Electrolytic gradient Al2O3-ZrO2 composite coatings on
Ti-6Al-4V implant alloys
S. J. Wu, S. K. Yen*
Department of Materials Engineering, National Chung Hsing University
250, Kuo Kuang Road, Taichung, 40254, Taiwan
Abstract
The bioceramic coatings on metallic implants have the potential to improve the
performance with respect to implant fixation, wear and/or corrosion resistance. In this
study, a novel method of electrolytic Al2O3-ZrO2 nanostructure composite coatings
were successfully conducted on F-136 Ti-6Al-4V alloy in the mixture of ZrO(NO3)2
and Al(NO3)3 aqueous solution. The uncoated and Al2O3-ZrO2 coated specimens were
evaluated by cycle polarization tests in Hank’s solution, wear tests (including dry and
wet) with UHMWPE (ultra-high molecular-weight polyethylene), scratch tests, and
surface morphology observations. Al2O3-ZrO2 coated specimens have shown the
better corrosion resistance than the uncoated specimen, such as higher corrosion
potential Ecorr, lower corrosion current density icorr, and higher protection potential Epp.
The nanostructure of Al2O3-ZrO2 composite films with a little gradient distribution
was confirmed by AFM observations and AES components depth profiles. The wear
loss of UHMWPE was dramatically reduced by the Al2O3-ZrO2 composite coating.
The nature passivation film of the uncoated was destroyed during the wear corrosion
test. In contrast, the Al2O3-ZrO2 coated maintained at the passive state. Al2O3-ZrO2
composite coating still remaining on the Ti-6Al-4V alloy after the scratch test
indicated the excellent adhesion between the coated film and the substrate.
Keywords: electrolytic coating, nanostructure Al2O3-ZrO2 composite, Ti-6Al-4V
alloy, UHMWPE, gradient, wear-corrosion.
Introduction
The main characteristics for biomaterials are mechanical properties,
biocompatibility, bioahesivity, and biodegradation. Based on the biocompatibility,
biomaterials can be divided into four types: nearly inert, porous, bioactive, and
resorbable. The implants of the total hip joint replacement (THR) are composed of
femoral stem (alloy), ball head (alloy or ceramic), shell (alloy), and acetabular cup
(polymer). Titanium alloys have been clinically used for dental and orthopaedic
applications owing to their advantages, such as light, high specific strength, lower
modulus of elasticity, greater corrosion resistance, and excellent biocompatibility.
Ti-6Al-4V ELI is the first titanium alloy registered as an implant material in the
ASTM standard (F-136-84). Ultra-high molecular weight polyethylene (UHMWPE)
is an organic polymer widely used as an acetabular cup in hip joint replacement, or
tibia plateau in knee joint replacement. Ti-6Al-4V/UHMWPE couple is one of the
most important biomaterials systems for total joint replacement prosthesis. When the
sliding surfaces of the joint components are made from titanium alloys, wear can load
to dirty of surrounding tissue and failure of the joint [1-2]. Variable amounts of plastic
and metal are released due to wear and wear-corrosion. Therefore, how to improve the
wear resistance of UHMWPE/alloy couple and reduce the volumes of wear debris,
hereby becomes a key factor for achieving the long life of orthopaedic joint
prostheses.
To solve the above problems, Boutin [3] in France started to develop a hip
prosthesis with cup and ball made of alumina ceramic at the beginning of 1970s.
Semlitsch et al. [4] have determined that the wear rate of polyethylene (PE) against
alumina ceramic is about 20 times lower than that of PE against Co-Cr-Mo alloy.
However, alumina ceramic exhibits to incline to the fragile and is sensitive to
microstructural cracks [5]. Recently, zirconia ceramic is being recognized for its high
strength and high toughness, making this material potentially suitable for the highly
loaded environments found in joint replacement [5-8]. Therefore, some authors have
recently proposed ceramic balls made of a mix of zirconium and aluminum oxides [9]
claiming against higher strengths and lower wear rates. Between the biocompatibility
of zirconia and that of alumina ceramics, there is no apparent difference in the
biological reaction in vivo [10-11]. In addition, the wear factor of UHMWPE against
ZrO2 is 40%-60% less than that against Al2O3 counterfaces and 10%-20% less than
that against SUS 316L metal counterfaces [12]. Also, some surface modification on
alloys may be helpful. An electrolytic coating of zirconia single layer successfully
conducted on pure titanium and Ti-6Al-4V substrate has shown the excellent adhesion
on the metal substrate and improved corrosion resistance [13-15]. Similar results were
also found on alumina and zirconia coated Co-Cr-Mo alloy [16-17].
