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Identification of factors affecting the aqueous corrosion properties of (Ti, Al) N-coated steel
Surfi,ce and Coatings Technology, 49 (1991) 353—358
353
Identification of factors affecting the aqueous corrosion properties of (Ti, Al)N-coated steel J. Aromaa, H. Ronkainen, A. Mahiout and S.-P. Hannula Technical Research c’entre of Finland (Valtion Teknillinen Tutkimuskeskus), Metallimiehenkuja 4, SF-02150, Espoo (Finland)
Abstract Carbides, borides and nitrides of transition metals are increasingly used as coating materials because of their high hardnesses and excellent wear resistances. However, these coatings can also potentially be used in applications requiring corrosion resistance because of their inherent stability under a variety of aqueous conditions. Thin film coatings based on these materials are generally produced by physical vapour deposition (PVD) methods. Such coatings often exhibit porosity which usually is thought to result from pinholes formed during deposition. If local defects form a direct path between the corrosive environment and the base material, the corrosion protection is lost. In this work the influence of different factors on the corrosion resistance of PVD (Ti, Al)N coatings on type ASP 23 high speed steel was studied. The factors evaluated were the thickness of the coating, the thickness and the type of the intermediate layer, the titanium-to-aluminium ratio of the coating and the number of heterogeneities in the coating. Electrochemical porosity measurements which are essential for the estimation of the corrosion resistance of coated components were made~inI N sulphuric acid solution at ambient temperature. In these conditions, corrosion of a coated steel part occurs mainly in the base material. The corrosion resistance was also evaluated with immersion tests. The results of this work suggest the dominating role of the inhomogeneities on the corrosion resistance of the PVD (Ti, Al)N and TiN coatings. Preliminary analysis of the nature of the inhomogeneities is made and their possible source is discussed.
1. Introduction Hard coatings obtained by physical vapour deposition (PVD) have been successfully used for several years in many applications requiring wear resistance. In particular TiN has improved the performance of tools and machine parts considerably. In recent years, further improvements have been obtained with alloyed coatings such as (Ti, Al)N which have shown even better performance in metal cutting than TiN coatings do [1—3]. Titanium-based ceramics such as TiN have inherently good corrosion properties too and, consequently, the possibility of using PVD coatings for corrosion protection has attracted interest (see for example refs. 4 and 5). In practice, the corrosion resistance of the coatings has been found to be relatively poor; this has generally been related to pinholes in the coatings [5]. High porosity can induce corrosion beneath the coating. The porosity of the coating depends on the coating material composition and on the course of manufacturing, Different methods for increasing the corrosion resistance of PVD coatings have been suggested including deposition of an intermediate metal layer [6—8],pulsing of nitrogen flow, rotation of specimen and separate sputtering [9] as well as post-deposition methods such as passivation [9, 10]. In this study the porosity and the corrosion behaviour of TiN and (Ti, Al)N coatings with different thicknesses and compositions of the coating and inter-
mediate layer were determined. TiN coatings were produced commercially and were mainly used as a reference material. Special attention was paid to the microstructural factors affecting the corrosion behaviour of the coatings.
2. Experimental details The substrate material was powder metallurgically produced ASP 23 high speed steel which was hardened to a Rockwell C hardness of 64 HRC. Samples were prepared to a surface roughness Ra of 0.04 ~.tm.The nominal composition of ASP 23 high speed steel is 1.15—1.25 wt.% C, 3.5—4.5 wt.% Cr, 4.6—5.3 wt.% Mo, 2.7—3.2 wt.% V, 5—6 wt.°AW and less than 1 wt.% Co. An ionization-assisted electron beam PVD method was used for the deposition of the coatings. (Ti, Al)N coatings were deposited by means of a dual-source coating unit. An electron beam was used for the evaporation of titanium block and resistive evaporation for the evaporation of aluminium wire. The samples were sputter cleaned for 45 mm prior to deposition. The total pressure during the deposition was 0.62—0.67 Pa, the substrate bias was 100 V and the current density was about 1.25 mA cm2. Titanium and aluminium were evaporated simultaneously for about 45 mm. An intermediate metal layer having a thickness of approximately 100 nm was deposited except when the influence
Elsevier Sequoia, Lausanne
J. Arornaa ci al. / Aqueous corrosion properties of (Ti, Al)N-coatecl steel
354
of the intermediate layer thickness itself was studied. The temperature during the deposition was about 400— 500 °C.The TiN coatings used in this study were cornmercially produced. The coatings were investigated using scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDXA). Both coating surfaces and crosssections were studied. The compositions of selected (Ti, AI)N coatings were determined by Rutherford backscattering spectroscopy (RBS). Coating thickness measurements were carried out using the ball crater method [111. A kerosene diamond suspension was used as the abrasive medium and the coating thickness was assessed using a standard formula based on measurement of the crater dimensions and the ball diameter [12]. Electrochemical measurements were applied to estimate the porosity and the general corrosion behaviour of the coated material. Measurements were carried out with a PARC 273 electrochemical measurement system with an automatic current interrupt iR drop compensation and using SOFTCORR 342 corrosion measurement software. Experiments were performed in 1 N (0.5 M) sulphuric acid solution at room temperature. The sample area varied between 0.2 and 1 cm2 and the cell volume was 150 ml. All potentials were measured with respect to a saturated calomel electrode (SCE). For each coated sample as well as for the substrate material the corrosion potential Ecorr, the polarization resistance R~,the Tafel slopes and the corrosion current density Jcorr were determined and the anodic polarization curve was measured. During corrosion potential measurements the sample was allowed to stay in solution until Ecorr remained within ±2 mY for a time of 5 mm. The polarization resistance was measured within a potential domain of Ecorr ±10 mY using a sweep rate of 2 mV mm Tafel curves were measured using a sweep rate of 10 mV mm and the calculation of the corrosion current density and Tafel slopes was made by a PARCALC Tafel analysis program included in the measurement software. Cyclic anodic polarization curves were measured from the corrosion potential to 2 threshold current +2000 mY (SCE) using 5 mA cm density. The sweep rate was 100 mY mm Experience has shown that in relatively good quality coatings no significant changes occur at the sample surface before the measurement on the anodic polarization curve [13, 14]. Typically three to five measurements on each type of sample were made. The reproducibility of the corrosion current density measurement is typically of the order of ±lO%. Porosity was calculated using the
growth of the corrosion pits. The specimens were mounted in non-conductive plastic, washed with alcohol and then covered with non-conductive lacquer so that the coating area exposed to the environment was between 0.2 and 1 cm2. The immersion tests were made at ambient temperature and the duration of the tests was 5 h for studying the initiation of corrosion and 24 h for evaluating the pit densities.
