ORIGINAL RESEARCH published: 16 April 2019 doi: 10.3389/fchem.2019.00241 Nanocrystalline Electrodeposited Fe-W/Al2O3 Composites: Effect of Alumina Sub-microparticles on the Mechanical, Tribological, and Corrosion Properties Aliona Nicolenco 1,2*, Antonio Mulone 3 , Naroa Imaz 4 , Natalia Tsyntsaru 1,2*, Jordi Sort 5,6 , Eva Pellicer 6 , Uta Klement 3 , Henrikas Cesiulis 1 and Eva García-Lecina 4 1 Department of Physical Chemistry, Vilnius University, Vilnius, Lithuania, 2 Institute of Applied Physics, Chisinau, Moldova, 3 Department of Industrial and Materials Science, Chalmers University of Technology, Gothenburg, Sweden, 4 CIDETEC, San Sebastián, Spain, 5 Departament de Física, Universitat Autònoma de Barcelona, Bellaterra, Spain, 6 Institució Catalana de Edited by: Recerca i Estudis Avançats, Barcelona, Spain Bongyoung Yoo, Hanyang University, South Korea In this study, nanocrystalline Fe-W alloy and Fe-W/Al2 O3 composite coatings with various Reviewed by: Anthony Peter O’Mullane, contents of sub-microsized alumina particles have been obtained by electrodeposition Queensland University of Technology, from an environmentally friendly Fe(III)-based electrolyte with the aim to produce a Australia novel corrosion and wear resistant material. The increase in volume fraction of Al2 O3 Yang Yang, University of Central Florida, in deposits from 2 to 12% leads to the grain refinement effect, so that the structure United States of the coatings change from nanocrystalline to amorphous-like with grain sizes below *Correspondence: 20 nm. Nevertheless, the addition of particles to the Fe-W matrix does not prevent the Aliona Nicolenco alionanicolenco@gmail.com development of a columnar structure revealed for all the types of studied coatings. Natalia Tsyntsaru The observed reduction in both hardness and elastic modulus of the Fe-W/Al2 O3 ashra_nt@yahoo.com composites is attributed to the apparent grain size refinement/amorphization and the nanoporosity surrounding the embedded Al2 O3 particles. In the presence of 12 vol% Specialty section: This article was submitted to of Al2 O3 in deposits, the wear rate decreases by a factor of 10 as compared to Fe- Electrochemistry, W alloy tested under dry friction conditions due to the lowering of tribo-oxidation. The a section of the journal Frontiers in Chemistry addition of alumina particles slightly increases the corrosion resistance of the coatings; Received: 21 January 2019 however, the corrosion in neutral chloride solution occurs through the preferential Accepted: 26 March 2019 dissolution of Fe from the matrix. The obtained results provide a possibility to integrate Published: 16 April 2019 the nanocrystalline Fe-W/Al2 O3 composite coatings into various systems working under Citation: dry friction conditions, for example, in high-temperature vacuum systems. Nicolenco A, Mulone A, Imaz N, Tsyntsaru N, Sort J, Pellicer E, Keywords: iron alloys, alumina, composite coatings, columnar growth, wear resistance Klement U, Cesiulis H and García-Lecina E (2019) Nanocrystalline Electrodeposited Fe-W/Al2 O3 Composites: Effect of INTRODUCTION Alumina Sub-microparticles on the Mechanical, Tribological, and Nanocrystalline Fe-W coatings with tunable functional properties can be electrodeposited from Corrosion Properties. environmentally friendly Fe(III)-based solution with a high current efficiency (up to 60–70%) Front. Chem. 7:241. (Nicolenco et al., 2017, 2018a). The W content in the alloys can be varied from a few at.% up doi: 10.3389/fchem.2019.00241 to 25 at.% by fine control of experimental parameters. Previous studies (Nicolenco et al., 2018a) Frontiers in Chemistry | www.frontiersin.org 1 April 2019 | Volume 7 | Article 241 Nicolenco et al. Electrodeposited Fe-W/Al2 O3 Composites have shown that alloys with low W content have a soft magnetic hardness) and wear (Rupert and Schuh, 2010; García-Lecina character and relatively high saturation magnetization, making et al., 2013; Bajwa et al., 2016). The studies performed mainly such alloys suitable for the design and fabrication of MEMS, on Ni-based coatings showed that nanocrystalline materials for example, read-write heads or actuators. On the other hand, with a higher ratio between hardness and elastic modulus, so- Fe-W alloy coatings with high W content (>20 at.%) are called elastic strain to failure, should better resist the plastic characterized by outstanding thermal stability (up to 600◦ C), deformation, hence, resulting in lower wear (Leyland and good mechanical properties in a wide range of temperatures (10 Matthews, 2000; Rupert and Schuh, 2010). Nevertheless, the ÷ 16 GPa) (Mulone et al., 2018a), and mirror-like smooth surface wear mechanism of Fe-containing coatings was ascribed to the (Ra < 100 nm) (Bobanova et al., 2009; Mulone et al., 2018b; combination of abrasive and adhesive wear considering tribo- Nicolenco et al., 2018a). Nevertheless, one of the drawbacks oxidation as the driving factor. The high wear rates of Fe- of Fe-W coatings is their rather low wear and corrosion W coatings could only be overcome by applying lubrication resistance that remains an important issue for the applications conditions (Bobanova et al., 2009; Nicolenco et al., 2018b). The of these nanoengineered materials as protective coatings and tribological study performed on Ni-Fe/Al2 O3 composites under smart interfaces. dry friction conditions has also shown the negative and prevalent Fabrication of composite materials is one of the common effect of Fe content in matrix on the wear resistance of the approaches, which enables to combine the useful properties of coatings, while the content of alumina particles had a minor effect the second-phase particles with those of the metallic matrix, thus (Starosta and Zielinski, 2004). rendering a novel material with the tailored characteristics. For The aim of our work is to electrodeposit nanocrystalline example, the co-deposition of transition metal sulfides (MoS2 Fe-W/Al2 O3 composite coatings with various contents of and WS2 ) in Ni-W and Ni-P coatings significantly reduced the incorporated sub-microsized alumina particles to improve the friction coefficient of the composites due to the specific layered wear and corrosion characteristics of the coatings and make them structure and high elasticity of the particles, which provide a suitable for various applications where aggressive conditions self-lubricating effect (Cardinal et al., 2009; García-Lecina et al., can exist. The effect of alumina particles’ co-deposition on the 2013; He Y. et al., 2016). Many studies have been conducted on composition and structure of the coatings, and their mechanical, the electrodeposition of metallic matrix composites with oxide tribological, and corrosion behavior is investigated. and carbide particles. Here, an improvement of the strength and corrosion resistance of the resulting composite coatings was obtained thanks to the high hardness, the oxidation and thermal EXPERIMENTAL PART resistance of the co-deposited particles (Hosseini et al., 2014; Anwar and Mulyadi, 2015; Bhogal et al., 2015; Bajwa et al., Fe-W/Al2 O3 composite coatings were electrodeposited from 