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Ion-beam-assisted coatings for corrosion protection studies
Materials Science and Engineering, A 116 (1989) 1-14
l
Ion-beam-assisted Coatings for Corrosion Protection Studies* W. ENSINGER and G. K. WOLF Physikalisch-Chemisches Institut, Universitiit Heidelberg, Im Neuenheimer Feld 500, 6900 Heidelberg (E R. G.) (Received September 16, 1988)
Abstract
For corrosion protection, modified surfaces or coatings prepared by ion implantation or ion beam mixing often are not sufficient for technical longterm applications. The main problem, i.e. the shallowness of the films, can be overcome by ionbeam-assisted deposition which offers the positive effects of ion beam treatment, e.g. high adhesion, low porosity, and a low process temperature without thickness limitation. The corrosion protection behaviour of a number of coatings, e.g. boron, chromium and their nitrides, chromium oxide, silicon, hard carbon and aluminium, are discussed. The results demonstrate ion-beam-assisted deposition to be appropriate for producing adherent coatings which are well suited to long-term protection against corrosion, and partly also against tribological deterioration, even for technical rough surfaces, 1. Introduction The main method for corrosion protection, apart from using corrosion-resistant bulk materials, is coating of the substrate material with a resistant layer. The application of corrosionresistant bulk materials often is restricted, because other properties, e.g. mechanical ones, do not meet the requirements. Sometimes the material simply is too expensive. In the age of conservation of resources the resistant coating is the appropriate method and techniques for producing such coatings and tailoring their properties are necessary. There are many techniques for production of corrosion-resistant coatings in use which have both advantages and disadvantages, The galvanic process, cheap but often burdening the environment with poisonous waste solutions, is very appropriate for conventional protecting *Paper presented at the Sixth International Conference on Surface Modification of Metals by Ion Beams, Riva del Garda, Italy, September 12-16, 1988. 0921-5093/89/$3.50
layers such as the well-known chromium layers. However, it does not allow production of layers other than of the group of the refractory metals from aqueous solution, although these (aluminium, titanium or tantalum) are very well suited to corrosion protection. Also most non-metals such as boron, silicon or carbon cannot be produced as films in this way. This restriction can be overcome by evaporation or physical vapour deposition. In this case, the adhesion of coating to substrate is often poor or, for better adhesion, high process temperatures are necessary which often cannot be tolerated by the substrate materials. Other processes such as chemical vapour deposition need high temperatures to be carried out or lead to contaminations of the layers etc. Another modern technique, laser melting, causes problems with reflecting metallic substrates or when there are large differences between melting or evaporation temperatures of coating and substrate materials and leads to an often undesired surface morphology. Many of these problems can be avoided with ion implantation which exhibits universality, strong adhesion, a low process temperature, very good controllability and reproducibility. With this technique, stable and also metastable alloys or compounds are available which can hardly be made with other techniques. In addition to crystalline phases, amorphous phases with special corrosion resistance properties may also be obtained. An intrinsic limitation of ion implantation, i.e. that only a certain percentage of alloying material can be implanted, can be overcome by ion beam mixing. After coating, e.g. by evaporation with a corrosion-resistant material such as the metal aluminium or the non-metal boron, the layer is mixed with the substrate, and a mixed phase (ion beam mixing) or an interphase which forms a transition from the pure layer to the substrate (interface mixing)is formed. Multilayers and sandwich layers are used to produce thicker, © Elsevier Sequoia/Printed in The Netherlands
homogeneously mixed coatings. With an appropriate reactive ion species, compounds such as oxides or nitrides are available. Both methods-ion implantation and ion beam mixing--are useful for basic corrosion studies as demonstrated in numerous publications (see refs. 1 and 2). From the latest conferences on ion beam technology, e.g. the International Conference on the Ion Beam Modification of Materials 1988 and the International Conference on Ion Implantation Technology 1988, it appears that the number of papers dealing with corrosion is recurrent and the interest is decreasing. The reason may be the limitation of the technique for practical applications which on the contrary results from t h e shallowness of the layer. Corrosion often consumes the thin coatings too fast, so that no longterm corrosion protection is possible, For corrosion processes with slow rates, ion implantation or ion beam mixing in special cases may be sufficient. An example for this is the case where the passivity of a metal is maintained by a very small amount of implanted material although in the adequate corrosive medium it usually corrodes actively with high rates. This is the case for titanium or stainless steel in strong acids, They can be protected by implanting platinum or palladium [3-6]. The few micrograms per square centimetre of implanted or mixed material are already sufficient for long-term protection. For active corrosion in slightly acidic media, e.g. unalloyed steel in a natural environment, such mechanisms mostly do not work. Here the protection effect of a coating with a thickness below 1/~m which can be produced by ion beam mixing is only of short duration,
A possibility for overcoming this limitation is ion-beam-assisted deposition (IBAD). This technique combines the coating process--evaporation or sputtering--with ion bombardment to a onestep process. Thick layers can be produced without losing the advantages of the ion beam treatment. With this technique it is possible to make successful technical applications. This will be shown by a number of examples. The following mainly deals with ion-beam-assisted evaporation for protection against wet corrosion.
2. Ion-beam-assisted deposition process control Controlling and engineering the IBAD process can be done in different ways (Fig. 1). An IBAD machine can be a combination of an implanter and an evaporation facility. In this case the atom and ion beams usually meet at right angles. Thus, two operational possibilities result. The substrate is either simultaneously coated and bombarded at 45 ° (or other correlating angles) or consecutively coated and bombarded in small steps, e.g. by evaporating a thin layer at 90 °, rotating the substrate holder and bombarding at 90 ° etc. The latter process is intermediate between the continuous IBAD process and single-step ion beam mixing. Most of the IBAD facilities used today are compact machines where the ion gun is directly fixed to the vacuum chamber. Thus the 90 ° alternating conception can be used or nearly parallel atom and ion beams with impact angles near to 90 ° are utilized. Atom and ion beam generation are independent of each other and can be varied over a wide rang e . Therefore the process can be carried out in
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PROCESSTIME PROCESSTIME (J DEPTH ]Fig. 2. Possibilities for I B A D processing: (a) cleaning and activating by sputtering; (b) interface mixing; (c) bombardment of growing film; (d) post-implantation; (e) different ion-to-atom ratios; (f) formation of muitilayer; (g) formation of alloys with two independent evaporators; (h) formation of compound/elem(nt multilayer. (a)-(d) Number of impacting particles vs. process time and (e)-(f) schematic depth profiles of coatings obtained.
many ways. Figure 2 shows a selection of possibilities. The abscissa represents the process time, and the ordinate the relative number of impacting or condensing particles. In Fig. 2(a) the simple case of substrate pre-cleaning and activating is shown. Sputtering leads to removal of contamination layers which may influence corrosion behaviour and adhesion. Nucleation sites for the condensing film are formed, and the surface is activated. The layer grows without being influenced by the ion beam. In Fig. 2(b) the bombardment is maintained until the film has reached a certain thickness. The layer and substrate are mixed, a corrosion-resistant phase may be formed, and interlocking leads to enhanced adhesion. In Fig. 2(c) the whole layer is formed under bombardment. Besides formation of an interphase, many properties of the layer which are important for corrosion protection are influenced. Carrying on the bombardment after the coating process (Fig. 2(d)) may be useful for reactive IBAD where a reactive gas such as nitrogen is used to increase its percentage in the layer. Thus post-bombardment influences stoichiometry, morphology, etc. Figure 2(e) shows different ion-to-atom ratios. For rare gas ions, such a variation leads to different properties of the layer; for reactive ions, differences in the composition add to it. Changing the evaporating element results in formation of sandwich layers as shown in Fig.
2(f). For corrosion protection this may be a nickel layer on steel followed by a chromium layer. Two independent evaporators (Fig. 2(g)) offer the possibility of depositing a binary alloy or compound or a ternary compound when using a reactive ion. The last example (Fig. 2(h)) again shows a method for producing a sandwich layer. In this case the ion species is changed during the process. As an example, a steel substrate is cleaned and activated with an argon beam, and then a Fe-Cr phase is formed by ion beam mixing of the condensing chromium. The interphase is thickened by a pure chromium layer growing under moderate argon bombardment. Then N2 is introduced into the ion gun instead of argon. A hard CrN layer is formed on top of the corrosion protection coating for tribological protection. This list could be continued, but it illustrates already the possibilities and the flexibility of the IBAD process for producing corrosion protection coatings. 3. Basic theory of corrosion of coatings Before dealing with some examples for corrosion protection with IBAD a short introduction into the basic theory of corrosion of coated metals is given in order to understand the results better. The corrosion products such as iron ions and salts are thermodynamically more stable than
the pure metals, and this is the driving force for corrosion. Therefore the metal tends to dissolve in the solution; it corrodes. The aqueous corrosion reaction of a metal can be separated into partial electrochemical reactions taking place simultaneously at the metallic surface. The anodic reaction is the oxidation of a metal: Me--' Me n + + neThe cathodic reaction depends on the solution. In acidic solutions it is hydrogen evolution: 2H ÷ + ne- ~ H2 In the presence of oxygen, oxygen reduction will take place: 02 + 4H ÷ + 4 e - ~ 2H20 In electrochemical equilibrium the counterreactions (metal reduction and hydrogen oxidation) take place at the same rate and cancel the reaction macroscopically. However, under corrosion conditions, one of the reactions predominates; metal is oxidized, and hydrogen gas is evolved. The net reaction in acids is Me + n H + ~ Me n* + ½nil 2 The metal corrodes under hydrogen evolution, Separating the metal from the aggressive environment with an inert coating stops the reaction and protects the metal. Another possibility is influencing one of the reactions, e.g. reducing the cathodic reaction, and thus also reducing the complementary reaction, in this case the metal dissolution, Corrosion of a coated material mostly starts at defects such as scratches or pores. Another problem can be local failure of adhesion. There are several reasons for such a failure, e.g. contaminations in the interface, gas bubbles, stress between layer and substrate, and incompatibility in any sense. Water is adsorbed at such defects and leads to corrosion. Small micropores can be the reason. Water penetrates the coating through these small channels driven by capillary forces, Then corrosion leads to formation of a solution, Osmosis may force more water through the pores, leading to formation of a bubble. The evolving hydrogen gas increases pressure with debonding of the film. Bubble formation also can be caused by electro-osmotic effects where a potential gradient resulting from a local element forces electrolyte through pores to the substrate, Aggressive ions such as chloride can penetrate
the layer, causing a strong corrosion attack. Depending on the electrolyte-layer-substrate combination, different types of corrosion can develop such as local elements, crevice corrosion and concentration elements. They lead to failure of the coating or to strong local corrosion of the substrate. It depends on the electrochemical mechanisms which metallic coatings are responsible for the reactions. They can be divided into two groups: the more and the less noble metals with respect to the substrate material. In the case of iron and steel the former are tin or silver, and also metals which are more noble and corrosion resistant than iron by formation of resistant oxides such as chromium and aluminium. Under normal conditions in neutral or acidic media, they corrode with much lower rates than iron because they are passive or immune and protect iron. However, the layers have to be free of flaws and pores as these will form a local element because of the standard potential differences between coating and substrate. In this case, the coating is a large-area cathode and the substrate contains local small-area anodes. High anodic corrosion current densities lead to undermining of the layer and formation of deep holes. Coatings which are less noble than iron, e.g. zinc and cadmium, behave the other way round. They are anodically polarized and protect the underlying iron by dissolving as sacrificial anode. In this case, only thick layers are useful because thin coatings are consumed too fast. The following examples deal with pure iron or steel as substrate and more noble cathodic coatings.
