Additive manufacturing of non-oxide ceramics such as silicon carbide is much less developed than oxides (such as alumina and zirconia). If adequately developed, it could open the way to different functionalities from those that can be achieved with oxide ceramics, particularly at high temperatures, with superior stability, high thermal conductivity, strong mechanical properties, and low density. A new approach based on Smart SiC precursors, developed by the IRCER institute in Limoges and by 3DCeram, could soon offer a commercially viable solution for SiC stereolithography.
The manufacture of complex carbide ceramic parts by stereolithography would widen the applications of these materials, in particular for structural parts (e.g. lightened structures operating under mechanical and/or thermal stresses and solar absorbers in thermodynamic concentrated solar systems). However, 3D printing silicon carbide materials with light-based AM technologies is among the most challenging feats you could think of. And yet (or perhaps because it is so challenging) some of the most advanced companies in ceramic 3D printing are now working on SiC 3D printing.
The IRCER laboratory (part of the University of Limoges/CNRS) is one of the most advanced facilities for the study of advanced ceramic materials anywhere in the world. Here, a team led by Thierry Chartier, Vincent Pateloup and Samuel Bernard, and supported by French ceramic SLA 3D printer manufacturer 3DCeram, are working on ways to optimize silicon carbide 3D printing on a Ceramaker 3D printer.
This latest development illustrates collaborative and close work between the scientific and industrial world. “The first results appear promising – the authors of the study write. The association ‘UV-Smart SiC laser stereolithography’ currently under development could play a decisive role in the development of this technology. The combination of Smart SiC precursors and the additive processes will reduce the manufacturing steps, making the process more energy-efficient and economically viable. This approach could thus achieve – after heat treatments – dense parts with highly complex shapes.”
The compromise between lightweight/durability/functionality constitutes is the key to demonstrate the relevance of carbide parts developed by coupling “stereolithography by UV laser-Smart SiC precursors” and thus accelerate their industrial deployment As examples, these materials should be able to provide an answer to energy and environmental challenges in air transport (such as the need to drastically reduce fuel consumption) as well as in energy production (for example in improved conversion efficiency at high temperature of Volumetric Solar Receiver (VSR) in Concentrated Solar Power (CSP) units and thus offer an energy alternative by exploiting solar energy – the authors of study concluded.
Why is SiC stereolithography so hard?
Stereolithography, as used UV polymerization of suspensions or reactive pastes containing ceramic powders, can only be used for materials with low absorption of UV radiation (350 to 400 nm), which is mainly the case of oxides such as alumina, zirconia, silica and hydroxyapatite. Silicon carbide has a very high UV light absorption rate (approximately 80% absorption at 355 nm), which makes standard ceramic stereolithography from a photocurable resin impossible.
One possible solution experimented by the authors of the study is the application of a coating on the SiC particles (core-shell structure) by a layer of non-transparent and non-absorbent material in the desired photocuring wavelength. On the other hand, they reported that “this can introduce a secondary phase in the final material, which can be harmful to the desired properties concerned, in particular at high temperatures. The conventional sintering additives of SiC in the liquid phase (i.e. Y2O3, Al2O3, etc.) are not feasible because they are mostly transparent and thus allow the radiation to pass through the coating and be absorbed by the SiC.”
A second approach that was tested and reported was the use of systems based on preceramic polymers: these materials can be shaped like polymers and then transformed into ceramic material after heat treatment, with precise control of the chemical composition of the final material. In this respect, shaping of preceramic polymers by stereolithography appears to be an extremely relevant approach.
According to the authors, “the coupling of the ceramic prepolymer with lithography-derived processes was developed in the mid-2000s for applications in the electromechanical microsystems (MEMS) market at high temperatures or in a corrosive environment. 2D and 3D ceramic nanostructures were built by lithography methods as well as by stereolithography with a resolution of several micrometers, using preceramic polymers or polymers mixed with ceramic powders. More complex ceramic 3D silicon carbide (Si-C-N) microstructures could be fabricated from an “acrylate” polysilazane (polymers in which silicon and nitrogen atoms alternate to form the basic backbone) followed by pyrolysis at 600 ° C under argon gas. This study, however, also demonstrated that the use of commercial preceramic polymers in lithography-based processes for the precise fabrication of 3D objects would result in 40% volume shrinkage. Too high to enable the precise fabrication of large parts.”