2016). However, the presence of second phase particles in the a glycolate-citrate Fe(III)-based electrolyte which contained coating is not a guarantee that the desired performance will be alumina particles in suspension. The base electrolyte was attained. Often the incorporation of particles causes porosity, composed of 1 M glycolic acid, 0.3 M citric acid, 0.1 M Fe2 (SO4 )3 , bad adhesion and weak interface bonding with the matrix. and 0.3 M Na2 WO4 . Bath pH was adjusted to 7.0 with NaOH All these effects can eventually reduce the overall performance at room temperature. Sub-microsized alumina particles (Alfa of the surface, particularly, corrosion resistance (Starosta and Aesar 42572) with the concentration of 25, 50, and 100 g L−1 Zielinski, 2004; Hu et al., 2009; Bajwa et al., 2016). Therefore, the were added to the base electrolyte and the suspensions were appropriate design of the composite system is essential. stirred at 300 rpm for 24 h prior to first electrodeposition in Composite coatings with alumina particles have been the order to hydrate the particles. Before starting the electroplating, most extensively studied among particles-reinforced composites. the electrolytes were placed in an ultrasonic bath for 10 min to In addition of its cost effectiveness, alumina has high chemical prevent agglomeration. stability and sufficiently high affinity to adsorb iron group Electrodeposition was performed in a typical three-electrode metal ions on its surface, thus facilitating the co-deposition cell, where a brass plate with an area of 4 cm2 was used as a of composites with Co, Ni and their alloys (Wu et al., working electrode, Ag/AgCl/KClsat as a reference electrode and 2004; Man et al., 2014). Well-dispersed composites with a platinized titanium mesh as a counter electrode. The substrates Al2 O3 particles typically show good corrosion resistance in were degreased in a hot commercial alkaline cleaner and activated the sulfate and chloride medium due to the partial blocking in H2 SO4 solution, followed by Ni-seed layer electrodeposition of corrosion pits by inert alumina nano- or microsized at 30 mA cm−2 for 1 min from sulfate-chloride bath operated particles (Bajwa et al., 2016). The addition of alumina particles at 65◦ C. The anode was placed parallel to the cathode at a can also contribute to the increased hardness and strength distance of 4.8 cm. The electrolyte volume was kept at 200 mL. of the coatings, as the particles restrain the grain growth The bath temperature was maintained at 65◦ C. Electrodeposition and provide a dispersion strengthening effect described by was carried out at a constant cathodic current density of 40 mA Orowan mechanism (Beltowska-Lehman et al., 2012; Man et al., cm−2 during 1 h, at a constant stirring rare of 200 rpm. 2014;Bajwa et al., 2016; Wasekar et al., 2016). The morphology of the coatings was investigated by a Carl Several works exist in the literature correlating the mechanical Zeiss Ultra Plus field emission scanning electron microscope characteristics of composite coatings with their tribological (Zeiss, Jena). The chemical composition was analyzed on cross- properties, giving relationships between the plasticity index (or sectional area with the energy dispersive X-ray spectroscopy Frontiers in Chemistry | www.frontiersin.org 2 April 2019 | Volume 7 | Article 241 Nicolenco et al. Electrodeposited Fe-W/Al2 O3 Composites FIGURE 1 | Effect of Al2 O3 particles’ concentration in the electrolyte on Al content in deposit (A) and on the composition of Fe-W matrix (B). The coatings were obtained at 200 rpm, 40 mA cm−2 , 65◦ C. (EDS) analysis tool AMETEK EDAX attached to the microscope coatings in order to avoid the influence from the resin. The operated at 15 kV. The particles size distribution and the metallographic preparation of the samples was performed by volume fraction of co-deposited Al2 O3 particles in the deposits mechanical polishing with a 1 µm finishing using a diamond was determined by the image analysis software ImageJ. It was suspension as the last step. Both hardness (H) and reduced determined that the majority of the alumina particles have elastic modulus (Er ) were derived from the initial part of the the mean diameter between 50 and 150 nm, although some unloading indentation segments using the method of Oliver and of the particles were forming agglomerates with 200–250 nm Phar (Oliver and Pharr, 2004). In order to characterize the elastic- (Figure S1), probably due to the high surface energy of the plastic response of the coatings under external deformation, the powder. The contact profilometer Surftest SJ-210 was used to plasticity index (Up /Utot ) was calculated as a ratio between plastic determinate the roughness (Ra ) of the coatings. A contact needle and total (plastic + elastic) energy during nanoindentation. The tip was scanning the surfaces horizontally along the 1.5 mm plastic energy was derived as the area between loading and length. The crystallographic structure and phase composition unloading curves, while the total energy was calculated as the of the obtained coatings were studied by means of X-ray area between the loading curve and the displacement axis. All the diffraction (XRD), using a Rigaku MiniFlex II diffractometer extracted parameters were statistically treated, and the average with Cu Kα radiation (λ = 1.54183 Å) operated at 30 kV values are reported. and 30 mA. The investigation of tribological behavior of electrodeposited The crystallographic orientation of the grains in the deposited coatings was carried out under dry friction conditions using coatings was examined by the Electron Back Scattered Diffraction ball-on-disk configuration sliding tests (CSM, model THT). technique (EBSD) in a Leo 1550 Gemini Scanning Electron A corundum ball of 6 mm diameter was the counter-body Microscope (SEM). The EBSD data were acquired with a Nordlys that moved against rigidly fixed coated samples for 500 m II detector (Oxford Instruments) using a step size of 25 nm. Post- with a rotation speed of 4 cm s−1 (rotation diameter 6 mm). processing of the acquired EBSD data was done with the HKL The applied load was 2 N. All the tests were performed in Channel 5 software (Oxford Instruments): noise reduction was ambient air at 20 ± 2◦ C and 55% relative humidity. After the performed by removal of wild spikes and extrapolation of non- sliding tests, the debris were removed from the surface with indexed points (5 nearest neighbors required). Metallographic dry cold air jet, and the samples were ultrasonically cleaned preparation of the samples was performed by mechanical in ethanol in order to remove the remaining adhered debris polishing with a 50 nm finishing using OP-S silica suspension as prior to measuring the wear track profiles. The specific wear the last step. rate was defined as wear volume per unit distance and unit The mechanical properties were evaluated by means of load (Bajwa et al., 2016). nanoindentation using Nanoindenter XP from MTS equipped Corrosion resistance of the Fe-W and Fe-W/Al2 O3 with a Berkovich pyramidal-shaped diamond tip under load- coatings was investigated by