Effect of carbon nanotubes incorporation on the properties of the PEO coatings formed on Mg-Mn-Ce alloy

. The properties of coatings formed on the MA8 magnesium alloy by the plasma electrolytic oxidation in electrolytes containing multi-walled carbon nanotubes in concentrations of 2, 4 and 6 g/l have been investigated. It was found that the introduction of multi-walled carbon nanotubes leads to an increase in the adhesive strength, microhardness and Young's modulus of the obtained layers.


Introduction
Magnesium alloys are one of the most common metallic construction materials used in modern aviation and mechanical engineering owing to their low density, high strength, and high vibration-damping properties. These qualities, combined with the simplicity of Mg casting and moulding, make magnesium alloys a promising solution for a wide range of engineering issues [1].
However, high chemical activity and low wear resistance significantly limit the application of magnesium alloys. The problem of preserving magnesium alloys from the effects of environmental conditions can be alleviated with various methods of protective coating creation, among which plasma electrolytic oxidation (PEO) can be marked as one of the most prospective [2]. PEO coatings are corrosion-resistant ceramic-like layers with great adhesion to the substrate and a developed surface. The protective properties of PEO coatings can be improved by their modification with various nanoparticles that expand the range of physico-chemical characteristics of the surface. Nanoparticles of various composition and shape are used to increase the hardness and wear resistance [3,4] of PEO coatings, provide photocatalytic [5], antibacterial [6], hydrophobic [7,8], adsorptive [9] and other specific features.
Multilayer carbon nanotubes (CNTs) have a high Young's modulus and tensile strength, which can be used to improve the mechanical properties of the PEO coatings. These synthetic nanoparticles are extensively used in the design of composite materials with enhanced durability against mechanical failure. The conductivity of CNTs provides opportunities for their application as electrodes for supercapacitors and as a material for creating nanometersized electronic devices [10]. Despite the profound studies of the electrical properties of CNTs and their applicability as reinforcement for composites, only a few publications are devoted to the influence of this nanomaterial on the properties of the PEO coatings. PEO coatings containing CNTs are of scientific significance as a method of improving the corrosion and wear resistance of magnesium-based surfaces for the purposes of weightsensitive modern engineering.

Materials and methods
Rectangular plates of a size of 15 mm × 20 mm × 2 mm made of the МА8 magnesium alloy (1.5 wt. % Mn; 0.15 wt. % Ce; Mg balance) were used as samples. To standardize the samples surface they have been polished with sanding paper of different grain sizes (600, 800, and 1200), washed with distilled water and alcohol.
The water solution of sodium fluoride (5 g/l) and silicate (15 g/l) was selected as a base electrolyte. The multi-walled CNTs with length of 2-4 μm, outer diameter of 15-40 nm, and inner diameter of 5-20 nm were used. The nanopowder concentration in the base electrolyte was equal to 0, 2, 4, and 6 g/l. The coatings obtained in the corresponding electrolytes were named C0, C2, C4, and C6, respectively. Sodium dodecyl sulfate at a concentration of 0.25 g/l was used as a stabilizer for the dispersed system.
The process of coatings formation was carried out using a plasma electrolytic oxidation unit. The frequency of the polarizing pulses was 300 Hz. All samples were processed in a two-stage bipolar PEO mode. In the first stage (200 s), the anodic component was fixed galvanostatically at a current density of 0.35 A/cm 2 , and the cathodic one was fixed potentiostatically at -30 V. During the second stage (600 s) the anodic component changed potentiodynamically from the current value of voltage to 200 V, and the cathodic one changed from -30 V to -10 V.
Microphotographs of the surface of the samples were obtained using the Sigma 300 (Carl Zeiss, Germany) scanning electron microscope (SEM). The morphology of the obtained coatings was studied using SEM images obtained in secondary electrons (SE) and backscattering electrons (BSE) detection regimes. The elemental composition of the surface layers was determined by energy dispersive spectroscopy (EDS) using the energy dispersive X-ray fluorescence spectrometer EDX-800HS (Shimadzu, Japan). Determination of microhardness of the coatings was carried out using a DUH-W201 dynamic ultramicrohardness tester (Shimadzu, Japan). Measurements of the microhardness were carried out on the coatings cross-sections using a Vickers indenter at a load of 50 mN. The adhesive properties of the surface layers were investigated by scratch testing using Revetest Scratch Tester (CSM Instruments, Switzerland). The experiments were performed with a Rockwell diamond indenter at a track length of 5 mm and a gradual increase in load from 1 up to 30 N with a loading rate of 10 N/min.

