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Elastic properties of Ni and Ni + Mo coatings electrodeposited on stainless steel substrate
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Surface & Coatings Technology 202 (2008) 2292 – 2296 www.elsevier.com/locate/surfcoat
Elastic properties of Ni and Ni + Mo coatings electrodeposited on stainless steel substrate M. Kubisztal a,⁎, J. Kubisztal a , A. Chrobak b , G. Haneczok a , A. Budniok a , J. Rasek a a
Institute of Materials Science, University of Silesia, 40-007 Katowice, Bankowa 12, Poland b Institute of Physics, University of Silesia, 40-007 Katowice, Uniwersytecka 4, Poland Available online 2 August 2007
Abstract Adhesion coefficient and Young's modulus of Ni and Ni + Mo coatings electrochemically deposited on stainless steel were examined by applying vibrating reed technique. It was shown that adhesion coefficient of the Ni coating slightly decreases (about 8%) with increasing layer thickness (5–40 μm). Young's modulus Ef of these coatings at room temperature was found to be about 130 GPa. The relative adhesion coefficient of the Ni layer decreases with increasing temperature (300–600 K) in relation to the thinnest examined layer (5 μm). Young's modulus of the Ni + Mo coatings decreases with increasing Mo content; for 9 wt.% of Mo Ef = 40 GPa and for 32 wt.% of Mo Ef = 23 GPa. © 2007 Elsevier B.V. All rights reserved. Keywords: Composite coatings; Galvanostatic deposition; Adhesion coefficient; Young's modulus
1. Introduction Nickel-based composite coatings electrochemically deposited on a steel substrate are known as materials of special properties like very good corrosive resistance in aggressive environments or high catalytic activity in many electrochemical processes — e.g. in a process of hydrogen evolution [1–4]. In a family of electrolytic composite coatings very interesting are those containing metals like Ti, Mo and W, which could not be directly codeposited in the form of ions using aqueous solutions. However, these metals can be introduced in a form of powders by galvanic embedding their particles into a metal matrix [5]. Molybdenum as a component of composite coatings seems to be of particular interest on account of its electrochemical properties. For example in a group of cathode materials Mo plays a role of an activator in hydrogen electroevolution. Additionally, Mo as a composite ingredient modifies morphology of the coating i.e. creates porous material with rough surface. On the one hand such properties are very advisable from electrochemical point of view (e.g. improve electrocatalytic activity) and cause that Ni + Mo composite coatings are ⁎ Corresponding author. Tel.: +48 32 359 12 47; fax: +48 32 359 21 33. E-mail address: [email protected] (M. Kubisztal). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.07.057
frequently used as electrode materials for hydrogen evolution [2,3]. On the other hand the modification of nickel layer by Mo deteriorates mechanical properties of the composite coatings and hinders their applications. Thereby, the main goal of the present paper is to study the changes of Young's modulus as a function of Mo content in Ni + Mo composite coatings by applying vibrating reed technique — a nondestructive material testing method widely used in [6–14]. This method allows also studying the adhesion coefficient γ (0 ≤ γ ≤ 1) defined as a fraction of mechanical energy transferred during vibrations from the substrate to the coating material [13]. 2. Examined material and experimental procedure The experiments were carried out for Ni and Ni + Mo composite coatings obtained by galvanostatic deposition on stainless steel substrate (cathode) of nominal chemical composition — Fe (70 wt.%), Cr (20 wt.%), Ni (10 wt.%), Mn (b 0.2 wt.%). The anode was made of platinum grid (size of 1 dm2). The deposition process was carried out at a temperature of 330 K (± 3 K) using a current density of 40–80 mA/cm2 and a deposition time of 5–50 min. Before each deposition the substrate was chemically etched using concentrated HCl solution and rinsed out in distilled water. Nickel coatings were obtained using the solution: NiSO4 7H2O (84 g/dm3),
