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Effect of oxygen implantation upon the corrosion resistance of the OT-4-0 titanium alloy
XUilFACE
COATINGS ELSEVIER
Surface and CoatingsTechnology96 (I997) 223-229
HglltdOL06?
Effect of oxygen implantation upon the corrosion resistance of the OT-4-0 titanium alloy D. K r u p a a , , j. B a s z k i e w i c z a, J. K o z u b o w s k i
a,
A. B a r c z b, G. G a w l i k °, J. J a g i e l s k i °, B. L a r i s c h a
a Department of Materials Science andEngineering, Warsaw University of Technology, Narbutta 85, 02-524 Warsaw, Poland b Institute of Electron Technology, Al. Lotnikdw 46, 02-668 Warsaw, Poland c Institute of Technology of Electronics Materials, WOlczyhska 133, 01-919 Warsaw, Poland d Bergakadekmie Freiberg Teehnische UnirersitSt Institut FzTr Werkstofftechni#, Gustav-Zeuner-str 5, 09596 Freiberg, Germany
Received 13 November 1996; accepted 25 March 1997
Abstract
The properties of the surface layer on the OT-4-0 titanium alloy after oxygen implantation were examined. Polished sampIes were implanted with doses of 5 x 1016 and i x 10I70 + cm -2. The O + ion energy was 50 keV. Transmission electron microscopy has been used to investigate the microstructure of the implanted layers formed on the OT-4-0. It was found that the implanted layers are nanocrystalline rutile (TiOa). Depth profiles of oxygen and titanium were investigated by SIMS and corrosion resistance was analysed by electrochemical techniques. The results indicate that the oxygen implantation increases the corrosion resistance of the OT-4-0 titanium alloy. © 1997 Elsevier Science S.A. Keywords: Oxygen implantation; Surface layers; Titanium alloys
1. Introduction
Because of their specific properties, titanium and its alloys are used in orthopaedics as biocompatible materials. These specific properties include a high resistance to corrosion, both general and pitting, good mechanical properties (such as resistance to fatigue) and compatibility with human tissue. The high corrosion resistance of titanium and its alloys is due to passive layers formed spontaneously on their surfaces. These oxide layers also improve the biocompatibility of the alloys since they reduce the rate of corrosion, and thereby the rate at which ions of certain metals pass from the alloy to the local environment. This is especially important with the ions of chromium, vanadium and aluminium which, on passing to the local environment, can irritate the muscles that surround the implant. The passive layers formed spontaneously are, however, very thin and thus liable to damage when subjected to frictional corrosion. We can expect that the corrosion resistance of titanium alloys will be increased when thicker oxide layers are formed on their surface. This can be done by:
* Corresponding author. 0257-8972/97/$17.00 © 1997ElsevierScienceS.A. All rights reserved. PIIS0257-8972(97)00107-2
(1) electrochemical oxidization; (2) glow-discharge oxidization; (3) implantation of oxygen ions. The effects of implantation of oxygen ions on the structure of the surface layers formed on titanium were described by Okabe et al. [1-4] who examined the mechanism of the formation of oxides during oxygen ion implantation into the titanium surface. The oxygen doses ranged from 2 x 10 Iv to 2 x 101Scm -2. X-ray diffraction (XRD) patterns of implanted titanium sheets were measured to estimate the formation of titanium oxides. These investigators have found that the kind of oxide formed depends on the oxygen dose: at lower doses, TiO was formed and at higher doses, TiO2. In Ref. [1], they also describe how the oxide formation mechanism depends on the implantation temperature. The oxygen doses used ranged from 2x1017 to 2 x l 0 1 S O ÷ cm -2, and the implantation temperature from - 6 0 to 300 °C. With the oxygen dose exceeding 1 x 10 zs O +, TiO formed at lower temperatures ( - 6 0 to - 3 0 ° C ) and TiO2 (futile) at higher (50-300°C). Moreover, when implanting at low temperatures, increased oxygen doses did not result in the TiO-TiO2 transformation, which suggests that TiO2 forms not only
D. Krupa et aL / Surface and Coatings Technology 96 (t997) 223-229
224
as a result of the oxygen implantation itself, but also under the thermal impact of the process. We can find in the literature a number of reports on the effect of oxygen ion implantation on the mechanical properties of titanium and its alloys [2, 5, 6], but no data are available concerning the effect of oxygen ion implantation upon the corrosion resistance of these materials. There are numerous reports on the corrosion resistance of titanium and its alloys in H z S O 4 , H s P O 4 and NaC1 environments [7-14]. The aim of the present study is to determine how the structure and the corrosion resistance of the passive layer formed on the surface of a titanium alloy are affected by the implantation of oxygen ions into this surface.
~.,-~::,'~,,'~'4..'..~~>,.-.~x;>~0 ,-.',//.,.:,'~i.;//,,6.~ .;' ,;~.~-<,:~ . ~
~?;~).:;/./:';.:L~Yx:/,-7~..,~r. ,:.>'E,~SUz <~,.;"~/~4,t~k~J U 2;
Fig. I. Microstr~cture o f the O T - 4 - 0 alloy, etched in a solution composed o f 16 cm ~ HNOs, 16 cm ~ and 60 cm ~ glycerine.