4. Corrosion measurement techniques For investigationsofelectrochemicalcorrosion processes, potential, current and polarization measurements are employed. The simplest method is measuring the corrosion potential of a specimen or its change with time. For special cases the currents flowing between anode and cathode can be measured. Polarization measurements are used most widely. With the help of an external current source a defined current or potential is established and the resulting potential or current is measured. To obtain current-potential plots a potentiostat is needed which electronically introduces a potential between the working and the reference electrode. The resulting current between working and counterelectrode is re-
corded. This can be done potentiostatically as a steady state measurement, or potentiodynamically when the potential is changed with time. Scanning the potential up from a start potential to a reverse potential and back again to the start potential is called cyclic voltammetry. Figure 3 shows a cyclic current density-potential curve for pure iron. The start potential is cathodic, i.e. the specimen works as cathode; hydrogen evolves, and oxygen is reduced. At the corrosion potential the current passes through zero. Then the specimen becomes anodic; iron is oxidized and dissolves. The highest anodic current is the so-called critical current ic, where passivation starts, i.e. a protecting oxide grows. The current is strongly reduced, and the specimen is passive. At high positive potentials, water decomposes and oxygen evolves. The shape of the plot and the current densities deliver informations about the electrochemical or corrosive behaviour of the specimen and about corrosion mechanisms. The critical current density is a criterion for the corrosion. Except for special cases, no quantitative data for corrosion rates can be derived from these curves, For quantitative dissolution data, immersion tests have to be carried out. If the samples are large enough, they can be weighed before and
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J Fig. 3. Potentiodynamic current-potential curve for pure iron in acetate buffer pH 5.6 at 25 °C (scan rate, 10 mV s- 1) in (a) the cathodic region (hydrogen evolution), (b) the anodic region (metal dissolution), (c) the passive region (passive corrosion) and (d) the oxygen evolution, where E~ is the corrosion potential, EF the flade potential (beginning of corrosion reduction by oxide formation), E s the potential at the beginning of oxygen evolution, lw the current density of hydrogen evolution, I s the current density of oxygen reduction, IK the critical current density (maximum iron dissolution) and Iv the passive current density.
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The corrosion potentials of the coated specimens are more noble; the corrosion currents are significantly reduced compared with those for the uncoated samples. The potentiostatic currenttime plots in Fig. 5 show that the anodic currents of the RIBAD samples remain very small. They are about two to three orders of magnitude lower than for Armco iron and one to two orders of magnitude lower than for Fe/B multilayers implanted with nitrogen ions even after immersion for several hours. This demonstrates such hard layers to be very well suited to both tribological and corrosion protection, The Naval Research Laboratory group in Washington prepared chromium oxide layers on AISI 52 100 steel using a metal ion beam for an IBAD corrosion protection layer [10]. A 45 ° arrangement was used for simultaneous bombardment of the condensing film. The samples were pre-implanted with 40 keV Cr + ions to clean the surface by sputtering, and to introduce a buried layer of chromium. Then evaporation of chromium was started with simultaneous bombardment with Cr + ions. During the process the sample was exposed to oxygen at 10- 5 Torr. Thus a coating 200 nm thick was prepared. The corrosion behaviour was studied in 1 N H2SO4 and in a 3 ppm solution of NaC1, both deaerated using potentiostatic polarization measurements. Figure 6 shows the anodic polarization curves in the salt solution. The open-circuit potential of the coated specimen was about 0.25 V more noble than that
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of the steel sample. The current densities were one to two orders of magnitude lower. Inspection of the corroded samples showed that the uncoated steel was heavily attacked whereas the Cr203 layer remained unattacked except for a number of pits coming from chlorine action at defects. A thicker layer is expected to give good long-term corrosion resistance. A Japanese group [11] coated low carbon steel with aluminium in a two-step procedure. An aluminium layer 30 nm thick was condensed on the substrate and then mixed with 30 keV Ar + ions. Afterwards this intermediate or starting layer was thickened to 1 #xm by evaporation. In this case the parallel beam arrangement shown in Fig. 1 was used; the ion impact angle was 90 °, and the atom angle accordingly smaller. Adhesion tests, i.e. tensile test and bending of the foil at 180 °, showed strong adhesion for the ion-beamtreated samples in contrast with the untreated samples where the coating was easily detached. The corrosion behaviour was tested in acetate buffer pH 5.0 using multisweep cyclic voltammetry. Comparison of the critical anodic current densities shows a marked decrease for the ionbeam-coated samples. The uncoated steel sample starts with a current density of about 10 mA cm-2 which increases with the number of potential sweep cycles and reaches 100 mA cm-2 after 20 cycles. The ion-beam-mixed layer starts with a current density of some 0.1 mA cm-2 and attains 2 mA cm- 2 after 50 cycles. This example again shows the value of IBAD for producing well-adhering corrosion protection coatings.
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6.1. Experimental details The I B A D machine at Heidelberg for the simultaneous process consists of a duoplasmatron ion source and an electron beam evaporator, The impact angles are 80 ° for both ions and atoms. Figure 7 shows a cross-section of the vacuum chamber. The whole assembly has been described in detail elsewhere [12, 13]. The electrochemical tests (cyclic voltammetry) and immersion tests were carried out in acetate buffer pH 5.6 or artificial sea water. The corrosion rates were determined using polarography and neutron activation analysis. In the former case, faradaic currents of the dissolved species and, in the latter, Y radiation of the activated dissolved material in the solution are used for quantitative measurements of the corrosion rates. Adhesion was tested with the scratch test with a 200 /~m Rockwell stylus. Another test was the pull-off (pin pull) test where a stamp is glued to the coating and pulled off perpendicular to the coating plane. Further information could be obtained from the bending test where the sample sheet is bent around cylinders with different radii until the coating is detached, Mostly pure iron and a low carbon (0.3 wt.%) steel served as substrate polished down to 1/.tm diamond paste or ground to obtain technical grade surfaces. Chromium, aluminium, titanium, boron, silicon and their nitrides and i-C layers
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were prepared at room temperature with thicknesses ranging from 50 nm to 10/am. Of these, some examples are chosen to show the corrosion protection effect.
