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Embedment of eutectic tungsten carbides in arc sprayed steel coatings
Surface & Coatings Technology 331 (2017) 153–162
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Embedment of eutectic tungsten carbides in arc sprayed steel coatings
MARK
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Wolfgang Tillmann, Leif Hagen , David Kokalj Institute of Materials Engineering, TU Dortmund University, Leonhard-Euler-Straße 2, 44227 Dortmund, Germany
A R T I C L E I N F O
A B S T R A C T
Keywords: Arc spraying Tungsten carbide External powder injection Steel coatings
Tungsten carbide reinforced deposits have already evolved into a predominant coating system in order to protect stressed surfaces against wear. Among thermal spraying processes, due to a high deposition rate, arc spraying is a promising process to manufacture cost-saving, wear resistant coatings. However, inherent process characteristics prevailing in arc spraying as well as the utilization of tungsten carbides, as a filling for cored wires, could lead to undesirable phase evolutions, which in turn provoke the degradation of the mechanical properties. The embedment of tungsten carbides into the surrounding metallic matrix is affected by metallurgical interactions with molten spray particles. Within the scope of this study, an external injection of tungsten carbides was applied in order to analyze the embedment of tungsten carbides in arc sprayed low alloyed steel. Accordingly, metallographic investigations were carried out, which address the reactive layer at the interface of embedded tungsten carbides to the surrounded iron-based matrix. Microstructural characteristics such as mechanical properties and phase composition were scrutinized by means of nanoindentation, energy dispersive X-ray spectroscopy, and X-ray diffraction, respectively. It was found that the embedment of tungsten carbides, which have been externally injected into the arc burning zone, differs from that obtained from deposits produced with the use of cored wire with tungsten carbide as filling. Thus, externally injected tungsten carbides are less inclined to form eta carbides due to dissolution, which again results in differences in the mechanical properties across the reactive layer.
1. Introduction The twin wire arc spraying (TWAS) process is a time saving [1] and energy efficient [2] technique, mainly used in surface refurbishments and maintenance applications [3]. Nevertheless, TWAS has the ability to deposit wear and corrosion resistant coatings in a simple and cost saving way due to the use of compressed air as atomization gas, and hard particle reinforced cored wires as feedstock, respectively. Within the field of wear protection, arc sprayed coatings, produced by utilizing cored wires with hard particle as a filling, are widely used to enhance the tribological and mechanical characteristics of surfaces. Due to their outstanding properties, tungsten carbide reinforced arc sprayed coatings are appropriate to protect stressed surfaces against wear. In terms of arc sprayed coatings, only few studies have discussed the use of tungsten carbide reinforced feedstock materials such as iron-based (FeCSiMn-WC/W2C, Fe3Al-WC, Fe-FeB-WC) [4–8] as well as nickel based alloys ((NiCrBSi, NiBSi)-WC/W2C, Ni-(WC-Co)) [3,9,10]. According to studies on tungsten carbide reinforced Fe-based arc sprayed coatings, a major objective was the focus on the tribological behavior [4,6–8]. It was found that a fine lamellar coating structure, provided by utilizing fine-grain hard material fillings, can lead to enhanced tribological coating characteristics [6].
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With respects to the feedstock, Ni-based and Co-based coatings cause some health and safety issues. Ni-based alloys are allergenic, labelled as suspect carcinogenic agents [11], and classed as hazardous powder materials (hazard statement according to the European Commission regulation EC 790/2009) [12]. Regarding the WC–Co feedstock, it is reported that the materials are also toxic when inhaled [13]. Moreover, the feedstock is listed in the “Report on Carcinogens” [14]. In terms of the compatibility of thermal spray coatings in the food production technology (according to EU and PDA standards), coatings containing Ni- and Co- might fail to meet the specific requirements within this field such as strict limitations concerning the contamination of the products with harmful or even toxic elements [15]. Regarding thermally sprayed coatings, Fe-based alloys have proven to be less hazardous when compared to Ni- and Co-based alloys. However, Febased alloys are known to be less corrosion resistant compared to Niand Co-based alloys. Over the last decades, it was already demonstrated that Fe-based amorphous coatings provide a corrosion resistance comparable to that of high-performance alloys such as Ni-based alloys [16]. As a result, some Fe-based materials already replace more expensive Co- or Ni-based alloys in various industry applications where the components are subjected to corrosion and wear [17]. It can be stated
Corresponding author. E-mail addresses: [email protected] (W. Tillmann), [email protected] (L. Hagen), [email protected] (D. Kokalj).
http://dx.doi.org/10.1016/j.surfcoat.2017.10.044 Received 24 June 2017; Received in revised form 13 October 2017; Accepted 14 October 2017 Available online 16 October 2017 0257-8972/ © 2017 Elsevier B.V. All rights reserved.
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combinations demonstrated the possibility to embed single particles. However, an insufficient wetting of particles was observed. As a result, an adequate adhesion could not be established for the insertion of steel particles in a Zn-based and Al-based matrix. Furthermore, the investigation showed that the injection of particles into the axially incoming gas stream has a significant influence on the atomization of the molten electrode tips. Accordingly, the gas flow was disturbed, resulting in a more heterogeneous microstructure and topographical irregularities. Another approach of external powder injection is executed by means of an injection just behind the arc burning zone. Barbezat and Warnecke [29] have a patent on this approach. With the use of a modified arc spraying device and spray torch, a powdered additive material is fed axially into the arc burning zone via the primary gas stream or through the additional injector behind the arc burning zone. A closer examination of this modification or the generated layer systems cannot be found in the literature. Paczkowski et al. [30] analyzed the applicability of producing particle-reinforced layer systems by means of an open wire arc spraying system. Accordingly, different MMCs were produced using a Cu-based matrix. Metallographic investigations revealed a homogeneous distribution and a form-fitting embedding of the hard particle phase in the surrounding Cu-based matrix. Metallurgical interactions across the interface of hard particles and the metallic matrix were not investigated. Nevertheless, it was found that the particle loading of the atomizing gas stream stabilized the plasma at the arc burning zone. Dubovoj et al. [31] further examined the production of composite materials by means of external powder injection into the high-temperature region of the arc burning zone. Various metal-polymer, metal-ceramic, and metal-glass composites were investigated with respect to their mechanical and physical properties. According to this study, a major objective was the wear behavior, adhesion, and thermal conductivity of the coatings. Metallurgical investigations on the embedment of hard particles into the surrounding metallic matrix were not taken into account. In contrast to the aforementioned examinations, in this study the manufacturing route of an external injection of tungsten carbides and their embedding into the metallic matrix across the deposits are investigated. For this purpose, the embedment of tungsten carbides in coatings produced by conventional cored wires is compared with that manufactured with exclusively externally injected tungsten carbides. Therefore, the transition zones between the tungsten carbides and surrounded iron-based matrix are fundamentally investigated by means of scanning electron microscopy and nanoindentation. By using X-ray diffraction, the occurred phases can be traced back to the manufacturing process and the embedment of the tungsten carbides.