2. Materials and methods
decreased in 2 mV decrements in the cathodic direction until a potential 20 mV below Eoorr was achieved. After each potential decrement, the electric current and potential were allowed to stabilize for 5 min. The polarization resistance (Rp) was calculated by the least squares method. The polarization resistance being measured, the samples were polarized in the anodic direction beginning from a potential of - 800 mV up to a potential of 5000mV. The potential variation rate was 100 mV min-1; the reference electrode was a saturated calomel electrode. The charge (Q) that passed in the anodic region during the sample polarization to a potential of 4000 mV was calculated. After the polarization, the samples were examined using an optical microscope. The Vickers hardness H~ of O+-implanted Ti was measured using a Fischerscope H-100 hardness tester with the load increased from 0.4 to 1000 mN and then alleviated (unloading). There were 30 measuring points (stops) during loading and 30 measuring points during the load alleviation. At all the indentation depths, the measuring time was 1 s. Five measurements were made for each sample and the standard deviation was calculated.
The chemical composition of the alloy examined is given in Table 1 and its microstructure is shown in Fig. 1. Disk-shaped test samples 23 mm in diameter and 2 mm high were mechanically polished on one side to a mirror finish. Then they were implanted with oxygen ions in doses of 5 x l 0 ~ 6 0 + c m -2 and 1 x 10 ~v O + cm -~. The O + ion energy was 50 keV and the implantation temperature did not exceed 70 °C. The implantation was carried out in a BALZERS MPB-202RP apparatus. The chemical composition of the surface layers obtained after implantation was examined by SIMS and the chemical composition profiles were determined using an argon beam of energy of 4 keV. The scanned surface area was about 1 mm a, and the rate at which the material was removed was 0.15 nm s- L Structural (TEM) examinations were made using a PHILIPS EM 300 transmission electron microscope. The TEM samples were cut by the electrospark method, and then one-side thinned using a Struers electrolytic thinner from the nonimplanted surface until a perforation occurred. The resistance to corrosion of the samples was examined in 3% NaC1 and 15% H 2 S O 4 solutions at a temperature of 20 °C using two methods: the linear polarization method (Stern's method) and the polarization curves method. The electrode to be examined was exposed to the test conditions for 24 h so that the corrosion potential E~or~ could stabilize. To define polarization resistance Rp the polarization was started from a potential about 20mV higher than E~o~ and the voltage was
3. Results 3.1. T E M results The initial state of the material to be implanted is characterized by the grain size within the range 0.3-5 gm and the presence of very small precipitates of uniden-
Table I The chemical composition of the OT-4-0 alloy (wt.%) C
Mn
Si
S
Cr
Ni
Cu
Co
A1
V
N
Fe
Ti
<0,01
0.7i
0.0I
0.00I
0.0I
0.03
0.002
0.01
0.67
0.04
0.0059
0,09
98
D. Krupa er al. / Surface and Coatings Technology 96 (1997) 223-229
tiffed phase. The material was not completly recrystallized as indicated by the presence of subgrains and the relatively high density of dislocations (Fig. 2). The implantation of oxygen ions results in a Ti20 (rutile) layer being formed irrespective of the oxygen dose applied (Fig. 3). This surface layer of oxide is composed of nanocrystaline ( ~ 10 nm) rutile grains, usually highly
225
textured in the area of the former matrix grains. This texture is probably a result of the topotactic growth of the oxide inside the matrix grain. The TEM results obtained differ from those reported by [1-4]. Okabe et al. observed the formation of TiO2 at implanted oxygen doses above 1 x 10 is O + cm -2, whereas in our experiment we identified the presence of TiO2 at lower
(a)
(b) Fig. 2. Microstructure and electron diffraction pattern of OT-4-0 non-impIanted alloy. The diffraction pattern (b) was taken from the upper part
of (a) and showsquasi singlecrystalpattern of upper part of three slightlymisorientedsubgrains.No reflectionsfrom the titaniumoxide are visible.
226
D. Krupa et al. / Surface and Coatings Technology 96 (]997) 223-229
Fig. 3. Microstructure and electron diffraction pattern of oxygen implanted OT-4-0 for the dose of 1 x 1017O ÷ cm -a. No matrix reflections are visible, only the reflection from textured rutile grains, Arched diffraction spots indicates prefered [00t] orientation. The presence of continuous rings on the diffraction pattern suggests that a certain number of randomly oriented rutile grain are also present.
oxygen doses. This different c o n d i t i o n s c a r r i e d out. O k a b e between 75 a n d 150
difference m a y be a t t r i b u t e d to the u n d e r which the i m p l a n t a t i o n was et al. used a b e a m o f an energy keV, whereas in o u r e x p e r i m e n t the
b e a m energy was 50 keV. The difference m a y also be due ~to the differences in the m e t h o d s used for the identification o f the phases present in the surface layer (TEM, XRD).
D. Krupa et aI. / Surface and Coatings Technology 96 (1997) 223-229
~o~
3.2. S I M S resuhs
The oxygen concentration depth profile in the surface layer produced by oxygen implantation at a dose of 1 x 10~70 + cm -a is shown in Fig. 4. The oxygen profile measured by the SIMS technique shows a substantial accumulation of oxygen on the surface and down to a depth of 50nm. This is not observed in the nonimplanted sample. The variations in the titanium profile, showing a similar trend to that of oxygen, indicate that the Ti atoms have been oxidized and thus their ionization probability has increased [15]. In the samples implanted with a smaller oxygen dose, similar changes are observed, but they are less pronounced. 3.3. Resistance
to
227
1
Q
3 ,,i I/i
,/"~/ 0,001 i -1000
10'00
20'00 30'00 POTENTIAL [rnV]
40'00
5000
Fig. 5. The anodic polarization curves measured for the O T - 4 - 0 alloy in a solution of 3% NaCI: (1) non-implanted specimen; (2) specimen implanted with a 5 x 1016 O + cm -2 dose; (3) specimen implanted with a i x 1017 O + cm -2 dose.
