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Transactions of the IMF The International Journal of Surface Engineering and Coatings
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Dry sliding tribological behaviour of bilayer Cr/Cr coatings obtained in sulphate Cr(III) baths G. Bikulčius, A. Češūnienė, T. Matijošius & A. Selskienė To cite this article: G. Bikulčius, A. Češūnienė, T. Matijošius & A. Selskienė (2018) Dry sliding tribological behaviour of bilayer Cr/Cr coatings obtained in sulphate Cr(III) baths, Transactions of the IMF, 96:3, 130-136, DOI: 10.1080/00202967.2018.1443422 To link to this article: https://doi.org/10.1080/00202967.2018.1443422
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TRANSACTIONS OF THE IMF, 2018 VOL. 96, NO. 3, 130–136 https://doi.org/10.1080/00202967.2018.1443422
Dry sliding tribological behaviour of bilayer Cr/Cr coatings obtained in sulphate Cr (III) baths G. Bikulčius, A. Češūnienė, T. Matijošius and A. Selskienė CPST (Center for Physical Sciences and Technology), Institute of Chemistry, Vilnius, Lithuania ABSTRACT
ARTICLE HISTORY
This study was intended to investigate the properties of Cr/Cr bilayer coatings. These coatings were deposited on a copper substrate by the DC electrodeposition method from Cr(III) sulphate baths with additions of formate-urea or glycine as complexing agents. Examination of the surface morphology of Cr coatings with SEM has shown that both single and bilayer Cr coatings obtained in the Cr(III) bath with formate-urea, and those obtained in the Cr(III) bath with glycine are cracked. It has been determined that the surface microhardness (HV) of bilayer Cr coatings obtained in the Cr(III) bath with glycine is higher compared with that of single-layer Cr coatings. Wear testing of the coatings was undertaken against an Al2O3 ball counterface (6 mm diameter) at 1N load. The results indicate that the friction coefficients (COF) of bilayer Cr/Cr coatings obtained in the Cr(III) bath with formate-urea increased from 0.2 to 0.5 compared with that of single-layer Cr coatings, while their wear resistance deteriorated. However, bilayer Cr/Cr coatings obtained in the Cr(III) bath with glycine exhibit wear resistance close to that of single coatings with COF equal to 0.05.
Received 25 September 2017 Accepted 8 December 2017
1. Introduction The trivalent chromium electrodeposition process is considered to be 500 to 1000 times less toxic than hexavalent chromium.1–3 The substitution of coatings obtained from the hexavalent chromium bath in a friction pair with the Cr coatings obtained from Cr(III) baths is conjectural, inasmuch as thick Cr coatings from Cr(III) baths cannot be developed easily.4 After deposition of some micrometres of coating from the latter, the deposition rate and current density quickly fall and then deposition stops.5 The maximum Cr thickness obtained from commercial trivalent baths is 0, the surface is flat with peaks (Figure 1(b)). The microhardness tests were performed on the surface of the coatings using a PTM-3 (Russia) set-up at a load of 20 g. The values of HV are the average of five indentations. Dry sliding ball-on-plate wear tests on coated samples were carried out in the laboratory atmosphere with a relative humidity of 30–40% at room temperature. For tribological measurements, a CSM Tribometer (Anton Paar, Switzerland) was used. A ball (Al2O3) of 6 mm was fixed stationary. The Cr specimen was mounted on a pre-installed tribometer module, which maintained a linear reciprocal motion of 4 mm amplitude, speed 2 cm s−1, and load 1 N. For plotting the graphs, each data point of the coefficient of friction was obtained by taking an arithmetical average of the modular values from the central 80% segment of the linear path. Wear profiles were measured after friction coefficient recording, and cross-section area of wear tracks was calculated from at least three measurements on different wear track locations within the same specimen. Specific wear rates (mm3 N−1 m−1) were calculated using the following equation: K = (Sl)/(Fd), where S is the average cross-section area of wear track in mm2; l is the length of wear track in mm; F is the load applied in N; d is the sliding distance in m. The most representative wear tracks were selected for comparison between samples in profilometry graphs.
of formate-urea (Figure 2(a)), while it is completely different in the case of glycine (Figure 2(b)). In both the cases, the authors failed to prevent emerging of cavities. It is anticipated that they emerged owing to abundant hydrogen evolution. Any adhesion problems both between Cr layers and between the Cr coating and substrate have not been observed.
