Electrosynthesis and performances of cobalt–ceria nanocomposite biocoatings

Abstract

Co–CeO₂ nanocomposite coatings were obtained by electrocodeposition of nanoparticulate cerium oxide bioceramic (25 nm mean diameter) into cobalt matrix on stainless steel support and the resulting properties were evaluated comparatively with pure cobalt layers. The surface and anticorrosion properties in the simulating body fluid (SBF) solution of the obtained Co–CeO₂ nanocomposite layers are characterised using scanning electron microscopy with energy dispersive spectroscopy system, ultrahigh microtopography, Vickers surface microhardness, linear polarisation and electrochemical impedance spectroscopy techniques. The results indicate that CeO₂ nanoparticles are uniformly dispersed into cobalt matrix and influence the surface morphology, microtopography, and microhardness and corrosion behaviour of nanocomposite coatings obtained. Coating thickness increases with increased current density as well as the surface roughness and microhardness. At the same time, the results obtained by corrosion tests show that when compared with the traditional polycrystalline cobalt films, the obtained Co–CeO₂ nanocomposite layers exhibit the enhanced corrosion resistance in the SBF solution.

Keywords

Nanocomposites Ceria nanoparticles Coating Corrosion Microhardness

Introduction

In recent years much attention has been focused on the research and development of metal matrix composite coatings because such materials offer outstanding mechanical and multifunctional properties. These composite coatings possess enhanced properties such as wear, corrosion and oxidation resistance, dispersion hardening or self-lubrication relative to pure metal, so that they can protect the metal substrates more effectively against severe environmental conditions during operation,1 and provide improved electrocatalytic activity.2,3^–10 The metal matrix composite can be produced through a number of routes including metal processing, powder metallurgy, electrodeposition techniques, etc.11 By electrodeposition technique the composite coatings are obtained during electrocodeposition of fine ceramic or polymer particles dispersed within a metallic matrix from metal electrolytic baths. There are reports on the incorporation of nanosized SiC, ZrO₂, Al₂O₃, TiO₂, La₂O₃ and CeO₂ in the nickel matrix forming composites.3,5,12^–19 The incorporated inert particles have played an important role in enhancing either the corrosion resistance or the wear resistance, many works being carried out on the Ni matrix. There are reports on the synthesis and properties of Ni–Al₂O₃ and Ni–ZrO₂ composite coatings. 5 6 5,6,13 However, to the best of our knowledge, there are no reports on the synthesis and properties of Co composite coatings containing an oxide powder such as CeO₂ and the performances characterisation of composite coatings obtained. Among the oxide ceramics, the silicon carbide, alumina and mixed alumina–zirconia systems are by far the most useful ceramics to prepare composite coatings. Because of their excellent mechanical properties, including strength, toughness and wear resistance, as well as the thermal and chemical stability, the alumina and zirconia composite coatings have been widely used in the engineering and biomedical fields for materials such as structural ceramics, thermal barrier coatings and total hip replacement.20 When organised well, the benefits of the individual components play a synergistic role in the final properties of the composite coatings obtained.21,22^–28 Metals are generally chosen for their inert qualities whereas ceramics may offer bioactivity, resorption as biomaterials or hardness, wear corrosion resistance as hard facing materials for industrial use. Combining cobalt as the metal matrix and cerium oxide as the dispersion phase, the resulting composite coating could give some interesting properties with possibilities to use them as biomaterials or wear corrosion resistant coatings. Moreover, there are very few references in the literature regarding the electrodeposition of ceramic particles in the cobalt matrix.29, 30 Many of studies are carried out on electrodeposited nickel matrix.13^–19,22–28

In the present work, efforts have been made to synthesise by electrocodeposition and to analyse the performance characteristics of the surface modified cobalt with CeO₂ nanosized bioceramic particles (mean diameter 25 nm), in order to obtain a coating as biocompatible material or industrial use presenting at the same time good adhesion to stainless steel support, microhardness and corrosion performances. In the literature very limited studies could be found in the dispersion of nanoinert particles in the cobalt matrix and its influence on the overall properties including corrosion resistance in synovial solutions.