that tungsten carbide reinforced Fe-based Metal Matrix Composites (MMCs) represent an environmentally friendly alternative. By means of arc spraying, tungsten carbide reinforced coatings are manufactured exclusively by using of cored wires. These cored wires mainly consist of a metallic (electrically conductive) outer sheath and cast tungsten carbides as a filling. With respect to cast tungsten carbides (eutectic mixture of WC-W2C), W2C has the largest share with 73–80 wt %. The stoichiometric WC has a carbon content of 6.13 wt%, whereas W2C has a lower carbon content of 3.61 wt% [18]. So far, the phase evolution of arc sprayed Fe-based WC-W2C reinforced coating systems has not yet been examined. A few studies investigated the microstructure formation and phase transformation of arc sprayed Ni-based WC-W2C reinforced coating systems [3,9]. According to these studies, eutectic tungsten carbides decarburized during spraying and a large amount of W dissolved into the Ni-rich matrix. Moreover, Ni also dissolved into WC-W2C and gathered around WCW2C grains to form fine WC-W2C–Ni composites. In [9] it was stated that these metallurgical interactions are attributed to the electrode phenomena during atomization, when the WC-W2C grains are wetted by the molten metal. It was mentioned that these metallurgical interactions can lead to a degradation of the mechanical properties. The stoichiometric WC decomposes above 3058 K to L + C (L = liquid melt) in an inert gas atmosphere or vacuum, respectively [19]. Exposed to oxygen, the decomposition starts at significantly lower temperatures of approximately 823 K. With W2C, congruent melting and the formation of liquid phases occur above a liquidus temperature of 3058 K. Two eutectic phases form during the cooling of the melt. A peritectic decomposition of W2C to W + WC occurs at a temperature of 1523 K. Eta carbides (e.g. M3W3C, M6W6C etc.) are mainly formed according to Kurlov et al. [20] with heterogeneous microstructures, especially in the interface of WC and the transitional metal. The stoichiometries as well as the matrix material influence the phase stability [21,22]. Besides the chemical composition of the metal matrix, the degree of dissolution of the tungsten carbide depends on type, shape, and size of the carbides. For instance, irregularly shaped particles are more prone to dissolve due to their tendency to heat up to higher temperatures by the exposure of a heat source [23]. Small sized particles tend to dissolve faster than coarser particles. Moreover, as reported by other researchers [24,25] eutectic tungsten carbides are more susceptible to react with the liquid melt than more thermally stable carbides such as mono crystalline carbides consisting of hexagonal WC. For tribologically stressed surfaces, it was found that the wear resistance of WC-composites depends, inter alia, on the embedment of hard phases [26,27]. Accordingly, the process control requires a good embedding and adhesion of hard particles in the layer composite, so that the contacting surface is not damaged due to breakouts of weakly embedded hard phases. In that respect, an insufficient adhesion of hard phases can lead to a damage of the surface integrity, the counter body or layer composite itself. Regarding the manufacturing route of arc sprayed coatings using cored wires, there is no possibility to deposit coatings with graded carbide content. The local reinforcement due to hard particles is determined by the amount of hard particles used as a filling. Accordingly, the amount of hard particles cannot be varied via the running wire feed. The hard particle content can thus not be locally changed. The use of an external powder injection could be one approach to vary the amount of hard material in the coating to locally strengthen functional layer areas (Fig. 1). Accordingly, the potential of using functionally graded coatings in the field of toolmaking for the sheet metal forming industry (e.g. deep drawing tool) was already emphasize within the scope of the Collaborative Research Centre SFB 708 (DFG; German Research Foundation). With regard to external hard particle injection in TWAS, initial approaches have already been described by Wilden et al. [28], who developed a modified arc spraying device in which powders are injected into the primary gas stream. Experiments using several material
2. Experimental 2.1. Approach In order to inject hard particles into the arc burning zone, a simple nozzle attachment was installed. Within this study, the Smart Arc 350 PPG arc spraying system (Fa. Oerlikon Metco, Switzerland) was utilized. A commercially available nozzle configuration with a cylindrical shape was used as an atomization nozzle inlet. The hard particles are radially injected into the arc burning zone next to the nozzle outlet. Fig. 2 visualizes the process of hard particle injection. The position and orientation of the injection needle can be adjusted depending on the injection parameters such as the powder feed rate and carrier gas flow. The experimental setup enables the required access and provides realtime recordings during the process by means of high-speed camera imaging, type Fastcam SA2 (Fa. Photron, Japan). Our objective is to ensure a sufficient wetting of injected hard particles at the molten wire tips, when the particles pass the realm of the molten pool of the steel wire tips. It has to be ensured that the radial powder injection needle does not become an obstacle within the axial gas flow. Moreover, since the structural components are mainly manufactured of electric 154
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Fig. 1. Approach of a functional graded microstructure by means of TWAS in the field of toolmaking for the sheet metal forming industry.
conductive materials, a flashover of the arcing from the electrode tip of the wires to the injection needle has to be avoided with a sufficient distance between the injector needle and wire tips.
High speed camera images enable the adjustment of the position settings (slope and distance) of the injection needle and the powder injection parameters. Based on the knowledge gained by these observations, both the carrier gas pressure (CGP) and the feeder disk velocity were kept at a constant level using 0.3 MPa and 60%, respectively. With respect to the setup, a powder feeder system of type Single 10C (Fa. Plasma-Technik, Switzerland) was utilized.
2.2. Substrate and feedstock material In this study, round 1.1191 (AISI-1045) steel specimens with dimensions of 40 mm × 6 mm were used as substrates. Prior to the spraying, the substrates were grit-blasted with corundum (EKF-14) and then cleaned for 10 min in an ultrasonic ethanol (C2H6O) bath. Two low-alloyed steel wires (Fe-0.68Si-0.43C-0.39Mn wt%, type Durmat AS811, Fa. Durum Verschleissschutz, Germany) served as feedstock. In terms of the external powder injection, cast tungsten carbides (CTC), which consist of a eutectic mixture of WC and W2C, (Fa. Durum Verschleissschutz, Germany) were used. Fig. 3 shows the morphology of the CTC used in this study. Particle size measurements by means of laser light scattering, using the particle analyzer S3500 (Fa. Microtrac, Pennsylvania), reveal a 50th percentile (dP50) of the sampling of 38.7 μm (dP10 = 22.3 μm, dP90 = 66.0 μm). The embedment of CTC across the deposit, produced by means of external injection, was compared to that obtained for the deposit that was manufactured using cored wires and CTC as a filling. Thus, an iron-based cored wire (outer sheath made of FeCMnSi) with 50 wt% of CTC as a filling (type Durmat AS-850, Fa. Durum Verschleissschutz, Germany) served as a reference. The CTC used for both experiments (within external particle injection; as filling in terms of cored wires) were taken from the same batch.
2.4. Analytic methods In order to investigate the produced coatings systems, the coated samples were metallographically prepared by using diamond grinding disks and polishing cloths with a diamond suspension (steps: 9 μm, 6 μm, 3 μm, and 1 μm). Cross-section images were taken by a field emission scanning electron microscope (FE-SEM) with secondary-electron and backscattered electron detectors type JSM-7001F (Fa. Jeol, Germany). Spot analyses by means of EDX and nanoindentation were conducted in order to determine the chemical composition and mechanical properties across the heterogeneous microstructure. Thus, mechanical properties such as the hardness (H) and Young's modulus (E) were analyzed for single phases such as eutectic carbides, reactive layers at the interface, and the surrounded matrix. Both values were determined by means of load and displacement sensing indentation, as demonstrated in a study by Oliver and Pharr [32], using the nanoindentation technique. The indents were conducted with the use of the nanoindentor type G200 (Fa. Agilent Technology, California) and a Berkovich indenter. Both values are evaluated on machined and polished (see above) cross-sections under penetration-control mode with a depth of 100 nm. A Poisson's ratio of ν = 0.3 was exerted for the evaluation. With respect to the spot analysis, the distribution of elements (C, Fe, O, and W) was evaluated by means of energy dispersive x-ray spectroscopy (EDX) with the commercially available INCA Software (Fa. Oxford Instruments, United Kingdom). The phase evolution in the produced coating systems was analyzed
2.3. Particle injection and coating deposition Spraying experiments were conducted using two different wire configurations (Section 2.2). The produced sample type using the solid wire (type AS-811) with an additional injection of CTC particles is referred to as SWC. Opposed to that, the sample type, which serves as reference (AS-850), is referred to as CWC. The different parameter settings used for the coating deposition are summarized in Table 1.
Fig. 2. Experimental setup with additional external powder injection.
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Fig. 3. Scanning electron microscope images and particle analyses showing the morphology (a) and particle size distribution (b) of the cast tungsten carbide used in this study.
particles reveal a polygonal shape. It can be clearly seen that they are heterogeneously distributed across the coating. As opposed to that, electron microscopy and quantitative EDX analyses reveal that the CWC sample is formed from a lamellar microstructure of Fe-rich and W-rich phases (Fig. 4b). The lamellar structure of W-rich hard phases (bright phases) asserts their completely molten state during spraying. In a few cases, large clusters of not fully fused W-rich hard phases are observed.
