500
T. Balusamy et al.
colour) is much pronounced in region II. The phase transformation is not limited to
region II rather it extends further below. In region III, hcp laths inside the fcc grains
are evident. EBSD patterns of the treated alloy have suggested that the transformation of fcc phase to hcp phase could occur even up to a depth of 150 μm from the top
surface. Obviously, a steady decrease in the amount of hcp phase from the surface
to the bulk is expected [85]. During SMAT, the low SFE of CoCrMo alloy is likely
to promote formation of stacking faults, grain refinement, a transformation of fcc
to hcp phase and formation of nanocrystalline structure [87]. The microstructure of
the subsurface could exert a strong influence on the wear and corrosion resistance of
the treated alloy. The evolution of subsurface microstructure as a function of applied
potential and its influence on the corrosion behavior of CoCrMo alloy in phosphatebuffered saline in presence of bovine serum albumin has been established by Wang
et al. [88].
SMAT of Co28Cr6Mo alloy using 3 mm Ø 100Cr6 alloy balls for 5, 15, and 25 min
has enabled an increase in hardness. The hardness profile indicates that the maximum
hardness value is reached at few microns from the top of the surface. The hardness
is decreased slowly until a depth of ~400 μm from the surface to the bulk [84].
The increment in the hardness of the Co28Cr6Mo alloy after SMAT is due to grain
refinement following plastic deformation. The increase in grain boundary density
could block the movement of dislocations and increase the hardness. SMAT could
also induce compressive residual stress due to the SPD. The maximum hardness value
observed at the subsurface further substantiates the influence of compressive residual
stress. Besides, strain-induced dislocations, twins, stacking faults and formation of
martensite phase could also contribute to the increase in hardness. Similar behavior
is also observed for other materials subjected to SP. Hence, it is clear the SMAT
is capable of inducing SPD of the Co28Cr6Mo alloy, and the grain refinement,
compressive residual stress and strain-induced dislocations are responsible for the
increase in hardness [84].
The random multidirectional impingement of the 3 mm Ø 100Cr6 alloy balls
increased the R a of the Co28Cr6Mo alloy after SMAT. After 15 min treatment, the
R a is increased and it ranges from 0.64 μm to 0.77 μm [19, 84]. It has been reported
earlier that scratches present on the femoral head could cause the counterpart to worn
out more [89]. Hence, the higher R a of the Co28Cr6Mo alloy after SMAT becomes
a major concern. Scratch testing performed under progressive load mode from 0 to
30 N indicates that the depth of penetration is much lower for Co28Cr6Mo alloy
subjected to SMAT when compared to the untreated alloy. This inference indicates
that SMAT is capable of increasing the wear resistance of Co28Cr6Mo alloy under
severe abrasive wear environment, which prevails under tribocorrosion conditions.
SMAT also improves the corrosion resistance of Co28Cr6Mo alloy in Ringer’s
solution [19]. The increase in R a after SMAT from 0.02 μm to 0.64 μm would
deleteriously influence the corrosion resistance. Nevertheless, the SPD induced by
SMAT facilitates the permeation of oxygen atoms and promotes the formation of
a passive oxide layer. The chemical composition of the passive oxide layer is also
considered to play a key role in improving the corrosion resistance. Based on XPS
analysis, cobalt oxide is identified as the main constituent of the passive oxide layer
T. Balusamy et al.
colour) is much pronounced in region II. The phase transformation is not limited to
region II rather it extends further below. In region III, hcp laths inside the fcc grains
are evident. EBSD patterns of the treated alloy have suggested that the transformation of fcc phase to hcp phase could occur even up to a depth of 150 μm from the top
surface. Obviously, a steady decrease in the amount of hcp phase from the surface
to the bulk is expected [85]. During SMAT, the low SFE of CoCrMo alloy is likely
to promote formation of stacking faults, grain refinement, a transformation of fcc
to hcp phase and formation of nanocrystalline structure [87]. The microstructure of
the subsurface could exert a strong influence on the wear and corrosion resistance of
the treated alloy. The evolution of subsurface microstructure as a function of applied
potential and its influence on the corrosion behavior of CoCrMo alloy in phosphatebuffered saline in presence of bovine serum albumin has been established by Wang
et al. [88].
SMAT of Co28Cr6Mo alloy using 3 mm Ø 100Cr6 alloy balls for 5, 15, and 25 min
has enabled an increase in hardness. The hardness profile indicates that the maximum
hardness value is reached at few microns from the top of the surface. The hardness
is decreased slowly until a depth of ~400 μm from the surface to the bulk [84].
The increment in the hardness of the Co28Cr6Mo alloy after SMAT is due to grain
refinement following plastic deformation. The increase in grain boundary density
could block the movement of dislocations and increase the hardness. SMAT could
also induce compressive residual stress due to the SPD. The maximum hardness value
observed at the subsurface further substantiates the influence of compressive residual
stress. Besides, strain-induced dislocations, twins, stacking faults and formation of
martensite phase could also contribute to the increase in hardness. Similar behavior
is also observed for other materials subjected to SP. Hence, it is clear the SMAT
is capable of inducing SPD of the Co28Cr6Mo alloy, and the grain refinement,
compressive residual stress and strain-induced dislocations are responsible for the
increase in hardness [84].
The random multidirectional impingement of the 3 mm Ø 100Cr6 alloy balls
increased the R a of the Co28Cr6Mo alloy after SMAT. After 15 min treatment, the
R a is increased and it ranges from 0.64 μm to 0.77 μm [19, 84]. It has been reported
earlier that scratches present on the femoral head could cause the counterpart to worn
out more [89]. Hence, the higher R a of the Co28Cr6Mo alloy after SMAT becomes
a major concern. Scratch testing performed under progressive load mode from 0 to
30 N indicates that the depth of penetration is much lower for Co28Cr6Mo alloy
subjected to SMAT when compared to the untreated alloy. This inference indicates
that SMAT is capable of increasing the wear resistance of Co28Cr6Mo alloy under
severe abrasive wear environment, which prevails under tribocorrosion conditions.
SMAT also improves the corrosion resistance of Co28Cr6Mo alloy in Ringer’s
solution [19]. The increase in R a after SMAT from 0.02 μm to 0.64 μm would
deleteriously influence the corrosion resistance. Nevertheless, the SPD induced by
SMAT facilitates the permeation of oxygen atoms and promotes the formation of
a passive oxide layer. The chemical composition of the passive oxide layer is also
considered to play a key role in improving the corrosion resistance. Based on XPS
analysis, cobalt oxide is identified as the main constituent of the passive oxide layer