1. No category

Sciknow Publications Ltd.
JMDS 2015, 2(1):1-10
DOI: 10.12966/jmds.06.01.2015
Journal of Manufacturing and Design Science
©Attribution 3.0 Unported (CC BY 3.0)
Micromachining and Electrochemical Nanopolishing of Titanium
Microchannels
Wayne Hung1,*, Dakota Brock1, Han Xiao2, Paul Lomeli3, and Everardo Granda4
1
Texas A&M University, College Station, Texas, USA
Lynntech, College Station, Texas, USA
3
Keysight Technologies, Santa Rosa, California, USA
4
Comimsa, Saltillo, Mexico
2
*Corresponding author (Email: [email protected])
Abstract - A hybrid technique to fabricate microchannels on pure titanium sheet was developed. Microdrilling and micromilling
with coated tools were utilized for bulk material removal and generating microchannels with different profiles and aspect ratios;
the machined microchannels were then electrochemically polished to remove burrs and other surface defects. Mirror-like
microchannels were resulted from polishing in an alcohol-based electrolyte at two-level current densities. The differences in
crystalline orientations, etching rates on each grain, and impurities in grain boundaries contributed to variance in surface finish
data; the average surface finish of polished specimens was about 300 nm on a large polycrystalline surface, and few nanometers
within a single grain.
Keywords – Microchannel, Electrochemical polishing, Titanium, Micromilling, Microdrilling, Surface finish.
1. Introduction
Titanium and alloys are noble materials with many
exceptional properties. The specific strength of commercially
pure alpha titanium is 122 MPa.cm3/g that is double the
specific strengths of 304, 316, and 316L stainless steels.
When alloying with aluminum and vanadium, the specific
strength of Ti 6Al 4V alloy is even quadruple those of
stainless steels. Titanium is also biocompatible and has
excellent chemical resistance due to its robust protective
oxide on the surface. For these reasons, titanium and its alloys
are used for many applications in aerospace, transportation,
medical, dental, pharmaceutical, chemical, petroleum, and
marine industries. The unique properties of titanium also lend
itself in sensor and microsensor applications. Components of
some microsensors and microfluidic devices require
microscale features and three-dimensional surfaces with
submicron finish. Fabrication of such components with high
aspect ratio microfeatures is still an engineering challenge.
Chemical etching and electrochemical machining produces
isotropic features, the expensive reactive ion etching
technique is extremely slow yet having re-depositing issue,
energetic beams such as laser, focused ion beam, or electron
beam can produce the shape but not the required surface finish.
It is proposed that high aspect ratio features on titanium can
first be removed by traditional machining, and then the
machined surfaces can be polished in a secondary process.
This paper presents our research study on the hybrid
micromachining and electrochemical polishing (ECP) of
titanium microchannels with more emphasis on the latter
process.
2. Literature Review
Smooth titanium surface is required for many engineering and
medical microcomponents. Since mechanical polishing is not
applicable for microscale features, many inspired researchers
have been exploring different techniques to manufacture
mirror-like microsurfaces. Among all possible techniques, the
electrochemical process is among the most promising
methods that can be applied to economically mass-produce
titanium microdevices.
Electrochemical machining (ECM) refers to removal of
material at high removal rate while the ECP process aims to
level microscale irregularity on a surface, therefore, increases
its smoothness and reflectivity. Both direct current (DC) and
pulsed DC can be used in ECM/ECP; a DC current gives high
removal rate while a high frequency pulsed DC current
contributes to the machining quality. When applying
ECM/ECP to titanium, which is known for its excellent
corrosion resistance, one of the challenges is to break up the
tough protective oxide before the substrate material can be
removed. Landolt (1987) concluded that a titanium oxide film
can be as thick as 40 nm, but it can be dissolved by either
applying a high voltage in an appropriate electrolyte, or
byadding hydrofluoric acid in electrolyte to de-passivate an
oxide surface. Table 1 summarizes different electrolytes and
2
Journal of Manufacturing and Design Science (2015) 1-10
operating conditions for different titanium and its alloys.