In this study, the characterization of electrolytic gradient Al2O3-ZrO2 composite
layer coating on Ti-6Al-4V alloy including the wear loss of counterpart UHMEPE,
and the corrosion resistance, adhesion, and wear-corrosion resistance of the coated
specimen is reported.
2. Experimental
2.1. Sample preparation
An ASTM F-136 Ti-6Al-4V sheet, as received, was used as a substrate of the
Al2O3-ZrO2 electrolytic gradient coating. Its nominal composition is given in Table 1.
The sheet thickness is 1.0 mm with a grain size of 5  2 m. The sheet was cut into
discs with a diameter of 14 mm for corrosion tests, and 28 mm for wear and scratch
tests. All specimens were polished to a mirror finish with 0.3 m Al2O3 powders, then
degreased by detergent and further ultrasonically cleaned in deionized water and
acetone, then dried by an N2 gas gun.
2.2. Electrolytic deposition and annealing
The electrolytic deposition of Al2O3-ZrO2 with gradient composition was
conducted in a naturally aerated solution of ZrO(NO3)2 and Al(NO3)3 aqueous
solution, at pH = 2.66 and a scanning cathodic potential from the range of -0.8 to -1.8
V (Ag/AgCl) with a scanning rate of 2 mV/sec by using an EG&G M273A
Potentio-stat and M352 software. The alloy disc was the cathode, graphite the anode,
and a saturated calomel the reference electrode. The above electrolytic condition was
the most efficient deposition in our experiment. The post-coated specimens were
naturally dried in the air and annealed at 823 K for 1h.
2.3. Scratch, wear, and wear-corrosion tests
Some specimens were tested by scratch using Model ST200/F TEER COATING
Limited (England) with a preload 1N, load speed 50 N/min, scratch speed 20 mm/min,
and the end load 50 N. Reciprocating wear test was also conducted by the above
instrument ST200/F. UHMWPE in diameter of 0.5 mm, finally polished with # 2000
SiC paper, was used as pin. The Al2O3-ZrO2 coated and uncoated specimens were
used as disks respectively, with a contact surface area of 0.2 mm2 and a load of 10 N
(or axial stress of 50 MPa), at the sliding distance of 1 mm per cycle with 30
cycle/min. The drive speed and room temperature (251oC) were kept constant
throughout the test and the duration is 1 hr. Frictional force between the UHMWPE
pin and the counterface was monitored by a strain gauge fixed on a leaf spring
attached to the transverse bar holding the PE pin. The wear-corrosion experiments
were performed in Hank’ solution at 37±1℃, using an Ag/AgCl reference electrode
and a platinum sheet as counter electrode. A potentio-stat EG&G PAR 273A was used
to measure the corrosion potential and current density. A polyvinyl chloride (PVC)
cell contains the test solution and a transmission shaft with a counterface connected to
the motor, as shown in Fig. 1. Wear-corrosion tests were performed in the pin-on-disk
configuration, with a counter rotation speed of 458 rpm, and a weight of 21.9 MPa.
The uncoated and Al2O3-ZrO2 composite coated discs(ψ22 mm) were used as the
counterpart and connected to the working electrode. The test duration was 3 h for a
total wear path length of 1250 m.
2.4. Corrosion test
All annealed specimens were potentiodynamically polarized with EG&G Model
273A M352 in aerated Hank’s solution with pH value of 7.1 which compositions are
given as: NaCl 8.00 g/L, CaCl 0.14 g/L, KCl 0.40 g/L, NaHCO3 0.35 g/L, Glucose
1.00 g/L, MgCl26H2O 0.10 g/L, KH2PO4 0.06 g/L, Mg2SO47H2O 0.06 g/L, and
Na2HPO4 0.06 g/L. The cyclic polarization test was from -0.80 to +0.80 V (Ag/AgCl),
then back to -0.80 V at a scanning rate of 0.166 mV/sec. The first oxidation-reduction
equilibrium potential E01 was derived when current density equals zero during the
applied voltage increased (forward cycle). At E01, the reduction current density is
equal to the oxidation current density. This exchange current density is defined as the
equilibrium current density i0. They are also called corrosion potential Ecorr and
corrosion density icorr respectively, when the oxidation is attributed to metal corrosion.