6.2. Results Figure 8 shows two Auger depth profiles of 100 nm boron on steel, one layer which had been merely deposited, and the second obtained under simultaneous bombardment with 6 keV Ar + ions. In the latter case the sample was pre-bombarded to clean it by sputtering and to create nucleation sites before the evaporation was started. Part of the condensing layer was mixed with the substrate. W h e n the layer grew thicker, no more mixing occurred because of the limited penetration depth of the ions, but the film still grew under gentle bombardment. As can be seen from these two profiles, the measurable differences between
the layers are minor. The extent of the mixed interface lies within the experimental uncertainty of the Auger depth profiling. Nevertheless there are enormous differences in adhesion and corrosion behaviour. When the ratio of mixing ions to condensing atoms is increased, the profiles change dramatically. Figure 9 shows two profiles of a 100 nm silicon layer on steel. The profile for the as-evaporated layer is similar to the boron profiles. The interface is marked by strong carbon and oxygen contaminations. The IBAD layer looks more like a surface compound, a diffusion layer or even an implantation profile with an anomalously high content of implanted material, Strong mixing by knock-on implantation, radiation-enhanced diffusion etc. leads to formation of a compound. Iron is already present at the surface of the specimen. Strong sputtering causes a evaporated 100
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reduced thickness of the layer compared with the simultaneously evaporated amount condensed on the thickness monitor not hit by the ion beam. This comparison illustrates the above-mentioned flexibility of the IBAD process. Ion and atom beams can be controlled independently from one another. Electrochemical measurements proved that the boron and silicon layers exhibited good corrosion resistance. For rough~,urfaces, IBAD was superior to conventional ion beam mixing. Details have been described elsewhere [13]. Figure 10 shows the results of the immersion tests for the silicon layers [14]. Corrosion rates are plotted vs. immersion time. Despite the roughness of the ground substrate surface (more than 1 pm) and the shallowness of the silicon IBAD layer the corrosion rate stayed low for about 3 weeks before normal corrosion of the steel started. The second plot shows a com-
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9 parison between untreated steel, as-evaporated boron layers, ion-beam-mixed (5 x 10 ]6 Ar + cm -2) boron layers and a 6 keV Ar + IBAD layer, Whereas the evaporated boron layer is detached after a few hours of immersion, post-bombardment and simultaneous bombardment significantly reduce corrosion for more than a month, The reason for the early failure of the as-evaporated layer clearly is insufficient adhesion and high stress. Water or solution enters the layer through micropores, undermines it and breaks the weak bonds between layer and substrate. Then stress causes detachment of the film. Ion beam treatment forms a stronger interface under creation of an interphase. This results in strongly enhanced adhesion. The pull-off test yields values of below 10 MPa for as-evaporated layers and for more than 80 MPa, the limit of the test, for ionbeam-treated layers. Figure 11 shows the results of the scratch test for silicon layers on steel. Ion beam mixing leads to an enhancement of adhesion. The highest value was found for IBAD. Contrary to the as-evaporated and ion beam mixed layers in the case of IBAD there was no clear failure to be found, probably because the degree of mixing was so high. No clear detachment could be observed, and the true value might even be higher, In the case of metalloids such as boron and silicon the interphase may be amorphous, rendering the specimens corrosion resistant. An amorphous structure often is less susceptible to corrosion than is a crystalline structure. There are fewer grain boundaries and other defects where
corrosion usually starts. The formation of local elements which are to be found in multiphase systems as a consequence of segregation phenomena are less possible in an amorphous singlephase structure. Another effect may be an electrochemical influence. Metalloids such as boron or silicon exhibit a much lower exchange current density for hydrogen evolution. As mentioned above, reduction of the cathodic reaction also reduces the anodic counter-reaction metal dissolution. Boron and borides suffer from high intrinsic stress. Therefore boron layers tend to detach when attacked mechanically or chemically. IBAD offers the possibility to change stress values of boron layers by changing the ion-to-atom ratio. Figure 12 contains results of stress measurements [15]. 200 nm of boron were condensed on a steel foil 50/~m thick, and the degree of bending opti L cally determined. Without bombardment there is high tensile stress often connected with cracking of the layer. Bombarding the condensing layer with 6 keV Ar + ions converts the tensile to compressive stress when the ion-to-atom ratio is higher than 0.001. Similar results were obtained for chromium [16, 17] and niobium [18] layers. This effect is attributed to structural rearrangement of film atoms shortly after deposition. The energy necessary for activating these processes is delivered by the energetic ions. A second mechanism is preferential sputtering of impurity atoms, resulting in higher purity of the layer [ 19]. A universal protective coating is diamond-like carbon (i-C and hydrogenated amorphous carbon bending
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10
(a-C: H)). It possesses high chemical resistance and hardness and low friction coefficients. It can act as corrosion protection as well as mechanical protection, In Fig. 13 the first three positive-going potential sweeps of a cyclic voltammogram of an i-C layer 100 nm thick prepared by evaporation of graphite under 6 keV Ar + ion bombardment on steel are plotted. The current densities are reduced by more than two orders of magnitude, indicating the corrosion protection effect of the layer. The anodic current peak in the region of - 0 . 4 V stems from iron dissolving through micropores and other defects. Immersion tests in acetate buffer and artificial sea water showed that the substrate corroded through micropores around the edges but the middle part of the specimen was unchanged over a long period. This means that thin i-C layers are very well suited to corrosion protection unless there are pinholes or other defects in the layer mostly coming_ from dust particles shading off the surface during the coating process. Figure 14 shows the results of a quantitative immersion test. In this case a layer 0.3 /~m thick was prepared. For the first 3 days there was no detectable corrosion. The layer remained hydrophilic. Then corrosion started at defects and undermined the layer. This again demonstrates that defects and porosity are the crucial point of corrosion protection with thin layers provided that adhesion is sufficient, Scratch tests and pull-off tests prove the good adhesion of the prepared i-C layers. Knoop microhardness tests give a value of 900 HK (load,
50 mN) compared with 400 HK for the untreated material. Although the indenter penetrates the layer and thus only an average value can be obtained, the values are typical for diamond-like and not for graphitic films. Optical band gap and transmission measurements confirm these results [20]. Nuclear reaction analysis gave a hydrogen content of 1-2 at.%. Chromium layers belong to the standard coatings for corrosion protection. Usually they are prepared galvanically. However, thin galvanic layers are strongly porous, and thick layers easily develop microcracks. For comparison with other coating materials, chromium layers 100 nm thick were prepared by evaporating chromium on steel, and by condensing under 6 keV Ar + ion bornbardment. Figures 15 and 16 show the cyclic voltammograms in buffered acetic acids. The layer deposited without ion beam treatment already shows a reduction of the anodic current densities compared with those for untreated steel. In the passive potential region beyond 0.5 V a double peak appears. Here chromium dissolves transpassively as chromate. The results for the IBAD specimens look different. The anodic critical current density is strongly reduced compared with the as-evaporated layers. The peak in the transpassive region is higher. In this plot, three different samples prepared in three different runs are shown. Here the high reproducibility of the process can be seen. Again the ion bombardment yields more corrosion-resistant films than the mere vapour deposition. Higher adhesion, lower
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microporosity and the formation of an Fe-Cr alloy are the reasons for the better behaviour, Figure 17 shows how the intrinsic stress of the film changes with ion-to-atom ratio [20]. This is one example out of a series where different rare gases with varying energies (3-15 keV) were used [15]. The bombardment changes tensile into compressive stress crossing the abscissa at an ionto-atom ratio of about 0.015 in the case of 6 keV Ar + ions. The higher the energy, the more the zero-stress point shifts to lower values, i.e. fewer ions are necessary for the same effect. These results agree very well with values obtained by other workers [16]. It is interesting to compare the value obtained for chromium with that for boron. In the case of boron the ion-to-atom ratio
needed for zero stress is about an order of magnitude lower. Obviously the effect is strongly dependent on mass and bonding characteristics of the layer element. Substituting the rare gases by nitrogen (RIBAD) yielded a chromium nitride layer. The Knoop microhardness (load, 100 mN) of a layer 3/~m thick was 2200 HK compared with 400 HK for the untreated steel. Immersion tests of a layer 2 jxm thick in artificial sea water and visual inspection showed the untreated steel to be heavily corroded after a few days under formation of rust and pits whereas on the coated sampie no sign of rust was visible even when the test w a s stopped after 52 days. The layer apparently works as a sacrificial anode which protects the substrate. Because atom and ion beams can be controlled independently, it is possible to determine precisely the nitrogen content. As mentioned above, in the surface region a high nitrogen-tochromium ratio, i.e. 50 to 50 can be adjusted in order to have a hard surface whereas towards the interface and the substrate the nitrogen content is reduced for better corrosion protection. The last example is aluminium. It is very well suited to corrosion protection of iron in neutral or slightly acidic solutions and could substitute the poisonous cadmium. Unfortunately the adhesion of vapour-condensed films is very poor, and the films tend to columnar growth. Mostly these films have low packing densities and contain pores and microvoids. Preparation of
12
IBAD aluminium films on different substrates were investigated in detail. The thickness ranged from 50 nm to 10 ktm. Mostly 6 keV Ar + ions were used for bombardment. Generally the results show that simultaneous bombardment of the condensing film lead to higher density, purity and reflectivity of the film, to higher adhesion and to lower porosity. In Fig. 18 the values of the anodic critical current density gained from cyclic voltammograms are represented. Untreated iron is compared with 1 Mm evaporated aluminium with and without pre-cleaning of the substrate by sputtering and with ion-beam-assisted evaporation. The critical current density of the as-evaporated layer is about 0.4 mA cm -2 compared with 11 mA cm-2 for untreated iron. Within 10 cycles the current reaches more than 1 mA cm-2. Sputter cleaning reduces the current density below 0.2 mA cm- 2. Because the film itself grows under the same conditions as the film without pre-cleaning and activating the surface, the effect has to be attributed to the pre-bombardment. If the growth mechanism is not influenced by the pre-bombardment, the purity of the interface and higher adhesion must be the reason for the markedly improved corrosion behaviour. This is proved by Auger depth profiling and by adhesion tests. Pull-off tests prove adhesion to increase strongly when the substrate is cleaned immediately before coating. Figures 19 and 20 show the test results for films of different thicknesses. Prebombardment of the substrate already yields pulloff values at least one order of magnitude higher than those for untreated samples.
The IBAD layers show the best performance. Not only is the initial value of the critical current density (Fig. 18) the lowest, but also the longterm behaviour is significantly better, as indicated by the fact that the current density is still low during the tenth cycle. The reason for this is higher density, lower porosity and the more finegrained structure of the bombarded film compared with the unbombarded film. Another reason might be the formation of an amorphous Fe-A1 interphase. The high adhesion values of IBAD layers are represented in Figs. 20 and 21. Bending the sample foil around a radius of 5 mm leads to detachment of the merely evaporated layer whereas the IBAD film remains unaffected. For evaluation of the technical usability of IBAD aluminium coatings, steel plates with technical grade surfaces, i.e. a roughness of several (MPa) PULL OFF FORCE
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Fig. 20. Pull-off test results of aluminium layer of various thicknesses on iron: 1, 1000 nm, evaporated after sputter cleaning; 2, 2000 nm, evaporated after sputter cleaning; 3 , 2 0 0 nm, evaporated under 6 keV A r + ion bombardment; 4, 400 nm, evaporated under 6 keV A r + ion bombardment; 5 , 5 0 0 nm, evaporated under 6 keV A r + ion bombardment; 6, 1000 nm, evaporated under 6 keV A r ÷ ion bombardment; 7, 2000 nm, evaporated under 6 keV A r + ion bombardment. A, failure of glue; - - , (adhesion) > (measured value).