1°l ./,,,,
" "Z
o,I
106-
i 2 -
i./'
0,01
corrosion
The results of the electrochemical analysis by the Stern method (Rp) and the values of the corrosion potential E~o~rare given in Table 2. This table also gives the amount of the electric charge that passed through the sample in the anodic region. This latter parameter was introduced in order to visualize better the differences between the values of the corrosion resistance within the alloy examined. The polarization curves for the alloy implanted at various oxygen doses are shown in Figs. 5 and 6.
%,,,....................,....-..,.'~/ /
.,."
/
,,,.i
,^\
\
..........
/
0.01:
g'io 5
3
Ti
/
/ r ---./ . ,
i-{ 0,001 -1000
t•--D3104'~ CO CO
~
~q
16o Depth
(rim)
260
1obo
aobo E [mV]
30'00
40'00
5000
Fig. 6. The anodic polarization curves measured for the O T - 4 - 0 alloy in a solution of 15% H2SO4: ( 1) non-implanted specimen; (2) specimen implanted with a 5 x 10 t 6 0 + cm -2 dose; and (3) specimen implanted with a 1 x I0 I 7 0 + cm -2 dose.
0-2
0-I
lo%
6
i
a6o
Fig. 4. Oxygen and titanium concentration depth profiles: O-i nonimplanted; 0 - 2 implanted with a dose of 1 x 101~ O + cm -2.
Based on the results obtained, we can conclude that the ions implanted into the surface of the OT-4-0 titanium alloy increase its corrosion resistance in both the environments used in the experiment. This can be inferred from the increased polarization resistance .Rp,
Table 2 Results of electrochemical experiments Specimen
Non-implanted Implanted dose of 5 x I016 O + cm -2 Implanted dose of I x I017 O ÷ cm -2
Eco= (mV)
Rp (Mg~ x cm 2)
Q (C cm a)
3% NaCI
15% H2SO 4
3% NaC1
15% H2SO 4
3% NaC1
15% I-I2SO 4
- i25 132 178
- 139 -78 17
4 14 28
0.35 0.95 3.9
0.705 0.266 0.043
1 0.93 0.553
228
D. Krupa et aL/ Surface and Coatings Technology 96 (J997) 223-229
the decreased amount of electric charge, and the changes in the polarization curves. It should be noted that this advantagous effect becomes stronger with increasing oxygen dose. In the 3% NaC1 environment, the polarization resistance increased by the factor of three to seven depending on the oxygen dose. This factor was even greater in the 15% H 2 S O 4 environment: at the lower oxygen dose, the polarization resistance increased three times, whereas at the higher dose, 11 times (Table 2). If we compare the values of the polarization resistance in the two environments, we see that, as can be expected, they are greater in 3% NaC1 than in 15% H 2 S O 4, This is evidence that in NaC1, the oxide layer is more resistant to corrosion. The decreased amount of the electric charge Q (C cm -2) also reflects the advantageous effect of the oxygen implantation. The amount of charge is associated with the degree of oxidization of the sample during the polarization. The smaller the charge, the more strongly the sample resists oxidization. The effect of implantation can also be seen in the polarization curves: for the implanted samples examined in both sodimn chloride and sulphuric acid environments, the values of the corrosion potential are shifted towards positive values (Table 2) and the curves are lowered, which means that the anodic current is smaller. In the 3% NaC1 environment, the anodic current is reduced within the entire potential range. The effect becomes stronger as the oxygen dose is increased. At smaller oxygen doses, the maximum of the current density is lower and shifted towards positive values. With increasing oxygen dose, the current density peak vanishes. Beginning from about 700mV, the anodic current increases, probably due to the growth of the oxide layer. In the 15% H 2 S O 4 environment, for the oxygenimplanted samples the anodic current begins to increase at about t000 mV and reaches a maximum between 3000 and 4000 mV depending on the oxygen dose. The current peaks are here higher for implanted samples than for non-implanted, and also shifted further towards positive potentials. Above a potential of 4 V, the anodic current magnitudes in small-dose implanted and nonimplanted samples are similar, whereas in samples implanted with a higher dose, the anodic current is reduced. The reduction in the anodic current observed after implantation in samples exposed to both environments is due to the increased thickness of the oxide layer. Armstrong and Quinn [13] report that the oxide layers, whose thickness does not exceed 1.5 nm, hinder the charge transfer at the beginning of the oxidization process when present on titanium surface. The oxide layers formed spontaneously on titanium are very thin: according to Schultze et al. [14], their thickness is 1.2 nm. The thickness of the solid oxide layer formed by oxygen ion implantation is greater - it is about
50 nm. We can suppose that this thicker layer hinders the charge transfer more strongly. This is reflected by the reduced amount of electric charge that passes through the titanium surface in implanted samples. The smallest value was observed in the samples implanted with the greater oxygen dose (1 x 1017 O + cm-2). The presence of the current density maximum on the polarization curves at a potential of about 3 V is, however, difficult to interpret. When examining the properties of the oxide layers deposited on pure titanium, Armstrong and Quinn [ 13] found a peak at about 2 V. They suggest that this peak is due to the modification of the oxide layer structure. The shift of the current density peak to about 3 V, observed in our experiment, can be explained in terms of the difference in the chemical composition of the materials examined in the two studies and, hence, different dopants present in the oxide layer. The oxide layers formed during implantation are colourless. Observations of the sample surfaces implanted and non-implanted after polarization have shown that the exposed surfaces have changed their colour; during polarization they become golden-brown. This change in colour is probably due to the change in thickness or in composition of the oxide layer. Observations of the sample surfaces using an optical microscope have shown that during the polarization, the grain boundaries are exposed there. This effect is observed in both implanted and non-implanted samples. 3.4. Hardness
How the surface hardness of the OT-4-0 alloy changes after subjecting the samples to oxygen ion implantation is shown in Fig. 7, where the hardness is plotted as a function of the distance down from the 8000
7000-
E 6000"
z. 5000W Z
Q < Z
4O00-
3000-
2000
0
0:2
014- 018 ola
~
t:2
1'.4
118 118
DEPTH [um]
Fig. 7. Hardness versus indentation depth profiles: (a) specimen nonimplanted; (b) specimen implanted with a 1 x l0 ~70 + cm -2 dose; (c) specimen implanted with a 5 x 1016 O ÷ cm -2 dose.