potentiodynamic polarization control mode on the cross-section of the coatings, previously and electrochemical impedance spectroscopy (EIS) techniques embedded in epoxy resin and polished to mirror-like appearance. under open circuit conditions. A 250 mL three-electrode Flat The maximum applied load was 50 mN to ensure that the Cell Kit (Princeton Applied Research, Oak Ridge, TN, USA) with lateral size of the imprint remained small compared to the a platinized titanium mesh and Ag/AgCl/NaCl(3M) electrodes total film thickness but still embraced both the matrix and the as counter and reference electrodes, respectively, was used to particles in the case of composite coatings. Fifty indentations conduct the experiments. The potential of the working electrode were placed in the middle of the cross-section area of the was measured against the reference electrode with a Luggin Frontiers in Chemistry | www.frontiersin.org 3 April 2019 | Volume 7 | Article 241 Nicolenco et al. Electrodeposited Fe-W/Al2 O3 Composites FIGURE 2 | SEM images of the top surface (A–D) and the cross-section polished with 1 µm finishing (E–H) of the composite coatings obtained with different concentration of alumina in electrolyte: 0 g L−1 (A,E), 25 g L−1 (B,F), 50 g L−1 (C,G), and 100 g L−1 (D,H). The average roughness of the surface is indicated in the corresponding images A–D. In inserts F–H is the volume percentage of embedded alumina particles determined by corresponding image analysis. Electrodeposition was carried out at 40 mA cm−2 , 65◦ C, 200 rpm. Haber capillary tip. The exposed area of the sample was 1 a scan rate of 1 mV s−1 . EIS measurements were conducted by cm2 . The tests were performed in 0.1 M NaCl solution at room applying sinusoidal voltage with 5 mV (vs. OCP) amplitude in temperature. The open circuit potential was recorded for 15 min the frequency range 10 kHz−0.01 Hz, and the fitting of data has followed by a linear potentiodynamic sweep from −1 to 0 V with been performed using Z-View software. Frontiers in Chemistry | www.frontiersin.org 4 April 2019 | Volume 7 | Article 241 Nicolenco et al. Electrodeposited Fe-W/Al2 O3 Composites The compositional analysis of the electrodeposited coatings was performed on the cross-section area where two different regions were selected for examination: a wider area embracing Al2 O3 particles, and a particle-free region (Fe-W matrix). Both regions were inspected at four different locations of each sample. The average atomic fraction of incorporated Al, and the Fe and W % contents in the matrix are shown in Figures 1A,B, respectively. It is seen that the content of aluminum (alumina particles) increases sharply with the increase of the particles concentration in electrolyte up to 100 g L−1 . The maximum percentage of incorporated alumina is obtained at the maximum studied concentration of particles in solution, as predicted by Guglielmi’s two-step adsorption model (Low et al., 2006; Wasekar et al., 2016). In its simplest form, the mechanism of alumina particles co- deposition with Fe-W alloy can be described as follows: in a first step, suspended alumina particles are transported from the bulk of the solution to the cathode through the Nernst diffusion layer, where they become loosely adsorbed at the cathode’s surface. It is supposed that the transportation of particles is not only due to convection force, but also due to the formation of ionic clouds (i.e., adsorption of charged metal complexes in the bulk electrolyte) on the particles surface, which move toward the cathode under electric field and thus drag the particles (Low et al., 2006; Wasekar et al., 2016). In a second step, the charge transfer reaction breaks the ionic shell surrounding the particles and thus they become irreversibly encapsulated into the growing metallic layer. Although the particles co-deposition mechanism has been studied by many authors and using different approaches, the Guglielmi’s model still remains one of the most commonly adopted to describe the behavior of the particles FIGURE 3 | XRD patterns of sub-microsized alumina powder (A); Fe-W during electrodeposition. It was actually validated for some coating (B), and Fe-W/Al2 O3 composite coatings obtained with different binary alloys composites with inert particles, such as Ni-Fe/Al2 O3 concentration of alumina in electrolyte: 25 g L−1 (C), 50 g L−1 (D), and 100 g (Starosta and Zielinski, 2004; Torabinejad et al., 2016), Ni-W/SiC L−1 (E). (Wasekar et al., 2016) and others (Low et al., 2006). Taking into account the electrochemically inert nature of alumina particles one can expect that the composition of the metallic matrix should not be affected by the particles RESULTS AND DISCUSSION concentration in the solution unless their incorporation does not cause an apparent decrease of the active cathode surface Effect of Alumina Particles on the area (Beltowska-Lehman et al., 2012). Indeed, as it is seen Composition, Morphology, and Structure from Figure 1B, the incorporation of alumina particles does not of the Fe-W Composite Coatings influence on the W content in Fe-W matrix. However, slightly Considerable attention in composite coatings electrodeposition lower W content was achieved within the matrix when the was directed primarily toward determination of optimum concentration of particles in solution reached 100 g L−1 . This conditions for their production, i.e., temperature, pH, current is probably due to the partial blocking of the cathode area by density, rotation speed, etc. (Low et al., 2006). However, it is non-conductive alumina, which resulted in an increased cathodic often difficult to generalize the findings due to the large difference current density of the alloy deposition and a shift of cathodic in experimental conditions. Therefore, in order to investigate polarization curve toward more negative potentials (Figure S2). the effect of sub-micron alumina particles on the composition, As a result, deposition of coatings from the bath containing morphology and structure of Fe-W composite coatings, the same 100 g L−1 of Al2 O3 occurs at slightly higher overpotential, that parameters as for electrodeposition of compact Fe-W alloys with is −1.32 V compared to −1.25 V recorded in the case of Fe-W high W content and high current efficiency were used, i.e., the alloy deposition at the same cathodic current density applied deposition conditions applied were 65◦ C, pH 7, 40 mA cm−2 , (Figure S3). Thus, the W content in Fe-W matrix attains 21 at.% and 200 rpm. The different content of alumina particles in when the concentration of particles does not exceed 50 g L−1 , and the deposits was achieved by variation of the concentration of 17 at.% at higher alumina particles concentrations, respectively. alumina particles in the bath. Therefore, despite the non-conductive nature of Al2 O3 particles, Frontiers in Chemistry | www.frontiersin.org 5 April 2019 | Volume 7 | Article 241 Nicolenco et al. Electrodeposited Fe-W/Al2 O3 Composites Remarkably, in the XRD patterns of Fe-W/ Al2 O3 composites (Figures 3C,D) the peaks corresponding to the Al2 O3 particles are not observed. It can be suggested that the amount of co- deposited alumina particles is insufficient to provide strong reflections. In fact, their amount ranges between 2 and 12 vol% (Figure 2) which roughly corresponds to <4 wt.