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The influence of titanium interlayers on the adhesion of PVD TiN coatings on oxidized stainless steel substrates
Surface and Coatings Technology, 58 (1993) 163—172
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The influence of titanium interlayers on the adhesion of PVD TiN coatings on oxidized stainless steel substrates* K. A. Pischow, L. Eriksson, E. Harju and A. S. Korhonen Laboratory of Processing and Heat Treatment of Materials, Helsinki University of Technology, Vuorimiehentie 2A, SF-02150 Espoo (Finland)
E. 0. Ristolainen Centrefor Chemical Analysis, Helsinki University of Technology, Vuorimiehentie 2A, SF-02]50 Espoo (Finland) (Received May 12, 1992; accepted in final form February 26, 1993)
Abstract It has been shown that the use of thin titanium interlayers improves the coating—substrate adhesion of physical vapour deposition (PVD) titanium nitride thin films on a stainless steel substrate. This improvement arises from a combination ofchemical gettering and mechanical compliance effects. The improved adhesion ofplasma-assisted chemical vapour deposition TiN coatings with increasing interlayer thickness has been shown to be largely attributable to the compliance effect (S. J. Bull, P. R. Chalker, C. F. Ayres and D. S. Rickersby, Mater. Sci. Eng. A, 139 (1991) 71). The development of practical methods to improve adhesion is hampered by the difficulties involved in quantitative measurements of the effect. To avoid the influence of the intrinsic and extrinsic parameters involved in scratch test and microhardness measurements, efforts have been made to apply fracture mechanical testing methods to the determination of the adhesion strength of the film on the substrate (S. Berg, S. W. Kim, V. Grajewski and E. Fromm, Mater. Sci. Eng. A, 139(1991) 345). In our study the influence of Ti interlayers on the adhesion of PVD TiN coatings on oxidized stainless steel substrates was investigated using a pull-off test for adhesion measurements and scanning tunnelling microscopy and secondary ion mass spectrometry for analysis of the fractured surfaces. It was shown that the thickness ofthe Ti layer must be chosen according to the thickness of the oxide layer. An excess ofTi leads to lower adhesion values due to failure in the Ti layer, while a shortage of Ti leads to unreacted oxide and minimum adhesion due to brittle fracture in the oxide layer, which was shown to be amorphous.
1. Introduction Good adhesion is a basic requirement for coatings in all applications and is especially important for thin hard coatings which are used in exacting applications such as tools, moulds, etc. Direct measurement of adhesion is, however, difficult. Thus the effects of factors such as sample preparation, pretreatments (e.g. sputtering) and possible interfacial layers are difficult to study. However, efforts have been made to apply fracture mechanical testing methods to the determination of an experimental value which would be more closely related to adhesion than commonly used scratch tests and microhardness measurements. Berg et al. [1] have presented an experimental technique which uses the modifled single-lap shear test in the determination of the adhesive strength of sputtered TiN coatings. However, although the reported preliminary results are promising, it is obvious that the stress distribution in the test is * Paper presented at the 19th International Conference on Metallurgical Coatings and Thin Films, San Diego, CA, USA, April
2—6, 1992.
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very complicated. Similarly, Filiaggi and Pilliar [2] have used .a modified short-bar test to measure the fracture toughness of plasma-sprayed ceramic—metal substrate interfaces. According to the authors, further refinement of this method is needed, in view of experimental compliance behaviour and potential or mixed mode stress intensities, to confirm the preliminary toughness values achieved. Improvements in the monitoring of mouthopening displacement are also required. Nevertheless, in the authors’ opinion this technique offers a potentially more sensitive means of evaluating the mechanical integrity of these metal—ceramic interfaces than is possible with the scratch test. Several factors affecting adhesion have been addressed in numerous papers, which all suffer from a lack of a standard system to measure adhesion. For example, the influence of the surface topography of the substrate on the adhesion of physical vapour deposition (PVD) TiN has been investigated by Precht and Sterma [3] using the scratch test; Liu et al. [4] have used a tensile test and scratch test to investigate the effects of the addition of yttrium on adhesion. The use of a thin titanium interlayer is widely accepted
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Elsevier Sequoia. All rights reserved
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Adhesion of PVD TiN coatings
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as a method for improving the adhesion of titanium nitride coatings produced by PVD methods. The influence of this Ti interlayer is often described as being twofold. Firstly, it can reduce the internal stresses at the interface region and, secondly, in some cases it can also increase the interface contact and lead to stronger chemical bonds. Cheng et a!. [5] have used transmission electron microscopy (TEM) and Auger to analyse the interface region of a TiN film deposited with a Ti interlayer on AISI M50 steel. Wen et a!. [6] and later Huang et a!. [7] have used TEM to analyse a titanium interlayer on stainless steel. After sputter cleaning they noticed substrate—Fe—Ti—-Ti—Ti2N---TiN transitions and no oxides are reported. Bull and Rickerby [8] have investigated TiN-sputter-ion-plated stainless steel samples with different Ti interlayers using a multipass scratch test. The test showed a maximum value of critical load L~at a sputtering time of 50 mm. The pull-off test is the most direct measurement of adhesion. Unfortunately, it is impossible to find a glue giving sufficient adhesion to pull off a good quality TiN coating. Nevertheless, it is effective in investigating the reasons for low adhesion values. Chen and Duh [9] have used this method in their investigation into the effects of a Ti interlayer on TiN coatings magnetron
occur in the epoxy layer. Poor adhesion in the absence of an interlayer has been explainedwas measurement by van made Stappen in which et al.the [10]failure as follows. did not In the absence of N atoms, Ti atoms can form stable bonds with the slightly oxidized surface, because TiO is more stable than Si02 and Cr203. Thus the titanium coating has strong chemical bonds to the substrate. If nitrogen is then added to the plasma, the N atoms form stable bonds with the freshly formed titanium layer. Thus the TiN layer is strongly bonded to the titanium underlayer, which acts as an adhesive between the oxide and TiN layers. In contrast, if a flux of Ti atoms and a flux of N atoms are allowed to impinge simultaneously on the oxidized stainless steel surface, the N atoms will be less
bonded. Epitaxial growth of thebonding, TiN coating is stable than strongly bonded. Cr203. CrN, A TiN which coating enhances which forms directly is less on the oxidized stainless steel substrate is therefore less in this case apparently insufficient for strong bonding. The authors have found the optimum thickness of the underlayer to be 0.l—0.2 ~.tm,but for the stainless steel the optimum thickness is more problematic. The aim of the present work is to study the effects of different Ti interlayers on the adhesion of PVD TiN coatings on an oxidized stainless steel substrate. Adhesion measurements are made by a pull-off test and the fracture surfaces are analysed using scanning tunnelling microscopy (STM), scanning electron microscopy (SEM) and secondary ion mass spectrometry (SIMS). The attempt to develop a direct method for adhesion testing is addressed.