Table 1 Parameter settings used for coating deposition. Spray parameter settings (sample type: SWC, CWC) Spray angle [°] Gun velocity [mm/s] Track pitch [mm]
90 200 5
Spray distance [mm] Overruns
110 2 passes
Current [A] Voltage [V] Primary gas pressure [MPa] (Compressed air)
Powder injection parameters (sample type: SWC) Carrier gas pressure [MPa] 0.3 Feeder disk velocity [%] (nitrogen) Powder feed rate [g/min] 318 Wire feed rate [g/min]
180 30 0.6
3.1.1. Embedment of CTC in SWC sample (external injection of CTC particles) For the SWC sample, the magnified view of different polygonallyshaped eutectic carbides reveals that their embedment into the surrounded matrix is characterized by a reactive layer as shown by a change of the chemical composition (Fig. 5). The microstructure inside the W-rich hard phase consists of eutectic phases (WC and W2C). Grain boundaries were formed between the eutectic clusters. The interface between the eutectic carbides and the surrounded matrix is characterized by a planar transition zone. These characteristics were detected for most eutectic carbides (Fig. 5a–c). With respect to the transition zone, it was revealed that the share of W declines and the amount of Fe increases with a growing distance from the eutectic carbides (Fig. 5a, d). Fig. 6 shows the H and E calculated by nanoindentation measurements in regular intervals at a certain area (48 μm × 38 μm, 500 indents) across embedded carbides (see Fig. 5b) with the transition zone of the CTC to the surrounded Fe-rich matrix. For the bright phases, an increased H and E of more than 23 GPa and 600 GPa was found, respectively. The widespread Fe-W-rich solid solution, scattered around the eutectic carbides, provide a reduced H and E (see Fig. 6b and c). Even more reduced values for H and E are observed for Fe-rich phases (grey phase) as well as interstitial oxides (dark phase), which are distributed around the Fe-W-rich solid solution. The nanoindents were accompanied by an EDX spot analyses in order to correlate the findings for H and E with the prevailing element concentration (Fig. 7). The occurrence of eutectic carbides was evaluated by EDX spot analyses (indent no. 356 and 358) as shown in Fig. 7d. Despite some traces of Fe, the results obtained from the EDX spot analyses suggest the absence
60 120
by means of X-ray diffraction. The experiments were carried out at beamline BL9 of the synchrotron light source DELTA [33] (Dortmund, Germany). The incident photon energy was 27 keV (wavelength λ = 0.4592 Å). The angle of incidence was set to 5° and a beam-size of 0.2 × 1.0 mm2 (v × h) was utilized. A MAR345 image plate detector was used in order to measure the scattered intensity. The diffraction patterns were obtained from the MAR images by using a FIT2D program package [34]. Subsequently, the 2 theta scale of the diffraction patterns was converted to a wavelength of λ = 1.5406 Å according to Cu-Kα. 3. Results 3.1. Microstructural characteristics Fig. 4 shows the cross-section of the produced SWC and CWC samples. The deposits consist of a heterogeneous microstructure. The variation of the brightness suggests an inhomogeneity in the chemical composition. Electron microscopy and quantitative EDX analyses reveal that the SWC sample is formed from a lamellar microstructure of Ferich phases with interstitial oxides (dark grey) (Fig. 4a). W-rich hard particles (bright phases) are scattered across the coatings and are surrounded by the Fe-based matrix (grey phases). The W-rich hard
Fig. 4. Scanning electron microscope images showing the cross-section of a) sample SWC and b) CWC.
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Fig. 5. Cross-section images taken by SEM microscopy showing the embedment of CTC across sample SWC: a)–c) at different spots; d) magnified view of the SEM image in a) and EDX spot analysis.
3.1.2. Embedment of CTC in CWC sample In magnified cross-section views using electron microscopy, a different embedding of tungsten carbides to the surrounding Fe-based matrix is observed for deposits produced utilizing cored wires. With respect to the CWC sample, the embedment of tungsten carbides is characterized by a distinctive reactive layer around the carbides as visible by a change of the chemical composition (Fig. 8a). Fig. 8 shows the H and E measured in regular intervals within a certain area (40 μm × 40 μm, 400 indents) of embedded carbides with their transition zone to the surrounded Fe-rich matrix. For the bright phase, an increased H and E of more than 26 GPa and 650 GPa is found,
of Fe in the bright phase. Clusters of eutectic carbides can be seen using electron microscopy. Nanoindentation using load and displacement techniques reveals that the eutectic carbides have a value for E and H of 703–707 GPa, and 25.2–25.8 GPa (indent no. 356 and 358), respectively. Regarding the reactive layer around the eutectic carbides, EDX spot analyses reveal that the transition matrix is mainly composed of Fe-(Fe,W)xC eutectics (indent no. 165 and 236) as shown in Fig. 7b. Nanoindentation measurements of these spots show that the value for E and H ranges between 283 and 290 GPa, and 10.1–15.7 GPa, respectively. In contrast, EDX spot analyses in the area of the dark phase (idents no. 268) confirm the presence of a Fe-rich phase (Fig. 7c).
Fig. 6. Cross-section images of sample SWC taken by SEM microscopy showing the a) site of interest for nanoindentation using load and displacement sensing indentation, and the results taken from the measurement with respect to b) the hardness and c) the Young's modulus across embedded eutectic carbides and its transitional zone.
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Fig. 7. SEM images showing indents conducted by nanoindentation and EDX spot analysis at sample SWC: a) overview; b)–d) details shown in a magnified view.
oxides (dark phase) distributed around the Fe-W-rich ternary phases, a reduced H and E was observed. In comparison to sample SWC, the reactive layer (light bright phase) scattered around the bright phase is more pronounced for the CWC sample. This is evidenced by the fact
respectively. The widespread Fe-W-rich ternary phases (light bright phase), scattered around the bright phase, provide a hardness ranging from 15 to 23 GPa. In terms of the E, a value between 300 and 500 GPa was determined. For the Fe-rich phases (grey phase) and interstitial
Fig. 8. Cross-section images of sample CWC taken by SEM microscopy showing the a) site of interest for nanoindentation using load and displacement sensing indentation, and the results taken from the measurement with respect to b) the hardness and c) the Young's modulus across embedded eutectic carbides and its transitional zone.
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Fig. 9. SEM images showing indents conducted by nanoindentation and EDX spot analysis in sample CWC: a) overview; b)–f) details shown in a magnified view.
that electron microscopic investigations reveal a chemical inhomogeneity while nanoindentation expose significant differences in the mechanical response. Quantitative analyses of the heterogeneous microstructure by means of EDX show that the bright phase mainly consists of W and C (indent no. 174 and 207). It can be stated that the microstructure inside the W-rich hard phase consists of eutectic phases (WC and W2C). Grain boundaries were formed between the eutectic clusters. These phases are referred as indent no. 174 (Fig. 9b) and 207 (Fig. 9d), reveal a H of 25.7 and 26.2 GPa, as well as a E of 711 and 793 GPa. EDX spot analyses reveal that the surrounded matrix around the W-rich hard phase is mainly composed of eta carbides such as M6C and M12C phases (M = Fe, W). The load and displacement sensing indentation of several spots, referred to as indent No. 185, 211, and 276 (Fig. 9c, e, f) shows that the E and H is between 494 and 513 GPa, and 19.6–19.7 GPa, respectively. It is conspicuous that the interface of the eutectic carbides to the surrounded matrix (mainly M6C and M12C) is characterized by globular clusters. Those globular clusters are also heterogeneously distributed in the surrounding matrix. Depending on the location of the spot analysis and interaction of the primary electron beam with the surrounding area, it cannot be distinguished whether the interface or the globular clusters consist of WC, W2C, or WC1 − x. Fig. 10. XRD patterns observed for sample CWC and SWC. The 2-Theta scale is scaled to an energy of 8 keV (Cu-Kα).
3.2. Phase evolution processes XRD pattern reveals the presence of elementary W and the intermetallic Fe2W phase. In addition, it is found that the SWC sample contains different Fe-C-phases such as FeC, Fe3C, Fe1.86C0.14, and Fe15.1C. For the CWC sample, a similar composition is assumed. However, the XRD pattern reveals that the presence of eta carbides such as FeW3C, Fe2W2C, Fe3W3C, and Fe6W6C is more pronounced in the CWC sample as higher diffraction intensities are observed in these crystalline compounds. This is supported by the fact that the reflection at approximately 59° (FeW3C, Fe3W3C) can only be seen in the CWC sample. In addition to the great diversity of Fe-C phases as described above, the CWC sample further contains Fe5C2. However, due to the environmental conditions and inherent process characteristics, such as a rapid solidification of molten spray particles, it can be stated that constitutional supercooling takes place, which leads
Fig. 10 shows XRD patterns of the CWC and SWC samples. Based on the diffraction interferences, it is revealed that both coatings exhibit almost the same phase composition due to the use of comparable feedstocks. Thus, both coatings consist of Fe, WC, and W2C, which are also present in the feedstock used within this study. It turns out that in the case of the SWC sample, the intensity of the Fe reflection is greater than for the CWC sample, which supports the assumption that the SWC sample consists of a significantly higher amount of Fe when compared to the CWC sample. The additional reflections in the diagram, obtained from the SWC and CWC samples versus those obtained from the feedstock (data not shown), indicate that new phases were formed. With respect to the XRD pattern of the SWC sample, the coating is mainly composed of eta carbides such as FeW3C, Fe2W2C, Fe3W3C, and Fe6W6C. Moreover, the 159
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Table 2 Hardness and Young's modulus of relevant carbides and other phases.
