Manufacturing Bits: March 21

Making harder windows
Using cubic silicon nitride materials, a team of researchers have developed a harder window that can sustain severe conditions.

There is a demand for harder and stronger windows in various applications, such as engines, ball bearings, cutting tools and other others. To enable this technology, researchers used materials based on transparent polycrystalline ceramics. One ceramic type is called cubic silicon nitride. This material has an optical transparency over a wide range of wavelengths.

Cubic silicon nitride is also the world’s third hardest material, next to diamond and cubic boron nitride, but it can withstand higher temperatures, according to Deutsches Elektronen-Synchrotron (DESY), a Research Center of the Helmholtz Association.

DESY and other researchers synthesized transparent samples of ceramic materials based on cubic silicon nitride. Silicon nitride has a hexagonal crystal structure. In the lab, the materials underwent a sintering process. This is a process of forming macroscopic structures using heat and pressure. At pressures above 130 thousand times the atmospheric pressure, silicon nitride transforms into a crystal structure with cubic symmetry.

A sample of transparent polycrystalline cubic silicon nitride. (Credit: Norimasa Nishiyama, DESY/Tokyo Tech)

This is called spinel-type or artificial spinel, which is used as transparent ceramic in the industry. “The transformation is similar to carbon that also has a hexagonal crystal structure at ambient conditions and transforms into a transparent cubic phase called diamond at high pressures,” said Norimasa Nishiyama from DESY who now is an associate professor at Tokyo Institute of Technology.

“Cubic silicon nitride is the hardest and toughest transparent spinel ceramic ever made,” Nishiyama said. “Cubic silicon nitride is the third hardest ceramic known, after diamond and cubic boron nitride. But boron compounds are not transparent, and diamond is only stable up to approximately 750 degrees Celsius in air. Cubic silicon nitride is transparent and stable up to 1,400 degrees Celsius.

“The raw material is cheap, but to produce macroscopic transparent samples we need approximately twice the pressure as for artificial diamonds,” Nishiyama said. “It is relatively easy to make windows with diameters of one to five millimeters. But it will be hard to reach anything over one centimeter.”

A TEM image of cubic silicon nitride. The grain size is about 150nm. (Credit: Norimasa Nishiyama, DESY/Tokyo Tech)

Chaotic carbon
The Massachusetts Institute of Technology (MIT) has discovered that chaotically arranged carbon atoms can produce stronger materials.

Polymer-derived carbon ceramics are strong materials. But the mechanical performances are not understood and unpredictable.

MIT has found a link between the random ordering of carbon atoms within a phenol-formaldehyde resin. Sometimes known as “SU-8,” this resin is a synthetic polymer. They are used for many molded products, such as billiard balls, lab countertops and others. It was also used in printed circuit boards (PCBs). But it has been largely replaced with epoxy resins and other materials.

In any case, researchers from MIT took phenol-formaldehyde resin. Then, the resin was baked at a high temperature in an inert gas. This process is called pyrolysis. As a result, researchers formed a disordered graphite-like carbon material, dubbed glassy carbon.

The measured “Vickers hardness” reached a peak at ∼4 GPa at 1000 °C. “PyCs studied here are shown to be the lightest super-hard materials, having Vickers hardness-to-density ratios that are comparable to super-hard carbides, oxides, nitrides, and phosphides,” according to a paper from MIT.

“These materials we’re working with, which are commonly found in SU-8 and other hydrocarbons that can be hardened using ultraviolet light, are really promising for making strong and light lattices of beams and struts on the nanoscale, which only recently became possible due to advances in 3-D printing,” said Itai Stein, a researcher from MIT. “But up to now, nobody really knew what happens when you’re changing the manufacturing temperature, that is, how the structure affects the properties. There was a lot of work on structure and a lot of work on properties, but there was no connection between the two. We hope that our study will help to shed some light on the governing physical mechanisms that are at play.”

Magnetic carbon
The Karlsruhe Institute of Technology (KIT) has produced micro- and nanostructured magnetic carbon.

With Freiburg University, researchers have provided polymers with tiny structures by means of lithography and converted them by pyrolysis. As a result, they obtained pyrolytic magnetic carbon (PMC). It is inexpensive and can be used for micro- and nano electromechanical systems (MEMS and NEMS).

Pyrolytic magnetic carbon (PMC): The model shows the unpaired electron spins (red) responsible for the magnetic properties. (Image: Swati Sharma)

“This pyrolytic magnetic carbon, PMC for short, is fundamentally different from glass-like carbon, the classical form of pyrolytic carbon. PMC possesses intrinsic magnetic properties, because it changes it microstructure during pyrolysis and forms unpaired electron spins,” explained Swati Sharma of KIT. “The more unpaired electron spins exist, the stronger are the magnetic properties.”

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