Electrodeposition of Co-B/SiC composite coatings: 2 Characterization and evaluation of wear volume and 3 hardness 4

: In the present paper, Co-B/SiC composite coatings were obtained via electrodeposition 16 from colloidal suspensions with different concentrations of SiC particles and subsequent heat 17 treatments at 350 °C. The composition, morphology and structure of the Co-B/SiC composite 18 coatings were analyzed using glow discharge spectrometry (GDS), scanning electron microscopy 19 (SEM) coupled with energy-dispersive spectroscopy (EDS) and X-ray diffraction (XRD). Hardness 20 and tribological properties were also studied. The results showed that an increase in the SiC 21 concentration in the colloidal suspensions resulted in both an increase in the SiC content and a 22 decrease in the B content in the obtained Co-B/SiC coatings. The Co-B/SiC coatings were adherent, 23 glossy and soft and exhibited a homogeneous composition in all thicknesses. By contrast, an increase 24 in the SiC particle content of the Co-B/SiC composite coating from 0 to 2.56 at.% SiC reduced the 25 hardness of the film from 680 to 360 HV and decreased the wear volume values from 1180 to 23 μ m 3 26 N -1 m -1 , respectively (that is, the wear resistance increased). Moreover, when the Co-B/SiC coatings 27 with SiC content ranging from 0 to 2.56 at.% SiC were subjected to a heat treatment process, the 28 obtained coating hardness values were in the range of 1200 to 1500 HV and the wear volume values 29 were in the range of 382 to 19 μ m 3 N -1 m -1 .


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Surface degradation is one of the main damages experienced by the metallic parts of machinery 34 exposed to high stress in hostile environments. The surface degradation of these components causes 35 a deterioration in their mechanical properties, such as their wear resistance and hardness, which 36 results in machinery malfunctions. For many decades, hard coatings such as cadmium (Cd), nickel 37 (Ni) or chromium (Cr) have been widely used to protect metal components and tools against wear.
in the metal matrix [2,3]. The insoluble micro o nano particles occluded in the metal matrix can be 45 nitrides, oxides and carbides (Si3N4, SiO2, Al2O3, TiO2, SiC, WC, graphite) and its function is to 46 increase the wear resistance and hardness of the coatings [4,5,6,7,8,9]. These phenomena are 47 mainly attributed to the hardening of the metal matrix by finely dispersed ceramic particles. Ogihara 48 et al. [10] reported that composite films with Ni-B as the matrix material and SiC particles as the 49 second phase exhibit hardness values of 845 HV without heat treatment and 1490 HV with heat 50 treatment. Balaraju and Seshadri showed that, when the content of Si3N4 particles occluded in a Ni-P 51 metal matrix is increased, the wear resistance of the Ni-P/Si3N4 coating increases substantially [11].

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Min-Chieh et al. [9] reported that the addition of SiC particles to a Ni-P metal matrix reduces the 53 residual stress of the deposits and therefore eliminates surface cracking. Also, several studies on 54 Ni/SiC composite coatings have reported significant improvement in the wear resistance when SiC 55 particles are added in the nickel matrix [12,13]. In another study, the addition of B4C particles to the 56 matrix of the metallic Ni-P(9 %) alloy was found to increase the wear resistance of Ni-P(9 %)/B4C 57 composites [14].

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Recently, nanocrystalline cobalt (Co) coatings and Co alloys have been identified as good 59 candidates for replacing the hard coatings of Cr, Cd or Ni because of their similar or improved 60 mechanical properties [15,16]. Additionally, Co is not considered a heavy metal that negatively 61 affects human health [17]. In a previous work [18], we reported the formation of Co-B hard coatings 62 with hardness values between 760 and 800 HV (very similar to those of hard Cr), depending on the 63 content of B in the coating.

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The novelty of the present work, consists in the preparation of Co-B/SiC composite coatings by 65 electrodeposition method due to the need of coatings with low friction and an improved wear 66 resistance for its application in fabrication of machines, parts and metal structures exposed to high 67 stress and severe erosion conditions. The aim was to elucidate the effect of incorporated SiC particles 68 in the Co-B metal matrix on the wear volume, friction coefficient, and hardness. The effect of 69 thermally treated Co-B/SiC composite coatings was also studied.

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This study was carried out with a Turbiscan analyser (mod. Lab Expert, Formulation Co.,   with an inter-electrode distance of 5 cm. AISI 1018 steel plates (2.5 × 5.5 cm 2 of exposed area) were 97 used as the cathodes and graphite plates were used as the anodes. The coatings were deposited under 98 galvanostatic conditions by applying 8.6 mA cm -2 for 56 min at 25 °C. The current density was selected 99 from additional tests (not presented here) using a Hull cell for each of the electrolytic baths.

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The deposited phases of the Co-B/SiC composite coatings were analyzed by X-ray diffraction 101 (XRD) using a Bruker diffractometer (model D8 Advance) in the Bragg-Brentano arrangement with 102 Cu Kα radiation (α = 1.54 Å). The 2θ range from 30° to 150° was recorded at a scan rate of 0.2° s -1 .

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The morphologies of the coatings were evaluated using scanning electron microscopy (SEM) (JEOL                        To understand the wear mechanism of the coatings, we analyzed the worn surface of the Co-

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B/SiC composite coatings with different SiC content by SEM. In the Co-B coatings (Fig. 9a), the 265 presence of torn patches and some detachment within the worn tracks is typical evidence of that a 266 plastic deformation was carried out in this process, which is indicating that the principal mechanism 267 is adhesive wear. The results show that increasing SiC particles content up to 2.5 at.% SiC (Fig. 9b)

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The microhardness results are shown in Fig. 10. As expected, the hardness of the coatings 289 increased considerably after they were thermally treated; hardness values from 1200 to 1500 HV were 290 obtained over the entire range of SiC concentrations (0 to 2.56 at.% SiC, respectively). Thus, the 291 presence of SiC particles in the coatings has little influence on their hardness.

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Therefore, the increase in hardness is mainly associated with the crystallization of the hard 296 intermetallic species Co3B, which occurs between 200 and 400 °C, which is further confirmed by XRD 297 patterns (Fig. 11). In Fig. 11 Table 1). Therefore, the heat treatment strongly influences the 311 hardness of the coatings due to the formation of the Co3B intermetallic species but has little influence 312 on the volume of wear and the coefficient of friction, which are mainly decreased by the incorporation 313 of the SiC particles into the metal matrix.