Table 1 . Electrolytes and parameters in ECM/ECP of titanium and its alloys
Type
Acid based
Alcohol-based
Acid and
alcohol based
Electrolyte
0.5M H2Cr2O4
0.5M H2Cr2O4 + 0.095M HF
700 mL/L ethanol + 300 mL/L
isopropyl alcohol + 60 g/L AlCl3 + 250
g/L ZnCl2
Reference
2
0-0.01 A/cm
Zwilling et al., 1999
DC 0.2 A/cm2
Takima et al., 2008
3M H2SO4 + methanol
8v @ 0 and -10°C
5M H2SO4 + methanol
5% HClOH + 53% ethylene glycol +
42% methanol
590 mL methanol + 350 mL 2-butoxy
ethanol + 60 mL 60% HClO4
8v @ -10°C
Madore et al., 1997
Chauvy et al., 2001
Jaeggi et al., 2005
Kern et al., 2007
Landolt et al., 2003
52v 1 min + 28v 13 min @ 15°C
Chen et al., 2005
42v DC
Tam et al., 1992
>5v
>5v
Jet ECM @200v DC, 45 mA
12-74 A/cm2
> 12v
Madore et al., 1997
Landolt et al., 2003
Lu et al., 2005
Thornton, 1992
Landolt et al., 2003
2.5-25 A/cm2
Bannard, 1976
64 A/cm2
Grenier, 1968
5M NaBr
Salt-based
Condition
0.09-0.55M NaNO3 + 0.94-1.16M NaBr
5M NaClO4
1-4M KBr
2M KBr + 0.25M Na2SO4
4M KBr + 1M KF
2M KBr + 2M NaCl
2M KBr + 0.5M KF + 2M NaCl
3.9M NaCl
1M NaCl + 0.1M Na3C6H5O7
Both ECM and ECP can be applied to the workpiece
surface directly or to a masked surface for though-mask
material removal. Landolt et al. (2003) presented two mass
transport mechanisms:
1) Dissolution product limited transport: the rate of
transport of dissolving (titanium) metal ions from the
anode into the bulk electrolyte is rate limiting. At the
limiting current a salt film is formed on the surface,
and the concentration of dissolving metal ions at the
surface corresponds to the saturation concentration
of the salt formed with the electrolyte anions.
2) Acceptor limited transport: the transport rate towards
the anode of acceptor species is rate limiting. The
acceptor species combines with the dissolving
(titanium) metal ions to form a complex species. At
the limiting current the surface concentration of the
acceptor is zero.
West et al. (1992) simulated the process by boundary
element method to predict the final shape of ECM’ed
titanium.Considering an opening with length 2L and mask
thickness d, when d >> L it was found that the process
produced an isotropic profile with significant undercut below
the mask; when d << L the profile started as a flat surface but
eventually evolved into a hemisphere. The simulation result
was later verified experimentally by the work of Madore and
Landolt (1997). These authors obtained hemispherical shapes
on titanium after ECM with 5M NaBr, or with a solution of
3M H2SO4 and methanol at 0°C, or with HF in direct chemical
etching. Surface pitting of titanium specimen was observed
when applying a voltage less than 2 volts, but smooth surface
when voltage was at least 5 volts when the current density
increased above 1A/cm2. Although good surface finish was
obtained, the material removal rate in acid/alcohol electrolyte
was only 20% of that with NaBr as the solute. Chemical
etching in HF produced a rough surface on polycrystalline
titanium due to different crystallographic orientations of
titanium grains. Bannard (1976) found that using bromide in
the solution reduced the breakdown potential considerably,
while chlorine improved the surface finish and reduced stray
current; however, addition of chlorine did not affect the
removal rate significantly. This finding agreed with Landolt’s
published work (2003) for measuring and ranking the resulted
current densities of different electrolytes for ECM of titanium.
The measured current densities associated with different
solutes were found in decreasing order for NaBr, NaCl,
NaClO4, NaOH, and NaNO3 respectively.
Although titanium can be electrochemically processed in
salt solutions or acids, some researchers sought alternative
Journal of Manufacturing and Design Science (2015) 1-10
electrolytes to avoid potential safety issues with hydrofluoric
acid (HF) and perchloric acid (HClO4). Tajima et al. (2008)
used an alcohol solution (700 mL/L ethanol + 300 mL/L
isopropyl alcohol + 60 g/L AlCl3 + 250 g/L ZnCl2) as
electrolyte to ECP titanium and alloys in both wrought and
cast conditions. The tests were performed at temperature
between 25-35°C while agitating the electrolyte since this
helped to remove a viscous layer forming on the anode
surface. The best surface finish was obtained when current
density was within 0.15-0.22 A/cm2 for both CP titanium and
Ti 6Al 4V. The mean roughness Ra less than 0.1 µm was
achieved for polished wrought materials, but 0.67-0.80 µm Ra
for cast titanium. They concluded that the rougher surface on
cast specimens were due to the needle-like iron compounds
from the cast impurity. The authors recommended two-step
process: mechanical polishing a cast surface following by a
fine polishing step using ECP process.
