Phenomenon of Super-Effective Field-Assisted Sintering Explained

High-density ceramic materials like zirconia can be hardened much faster and by much lower temperatures with two recently developed new techniques, Field-Assisted Sintering (FAST) and Selective Laser Sintering (SLS). While the effectiveness was shown experimentally in the past, a detailed uniform model and explanation is now being proposed by a researcher from North Carolina State University. The researcher presents evidence that a carefully calibrated electric field of about 100 Volt per centimeter can lead to significantly better results regarding density and internal tension of sintered parts.

Selective laser sintering uses short laser pulses of less than a second per point to melt ceramic powders like zirconia. SLS was already invented around 1980 but rapidly developed during the last decade due to widely available 3D-modelling, much higher precision parts like piezo-actuators and new substances like nano-materials. It is now used for many applications with high-performance ceramics which include technologies like body armor, fuel cells, spark plugs, nuclear rods, superconductors, and especially medical implants.

Biocompatible zirconia implants for example, which are twice as durable as titanium and with a greater density and hardness, as well as higher bending strength and mode of elasticity, became easier and cheaper to manufacture. The most advanced additive manufacturing machines for ceramics, also known as 3D-printers, that use SLS to produce individualized implants (and anything else) now reach resolutions down to just 30 micrometers and are expected to become even faster and more precise, developed and built by the SLS market-leader EOS Gmbh in Munich, Germany.

Sintering in general is the process needed to harden the materials, by 'baking' ceramic powders together, and involves the powders (such as yttria-stabilized zirconia) being pressed into the desired shape and then exposed to high heat until the powder particles are bound together into a solid, but slightly porous, material.

Dr. Jay Narayan, a professor of Materials Science and Engineering at NC State, looked at what exactly happens when combining an electric field (FAST) and selective-melt sintering, which allows sintering of yttria-stabilized zirconia at 800 degrees Celsius (C) instead of the usual 1450 C. The process create a material with no porosity at all, and all this in less than a second, compared to traditional sintering techniques that take four to five hours at 1450 C.

"This technique allows you to achieve 'theoretical density,' meaning it eliminates all of the porosity in the material," Narayan says in a release by NC State. "This increases the strength of the ceramic, as well as improving its optical, magnetic and other properties."

Applying an electric field to the material beforehand appears to be the key element that enables these astonishing advantages by changing the internal, atomic structure of the ceramic particles. Applied at approximately 100 volts per centimeter, the field "creates subtle changes in the material's 'grain boundaries' - where atoms from different crystals meet in the material. Namely, the field draws 'defects' to the grain boundary. These defects consist of vacancies (missing atoms) which can carry charges. The defects are negatively charged and draw current from the electric field to the area - which raises the temperature along the grain boundary," according to the release.

Because the temperature raises primarily along the grain boundaries due to the selectively applied electric current, the material can be effectively sintered at a much lower temperature, because sintering is generally achieved by fusing the crystals of ceramic powder together which requires melting of just the grain boundaries.

The new techniques combined save time and energy because not the whole mass of all the material needs to be brought to the melting point, with "pre-heating" of the grain boundaries with an electric field being the trick.

Papers, published in Scripta Materialia:

J. Narayan - Grain growth model for electric field-assisted processing and flash sintering of materials

Invited viewpoint paper: New mechanism for field-assisted processing and flash sintering of materials