Surface Nanostructuring of Metallic Materials …
505
depth until the end of the deformation depth. SMAT improved tribological behavior.
The refinement in grain size and a higher hardness are considered responsible for the
improvement in tribological behavior after SMAT. SMAT increased the corrosion
resistance of Ti and Ti alloys, NiTi alloy, and CoCrMo alloy in SBF. The decrease
in grain size, increase in grain boundary density, dilution of segregated alloying
elements at grain boundaries and an increase in the tendency of the treated material
to form an intact passive oxide layer with a higher thickness are responsible for
the observed improvement in corrosion resistance of Ti and Ti alloys, NiTi alloy
and CoCrMo alloy. The higher corrosion resistance of materials after SMAT would
decrease the extent of leaching of the toxic metal ions (aluminum and vanadium
ions from Ti6Al4V alloy, nickel ions from NiTi alloy and, cobalt and chromium ions
from CoCrMo alloy) and improve their biocompatibility. An improvement in both
the tribological properties as well as corrosion resistance would offer improvement
in the performance of CoCrMo alloy under conditions of tribocorrosion experienced
by hip and knee implants. An improvement in corrosion resistance of stainless steel
after SMAT is largely a function of the treatment conditions employed. An improvement in corrosion resistance is observed only for 304 stainless steels treated using
2 mm Ø balls. In contrast, SMAT using 5 and 8 mm Ø balls has led to a decrease
in corrosion resistance. The increase in surface roughness, a transformation of the
austenite to α’-martensite phase, higher extent of deformation and presence of a
larger number of defects/dislocations are responsible for the observed decrease in
corrosion resistance of 304 SS treated using 5 and 8 mm Ø balls when compared to
the untreated one in Ringer’s solution as well as 0.6 M NaCl. The grain refinement,
surface nanocrystallization, selective dissolution of Fe and enrichment of Cr at the
surface have increased the number of nucleation sites and promotes the formation
of an intact and highly protective passive oxide layer on 304 stainless steel treated
using 2 mm Ø balls when compared to the untreated one, in Ringer’s solution as
well as 0.6 M NaCl. The corrosion resistance is drastically affected for Mg and Mg
alloy after SMAT. The marginal improvement in corrosion resistance of AZ91D Mg
alloy after SMAT using 2 mm Ø balls is due to dissolution of β-Mg 17 Al 12 phase into
the matrix, which nullifies the formation of the galvanic cells. Hence, SMAT can be
considered as a useful surface treatment to improve the performance of Ti and Ti
alloys, stainless steels, NiTi alloy and CoCrMo alloy for biomedical applications.
8 Future Perspectives
Increase in surface roughness after SMAT deleteriously influence the corrosion resistance although it is beneficial for better wetting, an increase in surface energy, and
imparting a higher degree of hydrophilicity. Adapting a post-polishing stage can be
recommended to improve their surface finish. This would be particularly beneficial
for stainless steel and CoCrMo alloy implants. Electropolishing is a viable option
to achieve this. Large pulsed electron beam (LPEB) irradiation can be explored to
improve the surface finish. However, necessary precautions should be followed as
505
depth until the end of the deformation depth. SMAT improved tribological behavior.
The refinement in grain size and a higher hardness are considered responsible for the
improvement in tribological behavior after SMAT. SMAT increased the corrosion
resistance of Ti and Ti alloys, NiTi alloy, and CoCrMo alloy in SBF. The decrease
in grain size, increase in grain boundary density, dilution of segregated alloying
elements at grain boundaries and an increase in the tendency of the treated material
to form an intact passive oxide layer with a higher thickness are responsible for
the observed improvement in corrosion resistance of Ti and Ti alloys, NiTi alloy
and CoCrMo alloy. The higher corrosion resistance of materials after SMAT would
decrease the extent of leaching of the toxic metal ions (aluminum and vanadium
ions from Ti6Al4V alloy, nickel ions from NiTi alloy and, cobalt and chromium ions
from CoCrMo alloy) and improve their biocompatibility. An improvement in both
the tribological properties as well as corrosion resistance would offer improvement
in the performance of CoCrMo alloy under conditions of tribocorrosion experienced
by hip and knee implants. An improvement in corrosion resistance of stainless steel
after SMAT is largely a function of the treatment conditions employed. An improvement in corrosion resistance is observed only for 304 stainless steels treated using
2 mm Ø balls. In contrast, SMAT using 5 and 8 mm Ø balls has led to a decrease
in corrosion resistance. The increase in surface roughness, a transformation of the
austenite to α’-martensite phase, higher extent of deformation and presence of a
larger number of defects/dislocations are responsible for the observed decrease in
corrosion resistance of 304 SS treated using 5 and 8 mm Ø balls when compared to
the untreated one in Ringer’s solution as well as 0.6 M NaCl. The grain refinement,
surface nanocrystallization, selective dissolution of Fe and enrichment of Cr at the
surface have increased the number of nucleation sites and promotes the formation
of an intact and highly protective passive oxide layer on 304 stainless steel treated
using 2 mm Ø balls when compared to the untreated one, in Ringer’s solution as
well as 0.6 M NaCl. The corrosion resistance is drastically affected for Mg and Mg
alloy after SMAT. The marginal improvement in corrosion resistance of AZ91D Mg
alloy after SMAT using 2 mm Ø balls is due to dissolution of β-Mg 17 Al 12 phase into
the matrix, which nullifies the formation of the galvanic cells. Hence, SMAT can be
considered as a useful surface treatment to improve the performance of Ti and Ti
alloys, stainless steels, NiTi alloy and CoCrMo alloy for biomedical applications.
8 Future Perspectives
Increase in surface roughness after SMAT deleteriously influence the corrosion resistance although it is beneficial for better wetting, an increase in surface energy, and
imparting a higher degree of hydrophilicity. Adapting a post-polishing stage can be
recommended to improve their surface finish. This would be particularly beneficial
for stainless steel and CoCrMo alloy implants. Electropolishing is a viable option
to achieve this. Large pulsed electron beam (LPEB) irradiation can be explored to
improve the surface finish. However, necessary precautions should be followed as