant improvements over conventional hot press and hot isostatic press sintering.
Spark plasma sintering
(SPS) is a high-speed powder consolidation/sintering technology capable of
processing conductive and nonconductive materials. Theories on the SPS process
vary, but most commonly accepted is the micro-spark/plasma concept, which is
based on the electrical spark discharge phenomenon wherein a high-energy,
low-voltage pulse current momentarily generates spark plasma at high
temperatures (many thousands of °C) in fine local areas between particles.
SPS’ operational or “monitored” temperatures (200-2400°C) are commonly 200 to 500°C lower than with conventional
sintering, classifying SPS as a lower-temperature sintering technology.
Material processing (pressure and temperature rise and hold time) is completed
in short periods of approximately 5 to 25 minutes. The relatively low
temperatures combined with fast processing times ensure tight control over
grain growth and microstructure.
1. Basic configuration of an SPS machine.
SPS utilizes uniaxial force
and ON-OFF DC pulse energizing. The ON-OFF DC pulse voltage and current creates
spark discharge and Joule heat points between material particles (high-energy
pulses at the point of intergranular bonding). The high frequency transfers and
disperses the spark/Joule heat phenomena throughout the specimen, resulting in
a rapid and thorough heat distribution, high homogeneity and consistent
The initiation of the spark discharge in the gap between particles is assisted
by fine impurities and gases on and between the surfaces of the particles. The
spark discharge creates a momentary local high-temperature state of up to
10,000°C, causing vaporization of both the impurities and the surfaces of the
particles in the area of the spark. Immediately behind the area of
vaporization, the surfaces of the particles melt. Via electron draw during ON
TIME and the vacuum of OFF TIME, these liquidized surfaces are drawn together,
creating “necks.” The ongoing “radiant” Joule heat and pressure causes these necks
to gradually develop and increase. The radiant heat also causes plastic
deformation on the surface of the particles, which is necessary for
During the SPS process, heat is concentrated primarily on the surfaces of
the particles. Particle growth is limited due to the speed of the process and
the fact that only the surface temperature of the particles rises rapidly. The
entire process-from powder to finished bulk sample-is completed quickly, with
high uniformity and without changing the particles’ characteristics.
Force (pressure) plays an important and
predictable role in curbing particulate growth and influencing overall
densities, but in the SPS process, accurate manipulation of force can actually
enhance the process. Force multiplies spark initiation (diffusion) throughout
the sample as the material moves under pressure, especially during critical
out-gassing stages. Both too much and too little pressure can negatively
influence the process. In large samples where high density is required, force
is commonly increased in stages to enhance out-gassing and electrical
2. ON-OFF pulsed current path through the machine.
SPS is effective for any
powder material application, but interest is especially high for nanocrystalline
structures. Generally, super-fine materials have more surface area per volume
than the same material made with larger particles. This, along with the way
particles interact once compacted, “amplifies” material characteristics. In
theory, high-strength materials show an increase in strength, highly
wear-resistant materials show higher wear resistance, highly magnetic materials
show higher magnetism, and so on. Suitable applications include advanced
lightweight armament, guidance optics and ultra-high-strength tooling.
Nanomaterials haven’t seen much industrial commercialization to date because
conventional sintering technologies cause substantial particulate damage and
growth. However, since SPS technology can sinter nanocrystalline materials with
very little grain growth and negative particulate effect, the door is now open
to test and study new ideas in powder material applications. Many common
materials may well find new applications with substantially improved
SPS technology can sinter materials without the use of binders. Most
conventional powdered material sintering technologies require pre-forming and
binders, and in many cases, expensive binder removal processes. Binders weaken the part due to their
susceptibility to chemical wear, reduced hardness and strength, and oxidation
breakdown. The availability of binder-less material could be valuable in
numerous applications, including cobalt-less tungsten carbide, high-purity
ceramic fuel cells and ceramic optics.
3. ON-OFF pulsed current path through the powder.
The rapid spark diffusion
and the consistent heat throughout the part produces very little internal
stress within a part made with SPS. Many part failures occur due to internal
stress and micro-cracks caused by heat migration during conventional sintering.
SPS technology binds particles with electrical discharge energy evenly
throughout the part. The heat is simultaneously consistent at the outside of
the part and the center. Along with the relative speed of the process, this eliminates
much of the internal stress commonly seen in conventionally sintered parts.
SPS technology is capable of achieving nearly 100% theoretical density in
almost any metallurgical or ceramic material, including composites. When
ultra-high densities are required, some grain growth is necessary. Some
materials require binders that are added to facilitate density with less grain
growth, while other applications require very specific and uniform porosity. By
accurately controlling compression and temperature, SPS technology can control
material porosity while maintaining strong particle bonds throughout the shape.
“Net” and “near-net” shapes are also possible with SPS, going directly from
powder to finished part in one step. Currently, these shapes need to be
symmetrical and relatively simple; a complex shape would require secondary
SPS technology can produce true seamless bonding (dry and liquid phase
bonding). Because the SPS process draws particles together and offers the
ability to reach nearly 100% theoretical density, the process can produce bonded parts
that have no seam. This bonding phenomenon is possible between like and
dissimilar materials, though radically dissimilar materials require progressing
layers between the initial materials to account for different thermal stresses.
In the case of bonding rough surfaces, loose powder can be applied between the surfaces to ensure a solid
4. Example of a production concept.
As mentioned previously,
the SPS process utilizes high-amperage pulse DC current to generate spark plasma energy between
each particle. Physical compression can be up
to 300 tons and the chamber is under negative atmospheric pressure (vacuum)
with or without inert gas. Heat is concentrated on the surface area of each
particle, and every particle is equally and completely bonded with the
surrounding particles. Under high pressure, mild plastic deformation of the
particles ensures ultra-high density values, while high-porosity, fully bonded
materials can be achieved with lower pressure and less heat and time.
Pre-existing oxidation and contaminates are
vaporized from particle surfaces during sintering, providing for higher-purity
materials and stronger bonding between particles. SPS is capable of sintering
dissimilar materials without going to a liquid phase on the lower-temperature
materials. In the case of composite materials, high homogeneity is possible
even with lower densities. In the case of layered materials known as
functionally graded materials (FGMs), preformed layers remain consistent in
density and shape even if the materials have radically different sintering
Consideration of a powdered
metallurgical “recipe” requires metallurgical expertise, but the actual
operation of the machine is quite simple. Once the die set is loaded and the
temperature feedback system (thermocouple or pyrometer) is in place, the
operator programs the temperature and pressure ramp-up and hold settings. The
atmosphere (vacuum and/or inert gas) is set, the electrical settings (ON-OFF
times and frequency strategy) are programmed and then the data feedback
graphics are set up. SPS technology can also be combined with various forms of
production and automation systems, including multi-head, rotary, batch, and
conveyor systems. Robotic interface is also possible.
Finally, SPS operational
expenses are consistently 50 to 80% less than conventional sintering
technologies, due primarily to speed. In some applications, SPS technology has
been more than 20 times faster than conventional sintering technologies. Most
SPS applications only require minutes in comparison to the hours needed with
For additional information regarding spark
plasma sintering, contact the author at (847) 714-6689 or e-mail email@example.com.