Chen et al. (2005) combined acid with alcohol to
formulate their electrolyte (5% perchloric acid + 53%
ethylene glycol + 42% methanol). At 15°C and 200 rpm
stirring rate, the authors found that 15 minutes (900 s) would
be the optimal ECP time since a longer polishing time resulted
in grain boundary depletion and pitted grains.
Both ECM and ECP produce isotropic profile, i.e. aspect
ratio 0.5:1, after a long processing time. To achieve a higher
aspect ratio, Lu and Leng (2005) experimented with a jet-type
ECM system to drill deep microholes on a cylindrical titanium
surface. Using a 5M NaBr electrolyte at 280 kPa jet pressure,
0.73-1.17 m/s flow speed, 200 volts, and 45 mA at 3 mm
electrode gap, the authors fabricated microholes with
ϕ600-800 µm diameters and 1.3:1 aspect ratio. No material
was removed when an applied DC voltage was below 100
volts, but harmful arcing occurred when increasing the
voltage above 250 volts. Although relatively deep microholes
could be produced on flat or curve surfaces, the process was
somewhat limited due to stray current pitting, rough surface
finish and holes with tapered walls and rounded bottoms.
Microfeatures can also be fabricated in through-mask
ECM/ECP. In their research work, Kern et al. (2007)
fabricated a mask by conventional lithography process using
SU8 resist patterning with ultraviolet radiation or electron
beam, Chauvy et al. (2001) and Jaeggi et al. (2005) used laser
to pattern oxide film, while Zwilling et al. (1999) formed the
mask by anodizing thin oxide film on titanium. Using 3M
H2SO4 and methanol at -10°C as electrolyte, the researchers
produced perfect hemispherical profiles under circular mask
openings after 4000 s (1.1 hours). However, a square opening
in the mask caused low resolution wavy lines due to isotropic
etching effect.
Bannard (1976) and Chen et al. (2005) observed resulted
black stains on electrochemically polished titanium. The stain
can be removed by increasing the initial voltage to 53 volts for
one minute and then lowering it down to 28 volts, or
alternatively increase the electrolyte stirring rate. The latter
approach, however, can easily over-polish the titanium
specimens.
3
Conventional ECM/ECP and electroplating utilize DC
current to remove material at anode. Under this condition and
diffusion control mechanism, titanium may form a passive
oxide firm and hinder the process. Hydrofluoride acid can be
added in the electrolyte to de-passivate the surface. This
approach is less attractive due to safety concern when using
hydrofluoride acid. Other alternative approaches are to apply
a high current DC pulse at high electrolyte stirring rate, or
applying AC pulses to break up the oxide layer. Taylor and
colleagues (2014) first applied an anodic pulse to enhance the
mass transport and control current distribution, and following
with a reversal cathodic pulse to de-passivate the surface. An
off-time was added between pulses to facilitate the
replenishment of electrolyte, flushing of byproducts, and
effective cooling of the electrodes. The authors claimed that
ECM/ECP process using reversed AC pulses would improve
productivity since the resulted material removal rate was
higher comparing to the rate at DC current.
3. Experiment
Sheet rolling direction
7.07
R2
4
25
0.5 cm
2
7.07
Fig. 1. Design of titanium specimen. All dimensions are in
millimeters.
Commercially pure (CP) titanium (0.5-1 mm thick) and
304 stainless steel (1 mm thick) polycrystalline and
cold-rolled sheets were used in this study. A titanium sheet
was wire-electrical discharge machined (wire-EDM) to shape
the anodic specimens to double ended square tabs of 0.5 cm2
area (Fig. 1). The same process was used to fabricate the
cathodic stainless steel specimens to rectangular specimens of
10 x 25 mm. Some titanium specimens were hot glued at 75°C
onto a flat aluminum block and then micromilled or
microdrilled to form microchannels. An example of
microchannel geometry is shown in Fig. 2. Micromachining
was performing in minimum quantity lubrication (MQL) on a
Haas OM2 machine with 50,000 rpm air spindle (Table 2).
