Microstructure and mechanical properties of nanocrystalline Ni-Mo protective coatings

IOP Conference Series: Materials Science and Engineering Microstructure and mechanical properties of nanocrystalline Ni-Mo protective coatings To cite this article: A Bigos et al 2012 IOP Conf. Ser.: Mater. Sci. Eng. 32 012002 View the article online for updates and enhancements. Related content - Microstructural characterisation of electrodeposited coatings of metal matrix composite with alumina nanoparticles P Indyka, E Beltowska-Lehman and A Bigos - Surface morphology and electrochemical characterization of electrodeposited Ni Mo nanocomposites as cathodes for hydrogen evolution Elhachmi Guettaf Temam, Hachemi Ben Temam and Said Benramache - The application of VP-ESEM in microstructure analysis of ceramic macroporous scaffolds for bone tissue engineering A M Janus and M Faryna This content was downloaded from IP address 46.3.193.243 on 29/01/2018 at 18:38

Microstructure and mechanical properties of nanocrystalline Ni-Mo protective coatings A Bigos 1, E Beltowska-Lehman and P Indyka Polish Academy of Sciences, Institute of Metallurgy and Materials Science, 25 ul. Reymonta, PL-30059 Krakow, Poland E-mail: agnieszkabigos21@gmail.com Abstract. Nickel-molybdenum alloys are of interest due to their functional properties, such as high hardness, corrosion resistance, as well as low wear and friction coefficients. Thus, they provide an excellent alternative to hard chromium coatings obtained from toxic electrolytes. Characterized coatings were electrodeposited on the steel substrates, from an aqueous citrate complex solution, containing nickel and molybdenum salts, in a system with a rotating disk electrode (RDE). The effect of cathodic current density on microstructure, chemical and phase composition, and crystallite size of the coatings was determined by the scanning and transmission electron microscopy, energy-dispersive X-ray spectrometry, and X-ray diffraction. The influence of microstructure as well as chemical composition on functional properties of coatings were also determined. 1. Introduction Hard chromium coatings, characterised by enhanced functional properties, are widely used in many industrial applications. However, according to the European Union legal restriction (Directive: 2000/53/EC), they should be eliminated from the manufacturing process due to toxicity of electrolytes (containing carcinogenic hexavalent chromium) used for their preparation [1]. Ni-based alloys with refractory metals are proposed as a promising substitute for chromium coatings, characterized by good corrosion properties in many aggressive environments and catalytic activities for hydrogen evolution [2, 3]. Moreover, the addition of molybdenum to the nickel deposit should significantly improve their hardness, wear, and friction resistance. Among the methods for obtaining Ni-based coatings, electroplating is a relatively simple and low-cost technique, which enables uniform covering of substrates with defined shapes. In addition, it eliminates the problem of conventional thermal processes associated with large differences in the melting points of metals (e.g., Ni-1455 C, Mo-2620 C) and their limited mutual solubility [4]. The mechanism of electrodeposition process is still not clearly understood, despite a few hypotheses presented in the literature [5, 6]. It is known that pure Mo cannot be electrodeposited in a metallic state from an aqueous solution of its salts. The process is hindered by the formation of a molybdenum oxide layer of lower valences on the cathode surface. Only co-deposition of the alloy with iron-groups metals (Fe, Co, Ni) is possible. According to Brenner that phenomenon is called induced co-deposition [7]. 1 To whom any correspondence should be addressed. Published under licence by IOP Publishing Ltd 1

