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Structural and electrochemical examinations of PACVD TiO2 films in Ringer solution
Biomoteriols 0
SOl42-9612
PII
ELSEVIER
(96)
18 (1997)
789-794
1997
Elsevier Science Limited Printed in Great Britain. All rights reserved 014%9612/97/$17.00
00210-4
Structural and electrochemical examinations of PACVD Ti02 films in Ringer solution J. Gluszek, J. Masalski, P. Furman and K. Nitsch* institute of fnorganic Technology, Technical University of Wroc4’aw, Smoluchowskiego 25, 50-372 Wrocraw, Poland; ‘Institute of Electron Technology, Technical University of Wrocraw, Wybrzeie Wyspiahskiego 27, 50-370 Wrocraw, Poland The conditions and applying deposition
for obtaining
method
anatase
is created,
titanium
dioxide
120 h the coating resistivity
dioxide
stainless
crystallizing
coating,
of steel
superior
solution,
of the coating Science
does
Limited.
barrier
Titanium
Received 13 February
not cause
All rights
dioxide,
PACVD,
1995; accepted
titanium
tetrachloride
lattice.
that, during
During
properties
characteristics.
Titanium
corrosion
an increased
and general galvanic
assisted
the process,
exposure
protective
of the type 316L to pitting
removal
the substrates
It was established
in a tetragonal
in Ringer’s
adopts
from
steel of the type 316L by the plasma
have been determined.
Elsevier Keywords:
titanium
this to a surgical
vapour
titanium
dioxide
of the 316L steel
of this covering dioxide
covering
corrosion.
corrosion
and oxygen chemical
with the
improve.
After
increases
Any damage
of the substrate.
::,a
or partial 0
1997
reserved stainless
19 November
Titanium is a very reactive element (standard electrode potential -1.63V) and, due to its very strong chemical affinity to oxygen, ii. very easily produces a compact oxide film, ensuring high corrosion resistance of the metal. Titanium shows high corrosion resistance in physiological saline solution as well’, and its dissolution by-products such as titanium dioxide are generally well tolerated and do not appear to cause any undesirable reactions in patients. An outstandingly advantageous property of titanium is that when it forms a galvanic couple with a metal l~essnoble than itself, it (or titanium dioxide) undergoes strong cathode polarization and the potential of the galvanic couple is close to the potential of the less noble materia12.3. This means that, in the case of any damage to titanium dioxide film on steel substrate, one should not expect intensified substrate corrosion. Glow discharge plasma facilitates the course of chemical reactions and it is able to form a variety of superficial layers on different substrates. The main advantage of using the plasma is reduction in the temperature of films created from reactive substrates. This temperature reduction is particularly important for such processes in which increased temperature can cause undesirable structural changes of the substrate. When titanium dioxide film on austenitic steel is deposited using the conventional chemical vapour deposition (CVD) method in the temperature range of 600-9OO”C, the susceptibility of the steel substrate to intercrystalline corrosion increases4. The plasma method proves to be appropriate for obtaining a Ti02 covering on austenitic surgical steel of the type 316L. The literature abounds in articles
steel,
corrosion
1996
dealing with obtaining titanium dioxide with CVD and modified CVD methods on such substrate materials as glass, silicon, titanium, quartz, sapphire and sodium chloride. On the other hand, there is scanty information on how to apply titanium dioxide with the plasma method to a stainless steel substrate’.
EXPERIMENTAL A plasma assisted chemical vapour deposition (PACVD) device with a hot anode was used for TiOz film deposition6. Figure 1 is a diagrammatic representation of the plasma deposition system utilized in this study. The reactor chamber is constructed of stainless steel. Electric activation of the gas environment by direct current glow discharge is used. Applying electrical activation of the gas environment gives the possibility of producing TiOz films in TiC14 and O2 atmosphere over a wide range of temperatures. Using the hot anode prevents deposition of the reaction byproducts (mainly lower titanium chlorides) on the walls of the working chamber. TiOz films were deposited on stainless steel of type 316L. The specimens were discs of diameter 14 mm and thickness 1 mm. Surface treatment of the specimens consisted of grinding and finishing with abrasive paper of grain size 600. The specimens were degreased in acetone before being placed in the device for film deposition. In a typical deposition, the heating elements were heated to the desired temperature. The specimens were placed in the reactor and the chamber evacuated to a pressure of approximately 1O”Pa. The specimens
Correspondence to Dr J. Gluszek. 789
Biomaterials
1997, Vol. 18 No. 11
790
PACVD Ti02 films: J. GIuszek et a/.
