498
T. Balusamy et al.
remaining areas, the formation of galvanic cells is promoted, leading to the preferential dissolution of metal at the damaged area [80]. The corrosion of freshly exposed
alloy surface (after removal of the thin oxide film) leads to leaching of highly toxic
Co and Cr ions into the body fluid. Hence, the generation of metal debris due to wear,
the inflammation of surrounding tissues and leaching of highly toxic Co and Cr ions
into the body fluid becomes a major concern in using CoCrMo alloy for biomedical
applications [76]. Aseptic loosening of the hip implant is a major consequence of
tribocorrosion.
Improving the performance of CoCrMo alloy based hip implants under tribocorrosion conditions can be approached by several means. Thickening of the oxide
layer either by acid passivation or oxygen ion implantation is one of the options.
Formation of a surface coating is another option. However, adhesion of the coated
layer, tensile residual stress and delamination of the coating are the major limitations of many coatings. Deposition of a thin (3 μm thick), uniform and dense TiN
coating with a columnar structure by physical vapor deposition (PVD) at 550 °C
for 6 h is effective in preventing the release of Co and Cr ions from CoCrMo alloy
[81]. Nevertheless, unlike the as-polished CoCrMo alloy, the PVD TiN coated alloy
fails to exhibit bioactivity. PVD coatings, in general, possess a higher hardness and
stiffness. Hence, the compatibility between the PVD coating and CoCrMo alloy
becomes a major problem in high-stress contacts [79]. Plasma nitriding drastically
affected the wear resistance and corrosion resistance of CoCrMo alloy. The duplex
treatment involving nitriding as the first stage treatment followed by PVD of CrN
and (TiN/CrN) × 3 as the second stage treatment is also found to be ineffective in
offering a better wear resistance. PVD (TiN/CrN) × 3 multilayer coating has been
suggested as an alternative surface modification method to achieve a better wear
resistance for CoCrMo alloy [79]. Nitrogen ion implantation with 100 keV at a dose
of 4 × 10
17 N
+ /cm
2 is found to be effective in reducing the friction coefficient of
CoCrMo alloy from 0.35 to 0.15 under tribocorrosion conditions [82]. Much remains
to be explored on the surface modification of CoCrMo alloy to impart the desired
characteristics suitable for hip implant applications.
CoCrMo alloy subjected to mechanical polishing using successive grades (P240,
P400, P800 and P1200) silicon carbide coated abrasive papers, followed by successive fine polishing using a 6 μm and 1 μm diamond polishing compound exhibited
formation of ε-martensite and mechanical twins. However, the mechanically polished
(MP) surface has enabled the formation of a nanocrystalline layer during sliding wear
through high strain shear process and the prior formation of a martensite structure
and mechanical twins that help in achieving the nanocrystalline layer [83]. The extensive deformation, formation of ε-martensite, mechanical twins, and high dislocation
density together with the formation of nanocrystalline layer has offered a better wear
resistance for MP CoCrMo alloy. A decrease in the specific wear rate of the mechanically polished surface with an increase in normal load points out the advantage of
forming a nanocrystalline layer [83].
SMAT is a S
2 PD technique. The plastic deformation induced by SMAT leads to
refinement of coarse-grained grains down to nanosized grains. The ability to generate
a graded layer structure is another beneficial attribute of SMAT. The influence of
T. Balusamy et al.
remaining areas, the formation of galvanic cells is promoted, leading to the preferential dissolution of metal at the damaged area [80]. The corrosion of freshly exposed
alloy surface (after removal of the thin oxide film) leads to leaching of highly toxic
Co and Cr ions into the body fluid. Hence, the generation of metal debris due to wear,
the inflammation of surrounding tissues and leaching of highly toxic Co and Cr ions
into the body fluid becomes a major concern in using CoCrMo alloy for biomedical
applications [76]. Aseptic loosening of the hip implant is a major consequence of
tribocorrosion.
Improving the performance of CoCrMo alloy based hip implants under tribocorrosion conditions can be approached by several means. Thickening of the oxide
layer either by acid passivation or oxygen ion implantation is one of the options.
Formation of a surface coating is another option. However, adhesion of the coated
layer, tensile residual stress and delamination of the coating are the major limitations of many coatings. Deposition of a thin (3 μm thick), uniform and dense TiN
coating with a columnar structure by physical vapor deposition (PVD) at 550 °C
for 6 h is effective in preventing the release of Co and Cr ions from CoCrMo alloy
[81]. Nevertheless, unlike the as-polished CoCrMo alloy, the PVD TiN coated alloy
fails to exhibit bioactivity. PVD coatings, in general, possess a higher hardness and
stiffness. Hence, the compatibility between the PVD coating and CoCrMo alloy
becomes a major problem in high-stress contacts [79]. Plasma nitriding drastically
affected the wear resistance and corrosion resistance of CoCrMo alloy. The duplex
treatment involving nitriding as the first stage treatment followed by PVD of CrN
and (TiN/CrN) × 3 as the second stage treatment is also found to be ineffective in
offering a better wear resistance. PVD (TiN/CrN) × 3 multilayer coating has been
suggested as an alternative surface modification method to achieve a better wear
resistance for CoCrMo alloy [79]. Nitrogen ion implantation with 100 keV at a dose
of 4 × 10
17 N
+ /cm
2 is found to be effective in reducing the friction coefficient of
CoCrMo alloy from 0.35 to 0.15 under tribocorrosion conditions [82]. Much remains
to be explored on the surface modification of CoCrMo alloy to impart the desired
characteristics suitable for hip implant applications.
CoCrMo alloy subjected to mechanical polishing using successive grades (P240,
P400, P800 and P1200) silicon carbide coated abrasive papers, followed by successive fine polishing using a 6 μm and 1 μm diamond polishing compound exhibited
formation of ε-martensite and mechanical twins. However, the mechanically polished
(MP) surface has enabled the formation of a nanocrystalline layer during sliding wear
through high strain shear process and the prior formation of a martensite structure
and mechanical twins that help in achieving the nanocrystalline layer [83]. The extensive deformation, formation of ε-martensite, mechanical twins, and high dislocation
density together with the formation of nanocrystalline layer has offered a better wear
resistance for MP CoCrMo alloy. A decrease in the specific wear rate of the mechanically polished surface with an increase in normal load points out the advantage of
forming a nanocrystalline layer [83].
SMAT is a S
2 PD technique. The plastic deformation induced by SMAT leads to
refinement of coarse-grained grains down to nanosized grains. The ability to generate
a graded layer structure is another beneficial attribute of SMAT. The influence of