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1. Introduction
Engineering ceramics is an irreplaceable material used in the extreme working conditions due to their excellent mechanical properties, as well as thermal and chemical resistances (Richerson, 2005). However, traditional manufacturing methods for engineering ceramic parts have difficulties in meeting the requirements from the fast-developing modern technology because of their limitations, such as low sintering efficiency, limited complexity of fabricated structures and relatively low strength (Kingery et al., 1976; Rahaman, 2006). The limitations mostly arise from batching, forming, sintering and machining, which are the main processing steps in the traditional manufacturing methods. Therefore, it is significant to develop new processing methods for making full use of engineering ceramics.
Ceramic additive manufacturing (AM) is a newly developed method in recent decades and has been successfully demonstrated for its advantages with the use of “indirect” and “direct” methods. By the indirect method, a green body is first created from ceramic powders or slurry with a high content of an organic or inorganic binder, and then sintered and densified to eliminate binders. Because a high energy source is not needed, many indirect processing methods have been developed, such as stereo lithography (Yen, 2012; Licciulli et al., 2005), laminated object manufacturing (Park et al., 2000), selective laser sintering (SLS) (Harlan et al., 2001; Evans et al., 2005; Shahzad et al., 2014), fused deposition of ceramics (Allahverdi et al., 2001) and freeze-form extrusion fabrication (Huang et al., 2009), etc. Through these methods, complex geometries can be obtained, but limitations, such as impurities, porosities and shrinkage, still exist because the processing steps are the same as in the traditional methods. On the other hand, with the direct method, pure ceramic powders can be melted directly by a high energy source and then the melt pool solidifies to form a part. Due to the melting/solidification processes, fully densified ceramic parts with good performances are produced more easily and rapidly. Comparatively, few direct processing methods, e.g. laser-engineered net shaping (LENS) and selective laser melting (SLM), have been developed because of the need for a high energy source. Yves-Christian et al. (2010, Jan et al., 2013) manufactured ceramic parts from ZrO2/Al2O3 powders with an SLM system and obtained...