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Corrosion of zirconium boride and zirconium boron nitride coated steels
ELSEVIER
Surfaceand Coatings Technology71 (1995)60-66
Corrosion of zirconium boride and zirconium boron nitride coated steels M. Urgen ", A.F. (~aklr ", O.L. Eryllmaz a, C. Mitterer b a Technical University oflstanbul, Department of Metallurgical Engineering, 80626, AyazaSa, Istanbul, Turkey b Institutfiir Metallkunde und Werkstoffpr~ifung, Montanuniversitat, Franz-Josef-Strasse 18, A-8700 Leoben, Austria Received 10 September 1993; accepted in final form 26 May 1994
Abstract
The corrosion behaviour of sputtered zirconium boride and zirconium boron nitride coatings on carbon steel substrates was investigated. Coatings were produced employing non-reactive as well as reactive d.c. magnetron sputtering deposition using zirconium diboride targets. The copper decoration technique was used to make visible the types of coating defects and their distribution. To quantify the corrosion behaviour, potentiodynamic polarization measurements and electrochemical impedance spectroscopy were employed. It was found that the corrosion protection depends strongly on the porosity and adherence of the coatings. Zirconium boron nitride films are more promising in this aspect owing to their lower number of coating defects. Keywords: Zr-B; Zr-B-N; PVD coatings; Copper decorations; Corrosion properties
1. Introduction
Hard, wear-resistant coatings deposited by physical vapour deposition (PVD) techniques are well established for the protection of materials against wear. These coatings are mainly based on the nitrides and carbides of the transition metal elements. In addition, recently increasing interest is laid on the corresponding borides. The combination of the properties of these hard materials makes them a group of outstanding coating materials, e.g. for tribological or decorative applications, corrosion protective coatings, or diffusion barriers. While numerous investigations deal with the properties and tribological performance of hard coatings as a function of the deposition process and parameters, only a relatively limited number cover corrosion studies [ 1 ]. However, to enhance the service applications of hard coatings, it would be desirable to provide corrosion resistance of the coating material itself as well as corrosion protection for the substrate on which the coating is deposited. Therefore, a detailed understanding of the corrosion behaviour is necessary to avoid failure of the coating-substrate combination used in a given environment. In general, hard materials are noble with respect to steel substrates. Essential reasons responsible for corrosion of the coating-substrate system are defects extending through the coating and resulting from the deposition process or poor surface quality of the sub0010-8545/95/$09.50 © 1995 ElsevierScienceS.A. All rights reserved
SSD1 0010-8545(94)02316-1
strates. In addition, the coating structure with potential grain boundaries crossing the entire film, which permits diffusion to the substrate surface, may contribute in part to the corrosion behaviour [2]. It is well known that the presence of pinholes - - porosity in hard and wear resistant coatings - - changes their protective properties unfavourably [3,4]. Moreover, especially in coatings produced by arc PVD, macroparticle (droplet) type defects not only deteriorate the coated material's friction and decorative properties by producing extensive surface roughness [ 5,6] but also the corrosion protective properties as well [4,7,8]. In the literature, there are several approaches in relation to the determination of the defects extending through the coating, in hard wear-resistant ceramic coatings. One approach [ 9 - 1 1 ] is to estimate porosity from the electrochemical behaviour of the system, in which the sources of porosity are left unidentified. In an another approach [12] the area of visible defects such as 'pores, cracks and nodular cracks' on coated samples was estimated based on SEM observations without taking into account the invisible pores or pinholes. A third approach is the ferroxyl test [13], in which filter paper wetted by corrosive chloride solution and indicators for iron ions is put on the coated sample and the number of coloured stains formed on filter paper resulting from the dissolution of iron substrate from pinholes is counted; the number of stain spots gives a rough
61
M. (Jrgen et al. / Surface and Coatings Technology 71 (1995) 60 66
estimation of macro defects. A fourth approach is the decoration of the defect sites by noble metals such as platinum, gold and copper, and is valuable in making the defect sites visible. It has been applied to passive metals [14-16], metals covered with non-conductive coatings [ 17], anodized aluminium [ 18] and recently to hard wear-resistant coatings [7,8]. Examination of the decorated samples with SEM can give a detailed picture of the sources of porosity in hard and wearresistant coatings. The aim of this investigation was to evaluate the corrosion resistance of carbon steel substrates coated with zirconium boride and zirconium boron nitride. Earlier work indicated that these coatings may be interesting candidates for decorative and tribological applications [19-21], The copper decoration technique [7,8] was employed for the identification and evaluation of the various coating defects. The corrosion behaviour was investigated by a standard potentiodynamic polarization technique using sulphuric acid and NaC1 solutions. The polarization resistance of the samples was determined employing electrochemical impedance spectroscopy.