Eta carbides Fe2W2C [40] Fe3W3C [38–40] Fe6W6C [38,40] Fe21W2C6 [40] Others Fe [46,47] W [45,47] Fe2W [47] Fe3C [39,40,43]
Young's modulus E [GPa]
Hardness [GPa]
275 446–470 447 438
7.6 15.6–16.8 14.73–15.6 15.0
Tungsten carbides h-WC [40–42] WC [43,44] h-W2C [42] o-W2C [42] t-W2C [42] W2C [43–45]
141 400–410 370–373 306–311
Young's modulus E [GPa]
Hardness [GPa]
691–697 710 480 437 427 420–444
16–22 23.5
29.4
h-WC: hexagonal WC, h-W2C: hexagonal W2C, o-W2C: orthorhombic W2C, t-W2C: tetragonal W2C.
carbides such as Fe3W3C and Fe6W6C using first-principle calculations. Their calculations showed that all eta carbides are thermodynamically stable. However, the authors concluded that the most thermodynamically stable compound among the investigated eta carbides is Fe6W6C. This eta carbide is detected in both samples, but the formation is more pronounced in the CWC sample. At the same time, it is reported that all examined eta carbides are less stable than tungsten mono-carbides which consists of hexagonal WC (h-WC). Moreover, the authors studied the formation energies of serval eta carbides for some possible reaction routes with participation of h-WC and tungsten semi-carbide W2C. It was found out that the formation of eta carbides in reactions with the participation of W2C is preferable. As opposed to that, reactions that included the participation of h-WC are unlikely due to the fact that h-WC is highly stable. Thus, the replacement of W atoms by Fe is energetically unfavorable. Liu et al. [40] employed first principles calculations in order to investigate the mechanic and electronic properties of Fe2W2C, Fe3W3C, Fe6W6C, and Fe21W2C6 compounds. According to this study, the cohesive energy and formation enthalpy were used to evaluate the stability of these carbides. The calculations implied that all compounds are thermodynamically stable, and the stability sequence of the examined phases from the Fe–W–C ternary system forms the following order: Fe21W2C6 > Fe6W6C > Fe3W3C > Fe2W2C. The aforementioned Fe3W3C and Fe2W2C eta carbides occurred in both samples, whereby their formation is more pronounced in the CWC sample. This can be due to a higher amount of WC and W2C in this sample, conditioned by the manufacturing process, and the embedment of CTC. Concerning the CWC sample, a higher amount of partially molten CTC is observed compared to the SWC sample. Accordingly, the implementation of CTC insertion, either as an external injection or as filling of the electrode tips, affects the metallurgical interactions. Mechanical properties of the eta carbides mentioned above as well as other relevant phases of the Fe-W-C-system can be found in literature [38–47]. Accordingly, the hardness and Young's modulus of several eta carbides and other possible phases in the deposits are listed below (Table 2). With respect to the findings observed by load and displacement sensing indentation (Figs. 7 and 9) in this study, it seems obvious that WC makes the largest share of the embedded W-rich hard phase (bright phase). The obtained mechanical properties (Fig. 7: ident no. 356 and 358, and Fig. 9: ident no. 174 or 207) are in good accordance with the values (E, and H) found in the literature (Table 2). With respect to the CWC sample, the measured values for H and E, taken from the surrounding matrix (ident no. 185, 211, and 276) correspond well with the measurements of eta carbides such as Fe3W3C and Fe6W6C as reported in other studies (Table 2). With respect to their shear and bulk modulus, Fe3W3C is more brittle than other eta carbides in the examined compounds [40]. Summarized, the CWC sample shows a larger transition area between CTC and the iron matrix, which is due to a greater dwell time next to the arc burning zone and stronger melting degree of the CTC. Thus, a change of the blocky shaped starting-off CTC to a more
to a wide varying non-equilibrium state. Thus, the phase transformation processes could not be completely analyzed. 4. Discussion As observed from the XRD patterns, both coatings contain Fe, WC, and W2C, which serve as feedstock components. Since the solid wire configuration provides a higher Fe content, the XRD pattern of the SWC sample exhibits a greater intensity of the Fe reflection than for the CWC sample. Besides the eutectic WC and W2C phases, traces of elementary W were identified. It can be stated that the occurrence of elementary W is caused by the dissolution of eutectic carbides. Moreover, only a small amount of WC was detected. Within a study on the microstructure of arc sprayed Fe-FeB-WC coatings, He et al. [4] reported that during spraying the C-deficient W2C phase is formed as a result of oxidation and decarburization processes of WC. Compared to the deposit sprayed with the use of the cored wire using eutectic tungsten carbides as filling (sample CWC), the layer sprayed with the use of the solid wire and external injection of eutectic tungsten carbides (sample SWC) is less to form ternary carbides, which can be assigned to FeW3C, Fe2W2C, Fe3W3C, and Fe6W6C. WC-Fe composites were already investigated by numerous researchers [35–37]. Their work show different melting interactions of WC particles with the surrounding molten matrix. Research concerning the production of WC-reinforced iron based alloys by means of laser melt interjection conducted by Nascimento et al. [35] showed that small WC and W2C isles formed around the WC particles. It is reported that the partial dissolution of WC particles forms new microstructural features containing Fe-(Fe,W)xC eutectics with different amounts of W. In another study, Nascimento et al. [36] examined the different types of reaction zones formed close to the interface between the steel matrix and WC particle in more detail. In this observation, it is recognized that the metallurgical interactions depend on the trajectories of injected particles and their interaction with the laser beam. The authors therefore concluded that the interface created in a reaction between the steel melt and WC particle surface is impaired by the WC particle temperature. According to Zhou et al. [37], the solubility of W in Fe is also influenced by the WC particle size. Thus, the solubility of W to the surrounding Fe-matrix increases with a decreasing WC particle size. The process kinetics of the abovementioned laser assisted manufacturing techniques differs to that of an arc spraying process. In particular, such kinetics play a significant role in interdiffusion processes. The formation of eta carbides as found for both sample types within this study can be also estimated taking thermodynamic aspects into account. Thus, several authors have reported about the ground state properties of Fe–W–C ternary compounds either by theoretical calculations or with empirical methods [38–40]. Suetin et al. [38] examined the stability for eta
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(2001) 176–177. [11] H.C. Starck, AMPERIT Thermal Spray Powder Brochure, Available online at: https://www.hcstarck.com/hcs-admin/file/ae23e4b2316f58850131b30f364743f9. de.0/akt_04_2017_web.pdf , Accessed date: 10 October 2017. [12] Commission Regulation (EC) No 790/2009: amending, for the purposes of its adaptation to technical and scientific progress, Regulation (EC) No 1272/2008 of the European Parliament and of the Council on classification, labelling and packaging of substances and mixtures, Off. J. Eur. Union L235/1 (5.9.2009) Available online at: http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri= OJ:L:2009:235:0001:0439:EN:PDF. [13] S. Zimmermann, B. Gries, J. Fischer, E. Lugscheider (Eds.), Thermal Spray 2008: Crossing Borders: Proceedings of the International Thermal Spray Conference 2008, DVS Deutscher Verband für Schweißen, Düsseldorf, 2008. [14] U.S. Department of Health and Human Services, Cobalt–Tungsten Carbide: Powders and Hard Metals, 12th Report on Carcinogens, Available online at: http://ntp.niehs. nih.gov/?objectid=03C9AF75-E1BF-FF40-DBA9EC0928DF8B15. [15] A. Scrivani, N. Antolotti, S. Bertini, R. Groppetti, T. Gutema, A. Mangia, S. Mucchino, G. Viola, C. Coddet (Eds.), Thermal Spray: Meeting the Challenges of the 21st Century, ASM International, 1998, p. 1019 Ed.. [16] J.C. Farmer, et al., Corrosion resistance of thermally sprayed high-boron iron-based amorphous-metal coatings: Fe49. 7Cr17. 7Mn1. 9Mo7. 4W1. 6B15. 2C3. 8Si2. 