Material and methods

Electrochemical synthesis of Co–CeO₂ nanocomposite layers

The composite materials were obtained using dispersed nanosized CeO₂ particles (25 nm mean diameter) and cobalt plating electrolyte based on cobalt chloride and cobalt sulphate, using as support for coatings deposition stainless steel (304L). The compositions of the solution and operating parameters for electrodeposition are shown in Table 1. A bath solution without additives was used to avoid any reactions with the CeO₂ particles and ensure that no impurities are present. Analytical reagents and double distilled water were used to prepare the plating solutions. Before deposition, the electrolyte was stirred for 24 h to ensure a good dispersion avoiding the agglomeration of particles suspended in the electrolyte.

Table 1

Operating parameters for pure Co and Co–CeO₂ nanocomposite electrodeposition

Deposition parameters	Value
CoSO₄.7H₂O/g L⁻¹	300
CoCl₂.6H₂O/g L⁻¹	50
H₃BO₃/g L⁻¹	30
pH	4·5
CeO₂/g L⁻¹	0–30
Temperature/°C	25
Current density/mA cm⁻²	23–96
Stirring speed/rev min⁻¹	300
Electroplating time/min	15–60

The boric acid was used as a buffer to prevent pH changes of the electrolyte during the electrodeposition process. Particles were kept suspended with a magnetic stirrer and codeposited with cobalt resulting nanocomposite coatings of cobalt matrix with nanodispersed bioceramic of CeO₂ noted in this paper as Co–CeO₂.

A double electrode cell was employed to carry out electroplating experiments. The cathode, made of 304L stainless steel foils with an exposed area of 5 cm², was positioned in the vertical plane with anode. A similar dimension of pure cobalt foil was used as the anode. The distance between cathode and anode was fixed at 4 cm. Schematic illustration of the electrochemical deposition process is shown in Fig. 1.

Figure 1

Schematic electrochemical codeposition of nano-CeO₂ (25 nm) with cobalt to obtain Co–CeO₂ nanocomposite coatings:

Pure dispersed cerium oxide (CeO₂) was suspended in the electrolysis bath. The particles have had different shapes with an average particle size of 25 nm (Fig. 1a). The particles were kept in suspension by magnetic stirring at a rotation speed of 300 rev min⁻¹. The cobalt foil was used as anode to make sure that the cobalt ions concentration remains constant during the electrodeposition.

Before plating, the stainless steel cathode was polished with emery papers, rinsed in distilled water and ethylic alcohol. For comparison, cobalt films were also deposited in the same electroplating conditions but without addition of CeO₂ bioceramic particles.

Characterisation of Co–CeO₂ nanocomposite

The surface morphologies and the components of the coatings were studied, using scanning electron microscopy (SEM) (JEOL JSM-T220, Oxford Instrument) with energy dispersive X-ray (EDS) analysis attachment.

Thickness of pure cobalt and composite deposits were verified by measuring the weight before and after deposition and also by examining the cross-section of the electrodeposited coatings using a light microscopy (MeF3 – Visilog F5).

On every electroplated surface the Vickers microhardness was measured on a Metkon type microscope with an applied force value of 0·025 kg (chosen to have a good readable tip print and avoid the influence on steel support). The average of five microhardness measurements was reported for minimising error percentage. Coating roughness was measured with a high resolution optical sensor (three-dimensional measuring station with a high resolution optical sensor, STIL).

Electrochemical studies to investigate the corrosion behaviour of Co–CeO₂ nanocomposite layers and pure cobalt coatings were performed in three electrode cells using Hank simulating body fluid (SBF) as the solution having the composition present in Table 2.

Table 2

Composition of SBF solution (Hank) used for corrosion resistance investigation of Co–CeO₂ composite coatings

Components	Value
NaCl/g L⁻¹	8·0
KCl/g L⁻¹	0·4
CaCl₂/g L⁻¹	0·14
NaHCO₃/g L⁻¹	0·35
C₆H₆O₆/g L⁻¹	1
MgSO₄.7H₂O/g L⁻¹	0·2
KH₂PO₄.H₂O/g L⁻¹	0·1
Na₂HPO₄.7H₂O/g L⁻¹	0·06
pH	7·0