2. Experimental Carbon steel (Ck 35) discs of 30 mm diameter and 1 0 m m height were used as substrate materials. The surface of the substrates was prepared by wet grinding and then polishing with 1 gm diamond paste. Specimens were coated employing non-reactive (Zr-B) and reactive ( Z r - B - N ) d.c. magnetron sputtering using ZrB2. The addition of nitrogen caused a grain refinement resulting in a gradual shift of the film structure from crystalline (non-reactive deposition) to amorphous (reactive deposition at high nitrogen flow rates) [19-21]. Coating parameters and the thicknesses of the coatings are given in Table 1. Window glass substrates were also coated in the same manner as the steel substrates. The thickness of the coatings on glass substrates was approximately 1 txm. The thicknesses of the coatings were determined by examining the fracture surface of the coated samples with SEM. Coated specimens, after mechanically thin-
ning locally from the back and holding in liquid nitrogen for 5 min, were broken. 2.1. Decoration procedure
The principle of the decoration technique [7,8,14,15,17] is based on simple cementation. The coated materials are introduced into the solution containing copper ions which are noble relative to the steel substrate but base relative to nitride coatings. Copper preferentially deposits on defective, coating-free sites and decorates them while the substrate is oxidized to give away electrons that are consumed in the reduction process: Cu2+ +2e
--*Cu °
(1)
F e ~ F e 2+ + 2 e -
(2)
Zr B and Z r - B - N are both electronically conductive materials [22]. In order to check if copper precipitates on the coating material itself, a test was conducted. ZrB and Z r - B - N coated on glass (inert) substrates were dipped in copper sulphate decoration solution and the surfaces were examined by SEM. Copper decoration did not take place on ZrB and Z r - B - N coated on glass substrates. However, samples coated on steel substrates were easily copper decorated, indicating that only defects connecting the active substrate to the environment could be decorated. Before decoration, Z r - B and Zr B - N coated samples were degreased ultrasonically in 50% e t h e r + 5 0 % alcohol solution for 5 min. Decoration of the degreased specimens was conducted by dipping the specimens in copper sulfate solution at pH = 1 and containing 0.1 g 1 1 Cu2+, for 60s. After decoration, the decorated samples were immediately dipped in distilled water and then dried after washing with methanol. To optimize the decoration process, various copper concentrations and decoration times have been tested; however, the previous conditions were found to allow the decoration without heavy copper build-up. Decorated specimens were examined without any further treatment using a scanning electron microscope (Jeol T330).
Table 1 Zr-B and Zr-B-N coating parameters used Coating type
Sputtering power (W)
Total pressure (Pa)
Zr-B Zr-B N
600 600
1.1 1.1
Bias potential (V)
- -
- -
200 200
Substrate temperature (°C)
Argon f l o w rate (sccrn)
Nitrogen flow rate (sccm)
Coating thickness (rtm)
300 300
110 95
15
5 5
M. (]rgen et al. / Surface and Coatings Technology 71 (1995) 60-66
62
2.2. Electrochemical experiments A standard potentiodynamic polarization technique was used in the electrochemical experiments. The electrochemical measurement system consisted of a potentiostat (EG&G M273) controlled by an Apple IIe computer. The scan rates were 120 mV min-~ and 20 mV min 1 for the tests conducted in sulphuric acid and NaC1 solutions respectively. Experiments were conducted in nitrogen deaerated 1 N sulphuric acid and 3.5% NaC1 solutions at room temperature. The Zr-B and Z r - B - N coated steel samples were degreased ultrasonically in 50% ether+50% alcohol solution for 5min before the experiments. In order to determine the polarization resistance of the samples in 1 N sulphuric acid, electrochemical impedance spectroscopy (EIS) was used. The electrochemical impedance system consisted of a potensiostat (EG&G M273) and lock-in amplifier (EG&G 5315) controlled by an Apple IIe computer. Experiments were conducted at corrosion potential after waiting for 30 min for the stabilization of the corrosion potential. The a.c. potential amplitude and the frequency range were 10 mV and 105 Hz to 0.1 Hz respectively. All potentials were measured with respect to saturated calomel reference electrode (SCE).
ignited between the biased substrate and the wall of the vacuum chamber. The arc might evaporate the coating locally, resulting in craters. If the arc is ignited between the target and the wall of the chamber, target material might evaporate and incorporate into the growing film in the form of droplets. After decoration of the Zr-B coated steel samples it was observed that the peripheries of droplet-like particles and craters were heavily decorated with copper. In other studies conducted on arc PVD TiN and CrN coatings the droplets were also identified as important defect sites I-7,8]. Widespread string-type copper decoration, as stringers, was observed on ZrB coatings, revealing the inherent porosity of the coating (Figs. 2 and 3). Similar decoration characteristics were also observed on arc PVD TiN and CrN coated steels, indicating the presence of extensive porosity [7,8]. The major difference
3. Results and discussions
3.1. Decoration experiments The SEM image of the Zr-B and Z r - B - N coated steel samples showed that these coating were not free of defects. The defects appear as craters or droplets (Fig. ! ). Craters are formed during deposition as a result of arcs
Fig. 2. General view of copper-decorated Z r - B coated steel (white copper particles as stringers and as clusters in craters and at periphery of droplets).
Fig. 1. General view of Zr B coated steel with droplets,
Fig. 3. Copper decoration at the periphery of a droplet on Zr B coated steel: A, droplet; B, copper particles.
63
M. Urgen et al. / Surjace and Coatings Technology 71 (1995) 60 66
observed between decorated arc PVD CrN and TiN coated samples and ZrB coated samples was that in the case of the former coatings the decoration was more homogeneous and intense compared with ZrB coatings. In the case of Zr-B-N coated samples, only the peripheries of macroparticles and craters were decorated, indicating that these coatings were free of inherent porosity (Figs. 4 and 5). The presence of inherent porosity in Zr B coatings can be explained by the crystalline, fine columnar structure with voids formed at the grain boundaries and by the high hardness impeding stress relaxation [19 21]. The extremely fine-grained to amorphous structure of Zr-B N coatings prevents the formation of pinholes crossing the entire coating. In addition, stresses generated on the coating during the deposition process may be reduced, as indicated by the lower hardness.