Selected machined specimens were sectioned by wire EDM,
4
Journal of Manufacturing and Design Science (2015) 1-10
cold mounted in epoxy, hand ground, polished, and then
etched in Kroll’s reagent (90 mL distilled water, 6 mL nitric
acid, and 2 mL hydrofluoric acid) to reveal the subsurface
microstructure below the machined surface.
2
4
Tool feed
3
1
Fig. 2. Milling of microchannels. A microtool starts at position #1
and moves to #2, lifts up and restarts at #3 and then ends at #4.
from a Galvanostat model 273; the eDAQ software was used
to program and control an applied DC current while monitori
ng temperature of the electrolyte with a thermocouple.
The electrolyte was a 3:7 mixture of isopropyl alcohol and
ethanol, 60 g/L of AlCl3 and 300 g/L of ZnCl2. The
Galvanostat was programed to supply power in two
successive steps. The first step was either 1 A (2 A/cm2) for
15-60 s, or 0.5 A (1 A/cm2) for 60-1200 s to break an oxide
layer on titanium surface. The second step was 0.5 A (1 A/cm2)
for 60-600 s. After completing an ECP process, any residual
stain on a polished titanium specimen was cleaned in a
solution of 10% H3PO4 mixed with 1500 ppm CrO3 for 300s.
It was4then washed in isopropyl alcohol and dried in warm air.
The ECP'ed specimens were examined with the measuring
Olympus STM6 microscope, or the JEOL JSM-400 scanning
electron
microscope (SEM) that was integrated with an
1
energy dispersive X-ray (EDX) system. Additional surface
assessment was performed with the Digital Instrument atomic
force microscope (AFM) and compared with surface
measurement data from a Zygo white-light interferometer.
Both area and line surface finish data were recorded. The line
surface finish was measured in the direction perpendicular to
the rolling direction on a sheet.
Table 2. Material specification and micromachining parameters.
CP
Titanium
Micromilling
Microdrilling
Minimum
quantity
lubrication
Ti 0.08C 0.25O2 0.03N2 0.015H2 0.3Fe
345 MPa tensile strength, 275 MPa yield
strength, 20% elongation.
AlTiN coated and uncoated micrograin
tungsten carbide, 2 flutes, 0.198 mm
diameter, 30 µm depth of cut, 24 m/min
cutting speed, 0.1-0.3 µm/flute chip load.
AlTiN coated and uncoated micrograin
tungsten carbide, 2 flutes, 0.100-0.127 mm
diameter, 12-20 m/min cutting speed,
0.05-0.1 µm/flute chip load, progressive
pecking.
Nozzle 60°from tool axis, 30 mm away
from tool tip, Coollube lubricant, 414 kPa
(60 psi) air pressure.
Figure 3 illustrates the experimental setup. Both the
stainless steel cathode (#2) and titanium anode (#1) were
secured in a Teflon fixture at 5 mm apart. A stainless steel
setscrew tightened the titanium specimen at the middle stem
to avoid damaging to the square end tabs. Both anode and
cathode were submerged in fresh electrolyte (#6) in a glass
beaker (#5), which was positioned inside two tandem and
hollow rare earth Nd-Fe-B ring magnets (#7). Each ring
magnet (ϕ75 mm outside diameter, ϕ50 mm inside diameter,
and 25 mm thick) was wrapped in plastic sheet to minimize its
contact with corrosive fluids. The glass beaker and plastic
wrapped magnets were housed in an ice bath to keep the
electrolyte temperature in the range 0-10°C. The magnetic
strength of a magnet was measured in air with the AlphaLab
GM-1ST DCGaussmeter. Power to the ECP cell was supplied
Fig. 3. ECP experimental setup with (#1) titanium anode, (#2)
stainless steel cathode, (#3) thermocouple, (#4) reference electrode,
(#5) glass beaker, (#6) electrolyte, and (#7) ring magne.Result and
Discussion
Two steps were performed to fabricate microchannels that
required smooth surface finish and different geometrical
profiles. The first micromachining step was to remove the
bulk material and generate the desired microchannel shapes,
and the second finishing step was to polish the microchannel
surface and its surrounding.
3.1. Micromachining
The CP titanium material can be microdrilled and
micromilled with custom-coated microtools to achieve a
reasonable surface finish and different aspect ratios. An
Journal of Manufacturing and Design Science (2015) 1-10
uncoated tool, however, produced unacceptable burr and
subjected to high tool wear rate. Coating a tool with ~1.8 µm
thick AlTiN improved the tool performance significantly.