Results and discussion
The addition of CNTs to the electrolyte result in changes in the surface morphology of the obtained samples (Fig. 1). For the C6 sample, a developed surface occupied with fused nanoparticles agglomerates (Fig. 1, c, d) is observed, while the base PEO coating demonstrates more uniform surface (Fig. 1, a, b). The obtained samples also represent higher porosity compared to the base PEO layer, which can be attributed to the influence of nanoparticles presence on the electrical parameters of the PEO process which leads to the more frequent discharges.
Since the BSE regime of SEM is sensitive to the atomic number of the coating components, it helps to discern differences in phase and elemental composition of the studied layer. The areas coloured in darker tones correspond to the regions that demonstrate a weaker electrons scattering in comparison with the light-coloured regions. The weak electron scattering is inherent not only to pores and defects, but also to light elements such as carbon. In the presented images (Fig. 1), red-coloured circles highlight areas that scatter electrons weakly and have dark colour in BSE regime, but do not constitute a pore. The CNTs consist of carbon, which is the lightest element among all possible components of the PEO layer, therefore, highlighted areas presumably correspond to the CNTs agglomerates and spots of their inclusion in the coating surface. At the same time, similar areas are not observed on the surface of the base PEO coating. According to the EDS analysis results (Table 1), the concentration of carbon increases gradually from C0 to C6. Since C0 is a base PEO coating, the presence of carbon on its surface is attributable to organic contamination from the environment. On the other hand, C2, C4, and C6 include carbon due to the presence of the CNTs in their composition. The presence of Si, Na, and F stems from the electrolyte composition. Mechanical properties of the coatings were estimated by microhardness and Young's modulus values (Table 2). Samples containing nanoparticles showed higher microhardness and modulus of elasticity magnitudes; for instance, the Hµ value of the С4 sample is higher by 0.7 GPa in comparison with the value for the base PEO coating. Presumably the main reason for this phenomenon is the introduction of CNTs, which, as was noted above, have great mechanical characteristics due to their seamless structure. For example, CNTs Young's modulus reaches a value of 1 TPa according to various studies [11]. The mechanical characteristics of the coatings begin to decrease from C6 sample owing to its high porosity that was also noted for SEM images (Fig. 1). Scratch testing was performed to evaluate the adhesive properties of the obtained coatings. According to the analysis of the obtained data (Table 3, Figure 2), the LC2 load value increase with the concentration of nanoparticles in the electrolyte up until the С6 sample. This tendency can be explained by the increase in the resistance of a layers to deformation after introduction of CNTs, which is reflected in Young's modulus values as well. The adhesive properties start to deteriorate from the samples obtained in electrolytes with CNTs concentration above 4 g/ l, due to the high porosity and heterogeneity of the formed layers.

Summary
In this paper, composition, morphological features and mechanical properties of the PEO coatings containing multi-walled CNTs were studied. It was shown that the coatings formed in the electrolytes with a concentration of the CNTs of 4 g/l have the highest scratch test critical loads and Young's modulus values. It was found that the addition of the multi-walled CNTs leads to the increase in the microhardness magnitudes and the formation of a more developed surface of the PEO coatings. The value of microhardness was increased by 0.5-0.7 GPa, the value of Young's modulus by 22-25 GPa, which indicates a positive effect of the multi-walled CNTs on the mechanical properties of the coatings formed by the PEO method on Mg-Mn-Ce alloy.