Surface Nanostructuring of Metallic Materials …
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higher R a are likely to cause a deleterious influence on the corrosion resistance. The
microstructure of the SMAT NiTi indicates the formation of a nanocrystalline surface
with a partial amorphous structure. During SMAT, the SPD promotes permeation of
oxygen atoms, which is supported by the oxygen concentration of 26.7 at. % at the
top surface of the treated alloy. The increase in temperature facilitates the formation
of a passive oxide layer, mainly consisting of titanium oxide. Mott-Schottky analysis
reveals that the passive film formed on nickel is mainly a p-type semiconductor. In
contrast, the passive film formed on titanium and NiTi alloy exhibit the characteristics
of an n-type semiconductor. Based on the positive slope values observed in Mott–
Schottky plots, Jinlong et al. [75] have identified TiO 2 as the major constituent of
the passive film formed on NiTi alloy in 0.5 M H 2 SO 4 , 0.2 M NaCl and borate buffer
solution. Hence, it is clear that the nanocrystalline and amorphous surface structure
of the alloy and formation of a passive layer, which is rich in TiO 2 is responsible for
the observed improvement in corrosion resistance of SMAT NiTi alloy in 0.9% NaCl.
The enrichment of titanium oxide in the passive oxide layer and the higher corrosion
resistance of the SMAT NiTi alloy are likely to decrease the extent of leaching of the
toxic nickel ions and contribute to the improvement in the biocompatibility of the
alloy.
5 CoCrMo Alloy
CoCrMo alloys, both in the as-cast as well as wrought form, have been used as
orthopedic implant materials. They possess high strength, good mechanical properties, excellent wear resistance, good corrosion resistance, better biocompatibility,
high fatigue resistance and low elastic modulus. Due to these key attributes, CoCrMo
alloys are considered as the material of choice for the fabrication of metal-on-metal
total hip replacements [76]. CoCrMo alloy tends to form a spontaneous oxide film
(~4 nm thick) on its surface, primarily consisting of Cr 2 O 3 oxide along with minor
quantities of Co and Mo oxides, which is responsible for its excellent corrosion
resistance and better biocompatibility. CoCrMo alloy based hip joints experiences
a combination of both wear and corrosion, commonly referred to as tribocorrosion
[43, 76]. Microabrasion-corrosion has been identified as a major mode of failure of
CoCrMo alloy [77, 78].
Fortunately, the metal-on-metal (MOM) tribological contacts are more wearresistant when compared to metal-on-polyethylene (MOP) tribological contacts [79].
However, in MOM tribological contacts, the wear process breaks the naturally formed
thin oxide film on the CoCrMo alloy, resulting in the generation of metal debris, the
accumulation of which could cause inflammation in the surrounding tissues. It has
been reported that under conditions of infection, the concentration of protein in
synovial fluid is likely to increase to a large extent. The local temperature is also
expected to increase up to 50 °C. These conditions could promote localized breakdown of the passive oxide layer [80]. Since the damaged area is much smaller than the
497
higher R a are likely to cause a deleterious influence on the corrosion resistance. The
microstructure of the SMAT NiTi indicates the formation of a nanocrystalline surface
with a partial amorphous structure. During SMAT, the SPD promotes permeation of
oxygen atoms, which is supported by the oxygen concentration of 26.7 at. % at the
top surface of the treated alloy. The increase in temperature facilitates the formation
of a passive oxide layer, mainly consisting of titanium oxide. Mott-Schottky analysis
reveals that the passive film formed on nickel is mainly a p-type semiconductor. In
contrast, the passive film formed on titanium and NiTi alloy exhibit the characteristics
of an n-type semiconductor. Based on the positive slope values observed in Mott–
Schottky plots, Jinlong et al. [75] have identified TiO 2 as the major constituent of
the passive film formed on NiTi alloy in 0.5 M H 2 SO 4 , 0.2 M NaCl and borate buffer
solution. Hence, it is clear that the nanocrystalline and amorphous surface structure
of the alloy and formation of a passive layer, which is rich in TiO 2 is responsible for
the observed improvement in corrosion resistance of SMAT NiTi alloy in 0.9% NaCl.
The enrichment of titanium oxide in the passive oxide layer and the higher corrosion
resistance of the SMAT NiTi alloy are likely to decrease the extent of leaching of the
toxic nickel ions and contribute to the improvement in the biocompatibility of the
alloy.
5 CoCrMo Alloy
CoCrMo alloys, both in the as-cast as well as wrought form, have been used as
orthopedic implant materials. They possess high strength, good mechanical properties, excellent wear resistance, good corrosion resistance, better biocompatibility,
high fatigue resistance and low elastic modulus. Due to these key attributes, CoCrMo
alloys are considered as the material of choice for the fabrication of metal-on-metal
total hip replacements [76]. CoCrMo alloy tends to form a spontaneous oxide film
(~4 nm thick) on its surface, primarily consisting of Cr 2 O 3 oxide along with minor
quantities of Co and Mo oxides, which is responsible for its excellent corrosion
resistance and better biocompatibility. CoCrMo alloy based hip joints experiences
a combination of both wear and corrosion, commonly referred to as tribocorrosion
[43, 76]. Microabrasion-corrosion has been identified as a major mode of failure of
CoCrMo alloy [77, 78].
Fortunately, the metal-on-metal (MOM) tribological contacts are more wearresistant when compared to metal-on-polyethylene (MOP) tribological contacts [79].
However, in MOM tribological contacts, the wear process breaks the naturally formed
thin oxide film on the CoCrMo alloy, resulting in the generation of metal debris, the
accumulation of which could cause inflammation in the surrounding tissues. It has
been reported that under conditions of infection, the concentration of protein in
synovial fluid is likely to increase to a large extent. The local temperature is also
expected to increase up to 50 °C. These conditions could promote localized breakdown of the passive oxide layer [80]. Since the damaged area is much smaller than the