4, J. Mater. Res. 22 (2007) 2297–2311. [17] W.H. Liu, F.S. Shieu, W.T. Hsiao, Enhancement of wear and corrosion resistance of iron-based hard coatings deposited by high-velocity oxygen fuel (HVOF) thermal spraying, Surf. Coat. Technol. 249 (2014) 24–41. [18] V. Lovelock, Powder/processing/structure relationships in WC-Co thermal spray coatings: a review of the published literature, J. Therm. Spray Technol. 7 (1998) 357–373. [19] I.-T. Baumann, Hochverschleißfeste und konturnahe Werkzeugoberflächen durch Hochgeschwindigkeitsflammspritzverfahren, Dissertation TU Dortmund, 2012. [20] A.S. Kurlov, A.I. Gusev, Phase equilibria in the W–C system and tungsten carbides, Russ. Chem. Rev. 75 (2006) 617–636. [21] Y. Li, et al., First-principles study on the stability and mechanical property of eta M3W3C (M = Fe, Co, Ni) compounds, Physica B 405 (2010) 1011–1017. [22] Y. Liu, Y. Jiang, R. Zhou, J. Feng, First-principles calculations of the mechanical and electronic properties of Fe–W–C ternary compounds, Comput. Mater. Sci. 82 (2014) 26–32. [23] S. Kaierle, et al., Review on laser deposition welding: from micro to macro, Phys. Procedia 39 (2012) 336–345. [24] S.W. Huanga, et al., Abrasive wear performance and microstructure of laser clad WC/Ni layers, Wear 256 (2004) 1095–1105. [25] M. Jones, et al., The influence of carbide dissolution on the erosion-corrosion properties of cast tungsten carbide/Ni-based PTAW overlays, Wear 271 (2011) 1314–1324. [26] Z. Li, Y. Jiang, R. Zhou, D. Lu, R. Zhou, Dry three-body abrasive wear behavior of WC reinforced iron matrix surface composites produced by V-EPC infiltration casting process, Wear 262 (2007) 649–654. [27] R. Zhou, Y. Jiang, D. Lu, The effect of volume fraction of WC particles on erosion resistance of WC reinforced iron matrix surface composites, Wear 255 (2004) 134–138. [28] J. Wilden, J. Bergmann, S. Jahn, A new method for the production of particle reinforced coatings, Thermal Spray 2007: Global Coating Solutions, 2007, pp. 359–364. [29] G. Barbezat, C. Warnecke, United States Patent, No. US 7,019,249 B2, 2006. [30] G. Paczkowski, et al., Der neue alte Weg-Hochgeschwindigkeitslichtbogenspritzen eröffnet neue Perspektiven & Prozessdiagnostik beim offenen Drahtlichtbogenspritzen, Tagungsunterlagen 9. HVOF-Kolloquium, 2012, pp. 139–150. [31] A.N. Dubovoj, S.A. Prokudin, A.A. Karpetschenko, Therm. Spray Bull. 6 (2) (2013) 102–108. [32] W.C. Oliver, G.M. Pharr, An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments, J. Mater. Res. 7 (1992) 1564–1583. [33] C. Krywka, C. Sternemann, M. Paulus, N. Javid, R. Winter, A. Al-Sawalmih, S. Yi, D. Raabe, M. Tolan, The small-angle and wide-angle x-ray scattering setup at beamline BL9 of DELTA, J. Synchrotron Radiat. 14 (2007) 244–251. [34] A. Hammersley, S. Svensson, M. Hanfland, A. Fitch, D. Hausermann, Two-dimensional detector software: from real detector to idealised image or two-theta scan, Int. J. High Pressure Res. 144 (1996) 6235–6248. [35] A. Do Nascimento, V. Ocelik, M. Ierardi, J. De Hosson, Wear resistance of WC p/ Duplex Stainless Steel metal matrix composite layers prepared by laser melt injection, Surf. Coat. Technol. 202 (2008) 4758–4765. [36] A. Do Nascimento, V. Ocelik, M. Ierardi, J. De Hosson, Microstructure of reaction zone in WC p/duplex stainless steels matrix composites processing by laser melt injection, Surf. Coat. Technol. 202 (2008) 2113–2120. [37] S. Zhou, X. Dai, Microstructure evolution of Fe-based WC composite coating prepared by laser induction hybrid rapid cladding, Appl. Surf. Sci. 256 (2010) 7395–7399. [38] D. Suetin, I. Shein, A. Ivanovskii, Structural, electronic and magnetic properties of eta carbides (FE3W3C, FE6W6C, Co3W3C and Co6W6C) from first principles calculations, Physica B 404 (2009) 3544–3549.
roundly shape in the CWC sample is observed. In addition, the molten or partially molten state promotes the formation of new phases with the iron based matrix. The phase transformation processes (i.e. formation of eta carbides) in turn can lead to a degradation of the mechanical properties such as hardness. 5. Conclusion Within the scope of this study, an external injection of eutectic tungsten carbides was applied in a TWAS process in order to analyze the embedment of eutectic tungsten carbides in arc sprayed low alloyed steel. The embedment of eutectic tungsten carbides was compared with that obtained for deposits, manufactured by using cored wires and eutectic tungsten carbides as fillings. Both sample types are formed from a lamellar microstructure. W-rich hard phases scatter throughout the coating and are surrounded by a Fe-based matrix. Concerning the embedded W-rich hard phases which were inserted by means of external injection, electron microscopy and EDX analyses revealed a slight dissolution of eutectic carbides. Moreover, the interface of the eutectic carbide to the surrounded matrix is characterized by a planar transition zone. With respect to the embedded W-rich hard phases, which were inserted as filler material by using a cored wire, electron microscopy and EDX analyses showed a significant dissolution of eutectic tungsten carbides as a reactive layer was generated. Globular clusters that can be assigned to WC, W2C, or WC1-x precipitates were formed, typically at the interface between the polygonal shaped eutectic carbide and surrounding matrix, which comprise of composite carbides. XRD analyses confirmed that both sample types are mainly composed of WC, W2C as well as of eta carbides (M4C, M6C, and M12C), elementary W, and some other Fe-rich carbides. However, the formation of M4C, M6C, and M12C phases such as FeW3C, Fe2W2C, Fe3W3C, and Fe6W6C was less pronounced when inserting eutectic tungsten carbides via external injection during spraying. Acknowledgement The authors gratefully acknowledge the financial support of the DFG (German Research Foundation) within the Collaborative Research Centre SFB 708 subproject A1. The contributions of DURUM Verschleissschutz GmbH are gratefully acknowledged for supplying the feedstock material. The authors thank the DELTA machine group for providing the synchrotron radiation. References [1] A.P. Newbery, P.S. Grant, R.A. Neiser, The velocity and temperature of steel droplets during electric arc spraying, Surf. Coat. Technol. 195 (2005) 91–101. [2] I. Gedzevicius, A.V. Valiulis, Analysis of wire arc spraying process variables on coatings properties, J. Mater. Process. Technol. 175 (2006) 206–211. [3] P. Sheppard, H. Koiprasert, Effect of W dissolution in NiCrBSi–WC and NiBSi–WC arc sprayed coatings on wear behaviors, Wear 317 (2014) 194–200. [4] D.J. He, B.J. Fu, J.M. Jiang, X.J. Li, Microstructure and wear performance of arc sprayed Fe-FeB-WC coatings, J. Therm. Spray Technol. 17 (2008) 757–761. [5] W. Tillmann, B. Klusemann, J. Nebel, B. Svendsen, Analysis of the mechanical properties of an arc-sprayed WC-FeCSiMn coating: nanoindentation and simulation, J. Therm. Spray Technol. 20 (2011) 328–335. [6] W. Tillmann, W. Luo, J. Nebel, der Einfluss, Hartstoffkorngröße auf die tribologischen Eigenschaften gewalzter und geschliffener WSC-FeCSiMn-Schichten, Therm. Spray Bull. 4 (2011) 56–63. [7] W. Tillmann, W. Luo, U. Selvadurai, Wear analysis of thermal spray coatings on 3D surfaces, J. Therm. Spray Technol. 23 (2014) 245–251. [8] B. Xu, Z. Zhu, S. Ma, W. Zhang, W. Liu, Sliding wear behavior of Fe–Al and Fe–Al/ WC coatings prepared by high velocity arc spraying, Wear 257 (2004) 1089–1095. [9] P. Niranatlumpong, H. Koiprasert, Phase transformation of NiCrBSi–WC and NiBSi–WC arc sprayed coatings, Surf. Coat. Technol. 206 (2011) 440–445. [10] D.G. Atteridge, R. Davis, M. Scholl, G. Tewksbury, J. Therm. Spray Technol. 10
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W. Tillmann et al.
[44] M. Yandouzi, L. Ajdelsztajn, B. Jodoin, WC-based composite coatings prepared by the pulsed gas dynamic spraying process: effect of the feedstock powders, Surf. Coat. Technol. 202 (2008) 3866–3877. [45] H. Taimatsu, S. Sugiyama, Y. Kodaira, Synthesis of W2C by reactive hot pressing and its mechanical properties, Mater. Trans. 49 (2008) 1256–1261. [46] H. Berns, Stahl und Gusseisen, Springer Verlag, Berlin Heidelberg, 2013 (in german). [47] Q.Q. Ren, J.L. Fan, Y. Han, H.R. Gong, Structural, thermodynamic, mechanical, and magnetic properties of FeW system, J. Appl. Phys. 116 (2014) (093909-1 093909-9).