D. Krupa eta(. / Surface and Coatings Technology 96 (1997) 223-229
sample surface. We can see that the implanted ions do not change the hardness of the alloy. This is in agreement with the results obtained by P. Shioshansi and R. Oliver [5] who found that doses of 1 x 10 iv to 5 x 10 I 7 0 + cm -2 did not affect the hardness. A different result was reported by Saritas et al. [6] who observed a slight increase of hardness after the implantation of oxygen ions at a dose of 2 x 10 iv O + cm -2. The dependence of the titanium hardness on the implanted oxygen dose was examined by Okabe et aI. [2]. They found that at a dose of 2 x 1017 O + cm -2, the hardness increased only slightly compared with that of non-implanted samples. It was only between 4 x 10 iv and 6 x 10 iv O + cm -2 that a rapid increase in hardness was observed. A further increase in the oxygen dose, from 1 x l0 is to 2× 101so +, had no influence upon the hardness. Based on these reports, the fact that, in our experiments, the oxygen ion implantation did not affect the hardness of the O T - 4 - 0 alloy may be explained by the low oxygen dose.
4. Conclusions
(1) The implantation of oxygen ions into the surface of the O T - 4 - 0 alloy using doses of 5 x 1016 or 1 x 1017cm -2 results in a nanocrystalline TiO2 (futile) layer being formed on the surface. (2) The oxide layer formed by oxygen implantation increases the corrosion resistance of the alloy. (3) The increase of the corrosion resistance depends on
229
the oxygen dose applied. It appeared to be greater when the material was implanted with the higher of the oxygen doses used in the experiment. (4) At oxygen doses of 5 x 1016 and 1 x 1017 cm -2, the oxygen ion implantation does not affect the hardness of the O T - 4 - 0 alloy.
References [1] Y. Okabe, M. Iwaki, K. Takahashi, Mater. Sci. Eng. Al15 (i989) 79. [2] Y. Okabe, M. Iwaki, K. Takahashi, S. Ohira, B.V. Crist, Nucl. Instrum. Methods Phys. Res. B39 (1989) 619. [3] Y. Okabe, M. Iwaki, K. Takahashi, Nucl. Lnstrum. Methods Phys. Res. B61 (1991) 44. [4] Y. Okabe, T. Fujihana, M. Iwaki, B.V. Crist, Surf. Coat. Technol. 66 (1994) 384. [5] P. Sioshansi, R. Oliver, Mater. Res. Soc. Symp. Proc. 55 (1986) 237. [6] S. Saritas, R.P.M. Procter, W.A. Grant, Mater. Sci. Eng. 90 (1987) 297. [7] F. Alonso, A. Arizaga, S. Quainton, J.J. Ugarte, J.g. Viviente, J.I. Ofiate, Surf. Coat. Technol. 74/75 (1995) 986. [8] D. Krupa, E. Jezierska, J. Baszkiewicz, T. Wierzchofi, A. Barcz, Surf. Coat. TechnoI. 79 (1996) 240. [9] L.D. Arsov, Electrochimica Acta 27 (6) (1982) 663. [10] E.J. Kelly, Modern Aspects of Electrochemistry, Plenum Press, New York, 1982, p. 319. [11] L.D. Arsov, Electrochimica Acta 30 (12) (1985) 1645. [12] A.M. De Becdelievre, D. Fleche, J.D. Becdelievre, Electrochimica Acta 33 (8) (I988) I067. [13] N.R. Armstrong, R.K. Quinn, Surf. Sci. 67 (i977) 451. [14] J.W. Schultze, B. Danzfuss, O. Meyer, U. Stimming, Mater. Sci. Eng. 69 (1985) 232. [15] R.G. Wilson, F.A. Stevie, C.W. Magee, Secondary Ion Mass Spectrometry, John Wiley and Sons, NY, i989, p. 1.