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H3BO3 (8 g/dm3), CH3COONa (10 g/dm3), C6H5O7Na3 2H2O (30 g/dm3), NH4Cl (10 g/dm3). In order to obtain Ni + Mo composite coatings Mo powder in amount of 10 g/dm3 (100 mesh, Aldrich;) was added to the solution; in both cases pH of the suspension was in the range from 5.4 to 5.8. In order to maintain homogenous concentration of all ingredients during deposition process, the electrolyte was mixed with the rate of about 200 rpm. Surface morphology of the deposited coatings was determined by Hitachi S-4200(4) scanning electron microscope (SEM). Thicknesses af of the coatings were within the range of 5– 40 μm and 25–50 μm for Ni and Ni + Mo, respectively. Molybdenum content was determined by making use of X-ray diffraction method (Philips X'Pert; CuKα radiation). The quantitative analysis was performed by comparing the integrated intensity of the strongest lines of the mixture constituents (Mo (110) and Ni (111)) with the corresponding intensity of the pure phase [15]. Molybdenum content of the Ni + Mo composite coatings was in the range from 9 wt.% to 32 wt.%. Structural examinations and electrochemical properties of these kinds of coatings were already published in [2]. Measurements of Young's modulus were carried out using a vibrating reed apparatus (clamped — free geometry) operating in the frequency range of 10–250 Hz [11,14]. Square of the free vibrations frequency fs of a cantilever sample excited into flexural vibration is proportional to the Young's modulus of the examined material Es [11,14]: 2 2 an as Es 2 ð1Þ fs ¼ 2 pl 3qs
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3. Results and data analysis Fig. 1a shows a typical example of fs2(T ) and fc2(T ) obtained for stainless steel substrate and composite consisting of the substrate and the electrochemically deposited Ni coating (af = 28 μm). Similar curves fs2(T ) and fc2(T ) for the electrochemically deposited Ni + Mo coating (af = 25 μm) are presented in Fig. 1b. Let notice that for Ni coatings fc2(T ) N fs2(T ) while for Ni + Mo we have fc2(T ) b fs2(T ). Thereby, one can expect that the term γ2Ef of Ni + Mo coatings is smaller than γ2Ef for the Ni coatings. Note that Es is the same in both cases and ρf (Ni + Mo) b ρf (Ni) (see Eq. (2)). The measurements of fs2(T ) and fc2(T ) allow determining the term γ2Ef as a function of temperature by making use of Eq. (2). The coating thickness af and the density ρf entering into Eq. (2) were calculated by making use of analysis of the cross-section micrographs (stereoscopic microscope) and by determining the coating mass, respectively. The results obtained for Ni and Ni + Mo coatings are shown in Fig. 2a and b. In both cases for a given coating thickness the product γ2 Ef decreases with temperature due to a decrease of Ef and a possible change of
where αn2 is the numerical factor which for the first odd tone (n = 1) is 0.879; as, l and ρs are the sample thickness, the sample length and the material density, respectively. In the case of a composite sample consisting of a substrate and a layer the relationship between the free vibrations frequency of the composite fc and the substrate fs is [13]: fc2 fs2 af 3g2 Ef qf ¼ ð2Þ fs2 as Es qs where subscript f stands for films, s for substrate and c for composite. The coefficient γ, introduced by Wuttig et al. [13], is the so-called adhesion coefficient and represents the elastic energy transfer from the substrate to the layer and varies between 1 (perfect adhesion) and 0 (no adhesion). It is necessary to point out that the Young's modulus Ef which enters into Eq. (2) is not identical with that of the bulk material of the layer. The reason is that Ef is a modified Young's modulus corresponding to the uniaxial stiffness of the layer when constrain to deform with the Poisson's ratio of the substrate rather than with its own unconstrained value [14]. The measurements of the free vibrations frequency were carried out in the following manner: i) the frequency fs (substrate) was measured as a function of temperature in the range of 300–600 K with the heating rate of 3 K/min, and ii) the frequency fc (composite, i.e. the same substrate + layer) was measured with the same heating rate.
Fig. 1. Square of frequency f2s and f2c versus temperature determined for: a) Ni coating (af = 28 μm) and b) Ni + Mo coating (af = 25 μm, Mo content 9 wt.%).
