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TiN multilayer coated tool steel
ELSEVIER
Surt’ace and Coatings Technology 99 (1998) 191-196
Corrosion behaviour of Ti/TiN multilayer coated tool steel M. Herranen a**, U. Wiklund b, J.-O. Carlsson a, S. Hogmark b ’ Depastment of Inorganic Chemistry, The kngstr&n Laboratory, Uppsala University, Box 538, S-751 21 Uppsala, Slr,eden b Department of Materials Science, The kngstrOm Labosatory, Uppsala University, Box 534, S-751 21 Uppsala, Sweden
Received6 June 1997;accepted8 August 1997
Abstract The corrosion behaviour of Ti/TiN multilayer coated tool steelhas beeninvestigatedby potentiodynamic measurements in 0.1 M H,S04 and comparedwith singlelayer coatingsof Ti and TiN, respectively.All the coatingshad a total thicknessof about 1 urn. Two multilayer coatingsof different sublayerthicknesses were investigatedi.e. 0.5 and 0.08 urn. An improved protection of the substratewas observedby using multilayer coatingswith thick sublayers.To obtain good protection of the substrate,the columnar structure of ion plated TiN should be avoided. This is achievedby introducing Ti sublayerswith a higher degreeof perfection which hinder columnarfilm growth. The introduction of a few thick sublayerswasmore efficient and resultedin better corrosion protection than many thin sublayers.0 1998ElsevierScienceS.A. Keywords:
Anodic oxidation; PVD; TilTiN multilayers
1. Introduction TiN has found many technical application areas due
to its chemical stability, hardness and good adhesion to most steel substrates. A problem with TIN coatings on an active substrate such as mild steel is, however, the poor corrosion resistance due to the presence of pores and pinholes, through which the substrate can be exposed to the environment. The overall performance of TiN coatings is therefore also strongly dependent on the porosity and microstructure of the deposited film. Several methods can be used to prepare the coatings: chemical vapour deposition (CVD), ion plating, reactive sputtering, etc. The coating technique and the process parameters influence the microstructure and the defect content considerably [ 1,2]. A recent improvement in coatings technology is the introdution of multilayered coatings. With these coatings, the tribological properties may be markedly improved [3,4]. A reduced grain size and a correspondingly large number of interfaces will increase both the hardness [5,6] and toughness [4,7-91 of the coatings. The relatively low compressive residual stress of multilayers, as compared to that of single layer coatings, is also beneficial for the adhesion of the coating. * Correspontine author. 0257-8972/98/$19.00 D 1998 Elsevier Science B.V. All rights reserved. PII
SO257-8972(97)00525-2
A multilayered coating will have lower porosity than a single layer coating, since the open structure, reaching from the surface to the substrate, will be interrupted by repeated nucleation at the interfaces between sublayers [8]. In addition to the mentioned effect on the mechanical performance, which have been the main focus of earlier work, an improved corrosion resistance of multilayer coatings in aqueous solutions has also been observed. Apart from the studies by Hiibler et al. [ 10, 111: there is, however, limited information about the corrosion behaviour of multilayers. In this study corrosion properties of ion plated Ti/TiN multilayer coatings deposited on steel substrates have been investigated. Comparison was made with single layer coatings of titanium and titanium nitride, respectively.
2. Experimental 2.1. Coating deposition
The substrate used was a powder metallurgical tool steel, ASP2030, frequently used in coated metal working tools. The nominal chemical composition (in wt.%) of this steel is 1.28 C, 4.2 Cr, 5.0 MO, 6.4 W, 3.1 V, 8.5 Co. The steel is heat treated by austenitization at
1180“C, followed by tempering for 3 x 1 h at 560” C.