More than 100 holes of ϕ100-127 µm with deep aspect ratio
of 10:1 were fabricated with a single AlTiN-coated microdrill.
Different channel profiles were successfully milled with
either a flat-end or spherical-end microtool. Figure 4 shows
cross sectional views of two micromilled channels with aspect
ratio of 0.5:1 (shallow and isotropic) and 2.2:1 (deep and
anisotropic) respectively.
100 µ m
(a)
100 µ m
(b)
Fig. 4. Sectional views of machined microchannels with (a)
isotropic 0.5:1 aspect ratio and (b) anisotropic 2.2:1 aspect ratio.
Micromilling with ϕ198 µm, 24 m/min, 0.1 µm/flute, in MQL.
An experimental investigation was attempted to directly
micromachine a microchannel to achieve nanoscale surface
roughness. Berestovskyi et al. (2014) developed a theoretical
equation to predict surface finish of ball-end milled surface:
𝑓2
𝑅𝑎 = 0.2423
𝐷
Where Ra : average surface finish (µm)
f : chip load (µm/flute)
D : tool diameter (µm)
(1)
Wang and Chang (2004) showed that the theoretical surface
finish machined with a flat-end mill can be estimated by:
1
𝑅𝑎 = 𝑓𝑡𝑎𝑛𝛼
(2)
4
Where Ra : average surface finish (µm)
f : chip load (µm/flute)
α : concavity (disk) angle (°)
Both equations (1-2) predict that a small chip load is the
necessary condition, but not the sufficient condition, for a
5
very smooth surface finish. Each model development assumes
an ideal tool with sharp cutting edges cutting on homogenous
and isotropic material, and there is no chip smearing and
built-up-edge forming during machining.
Assuming ideal micromilling conditions, milling with a
sharp microtool (ϕ127 µm, 2 flutes, 2.5°concavity angle) at
chip load of 0.1µm/flute, equations (1-2) predict the resulted
surface finish of 1.9x10-5 µm for ball-end milling and 1.1x10-3
µm for flat-end milling. The best surface finish of ball-end
milled microchannels, however, was obtained in the range of
0.3-0.5 µm Ra. The actual surface finish is 4 orders of
magnitude higher than the theoretical value predicted by
equation (1). Although equations (1-2) have been validated
with experimental data in macromachining, they do not
accurately predict surface finish in micromachining. Such
“size-effect” is due to micromilling at shallow axial and radial
depths of cut with a realistic tool having finite edge sharpness.
At such condition the workpiece material (i) is partially
plowed underneath a milling tool, (ii) is smeared by
built-up-edges (BUEs) welded to the tool cutting edges,
therefore results in rougher surface finish (Berestovskyi et al.,
2014). Evidence of BUE smearing and residual burr is shown
in Fig. 5a.
The surface finish is improved significantly when (i) using
suitable coating material such as AlTiN to minimize BUE
formation and reducing tool wear, and (ii) applying high
velocity micromist to blow away tiny chips while lubricating
the rake surfaces for effective micromachining (Berestovskyi
et al., 2014). Micromilling with an AlTiN coated microtool in
micromist is performed for all microchannels in this study.
Surface finish of microchannel fabricated with coated tool in
micromist, although better than that with uncoated tool, was
still much higher than the theoretical values predicted from
equations (1-2).
Microstructural examination of micromachined and
chemically etched subsurface shows no evidence of
microcrack, and no significant grain distortion and plastic
deformation due to machining (Fig. 5b). With insignificant
machining defect and assuming low residual stress after
micromachining, it should not be a concern for dimensional
instability and reliability of these microchannels.
A suitable polishing process after bulk micromaching is
required since it is unrealistic to achieve a nanoscale surface
finish on titanium by micromilling alone.
6
Journal of Manufacturing and Design Science (2015) 1-10
25 µ m
Burr
a
b
c
d
Tool
feed
25 µ m
(a)
Tool feed
50 µ m
(b)
Fig. 5. (a) Burr and smearing on a micromilled channel, and (b)
microstructure of machined subsurface. Notice no visible grain
distortion along the micromachined surface. Micromilling with
ϕ127µm tool, 10 m/min, 0.05 µm/flute, in MQL.