%. This is, however, in the limit of X-Ray diffraction in a multi-component materials, where at least 2–3 wt.% is needed to have clear diffraction peaks (Newman et al., 2015). The corresponding XRD pattern of the Fe-W alloy (Figure 3B) illustrates that the matrix of the composite coatings is composed of a crystalline body-centered cubic (bcc) single phase Fe(W) solid solution. In fact, the identified peaks are shifted to lower angles as compared to the characteristic peaks of bcc Fe due to the expansion of Fe lattice parameter with incorporation of bigger W atoms. However, the appearance of a broad shoulder at ∼43◦ indicates that the coating contains an amorphous-like fraction. It is worth noticing that appearance of crystalline peaks in Fe-W alloy having ∼ 20 at.% of W is rather unusual because at the W contents above ∼16 at.% the transformation to amorphous structure typically occurs FIGURE 4 | SEM micrograph of a fracture surface of the Fe-W coating (Nicolenco et al., 2018a). In fact, the deposition of Fe-W showing a fibrous structure. alloys previously reported by different authors (Bobanova et al., 2009; Wang et al., 2016) was performed under stagnant conditions, while the effect of the hydrodynamic conditions the current efficiency and the deposition rate remained rather on the structure development during electrodeposition of this unchanged as compared to Fe-W alloy deposition. These findings type of alloy has not been investigated yet. It was, however, are consistent with other studies emphasizing the effect of observed that under stirring conditions the adsorption alumina particles on the composition of binary alloy metallic state of an intermediate electroactive complex and the pH matrix (Yari and Dehghanian, 2013; Wasekar et al., 2016). of the near-electrode layer are modified as compared to The morphology of Fe-W alloy and Fe-W/Al2 O3 composite stagnant conditions, thus leading to the growth of coatings coatings electrodeposited at the same conditions was analyzed by with coarser grains (i.e., having stronger X-ray reflection) SEM and the representative images of the coatings surface are (Belevskii et al., 2010). shown in Figures 2A–D (left). SEM images of Fe-W alloy surface By increasing the content of alumina particles, a gradual depict a smooth, cracks-free globular structure with the average variation in the microstructure is observed. The intensity of the roughness of ∼80 nm (Figure 2A). The addition of particles Fe(W) crystalline peaks is reduced with the incorporation of does not change the morphology of the coatings. However, a small amount of Al2 O3 particles and only one broad peak the surface roughness increases gradually up to 300 nm due to is observed with further increase in concentration of alumina protrusion of Al2 O3 particles (Figures 2B–D). The panels e-g in solution and in the coating (Figures 3C,D). Typically, the of Figure 2 (right) illustrate the corresponding cross-sectional broadening of XRD peaks indicates the decrease in grain size images of the studied samples. Remarkably, the distribution of that is commonly observed with the incorporation of second Al2 O3 across the whole thickness of the coatings is uniform; phase particles. The particles can restrain grain growth in although, the particles tend to incorporate in form of the two ways: (i) disordering the regular structure providing more small agglomerates with ∼200 nm size. The volume percentage active sites for crystal nucleation and hence, by themselves, of embedded alumina particles was determined by the image limiting the crystallites growth (Yilmaz et al., 2014; Bhogal analysis of the corresponding cross-sectional surface of the et al., 2015); (ii) causing a shift of overpotential, thus leading coatings. According to that, the volume fraction of the Al2 O3 in to compositional variations in metallic matrix, which in deposit increases linearly from ∼3 vol% to a maximum 12 vol% turn causes a decrease (or increase) in the average grain with the increase in particles concentration in the bath up to 100 g size (Beltowska-Lehman et al., 2012; Yari and Dehghanian, L−1 . All the coatings have approximately the same thickness, 2013), that is the typical case of W alloys with iron group about 14–15 µm, which confirms the minor effect of alumina metals (Tsyntsaru et al., 2012). As has been discussed above, particles on the deposition rate of the composites. the incorporation of alumina particles does not change The microstructural changes caused by addition of Al2 O3 significantly the W content in Fe-W matrix. Therefore, it is particles to the Fe-W matrix were investigated by XRD possible to separate the direct contribution of reinforcement and EBSD techniques. Figure 3A shows the XRD pattern particles to the grain refinement of the composite from the of the sub-microsized alumina particles where the main amorphization of the electrodeposited matrix due to the W peaks corresponding to rhombohedral Al2 O3 were indexed. content variation. Frontiers in Chemistry | www.frontiersin.org 6 April 2019 | Volume 7 | Article 241 Nicolenco et al. Electrodeposited Fe-W/Al2 O3 Composites FIGURE 5 | SEM micrograph of the polished cross-section of the Fe-W coating for EBSD analysis (A). The red dashed box is highlighting the area from where the EBSD maps were acquired: phase map (B), orientation map in inverse pole figure coloring along growth direction with the corresponding code (C), and band contrast map (D). In the phase map the blue grains belong to the Fe(W) phase. SEM micrograph of the polished cross-section of the sample deposited with 100 g L−1 of Al2 O3 particles for EBSD analysis (E) with a band contrast map (F) and phase map (G). In the phase map the green grains belong to the Al2 O3 phase. The zero solutions are indicated as white pixel in both the orientation and phase maps, as dark pixel in the band contrast map. To get further insight into the structure of the coatings, shown in the SEM image in Figure 5A, the 50 nm polishing EBSD analyses were performed on selected areas of the of the cross-section reveals the presence of pillars of various coatings cross-sections. During metallographic preparation of thicknesses. EBSD analysis performed at these areas shows that the samples the formation of columns growing perpendicular such pillars consist of several sub-micron/nano Fe(W) grains, to the substrate plane was noted in the fracture surface presented in blue in the phase map of Figure 5B. The majority of the Fe-W alloys (Figure 4). A previous study suggested of Fe(W) grains have a diameter between 50 and 250 nm. The the formation of nanofibers in Ni-W and Co-W alloys Fe(W) grains are characterized by random texture, as there is no rich in W electrodeposited onto copper substrate (Donten predominant color in the orientation map shown in Figure 5C. A et al., 2003), while another study on Co-W alloys revealed large fraction of zero solutions, that is 71%, is found in between a columnar structure only for the coatings with low W the pillars. Zero solutions are shown in the phase and orientation content, i.e., around 3 at.