3. Results and discussion 3.1. Coating structure The structure of the coatings studied was slightly columnar and dense, except that some commercial TiN coatings were clearly columnar and the thicker TiN coatings had a layered structure. SEM inspection showed that all the coatings contained some kinds of surface defect. These consisted of scratches or deep grooves resulting from machining of the substrate material, dents or crevices with sharp edges (these were noticed only on the commercial TiN coatings) and various small nodules or particles. Because of the small thickness of the film the scratches resulting from machining of the substrate material are often clearly visible. The coating, however, does deposit well over these scratches. All samples had a variable density of particle-like or nodular heterogeneities on the surface. Either round nodules with a diameter of 1—10 ~tm or irregular sharp particles with a size of about 5—10 ~.tmwere detected. The coating composition and the deposition method seemed to affect the size, shape and number of defects. The aluminium-to-titanium evaporation ratios of 0.6, 1.2 and 2.4 were found to result in coating compositions of Ti 0 3A102N05, Ti0 15Al0 1N055 and Ti0 1A104N05 respectively, as measured by RBS.
- ~.
-‘
-
.
measured R~and given in ref. 15. Ecorr values according to the method Immersion tests were carried out in aerated 1 N (0.5 M) sulphuric acid to detect the initiation and
3.2. Electrochemical measurements The meaning of the term porosity of a coating is ambiguous. However, from the corrosion point of view, the relative area of the reacting base material accessible via cracks, during voids etc. either existing in the coating or produced exposure is important. Therefore, when studying the corrosion resistance of a coated system, it is convenient to define the porosity of the coating as the ratio of the exposed area of the base material to the total area of the sample. The porosity of a coating/substrate system can be estimated if the electrochemical properties of the base material and the coating differ sufficiently. The base material ASP 23 was found to dissolve rapidly in 1 N H 2S04 with 2 a corrosion current density Jcorrnitride = 12.6 coating mA cm is and Ecor. = —428 ± 10 mY. Since the inactive at low overpotentials [14—16], the measured corrosion current results from the corroding base
J. Aromaa ci a!.
material. Because it was found that
/ Aqueous corrosion properties of
(Ti, Al)N-coc:ted steel
355
3.4. Effect of the intermediate layer When depositing nitride films a thin metallic intermemeasurements, the fcorr values can be taken as represen- diate layer is usually deposited on the base material tative of the porosities of the coated samples. The before deposition of the nitride film. The main purpose porosity values calculated from R~and Ecorr data ac- of this layer is to improve the adhesion of the coating. cording to the method given in ref. 15 showed similar By varying the composition and thickness of the intertendencies as the fcor. values so that only the Jcorr values mediate layer it may be possible to improve the corrowill be presented in the following. sion resistance of the coated material. In this study the The anodic polarization curves of the coated samples thickness of the intermediate layer was varied from showed similar features to those of the base material approximately 50 to 200 nm and both pure titanium except that the current densities were orders of magni- and aluminium intermediate layers were deposited. tude lower. Some examples of the curves as well as R~ Figure 2 shows the effect of varying the intermediate and Ecorr values can be found in ref. 17. layer composition and thickness on the corrosion current density. These results suggest that the increase in 3.3. Effect of coating thickness the metallic intermediate layer thickness at least above Figure 1 shows the measured corrosion current densi- 100 nm is slightly detrimental. The negative effect of ties for TiN coatings deposited under the same condi- increasing layer thickness is clearer for aluminium than tions and for (Ti, Al)N coatings having somewhat for titanium. This suggests that the porosity of the different deposition parameters and compositions. Most coating is increased with increasing intermediate layer TiN samples had coating thicknesses in the 4—7 iim thickness. The use of a metallic nickel and chromium range and one experimental specimen had a coating intermediate layer has earlier been found to improve thickness of 18 l.tm. The coating thickness of the the corrosion resistance of PYD coatings [61. (Ti, Al)N samples varied between 2 and 5 ~tm. These results show that the corrosion resistance can 3.5. Effect of coating composition be somewhat increased by increasing the coating thickEven though the (Ti, Al)N coatings studied have ness, at least within the range studied. However, it been found to be essentially of a single-phase nature should be pointed out that for applications requiring [19], it was expected that the coating composition wear resistance the thickness of a nitride coating gener- would have an effect on the electrochemical properties ally cannot be increased over 5—6 ~tm because of the of the coating/substrate system. The results on the effect brittleness of the coating [181. The large scatter in the of the aluminium-to-titanium evaporation ratio on the measured corrosion current densities is characteristic of corrosion current density of the coating are shown in the studied coatings, especially the thin coatings for Fig. 3. The results indicate that coatings with a high which the scatter in porosity is large even for coatings aluminium-to-titanium evaporation ratio, i.e. aludeposited under the same conditions. This scatter minium-rich coatings, have a slightly higher porosity reflects strong variations in the coating properties from and consequently worse corrosion resistance than the one site to another in the same sample. titanium-rich coatings. However, the scatter in fcor. Eeorr
values did not
vary systematically with Jcorr within the accuracy of the
S Ti +
OAI
OTiN +
+ (Ti,A1)N
\
102
0 ~
~2
8
u
0
$
10’
~
0
10
‘._.......86
8 0
100
.