2.5. SEM, AFM, and AES
The surface morphology of coated specimen after scratch, or wear test was
observed by scanning electron microscopy/energy depressive spectroscopy
(SEM/EDS, JEOL JSM-5400 Japan). The nanostructure of Al2O3-ZrO2 composite
film was clarified by the observation using atomic force microscopy (AFM, Digital
instrument NS3a controller with D3100 stage), with an attempt to evaluate the surface
morphology of thin films in terms of the three-dimensional roughness, obtained by the
image analysis. The component depth profiles were plotted using an Auger electron
spectroscopy (AES Fison VG Microlab 310D). Specimens were sputtered by 5 KeV
Ar+ with a beam current of 0.05 A, and a sputtering rate about 1 nm s-1.
3. Results and Discussion
3.1. Corrosion resistance
From polarization curves as shown in Fig. 2, the first redox potential E1 (or
corrosion potential Ecorr), the second redox potential E2 (or protective potential),
equilibrium current i0 (or corrosion current), cathodic polarization slope c, anodic
polarization slope a are analyzed, as given in Table 2. Obviously, the Al2O3-ZrO2
coated specimen have shown the better corrosion resistance than the uncoated
specimen, such as higher corrosion potential Ecorr, lower corrosion current density icorr,
and higher protection potential Epp. In other words, the Al2O3-ZrO2 composite coated
film is more inert than the nature passive film of a Ti-6Al-4V alloy.
3.2. Adhesion
The friction versus load curve of scratch test on the Al2O3-ZrO2 coated specimen
reveals a smooth line with a lower slope about 0.14 of the first stage, and the higher
slope about 0.79 of the second, as shown in Fig. 3. The surface morphology and EDS
mapping of the Al2O3-ZrO2 coated specimen after scratch tests are shown in Fig. 4.
EDS mappings of elements Al and Zr indicate that a complete Al2O3-ZrO2 composite
film is still found at the end of scratch stage with a load of 50 N. The first stage (0 N 25 N) is a smooth region without cracking the coated film and the second stage (25 N
- 50 N) is more striated since the coated film was broken to let pin plow the substrate.
The plastic deformation of the substrate resulted in increasing wear slope (friction
coefficient). However, the most of Al2O3-ZrO2 composite film was still found on the
scratch end of the coated specimen by EDS mapping. The excellent adhesion between
the coated and the substrate was concluded.
3.3. Wear and wear-corrosion tests
The wear loss of UHMWPE after the pin-on-disk wear test is given in Table 3.
The wear loss of UHMWPE countered by the Al2O3-ZrO2 specimen is obviously less
than that by the uncoated specimen. 79.23% reduction of the wear loss, which is
found on the coated specimen compared with the uncoated one, may be resulted from
the improved mechanical properties of coated film such as hardness from 323 to 483
(Hv). Also, the roughness of the uncoated specimen reduced by the coating, as listed
in Table 3, was the other factors on reducing wear loss of UHMWPE. Similar to Fig. 4,
a complete Al2O3-ZrO2 composite film, which was still found after the wear test,
reconfirmed the excellent adhesion of coated film on Ti alloy.
The corrosion potential of the Al2O3-ZrO2 coated and uncoated were varied with
wear-corrosion testing time, as shown in Fig. 5. When a load of 21.9 MPa is applied
(using a PE pin counterface), the corrosion potential dropped from -120 to - 610 mV
(Ag/AgCl), and became unstable. After the wear load was removed, the original
potential was regained little by little. On the contrary, the corrosion potential of
Al2O3-ZrO2 coated was kept at 100 mV and with little striation. These results indicate
that passivation film of the uncoated was destroyed but that of the Al2O3-ZrO2 coated
remained stable during the wear corrosion test.
The effect of wear-corrosion on anodic dissolution current of the uncoated and the
Al2O3-ZrO2 coated at an imposed potential of + 0.5 V (SCE) is shown in Fig. 6.
Before abrasion, the anodic dissolution currents of the uncoated and the Al2O3/ZrO2
coated were 7.1 and 0.9 μA/cm2, respectively. When the wear was on, the current
density of the uncoated was increased to 63 μA/cm2, and back down to 8 μA/cm2
when the wear off. In contrast, the current density of the Al2O3-ZrO2 coated remained
constant during the test. A lower and more stable anodic dissolution current density
was found for the Al2O3-ZrO2 coated than the uncoated. It is conclusive that the
Al2O3-ZrO2 coated specimen is much more wear-corrosion resistance than the
uncoated.
3.4. AFM image and Component Profiles
AFM measurement revealed this tendency of the surface structure more clearly
than SEM. Fig. 7 shows the AFM image for Al2O3-ZrO2 composite film. The detailed
profiles of surface, within a 1x1 μm2 image window, are observed in this figure. The
mean roughness of the surface roughness is the range of 0.952 nm. This value is small
compared with the peak size parallel to the plane, which may be considered as the
particle size about 30 nm. In other word, the coated film was composed of Al2O3-ZrO2
composite particles with size about 30 nm and finally revealed the surface roughness
of 0.952 nm.
