Surface Nanostructuring of Metallic Materials for Implant Applications

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AbstractNanostructured metallic materials are progressively investigated for numerous biomedical implant applications due to their superior mechanical properties, biocompatibility and, ability to promote cell adhesion and proliferation. Several materials processing routes have been explored to prepare bulk and surface nanostructured metals/alloys. Surface nanostructuring by changing the surface chemistry through deposition or diffusion processes is limited by porosity and contamination besides uncertainty in achieving a good bonding between substrate and coating. The surface nanostructuring process must enable an improvement in the overall properties of materials such as high hardness and strength, higher thermal expansion coefficient, improved tribological properties, and better fatigue properties, etc., to avoid premature failure of implant materials. In this perspective, surface severe plastic deformation (S2PD) processes assume significance as they could impart the desired characteristics to a variety of metals and alloys by grain refinement mechanism, without changing the overall composition and/or phases present in the material. The improvement in properties is fundamentally derived from grain refinement mechanism by the introduction of a large amount of defects/strain. Surface mechanical attrition treatment (SMAT) is an effective way of inducing localized plastic deformation (S2PD method) that results in grain refinement down to nanometer scale without changing the chemical composition of the material. It is a very promising method to produce functionally gradient materials, in which the nanocrystalline surface layer provides suitable surface properties while the coarse-grained matrix provides the ductility. SMAT provides desirable surface topography and increases the average surface roughness. The surface topography of materials determines its hydrophilic or hydrophobic nature. For implants, a hydrophilic surface is considered to be more desirable than a hydrophobic one because of its better interaction with biological fluids, cells, and tissues. In addition, generating the desired surface topography could provide significant enhancement in osteoblast adhesion, proliferation, maturation, and mineralization. SMAT decreases the grain size, induces compressive residual stress, microstrain, defects/dislocations, and phase transformation, all of which enable a significant improvement in hardness, fatigue resistance, and tribological properties of materials. In addition, the extent of grain refinement, extent of deformation, extent of change in surface roughness, phase transformation, residual stress, microstrain, and defect/dislocation density could influence the performance of materials subjected to SMAT. This could ultimately influence the biological performance of the implant materials. The present chapter aims to provide detailed coverage of SMAT of stainless steel, Ti alloys, Ni-Ti alloy, CoCrMo alloy, and how the nanostructured surface enables an improvement in the characteristic properties that are suitable for biomedical applications.KeywordsMetallic biomaterialsSurface nanocrystallizationSurface mechanical attrition treatmentMechanical propertiesCorrosionCell response