Structure and Bulk Properties of Silicon Nitride

Silicon nitride (Si3N4) is an engineered ceramic, with intensive research supporting its favorable mechanical properties under extreme thermochemical and wear conditions. Major investigations into the material during the 1970s led to the development of several fully dense formulations using various additives to achieve desired properties. In addition, Si3N4 can be manufactured in porous versions as well. Dense Si3N4 can be made by mixing sintering additives, such as magnesium-, aluminum-, or yttrium-oxide with silicon or silicon nitride powder, followed by either nitriding, sintering, hot-pressing, or hot-isostatic pressing at high temperatures. The favorable physical, mechanical, and thermal properties of dense silicon nitrides translate into superior resistance to machining damage and improved fatigue behavior, as well as excellent thermal shock resistance. Unlike oxide ceramics such as zirconia and alumina, Si3N4 is less susceptible to crack propagation in laboratory testing. This behavior can be attributed to mechanisms that inhibit stress-induced chemical corrosion and crack growth. Because of its excellent wear resistance, Si3N4 is used in bearings and engine components in many industrial, automotive, and aircraft applications. Favorable wear and strength data have shown that Si3N4 bearings in prosthetic hips may be superior to legacy bioceramics, such as alumina. Furthermore, Si3N4 is very stable and resistant to oxidation and corrosion in aqueous fluids, such as the in vivo environment of biomedical implants. Compared to metal implants, Si3N4 encourages cell adhesion, normal proliferation, and differentiation, without toxicity, and it possesses a unique surface chemistry that discourages bacterial adhesion. The latter property is highly desirable in medical implants. Further research will lead to expanded applications of Si3N4 as a bioceramic, by itself, and in combination with other materials, such as biopolymers.