The aim of this study is to produce highly dense Si 3 N 4 based composites with good mechanical properties and bioactivity. Si 3 N 4 ceramics without using sintering aids, Si 3 N 4-HA and Si 3 N 4-HA-GNP based composites have been produced by spark plasma sintering (SPS) at temperatures of 1525-1550°C. The effect of reinforcement type and content on the densification behavior, phase analysis, microstructural development, mechanical properties, and in-vitro bioactivity behavior of Si 3 N 4 were systematically investigated. Monolithic Si 3 N 4 that contains a high amount of β-Si 3 N 4 phase (~87 wt%) was produced by nearly full densification (~99%). Hydroxyapatite (HA) was used as a starting powder during the preparation of binary and triple composites to provide bioactivity to Si 3 N 4 , and after sintering, HA transformed into tricalcium phosphate (β-TCP and α-TCP) polymorphs. The incorporation of GNPs had a positive effect on the stability of β-TCP phases at higher sintering temperatures. The improvement in indentation fracture toughness of the samples with GNP reinforcement was mainly attributable to pull-out and crack deflection mechanisms. In-vitro bioactivity of GNP added composites enhanced with increasing α-TCP content. More calcium phosphate-based particle formation was observed in Si 3 N 4-HA-GNP composites compared to the Si 3 N 4-HA.

Effects of MoCl 3 and NiCl 2 , originally incorporated as coloring agent, on the cellular response of 3 mol% yttria stabilized zirconia (3Y-TZP) ceramics was investigated. MoCl 3 and NiCl 2 -MoCl 3 incorporated, tooth-colored 3Y-TZP ceramics were produced through cold isostatic pressing at 100 MPa followed by pressureless sintering at 1450°C for 2 h. Aging was performed on the sintered ceramics using distilled water in a reactor at 134°C at 2.3 bar pressure for 2 h. The phases developed during different stages of processing were identified by X-ray diffraction (XRD) analysis. In vitro cell culture studies were carried out using L929 fibroblast cell line. The cell viability and proliferation studies revealed that none of the specimens showed cytotoxicity with respect to coloring. Confocal laser scanning microscope (CLSM) analyses suggested that all of the specimens exhibited good in vitro cytocompatibility. Enhancement in cell attachment, adhesion, and proliferation was observed in all specimens via scanning electron microscope (SEM) analysis. Although the coloring process did not improve the proliferation performance of the aged specimens, the incorporation of transition metals enhanced the in vitro performance of 3Y-TZP ceramics.

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The influence of two different spark plasma sintering-based processing routes (i.e., reactive SPS (RSPS) and non-reactive SPS) on the properties of TaB2-TaC composites was investigated. Ta2O5 and B4C powders were used as starting materials in the RSPS method, and synthesis and densification of TaB2-TaC composites were accomplished in a facile single step. The effect of sintering temperature and time on the microstructure and densification of the in-situ RSPS were investigated. The obtained results were compared with non-reactive spark plasma sintered TaB2-TaC composites. The highest densification (~ 99.5 %) was achieved for the TaB2-TaC composite with 6.64 vol% TaC after reactive sintering at 1550 °C under 40 MPa with a 5 min holding time. Although lower SPS temperature was used in the RSPS method, better densification and higher Vickers hardness were obtained compared to the non-reactive SPS. While platelet-shaped TaC formation was observed in both processes, the average grain size was smaller in the sample produced by the RSPS method. On the other hand, no significant difference was detected in fracture toughness and oxidation behavior of the composites produced by RSPS and non-reactive SPS.