Topographic fidelity of Ti-O film deposited onto Ti-6Al-4V alloy substrate to its surface by reactive DC sputtering

Materials Research Bulletin, Vol. 32, No. 10, pp. 1419-1426, 1997 Copyright 0 1997 Elsevier Science Ltd Printed in the USA. All rights reserved 00255408/97 $17.00 + .oO

Pergamon

PII SO0255408(97)00121-9

TOPOGRAPHIC FIDELITY OF Ti-0 FILM DEPOSITED ONTO Ti-6A1-4V ALLOY SUBSTRATE TO ITS SURFACE BY REACTIVE DC SPUTTERING

T. Sonoda* and M. Kato National

Industrial

Research Institute

(Received

of Nagoya

l-l,

Hirate-cho,

Kita-ku, Nagoya, Japan

(Refereed) March 21, 1997; accepted April 4, 1997)

ABSTRACT Coating of Ti-6A1-4V alloy with Ti-0 film by reactive DC sputtering in Ar-0, gas mixtures, in order to improve the biocompatibility of the alloy, was examined. The effects of oxygen content in the gas mixtures on the formation of Ti-0 film were investigated. Under visual observation, the films deposited under various oxygen contents appeared to be uniform and adhesive. Under SE&l, the surface of the Ti-0 films had fine particles dispersed on a smooth accumulated deposit. Based on observation of the accumulated deposit, we expected that the addition of oxygen to sputter gas might improve the adhesion between the deposited films and the alloy substrates in this study. By analyzing the surface roughness of obtained films, we found that the addition of oxygen to sputter gas improved topographic fidelity of the film to the surface of the alloy substrate onto which it was deposited and that the topographic fidelity depended on the oxygen content of the gas mixture. According to AES, the Ti/O ratio in depth direction was nearly constant in each of the films and oxygen concentration increased with increasing oxygen content of the sputter gas. The Vickers hardness of the films increased almost linearly with increasing oxygen content, and the maximum hardness reached over Hv1600 for the film deposited under the oxygen flow rate of 3.0 mL./min. This confirmed that coating of Ti-6Al-4V alloy with Ti-0 film improved the hardness of the alloy. Further, the formation of titanium oxides or suboxides in the Ti-0 film was assumed because of its high hardness and high oxygen concentration. copyright 0 1997 Elsevier science Ltd KEYWORDS: A. oxides, A. thin films, B. sputtering, ties, D. microstructure ~____ *To whom correspondence should be addressed. 1419

D. mechanical

proper-

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INTRODUCTION Ti-6Al-4V alloy is attracting attention as a dental material because its excellent mechanical properties, corrosion resistance, and superplasticity enable denture bases of complicated shapes to be formed. However, this alloy contains aluminum and vanadium, which are potentially harmful to human bodies [ 1,2]. Actual use of the alloy requires prevention of direct contact with biological tissues [3-51. On the other hand, pure titanium has been utilized as a biomaterial in the dental and orthopedic fields. It has been used in denture clasps, artificial dental roots, artificial joints, and fixing plates and screws for broken bones [6-81. Such prostheses and implants exhibit excellent biocompatibility [9-131. In a previous study [14], we developed a method of coating the alloy surface with pure titanium film of excellent biocompatibility as a barrier layer by DC sputtering in argon atmosphere, in order to improve the dental applicability of the alloy. The pure titanium barrier layer prevents harmful substances from leaching out to biological tissues surrounding the prosthesis. However, the pure titanium layer is soft and easily suffers mechanical damage. It is necessary to improve the hardness of the barrier layer so that the layer should resist such damage and thus prevent exposure of the biological tissues to the alloy. It has been observed that titanium oxide has high hardness and its coatings enhance the bonding of titanium alloy implants to living bone [ 15-181. The topographic compatibility of the surface of prostheses or implants with biological tissues surrounding them is also an important consideration. Because human bodies are highly sensitive to touch, they may sense a prosthesis or implant as foreign matter unless the topography of its surface is compatible with the biological tissues surrounding it. Therefore, it is also necessary to improve the topographic fidelity of the barrier layer to the surface of the prosthesis or implant so that the surface topography of the barrier layer is congruent with that of the prothesis or implant. In the present work, we performed coating of Ti-6Al-4V alloy substrate with Ti-0 film by reactive DC sputtering in At-O, gas mixtures, expecting solution hardening by oxygen dissolving into titanium and the formation of oxide phases [ 19-211. The intended result was improvement in the hardness and the biocompatibility of the barrier layer. Furthermore, we investigated the effects of oxygen content in the atmosphere for reactive DC sputtering on the formation of the Ti-0 film as well as its topographic fidelity to the alloy surface, by analyzing surface morphology, surface roughness, microhardness, composition, and structure.