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Fig. 2. The term γ2Ef versus temperature determined for: a) Ni and b) Ni + Mo coatings.
taken as a reference one. It allows studying the dependence of the adhesion on coating thickness and also on some external parameters e.g. temperature. Assuming that the values of Ef are the same for all examined Ni coatings one can determine the relative adhesion coefficient γr versus temperature in relation to the thinnest layer with af = 5 μm (the reference layer). The results are presented in Fig. 3 where in the inset γr versus af at room temperature is also shown. It can be noticed that for Ni coatings γr decreases with increasing temperature which means that the adhesion of the examined coating slightly deteriorates (about 8%) in relation to the Ni coating with af = 5 μm. The assumption that the adhesion coefficient (not relative) at room temperature for the reference layer (af = 5 μm) equals one (γ = 1) corresponds to the value of Ef = 130 GPa. This means that the Young's modulus for the electrochemically deposited Ni layer on stainless steel at room temperature is considerably lower than the Young's modulus of Ni bulk material i.e. 210 GPa [16]. As it can be noticed from Fig. 3 γr slightly decreases with increasing af. The total change of γr does not exceed 8% while the error limit of the determination of the adhesion coefficient is about 3%. The result presented in Fig. 3 i.e. the dependence of the adhesion coefficient on the coating's thickness allows determining the Young's modulus of Ni + Mo coating electrochemically deposited on stainless steel. In the first approximation one can assume that the adhesion coefficient of Ni + Mo coating is the same as the adhesion of Ni coating. Indeed, Mo particles essentially have no contact with the substrate which was confirmed using scanning electron microscopy observations and cause only a change of the Young's modulus of the coating material. Taking into account this fact one can calculate Ef at room temperature for different Mo content by making use of Eq. (2) and the values of the adhesion coefficient determined for Ni coatings (see Fig. 3). The results obtained are shown in Fig. 4. It can be seen that Ef decreases with increasing Mo content.
the adhesion coefficient with temperature. For Ni coating the adhesion coefficient at a given temperature also decreases with increasing af as the modulus Ef is the same for different samples. However, in the case of the Ni + Mo composite coatings γ2Ef values depend on both af and Mo content whereas Ef is independent on coating thickness at least in the examined range. As it was already mentioned the modulus Ef does not correspond to the modulus of the bulk material [14], so the determination of the adhesion coefficient γ and the modulus Ef independently requires additional measurements or information. In spite of these difficulties some important data can be obtained. For samples with the same value of Ef (the same technique for coating deposition) one can define the so-called relative adhesion coefficient γr defined by the relation: g2r ¼
g22 Ef g22 ¼ g21 Ef g21
ð3Þ
According to this equation γr represents a change of the adhesion coefficient of a certain layer in relation to the layer
Fig. 3. The relative adhesion coefficient γr versus temperature for Ni coating (recalculated data of Fig. 2a; Ni coating with af = 5 μm is taken as the reference); The inset presents γr versus af at room temperature.
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4. Discussion The experimental results presented in the preceding section show that the vibrating reed technique is fully applicable to the described problem. It allows studying mechanical properties of the coating materials by determining the term γ2Ef as a function of temperature. Moreover, under some additional assumptions it allows also to determine the Young's modulus Ef and the adhesion coefficient γ as a function of temperature, coating thickness and/or chemical composition of the examined layer. The main result of the present paper is shown in Fig. 2a and b, where γ2Ef is plotted versus temperature for Ni and Ni + Mo coatings, respectively. The data analysis is based on two assumptions: i) the adhesion coefficient of the Ni + Mo coatings is the same as for the Ni coatings on the same substrate, and ii) the adhesion coefficient of the thinnest layer (af = 5 μm) equals one. The first assumption results from the fact that in the applied deposition technique Mo particles are embedded in the Ni matrix [1–3]. According to Fig. 3 the observed decrease of the adhesion coefficient with the coating thickness is rather weak and in the range of 5 b af b 40 μm it does not exceed 8%. So, an unavoidable error resulting from the adhesion correction of Ni + Mo composite coatings is practically negligible. The second assumption plays a role of a scaling factor and should be commented. Let notice that γ = 1 leads to the value of the Young's modulus of Ni coating at room temperature Ef = 130 GPa. As it was already mentioned this value is considerably lower than the Young's modulus of the bulk Ni i.e. 210 GPa. If we take Ef = 210 GPa (which is surely not correct) than for the thinnest layer γ = 0.78 instead of 1. In this case the data presented in the inset of Fig. 3 γr = f(af) and the data in Fig. 4 Ef = f(wt.% Mo) should be multiplied by 0.78 and 1.61 respectively. The relative relations are the same. It is also known that Ni coatings obtained by applying galvanostatic method are characterized by a rough and uneven surface which indicates that the Young's modulus of the layer material cannot be identified with its value of the bulk Ni. The surface
Fig. 4. Young's modulus at room temperature of Ni + Mo coatings electrochemically deposited on stainless steel substrate versus Mo content.