192
hf. Herrmen
rr al. / Swjbce
and Conrings
The substrates were polished to mirror finish. Prior to deposition, the substrates were ultrasonically cleaned in trichloroethylene, acetone and alcohol, and were finally dried in clean Nz gas. The coatings were deposited using triode ion plating with high plasma density in a Balzers BAI 640R equipment. The substrates are mounted on a rotating substrate holder in the roof of the chamber, and titanium is evaporated by electron beam heating from a crucible at the bottom of the chamber. During the deposition process, the chamber pressure was kept below 10m6 mbar. The substrates were heated to a temperature of 400” C and kept there for 1 h to ensure a homogeneous temperature throughout the substrates. This temperature prevailed during the remaining steps in the process. As final cleaning of the substrates, 15 min sputter etching was used during which argon ions were accelerated towards the substrates by an applied bias voltage of -200 V. During the deposition, the substrate bias was lowered to - 110 V. While depositing Ti layers, the electron gun emission was held constant, and only argon gas was supplied to the plasma in order to establish soft etching during the deposition. While depositing TiN layers, nitrogen gas was also supplied, and the electron gun emission and hence the titanium evaporation rate was regulated in order to maintain a constant Nz partial pressure. The deposition rates of the Ti and TIN phases are approximately 0.05 and 0.10 pm min-‘, respectively, After deposition, the substrates were cooled in helium gas for 20 min. Normally, when depositing TIN or other ceramic coatings for tooling applications, a metallic adhesion layer is deposited onto the substrate before the actual coating is deposited. In this work, this interlayer was omitted in order to study the influence of the coating itself on the corrosion properties. In addition to the two multilayer coatings, single layer coatings of Ti and TIN were also deposited. The multilayers differed in multilayer period. Multilayer A had a short period (approximately 0.08 pm), while multilayer B had a long period of 0.50 pm. In the latter case the coating consisted of only two sublayers. A total thickness of 1 pm was aimed at in all cases. Details of the multilayers as well as the single layer coatings are listed in Table 1.
Technolog~~ 99 (1998j
191-196
2.2. Cl~nrncte~ixtion
Film thickness was measured using SEM on fracture cross-sections of one sample from each batch. Structural characterization was done by X-ray diffraction (XRD) using CuKcl radiation to determine the phase content and texture. Surface morphology of the films were observed by scanning force microscopy (TopoMetrix TMX2000). Root-mean-square (r.m.s.) roughness values were computed by software (TopoMetrix SPMLab V.3.06) from the square root of the power spectral density, evaluated over aU300 x 300 pixel points in the image. At least six images were employed on each type of coating. 2.3. Cowosion testing
The corrosion properties of the coatings were evaluated by potentiodynamic scans in 0.1 M H,SO,. The samples were attached onto copper holders with silver glue and embedded in an epoxy resin. After hardening, the samples were ultrasonically cleaned in acetone for 20 min. Thereafter, they were rinsed with Millipore water and dried in air. The experimental set-up consisted of a conventional three-electrode cell. The working electrode was a rotating disc with a rotation speed of 2000 rpm. A saturated calomel electrode (SCE) was used as reference electrode and a platinum spiral served as counter electrode. The electrolyte used was 0.1 M H+O,, prepared from reagent grade chemicals and Mlllipore water. The electrolyte was deaerated by a 20 min water aspiration followed by a 20 min nitrogen purge, immediately before the sample was immersed. The nitrogen purge was continued throughout the experiment. All measurements were performed at 2.5 “C. Before starting the recording of the anodic polarization, the corrosion potential was registered for about 1 h. The anodic polarization curves were registered from the corrosion potential up to 2 V with a scan rate of 0.2 mV s-l. 3. Results
SEM and SFM were used to examine the morphology of the coatings. Fig. la and b show fracture cross-
Table 1 Characteristics of the different coatings
Composition Base layer/top layer Total film thickness (vm) Nominal multilayer period (pm) Roughness r.m.s. (nm)