3.2. Morphology, roughness analysis and microhardness SEM micrographs from the surface of the Cu substrate and single- and bilayer Cr coatings obtained in the Cr(III) baths are presented in Figure 3. On the surface of the prepared substrate, marks of mechanical polishing (Cu/Cu’) can be seen. The morphology of ASL, BSL and CBL coatings obtained in the Cr(III) bath with formate-urea has a typical nodular structure. Moreover, cracks were observed in all the cases, in contrast to Cr coating deposited in the Cr(III) bath with formateurea on the stainless steel substrate.28 Similar results were obtained while depositing ESL, DSL and FBL Cr coatings
3. Results and discussion 3.1. Interface It is known27 that Cr coatings obtained in Cr(III) baths compared with those deposited in Cr(VI) baths are characterised by tolerance to the interruption of DC, and in particular the sulphate-based trivalent Cr bath is not susceptible to current interruption. It should be noted that an attempt to form bilayer Cr coatings in Cr plating electrolytes by only turning on and off DC has not met with success. For example, the bilayer Cr coating formed in the Cr(III) formateurea electrolyte exhibits poor adhesion to the Cr substrate which manifests itself as emerging blisters. Electrochemical fabrication of bilayer Cr coatings is shown in Figure 2. This figure clearly demonstrates that the interface formed between the first and second Cr layers depends on the nature of complexing agent added to the Cr(III) plating electrolyte. The interface akin to ‘weld juncture’ is formed in the case
Figure 2. Interfaces of bilayer Cr coatings obtained in the Cr(III) baths: (a) formate-urea (CBL) and (b) glycine (FBL).
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Figure 3. SEM images of surfaces: Cu substrate and Cr coatings obtained in the baths with formate-urea: single-layer ASL and BSL and bilayer CBL; and with glycine: single-layer ESL and DSL and bilayer FBL as deposited.
from the Cr(III) bath with glycine. It is thought the microcracks during electrodeposition occurred due to the adsorption of hydrogen gas. It is known29 that Ra gives a very good overall description of height variations but does not give any information on waviness, and it is not sensitive to small changes in profile. Rsk is defined as skewness and is sensitive on occasional deep valleys or high peaks. Zero skewness reflects in
symmetrical height distribution, while positive or negative skewness describes surfaces with high peaks or filled valleys and with deep scratches or loss of peaks, respectively. Thus, it is very important to evaluate both the parameters. It is of interest to compare the Cu substrate to Cr coatings in terms of their Ra and Rsk parameters. In Figure 3, mechanical polishing marks on the surface of Cu substrate can be seen. The roughness parameters of such a surface were equal to
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Figure 4. Surface microhardness (HV) of Cr single and bilayer coatings obtained in the baths with formate-urea: ASL, BSL, CBL and with glycine DSL, ESL, FBL as deposited.
0.58 µm. Both the Ra parameters of the ASL and BSL coatings deposited on the Cu substrate in the Cr(III) formate-urea bath were approximately twice as great as that of Cu substrate, whereas the Ra of the bilayer CBL coating assumes a value intermediate between those of ASL and BSL coatings. Parameter Ra of Cr coatings obtained from the Cr(III) bath with glycine behaves analogously. In the case when the single-layer ASL Cr coating is deposited on the Cu substrate with negative Rsk (−0.77), the surface parameter of this coating assumes a positive value (0.59) (Figure 3). This suggests that the single ASL Cr coating does not replicate the morphology of the Cu substrate surface. When the BSL Cr coating is deposited on the Cu substrate, Rsk assumes a positive value (0.75), which suggests that the surface morphology of single BSL Cr coating does not replicate the morphology of the Cu substrate. Once the BSL single Cr coating was deposited on the ASL Cr coating, a bilayer CBL coating, whose Rsk is positive (as in the case of the BSL Cr coating), was fabricated. The value of Rsk of the DSL Cr coating deposited on the Cu substrate is also positive (0.13), meanwhile the Rsk of the ESL Cr coating already assumes a negative value (−0.31), i.e. replicates the morphology of Cu substrate. Eventually after the fabrication of the FBL bilayer Cr coating, it becomes clear that upon deposition of ESL on DSL, differently from ESL deposited on the Cu substrate, Rsk assumes a positive (0.26) value. The reason of this is still unclear and remains yet to be determined. Surface microhardness (HV) values of Cr single- and bilayer coatings obtained from the baths with formate-urea ASL, BSL, CBL and with glycine DSL, ESL, FBL as deposited are shown in Figure 4 for comparative purposes. It is clear that the hardness of the bilayer Cr coating obtained from the baths with formateurea assumes an HV value intermediate between those of ASL and BSL. An analogous tendency was also observed for roughness parameter Ra (Figure 3). Meanwhile, the HV of Cr coatings obtained from the baths with glycine behaves in a different manner. From this bath, the bilayer (FBL) coating exhibits the highest HV. The results obtained are in good agreement with the results reported previously.30–33 The different HV behaviour of Cr coatings cannot be explained unambiguously. It is obvious that in each specific case coating HV will depend on a wide range of factors, for example, on the composition of the electrolyte and the
Figure 5. Friction coefficient (μ) of Cr single- and bilayer coatings as a function of the number of friction cycles: Cr coatings obtained in the baths with formateurea: ASL, BSL, CBL and with glycine DSL, ESL, FBL as deposited.