Fig. 5. Surface morphology of the electrodeposited coatings on stainless steel, a) Ni and b) Ni + Mo (×100).
morphology of the examined electrodeposited Ni and Ni + Mo coatings on stainless steel are presented in Fig. 5a and b, respectively. According to Fig. 5b one can conclude that the codeposition of Mo particles with Ni ions causes an increase in surface roughness by creating some pores. Therefore, on the one hand molybdenum particles improve electrochemical properties of the electrode material and on the other hand cause a significant deterioration of their elastic properties. It should be emphasized that irrespective of the arguments presented above it can be stated that Mo as an addition to Ni coating causes a considerable drop in the Young's modulus. According to Fig. 4 the value of Ef for the Ni coating is more than three times higher than Ef for the Ni + Mo coating with 9 wt.% of Mo. From a technological point of view irrespective of any additional assumptions, the term γ2Ef has important practical meaning and essentially gives information about elastic properties of the coating material. Indeed, for a given substrate the transfer of elastic energy from the substrate to the layer is described by the factor γ2Ef. From this point of view γ2Ef should be treated as an apparent Young's modulus of the coating material. According to Fig. 2b the porosity of Ni + Mo layer causes a decrease of γ2Ef at least 3 times in relation to Ni coating.
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The idea of determination of the so-called relative adhesion coefficient γr defined by Eq. (3) is very useful in application to the coatings on any substrate. In addition it allows determining the temperature dependence of the adhesion coefficient in relation to a reference layer. From Fig. 3 it can be concluded that with increasing temperature the adhesion of a thicker coating gets worse in relation to a thinner one. This relation allows optimizing the technique of the layer deposition and improving elastic properties of the final product. 5. Conclusions The main conclusions of the present paper can be summarized as follows: i) the adhesion coefficient of the Ni coating electrochemically deposited on stainless steel slightly decreases (about 8%) with increasing coating thickness af (5– 40 μm). Young's modulus of these coatings at room temperature is found to be 130 GPa, ii) the relative adhesion coefficient of the Ni coating electrochemically deposited on stainless steel decreases with temperature in relation to the thinnest examined layer (5 μm), and iii) Young's modulus of Ni + Mo coating electrochemically deposited on stainless steel strongly decreases with increasing Mo content; in the range of 9–32 wt.% it decreases from 40 to 23 GPa.
