substrate systems

Surfaceand Coatings Technology80 (1996) 35-48

The influence of ion irradiation during film growth on the chemical stability of film/substrate systems W. Ensingerl Universitiit Augsburg, Institut fiir Physik, Memmingerstrasse 6, 86159 Augsburg, Germany

Abstract The present review deals with aqueous corrosion of metal substrates coated with thin films deposited from the vapour phase under concurrent bombardment with energetic ions (ion beam assisted deposition, IBAD). The influence of the main deposition process parameters such as the ratio of impinging ions to condensing atoms, the ion energy, and ion incidence angle is discussed. The features of corrosion resistant elemental films such as aluminium, titanium, and niobium, and ceramic films such as titanium nitride, silicon nitride, and aluminium oxide on steel and aluminium, tested in dilute acids and salt brines, are described. The results show that the most important characteristics of corrosion resistant IBAD thin films are low microporosity and high adhesion. keywords: Corrosion protection; Chemical stability; Ion beam assisted deposition; Microporosity;

1. Introduction Physical vapour deposition provides the possibility of protecting materials from corrosion by a variety of inert elemental or compound coatings. Apart from their chemical character, the protective power of the films depends strongly on their morphology and on the structure and composition of the transition zone between film and substrate. These can be influenced markedly by bombardment with energetic particles during film growth. Ion irradiation causes a number of effects among which are densification of the film with a reduction in the microporosity and change in crystal size and shape. These influence the corrosion stability. When ions from reactive elements are used they may additionally cause a chemical change. An elemental film may be converted to a compound film with an increase in chemical stability. In the present contribution, the aqueous corrosion of metal substrates coated with thin films deposited under concurrent bombardment with energetic ions (ion beam assisted deposition, IBAD) is dealt with. The influence of the deposition process parameters such as ion energy, ion incidence angle and ratio of impinging ions to condensing atoms is discussed. The features of corrosion l Tel: +49 8215983 445; fax: +49 8215983 425;

e-mail:[email protected]. 0257-8972/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved

Coatings

resistant films of metals, oxides and nitrides on metal substrates in dilute acids and salt brines are described. For a better understanding of the effects of ion irradiation, in the following section a short introduction to corrosion measurement techniques and the fundamentals of film deposition under ion bombardment is given.

2. Principles of ion beam assisted thin film deposition During ion beam assisted thin film deposition, a variety of processes take place, mainly on the surface and in the near-surface region of the growing film. Fig. 1 shows a schematic diagram of the most important processes. Gas phase reactions play a minor role for the range of pressure used for the IBAD process, which is usually between 10-j and lo-’ Pa. The mean free paths are larger than the distances between the ion or vapour source and the substrate. What may play a role is neutralization of ions by charge exchange with neutral gas molecules or atoms. This may have an influence on the films when they are insulating. Ions may cause charge up and radiation damage, neutrals do not. The film material, usually atoms, is condensed from the vapour phase. At low temperatures, the adatoms have a very low mobility and stick to the place where they are deposited. Ion bombardment leads to an increase in adatom mobility. The adatoms obtain a higher energy

W. Ensinger/Surface and Coatings Technology 80 (1996) 35-48

eeeeeeeeeeeeeeeeeeeeeeee eeeeeeeeeeeeeeeeeeeeeeee 808098800888888088888888

substrate

l eeoeeeeeeoeeeeeeoeeeeoe e 8 0 8 8 e 8 8 8 8 8 8 0 8 8 0 0 8 8 0 8 8 8 8 800800080008800800080000 l oooooooooeooooooooooooo 0~0@00000000000@0000000~ l oooooeoeoeoooeoeoeoeoeo OChan”eling l 0 l 0 l 0 l 0 e lattice defects :: 0 0 l collision cascades 0 e::o l 0 0

0

0

0

0

0 0

0 0

0 0

0 0

transition

zone

alloyfilm

0 chemical reactions

neutralisation

(IO

Ions = 105 eV)

gas molecules

(0.03 ev)

atoms

(0.15 /

1 - 20

eV)

Fig. 1. Processes which take place during ion beam assisted deposition.

and mobility and are able to move over the surface before they settle down. This leads to different crystal growth and, as a result, to a different structure. Another surface effect of ion bombardment acts on desorbed gases. Loosely bound molecules may be desorbed. This influences the composition of the film and its purity. Reactive gases such as oxygen, nitrogen and hydrocarbons may be activated and the reactivity of the system gas molecule-deposited atom may be enhanced. This enhanced chemical reactivity may lead to the formation of compound. The ions penetrate into the lattice of the growing material. They lose their energy by two main processes, inelastic collisions with electrons and elastic collisions with atoms. The atoms are knocked out of their lattice positions. Thus, material transport both in the direction of the incident ion beam and in other directions can take place. Knock-on implantation, forward sputtering and re-sputtering are direct results of this effect. Some of the knocked-on atoms possess energies high enough to lead to secondary collisions etc. In this way, collision cascades develop. The resulting strong atomic motion along the trajectory of the ions

leads to a rearrangement of the lattice. Radiation damage such as point defects, interstitials and defect agglomerates are created. Part of the damage is annealed during the process, and part remains. When a high number of defects is created, they can agglomerate to two- and three-dimensional defects such as dislocation loops. Secondary effects are created such as increased mobility of species in the lattice, radiation-enhanced diffusion, and phase changes, e.g. by radiation-enhanced segregation. Collision cascades may be directed away from the direction of ion incidence. If near-surface collisions are energetic enough, they may lead to the removal of atoms from the outer atomic layers. This re-sputtering leads to a reduction in growth velocity. It may also lead to structural changes, e.g. development of texture, and to changes in composition when the different components of the film have different sputtering yields. When the ions enter the lattice of the growing film along lattice planes or channels, they are able to penetrate deeply into the lattice without creating damage by atomic collisions. In this case, the ions lose their energy by electronic excitation. Radiation damage in connection

W. EmingerJSwjace

and Coatings

with atomic collisions, including knock-on implantation, creation of point defects and sputtering, is strongly reduced. The most important process parameters of IBAD are the arrival ratio, the ion incidence angle and the ion energy. The arrival or transport or I/A ratio is the ratio of the number I of impinging ions to the number A of condensing atoms. It determines how much energy or momentum is transferred to the growing film by the ion beam. The ion incidence angle is the angle at which the beam of ions, which usually are moving in parallel, impinges onto the surface plane of the growing film. Finally, the ion energy is of importance. It is determined by the acceleration voltage cf the extraction system of the ion source and the charge state of the ions (usually most of the ions are singly charged). The ions are given an energy with a very narrow spread so that the ion beam can be treated as monoenergetic. These parameters are independent of each other and can be selected over a wide range. This allows for particularly good control of the film properties. Further details and examples of IBAD can be found in a number of review articles, e.g. [l-4].