TiN
Ti
A
B
TiN
Ti
1.0*0.1
1.0+0.1
s5*15
4015
TiiTiN Ti/TiN 1.1iO.l 0.08 75110
Ti/TiN TiiTiN 1.2,O.l 0.50 6oE-5
A4, Herranen
et al. / Surface
and Coatings
Technology
99 (1998)
images of fracture
cross sections
193
b)
4 Fig. I. SEM
191-196
of the multilayer
coatings:
(a) batch
A with short
period;
(b) batch
B with long period
sections of the two different types of multilayered coatings. The short period multilayer structure of batch A is seen as light Ti layers and dark TIN layers. In the multilayer with long period. an additional contrast between the two materials is evident. The smooth areas are brittle fractured TIN layers, and the areas in between are heavily deformed Ti layers. The difference in the surface morphology between the various coatings is displayed in Fig. 2a-d. The single layer TiN coatings (Fig. 2a) had the typical crater-like appearance that reflects the carbide pattern of the substrate surface [ 121. The grain size of the TIN coating was less than 50 nm, and the crater-like structures had a diameter of 600-1000 nm. The grain size of the single layer Ti coatings (Fig. 2b) was significantly larger (300-500 nm). The two multilayers with TiN on top (Fig. 2c and d) had a similar appearance, with small grains (~50 nm) on the surface. The influence of the multilayer structure on the surface morphology is clearly seen when comparing the appearance of the multilayer with the short period (batch A) in Fig. 2c, with that of the multilayer with long period, Fig. 2d. The multilayer with the long period resembles the surface of the TIN coating, while the fine-grained thin layer in Fig. 2c reflects the underlying coarse Ti layer in Fig. 2b. XRD also revealed the difference in grain size. Diffractograms recorded for the single layer Ti coating, displayed narrow peaks with a strong [002]-texture. For the TiN coating, only peaks corresponding to TIN, osbornite (JCPDS-file 3%1420), and a trace of Ti were found. The multilayer with the long period displayed a strong [002]-texture for the Ti and a strong [ 11 l]-texture for the TiN. Diffractograms for the multilayer with the shorter multilayer period displayed no preferential orientation with respect to Ti and TIN.
corrosion potential recorded after 30 min immersion in the electrolyte was essentially the same for all samples and close to the value of the pure substrate (see Fig. 3). The time required to reach a stable corrosion potential varied, however, from a few minutes to about 30 min for the different batches. A stable value of -450 & 2 mV was recorded for the single layer TIN coatings within a few minutes. The initial corrosion potential for the multilayer coatings was above 250 mV, but dropped to -450 mV after less than 15 min. A stable corrosion potential of -455 mV was recorded for the substrate in the electrolyte used. Polarization curves of the different coatings are displayed in Fig. 3. The single layer TIN coating showed poor corrosion resistance and the protective properties were completely lost above 200 mV, as indicated by the high current densities recorded and displayed in Fig. 3. Severe undermining of the coating was observed, and finally the coating peeled off the substrate. The current density recorded above 250 mV was comparable with the current densities obtained for a pure steel substrate. For the multilayers, the current densities at all potentials are reduced by one or two orders of magnitude compared to the steel substrate. The batch B coatings which have a long multilayer period show the lowest values for the critical current density and also a lower amount of corrosion sites, while the batch A coatings have values significantly higher. The lowest values were recorded for the single layer Ti coatings. Corrosion data for the different samples are summarized in Table 2. The decrease in the current density reflects the reduced porosity of the different coatings.
3.2. Corrosion properties
The poor corrosion resistance provided by the single layer TIN coating is attributed to its columnar structure. In acid solution, rapid corrosion of steel coated with TIN takes place at pores and pinholes in the coating
Already the low corrosion potential indicated the presence of defects and pores in the coatings. The
4. Discussion
5w
b)
c)
d)
Fig. 2. Typical surface morphologies of the diirerent coatings. (a) TN, (b) Ti, (c) batch A - period 0,08 pm: and (d) batch B - period 0.5 pm.