8 Pc
of thermal expansion for TiOz is 8 x 10-6”C-‘s, while the coefficient for steel of the type 17Cr14Ni2Mo is two-fold greater (16-18 x 10-““C~l). Figure 3 shows that titanium dioxide film deposited on stainless steel carries irregularities on the substrate surface. It was determined (Figure 4) that the coating contained mainly oxide and titanium atoms. The 2.6 keV peak in the spectrum shows the presence of chlorine. Some
7
4
1
2
Figure 1 Diagram of measuring set for PACVD processes with a hot anode. 1, Resistance furnace; 2, working chamber; 3, heating elements; 4, treated substrates; 5, high-voltage power supply; 6, gas substrates dispenser; 7, current pass; 8, vacuum system. were then cleaned of organic contaminants by striking an argon plasma for 10min. At the end of the argon plasma treatment, Tic& and O2 flow rates were set at the desired values. As a result of a series of experiments, parameters of effective reaction for obtaining the films were determined as follows: reaction temperature, 300°C; working pressure, 10’ Pa; flow speed: 02, 3.7 x 10e3 molmin-‘; TiCl*, 2.6 x 10m4 to 6.6 x 10m4mol min-‘; deposition time, 30min. The measurement of TiOz thickness was performed on a kalotester device, made by Bernex. methods were used for Electron microscopic determining the structure and the contents. The following devices were applied: scanning electron microscope stereoscan 180 (Cambridge Instruments) equipped with X-ray microanalysis set (Link Systems), scanning electron microscope SEM 515 (Philips) with semiconductor X-ray detector (Edax) and transmission electron microscope EM301 (Philips). Eletrochemical examinations with the use of impedance spectroscopy and polarization examinations were made using a Schlumberger set (EC1 1286, FRA 1255)‘. Computer programs: ‘imp-opt v. 3.0’ and ‘~01-1286’ were used for measurements and ‘Equivalent Circuit v. 4.51’ made by B. Boukamp was applied for the impedance spectra analysis. Galvanic currents were determined by means of methods described in previous publications’* 3.
Figure 2
Microstructure
of Ti02 film on steel of type 316L.
Flgure 3
Microstructure
of TiOp film on steel of type 316L.
RESULTS AND DISCUSSION
Ti Ka
TiOz film structure Titanium dioxide deposition on the steel of the type 316L was composed of two layers. The first layer (1 pm thick) was strongly adherent to the substrate material and the second layer, appearing on the first one, was thicker, cracked and of weak adhesion to the substrate. The two-layer structure of the covering probably resulted from the fact that titanium dioxide, strongly adherent to the substrate and showing low electrical conductivity, caused electrical interlocking and the deposition process was inhibited. The cracks, clearly seen in Figure 2, can result from different thermal expansions of TiOz and the substrate. The coefficient Biomaterials
1997, Vol. 18 No. 11
2.0 OlNT
Figure 4 X-ray substrate.
4.0
6.0 o.ooKBv
spectrum
8.0 1CkV/ch
B EDAX
of TiOp film on the 316L steel
791
PACVD TiOp films: J. Wuszek et al.
other lower peaks stem from iron, chromium and nickel atoms in the substrate. The structure of the coating carefully scratched from the steel is characteristic of a polycrystal. Diffraction patterns of different fragments of the film provide additional proof of its polycrystalline form. They showed configuration and intensity close to each other, which proved to be the homogeneous polycrystalline structure of the oxide. from the Interplane distances, determined measurements of diffraction ring radii, allow one to state that the coating under examination constitutes anatase, crystallizing in tetragonal form.
Electrochemical
---__..-
~--T
1 E+7
++++$+*
;
Electrochemical examinations comprised polarization and impedance measurements and the measurements of galvanic currents. Three kinds of specimens were used in the examinations: 1. Specimens with TiOz film obtained with the PACVD method, denoted as 316L + TiOz. 2. Specimens which were subject to argon etching in glow discharge conditions, denoted by 316-ar. 3. Ground steel specimens, denoted by 316L. The examinations were carried out in oxygen-saturated Ringer solution (NaCl, 8.6 g dmp3; KCl, 0.3 g dme3; CaClz, 0.48g dm-3). The temperature of the solution during experiments was 37°C.
Polarization measurements Before specimen 316L + TiOz was placed in the electrochemical cell, the titanium oxide layer, weakly adhering to the substrate, was removed from its surface with a soft brush. Polarization speed dE/dt = 1 mVs_’ was applied, starting from cathode potentials and ending in anode potentials. Figure 5 presents cathode and anode polarization curves for the
El -
31 6L+Ti02
---316L ---316L-ar
10X
E 09 0
4h
-:-
A
24h
-:-
0
1Omin (316L+Ar)
lE+3 1E+Z
examinations
2mc
lE+S
1E+l
:u’
Ih
-:-
0
24h
-:-
lE+O lE+O lE+l
’
lE+2 lE+3 lE+4 lE+5 lE+6 lE+7 lE+6 2 real (ohm)
Figure 6 Impedance of the specimens 316L-ar and 316L + TiOp on a double-logarithmic coordinate system after different exposure times in Ringer solution.
following: just prepared substrate (specimen of type 316L); substrate subjected to ion etching in argon plasma (specimen of type 316L-ar); substrate covered with TiOz film (specimen of type 316L+TiOJ after 120h exposure in Ringer solution. The corrosion potential for the specimen of type 316L-ar is slightly higher than that for the specimen of type 316L. It may be suggested that the oxide film is created as a result of ion etching in argon plasma and then contact of the etched specimen with atmospheric oxygen, as a result of lower anode current density values. Higher cathode current densities for the specimen type 316L-ar compared to the specimen type 316L may be connected with the increase in the area of the specimen during the ion etching process. Pitting corrosion resistance, expressed as the potential value for which an abrupt change of the current on the anode curve is observed, is actually the same for both the steel substrate alone and the steel substrate subjected to ion etching. The polarization curve for the specimen type 316L + TiOz differs considerably from the curves for specimens 316L and 316L-ar. The steel of type 316L with TiOz coating does not undergo pitting corrosion (even when the potential is +3V); the speed of the anode process (thus approximately the speed of corrosion) for the potential values near the corrosion potential is lOO1000 times less than the speed of the anode process on the steel substrate alone. Such an extreme decrease in the speed of the anode and cathode processes is connected with the presence of the barrier layer, strongly inhibiting ion transport in the cathode and anode directions.