2. Experimental procedure 2.1. Adhesion measurements Adhesion measurements were made with a 100 kN electronic tensile testing machine. The special design of the test specimens is shown in Fig. 1. A thin (5-40 nm) carbon film (marked black in Fig. I) was first evaporated on the specimen surface area in order to produce a notch effect. This carbon film was protected with a shield when the substrate surface was sputtered before deposition. The TiN coatings were then deposited by triode ion-plating equipment. After the deposition the counterpiece was glued to the coated sample surface. Efforts were made to protect the carbon film with a thin gold layer. The adhesion of TiN coatings with different Ti interlayers was tested as before, but without carbon notches.
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from 20 to 40 mm (Fig. 2). According to Gröning et a!.
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[12], the oxide layer should be mainly Cr203 at this temperature. The amorphous structure of the oxide layer at short oxidation times of 6 and 30 mm can also be
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stated from the STM scans in Fig. 3. In contrast, at 600 mm oxidation time the oxide layer seems to be crystalline.
3. Results
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Fig. 2. X-ray diffraction dnalysis of two samples oxidized for (a) 20 and (b) 40 mm at 600 C. Only the peaks of y-Fe are visible,
3.1. Adhesion measurements The first experiments with samples prepared by the aforementioned method showed a very heavy sputtering attack on the sample surface around the opening in the shield. Moreover, this effect produced a drift of carbon
2.2. STM ana!ysis The scanning tunnelling microscope used was a Struers Tunnelscope 2400 with a short scanning tube for scanning within a 6 ~.tmx 6 tim area with a lateral resolution of up to 0.1 A. The bias voltage was adjustable from 10 mY to 5 V in steps of 1 mY. The tunnel current could be varied from 0.01 to 10 nA in steps of 10 pA. During scanning the bias voltage and the tunnelling current were lower and kept constant. They were 0.1 V and 1 nA respectively. 2.3. SIMS analysis Depth profiling was performed on a VG IX7OS double-focusing magnetic sector secondary ion mass spectrometer using an O~primary beam at 6 keV impact energy. A primary ion current of 100 nA was used with a spot size of 1000 nm. A mass resolution of 1000 was used in this study. Profiling 14N~,50Ti4, with the IX7OS was per52Cr~’,56Fe’~ formed by detecting and 64TiO” ions. positive 2.4. TiN deposition TiN films were deposited by an industrial-size reactive triode ion-plating apparatus which has been described elsewhere [11]. Ti layers of 30, 300 and 600nm were deposited next to the substrate before adding nitrogen to the plasma. Coating parameters were chosen based on our previous experience in achieving stoichiometric TiN with good adhesive and wear-resistant properties. 2.5. Oxidation of the samples The oxidation of the samples occurred in an ambient air atmosphere at a temperature of 600 °C.X-ray diffraction measurements of the oxidized surfaces showed only peaks for the austenitic structure of the substrate. This indicates strongly that the oxide layer is amorphous. The intensity of the austenite (Ill) reflection drops from 912 to 729 counts when the oxidization time increases
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Fig. 4. Adhesion forces achieved in the tensile test of TiN coatings on stainless steel substrates with various Ti interlayers and oxidation times of the substrates: (a) the different failure modes: (h) the correlation between adhesion force and oxidation time for a 30 nm Ti interlayer. F = 48.302 + —~26.558 log(t), R 2 = 0.970.
(d Fig. 3(c) and (d). STM scans of oxidized stainless steel samples: (a) 6 mm, 600 C, amorphous surface, picture area 6000 nm x 6000 nm; (b) as (a) but picture area I000nm x l000nm; (c) 30mm, 600”C, amorphous surface, picture area 6000 nm x 6000 nm; (d) 600 mi 600 C crystalline surface, picture area 6000 nm x 6000 nm.
in the vicinity of the edge of the shield opening. This then leads to a discontinuity and cracking of the Ti/TiN coating in the most critical area where the carbonprecoated surface changes to the sputtered surface to be tested. For the moment we are not able to solve the problems arising in the preparation of a suitable notch for use in fracture mechanical adhesion measurements. The adhesion of a TiN coating with different Ti interlayers on oxidized stainless steel surfaces was tested with the same kind of specimens without carbon notches. The limiting factor in this case is the strength of the glue, which was about 20 kN with a large process-
sensitive variation. The results of the measurements are given in Table 1. The forces measured are plotted against the oxidation time in Fig. 4(a), in which the different failure modes have also been marked. On plotting the forces measured for the samples with a 30 nm Ti interlayer against the oxidation time a good fit to a logarithmic curve will be achieved, as can be seen in Fig. 4(b). Sample 1 9 has been omitted from these data since its fracture surface is commensurate with those achieved by much higher stresses. The reason for the low value might be improper mounting of the sample in the tensile test machine. 3.2. Analysis ofthe fracture surftwes After the tensile tests the fracture surfaces were studied by optical microscopy, SEM, STM and SIMS. 3.2.1. Low adhesion areas In areas with low adhesion the structure of the fracture surface on the coating side is composed of smooth, almost round areas divided by cracks, as can be seen in Fig. 5(a) for sample 20. On the substrate side of sample
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TABLE I. Adhesion results Sample
Time
(mm) 4 5 6 7 8 9 10 11 12 13 l3b 14 19 20 21 22 23 24 32 37 40
2 6 30 60 600 720 60 100 40 20 20 —
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Oxidation temperature (SC)
Ti (nm)
TiN (nm)
Quality
600 600 600 600 600 600 600 600 600 600 600 600 600 600 600 600 600 600 600 600 600
600 300 300 300 300 600 600 600 600 600 600 600 30 30 30 30 30 30 300 600 600
2400 2700 2700 2700 2700 2400 2400 2400 2400 2400 2400 2400 2970 2970 2970 2970 2970 2970 2700 2400 2400
Decohesion 0% Decohesion 30% Decohesion 40% Decohesion 100% Decohesion I % Decohesion 0% Decohesion 80% Decohesion I00%~’ Decohesion 1% Decohesion <1% Decohesion <1% Decohesion 0% Good appearance Good appearance Good appearance Good appearance Good appearance Decohesion 90% Good appearance Good appearance Good appearance
aGlue failure. bDecohesion along Ti(N) interlayer.