COATINGS ELSEVIER
Surface and CoatingsTechnology96 (I997) 223-229
HglltdOL06?
Effect of oxygen implantation upon the corrosion resistance of the OT-4-0 titanium alloy D. K r u p a a , , j. B a s z k i e w i c z a, J. K o z u b o w s k i
a,
A. B a r c z b, G. G a w l i k °, J. J a g i e l s k i °, B. L a r i s c h a
a Department of Materials Science andEngineering, Warsaw University of Technology, Narbutta 85, 02-524 Warsaw, Poland b Institute of Electron Technology, Al. Lotnikdw 46, 02-668 Warsaw, Poland c Institute of Technology of Electronics Materials, WOlczyhska 133, 01-919 Warsaw, Poland d Bergakadekmie Freiberg Teehnische UnirersitSt Institut FzTr Werkstofftechni#, Gustav-Zeuner-str 5, 09596 Freiberg, Germany
Received 13 November 1996; accepted 25 March 1997
Abstract
The properties of the surface layer on the OT-4-0 titanium alloy after oxygen implantation were examined. Polished sampIes were implanted with doses of 5 x 1016 and i x 10I70 + cm -2. The O + ion energy was 50 keV. Transmission electron microscopy has been used to investigate the microstructure of the implanted layers formed on the OT-4-0. It was found that the implanted layers are nanocrystalline rutile (TiOa). Depth profiles of oxygen and titanium were investigated by SIMS and corrosion resistance was analysed by electrochemical techniques. The results indicate that the oxygen implantation increases the corrosion resistance of the OT-4-0 titanium alloy. © 1997 Elsevier Science S.A. Keywords: Oxygen implantation; Surface layers; Titanium alloys
1. Introduction
Because of their specific properties, titanium and its alloys are used in orthopaedics as biocompatible materials. These specific properties include a high resistance to corrosion, both general and pitting, good mechanical properties (such as resistance to fatigue) and compatibility with human tissue. The high corrosion resistance of titanium and its alloys is due to passive layers formed spontaneously on their surfaces. These oxide layers also improve the biocompatibility of the alloys since they reduce the rate of corrosion, and thereby the rate at which ions of certain metals pass from the alloy to the local environment. This is especially important with the ions of chromium, vanadium and aluminium which, on passing to the local environment, can irritate the muscles that surround the implant. The passive layers formed spontaneously are, however, very thin and thus liable to damage when subjected to frictional corrosion. We can expect that the corrosion resistance of titanium alloys will be increased when thicker oxide layers are formed on their surface. This can be done by:
* Corresponding author. 0257-8972/97/$17.00 © 1997ElsevierScienceS.A. All rights reserved. PIIS0257-8972(97)00107-2
(1) electrochemical oxidization; (2) glow-discharge oxidization; (3) implantation of oxygen ions. The effects of implantation of oxygen ions on the structure of the surface layers formed on titanium were described by Okabe et al. [1-4] who examined the mechanism of the formation of oxides during oxygen ion implantation into the titanium surface. The oxygen doses ranged from 2 x 10 Iv to 2 x 101Scm -2. X-ray diffraction (XRD) patterns of implanted titanium sheets were measured to estimate the formation of titanium oxides. These investigators have found that the kind of oxide formed depends on the oxygen dose: at lower doses, TiO was formed and at higher doses, TiO2. In Ref. [1], they also describe how the oxide formation mechanism depends on the implantation temperature. The oxygen doses used ranged from 2x1017 to 2 x l 0 1 S O ÷ cm -2, and the implantation temperature from - 6 0 to 300 °C. With the oxygen dose exceeding 1 x 10 zs O +, TiO formed at lower temperatures ( - 6 0 to - 3 0 ° C ) and TiO2 (futile) at higher (50-300°C). Moreover, when implanting at low temperatures, increased oxygen doses did not result in the TiO-TiO2 transformation, which suggests that TiO2 forms not only
D. Krupa et aL / Surface and Coatings Technology 96 (t997) 223-229
224
as a result of the oxygen implantation itself, but also under the thermal impact of the process. We can find in the literature a number of reports on the effect of oxygen ion implantation on the mechanical properties of titanium and its alloys [2, 5, 6], but no data are available concerning the effect of oxygen ion implantation upon the corrosion resistance of these materials. There are numerous reports on the corrosion resistance of titanium and its alloys in H z S O 4 , H s P O 4 and NaC1 environments [7-14]. The aim of the present study is to determine how the structure and the corrosion resistance of the passive layer formed on the surface of a titanium alloy are affected by the implantation of oxygen ions into this surface.
~.,-~::,'~,,'~'4..'..~~>,.-.~x;>~0 ,-.',//.,.:,'~i.;//,,6.~ .;' ,;~.~-<,:~ . ~
~?;~).:;/./:';.:L~Yx:/,-7~..,~r. ,:.>'E,~SUz <~,.;"~/~4,t~k~J U 2;
Fig. I. Microstr~cture o f the O T - 4 - 0 alloy, etched in a solution composed o f 16 cm ~ HNOs, 16 cm ~ and 60 cm ~ glycerine.