0
10
10
20
Thickness/jsm Fig. 1. Corrosion current density in I N H,S04 as a function of TiN and (Ti, Al)N coating thickness.
100 150 200 Inte*~mediatelayer thickness / nm
50
Fig. 2. Corrosion current density in I N H
250
2S04 as a function of intermediate aluminium or titanium layer thickness. The coating thickness is approximately 4 .im and the aluminium-to-titanium evaporation ratio 1.1.
J. Aro,naa ci al. / Aqueous corrosion properties of (Ti, Al)N—coc,ted stc’el
356 10~
tors other than the composition of the coating dominate
(+)
the corrosion resistance of the (Ti, Al)N coatings.
(+) 10~
Immersion tests Immersion tests were carried out to detect the initiation and growth of the corrosion pits in order to clarify the nature of the occurrence of the corrosion. All the samples 3.6.
+ +
~
10
2
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+
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+
+
+ +
.
+ + +
.
2 Al/Ti evaporation ratio
Fig. 3. Corrosion current density in I N H,S0
studied showed visible corrosion damage after immersion for only a few hours. In all cases where the initiation site could be identified, corrosion was found to start at some clearly visible local inhomogeneity or damage in the coating. Rapid gas evolution from these sites was noticed during immersion. Figures 4(a), 4(b), 4(c) and 4(d) show
between different samples having a similar aluminium-totitanium evaporation ratio is large again and, on the basis of these results only, the effect of the composition cannot fully be clarified. This indicates that fac-
examples of the surfaces of a (Ti, A1)N sample and a TiN sample On as-coated before and samples after a(Figs. 5 h immersion 4(a) andtest 4(b)), respectively. coating inhomogeneities are clearly visible. During the immersion tests, corrosion was found to initiate mainly on these inhomogeneities and to propagate under the coating in the substrate until the unsupported coating collapses as shown in Figs. 4(c) and 4(d). The initiation sites of the
(a)
(b)
(c)
(d)
4 as a function of the aluminium-to-titanium 4pm. evaporation ratio. The coating thickness varies from 3 to
Fig. 4. (a) An as-coated (Ti, AI)N sample and (b) an as-coated TiN sample, (c) a (Ti, Al)N-coated sample after a 5 h immersion test and (d) a TIN-coated sample after a 5 h immersion test. The initiation site of the corrosion failure in the middle of the collapsed areas should be noted.
J. Aromac: et a!. / Aqueous corrosion propertie.s of (Ti, Al)N-coated steel
failures are in the middle of the collapsed coating areas and are marked with an arrow. In brief, the primary corrosion mechanism of the (Ti, Al)N- and TiN-coated steel samples was as follows. First the base material dissolves at a coating defect and a corrosion pit is formed under the coating. In the second phase the pit grows until the coating collapses or, since the base material next to the interface may corrode more rapidly, the coating delaminates and the corrosion is further accelerated. Because the vast majority of the corrosion initiation sites that could be identified after the immersion tests were related to visible coating inhomogeneities, more attention was paid to these. Role of surface defects in corrosion The effect of the density of surface inhomogeneities on the coating porosity was studied more closely comparing SEM micrographs taken before and after the measurement of the corrosion current density. These evaluations are tedious and time consuming and the number of coatings selected for these experiments had to be kept small. Micrographs were taken using a magnification of 100 x which is high enough to show the irregularities on the coating surface but low enough to cover a relatively large area of the sample surface. The data related to these experiments are given in Table 1 for both TiN and (Ti, Al)N coatings. These data indicate qualitatively that the number of inhomogeneities (defect density), density of the failures (pit density) and the measured corrosion current Jcorr as well as the calculated porosity are interrelated such that, for a given type of coating, both the density of pits and concomitantly the measured corrosion current density (and porosity) increase with the increasing density of coating inhomogeneities. It should be pointed out that the data do not take into account the type and size of corrosion damage which varies in different types of coating. Therefore, a direct comparison of pit densities and measured corrosion current densities should not be made. In any case, only a fraction of the coating surface defects or “inhomogeneities” serve as an initiation site for corrosion. SEM inspection and EDXAs of the sample surfaces before and after immersion tests could not clarify why 3.7.
357
some of the “inhomogeneities” lead to initiation of base material corrosion and others not. Within the accuracy obtainable with EDXAs no difference between the coating and the defect could be detected from the surface, suggesting that the clusters or particles were areas of distorted coating growth. Therefore samples were moulded in plastic, cut and polished to prepare crosssections of the coatings. Cross-sections were inspected with SEM to find the defects which were then subjected to EDXA using element concentration maps of the primary elements of the system (aluminium, titanium, tungsten and iron). Making a cross-section of a defect was found difficult because the coating had a tendency to crack at the defect during the sample preparation. Figure 5 shows an example of a cross-section of (Ti, Al)N coating with an “inhomogeneity” and the corresponding concentration maps. The particle in this case seems to have a nucleus of iron and tungsten and it is coated with titanium and aluminium, obviously with the deposited (Ti, Al)N coating. The coating is clearly damaged at the particle edges, and it is obvious that these open paths allow free transfer of the corrosive to the base material surface. Other cases were found where the nodule did not extend through the coating. This may explain why only some of the nodules act as initiation sites for corrosion and why the corrosion resistance seems to increase with increasing coating thickness. It seems reasonable that the number of inhomogeneities extending through the coating decreases as the coating thickness or th~enumber of layers is increased. 3.8. Formation of surface defects Nodular inhomogeneities could be found on both sides of the disc specimen. This indicates that such inhomogeneities are formed from the plasma during the deposition process. The side which faced down towards the filament and evaporation sources had many more nodules than the other side which was in shadow. It thus seems that the source-to-substrate spacing plays a role in the formation of these nodules and that the nodules probably result from the distorted growth of coating. It is possible that they may also contain solid impurities and droplets which originate from coating
TABLE I. Density of nodular “inhomogeneities”, corrosion pits, corrosion current density, corrosion potential and calculated porosity of some TiN and (Ti, Al)N coatings, together with the coating thicknesses and aluminium-to-titanium evaporation ratios Coating
Thickness (jim)
Al-to-Ti ratio
Defect density 2) (cm
Pit density (cm2)
(Ti, Al)N (Ti,Al)N (Ti, AI)N TiN TiN TiN
3.3 3.3 3.4 18 4.2 5.3
0.56 1.40 2.40
1000 1200 6700 7000 10000 18000
700 1100 2300 2 93 950
./~.orr
(jiA cm2) 7.8 6.6 28 8 24 35
E=rr
(mY (SCE))
Porosity [15]
—413 —421 —427 —392 —408 —425
6.88 x 7.l4x 2.70 x I.22x 3.28 x 3.51 x
l0~ lO~ lO~ lO~~ I0~ lO~
358
J. Aromaa ci a!. / Aqueous corrosion properties of (Ti, Al)N-coated steel
the failures and the measured corrosion current are interrelated such that, for a given type of coating, both the density of pits and concomitantly the measured corrosion current density increase with increasing density of coating defects.