EXPERIMENTAL A planar magnetron sputtering system (ANELVA Corp. type SPF-210H) with a stainless steel chamber 200 mm in diameter and 130 mm in height was used. The planar target used for this study was a 99.99 mass% pure titanium disk 100 mm in diameter. Ti-6Al-4V alloy substrates (as-received, 13 X 9 mm2, 0.55 mm thick) cleaned with organic solvent were mounted on a water-cooled substrate holder. Reactive sputtering to deposit Ti-0 films was carried out in gas mixtures of argon (purity 99.998 ~01%) and oxygen (purity 99.6 ~01%). The sputtering conditions were as follows. Discharge voltage and current were fixed at 350 V and 1 A, respectively. The argon flow rate was fixed at 1.4 ml/mm. The oxygen flow rate was varied, with the maximum defined as just before the so-called critical oxygen flow rate [22] under its sputtering condition, to make sure of stable discharge conditions. Thus, the

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TITANIUM

1421

OXIDE FILM

Smm

Appearance

FIG. of the surface of films deposited

1 in Ar (a) and in Ar-0,

gas mixtures

(b).

oxygen flow rate ranged from 0 to 3.0 mL/min. Deposition time for sputtering under each oxygen flow rate was fixed at 20 min. The procedure of reactive DC sputtering was as follows. First, argon gas was introduced at the desired flow rate into the chamber which was evacuated to 1 X lo-* Pa or less in advance. Next, the pre-sputtering between the target and the shutter in the atmosphere of pure argon under the desired discharge voltage and current was carried out to clean the target surface. Then, oxygen gas was admitted into the chamber and the oxygen flow rate was gradually increased up to the desired flow rate, withholding the pre-sputtering. While oxygen gas was flowing into the chamber, gas pressure was held at the desired discharge voltage and current to prevent the oxygen gas from changing the discharge conditions. Consequently, the gas pressure during the reactive sputtering was increased from 1.0 to 1.6 Pa as the oxygen flow rate increased from 0 to 3.0 n-L/mm, by adjusting the exhaust through the main gas valve. After discharge voltage, current, and gas pressure achieved steady state, the shutter was removed to allow deposition of Ti-0 film onto the substrate. Thickness of the films obtained was measured by tracing the substrate-film step with a surface roughness tester [23]. SEM images were used to study the surface morphology of the films. The surface roughness of the films was measured using the surface roughness tester under 4 mm traversing length. In-depth profiles of the chemical composition (Ti/O ratio) of the film were analyzed by Auger electron spectroscopy (AES) with an ion sputter etching method which used an argon ion beam. Hardness measurements were performed with a Vickers h.ardness tester under a 10 g load.

FIG. 2 SEM images of the surface of films deposited in Ar (a) and in Ar-G,

gas mixtures

(b).

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FIG. 3 Examples of surface roughness curve for films deposited in Ar (a) and in Ar-02 gas mixtures (b). RESULTS AND DISCUSSION Appearance and Surface Morphology. The films deposited under various oxygen flow rates appeared to be uniform and adhesive. All films were approximately 3 p.m in thickness. Their color tone scarcely varied with the oxygen flow rate, while their gloss depended on whether oxygen gas was mixed into the atmosphere for sputtering. The surface of Ti-0 films deposited in Ar-G, gas mixtures had slightly more gloss than pure titanium film deposited in Ar. The patterns of minute scratches originally on the surface of the as-received Ti6Al-4V alloy substrates were observed more clearly on the Ti-0 films deposited in Ar-0, gas mixtures than on the pure titanium films deposited in Ar gas, as shown in Figure 1.

0.6 2 s 2

I

I

I

I

0.4

5):

4 P

8 & 0.2 e P e 0

‘I,8

I 0

=

p

I

p

P-

I

2 1 Oxygenflow rate (mUnin)

3

FIG. 4 Average roughness R, of the surfaces of Ti-6Al-4V alloy substrate (0) and of Ti-0 films deposited in Ar-G, gas mixtures with various oxygen contents regulated by adjusting the oxygen flow rate (0).

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TITANIUM OXIDE FILM

200

1423

400

Depth (number of sputtering) FIG. 5 AES in-depth profiles of the Ti-G film deposited in Ar-G, 1.8 mL/min).