2. Materials and methods
decreased in 2 mV decrements in the cathodic direction until a potential 20 mV below Eoorr was achieved. After each potential decrement, the electric current and potential were allowed to stabilize for 5 min. The polarization resistance (Rp) was calculated by the least squares method. The polarization resistance being measured, the samples were polarized in the anodic direction beginning from a potential of - 800 mV up to a potential of 5000mV. The potential variation rate was 100 mV min-1; the reference electrode was a saturated calomel electrode. The charge (Q) that passed in the anodic region during the sample polarization to a potential of 4000 mV was calculated. After the polarization, the samples were examined using an optical microscope. The Vickers hardness H~ of O+-implanted Ti was measured using a Fischerscope H-100 hardness tester with the load increased from 0.4 to 1000 mN and then alleviated (unloading). There were 30 measuring points (stops) during loading and 30 measuring points during the load alleviation. At all the indentation depths, the measuring time was 1 s. Five measurements were made for each sample and the standard deviation was calculated.
The chemical composition of the alloy examined is given in Table 1 and its microstructure is shown in Fig. 1. Disk-shaped test samples 23 mm in diameter and 2 mm high were mechanically polished on one side to a mirror finish. Then they were implanted with oxygen ions in doses of 5 x l 0 ~ 6 0 + c m -2 and 1 x 10 ~v O + cm -~. The O + ion energy was 50 keV and the implantation temperature did not exceed 70 °C. The implantation was carried out in a BALZERS MPB-202RP apparatus. The chemical composition of the surface layers obtained after implantation was examined by SIMS and the chemical composition profiles were determined using an argon beam of energy of 4 keV. The scanned surface area was about 1 mm a, and the rate at which the material was removed was 0.15 nm s- L Structural (TEM) examinations were made using a PHILIPS EM 300 transmission electron microscope. The TEM samples were cut by the electrospark method, and then one-side thinned using a Struers electrolytic thinner from the nonimplanted surface until a perforation occurred. The resistance to corrosion of the samples was examined in 3% NaC1 and 15% H 2 S O 4 solutions at a temperature of 20 °C using two methods: the linear polarization method (Stern's method) and the polarization curves method. The electrode to be examined was exposed to the test conditions for 24 h so that the corrosion potential E~or~ could stabilize. To define polarization resistance Rp the polarization was started from a potential about 20mV higher than E~o~ and the voltage was
3. Results 3.1. T E M results The initial state of the material to be implanted is characterized by the grain size within the range 0.3-5 gm and the presence of very small precipitates of uniden-
Table I The chemical composition of the OT-4-0 alloy (wt.%) C
Mn
Si
S
Cr
Ni
Cu
Co
A1
V
N
Fe
Ti
<0,01
0.7i
0.0I
0.00I
0.0I
0.03
0.002
0.01
0.67
0.04
0.0059
0,09
98
D. Krupa er al. / Surface and Coatings Technology 96 (1997) 223-229
tiffed phase. The material was not completly recrystallized as indicated by the presence of subgrains and the relatively high density of dislocations (Fig. 2). The implantation of oxygen ions results in a Ti20 (rutile) layer being formed irrespective of the oxygen dose applied (Fig. 3). This surface layer of oxide is composed of nanocrystaline ( ~ 10 nm) rutile grains, usually highly
225
textured in the area of the former matrix grains. This texture is probably a result of the topotactic growth of the oxide inside the matrix grain. The TEM results obtained differ from those reported by [1-4]. Okabe et al. observed the formation of TiO2 at implanted oxygen doses above 1 x 10 is O + cm -2, whereas in our experiment we identified the presence of TiO2 at lower
(a)
(b) Fig. 2. Microstructure and electron diffraction pattern of OT-4-0 non-impIanted alloy. The diffraction pattern (b) was taken from the upper part
of (a) and showsquasi singlecrystalpattern of upper part of three slightlymisorientedsubgrains.No reflectionsfrom the titaniumoxide are visible.
226
D. Krupa et al. / Surface and Coatings Technology 96 (]997) 223-229
Fig. 3. Microstructure and electron diffraction pattern of oxygen implanted OT-4-0 for the dose of 1 x 1017O ÷ cm -a. No matrix reflections are visible, only the reflection from textured rutile grains, Arched diffraction spots indicates prefered [00t] orientation. The presence of continuous rings on the diffraction pattern suggests that a certain number of randomly oriented rutile grain are also present.
oxygen doses. This different c o n d i t i o n s c a r r i e d out. O k a b e between 75 a n d 150
difference m a y be a t t r i b u t e d to the u n d e r which the i m p l a n t a t i o n was et al. used a b e a m o f an energy keV, whereas in o u r e x p e r i m e n t the
b e a m energy was 50 keV. The difference m a y also be due ~to the differences in the m e t h o d s used for the identification o f the phases present in the surface layer (TEM, XRD).
D. Krupa et aI. / Surface and Coatings Technology 96 (1997) 223-229
~o~
3.2. S I M S resuhs
The oxygen concentration depth profile in the surface layer produced by oxygen implantation at a dose of 1 x 10~70 + cm -a is shown in Fig. 4. The oxygen profile measured by the SIMS technique shows a substantial accumulation of oxygen on the surface and down to a depth of 50nm. This is not observed in the nonimplanted sample. The variations in the titanium profile, showing a similar trend to that of oxygen, indicate that the Ti atoms have been oxidized and thus their ionization probability has increased [15]. In the samples implanted with a smaller oxygen dose, similar changes are observed, but they are less pronounced. 3.3. Resistance
to
227
1
Q
3 ,,i I/i
,/"~/ 0,001 i -1000
10'00
20'00 30'00 POTENTIAL [rnV]
40'00
5000
Fig. 5. The anodic polarization curves measured for the O T - 4 - 0 alloy in a solution of 3% NaCI: (1) non-implanted specimen; (2) specimen implanted with a 5 x 1016 O + cm -2 dose; (3) specimen implanted with a i x 1017 O + cm -2 dose.