Acknowledgments The authors are grateful to Mr. Simo Varjus for carrying out the deposition experiments, to Mr. Arto Kukkonen and Mr. Tom Gustafsson for the SEM analyses and to Mr. Jaakko Saarilahti for the RBS analyses.
References
Fig. 5. A cross-section of a (Ti, Al)N sample and corresponding concentration maps of aluminium, titanium, iron and tungsten.
chamber walls and fixtures, evaporation material sources, filaments etc. Nodular surface morphology of stainless steel films has been found to depend on the source-to-substrate spacing and, when formed, to decrease the corrosion resistance of the films [20]. Furthermore, several of those nodules or particles that were found to serve as initiation sites for corrosion in our work seemed to have formed on a surface scratch. Thus the surface roughness of the substrate may also play a role in the formation of the defects. However, more work is needed to determine statistically the nature of particularly those heterogeneities which result in the initiation of corrosion.
4. Conclusions The aqueous corrosion resistance ofTiN and (Ti, Al)N coating/steel substrate system can be somewhat increased by increasing the coating thickness, but the thickness and the composition of an intermediate aluminium or titanium layer as well as the composition of the (Ti, Al)N coating have only a minor effect on the resistance. Clearly visible local coating defects serve as initiation sites for corrosion. The number of defects, the density of
I W-D. Münz, J. Vac. Sci. Technol. A, 4 (1986) 2717. 2 0. Knotek, W-D. Münz and T. Leyendecker, J. Vac. Sci. Technol. A. 5(1987) 2173. 3 0. Knotek, F. Löffler and A. Barimani, VDJ-Z., 132 (1990) 65. 4 A. Erdemir, W. B. Carter and R. F. Hochman, Mater. Sci. Eng., 69(1985) 89. 5 A. S. Korhonen, J. M. Molarius, E. Harju and 0. Forsén. Recent advances in the development of wear and corrosion resistant PYD-nitride coatings, 4th mt. c~onfon Surface Modification Techniques. Paris, November 6—9, 1990. 6 A. Leyland, H. J. Park and A. Matthews, Surf Coat. Technol., 43—44 (1990) 481. 7 J. Molarius, Structure and properties of titanium nitride and zirconium nitride thin films prepared by reactive ion plating, Dissertation, Helsinki University of Technology, Espoo, 1987. 8 M. Tavi, J. Aromaa, 0. Forsén and S. Yläsaari, Proc. 12th World Congr. on Surface Finishing. Paris, October 4—7, 1988, AITE, Paris, ~ 955—64. 9 U. K. Wiiala, I. M. Penttinen. A. S. Korhonen, J. Aromaa and E. Ristolainen, Surf Coat. Technol.. 41(1990) 191. 10 N. Nishida, U.S. Patent 4 226 082, October 7, 1980. II J. Valli, J. Palojärvi and U. Mäkelä, VTT Res. Notes 435, 1985, 22 pp. (Technical Research Centre of Finland). 12 H. Ronkainen, S. Varjus, K. Holmberg, K. Fancey, A. R. Pace, A. Matthews. B. Matthes and E. Broszeit, in D. Dowson, C. M. Taylor and M. Gode(eds.), Mechanics of Coatings, Proc. 17th Leeds—Lyon Symp. on Tribology, Elsevier, Amsterdam, 1990, pp. 453—463. 13 J. Aromaa, J. Kukkonen, 0. Forsén and S. Yläsaari, Proc. 11th mt. Congr. on Metallic Corrosion, Vol. 2, Florence. April 2—6, 1990, Associazione Italiana di Metallurgia, Milano, pp. 357—364. 14 M. Tavi, 0. Forsén and J. Aromaa, Mater. Sci. Forum, 44—45(1989) 15. 15 B. Elsener, A. Rota and H. Böhni, Mater. Sci. Forum, 44—45(1989) 29. 16 Y. Massiani, A. Medjahed, P. Gravier, L. Argéme and L. Fedrizzi, Thin Solid Films, 191 (1990) 305. 17 B. Matthes, E. Broszeit, J. Aromaa, H. Ronakainen, 5.-P. Hannula, A. Leyland and A. Matthews, Surf Coat. Technol., (1991). 18 E. Posti, and 1. Nieminen, Mater. Manuf Processes, 4 (1989) 19 239—252. H. Ronkainen, I. Nieminen, K. Holmberg, A. Leyland, A. Matthews, B. Matthes and E. Broszeit, Surf Coat. Technol., 49 (1991) 468—473. 20 B. A. Okorie, Thin Solid Films, 130 (1985) 237.