conditions of coating deposition, coating thickness and residual stress. In addition, the HV of bilayer coatings will also depend on the distribution of residual stress over the layers.
3.3. Analysis of friction coefficient The coefficients of friction for six different Cr coatings are shown in Figure 5. As can be seen, the friction coefficients of the ASL and BSL coating deposited in the Cr(III) bath with formate-urea on the Cu substrate after 10 cycles are 0.32
Figure 6. Profile of wear scars of Cr single- and bilayer coatings obtained in the baths with formate-urea (ASL, BSL and CBL) and with glycine (DSL, ESL and FBL) as deposited.
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FBL bilayer Cr coating are about seven times as high as compared with those of the ESL single Cr coatings (Figure 6(b)). Figure 7 illustrates the dependence of the specific wear rate of Cr single and Cr bilayer coatings on the nature of the chromium plating electrolyte. It can be seen that irrespective of whether the Cr(III) electrolyte bath contains formate-urea or glycine, the specific wear rates of bilayer Cr coatings CBL and FBL compared with those of single Cr coatings ASL, BSL and DSL, ESL are worse. Comparison of the specific wear rate of bilayer coatings of CBL with that of FBL shows that the specific wear rate of CBL is significantly higher than that of FBL. This can be explained by the different interfaces.
3.5. Worn surface morphology Figure 7. Comparison of specific wear rate of Cr single- and bilayer coatings obtained in the baths with formate-urea (ASL, BSL and CBL) and with glycine (DSL, ESL and FBL) as deposited.
and 0.20, respectively. The bilayer CBL coating friction coefficient is 0.50. The friction coefficients of the DSL, ESL and FBL coatings obtained in Cr(III) bath with glycine are completely different. Here friction coefficients of single DSL and bilayer FBL coatings at 10 cycles are about 0.05. The friction coefficient of the ESL coating is equal to 0.41. DSL and FBL coatings, in contrast to the other coatings, retain low friction coefficients even after 100 cycles.
3.4. Analysis of wear depth profile and specific wear rate Profiles of wear scars of Cr single-layer ASL, BSL, DSL and ESL, and bilayer CBL and FBL coatings after the 1000 cycles are presented in Figure 6. The ball wear rate was not measured, since the wear scars were too small and smooth to be observed. The depth profiles of wear scars of CBL bilayer Cr coatings are twice as high as compared with those of BSL single Cr coatings, 10.0 and 5.0 μm, respectively. (Figure 6(a)). The depth profiles of wear scars of the
Figure 8. SEM images of the worn surface of Cr single- and Cr bilayer coatings obtained in the baths with formate-urea (ASL, BSL and CBL) and with glycine (DSL, ESL and FBL) as deposited.