3.2. Electrochemical polishing
The as-received titanium sheets are filled with manufacturing
related surface defects. In addition to an oxide layer on the
surface, other visible surface microdefects include inclusions,
pitting cavities, microcracks, and rolling marks (Fig. 6).
Micromilling would remove these defects when machining
microchannels, but a long polishing time is still required to
polish the surface areas outside of the machined
microchannels.
The mass transport rate indicates the effectiveness of an
ECP process. During the process, titanium ions are generated
and diffuse away from the anode surface while acceptors in
electrolyte must reach the anode surface to continue the
chemical reaction. Such motions can be accelerated by a
continuous flowing of electrolyte to replenish fresh acceptors
to anode surface and to flush away saturated metallic ions and
other ECP byproducts. Bannard (1976) used two tandem
pumps for effective flowing of electrolyte at Reynolds
number of 20,000 and probably caused a turbulent flow in the
small channel. Tam et al. (1992), however, experimentally
observed that a turbulent flow would degrade the surface
finish.
Fig. 6. Scanning electron image of as-received titanium sheet.
Surface defects include (a) inclusion, (b) crack, (c) rolling mark, and
(d) pitting cavity.
Traditional laboratory utilizes a fluid mixer in which a
rotating magnet drives a Teflon coated magnet bar,
submerged and sank to the bottom of a beaker, and rotates it in
synchronized motion. Such mechanical mixer is used
satisfactory in fluid mixing, but is not effective as an
electrolyte stirrer for electrochemical polishing of a large
specimen. A submerged magnet, stirring from the bottom of
an electrolyte container, generates a gradient of electrolyte
speeds in the vertical direction. With heavy metal ions and
byproducts accumulated in the electrolyte, the speed variation
would affect the uniformity of polished surface since the
specimen area near the stirring magnet will be over-polished
or even eroded while the higher area is not. An over-polished
step as deep as 14 µm at the specimen bottom was observed
when using a mechanical fluid stirrer in this study.
To improve the surface uniformity, a hollow magnet was
used to induce a more uniform electrolyte stirring. Referring
to the experimental setup in Fig. 3, when applying a voltage
between anode and cathode, the positive titanium ions move
from anode toward cathode in +x direction while they are
inside a magnetic field aligning along the +z direction. The
ions subject to weak diffusion and Coulomb forces (in +x
direction) and a strong magnetic force (in -y direction). The
dominating magnetic force induces self-stirring motion that
forces ions and electrolyte to flow parallel to the surfaces of
both electrodes. The stirring force is uniform due to (i) the
constant magnetic field within the ring magnet, (ii) uniform
ion speed when leaving the anode surface, and (iii) the same
charge of all titanium ions. Recall that the magnitude and
direction of this magnetic force on each ion is governed by the
cross product of ion velocity and magnetic field vectors:
𝐹 = 𝑞𝑉 x𝐵
Where
F: magnetic force (N)
q: charge of titanium ion (Coulomb)
V: speed of an ion leaving anode (m/s)
(3)
Journal of Manufacturing and Design Science (2015) 1-10
B: effective magnetic strength (Tesla = 104 Gauss)
The self-inducing magnetic force was strong enough to
stir the electrolyte in uniform circular motion in the
cylindrical beaker. The fast flowing rate of electrolyte (i)
helps to remove the viscous layer between the two electrodes,
(ii) brings the fresh electrolyte and acceptors to anode surface,
therefore (iii) increases the reaction rate (Landolt et al., 2003).
A long polishing time, however, slightly affected uniformity
of the polished surface. The circulating metallic ions and
metallic compounds impacted and eroded the leading edges of
both electrodes. This issue can be solved by filtering out the
abrasive byproducts in the electrolyte.
Magnetic Strength (Gauss)
4000
3000
2000
Calculated
Measured
1000
0
0
50
100
150
Distance from center (mm)
200
(a)
Magnetic strength (Gauss)
1000
500
0
Calculated
Measured
-500
-1000
-1500
-25
0
25
50
75
100
125
150
Distance from center (mm)
(b)
Fig. 7. Axial magnetic field strength of Nd-Fe-B magnets: (a) solid
disk ϕ75 mm, 25 mm thick, (b) ring magnet with ϕ75 mm outside,
ϕ50 mm inside, 25 mm thick.
Both ring and disk magnets were considered in this study.