3. Corrosion measurement techniques for thin films For evaluation of the corrosion behaviour of film/ substrate systems, three main kinds of technique can be found in the literature. The most simple technique is to expose a sample to a corrosive medium and detect the development of defects or corrosion products under the microscope or using another method. With special techniques, this procedure can be quantified to a certain extent. Another method is to carry out quantitative dissolution measurements. The weight loss of the material into the corrosive solution can be determined analytically. One of the problems with such a method is that thin films usually corrode non-uniformly and it is therefore difficult to obtain quantitative data per area. The most widely used techniques for corrosion measurements of coatings are electrochemical techniques. Different kinds of potential, current or polarization measurements are carried out. In the literature, mainly polarization measurements can be found. In general, they allow for rapid evaluation of the corrosion behaviour and sometimes give insight into corrosion mechanisms. However, they are artificial laboratory tests and predictions of the long-term behaviour in field tests or in applications usually cannot be made. In a polarization test, the sample is given a defined potential with respect to a reference electrode. The resulting current between the sample and a counter electrode is recorded. This can be done potentiostatically where the system is given time to come to a steady state.

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The other method is the potentiodynamic technique where the potential is changed with time. In this way, a single potential ramp can be run, or a multicycling method can be employed where the potential is scanned repeatedly from a starting potential to a reverse potential and back. This technique is called cyclic voltammetry. With these techniques, different data can be obtained. One parameter is the corrosion potential (also the rest potential or open-circuit potential) which the sample naturally adopts in a corrosive environment. Further, it can be determined in which potential region the sample is protected for example by a passive film, and where it corrodes actively with high dissolution rates. When localized corrosion is investigated, the breakthrough or pitting potential can be determined where the natural protective film of a material breaks down locally owing to attack of a corrosive species. For inert coatings which themselves do not corrode, at least in certain potential ranges, polarization measurements yield the electrical currents from dissolution of the substrate material through micropores. This current value, usually the maximum current within a potential range, is one of the main values gained from currentpotential measurements for characterizing coating/substrate systems. Further details of corrosion measurement techniques can be found e.g. in Refs. [ 5,6]. The following discussion shows that some of the above described effects of ion irradiation during film growth are useful for creating films with microstructures which are particularly suitable for corrosion protection. It is divided in three sections dealing with different coating materials. With a selection of examples from the literature the most important processes and effects of IBAD of corrosion protective films are discussed.

4. Metal films 4.1. Gold films In one of the early corrosion studies on IBAD, Nandra et al. carried out a basic investigation of the influence of ion bombardment on the corrosion protection ability of evaporated thin noble metal films [7]. Copper samples were coated with gold vaporized by resistance heating. Simultaneously, a beam of argon ions with energies varied from 0.8 to 6 keV was directed onto the samples. In addition to the ion energy, the ratio I/A was varied over one order of magnitude by changing the deposition rate and/or the ion current density. Prior to deposition, the sample surfaces were cleaned by ion irradiation. The objective of the study was to determine the adhesion of the film to the substrate and the porosity of the films. Each sample was shielded on one side from the ion beam, so that a direct comparison of deposition

W. EnsingerjSurface and Coatings Technology 80 (1996) 35-48

38

Table 1 Effect of ion energy and I/A ratio ([Ar’]/[Au]) Ion energy (keV) 0.8 1 1.5 5 6

I/A=O.O23

1.

on the porosity of gold films on copper (after Ref. [7]) I/A=O.O38

I!z + +

I/A=O.O46

+ +

I/A= 0.076

I/A = 0.098

I/A=O.15

I/A=O.23

Ik f f

?I It

It

f

+

f No significant difference between ion bombarded and non-bombarded sample, + reduction in porosity due to ion bombardment,

with ion bombardment and without ion bombardment was possible. Pull-tests showed that films deposited without pre-cleaning by ion bombardment exhibited very poor adhesion. For all the samples cleaned by sputtering prior to coating, the limit of the adhesion test was exceeded (69 MPa). This result shows that cleaning the substrate in vacua shortly before coating is an important step in the IBAD process. The porosity was determined quantitatively with an electrographic test. A wet filter paper enriched with cadmium sulphide was placed on the sample. An electrode was pressed onto the paper, then an electrical current was passed through it. Copper ions from the substrate which passed through the gold film via pores caused blackening of the sulphide paper. Each black spot developed in this way represented a pore. In table 1 the results of this test are compiled. It turns out that generally ion energies below 1.5 keV did not lead to a significant reduction in porosity. For higher energies, there was a reduction when the I/A ratio exceeded a certain value. From this the authors conclude that there is a threshold value both for the ion energy and the I/A ratio below which no reduction in microporosity can be obtained. The densification of the films is ascribed to structural changes arising from momentum transfer from the ions to the atoms. Too small a momentum transfer, due to either too low a number of ions per atom or too low energy of the ions, may not produce a significant overall effect with respect to porosity.

ions which penetrate through the aluminium film via pores. At first order, this critical current is a measure of the porosity of the films and of their corrosion protection effect. In Table 2 the critical currents for different potential cycles of iron coated with aluminium under different conditions are listed. The data show that aluminium deposited onto the untreated substrate without any ion irradiation shows higher iron dissolution currents than aluminium deposited onto a substrate treated by ions prior to deposition. The lowest values, however, were obtained when the ion bombardment was carried out during the entire process. This shows that both the substrate condition and the porosity of the film itself can be improved by ion bombardment. In the case of localized corrosion at pores, the substrate material is attacked through the pores. This is shown schematically in Fig. 2. The coating is undermined and lifted off. More of the substrate surface comes into contact with the corrosive solution. The corroded area

4.2. Aluminiumjilms

The values of the first, second and tenth potential cycle are compared for samples coated with aluminium without ion irradiation, coated with aluminium without ion irradiation but with pre-cleaning by ion bombardment prior to deposition, and coated with aluminium with ion bombardment prior to and during deposition,