1°l ./,,,,
" "Z
o,I
106-
i 2 -
i./'
0,01
corrosion
The results of the electrochemical analysis by the Stern method (Rp) and the values of the corrosion potential E~o~rare given in Table 2. This table also gives the amount of the electric charge that passed through the sample in the anodic region. This latter parameter was introduced in order to visualize better the differences between the values of the corrosion resistance within the alloy examined. The polarization curves for the alloy implanted at various oxygen doses are shown in Figs. 5 and 6.
%,,,....................,....-..,.'~/ /
.,."
/
,,,.i
,^\
\
..........
/
0.01:
g'io 5
3
Ti
/
/ r ---./ . ,
i-{ 0,001 -1000
t•--D3104'~ CO CO
~
~q
16o Depth
(rim)
260
1obo
aobo E [mV]
30'00
40'00
5000
Fig. 6. The anodic polarization curves measured for the O T - 4 - 0 alloy in a solution of 15% H2SO4: ( 1) non-implanted specimen; (2) specimen implanted with a 5 x 10 t 6 0 + cm -2 dose; and (3) specimen implanted with a 1 x I0 I 7 0 + cm -2 dose.
0-2
0-I
lo%
6
i
a6o
Fig. 4. Oxygen and titanium concentration depth profiles: O-i nonimplanted; 0 - 2 implanted with a dose of 1 x 101~ O + cm -2.
Based on the results obtained, we can conclude that the ions implanted into the surface of the OT-4-0 titanium alloy increase its corrosion resistance in both the environments used in the experiment. This can be inferred from the increased polarization resistance .Rp,
Table 2 Results of electrochemical experiments Specimen
Non-implanted Implanted dose of 5 x I016 O + cm -2 Implanted dose of I x I017 O ÷ cm -2
Eco= (mV)
Rp (Mg~ x cm 2)
Q (C cm a)
3% NaCI
15% H2SO 4
3% NaC1
15% H2SO 4
3% NaC1
15% I-I2SO 4
- i25 132 178
- 139 -78 17
4 14 28
0.35 0.95 3.9
0.705 0.266 0.043
1 0.93 0.553
228
D. Krupa et aL/ Surface and Coatings Technology 96 (J997) 223-229
the decreased amount of electric charge, and the changes in the polarization curves. It should be noted that this advantagous effect becomes stronger with increasing oxygen dose. In the 3% NaC1 environment, the polarization resistance increased by the factor of three to seven depending on the oxygen dose. This factor was even greater in the 15% H 2 S O 4 environment: at the lower oxygen dose, the polarization resistance increased three times, whereas at the higher dose, 11 times (Table 2). If we compare the values of the polarization resistance in the two environments, we see that, as can be expected, they are greater in 3% NaC1 than in 15% H 2 S O 4, This is evidence that in NaC1, the oxide layer is more resistant to corrosion. The decreased amount of the electric charge Q (C cm -2) also reflects the advantageous effect of the oxygen implantation. The amount of charge is associated with the degree of oxidization of the sample during the polarization. The smaller the charge, the more strongly the sample resists oxidization. The effect of implantation can also be seen in the polarization curves: for the implanted samples examined in both sodimn chloride and sulphuric acid environments, the values of the corrosion potential are shifted towards positive values (Table 2) and the curves are lowered, which means that the anodic current is smaller. In the 3% NaC1 environment, the anodic current is reduced within the entire potential range. The effect becomes stronger as the oxygen dose is increased. At smaller oxygen doses, the maximum of the current density is lower and shifted towards positive values. With increasing oxygen dose, the current density peak vanishes. Beginning from about 700mV, the anodic current increases, probably due to the growth of the oxide layer. In the 15% H 2 S O 4 environment, for the oxygenimplanted samples the anodic current begins to increase at about t000 mV and reaches a maximum between 3000 and 4000 mV depending on the oxygen dose. The current peaks are here higher for implanted samples than for non-implanted, and also shifted further towards positive potentials. Above a potential of 4 V, the anodic current magnitudes in small-dose implanted and nonimplanted samples are similar, whereas in samples implanted with a higher dose, the anodic current is reduced. The reduction in the anodic current observed after implantation in samples exposed to both environments is due to the increased thickness of the oxide layer. Armstrong and Quinn [13] report that the oxide layers, whose thickness does not exceed 1.5 nm, hinder the charge transfer at the beginning of the oxidization process when present on titanium surface. The oxide layers formed spontaneously on titanium are very thin: according to Schultze et al. [14], their thickness is 1.2 nm. The thickness of the solid oxide layer formed by oxygen ion implantation is greater - it is about
50 nm. We can suppose that this thicker layer hinders the charge transfer more strongly. This is reflected by the reduced amount of electric charge that passes through the titanium surface in implanted samples. The smallest value was observed in the samples implanted with the greater oxygen dose (1 x 1017 O + cm-2). The presence of the current density maximum on the polarization curves at a potential of about 3 V is, however, difficult to interpret. When examining the properties of the oxide layers deposited on pure titanium, Armstrong and Quinn [ 13] found a peak at about 2 V. They suggest that this peak is due to the modification of the oxide layer structure. The shift of the current density peak to about 3 V, observed in our experiment, can be explained in terms of the difference in the chemical composition of the materials examined in the two studies and, hence, different dopants present in the oxide layer. The oxide layers formed during implantation are colourless. Observations of the sample surfaces implanted and non-implanted after polarization have shown that the exposed surfaces have changed their colour; during polarization they become golden-brown. This change in colour is probably due to the change in thickness or in composition of the oxide layer. Observations of the sample surfaces using an optical microscope have shown that during the polarization, the grain boundaries are exposed there. This effect is observed in both implanted and non-implanted samples. 3.4. Hardness
How the surface hardness of the OT-4-0 alloy changes after subjecting the samples to oxygen ion implantation is shown in Fig. 7, where the hardness is plotted as a function of the distance down from the 8000
7000-
E 6000"
z. 5000W Z
Q < Z
4O00-
3000-
2000
0
0:2
014- 018 ola
~
t:2
1'.4
118 118
DEPTH [um]
Fig. 7. Hardness versus indentation depth profiles: (a) specimen nonimplanted; (b) specimen implanted with a 1 x l0 ~70 + cm -2 dose; (c) specimen implanted with a 5 x 1016 O ÷ cm -2 dose.