Impedance measurements I
i (Alcmz) Figure 5 specimens solution.
Cathode
and
anode
of types 316L, 316L-ar
polarization curves for and 316L + Ti02 in Ringer
The wide range of impedance component values for specimens of types 316L-ar and 316L+TiOz did not permit the measurement results to be shown on one plot on the complex plane (Nyquist). Therefore, they were presented as a double-logarithmic coordinate system (Figure 6). It can be seen that the TiOz coating Biomaterials
1997, Vol. 18 No. 11
792
PACVD
lE-10
IE-11
Capacitance and conductance equivalent circuit.
parallel
frequency
characteristics
of the steel of type 316L causes a considerable increase in impedance (approximately five orders). This arises conductance frequency from capacitance and characteristics (Figure 7). At the start of exposure of the specimen type 316L + TiOz in Ringer solution, the conductivity of the coating film increases. This is probably caused by electrolyte penetration of cracks, fissures and pinholes in the film, thus producing ion conductivity paths. After 4 h the conductivity decreases and then the oxide layers on the steel substrate are tightened. The properties of TiOz film on surgical steel depend upon the exposure time in Ringer solution. A few electrical circuit models were examined in the analysis of the impedance data. In the equivalent circuit, proposed as a model of two-layer coating of the specimen type 316L+TiOz, the presence of two
TiO P-x) 7
Biomaterials
Electrical
of specimens
IODO
1
of type
317L+TiOP
1997, Vol. 18 No. 11
and a model
of the specimen
in Ringer
IODDO
solution
in a
constant-phase elements was assumed (CPEl and CPE2), along with resistor R (Figure 8). The admittance of the CPE is given by the formula Y;(w) = Q . (j~)“~, This is a very general dispersion formula. If R = 0, CPE is a resistor, if n = 0.5 it is Warburg type behaviour, and it is an ideal capacitor if n = 1. Table 1 shows the values of parameters of elements of the electrical equivalent circuit for the specimen 316L+ TiOa for different exposure times in Ringer solution. For 120 h exposure time, the coefficients n of constant-phase elements are unity. This means that the elements are simply capacitors and the film forms a barrier layer. Determination of the potential of flat bands and the volume concentration of charge carriers, based on Mott-Schottky’s equation, is impossible. The influence of the potential on the capacity was not observed
I
circuit
96h
i
passive layer
equivalent
et al.
-/r
R,
Figure 8
J. Gluszek
t
$
0
Figure 7
TiO, films:
316L+TiOp
surface.
c2
PACVD
TiOn films:
Table 1
Electrical
J. Wuszek equivalent
793
et al. circuit
parameters
as a function
of exposure
r (h)
Ql
nl
R
1 4 48 120
4.73 x 10-g 9.1 x lo-” 1.2 x lo-‘0 2 x lO_‘O
0.40 0.81 1 1
3.16 9.60 2.22 1.38
x x x x
lo7 lo5 lo7 10’
_-.----
c? C3 n +
zlE+6 r
x
0
v 0 -ii-
: .-E N
time used to describe
the experimental
data
QP
n2
1.06 x 1O-7 3.13 x 10-r 2.36 x lo-* 4.0 x 10-g
0.52 0.29 0.49 1
150
2500mV 2000mV 15oOmV
130
.. .-.A
1OOOmV
A
6OOmV
ii0
OmV
a
-500mV -lOOOmV 90
E WI
Am
lE+5
m
70 1000
IOWO
frequency Figure 9 component exposure potentials.
IOOWO
IoooooO
(Hz)
The influence of frequency on the of the specimen of type 316L+Ti02 in Ringer solution for different
50 capacitance after 120 h polarization
Galvanic
currents
An attempt was made to determine the E-i relationship for the following two galvanic couples: 1. Galvanic couple and steel of the surface. 2. Galvanic couple steel of type 3161,
of two specimens (316L+ TiOJ type 316L with a just-renewed of two specimens (316L-ar) with a just-renewed surface.
I
1
I
I
I
I
1
20
30
40
50
80
70
80
90
i (nAkm2) Figure 10
within the potential range from -1000 to 2500 mV for a wide range of frequencies (Figure 9). It may be caused by the considerable thickness of the 316L + TiOz film, which amounts to 1 pm. In Ref. 9, the influence of polarization voltage on the capacity was observed for polycrystalline TiOz specimens of thickness approximately 20 ti.mes less. In Ref. 10, the authors who examined polycrystalline TiOz of thickness about 1 pm stated that the potential has some influence on the slope and shape of Bode plots, but the process conditions, the substrate and the solution were different from those applied in this work. Thus, the thickness seems not to be the main reason for the lack of influence of the potential on the capacity, but the method of coating formation” and barrier properties of the coating”.
I
10
Galvanic couple between 316L-ar and 316L.
