The extremely high melting point of many ceramics adds challenges to additive manufacturing as compared with metals and polymers. At the same time, because ceramics cannot be cast or machined easily, 3D printing enables a big leap in geometrical flexibility. This is certainly true for silicon carbide (SiC) ceramics. And bound silicon carbide 3D printing is starting to really shine.
Also known as carborundum (due to the interesting story of its discovery – as per Wikipedia), SiC contains silicon and carbon. As a powder, it can be sintered to form very hard ceramics that are widely used in applications requiring high endurance, such as car brakes, car clutches and ceramic plates in bulletproof vests. It is also a semiconductor so it can be used in electronics devices that operate at high temperatures or high voltages, or both.
Silicon carbide (SiC) based materials are by far the most important carbide ceramics. Diverse types are manufactured, depending on the intended purpose, but all are characterized by the typical properties of silicon carbide, such as being an extremely hard, heat resistant, abrasion resistant, chemical resistant, and thermally conductive material. These properties mentioned above are more or less pronounced in the different varieties of the material. Depending on the manufacturing technique, it is necessary to distinguish between self-bonded and second-phase bonded silicon carbide ceramics, as well as between open-porous and dense types such as, for example reaction-bonded and silicon-bonded silicon carbides (RBSiC and SISiC).
However, silicon carbide is very difficult to manufacture by subtractive methods. Fully dense, sintered varieties of SiC can cost $400 per cubic inch just for raw material. The powdered variety, however, may be purchased by the boxcar for around $2.00/pound.
Enter Additive Manufacturing
SiC exists in nature but it is extremely rare, however, it can be produced artificially and has many advanced industrial uses from the coating on nuclear fuel to producing single photons used in quantum physics applications. One of its most common uses in industrial manufacturing is automotive parts, such as brake discs as well as electronic devices. As AM companies and AM adopters explore higher and higher performance materials to process additively, a few firms have begun exploring silicon carbide 3D printing by powder bed, extrusion and photopolymerization-type process. The common denominator is that these processes mix SiC powder with a polymer used as a binder, in order to create complex 3D geometries, in a way that is very similar to what is happening with bound metal printing processes.
One of the first companies to explore silicon carbide 3D printing was HRL Laboratories. Through its CAM (Center for Additive Materials) facility, HRL developed preceramic resin systems that can be cured with ultraviolet light in commercially available stereolithography 3D printers or through a patterned mask. Polymer structures with complex shape can be formed and then pyrolyzed to a ceramic with uniform shrinkage and virtually no porosity. HRL used silicon oxycarbide to fabricate structures that exhibit high strength and withstand temperatures up to 1700C.
Other ways to process silicon carbide, demonstrated by the University of Texas (UT), include a hybrid powder bed-based approach which combines selective laser sintering and binder jetting. The process involves laser sintering a SiC powder mixed with binder compounds (green part), carbonization of the binder (brown part) and reactive infiltration of liquid silicon.
Taking silicon carbide 3D printing to market
Now two companies, in particular, have begun offering silicon carbide 3D printing as an option. They are not additive manufacturing companies but rather materials (primarily carbon) experts and they have formed partnerships with AM companies to fine-tune the AM process. SGL, one of the world’s leading manufacturers of carbon-based products, introduced the SICAPRINT range of additively manufactured silicon carbide parts.
Using ExOne’s binder jetting additive manufacturing technology, SGL produces complex silicon carbide parts that could with geometries such as undercuts, cavities or variable wall thicknesses that are difficult or impossible to produce using traditional manufacturing methods. On a material level, the SICAPRINT product family is suitable for corrosive, abrasive and high-temperature applications as well as ballistic protection applications. The SGL-ExOne process includes a post-processing options (e.g. infiltration with polymers or liquid silicon) which can even tailor the material properties of the 3D printed silicon carbide to individual needs.
SGL offers two options: SICAPRINT P is based on polymer infiltration and SICAPRINT Si is based on liquid silicon infiltration. The material properties can also be significantly changed by further processing steps (e.g. densification with carbon).
Recently Schunk Carbon Technology – a division of the Global Schunk Group specializing in the development, manufacture and application of carbon and ceramic solutions – also started taking an interest to 3D printing for carbon-based materials. In June it even began exploring composite extrusion 3D printing through a collaboration with Russian company Anisoprint. The company’s activities in the SiC segment are centered on the IntrinSiC range of 3D printed components. Schunk uses a – yet undisclosed – powder bed AM technology to produce components made of reaction bonded, silicon infiltrated silicon carbide (RBSiC) material, which is nearly as hard as diamonds.
Again, using 3D printing enables Schunk to produce parts that are larger and more complex then with traditional methods, with geometries that can now include undercuts and cavities. AM enables Schunk engineers to do produce parts directly from a CAD model and do away with time-consuming production of casting molds. THe RBSiC material also stands out for its thermal shock resistance, ensuring a high degree of oxidation and corrosion resilience, high bending strength and creep resistance as well as for its low density.