113,141. A TIN coating can provide protection only if the substrate is easily passivated [ 131. The corrosion potential recorded for pure TIN in 0.05 M HZSOJ using an inert substrate is 240 mV [ 141. This is close to the values recorded for the multilayer coatings during the first 5-15 min of immersion. Modification of the TIN layer which leads to loss of the columnar structure improves the corrosion resistance of the coating [ 151. Anodic polarization curves for PVD TIN on steel have been investigated in 0.5 M H$O., by Yan and Yang [16]. They obtained critical current densities of about 0.1 A cm-‘. This is of the same order as the present results for the single layer TIN coatings, but a direct comparison cannot be made. due to the higher concentration and the unknown scan rate used in their work. Ti/TiN multilayer coatings improve the corrosion protection of the substrate significantly. The columnar structure is disordered and the coating has less pores and pinholes. Multilayering also has the effect of reduc-
ing the grain size, which enhances hardness axd toughness. According to Wen et al. [6J, the grain size in the Ti/TiN multilayers decreases as the multilayer period is reduced. However, to achieve a Ti/TiN multilayer with mechanical properties comparable to those of a single layer TiN coating, they concluded that the multilayer wavelength has to be short, approximately lo-50 run. The multilayer coatings with short wavelength studied in this work. however. displayed a less dense structure than the multilayer with long wavelength. A few, relatively thick Ti layers with large crystal&es appear to be more efficient in preventing columnar film growth, with the risk of open structures reaching from the surface to the substrate, than a larger number of thin Ti layers. Htibler et al. [lo? 1l] studied the corrosion behaviour in oxygen-saturated acetate buffer of multilayer coatings produced by PVD and ion-beam-assisted deposition ( IBAD). They used cyclic voltammetry. An increased thickness of Ti layers improved the corrosion resistance:
Al. Henmen
et al. / Swface
and Coatings
Technology
99 (1598)
191-196
195
t
Fig. 3. Anodic polarization curves for the various coatings in 0.1 M H,SO,. Scan rate 0.2 mV s-‘. For the TiN coating the curve is only displayed to about 200 mV. Above 250 mV, a current denaity equal to that of rhe substrate was recorded, thus indicating complete removal of the TiN coating. Table 2 Corrosion parameters for the different coatings
Corrosion potential” (mV) tw.ws SCE Corrosion current density (pA cm ‘) Critical current density (PA cm-?)
TiN
Ti
A
B
-45222 3 > 500 000
-4lOi2 3 IO-20
-44312 6-9 20 000
-423* 10 5-l 60-100
“Measured 1 h after immersion in the electrolyte and the starting point of the anodic polarization curve.
and the best results were found for multilayers with a Ti/TiN thickness ratio of 2. Multilayer coatings with a graded composition interface have a more protective character than those with a sharp interface [ 171. This is due to lower porosity of the films and reduced interface stresses. The [ 11 l] texture usually found in ion plated TIN coatings is not present in the single layer TIN coatings investigated here, possibly due to the absence of an adhesion layer. The conceivable influence this may have on the ability to protect the substrate from corrosion has not been studied in the present work.
of the substrate as compared to a single layer TiN coating. Utilizing a coating with a large number of thin Ti layers retains the mechanical properties, but also the columnar structure of TIN, and consequently does not provide as good corrosion protection.
Acknowledgement Erasteel Kloster AB is recognized for providing the substrate material. Financial support by the Swedish Natural Science Research Council (NFR) and the National Swedish Board for Technical and Industrial Development (NUTEK) is gratefully acknowledged.
5. Conclusions The ability of a coating to provide corrosion protection of the underlying substrate is closely related to its microstructure. To achieve good protection. columnar film growth, such as that of ion plated TiN, with open paths to the substrate. has to be avoided. On some expense of the mechanical strength, a few thick Ti layers can be applied to hinder columnar growth. This results in a coating that provides improved corrosion resistance
References [I] R.F. Bunshah (Ed.), Handbook of Deposition Technologies for Films and Coatings: Science, Technology and Applications, 2nd ed., Park Ridge. Noyes, NJ, 1994. [2] Korhonen, A.S., Molarius. J.M., Harju, E. For& 0. In: T.S. Sudarshan, D.G. Bhat, M. Jeadin (Eds.), Surface Modification Technologies IV. TMS-AIME, Warrendale, PA, 1991. [3] S.J. Bull, A.M. Jones, Surf. Coat. Technol. 78 (1996) 173.
196
M Hemnm
er al. / Surface
and Coatings
[4] M. Larsson, M. Bromark, P. Hedenqvist, S. Hogmark, Surf. Coat. Technol. 91 (1997) 43. [5] K.K. Shih, D.B. Dove, Appl. Phys. Lett. 61 ( 1992) 654. [6] L.S. Wen, R.F. Huang, L.P. Guo, J. Gong, T.Y. Wei, Y.Z. Chuang, J. Magnet. Magnet. Mater. 126 (1993) 200. [7] Wiklund, U., Hedenqvist. P., Hogmark, S., Surf. Coat. Technol. (in press). [S] M. Larsson, P. Bromark, P Hedenqvist, S. Hogmark, Surf. Coat. Technol. 7617 i 1995) 202. [9] A. Leyland, A. Matthews, Surf. Coat. Technol. 70 (1994) 19. [lo] R. Htibler, A. SchrBer, W. Ensinger, G.K. Wolf, W.H. Schreiner, I.J.R. Baumvol, Surf. Coat. Technol. 60 (1993) 561.