Owing to its protective natural oxide film, aluminium is chemically stable in water when it is neutral and does not contain aggressive species, An aluminium film is able to protect iron from corrosion when it is dense and adhering. In a model system study, Ensinger and Wolf coated iron with aluminium by electron beam evaporation under concurrent bombardment with 6 keV argon ions [ 51. To evaluate the corrosion behaviour, the samples were subjected to polarization measurements in dilute buffered acetic acid of pH 5.6. The potential was scanned cyclically from - 1.2 V SCE to t 1.2 V. Around -0.4 V SCE the anodic currents show a maximum in iron dissolution. This dissolution current is due to iron

Table 2 Maximum anodic current densities from iron dissolution of aluminium coated iron samples Fe substrate coated with Al

1st cycle (mA cms2)

2nd cycle (mA cm-‘)

10th cycle (mA cmw2)

Without ion irradiation After pre-irradiation Under simultaneous ion irradiation

0.40 0.19 0.09

0.60 0.24 0.15

1.07 0.75 0.30

a

b corr. solution

C blister

substrate

Fig. 2. Schematic presentation of the corrosion of a film/substrate system via micropores: (a) invasion of corrosive solution by capillary action, (b) corrosion at the interface between coating and film at the pore, and (c) delamination of the film.

W. Erhger/Surface

and Coatings

Teclmology

80 (1996)

I/A

39

35-48

0

Fig. 3. (a) Relative number of pores N/N* and average radius 7/T* of the pores of 0.5 urn thick aluminium coatings on low-alloy steel as a function of the I/A ratio; ion energy 12 keV, ion incidence angle 10”. (b) Relative number of pores N/N* as a function of angle 0 of ion incidence; ion energy 6 keV, I/A ratio 0.01, from Ref. [8].

increases. Accordingly, one of the most important parameters for describing the corrosion of coating/substrate systems where the coating itself does not corrode is the surface area of corroding substrate material as it develops with time. Enders et al. have developed a mathematical description of this process [S]. They describe the corrosion rate as an increase in corroding surface area with time, dS/dt. The following formula is derived from their considerations: S(t) = nN[F, t kt]‘, with N being the number of circular-shaped pores with average radius i;, and k being the radius growth constant. With this equation the relative number of coating defects and their average diameter can be calculated. The corrosion area is derived from the maximum iron dissolution current density from polarization curves. Enders et al. applied these theoretical considerations to 0.5 pm thick aluminium coatings prepared by evaporation of aluminium on polished low alloy steel under argon ion bombardment. They varied the I/A ratio, the ion energy and the angle of ion incidence. The corrosion behaviour was determined in buffered acetic acid at pH 5.6 by cyclic voltammetry. The result in Fig. 3(a) shows that the relative number of pores decreases initially with I/A and then increases again. For the radius of the pores the opposite trend is observed. These findings can be explained by the influence of ion irradiation on thin film growth. The deposition of energy and momentum into the growing film leads to densification. After reaching a maximum, the excess energy and momentum produce increasing damage of the lattice with a reduction in density and an increase in porosity. These effects are also strongly correlated with the angle 0 of ion incidence. Fig. (3(b) shows how the number of pores is a strong function of this angle. The energy and momentum transfer depends geometrically on 0. At an angle of around 40” a distinct minimum in the number of pores is observed. Films deposited at this ion incidence angle show maximum corrosion stability. Some of the aluminium alloys are suitable for industrial application owing to their high mechanical

strength. This in combination with their low weight means they are of particular interest for the aerospace industry. Often, these alloys contain copper and other components which render them susceptible to localized corrosion when aggressive anions are present in the environment. In this case, a pure aluminium coating can protect the substrate material. The deposition of dense aluminium films is difficult, particularly when the substrate must not experience high temperatures. Ensinger et al. report the deposition of 2 and 5 urn thick films of aluminium onto aluminium-copper-magnesium alloys by evaporation of aluminium under 6 keV argon ion irradiation [ 91. The samples were subjected to salt spray and immersion tests in NaCl solution. Under these conditions, the uncoated alloy suffers from severe pitting corrosion. Optical inspection and metallographic crosssectioning after the tests revealed that the coated samples developed less and smaller pits than the uncoated samples. Fig. 4 shows for comparison cross-sections of an uncoated and a 5 urn aluminium coated sample after a 500 h test with alternating wet and dry cycling in 3.5% NaCl solution. The untreated sample shows deep and wide pits, the coated sample is still intact. In this study, the low process temperature is remarkable. The samples had to be kept below 150 “C to avoid structural changes

_.

.-

.i’?A-*

?’

Fig. 4. Cross-sectional micrographs of an uncoated (top) and an aluminium-coated (bottom) aluminium alloy sample after 500 h corrosion testing (wet-dry cycling in NaCl solution).

40

W. EnsingesJSurface ad Coatings Technology 80 (1996) 35-48

with degradation in mechanical and chemical stability. The good film growth with low microporosity and high adhesion despite the low process temperature is due to ion beam effects. 4.3. Transition metaljilms: chromium, titanium

930 700

zirconium, niobium, niobium-

A particularly severe type of corrosion of aluminium alloys can occur when they are under strain. Stress cracking leads to mechanical failure. corrosion Emmerich et al. investigated the effect of ion beam treatments of an aluminium alloy (7475-T651) under stress corrosion conditions [lo]. Among other treatments, the cylindrical samples were coated with 0.1 urn thick zirconium films under bombardment with 0.5 keV argon ions. The tests were performed in wet-dry cycling by alternating immersion of the samples in aqueous 3.5% NaCl solution under a constant strain of 100 MPa. The time until fracture was recorded. The uncoated alloy showed an average lifetime of 39 h; the coated alloy had a lifetime of 260 h. Crystallographic examination showed that the cracks which led to fracture initiated from the ground of pits and also from places on the uncorroded surface. Propagation of the cracks occurred intergranularly. The zirconium films were nanocrystalline or partially amorphous and very smooth. These features reduce the possibility of intergranular attack. In addition to zirconium, some of the other transition metals are particularly suitable for corrosion protection, such as chromium, titanium, tantalum and niobium. Hsieh et al. deposited 2.5 ym thick coatings of niobium onto AISI 316L stainless steel by electron beam evaporation under argon ion irradiation with ion energies of 0.25 and 0.5 keV [ 111. Potentiodynamic measurements carried out in 3% aerated NaCl solution showed that the uncoated steel started pitting at t 0.2 V vs. SCE. A sample with a niobium film deposited without ion bombardment showed a pitting potential of +0.18 V. For steel coated with niobium under an I/A ratio of 0.4 the pitting potential was shifted to f0.22 V. I/A ratios of 0.68 and 0.8 led to coatings which suffered no pitting at all and behaved similarly to bulk niobium. Scanning electron microscopic investigations showed that the film grown without ion bombardment had a columnar structure with open boundaries. A similar structure was observed for the film formed under moderate ion bombardment. More intense ion irradiation led to a dense appearance of the films. This structure is consistent with the electrochemical results. Hsieh et al. extended these studies to niobium-chromium alloy films which were formed by co-evaporation of niobium and chromium under argon ion bombardment with an energy of 0.25 keV [ 121. Different compositional ratios were compared for a constant I/A ratio with I/A= [Ar’]/[Nb t Cr] =0.68. In Fig. 5 anodic polarization

