Non-linear mechanical properties and dynamic response of silicon nitride bioceramic

Abstract

Natural spinal tissues have unique dynamic properties and it is crucial to investigate the full dynamic response of spinal implants and related biomedical materials to ensure compatibility with the host tissue. Silicon nitride is a promising bioceramic for spinal implants. In this study, the mechanical properties and dynamic response of silicon nitride were comprehensively evaluated using novel and efficient testing methods, at both the material and structural level. The correlation between mechanical properties and porosity was investigated and the results showed that Young's modulus and compressive strength decreased non-linearly as porosity increased. Both the compressive strength (100.35±3.39MPa) and fracture toughness (1.06±0.06MPa⋅m1/2) of the porous silicon nitride samples (~70% porosity) can be considered sufficient for load bearing purposes as a substitute for trabecular bone. Moreover, the results from a drop tower test showed that the average ultimate strength of the scaffolds under an impact loading was significantly lower than the strength under quasi-static compression. Nevertheless, this value is still comparable to that of trabecular bone. Regarding damping capacity, the results of free damped vibration tests of cantilever beams showed that a silicon nitride beam has a higher damping ratio (0.17%±0.01%) than a zirconia beam (0.13%±0.01%), which indicates that ceramics with similar Young's moduli can have different energy dissipating capacities. However, both dense and porous silicon nitride showed a low energy dissipation, substantially lower than natural spinal tissues. Overall, dense silicon nitride possesses a high stiffness, and altering the porosity of silicon nitride is one option to tailor its mechanical properties towards those of trabecular bone, however, more effort is required to increase its damping capacity, if this is a desired trait.