The worn areas on the surface of single- and bilayer coatings were further analysed by utilising SEM analysis (Figure 8). It is known34 that the wear mechanism could be inferred by the wear scar morphology. According to this study,34 the wear between single Cr coatings ASL and BSL obtained in the Cr (III) electrolyte bath with formate-urea and Al2O3 counterbody proceeds by adhesion wear mechanisms. In the case of bilayer CBL, wear proceeds by the adhesion/abrasive wear mechanism. As for the Cr coatings obtained from the Cr(III) bath with glycine, they behave differently. The wear behaviour of DSL and ESL coatings is different to that of the electrodeposited FBL, because they are softer and more ductile than the FBL coating. DSL and ESL coating wear proceeds according to the adhesion/plastic deformation wear mechanism. For example, when the hardness values of Cr coating surface of DSL and ESL are 181 kg mm−2 and 133 kg mm−2, respectively (Figure 4), adhesion/plastic deformation can be observed (Figure 8), while at the coating microhardness (FBL) of 405 kg mm−2 (Figure 4), the wear process proceeds according to the adhesion wear mechanism (Figure 8). It is very important to recognise two variations of wear: dry wear and wear under lubrication. According to the data obtained from earlier studies,35,36 the dry test coefficient of friction is lower when roughness is high, while the coefficient of friction is lower when roughness is low for the lubricated test. Friction also tends to get lower when parameter Rsk gets more negative for lubricated tests. The values of the friction coefficient obtained under dry wear in the present study are not in accordance with the above statement. In the opinion of the present authors, this can be explained by the fact that all the coatings obtained differed from one another. On the other hand, what is more important: the lowest possible COF or specific wear rate of Cr? It is axiomatic that preference is given to coatings exhibiting both the lowest COF and the lowest specific wear rate of Cr. Our results suggest that the single-layer ESL coating, whose friction coefficient and specific wear rate are 0.05 and 0.8 mm3 N−1 m−1, respectively, represents the optimal choice. Although wear resistance under dry sliding conditions of bilayer Cr coatings compared with that of single-layer Cr coatings is not higher, nevertheless employment of bilayer coatings may be advisable under lubrication conditions. The more so that for lubricated sliding contact, surfaces with more negative Rsk, in which the surface was relatively flat with many deep valleys, have been reported as resulting in low friction.25 Thus, of all the results obtained from this study, the bilayer FBL Cr coatings obtained in the Cr(III) bath with glycine are best suited for this purpose.
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4. Conclusions In this study, the surface morphology, surface roughness, surface microhardness and wear behaviour of Cr single- and Cr bilayer coatings have been investigated. (1) Cr bilayer coatings were fabricated by electrodeposition of Cr on the Cr layer deposited in the Cr(III) sulphate bath. (2) The structure of the interface of bilayer Cr coatings depends on the nature of the complexing agent in the Cr(III) electrolyte. (3) Cr coatings had cracks regardless of whether they were single- or bilayer ones and irrespective of whether formate-urea or glycine were used as additives. (4) The surface roughness of bilayer Cr coatings obtained in the Cr(III) bath with formate-urea and that of the coatings obtained in the Cr(III) bath with glycine is medium compared with the roughness of single-layer coatings. (5) The surface microhardness (HV) of bilayer Cr coatings obtained in the Cr(III) bath with glycine is higher compared with that of single-layer Cr coatings. (6) Both single-layer (DSL) and bilayer (FBL) Cr coatings obtained in the Cr(III) bath with glycine exhibit similar tribological characteristics. (7) Bilayer (FBL) Cr coatings obtained in the Cr(III) bath with glycine, compared with the single-layer (DSL) Cr coating, exhibit a lower coefficient of friction, but a higher specific wear rate.
Acknowledgements The authors wish to thank Dr Irena Mirvienė for improving the language of this manuscript.
Disclosure statement No potential conflict of interest was reported by the authors.
References 1. L. Li, G. P. Swain, A. Howell, D. Woodbury and G. M. Swain: J. Electrochem. Soc., 2011, 158, C274. 2. L. Li, K. P. Doran and G. M. Swain: J. Electrochem. Soc., 2013, 160, C396. 3. L. Li, A. L. Desouza and G. M. Swain: J. Electrochem. Soc., 2014, 161, C246. 4. M. S. Doolabli, S. K. Sadrnezhaad and D. S. Doolabli: Anti-Corros. Meth. Mater., 2014, 61, 205–214. 5. infohouse.p2ric.org/ref/29/28140.pdf. 6. V. Protsenko, V. Gordiienko, T. Butyrina, E. Vasil’eva and F. Danilov: Turkish J. Chem., 2014, 38, 50–55.