-500’ -6

’ .7

1 -6

1 -5

LOG

I (A/cm*)

.4

1 -3

Fig. 5. Current-potential curves of stainless steel: uncoated SS, and steel coated with niobium-chromium films of different composition [Nb]/[Cr]. For comparison, the curves of pure niobium and pure chromium are included. Curve 1, non-irradiated, Nb:Cr 50:50; curve 2, Nb:Cr 50:50, I/A=O.4; curve 3, Nb:Cr 50:50, I/,4=0.68; curve 4, Nb:Cr 70:30, I/A=O.68; curve 5, Nb:Ct 30:70, I/.4=0.68 (from Ref. [ 121).

curves of uncoated steel, steel coated with different films, and bulk chromium and niobium are compared. The uncoated steel exhibits low currents until, with a sudden increase at around 0.2 V SCE, pitting corrosion starts. The curve for the niobium-chromium 50/50 coating deposited without ion irradiation is anodically shifted. Ion irradiation leads to another shift in the anodic direction. The curve for niobium-chromium 30/70 coating is closer to that for uncoated steel, but meets the other curves at above 0.5 V SCE. The niobium-rich niobium-chromium 70/30 film resembles in its behaviour the bulk niobium specimen, although the currents are almost an order of magnitude lower. With this composition, the current remains at a constant value of around 10m6 A cmw2, even up to the highest anodic potentials. The authors ascribe the strong protective power of this coating to the formation of a highly corrosion resistant niobium-rich oxide layer. The morphology of the differently prepared coatings was studied by scanning electron microscopy. Films of the non-alloyed metals chromium and niobium grew in a columnar structure. Columnar growth renders the films intrinsically porous and poorly corrosion resistant. Alloying with 50% chromium by co-evaporation, but without ion bombardment, leads to a reduction in the distinct columnar growth. The films become more dense. Ion irradiation leads to the formation of coatings which appear smooth and featureless. In terms of morphology, this is the best condition for effective corrosion protection. Matsuura et al. coated a titanium-nickel alloy with titanium film to prevent nickel dissolution [ 131. Shape memory alloys such as nickel-titanium are increasingly being used for biomedical applications such as implants. In contact with human tissue and corrosive body liquid, this material tends to release nickel. Nickel is physiologically harmful. Therefore, several coatings have been used

TV’. EnsingerjSusface

and Coatings

to prevent nickel corrosion, such as TIN formed by ion plating, TiCN formed by chemical vapour deposition or titanium oxides formed by thermal oxidation [ 131. However, all of these coatings failed, presumably because of the severe corrosion conditions and poor coverage, particularly on rough surfaces, edges, scratched areas and shape-recovered surfaces. Matsuura et al. deposited 3 urn thick titanium films by titanium evaporation under titanium ion bombardment. In order to simulate the damage which occurs in application, the samples were indented with a Vickers diamond or scratched. Additionally, some samples were prepared with an edge with incomplete coverage. Prior to testing, the samples were bent to an angle of 45” and then their shape recovered by dipping into hot water. Scanning electron microscopy showed that the surfaces of the films were smooth. No columnar structure could be detected and no crack had developed on bending. After bending, polarization measurements were carried out with a potential scan rate of 0.5 mV s-l in 0.9% NaCl solution at a temperature of 310 K. Fig. 6 shows the currentpotential curves for the differently treated samples. On scanning the potential in the positive direction, the uncoated samples show a passive region with low currents and a steep increase at around 0.5 V SCE when passivity breaks down and nickel dissolves rapidly. By

TiNi,

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80 (1996)

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41

contrast, the titanium film shifts the onset of the current increase considerably into the anodic direction, despite the macroscopic damage caused by indenting or scratching. This result is confirmed by long-term immersion tests. The samples were exposed to physiological sodium chloride solution at human body temperature for 3 months. After that, the amount of dissolved nickel was determined by atomic absorption spectrometry. The nickel concentration was found to be low enough for biomedical application. The authors ascribe the superior performance of the titanium films to their low microporosity, strong adhesion to the substrate and comparatively high elasticity. The latter properties are particularly important for systems which are bent in application such as the shapememory materials.

5. Oxide films Apart from noble metals which are thermodynamically protected and inert in most environments, most of corrosion resistant metals such as those discussed in the foregoing section are protected by a thin inert oxide film. Consequently, these oxide films are suitable for efficient corrosion protection, provided they can be deposited with sufficiently low porosity and high adhesion. Before discussing the effectiveness of oxide films as corrosion protection in conjunction with a substrate material the stability of the films themselves in terms of gas absorption is described. Studies in this field have mainly been carried out for forming optical films. They started more than a decade ago and are among the first studies to reveal the effect of ion bombardment on corrosion. 5.1. Stability of optical coatings

-1

0 current

1 density

2 log i / PA/cm2

3

4

3TiNi, coated

-2

Techology

with Ti

I,

1

-1

0 current

.,,/,I,,

1 density

2

3

4

log i / pA/cmz

Fig. 6. Polarization curves for NiTi alloy after damage by VT indenting, ST scratching, or with an edge EG; adapted from Ref. [ 131.