3.2. Electrochemical experiments The cathodic and the anodic electrochemical behaviour of uncoated steel substrate, Zr B and Zr-B-N coated steel samples was investigated in 1 N sulphuric acid (Fig. 6). The corrosion potential of the Zr-B coated samples was close to the corrosion potential of the substrate. During the cathodic polarization, Zr--B coating on the steel samples blistered and almost a third of the coating was peeled off. During the subsequent anodic polarization, the behaviour of the coated steel did not show significant difference from the anodic behaviour of the uncoated steel. Therefore the polarization curve represented the electrochemical behaviour of the partially coated substrate material (Fig. 6). The debonding produced during cathodic polarization of the coated samples may be an indication of the low adhesion of these coatings to the substrate. Zr-B-N coated samples showed far better electrochemical corrosion behaviour when compared with Zr B coated and uncoated samples by decreasing the active corrosion rate, the critical current density %r, and the minimum passive current imi, (Table 2). Cathodic debonding was not observed in these coatings. However, the effect of the substrate steel was still observed on the anodic polarization curve: an active passive transition region in the same potential range as the substrate steel was present (Fig. 6), indicating that the coatings were not capable of covering the surface totally without physical discontinuities. Another set of experiments was conducted in 1 N sulphuric acid without cathodically polarizing the coated samples in order to verify the hydrogen debonding of
Fig. 4. Copper decoration as cluster at the periphery of a partially fallen droplet on a Zr B N coated steel: A, droplet; B, copper particles.
1100 I
600
.
~. 400
Substrate-
-900
....... I ....... I ...,...I
100
101
102
........ I ....... I
103
104
Current Density, i [ I~A/cm Fig. 5. Copper decoration of a crater on Zr B N coated steel: K, crater; B, copper particles,
105
.......
106
2]
Fig. 6. Cathodic and anodic polarization curves of Zr B, Z r - B - N coated steel samples and steel substrate in 1 N H2SO4.
64
M. (~rgenet aL / Surface and Coatings Technology 71 (1995) 60-66
Table 2 Corrosion potentials (Eeoc), critical current densities (ier) and minimum passive current densities of the substrate, Z r - B and Z r - B N coated samples obtained in 1 N sulphuric acid i= (mA cm -z)
imi. (mA cm 2)
- 534 - 504
266 1.93
3 0.10
- 556 -494
67.5 0.58
2 0.015
- 525
0.60
0.05
Eeorr VS SCE
Substrate Zr-B Zr-B (cathodically polarized) Zr-B-N Zr-B-N (cathodically polarized)
Table 3 Corrosion potential, polarization resistance of the substrate, and corrosion potentials, polarization resistance, percentage porosity of the Z r - B and Z r - B - N coated steel samples in 1 N sulphuric acid E¢o= vs SCE
Rp (~
Substrate Zr-B Zr-B-N
- 534 - 504 - 494
Porosity
cm-2)
9.2 532 12250
1600
1100
Substrate
g
600
g ~ -400
-9oo
....... I ,. ..... I ,,,,...I
10 o
101
102
....... I ........ I .....
103
104
105
106
Current Density, i [ I~ A / am 2 I Fig. 7. Anodic polarization curves of Zr B and Z r - B - N coated steel samples (without cathodic polarization) and cathodic+anodic polarization curve of steel substrate in 1 N H2SO 4.
0.67 0.021
lated (Table 3) using the following equation [10]: F = ( R v , m / R p ) 10-1AE.... Ilba
the Zr-B coated samples. Coating debonding was not observed during anodic polarization, as expected (Fig. 7). Under similar experimental conditions, Z r - B - N coated samples exhibited better corrosion behaviour than Zr-B coated samples, which was revealed both by lower icr and imin (Table 2). To determine the polarization resistance of the samples at the corrosion potential without disturbing (polarizing) the system significantly, the electrochemical impedance spectroscopy (EIS) technique was used. The polarization resistance Rp of the samples in 1 N sulphuric acid obtained by EIS is given in Table 3. The R v values are determined from Bode plots (Fig. 8). The Rp values showed significant differences between the substrate and coated samples. The Rp value of the Z r - B - N coated sample was 20-fold higher than that of the Zr-B coated samples, showing the better protective properties of the Z r - B - N coatings. The percentage of porosity in the coatings was calcu-
(%)
(3)
In Eq. (3), F is the total coating porosity, Rp, m is the polarization resistance of the substrate, Rp is the measured polarization resistance of the coated steel, AEeorr is the difference between the corrosion potentials of the substrate and the coated steel, and ba is the anodic Tafel slope of the substrate material (75 mV/decade, obtained from the anodic polarization curve). The percentages of porosity calculated by using a similar electrochemical approach were 0.014% for PVD coated and 0.4% for CVD coated TiN [9]. Depending on the immersion time of the samples in the corrosive solution, the porosity had changed between 0.1% and 0.4% for CVD coated TiN [ 11 ]. As a function of the surface roughness of the substrate, the porosity had changed between 0.29% and 2.42% for arc PVD coated TiN and 0.06%-0.26% for arc PVD coated CrN [23]. The percentage of porosity for Z r - B - N coatings determined in this study gave porosity (0.021%) comparable with that of the PVD coated TiN but it was lower than the calculated porosity of both arc PVD coated TiN and CrN. However, the percentage of porosity determined for ZrB coatings (0.67%) was higher than that of PVD coated TiN and arc PVD coated CrN. The results of the anodic polarization experiments conducted in 3.5% NaC1 solution are given in Fig. 9. Both Zr-B and Z r - B - N coatings improved the pitting behaviour of the substrate. However, as the experiments conducted in sulphuric acid previously showed, because of the defective nature of the coating, the elimination of pitting was not possible. These results are in accordance with the findings of a study conducted with Zr-B and Z r - B - N coated stainless steels in a similar medium [24].