(a) Fig. 5(a) and (b).
(b)
Force (kN)
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7.8 16.9 17.4 a
6.8 5.5 21.2 13.2 11.5 10.65 6.3 8.0
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Adhesion of PVD TiN coatings
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Fig. 5(c) and (d). Optical micrographs of the different fracture surface types. Sample 20, oxidation time 40 mm, 30 nm Ti, lo~adhesion ) = 5.5 kN. (a) The coating side shows a smooth oxide surface. (b) On the substrate side only some traces of coating can be seen bum there is a brownish oxide layer clearly visible. Sample 37, oxidation time 10 mm, 600 nm Ti, low adhesion F=6.3 kN. (c) The coating side shows a Ti layer and small areas of TiN on the cracked area. (d) The substrate side also shows a white shiny Ti layer and TiN can only he seen on cracks. (Original magnification 500 x
20 (Fig. 5(b)) only some traces of the coating are visible, although the brownish oxide layer is clearly visible. The same kind of fracture surface can also be seen when the decohesion occurs through a pure Ti interlayer in sample 37 leaving a white shiny Ti surface on the substrate side (Fig. 5(c)) and on the coating side (Fig. 5(d)). STM scans from the coating side show severe distortions like those generally seen on titanium oxide surfaces. 50Ti~, SIMS depth profiles of the elements (‘4N 52Cr~,56Fe~and 64TiO~)of the coating side of sample 20 are shown in Fig. 6. The concentration of the elements changes slightly at the interface. At the start of sputtering the 52Cr~,56Fe~ and 64TiO~ signals are strong; the chromium and iron start to decrease, while the titanium oxide starts to increase slowly. This means that the phases from the fractured surface to the TiN coating are chromium oxide and iron oxide with some titanium oxide. The percentage of the first two decreases sharply at the same time as the percentage of the titanium oxide increases. The profile of 14N ~ shows the formation of the titanium nitride layer. The concentration of nitrogen increases slowly towards the stable coating layer. Thermal stresses caused failure of the coating on sample 24. Decohesion occurred along the oxide interface leaving a brown oxide layer on the substrate surface with 10% of the coating removed along the Ti/TiN interlayer. ~,
3.2.2. Medium adhesion areas In areas with medium adhesion (Fig. 7(a), sample 12) the fracture surface on the coating side shows a smooth
oxide surface with brighter areas where the TiN interface is visible. On the substrate side (Fig. 7(b)) the stainless steel surface appears clear and is partly covered with small fragments of TiN coating showing the typical golden colour of the near-stoichiometric titanium nitride. Energy-dispersive X-ray (EDX) analyses in the scanning electron microscope showed Cr, Fe and low Ti in the brownish areas and high Ti without any Cr or Fe in the bright ones. STM scans from the oxide part of the coating side show the same kind of behaviour as before, although in the brighter areas the Ti interlayer consists of small grains with an approximate length of 400 nm, as can be seen in Fig. 8.
3.2.3. High adhesion areas Figure 9 shows the fracture surfaces of sample I 3, which is typical of higher adhesion. On the coating side the fracture plane seems to intersect different layers and only in some parts has the decohesion occurred along the oxide surface. On the substrate side the stainless steel surface again appears clear and the amount of remaining TiN coating increases. It is typical of the remainder of the coating parts that the fracture plane seems to intersect different layers and even the columnar TiN grains. This can also be said of the SEM image in Fig. 10, where the different fracture surfaces are marked. Energy-dispersive spectroscopy (EDS) analyses made from this sample showed three different areas. Firstly, in area A there is no Ti but strong Cr and Fe, identifying
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The oxide layers the samples used for adhesion measurements were on shown to be amorphous to both X-ray diffraction and STM. Distortion-free scans were achieved immediately from the first scan and the appearance of the scans did not change markedly even with prolonged scanning times. Very similar STM scans of a chemically inert Cr203 surface fabricated from Cr(CO)6 by chemical vapour deposition have been described by Perkins et a!. [13]. However, they used a 1 h scanning time or nitrogen flow to achieve distortion-free scans. At longer oxidation times the oxide layer seems to be crystalline. Czerwiec et a!. [14] have reported an amorphous interface layer between a sputter-deposited TiN coating and a high speed steel (HSS) substrate when the substrate temperature is lower than 620 K. This is consistent with our ofresults oxide layer failure when the thickness the Tishowing layer is inadequate. The identification of different fracture surfaces was possible by combining the information received from STM, SEM and SIMS analyses and the results achieved seem to be in good agreement with the excellent TEM images published by Hultman et a!. in various papers The SIMS profiling from the coating side of sample 20 showing titanium oxides leads us to the conclusion that is reducing part of in theorder chromium oxide. possible Further SIMSTi analysis is needed to confirm changesintheoxidationstateofchromiumatthe interface. Both SIMS profiling and SEM EDX analysis .
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Fig. 6. SIMS depth profiles from the fracture surface to the TiN coating.
confirmed titanium on the coating-side fracture surface but none at all or only traces on the substrate side at low adhesion values, showing the importance of the
this area as substrate. Secondly, in area B there is strong Ti but still clear Cr and Fe, which is typical of the Ti interlayer. Thirdly, in areas C and D there is strong Ti without any Cr or Fe, which points to TiN. The situation was much the same in sample 19, where the fracture surface resembles that typical of higher adhesion, as can be seen from the SEM image in Fig. 11, where the same three different fracture surface types can be found. In Fig. 12 an STM scan of the fracture surface is presented showing the corresponding surfaces. The distance between surfaces A and B is roughly 25 nm, between B and C close to 40 nm and between the two C surfaces over 50 nm. Figure 13 shows an STM image of the surface where the fracture has occurred along the Ti interlayer—TiN interface. The grain size varies from a few hundred nanometres to about 800 nm. Figure 14 presents a typical stainless steel surface of this sample where the appearance of the surface is determined by the passive layer.
chemical compliance of the Ti for adhesion. Other processes can also still be involved as has been shown by Helmersson et a!. [16]. They have found an increase in adhesion of TiN-coated HSS samples at temperatures where the iron oxide on the sample surface transforms from Fe2O3 and Fe3O4 to FeO. This increase in adhesion is explained as being caused by a decrease in interfacial energy due to the better match of FeO and TiN lattices. When oxidized stainless steel samples are deposited with TiN and a Ti intermediate layer is applied, three different interfaces must be considered: firstly, a stainless steel—oxide interface (A); secondly, an oxide— titanium interface (Al) where the reaction 2Cr2O3+3Ti—e3TiO2+4Cr is believed to occur; and thirdly, a titanium—titanium nitride interface (B) where Ti changes gradually to TiN. All three of these interfaces are possible places for cohesive failure of the coating. In our studies failures along all the interfaces were noticed. When the Ti layer is not sufficiently thick, as in sample 20, the fracture occurs through the oxide layer.