% (Tsyntsaru et al., 2013). This maps as white pixels, while in the band contrast map as dark implies that the columnar structure is strictly determined pixels. The band contrast map shown in Figure 5D is providing by the electrochemical and hydrodynamic conditions, whose information about the quality of the EBSD patterns acquired modification may greatly change the nucleation and growth from the analyzed area: crystalline regions appear bright, while processes. As a matter of fact, no columnar structure was revealed areas providing poor quality patterns (e.g., grain boundaries, in Fe-W alloys electrodeposited under stagnant conditions amorphous phase, or deformed grains) appear dark. Hence, the (Nicolenco et al., 2018a). absence of EBSD pattern in between the Fe(W) pillars could be Figure 5A shows a secondary electron image of the polished related to an amorphous phase, or to a nanocrystalline phase cross-sections of the sample deposited with 0 g L−1 of Al2 O3 with sizes below the EBSD detection limits, that is 20 nm. Such particles (Fe-W alloy) and the red dashed box defines the high fraction of zero solutions would be expected considering area where the EBSD phase map (Figure 5B), orientation map the broad shoulder in XRD pattern of the sample, as can be seen (Figure 5C), and band contrast map (Figure 5D) were acquired. in Figure 3B. A Fe(W) solid solution phase (i.e., bcc cubic cell with 2.9231 Å Figure 5E shows a secondary electron image of the polished lattice constant), generated from the XRD results (Figure 3B) cross-sections of the sample deposited with 100 g L−1 of Al2 O3 was used as the reference phase for the EBSD indexing. As particles and the red dashed box defines the area where the EBSD Frontiers in Chemistry | www.frontiersin.org 7 April 2019 | Volume 7 | Article 241 Nicolenco et al. Electrodeposited Fe-W/Al2 O3 Composites FIGURE 6 | Load-displacement curves (A) and extracted dependence of hardness (B), elastic modulus (C), and plasticity index (D) as a function of the particles concentration in the bath. In insert (B) is the representative SEM image of the indentation imprint on the cross-section area of Fe-W/Al2 O3 composite with lowest particles concentration. band contrast map (Figure 5E) and phase map (Figure 5G) were Effect of Alumina Particles on Mechanical acquired. As revealed from the SEM image of the polished Properties of Fe-W Composite Coatings cross-section, a columnar structure is found also in the Fe- The mechanical properties (hardness, reduced elastic modulus, W/Al2 O3 composite sample. The size of the columns, both plasticity index, and elastic strain to failure) of the Fe-W and Fe- in thickness and length, appears smaller as compared to the W/Al2 O3 coatings were obtained by nanoindentation on cross- columns found for the sample deposited with 0 g L−1 of Al2 O3 section and the representative load–displacement curves are (see Figures 5A,D). Their extension along the cross-section shown in Figure 6A. It can be seen that compared to the Fe-W appears to be limited by the presence of the alumina particles. matrix, the composite coatings show rather complex behavior. However, as shown in the band contrast map (Figure 5F), it The hardness values extracted from the load-displacement was not possible to acquire any EBSD pattern from the areas in curves show that with the incorporation of small amounts of the cross-section that were including the columnar structures. particles into the Fe-W matrix the hardness increases sharply Only areas containing Al2 O3 particles were correctly indexed from ∼10 GPa to ∼15 GPa (Figure 6B). It is commonly (the grains of the Al2 O3 particles are shown with a green observed that the addition of particles improves the hardness color in the phase map in Figure 5G). These results are in of the coatings due to the two synergistic effects: (i) dispersion- agreement with the XRD findings (Figure 3E) regarding the strengthening, related to the presence of mechanically hard presence of an amorphous phase, or of a nanocrystalline phase fine particles retarding the plastic deformation of the matrix, with smaller grain sizes compared to the Fe-W alloy. Hence, and (ii) grain refinement, resulting in an increased number as observed from XRD and EBSD results, the incorporation of grain boundaries able to stop the dislocation motions. The of Al2 O3 particles in the coatings leads to a reduction of the former mechanism is described by the Orowan’s equation, grain size but it does not prevent the development of columnar which assumes that the dislocations bend out when passing the growth. Based on these observations one can infer the inherent obstacles (particles), thus creating some resisting force which porosity between the columnar Fe-W matrix and incorporated leads to improved hardness of the material. However, as the spherical shape Al2 O3 particles, which is however challenging to concentration of incorporated particles further increases, then confirm experimentally. a continuous decrease in hardness is observed (Figure 6B), Frontiers in Chemistry | www.frontiersin.org 8 April 2019 | Volume 7 | Article 241 Nicolenco et al. Electrodeposited Fe-W/Al2 O3 Composites so the hardness of the composite coating with the highest particles concentration is even lower than that of pure Fe- W matrix. This can be attributed to the apparent grain size refinement/amorphization confirmed by XRD and EBSD analyses (Figures 3, 5). The mechanical behavior of a composite is then governed by the inverse Hall-Petch relation, which describes the decrease of hardness as being due to grain boundary sliding and triple junctions (i.e., plastic deformation mechanisms different than dislocation motion), similar as it was observed elsewhere (Giallonardo et al., 2011; Wasekar et al., 2016). Contrary to what is observed for the hardness, the trend in elastic modulus is more unexpected. Figure 6C reveals that Er reduces with the incorporation of a small amount of particles (25 g L−1 Al2 O3 particles in the bath). This is in contradiction with a large number of studies showing the increase in elastic modulus with the addition of particles (Beltowska- Lehman et al., 2012; García-Lecina et al., 2013; Mahidashti et al., 2018). Often, the so-called rule of mixtures is used to infer the strengthening of composite materials containing FIGURE 7 | Evolution of the coefficient of friction of Fe-W (A) and Fe-W/Al2 O3 reinforcing fibers (i.e., with columnar microstructure), which composite coatings obtained with different concentration of alumina in can roughly predict the elasticity of the material if the load electrolyte: 25 g L−1 (B), 50 g L−1 (C), and 100 g L−1 (D) tested under dry is applied parallel to the fibers direction (isostrain conditions). friction conditions at 2 N load, 4 cm s−1 , 500 m. Taking into account the columnar structure of Fe-W revealed in Figures 4, 5 and the rather spherical (non-columnar) shape of the reinforcing particles, it can be expected that applying the Effect of Alumina Particles on Tribological load on