Surface Nanostructuring of Metallic Materials …
493
lower mean coefficient of friction when compared to that of CO Ti. The oxide layer
with rutile phase on SO Ti improves biocompatibility. The higher surface energy,
presence of a crystalline rutile layer promotes cell adhesion and spreading on SO
Ti. SMAT is also used as a pretreatment for diffusion coatings such as boriding and
nitriding [30, 63–65] as well as for phosphate conversion coatings [66, 67].
4 NiTi Alloy
Nickel-titanium shape memory alloys (NiTi SMAs) have received considerable attention due to their superior shape memory properties, superelasticity, high damping
capacity, high fatigue resistance, good biocompatibility and corrosion resistance.
NiTi alloys are widely used for biomedical applications, particularly for bone fraction fixation plates, stents, orthodontic wires, spine correction rods and staples. In
spite of its good biocompatibility, the major concern with the use of NiTi alloys for
biomedical applications is leaching of nickel ions into the body fluid, which could
cause inflammation of the surrounding tissues and induce allergic reactions [68].
Since NiTi based biomedical devices are likely to experience a high cycle loading
and stress, it is expected that they should possess excellent strength, fatigue resistance,
corrosion resistance and wear resistance. Surface modification is a viable option to
impart the desired characteristics of the NiTi alloys. A variety of surface treatment
methods are explored to improve the performance of NiTi alloys. These include
simple surface treatments such as polishing, electropolishing, surface passivation by
chemical and thermal oxidation, nitriding and deposition of titanium oxide, titanium
nitride, calcium phosphate, polymeric and bioactive coatings [69]. Modification of
the surface of the Ni-Ti alloy tends to improve the corrosion and wear resistance,
which would decrease the extent of leaching of the toxic nickel ions from the alloy
into the human body environment.
SMAT is a S
2 PD method that facilitates surface nanocrystallization without
changing the chemical composition of the material. The multidirectional impingement of the balls at a very high strain rate enables refinement of the coarse grains
to nano-sized grains at the surface along with the formation of a graded layer structure. The compressive residual stress induced during SMAT and the high density of
dislocations helped to improve their hardness, mechanical properties and tribological
behavior. The increase in grain boundary density also promotes the formation of a
passive oxide layer which improves the corrosion resistance. The influence of SMAT
on the microstructure, grain size, surface roughness, hardness, tribological behavior
and corrosion resistance of NiTi alloy is addressed in the following paragraphs.
The effect of SMAT on the microstructure, phase content, hardness, wear resistance and corrosion resistance of NiTi alloys is extensively studied by Hu et al. [11,
70–73]. The grain size of the coarse-grained NiTi alloy ranges from 40 to 80 μm.
After SMAT using 2 mm Ø stainless steel balls for 60 min, the NiTi alloy is severely
deformed with a deformation thickness of ~200 μm [11].
493
lower mean coefficient of friction when compared to that of CO Ti. The oxide layer
with rutile phase on SO Ti improves biocompatibility. The higher surface energy,
presence of a crystalline rutile layer promotes cell adhesion and spreading on SO
Ti. SMAT is also used as a pretreatment for diffusion coatings such as boriding and
nitriding [30, 63–65] as well as for phosphate conversion coatings [66, 67].
4 NiTi Alloy
Nickel-titanium shape memory alloys (NiTi SMAs) have received considerable attention due to their superior shape memory properties, superelasticity, high damping
capacity, high fatigue resistance, good biocompatibility and corrosion resistance.
NiTi alloys are widely used for biomedical applications, particularly for bone fraction fixation plates, stents, orthodontic wires, spine correction rods and staples. In
spite of its good biocompatibility, the major concern with the use of NiTi alloys for
biomedical applications is leaching of nickel ions into the body fluid, which could
cause inflammation of the surrounding tissues and induce allergic reactions [68].
Since NiTi based biomedical devices are likely to experience a high cycle loading
and stress, it is expected that they should possess excellent strength, fatigue resistance,
corrosion resistance and wear resistance. Surface modification is a viable option to
impart the desired characteristics of the NiTi alloys. A variety of surface treatment
methods are explored to improve the performance of NiTi alloys. These include
simple surface treatments such as polishing, electropolishing, surface passivation by
chemical and thermal oxidation, nitriding and deposition of titanium oxide, titanium
nitride, calcium phosphate, polymeric and bioactive coatings [69]. Modification of
the surface of the Ni-Ti alloy tends to improve the corrosion and wear resistance,
which would decrease the extent of leaching of the toxic nickel ions from the alloy
into the human body environment.
SMAT is a S
2 PD method that facilitates surface nanocrystallization without
changing the chemical composition of the material. The multidirectional impingement of the balls at a very high strain rate enables refinement of the coarse grains
to nano-sized grains at the surface along with the formation of a graded layer structure. The compressive residual stress induced during SMAT and the high density of
dislocations helped to improve their hardness, mechanical properties and tribological
behavior. The increase in grain boundary density also promotes the formation of a
passive oxide layer which improves the corrosion resistance. The influence of SMAT
on the microstructure, grain size, surface roughness, hardness, tribological behavior
and corrosion resistance of NiTi alloy is addressed in the following paragraphs.
The effect of SMAT on the microstructure, phase content, hardness, wear resistance and corrosion resistance of NiTi alloys is extensively studied by Hu et al. [11,
70–73]. The grain size of the coarse-grained NiTi alloy ranges from 40 to 80 μm.
After SMAT using 2 mm Ø stainless steel balls for 60 min, the NiTi alloy is severely
deformed with a deformation thickness of ~200 μm [11].