4. Comparison of results of decoration and electrochemical experiments The electrochemical behaviour of Zr-B and Z r - B - N coated steel samples in 1 N sulphuric acid and 3.5% NaC1 showed that none of the coatings was capable of covering the surface totally without physical discontinu-
65
M. Urgen et al. / Surface and Coatings Technology 71 (1995) 60-66
• oOoO~oo~qJin~oo 0•
R"
E
•
o
SubstratezrB 1
L'z'",..
J~ 0 N
•
• • •••tt•t/k/tAAhtt, A4k• •
•
•
o
d 0 a0
"(3 0 E
• mmmmmnnmlllmmmm
•
•
•
•
•
• •
• •
I
|
0
1
0
-1
•
•
m
I
I
I
2
3
4
m
LogFrequency,[Hz]
m
m
5
Fig. 8. Bode plots of Zr-B, Zr-B N coated steel samples and steel substrate in 1 N H2SO4. -300 I
'
'
'
The decoration experiments not only supported the findings of the electrochemical tests, but they also made visible the types of defect and their distribution at the coating surface. A quantitative assessment of the porosity using the decoration experiment results was not attempted because of the uneven distribution of copper-decorated sites.
'
I
-400 I
~
ZrBN
-500
5. Conclusions o
i
o,e
-700 ~
-800 L 0
60
~ 120
180
240
300
C u r r e n t Density, i [ p. A / c m 2 ) Fig. 9. Cyclic polarization curves of Zr B, Zr-B N coated steel samples and steel substrate in 3.5% NaC1.
ities, which were made clearly visible by the copper decoration experiments. Z r - B - N coatings protected the substrate better than Zr-B coatings. The porosity of the Z r - B - N layers was at least 30-fold lower. The critical current densities and the polarization resistances of the coated samples were actually related to the critical current density and the polarization resistance of the exposed substrate. Hence, the lower critical current density and higher Rp of the Z r - B - N coated samples showed that they are less defective than Zr-B coated samples.
(1) Zr-B and Zr-B-N layers on steel substrate exhibited various types of defect, which decreased the protective properties of the coating. The copper decoration technique could be applied successfully to identify and make visible defects extending through the coating to the steel substrate. (2) Z r - B - N coated steel showed better corrosion resistance than the Zr-B coated steel because of the lower number of total coating defects. However, the 'small anodic-large cathodic' area relation on Z r - B - N may provoke severe localized corrosion. (3) The corrosion protection properties of these coatings depend on their porosity. Z r - B - N coatings are more promising in this aspect while they do not possess inherent porosity. Elimination of droplets and craters in Z r - B - N coatings could most probably produce corrosion-protective coatings.
References [1] H.A. Jehn and M.E. Baumgartner, Surf Coat. Technol., 54/55 (1992) 108.
66
M. (]rgen et al. / Surface and Coatings Technology 71 (1995) 60-66
[2] L. van Leaven, M.N. Alias and R. Brown, Surf Coat. Technol., 53 (1992) 25. [3] M.J. Park, A. Leyland and A. Matthews, Surf. Coat. Technol., 43/44 (1990) 481. [4] M. Nishibori, Surf. Coat. Technol., 52 (1992) 229. [5] H. Randbawa, Surf. Coat. Technol., 36 (1988) 829. [6] C.N. Tai and E.S. Koh, Surf. Coat. Technol., 43/44 (1990) 324. [7] M. 10rgen, A.F. (~aklr, O.L. Eryllmaz and K. Tokmano(glu, in Proc. 3rd Int. Ceramic Cong, 19 23 October 1992, Istanbul, p. 530. [8] M. Urgen, A.F. (~aklr and O.L. Eryllmaz, in Proc. 3rd EAST Conf. 18 19 November 1993, Schwgibisch Gmfind., Eugen G. Leuze Verlag, Salgau, in press. [9] B. Elsener, A. Rota and H. B6hni, Mater. Sci. Forum, 44/45 (1989) 28. [10] J. Pippo, B. Elsener and H. B6hni, Mater. Sei. Forum, 111-112 (1992) 219. [11] J. Pippo, B. Elsener and H. B6hni, Surf. Coat. Technol., 61 (1993) 43. [ 12] I.M. Penttinen, A.S. Korhonen, E. Harju, M.A. Turkia, O. Forsen and E.O. Ristolainen, Surf. Coat. Technol., 50 (1992) 161.
[13] J. Munemasa and T. Kumakiri, Surf Coat. Technol., 49 (1991) 496, [14] N.A. Nielsen, Corrosion, 20 (1964) 104t. [15] M. 1Jrgen, Corrs. Sci., 33 (1992) 1179. [16] E.M. Baker and A.M. Rente, Trans. Am. Electrochem. Soc., 54 (1928) 337. [173 I.M. Notter and D.R. Gabe, Corros. Rev., 10 (1992) 217. [18] ISO Standard, ISO 2085-1976 (E). [19] C. Mitterer, A. ~bleis and R. Ebner, Mater. Sci. Eng., A140 (1991) 670. [20] C. Mitterer, P. Losbichler, W.S.M. Werner, H. St6ri and J. Barounig, Surf. Coat. Technol., 54/55 (1992) 329. [21] E. Brandstetter, C. Mitterer and R. Ebner, Thin Solid Films, 201 (1991) 123. [22] J.R. Shappirio and J.J. Finnegan, Thin Solid Films, 107 (1983) 81. [23] M. l~lrgen, A.F. (~aklr and O.L. Eryllmaz, Surf. Coat. Technol, submitted for publication (1994). [24] A. Ubleis, C. Mitterer and R. Ebner, Surf. Coat. Technol., 60 (1993) 571.