316-ar-316L (Figure 20). Connecting the specimens 316L +TiOz by resistors of decreasing resistance values gave rise to large oscillations of 316L +TiOz electrode potential, and it was impossible to determine the electrode potential accurately. The potential of the other electrode (316L) and the current flowing in the circuit did not show large oscillations. Such behaviour of the specimen (316L+TiOz) comes from the dielectric properties or a thick barrier layer of titanium oxide on steel of type 316L. Figure 11 shows changes of the currents with time for both galvanic couples. The currents in galvanic couples are significantly lower than the corresponding current densities registered on anode polarization curves (Figure 5) in the passive area. This galvanic couple may be considered to be safe as far as corrosion is concerned, i.e. it is guaranteed that the increased substrate corrosion caused by any layer damage will not occur. In the case of the galvanic couple 316L +Ti02-316L, extremely low values of short circuit current are observed. They account for ca 20 pAcm_‘, which corresponds to a corrosion rate of ca lo-'mm year-l.
and
Those galvanic couples were to simulate the behaviour of the systems in conditions of coating defects (cracks, scratches, etc.). We managed only to determine relationships for the galvanic couple
CONCLUSIONS The PACVD method enables us to produce a titanium dioxide coating on stainless steel of type 316L from the substrates titanium tetrachloride and oxygen. The Biomaterials 1997, Vol. 18 No. 11
PACVD TiOp films: J. GIuszek
794
et al.
ACKNOWLEDGEMENTS The authors would like to thank KBN (Scientific Research Committee), Poland for the financial support. The research was done as a part of a grant (Contract No. 334059203. P4).
b-0-0-+0-0-0--0-0-0-0-0-0-0-0 lE-87
REFERENCES
lE-9s I(A/cm?
.
1.
IE-IO
2. .
A-A+A,\A_A-A_A,AA--a-A-A~A
ISA'
3.
lE-11:
IE-12 0
I 30
I 90
I 60
I 120
4. 150
t (min)
Figure 11 Changes of short circuit currents with time for galvanic couples: 316L + TiO,-316L, lower curve; 316L-ar316L, upper curve.
conditions for the process to be effective are as follows: reaction temperature, 300°C; working pressure, 1O’Pa: flows: 02, 3.7 x 10m3 molmin-‘; TiC14, 2.6 x 10m4 to min-l; 3.7 x lop3 mol 6.6 x 10m4mol min-l; Ar, reaction time, 30min; time of exposure in argon plasma, 10min. The film produced in the abovementioned conditions is anatase and crystallizes in the form of a tetragonal structure. During the exposure of steel of type 316L with titanium dioxide coating in Ringer solution, protective properties of the coating improve with time. After about 120 h, the layer has a barrier character. Titanium dioxide film protects surgical steel of type 316L outstandingly well against pitting corrosion and decreases its corrosion rate approximately 100-1000 times. TiOz film possesses one very advantageous property. Its damage or partial removal from steel substrate does not cause any increase in substrate corrosion. The short circuit current density of such a galvanic couple is extremely low (20 pA cm-‘).
Biomaterials 1997, Vol.
18
No. 11
5.
6.
7.
8. 9.
10.
11.
12.
Gluszek, J. and Masalski, J., Galvanic coupling of 316L
steel with titanium, niobium, and tantalum in Ringer’s solution. Br. Corros. J., 1992, 27, 135-138. Shalaby, L. A., Galvanic coupling of Ti with Cu and Al
alloys in chloride media. Corros. Sci., 1971, 11, 767778. Gluszek, J., Jedrkowiak, J., Markowski, J. and Masalski, J., Galvanic couples of 316L steel with Ti and ion plated Ti and TiN coatings in Ringer’s solutions. Biomaterials, 11, 330-335. Sedriks, A. J., Corrosion of Stainless Steels. John Wiley and Sons, New York, 1979, p. 110. Battiston, A., Gerbasi, R., Porchia, M. and Marigo, A., Influence of substrate on structural properties of TiOz thin films obtained via MOCVD. Thin Solid Films, 1994,239,186-191. Michalski, J. and Wierzchon, T., Apparatus for obtaining surface coatings. Polish Patent Application, W36560,46560,1988. Gluszek, J. and Masalski, J., The influence of variations in pH of Ringer’s solution on corrosion behaviour of surgical steel. In Progress in the Understanding and Prevention of Corrosion, ed. J.M. Costa and A.D. Mercer. The Institute of Materials, London, 1993, pp. 1289-1296. Hayashi, S. and Hirai, T., Chemical vapour deposition of rutile films. I. Crystal Growth, 1976, 36, 157-164. Nogami, G. and Ogawa, Y., Electrochemical properties of polycrystalline TiOz electrodes prepared by anodic oxidation. I. Electrochem. Sot., 1988, 12, 3008-3015. Pyun, S., Yang, T. and Yoon, Y., Impedance analysis of plasma-enhanced chemical-vapour-deposited TiOz film in 0.1 M NaOH solution near the flat-band potential. I. Alloys Compounds, 1994,205,53-57. Chazalviel, J.N., Surface methoxylation as the key factor for the good performance on N-Si methanol photoelectrochemical cells. I. Electroanal. Chem., 1987,233,37-48. Kelly, J. J. and Notten, P. H. L., Surface charging effect during photoanodic dissolution of n-GaAs electrodes. J. Electrochem. Sot., 1983, 130, 2452-2459.
