Additive ManufacturingCeramic Additive Manufacturing

Novel process by HRL Laboratories creates 3D printed ceramic matrix composites

Material and process sweet spot for the free-form fabrication of tough ceramic parts

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HRL Laboratories, LLC, one of the most innovative labs working on ceramics 3D printing, has developed a novel method used to additively manufacture components made from fracture-resistant ceramic matrix composites (CMCs).  Published in The Journal of the American Ceramic Society, this technique makes possible a new range of complex designs with these durable materials.

Ceramic parts resist corrosion and wear, and have excellent high-temperature capabilities that make them desirable for application in propulsion and energy generation systems as well as chemical processing equipment and medical implants. However, their use has been limited by the difficulties of shaping ceramic materials, a problem solved by 3D printing. With properties commensurate with traditionally processed technical ceramics, HRL’s technique allows free-form fabrication of high-performance CMC components.

Ceramic matrix composites (CMCs) are a subgroup of composite materials as well as a subgroup of ceramics. They consist of ceramic fibers embedded in a ceramic matrix. Both the matrix and the fibers can consist of any ceramic material, whereby carbon and carbon fibers can also be considered a ceramic material.

CMCs were developed to overcome the problems associated with the conventional technical ceramics, like alumina, silicon carbide, aluminum nitride, silicon nitride or zirconia, which they fracture easily under mechanical or thermo-mechanical loads because of cracks initiated by small defects or scratches. Fibers can increase flexibility – as in any composite material – but of course, add a new set of challenges to manufacturing parts.

ceramic matrix composites
HRL achieved the “sweet spot,” determining the processing bounds for the most durable CMCs. © 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, first author on the current paper. “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.”

A ceramic fabrication technique must result in as low a void fraction as possible to not compromise the final component. If too high a fraction of the reinforcement element is added, the elements will impinge upon surpassing their packing limit and the part is subsequently weakened by the process. Through careful measurements and characterization, HRL achieved the “sweet spot,” determining the processing bounds for the most durable CMCs.

Continuing from their 3D printed ceramic technique published in Science in 2016, the HRL team utilized UV curable preceramic polymers. A digital light projection printer cures the polymer, printing parts in a rapid layer-by-layer method. In the current experiments, a siloxane-based preceramic resin was reinforced with inert particles. The printed polymer parts were then converted to silicon oxycarbide (SiOC), a glassy ceramic, through an extreme heating process called pyrolysis.

ceramic matrix composites
Continuing from their 3D-printed ceramic technique published in Science in 2016, the HRL team utilized UV curable preceramic polymers. © 2020 HRL Laboratories.

“Finding the ideal process to include these reinforcement elements was quite challenging and eluded us for some time,” said Ekaterina “Katya” Stonkevitch, HRL engineer and second author. “Through a detailed study and careful inspection for defects using light and electron microscopy, we were able to identify the right processing conditions. With that information we printed additional parts to do mechanical testing on, and that’s when we figured out our fracture toughness and mechanical strength. We also discovered that with the reinforced material we could print parts thicker than before.”

The paper’s citation is: O’Masta, MR, Stonkevitch, E, Porter, KA, Bui, PP, Eckel, ZC, Schaedler, TA. Additive manufacturing of polymer‐derived ceramic matrix composites. J Am Ceram Soc. 2020; 00: 1– 12.

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