Surface Nanostructuring of Metallic Materials …
479
intergranular attack [37]. For NC 316L stainless steel, the fractured surface within the
pit appears to be rough, which is a clear indication of the occurrence of ductile mode
of fatigue crack propagation (Fig. 9c). Since nanocrystalline grains have a lower
capacity to store dislocations, unlike its microcrystalline counterpart the NC 316L
stainless steel is less prone to intergranular corrosion [38]. Compressive residual
stress induced during SMAT reduces the crack propagation in NC 316L stainless
steel. As the ductile mode of fatigue crack propagation requires more energy than
the brittle form, the crack propagation is retarded to a large extent in NC 316L
stainless steel. Hence, the formation of a thicker oxide layer, a lesser amount of
dislocation pile-up, compressive residual stress, requirement of higher energy for
crack propagation, reduction in cathodic reactions (H 2 evolution) and trapping of
hydrogen at the grain boundaries exert a synergistic effect in improving the corrosion
fatigue strength of NC 316L stainless steel.
The adhesion and proliferation of MC3T3-E1 subclone 4 osteoblast cells on
untreated 316L stainless steel (MC) and those subjected to SMAT using 5.5 mm
Ø steel balls at 50 Hz for 15 min (NC) is studied by Bahl et al. [24]. The cell attachment and proliferation are measured using WST-1 assay at 1 day and 3 days. The
extent of protein adsorption, cell attachment and proliferation largely depends on
the degree of hydrophilicity of the surface. A hydrophilic surface is considered to be
desired than a hydrophobic one for implant applications due to its ability to promote a
better interaction of the implant with the biological fluids, cells and tissues [39, 40]. In
general, the higher the surface roughness, the lower is the water contact angle, higher
is the surface energy and higher is the degree of hydrophilicity. However, Bahl et al.
[24] have suggested that factors other than R a could also contribute to improvement
in cell attachment and proliferation. According to them, the electronic properties
of oxide film formed on NC 316 L stainless steel plays a critical role than surface
roughness as the R a of both MC and NC 316L stainless steels are highly comparable.
The NC 316L stainless steel having a higher charge carrier density lowers the electron work function of the oxide film and the net positive charge developed on the
oxide layer favors cell adhesion and proliferation (Fig. 10). The development of a
novel “net-like” nanostructured surface on 316L stainless steel by SMAT using 5 mm
Ø chrome steel balls for 2 min exhibits significant enhancement in the attachment,
spreading and proliferation of human osteoblast cells (Saos-2) when compared to the
coarse-grained untreated stainless steel [41]. The high density of grain boundaries in
the “net-like” nanostructured surface is considered to be responsible for the enhanced
protein adsorption and higher cell attachment.
3 Ti and Ti Alloys
Titanium and titanium alloys have received considerable attention due to their high
strength to weight ratio, better mechanical properties, formability, machinability, low
toxicity, low corrosion rate and good biocompatibility. These attributes make them
suitable for orthopedic and dental implants, particularly for load-bearing implants
479
intergranular attack [37]. For NC 316L stainless steel, the fractured surface within the
pit appears to be rough, which is a clear indication of the occurrence of ductile mode
of fatigue crack propagation (Fig. 9c). Since nanocrystalline grains have a lower
capacity to store dislocations, unlike its microcrystalline counterpart the NC 316L
stainless steel is less prone to intergranular corrosion [38]. Compressive residual
stress induced during SMAT reduces the crack propagation in NC 316L stainless
steel. As the ductile mode of fatigue crack propagation requires more energy than
the brittle form, the crack propagation is retarded to a large extent in NC 316L
stainless steel. Hence, the formation of a thicker oxide layer, a lesser amount of
dislocation pile-up, compressive residual stress, requirement of higher energy for
crack propagation, reduction in cathodic reactions (H 2 evolution) and trapping of
hydrogen at the grain boundaries exert a synergistic effect in improving the corrosion
fatigue strength of NC 316L stainless steel.
The adhesion and proliferation of MC3T3-E1 subclone 4 osteoblast cells on
untreated 316L stainless steel (MC) and those subjected to SMAT using 5.5 mm
Ø steel balls at 50 Hz for 15 min (NC) is studied by Bahl et al. [24]. The cell attachment and proliferation are measured using WST-1 assay at 1 day and 3 days. The
extent of protein adsorption, cell attachment and proliferation largely depends on
the degree of hydrophilicity of the surface. A hydrophilic surface is considered to be
desired than a hydrophobic one for implant applications due to its ability to promote a
better interaction of the implant with the biological fluids, cells and tissues [39, 40]. In
general, the higher the surface roughness, the lower is the water contact angle, higher
is the surface energy and higher is the degree of hydrophilicity. However, Bahl et al.
[24] have suggested that factors other than R a could also contribute to improvement
in cell attachment and proliferation. According to them, the electronic properties
of oxide film formed on NC 316 L stainless steel plays a critical role than surface
roughness as the R a of both MC and NC 316L stainless steels are highly comparable.
The NC 316L stainless steel having a higher charge carrier density lowers the electron work function of the oxide film and the net positive charge developed on the
oxide layer favors cell adhesion and proliferation (Fig. 10). The development of a
novel “net-like” nanostructured surface on 316L stainless steel by SMAT using 5 mm
Ø chrome steel balls for 2 min exhibits significant enhancement in the attachment,
spreading and proliferation of human osteoblast cells (Saos-2) when compared to the
coarse-grained untreated stainless steel [41]. The high density of grain boundaries in
the “net-like” nanostructured surface is considered to be responsible for the enhanced
protein adsorption and higher cell attachment.
3 Ti and Ti Alloys
Titanium and titanium alloys have received considerable attention due to their high
strength to weight ratio, better mechanical properties, formability, machinability, low
toxicity, low corrosion rate and good biocompatibility. These attributes make them
suitable for orthopedic and dental implants, particularly for load-bearing implants