SOl42-9612
PII
ELSEVIER
(96)
18 (1997)
789-794
1997
Elsevier Science Limited Printed in Great Britain. All rights reserved 014%9612/97/$17.00
00210-4
Structural and electrochemical examinations of PACVD Ti02 films in Ringer solution J. Gluszek, J. Masalski, P. Furman and K. Nitsch* institute of fnorganic Technology, Technical University of Wroc4’aw, Smoluchowskiego 25, 50-372 Wrocraw, Poland; ‘Institute of Electron Technology, Technical University of Wrocraw, Wybrzeie Wyspiahskiego 27, 50-370 Wrocraw, Poland The conditions and applying deposition
for obtaining
method
anatase
is created,
titanium
dioxide
120 h the coating resistivity
dioxide
stainless
crystallizing
coating,
of steel
superior
solution,
of the coating Science
does
Limited.
barrier
Titanium
Received 13 February
not cause
All rights
dioxide,
PACVD,
1995; accepted
titanium
tetrachloride
lattice.
that, during
During
properties
characteristics.
Titanium
corrosion
an increased
and general galvanic
assisted
the process,
exposure
protective
of the type 316L to pitting
removal
the substrates
It was established
in a tetragonal
in Ringer’s
adopts
from
steel of the type 316L by the plasma
have been determined.
Elsevier Keywords:
titanium
this to a surgical
vapour
titanium
dioxide
of the 316L steel
of this covering dioxide
covering
corrosion.
corrosion
and oxygen chemical
with the
improve.
After
increases
Any damage
of the substrate.
::,a
or partial 0
1997
reserved stainless
19 November
Titanium is a very reactive element (standard electrode potential -1.63V) and, due to its very strong chemical affinity to oxygen, ii. very easily produces a compact oxide film, ensuring high corrosion resistance of the metal. Titanium shows high corrosion resistance in physiological saline solution as well’, and its dissolution by-products such as titanium dioxide are generally well tolerated and do not appear to cause any undesirable reactions in patients. An outstandingly advantageous property of titanium is that when it forms a galvanic couple with a metal l~essnoble than itself, it (or titanium dioxide) undergoes strong cathode polarization and the potential of the galvanic couple is close to the potential of the less noble materia12.3. This means that, in the case of any damage to titanium dioxide film on steel substrate, one should not expect intensified substrate corrosion. Glow discharge plasma facilitates the course of chemical reactions and it is able to form a variety of superficial layers on different substrates. The main advantage of using the plasma is reduction in the temperature of films created from reactive substrates. This temperature reduction is particularly important for such processes in which increased temperature can cause undesirable structural changes of the substrate. When titanium dioxide film on austenitic steel is deposited using the conventional chemical vapour deposition (CVD) method in the temperature range of 600-9OO”C, the susceptibility of the steel substrate to intercrystalline corrosion increases4. The plasma method proves to be appropriate for obtaining a Ti02 covering on austenitic surgical steel of the type 316L. The literature abounds in articles
steel,
corrosion
1996
dealing with obtaining titanium dioxide with CVD and modified CVD methods on such substrate materials as glass, silicon, titanium, quartz, sapphire and sodium chloride. On the other hand, there is scanty information on how to apply titanium dioxide with the plasma method to a stainless steel substrate’.
EXPERIMENTAL A plasma assisted chemical vapour deposition (PACVD) device with a hot anode was used for TiOz film deposition6. Figure 1 is a diagrammatic representation of the plasma deposition system utilized in this study. The reactor chamber is constructed of stainless steel. Electric activation of the gas environment by direct current glow discharge is used. Applying electrical activation of the gas environment gives the possibility of producing TiOz films in TiC14 and O2 atmosphere over a wide range of temperatures. Using the hot anode prevents deposition of the reaction byproducts (mainly lower titanium chlorides) on the walls of the working chamber. TiOz films were deposited on stainless steel of type 316L. The specimens were discs of diameter 14 mm and thickness 1 mm. Surface treatment of the specimens consisted of grinding and finishing with abrasive paper of grain size 600. The specimens were degreased in acetone before being placed in the device for film deposition. In a typical deposition, the heating elements were heated to the desired temperature. The specimens were placed in the reactor and the chamber evacuated to a pressure of approximately 1O”Pa. The specimens
Correspondence to Dr J. Gluszek. 789
Biomaterials
1997, Vol. 18 No. 11
790
PACVD Ti02 films: J. GIuszek et a/.
