3D printing has attracted considerable interest from researchers, industry and the media over the last decade. It allows users to produce an extraordinary range of structures at the click of a button. The inherent freedom of 3D printing lends itself to bespoke applications, such as biomedical devices, where parts could be designed and tailored to an individual patient's body. Also, big advantages can also be found in industries based around short production runs where tooling accounts for a large proportion of a part's cost.
For a number of these applications ceramic materials and ceramic composites are particularly desirable, such as in the batch production of high temperature aerospace components or the printing of unique bone prosthetics. Furthermore there are a number of difficulties in producing complex, detailed ceramic parts using traditional methods, as the materials cannot be cast due to their high melting points. 3D printing presents a way to overcome these barriers.
Not only can 3D printing assist in making complex shapes, but also it is emerging that it can also be used to tailor complex microstructures. During the 3D printing process known as robocasting, a filament of material is extruded and deposited in a location determined by a CAD model, layer by layer, until a part is complete. The extrusion forces can act on the shear thinning paste to form preferential flow structures. If the paste consists of highly anisotropic particles, such as fibres, then they can align to a very high degree in the printing direction. This allows parts to be built with properties programmed and controlled at the CAD stage; for example the build path can be designed such that the fibres are aligned parallel to force lines to give strength and stiffness where it is most needed in a part.
The cover image is what results when this same process is used to align platelets during 3D printing. Aside from aligning in the printing direction, we report for the first time a second degree or alignment where the platelets form concentric rings perpendicular to the printing nozzle's walls, as can be seen in the image which shows a single printed filament of platelet paste. This is a result of the velocity gradient experienced by the paste during extrusion through the 250 μm channel of the printing nozzle. This alignment can be used to create parts with microstructures similar to a number of natural hierarchical materials, such as bone, enamel and nacre.
The platelets are then bonded by sintering, and this porous preform is infiltrated to create composites. Using these methods we have created polymer, metal and ceramic matrix composites with high strength and toughness as well as a number of other promising properties. They exhibit stable crack growth as well as a range of toughening mechanisms which can be programmed at the CAD level by controlling the tool path. For example, bars printed with the tool path printing along the width of the test bar show macroscopic crack deflection around the perimeter of the filaments, following the platelet contours, while bars printed along the length of the test bar exhibit extensive platelet pull-out and bridging.
This is just one example of the ways in which 3D printing can drive progress in novel and unexpected areas. As new printing techniques emerge, and older ones mature, it is likely that many new innovations will be revealed, allowing additive manufacturing to take its rightful place at the centre of modern industrial development.
The authors would like to acknowledge the Centre of Advanced Structural Ceramics Industrial Consortium and the EPSRC for funding this work.