In order to minimise the influence of the coating thickness on the electrochemical measurement parameters, the sample thickness was maintained at 23±2 μm for both types Co–CeO₂ and pure cobalt layers. The CeO₂ dispersed phase in the Co–CeO₂ composite layer was estimated at 15 wt-% after analysis. The sample was loaded in the Teflon sample holder and the exposed surface area to the corrosive medium was 1 cm². Platinum grid of 1 cm² area was used as the counter electrode and Ag–AgCl (saturated KCl) was used as the reference electrode. The corrosion resistance of the obtained Co–CeO₂ nanocomposite films and the pure Co layer was evaluated by the electrochemical impedance spectroscopy (EIS) and polarisation measurement, using the Solartron electrochemical workstation (1286 potentiostat coupled with 1255 frequency response analyser). The sample was kept in SBF solution for 12 h in order to establish the open circuit potential E_OCP or the steady state potential. The EIS measurements were conducted in a frequency range from 100 kHz to 0·01 Hz with 10 mV sinusoidal signal amplitude versus open circuit potential E_OCP. The spectrum was recorded with a data density of 5 points per decade. After each experiment the impedance data were displayed as Nyquist plot. The Nyquist plot is a plot of real impedance (real Z) versus imaginary impedance (imaginary Z). Each experiment was repeated at least three times to verify the reproducibility of the results. The acquired data were curve fitted and analysed using Z_View programme. After EIS measurements the system was allowed to attain the open circuit potential and a potentiodynamic curve was acquired.

Results and discussion

Preparation of Co–CeO₂ composite layers

After immersing the nano-CeO₂ dispersed phase (Fig. 1a) into cobalt electrolyte, it is supposed that cobalt ions are adsorbed on cerium oxide particles (Fig. 1b) being further reduced to cathode surface resulting in entrapped particles into electrodeposited cobalt matrix (Fig. 1c). The cobalt electrocrystallises and forms the metal matrix which includes the CeO₂ particles to form Co–CeO₂ nanocomposite coating as it is shown schematically in Fig. 1 and noted in this paper as Co–CeO₂. In aqueous solutions, cobalt is present in the form of divalent cobalt ions with octahedral coordination, having four equatorial and two axial coordination sites, each occupied by water molecules. By adding CeO₂ particles in the electrolyte, some ions could be adsorbed on their surfaces, transported in the electrolyte and further entrapped into cobalt growing electrodeposited coating (Fig. 1b), following the equation (1) (1) Incorporation of particles into the growing cobalt film results in a perturbation of cobalt growth and surface morphology of nanocomposite layers.

Influence of CeO₂ nanometric particles on coating thicknesses

The coating thicknesses obtained at different current densities, determined by weighing the cathode before and after deposition or codeposition, are presented in Fig. 2. From Fig. 2 it could be noticed that by increasing the current density and maintaining the deposition time constant, the thickness of the deposited layers also increases. The Co–CeO₂ nanocomposite coatings show a larger increase in thickness as compared with the pure cobalt layer at the same current density. This proves a higher current efficiency by adding the nanometric cerium oxide in the cobalt electrolyte to obtain Co–CeO₂ nanocomposite coatings. By calculating the cathode current efficiency, it was found a mean value of 92% for pure cobalt electrodeposition and a higher mean value of 96% for Co–CeO₂ composite codeposition.

Figure 2

Thicknesses of layers versus current density obtained by: 1: pure cobalt electrodeposition; 2: Co–CeO₂ codeposition of nanocomposite layers at 20 g L⁻¹ of cerium oxide in cobalt electrolyte

SEM–EDS analysis of nanocomposite coatings

Figure 3 Figure 4 Figures 3–5 compare pure cobalt layers and Co–CeO₂ nanocomposite coatings, carried out with 20 g L⁻¹ of CeO₂ in the plating bath at different current densities. The cobalt electroreduction is perturbed by nanometric CeO₂ bioceramic particles embedded in the coating during the electroplating process as could be seen on SEM surface morphologies at different current densities.

Figure 3

Images (SEM) of surface morphologies of electrodeposited layers obtained at 23 mA cm⁻²:

Figure 4

Images (SEM) of surface morphologies of electrodeposited layers obtained at 48 mA cm⁻²:

Figure 5

Images (SEM) of surface morphologies of electrodeposited layers obtained at 72 mA cm⁻²:

From Figure 3 Figure 4 Figs. 3–5, it could be observed that the pure Co surface remains regular with the increase in the current density (Figs. 3b, 4b and 5b), whereas the composite surface changes slightly from a slightly agglomerated surface to a more non-agglomerated surface with a spindle-like structure (Figs. 3a, 4a and 5a). Changes in cobalt matrix surface morphology compared with those of pure cobalt electrodeposits provide information on the effect of CeO₂ dispersed phase in the electrocodeposition process of composite layers. Scanning electron microscopy–EDS analysis gave the information on the cerium oxide particles content embedded in the cobalt matrix and the mapping mode on the cerium element shows good uniformity of the dispersed CeO₂ phase in the composite coatings as it is shown in Fig. 6.

