Wollastonite–Silicon Nitride Ceramic Composites: Microstructure, Mechanical Performance and Bioactivity

Silicon nitride (Si3N4) is an advanced structural ceramic with considerable potential for load-bearing biomedical applications owing to its excellent mechanical properties and favorable biological response. In this study, the effect of wollastonite (CaSiO3) addition on the microstructure, mechanical performance, and in vitro bioactivity of Si3N4–CaSiO3 ceramic composites was investigated. Composites containing 5–30 wt.% wollastonite were prepared by pressureless sintering at 1800 °C for 1 h and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), density measurements, nanoindentation, Vickers hardness, fracture toughness, compressive strength, and simulated body fluid (SBF) immersion tests. Increasing the wollastonite resulted in progressive densification up to 20 wt.% CaSiO3, resulting in a maximum relative density of 96%, together with complete α→β-Si3N4 transformation and the development of elongated β-Si3N4 grains. The composition containing 20 wt.% wollastonite exhibited optimum mechanical performance, achieving a hardness of approximately 13 GPa, fracture toughness of ~5.5 MPa·m1/2, and compressive strength of ~2840 MPa. The results demonstrate that wollastonite plays a multifunctional role in Si3N4 ceramic composites by promoting densification during sintering and improving in vitro bioactivity while maintaining high mechanical performance. These findings highlight the potential of Si3N4–CaSiO3 ceramic composites as promising bioactive structural materials for load-bearing orthopedic and dental applications.