Surface Nanostructuring of Metallic Materials …
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formed on plastically deformed Co28Cr6Mo alloy in SBF [90]. Hence, it is clear
that the ability of the treated alloy to form a stable passive film is responsible for the
observed improvement in corrosion resistance.
The higher R a of the Co28Cr6Mo alloy after SMAT causes a deleterious influence
on both the wear and corrosion resistance. Hence, to achieve better wear and corrosion
resistance, post-polishing of the treated alloy is warranted. Demangel et al. [84] have
explored polishing using a brush, fluid containing grinding media and polishing
cloth. Polishing using brushes removed only the superficially attached particles.
Polishing using fluid containing grinding media removed some material and provided
a reasonable surface finish. The use of a polishing cloth offered a relatively better
surface finish. In scratch testing, the polished samples show a 30–60% decrease in
penetration depth and among the polished ones the penetration depth is much lower
for the cloth polished sample. Tchana Nkonta et al. [19] have used a proprietary
biomedical polishing treatment of Co28Cr6Mo alloy after SMAT which enabled a
50% decrease in R a from 0.64 μm to 0.30 μm. The decrease in R a also enabled
an increase in corrosion resistance. Hence, it is evident that adopting a suitable
post-polishing treatment is likely to improve both the tribological and corrosion
performance of CoCrMo alloys subjected to SMAT. In addition, the mean depth,
extent of material removal and mean flatness of the finished surface should also
be considered during post-polishing treatment. According to Demangel et al. [84],
polishing using both brush and fluid could remove only a limited amount of material
(up to 2–12 μm). However, the mean flatness is not improved much after polishing. In
contrast, the mean flatness is significantly improved for cloth polishing. Nevertheless,
the extent of material removal is much higher (up to 34 μm). The extent of material
removal during post-polishing treatment should be in line with thickness of the
deformed layer after SMAT. Otherwise, the beneficial influence of SMAT cannot be
realized.
6 Mg and Mg Alloys
Magnesium and its alloys are considered suitable for the development of degradable
implants due to their non-toxic nature, biodegradability and biocompatibility. The
dissolution of Mg is not likely to cause any adverse side effects. Since the specific
density and Young’s modulus of Mg are closer to bone it could reduce stress shielding
effect at the bone/implant interface. The use of Mg-based degradable implants solves
many of the problems associated with permanent implants such as restenosis, thrombosis and, inability to adapt to the growth and changes in human body, etc., and,
eliminate the need for a second surgery. However, the rapid corrosion of Mg and its
alloys in physiological environment undermines their mechanical integrity. Corrosion of Mg and its alloys results in the generation of a large volume of hydrogen gas
and increase the local pH of body fluid. Accumulation of the hydrogen bubbles near
the implant could lead to necrosis of tissues while the local increase in pH could lead
to alkaline poisoning. Alloying could improve mechanical integrity. Nevertheless, the
501
formed on plastically deformed Co28Cr6Mo alloy in SBF [90]. Hence, it is clear
that the ability of the treated alloy to form a stable passive film is responsible for the
observed improvement in corrosion resistance.
The higher R a of the Co28Cr6Mo alloy after SMAT causes a deleterious influence
on both the wear and corrosion resistance. Hence, to achieve better wear and corrosion
resistance, post-polishing of the treated alloy is warranted. Demangel et al. [84] have
explored polishing using a brush, fluid containing grinding media and polishing
cloth. Polishing using brushes removed only the superficially attached particles.
Polishing using fluid containing grinding media removed some material and provided
a reasonable surface finish. The use of a polishing cloth offered a relatively better
surface finish. In scratch testing, the polished samples show a 30–60% decrease in
penetration depth and among the polished ones the penetration depth is much lower
for the cloth polished sample. Tchana Nkonta et al. [19] have used a proprietary
biomedical polishing treatment of Co28Cr6Mo alloy after SMAT which enabled a
50% decrease in R a from 0.64 μm to 0.30 μm. The decrease in R a also enabled
an increase in corrosion resistance. Hence, it is evident that adopting a suitable
post-polishing treatment is likely to improve both the tribological and corrosion
performance of CoCrMo alloys subjected to SMAT. In addition, the mean depth,
extent of material removal and mean flatness of the finished surface should also
be considered during post-polishing treatment. According to Demangel et al. [84],
polishing using both brush and fluid could remove only a limited amount of material
(up to 2–12 μm). However, the mean flatness is not improved much after polishing. In
contrast, the mean flatness is significantly improved for cloth polishing. Nevertheless,
the extent of material removal is much higher (up to 34 μm). The extent of material
removal during post-polishing treatment should be in line with thickness of the
deformed layer after SMAT. Otherwise, the beneficial influence of SMAT cannot be
realized.
6 Mg and Mg Alloys
Magnesium and its alloys are considered suitable for the development of degradable
implants due to their non-toxic nature, biodegradability and biocompatibility. The
dissolution of Mg is not likely to cause any adverse side effects. Since the specific
density and Young’s modulus of Mg are closer to bone it could reduce stress shielding
effect at the bone/implant interface. The use of Mg-based degradable implants solves
many of the problems associated with permanent implants such as restenosis, thrombosis and, inability to adapt to the growth and changes in human body, etc., and,
eliminate the need for a second surgery. However, the rapid corrosion of Mg and its
alloys in physiological environment undermines their mechanical integrity. Corrosion of Mg and its alloys results in the generation of a large volume of hydrogen gas
and increase the local pH of body fluid. Accumulation of the hydrogen bubbles near
the implant could lead to necrosis of tissues while the local increase in pH could lead
to alkaline poisoning. Alloying could improve mechanical integrity. Nevertheless, the