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Fig. 22. Step coverage and coverage of rough surfaces by evaporation under ion bombardment.
pores takes place. Energy and momentum transferred from impacting ions to condensing atoms lead to rearrangement of the surface structure by enhanced surface and bulk diffusion. Void and pore formation is prevented by forward sputtering. Figure 22 is a schematic representation of the step coverage and the higher degree of general coverage at rough surfaces by simultaneous bombardment and evaporation. A gradual transition from substrate to layer reduces stress over a longer distance compared with sharp short-range transitions. Often a new phase is formed in the interface which can be corrosion resistant or act as "glue" between substrate and layer. Sputtering of the substrate surface before coating provides cleaning and activating of bonds. Thus a direct contact between layer and substrate with better adhesion is possible. Fig. 21. Scanning electron photograph of aluminium layer on rolled iron sheets (technical surface quality) after bending around a cylinder with a 5 mm radius: (a) evaporated; (b) evaporated under Ar ÷ ion bombardment.
microns, were coated and immersed in artificial sea water [21]. Under these technical conditions, rather thick layers are necessary to provide sufficientprotection, Uncoated steel is heavily attacked within 1 day. The whole surface is covered with rust spots within 2-3 days. A 0.5 /~m aluminium layer which could already be called thick under ionbeam-mixing conditions protects the steel for 2 weeks. A layer 2/~m thick stands the corrosion attack for more than a month. A sample coated with 8 ~m aluminium showed no rust even after exposure for 52 days when the test was stopped, With an evaporation rate of 6 nm s-1, this layer grew within less than ½h. There are several reasons for the good corrosion protection. As mentioned above, corrosion starts at defects and micropores. Osmosis and evolution of hydrogen cause blisters and lift off the coating. Large adhesive forces prevent this process. Also the closure of voids and micro-
7. Summarizing remarks
Ion implantation and ion beam mixing can only modify layers or coatings of low thickness. Therefore, only in the case of very small corrosion rates and when the coating material itself is not consumed too fast, can they be chosen for technical application. This is often the case when the cathodic partial corrosion reaction of suitable metals is affected. For other purposes, especially when thicker coatings are necessary, IBAD was shown to be appropriate for producing coatings with high adhesion and low microporosity suited to longterm corrosion protection. The flexibility and versatility of the process offers the possibility of tailoring protective layers. Pure coatings can be produced as well as alloys or compounds with special properties or multilayers useful for both tribological and corrosion protection. References 1 c. R. Clayton, Nucl. 865.
Instrum. Methods, 182-18,7
(1981)
14 2 H. Ferber and G. K. Wolf, Mater. Sci. Eng., 90 (1987) 213. 3 G. K. Hubler and E. McCafferty, Corros. Sci., 20 (1980) 103. 4 P. Munn and G. K. Wolf, Mater. Sci. Eng., 69 (1985) 303. 5 B.R. Appleton, E. J. Kelly, C. W. White, N. G. Thompson and B. D. Lichter, Nucl. Instrum. Methods, 182-183 ( 1981 ) 991. 6 W. Ensinger, A. Meger and G. K. Wolf, Proc. 8th Eur. Congr. on Corrosion, Nice, 1985, Centre Franqais de la Corrosion, Paris, p. 71. 7 M. Elena, L. Fedrizzi, V. Zanini, M. Sarkar, L. Guzman and P. L. Bonora, Nucl. lnstrum. Methods B, 19-20 (1987) 247. 8 L. Guzman, F. Giacomozzi, B. Margesin, L. Calliari, L. Fedrizzi, P. M. Ossi and M. Scotoni, Mater. Sci. Eng., 90(1987)349. 9 B. Margesin, F. Giacomozzi, L. Guzman, G. Lazzari and V. Zanini, NucL Instrum. Methods B, 21 (1987) 566. 10 E. McCafferty, G. K. Hubler, P. M. Natishan, P.G. Moore, R. A. Kant and B. D. Sartwell, Mater. Sci. Eng., 86 (1987) 1.
11 Y. Andoh, Y. Suzuki, K. Matsuda, M. Satou and E Fujimoto, Nucl. Instrum. Methods B, 6 (1985) 111. 12 G. K. Wolf, K. Zucholl, M. Barth and W. Ensinger, Nucl. lnstrum. Methods B, 21 (1987)570. 13 W. Ensinger, M. Barth and G. K. Wolf, Nucl. Instrum. Methods B, 32 (1988) 104. 14 A. Schr6er, Diplomarbeit, UniversitStHeidelberg, 1988. 15 M. Barth and G. K. Wolf, unpublished results, 1988. 16 D.W. Hoffman and M. R. Gaerttner, J. Vac. Sci. Technol., 17 ( 1) (1980) 425. 17 E. H. Hirsch, D. K. Varga, Thin Solid Films, 69 (1980) 99. 18 J. J. Cuomo, J. M. E. Harper, C. R. Guarnieri, D. S. Yee, C.J. Attanasio, J. Angilello and R. J. Hammond, J. Vac. Sci. Technol., 20 (1982) 349. 19 J. M. E. Harper, J. J. Cuomo, R. J. Gambino and H. R. Kaufman, in O. Auciello and R. Kelly (eds.), Ion Modification of Surfaces, Elsevier, Amsterdam, 1984, p. 127. 20 G. K. Wolf, M. Barth and W. Ensinger, Nucl. Instrum. MethodsB, 37-38 (1989)682. 21 W. Fischer, Diplomarbeit, Universit~itHeidelberg, 1988.
l
Ion-beam-assisted Coatings for Corrosion Protection Studies* W. ENSINGER and G. K. WOLF Physikalisch-Chemisches Institut, Universitiit Heidelberg, Im Neuenheimer Feld 500, 6900 Heidelberg (E R. G.) (Received September 16, 1988)
Abstract
For corrosion protection, modified surfaces or coatings prepared by ion implantation or ion beam mixing often are not sufficient for technical longterm applications. The main problem, i.e. the shallowness of the films, can be overcome by ionbeam-assisted deposition which offers the positive effects of ion beam treatment, e.g. high adhesion, low porosity, and a low process temperature without thickness limitation. The corrosion protection behaviour of a number of coatings, e.g. boron, chromium and their nitrides, chromium oxide, silicon, hard carbon and aluminium, are discussed. The results demonstrate ion-beam-assisted deposition to be appropriate for producing adherent coatings which are well suited to long-term protection against corrosion, and partly also against tribological deterioration, even for technical rough surfaces, 1. Introduction The main method for corrosion protection, apart from using corrosion-resistant bulk materials, is coating of the substrate material with a resistant layer. The application of corrosionresistant bulk materials often is restricted, because other properties, e.g. mechanical ones, do not meet the requirements. Sometimes the material simply is too expensive. In the age of conservation of resources the resistant coating is the appropriate method and techniques for producing such coatings and tailoring their properties are necessary. There are many techniques for production of corrosion-resistant coatings in use which have both advantages and disadvantages, The galvanic process, cheap but often burdening the environment with poisonous waste solutions, is very appropriate for conventional protecting *Paper presented at the Sixth International Conference on Surface Modification of Metals by Ion Beams, Riva del Garda, Italy, September 12-16, 1988. 0921-5093/89/$3.50
layers such as the well-known chromium layers. However, it does not allow production of layers other than of the group of the refractory metals from aqueous solution, although these (aluminium, titanium or tantalum) are very well suited to corrosion protection. Also most non-metals such as boron, silicon or carbon cannot be produced as films in this way. This restriction can be overcome by evaporation or physical vapour deposition. In this case, the adhesion of coating to substrate is often poor or, for better adhesion, high process temperatures are necessary which often cannot be tolerated by the substrate materials. Other processes such as chemical vapour deposition need high temperatures to be carried out or lead to contaminations of the layers etc. Another modern technique, laser melting, causes problems with reflecting metallic substrates or when there are large differences between melting or evaporation temperatures of coating and substrate materials and leads to an often undesired surface morphology. Many of these problems can be avoided with ion implantation which exhibits universality, strong adhesion, a low process temperature, very good controllability and reproducibility. With this technique, stable and also metastable alloys or compounds are available which can hardly be made with other techniques. In addition to crystalline phases, amorphous phases with special corrosion resistance properties may also be obtained. An intrinsic limitation of ion implantation, i.e. that only a certain percentage of alloying material can be implanted, can be overcome by ion beam mixing. After coating, e.g. by evaporation with a corrosion-resistant material such as the metal aluminium or the non-metal boron, the layer is mixed with the substrate, and a mixed phase (ion beam mixing) or an interphase which forms a transition from the pure layer to the substrate (interface mixing)is formed. Multilayers and sandwich layers are used to produce thicker, © Elsevier Sequoia/Printed in The Netherlands
homogeneously mixed coatings. With an appropriate reactive ion species, compounds such as oxides or nitrides are available. Both methods-ion implantation and ion beam mixing--are useful for basic corrosion studies as demonstrated in numerous publications (see refs. 1 and 2). From the latest conferences on ion beam technology, e.g. the International Conference on the Ion Beam Modification of Materials 1988 and the International Conference on Ion Implantation Technology 1988, it appears that the number of papers dealing with corrosion is recurrent and the interest is decreasing. The reason may be the limitation of the technique for practical applications which on the contrary results from t h e shallowness of the layer. Corrosion often consumes the thin coatings too fast, so that no longterm corrosion protection is possible, For corrosion processes with slow rates, ion implantation or ion beam mixing in special cases may be sufficient. An example for this is the case where the passivity of a metal is maintained by a very small amount of implanted material although in the adequate corrosive medium it usually corrodes actively with high rates. This is the case for titanium or stainless steel in strong acids, They can be protected by implanting platinum or palladium [3-6]. The few micrograms per square centimetre of implanted or mixed material are already sufficient for long-term protection. For active corrosion in slightly acidic media, e.g. unalloyed steel in a natural environment, such mechanisms mostly do not work. Here the protection effect of a coating with a thickness below 1/~m which can be produced by ion beam mixing is only of short duration,
A possibility for overcoming this limitation is ion-beam-assisted deposition (IBAD). This technique combines the coating process--evaporation or sputtering--with ion bombardment to a onestep process. Thick layers can be produced without losing the advantages of the ion beam treatment. With this technique it is possible to make successful technical applications. This will be shown by a number of examples. The following mainly deals with ion-beam-assisted evaporation for protection against wet corrosion.