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Fig. 7. Optical micrographs of the different fracture surface types. Sample 12. oxidation time 40 mm. 600 nm Ti, medium adhesion F=7.8 kN. (a) The coating side shows a smooth oxide surface with brighter areas ~hcre a TiN interface is visible. (h) On the substrate side the stainless steel surface is clear and covered with small fragments of TiN coating shossing the typical yellowish colour of TiN. (Original magnification 500 x
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time. A similar decrease in adhesion values has been reported for a 300 nm Ti interlayer by Valli et a!. [17]. The samples showed a brittle decohesion mode in all cases where the adhesion was low enough to allow the coating to be pulled off. This is consistent with the results achieved by Bull [18] showing that in stainless steel both brittle and ductile behaviour could occur depending on the oxide layer on the surface. Measured critical loads were reported to be much higher for ductile failure than for brittle failure. The author also explains that the effects of the Ti interlayer are partly due to the enhancement of ductile failure at the coating—substrate interface.
5. Summary and conclusions
Fig. 8. STM image showing the Ti interlayer of small grains with an approximate length of 400 nm.
Very low adhesion was also measured for sample 37 where fracture occurred through a Ti layer, because the deposited (600 nm thick) Ti was too thick compared with the thin oxide layer after only 10 mm oxidation
A tensile test was used for adhesion measurements. It proved impossible to produce a notch effect using carbon evaporation because of the strong sputtering effect in the coating process used. However, the effect of the Ti interlayers on the adhesion of TiN coatings on oxidized stainless steel substrates was measured using a tensile test. The strength of the glue was the upper limit for the adhesion measurements. It was shown that the effect of a Ti interlayer on adhesion is due to the chemical compliance and that the thickness of the Ti layer has an optimum value corre-
K. A. Pischow et al.
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Adhesion of PVD TiN coatings
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(h)
Fig. 9. Optical micrographs of the different fracture surface types. Sample 13, oxidation time 20 mm, 600 nm Ti, high adhesion F= 17.4 kN. (a) The coating side shows a typical fracture pattern which penetrates different layers and also goes through the columnar TiN grains. The dark area is glue. (b) The substrate surface is a clear stainless steel surface and the remaining coating parts show irregular crack surfaces penetrating the different layers. (Original magnification 500 x
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fig. 10. SLM image of sample 13 showing the remaining coating
layers on the stainless steel substrate. The stainless steel is marked A, the Ti interlayer B, the TiN crack surfaces C and the TiN top surface D.
sponding to the oxide layer thickness, so that Ti can diffuse through the whole oxide layer to the substrate surface. If the Ti layer is too thick, adhesion decreases and fracture occurs in the Ti layer. In contrast, if the Ti layer is too thin, the failure occurs in the oxide layer. A straightforward pull-off test proved to be useful, not in the determination of the highest values of adhesion, where it failed as expected, but in giving valuable
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Fig. II. SEM image of sample 19 with the same kind of fracture surface as sample 13. The layers are marked as in Fig. 10.
information on the effects of different modifications of intermediate layers when used in combination with STM.
Acknowledgments The authors would like to thank E. Nykãnen for X-ray diffraction analyses and R. Suominen for taking the SEM pictures.
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Fig. 12. STM image of sample ) showing an area of 3000 nm x 3000 nm with the same kind ol fraciure surface as sample 13. The layers are marked as in Fig. 9. Tunnelling bias t).l V. current I nA.
Fig. 14. STM image of sample I 9 sho~~ ing ,i typical oxidited stainless steel surface. The picture area is 6000 nm x 6000 nm. Tunnelling bias 0.1 V. current I nA.
References I S. Berg, S. W. Kim, V. Grajewski and E. Fromm, Mater. Sci. Eng. A, 139 (1991) 345. 2 M. J. Filiaggi and R. M. Pilliar, J. Mater. Sd., 26 (1991) 5383. 3 W. Precht and F. Sterma, Vacuum, 41(1990) 2223. 4 C. Liu, W. Wu, Z. Yu and Z. Jin, Thin Solid Films, 207 (1992) 98. 5 C. C. Cheng, A. Erdemir and G. R. Fenske, Surf Coat. Technol.. 39—40 (1989) 365. ~
~
6 (1986)2682. X. Jiang and C. Y. Si, J. Vac. Sci. Technol. A. 4 7 Coat. R. F. Huang, S. Wen, L. Technol.,L.50(1991) 97. P. Guo, J. Gong and B. H. Yu, Surf
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8 S. J. Bull and D. S. Riekerby, Thin Solid Films, 181 (1989) 545—553. 9 Y.-I. Chen and J.-G. Duh, Surf Coat. Technol., 46 (1991) 371. 10 M. van Stappen, B. Malliet, L. De Schepper, L. M. Stals, J. P. Celis and J. R. Roos, Surf Eng., 5 (1989) 305. 11 J. M. Molarius, Dissertation Thesis, Helsinki University of
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fig. 13. STM image of sample 10 shov~ingUie Ti interlaser surface, The picture area is 6000 nm v ~~)(i() nm and the mixed grain size varies from a few hundred nanometres to about 800 nm. Tunnelling bias 0.1 V, current I nA.
12 P. Groning, S. Nowak and L. Schlapbach, Appl. Surf. Si’i., 52 (1991) 333. 13 F. K. Perkins, C. Hwang and M. Onellion. Thin Solid Films, 198 14 T. Czerwiec, K. Anoun. M. Remy and H. Michel, Mater, Sci. Eng. A, 139 (1991) 276. IS L. Huitman, G. Hâkansson, U. Wahlström, J.-E. Sundgren, I. Petrov, F. Adibi and J. E. Greene, Thin Solid Films, 205 (1991) 153. 16 U. Helmersson, B. 0. Johansson, J.-E. Sundgren, H. T. G. Hentzell and P. Billgren, 2. Vac. Sci. Technol. A, 3 (1985) 308. 17 J. Valli, U. Mäkelä, A. Matthews and V. Murawa. J. Vac. Sci. Technol. A, 3 (1985) 2411. 18 S. J. Bull, Surf. Coat. Technol., 50 (1991) 25.