The glass beaker was either inserted inside the ring magnet
(Fig. 3) or positioned on top of a solid disk magnet. Figure 7
compares the magnetic strengths (in air) of the rare earth
magnets as a function of radial distance from its center. The
theoretical magnetic strengths can be obtained from several
online resources (Dextermag; Kjmagnetics; Arnoldmagnetics,
2014). Because the magnetic strength of a solid disk drops
drastically away from its center (Fig. 7a), the position of the
anode in an ECP cell must be precisely located for repeatable
polishing results. The ring magnet is preferred due to its more
7
uniform magnetic field inside its inner diameter (Fig. 7b). A
slight variation of electrode position inside the uniform
magnetic field should neither affect the stirring rate nor
uniformity of polished specimen.
A series of chemical reactions happen and new titanium
oxide is formed during the process. At the anode, the titanium
atoms are ionized according to:
Ti → Ti4+ + 4e(4)
The hydroxide evolution also occurs at cathode:
4H2 O + 4e- → 2H2 + 4(OH)(5)
And new titanium oxide forms when combining with the
hydroxide ions:
Ti + 4H2 O → titanium oxide + 2H2
(6)
Chen et al. (2005) reported that different titanium oxides
can be formed depending on the pH level and applied voltage:
TiO2, TiO3, Ti3O5, Ti(OH)2, Ti(OH)3, and Ti(OH)4. At a low
current density in ECP process, the resulted surface finish was
inconsistent perhaps due to different oxide type and thickness
on a specimen. Reasonable consistency can be achieved when
the original titanium oxide layer from an as-received
specimen is first dissolved at a high current density; this
reaction then reveals titanium asperities for subsequent
electrochemical polishing at a lower current density. Four
different specimens were tested for repeatability of polished
results. Table 3 lists selected tests and the optimal conditions
for polishing titanium specimens.
Surface finish of an area is commonly higher than that
along a line since the former covers more data points and
reflects the true surface topography. A non-contact surface
measuring technique, although is preferred over the contact
technique, is sensitive to contaminants on a surface. The
root-mean-square surface finish value (Rrms) is always higher
than that for an averaged finish (Ra).
Different non-contact techniques were used in this study
to characterize a polished specimen: SEM, optical microscopy,
interferometry, and AFM. The non-contact techniques are
preferred over traditional profilometry technique since a
mechanical stylus of a profilometer would (i) scratch a
polished surface and might not truly provide the surface finish
data, and (ii) provide only surface finish along a straight line.
The non-contact methods, however, are more challenging
when examining a nanofinished surface. Since a featureless
polished surface does not provide spatial contrast in SEM, a
microscratch was intentionally produced on a polished
specimen to enhance the contrast against the polished surface
(Fig. 8). In optical microscopy, a mirror-like surface was too
reflective to be observed unless illuminated with polarized
light. Different shades of multiple grains were distinguishable
due to the difference in grain crystalline orientations and
etching rates (Fig. 9). Interferometry was the preferred tool to
quickly capture the surface 3D profile and quantify the
surface integrity of a polished surface. As expected, surface
finish of an entire area is usually higher than the surface finish
measuring along a line, or a circle. The average 2x2 mm area
surface finish of the four polished samples (after 60 s at 2
8
Journal of Manufacturing and Design Science (2015) 1-10
A/cm2 and 300 s after 1 A/cm2) is 316 nm Rrms, and the
average line surface finish is 202 nm Ra (Table 3). Measuring
with an AFM not only takes a long time yet it only produces
accurate result from a very small area. The AFM image in Fig.
10 shows three adjacent grains with grain boundaries in
between. The area surface finish on 12 x 12 µm area was 42
nm Rrms when including the grains and their grain boundaries,
but the area surface finish reduced to 1-10 nm Rrms when
measuring within a single grain.
Grain
boundary
Table 3. Surface finish of polished titanium, measured with white
light interferometry.
ECP parameters
15v, 660s
@0.02A/cm2
Mechanical stirring
45s @2A/cm2 +
240s @1A/cm2
Magnetic stirring
60s @2A/cm2 + 300s
@1A/cm2
Magnetic stirring
10 µ m
2x2 mm area
surface finish
Rrms (nm)
2 mm line
surface finish
Ra (nm)
1424
--
845
--
231, 423, 302,
310
(316 average)
170, 270, 150,
220
(202 average)
Intentional scratch
after polishing
Fig. 8. Scanning electron image of polished titanium. ECP 45s at
2A/cm2, and then 180s at 1A/cm2.
20 µ m
Fig. 9. Polarized-light enhanced optical image of polished titanium.