2. Ion-beam-assisted deposition process control Controlling and engineering the IBAD process can be done in different ways (Fig. 1). An IBAD machine can be a combination of an implanter and an evaporation facility. In this case the atom and ion beams usually meet at right angles. Thus, two operational possibilities result. The substrate is either simultaneously coated and bombarded at 45 ° (or other correlating angles) or consecutively coated and bombarded in small steps, e.g. by evaporating a thin layer at 90 °, rotating the substrate holder and bombarding at 90 ° etc. The latter process is intermediate between the continuous IBAD process and single-step ion beam mixing. Most of the IBAD facilities used today are compact machines where the ion gun is directly fixed to the vacuum chamber. Thus the 90 ° alternating conception can be used or nearly parallel atom and ion beams with impact angles near to 90 ° are utilized. Atom and ion beam generation are independent of each other and can be varied over a wide rang e . Therefore the process can be carried out in
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many ways. Figure 2 shows a selection of possibilities. The abscissa represents the process time, and the ordinate the relative number of impacting or condensing particles. In Fig. 2(a) the simple case of substrate pre-cleaning and activating is shown. Sputtering leads to removal of contamination layers which may influence corrosion behaviour and adhesion. Nucleation sites for the condensing film are formed, and the surface is activated. The layer grows without being influenced by the ion beam. In Fig. 2(b) the bombardment is maintained until the film has reached a certain thickness. The layer and substrate are mixed, a corrosion-resistant phase may be formed, and interlocking leads to enhanced adhesion. In Fig. 2(c) the whole layer is formed under bombardment. Besides formation of an interphase, many properties of the layer which are important for corrosion protection are influenced. Carrying on the bombardment after the coating process (Fig. 2(d)) may be useful for reactive IBAD where a reactive gas such as nitrogen is used to increase its percentage in the layer. Thus post-bombardment influences stoichiometry, morphology, etc. Figure 2(e) shows different ion-to-atom ratios. For rare gas ions, such a variation leads to different properties of the layer; for reactive ions, differences in the composition add to it. Changing the evaporating element results in formation of sandwich layers as shown in Fig.
2(f). For corrosion protection this may be a nickel layer on steel followed by a chromium layer. Two independent evaporators (Fig. 2(g)) offer the possibility of depositing a binary alloy or compound or a ternary compound when using a reactive ion. The last example (Fig. 2(h)) again shows a method for producing a sandwich layer. In this case the ion species is changed during the process. As an example, a steel substrate is cleaned and activated with an argon beam, and then a Fe-Cr phase is formed by ion beam mixing of the condensing chromium. The interphase is thickened by a pure chromium layer growing under moderate argon bombardment. Then N2 is introduced into the ion gun instead of argon. A hard CrN layer is formed on top of the corrosion protection coating for tribological protection. This list could be continued, but it illustrates already the possibilities and the flexibility of the IBAD process for producing corrosion protection coatings. 3. Basic theory of corrosion of coatings Before dealing with some examples for corrosion protection with IBAD a short introduction into the basic theory of corrosion of coated metals is given in order to understand the results better. The corrosion products such as iron ions and salts are thermodynamically more stable than
the pure metals, and this is the driving force for corrosion. Therefore the metal tends to dissolve in the solution; it corrodes. The aqueous corrosion reaction of a metal can be separated into partial electrochemical reactions taking place simultaneously at the metallic surface. The anodic reaction is the oxidation of a metal: Me--' Me n + + neThe cathodic reaction depends on the solution. In acidic solutions it is hydrogen evolution: 2H ÷ + ne- ~ H2 In the presence of oxygen, oxygen reduction will take place: 02 + 4H ÷ + 4 e - ~ 2H20 In electrochemical equilibrium the counterreactions (metal reduction and hydrogen oxidation) take place at the same rate and cancel the reaction macroscopically. However, under corrosion conditions, one of the reactions predominates; metal is oxidized, and hydrogen gas is evolved. The net reaction in acids is Me + n H + ~ Me n* + ½nil 2 The metal corrodes under hydrogen evolution, Separating the metal from the aggressive environment with an inert coating stops the reaction and protects the metal. Another possibility is influencing one of the reactions, e.g. reducing the cathodic reaction, and thus also reducing the complementary reaction, in this case the metal dissolution, Corrosion of a coated material mostly starts at defects such as scratches or pores. Another problem can be local failure of adhesion. There are several reasons for such a failure, e.g. contaminations in the interface, gas bubbles, stress between layer and substrate, and incompatibility in any sense. Water is adsorbed at such defects and leads to corrosion. Small micropores can be the reason. Water penetrates the coating through these small channels driven by capillary forces, Then corrosion leads to formation of a solution, Osmosis may force more water through the pores, leading to formation of a bubble. The evolving hydrogen gas increases pressure with debonding of the film. Bubble formation also can be caused by electro-osmotic effects where a potential gradient resulting from a local element forces electrolyte through pores to the substrate, Aggressive ions such as chloride can penetrate
the layer, causing a strong corrosion attack. Depending on the electrolyte-layer-substrate combination, different types of corrosion can develop such as local elements, crevice corrosion and concentration elements. They lead to failure of the coating or to strong local corrosion of the substrate. It depends on the electrochemical mechanisms which metallic coatings are responsible for the reactions. They can be divided into two groups: the more and the less noble metals with respect to the substrate material. In the case of iron and steel the former are tin or silver, and also metals which are more noble and corrosion resistant than iron by formation of resistant oxides such as chromium and aluminium. Under normal conditions in neutral or acidic media, they corrode with much lower rates than iron because they are passive or immune and protect iron. However, the layers have to be free of flaws and pores as these will form a local element because of the standard potential differences between coating and substrate. In this case, the coating is a large-area cathode and the substrate contains local small-area anodes. High anodic corrosion current densities lead to undermining of the layer and formation of deep holes. Coatings which are less noble than iron, e.g. zinc and cadmium, behave the other way round. They are anodically polarized and protect the underlying iron by dissolving as sacrificial anode. In this case, only thick layers are useful because thin coatings are consumed too fast. The following examples deal with pure iron or steel as substrate and more noble cathodic coatings.
4. Corrosion measurement techniques For investigationsofelectrochemicalcorrosion processes, potential, current and polarization measurements are employed. The simplest method is measuring the corrosion potential of a specimen or its change with time. For special cases the currents flowing between anode and cathode can be measured. Polarization measurements are used most widely. With the help of an external current source a defined current or potential is established and the resulting potential or current is measured. To obtain current-potential plots a potentiostat is needed which electronically introduces a potential between the working and the reference electrode. The resulting current between working and counterelectrode is re-
corded. This can be done potentiostatically as a steady state measurement, or potentiodynamically when the potential is changed with time. Scanning the potential up from a start potential to a reverse potential and back again to the start potential is called cyclic voltammetry. Figure 3 shows a cyclic current density-potential curve for pure iron. The start potential is cathodic, i.e. the specimen works as cathode; hydrogen evolves, and oxygen is reduced. At the corrosion potential the current passes through zero. Then the specimen becomes anodic; iron is oxidized and dissolves. The highest anodic current is the so-called critical current ic, where passivation starts, i.e. a protecting oxide grows. The current is strongly reduced, and the specimen is passive. At high positive potentials, water decomposes and oxygen evolves. The shape of the plot and the current densities deliver informations about the electrochemical or corrosive behaviour of the specimen and about corrosion mechanisms. The critical current density is a criterion for the corrosion. Except for special cases, no quantitative data for corrosion rates can be derived from these curves, For quantitative dissolution data, immersion tests have to be carried out. If the samples are large enough, they can be weighed before and
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....,
-5-
after immersion. In the case of small specimens, short immersion times or small corrosion rates, determination of rates by weighing is not pussible. The dissolved quantities have to be analysed. Because these are microscopic quantities, trace analytical methods such as spectrophotometric or electrochemical techniques have to be used. Unfortunately, until now, it is almost only electrochemical measurements which are to be found in the literature for ion-beam-modified specimens. However, in this paper, some quantitative dissolution data will be given. In the following section, some examples from several groups for corrosion protection by IBAD layers are presented. After a short description of the experimental set-ups used by the authors' group in Heidelberg, some results are presented. 5. Examples of ion-beam-assisted deposition layers for corrosion protection The ion beam group of lstituto per la Ricerca Scientifica e Tecnologica in Trento was one of the first working in the field of IBAD for corrosion protection. They prepared boron layers with IBAD and chromium and boron nitride films with reactive ion-beam-assisted (enhanced) deposition (RIBAD) [7-9]. A 90 ° atom-ion beam arrangement was used. The polished specimens were pre-implanted, coated at 90 °, then rotated, bombarded at 90 ° etc. in a sequence of several steps. Figure 4 represents an Auger depth profile which shows the formation of an F e - C r - N alloy with variable composition. The electrochemical behaviour was studied in 0.5 M H 2 S O 4 and 1 M NaC1 (pH 4) using the above-mentioned measurement techniques.
