Abstract:

Ceramics are a series of inorganic nonmetallic materials with a long history and has been extensively used. They have been playing a vital role in the human civilization process. Nowadays, as one of the three pillars in the material systems for modern industry, ceramics with excellent physical and chemical properties have been increasingly used and researched and become indispensable in the high-tech application fields such as machinery and electronics, energy and environmental protection, aerospace and biomedical industries, especially for the advanced ceramics with complex and integrated structural and functional properties. However, due to their inherent high hardness and brittleness, when manufacturing ceramic products with highly complex three-dimensional shapes or customized structures and functions, the traditional molding and machining technologies often face certain technical limitations with great difficulty and long production cycle.
The emergence of additive manufacturing (AM) paves a new way to break through above limitations. AM technology is earliest known as freeform fabrication (FFF) or rapid prototyping (RP) technology, and was gradually popular with the general public and is now commonly known as 3D printing technology in this century. ‘Stereolithography (SL)’ photopolymerization additive manufacturing technology suitable for organic resin solutions and ‘Selective Laser Melting (SLM)’ additive manufacturing technology for metal powders, were invented in the US and Germany in the 1980s and 1990s, respectively. These two epoch-making technologies are the most significant representatives of all AM technologies. At present, over ten types of additive manufacturing technologies have been developed for various raw materials. However, compared with organic and metallic materials, ceramics generally possess lower physical and chemical activities and higher melting points; some AM processes used for organic and metallic materials cannot be directly applied to ceramic materials. Nevertheless, currently most of the ceramic AM technologies are derived from that made for organic and metallic materials. This makes the development of AM technologies for ceramic materials difficult, and thus its development history is relatively shorter. However, the magic and charm of AM lie in its unique flexibility of manufacturing highly complex shapes, as well as the advantages of customized structures and functions, which extensively attract worldwide researchers to investigate AM of ceramic materials and their applications.
In recent years, a number of excellent research groups and industrial organizations have sprung up in the field of ceramic AM. According to the statistics preliminary generated from the 1^st National Forum on Additive Manufacturing of Ceramics (FAME2021) initiated by Prof. CHEN Zhangwei of Shenzhen University in July 2021, there are approximately over 60 institutes in China specializing in the research of ceramic AM and applications, while more than 20 industrial manufacturers are involved in the development and fabrication of materials, printing process equipment, and post-processing equipment that are relevant to ceramic AM technology. Among them, the majority of research focuses on the AM of photopolymerization (including SL and Digital Light Processing (DLP)) or Direct Ink Writing (DIW) using the mixtures of ceramic powders and resins/adhesives. The rest mainly utilizes ceramic powders and high-power lasers such as Selective Laser Sintering (SLS), Laser Directed Energy Deposition (LDED), and other processes for direct AM of ceramics. In terms of the types of ceramic materials involved, most research focuses on oxide ceramic materials, such as SiO₂, ZrO₂, Al₂O₃, and their mixed or multiphase materials, as well as PZT, BTO, TCP, and other advanced ceramics. The main application directions include load-bearing components and functional parts, such as catalytic carriers, casting molds, heat insulation, piezoelectric, sensors, artificial bones, dentistry, ultra-high temperature parts, optics, and other fields. Besides, in recent years, researchers have also turned to AM of non-oxide ceramics such as SiC, Si₃N₄, AlN, and even more complex Polymer-Derived Ceramics (PDCs) that generate polynary ceramics, and substantial progress has been achieved.
In general, the ceramic AM processes take ceramic-based materials as ‘inks’ and focused energies such as light energy, mechanical energy and heat energy as ‘pens’, to ‘draw’ a variety of complex ceramic devices with integrated structures and functions, which is similar with the Chinese fairy tale of ‘Ma Liang the Magic Brush’. It is noteworthy that the outcome of ‘Ma Liang the Magic Brush’ was ‘What You Draw Is What You Get’. In our opinion, this is exactly the ultimate goal of AM or 3D printing, namely ‘What You Print Is What You Get’. There are still numerous challenges to overcome to achieve ‘What You Print Is What You Get’ in the field of ceramic AM. Due to the complicated material properties of ceramics, the process of shaping by various AM technologies involves the preparation of the material feedstock systems, the adaptation of forming process, and the optimization of heat treatment and post-treatment process. Therefore, material selection and controls over the forming process, deformation and defect, structures and properties, and other aspects require overall investigation and careful balance.
In the second half of 2021, while the conference of FAME2021 was held, the Editorial Board of the Journal of Inorganic Materials invited Prof. LU Jian from the City University of Hong Kong (CUHK) and Prof. CHEN Zhangwei from Shenzhen University (SZU) as guest editors to organize this Special Issue (SI) themed ‘Additive Manufacturing of Inorganic Materials’. Prof. WU Jiamin from Huazhong University of Science and Technology (HUST) also contributed to the organization of the SI. This SI focuses on some of the latest research outcomes and review articles in the field of ceramic AM in China, representing the frontier progress of ceramic AM research in China. Due to the limitations of time and space, some excellent work cannot be included in this SI in a timely manner. We hope that the SI can provide a useful reference for promoting the research and application development of ceramic AM in China. With ceramics as inks and focused energies as pens, would ceramic AM mark the new chapter like ‘Ma Liang the Magic Brush’, manifesting a significant leap from the processing and engineering research of ‘Accumulating sands to form a pagoda’ to the popularized application of high added value of ‘Turning stones into gold by touching’? We believe that with the unremitting efforts and progress made by researchers worldwide, this dream will eventually come true in the near future!