[39] Y. Li, et al., First-principles study on the stability and mechanical property of eta M3W3C (M = Fe, Co, Ni) compounds, Physica B 405 (2010) 1011–1017. [40] Y. Liu, Y. Jiang, R. Zhou, J. Feng, First-principles calculations of the mechanical and electronic properties of Fe–W–C ternary compounds, Comput. Mater. Sci. 82 (2014) 26–32. [41] T. Dash, B. Nayak, Preparation of WC–W2C composites by arc plasma melting and their characterisations, Ceram. Int. 39 (2013) 3279–3292. [42] Y. Liu, Y. Jiang, J. Feng, Mechanical properties and chemical bonding characteristics of WC and W2C compounds, Ceram. Int. 40 (2014) 2891–2899. [43] T. Gerthsen, Chemie für den Maschinenbau, Universitätsverlag Karlsruhe, Karlsruhe, 2006 (in german).
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Embedment of eutectic tungsten carbides in arc sprayed steel coatings
MARK
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Wolfgang Tillmann, Leif Hagen , David Kokalj Institute of Materials Engineering, TU Dortmund University, Leonhard-Euler-Straße 2, 44227 Dortmund, Germany
A R T I C L E I N F O
A B S T R A C T
Keywords: Arc spraying Tungsten carbide External powder injection Steel coatings
Tungsten carbide reinforced deposits have already evolved into a predominant coating system in order to protect stressed surfaces against wear. Among thermal spraying processes, due to a high deposition rate, arc spraying is a promising process to manufacture cost-saving, wear resistant coatings. However, inherent process characteristics prevailing in arc spraying as well as the utilization of tungsten carbides, as a filling for cored wires, could lead to undesirable phase evolutions, which in turn provoke the degradation of the mechanical properties. The embedment of tungsten carbides into the surrounding metallic matrix is affected by metallurgical interactions with molten spray particles. Within the scope of this study, an external injection of tungsten carbides was applied in order to analyze the embedment of tungsten carbides in arc sprayed low alloyed steel. Accordingly, metallographic investigations were carried out, which address the reactive layer at the interface of embedded tungsten carbides to the surrounded iron-based matrix. Microstructural characteristics such as mechanical properties and phase composition were scrutinized by means of nanoindentation, energy dispersive X-ray spectroscopy, and X-ray diffraction, respectively. It was found that the embedment of tungsten carbides, which have been externally injected into the arc burning zone, differs from that obtained from deposits produced with the use of cored wire with tungsten carbide as filling. Thus, externally injected tungsten carbides are less inclined to form eta carbides due to dissolution, which again results in differences in the mechanical properties across the reactive layer.
1. Introduction The twin wire arc spraying (TWAS) process is a time saving [1] and energy efficient [2] technique, mainly used in surface refurbishments and maintenance applications [3]. Nevertheless, TWAS has the ability to deposit wear and corrosion resistant coatings in a simple and cost saving way due to the use of compressed air as atomization gas, and hard particle reinforced cored wires as feedstock, respectively. Within the field of wear protection, arc sprayed coatings, produced by utilizing cored wires with hard particle as a filling, are widely used to enhance the tribological and mechanical characteristics of surfaces. Due to their outstanding properties, tungsten carbide reinforced arc sprayed coatings are appropriate to protect stressed surfaces against wear. In terms of arc sprayed coatings, only few studies have discussed the use of tungsten carbide reinforced feedstock materials such as iron-based (FeCSiMn-WC/W2C, Fe3Al-WC, Fe-FeB-WC) [4–8] as well as nickel based alloys ((NiCrBSi, NiBSi)-WC/W2C, Ni-(WC-Co)) [3,9,10]. According to studies on tungsten carbide reinforced Fe-based arc sprayed coatings, a major objective was the focus on the tribological behavior [4,6–8]. It was found that a fine lamellar coating structure, provided by utilizing fine-grain hard material fillings, can lead to enhanced tribological coating characteristics [6].
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With respects to the feedstock, Ni-based and Co-based coatings cause some health and safety issues. Ni-based alloys are allergenic, labelled as suspect carcinogenic agents [11], and classed as hazardous powder materials (hazard statement according to the European Commission regulation EC 790/2009) [12]. Regarding the WC–Co feedstock, it is reported that the materials are also toxic when inhaled [13]. Moreover, the feedstock is listed in the “Report on Carcinogens” [14]. In terms of the compatibility of thermal spray coatings in the food production technology (according to EU and PDA standards), coatings containing Ni- and Co- might fail to meet the specific requirements within this field such as strict limitations concerning the contamination of the products with harmful or even toxic elements [15]. Regarding thermally sprayed coatings, Fe-based alloys have proven to be less hazardous when compared to Ni- and Co-based alloys. However, Febased alloys are known to be less corrosion resistant compared to Niand Co-based alloys. Over the last decades, it was already demonstrated that Fe-based amorphous coatings provide a corrosion resistance comparable to that of high-performance alloys such as Ni-based alloys [16]. As a result, some Fe-based materials already replace more expensive Co- or Ni-based alloys in various industry applications where the components are subjected to corrosion and wear [17]. It can be stated
Corresponding author. E-mail addresses: [email protected] (W. Tillmann), [email protected] (L. Hagen), [email protected] (D. Kokalj).
http://dx.doi.org/10.1016/j.surfcoat.2017.10.044 Received 24 June 2017; Received in revised form 13 October 2017; Accepted 14 October 2017 Available online 16 October 2017 0257-8972/ © 2017 Elsevier B.V. All rights reserved.
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combinations demonstrated the possibility to embed single particles. However, an insufficient wetting of particles was observed. As a result, an adequate adhesion could not be established for the insertion of steel particles in a Zn-based and Al-based matrix. Furthermore, the investigation showed that the injection of particles into the axially incoming gas stream has a significant influence on the atomization of the molten electrode tips. Accordingly, the gas flow was disturbed, resulting in a more heterogeneous microstructure and topographical irregularities. Another approach of external powder injection is executed by means of an injection just behind the arc burning zone. Barbezat and Warnecke [29] have a patent on this approach. With the use of a modified arc spraying device and spray torch, a powdered additive material is fed axially into the arc burning zone via the primary gas stream or through the additional injector behind the arc burning zone. A closer examination of this modification or the generated layer systems cannot be found in the literature. Paczkowski et al. [30] analyzed the applicability of producing particle-reinforced layer systems by means of an open wire arc spraying system. Accordingly, different MMCs were produced using a Cu-based matrix. Metallographic investigations revealed a homogeneous distribution and a form-fitting embedding of the hard particle phase in the surrounding Cu-based matrix. Metallurgical interactions across the interface of hard particles and the metallic matrix were not investigated. Nevertheless, it was found that the particle loading of the atomizing gas stream stabilized the plasma at the arc burning zone. Dubovoj et al. [31] further examined the production of composite materials by means of external powder injection into the high-temperature region of the arc burning zone. Various metal-polymer, metal-ceramic, and metal-glass composites were investigated with respect to their mechanical and physical properties. According to this study, a major objective was the wear behavior, adhesion, and thermal conductivity of the coatings. Metallurgical investigations on the embedment of hard particles into the surrounding metallic matrix were not taken into account. In contrast to the aforementioned examinations, in this study the manufacturing route of an external injection of tungsten carbides and their embedding into the metallic matrix across the deposits are investigated. For this purpose, the embedment of tungsten carbides in coatings produced by conventional cored wires is compared with that manufactured with exclusively externally injected tungsten carbides. Therefore, the transition zones between the tungsten carbides and surrounded iron-based matrix are fundamentally investigated by means of scanning electron microscopy and nanoindentation. By using X-ray diffraction, the occurred phases can be traced back to the manufacturing process and the embedment of the tungsten carbides.