cross-section, i.e., perpendicular to the Fe-W fibrous, the Properties of Fe-W Composite Coatings reinforcing effect of the particles would be less pronounced than The investigation of tribological behavior of electrodeposited Fe- in isostrain conditions. Nonetheless, even if the microstructure W and Fe-W/Al2 O3 composite coatings was carried out using of Fe-W/Al2 O3 composite does not correspond to the ideal ball-on-disk configuration sliding tests where conditions were the “isostrain” conditions, an increase of the Young’s modulus of same for all the tested tribo-pairs. The variation of the coefficient the composite with respect to that of the matrix should be of friction (COF) recorded during the tests as a function of expected, since the Al2 O3 particles possess a Young’s modulus sliding distance is shown in Figure 7. The COF of the Fe-W larger than the Fe-W matrix. However, other factors like loose alloys and composite coatings with low particles concentration Al2 O3 particles-matrix interfaces (Zhou et al., 2016) and, more is high, ∼0.8–0.9, while it decreases to ∼0.6 for the composite in particular, nanoporosity surrounding the embedded particles coating with the highest particles concentration. Typically, the could explain the reduction of Er . Actually, a strong dependence high values of COF are ascribed to the presence of microscopic of the Young’s modulus on porosity in nanoporous materials is irregularities of materials surfaces. Taking into account that the well-known and it has been confirmed by nanoindentation and initial surface roughness of both Fe-W coatings and alumina finite element simulations in a variety of works (Ramakrishnan counter body was in order of hundred nanometers, such high and Arunachalam, 1993; Gibson and Ashby, 1999; Pellicer et al., values of the COF can be attributed to the presence of adhered 2012). Higher porosity causes a decrease of the reduced Young’s debris which increase the asperity contact between two bodies modulus during nanoindentation (Esqué-de los Ojos et al., 2016). in contact. When the amount of incorporated particles increases, since The nature of debris particles was investigated by SEM/EDS alumina has a very high Young’s modulus (300–400 GPa) the net techniques on the resulted wear tracks (Figure 8). Indeed, the effect is that Er increases but remains always lower than for the debris particles were found to be distributed within the whole pure Fe-W alloy. length of the wear track accumulating mainly in piles. The The plasticity index was also derived from the oxygen content in these regions is increased up to 50 at.% which nanoindentation data in order to provide further insight corresponds to the oxygen content in mixed iron oxide Fe3 O4 . into mechanical properties of electrodeposited composite This is consistent with the previous studies on the tribological coatings. Thus, the Up /Utot reduces with the incorporation behavior of electrodeposited Fe-W alloys which showed that of small amount of particles (Figure 6D), indicating rather these coatings undergo severe oxidation during fretting (tribo- elastic nature of the composites and their improved resistance oxidation) (Bobanova et al., 2009; Nicolenco et al., 2018b). The to permanent deformation in contrast to rather plastic nature tribo-oxidation phenomenon can be explained as follows: when of Fe-W matrix. An increase in plasticity observed in the two surfaces slide together, the friction work is turned into composites with higher particles concentration is most likely due thermal energy, which tends to maximize the potential energy to the grain refinement which promotes branching of the shear at the interface. Since the maximized thermal energy is naturally bands in nanocrystalline/amorphous matrix (Figure 6D). an unstable state, the oxidation itself is a form of intrinsic Frontiers in Chemistry | www.frontiersin.org 9 April 2019 | Volume 7 | Article 241 Nicolenco et al. Electrodeposited Fe-W/Al2 O3 Composites FIGURE 8 | SEM images of the worn surface of the Fe-W (A) and Fe-W/Al2 O3 coatings obtained with different concentration of alumina in electrolyte: 25 g L−1 (B), 50 g L−1 (C), and 100 g L−1 (D) presented along with the corresponding wear track depth profiles. response of the material to recover the system equilibrium under alloy matrix. It is also comparable to the wear rate of annealed friction conditions (Abdel-Aal, 2003). Thus, the formation of Fe-W alloy and even electrodeposited hard chromium coatings iron oxides during dry friction can be considered a driving tested by using a similar set-up and dry friction conditions factor determining the resultant tribological behavior of the Fe- applied (Mulone et al., 2019). Thus, in annealed Fe-W alloys W coatings (COF, wear depth/volume, cracks initiation, plastic the reduction in the tribooxidation was achieved mainly due deformations, etc.). As it can be seen from Figure 8C, the to the phase transformation, that is, the formation of Fe2 W presence of micro-sized Al2 O3 particles significantly reduce the and FeWO4 hard phases which are not prone to oxidation. area of the exposed Fe-W matrix, thus reducing the asperity Hence, electrodeposited Fe-W/Al2 O3 composite coatings contact between two sliding surfaces what results in a lower COF are getting advantageous in terms of fabrication (one-step and wear depths (Figure 8). electrodeposition, no post-treatment required, environmentally- The wear rate of Fe-W alloy and Fe-W/Al2 O3 composite friendly bath composition) and sustainability (suitable for coatings was calculated by integration the cross-section area chromium replacement). Taking into account the outstanding of the wear tracks shown in inserts Figure 8 and the obtained thermal resistance of the amorphous-like Fe-W deposits (Mulone results are shown in Figure 9. The interpretation of the depth et al., 2018a) (up to 600◦ C) and remarkable hardness of these profiles was sometimes not straightforward due to the presence coatings (10–16 GPa), the novel Fe-W/Al2 O3 composites can be of debris which remained adhered in the wear track even after considered as alternative appealing materials for a wide range ultrasound cleaning that increased the error in calculation of the of technically-demanding applications, such as components and wear depth and wear rate. Nevertheless, it is clearly seen that mechanisms where lubrication is recognized as ineffective or the addition of Al2 O3 particles reduces the specific wear rate, inapplicable, e.g., in high-temperature vacuum systems. i.e., increases the wear resistance of the coatings under the tested Many researchers address the importance of the mechanical conditions. The lowest wear rate (1.8·10−6 mm3 N−1 m−1 ) was characteristics for the good wear resistance and show a linear obtained for the sample with the highest particles concentration, relationship between the hardness (or H/E ratio) and wear for which is an order of magnitude lower than that of pure Fe-W different polycrystalline materials like Ni-W (Sriraman et al., Frontiers in Chemistry | www.frontiersin.org 10 April 2019 | Volume 7 | Article 241 Nicolenco et al. Electrodeposited Fe-W/Al2 