Figure 6

Scanning electron microscopy–EDS analysis of Co–CeO₂ composite layer showing presence and uniformity of nanometric cerium oxide in cobalt matrix:

From Fig. 6a, the X-ray spectrum analysis of nanocomposite layer with cerium could be observed indicating the presence of nano-CeO₂ on the SEM image performed on Co–CeO₂ nanocomposite layer (Fig. 6b). Good uniformity of nano-CeO₂ dispersion is shown in Fig. 6c by X-ray elemental mapping on the SEM image.

Microhardness of Co–CeO₂ nanocomposite and pure Co layers

The microhardness of Co–CeO₂ nanocomposite and pure cobalt layers has been determined by optical microscopy, using the Vickers method on a Metkon type microscope on the sample surface. The average of five microhardness measurements was reported for minimising error percentage. As expected there was an increase in the microhardness of the Co composite coatings after the addition of CeO₂ particles (Fig. 7). The highest microhardness was exhibited by the composite electrodeposited at the highest current density of 96 mA cm⁻².

Figure 7

Microhardness of electrodeposited layers versus current density: 1: pure Co surfaces; 2: Co–CeO₂ nanocomposite layers

Nanocomposite coatings, Co–CeO₂ (25 nm) revealed a microhardness between 446 and 518 HV0·025 compared with pure cobalt layers which allow the values between 304 and 319 HV0·025. As it is known from classical metallurgy, hindering the dislocation movement could increase the hardness of a material by lattice distortions (the grain boundary density or the small grain size). The benefit of oxide (chromium oxide) in the microstructure of cobalt chromium alloys was observed by some authors in the spark plasma sintering of cobalt and chromium fine powders,31 as a new alternative method to improve the performances of alloy implants. This chromium oxide in the microstructure of the alloy can be more beneficial than carbides and the hardness of the alloy becomes closer to that of ceramics. Also its lower density enables the alloy to be lighter. The chemical stability of the oxide ensures that it remains intact and due to its insolubility in water, no carcinogenic or toxic reactions will occur.31 In our research results the cerium oxide nanoparticles embedded into cobalt matrix during the electroplating process can act as dispersion hardeners that limit dislocation movement. This assessment could confirm the differences in surface morphologies of the two types of coatings and good effect in improving microhardness of added CeO₂ nanoparticles to cobalt matrix.

Surface microtopography of Co–CeO₂ nanocomposite coatings

Surface microtopography of composite coatings could be an important factor for further engineering and applications of such materials, especially in implant applications for cells adhesion. The surface topography of artificial materials is known to play a significant role in interactions with biological systems such as proteins and cells.32, 33 Furthermore in a new technique using electrohydrodynamic print patterning that generates ordered topographies using proven biomaterials and composites,34 the authors notice that the topography, which includes the surface features and the features themselves, is a crucial physical cue for cells, influencing cell adhesion, proliferation and differentiation and should be considered when designing biomedical architectures.

Therefore, roughness plays an important role in determining how a real object will interact with its environment. Three-dimensional surface microtopography of pure cobalt and Co–CeO₂ composite coatings were measured with an ultrahigh microtopography (STIL) with a high resolution optical sensor and further treated with an image surface analyser (Surface Map) software. The results presented in Fig. 8 show a slow increase in surface roughness with the current density for pure cobalt electrodeposited layers as well as for Co–CeO₂ electrodeposited composite layers.

Figure 8

Variation of three-dimensional surface microtopography (Sa) with current density applied in electroplating process of: 1: pure Co layer; 2: Co–CeO₂ (20 g L⁻¹) nanocomposite coating

Surface roughness of Co–CeO₂ nanocomposite layers is increasing from 0·17 to 0·30 μm, being higher as compared with the pure cobalt layer at the same current density (Fig. 8).