8 Pc
of thermal expansion for TiOz is 8 x 10-6”C-‘s, while the coefficient for steel of the type 17Cr14Ni2Mo is two-fold greater (16-18 x 10-““C~l). Figure 3 shows that titanium dioxide film deposited on stainless steel carries irregularities on the substrate surface. It was determined (Figure 4) that the coating contained mainly oxide and titanium atoms. The 2.6 keV peak in the spectrum shows the presence of chlorine. Some
7
4
1
2
Figure 1 Diagram of measuring set for PACVD processes with a hot anode. 1, Resistance furnace; 2, working chamber; 3, heating elements; 4, treated substrates; 5, high-voltage power supply; 6, gas substrates dispenser; 7, current pass; 8, vacuum system. were then cleaned of organic contaminants by striking an argon plasma for 10min. At the end of the argon plasma treatment, Tic& and O2 flow rates were set at the desired values. As a result of a series of experiments, parameters of effective reaction for obtaining the films were determined as follows: reaction temperature, 300°C; working pressure, 10’ Pa; flow speed: 02, 3.7 x 10e3 molmin-‘; TiCl*, 2.6 x 10m4 to 6.6 x 10m4mol min-‘; deposition time, 30min. The measurement of TiOz thickness was performed on a kalotester device, made by Bernex. methods were used for Electron microscopic determining the structure and the contents. The following devices were applied: scanning electron microscope stereoscan 180 (Cambridge Instruments) equipped with X-ray microanalysis set (Link Systems), scanning electron microscope SEM 515 (Philips) with semiconductor X-ray detector (Edax) and transmission electron microscope EM301 (Philips). Eletrochemical examinations with the use of impedance spectroscopy and polarization examinations were made using a Schlumberger set (EC1 1286, FRA 1255)‘. Computer programs: ‘imp-opt v. 3.0’ and ‘~01-1286’ were used for measurements and ‘Equivalent Circuit v. 4.51’ made by B. Boukamp was applied for the impedance spectra analysis. Galvanic currents were determined by means of methods described in previous publications’* 3.
Figure 2
Microstructure
of Ti02 film on steel of type 316L.
Flgure 3
Microstructure
of TiOp film on steel of type 316L.
RESULTS AND DISCUSSION
Ti Ka
TiOz film structure Titanium dioxide deposition on the steel of the type 316L was composed of two layers. The first layer (1 pm thick) was strongly adherent to the substrate material and the second layer, appearing on the first one, was thicker, cracked and of weak adhesion to the substrate. The two-layer structure of the covering probably resulted from the fact that titanium dioxide, strongly adherent to the substrate and showing low electrical conductivity, caused electrical interlocking and the deposition process was inhibited. The cracks, clearly seen in Figure 2, can result from different thermal expansions of TiOz and the substrate. The coefficient Biomaterials
1997, Vol. 18 No. 11
2.0 OlNT
Figure 4 X-ray substrate.
4.0
6.0 o.ooKBv
spectrum
8.0 1CkV/ch
B EDAX
of TiOp film on the 316L steel
791
PACVD TiOp films: J. Wuszek et al.
other lower peaks stem from iron, chromium and nickel atoms in the substrate. The structure of the coating carefully scratched from the steel is characteristic of a polycrystal. Diffraction patterns of different fragments of the film provide additional proof of its polycrystalline form. They showed configuration and intensity close to each other, which proved to be the homogeneous polycrystalline structure of the oxide. from the Interplane distances, determined measurements of diffraction ring radii, allow one to state that the coating under examination constitutes anatase, crystallizing in tetragonal form.
Electrochemical
---__..-
~--T
1 E+7
++++$+*
;
Electrochemical examinations comprised polarization and impedance measurements and the measurements of galvanic currents. Three kinds of specimens were used in the examinations: 1. Specimens with TiOz film obtained with the PACVD method, denoted as 316L + TiOz. 2. Specimens which were subject to argon etching in glow discharge conditions, denoted by 316-ar. 3. Ground steel specimens, denoted by 316L. The examinations were carried out in oxygen-saturated Ringer solution (NaCl, 8.6 g dmp3; KCl, 0.3 g dme3; CaClz, 0.48g dm-3). The temperature of the solution during experiments was 37°C.
Polarization measurements Before specimen 316L + TiOz was placed in the electrochemical cell, the titanium oxide layer, weakly adhering to the substrate, was removed from its surface with a soft brush. Polarization speed dE/dt = 1 mVs_’ was applied, starting from cathode potentials and ending in anode potentials. Figure 5 presents cathode and anode polarization curves for the
El -
31 6L+Ti02
---316L ---316L-ar
10X
E 09 0
4h
-:-
A
24h
-:-
0
1Omin (316L+Ar)
lE+3 1E+Z
examinations
2mc
lE+S
1E+l
:u’
Ih
-:-
0
24h
-:-
lE+O lE+O lE+l
’
lE+2 lE+3 lE+4 lE+5 lE+6 lE+7 lE+6 2 real (ohm)
Figure 6 Impedance of the specimens 316L-ar and 316L + TiOp on a double-logarithmic coordinate system after different exposure times in Ringer solution.
following: just prepared substrate (specimen of type 316L); substrate subjected to ion etching in argon plasma (specimen of type 316L-ar); substrate covered with TiOz film (specimen of type 316L+TiOJ after 120h exposure in Ringer solution. The corrosion potential for the specimen of type 316L-ar is slightly higher than that for the specimen of type 316L. It may be suggested that the oxide film is created as a result of ion etching in argon plasma and then contact of the etched specimen with atmospheric oxygen, as a result of lower anode current density values. Higher cathode current densities for the specimen type 316L-ar compared to the specimen type 316L may be connected with the increase in the area of the specimen during the ion etching process. Pitting corrosion resistance, expressed as the potential value for which an abrupt change of the current on the anode curve is observed, is actually the same for both the steel substrate alone and the steel substrate subjected to ion etching. The polarization curve for the specimen type 316L + TiOz differs considerably from the curves for specimens 316L and 316L-ar. The steel of type 316L with TiOz coating does not undergo pitting corrosion (even when the potential is +3V); the speed of the anode process (thus approximately the speed of corrosion) for the potential values near the corrosion potential is lOO1000 times less than the speed of the anode process on the steel substrate alone. Such an extreme decrease in the speed of the anode and cathode processes is connected with the presence of the barrier layer, strongly inhibiting ion transport in the cathode and anode directions.
