SiC is a covalently bonded ceramic which have a high refractoriness and will decompose to volatile Si-species and graphite at temperatures between 2300 and 2500℃ . This leads to some issues regarding manufacturing like the ability to maximize the utilization of the material in terms of sustainability. Often parts produced by traditional methods, need both green machining and post machining (after sintering), which lead to additional material loss. For metals, and some ceramics, a promising process route is 3D-printing, especially where small and complex parts are required. 3D-printing also gives the option to utilize left-over materials, which further reduce the cost of manufacturing since less material over time is needed. Another important aspect is that 3D-printing techniques can make parts that need little to none surface finish, which reduce material loss and cost even more.
In this project the focus is on Digital Light Processing (DLP), a 3D-printing technique adapted to stable dispersions of ceramic particles and a photomonomer. The choice of DLP was motivated by its high processing speed, exceptional good surface quality and the best technique when it comes to high-rate material utilization. Unexposed dispersion is also easily recycled for further 3D-prints.
The photomonomer (PM) in the dispersion is optical active in the sense that light of certain wavelength triggers polymerization. The polymerization of PM forms a solid three-dimensional polymer network with integrated SiC-particles and constitutes the green body after 3D-printing. A decisive property for the success of applying DLP is the penetration depth of the incoming light into the SiC-based dispersion. From a production rate point of view a large penetration depth is beneficial. (larger penetration depth means shorter printer time)
DLP is a technique based on a light source that projects a certain wavelength (usually by a laser in the UV-range) able to trigger polymerisation of the photomonomer. The dispersion is composed a homogeneous mixture of photomonomers and ceramic particles, and laser exposure will form a solid composite of ceramic particles surrounded by a cross-linked organic network. A mesh speeds up the process enabling the laser to expose all points making up one layer simultaneously. After one layer is cured the platform is lowered/elevated the same distance as the thickness of the printed layer (this is referred to as penetration depth). The curing depth is dependent on ceramic particle size, volume fraction of particles, light exposure power and the refractive index difference between the photomonomer and ceramic particles.
The solid loading of the dispersion is an important parameter. Experience from slip-casting and injection moulding recommend solid loading around 70 % to avoid formation of cracks and pores in the final sintered body. However, penetration depth decreases significantly with solid loading due to enhanced reflection, scattering and absorption, while the viscosity increases. Hence, an optimized dispersion will be a trade-off between penetration depth and solid loading as well as viscosity.
The difference in refractive index is crucial because ceramic particles which exhibit larger light absorption, can inhibit curing. Light scattering is also an issue that must be addressed, and this takes place even if the ceramic particles themselves are transparent. If the amount of scattering is high, it will result in a lower curing depth. After the part is cured, de-binding and sintering takes place to remove organic components and densifing the cermic particles
