The Surface Chemistry of Silicon Nitride

Silicon nitride components are produced from fine high-purity submicron powders formed into near-net-shape green bodies by colloidal processes. The green bodies are then densified by sintering using tightly controlled manufacturing steps that involve important chemical reactions and phase changes. The basic chemistry required to manufacture high-strength industrial silicon nitride ceramics with these methods is well understood. Pertinent to biomedical applications of silicon nitride, recent advances in our knowledge of the material’s biochemistry have helped expand our understanding of why silicon nitride exhibits its unique bio-functionality in vivo. Silicon nitride undergoes passive oxidation at room temperature, leading to a graded oxynitride layer on an implant. Investigational tools such as electron microscopy, X-ray-induced Auger electron spectroscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectrometry have confirmed a complex scenario of several interacting chemical groups at the surface. Specifically, a balance between silanol groups and silicic acid contributes to the surface bioactivity of silicon nitride and provides investigators with a tool to modulate the biomedical properties of the material. When exposed to moist air, silicon nitride powders undergo a series of surface hydrolytic reactions, resulting in the release of trace quantities of ammonia, silica, and other silicate species. The leaching rate of the oxidized surface layer is pH-dependent. Together, these dynamic surface properties provide a foundational understanding of why the material, upon interacting with biological fluids, shows remarkable biocompatibility, as well as clinically significant osteogenic and antipathogenic attributes. This knowledge is of value to biomaterial scientists who seek to design medical implants with properties tailored for specific biological applications.