Surface Nanostructuring of Metallic Materials …
469
Metallic biomaterials are widely used for the fabrication of implants. The high
strength and resistance to fracture provide them a reliable long-term implant performance in major load-bearing situations. Stainless steels, CP-Ti, Ti6Al4V alloy, βphase Ti alloy, NiTi alloy and CoCrMo alloy are the most commonly used metallic
materials for orthopedic implant applications. Among them, stainless steels, CP-Ti,
β-phase Ti alloy and NiTi alloy are primarily used for osteosynthesis applications.
CoCrMo alloy is mainly used for joint arthroplasty application whereas Ti6Al4V
alloy is used for both for joint arthroplasty as well as osteosynthesis applications
[22]. The present chapter aims to provide detailed coverage of SMAT of stainless
steel, Ti and Ti alloys, Ni-Ti alloy, and CoCrMo alloy and how the nanostructured
surface enables an improvement in the characteristic properties that are suitable for
biomedical applications.
2 Stainless Steels
Austenitic stainless steels are widely used for biomedical implant and device manufacturing purposes due to their good mechanical properties, better corrosion resistance, acceptable biocompatibility, ease of fabrication. In general, AISI 316 LVM
stainless steel (ASTM F138 and F139 standards) is the recommended material for
orthopedic applications, particularly for load-bearing implants and stents. AISI 304
stainless steel is used in orthodontics such as archwires, brackets, and screws. The
major causes of failure of stainless steel implants are due to stress shielding effect,
poor osseointegration and corrosion fatigue. Generation of wear debris, accumulation of wear debris at the surrounding tissues causing inflammation, toxicity of metal
ions leached into body fluid are other associated problems. Most of the failures are
surface related. The interaction between the implant and the surrounding tissues
determines the overall performance of the implant. The surface properties such as
surface roughness, wettability, surface energy and surface chemistry are the key
factors that determine the interaction between the host tissue and the implant immediately after the implantation. Absorption of proteins is the basic step in promoting
cell adhesion and proliferation. Implant surfaces with a suitable surface profile could
increase the degree of hydrophilicity and promote cell adhesion and proliferation.
Hence, engineering the surface of implant materials is critical for the development of
next-generation orthopedic implants. SMAT enables the formation of a nanostructured surface layer on metallic materials and improves their overall properties and
performance. The effect of SMAT on the microstructure, grain size, phase content,
phase transformation, compressive residual stress, surface roughness, wettability and
corrosion resistance of stainless steels are addressed in the following paragraphs.
The evolution of the microstructure of stainless steel during SMAT is addressed
by Lu and Lu [1, 3] and Tao et al. [4]. SMAT effectively induces localized plastic
deformation that results in grain refinement down to the nanometer scale at the
surface of austenitic stainless steel. It is well known that plastic deformation of
metallic materials occurs by slip and twinning mechanisms. In metallic materials
469
Metallic biomaterials are widely used for the fabrication of implants. The high
strength and resistance to fracture provide them a reliable long-term implant performance in major load-bearing situations. Stainless steels, CP-Ti, Ti6Al4V alloy, βphase Ti alloy, NiTi alloy and CoCrMo alloy are the most commonly used metallic
materials for orthopedic implant applications. Among them, stainless steels, CP-Ti,
β-phase Ti alloy and NiTi alloy are primarily used for osteosynthesis applications.
CoCrMo alloy is mainly used for joint arthroplasty application whereas Ti6Al4V
alloy is used for both for joint arthroplasty as well as osteosynthesis applications
[22]. The present chapter aims to provide detailed coverage of SMAT of stainless
steel, Ti and Ti alloys, Ni-Ti alloy, and CoCrMo alloy and how the nanostructured
surface enables an improvement in the characteristic properties that are suitable for
biomedical applications.
2 Stainless Steels
Austenitic stainless steels are widely used for biomedical implant and device manufacturing purposes due to their good mechanical properties, better corrosion resistance, acceptable biocompatibility, ease of fabrication. In general, AISI 316 LVM
stainless steel (ASTM F138 and F139 standards) is the recommended material for
orthopedic applications, particularly for load-bearing implants and stents. AISI 304
stainless steel is used in orthodontics such as archwires, brackets, and screws. The
major causes of failure of stainless steel implants are due to stress shielding effect,
poor osseointegration and corrosion fatigue. Generation of wear debris, accumulation of wear debris at the surrounding tissues causing inflammation, toxicity of metal
ions leached into body fluid are other associated problems. Most of the failures are
surface related. The interaction between the implant and the surrounding tissues
determines the overall performance of the implant. The surface properties such as
surface roughness, wettability, surface energy and surface chemistry are the key
factors that determine the interaction between the host tissue and the implant immediately after the implantation. Absorption of proteins is the basic step in promoting
cell adhesion and proliferation. Implant surfaces with a suitable surface profile could
increase the degree of hydrophilicity and promote cell adhesion and proliferation.
Hence, engineering the surface of implant materials is critical for the development of
next-generation orthopedic implants. SMAT enables the formation of a nanostructured surface layer on metallic materials and improves their overall properties and
performance. The effect of SMAT on the microstructure, grain size, phase content,
phase transformation, compressive residual stress, surface roughness, wettability and
corrosion resistance of stainless steels are addressed in the following paragraphs.
The evolution of the microstructure of stainless steel during SMAT is addressed
by Lu and Lu [1, 3] and Tao et al. [4]. SMAT effectively induces localized plastic
deformation that results in grain refinement down to the nanometer scale at the
surface of austenitic stainless steel. It is well known that plastic deformation of
metallic materials occurs by slip and twinning mechanisms. In metallic materials