AES component depth profiles are plotted in Fig. 8. It is confirmed that the
deposited film is composed of zirconium and aluminum oxides composite layer. AES
components depth profiles of Al2O3-ZrO2 composite films revealed a little gradient
distribution, which was attributed to the scanning applied voltage.
4. Summary and conclusions
A novel electrolytic coating method of Al2O3-ZrO2 composite coating has
successfully been conducted on an ASTM F-136 Ti-6Al-4V alloy to investigate its
characteristics. Through the annealing, polarization tests, surface observations, wear,
wear-corrosion, and scratch tests, several conclusions are drawn:
1. The Al2O3-ZrO2 composite film revealed a nanostructure with a little gradient
component distribution and the excellent adhesion between the Al2O3-ZrO2
composite coating and Ti-6Al-4V alloy was found.
2. The Al2O3-ZrO2 coated specimen showed the more corrosion resistance than the
uncoated specimen, such as the higher corrosion potential Ecorr, higher protection
potential Epp and lower corrosion current density icorr in Hank’s solution.
3. The roughness of the uncoated was further reduced by the Al2O3-ZrO2 coating and
so was the wear loss of UHMWPE.
4. The natural passivation film of the uncoated was destroyed during the wear
corrosion test but that of the Al2O3-ZrO2 coated was not. Also, a higher (more noble)
corrosion potential, and a lower and more stable anodic dissolution current density
was found for the coated than the uncoated.
Acknowledgements
The authors are grateful for the support of this research by National Science
Council, Republic of China under contract No. NSC 91-2213-E-005-014.
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Element Al
V
Fe
O
N
C
Y
H
Wt ﹪
3.86
0.12
0.12
0.019
0.022
0.001
0.0032 Bal.
6.04
Ti
Table 1. The nominal composition of Ti-6Al-4V alloy.
Uncoated
Al2O3-ZrO2
coated
E1 (Ecorr) i0 (icorr)
[mV]
[nA/cm2]
-301.1
1000
14.06
6
E2 (Epp)
[mV]
177.7
c
[V]
0.2521
a
[V]
0.2078
438.9
0.3227
0.6792
Table 2. List of the first redox potential E1 (or Ecorr), equilibrium current density i0 (or
icorr), the second redox potential E2 (or Epp), cathodic polarization slope c, and anodic
polarization slope a, derived from the polarization test in Hank’s solution.
Counterpart
Uncoated Ti-6Al-4V
Al2O3- ZrO2 coated
Hardness (Hv)
323
483
Surface roughness (nm)
42
0.95
S1
S2
S3
S1
S2
Weight before test (mg) 1232.25 1245.39 1239.50 1235.06 1245.09
Weight after test (mg)
Weight loss (mg)
Average (mg)
1232.14 1245.26 1239.35 1235.02 1245.06
0.11
0.13
0.15
0.13 ± 0.02
0.03
S3
1219.05
1219.03
0.03
0.02
0.027 ± 0.003
Table 3. Weight loss of UHMWPE after the reciprocating wear test, using the
uncoated and Al2O3-ZrO2 coated as the counterparts.
Fig.1. Illustration of wear-corrosion test. (R: reference electrode (Ag/AgCl), C:
counter electrode (Pt), W: working electrode, H: Hank’s solution, P: UHMWPE pin, S:
specimen)
Fig. 2. Polarization curves of uncoated specimen and the Al2O3-ZrO2 composite
coated annealed at 873 K tested in Hank's solution (pH=7.1).
Fig. 3. Friction force versus load diagram of the scratch test on the Al2O3-ZrO2 coated
specimen.
Fig. 4. SEM observations and EDS mappings of the Al2O3-ZrO2 coated specimens, at
a scratch load of 50 N.
Fig.5. Ecorr of the Al2O3-ZrO2 composite coated and uncoated specimens during
wear-corrosion tests.
Fig. 6. Effects of wear-corrosion on anodic dissolution current density of the uncoated
and Al2O3-ZrO2 composite coated specimens at an imposed potential of +0.5 V
(SCE).
Fig.7. AFM image of the Al2O3-ZrO2 composite film (Z range: 14.475 nm, Ra: 0.952
nm, Rms: 1.203 nm).
Fig.8. AES component depth profiles of the Al2O3-ZrO2 composite film.