353
Identification of factors affecting the aqueous corrosion properties of (Ti, Al)N-coated steel J. Aromaa, H. Ronkainen, A. Mahiout and S.-P. Hannula Technical Research c’entre of Finland (Valtion Teknillinen Tutkimuskeskus), Metallimiehenkuja 4, SF-02150, Espoo (Finland)
Abstract Carbides, borides and nitrides of transition metals are increasingly used as coating materials because of their high hardnesses and excellent wear resistances. However, these coatings can also potentially be used in applications requiring corrosion resistance because of their inherent stability under a variety of aqueous conditions. Thin film coatings based on these materials are generally produced by physical vapour deposition (PVD) methods. Such coatings often exhibit porosity which usually is thought to result from pinholes formed during deposition. If local defects form a direct path between the corrosive environment and the base material, the corrosion protection is lost. In this work the influence of different factors on the corrosion resistance of PVD (Ti, Al)N coatings on type ASP 23 high speed steel was studied. The factors evaluated were the thickness of the coating, the thickness and the type of the intermediate layer, the titanium-to-aluminium ratio of the coating and the number of heterogeneities in the coating. Electrochemical porosity measurements which are essential for the estimation of the corrosion resistance of coated components were made~inI N sulphuric acid solution at ambient temperature. In these conditions, corrosion of a coated steel part occurs mainly in the base material. The corrosion resistance was also evaluated with immersion tests. The results of this work suggest the dominating role of the inhomogeneities on the corrosion resistance of the PVD (Ti, Al)N and TiN coatings. Preliminary analysis of the nature of the inhomogeneities is made and their possible source is discussed.
1. Introduction Hard coatings obtained by physical vapour deposition (PVD) have been successfully used for several years in many applications requiring wear resistance. In particular TiN has improved the performance of tools and machine parts considerably. In recent years, further improvements have been obtained with alloyed coatings such as (Ti, Al)N which have shown even better performance in metal cutting than TiN coatings do [1—3]. Titanium-based ceramics such as TiN have inherently good corrosion properties too and, consequently, the possibility of using PVD coatings for corrosion protection has attracted interest (see for example refs. 4 and 5). In practice, the corrosion resistance of the coatings has been found to be relatively poor; this has generally been related to pinholes in the coatings [5]. High porosity can induce corrosion beneath the coating. The porosity of the coating depends on the coating material composition and on the course of manufacturing, Different methods for increasing the corrosion resistance of PVD coatings have been suggested including deposition of an intermediate metal layer [6—8],pulsing of nitrogen flow, rotation of specimen and separate sputtering [9] as well as post-deposition methods such as passivation [9, 10]. In this study the porosity and the corrosion behaviour of TiN and (Ti, Al)N coatings with different thicknesses and compositions of the coating and inter-
mediate layer were determined. TiN coatings were produced commercially and were mainly used as a reference material. Special attention was paid to the microstructural factors affecting the corrosion behaviour of the coatings.
2. Experimental details The substrate material was powder metallurgically produced ASP 23 high speed steel which was hardened to a Rockwell C hardness of 64 HRC. Samples were prepared to a surface roughness Ra of 0.04 ~.tm.The nominal composition of ASP 23 high speed steel is 1.15—1.25 wt.% C, 3.5—4.5 wt.% Cr, 4.6—5.3 wt.% Mo, 2.7—3.2 wt.% V, 5—6 wt.°AW and less than 1 wt.% Co. An ionization-assisted electron beam PVD method was used for the deposition of the coatings. (Ti, Al)N coatings were deposited by means of a dual-source coating unit. An electron beam was used for the evaporation of titanium block and resistive evaporation for the evaporation of aluminium wire. The samples were sputter cleaned for 45 mm prior to deposition. The total pressure during the deposition was 0.62—0.67 Pa, the substrate bias was 100 V and the current density was about 1.25 mA cm2. Titanium and aluminium were evaporated simultaneously for about 45 mm. An intermediate metal layer having a thickness of approximately 100 nm was deposited except when the influence
Elsevier Sequoia, Lausanne
J. Arornaa ci al. / Aqueous corrosion properties of (Ti, Al)N-coatecl steel
354
of the intermediate layer thickness itself was studied. The temperature during the deposition was about 400— 500 °C.The TiN coatings used in this study were cornmercially produced. The coatings were investigated using scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDXA). Both coating surfaces and crosssections were studied. The compositions of selected (Ti, AI)N coatings were determined by Rutherford backscattering spectroscopy (RBS). Coating thickness measurements were carried out using the ball crater method [111. A kerosene diamond suspension was used as the abrasive medium and the coating thickness was assessed using a standard formula based on measurement of the crater dimensions and the ball diameter [12]. Electrochemical measurements were applied to estimate the porosity and the general corrosion behaviour of the coated material. Measurements were carried out with a PARC 273 electrochemical measurement system with an automatic current interrupt iR drop compensation and using SOFTCORR 342 corrosion measurement software. Experiments were performed in 1 N (0.5 M) sulphuric acid solution at room temperature. The sample area varied between 0.2 and 1 cm2 and the cell volume was 150 ml. All potentials were measured with respect to a saturated calomel electrode (SCE). For each coated sample as well as for the substrate material the corrosion potential Ecorr, the polarization resistance R~,the Tafel slopes and the corrosion current density Jcorr were determined and the anodic polarization curve was measured. During corrosion potential measurements the sample was allowed to stay in solution until Ecorr remained within ±2 mY for a time of 5 mm. The polarization resistance was measured within a potential domain of Ecorr ±10 mY using a sweep rate of 2 mV mm Tafel curves were measured using a sweep rate of 10 mV mm and the calculation of the corrosion current density and Tafel slopes was made by a PARCALC Tafel analysis program included in the measurement software. Cyclic anodic polarization curves were measured from the corrosion potential to 2 threshold current +2000 mY (SCE) using 5 mA cm density. The sweep rate was 100 mY mm Experience has shown that in relatively good quality coatings no significant changes occur at the sample surface before the measurement on the anodic polarization curve [13, 14]. Typically three to five measurements on each type of sample were made. The reproducibility of the corrosion current density measurement is typically of the order of ±lO%. Porosity was calculated using the
growth of the corrosion pits. The specimens were mounted in non-conductive plastic, washed with alcohol and then covered with non-conductive lacquer so that the coating area exposed to the environment was between 0.2 and 1 cm2. The immersion tests were made at ambient temperature and the duration of the tests was 5 h for studying the initiation of corrosion and 24 h for evaluating the pit densities.