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The influence of titanium interlayers on the adhesion of PVD TiN coatings on oxidized stainless steel substrates* K. A. Pischow, L. Eriksson, E. Harju and A. S. Korhonen Laboratory of Processing and Heat Treatment of Materials, Helsinki University of Technology, Vuorimiehentie 2A, SF-02150 Espoo (Finland)
E. 0. Ristolainen Centrefor Chemical Analysis, Helsinki University of Technology, Vuorimiehentie 2A, SF-02]50 Espoo (Finland) (Received May 12, 1992; accepted in final form February 26, 1993)
Abstract It has been shown that the use of thin titanium interlayers improves the coating—substrate adhesion of physical vapour deposition (PVD) titanium nitride thin films on a stainless steel substrate. This improvement arises from a combination ofchemical gettering and mechanical compliance effects. The improved adhesion ofplasma-assisted chemical vapour deposition TiN coatings with increasing interlayer thickness has been shown to be largely attributable to the compliance effect (S. J. Bull, P. R. Chalker, C. F. Ayres and D. S. Rickersby, Mater. Sci. Eng. A, 139 (1991) 71). The development of practical methods to improve adhesion is hampered by the difficulties involved in quantitative measurements of the effect. To avoid the influence of the intrinsic and extrinsic parameters involved in scratch test and microhardness measurements, efforts have been made to apply fracture mechanical testing methods to the determination of the adhesion strength of the film on the substrate (S. Berg, S. W. Kim, V. Grajewski and E. Fromm, Mater. Sci. Eng. A, 139(1991) 345). In our study the influence of Ti interlayers on the adhesion of PVD TiN coatings on oxidized stainless steel substrates was investigated using a pull-off test for adhesion measurements and scanning tunnelling microscopy and secondary ion mass spectrometry for analysis of the fractured surfaces. It was shown that the thickness ofthe Ti layer must be chosen according to the thickness of the oxide layer. An excess ofTi leads to lower adhesion values due to failure in the Ti layer, while a shortage of Ti leads to unreacted oxide and minimum adhesion due to brittle fracture in the oxide layer, which was shown to be amorphous.
1. Introduction Good adhesion is a basic requirement for coatings in all applications and is especially important for thin hard coatings which are used in exacting applications such as tools, moulds, etc. Direct measurement of adhesion is, however, difficult. Thus the effects of factors such as sample preparation, pretreatments (e.g. sputtering) and possible interfacial layers are difficult to study. However, efforts have been made to apply fracture mechanical testing methods to the determination of an experimental value which would be more closely related to adhesion than commonly used scratch tests and microhardness measurements. Berg et al. [1] have presented an experimental technique which uses the modifled single-lap shear test in the determination of the adhesive strength of sputtered TiN coatings. However, although the reported preliminary results are promising, it is obvious that the stress distribution in the test is * Paper presented at the 19th International Conference on Metallurgical Coatings and Thin Films, San Diego, CA, USA, April
2—6, 1992.
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very complicated. Similarly, Filiaggi and Pilliar [2] have used .a modified short-bar test to measure the fracture toughness of plasma-sprayed ceramic—metal substrate interfaces. According to the authors, further refinement of this method is needed, in view of experimental compliance behaviour and potential or mixed mode stress intensities, to confirm the preliminary toughness values achieved. Improvements in the monitoring of mouthopening displacement are also required. Nevertheless, in the authors’ opinion this technique offers a potentially more sensitive means of evaluating the mechanical integrity of these metal—ceramic interfaces than is possible with the scratch test. Several factors affecting adhesion have been addressed in numerous papers, which all suffer from a lack of a standard system to measure adhesion. For example, the influence of the surface topography of the substrate on the adhesion of physical vapour deposition (PVD) TiN has been investigated by Precht and Sterma [3] using the scratch test; Liu et al. [4] have used a tensile test and scratch test to investigate the effects of the addition of yttrium on adhesion. The use of a thin titanium interlayer is widely accepted
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as a method for improving the adhesion of titanium nitride coatings produced by PVD methods. The influence of this Ti interlayer is often described as being twofold. Firstly, it can reduce the internal stresses at the interface region and, secondly, in some cases it can also increase the interface contact and lead to stronger chemical bonds. Cheng et a!. [5] have used transmission electron microscopy (TEM) and Auger to analyse the interface region of a TiN film deposited with a Ti interlayer on AISI M50 steel. Wen et a!. [6] and later Huang et a!. [7] have used TEM to analyse a titanium interlayer on stainless steel. After sputter cleaning they noticed substrate—Fe—Ti—-Ti—Ti2N---TiN transitions and no oxides are reported. Bull and Rickerby [8] have investigated TiN-sputter-ion-plated stainless steel samples with different Ti interlayers using a multipass scratch test. The test showed a maximum value of critical load L~at a sputtering time of 50 mm. The pull-off test is the most direct measurement of adhesion. Unfortunately, it is impossible to find a glue giving sufficient adhesion to pull off a good quality TiN coating. Nevertheless, it is effective in investigating the reasons for low adhesion values. Chen and Duh [9] have used this method in their investigation into the effects of a Ti interlayer on TiN coatings magnetron
occur in the epoxy layer. Poor adhesion in the absence of an interlayer has been explainedwas measurement by van made Stappen in which et al.the [10]failure as follows. did not In the absence of N atoms, Ti atoms can form stable bonds with the slightly oxidized surface, because TiO is more stable than Si02 and Cr203. Thus the titanium coating has strong chemical bonds to the substrate. If nitrogen is then added to the plasma, the N atoms form stable bonds with the freshly formed titanium layer. Thus the TiN layer is strongly bonded to the titanium underlayer, which acts as an adhesive between the oxide and TiN layers. In contrast, if a flux of Ti atoms and a flux of N atoms are allowed to impinge simultaneously on the oxidized stainless steel surface, the N atoms will be less
bonded. Epitaxial growth of thebonding, TiN coating is stable than strongly bonded. Cr203. CrN, A TiN which coating enhances which forms directly is less on the oxidized stainless steel substrate is therefore less in this case apparently insufficient for strong bonding. The authors have found the optimum thickness of the underlayer to be 0.l—0.2 ~.tm,but for the stainless steel the optimum thickness is more problematic. The aim of the present work is to study the effects of different Ti interlayers on the adhesion of PVD TiN coatings on an oxidized stainless steel substrate. Adhesion measurements are made by a pull-off test and the fracture surfaces are analysed using scanning tunnelling microscopy (STM), scanning electron microscopy (SEM) and secondary ion mass spectrometry (SIMS). The attempt to develop a direct method for adhesion testing is addressed.