Surfaceand Coatings Technology71 (1995)60-66
Corrosion of zirconium boride and zirconium boron nitride coated steels M. Urgen ", A.F. (~aklr ", O.L. Eryllmaz a, C. Mitterer b a Technical University oflstanbul, Department of Metallurgical Engineering, 80626, AyazaSa, Istanbul, Turkey b Institutfiir Metallkunde und Werkstoffpr~ifung, Montanuniversitat, Franz-Josef-Strasse 18, A-8700 Leoben, Austria Received 10 September 1993; accepted in final form 26 May 1994
Abstract
The corrosion behaviour of sputtered zirconium boride and zirconium boron nitride coatings on carbon steel substrates was investigated. Coatings were produced employing non-reactive as well as reactive d.c. magnetron sputtering deposition using zirconium diboride targets. The copper decoration technique was used to make visible the types of coating defects and their distribution. To quantify the corrosion behaviour, potentiodynamic polarization measurements and electrochemical impedance spectroscopy were employed. It was found that the corrosion protection depends strongly on the porosity and adherence of the coatings. Zirconium boron nitride films are more promising in this aspect owing to their lower number of coating defects. Keywords: Zr-B; Zr-B-N; PVD coatings; Copper decorations; Corrosion properties
1. Introduction
Hard, wear-resistant coatings deposited by physical vapour deposition (PVD) techniques are well established for the protection of materials against wear. These coatings are mainly based on the nitrides and carbides of the transition metal elements. In addition, recently increasing interest is laid on the corresponding borides. The combination of the properties of these hard materials makes them a group of outstanding coating materials, e.g. for tribological or decorative applications, corrosion protective coatings, or diffusion barriers. While numerous investigations deal with the properties and tribological performance of hard coatings as a function of the deposition process and parameters, only a relatively limited number cover corrosion studies [ 1 ]. However, to enhance the service applications of hard coatings, it would be desirable to provide corrosion resistance of the coating material itself as well as corrosion protection for the substrate on which the coating is deposited. Therefore, a detailed understanding of the corrosion behaviour is necessary to avoid failure of the coating-substrate combination used in a given environment. In general, hard materials are noble with respect to steel substrates. Essential reasons responsible for corrosion of the coating-substrate system are defects extending through the coating and resulting from the deposition process or poor surface quality of the sub0010-8545/95/$09.50 © 1995 ElsevierScienceS.A. All rights reserved
SSD1 0010-8545(94)02316-1
strates. In addition, the coating structure with potential grain boundaries crossing the entire film, which permits diffusion to the substrate surface, may contribute in part to the corrosion behaviour [2]. It is well known that the presence of pinholes - - porosity in hard and wear resistant coatings - - changes their protective properties unfavourably [3,4]. Moreover, especially in coatings produced by arc PVD, macroparticle (droplet) type defects not only deteriorate the coated material's friction and decorative properties by producing extensive surface roughness [ 5,6] but also the corrosion protective properties as well [4,7,8]. In the literature, there are several approaches in relation to the determination of the defects extending through the coating, in hard wear-resistant ceramic coatings. One approach [ 9 - 1 1 ] is to estimate porosity from the electrochemical behaviour of the system, in which the sources of porosity are left unidentified. In an another approach [12] the area of visible defects such as 'pores, cracks and nodular cracks' on coated samples was estimated based on SEM observations without taking into account the invisible pores or pinholes. A third approach is the ferroxyl test [13], in which filter paper wetted by corrosive chloride solution and indicators for iron ions is put on the coated sample and the number of coloured stains formed on filter paper resulting from the dissolution of iron substrate from pinholes is counted; the number of stain spots gives a rough
61
M. (Jrgen et al. / Surface and Coatings Technology 71 (1995) 60 66
estimation of macro defects. A fourth approach is the decoration of the defect sites by noble metals such as platinum, gold and copper, and is valuable in making the defect sites visible. It has been applied to passive metals [14-16], metals covered with non-conductive coatings [ 17], anodized aluminium [ 18] and recently to hard wear-resistant coatings [7,8]. Examination of the decorated samples with SEM can give a detailed picture of the sources of porosity in hard and wearresistant coatings. The aim of this investigation was to evaluate the corrosion resistance of carbon steel substrates coated with zirconium boride and zirconium boron nitride. Earlier work indicated that these coatings may be interesting candidates for decorative and tribological applications [19-21], The copper decoration technique [7,8] was employed for the identification and evaluation of the various coating defects. The corrosion behaviour was investigated by a standard potentiodynamic polarization technique using sulphuric acid and NaC1 solutions. The polarization resistance of the samples was determined employing electrochemical impedance spectroscopy.