3. Results and discussion 3.1. Coating structure The structure of the coatings studied was slightly columnar and dense, except that some commercial TiN coatings were clearly columnar and the thicker TiN coatings had a layered structure. SEM inspection showed that all the coatings contained some kinds of surface defect. These consisted of scratches or deep grooves resulting from machining of the substrate material, dents or crevices with sharp edges (these were noticed only on the commercial TiN coatings) and various small nodules or particles. Because of the small thickness of the film the scratches resulting from machining of the substrate material are often clearly visible. The coating, however, does deposit well over these scratches. All samples had a variable density of particle-like or nodular heterogeneities on the surface. Either round nodules with a diameter of 1—10 ~tm or irregular sharp particles with a size of about 5—10 ~.tmwere detected. The coating composition and the deposition method seemed to affect the size, shape and number of defects. The aluminium-to-titanium evaporation ratios of 0.6, 1.2 and 2.4 were found to result in coating compositions of Ti 0 3A102N05, Ti0 15Al0 1N055 and Ti0 1A104N05 respectively, as measured by RBS.
- ~.
-‘
-
.
measured R~and given in ref. 15. Ecorr values according to the method Immersion tests were carried out in aerated 1 N (0.5 M) sulphuric acid to detect the initiation and
3.2. Electrochemical measurements The meaning of the term porosity of a coating is ambiguous. However, from the corrosion point of view, the relative area of the reacting base material accessible via cracks, during voids etc. either existing in the coating or produced exposure is important. Therefore, when studying the corrosion resistance of a coated system, it is convenient to define the porosity of the coating as the ratio of the exposed area of the base material to the total area of the sample. The porosity of a coating/substrate system can be estimated if the electrochemical properties of the base material and the coating differ sufficiently. The base material ASP 23 was found to dissolve rapidly in 1 N H 2S04 with 2 a corrosion current density Jcorrnitride = 12.6 coating mA cm is and Ecor. = —428 ± 10 mY. Since the inactive at low overpotentials [14—16], the measured corrosion current results from the corroding base
J. Aromaa ci a!.
material. Because it was found that
/ Aqueous corrosion properties of
(Ti, Al)N-coc:ted steel
355
3.4. Effect of the intermediate layer When depositing nitride films a thin metallic intermemeasurements, the fcorr values can be taken as represen- diate layer is usually deposited on the base material tative of the porosities of the coated samples. The before deposition of the nitride film. The main purpose porosity values calculated from R~and Ecorr data ac- of this layer is to improve the adhesion of the coating. cording to the method given in ref. 15 showed similar By varying the composition and thickness of the intertendencies as the fcor. values so that only the Jcorr values mediate layer it may be possible to improve the corrowill be presented in the following. sion resistance of the coated material. In this study the The anodic polarization curves of the coated samples thickness of the intermediate layer was varied from showed similar features to those of the base material approximately 50 to 200 nm and both pure titanium except that the current densities were orders of magni- and aluminium intermediate layers were deposited. tude lower. Some examples of the curves as well as R~ Figure 2 shows the effect of varying the intermediate and Ecorr values can be found in ref. 17. layer composition and thickness on the corrosion current density. These results suggest that the increase in 3.3. Effect of coating thickness the metallic intermediate layer thickness at least above Figure 1 shows the measured corrosion current densi- 100 nm is slightly detrimental. The negative effect of ties for TiN coatings deposited under the same condi- increasing layer thickness is clearer for aluminium than tions and for (Ti, Al)N coatings having somewhat for titanium. This suggests that the porosity of the different deposition parameters and compositions. Most coating is increased with increasing intermediate layer TiN samples had coating thicknesses in the 4—7 iim thickness. The use of a metallic nickel and chromium range and one experimental specimen had a coating intermediate layer has earlier been found to improve thickness of 18 l.tm. The coating thickness of the the corrosion resistance of PYD coatings [61. (Ti, Al)N samples varied between 2 and 5 ~tm. These results show that the corrosion resistance can 3.5. Effect of coating composition be somewhat increased by increasing the coating thickEven though the (Ti, Al)N coatings studied have ness, at least within the range studied. However, it been found to be essentially of a single-phase nature should be pointed out that for applications requiring [19], it was expected that the coating composition wear resistance the thickness of a nitride coating gener- would have an effect on the electrochemical properties ally cannot be increased over 5—6 ~tm because of the of the coating/substrate system. The results on the effect brittleness of the coating [181. The large scatter in the of the aluminium-to-titanium evaporation ratio on the measured corrosion current densities is characteristic of corrosion current density of the coating are shown in the studied coatings, especially the thin coatings for Fig. 3. The results indicate that coatings with a high which the scatter in porosity is large even for coatings aluminium-to-titanium evaporation ratio, i.e. aludeposited under the same conditions. This scatter minium-rich coatings, have a slightly higher porosity reflects strong variations in the coating properties from and consequently worse corrosion resistance than the one site to another in the same sample. titanium-rich coatings. However, the scatter in fcor. Eeorr
values did not
vary systematically with Jcorr within the accuracy of the
S Ti +
OAI
OTiN +
+ (Ti,A1)N
\
102
0 ~
~2
8
u
0
$
10’
~
0
10
‘._.......86
8 0
100
.