2. Experimental procedure 2.1. Adhesion measurements Adhesion measurements were made with a 100 kN electronic tensile testing machine. The special design of the test specimens is shown in Fig. 1. A thin (5-40 nm) carbon film (marked black in Fig. I) was first evaporated on the specimen surface area in order to produce a notch effect. This carbon film was protected with a shield when the substrate surface was sputtered before deposition. The TiN coatings were then deposited by triode ion-plating equipment. After the deposition the counterpiece was glued to the coated sample surface. Efforts were made to protect the carbon film with a thin gold layer. The adhesion of TiN coatings with different Ti interlayers was tested as before, but without carbon notches.
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from 20 to 40 mm (Fig. 2). According to Gröning et a!.
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[12], the oxide layer should be mainly Cr203 at this temperature. The amorphous structure of the oxide layer at short oxidation times of 6 and 30 mm can also be
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stated from the STM scans in Fig. 3. In contrast, at 600 mm oxidation time the oxide layer seems to be crystalline.
3. Results
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Fig. 2. X-ray diffraction dnalysis of two samples oxidized for (a) 20 and (b) 40 mm at 600 C. Only the peaks of y-Fe are visible,
3.1. Adhesion measurements The first experiments with samples prepared by the aforementioned method showed a very heavy sputtering attack on the sample surface around the opening in the shield. Moreover, this effect produced a drift of carbon
2.2. STM ana!ysis The scanning tunnelling microscope used was a Struers Tunnelscope 2400 with a short scanning tube for scanning within a 6 ~.tmx 6 tim area with a lateral resolution of up to 0.1 A. The bias voltage was adjustable from 10 mY to 5 V in steps of 1 mY. The tunnel current could be varied from 0.01 to 10 nA in steps of 10 pA. During scanning the bias voltage and the tunnelling current were lower and kept constant. They were 0.1 V and 1 nA respectively. 2.3. SIMS analysis Depth profiling was performed on a VG IX7OS double-focusing magnetic sector secondary ion mass spectrometer using an O~primary beam at 6 keV impact energy. A primary ion current of 100 nA was used with a spot size of 1000 nm. A mass resolution of 1000 was used in this study. Profiling 14N~,50Ti4, with the IX7OS was per52Cr~’,56Fe’~ formed by detecting and 64TiO” ions. positive 2.4. TiN deposition TiN films were deposited by an industrial-size reactive triode ion-plating apparatus which has been described elsewhere [11]. Ti layers of 30, 300 and 600nm were deposited next to the substrate before adding nitrogen to the plasma. Coating parameters were chosen based on our previous experience in achieving stoichiometric TiN with good adhesive and wear-resistant properties. 2.5. Oxidation of the samples The oxidation of the samples occurred in an ambient air atmosphere at a temperature of 600 °C.X-ray diffraction measurements of the oxidized surfaces showed only peaks for the austenitic structure of the substrate. This indicates strongly that the oxide layer is amorphous. The intensity of the austenite (Ill) reflection drops from 912 to 729 counts when the oxidization time increases
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Fig. 4. Adhesion forces achieved in the tensile test of TiN coatings on stainless steel substrates with various Ti interlayers and oxidation times of the substrates: (a) the different failure modes: (h) the correlation between adhesion force and oxidation time for a 30 nm Ti interlayer. F = 48.302 + —~26.558 log(t), R 2 = 0.970.
(d Fig. 3(c) and (d). STM scans of oxidized stainless steel samples: (a) 6 mm, 600 C, amorphous surface, picture area 6000 nm x 6000 nm; (b) as (a) but picture area I000nm x l000nm; (c) 30mm, 600”C, amorphous surface, picture area 6000 nm x 6000 nm; (d) 600 mi 600 C crystalline surface, picture area 6000 nm x 6000 nm.
in the vicinity of the edge of the shield opening. This then leads to a discontinuity and cracking of the Ti/TiN coating in the most critical area where the carbonprecoated surface changes to the sputtered surface to be tested. For the moment we are not able to solve the problems arising in the preparation of a suitable notch for use in fracture mechanical adhesion measurements. The adhesion of a TiN coating with different Ti interlayers on oxidized stainless steel surfaces was tested with the same kind of specimens without carbon notches. The limiting factor in this case is the strength of the glue, which was about 20 kN with a large process-
sensitive variation. The results of the measurements are given in Table 1. The forces measured are plotted against the oxidation time in Fig. 4(a), in which the different failure modes have also been marked. On plotting the forces measured for the samples with a 30 nm Ti interlayer against the oxidation time a good fit to a logarithmic curve will be achieved, as can be seen in Fig. 4(b). Sample 1 9 has been omitted from these data since its fracture surface is commensurate with those achieved by much higher stresses. The reason for the low value might be improper mounting of the sample in the tensile test machine. 3.2. Analysis ofthe fracture surftwes After the tensile tests the fracture surfaces were studied by optical microscopy, SEM, STM and SIMS. 3.2.1. Low adhesion areas In areas with low adhesion the structure of the fracture surface on the coating side is composed of smooth, almost round areas divided by cracks, as can be seen in Fig. 5(a) for sample 20. On the substrate side of sample
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TABLE I. Adhesion results Sample
Time
(mm) 4 5 6 7 8 9 10 11 12 13 l3b 14 19 20 21 22 23 24 32 37 40
2 6 30 60 600 720 60 100 40 20 20 —
20 40 10 20 30 60 60 10 60
Oxidation temperature (SC)
Ti (nm)
TiN (nm)
Quality
600 600 600 600 600 600 600 600 600 600 600 600 600 600 600 600 600 600 600 600 600
600 300 300 300 300 600 600 600 600 600 600 600 30 30 30 30 30 30 300 600 600
2400 2700 2700 2700 2700 2400 2400 2400 2400 2400 2400 2400 2970 2970 2970 2970 2970 2970 2700 2400 2400
Decohesion 0% Decohesion 30% Decohesion 40% Decohesion 100% Decohesion I % Decohesion 0% Decohesion 80% Decohesion I00%~’ Decohesion 1% Decohesion <1% Decohesion <1% Decohesion 0% Good appearance Good appearance Good appearance Good appearance Good appearance Decohesion 90% Good appearance Good appearance Good appearance
aGlue failure. bDecohesion along Ti(N) interlayer.