The optical properties of coatings, such as the absorption and refractive index, depend strongly on the microstructure. If the structure is not sufficiently dense, the coatings will appear unstable when they are in operation and exposed to the atmosphere which contains water molecules. These are absorbed by capillary action. As a result, the optical properties are changed because the properties of water are superimposed on those of the film. Under normal physical vapour deposition conditions, dielectric films will mostly develop a columnar structure. Water can penetrate into the film along the open column boundaries. This can be avoided by ion bombardment during film growth. The first results on optical films with low microporosity were, to the author’s best knowledge, reported by Martin et al. [ 141. They prepared dense films of the oxides of silicon, titanium and zirconium by evaporation

W. EnsingerjSmface and Coatings Technology 80 (1996) 35-48

0.7L 400 (al

I

I

I

I 600

500 WAVELENGTH

, 700

(nm)

400

600

500 WAVELENGTH

(b)

700

hm)

Fig. 7. Spectral transmittance of ZrO, film deposited, (a) without ion irradiation, (b) under oxygen ion irradiation. Vacuum, measured immediately after deposition; air, measured after exposure to humid atmosphere; from Ref. [ 151.

of the oxide under simultaneous argon ion bombardment [15]. Measurements of ZrO, coatings showed that the film quality could be improved by both argon and oxygen ion irradiation. For 1.2 keV oxygen ion irradiation, an I/A ratio of 0.28 for [O,‘]/[ZrO,] gave dense and stable films. Fig. 7 shows the effect of oxygen ion irradiation on the stability of zirconium dioxide exposed to humid air. The transmittance over the visible spectral range is compared for films deposited with and without ion assistance shortly after preparation and after exposure to humid air. Whereas the ion bombarded films remain stable, which indicates that no water has been absorbed, the non-irradiated films show a shift in the transmittance curve to longer wavelengths. Water absorption, which is shown indirectly by the change in optical properties, was measured directly by nuclear reaction analysis of the water component hydrogen. Depth profiling revealed a much higher content for the non-irradiated oxide compared with the irradiated oxide. 5.2. Oxide coatings (alumina, zirconin, chromin) fof corrosion protection

Gibson reports on the densification of Al,O, films by ion bombardment for application as a hermetic barrier for magneto-optic media [ 161. These media are highly sensitive to oxidation. A typical example is the TbFeCobased alloy. Terbium is oxidized rapidly when it comes into contact with oxygen or water vapour. Gibson carried out studies on this alloy and, as a model system, also on layers of pure iron. A substrate was coated with iron films and then alumina layers were deposited on

top under ion bombardment with different irradiation intensities. The samples were subjected to repeated humidity-temperature cycling. With a microreflectometer based on a commercial compact disc player, the surface was scanned for defects. Fig. 8 shows the increase in defects after 100 h testing as a function of ion irradiation intensity in terms of momentum per atom. Two effects can be seen from this result: pre-cleaning by ion bombardment is beneficial, and ion beam treated films exhibit a lower defect density and higher corrosion stability than untreated films. Within the investigated range, more than one order of magnitude in improvement could be obtained. Martin et nl. deposited A1203, SiOz and ZrO, coatings under oxygen ion assistance onto silver and aluminium mirrors [ 171. An example of the application of such

E

,

No

Ion

Pre-Clam

AIzOJ

8

on Conv.

Fe Films

5 0.1 : : b C I$? 0.01

PI 2 .E 0.001

Fig. 8. Increase in defect area of alumina films on iron after humiditytemperature cycling as a function of the momentum transferred from the ions to the film; from Ref. [ 161.

W EnsingerlSusface and Coafings Techrzology 80 (1996) 35-48

systems is large astronomical mirrors. The protective effect of the layers was determined in a laboratory test by exposing it to an etching agent and monitoring the etch rate by determining the increase in transmittance. The coated aluminium mirror was immersed in a cell in 0.2 M NaOH solution; the silver mirror was immersed in 1 M HNO,. The optical transmittance was observed in situ by passing a light beam through the cell onto a photomultiplier. The unprotected mirror would have been removed in 1 min. In Fig. 9 the optical transmittance is depicted as a function of the immersion time in the etch agent. For both aluminium and silver, ion assistance leads to films with considerably improved durability. The best result was obtained with a ZrOz layer only 0.12 urn thick. Owing to their chemical intertness, oxide films are suitable not only for protection in humid atmosphere, but also for reducing corrosion in aggressive liquid

/ A1203

(ions)

?

I

i

ZrO, / 0

\ 2

, 4

i,

(ions)

/

li

111 6 810

/ 20

.$J

I 60

I

/I,

II 100

I 200

I 400

I

/

i

TIME(MIN)

,

I

I

I/l

OVERCOATED

1 I

SILVER

I 1

SO2

(no

ions)

43

media such as salt brines. For this purpose, McCafferty et al. deposited 0.2 pm thick chromium oxide films onto 52100 steel by chromium evaporation under chromium ion bombardment in an atmosphere of backhlled oxygen [18]. The coating provided some protection against corrosion in 3 ppm sodium chloride solution and in 1 N sulphuric acid, as indicated by reduced anodic currents in potentiostatic polarization measurements. In a study on aluminium substrates, they deposited 0.5 urn thick tantalum oxide films by tantalum evaporation under concurrent bombardment with 0.25 and 0.5 keV oxygen ions [19]. Fig. 10 shows the corresponding potentiostatic anodic current-potential curves for 0.1 M NaCl solution. Uncoated aluminium stays passive with low current densities until at around -0.7 V vs. SCE a steep increase in current indicates the onset of pitting. The Ta205 coating formed at the lower ion energy shows a pitting potential of -0.35 V with a difference AEpit in pitting potential of 0.4 V in comparison with uncoated aluminium. An increase in the ion energy shifts Eri, considerably in the anodic direction, with Eri, = t 0.3 V and AEpit = + 1.0 V. Emmerich et al. prepared 1 nm thick aluminium oxide films on polished pure aluminium substrates by aluminium evaporation under oxygen ion irradiation [ 201. The effects of several I/A ratios and two different ion energies were investigated. The resistance of the film against pitting corrosion was tested in 2% NaCl solution by polarization measurements with a potential scan rate of 0.05 mV s-r. Some of the current-potential plots are shown in Fig. 11. The top figure compares uncoated aluminium using two samples which were coated by evaporation of aluminium in an oxygen atmosphere (reactive evaporation) without ion bombardment. The uncoated material shows strong pitting corrosion behaviour of aluminium in a corrosive solution with high chloride concentration. As soon as the anodic potential region is reached, rapid dissolution starts. The oxide coating reduces the anodic currents strongly. The curves in the middle of Fig. 11 show the influence of oxygen ion energy for the same process parameters (I/A =0.6). Whereas bombardment with 0.5 keV ion energy hardly

I

ZrO, b

(ions) /I

1

I

III

2

4

6 810

I

I

I

20

40

60

II 100

I

I

200

400

TIME(MIN)

Fig. 9. Dissolution measurements via determination of the transmittance of aluminium and silver mirrors with protective oxide iilms in sodium hydroxide or nitric acid; from Ref. [ 171.