0
10
10
20
Thickness/jsm Fig. 1. Corrosion current density in I N H,S04 as a function of TiN and (Ti, Al)N coating thickness.
100 150 200 Inte*~mediatelayer thickness / nm
50
Fig. 2. Corrosion current density in I N H
250
2S04 as a function of intermediate aluminium or titanium layer thickness. The coating thickness is approximately 4 .im and the aluminium-to-titanium evaporation ratio 1.1.
J. Aro,naa ci al. / Aqueous corrosion properties of (Ti, Al)N—coc,ted stc’el
356 10~
tors other than the composition of the coating dominate
(+)
the corrosion resistance of the (Ti, Al)N coatings.
(+) 10~
Immersion tests Immersion tests were carried out to detect the initiation and growth of the corrosion pits in order to clarify the nature of the occurrence of the corrosion. All the samples 3.6.
+ +
~
10
2
÷
÷4 + +
+
+•~
to’ 100
+
+
+ +
.
+ + +
.
2 Al/Ti evaporation ratio
Fig. 3. Corrosion current density in I N H,S0
studied showed visible corrosion damage after immersion for only a few hours. In all cases where the initiation site could be identified, corrosion was found to start at some clearly visible local inhomogeneity or damage in the coating. Rapid gas evolution from these sites was noticed during immersion. Figures 4(a), 4(b), 4(c) and 4(d) show
between different samples having a similar aluminium-totitanium evaporation ratio is large again and, on the basis of these results only, the effect of the composition cannot fully be clarified. This indicates that fac-
examples of the surfaces of a (Ti, A1)N sample and a TiN sample On as-coated before and samples after a(Figs. 5 h immersion 4(a) andtest 4(b)), respectively. coating inhomogeneities are clearly visible. During the immersion tests, corrosion was found to initiate mainly on these inhomogeneities and to propagate under the coating in the substrate until the unsupported coating collapses as shown in Figs. 4(c) and 4(d). The initiation sites of the
(a)
(b)
(c)
(d)
4 as a function of the aluminium-to-titanium 4pm. evaporation ratio. The coating thickness varies from 3 to
Fig. 4. (a) An as-coated (Ti, AI)N sample and (b) an as-coated TiN sample, (c) a (Ti, Al)N-coated sample after a 5 h immersion test and (d) a TIN-coated sample after a 5 h immersion test. The initiation site of the corrosion failure in the middle of the collapsed areas should be noted.
J. Aromac: et a!. / Aqueous corrosion propertie.s of (Ti, Al)N-coated steel
failures are in the middle of the collapsed coating areas and are marked with an arrow. In brief, the primary corrosion mechanism of the (Ti, Al)N- and TiN-coated steel samples was as follows. First the base material dissolves at a coating defect and a corrosion pit is formed under the coating. In the second phase the pit grows until the coating collapses or, since the base material next to the interface may corrode more rapidly, the coating delaminates and the corrosion is further accelerated. Because the vast majority of the corrosion initiation sites that could be identified after the immersion tests were related to visible coating inhomogeneities, more attention was paid to these. Role of surface defects in corrosion The effect of the density of surface inhomogeneities on the coating porosity was studied more closely comparing SEM micrographs taken before and after the measurement of the corrosion current density. These evaluations are tedious and time consuming and the number of coatings selected for these experiments had to be kept small. Micrographs were taken using a magnification of 100 x which is high enough to show the irregularities on the coating surface but low enough to cover a relatively large area of the sample surface. The data related to these experiments are given in Table 1 for both TiN and (Ti, Al)N coatings. These data indicate qualitatively that the number of inhomogeneities (defect density), density of the failures (pit density) and the measured corrosion current Jcorr as well as the calculated porosity are interrelated such that, for a given type of coating, both the density of pits and concomitantly the measured corrosion current density (and porosity) increase with the increasing density of coating inhomogeneities. It should be pointed out that the data do not take into account the type and size of corrosion damage which varies in different types of coating. Therefore, a direct comparison of pit densities and measured corrosion current densities should not be made. In any case, only a fraction of the coating surface defects or “inhomogeneities” serve as an initiation site for corrosion. SEM inspection and EDXAs of the sample surfaces before and after immersion tests could not clarify why 3.7.
357
some of the “inhomogeneities” lead to initiation of base material corrosion and others not. Within the accuracy obtainable with EDXAs no difference between the coating and the defect could be detected from the surface, suggesting that the clusters or particles were areas of distorted coating growth. Therefore samples were moulded in plastic, cut and polished to prepare crosssections of the coatings. Cross-sections were inspected with SEM to find the defects which were then subjected to EDXA using element concentration maps of the primary elements of the system (aluminium, titanium, tungsten and iron). Making a cross-section of a defect was found difficult because the coating had a tendency to crack at the defect during the sample preparation. Figure 5 shows an example of a cross-section of (Ti, Al)N coating with an “inhomogeneity” and the corresponding concentration maps. The particle in this case seems to have a nucleus of iron and tungsten and it is coated with titanium and aluminium, obviously with the deposited (Ti, Al)N coating. The coating is clearly damaged at the particle edges, and it is obvious that these open paths allow free transfer of the corrosive to the base material surface. Other cases were found where the nodule did not extend through the coating. This may explain why only some of the nodules act as initiation sites for corrosion and why the corrosion resistance seems to increase with increasing coating thickness. It seems reasonable that the number of inhomogeneities extending through the coating decreases as the coating thickness or th~enumber of layers is increased. 3.8. Formation of surface defects Nodular inhomogeneities could be found on both sides of the disc specimen. This indicates that such inhomogeneities are formed from the plasma during the deposition process. The side which faced down towards the filament and evaporation sources had many more nodules than the other side which was in shadow. It thus seems that the source-to-substrate spacing plays a role in the formation of these nodules and that the nodules probably result from the distorted growth of coating. It is possible that they may also contain solid impurities and droplets which originate from coating
TABLE I. Density of nodular “inhomogeneities”, corrosion pits, corrosion current density, corrosion potential and calculated porosity of some TiN and (Ti, Al)N coatings, together with the coating thicknesses and aluminium-to-titanium evaporation ratios Coating
Thickness (jim)
Al-to-Ti ratio
Defect density 2) (cm
Pit density (cm2)
(Ti, Al)N (Ti,Al)N (Ti, AI)N TiN TiN TiN
3.3 3.3 3.4 18 4.2 5.3
0.56 1.40 2.40
1000 1200 6700 7000 10000 18000
700 1100 2300 2 93 950
./~.orr
(jiA cm2) 7.8 6.6 28 8 24 35
E=rr
(mY (SCE))
Porosity [15]
—413 —421 —427 —392 —408 —425
6.88 x 7.l4x 2.70 x I.22x 3.28 x 3.51 x
l0~ lO~ lO~ lO~~ I0~ lO~
358
J. Aromaa ci a!. / Aqueous corrosion properties of (Ti, Al)N-coated steel
the failures and the measured corrosion current are interrelated such that, for a given type of coating, both the density of pits and concomitantly the measured corrosion current density increase with increasing density of coating defects.
