Heat-resistant ceramics are useful for making components such as engine hot parts, rocket nozzles, and nose cones that have to contend with high temperatures or extreme environments. The trouble is it’s not at all easy to cast or machine these heat-stable engineering ceramics into the necessary complex shapes.
In recent years, 3D-printing processes have been developed that enable much greater geometrical flexibility in fabricating ceramics. But whether the process deposits photosensitive resins that contain ceramic particles, jets binders onto ceramic particles, or fuses beds of ceramic powder with lasers, current additive manufacturing (AM) methods are limited by slow fabrication rates. Plus, they are often followed by a time-consuming binder-removal process. In any case, the physical properties of the final components are not optimal, yielding unreliable, low-strength parts that suffer from residual porosity, cracks, and/or inhomogeneities.
A new AM technique developed at HRL Laboratories, an R&D lab in Malibu, CA, that is jointly owned by General Motors and Boeing, has demonstrated the ability to make high-strength ceramic components featuring complex geometries more easily and rapidly. HRL’s Senior Chemical Engineer Zak Eckel and Senior Chemist Chaoyin Zhou have invented a polymer resin formulation that can be 3D-printed into green parts with complex geometries and then fired in a furnace where they pyrolyze with uniform shrinkage into high-density ceramics.
“With our new 3D-printing process we can take full advantage of the many desirable properties of this silicon oxycarbide ceramic, including high hardness, strength, and temperature capability as well as resistance to abrasion and corrosion,” said HRL program manager Tobias Schaedler when the new technology was unveiled. Such cellular ceramic materials are of interest for the core of lightweight, load-bearing ceramic sandwich panels for high-temperature applications—for example, in hypersonic vehicles and jet engines.
Printing preceramic monomers
“We go straight from printing the preceramic polymer to fully dense parts,” Eckel said. “The first method is stereolithography, where we solidify, polymerize a special ultraviolet (UV) curable preceramic resin and a UV photo initiator with a laser to form complex shapes, but this still takes hours or even days.”
That’s why the HRL team focused as well on a home-grown technique that produces green parts much more quickly in larger volumes. As part of a decade-long DARPA contract to develop lightweight, high-strength materials, he explained, researchers had developed a way “to funnel the UV light all the way down to the bottom” of the precursor resin tank, allowing much faster builds.
The trick is to solidify material by shining a UV lamp simultaneously through the holes in a lithographic mask while at the same time collimating the light within the illuminated shafts to harden all the way to the bottom. In this “self-propagating photopolymer waveguide method” the light penetrates via a waveguide effect based on successive downward reflections off the internal surfaces of the resin column. This process has created uniquely lightweight but strong truss structures, for instance.
“We produced an ultralight nickel microlattice that for awhile was the world’s lightest material; now it’s the world’s lightest metal material.”
Multiple ceramic recipes
Today they’re applying the alternative additive fabrication technique to high-temperature ceramic components.
Both UV hardening processes can ultimately produce many different ceramic materials, but for a start the team has demonstrated a silicon oxycarbide ceramic shaped into an intricately porous, lightweight structure that can withstand ultrahigh temperatures in excess of 1700°C (3092°F) and exhibits strength ten times higher than similar cellular ceramic materials, Eckel said.
“Technically, the amorphous glass microstructure is sort of a hybrid of glass and carbides; at the nanoscale it’s segregated into tiny silicon oxide regions surrounded by graphite layers,” he explained.
“We’re leveraging a certain special chemistry here,” Eckel continued. “Preceramic polymers and polymer-derived ceramics are pretty common. This class of materials was first developed in the 1960s.”
When heat treated to 1000°C (1832°F) under an inert atmosphere such as argon, they pyrolyze, forming many ceramic compounds including silicon carbide, silicon nitride, boron nitride, aluminum nitride, and various carbonitrides. At the same time, volatile chemical species such as methane, hydrogen, carbon dioxide, water, and hydrocarbons “cook off,” leaving the mostly densified, shrunken-down ceramic shape behind.
By attaching various organic molecular groups to an inorganic silicon- or carbon-based backbone such as a siloxane, silazane, or carbosilane, the research team can formulate the UV-active pre-ceramic monomers that crosslink strongly when suitably illuminated.
In the test reported in their paper, “Additive Manufacturing of Polymer Derived Ceramics” in the January 1st issue of Science magazine, the silicon oxycarbide precursor pattern experienced a substantial 42% mass loss and 30% linear shrinkage during conversion in the furnace. But the team described the shrinkage as “remarkably uniform,” almost like the storied shrunken heads of South Sea headhunters of yore, the relative proportions of the shrunken objects’ features remain the same.
The NRL team has used the ceramic fab technology to produce thin-element truss structures—demoing multiple microstructures, honeycombs, re-entrant honeycombs—that exhibit surprising degrees of flexibility. They’ve also built everything from corkscrews to rocket nozzles, missile nose cones, gas turbine engine blades, and micro impellers.
Additive manufacturing of such polymer-derived ceramic materials is not only of interest for propulsion components for jet engines and hypersonic vehicles, but thermal protection systems, porous burners, microelectromechanical systems, and electronic device packaging as well. HRL said that it is looking for a commercialization partner for this technology.