Impedance measurements I
i (Alcmz) Figure 5 specimens solution.
Cathode
and
anode
of types 316L, 316L-ar
polarization curves for and 316L + Ti02 in Ringer
The wide range of impedance component values for specimens of types 316L-ar and 316L+TiOz did not permit the measurement results to be shown on one plot on the complex plane (Nyquist). Therefore, they were presented as a double-logarithmic coordinate system (Figure 6). It can be seen that the TiOz coating Biomaterials
1997, Vol. 18 No. 11
792
PACVD
lE-10
IE-11
Capacitance and conductance equivalent circuit.
parallel
frequency
characteristics
of the steel of type 316L causes a considerable increase in impedance (approximately five orders). This arises conductance frequency from capacitance and characteristics (Figure 7). At the start of exposure of the specimen type 316L + TiOz in Ringer solution, the conductivity of the coating film increases. This is probably caused by electrolyte penetration of cracks, fissures and pinholes in the film, thus producing ion conductivity paths. After 4 h the conductivity decreases and then the oxide layers on the steel substrate are tightened. The properties of TiOz film on surgical steel depend upon the exposure time in Ringer solution. A few electrical circuit models were examined in the analysis of the impedance data. In the equivalent circuit, proposed as a model of two-layer coating of the specimen type 316L+TiOz, the presence of two
TiO P-x) 7
Biomaterials
Electrical
of specimens
IODO
1
of type
317L+TiOP
1997, Vol. 18 No. 11
and a model
of the specimen
in Ringer
IODDO
solution
in a
constant-phase elements was assumed (CPEl and CPE2), along with resistor R (Figure 8). The admittance of the CPE is given by the formula Y;(w) = Q . (j~)“~, This is a very general dispersion formula. If R = 0, CPE is a resistor, if n = 0.5 it is Warburg type behaviour, and it is an ideal capacitor if n = 1. Table 1 shows the values of parameters of elements of the electrical equivalent circuit for the specimen 316L+ TiOa for different exposure times in Ringer solution. For 120 h exposure time, the coefficients n of constant-phase elements are unity. This means that the elements are simply capacitors and the film forms a barrier layer. Determination of the potential of flat bands and the volume concentration of charge carriers, based on Mott-Schottky’s equation, is impossible. The influence of the potential on the capacity was not observed
I
circuit
96h
i
passive layer
equivalent
et al.
-/r
R,
Figure 8
J. Gluszek
t
$
0
Figure 7
TiO, films:
316L+TiOp
surface.
c2
PACVD
TiOn films:
Table 1
Electrical
J. Wuszek equivalent
793
et al. circuit
parameters
as a function
of exposure
r (h)
Ql
nl
R
1 4 48 120
4.73 x 10-g 9.1 x lo-” 1.2 x lo-‘0 2 x lO_‘O
0.40 0.81 1 1
3.16 9.60 2.22 1.38
x x x x
lo7 lo5 lo7 10’
_-.----
c? C3 n +
zlE+6 r
x
0
v 0 -ii-
: .-E N
time used to describe
the experimental
data
QP
n2
1.06 x 1O-7 3.13 x 10-r 2.36 x lo-* 4.0 x 10-g
0.52 0.29 0.49 1
150
2500mV 2000mV 15oOmV
130
.. .-.A
1OOOmV
A
6OOmV
ii0
OmV
a
-500mV -lOOOmV 90
E WI
Am
lE+5
m
70 1000
IOWO
frequency Figure 9 component exposure potentials.
IOOWO
IoooooO
(Hz)
The influence of frequency on the of the specimen of type 316L+Ti02 in Ringer solution for different
50 capacitance after 120 h polarization
Galvanic
currents
An attempt was made to determine the E-i relationship for the following two galvanic couples: 1. Galvanic couple and steel of the surface. 2. Galvanic couple steel of type 3161,
of two specimens (316L+ TiOJ type 316L with a just-renewed of two specimens (316L-ar) with a just-renewed surface.
I
1
I
I
I
I
1
20
30
40
50
80
70
80
90
i (nAkm2) Figure 10
within the potential range from -1000 to 2500 mV for a wide range of frequencies (Figure 9). It may be caused by the considerable thickness of the 316L + TiOz film, which amounts to 1 pm. In Ref. 9, the influence of polarization voltage on the capacity was observed for polycrystalline TiOz specimens of thickness approximately 20 ti.mes less. In Ref. 10, the authors who examined polycrystalline TiOz of thickness about 1 pm stated that the potential has some influence on the slope and shape of Bode plots, but the process conditions, the substrate and the solution were different from those applied in this work. Thus, the thickness seems not to be the main reason for the lack of influence of the potential on the capacity, but the method of coating formation” and barrier properties of the coating”.
I
10
Galvanic couple between 316L-ar and 316L.