7. M. S. Chandrasekar and Malathy Pushpavanam: Electrochimica Acta, 2008, 53, 3313–3322. 8. A. Liang, Y. Li, H. Liang, L. Ni and J. Zhang: Mater. Lett., 2017, 189, 221–224. 9. F. I. Danilov, V. S. Protsenko, T. E. Butyrina, V. A. Krasinskaia, A. S. Baskevich, S. C. Kwonb and J. Y. Lee: Protect. Metals Phys. Chem. Surf., 2011, 47, 598–606. 10. L. Sziraki, E. Kuzman, K. Papp, C. U. Chisholm, M. R. El-Sharif and K. Havancsak: Mater. Chem. Phys., 2012, 133, 1092–1100. 11. A. W. Ruff and D. S. Lashmore: Wear, 1991, 151, 245–53. 12. E. L. Courtright and J. W. Patten: Wear behaviour in thin layered Au/SS and Au/Mo coatings, Office of Naval Research, Battelle, Pacific Northwest Laboratories, Technical Report, Washington, 1982. 13. S. Norose and T. Sasada. Anti-wear property of metal laminates. Proc. Japan, International Tribology Conference, Japanese Society of Tribologists, Nagoya; 1990. 689–694. 14. X. Deng, N. Chawla, K. Chawla, M. Koopman and J. Chu: Adv. Eng. Mater., 2005, 7, 1099–108. 15. Y. Pauleau and F. Thiery: Surf. Coat. Technol., 2004, 180–181, 313–322. 16. G. Portu, L. Micele, D. Prandstraller, G. Palombarini and G. Pezzotti: Wear, 2006, 260, 1104–1111. 17. F. Toschi, C. Melandri, P. Pinasco, E. Roncari, S. Guicciardi and G. De Portu: J. Am. Ceram. Soc., 2003, 86, 1547–53. 18. D. F. Arias, A. Gómez, J. M. Vélez, R. M. Souza and J. J. Olaya: Mater. Chem. Phys., 2015, 160, 131–140. 19. A. Hadipour and S. M. Monirvaghefi: Surf. Eng., 2015, 31, 666–672. 20. S. Survilienė, V. Jasulaitienė, O. Nivinskienė and A. Češunienė: Appl. Surf. Sci., 2007, 253, 6738–6743. 21. S. Survilienė, A. Lisowska-Oleksiak, A. Selskis and A. Češunienė: Trans. IMF, 2006, 84, 241–245. 22. S. Survilienė, V. Jasulaitienė, A. Češūnienė and A. Lisowska-Oleksiak: Solid State Ionics, 2008, 179, 222–227. 23. S. Survilienė, A. O. Češūnienė, A. Selskis and R. Butkienė: Trans. IMF, 2013, 91, 24–31. 24. V. S. Protsenko and F. I. Danilov: Clean Technol. Environ. Policy, 2014, 16, 1201–1206. 25. M. Sedlaček, B. Podgornik and J. Vižintin: Tribol. Internat., 2012, 48, 102–112. 26. Jun Feng Su, Xueyuan Nie, and Henry Hu: J. Vac. Sci. Technol. A, 2012, 30(6), 1–9. 27. http://infohouse.p2ric.org/ref/25/24598.pdf 28. G. Bikulčius, A. Češunienė, A. Selskienė, V. Pakštas and T. Matijošius: Surf. Coat. Technol., 2017, 315, 130–138. 29. E. S. Gadelmawla, M. M. Koura, T. M. A. Maksoud, I.M. Elewa and H.H. Soliman: J. Mater. Proc. Tech., 2002, 123, 133–145. 30. M. Nordin, M. Larson and S. Hogmark: Surf. Coat. Technol., 1998, 106, 234–241. 31. Y. Y. Chang, D. Y. Wang and C. Y. Hung: Surf. Coat. Technol., 2005, 200, 1702–1708. 32. Y. Y. Chang, S. J. Yang and D. Y. Wang: Surf. Coat. Technol., 2006, 201, 4209–4214. 33. S. M. Yang, Y. Y. Chang, D. Y. Lin, D. Y. Wang and W. T. Wu: Surf. Coat. Technol., 2008, 202, 2176–2181. 34. N. Suh: Wear, 1973, 25, 111–124. 35. M. Sedlaček, B. Podgornik and J. Vižintin: Wear, 2009, 266, 482–487. 36. Sh. Zhu and P. Huang: Tribol. Internat., 2017, 109, 10–18.