2. Experimental Carbon steel (Ck 35) discs of 30 mm diameter and 1 0 m m height were used as substrate materials. The surface of the substrates was prepared by wet grinding and then polishing with 1 gm diamond paste. Specimens were coated employing non-reactive (Zr-B) and reactive ( Z r - B - N ) d.c. magnetron sputtering using ZrB2. The addition of nitrogen caused a grain refinement resulting in a gradual shift of the film structure from crystalline (non-reactive deposition) to amorphous (reactive deposition at high nitrogen flow rates) [19-21]. Coating parameters and the thicknesses of the coatings are given in Table 1. Window glass substrates were also coated in the same manner as the steel substrates. The thickness of the coatings on glass substrates was approximately 1 txm. The thicknesses of the coatings were determined by examining the fracture surface of the coated samples with SEM. Coated specimens, after mechanically thin-
ning locally from the back and holding in liquid nitrogen for 5 min, were broken. 2.1. Decoration procedure
The principle of the decoration technique [7,8,14,15,17] is based on simple cementation. The coated materials are introduced into the solution containing copper ions which are noble relative to the steel substrate but base relative to nitride coatings. Copper preferentially deposits on defective, coating-free sites and decorates them while the substrate is oxidized to give away electrons that are consumed in the reduction process: Cu2+ +2e
--*Cu °
(1)
F e ~ F e 2+ + 2 e -
(2)
Zr B and Z r - B - N are both electronically conductive materials [22]. In order to check if copper precipitates on the coating material itself, a test was conducted. ZrB and Z r - B - N coated on glass (inert) substrates were dipped in copper sulphate decoration solution and the surfaces were examined by SEM. Copper decoration did not take place on ZrB and Z r - B - N coated on glass substrates. However, samples coated on steel substrates were easily copper decorated, indicating that only defects connecting the active substrate to the environment could be decorated. Before decoration, Z r - B and Zr B - N coated samples were degreased ultrasonically in 50% e t h e r + 5 0 % alcohol solution for 5 min. Decoration of the degreased specimens was conducted by dipping the specimens in copper sulfate solution at pH = 1 and containing 0.1 g 1 1 Cu2+, for 60s. After decoration, the decorated samples were immediately dipped in distilled water and then dried after washing with methanol. To optimize the decoration process, various copper concentrations and decoration times have been tested; however, the previous conditions were found to allow the decoration without heavy copper build-up. Decorated specimens were examined without any further treatment using a scanning electron microscope (Jeol T330).
Table 1 Zr-B and Zr-B-N coating parameters used Coating type
Sputtering power (W)
Total pressure (Pa)
Zr-B Zr-B N
600 600
1.1 1.1
Bias potential (V)
- -
- -
200 200
Substrate temperature (°C)
Argon f l o w rate (sccrn)
Nitrogen flow rate (sccm)
Coating thickness (rtm)
300 300
110 95
15
5 5
M. (]rgen et al. / Surface and Coatings Technology 71 (1995) 60-66
62
2.2. Electrochemical experiments A standard potentiodynamic polarization technique was used in the electrochemical experiments. The electrochemical measurement system consisted of a potentiostat (EG&G M273) controlled by an Apple IIe computer. The scan rates were 120 mV min-~ and 20 mV min 1 for the tests conducted in sulphuric acid and NaC1 solutions respectively. Experiments were conducted in nitrogen deaerated 1 N sulphuric acid and 3.5% NaC1 solutions at room temperature. The Zr-B and Z r - B - N coated steel samples were degreased ultrasonically in 50% ether+50% alcohol solution for 5min before the experiments. In order to determine the polarization resistance of the samples in 1 N sulphuric acid, electrochemical impedance spectroscopy (EIS) was used. The electrochemical impedance system consisted of a potensiostat (EG&G M273) and lock-in amplifier (EG&G 5315) controlled by an Apple IIe computer. Experiments were conducted at corrosion potential after waiting for 30 min for the stabilization of the corrosion potential. The a.c. potential amplitude and the frequency range were 10 mV and 105 Hz to 0.1 Hz respectively. All potentials were measured with respect to saturated calomel reference electrode (SCE).
ignited between the biased substrate and the wall of the vacuum chamber. The arc might evaporate the coating locally, resulting in craters. If the arc is ignited between the target and the wall of the chamber, target material might evaporate and incorporate into the growing film in the form of droplets. After decoration of the Zr-B coated steel samples it was observed that the peripheries of droplet-like particles and craters were heavily decorated with copper. In other studies conducted on arc PVD TiN and CrN coatings the droplets were also identified as important defect sites I-7,8]. Widespread string-type copper decoration, as stringers, was observed on ZrB coatings, revealing the inherent porosity of the coating (Figs. 2 and 3). Similar decoration characteristics were also observed on arc PVD TiN and CrN coated steels, indicating the presence of extensive porosity [7,8]. The major difference
3. Results and discussions
3.1. Decoration experiments The SEM image of the Zr-B and Z r - B - N coated steel samples showed that these coating were not free of defects. The defects appear as craters or droplets (Fig. ! ). Craters are formed during deposition as a result of arcs
Fig. 2. General view of copper-decorated Z r - B coated steel (white copper particles as stringers and as clusters in craters and at periphery of droplets).
Fig. 1. General view of Zr B coated steel with droplets,
Fig. 3. Copper decoration at the periphery of a droplet on Zr B coated steel: A, droplet; B, copper particles.
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M. Urgen et al. / Surjace and Coatings Technology 71 (1995) 60 66
observed between decorated arc PVD CrN and TiN coated samples and ZrB coated samples was that in the case of the former coatings the decoration was more homogeneous and intense compared with ZrB coatings. In the case of Zr-B-N coated samples, only the peripheries of macroparticles and craters were decorated, indicating that these coatings were free of inherent porosity (Figs. 4 and 5). The presence of inherent porosity in Zr B coatings can be explained by the crystalline, fine columnar structure with voids formed at the grain boundaries and by the high hardness impeding stress relaxation [19 21]. The extremely fine-grained to amorphous structure of Zr-B N coatings prevents the formation of pinholes crossing the entire coating. In addition, stresses generated on the coating during the deposition process may be reduced, as indicated by the lower hardness.