References [1] B. Łosiewicz, A. Stępień, D. Gierlotka, A. Budniok, Thin Solid Films 349 (1999) 43. [2] J. Kubisztal, A. Budniok, A. Lasia, Int. J. Hydrogen Energy 32 (2007) 1211. [3] R.K. Shervedani, A. Lasia, J. Electrochem. Soc. 145 (1998) 2219. [4] M. Kowalewska, M. Trzaska, Arch. Mater. Sci. 25 (2004) 553. [5] J.P. Celis, J.R. Roos, C. Buelens, J. Fransaer, Trans. Inst. Met. Finish. 69 (1991) 133. [6] S. Etienne, Z. Ayadi, M. Nivoit, J. Montagnon, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 370 (2004) 181. [7] H. Mizubayashi, Y. Yoshihara, S. Okuda, Phys. Status Solidi, A Appl. Res. 129 (1992) 475. [8] Y. Nishino, S. Asano, Phys. Status Solidi, A Appl. Res. 139 (1993) K97. [9] Y. Nishino, K. Tanahashi, S. Asano, Philos. Mag., A 71 (1995) 139. [10] Z.S. Li, Q.F. Fang, S. Veprek, S.Z. Li, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 370 (2004) 186. [11] H. Mizubayashi, T. Yamaguchi, J. Alloys Compd. 211/212 (1994) 442. [12] F.A. List, R.A. Mckee, Thin Solid Films 151 (1987) 17. [13] Q. Su, S.Z. Hua, M. Wuttig, J. Adhes. Sci. Technol. 8 (1994) 625. [14] M. Weller, in: R. Schaller, G. Fantozzi, G. Gremaud (Eds.), Mechanical Spectroscopy Q− 1 2001, vol. 366–368, TTP, Uetikon-Zuerich, 2001, p. 549. [15] J.M. Hanchar, K.L. Nagy, P. Fenter, R.J. Finch, D.J. Beno, N.C. Sturchio, Am. Mineral. 85 (2000) 1217. [16] M. Jeżewski, J. Kalisz, Physical properties tables, ed.(PWN, Warszawa, 1997), p.67.
Surface & Coatings Technology 202 (2008) 2292 – 2296 www.elsevier.com/locate/surfcoat
Elastic properties of Ni and Ni + Mo coatings electrodeposited on stainless steel substrate M. Kubisztal a,⁎, J. Kubisztal a , A. Chrobak b , G. Haneczok a , A. Budniok a , J. Rasek a a
Institute of Materials Science, University of Silesia, 40-007 Katowice, Bankowa 12, Poland b Institute of Physics, University of Silesia, 40-007 Katowice, Uniwersytecka 4, Poland Available online 2 August 2007
Abstract Adhesion coefficient and Young's modulus of Ni and Ni + Mo coatings electrochemically deposited on stainless steel were examined by applying vibrating reed technique. It was shown that adhesion coefficient of the Ni coating slightly decreases (about 8%) with increasing layer thickness (5–40 μm). Young's modulus Ef of these coatings at room temperature was found to be about 130 GPa. The relative adhesion coefficient of the Ni layer decreases with increasing temperature (300–600 K) in relation to the thinnest examined layer (5 μm). Young's modulus of the Ni + Mo coatings decreases with increasing Mo content; for 9 wt.% of Mo Ef = 40 GPa and for 32 wt.% of Mo Ef = 23 GPa. © 2007 Elsevier B.V. All rights reserved. Keywords: Composite coatings; Galvanostatic deposition; Adhesion coefficient; Young's modulus
1. Introduction Nickel-based composite coatings electrochemically deposited on a steel substrate are known as materials of special properties like very good corrosive resistance in aggressive environments or high catalytic activity in many electrochemical processes — e.g. in a process of hydrogen evolution [1–4]. In a family of electrolytic composite coatings very interesting are those containing metals like Ti, Mo and W, which could not be directly codeposited in the form of ions using aqueous solutions. However, these metals can be introduced in a form of powders by galvanic embedding their particles into a metal matrix [5]. Molybdenum as a component of composite coatings seems to be of particular interest on account of its electrochemical properties. For example in a group of cathode materials Mo plays a role of an activator in hydrogen electroevolution. Additionally, Mo as a composite ingredient modifies morphology of the coating i.e. creates porous material with rough surface. On the one hand such properties are very advisable from electrochemical point of view (e.g. improve electrocatalytic activity) and cause that Ni + Mo composite coatings are ⁎ Corresponding author. Tel.: +48 32 359 12 47; fax: +48 32 359 21 33. E-mail address: [email protected] (M. Kubisztal). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.07.057
frequently used as electrode materials for hydrogen evolution [2,3]. On the other hand the modification of nickel layer by Mo deteriorates mechanical properties of the composite coatings and hinders their applications. Thereby, the main goal of the present paper is to study the changes of Young's modulus as a function of Mo content in Ni + Mo composite coatings by applying vibrating reed technique — a nondestructive material testing method widely used in [6–14]. This method allows also studying the adhesion coefficient γ (0 ≤ γ ≤ 1) defined as a fraction of mechanical energy transferred during vibrations from the substrate to the coating material [13]. 2. Examined material and experimental procedure The experiments were carried out for Ni and Ni + Mo composite coatings obtained by galvanostatic deposition on stainless steel substrate (cathode) of nominal chemical composition — Fe (70 wt.%), Cr (20 wt.%), Ni (10 wt.%), Mn (b 0.2 wt.%). The anode was made of platinum grid (size of 1 dm2). The deposition process was carried out at a temperature of 330 K (± 3 K) using a current density of 40–80 mA/cm2 and a deposition time of 5–50 min. Before each deposition the substrate was chemically etched using concentrated HCl solution and rinsed out in distilled water. Nickel coatings were obtained using the solution: NiSO4 7H2O (84 g/dm3),