Technology
99 (1998)
191-196
[ 1I] R. Htibler. A. Schriier, W. Ensinger, G. Wolf, F.C. Stedile, W.H. Schreiner, I.J.R. Baumvol. J. Vat. Sci. Technol. A 11 (1993) 451. [12] M. Bromark, M. Larsson, P. Hedenqvist, M. Olsson, S. Hogmark, E. Bergmann. Surf. Engng 10 (1994j 205. 1131 B. Elsener, A. Rota, H. Biihni, Mater. Sci. Forum 4445 (1989) 29. [ 141 M. Tavi, 0. For&, J. Aromaa, IMater. Sci. Forum 4445 (198g-15. [I51 R. Htibler, L. Alberts, G.K. Wolf, Surf. Coat. Technol. 60 ( 1993) 549. [ 161 P. Yang, S.-Z. Yang, Thin Solid Films 232 (1993) 204. [ 171 L.A.S. Reis, D.S. Azambuja, I.J.R. Bsumvol, Surf. Coat. Technol. 89(1997) 114.
Surt’ace and Coatings Technology 99 (1998) 191-196
Corrosion behaviour of Ti/TiN multilayer coated tool steel M. Herranen a**, U. Wiklund b, J.-O. Carlsson a, S. Hogmark b ’ Depastment of Inorganic Chemistry, The kngstr&n Laboratory, Uppsala University, Box 538, S-751 21 Uppsala, Slr,eden b Department of Materials Science, The kngstrOm Labosatory, Uppsala University, Box 534, S-751 21 Uppsala, Sweden
Received6 June 1997;accepted8 August 1997
Abstract The corrosion behaviour of Ti/TiN multilayer coated tool steelhas beeninvestigatedby potentiodynamic measurements in 0.1 M H,S04 and comparedwith singlelayer coatingsof Ti and TiN, respectively.All the coatingshad a total thicknessof about 1 urn. Two multilayer coatingsof different sublayerthicknesses were investigatedi.e. 0.5 and 0.08 urn. An improved protection of the substratewas observedby using multilayer coatingswith thick sublayers.To obtain good protection of the substrate,the columnar structure of ion plated TiN should be avoided. This is achievedby introducing Ti sublayerswith a higher degreeof perfection which hinder columnarfilm growth. The introduction of a few thick sublayerswasmore efficient and resultedin better corrosion protection than many thin sublayers.0 1998ElsevierScienceS.A. Keywords:
Anodic oxidation; PVD; TilTiN multilayers
1. Introduction TiN has found many technical application areas due
to its chemical stability, hardness and good adhesion to most steel substrates. A problem with TIN coatings on an active substrate such as mild steel is, however, the poor corrosion resistance due to the presence of pores and pinholes, through which the substrate can be exposed to the environment. The overall performance of TiN coatings is therefore also strongly dependent on the porosity and microstructure of the deposited film. Several methods can be used to prepare the coatings: chemical vapour deposition (CVD), ion plating, reactive sputtering, etc. The coating technique and the process parameters influence the microstructure and the defect content considerably [ 1,2]. A recent improvement in coatings technology is the introdution of multilayered coatings. With these coatings, the tribological properties may be markedly improved [3,4]. A reduced grain size and a correspondingly large number of interfaces will increase both the hardness [5,6] and toughness [4,7-91 of the coatings. The relatively low compressive residual stress of multilayers, as compared to that of single layer coatings, is also beneficial for the adhesion of the coating. * Correspontine author. 0257-8972/98/$19.00 D 1998 Elsevier Science B.V. All rights reserved. PII
SO257-8972(97)00525-2
A multilayered coating will have lower porosity than a single layer coating, since the open structure, reaching from the surface to the substrate, will be interrupted by repeated nucleation at the interfaces between sublayers [8]. In addition to the mentioned effect on the mechanical performance, which have been the main focus of earlier work, an improved corrosion resistance of multilayer coatings in aqueous solutions has also been observed. Apart from the studies by Hiibler et al. [ 10, 111: there is, however, limited information about the corrosion behaviour of multilayers. In this study corrosion properties of ion plated Ti/TiN multilayer coatings deposited on steel substrates have been investigated. Comparison was made with single layer coatings of titanium and titanium nitride, respectively.