(a) Fig. 5(a) and (b).
(b)
Force (kN)
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7.8 16.9 17.4 a
6.8 5.5 21.2 13.2 11.5 10.65 6.3 8.0
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Fig. 5(c) and (d). Optical micrographs of the different fracture surface types. Sample 20, oxidation time 40 mm, 30 nm Ti, lo~adhesion ) = 5.5 kN. (a) The coating side shows a smooth oxide surface. (b) On the substrate side only some traces of coating can be seen bum there is a brownish oxide layer clearly visible. Sample 37, oxidation time 10 mm, 600 nm Ti, low adhesion F=6.3 kN. (c) The coating side shows a Ti layer and small areas of TiN on the cracked area. (d) The substrate side also shows a white shiny Ti layer and TiN can only he seen on cracks. (Original magnification 500 x
20 (Fig. 5(b)) only some traces of the coating are visible, although the brownish oxide layer is clearly visible. The same kind of fracture surface can also be seen when the decohesion occurs through a pure Ti interlayer in sample 37 leaving a white shiny Ti surface on the substrate side (Fig. 5(c)) and on the coating side (Fig. 5(d)). STM scans from the coating side show severe distortions like those generally seen on titanium oxide surfaces. 50Ti~, SIMS depth profiles of the elements (‘4N 52Cr~,56Fe~and 64TiO~)of the coating side of sample 20 are shown in Fig. 6. The concentration of the elements changes slightly at the interface. At the start of sputtering the 52Cr~,56Fe~ and 64TiO~ signals are strong; the chromium and iron start to decrease, while the titanium oxide starts to increase slowly. This means that the phases from the fractured surface to the TiN coating are chromium oxide and iron oxide with some titanium oxide. The percentage of the first two decreases sharply at the same time as the percentage of the titanium oxide increases. The profile of 14N ~ shows the formation of the titanium nitride layer. The concentration of nitrogen increases slowly towards the stable coating layer. Thermal stresses caused failure of the coating on sample 24. Decohesion occurred along the oxide interface leaving a brown oxide layer on the substrate surface with 10% of the coating removed along the Ti/TiN interlayer. ~,
3.2.2. Medium adhesion areas In areas with medium adhesion (Fig. 7(a), sample 12) the fracture surface on the coating side shows a smooth
oxide surface with brighter areas where the TiN interface is visible. On the substrate side (Fig. 7(b)) the stainless steel surface appears clear and is partly covered with small fragments of TiN coating showing the typical golden colour of the near-stoichiometric titanium nitride. Energy-dispersive X-ray (EDX) analyses in the scanning electron microscope showed Cr, Fe and low Ti in the brownish areas and high Ti without any Cr or Fe in the bright ones. STM scans from the oxide part of the coating side show the same kind of behaviour as before, although in the brighter areas the Ti interlayer consists of small grains with an approximate length of 400 nm, as can be seen in Fig. 8.
3.2.3. High adhesion areas Figure 9 shows the fracture surfaces of sample I 3, which is typical of higher adhesion. On the coating side the fracture plane seems to intersect different layers and only in some parts has the decohesion occurred along the oxide surface. On the substrate side the stainless steel surface again appears clear and the amount of remaining TiN coating increases. It is typical of the remainder of the coating parts that the fracture plane seems to intersect different layers and even the columnar TiN grains. This can also be said of the SEM image in Fig. 10, where the different fracture surfaces are marked. Energy-dispersive spectroscopy (EDS) analyses made from this sample showed three different areas. Firstly, in area A there is no Ti but strong Cr and Fe, identifying
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The oxide layers the samples used for adhesion measurements were on shown to be amorphous to both X-ray diffraction and STM. Distortion-free scans were achieved immediately from the first scan and the appearance of the scans did not change markedly even with prolonged scanning times. Very similar STM scans of a chemically inert Cr203 surface fabricated from Cr(CO)6 by chemical vapour deposition have been described by Perkins et a!. [13]. However, they used a 1 h scanning time or nitrogen flow to achieve distortion-free scans. At longer oxidation times the oxide layer seems to be crystalline. Czerwiec et a!. [14] have reported an amorphous interface layer between a sputter-deposited TiN coating and a high speed steel (HSS) substrate when the substrate temperature is lower than 620 K. This is consistent with our ofresults oxide layer failure when the thickness the Tishowing layer is inadequate. The identification of different fracture surfaces was possible by combining the information received from STM, SEM and SIMS analyses and the results achieved seem to be in good agreement with the excellent TEM images published by Hultman et a!. in various papers The SIMS profiling from the coating side of sample 20 showing titanium oxides leads us to the conclusion that is reducing part of in theorder chromium oxide. possible Further SIMSTi analysis is needed to confirm changesintheoxidationstateofchromiumatthe interface. Both SIMS profiling and SEM EDX analysis .
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Fig. 6. SIMS depth profiles from the fracture surface to the TiN coating.
confirmed titanium on the coating-side fracture surface but none at all or only traces on the substrate side at low adhesion values, showing the importance of the
this area as substrate. Secondly, in area B there is strong Ti but still clear Cr and Fe, which is typical of the Ti interlayer. Thirdly, in areas C and D there is strong Ti without any Cr or Fe, which points to TiN. The situation was much the same in sample 19, where the fracture surface resembles that typical of higher adhesion, as can be seen from the SEM image in Fig. 11, where the same three different fracture surface types can be found. In Fig. 12 an STM scan of the fracture surface is presented showing the corresponding surfaces. The distance between surfaces A and B is roughly 25 nm, between B and C close to 40 nm and between the two C surfaces over 50 nm. Figure 13 shows an STM image of the surface where the fracture has occurred along the Ti interlayer—TiN interface. The grain size varies from a few hundred nanometres to about 800 nm. Figure 14 presents a typical stainless steel surface of this sample where the appearance of the surface is determined by the passive layer.
chemical compliance of the Ti for adhesion. Other processes can also still be involved as has been shown by Helmersson et a!. [16]. They have found an increase in adhesion of TiN-coated HSS samples at temperatures where the iron oxide on the sample surface transforms from Fe2O3 and Fe3O4 to FeO. This increase in adhesion is explained as being caused by a decrease in interfacial energy due to the better match of FeO and TiN lattices. When oxidized stainless steel samples are deposited with TiN and a Ti intermediate layer is applied, three different interfaces must be considered: firstly, a stainless steel—oxide interface (A); secondly, an oxide— titanium interface (Al) where the reaction 2Cr2O3+3Ti—e3TiO2+4Cr is believed to occur; and thirdly, a titanium—titanium nitride interface (B) where Ti changes gradually to TiN. All three of these interfaces are possible places for cohesive failure of the coating. In our studies failures along all the interfaces were noticed. When the Ti layer is not sufficiently thick, as in sample 20, the fracture occurs through the oxide layer.