M. Kubisztal et al. / Surface & Coatings Technology 202 (2008) 2292–2296
H3BO3 (8 g/dm3), CH3COONa (10 g/dm3), C6H5O7Na3 2H2O (30 g/dm3), NH4Cl (10 g/dm3). In order to obtain Ni + Mo composite coatings Mo powder in amount of 10 g/dm3 (100 mesh, Aldrich;) was added to the solution; in both cases pH of the suspension was in the range from 5.4 to 5.8. In order to maintain homogenous concentration of all ingredients during deposition process, the electrolyte was mixed with the rate of about 200 rpm. Surface morphology of the deposited coatings was determined by Hitachi S-4200(4) scanning electron microscope (SEM). Thicknesses af of the coatings were within the range of 5– 40 μm and 25–50 μm for Ni and Ni + Mo, respectively. Molybdenum content was determined by making use of X-ray diffraction method (Philips X'Pert; CuKα radiation). The quantitative analysis was performed by comparing the integrated intensity of the strongest lines of the mixture constituents (Mo (110) and Ni (111)) with the corresponding intensity of the pure phase [15]. Molybdenum content of the Ni + Mo composite coatings was in the range from 9 wt.% to 32 wt.%. Structural examinations and electrochemical properties of these kinds of coatings were already published in [2]. Measurements of Young's modulus were carried out using a vibrating reed apparatus (clamped — free geometry) operating in the frequency range of 10–250 Hz [11,14]. Square of the free vibrations frequency fs of a cantilever sample excited into flexural vibration is proportional to the Young's modulus of the examined material Es [11,14]: 2 2 an as Es 2 ð1Þ fs ¼ 2 pl 3qs
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3. Results and data analysis Fig. 1a shows a typical example of fs2(T ) and fc2(T ) obtained for stainless steel substrate and composite consisting of the substrate and the electrochemically deposited Ni coating (af = 28 μm). Similar curves fs2(T ) and fc2(T ) for the electrochemically deposited Ni + Mo coating (af = 25 μm) are presented in Fig. 1b. Let notice that for Ni coatings fc2(T ) N fs2(T ) while for Ni + Mo we have fc2(T ) b fs2(T ). Thereby, one can expect that the term γ2Ef of Ni + Mo coatings is smaller than γ2Ef for the Ni coatings. Note that Es is the same in both cases and ρf (Ni + Mo) b ρf (Ni) (see Eq. (2)). The measurements of fs2(T ) and fc2(T ) allow determining the term γ2Ef as a function of temperature by making use of Eq. (2). The coating thickness af and the density ρf entering into Eq. (2) were calculated by making use of analysis of the cross-section micrographs (stereoscopic microscope) and by determining the coating mass, respectively. The results obtained for Ni and Ni + Mo coatings are shown in Fig. 2a and b. In both cases for a given coating thickness the product γ2 Ef decreases with temperature due to a decrease of Ef and a possible change of
where αn2 is the numerical factor which for the first odd tone (n = 1) is 0.879; as, l and ρs are the sample thickness, the sample length and the material density, respectively. In the case of a composite sample consisting of a substrate and a layer the relationship between the free vibrations frequency of the composite fc and the substrate fs is [13]: fc2 fs2 af 3g2 Ef qf ¼ ð2Þ fs2 as Es qs where subscript f stands for films, s for substrate and c for composite. The coefficient γ, introduced by Wuttig et al. [13], is the so-called adhesion coefficient and represents the elastic energy transfer from the substrate to the layer and varies between 1 (perfect adhesion) and 0 (no adhesion). It is necessary to point out that the Young's modulus Ef which enters into Eq. (2) is not identical with that of the bulk material of the layer. The reason is that Ef is a modified Young's modulus corresponding to the uniaxial stiffness of the layer when constrain to deform with the Poisson's ratio of the substrate rather than with its own unconstrained value [14]. The measurements of the free vibrations frequency were carried out in the following manner: i) the frequency fs (substrate) was measured as a function of temperature in the range of 300–600 K with the heating rate of 3 K/min, and ii) the frequency fc (composite, i.e. the same substrate + layer) was measured with the same heating rate.