ECP 60s at 2A/cm2, and then 300s at 1A/cm2.
Fig. 10. Atomic force microscopic image of adjacent polished
grains. 12x12 µm area surface finish 42 nm Rrms across grains, and
1-10 nm Rrms within a grain.
The polished surface finish of titanium achieved in this
study (1-316 nm area Rrms) is either equivalent or better when
comparing with published data. In their research, Landolt et al.
(2003) reported that surface finish of titanium polished by
electrochemical (10-900 nm Rrms) was much better than that
when polished mechanically (500-900 Rrms). When
processing titanium at a higher current density above 14
A/cm2 for higher material removal rate, the surface finish was
compromised and reported in the range 410-1020 nm Ra
(Thornton, 1992). The best surface finish of 800 nm area R a
was also reported by Tam et al. (1992). Tajima el al. (2008)
used similar electrolyte to polish CP titanium at current
densities in the range 0.15-0.21 A/cm2 for 15 minutes (900s).
A very smooth and reflective surface finish of 30 nm R a
resulted for wrought titanium but a rougher surface of
670-800 nm Ra was obtained on cast titanium specimens. The
surface measurement, performed with a contact-type
profilometer by these authors, might be different from those
obtained with non-contact techniques.
Some black stains were occasionally observed on polished
titanium specimens. Similar observations were also reported
by Bannard (1976) and Chen et al. (2005). Energy dispersive
X-ray analysis of several black stains were obtained and
compared with that from a titanium BUE that “welded” onto
an uncoated tungsten carbide (WC) microtool during a
parallel microdrilling study. The latter spectrum shows both
the titanium and tungsten peaks as expected, but the EDX
spectrum of a black stain contains titanium peaks and an
obvious carbon peak (Fig. 11). Although the root cause of
such black stain was yet to be found, the contaminating stain
can be removed by a post ECP cleaning with a solution of
phosphoric acid (H3PO4) and chromium trioxide (CrO3).
A mirror-like microchannel after micromachining and
polishing is shown in Fig. 12. After micromilling and
polishing, the surface finish of a microchannel measuring
along the channel's axis agreed with line surface finish outside
of the channel. This indicated uniform polishing at the sheet
surface and within a channel profile for a channel with 0.5:1
aspect ratio. It is yet to find out if similar result can be
achieved for microchannels with higher aspect ratios. In other
words, it is a concern that an electrolyte might not enter into
Journal of Manufacturing and Design Science (2015) 1-10
the bottom of a very deep channel with narrow width.
9
Rrms on 12x12 µm area across multiple grains, and
1-10 nm Rrms within a single grain.
Ti
Acknowledgement
W
C
Ti
W
Fig. 11. Comparison of energy dispersive spectra of a black stain
after ECP of titanium, and that of a titanium chip on uncoated WC
microdrill.
100 µ m
Fig. 12. Mirror-like microchannels after micromilling and polishing
4. Conclusion
It is unrealistic to mass produce nanofinished and deep
microchannels by milling process alone. A maskless hybrid
technique was developed by combining both micromachining
and electrochemical nanopolishing for effective fabrication of
microchannels on commercially pure titanium sheets. This
study shows:
1) Microchannels with different profiles and aspect
ratios can be fabricated by microdrilling and
micromilling to remove the bulk material. Burrs,
machining marks, and other surface defects can then
be electrochemical polished to mirror-like finish.
2) The environmentally friendly alcohol-based
electrolyte is preferred over acid-base electrolyte in
electrochemical polishing of titanium. A two-step
process is proposed: the first step at a high current
density of 2 A/cm2 for breaking up any surface
oxides, and the second step at a lower current density
of 1 A/cm2 for polishing.
3) Uniform polishing can be achieved when stirring an
electrolyte with self-induced electromagnetic force.
4) Non-contact surface finish measurement is preferred
since it does not degrade the polished surface. The
results, however, depend on local and specific areas.
The average surface finish of optimally polished
specimens is 316 nm Rrms on 2x2 mm area, 42 nm
This work is funded by Agilent University Research
Corporate grants (#512433, #2847). Help from Professor
Manuel Soriaga, Mr. Kyle Cummims, and Mr. Brennon
Sessions of Chemistry Department at Texas A&M University
is much appreciated. The authors would like to thank Unist,
Haas Automation, Performance Microtools, and Swiss-Tek
Coating for their kind support.
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