2. Experimental 2.1. Coating deposition
The substrate used was a powder metallurgical tool steel, ASP2030, frequently used in coated metal working tools. The nominal chemical composition (in wt.%) of this steel is 1.28 C, 4.2 Cr, 5.0 MO, 6.4 W, 3.1 V, 8.5 Co. The steel is heat treated by austenitization at
1180“C, followed by tempering for 3 x 1 h at 560” C.
192
hf. Herrmen
rr al. / Swjbce
and Conrings
The substrates were polished to mirror finish. Prior to deposition, the substrates were ultrasonically cleaned in trichloroethylene, acetone and alcohol, and were finally dried in clean Nz gas. The coatings were deposited using triode ion plating with high plasma density in a Balzers BAI 640R equipment. The substrates are mounted on a rotating substrate holder in the roof of the chamber, and titanium is evaporated by electron beam heating from a crucible at the bottom of the chamber. During the deposition process, the chamber pressure was kept below 10m6 mbar. The substrates were heated to a temperature of 400” C and kept there for 1 h to ensure a homogeneous temperature throughout the substrates. This temperature prevailed during the remaining steps in the process. As final cleaning of the substrates, 15 min sputter etching was used during which argon ions were accelerated towards the substrates by an applied bias voltage of -200 V. During the deposition, the substrate bias was lowered to - 110 V. While depositing Ti layers, the electron gun emission was held constant, and only argon gas was supplied to the plasma in order to establish soft etching during the deposition. While depositing TiN layers, nitrogen gas was also supplied, and the electron gun emission and hence the titanium evaporation rate was regulated in order to maintain a constant Nz partial pressure. The deposition rates of the Ti and TIN phases are approximately 0.05 and 0.10 pm min-‘, respectively, After deposition, the substrates were cooled in helium gas for 20 min. Normally, when depositing TIN or other ceramic coatings for tooling applications, a metallic adhesion layer is deposited onto the substrate before the actual coating is deposited. In this work, this interlayer was omitted in order to study the influence of the coating itself on the corrosion properties. In addition to the two multilayer coatings, single layer coatings of Ti and TIN were also deposited. The multilayers differed in multilayer period. Multilayer A had a short period (approximately 0.08 pm), while multilayer B had a long period of 0.50 pm. In the latter case the coating consisted of only two sublayers. A total thickness of 1 pm was aimed at in all cases. Details of the multilayers as well as the single layer coatings are listed in Table 1.
Technolog~~ 99 (1998j
191-196
2.2. Cl~nrncte~ixtion
Film thickness was measured using SEM on fracture cross-sections of one sample from each batch. Structural characterization was done by X-ray diffraction (XRD) using CuKcl radiation to determine the phase content and texture. Surface morphology of the films were observed by scanning force microscopy (TopoMetrix TMX2000). Root-mean-square (r.m.s.) roughness values were computed by software (TopoMetrix SPMLab V.3.06) from the square root of the power spectral density, evaluated over aU300 x 300 pixel points in the image. At least six images were employed on each type of coating. 2.3. Cowosion testing
The corrosion properties of the coatings were evaluated by potentiodynamic scans in 0.1 M H,SO,. The samples were attached onto copper holders with silver glue and embedded in an epoxy resin. After hardening, the samples were ultrasonically cleaned in acetone for 20 min. Thereafter, they were rinsed with Millipore water and dried in air. The experimental set-up consisted of a conventional three-electrode cell. The working electrode was a rotating disc with a rotation speed of 2000 rpm. A saturated calomel electrode (SCE) was used as reference electrode and a platinum spiral served as counter electrode. The electrolyte used was 0.1 M H+O,, prepared from reagent grade chemicals and Mlllipore water. The electrolyte was deaerated by a 20 min water aspiration followed by a 20 min nitrogen purge, immediately before the sample was immersed. The nitrogen purge was continued throughout the experiment. All measurements were performed at 2.5 “C. Before starting the recording of the anodic polarization, the corrosion potential was registered for about 1 h. The anodic polarization curves were registered from the corrosion potential up to 2 V with a scan rate of 0.2 mV s-l. 3. Results
