One of the main interests of ceramic 3D printing in aerospace field is to use this technology to manufacture ceramic matrix composites, which have obvious performance advantages over superalloys and much lower density. For aerospace applications, the focus is on the development of SiC matrix composite combustion chamber lining, premixed fuel nozzle shield, turbine guide vane, nozzle and other engine components.

The conventional manufacturing methods of these components are based on the infiltration of preforms, such as chemical vapor infiltration, polymer infiltration pyrolysis, melt infiltration and hybrid methods combining these processes. These methods are usually slow, involving multiple steps and a lot of post-processing. 3D printing will be simpler and can achieve free and complex geometry, including sharp changes in section size, and the manufacture of hybrid and multifunctional composites. In addition, 3D printing can reduce production steps and shorten production time, so as to reduce costs. The main challenge lies in the mixing of fiber reinforced materials, the complete densification of parts and the optimization of process and properties.

Key features of several mainstream ceramic 3D printing processes

At present, several ceramic 3D printing processes are exploring the application of military aircraft, such as SLA, DLP, direct ink write (DIW), Selective Laser Sintering and aerosol jet (AJP).

1.

SLA and DLP have similar principles. Ceramic powder is added to the resin to obtain ceramic slurry for 3D printing. Compared with other technologies, the technology based on light curing is suitable for preparing large parts with high precision and complex shape. However, the requirements for slurry are generally high. For example, the slurry needs to have high solid content and low density. At the same time, ceramic particles need to be evenly dispersed in the resin. Moreover, the equipment used in this method is expensive and the manufacturing cost is high. Several international well-known ceramic 3D printing equipment manufacturers adopt this forming technology.

2.

Direct ink write (DIW) technology is to mix ceramic powder with various organics to make ceramic ink, and then print it on the forming plane by printer to form ceramic blank. For inkjet printing technology, the preparation of ceramic ink is the key. This requires that the ceramic powder can be well and evenly dispersed in the ink, with appropriate viscosity, surface tension and conductivity, fast drying rate and as high solid content as possible. At present, the difficulty of this technology is that the solid content in the ink is too low, which will lead to the low density of the ceramic body, and excessive increase of the solid content will make it difficult to spray the ink. Previously, researchers used this technology to prepare Si3N4 ceramic gear blank, with density of 3.18 g / cm3, fracture toughness of 4.4mpa • M1 / 2 and compressive strength of 600MPa. It can be seen that the products obtained by inkjet printing technology have good mechanical properties. This also shows that inkjet printing technology has great potential in the production of high-performance silicon nitride ceramics.

3.

binder jetting is to selectively spray binder on the powder bed and obtain the final ceramic body through layer-by-layer manufacturing. This technology has great advantages in preparing porous ceramic parts, but its forming accuracy is poor and the surface is rough, which is closely related to powder composition, particle size, fluidity and wettability. In the manufacturing process, the size and surface accuracy of the obtained blank can be improved by controlling the humidity of the powder layer. The density of parts prepared by binder jetting forming method is generally low, and subsequent processes are usually required to improve its density. For example, cold isostatic pressing and high-pressure infiltration treatment before sintering can significantly improve the compactness of sintered products, but also reduce the productivity. Ti3SiC2 ceramics were prepared by binder jetting technology, followed by infiltration of silicon melt and aluminum silicon alloy. The density of the composite can reach 4.1g/cm3. The maximum bending strength of this fully dense material is 233 MPa and its mechanical properties are good. binder jetting technology provides a new scheme for the preparation of ceramic composites.

4.

Selective Laser Sintering (SLS) uses laser to selectively scan the surface of the powder bed, so that the powder materials are heated, melted and bonded together, and finally form the green body. The sintering temperature of ceramic materials is very high, so it is difficult to sinter directly. At present, ceramic materials can only be sintered by indirect laser selective sintering method. The method is to cover the surface of ceramic particles with low melting point organic binder, and then laser only melts the organic binder to combine the ceramic particles with each other. Although the improved forming process is relatively simple, due to the high content of organic polymer binder, the resulting green body density is low, loose and porous. Therefore, subsequent treatment is usually required to improve the density, such as isostatic pressing treatment, infiltration technology, etc.

The military aircraft industry is also exploring 3D printing to improve combat readiness and affordability. Ceramic 3D printing has proved to be used to produce metal casting cores and molds, which greatly saves time and cost. In this regard, ceramic 3D printing can be used to mold traditional parts and manufacture new wings with improved cooling channel designs, which cannot be manufactured by traditional methods.

Ceramic 3D printing is also regarded as a disruptive innovative technology used in extreme environments, which can meet the needs of high-temperature materials (such as ultra-high temperature ceramics) and complex geometry. However, there is a lack of 3D printing process that can be produced at low cost and on a large scale to produce high-strength and damage resistant ceramics. An attractive area of early ceramic additive manufacturing is the low-cost engine development of small UAV, which can significantly improve the performance of the engine. In these applications, the higher component failure risk has a relatively insignificant impact, which can be regarded as a test platform for prototyping and accelerating iteration.