316-ar-316L (Figure 20). Connecting the specimens 316L +TiOz by resistors of decreasing resistance values gave rise to large oscillations of 316L +TiOz electrode potential, and it was impossible to determine the electrode potential accurately. The potential of the other electrode (316L) and the current flowing in the circuit did not show large oscillations. Such behaviour of the specimen (316L+TiOz) comes from the dielectric properties or a thick barrier layer of titanium oxide on steel of type 316L. Figure 11 shows changes of the currents with time for both galvanic couples. The currents in galvanic couples are significantly lower than the corresponding current densities registered on anode polarization curves (Figure 5) in the passive area. This galvanic couple may be considered to be safe as far as corrosion is concerned, i.e. it is guaranteed that the increased substrate corrosion caused by any layer damage will not occur. In the case of the galvanic couple 316L +Ti02-316L, extremely low values of short circuit current are observed. They account for ca 20 pAcm_‘, which corresponds to a corrosion rate of ca lo-'mm year-l.
and
Those galvanic couples were to simulate the behaviour of the systems in conditions of coating defects (cracks, scratches, etc.). We managed only to determine relationships for the galvanic couple
CONCLUSIONS The PACVD method enables us to produce a titanium dioxide coating on stainless steel of type 316L from the substrates titanium tetrachloride and oxygen. The Biomaterials 1997, Vol. 18 No. 11
PACVD TiOp films: J. GIuszek
794
et al.
ACKNOWLEDGEMENTS The authors would like to thank KBN (Scientific Research Committee), Poland for the financial support. The research was done as a part of a grant (Contract No. 334059203. P4).
b-0-0-+0-0-0--0-0-0-0-0-0-0-0 lE-87
REFERENCES
lE-9s I(A/cm?
.
1.
IE-IO
2. .
A-A+A,\A_A-A_A,AA--a-A-A~A
ISA'
3.
lE-11:
IE-12 0
I 30
I 90
I 60
I 120
4. 150
t (min)
Figure 11 Changes of short circuit currents with time for galvanic couples: 316L + TiO,-316L, lower curve; 316L-ar316L, upper curve.
conditions for the process to be effective are as follows: reaction temperature, 300°C; working pressure, 1O’Pa: flows: 02, 3.7 x 10m3 molmin-‘; TiC14, 2.6 x 10m4 to min-l; 3.7 x lop3 mol 6.6 x 10m4mol min-l; Ar, reaction time, 30min; time of exposure in argon plasma, 10min. The film produced in the abovementioned conditions is anatase and crystallizes in the form of a tetragonal structure. During the exposure of steel of type 316L with titanium dioxide coating in Ringer solution, protective properties of the coating improve with time. After about 120 h, the layer has a barrier character. Titanium dioxide film protects surgical steel of type 316L outstandingly well against pitting corrosion and decreases its corrosion rate approximately 100-1000 times. TiOz film possesses one very advantageous property. Its damage or partial removal from steel substrate does not cause any increase in substrate corrosion. The short circuit current density of such a galvanic couple is extremely low (20 pA cm-‘).
Biomaterials 1997, Vol.
18
No. 11
5.
6.
7.
8. 9.
10.
11.
12.
Gluszek, J. and Masalski, J., Galvanic coupling of 316L
steel with titanium, niobium, and tantalum in Ringer’s solution. Br. Corros. J., 1992, 27, 135-138. Shalaby, L. A., Galvanic coupling of Ti with Cu and Al
alloys in chloride media. Corros. Sci., 1971, 11, 767778. Gluszek, J., Jedrkowiak, J., Markowski, J. and Masalski, J., Galvanic couples of 316L steel with Ti and ion plated Ti and TiN coatings in Ringer’s solutions. Biomaterials, 11, 330-335. Sedriks, A. J., Corrosion of Stainless Steels. John Wiley and Sons, New York, 1979, p. 110. Battiston, A., Gerbasi, R., Porchia, M. and Marigo, A., Influence of substrate on structural properties of TiOz thin films obtained via MOCVD. Thin Solid Films, 1994,239,186-191. Michalski, J. and Wierzchon, T., Apparatus for obtaining surface coatings. Polish Patent Application, W36560,46560,1988. Gluszek, J. and Masalski, J., The influence of variations in pH of Ringer’s solution on corrosion behaviour of surgical steel. In Progress in the Understanding and Prevention of Corrosion, ed. J.M. Costa and A.D. Mercer. The Institute of Materials, London, 1993, pp. 1289-1296. Hayashi, S. and Hirai, T., Chemical vapour deposition of rutile films. I. Crystal Growth, 1976, 36, 157-164. Nogami, G. and Ogawa, Y., Electrochemical properties of polycrystalline TiOz electrodes prepared by anodic oxidation. I. Electrochem. Sot., 1988, 12, 3008-3015. Pyun, S., Yang, T. and Yoon, Y., Impedance analysis of plasma-enhanced chemical-vapour-deposited TiOz film in 0.1 M NaOH solution near the flat-band potential. I. Alloys Compounds, 1994,205,53-57. Chazalviel, J.N., Surface methoxylation as the key factor for the good performance on N-Si methanol photoelectrochemical cells. I. Electroanal. Chem., 1987,233,37-48. Kelly, J. J. and Notten, P. H. L., Surface charging effect during photoanodic dissolution of n-GaAs electrodes. J. Electrochem. Sot., 1983, 130, 2452-2459.