Spark Plasma Sintering (SPS) is a sintering technique that involves the simultaneous application of uniaxial pressure and high-intensity, low-voltage, pulsed current. The mechanism of SPS can be summarized into four main stages: vacuum creation, pressure application, resistance heating, and cooling. During the process, a local high-temperature state is generated momentarily by a spark discharge between particles, leading to accelerated sintering densification and the formation of a high-quality sintered body.
1. Vacuum Creation: The first stage of SPS involves removing gases and creating a vacuum. This step is crucial to prevent any gas inclusions within the sintered material, which could compromise its integrity and properties. By evacuating the atmosphere, the process ensures that the subsequent stages occur in a controlled and clean environment.
2. Pressure Application: In the second stage, pressure is applied. This uniaxial pressure is a key component of the SPS process, as it helps in the consolidation of the material particles. The pressure aids in the reduction of the interparticle distance and promotes the formation of necks between particles, which is essential for sintering.
3. Resistance Heating: The third stage involves resistance heating, where the material is heated by a current flowing directly through it. The pulsed DC current generates Joule heat within the material, leading to rapid and uniform heating. This heating mechanism is distinct from conventional furnace heating, as it allows for precise control over the temperature and heating rate. The high-intensity, low-voltage pulses also create a spark discharge at the contact points between particles, generating a local high-temperature state that facilitates the sintering process.
4. Cooling: The final stage is cooling, where the sintered material is allowed to cool down under controlled conditions. This stage is important to prevent any unwanted phase transformations or cracking that could occur if the material cools too quickly.
The SPS process is believed to involve several unique phenomena, such as the self-heating effect generated by the discharge between powder particles and the strengthening of bulk and grain boundary diffusion. These effects contribute to the acceleration of the sintering densification process, allowing for the production of high-quality sintered bodies at lower temperatures and in a relatively short time.
Despite the widespread use of SPS, there is still ongoing research to fully understand the intermediate processes of sintering and to clarify the role of plasma and electric discharges during the process. The term "Spark Plasma Sintering" itself has been disputed, as the presence of plasma has not been unequivocally proven. Nevertheless, SPS remains a valuable technique for the sintering of various materials, offering advantages in terms of speed, efficiency, and control over the sintering process.
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