Surface Nanostructuring of Metallic Materials …
491
aged at 650 °C for 6 h. However, for TLM alloy aged at 450 °C for 6 h, consisting
of 27% nano-sized α cobbles with higher strength, the extent of nanocrystallization
is relatively less. From these observations, one can draw out that the morphology
and volume fraction of the α phase as well as the strength of the alloy based on the
distribution of α cobbles/needles determine the nanocrystallization of the aged TLM
alloy by SMAT.
Since surface nanocrystallization of Ti and its alloys by SMAT is beneficial in
improving the hardness, mechanical properties, corrosion resistance, bioactivity and
cell growth, many duplexes or hybrid treatment approaches involving SMAT as a
pretreatment are explored. Zhang and Han [58] have used SMAT pretreatment for
anodizing of Ti. In their study, Ti is subjected to SMAT using 5 mm Ø zirconia
balls at 50 Hz for 60 min. Both untreated and SMAT Ti are anodized using glycol
containing 0.075 M NH 4 F and 0.02 vol. % H 2 O at 60 V for 15–240 min at room
temperature. Subsequently, the anodized Ti samples are annealed in Ar atmosphere
at 450 °C for 3 h. Surface nanocrystallization of Ti by SMAT fails to exhibit any
significant change in the morphology of the TiO 2 nanotubes as well as its phase
composition, both of them being amorphous TiO 2 . The thickness of the anodized
layer exhibits an obvious increase with an increase in anodizing time from 15 to
240 min. Nevertheless, irrespective of the treatment time, the anodized layer formed
on SMAT Ti is relatively thicker, which is further substantiated by a higher current
density recorded in the current-time transients. The formation of anodic oxide film
on Ti is governed by the electrochemical reaction and dictated by the field-aided
ionic transport of O
2− and Ti
4+ . The grain refinement, increase in grain boundary
density and high dislocation density induced by SPD during SMAT provide additional
pathways for the rapid diffusion of O
2− and promote the reaction between O
2− and
Ti
4+ . Hence, it is clear that surface nanocrystallization of Ti by SMAT is beneficial
for the growth of TiO 2 nanotubes.
The use of SMAT as a pretreatment for anodizing of Ti is also reported by Azadmanjiri et al. [57]. In their study, Ti is subjected to SMAT using 3 mm Ø zirconia
balls at 20 kHz for 20–120 min. Both the untreated and SMAT Ti are anodized using
1 M (NH 4 ) 2 SO 4 containing 0.5 wt% NH 4 F at 20 V for 2 h at room temperature.
SMAT increased the reactivity of Ti during anodizing, which results in the formation of TiO 2 nanotubes with a higher length (18 μm) and diameter (70 nm) when
compared to those formed on its coarse-grained untreated counterpart with a length
and diameter of 10 μm and 40 nm, respectively (Fig. 14). The high density of grain
boundaries and dislocations induced by SMAT increased the reaction rate and ion
diffusion coefficient during anodization. Hence, it is evident that SMAT can be used
as an effective pretreatment for the development of TiO 2 nanotubes with a suitable
length and diameter for drug delivery applications.
SMAT is used as a pretreatment for Ti to assist the formation of a crystalline
nanoporous titania by chemical oxidation and calcination [59]. In this study, Ti
is subjected to SMAT using 8 mm Ø stainless steel balls at 50 Hz for 60 min.
The untreated and SMAT Ti are immersed in 30 wt% H 2 O 2 at 25 °C for 24 h,
dried at 40 °C for 6 h and calcined in an air atmosphere at 600 °C for 1 h. The
grain refinement, increase in grain boundary density and high dislocation density
491
aged at 650 °C for 6 h. However, for TLM alloy aged at 450 °C for 6 h, consisting
of 27% nano-sized α cobbles with higher strength, the extent of nanocrystallization
is relatively less. From these observations, one can draw out that the morphology
and volume fraction of the α phase as well as the strength of the alloy based on the
distribution of α cobbles/needles determine the nanocrystallization of the aged TLM
alloy by SMAT.
Since surface nanocrystallization of Ti and its alloys by SMAT is beneficial in
improving the hardness, mechanical properties, corrosion resistance, bioactivity and
cell growth, many duplexes or hybrid treatment approaches involving SMAT as a
pretreatment are explored. Zhang and Han [58] have used SMAT pretreatment for
anodizing of Ti. In their study, Ti is subjected to SMAT using 5 mm Ø zirconia
balls at 50 Hz for 60 min. Both untreated and SMAT Ti are anodized using glycol
containing 0.075 M NH 4 F and 0.02 vol. % H 2 O at 60 V for 15–240 min at room
temperature. Subsequently, the anodized Ti samples are annealed in Ar atmosphere
at 450 °C for 3 h. Surface nanocrystallization of Ti by SMAT fails to exhibit any
significant change in the morphology of the TiO 2 nanotubes as well as its phase
composition, both of them being amorphous TiO 2 . The thickness of the anodized
layer exhibits an obvious increase with an increase in anodizing time from 15 to
240 min. Nevertheless, irrespective of the treatment time, the anodized layer formed
on SMAT Ti is relatively thicker, which is further substantiated by a higher current
density recorded in the current-time transients. The formation of anodic oxide film
on Ti is governed by the electrochemical reaction and dictated by the field-aided
ionic transport of O
2− and Ti
4+ . The grain refinement, increase in grain boundary
density and high dislocation density induced by SPD during SMAT provide additional
pathways for the rapid diffusion of O
2− and promote the reaction between O
2− and
Ti
4+ . Hence, it is clear that surface nanocrystallization of Ti by SMAT is beneficial
for the growth of TiO 2 nanotubes.
The use of SMAT as a pretreatment for anodizing of Ti is also reported by Azadmanjiri et al. [57]. In their study, Ti is subjected to SMAT using 3 mm Ø zirconia
balls at 20 kHz for 20–120 min. Both the untreated and SMAT Ti are anodized using
1 M (NH 4 ) 2 SO 4 containing 0.5 wt% NH 4 F at 20 V for 2 h at room temperature.
SMAT increased the reactivity of Ti during anodizing, which results in the formation of TiO 2 nanotubes with a higher length (18 μm) and diameter (70 nm) when
compared to those formed on its coarse-grained untreated counterpart with a length
and diameter of 10 μm and 40 nm, respectively (Fig. 14). The high density of grain
boundaries and dislocations induced by SMAT increased the reaction rate and ion
diffusion coefficient during anodization. Hence, it is evident that SMAT can be used
as an effective pretreatment for the development of TiO 2 nanotubes with a suitable
length and diameter for drug delivery applications.
SMAT is used as a pretreatment for Ti to assist the formation of a crystalline
nanoporous titania by chemical oxidation and calcination [59]. In this study, Ti
is subjected to SMAT using 8 mm Ø stainless steel balls at 50 Hz for 60 min.
The untreated and SMAT Ti are immersed in 30 wt% H 2 O 2 at 25 °C for 24 h,
dried at 40 °C for 6 h and calcined in an air atmosphere at 600 °C for 1 h. The
grain refinement, increase in grain boundary density and high dislocation density