Fig. 1. Square of frequency f2s and f2c versus temperature determined for: a) Ni coating (af = 28 μm) and b) Ni + Mo coating (af = 25 μm, Mo content 9 wt.%).
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Fig. 2. The term γ2Ef versus temperature determined for: a) Ni and b) Ni + Mo coatings.
taken as a reference one. It allows studying the dependence of the adhesion on coating thickness and also on some external parameters e.g. temperature. Assuming that the values of Ef are the same for all examined Ni coatings one can determine the relative adhesion coefficient γr versus temperature in relation to the thinnest layer with af = 5 μm (the reference layer). The results are presented in Fig. 3 where in the inset γr versus af at room temperature is also shown. It can be noticed that for Ni coatings γr decreases with increasing temperature which means that the adhesion of the examined coating slightly deteriorates (about 8%) in relation to the Ni coating with af = 5 μm. The assumption that the adhesion coefficient (not relative) at room temperature for the reference layer (af = 5 μm) equals one (γ = 1) corresponds to the value of Ef = 130 GPa. This means that the Young's modulus for the electrochemically deposited Ni layer on stainless steel at room temperature is considerably lower than the Young's modulus of Ni bulk material i.e. 210 GPa [16]. As it can be noticed from Fig. 3 γr slightly decreases with increasing af. The total change of γr does not exceed 8% while the error limit of the determination of the adhesion coefficient is about 3%. The result presented in Fig. 3 i.e. the dependence of the adhesion coefficient on the coating's thickness allows determining the Young's modulus of Ni + Mo coating electrochemically deposited on stainless steel. In the first approximation one can assume that the adhesion coefficient of Ni + Mo coating is the same as the adhesion of Ni coating. Indeed, Mo particles essentially have no contact with the substrate which was confirmed using scanning electron microscopy observations and cause only a change of the Young's modulus of the coating material. Taking into account this fact one can calculate Ef at room temperature for different Mo content by making use of Eq. (2) and the values of the adhesion coefficient determined for Ni coatings (see Fig. 3). The results obtained are shown in Fig. 4. It can be seen that Ef decreases with increasing Mo content.
the adhesion coefficient with temperature. For Ni coating the adhesion coefficient at a given temperature also decreases with increasing af as the modulus Ef is the same for different samples. However, in the case of the Ni + Mo composite coatings γ2Ef values depend on both af and Mo content whereas Ef is independent on coating thickness at least in the examined range. As it was already mentioned the modulus Ef does not correspond to the modulus of the bulk material [14], so the determination of the adhesion coefficient γ and the modulus Ef independently requires additional measurements or information. In spite of these difficulties some important data can be obtained. For samples with the same value of Ef (the same technique for coating deposition) one can define the so-called relative adhesion coefficient γr defined by the relation: g2r ¼
g22 Ef g22 ¼ g21 Ef g21
ð3Þ
According to this equation γr represents a change of the adhesion coefficient of a certain layer in relation to the layer
Fig. 3. The relative adhesion coefficient γr versus temperature for Ni coating (recalculated data of Fig. 2a; Ni coating with af = 5 μm is taken as the reference); The inset presents γr versus af at room temperature.
