Powder-based methods usually start with a powder bed in which powder granules with good flowability are spread out and bound together in layers. For the most part, the resulting components exhibit porous structures.
With suspension- or feedstock-based methods, the starting materials take the form of suspensions, pastes, inks, or semi-finished products, such as thermoplastic feedstocks, green films, or filaments. Because the particle distribution of the powder in a suspension is more homogeneous than in a powder bed, these shaping methods yield higher green densities, which result in sintered components with denser microstructures and lower surface roughness levels.
Typical to all additive manufacturing methods used for producing ceramic components is the need for post-AM thermal treatment steps such as debinding and sintering, which lend the ceramic component its final ceramic properties.
The best-known additive manufacturing method is binder jetting. As in a conventional inkjet printer, a liquid is dispensed onto the powder layer through a print head, whereby the interaction between liquid, powder, and binder results in pointwise solidification. The binder can be situated either in the liquid or in the powder. The densities of the printed green bodies are relatively low. The method thus offers advantages in applications in which porosity is explicitly required – for example, in bioactive ceramic structures made of hydroxyapatite. Components can also be manufactured for filtration applications and catalyst support structures or for complex ceramic cores and molds for precision casting. A wide range of materials – oxide and non-oxide ceramics as well as glass, hardmetals, and metals – can be processed in powder form by binder jetting.
In selective laser sintering, the powder particles are glued together by a laser beam treatment. The starting point is a powder layer applied with a doctor blade. To yield a dense material microstructure, the ceramic powder contains a liquid phase-forming component (e.g., Al2O3/SiO2).
In addition, laser sintering, like all other additive methods, can only be used for shaping of the ceramic green body. In this way, for instance, complex SiC parts can be produced and then converted to SiSiC by liquid Si infiltration. The material properties of the parts are comparable to those achievable with conventional technologies (pressing, green machining, and finishing).
This method, adapted especially for additive manufacturing of ceramics (CerAm VPP), works according to the so-called digital light processing principle. For this, Fraunhofer IKTS uses the CeraFab7500 and the CeraFab8500 system from Lithoz GmbH.
As in stereolithography, free radical polymerization of the binder system takes place with light of a defined wavelength, causing the suspension to solidify. Via a DLP module, the suspension is selectively irradiated with blue light, whereby all areas to be cross-linked on a given plane are exposed at the same time. The productivity is hence high. Achievable densities following conventional thermal treatment of the AM green bodies are at least 99.4 % of theoretical density for Al2O3 and at least 99.0 % for ZrO2.
Multi material jetting is based on a technology and equipment uniquely developed at Fraunhofer IKTS to overcome important limitations in existing methods. Focus is on the manufacturing of large ceramic green bodies, functionalization through use of various materials, and a significant increase in build speed.
The advantage of MMJ over other methods is that it can be used to make multi-component and/or graded parts regardless of the selected material. The method is based on use of particle-filled thermoplastic mixtures with low melting points (80–100 °C) as well as relatively low starting mixture viscosities. Analogously to the fused deposition modeling approaches used for polymers, this method involves application of the material not over the entire surface, but instead only in the required spots. Multiple heatable dispensing units controlled in all three spatial directions move over a fixed platform. The thermoplastic mixture is heated until it is in a flowable state, is deposited at the desired position, and solidifies immediately on cooling.
Solidification takes place nearly independently of the physical properties of the powders used. Several supply containers and dispensing units can be used for localized deposition of different materials, including supporting structures, in a part.
MMJ pushes back the technological boundaries of additive manufacturing for ceramics and can hence considerably expand the range of possible applications in various target industries.
Like MMJ, fused filament fabrication is based on highly filled thermoplastic mixtures, but the viscosities are much higher. These mixtures are supplied as filaments to the individual print heads, melted, and deposited as strands. The material solidifies on cooling. Through the selective application of materials via the various print heads, diverse material and property gradients can be produced in the part. The relatively low resolutions associated with this method are compensated for by the very large build chamber, high productivity, and range of materials that can be handled. In addition, ceramic fibers can be integrated into the filaments to enable additive manufacturing of CMCs.
Laminated object manufacturing was originally developed for build-up of electroceramic components (multilayer capacitors and stack actuators) and for production of ceramic multilayer substrates (LTCC, HTCC) in microelectronics. In the context of additive manufacturing, the LOM technology is used for building up three-dimensional microcomponents with integrated functionality. The basis for this is provided by (glass-)ceramic tapes produced by continuous tape casting. Fraunhofer IKTS has various casting machines available for producing these tapes.
Following mechanical or laser-based structuring of the individual layers, the layers are printed with functional pastes or inks (see “Functionalization” section) and laminated together. During subsequent co-firing, i.e., sintering of all materials contained in the multilayer in a single step, the desired component properties are obtained with a concomitant volume shrinkage. Special attention must be paid to compatibility between materials with respect to such aspects as shrinkage, sintering atmosphere, chemical compatibility, and thermal expansion.