3.2. Electrochemical experiments The cathodic and the anodic electrochemical behaviour of uncoated steel substrate, Zr B and Zr-B-N coated steel samples was investigated in 1 N sulphuric acid (Fig. 6). The corrosion potential of the Zr-B coated samples was close to the corrosion potential of the substrate. During the cathodic polarization, Zr--B coating on the steel samples blistered and almost a third of the coating was peeled off. During the subsequent anodic polarization, the behaviour of the coated steel did not show significant difference from the anodic behaviour of the uncoated steel. Therefore the polarization curve represented the electrochemical behaviour of the partially coated substrate material (Fig. 6). The debonding produced during cathodic polarization of the coated samples may be an indication of the low adhesion of these coatings to the substrate. Zr-B-N coated samples showed far better electrochemical corrosion behaviour when compared with Zr B coated and uncoated samples by decreasing the active corrosion rate, the critical current density %r, and the minimum passive current imi, (Table 2). Cathodic debonding was not observed in these coatings. However, the effect of the substrate steel was still observed on the anodic polarization curve: an active passive transition region in the same potential range as the substrate steel was present (Fig. 6), indicating that the coatings were not capable of covering the surface totally without physical discontinuities. Another set of experiments was conducted in 1 N sulphuric acid without cathodically polarizing the coated samples in order to verify the hydrogen debonding of
Fig. 4. Copper decoration as cluster at the periphery of a partially fallen droplet on a Zr B N coated steel: A, droplet; B, copper particles.
1100 I
600
.
~. 400
Substrate-
-900
....... I ....... I ...,...I
100
101
102
........ I ....... I
103
104
Current Density, i [ I~A/cm Fig. 5. Copper decoration of a crater on Zr B N coated steel: K, crater; B, copper particles,
105
.......
106
2]
Fig. 6. Cathodic and anodic polarization curves of Zr B, Z r - B - N coated steel samples and steel substrate in 1 N H2SO4.
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M. (~rgenet aL / Surface and Coatings Technology 71 (1995) 60-66
Table 2 Corrosion potentials (Eeoc), critical current densities (ier) and minimum passive current densities of the substrate, Z r - B and Z r - B N coated samples obtained in 1 N sulphuric acid i= (mA cm -z)
imi. (mA cm 2)
- 534 - 504
266 1.93
3 0.10
- 556 -494
67.5 0.58
2 0.015
- 525
0.60
0.05
Eeorr VS SCE
Substrate Zr-B Zr-B (cathodically polarized) Zr-B-N Zr-B-N (cathodically polarized)
Table 3 Corrosion potential, polarization resistance of the substrate, and corrosion potentials, polarization resistance, percentage porosity of the Z r - B and Z r - B - N coated steel samples in 1 N sulphuric acid E¢o= vs SCE
Rp (~
Substrate Zr-B Zr-B-N
- 534 - 504 - 494
Porosity
cm-2)
9.2 532 12250
1600
1100
Substrate
g
600
g ~ -400
-9oo
....... I ,. ..... I ,,,,...I
10 o
101
102
....... I ........ I .....
103
104
105
106
Current Density, i [ I~ A / am 2 I Fig. 7. Anodic polarization curves of Zr B and Z r - B - N coated steel samples (without cathodic polarization) and cathodic+anodic polarization curve of steel substrate in 1 N H2SO 4.
0.67 0.021
lated (Table 3) using the following equation [10]: F = ( R v , m / R p ) 10-1AE.... Ilba
the Zr-B coated samples. Coating debonding was not observed during anodic polarization, as expected (Fig. 7). Under similar experimental conditions, Z r - B - N coated samples exhibited better corrosion behaviour than Zr-B coated samples, which was revealed both by lower icr and imin (Table 2). To determine the polarization resistance of the samples at the corrosion potential without disturbing (polarizing) the system significantly, the electrochemical impedance spectroscopy (EIS) technique was used. The polarization resistance Rp of the samples in 1 N sulphuric acid obtained by EIS is given in Table 3. The R v values are determined from Bode plots (Fig. 8). The Rp values showed significant differences between the substrate and coated samples. The Rp value of the Z r - B - N coated sample was 20-fold higher than that of the Zr-B coated samples, showing the better protective properties of the Z r - B - N coatings. The percentage of porosity in the coatings was calcu-
(%)
(3)
In Eq. (3), F is the total coating porosity, Rp, m is the polarization resistance of the substrate, Rp is the measured polarization resistance of the coated steel, AEeorr is the difference between the corrosion potentials of the substrate and the coated steel, and ba is the anodic Tafel slope of the substrate material (75 mV/decade, obtained from the anodic polarization curve). The percentages of porosity calculated by using a similar electrochemical approach were 0.014% for PVD coated and 0.4% for CVD coated TiN [9]. Depending on the immersion time of the samples in the corrosive solution, the porosity had changed between 0.1% and 0.4% for CVD coated TiN [ 11 ]. As a function of the surface roughness of the substrate, the porosity had changed between 0.29% and 2.42% for arc PVD coated TiN and 0.06%-0.26% for arc PVD coated CrN [23]. The percentage of porosity for Z r - B - N coatings determined in this study gave porosity (0.021%) comparable with that of the PVD coated TiN but it was lower than the calculated porosity of both arc PVD coated TiN and CrN. However, the percentage of porosity determined for ZrB coatings (0.67%) was higher than that of PVD coated TiN and arc PVD coated CrN. The results of the anodic polarization experiments conducted in 3.5% NaC1 solution are given in Fig. 9. Both Zr-B and Z r - B - N coatings improved the pitting behaviour of the substrate. However, as the experiments conducted in sulphuric acid previously showed, because of the defective nature of the coating, the elimination of pitting was not possible. These results are in accordance with the findings of a study conducted with Zr-B and Z r - B - N coated stainless steels in a similar medium [24].