that tungsten carbide reinforced Fe-based Metal Matrix Composites (MMCs) represent an environmentally friendly alternative. By means of arc spraying, tungsten carbide reinforced coatings are manufactured exclusively by using of cored wires. These cored wires mainly consist of a metallic (electrically conductive) outer sheath and cast tungsten carbides as a filling. With respect to cast tungsten carbides (eutectic mixture of WC-W2C), W2C has the largest share with 73–80 wt %. The stoichiometric WC has a carbon content of 6.13 wt%, whereas W2C has a lower carbon content of 3.61 wt% [18]. So far, the phase evolution of arc sprayed Fe-based WC-W2C reinforced coating systems has not yet been examined. A few studies investigated the microstructure formation and phase transformation of arc sprayed Ni-based WC-W2C reinforced coating systems [3,9]. According to these studies, eutectic tungsten carbides decarburized during spraying and a large amount of W dissolved into the Ni-rich matrix. Moreover, Ni also dissolved into WC-W2C and gathered around WCW2C grains to form fine WC-W2C–Ni composites. In [9] it was stated that these metallurgical interactions are attributed to the electrode phenomena during atomization, when the WC-W2C grains are wetted by the molten metal. It was mentioned that these metallurgical interactions can lead to a degradation of the mechanical properties. The stoichiometric WC decomposes above 3058 K to L + C (L = liquid melt) in an inert gas atmosphere or vacuum, respectively [19]. Exposed to oxygen, the decomposition starts at significantly lower temperatures of approximately 823 K. With W2C, congruent melting and the formation of liquid phases occur above a liquidus temperature of 3058 K. Two eutectic phases form during the cooling of the melt. A peritectic decomposition of W2C to W + WC occurs at a temperature of 1523 K. Eta carbides (e.g. M3W3C, M6W6C etc.) are mainly formed according to Kurlov et al. [20] with heterogeneous microstructures, especially in the interface of WC and the transitional metal. The stoichiometries as well as the matrix material influence the phase stability [21,22]. Besides the chemical composition of the metal matrix, the degree of dissolution of the tungsten carbide depends on type, shape, and size of the carbides. For instance, irregularly shaped particles are more prone to dissolve due to their tendency to heat up to higher temperatures by the exposure of a heat source [23]. Small sized particles tend to dissolve faster than coarser particles. Moreover, as reported by other researchers [24,25] eutectic tungsten carbides are more susceptible to react with the liquid melt than more thermally stable carbides such as mono crystalline carbides consisting of hexagonal WC. For tribologically stressed surfaces, it was found that the wear resistance of WC-composites depends, inter alia, on the embedment of hard phases [26,27]. Accordingly, the process control requires a good embedding and adhesion of hard particles in the layer composite, so that the contacting surface is not damaged due to breakouts of weakly embedded hard phases. In that respect, an insufficient adhesion of hard phases can lead to a damage of the surface integrity, the counter body or layer composite itself. Regarding the manufacturing route of arc sprayed coatings using cored wires, there is no possibility to deposit coatings with graded carbide content. The local reinforcement due to hard particles is determined by the amount of hard particles used as a filling. Accordingly, the amount of hard particles cannot be varied via the running wire feed. The hard particle content can thus not be locally changed. The use of an external powder injection could be one approach to vary the amount of hard material in the coating to locally strengthen functional layer areas (Fig. 1). Accordingly, the potential of using functionally graded coatings in the field of toolmaking for the sheet metal forming industry (e.g. deep drawing tool) was already emphasize within the scope of the Collaborative Research Centre SFB 708 (DFG; German Research Foundation). With regard to external hard particle injection in TWAS, initial approaches have already been described by Wilden et al. [28], who developed a modified arc spraying device in which powders are injected into the primary gas stream. Experiments using several material
2. Experimental 2.1. Approach In order to inject hard particles into the arc burning zone, a simple nozzle attachment was installed. Within this study, the Smart Arc 350 PPG arc spraying system (Fa. Oerlikon Metco, Switzerland) was utilized. A commercially available nozzle configuration with a cylindrical shape was used as an atomization nozzle inlet. The hard particles are radially injected into the arc burning zone next to the nozzle outlet. Fig. 2 visualizes the process of hard particle injection. The position and orientation of the injection needle can be adjusted depending on the injection parameters such as the powder feed rate and carrier gas flow. The experimental setup enables the required access and provides realtime recordings during the process by means of high-speed camera imaging, type Fastcam SA2 (Fa. Photron, Japan). Our objective is to ensure a sufficient wetting of injected hard particles at the molten wire tips, when the particles pass the realm of the molten pool of the steel wire tips. It has to be ensured that the radial powder injection needle does not become an obstacle within the axial gas flow. Moreover, since the structural components are mainly manufactured of electric 154
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Fig. 1. Approach of a functional graded microstructure by means of TWAS in the field of toolmaking for the sheet metal forming industry.
conductive materials, a flashover of the arcing from the electrode tip of the wires to the injection needle has to be avoided with a sufficient distance between the injector needle and wire tips.
High speed camera images enable the adjustment of the position settings (slope and distance) of the injection needle and the powder injection parameters. Based on the knowledge gained by these observations, both the carrier gas pressure (CGP) and the feeder disk velocity were kept at a constant level using 0.3 MPa and 60%, respectively. With respect to the setup, a powder feeder system of type Single 10C (Fa. Plasma-Technik, Switzerland) was utilized.
2.2. Substrate and feedstock material In this study, round 1.1191 (AISI-1045) steel specimens with dimensions of 40 mm × 6 mm were used as substrates. Prior to the spraying, the substrates were grit-blasted with corundum (EKF-14) and then cleaned for 10 min in an ultrasonic ethanol (C2H6O) bath. Two low-alloyed steel wires (Fe-0.68Si-0.43C-0.39Mn wt%, type Durmat AS811, Fa. Durum Verschleissschutz, Germany) served as feedstock. In terms of the external powder injection, cast tungsten carbides (CTC), which consist of a eutectic mixture of WC and W2C, (Fa. Durum Verschleissschutz, Germany) were used. Fig. 3 shows the morphology of the CTC used in this study. Particle size measurements by means of laser light scattering, using the particle analyzer S3500 (Fa. Microtrac, Pennsylvania), reveal a 50th percentile (dP50) of the sampling of 38.7 μm (dP10 = 22.3 μm, dP90 = 66.0 μm). The embedment of CTC across the deposit, produced by means of external injection, was compared to that obtained for the deposit that was manufactured using cored wires and CTC as a filling. Thus, an iron-based cored wire (outer sheath made of FeCMnSi) with 50 wt% of CTC as a filling (type Durmat AS-850, Fa. Durum Verschleissschutz, Germany) served as a reference. The CTC used for both experiments (within external particle injection; as filling in terms of cored wires) were taken from the same batch.
2.4. Analytic methods In order to investigate the produced coatings systems, the coated samples were metallographically prepared by using diamond grinding disks and polishing cloths with a diamond suspension (steps: 9 μm, 6 μm, 3 μm, and 1 μm). Cross-section images were taken by a field emission scanning electron microscope (FE-SEM) with secondary-electron and backscattered electron detectors type JSM-7001F (Fa. Jeol, Germany). Spot analyses by means of EDX and nanoindentation were conducted in order to determine the chemical composition and mechanical properties across the heterogeneous microstructure. Thus, mechanical properties such as the hardness (H) and Young's modulus (E) were analyzed for single phases such as eutectic carbides, reactive layers at the interface, and the surrounded matrix. Both values were determined by means of load and displacement sensing indentation, as demonstrated in a study by Oliver and Pharr [32], using the nanoindentation technique. The indents were conducted with the use of the nanoindentor type G200 (Fa. Agilent Technology, California) and a Berkovich indenter. Both values are evaluated on machined and polished (see above) cross-sections under penetration-control mode with a depth of 100 nm. A Poisson's ratio of ν = 0.3 was exerted for the evaluation. With respect to the spot analysis, the distribution of elements (C, Fe, O, and W) was evaluated by means of energy dispersive x-ray spectroscopy (EDX) with the commercially available INCA Software (Fa. Oxford Instruments, United Kingdom). The phase evolution in the produced coating systems was analyzed
2.3. Particle injection and coating deposition Spraying experiments were conducted using two different wire configurations (Section 2.2). The produced sample type using the solid wire (type AS-811) with an additional injection of CTC particles is referred to as SWC. Opposed to that, the sample type, which serves as reference (AS-850), is referred to as CWC. The different parameter settings used for the coating deposition are summarized in Table 1.
Fig. 2. Experimental setup with additional external powder injection.
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Fig. 3. Scanning electron microscope images and particle analyses showing the morphology (a) and particle size distribution (b) of the cast tungsten carbide used in this study.
particles reveal a polygonal shape. It can be clearly seen that they are heterogeneously distributed across the coating. As opposed to that, electron microscopy and quantitative EDX analyses reveal that the CWC sample is formed from a lamellar microstructure of Fe-rich and W-rich phases (Fig. 4b). The lamellar structure of W-rich hard phases (bright phases) asserts their completely molten state during spraying. In a few cases, large clusters of not fully fused W-rich hard phases are observed.
