Manufacturing Bits: Sept. 22

Hairy nanoparticles; free-form ceramics; stronger SiC.


Hairy nanoparticles
The U.S. Air Force Research Laboratory is developing a new type of material called preceramic polymer-grafted nanoparticles or “hairy nanoparticles” (HNP).

HNPs can be used to manufacture a new class of aircraft parts made of ceramic composite materials. An HNP is a hybrid material. It is based on a polymer shell, which is bound to a nanoparticle core, according to the Wright-Patterson Air Force Base.

For this, researchers from Wright-Patterson Air Force Base have devised a specialized polymer, which is called a pre-ceramic polymer. This type of polymer can be used in the formation of high-performance ceramic fibers and composites.

Members of the Ceramic Materials & Processing Research Team, from left to right: C Thompson, Dr. D. Street, Dr. K. Martin & Dr. M. Dickerson. (U.S. Air Force photo by K. Schlesinger)

The polymer consists of a chain of repeating tiny molecules. In this case, the polymer has a backbone of silicon and carbon repeats. This structure resembles tiny “hairs” around the nanoparticle, according to researchers.

When heated to higher temperatures, the silicon and carbon chemistry is converted into a hard silicon carbide ceramic. “The special polymer used in our process is what sets our work apart,” said project lead Matthew Dickerson. “Researchers have made these sort of hairy nanoparticles in the past, but they’ve used organic polymers like polystyrene. Our polymer is different; it’s inorganic because it contains silicon. It’s a bit like silicones (caulk), which have a backbone of silicon and oxygen repeats, but ours has a backbone of silicon and carbon repeats.

“Ceramic composites are used for high-temperature US Air Force applications that benefit from materials that are lower in density than metals, including jet engine and hypersonic vehicle components,” said Dickerson. “The HNPs we synthesized are envisioned for those type of applications.”

Ming-Jen Pan, program officer at AFOSR, added: “This research is a major technological advancement in the synthesis of ceramic nanocomposites. It provides unprecedented control of the nanostructure of hybrid materials.”

Free-form ceramics
Using a novel 3D printer, HRL Laboratories has developed components made from fracture-resistant ceramic matrix composites.

Ceramic parts resist corrosion and wear. They also have high-temperature capabilities. This in turn makes them attractive for application in propulsion and energy generation systems, chemical processing equipment and medical implants.

The challenge is to develop ceramic materials with custom shapes. HRL’s technique allows the free-form fabrication of high-performance ceramic components with shapes.

HRL Laboratories reports a novel method to 3D-print components from fracture-resistant ceramic matrix composites. © 2020 HRL Laboratories.

“All ceramic parts, whether traditionally processed or 3D printed, have small defects, such as tiny voids, that arise during processing, handling, and service,” said HRL researcher Mark O’Masta. “The problem is when stress is applied to that region, the defect can become an uncontrolled crack that results in catastrophic failure of the entire part. It basically crumbles. Adding a ceramic reinforcement to a ceramic matrix is a common method to create defect-tolerant parts. The challenge we addressed in this project was integrating this toughening solution with our 3D-printing process. We can now add these reinforcements in large volume fractions to significantly toughen our 3D-printed ceramic parts. We’ve essentially made a brittle monolithic material into a durable composite. As an extra benefit, adding reinforcements relaxed some of the processing constraints.”

Stronger SiC
Russia’s National University of Science and Technology MISIS (NUST MISIS) has developed way to increase the fracture toughness of silicon carbide (SiC) by 1.5 times.

This in turn enables stronger refractory parts, including those for aircraft construction.

SiC, a compound of silicon and carbon, is a hard material. SiC is used in various industries, such as semiconductors, construction materials, abrasives and refractory materials.

Silicon carbide ceramics works well in the compression mode. But the technology is prone to structural defects. Therefore, the material “often has low tensile and bending strengths, as well as low crack resistance,” according to NUST MISIS.

In response, researchers are exploring a combustion-based ceramic matrix composite processing technique. This involves the deposition of single-crystal SiC nanowires on the surface of carbon fibers.

In other words, NUST MISIS has found a way to improve sintering ability and increase the toughness of SiC ceramics using high-temperature synthesis. “The synthesis has been carried out at several stages. First, powders of silicon, carbon, as well as tantalum and PTFE have been mixed in a planetary mill, then the resulting mixture has been burned in a reactor,” according to researchers. “Nanofibers have been formed during the combustion process. At the last stage, the product has been sintered in a vacuum oven.”

“Thanks to the effect of the combined addition of tantalum and PTFE, we were able to synthesize a material with a silicon carbide matrix reinforced with silicon carbide nanofibers. These nanofibers activate the sintering of the ceramic and increase the sintered material strength characteristics since they serve as a barrier to fracture propagation” said Stepan Vorotilo from SHS Center in NUST MISIS.

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