## 1 Introduction  
The history of silicon nitride (Si\({}_{3}\)N\({}_{4}\)) ceramics began about 150 years ago, with [PERSON] and [PERSON] developing silicon nitride synthetically in 1857 [1], even though naturally occurring nierite minerals, such as \(\alpha\)-Si\({}_{3}\)N\({}_{4}\) and \(\beta\)-Si\({}_{3}\)N\({}_{4}\), have been found during later, detailed analyses of particles of meteoritic rock [2]. Silicon nitride currently plays a dominant role in the field of nonoxide ceramics, and exhibits a combination of excellent material properties, such as high toughness and strength at high temperatures, excellent thermal shock resistance and good chemical resistance, which is unmatched by other ceramics [3]. Due to these superior material properties, silicon nitride ceramics became popular in the 1950s, e.g., for use in thermocouple tubing [4]. Owing to these properties, in particular thermal conductivity and thermal and corrosion resistance, silicon nitride-based ceramics can also be used as base material for thermal conductors, gas turbines, and ball bearings [5]. Apart from technical applications silicon nitride-based ceramics have also been used as material for medical devices (e.g., spinal cages). It was shown that the material shows good osseointegration and stimulated cell differentiation as well as osteoblastic activity which resulted in accelerated bone ingrowths compared to poly (ether ether ketone) (PEEK) [6]. Furthermore, silicon nitride was also evaluated regarding its anti-infective behavior when used for implants. [PERSON] et al. compared the bacterial growth on calvarial implants in rats of silicon nitride to implants made of either titanium or PEEK [7]. They observed bacteria in 88% of the PEEK implants and 12% of the titanium implants whereas no bacteria were present adjacent to the silicon nitride implants. Similar results have also been observed by [PERSON] et al. in tibial implants [8]. These properties, together with its excellent biocompatibility and mechanical strength, make silicon nitride to a very attractive material for biomedical engineering [9]. The combination of load-bearing ability and induction of cell differentiation makes it superior to classical bone replacement materials like calcium phosphates. Additionally, the bactericidal behavior could drastically reduce infection associated implant failures or necessary revision surgeries, which have quadrupled in the last 20 years [10]. The solid phase sintering of pure silicon nitride ceramics is generally not feasible due to decomposition processes and low diffusion coefficients. Based on that reasoning, sintering additives are needed to allow for consolidation by sintering. The first sintering aid investigated for the densification of Si\({}_{3}\)N\({}_{4}\) ceramics was magnesium oxide (MgO). Subsequently, other materials such as alumina (Al\({}_{2}\)O\({}_{3}\)), yttria (Y\({}_{2}\)O\({}_{3}\)), zirconia (ZrO\({}_{2}\)) and ceria (CeO\({}_{2}\)) were used to further promote densification. Overall, the yttria-alumina (Y\({}_{2}\)O\({}_{3}\)-Al\({}_{2}\)O\({}_{3}\)) system is the most widely applied sintering aid system [11]. An effective method for increasing the creep and oxidation resistance of silicon nitride is the addition of large amounts of sintering additives to the silicon nitride structure in the grain boundary phase, the formed mixed crystals being known as SiAlONs [12; 13]. SiAlON ceramics are the solid solution of silicon nitride and the sintering additives alumina (Al\({}_{2}\)O\({}_{3}\)), yttria (Y\({}_{2}\)O\({}_{3}\)) and aluminum nitride (AIN). In the early 1970s, the formation of this solid solution that has the same structural form as silicon nitride has been reported in Japan and in the UK parallel [14]. Since the SiAlON powders contain the sintering aids, the sintering additives were not blended additionally in this work. Conventional methods for manufacturing intricate ceramic bodies demand difficult and costly production steps.