80
J Fig. 3. Potentiodynamic current-potential curve for pure iron in acetate buffer pH 5.6 at 25 °C (scan rate, 10 mV s- 1) in (a) the cathodic region (hydrogen evolution), (b) the anodic region (metal dissolution), (c) the passive region (passive corrosion) and (d) the oxygen evolution, where E~ is the corrosion potential, EF the flade potential (beginning of corrosion reduction by oxide formation), E s the potential at the beginning of oxygen evolution, lw the current density of hydrogen evolution, I s the current density of oxygen reduction, IK the critical current density (maximum iron dissolution) and Iv the passive current density.
°°°1 < o 4oJi ..J ~ 20~ "
"" N
/ '
~)~< ~(~
>. C._._ [ ..~ , . . . . .e~. .'~. . . .2. . 10 20 SPUTTERING
................. "~
~.~
" ........ .....
30
40
TIME
(min)
50
Fig. 4. Auger depth profile of RIBAD CrN x layer on iron [8].
The corrosion potentials of the coated specimens are more noble; the corrosion currents are significantly reduced compared with those for the uncoated samples. The potentiostatic currenttime plots in Fig. 5 show that the anodic currents of the RIBAD samples remain very small. They are about two to three orders of magnitude lower than for Armco iron and one to two orders of magnitude lower than for Fe/B multilayers implanted with nitrogen ions even after immersion for several hours. This demonstrates such hard layers to be very well suited to both tribological and corrosion protection, The Naval Research Laboratory group in Washington prepared chromium oxide layers on AISI 52 100 steel using a metal ion beam for an IBAD corrosion protection layer [10]. A 45 ° arrangement was used for simultaneous bombardment of the condensing film. The samples were pre-implanted with 40 keV Cr + ions to clean the surface by sputtering, and to introduce a buried layer of chromium. Then evaporation of chromium was started with simultaneous bombardment with Cr + ions. During the process the sample was exposed to oxygen at 10- 5 Torr. Thus a coating 200 nm thick was prepared. The corrosion behaviour was studied in 1 N H2SO4 and in a 3 ppm solution of NaC1, both deaerated using potentiostatic polarization measurements. Figure 6 shows the anodic polarization curves in the salt solution. The open-circuit potential of the coated specimen was about 0.25 V more noble than that
// (F"A cm-l) 2 10
of the steel sample. The current densities were one to two orders of magnitude lower. Inspection of the corroded samples showed that the uncoated steel was heavily attacked whereas the Cr203 layer remained unattacked except for a number of pits coming from chlorine action at defects. A thicker layer is expected to give good long-term corrosion resistance. A Japanese group [11] coated low carbon steel with aluminium in a two-step procedure. An aluminium layer 30 nm thick was condensed on the substrate and then mixed with 30 keV Ar + ions. Afterwards this intermediate or starting layer was thickened to 1 #xm by evaporation. In this case the parallel beam arrangement shown in Fig. 1 was used; the ion impact angle was 90 °, and the atom angle accordingly smaller. Adhesion tests, i.e. tensile test and bending of the foil at 180 °, showed strong adhesion for the ion-beamtreated samples in contrast with the untreated samples where the coating was easily detached. The corrosion behaviour was tested in acetate buffer pH 5.0 using multisweep cyclic voltammetry. Comparison of the critical anodic current densities shows a marked decrease for the ionbeam-coated samples. The uncoated steel sample starts with a current density of about 10 mA cm-2 which increases with the number of potential sweep cycles and reaches 100 mA cm-2 after 20 cycles. The ion-beam-mixed layer starts with a current density of some 0.1 mA cm-2 and attains 2 mA cm- 2 after 50 cycles. This example again shows the value of IBAD for producing well-adhering corrosion protection coatings.
a / t/1
b
t
~
I
lO 3
E
I I
I
\, ~ ~.
Fig. 5.
Current
polarization (at
vs. -600
~'..
,
.,. .....
~,
6
TIME
(103s)
lOi
c/.I /
/. / " t
Z
•
klJ r-7
with
"# 215
respect
10°
~-
27
Z w
] lO "~
I
r'r"
time plot for potentiostatic anodic mV
~--
coo
A-+'-
o
. . .
lO a
zL
"
I,--~/ . ~2
<:~
to
a
saturated
calomel electrode (SCE)) of various samples in 1 M NaC1 (pH 4) solution: (a) Armco iron; (b) Fe/B/Fe/B ... on iron, implanted with nitrogen; (c) BN obtained by RIBAD on iron; (d) Cr2N obtained by RIBAD on iron [8].
~
]
]°~ -0.8
J -o. 5
-0.2
POTENT
o. 1
o. 4
o. 7
1.0
I AL(V)
Fig. 6. Anodic polarization curves for steel (m) and Cr203coated steel (o) in salt solution [10].
G
^
0
~
evaporated
~ ~
°
;IH,
I00 nrn
~oo.i~
=t [~1
t,,
'°i_ - °X
?
.o
,
(a)
Fig. 7. Schematic cross-section of the IBAD vacuum chamber: A, Faraday cup; B, shutter; C, thickness monitor;
i00
nm
B0.
60'
6. Selected results obtained at Heidelberg
University
IBAD
100
D, substrate; E, gas valve; F, cooled rotating substrate holder; G, ion current monitor; H, electron beam evaporator; I, glow discharge electrode; K, ion beam; L, defocusing electrode; M, pump.
=t In] 40'
6.1. Experimental details The I B A D machine at Heidelberg for the simultaneous process consists of a duoplasmatron ion source and an electron beam evaporator, The impact angles are 80 ° for both ions and atoms. Figure 7 shows a cross-section of the vacuum chamber. The whole assembly has been described in detail elsewhere [12, 13]. The electrochemical tests (cyclic voltammetry) and immersion tests were carried out in acetate buffer pH 5.6 or artificial sea water. The corrosion rates were determined using polarography and neutron activation analysis. In the former case, faradaic currents of the dissolved species and, in the latter, Y radiation of the activated dissolved material in the solution are used for quantitative measurements of the corrosion rates. Adhesion was tested with the scratch test with a 200 /~m Rockwell stylus. Another test was the pull-off (pin pull) test where a stamp is glued to the coating and pulled off perpendicular to the coating plane. Further information could be obtained from the bending test where the sample sheet is bent around cylinders with different radii until the coating is detached, Mostly pure iron and a low carbon (0.3 wt.%) steel served as substrate polished down to 1/.tm diamond paste or ground to obtain technical grade surfaces. Chromium, aluminium, titanium, boron, silicon and their nitrides and i-C layers
2o. c.j~....,~ 06~ (b)
5
,t ...... l0
.~
15
20
sputtercharge
25
30
[mC]
Fig. 8. Auger depth profiles of 100 nm boron on steel:
(a) evaporated; (b) evaporated under 6 keV A r + ion bombardment.
were prepared at room temperature with thicknesses ranging from 50 nm to 10/am. Of these, some examples are chosen to show the corrosion protection effect.
6.2. Results Figure 8 shows two Auger depth profiles of 100 nm boron on steel, one layer which had been merely deposited, and the second obtained under simultaneous bombardment with 6 keV Ar + ions. In the latter case the sample was pre-bombarded to clean it by sputtering and to create nucleation sites before the evaporation was started. Part of the condensing layer was mixed with the substrate. W h e n the layer grew thicker, no more mixing occurred because of the limited penetration depth of the ions, but the film still grew under gentle bombardment. As can be seen from these two profiles, the measurable differences between
the layers are minor. The extent of the mixed interface lies within the experimental uncertainty of the Auger depth profiling. Nevertheless there are enormous differences in adhesion and corrosion behaviour. When the ratio of mixing ions to condensing atoms is increased, the profiles change dramatically. Figure 9 shows two profiles of a 100 nm silicon layer on steel. The profile for the as-evaporated layer is similar to the boron profiles. The interface is marked by strong carbon and oxygen contaminations. The IBAD layer looks more like a surface compound, a diffusion layer or even an implantation profile with an anomalously high content of implanted material, Strong mixing by knock-on implantation, radiation-enhanced diffusion etc. leads to formation of a compound. Iron is already present at the surface of the specimen. Strong sputtering causes a evaporated 100
nm
reduced thickness of the layer compared with the simultaneously evaporated amount condensed on the thickness monitor not hit by the ion beam. This comparison illustrates the above-mentioned flexibility of the IBAD process. Ion and atom beams can be controlled independently from one another. Electrochemical measurements proved that the boron and silicon layers exhibited good corrosion resistance. For rough~,urfaces, IBAD was superior to conventional ion beam mixing. Details have been described elsewhere [13]. Figure 10 shows the results of the immersion tests for the silicon layers [14]. Corrosion rates are plotted vs. immersion time. Despite the roughness of the ground substrate surface (more than 1 pm) and the shallowness of the silicon IBAD layer the corrosion rate stayed low for about 3 weeks before normal corrosion of the steel started. The second plot shows a com-
100
80, corrosion raLe [~g/em 2*min]
80,
15
at [7,1 10, 20,
0 ~ 0
N8
5. Si IBAD
0 ....
~
"
~.
"
~
(a)
o IBAD
i00
nm
I00" (bl 20, Fe 80.
Si
15,
B evaporated EN 8
10.