I

ble-o4F,“““““,,,,,‘,.~ 0.6

0.2

-0.2

Potential

-0.6

-1

-1.4

(Vsce)

Fig. 10. Anodic polarization curves for untreated aluminium and for aluminium coated with 0.5 urn thick Ta,O, films prepared with 250 or 500 eV ion energy; from Ref. [ 191.

W, Ensinger/Surface rind Coatings Technology 80 (1996) 35-48 2

1 Al-substrate/

The authors determined the structure and composition of the alumina films and found that they have a stoichiometric composition and are amorphous. An amorphous structure renders thesefilms particularly suitable as there are no grain boundaries where corrosive attack could start. In addition, for many casesit is advantageous that alumina is non-conductive. Thus, the formation of galvanic cells between film and substrate is prevented.

/’

6. Nitride films

In addition to the oxides, the nitrides of the active transition metals and of someof the main group elements suchas aluminium and silicon are chemically very stable and hence basically suitable for corrosion protection. 6.1. Silicon nitride

-0.8 2

,1 -0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.3

-0.2

6 7 -0.8

-0.7

-0.6 -0.5 -0.4 potential vs. SCE / V

Fig. 11. Polarization curves of alumina films on aluminium; uncoated aluminium, and aluminium coated with alumina deposited without ion irradiation; centre, coated with alumina with two different ion energies; bottom, coated with alumina with two different irradiation intensities; from Ref. [20].

top, film films films

improves the corrosion behaviour in comparison with the films deposited without ion irradiation, an energy of 1 keV leads to a reduction of almost an order of magnitude in current in the anodic region beyond -0.6 V. Apparently, a certain amount of energy input or momentum transfer from the ions to the growing film is needed to obtain a reduction in porosity. With an ion energy of 1 keV at an I/A ratio equal to 0.6, each aluminium atom obtains on average an energy of 600 eV. The bottom figure shows that this amount of transferred energy can also be obtained when the I/A ratio is increased for a constant ion energy. At 0.5 keV, an increase from I/A=O.6 to I/A= 1.3 again reduces the anodic current density considerably. For both cases,the increase in energy and the increase in I/A ratio, the energy deposited per atom has roughly been doubled.

Natishan et al. prepared S&N4 films by silicon evaporation under 0.5 keV nitrogen ion bombardment [Zl]. Stoichiometric films of different thicknessescould be obtained by calibrating the deposition conditions using Rutherford backscattering analysis.The substrateswere polished pure aluminium. The corrosive medium was a deaerated aqueous solution of 0.1 M NaCl of pH 5.8. The samples were immersed in the solution for 24 h. The pitting potential was determined potentiostatically by anodically stepping the potential from the open circuit potential in stepsof 50 mV. After reaching steadystate values the current density was recorded. In Fig. 12 the polarization curves of uncoated and coated aluminium are presented.The coating shiftsthe pitting potential to more positive values. This shift increaseswith increasing film thickness.A film only 0.01 urn thick shifts the pitting potential 0.3 V in the anodic direction. The highest value, 0.75 V, is obtained for the 2 urn thick coating. Additionally, the passive current densities are reduced by the sameorder. Baba et nl. deposited silicon nitride films under similar experimental conditions onto

Polential (Vsce) Fig. 12. Current-potential curves of aluminium, uncoated and coated with IBAD Si3N4 films of different thicknesses; from Ref. [21].

W. EmingerJSwface and Coatings Technology 80 (1996) 35-48

0

2

4

8

6

coating shows a marked reduction in dissolution current, the currents of the as-deposited S&N, film are partially of the order of or above those of the uncoated material. This may be caused by a higher porosity of silicon nitride compared with chromium nitride. A considerable improvement could be achieved for silicon nitride coated samples by heating to 800 “C in air for 47 h. The increase in dissolution current stayed very low, and even after 40 potential cycles, only very little defects could be found on the sample surfaces. The authors speculate that oxidation and further interlocking between film and substrate may be the reasons for the beneficial effect.

10

transport ratio SUN Fig. 13. Current densities of austenitic stainless steel coated with silicon nitride films, taken from polarization curves at a potential of 1.2 V vs. Ag/AgCl after 100 potential cycles in 5% sulphuric acid; from Ref. [22].

austenitic stainless steel AISI 316L [22]. The samples were tested in aerated 5% sulphuric acid by potentiodynamic polarization measurements. In Fig. 13 the substrate dissolution current densities of the 100th potential cycle at a potential of 1.2 V vs. Ag/AgCl are plotted as a function of the I/A ratio. The currents exhibit a minimum at [Si]/[N] =4. The authors ascribe this result to an optimum with respect to microporosity and adhesion. Liu et al. treated the intermetallic compound N&Al (O.lB), which is a candidate as construction material for aerospace applications, with different ion beam techniques including IBAD of 1 pm thick chromium nitride and 1 pm thick silicon nitride [23]. By contrast to the above mentioned studies, with 14 and 20 keV respectively, the ion energy was in the high-energy regime. With an I/A ratio N:Si= 1:0.79, stoichiometric S&N4 could be obtained. The corrosion behaviour was determined in 1 N sulphuric acid by potentiodynamic polarization measurements at a scan rate of 10 mV s-l. In Fig. 14 the critical current density is depicted as a function of the number of scan cycles. Whereas the Cr,N

Ni3AI (0.1 B) 30

I

uncoated

coated with 1 km of: Si3N4

SisN4, oxidized ________________________________________----~

0

10 potential

20 cycles

45

30

[nl

Fig. 14. Anodic dissolution current densities of Ni,Al (O.iB); adapted from Ref. [ 231.