4. Comparison of results of decoration and electrochemical experiments The electrochemical behaviour of Zr-B and Z r - B - N coated steel samples in 1 N sulphuric acid and 3.5% NaC1 showed that none of the coatings was capable of covering the surface totally without physical discontinu-
65
M. Urgen et al. / Surface and Coatings Technology 71 (1995) 60-66
• oOoO~oo~qJin~oo 0•
R"
E
•
o
SubstratezrB 1
L'z'",..
J~ 0 N
•
• • •••tt•t/k/tAAhtt, A4k• •
•
•
o
d 0 a0
"(3 0 E
• mmmmmnnmlllmmmm
•
•
•
•
•
• •
• •
I
|
0
1
0
-1
•
•
m
I
I
I
2
3
4
m
LogFrequency,[Hz]
m
m
5
Fig. 8. Bode plots of Zr-B, Zr-B N coated steel samples and steel substrate in 1 N H2SO4. -300 I
'
'
'
The decoration experiments not only supported the findings of the electrochemical tests, but they also made visible the types of defect and their distribution at the coating surface. A quantitative assessment of the porosity using the decoration experiment results was not attempted because of the uneven distribution of copper-decorated sites.
'
I
-400 I
~
ZrBN
-500
5. Conclusions o
i
o,e
-700 ~
-800 L 0
60
~ 120
180
240
300
C u r r e n t Density, i [ p. A / c m 2 ) Fig. 9. Cyclic polarization curves of Zr B, Zr-B N coated steel samples and steel substrate in 3.5% NaC1.
ities, which were made clearly visible by the copper decoration experiments. Z r - B - N coatings protected the substrate better than Zr-B coatings. The porosity of the Z r - B - N layers was at least 30-fold lower. The critical current densities and the polarization resistances of the coated samples were actually related to the critical current density and the polarization resistance of the exposed substrate. Hence, the lower critical current density and higher Rp of the Z r - B - N coated samples showed that they are less defective than Zr-B coated samples.
(1) Zr-B and Zr-B-N layers on steel substrate exhibited various types of defect, which decreased the protective properties of the coating. The copper decoration technique could be applied successfully to identify and make visible defects extending through the coating to the steel substrate. (2) Z r - B - N coated steel showed better corrosion resistance than the Zr-B coated steel because of the lower number of total coating defects. However, the 'small anodic-large cathodic' area relation on Z r - B - N may provoke severe localized corrosion. (3) The corrosion protection properties of these coatings depend on their porosity. Z r - B - N coatings are more promising in this aspect while they do not possess inherent porosity. Elimination of droplets and craters in Z r - B - N coatings could most probably produce corrosion-protective coatings.
References [1] H.A. Jehn and M.E. Baumgartner, Surf Coat. Technol., 54/55 (1992) 108.
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M. (]rgen et al. / Surface and Coatings Technology 71 (1995) 60-66
[2] L. van Leaven, M.N. Alias and R. Brown, Surf Coat. Technol., 53 (1992) 25. [3] M.J. Park, A. Leyland and A. Matthews, Surf. Coat. Technol., 43/44 (1990) 481. [4] M. Nishibori, Surf. Coat. Technol., 52 (1992) 229. [5] H. Randbawa, Surf. Coat. Technol., 36 (1988) 829. [6] C.N. Tai and E.S. Koh, Surf. Coat. Technol., 43/44 (1990) 324. [7] M. 10rgen, A.F. (~aklr, O.L. Eryllmaz and K. Tokmano(glu, in Proc. 3rd Int. Ceramic Cong, 19 23 October 1992, Istanbul, p. 530. [8] M. Urgen, A.F. (~aklr and O.L. Eryllmaz, in Proc. 3rd EAST Conf. 18 19 November 1993, Schwgibisch Gmfind., Eugen G. Leuze Verlag, Salgau, in press. [9] B. Elsener, A. Rota and H. B6hni, Mater. Sci. Forum, 44/45 (1989) 28. [10] J. Pippo, B. Elsener and H. B6hni, Mater. Sei. Forum, 111-112 (1992) 219. [11] J. Pippo, B. Elsener and H. B6hni, Surf. Coat. Technol., 61 (1993) 43. [ 12] I.M. Penttinen, A.S. Korhonen, E. Harju, M.A. Turkia, O. Forsen and E.O. Ristolainen, Surf. Coat. Technol., 50 (1992) 161.
[13] J. Munemasa and T. Kumakiri, Surf Coat. Technol., 49 (1991) 496, [14] N.A. Nielsen, Corrosion, 20 (1964) 104t. [15] M. 1Jrgen, Corrs. Sci., 33 (1992) 1179. [16] E.M. Baker and A.M. Rente, Trans. Am. Electrochem. Soc., 54 (1928) 337. [173 I.M. Notter and D.R. Gabe, Corros. Rev., 10 (1992) 217. [18] ISO Standard, ISO 2085-1976 (E). [19] C. Mitterer, A. ~bleis and R. Ebner, Mater. Sci. Eng., A140 (1991) 670. [20] C. Mitterer, P. Losbichler, W.S.M. Werner, H. St6ri and J. Barounig, Surf. Coat. Technol., 54/55 (1992) 329. [21] E. Brandstetter, C. Mitterer and R. Ebner, Thin Solid Films, 201 (1991) 123. [22] J.R. Shappirio and J.J. Finnegan, Thin Solid Films, 107 (1983) 81. [23] M. l~lrgen, A.F. (~aklr and O.L. Eryllmaz, Surf. Coat. Technol, submitted for publication (1994). [24] A. Ubleis, C. Mitterer and R. Ebner, Surf. Coat. Technol., 60 (1993) 571.