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Fig. 7. Optical micrographs of the different fracture surface types. Sample 12. oxidation time 40 mm. 600 nm Ti, medium adhesion F=7.8 kN. (a) The coating side shows a smooth oxide surface with brighter areas ~hcre a TiN interface is visible. (h) On the substrate side the stainless steel surface is clear and covered with small fragments of TiN coating shossing the typical yellowish colour of TiN. (Original magnification 500 x
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time. A similar decrease in adhesion values has been reported for a 300 nm Ti interlayer by Valli et a!. [17]. The samples showed a brittle decohesion mode in all cases where the adhesion was low enough to allow the coating to be pulled off. This is consistent with the results achieved by Bull [18] showing that in stainless steel both brittle and ductile behaviour could occur depending on the oxide layer on the surface. Measured critical loads were reported to be much higher for ductile failure than for brittle failure. The author also explains that the effects of the Ti interlayer are partly due to the enhancement of ductile failure at the coating—substrate interface.
5. Summary and conclusions
Fig. 8. STM image showing the Ti interlayer of small grains with an approximate length of 400 nm.
Very low adhesion was also measured for sample 37 where fracture occurred through a Ti layer, because the deposited (600 nm thick) Ti was too thick compared with the thin oxide layer after only 10 mm oxidation
A tensile test was used for adhesion measurements. It proved impossible to produce a notch effect using carbon evaporation because of the strong sputtering effect in the coating process used. However, the effect of the Ti interlayers on the adhesion of TiN coatings on oxidized stainless steel substrates was measured using a tensile test. The strength of the glue was the upper limit for the adhesion measurements. It was shown that the effect of a Ti interlayer on adhesion is due to the chemical compliance and that the thickness of the Ti layer has an optimum value corre-
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Adhesion of PVD TiN coatings
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(h)
Fig. 9. Optical micrographs of the different fracture surface types. Sample 13, oxidation time 20 mm, 600 nm Ti, high adhesion F= 17.4 kN. (a) The coating side shows a typical fracture pattern which penetrates different layers and also goes through the columnar TiN grains. The dark area is glue. (b) The substrate surface is a clear stainless steel surface and the remaining coating parts show irregular crack surfaces penetrating the different layers. (Original magnification 500 x
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fig. 10. SLM image of sample 13 showing the remaining coating
layers on the stainless steel substrate. The stainless steel is marked A, the Ti interlayer B, the TiN crack surfaces C and the TiN top surface D.
sponding to the oxide layer thickness, so that Ti can diffuse through the whole oxide layer to the substrate surface. If the Ti layer is too thick, adhesion decreases and fracture occurs in the Ti layer. In contrast, if the Ti layer is too thin, the failure occurs in the oxide layer. A straightforward pull-off test proved to be useful, not in the determination of the highest values of adhesion, where it failed as expected, but in giving valuable
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Fig. II. SEM image of sample 19 with the same kind of fracture surface as sample 13. The layers are marked as in Fig. 10.
information on the effects of different modifications of intermediate layers when used in combination with STM.
Acknowledgments The authors would like to thank E. Nykãnen for X-ray diffraction analyses and R. Suominen for taking the SEM pictures.
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Fig. 12. STM image of sample ) showing an area of 3000 nm x 3000 nm with the same kind ol fraciure surface as sample 13. The layers are marked as in Fig. 9. Tunnelling bias t).l V. current I nA.
Fig. 14. STM image of sample I 9 sho~~ ing ,i typical oxidited stainless steel surface. The picture area is 6000 nm x 6000 nm. Tunnelling bias 0.1 V. current I nA.
References I S. Berg, S. W. Kim, V. Grajewski and E. Fromm, Mater. Sci. Eng. A, 139 (1991) 345. 2 M. J. Filiaggi and R. M. Pilliar, J. Mater. Sd., 26 (1991) 5383. 3 W. Precht and F. Sterma, Vacuum, 41(1990) 2223. 4 C. Liu, W. Wu, Z. Yu and Z. Jin, Thin Solid Films, 207 (1992) 98. 5 C. C. Cheng, A. Erdemir and G. R. Fenske, Surf Coat. Technol.. 39—40 (1989) 365. ~
~
6 (1986)2682. X. Jiang and C. Y. Si, J. Vac. Sci. Technol. A. 4 7 Coat. R. F. Huang, S. Wen, L. Technol.,L.50(1991) 97. P. Guo, J. Gong and B. H. Yu, Surf
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8 S. J. Bull and D. S. Riekerby, Thin Solid Films, 181 (1989) 545—553. 9 Y.-I. Chen and J.-G. Duh, Surf Coat. Technol., 46 (1991) 371. 10 M. van Stappen, B. Malliet, L. De Schepper, L. M. Stals, J. P. Celis and J. R. Roos, Surf Eng., 5 (1989) 305. 11 J. M. Molarius, Dissertation Thesis, Helsinki University of
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fig. 13. STM image of sample 10 shov~ingUie Ti interlaser surface, The picture area is 6000 nm v ~~)(i() nm and the mixed grain size varies from a few hundred nanometres to about 800 nm. Tunnelling bias 0.1 V, current I nA.
12 P. Groning, S. Nowak and L. Schlapbach, Appl. Surf. Si’i., 52 (1991) 333. 13 F. K. Perkins, C. Hwang and M. Onellion. Thin Solid Films, 198 14 T. Czerwiec, K. Anoun. M. Remy and H. Michel, Mater, Sci. Eng. A, 139 (1991) 276. IS L. Huitman, G. Hâkansson, U. Wahlström, J.-E. Sundgren, I. Petrov, F. Adibi and J. E. Greene, Thin Solid Films, 205 (1991) 153. 16 U. Helmersson, B. 0. Johansson, J.-E. Sundgren, H. T. G. Hentzell and P. Billgren, 2. Vac. Sci. Technol. A, 3 (1985) 308. 17 J. Valli, U. Mäkelä, A. Matthews and V. Murawa. J. Vac. Sci. Technol. A, 3 (1985) 2411. 18 S. J. Bull, Surf. Coat. Technol., 50 (1991) 25.