gas mixture (oxygen flow rate =

Therefore, we assumed that the Ti-0 films might be deposited onto the alloy substrate with higher to:pographic fidelity to the surface, compared to the pure titanium film. Figure 2 shows typical SEM images of the surfaces of the films obtained in this study. The surface morphology appeared to depend on whether oxygen gas was mixed into the atmosphere for sputtering. The surface of the pure titanium film deposited in Ar was observed to consist of uniformly accumulated submicron-sized particles. On the other hand, Ti-G films deposited in Ar-G, gas mixtures exhibited similar surface morphology to each other: their surfaces were observed to consist of micron-size particles on an accumulated deposit. Upon more detailed observation of the topography of the accumulated deposit, pits and bumps on the order of several microns were found. These might reflect pits and bumps existing originally on the substrate surface. Thus, sputtered substances might trace the topography of the surface as they accumulate on the substrate during deposition in Ar-G, gas mixtures. This is consismnt with results from the appearance of Ti-G films (Fig. 1) and lends support to the assumption that the Ti-G films might be deposited onto the alloy substrate with higher topographic fidelity to the substrate surface, compared to the pure titanium film. The accumulated deposit also exhibited adhesiveness to the substrate surface. According to Greene et al. [24], the addition of oxygen to sputter gas increases the adhesion of deposited film to its substrate. Therefore, we expected that the addition of oxygen to sputter gas might improve the adhesion between the deposited films and the alloy substrates, as demonstrated in this study.

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0

0

Dependence of oxygen concentrations mixed oxygen flow rate.

1

2 s Oxygen flow rate (mL/min) FIG. 6 (at%), in films deposited

in Ar-0,

gas mixtures,

on

Surface Roughness. Surface roughness curves are shown in Figure 3 for the pure titanium film deposited in Ar and the Ti-0 film deposited in Ar-0, gas mixture, shown in Figures 1 and 2. The surface roughness curve of the Ti-0 film in which many pits and bumps were detected is “rough,” compared with that of the pure titanium film. This supports the results from the appearances as well as the SEM images of the films. Average roughness R, was measured to investigate the effects of oxygen content in sputter gas on the surface roughness of the Ti-0 films. Figure 4 shows average roughness R, of the surfaces of Ti-0 films deposited in Ar-0, gas mixtures under various oxygen flow rates, compared with that of the Ti-6Al-4V alloy substrate surface. R, values of the films obtained with oxygen added to sputter gas were similar to that for the alloy substrate. The film with the R, value closest to that of the substrate was the one deposited at the oxygen flow rate of 1.0 mL/min. This implies that the addition of oxygen to sputter gas promotes topographic fidelity of the deposited films to the alloy substrate surface and that the topographic fidelity depends on the oxygen content in the gas mixture. On the other hand, it is possible that topographic fidelity of deposited film depends on the deposition rate. However, the effects of oxygen are considered to be more important than those of deposition rate in this study, because the deposition rate scarcely changed during the present work. Composition. AES in-depth profiles for the Ti-0 film deposited in Ar-0, gas mixture under the oxygen flow rate of 1.8 mL/min are shown in Figure 5. The AES signals for the analysis were Ti-LMM (480 eV), 0-KLL (510 eV), and Al-KLL (1396 eV). A specific increase of aluminum concentration (at%) was detected along the depth direction of the sample. This implies the position of the interface between the film and the substrate in depth direction. The concentrations of titanium and oxygen were nearly constant in depth direction, that is, the Ti/O ratio was nearly constant in depth direction of the film. The in-depth profiles for the films obtained under the other oxygen flow rates were similar to those obtained under the

mANIUM

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1425

OXIDE FILM

I

I

2500 2000.

Loo2 c 1000 -3 g

0

4

‘I 0

I

1 Ox&m flow Lx

I @/A)

FIG. 7 Vickers hardness of bulky Ti-based materials [Ti-6Al-4V (A), TiNi (m), and pure Ti (O)] and of Ti-0 films deposited in Ar-0, gas mixtures with various oxygen contents regulated by adjusting the oxygen flow rate (0). oxygen flow rate of 1.8 mL/min, although their Ti/O ratios varied, depending on the oxygen flow rate. The Ti/O ratio of each of these film also was found to be nearly constant in depth direction of the film. Figure 6 shows oxygen concentration in the film obtained under each oxygen flow rate, which was derived from the results of the AES analysis. The oxygen concentration depended on oxygen content added to the sputter gas and increased with increase of the oxygen content of the gas mixture.

Microhardness. Figure 7 shows the Vickers hardness of Ti-0 films deposited in Ar-0, gas mixtures under various oxygen flow rates, compared with that of bulky Ti-based materials, namely, Ti-6Al-4V, TiNi, and pure titanium. The hardness of pure titanium film deposited in Ar was almost the same as, or lower than, that of the bulky Ti-based materials, while that of the Ti-0 films deposited in Ar-0, gas mixtures was higher than that of the bulky materials. This confirmed that coating the Ti-6Al-4V alloy with Ti-0 film improved the hardness of the alloy. Further, the Vickers hardness of the films increased almost linearly with increase of oxygen content in the sputter gas. The maximum hardness, over Hv1600, was reached by the film deposited under the oxygen flow rate of 3.0 rnLJmin, in which the oxygen concentration was about 50 at% (Fig. 6). The formation of titanium oxides or suboxides; in the film was assumed because of its high hardness and high oxygen concentration. REFERENCES 1. 2. 3. 4.

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