Corrosion behaviour of Co–CeO₂ nanocomposite layers in SBF solution

The EIS and potentiodynamic measurements for the corrosion behaviour were performed in a three electrode cell in the SBF solution having the composition presented in the Table 2. The results are presented in Fig. 9 for Co–CeO₂ nanocomposite coating and Fig. 10 for pure cobalt layer, where the points represent the experimental data and the line represents the fitting curve. The acquired data were curve fitted and analysed using Z_View programme.

Figure 9

Electrochemical impedance spectroscopy Nyquist plot of experimental measurements and fitting impedance of Co–CeO₂ nanocomposite coatings after 12 h of immersion in SBF solution

Figure 10

Electrochemical impedance spectroscopy Nyquist plot of experimental measurements and fitting impedance of pure cobalt coatings after 12 h of immersion in SBF solution

As shown in Figs. 9 and 10, the EIS measurements evidenced the higher corrosion resistance of the Co–CeO₂ nanocomposite layers. Indeed the codeposition of the CeO₂ nanoparticles into cobalt matrix improves the anticorrosion properties in the SBF solution.

The Nyquist plots of Co and Co–CeO₂ coatings appeared as single semicircles. The EIS experimental data of pure cobalt layer could be fitted with a simple equivalent circuit model (Randles circuit) where instead that charge transfer resistance is the polarisation resistance, R₁ of pure cobalt coating (Fig. 11a). Based on the above analysis, the EIS plot shown in Fig. 10 is analysed using the elements of the electrochemical equivalent circuit, resulting from fitting line which calculates for polarisation resistance of pure Co layer, a value of R₁ = 1·11 kΩ cm².

Figure 11

Equivalent circuit used to fit EIS experimental results for corrosion behaviour in SBF solution:

More complex equivalent circuit was necessary and proposed to fit the experimental impedance data of the Co–CeO₂ nanocomposite layer (Fig. 11b).

The proposed equivalent circuit elements are as follows: R_e is the electrolyte resistance between the reference electrode and the working electrode (the tested sample with pure cobalt or Co–CeO₂ nanocomposite coatings), CPE₁ is the double layer capacitance depending on frequency of cobalt, CPE₂ is the double layer capacitance depending on frequency of insulating CeO₂ bioceramic particles included into cobalt matrix or possible formed complex passive layer, R₁ is the polarisation resistance of cobalt in pure cobalt layer or Co–CeO₂ nanocomposite coating, R₂ is the resistance of insulating CeO₂ bioceramic particles included into cobalt matrix in the case of nanocomposite coating or possible formed complex passive layer.

In this case the electrochemical impedance could be described by the following equation (2) The value of β determines the amplitude of the depressed impedance semicircle and the importance of the discrepancy between the Randles circuit model with a pure capacitor C_dl and the equivalent circuit with the CPE instead of C_dl. The borderline case of the Randles circuit is found if β = 1. CPEs are constant phase elements, accounting for the fact that the centres of the capacitive arcs of the impedance circle are under the axis of real part. This feature of capacitive arcs is encountered in all electrochemical impedance studies performed on inhomogeneous surfaces and has given rise to extensive studies. CPE are not pure capacitors but components depending on frequency. Based on the above analysis, the EIS plots shown in Fig. 10 are analysed using the elements of the electrochemical equivalent circuit, presented in Fig. 11b resulting from the fitting line a polarisation resistance value of Co–CeO₂ nanocomposite layer of 11·9 kΩ cm² as the sum between R₁+R₂ having a 10 times higher value as compared with the pure cobalt layer. The results confirm the improved anticorrosion properties of cobalt by adding nano-CeO₂ bioceramic particles into the coating during the electrocodeposition process. The CeO₂ nanoparticles dispersed into cobalt matrix act as insulating points on composite surface interacting probably with cobalt oxide passive layer improving therefore the overall surface anticorrosion properties of Co–CeO₂ coating.

Similar equivalent circuits were proposed to interpret the corrosion behaviour through EIS experimental data for composite coatings in nickel matrix having silicon carbide as reinforcing phase,2, 4 or copper matrix with the silicon carbide dispersed phase.35

After EIS measurements the system was allowed to attain open circuit potential and a polarisation potentiodynamic measurement was performed with a sweep rate of potential scan at 5 mV s⁻¹. The diagrams are presented in Fig. 12 for both Co–CeO₂ and pure cobalt layers.