40
20
o
(b)
5
o~s-
i -I~5-
~,
~5
5
a'5 ,
sputtercharge [mC]
Fig. 9. Auger depth profiles of 100 nm silicon on steel: (a) evaporated; (b) evaporated under 6 k e V A r ÷ ion bombardment,
01--I
0
7
I
t
14
21
i
e8 immersion time [d] .
3'5
Fig. 10. Immersion test results of low carbon steel coated with silicon and boron layers in acetate buffer of p H 5.5.
9 parison between untreated steel, as-evaporated boron layers, ion-beam-mixed (5 x 10 ]6 Ar + cm -2) boron layers and a 6 keV Ar + IBAD layer, Whereas the evaporated boron layer is detached after a few hours of immersion, post-bombardment and simultaneous bombardment significantly reduce corrosion for more than a month, The reason for the early failure of the as-evaporated layer clearly is insufficient adhesion and high stress. Water or solution enters the layer through micropores, undermines it and breaks the weak bonds between layer and substrate. Then stress causes detachment of the film. Ion beam treatment forms a stronger interface under creation of an interphase. This results in strongly enhanced adhesion. The pull-off test yields values of below 10 MPa for as-evaporated layers and for more than 80 MPa, the limit of the test, for ionbeam-treated layers. Figure 11 shows the results of the scratch test for silicon layers on steel. Ion beam mixing leads to an enhancement of adhesion. The highest value was found for IBAD. Contrary to the as-evaporated and ion beam mixed layers in the case of IBAD there was no clear failure to be found, probably because the degree of mixing was so high. No clear detachment could be observed, and the true value might even be higher, In the case of metalloids such as boron and silicon the interphase may be amorphous, rendering the specimens corrosion resistant. An amorphous structure often is less susceptible to corrosion than is a crystalline structure. There are fewer grain boundaries and other defects where
corrosion usually starts. The formation of local elements which are to be found in multiphase systems as a consequence of segregation phenomena are less possible in an amorphous singlephase structure. Another effect may be an electrochemical influence. Metalloids such as boron or silicon exhibit a much lower exchange current density for hydrogen evolution. As mentioned above, reduction of the cathodic reaction also reduces the anodic counter-reaction metal dissolution. Boron and borides suffer from high intrinsic stress. Therefore boron layers tend to detach when attacked mechanically or chemically. IBAD offers the possibility to change stress values of boron layers by changing the ion-to-atom ratio. Figure 12 contains results of stress measurements [15]. 200 nm of boron were condensed on a steel foil 50/~m thick, and the degree of bending opti L cally determined. Without bombardment there is high tensile stress often connected with cracking of the layer. Bombarding the condensing layer with 6 keV Ar + ions converts the tensile to compressive stress when the ion-to-atom ratio is higher than 0.001. Similar results were obtained for chromium [16, 17] and niobium [18] layers. This effect is attributed to structural rearrangement of film atoms shortly after deposition. The energy necessary for activating these processes is delivered by the energetic ions. A second mechanism is preferential sputtering of impurity atoms, resulting in higher purity of the layer [ 19]. A universal protective coating is diamond-like carbon (i-C and hydrogenated amorphous carbon bending
(N) CRITICALLOAD
of
substrate (mm) 3
16 compressive
stress
B
* w
, # 4
0
~
~
-
l
t
-
- 2
3
#
2
tensile stress
4
Fig. 11. Scratch test results for 100 n m silicon layers on steel (critical load): curve 1, as evaporated; curve 2, b o m b a r d e d with 120 keV, 1 x 10 j~ A r + cm 2; curve 3, b o m b a r d e d with 120 keV, 5 x 10 I~ A r + cm-2; curve 4, in situ b o m b a r d e d with 6 keV A r + ions.
3
0,0010
0,0100
O, ] 000
6 KEV a r g o n / b o r o n Fig. 12. Stress measurements of b o r o n layers, bending of substrate vs. ion-to-atom ratio.
10
(a-C: H)). It possesses high chemical resistance and hardness and low friction coefficients. It can act as corrosion protection as well as mechanical protection, In Fig. 13 the first three positive-going potential sweeps of a cyclic voltammogram of an i-C layer 100 nm thick prepared by evaporation of graphite under 6 keV Ar + ion bombardment on steel are plotted. The current densities are reduced by more than two orders of magnitude, indicating the corrosion protection effect of the layer. The anodic current peak in the region of - 0 . 4 V stems from iron dissolving through micropores and other defects. Immersion tests in acetate buffer and artificial sea water showed that the substrate corroded through micropores around the edges but the middle part of the specimen was unchanged over a long period. This means that thin i-C layers are very well suited to corrosion protection unless there are pinholes or other defects in the layer mostly coming_ from dust particles shading off the surface during the coating process. Figure 14 shows the results of a quantitative immersion test. In this case a layer 0.3 /~m thick was prepared. For the first 3 days there was no detectable corrosion. The layer remained hydrophilic. Then corrosion started at defects and undermined the layer. This again demonstrates that defects and porosity are the crucial point of corrosion protection with thin layers provided that adhesion is sufficient, Scratch tests and pull-off tests prove the good adhesion of the prepared i-C layers. Knoop microhardness tests give a value of 900 HK (load,
50 mN) compared with 400 HK for the untreated material. Although the indenter penetrates the layer and thus only an average value can be obtained, the values are typical for diamond-like and not for graphitic films. Optical band gap and transmission measurements confirm these results [20]. Nuclear reaction analysis gave a hydrogen content of 1-2 at.%. Chromium layers belong to the standard coatings for corrosion protection. Usually they are prepared galvanically. However, thin galvanic layers are strongly porous, and thick layers easily develop microcracks. For comparison with other coating materials, chromium layers 100 nm thick were prepared by evaporating chromium on steel, and by condensing under 6 keV Ar + ion bornbardment. Figures 15 and 16 show the cyclic voltammograms in buffered acetic acids. The layer deposited without ion beam treatment already shows a reduction of the anodic current densities compared with those for untreated steel. In the passive potential region beyond 0.5 V a double peak appears. Here chromium dissolves transpassively as chromate. The results for the IBAD specimens look different. The anodic critical current density is strongly reduced compared with the as-evaporated layers. The peak in the transpassive region is higher. In this plot, three different samples prepared in three different runs are shown. Here the high reproducibility of the process can be seen. Again the ion bombardment yields more corrosion-resistant films than the mere vapour deposition. Higher adhesion, lower
Iglil [A cm'2]- 1;0
20.
-0.5
±0
.
.
"0.5 .
E Iv ! SCE "1.0
corrosion rate [ l u g / c m i *min]
.
15.
-2-
-3-
EN
8
I0. 3 t~
-4""""~
- 5-
/ 2\
i ,r ~.~ >%~ i / b,
%~,,
~'\
17",",, .o
i-C IBAD
t
/
"-
,, ,"
";J
..... _::::J
1'~"!1/ \ " \ " / /
,,,,',/
.
Fig. ] 3. First three positive-going sweeps of cyclic voltamm o g r a m of 100 n m I B A D i-C on steel in an acetate buffer of p H 5.6.
o
0
7 II414ERSI011 TIME
, 14
(DAYS)
Fig. 14. I m m e r s i o n test results of i-C layers in an acetate buffer of p H 5.6.
II
Iglil [A cm-~l-t;o
-o.s
*-o
I
-o~
I
E [V] SCE -1.o
bending
I
I
-2-
1.5
-3'
0,5 I-~'t~
-4
t
of
subs[rate (ram)
',
s
0
compressive stress
I
I
I
I :::;:
i~
~1
I
I
I : :::
k ;
--0,5 ]
-5
-[,5 tensile stress -6-
2
Fig. 15. Cyclic voltammograms of chromium, evaporated on steel, in an acetate buffer of pH 5.6.
6 kEY
0.010 argon//chromium
O, lO
Fig. 17. Stress measurements of chromium layers, bending of substrate v s . ion-to-atom ratio.
Ig lil [A cm'2]- 1;0
E [vl SCE -0.5
±0
*0.5
'
z , ' _ / /
-2~
"1.0
/
-3-
~
_
-4ii]
.....
Fig. 16. Cyclic volammograms of IBAD chromiumon steel, in an acetate buffer of pH 5.6.