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4. Discussion The experimental results presented in the preceding section show that the vibrating reed technique is fully applicable to the described problem. It allows studying mechanical properties of the coating materials by determining the term γ2Ef as a function of temperature. Moreover, under some additional assumptions it allows also to determine the Young's modulus Ef and the adhesion coefficient γ as a function of temperature, coating thickness and/or chemical composition of the examined layer. The main result of the present paper is shown in Fig. 2a and b, where γ2Ef is plotted versus temperature for Ni and Ni + Mo coatings, respectively. The data analysis is based on two assumptions: i) the adhesion coefficient of the Ni + Mo coatings is the same as for the Ni coatings on the same substrate, and ii) the adhesion coefficient of the thinnest layer (af = 5 μm) equals one. The first assumption results from the fact that in the applied deposition technique Mo particles are embedded in the Ni matrix [1–3]. According to Fig. 3 the observed decrease of the adhesion coefficient with the coating thickness is rather weak and in the range of 5 b af b 40 μm it does not exceed 8%. So, an unavoidable error resulting from the adhesion correction of Ni + Mo composite coatings is practically negligible. The second assumption plays a role of a scaling factor and should be commented. Let notice that γ = 1 leads to the value of the Young's modulus of Ni coating at room temperature Ef = 130 GPa. As it was already mentioned this value is considerably lower than the Young's modulus of the bulk Ni i.e. 210 GPa. If we take Ef = 210 GPa (which is surely not correct) than for the thinnest layer γ = 0.78 instead of 1. In this case the data presented in the inset of Fig. 3 γr = f(af) and the data in Fig. 4 Ef = f(wt.% Mo) should be multiplied by 0.78 and 1.61 respectively. The relative relations are the same. It is also known that Ni coatings obtained by applying galvanostatic method are characterized by a rough and uneven surface which indicates that the Young's modulus of the layer material cannot be identified with its value of the bulk Ni. The surface
Fig. 4. Young's modulus at room temperature of Ni + Mo coatings electrochemically deposited on stainless steel substrate versus Mo content.
Fig. 5. Surface morphology of the electrodeposited coatings on stainless steel, a) Ni and b) Ni + Mo (×100).
morphology of the examined electrodeposited Ni and Ni + Mo coatings on stainless steel are presented in Fig. 5a and b, respectively. According to Fig. 5b one can conclude that the codeposition of Mo particles with Ni ions causes an increase in surface roughness by creating some pores. Therefore, on the one hand molybdenum particles improve electrochemical properties of the electrode material and on the other hand cause a significant deterioration of their elastic properties. It should be emphasized that irrespective of the arguments presented above it can be stated that Mo as an addition to Ni coating causes a considerable drop in the Young's modulus. According to Fig. 4 the value of Ef for the Ni coating is more than three times higher than Ef for the Ni + Mo coating with 9 wt.% of Mo. From a technological point of view irrespective of any additional assumptions, the term γ2Ef has important practical meaning and essentially gives information about elastic properties of the coating material. Indeed, for a given substrate the transfer of elastic energy from the substrate to the layer is described by the factor γ2Ef. From this point of view γ2Ef should be treated as an apparent Young's modulus of the coating material. According to Fig. 2b the porosity of Ni + Mo layer causes a decrease of γ2Ef at least 3 times in relation to Ni coating.
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The idea of determination of the so-called relative adhesion coefficient γr defined by Eq. (3) is very useful in application to the coatings on any substrate. In addition it allows determining the temperature dependence of the adhesion coefficient in relation to a reference layer. From Fig. 3 it can be concluded that with increasing temperature the adhesion of a thicker coating gets worse in relation to a thinner one. This relation allows optimizing the technique of the layer deposition and improving elastic properties of the final product. 5. Conclusions The main conclusions of the present paper can be summarized as follows: i) the adhesion coefficient of the Ni coating electrochemically deposited on stainless steel slightly decreases (about 8%) with increasing coating thickness af (5– 40 μm). Young's modulus of these coatings at room temperature is found to be 130 GPa, ii) the relative adhesion coefficient of the Ni coating electrochemically deposited on stainless steel decreases with temperature in relation to the thinnest examined layer (5 μm), and iii) Young's modulus of Ni + Mo coating electrochemically deposited on stainless steel strongly decreases with increasing Mo content; in the range of 9–32 wt.% it decreases from 40 to 23 GPa.
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