D. Krupa eta(. / Surface and Coatings Technology 96 (1997) 223-229
sample surface. We can see that the implanted ions do not change the hardness of the alloy. This is in agreement with the results obtained by P. Shioshansi and R. Oliver [5] who found that doses of 1 x 10 iv to 5 x 10 I 7 0 + cm -2 did not affect the hardness. A different result was reported by Saritas et al. [6] who observed a slight increase of hardness after the implantation of oxygen ions at a dose of 2 x 10 iv O + cm -2. The dependence of the titanium hardness on the implanted oxygen dose was examined by Okabe et aI. [2]. They found that at a dose of 2 x 1017 O + cm -2, the hardness increased only slightly compared with that of non-implanted samples. It was only between 4 x 10 iv and 6 x 10 iv O + cm -2 that a rapid increase in hardness was observed. A further increase in the oxygen dose, from 1 x l0 is to 2× 101so +, had no influence upon the hardness. Based on these reports, the fact that, in our experiments, the oxygen ion implantation did not affect the hardness of the O T - 4 - 0 alloy may be explained by the low oxygen dose.
4. Conclusions
(1) The implantation of oxygen ions into the surface of the O T - 4 - 0 alloy using doses of 5 x 1016 or 1 x 1017cm -2 results in a nanocrystalline TiO2 (futile) layer being formed on the surface. (2) The oxide layer formed by oxygen implantation increases the corrosion resistance of the alloy. (3) The increase of the corrosion resistance depends on
229
the oxygen dose applied. It appeared to be greater when the material was implanted with the higher of the oxygen doses used in the experiment. (4) At oxygen doses of 5 x 1016 and 1 x 1017 cm -2, the oxygen ion implantation does not affect the hardness of the O T - 4 - 0 alloy.
References [1] Y. Okabe, M. Iwaki, K. Takahashi, Mater. Sci. Eng. Al15 (i989) 79. [2] Y. Okabe, M. Iwaki, K. Takahashi, S. Ohira, B.V. Crist, Nucl. Instrum. Methods Phys. Res. B39 (1989) 619. [3] Y. Okabe, M. Iwaki, K. Takahashi, Nucl. Lnstrum. Methods Phys. Res. B61 (1991) 44. [4] Y. Okabe, T. Fujihana, M. Iwaki, B.V. Crist, Surf. Coat. Technol. 66 (1994) 384. [5] P. Sioshansi, R. Oliver, Mater. Res. Soc. Symp. Proc. 55 (1986) 237. [6] S. Saritas, R.P.M. Procter, W.A. Grant, Mater. Sci. Eng. 90 (1987) 297. [7] F. Alonso, A. Arizaga, S. Quainton, J.J. Ugarte, J.g. Viviente, J.I. Ofiate, Surf. Coat. Technol. 74/75 (1995) 986. [8] D. Krupa, E. Jezierska, J. Baszkiewicz, T. Wierzchofi, A. Barcz, Surf. Coat. TechnoI. 79 (1996) 240. [9] L.D. Arsov, Electrochimica Acta 27 (6) (1982) 663. [10] E.J. Kelly, Modern Aspects of Electrochemistry, Plenum Press, New York, 1982, p. 319. [11] L.D. Arsov, Electrochimica Acta 30 (12) (1985) 1645. [12] A.M. De Becdelievre, D. Fleche, J.D. Becdelievre, Electrochimica Acta 33 (8) (I988) I067. [13] N.R. Armstrong, R.K. Quinn, Surf. Sci. 67 (i977) 451. [14] J.W. Schultze, B. Danzfuss, O. Meyer, U. Stimming, Mater. Sci. Eng. 69 (1985) 232. [15] R.G. Wilson, F.A. Stevie, C.W. Magee, Secondary Ion Mass Spectrometry, John Wiley and Sons, NY, i989, p. 1.