Acknowledgments The authors are grateful to Mr. Simo Varjus for carrying out the deposition experiments, to Mr. Arto Kukkonen and Mr. Tom Gustafsson for the SEM analyses and to Mr. Jaakko Saarilahti for the RBS analyses.
References
Fig. 5. A cross-section of a (Ti, Al)N sample and corresponding concentration maps of aluminium, titanium, iron and tungsten.
chamber walls and fixtures, evaporation material sources, filaments etc. Nodular surface morphology of stainless steel films has been found to depend on the source-to-substrate spacing and, when formed, to decrease the corrosion resistance of the films [20]. Furthermore, several of those nodules or particles that were found to serve as initiation sites for corrosion in our work seemed to have formed on a surface scratch. Thus the surface roughness of the substrate may also play a role in the formation of the defects. However, more work is needed to determine statistically the nature of particularly those heterogeneities which result in the initiation of corrosion.
4. Conclusions The aqueous corrosion resistance ofTiN and (Ti, Al)N coating/steel substrate system can be somewhat increased by increasing the coating thickness, but the thickness and the composition of an intermediate aluminium or titanium layer as well as the composition of the (Ti, Al)N coating have only a minor effect on the resistance. Clearly visible local coating defects serve as initiation sites for corrosion. The number of defects, the density of
I W-D. Münz, J. Vac. Sci. Technol. A, 4 (1986) 2717. 2 0. Knotek, W-D. Münz and T. Leyendecker, J. Vac. Sci. Technol. A. 5(1987) 2173. 3 0. Knotek, F. Löffler and A. Barimani, VDJ-Z., 132 (1990) 65. 4 A. Erdemir, W. B. Carter and R. F. Hochman, Mater. Sci. Eng., 69(1985) 89. 5 A. S. Korhonen, J. M. Molarius, E. Harju and 0. Forsén. Recent advances in the development of wear and corrosion resistant PYD-nitride coatings, 4th mt. c~onfon Surface Modification Techniques. Paris, November 6—9, 1990. 6 A. Leyland, H. J. Park and A. Matthews, Surf Coat. Technol., 43—44 (1990) 481. 7 J. Molarius, Structure and properties of titanium nitride and zirconium nitride thin films prepared by reactive ion plating, Dissertation, Helsinki University of Technology, Espoo, 1987. 8 M. Tavi, J. Aromaa, 0. Forsén and S. Yläsaari, Proc. 12th World Congr. on Surface Finishing. Paris, October 4—7, 1988, AITE, Paris, ~ 955—64. 9 U. K. Wiiala, I. M. Penttinen. A. S. Korhonen, J. Aromaa and E. Ristolainen, Surf Coat. Technol.. 41(1990) 191. 10 N. Nishida, U.S. Patent 4 226 082, October 7, 1980. II J. Valli, J. Palojärvi and U. Mäkelä, VTT Res. Notes 435, 1985, 22 pp. (Technical Research Centre of Finland). 12 H. Ronkainen, S. Varjus, K. Holmberg, K. Fancey, A. R. Pace, A. Matthews. B. Matthes and E. Broszeit, in D. Dowson, C. M. Taylor and M. Gode(eds.), Mechanics of Coatings, Proc. 17th Leeds—Lyon Symp. on Tribology, Elsevier, Amsterdam, 1990, pp. 453—463. 13 J. Aromaa, J. Kukkonen, 0. Forsén and S. Yläsaari, Proc. 11th mt. Congr. on Metallic Corrosion, Vol. 2, Florence. April 2—6, 1990, Associazione Italiana di Metallurgia, Milano, pp. 357—364. 14 M. Tavi, 0. Forsén and J. Aromaa, Mater. Sci. Forum, 44—45(1989) 15. 15 B. Elsener, A. Rota and H. Böhni, Mater. Sci. Forum, 44—45(1989) 29. 16 Y. Massiani, A. Medjahed, P. Gravier, L. Argéme and L. Fedrizzi, Thin Solid Films, 191 (1990) 305. 17 B. Matthes, E. Broszeit, J. Aromaa, H. Ronakainen, 5.-P. Hannula, A. Leyland and A. Matthews, Surf Coat. Technol., (1991). 18 E. Posti, and 1. Nieminen, Mater. Manuf Processes, 4 (1989) 19 239—252. H. Ronkainen, I. Nieminen, K. Holmberg, A. Leyland, A. Matthews, B. Matthes and E. Broszeit, Surf Coat. Technol., 49 (1991) 468—473. 20 B. A. Okorie, Thin Solid Films, 130 (1985) 237.