microporosity and the formation of an Fe-Cr alloy are the reasons for the better behaviour, Figure 17 shows how the intrinsic stress of the film changes with ion-to-atom ratio [20]. This is one example out of a series where different rare gases with varying energies (3-15 keV) were used [15]. The bombardment changes tensile into compressive stress crossing the abscissa at an ionto-atom ratio of about 0.015 in the case of 6 keV Ar + ions. The higher the energy, the more the zero-stress point shifts to lower values, i.e. fewer ions are necessary for the same effect. These results agree very well with values obtained by other workers [16]. It is interesting to compare the value obtained for chromium with that for boron. In the case of boron the ion-to-atom ratio
needed for zero stress is about an order of magnitude lower. Obviously the effect is strongly dependent on mass and bonding characteristics of the layer element. Substituting the rare gases by nitrogen (RIBAD) yielded a chromium nitride layer. The Knoop microhardness (load, 100 mN) of a layer 3/~m thick was 2200 HK compared with 400 HK for the untreated steel. Immersion tests of a layer 2 jxm thick in artificial sea water and visual inspection showed the untreated steel to be heavily corroded after a few days under formation of rust and pits whereas on the coated sampie no sign of rust was visible even when the test w a s stopped after 52 days. The layer apparently works as a sacrificial anode which protects the substrate. Because atom and ion beams can be controlled independently, it is possible to determine precisely the nitrogen content. As mentioned above, in the surface region a high nitrogen-tochromium ratio, i.e. 50 to 50 can be adjusted in order to have a hard surface whereas towards the interface and the substrate the nitrogen content is reduced for better corrosion protection. The last example is aluminium. It is very well suited to corrosion protection of iron in neutral or slightly acidic solutions and could substitute the poisonous cadmium. Unfortunately the adhesion of vapour-condensed films is very poor, and the films tend to columnar growth. Mostly these films have low packing densities and contain pores and microvoids. Preparation of
12
IBAD aluminium films on different substrates were investigated in detail. The thickness ranged from 50 nm to 10 ktm. Mostly 6 keV Ar + ions were used for bombardment. Generally the results show that simultaneous bombardment of the condensing film lead to higher density, purity and reflectivity of the film, to higher adhesion and to lower porosity. In Fig. 18 the values of the anodic critical current density gained from cyclic voltammograms are represented. Untreated iron is compared with 1 Mm evaporated aluminium with and without pre-cleaning of the substrate by sputtering and with ion-beam-assisted evaporation. The critical current density of the as-evaporated layer is about 0.4 mA cm -2 compared with 11 mA cm-2 for untreated iron. Within 10 cycles the current reaches more than 1 mA cm-2. Sputter cleaning reduces the current density below 0.2 mA cm- 2. Because the film itself grows under the same conditions as the film without pre-cleaning and activating the surface, the effect has to be attributed to the pre-bombardment. If the growth mechanism is not influenced by the pre-bombardment, the purity of the interface and higher adhesion must be the reason for the markedly improved corrosion behaviour. This is proved by Auger depth profiling and by adhesion tests. Pull-off tests prove adhesion to increase strongly when the substrate is cleaned immediately before coating. Figures 19 and 20 show the test results for films of different thicknesses. Prebombardment of the substrate already yields pulloff values at least one order of magnitude higher than those for untreated samples.
The IBAD layers show the best performance. Not only is the initial value of the critical current density (Fig. 18) the lowest, but also the longterm behaviour is significantly better, as indicated by the fact that the current density is still low during the tenth cycle. The reason for this is higher density, lower porosity and the more finegrained structure of the bombarded film compared with the unbombarded film. Another reason might be the formation of an amorphous Fe-A1 interphase. The high adhesion values of IBAD layers are represented in Figs. 20 and 21. Bending the sample foil around a radius of 5 mm leads to detachment of the merely evaporated layer whereas the IBAD film remains unaffected. For evaluation of the technical usability of IBAD aluminium coatings, steel plates with technical grade surfaces, i.e. a roughness of several (MPa) PULL OFF FORCE
10 8
6
COATINGTHICKNESS(nm)
4 ~
R
~
~
~
~
~
~ o
2 ~ 0
~
~'~
~
~
~
[~
Fig. 19. Pull-off test results of aluminium layers of different thicknesses evaporated on iron. (MPa) PULL OFF FORCE 100.
~1.o
m •
~
" 1.2.10. POTENTIAL SWEEP
~,
U./."
///, ,,/,4
~
~-;~-; ;;;,;
"
///~
~-///,'~
".-<./..~
v-w//.q
f,;~$
~'//A
;;;;
~..~
H,
,H
....
1.-//.,i
9,//
,//,
,/'/;
//,4
5
6
20.
~
//~
0.5
:c f~c:
1
0 1
4
Fig. 18. Anodic critical current densities, obtained from cyclic voltammograms in an acetate buffer of p H 5.6: curve 1, iron uncoated; curve 2, 1 /~m aluminium, evaporated; curve 3, 1 Mm aluminium, evaporated after sputter cleaning; curve 4, 1 Mm aluminium, evaporated under ion bombardment.
:~
60.
2
........ ~,;~.; 3
~ ~,/~
~ y/~/// ,///
Villi
4
7
Fig. 20. Pull-off test results of aluminium layer of various thicknesses on iron: 1, 1000 nm, evaporated after sputter cleaning; 2, 2000 nm, evaporated after sputter cleaning; 3 , 2 0 0 nm, evaporated under 6 keV A r + ion bombardment; 4, 400 nm, evaporated under 6 keV A r + ion bombardment; 5 , 5 0 0 nm, evaporated under 6 keV A r + ion bombardment; 6, 1000 nm, evaporated under 6 keV A r ÷ ion bombardment; 7, 2000 nm, evaporated under 6 keV A r + ion bombardment. A, failure of glue; - - , (adhesion) > (measured value).
13 ~<
)
IONS
ATOMS
' ".
//.U
Fig. 22. Step coverage and coverage of rough surfaces by evaporation under ion bombardment.
pores takes place. Energy and momentum transferred from impacting ions to condensing atoms lead to rearrangement of the surface structure by enhanced surface and bulk diffusion. Void and pore formation is prevented by forward sputtering. Figure 22 is a schematic representation of the step coverage and the higher degree of general coverage at rough surfaces by simultaneous bombardment and evaporation. A gradual transition from substrate to layer reduces stress over a longer distance compared with sharp short-range transitions. Often a new phase is formed in the interface which can be corrosion resistant or act as "glue" between substrate and layer. Sputtering of the substrate surface before coating provides cleaning and activating of bonds. Thus a direct contact between layer and substrate with better adhesion is possible. Fig. 21. Scanning electron photograph of aluminium layer on rolled iron sheets (technical surface quality) after bending around a cylinder with a 5 mm radius: (a) evaporated; (b) evaporated under Ar ÷ ion bombardment.
microns, were coated and immersed in artificial sea water [21]. Under these technical conditions, rather thick layers are necessary to provide sufficientprotection, Uncoated steel is heavily attacked within 1 day. The whole surface is covered with rust spots within 2-3 days. A 0.5 /~m aluminium layer which could already be called thick under ionbeam-mixing conditions protects the steel for 2 weeks. A layer 2/~m thick stands the corrosion attack for more than a month. A sample coated with 8 ~m aluminium showed no rust even after exposure for 52 days when the test was stopped, With an evaporation rate of 6 nm s-1, this layer grew within less than ½h. There are several reasons for the good corrosion protection. As mentioned above, corrosion starts at defects and micropores. Osmosis and evolution of hydrogen cause blisters and lift off the coating. Large adhesive forces prevent this process. Also the closure of voids and micro-
7. Summarizing remarks
Ion implantation and ion beam mixing can only modify layers or coatings of low thickness. Therefore, only in the case of very small corrosion rates and when the coating material itself is not consumed too fast, can they be chosen for technical application. This is often the case when the cathodic partial corrosion reaction of suitable metals is affected. For other purposes, especially when thicker coatings are necessary, IBAD was shown to be appropriate for producing coatings with high adhesion and low microporosity suited to longterm corrosion protection. The flexibility and versatility of the process offers the possibility of tailoring protective layers. Pure coatings can be produced as well as alloys or compounds with special properties or multilayers useful for both tribological and corrosion protection. References 1 c. R. Clayton, Nucl. 865.
Instrum. Methods, 182-18,7
(1981)
14 2 H. Ferber and G. K. Wolf, Mater. Sci. Eng., 90 (1987) 213. 3 G. K. Hubler and E. McCafferty, Corros. Sci., 20 (1980) 103. 4 P. Munn and G. K. Wolf, Mater. Sci. Eng., 69 (1985) 303. 5 B.R. Appleton, E. J. Kelly, C. W. White, N. G. Thompson and B. D. Lichter, Nucl. Instrum. Methods, 182-183 ( 1981 ) 991. 6 W. Ensinger, A. Meger and G. K. Wolf, Proc. 8th Eur. Congr. on Corrosion, Nice, 1985, Centre Franqais de la Corrosion, Paris, p. 71. 7 M. Elena, L. Fedrizzi, V. Zanini, M. Sarkar, L. Guzman and P. L. Bonora, Nucl. lnstrum. Methods B, 19-20 (1987) 247. 8 L. Guzman, F. Giacomozzi, B. Margesin, L. Calliari, L. Fedrizzi, P. M. Ossi and M. Scotoni, Mater. Sci. Eng., 90(1987)349. 9 B. Margesin, F. Giacomozzi, L. Guzman, G. Lazzari and V. Zanini, NucL Instrum. Methods B, 21 (1987) 566. 10 E. McCafferty, G. K. Hubler, P. M. Natishan, P.G. Moore, R. A. Kant and B. D. Sartwell, Mater. Sci. Eng., 86 (1987) 1.
11 Y. Andoh, Y. Suzuki, K. Matsuda, M. Satou and E Fujimoto, Nucl. Instrum. Methods B, 6 (1985) 111. 12 G. K. Wolf, K. Zucholl, M. Barth and W. Ensinger, Nucl. lnstrum. Methods B, 21 (1987)570. 13 W. Ensinger, M. Barth and G. K. Wolf, Nucl. Instrum. Methods B, 32 (1988) 104. 14 A. Schr6er, Diplomarbeit, UniversitStHeidelberg, 1988. 15 M. Barth and G. K. Wolf, unpublished results, 1988. 16 D.W. Hoffman and M. R. Gaerttner, J. Vac. Sci. Technol., 17 ( 1) (1980) 425. 17 E. H. Hirsch, D. K. Varga, Thin Solid Films, 69 (1980) 99. 18 J. J. Cuomo, J. M. E. Harper, C. R. Guarnieri, D. S. Yee, C.J. Attanasio, J. Angilello and R. J. Hammond, J. Vac. Sci. Technol., 20 (1982) 349. 19 J. M. E. Harper, J. J. Cuomo, R. J. Gambino and H. R. Kaufman, in O. Auciello and R. Kelly (eds.), Ion Modification of Surfaces, Elsevier, Amsterdam, 1984, p. 127. 20 G. K. Wolf, M. Barth and W. Ensinger, Nucl. Instrum. MethodsB, 37-38 (1989)682. 21 W. Fischer, Diplomarbeit, Universit~itHeidelberg, 1988.