Only very recently the use of stereolithography and preceramic polymers was demonstrated in the production of larger 3D ceramic parts, which represent the key use for silicon carbide. The first studies were carried out at HRL laboratory by Prof. T. A. Schaedler and Padova University by Prof. Paolo Colombo. These studies were performed using a mixture of a commercial polysiloxane (silicone), a photopolymerizable resin (methacrylate-based) and a photoinitiator (camphorquinone-amine system) to form 3D silicon oxycarbide (Si-O-C) parts.
However, these silicon oxycarbides do not satisfy high-temperature applications (typically for civil and military aviation) because of the presence of free carbon, which induces carboreduction reactions generating volatile species beyond 1000 °C. In addition, the removal of the resin and the transformation of the PDC into ceramic resulted in a very large volume shrinkage of 71%.
Another more recent example relates to the manufacture of SiC-type ceramics from one SiC precursor mixed with an acrylate resin with a photoinitiator. Pyrolysis carried out at 1300 °C under argon gas leads to a silicon carbide (SiC) with a high proportion of free carbon, which may be problematic for high-temperature applications in an oxidizing atmosphere, and a high volume shrinkage.
The IRCER-3DCeram solution
The authors conclude that “the coupling “stereolithography-Smart SiC” is a promising way to lift the lock-related to the low reactivity of a system based on highly absorbing SiC particles in the UV field. The use of commercial prepolymers, associated with a photopolymerizable resin, leads to very low yields (very high shrinkage) with significant levels of free carbon (13-20 at%) and oxygen (8-15 at. %), which compromise the manufacture of dense pieces of large size with good dimensional resolution by stereolithography and which is detrimental to the stability of the materials in temperature.
Based on this state of the art, the work in progress between IRCER and 3DCeram focuses on two different approaches, a first based on custom-specific prepolymers and the second on a new chemistry of the suspensions used in stereolithography only adapted to focused laser insolation. Both approaches are made on Ceramaker machines. These studies constitute a real scientific and technological challenge and should lead in the medium term to the feasibility of 3D SiC parts by stereolithography on a Ceramaker machine.
Why is SiC stereolithography so relevant?
Stereolithography, a term that today includes a number of light-based 3D printing processes, was the first additive manufacturing process to emerge. Today, the stereolithography process is part of a set of processes to consider for the development of structural parts based on advanced ceramic materials, layer by layer, from a photocurable system consisting of a dispersion of ceramic particles in a photosensitive resin.
While the overall market opportunity for 3D printed silicon carbide parts is still relatively small, the possibilities offered by this material – one of the most widely used ceramics – are very significant and could be fully exploited through the implementation of AM capabilities. Since SiC is best when its impressive mechanical properties are used for complex, advanced parts, today the market for silicon carbide parts is limited by the relatively high cost of producing complex SiC parts using tools and by the extremely high costs of producing complex SiC parts subtractively from a solid.
Stereolithography is the only additive process that allows the direct manufacture of dense ceramic parts with complex architectures, high dimensional resolution, good surface finish and properties similar to those obtained by conventional processes. The benefits offered by additive manufacturing become even more relevant for the hardest ceramic materials, which are extremely difficult to process by traditional formative and subtractive methods. These include complexity, optimization by CAE simulation, lightweighting, reducing the number of components in assemblies, improved thermal and/or flow optimization in a chemical/thermal reactor, to name a few. In addition, the dimensional resolution that can be achieved in stereolithography makes it possible, for most applications, to avoid expensive and difficult machining, which is a definite advantage for carbide-type ceramics.