Learning Objectives
By the end of this section, students will be able to:
- Understand the range of engineered ceramic materials available for AM.
- Understand the range of other materials available for AM.
- Understand the different feedstocks of those materials, and how they relate to the different AM processes and the challenges associated with AM of ceramic materials.
- Understand the relationships between feedstocks, AM processing, AM post-processing, and the resultant AM part.
Ceramics represent one of the most challenging and least widely-developed category of engineered AM materials. Because conventional ceramic processing uses powders, feedstocks are widely available. The most common ceramic processes are BJP and MJ, with the polymer binder driven off during sintering, with VP processes becoming available. Because the ceramics are generally stable oxides, carbides, and nitrides, flammability concerns are minimal, although inhalation safety must be considered.
The processing path for a ceramic part made using BJP would also be shorter than a typical structural metallic part, as provided below:
- Build green part.
- Remove green part from machine and excess powder.
- Analyze log file from machine.
- Remove binder using thermal or chemical methods, resulting in a brown part.
- Sinter brown part to target density, checking furnace log file and taking simple dimensions as a quick cycle check, with generally no final heat treatment required.
- Lot acceptance testing.
- Interface machining. For the mirror, this would include polishing of the reflective surface.
- Part acceptance testing. Possibly radiographic to look for large discontinuities.
- Chemical treatments, especially application of the reflective coating.
While fully densified technical ceramics remain a challenge, ceramic SLA resins and other type of photoceramic approaches are currently being deployed. Ceramic-filled resins are being sold by a number of vendors. These resins can be printed in an SLA-style printer at high resolution and fired using traditional ceramic processing approaches. Currently, there are a number of groups pursuing the best methods for producing densified ceramics from pastes and other photoceramic feedstocks.
In addition to conventional engineering materials such polymers, metals, and ceramics, other material systems are being used in AM. One of the first of these was heavy gage paper used in the original sheet lamination process, called Laminate Object Modeling (LOM). This process used adhesively backed or epoxy impregnated paper that was bonded to the previous layer. The outline of the part for that layer was then cut with a laser, and the excess material cut into 1cm x 1cm x 1cm cubes. After completion of the build, the cubes were broken off, and the part remained. This process was used for prototypes, visualization, and simple sheet metal forming tools. This type of sheet lamination has been extended to carbon fiber paper and a number of other type of sheet stock with various binders.
Concrete is another common material that is starting to be used in AM. Large-scale extrusion-type machines are being built that deposit concrete to build structures, eliminating the need for formwork. An example of housing made this way is shown in Figure 3.9. Finally, sand with a polymer binder is used in BJP to make casting molds in a form of indirect AM. Indirect versus direct AM gives designers and engineers choices on how to approach processing for different types of materials. Innovations in indirect AM and hybrid processes and assemblies have been useful in diffusing new design and processing flexibility across a number of important applications.
Clearly, new materials are being brought into the AM sphere at a quick pace and materials innovations are one of the cornerstones of a growing AM industry as more product lines are being disrupted by AM strategies. While rapid prototyping started with brittle, unusable polymers, the industry has progressed to engineering metals and polymers that have long-lifetime applications. Work continues to additively manufacture high-value ceramic and composite components. While there have been some important breakthroughs in these areas, there is much left to do to industrialize many ceramic and composite parts.