Microstructure and chemical composition are the main factors influencing the functional properties of electrodeposited coatings. Thus, for optimising electroplating parameters to produce adherent and uniform electrodeposits with appropriate micro-mechanical properties, the impact of chemical composition on microstructure changes should be discussed in detail. Hence, the aim of the present study was to determine the correlation of the microstructural characteristics, chemical (influence of molybdenum content) and phase compositions with coating mechanical and tribological properties such as microhardness, Young modulus, wear resistance, and roughness. To achieve this goal, the scanning electron microscopy (SEM), transmission electron microscopy (TEM), as well as X-ray diffraction (XRD) techniques for the analysis of Ni-Mo deposits were applied. 2. Experimental details Micrometre-thick Ni-Mo alloys were electrochemically deposited from the citrate bath solution containing NiSO 4, Na 2 MoO 4 of ph 8.5 (adjusted by an addition of ammonia). No detergents, wetting agent, or brightener were used. The cathode was a low carbon steel disc of 0.028 dm 2, rotating at 640 rpm, whereas a 0.05 dm 2 Pt spiral was used as an anode. Electrolysis was carried out in a 0.75 dm 3 cell in a model system with a rotating disk electrode (figure 1). Current density was controlled by a PAR 273A potentiostat/galvanostat with the correction of an ohmic drop by the current interrupt method. Ni-Mo alloys were deposited at room temperature under galvanostatic conditions in the range of current density 0.5-5 A/dm 2. Figure 1. Experimental system with a rotating disk electrode (RDE). The microstructure of the deposits was characterized by electron microscopy techniques: scanning FEI ESEM XL30 (accelerating voltage conditions of 20 kv), and transmission FEI Tecnai G 2 (200 kv), equipped with an Edax energy-dispersive X-ray spectrometer. The thin foils for TEM investigation were prepared by focussed ion beam (FEI Quanta 2D Dual Beam with Omniprobe lift-out system). Concentrations of elements were determined by energy-dispersive X-ray spectroscopy (EDS) in the SEM equipped with an Edax Gemini 4000 X-ray spectrometer. ZAF correction was used to improve the accuracy of X-ray microanalysis. Phase composition and crystallite size measurements were carried out by X-ray diffraction, using a Co K α line (diffractometer Philips PW 1710 with X'Pert system). The surface roughness measurements were conducted by TOPO 01P profilometer from the sample area of 1.2 mm x 1.2 mm. The coating hardness was examined using the instrumented indentation technique according to the ISO 14577-1 standard (Micro-Combi-Tester produced by CSEM Instrument). The measurements were performed with a Vickers diamond indenter (for each coating at least 7 indentations) under the load of 50 mn applied at 100 mn/s. The duration time at maximum load was 5 seconds. The wear properties of electrodeposits were evaluated using a ball-on-disk apparatus with alumina balls of 1 mm in diameter as the counter body. The investigations were conducted at established test parameters: wear track of 5 mm diameter, rotation speed of 60 rpm, normal load of 1 N and 20000 as the total number of cycles. 2

3. Results and discussion The process of electrochemical deposition is determined mainly by cathodic current density. That crucial operating parameter controls the chemical composition and microstructure as well as the rate of the electrodeposition process and current efficiency. Mo, wt% 24 22 20 18 16 14 12 10 8 6 4 0 1 2 3 4 5 j, A/dm 2 Figure 2. Mo content (wt%) in function of current density. Figure 2 shows the dependence of the chemical composition of the coatings (determined by the EDS technique) on an applied current density. As can be seen, an increase of current density is strictly correlated with decrease in Mo content (from about 22 to 5 wt%) (table 1). Table 1. Chemical composition of Ni-Mo alloys electrodeposited at different current densities determined by EDS (analyses were performed at 20 kv). wt% j, A/dm 2 0.5 1.5 2.5 3 3.5 4 4.5 5 Mo 22.2±0.4 21.0±0.4 19.0±0.8 6.9±0.3 5.6±0.2 5.4±0.2 5.1±0.2 4.9±0.5 Ni 77.8±1.6 79.0±1.6 81.0±1.6 93.1±1.9 94.4±1.9 94.6±1.9 94.9±1.9 95.1±1.9 The SEM cross-section examination revealed that all deposits (regardless of the current density at which they were deposited) were crack free, homogenous, and well adherent to the steel substrate as presented in figure 3. 1.5 A/dm 2 4.5 A/dm 2 Ni-Mo Ni-Mo steel substrate steel substrate Figure 3. SEM images of cross-section of metallic Ni-Mo coatings electrodeposited at 1.5 A/dm 2 and 4.5 A/dm 2, at an RDE speed of 640 rpm. 3

However, scanning electron microscopy observations of the Ni-Mo surface show marked influence of current density on the microstructure of coatings (figure 4). 1.5 A/dm 2 21 ± 0.4 wt% Mo 3 A/dm 2 6.9 ± 0.3 wt% Mo 2 5.1 ± 0.2 wt% Mo 4.5 A/dm Figure 4. SEM (SE) images of surface morphology and EDS spectra of Ni-Mo coatings electrodeposited at 1.5 A/dm 2, 3 A/dm 2, and 4.5 A/dm 2, at an RDE speed of 640 rpm. As seen, the molybdenum content in deposited alloys strongly affects the surface topography. In the first range of low current densities (0.5-2.5 A/dm 2 ) the Mo content is relatively high (about 20-22 wt%) and coatings are characterized by nodular morphology. When the density range increases to 2.5-3.5 A/dm 2 the size of the nodules are considerably reduced, which correlates with the 4