O3 Composites density (jcorr ) was calculated from the presented polarization curves. Usually, the corrosion current is determined by the extrapolation of the rectilinear dependences of log j vs. E to Ecorr in the Tafel region. However, due to the peculiarities of cathodic hydrogen evolution and active anodic metal dissolution the cathodic and anodic branches of recorded voltammograms are asymmetric (Figure 10A), what makes the standard jcorr determination not reliably applicable. Therefore, the values of jcorr were obtained by using Allen-Hickling equation which enables to accurately estimate the corrosion current from the data obtained in a relatively narrow potential range close to Ecorr (Kublanovsky et al., 2008; Vernickaite et al., 2016). The obtained results show that the corrosion current density for all the studied samples is the same order, independently of the content of co-deposited alumina particles (Table 1). In the Figure 10B, each point in the plot corresponds to the measured impedance at a certain frequency, while the continuous line presents the best-fit data obtained by using an equivalent circuit shown in insert Figure 10B. The used equivalent circuit FIGURE 9 | The wear rate of Fe-W alloy and Fe-W/Al2 O3 composite coatings for fitting of EIS data supposes the complicated mechanism of at dry friction conditions at 2 N load, 4 cm s−1 , 500 m. alloy corrosion, which perhaps involves intermediate stages with the adsorption of corrosion products (Vernickaite et al., 2016). The equivalent circuit used to simulate the EIS consisted of Rs –solution resistance (uncompensated resistance); Rct –charge 2006), Ni-P (Jeong et al., 2003), and different type of composites transfer resistance; Cdl –a constant phase element having meaning (Leyland and Matthews, 2000; Shi et al., 2006; García-Lecina of double layer capacitance of the electrolyte at alloy surface; et al., 2013; Man et al., 2014; Bajwa et al., 2016). It is assumed and Cd which is adsorption-related capacitance of intermediates. in these cases that the abrasive wear originates from plastic Accordingly, circuit Rct Cdl describes the Faraday process, i.e., deformation. Therefore, materials with higher H (or H/E ratio) the resistance and capacities of the layer against electrochemical should better resist the plastic deformations and thus have lower reaction, and Cad represents the process of blocking the wear. However, in the Fe-W system the tribological behavior coating’s surface by adsorbed species. Generally, the higher the is primarily governed by the tribo-oxidation (Mulone et al., charge transfer resistance Rct , the greater corrosion resistance 2019), i.e., the chemical reactivity of the material and the nature of the system. Thus, according to Table 1, the lowest corrosion of the oxidation products formed are the determining factors. resistance was obtained for the Fe-W coating without particles. Nevertheless, the high hardness of Fe-W/Al2 O3 composites The highest values Rct are obtained for the composite coatings may result in an excessive coating brittleness contributing to produced from the bath with 50 and 100 g L−1 of particles (6 and the cracks propagation inside the wear tracks as depicted 12 vol% Al2 O3 , respectively). in Figures 8B,C. Noticeably, the Rct obtained for Fe-W coatings electrodeposited from glycolate-citrate electrolyte and tested Effect of Alumina Particles on Corrosion in chloride medium can be roughly comparable to other Properties of Fe-W Composite Coatings Fe-containing deposits described in literature: Fe-W alloy Corrosion of Fe-W alloys and composites was induced by using electrodeposited from citrate bath and tested in sulfate-chloride 0.1 M NaCl solution, which is commonly used to simulate marine medium (380.7  cm2 ) (Vernickaite et al., 2015), Ni-Fe-W alloy environment. The obtained potentiodynamic polarization curves tested in similar corrosion medium (207.0  cm2 ) (Sriraman and Nyquist plots are presented in Figures 10A,B, respectively, et al., 2007) and with Ni-Fe/Al2 O3 composite in sodium and the extracted parameters are given in the Table 1. It can sulfate (∼250  cm2 ) (Starosta and Zielinski, 2004); whereas be seen from Figure 10A that the Fe-W based coatings exhibit the corrosion resistance of Co-W and Ni-W alloys in neutral a very similar behavior with an active anodic metal dissolution mediums is much higher, i.e., 1,200 and 6,770  cm2 , respectively that do not transfer into passive state. With the increase in (Sriraman et al., 2007; Vernickaite et al., 2016). Taking this into particles concentration the corrosion potential (Ecorr ) slightly account, and the small variations in corrosion currents obtained shifts toward more negative potentials. Usually, the lowering of for investigated Fe-W composite coatings, one could suspect corrosion potential indicates a higher tendency of the electrode the detrimental and prevalent effect of Fe dissolution from to be dissolved in an electrolyte. Nevertheless, different hydrogen the coating. overvoltage on Fe-W surface with different alumina contents The surface of investigated Fe-W coatings after corrosion may vary without a direct correlation with the corrosion rate. test is cracked and the cracks propagation is along the Moreover, accumulation of corrosion products on the surface grain boundaries as it can be observed in Figure 11A. The could also cause the lower values of Ecorr . The corrosion current corrosion medium can penetrate faster through these cracks, thus Frontiers in Chemistry | www.frontiersin.org 11 April 2019 | Volume 7 | Article 241 Nicolenco et al. Electrodeposited Fe-W/Al2 O3 Composites FIGURE 10 | Linear sweep voltammograms (A) and EIS Nyquist plots (B) for Fe-W and Fe-W/Al2 O3 coatings electrodeposited under similar conditions and examined in 0.1 M NaCl solution at OCP. In insert (B) is the electrochemical circuit used for data fitting. Rs , solution resistance; Rct , charge transfer resistance; Cdl is a constant phase element having meaning of the double layer capacitance of alloy surface and Cad is adsorption-related capacitance of intermediates. TABLE 1 | Extracted corrosion parameters from E vs. j plots shown in Figure 10A and corresponding equivalent circuit parameters determined by fitting the impedance spectra of electrodeposited Fe-W and Fe-W/Al2 O3 coatings (Figure 10B) in 0.1 M NaCl medium at room temperature. Coating –Ecorr , V –jcorr , A/cm2 Rct ,  cm2 Cdl , F/ cm2 n(Cdl ) Cad , F/ cm2 n(Cd ) Rs ,  cm2 Fe-W 0.795 5 × 10−4 355.2 1.54 × 10−3 0.80 6.36 × 10−2 0.89 8.8 Fe-W+25 g/L Al2 O3 0.804 3 × 10−4 569.1 8.61 × 10−4 0.85 5.66 × 10−2 0.95 6.9 Fe-W+50 g/L Al2 O3 0.823 3 × 10−4 843.8 1.55 × 10−3 0.71 6.92 × 10−2 0.98 31.2 Fe-W+100 g/L Al2 O3 0.837 3 × 10−4 795.1 8.18 × 10−4 0.81 3.32 × 10−2 0.85 45.6 “n”—is the exponent in the equation for the constant phase element that measures how far the interface is from an ideal capacitor (0 < n < 1). accelerating the dissolution of Fe-W matrix. The EDS analysis with concentrations ranging from 25 to 100 g L−1 to the Fe-W of corroded surface of the studied coatings showed a significant plating bath did not influence the W content in Fe-W matrix increase in both oxygen content (up to 50 at.