SEM and SFM were used to examine the morphology of the coatings. Fig. la and b show fracture cross-
Table 1 Characteristics of the different coatings
Composition Base layer/top layer Total film thickness (vm) Nominal multilayer period (pm) Roughness r.m.s. (nm)
TiN
Ti
A
B
TiN
Ti
1.0*0.1
1.0+0.1
s5*15
4015
TiiTiN Ti/TiN 1.1iO.l 0.08 75110
Ti/TiN TiiTiN 1.2,O.l 0.50 6oE-5
A4, Herranen
et al. / Surface
and Coatings
Technology
99 (1998)
images of fracture
cross sections
193
b)
4 Fig. I. SEM
191-196
of the multilayer
coatings:
(a) batch
A with short
period;
(b) batch
B with long period
sections of the two different types of multilayered coatings. The short period multilayer structure of batch A is seen as light Ti layers and dark TIN layers. In the multilayer with long period. an additional contrast between the two materials is evident. The smooth areas are brittle fractured TIN layers, and the areas in between are heavily deformed Ti layers. The difference in the surface morphology between the various coatings is displayed in Fig. 2a-d. The single layer TiN coatings (Fig. 2a) had the typical crater-like appearance that reflects the carbide pattern of the substrate surface [ 121. The grain size of the TIN coating was less than 50 nm, and the crater-like structures had a diameter of 600-1000 nm. The grain size of the single layer Ti coatings (Fig. 2b) was significantly larger (300-500 nm). The two multilayers with TiN on top (Fig. 2c and d) had a similar appearance, with small grains (~50 nm) on the surface. The influence of the multilayer structure on the surface morphology is clearly seen when comparing the appearance of the multilayer with the short period (batch A) in Fig. 2c, with that of the multilayer with long period, Fig. 2d. The multilayer with the long period resembles the surface of the TIN coating, while the fine-grained thin layer in Fig. 2c reflects the underlying coarse Ti layer in Fig. 2b. XRD also revealed the difference in grain size. Diffractograms recorded for the single layer Ti coating, displayed narrow peaks with a strong [002]-texture. For the TiN coating, only peaks corresponding to TIN, osbornite (JCPDS-file 3%1420), and a trace of Ti were found. The multilayer with the long period displayed a strong [002]-texture for the Ti and a strong [ 11 l]-texture for the TiN. Diffractograms for the multilayer with the shorter multilayer period displayed no preferential orientation with respect to Ti and TIN.
corrosion potential recorded after 30 min immersion in the electrolyte was essentially the same for all samples and close to the value of the pure substrate (see Fig. 3). The time required to reach a stable corrosion potential varied, however, from a few minutes to about 30 min for the different batches. A stable value of -450 & 2 mV was recorded for the single layer TIN coatings within a few minutes. The initial corrosion potential for the multilayer coatings was above 250 mV, but dropped to -450 mV after less than 15 min. A stable corrosion potential of -455 mV was recorded for the substrate in the electrolyte used. Polarization curves of the different coatings are displayed in Fig. 3. The single layer TIN coating showed poor corrosion resistance and the protective properties were completely lost above 200 mV, as indicated by the high current densities recorded and displayed in Fig. 3. Severe undermining of the coating was observed, and finally the coating peeled off the substrate. The current density recorded above 250 mV was comparable with the current densities obtained for a pure steel substrate. For the multilayers, the current densities at all potentials are reduced by one or two orders of magnitude compared to the steel substrate. The batch B coatings which have a long multilayer period show the lowest values for the critical current density and also a lower amount of corrosion sites, while the batch A coatings have values significantly higher. The lowest values were recorded for the single layer Ti coatings. Corrosion data for the different samples are summarized in Table 2. The decrease in the current density reflects the reduced porosity of the different coatings.