6.2. Titanium nitride Titanium nitride formed by plasma-based physical vapour deposition techniques is widely used in industrial application for wear protection. TiN tends to columnar growth. This does not necessarily affect the tribological performance negatively, but in cases where corrosive attack is involved it may be detrimental. Several groups are therefore working on reducing the microporosity of TiN without losing its wear properties. Baba and Hatada [24] deposited TiN under bombardment with 2 keV nitrogen ions onto austenitic stainless steel (AISI 316L). The corrosion behaviour was tested in aerated 5% sulphuric acid at 30 “C by potentiodynamic polarization at 10 mV SK’. Owing to the protective power of the alloyed chromium, the steel did not show a passive-active transition such as iron would, but at high anodic potentials the currents increased owing to transpassive dissolution of chromium as chromate. This does not represent the normal corrosion conditions, but determination of these dissolution currents gives an indication of the porosity of the coated material. By contrast with uncoated specimens, TiN coated specimens exhibited low currents in this potential region indicating a protective effect. With increasing number of potential scans the currents increased gradually. From Fig. 15 the influence of the I/A ratio on the development of the dissolution currents with the number of scan cycles can be seen. With increasing I/A the currents decrease, indicating reduced porosity. A minimum is reached at I/A=4, then the performance becomes worse again. Besides appropriate selection of the I/A ratio, the angle of ion incidence plays a major role in structural density. Ensinger reports on the preparation of TiN by reactive evaporation of titanium in an atmosphere of backfilled nitrogen under irradiation with 12 keV argon ions with different angles of ion incidence [25]. The substrates were low-alloy steel. They were tested by cyclic polarization in buffered acetic acid at pH 5.6. The relative critical current densities of iron dissolution through pores of the 50th potential scan are shown in Fig. 16. The film with an ion incidence angle of 0” (parallel to surface normal) was set equal to unity for

W. Ensinger/Swface

and Coatings

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80 (1996)

1.5

r

1 TiMoN coated 2 TIN coated 3

0

10

20

30

+C 50 Cycles / I,

GO

70

15

40

angle of ion Incidence

uncoated

80

Fig. 15. Anodic dissolution current densities of TiN coated stainless steel at 1.3 V vs. Ag/AgCl as a function of the number of potential cvcles of polarization measurements: @ uncoated substrate. X titanium ’ deposited without ion bombardment; from Ref. [24].

0

3.7-48

55 [degl

Fig. 16. Dissolution currents of TiN films on steel for different angles of ion incidence.

normalization. The comparison shows that an increase in ion bombardment angle leads to a remarkable decrease in dissolution current. The lowest porosity was obtained for an angle of 40”. At higher angles, the currents increased slightly again. The change in angle of ion incidence is accompanied by disruption of the distinct columnar growth and preferential [loo] crystal orientation. When columnar growth is avoided, there are no longer open grain boundaries which render the film intrinsically porous. Another way to change the corrosion resistance of titanium-based nitride films is to alloy titanium with another transition metal. Li et al. 1261 deposited TiN and TiMoN films onto a hard material alloy (YG8). The ternary Clm was formed with the to-date still rare technique of multi-ion beam assisted deposition, including a metal ion beam. Titanium was sputter deposited onto the substrate whilst being irradiated with 40 keV nitrogen ions to form the nitride and additionally with a beam of molybdenum ions from the same ion source. The polarization curves obtained in 0.5 M sulphuric acid in Fig. 17 show that the addition of molybdenum leads to a decrease in current density over the whole potential

current

density

log I I p&m:!

Fig. 17. Polarization curves of YG8 alloy in 0.5 M sulphuric acid; adapted from Ref. [26].

range. Both in the region of anodic dissolution and in the transpassive region, the dissolution currents are reduced by an order of magnitude. By contrast, the TiN film is only able to reduce the transpassive current. In a literature study, Ensinger et al. compared the corrosion performance of various coating materials, including TiN obtained by plasma-based physical vapour deposition, with ion beam assisted deposited films [9]. In general, the results show that IBAD coatings have similar corrosion protection power to other physically vapour deposited coatings, but are superior when the substrate temperatures have to be kept low. This is the case for heat treated steels and aluminium alloys. Here, a niche for industrial application of IBAD is seen. 6.3. Industrial application of ion beam assisted deposited TiN and ZrN IBAD hard material nitrides with their high stability against corrosion and wear have been used successfully in industry for protecting razor blades from tribological and corrosive attack since 1987 [27]. In this particular case, the decorative appearance of the nitrides also played a role. Miyano and Kitamura from Matsushita Electric Works in Osaka developed a process line where razor blades of electrical shavers are coated with goldencoloured titanium nitride [28]. The substrate material is martensitic stainless steel (AISI 410). The coiled material is fed from a reel into the processing zone of the IBAD facility and recoiled on a second reel. Prior to coating it passes an ion source where it is cleaned by ion bombardment. Prior to application, the protective effect of the coating was evaluated by electrochemical measurements in 3% neutral sodium chloride solution. In this environment, stainless steel may suffer from pitting corrosion. Untreated steel was compared with steel coated using IBAD and with steel coated using conventional ion plating methods, such as activated reactive evaporation

K

EnsingerJ&rface

and Coatings

with hollow cathode discharge. Samples with films only 0.1 pm thick were immersed in the salt solution. After reaching a constant rest potential, the samples were polarized potentiodynamically at a scan rate of 1 mV s-l. In Fig. 18 the polarization curves are compared. In the cathodic region, the samples exhibit different current densities with the IBAD sample showing the lowest value. In the anodic region where dissolution of the substrate material takes place through pores in the film or at pits at the substrate material, the samples initially show low currents. They are passive. When the potential is scanned further in the anodic direction, the samples start to corrode locally. The onset of pitting corrosion can be read from the sudden increase in current. The result shows that the ion plated films are not dense and lead to early corrosive failure of the material. By contrast, the pitting potential of the IBAD film is shifted considerably to more positive values, indicating increased corrosion resistance. This result was supported by long-term immersion tests in combination with X-ray microanalysis of corrosion products on the surface. Ion plated steel showed rapid development of iron-based corrosion products. The IBAD sample stayed unstained for a considerably longer time. Based on the above results on IBAD films, the success of this system can be ascribed to the following reasons. The surface of the material is cleaned by sputtering just before coating. This leads to enhanced adhesion of the film and to reduced porosity. By ion mixing, a broad and presumably corrosion resistant transition region between the substrate material and the film is formed. Again, adhesion is enhanced and corrosion is reduced. Finally, ion bombardment during deposition leads to a reduction in film porosity. This effect is optimized, either by intention or by chance, as, presumably for constructural reasons, an angle of ion incidence off the surface normal has been used. From the schematic

Scanning Rate : 3160 mV/min

Technology

80 (1996)

35-48

47

drawing of the apparatus published in Ref. [28], an angle between 45” and 60” can be estimated. Thus, optimized conditions have been created for a corrosion resistant film which is at the same time hard and wear resistant and decorative, owing to its golden appearance. The coated shaver blades are more corrosion resistant than normal blades against sweat and can be cleaned with water and even used for wet shaving. Similar results are reported by Kiyama et al. from Sanyo Electric Corp. in Osaka [29]. They coat electroformed nickel for shaver blades with well adhering hard and corrosion resistant zirconium nitride films. In comparative measurements, the corrosion lifetime of a ZrN coated nickel foil was around three times higher than that of a ruthenium coated foil.

