496
T. Balusamy et al.
The wear resistance of untreated and SMAT NiTi alloy is evaluated by Hu et al.
[11] under dry sliding conditions. At a normal applied load of 7 N, untreated coarsegrained NiTi alloy exhibits a rapid increase in friction coefficient within a few
seconds, which eventually reached a steady state. In contrast, the transient time for
the initial sharp increase in friction coefficient is ~2 min for the treated NiTi alloy.
The difference is due to the higher R a of the SMAT NiTi alloy when compared to the
untreated alloy. When the applied load is <9 N, SMAT NiTi exhibits a lower steadystate friction coefficient when compared to its coarse-grained counterpart. However,
when the load is higher than >9 N, the difference in friction coefficient between the
untreated and SMAT NiTi alloy becomes narrow. The higher hardness of the SMAT
NiTi alloy achieved by grain refinement following SPD and a decrease in the contact
area has enabled a lower friction coefficient for the treated alloy. Irrespective of the
applied normal load in the range of 5–15 N, both the wear depth and wear volume
of SMAT NiTi are lower than that of the coarse-grained untreated alloy.
The mechanism of wear appears to be similar for both untreated and SMAT
NiTi alloy, which is governed by delamination wear. However, for any given load,
the extent of delamination is much more severe on the untreated alloy. The higher
hardness of SMAT NiTi alloy has enabled a decrease in the degree of delamination.
For any given load, more debris is found to be present on the worn surface of the
SMAT NiTi alloy than its untreated counterpart. The higher hardness of the SMAT
NiTi alloy makes the fragments brittle. When these fragments get trapped between
the mating surfaces (SMAT NiTi alloy and WC ball), their brittle nature makes them
break further apart into fine debris. Being highly reactive, these particles could be
easily oxidized. The wear debris could provide either a lubricative effect, reducing the
friction coefficient or they might play a decisive role if the particles become abrasive.
The lower difference in friction coefficient between the untreated and SMAT NiTi
alloy from 11 to 15 N is due to the generation of a larger amount of debris from the
treated alloy.
The corrosion behavior of SMAT NiTi alloy in 0.9% NaCl is evaluated by Hu
et al. [70]. Untreated coarse-grained NiTi alloy exhibits a typical active-passive
transition in the anodic branch of the polarization curve. In contrast, this activepassive transition is absent for SMAT NiTi alloy. The grain boundaries and triple
junctions formed during SMAT of NiTi alloy are highly active and during corrosion,
they tend to dissolve much faster and promote nucleation of a passive oxide layer.
The fluctuations observed in the OCP-time curve of SMAT NiTi alloy is due to the
dissolution-formation of passive film. When compared to the untreated NiTi alloy,
the E corr exhibits a positive shift from +58 mV vs. SCE to +206 mV vs. SCE and a
decrease in i corr from 4 × 10
−8 A/cm
2 to 4 × 10
−9 A/m
2 for the SMAT NiTi alloy.
The |Z| at 0.1 Hz derived from the Bode impedance plots is much higher for SMAT
NiTi alloy, implying the formation of a thicker passive oxide layer.
NiTi alloy subjected to SMAT offered a better corrosion resistance in 0.9% NaCl
when compared to its untreated coarse-grained counterpart. The microstructure,
surface roughness, tendency to form a passive oxide layer and composition of the
passive oxide layer determine the corrosion protection ability of materials subjected
to SMAT. The R a of NiTi alloy is increased after SMAT. In general, surfaces with a
T. Balusamy et al.
The wear resistance of untreated and SMAT NiTi alloy is evaluated by Hu et al.
[11] under dry sliding conditions. At a normal applied load of 7 N, untreated coarsegrained NiTi alloy exhibits a rapid increase in friction coefficient within a few
seconds, which eventually reached a steady state. In contrast, the transient time for
the initial sharp increase in friction coefficient is ~2 min for the treated NiTi alloy.
The difference is due to the higher R a of the SMAT NiTi alloy when compared to the
untreated alloy. When the applied load is <9 N, SMAT NiTi exhibits a lower steadystate friction coefficient when compared to its coarse-grained counterpart. However,
when the load is higher than >9 N, the difference in friction coefficient between the
untreated and SMAT NiTi alloy becomes narrow. The higher hardness of the SMAT
NiTi alloy achieved by grain refinement following SPD and a decrease in the contact
area has enabled a lower friction coefficient for the treated alloy. Irrespective of the
applied normal load in the range of 5–15 N, both the wear depth and wear volume
of SMAT NiTi are lower than that of the coarse-grained untreated alloy.
The mechanism of wear appears to be similar for both untreated and SMAT
NiTi alloy, which is governed by delamination wear. However, for any given load,
the extent of delamination is much more severe on the untreated alloy. The higher
hardness of SMAT NiTi alloy has enabled a decrease in the degree of delamination.
For any given load, more debris is found to be present on the worn surface of the
SMAT NiTi alloy than its untreated counterpart. The higher hardness of the SMAT
NiTi alloy makes the fragments brittle. When these fragments get trapped between
the mating surfaces (SMAT NiTi alloy and WC ball), their brittle nature makes them
break further apart into fine debris. Being highly reactive, these particles could be
easily oxidized. The wear debris could provide either a lubricative effect, reducing the
friction coefficient or they might play a decisive role if the particles become abrasive.
The lower difference in friction coefficient between the untreated and SMAT NiTi
alloy from 11 to 15 N is due to the generation of a larger amount of debris from the
treated alloy.
The corrosion behavior of SMAT NiTi alloy in 0.9% NaCl is evaluated by Hu
et al. [70]. Untreated coarse-grained NiTi alloy exhibits a typical active-passive
transition in the anodic branch of the polarization curve. In contrast, this activepassive transition is absent for SMAT NiTi alloy. The grain boundaries and triple
junctions formed during SMAT of NiTi alloy are highly active and during corrosion,
they tend to dissolve much faster and promote nucleation of a passive oxide layer.
The fluctuations observed in the OCP-time curve of SMAT NiTi alloy is due to the
dissolution-formation of passive film. When compared to the untreated NiTi alloy,
the E corr exhibits a positive shift from +58 mV vs. SCE to +206 mV vs. SCE and a
decrease in i corr from 4 × 10
−8 A/cm
2 to 4 × 10
−9 A/m
2 for the SMAT NiTi alloy.
The |Z| at 0.1 Hz derived from the Bode impedance plots is much higher for SMAT
NiTi alloy, implying the formation of a thicker passive oxide layer.
NiTi alloy subjected to SMAT offered a better corrosion resistance in 0.9% NaCl
when compared to its untreated coarse-grained counterpart. The microstructure,
surface roughness, tendency to form a passive oxide layer and composition of the
passive oxide layer determine the corrosion protection ability of materials subjected
to SMAT. The R a of NiTi alloy is increased after SMAT. In general, surfaces with a