3.2. Corrosion properties
The poor corrosion resistance provided by the single layer TIN coating is attributed to its columnar structure. In acid solution, rapid corrosion of steel coated with TIN takes place at pores and pinholes in the coating
Already the low corrosion potential indicated the presence of defects and pores in the coatings. The
4. Discussion
5w
b)
c)
d)
Fig. 2. Typical surface morphologies of the diirerent coatings. (a) TN, (b) Ti, (c) batch A - period 0,08 pm: and (d) batch B - period 0.5 pm.
113,141. A TIN coating can provide protection only if the substrate is easily passivated [ 131. The corrosion potential recorded for pure TIN in 0.05 M HZSOJ using an inert substrate is 240 mV [ 141. This is close to the values recorded for the multilayer coatings during the first 5-15 min of immersion. Modification of the TIN layer which leads to loss of the columnar structure improves the corrosion resistance of the coating [ 151. Anodic polarization curves for PVD TIN on steel have been investigated in 0.5 M H$O., by Yan and Yang [16]. They obtained critical current densities of about 0.1 A cm-‘. This is of the same order as the present results for the single layer TIN coatings, but a direct comparison cannot be made. due to the higher concentration and the unknown scan rate used in their work. Ti/TiN multilayer coatings improve the corrosion protection of the substrate significantly. The columnar structure is disordered and the coating has less pores and pinholes. Multilayering also has the effect of reduc-
ing the grain size, which enhances hardness axd toughness. According to Wen et al. [6J, the grain size in the Ti/TiN multilayers decreases as the multilayer period is reduced. However, to achieve a Ti/TiN multilayer with mechanical properties comparable to those of a single layer TiN coating, they concluded that the multilayer wavelength has to be short, approximately lo-50 run. The multilayer coatings with short wavelength studied in this work. however. displayed a less dense structure than the multilayer with long wavelength. A few, relatively thick Ti layers with large crystal&es appear to be more efficient in preventing columnar film growth, with the risk of open structures reaching from the surface to the substrate, than a larger number of thin Ti layers. Htibler et al. [lo? 1l] studied the corrosion behaviour in oxygen-saturated acetate buffer of multilayer coatings produced by PVD and ion-beam-assisted deposition ( IBAD). They used cyclic voltammetry. An increased thickness of Ti layers improved the corrosion resistance:
Al. Henmen
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Fig. 3. Anodic polarization curves for the various coatings in 0.1 M H,SO,. Scan rate 0.2 mV s-‘. For the TiN coating the curve is only displayed to about 200 mV. Above 250 mV, a current denaity equal to that of rhe substrate was recorded, thus indicating complete removal of the TiN coating. Table 2 Corrosion parameters for the different coatings
Corrosion potential” (mV) tw.ws SCE Corrosion current density (pA cm ‘) Critical current density (PA cm-?)
TiN
Ti
A
B
-45222 3 > 500 000
-4lOi2 3 IO-20
-44312 6-9 20 000
-423* 10 5-l 60-100
“Measured 1 h after immersion in the electrolyte and the starting point of the anodic polarization curve.
and the best results were found for multilayers with a Ti/TiN thickness ratio of 2. Multilayer coatings with a graded composition interface have a more protective character than those with a sharp interface [ 171. This is due to lower porosity of the films and reduced interface stresses. The [ 11 l] texture usually found in ion plated TIN coatings is not present in the single layer TIN coatings investigated here, possibly due to the absence of an adhesion layer. The conceivable influence this may have on the ability to protect the substrate from corrosion has not been studied in the present work.
of the substrate as compared to a single layer TiN coating. Utilizing a coating with a large number of thin Ti layers retains the mechanical properties, but also the columnar structure of TIN, and consequently does not provide as good corrosion protection.
Acknowledgement Erasteel Kloster AB is recognized for providing the substrate material. Financial support by the Swedish Natural Science Research Council (NFR) and the National Swedish Board for Technical and Industrial Development (NUTEK) is gratefully acknowledged.
5. Conclusions The ability of a coating to provide corrosion protection of the underlying substrate is closely related to its microstructure. To achieve good protection. columnar film growth, such as that of ion plated TiN, with open paths to the substrate. has to be avoided. On some expense of the mechanical strength, a few thick Ti layers can be applied to hinder columnar growth. This results in a coating that provides improved corrosion resistance
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