Feb 22 2017
Sandia National Laboratories researcher Pylin Sarobol looks at samples of carbide coatings as she stands in front of a deposition chamber. Sarobol and colleagues are working on a process to lay down ceramic coatings kinetically, avoiding the high temperatures that otherwise would be required. (Photo by Randy Montoya)
An elegant approach for ultrafine-grained ceramic coatings has been explained by researcher Pylin Sarobol in a rather inelegant manner - sub-micron particles splatting onto a surface.
A keypart of a Sandia National Laboratories project involves using the splatting action to lay down ceramic coatings kinetically. Sarobol and her colleagues were able to avoid the high temperatures required to process ceramics like barium titanate and alumina by making high-velocity submicron ceramic particles splat onto surfaces at room temperature.
Microelectronics design and fabrication becomes more flexible when coating is done at room temperature. This could one day pave the way for improved, less expensive microelectronics parts that underpin advanced technology. The kinetic process creates nanocrystalline films that are highly strong and could be used as protective coatings against corrosion, wear, oxidation, and related others.
Sarobol, who is involved in additive manufacture and coatings, said it is tough to combine ceramic coatings and similar hard materials and then incorporate them into devices with materials that have comparatively low melting temperatures.
Since ceramic components are manufactured at temperatures of approximately 1,300 °F (700 °C) or more, it can be tough to merge them with certain materials that have specific functions within mechanical and electrical devices. For instance, existing tiny waveguides require micro-machining out a miniature piece of electromagnetic material and sticking it onto another material.
The ability to put down ceramics at room temperature means you can process ceramics and lower-melting temperature materials at the same time. You can now put ceramics on copper, for example. Before you had to make the ceramics first, then put the copper down on it. This process is really about being able to integrate materials, especially ceramics, with other materials.
Pylin Sarobol, Researcher, Sandia National Laboratory
It opens up new possibilities for fabrication - electrical circuits combining miniature sensors or capacitors, or hybrid materials. “You can imagine spraying functional materials onto a circuit board rather than high-temperature processing, followed by tedious manual assembly,” Sarobol said.
Taking Advantage of Kinetic Energy and Materials Properties
Instead of heat, aerosol deposition applies kinetic energy and special material properties available at micro- and nano-scales.
There is still a lot to learn regarding the process.
We really need to spend the time to understand the process parameters, how they relate to the resulting microstructures and to the final material properties that we need. When we think about designing a new device, we need to keep the relationship of structure-processing-properties in mind and allow ourselves time to perform the research, the optimization, and understand how we can make the properties of coatings better.
Pylin Sarobol, Researcher, Sandia National Laboratory
Room-temperature microscale coatings will not be a universal solution, however, since the process creates nanocrystalline structures - not suitable for coatings used in applications such as micro-motors, micro-actuators, or capacitors that require large grain structure for enhanced device operation, she said.
“The aerosol-deposited coatings are made up of tiny, 20 nanometer crystals that we often call crystallites or grains,” Sarobol said. “When we heat our coatings, these tiny crystals grow and the properties change. By controlling the crystallite size, we can tune the properties in predictable ways to make more functional devices” for various applications.
Very few places in the world work on such room temperature, kinetic coating methods. Sarobol’s preliminary research was used as principal investigator for a two-year project, “Room Temperature Solid-State Deposition of Ceramics,” that lasted up to March 2016. It resulted in better understanding of the standard building blocks of coatings and the scientific elements involved in the process.
The next step was about optimizing the process, widening the materials that can be fabricated, and developing them for probable applications, which could take some years.
Summarizing how it works in a nutshell: In aerosol deposition, a nozzle speed up submicron particles suspended in a gas toward the surface. Particles make contact and stick, adding up to a coating layer by layer.
A tip is to apply submicron particles (50 times smaller than a human hair’s thickness), which would allow researchers to make use of materials properties located only at small scales and to trigger plastic deformation in the aerosol particles.
Plastic deformation, or plasticity, is a method to make a substance to permanently change shape or size due to applied stress. It is the plasticity of submicron particles that results in consolidation of succeeding deposition layers and produces the continuous surface that layers are constructed upon.
Another tip recommends deposition in a vacuum, which helps to lower the impacts of reflected gases on the flying particles. Reflection of the high-velocity carrier gas from the deposition substrate can generate “bow shock”, a gas boundary layer that is tough for the tiniest of particles to enter.
However, in a vacuum, reflected gases are diffused so that the bow shock layer is thinner. The smaller particles moving fast possess high momentum and can pass through the thin bow shock layer. Without a vacuum, the bow shock layer is large and particles do not possess the sufficient momentum to enter into the substrate.
Plastic Deformation Critical to Process
It is important to realize material plastic deformation by maintaining the particle kinetic energy via the bow shock layer. Without plastic deformation, there is neither sticking nor coating.
When the substrate or another layer is impacted by a particle, it plastically deforms and changes shape by a process called as dislocation nucleation and slip. Sarobol’s team found that particles had nanofractures that make them “lay down onto a substrate like splatting cookie dough, forming a pancake-shaped grain.”
The subsequent particle that hits and deforms, compresses down the original layer, forming an even tighter bond.
So you have both the materials deformation or shape change and fracturing without fragmentation, and finally the tamping from subsequent particles to help build the coating.
Pylin Sarobol, Researcher, Sandia National Laboratory
Those mechanisms make several layers achievable, forming coatings that measure tens of microns in thickness. “We have made nickel coatings as thick as 40 microns, and in literature I’ve seen reports of up to about 80 microns for ceramics,” Sarobol said.
Using the technique, the team has effectively deposited numerous materials, including nickel, barium titanate, copper, titanium dioxide, aluminum oxide, carbide compounds. Probable applications for only this short list of materials include resistors, capacitors, electrical contacts, inductors, and wear surfaces.
An appealing application of barium titanate films is electric field management in high-voltage systems. High-voltage capacitors, for instance, tend to fail where the dielectric material (barium titanate) makes contact the copper electrode and air, forming a three-material junction.
“If you spray on barium titanate at this junction, you open up the possibility of higher power capacitors,” Sarobol said. “There’s much more to do before we achieve good enough properties for that.”
Other researchers are keen about protective coatings, electrical contacts, or consolidating fragile and intermetallic compounds for the first time.
The process also covers the microscale space between two proven technologies, thermal spray technology and thin films. Thin films are coating layers that range in size from nanometers to a few microns. They can be defined into precision electrical circuits, and are patterned via photolithography methods instead of conventional printed circuit boards. Thermal spray technology can manufacture coatings beginning at approximately 50 μm up to a few centimeters.
“This can bridge that missing gap, where you can start to deposit hundreds of nanometers of materials up to a hundred microns,” Sarobol said.