Functional properties of silicon nitride based materials for joint applications

Abstract: Total hip and total knee replacements are generally successful procedures for patients suffering with pain due to bone fracture or diseases affecting the joints. However, the materials that are commonly used still have limitations. In particular, corrosion products and wear debris may give rise to negative body reactions.In this thesis, silicon nitride based materials were investigated for use in joint implants, namely as a coating for e.g. femoral heads and the metallic modular taper junction in hip implants, as well as a bulk bioceramic for joint applications. One of the main advantages of these materials is the potential to dissolve slowly in aqueous solutions, releasing only biocompatible ions.To understand the mechanical and wear properties of these materials, thin film coatings were deposited using magnetron-based techniques onto Si wafers and a CoCrMo alloy, the latter frequently used in biomedical implants. Coatings up to 8.8 µm thick were deposited on 2D flat discs as well as full 3D implant heads, following a CrN interlayer for improved adhesion. The chemical composition, microstructure, surface roughness, adhesion, wear resistance, and dissolution properties of the coatings were evaluated as a function of substrate rotation, bias voltage, target power as well as the addition of different elements.Results show that it is possible to produce coatings with mechanical properties and a wear performance similar to bulk ceramics and other ceramic coatings already evaluated in vivo. It was evident that a high coating density is needed, to avoid premature failure in an in vivo environment. The coating density, and stability over time in solution, was found to increase when a higher target power and process heating were used.New bulk silicon nitride materials containing only biocompatible additives, were evaluated for potential use in joint applications by wear tests for the first time, showing very low wear rates of the counter material.Silicon nitride coatings and bulk materials tested in this work showed promising results for further investigation and a basis for future application in joint bearings.