Figure 12

Potntiodyamic measurements in SBF solution performed with scan rate of 5 mV s⁻¹ of potential from −1 to −0·1 V versus Ag–AgCl for: 1: Co–CeO₂ nanocomposite layer; 2: pure cobalt layer

As shown in Fig. 12 the Co–CeO₂ nanocomposite layers confirm the better corrosion resistance compared with pure cobalt coating having a larger passive domain and a lower passivation current density. No results regarding the corrosion behaviour of Co–CeO₂ nanocomposite coatings in SBF solution were found in the literature.

Conclusion

Electrocodeposition of bioceramic CeO₂ nanometric particles in the cobalt matrix was investigated with the main aim to evaluate the effect of nanoparticles in the electrolyte during the electoreduction of cobalt and performance investigation of Co–CeO₂ nanocomposite layers obtained.

The morphology of Co–CeO₂ nanocomposite layers is changed as compared with pure Co layers obtained at the same current densities. Good uniformity of nanometric cerium oxide dispersed particles into cobalt matrix is obtained and is proved by SEM–EDS cerium mapping on Co–CeO₂ nanocomposite surfaces.

The Co–CeO₂ composite coatings are uniform and well bonded to the steel substrate, the thicknesses increase with increasing current density and the microparticulates affect the electrodepositing process of the Co coating, by a slow increase in the Co–CeO₂ nanocomposite coating thickness compared with pure Co obtained at the same current density, meaning probably a slow increase in current efficiency.

The Co–CeO₂ nanocomposite coatings exhibit better corrosion resistance compared to plain Co, observed from higher polarisation resistance (∼10 times) on electrochemical spectroscopy and potentiodynamic diagrams plotted in the SBF solution.

The present study has shown that by adding nano-CeO₂ bioceramic dispersed particles to cobalt by electrocodeposition the synergistic combination of corrosion resistance along with improved microhardness was imparted to the cobalt matrix. Because of the combined corrosion resistance and higher microhardness, Co–CeO₂ nanocomposite layers may find a lot of engineering applications.

Overall, our work demonstrates the possibility to obtain Co–CeO₂ nanocomposite coatings by the electrocodeposition process of CeO₂ nanoparticles with cobalt. The benefits of nanocerium oxide addition in the cobalt films are: good uniformity of dispersed phase into cobalt matrix, higher microhardness of nanocomposite layers obtained and improved corrosion resistance in the SBF solution.

Footnotes

Acknowledgements

The Romanian National Research Authority (ANCS) is gratefully acknowledged for the support of this work as a Bilateral Project Romania– France, Programme Humbert Curien (PHC) – Brancusi: ‘Nano-structured coatings for functionalised surfaces’, no. 19603 PC (contract no. ANCS 214/2009-2010). Furthermore, the work is sustained by the bilateral research agreement between Research (Competences) Centre Interfaces – Tribocorrosion and Electrochemical Systems (CC-ITES) from Dunarea de Jos University of Galati (RO) and Chemical and Material Engineering Laboratory (LGPM) from Ecole Centrale Paris (F).

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Electrosynthesis and performances of cobalt–ceria nanocomposite biocoatings

Abstract

Keywords

Introduction

Material and methods

Electrochemical synthesis of Co–CeO2 nanocomposite layers

Characterisation of Co–CeO2 nanocomposite

Results and discussion

Preparation of Co–CeO2 composite layers

Influence of CeO2 nanometric particles on coating thicknesses

SEM–EDS analysis of nanocomposite coatings

Microhardness of Co–CeO2 nanocomposite and pure Co layers

Surface microtopography of Co–CeO2 nanocomposite coatings

Corrosion behaviour of Co–CeO2 nanocomposite layers in SBF solution

Conclusion

Footnotes

Acknowledgements

References

Electrochemical synthesis of Co–CeO₂ nanocomposite layers

Characterisation of Co–CeO₂ nanocomposite

Preparation of Co–CeO₂ composite layers

Influence of CeO₂ nanometric particles on coating thicknesses

Microhardness of Co–CeO₂ nanocomposite and pure Co layers

Surface microtopography of Co–CeO₂ nanocomposite coatings

Corrosion behaviour of Co–CeO₂ nanocomposite layers in SBF solution