7. Summarizing remarks A variety of studies on coatings deposited from the vapour phase under simultaneous irradiation with a beam of highly energetic ions have shown that ion bombardment can lead to microstructural changes in the film and the interface between film and substrate which render the coating/substrate system more corrosion resistant. For optimized corrosion protection, the main parameters of ion beam assisted deposition, namely ion-to-atom arrival ratio, ion incidence angle and ion energy have to be selected appropriately. The phenomena which lead to improved corrosion resistance are complex. Two main effects can be found for ion irradiated coatings. One is an increased structural density with reduced porosity; the other is enhanced adhesion between the coating and the substrate. Both are required to prevent failure of the coating. Apart from chemical effects which depend on the system, it is mainly the physical effects of energy and momentum transfer from the ions to film-forming atoms which lead to the beneficial changes in thin film microstructure. From the above examples, it is clear that ion beam assisted film deposition, provided there are no economical obstacles, is suitable for industrial application for increasing the corrosion resistance of a variety of materials.

Acknowledgment Voltage(vs SCE) / mV

The author gratefully acknowledges Professors G.K. Wolf and Y. Pauleau, who initiated this review.

References Fig. 18. Polarization curves of TiN coated martensitic stainless steel in 3% NaCl solution at 25” C; 1, IBAD; 2, ion plated; 3, uncoated; n denotes the corresponding rest potential; from Ref. [28].

[l]

J.J. Cuomo, SM. Rossnagel and H.R. Kaufman (eds.), Handbook of Ion Beam Processing and Technology, Noyes, Park Ridge, NJ, 1989.

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PI F.A. Smidt, Int. Mater. Rev., 35(2) (1990) 61. G.K. Wolf and W. Ensinger, Nucl. Instrum. Methods Phys. Res. B59-69 (1991) 173. c41 R. Emmerich, B. Enders and W. Ensinger, Ion beam assisted deposition of thin films and coatings. In T.S. Sudarshan and J.F. Braza (eds.), Surface Mod$cation Technologies VI, TMS, Warrendale, PA, 1993, p. 811. 151 W. Ensinger and G.K. Wolf, Mater. Sci. Eng. Al 16 (1989) 1. C-51 G.K. Wolf and R. Emmerich, in Y. Pauleau (ed.), Materials and Processes for Surface and Interface Engineering, NATO-AS1 Series E, Applied Sciences, Vol. 290, Kluwer, Dordrecht, 1995, p. 565. c71 S.S. Nandra, F.G. Wilson and CD. DesForges, Thin Solid Films, 107 (1983) 335. I31 B. Enders, S. KrauB, K. Baba and G.K. Wolf, Surf. Coat. Technol., 74-75 (1995) 959. W. Ensinger, A. Schrijer and G.K. Wolf, Nucl. Instrum. Methods PI Phys. Res. B,80-81 (1993) 445. I301 R. Emmerich, G.K. Wolf, H. Buhl and R. Braun, Surf. Coat. Technoi., 74-75 (1995) 1043. El11 J.H. Hsieh, R. Lee, R.A. Erck, G.R. Fenske, Y.Y. Su, M. Marek and R.F. Hochman, Surf. Coat. Technol., 49 (1991) 83. [121 J.H. Hsieh, W. Wu, R.A. Erck, G.R. Fenske, Y.Y. Su and M. Marek, Surf. Coat. Technol., 51 (1992) 212. Cl31 M. Matsuura, N. Chida, H. Fujinuma, H. Ishikawa, M. Yoshinari and T. Sumii, lonics, 9 (1991) 31. Cl41 P.J. Martin, H.A. MacLeod, R.P. Netterfield, C.G. Pacey and W.G. Sainty, Appl. Opt., 22 (1983) 178. c31

P.J. Martin, R.P. Netterlield and W.G. Sainty, J. Appl. Phys., 55 (1984) 235. 1161 U.J. Gibson, Mater. Res. Sot. Symp. Proc., Vol. 223, Materials Research Society, Pittsburgh, PA, 1991, p. 263. Cl71 P.J. Martin, R.P. Netterfield, W.G. Sainty and C.G. Pacey, J. Vat. Sci. Technol., A,2 (1984) 341. El81 E. McCafferty, G.K. Hubler, P.M. Natishan, P.G. Moore, R.A. Kant and B.D. Sartwell, Muter. Sci. Eng., 86 (1987) 1. Cl91 E. McCafferty, P.M. Natishan and G.K. Hubler, Nuc2. Instrtrm. Methods Phys. Res. B, 56-57 (1991) 639. c201 R. Emmerich, B. Enders, G.K. Wolf, J. Kudela, P. Lukac, K. Baba and R. Hatada, in J. Williams (ed.), Proc. Int. Con& on Ion Beam Modification of Materials, Canberra, Febwary 1995, North Holland, in press. WI P.M. Natishan, E. McCafferty, E.P. Donovan, D.W. Brown and G.K. Hubler, St16 Coat. Technol., 51 (1992) 30. c221 K. Baba, R. Hatada, R. Emmerich, B. Enders and G.K. Wolf, Nucl. Instr. Meth. Phys. Res., B106 (1995) 106. 1231 X.H. Liu, S. Zou, S. Taniguchi, Q, Fang, S. Kalbitzer, A. SchrBer, W. Fischer, M. Barth, W. Ensinger and G.K. Wolf, Nucl. Instrum. Methods Phys. Res. B, 59-60 (1991) 851. [241 K. Baba and R. Hatada, Sut$ Coat. Technol., 65 (1994) 368. ~251 W. Ensinger, Surf. Coat. Technol., 66 (1994) 90. WI G. Li, X. Su, Z. Gong, B. Liu, X. Wen and T. Ma, Sur& Coat. Technol., 66 (1994) 350. 1271 M. Iwaki, Mater. Sci. Eng., Al 15 (1989) 369. iY1 T. Miyano and H. Kitamura, Str$ Coat. Technol., 65 (1994) 179. ~291 S. Kiyama, H. Hirano, Y. Domoto, K. Kuramoto, R. Suzuki and M. Osumi, Nucl. Instrum. Methods Phys. Res. B, SO-81 (1993) 1388. El51