%) and the W and the deposition rate remains also virtually unchanged. The content (up to 28 at.%), which is in accordance with other studies content of co-deposited Al2 O3 increased linearly from 2 to 12 performed on W alloys with Fe group metals (Wang et al., 2014; vol% with the increase in particles concentration in the bath. He Y. et al., 2016; Vernickaite et al., 2016). This indicates that the The obtained Fe-W and Fe-W/Al2 O3 composite coatings were corrosion process occurs through the preferential dissolution of characterized by a columnar growth structure. In the Fe-W Fe from the matrix, which leads to the formation of iron oxides coating such columns consisted of several sub-microns and nano and hydroxides at the electrode surface and to release of free Fe2+ Fe(W) grains. As observed from XRD and EBSD results, the ions into solution (Sriraman et al., 2007; He Y. et al., 2016). It incorporation of alumina particles in the Fe-W/Al2 O3 composite is supposed that during corrosion process W preferentially form coatings led to a reduction of the grain size and amorphization, oxides, or can be also released into solution in form of WO2− 4 but it did not prevent the development of columnar growth. ion. Thus, FeWO4 and Fe2 (WO4 )3 compounds could be found These structural changes partially contributed to an increase in in corrosion products, as shown in He et al. (2016). Slightly measured hardness up to 15 GPa obtained with the addition increased corrosion resistance shown by EIS can be attributed to of small amount of particles (25 g L−1 in the bath, 2 vol% in the presence of alumina particles which partially block the surface deposit), followed by a decrease below 10 GPa for the coating and hinder the propagation of corrosion cracks, thus no cracks with maximum volume fraction of alumina. The dependence are evidenced on the surface and cross-section of the composite of the elastic modulus on the Al2 O3 particles concentration coatings after corrosion test (Figure 11B). is not the typical one expected for composite coatings with reinforcing particles. For low particles concentration the Young’s CONCLUSIONS modulus decreases by around 40% and it increases with further increase of the particles concentration, but still remains lower The electrodeposition of novel Fe-W/Al2 O3 composite coatings than for the pure Fe-W alloy. The observed reduction in Young’s with various alumina concentrations has been carried out from modulus could be explained by the presence of nanopores an Fe(III)-based electrolyte. The addition of alumina particles surrounding the particles, among other possible reasons. A Frontiers in Chemistry | www.frontiersin.org 12 April 2019 | Volume 7 | Article 241 Nicolenco et al. Electrodeposited Fe-W/Al2 O3 Composites Electrodeposited Fe-W and Fe-W/Al2 O3 composite coating show similar corrosion behavior in 0.1 NaCl medium. The corrosion occurs via the formation of Fe and W oxygen compounds and preferential Fe dissolution from the matrix independently from alumina particles concentration. Nevertheless, the presence of Al2 O3 in the deposits results in a slightly increase corrosion resistance (up to ∼800  cm2 ) obtained by EIS due to the partial blocking of the surface and hindering the propagation of corrosion cracks. The results presented in this work provide a possibility to integrate the nanocrystalline Fe-W/Al2 O3 composite coatings into various mechanical systems working under dry friction, and in particular those working at elevated temperatures. AUTHOR CONTRIBUTIONS The electrodeposition of the coatings, wear, and corrosion measurements were performed by AN together with NI, under the supervision of NT, HC, and EG-L. The structural characterization was performed by AM under the supervision of UK. The mechanical characterization was performed by JS and EP. Writing-original draft preparation by AN and AM. All the authors contributed to the writing-review and editing. FUNDING This work has been funded by the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement N◦ 642642 (SELECTA), H2020 SMARTELECTRODES project (No.778357) and Research Council of Lithuania No 09.3.3-LMT-K-712-08-0003. Partial financial support from the Generalitat de Catalunya (2017-SGR- 292 project) and the Spanish Government (MAT2017-86357- FIGURE 11 | SEM images of Fe-W (A) and Fe-W+100 g/L Al2 O3 coatings (B) C3-1-R and MAT2017-86357-C3-2-R) is also acknowledged. after corrosion test in 0.1 M NaCl at room temperature. In inserts are the The authors also thank the Departamento de Desarrollo corresponding cross-sectional images of the samples. Económico y Competitividad of the Basque government (ELKARTEK, KK-2017/00096, KK-2018/00108) for partially superior tribological behavior, the lowest wear rate of 1.8·10−6 supporting this research. mm3 N−1 m−1 , was obtained for the Fe-W/Al2 O3 composite coating with the highest particles concentration (100 g L−1 in SUPPLEMENTARY MATERIAL the bath, 12 vol% in deposit). The wear rate was reduced by an order of magnitude as compared to the pure Fe-W The Supplementary Material for this article can be found alloy matrix mainly due to the reduction of the exposed online at: https://www.frontiersin.org/articles/10.3389/fchem. area of the Fe-W matrix and lowering the tribo-oxidation. 2019.00241/full#supplementary-material REFERENCES film coatings for industrial applications. Acta Metallurg. Sinica 29, 8–16. doi: 10.1007/s40195-015-0354-1 Abdel-Aal, H. A. (2003). On the interdependence between kinetics of friction- Belevskii, S. S., Cesiulis, H., Tsyntsaru, N., I., and Dikusar, A. I. (2010). 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Pulsed electrodeposition and mechanical properties of Conflict of Interest Statement: The authors declare that the research was Ni-W/SiC nano-composite coatings. Mater. Design 112, 140–150. conducted in the absence of any commercial or financial relationships that could doi: 10.1016/j.matdes.2016.09.070 be construed as a potential conflict of interest. Wu, G., Li, N., Wang, D. L., Zhou, D. R., Xu, B. Q., and Mitsuo, K. (2004). Effect of α-Al2O3particles on the electrochemical codeposition of Copyright © 2019 Nicolenco, Mulone, Imaz, Tsyntsaru, Sort, Pellicer, Klement, Co-Ni alloys from sulfamate electrolytes. Mater. Chem. Phys. 87, 411–419. Cesiulis and García-Lecina. This is an open-access article distributed under the doi: 10.1016/j.matchemphys.2004.06.016 terms of the Creative Commons Attribution License (CC BY). The use, distribution Yari, S., and Dehghanian, C. (2013). Deposition and characterization of or reproduction in other forums is permitted, provided the original author(s) and nanocrystalline and amorphous Ni-W coatings with embedded alumina the copyright owner(s) are credited and that the original publication in this journal nanoparticles. Ceram. Int. 39, 7759–7766. doi: 10.1016/j.ceramint.2013. is cited, in accordance with accepted academic practice. No use, distribution or 03.033 reproduction is permitted which does not comply with these terms. Frontiers in Chemistry | www.frontiersin.org 15 April 2019 | Volume 7 | Article 241