EMAS 2011: 12th European Workshop on Modern Developments in Microbeam Analysis IOP Publishing IOP Conf. Series: Materials Science and Engineering 32 (2012) 012002 doi:10.1088/1757-899x/32/1/012002 decrease in Mo content from about 19 to 6 wt%. At a current density above 3.5 A/dm2, coatings have smooth and compact morphology with a relatively low content of Mo (about 5 wt%). The EDS spectra of exemplified deposits obtained at three different current densities (figure 4) confirm that the complete reduction of Mo(VI) complex ions occurs at the steel cathode. Indeed, the oxygen content in such coatings is at the background level. The obtained Ni-Mo coatings do not contain oxides. Moreover, deposit morphology change is correlated with the decrease of the roughness coefficient Ra from a value of 0.27 µm to 0.15 µm up to 3 A/dm2 (figure 5). a) b) c) Figure 5. 3D surface profile of Ni-Mo coatings deposited at different current densities a) 1.5 A/dm2, b) 3 A/dm2 and c) 5 A/dm2. TEM observations of the Ni-Mo cross-section prepared by FIB technique showed that they are characterized by an equiaxial nanocrystalline microstructure with a grain size below 20 nm (for coatings deposited at 5 A/dm2) (figure 6). The selected area electron diffraction pattern phase analysis indicated that the coatings revealed an α-ni(mo) solid solution [8, 9]. Ni-Mo steel substrate 5 A/dm2 Figure 6. TEM images (bright field with the electron diffraction pattern (SAED) left, high resolution right) of an example of Ni-Mo coatings obtained at a current density of 5 A/dm2. 5

XRD shows that with increasing molybdenum content in the electrodeposits, the lines of the patterns increasingly broadened, their intensities decreased, and the (111) preferred orientation became less pronounced. The grain size (the size of the coherent domains) was calculated from the line broadening, using Scherrer s equation. They decrease slightly (from 10 to 4 nm) when the molybdenum content increases from about 5 to 22 wt% (Figure 7). Ni 111 Fe 110 Ni 200 Fe 200 Ni 220 Fe 211 Ni 311 counts 5 A/dm 2 4 A/dm 2 3 A/dm 2 2.5 A/dm 2 1.5 A/dm 2 40 50 60 70 80 90 100 110 120 2θ [deg] Figure 7. XRD pattern of Ni-Mo coatings deposited at different current densities (1.5-5 A/dm 2 ). Microstructure and chemical composition of electrodeposited Ni-Mo alloys are closely related to their mechanical properties [10]. Microhardness and elastic modulus values, analyzed by microindentation test, rise (from 6.6 to 7.8 GPa and 184 to 258 MPa, respectively) with an increase of current density up to 3 A/dm 2. Further rise of this operating parameter leads to stabilisation of their values till 4 A/dm 2 (7.6 GPa and 182 GPa) and slightly drop above 5 A/dm 2. The deformation mechanism and adhesion to the steel substrate were investigated by scratch test. For coating deposited under low current density, any form of cracks and delimitations were observed up to a maximal load 30 N. For stiffer coatings prepared with 3.5 A/dm 2, the first cohesive cracks appeared when the load exceeded 18 N, which correlates with an increased value of the elastic modulus. At current densities above 4 A/dm 2, again a plastic character of deformation was observed. Results of tribological tests indicated that a significant increase of wear resistance was found for electrodeposits prepared under current densities higher than 3 A/dm 2. Moreover, the friction coefficient is the lowest (0.23) for the Ni-Mo alloys obtained with high current densities. 4. Conclusion - Electrodeposition of alloys from the designed citrate bath solution containing considerable excess of Ni(II) relative to Mo(VI) content results in obtaining a good quality Ni-Mo coatings: crack free, compact, and well adherent to the steel substrate. - Cathodic current density is an operating parameter which controls the efficiency of the electrodeposition process as well as the chemical composition and microstructure of coatings. - The nanocrystalline Ni-Mo coatings of smooth surface microstructure and relatively low molybdenum content, obtained at the highest examined current densities, are characterized by the best micromechanical and tribological properties. Thus, the Ni-Mo alloys obtained under these conditions are promising materials from the viewpoint of their potential application as an alternative to hard chromium coatings. 6

Acknowledgments The results presented in this paper have been obtained within the project "KomCerMet" (contract no POIG.01.03.01-14-013/08-00 with the Polish Ministry of Science and Higher Education) in the framework of the Operational Programme Innovative Economy 2007-2013. References [1] Brooman E W 2000 Met. Finish. 6 38-43 [2] Ghahin G E 1998 Plat. Surf. Finish. 8 8-14 [3] Hille R 2002 Trends Biochem. Sci. 7 360-7 [4] Massalski T B 1990 Binary alloy phase diagrams (The United States of America: ASM International) [5] Chassaing E, Yu Quang K and Wiart R 1989 J. Appl. Electrochem. 19 839-44 [6] Podlaha E J and Landolt D 1996 J. Electrochem. Soc. 143 893-9 [7] Brenner A 1963 Electrodeposition of alloys (New York, London: Academic Press) [8] Labar J L 2008 Microsc. Microanal. 14 287-95 [9] Labar J L 2009 Microsc. Microanal. 15 20-9 [10] Kot M, Bełtowska-Lehman E, Bigos A, Indyka P, Morgiel J and Rakowski W 2010 Mater. Eng. 3 373-6 7