Table 1 Parameter settings used for coating deposition. Spray parameter settings (sample type: SWC, CWC) Spray angle [°] Gun velocity [mm/s] Track pitch [mm]
90 200 5
Spray distance [mm] Overruns
110 2 passes
Current [A] Voltage [V] Primary gas pressure [MPa] (Compressed air)
Powder injection parameters (sample type: SWC) Carrier gas pressure [MPa] 0.3 Feeder disk velocity [%] (nitrogen) Powder feed rate [g/min] 318 Wire feed rate [g/min]
180 30 0.6
3.1.1. Embedment of CTC in SWC sample (external injection of CTC particles) For the SWC sample, the magnified view of different polygonallyshaped eutectic carbides reveals that their embedment into the surrounded matrix is characterized by a reactive layer as shown by a change of the chemical composition (Fig. 5). The microstructure inside the W-rich hard phase consists of eutectic phases (WC and W2C). Grain boundaries were formed between the eutectic clusters. The interface between the eutectic carbides and the surrounded matrix is characterized by a planar transition zone. These characteristics were detected for most eutectic carbides (Fig. 5a–c). With respect to the transition zone, it was revealed that the share of W declines and the amount of Fe increases with a growing distance from the eutectic carbides (Fig. 5a, d). Fig. 6 shows the H and E calculated by nanoindentation measurements in regular intervals at a certain area (48 μm × 38 μm, 500 indents) across embedded carbides (see Fig. 5b) with the transition zone of the CTC to the surrounded Fe-rich matrix. For the bright phases, an increased H and E of more than 23 GPa and 600 GPa was found, respectively. The widespread Fe-W-rich solid solution, scattered around the eutectic carbides, provide a reduced H and E (see Fig. 6b and c). Even more reduced values for H and E are observed for Fe-rich phases (grey phase) as well as interstitial oxides (dark phase), which are distributed around the Fe-W-rich solid solution. The nanoindents were accompanied by an EDX spot analyses in order to correlate the findings for H and E with the prevailing element concentration (Fig. 7). The occurrence of eutectic carbides was evaluated by EDX spot analyses (indent no. 356 and 358) as shown in Fig. 7d. Despite some traces of Fe, the results obtained from the EDX spot analyses suggest the absence
60 120
by means of X-ray diffraction. The experiments were carried out at beamline BL9 of the synchrotron light source DELTA [33] (Dortmund, Germany). The incident photon energy was 27 keV (wavelength λ = 0.4592 Å). The angle of incidence was set to 5° and a beam-size of 0.2 × 1.0 mm2 (v × h) was utilized. A MAR345 image plate detector was used in order to measure the scattered intensity. The diffraction patterns were obtained from the MAR images by using a FIT2D program package [34]. Subsequently, the 2 theta scale of the diffraction patterns was converted to a wavelength of λ = 1.5406 Å according to Cu-Kα. 3. Results 3.1. Microstructural characteristics Fig. 4 shows the cross-section of the produced SWC and CWC samples. The deposits consist of a heterogeneous microstructure. The variation of the brightness suggests an inhomogeneity in the chemical composition. Electron microscopy and quantitative EDX analyses reveal that the SWC sample is formed from a lamellar microstructure of Ferich phases with interstitial oxides (dark grey) (Fig. 4a). W-rich hard particles (bright phases) are scattered across the coatings and are surrounded by the Fe-based matrix (grey phases). The W-rich hard
Fig. 4. Scanning electron microscope images showing the cross-section of a) sample SWC and b) CWC.
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Fig. 5. Cross-section images taken by SEM microscopy showing the embedment of CTC across sample SWC: a)–c) at different spots; d) magnified view of the SEM image in a) and EDX spot analysis.
3.1.2. Embedment of CTC in CWC sample In magnified cross-section views using electron microscopy, a different embedding of tungsten carbides to the surrounding Fe-based matrix is observed for deposits produced utilizing cored wires. With respect to the CWC sample, the embedment of tungsten carbides is characterized by a distinctive reactive layer around the carbides as visible by a change of the chemical composition (Fig. 8a). Fig. 8 shows the H and E measured in regular intervals within a certain area (40 μm × 40 μm, 400 indents) of embedded carbides with their transition zone to the surrounded Fe-rich matrix. For the bright phase, an increased H and E of more than 26 GPa and 650 GPa is found,
of Fe in the bright phase. Clusters of eutectic carbides can be seen using electron microscopy. Nanoindentation using load and displacement techniques reveals that the eutectic carbides have a value for E and H of 703–707 GPa, and 25.2–25.8 GPa (indent no. 356 and 358), respectively. Regarding the reactive layer around the eutectic carbides, EDX spot analyses reveal that the transition matrix is mainly composed of Fe-(Fe,W)xC eutectics (indent no. 165 and 236) as shown in Fig. 7b. Nanoindentation measurements of these spots show that the value for E and H ranges between 283 and 290 GPa, and 10.1–15.7 GPa, respectively. In contrast, EDX spot analyses in the area of the dark phase (idents no. 268) confirm the presence of a Fe-rich phase (Fig. 7c).
Fig. 6. Cross-section images of sample SWC taken by SEM microscopy showing the a) site of interest for nanoindentation using load and displacement sensing indentation, and the results taken from the measurement with respect to b) the hardness and c) the Young's modulus across embedded eutectic carbides and its transitional zone.
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Fig. 7. SEM images showing indents conducted by nanoindentation and EDX spot analysis at sample SWC: a) overview; b)–d) details shown in a magnified view.
oxides (dark phase) distributed around the Fe-W-rich ternary phases, a reduced H and E was observed. In comparison to sample SWC, the reactive layer (light bright phase) scattered around the bright phase is more pronounced for the CWC sample. This is evidenced by the fact
respectively. The widespread Fe-W-rich ternary phases (light bright phase), scattered around the bright phase, provide a hardness ranging from 15 to 23 GPa. In terms of the E, a value between 300 and 500 GPa was determined. For the Fe-rich phases (grey phase) and interstitial
Fig. 8. Cross-section images of sample CWC taken by SEM microscopy showing the a) site of interest for nanoindentation using load and displacement sensing indentation, and the results taken from the measurement with respect to b) the hardness and c) the Young's modulus across embedded eutectic carbides and its transitional zone.
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Fig. 9. SEM images showing indents conducted by nanoindentation and EDX spot analysis in sample CWC: a) overview; b)–f) details shown in a magnified view.
that electron microscopic investigations reveal a chemical inhomogeneity while nanoindentation expose significant differences in the mechanical response. Quantitative analyses of the heterogeneous microstructure by means of EDX show that the bright phase mainly consists of W and C (indent no. 174 and 207). It can be stated that the microstructure inside the W-rich hard phase consists of eutectic phases (WC and W2C). Grain boundaries were formed between the eutectic clusters. These phases are referred as indent no. 174 (Fig. 9b) and 207 (Fig. 9d), reveal a H of 25.7 and 26.2 GPa, as well as a E of 711 and 793 GPa. EDX spot analyses reveal that the surrounded matrix around the W-rich hard phase is mainly composed of eta carbides such as M6C and M12C phases (M = Fe, W). The load and displacement sensing indentation of several spots, referred to as indent No. 185, 211, and 276 (Fig. 9c, e, f) shows that the E and H is between 494 and 513 GPa, and 19.6–19.7 GPa, respectively. It is conspicuous that the interface of the eutectic carbides to the surrounded matrix (mainly M6C and M12C) is characterized by globular clusters. Those globular clusters are also heterogeneously distributed in the surrounding matrix. Depending on the location of the spot analysis and interaction of the primary electron beam with the surrounding area, it cannot be distinguished whether the interface or the globular clusters consist of WC, W2C, or WC1 − x. Fig. 10. XRD patterns observed for sample CWC and SWC. The 2-Theta scale is scaled to an energy of 8 keV (Cu-Kα).
3.2. Phase evolution processes XRD pattern reveals the presence of elementary W and the intermetallic Fe2W phase. In addition, it is found that the SWC sample contains different Fe-C-phases such as FeC, Fe3C, Fe1.86C0.14, and Fe15.1C. For the CWC sample, a similar composition is assumed. However, the XRD pattern reveals that the presence of eta carbides such as FeW3C, Fe2W2C, Fe3W3C, and Fe6W6C is more pronounced in the CWC sample as higher diffraction intensities are observed in these crystalline compounds. This is supported by the fact that the reflection at approximately 59° (FeW3C, Fe3W3C) can only be seen in the CWC sample. In addition to the great diversity of Fe-C phases as described above, the CWC sample further contains Fe5C2. However, due to the environmental conditions and inherent process characteristics, such as a rapid solidification of molten spray particles, it can be stated that constitutional supercooling takes place, which leads
Fig. 10 shows XRD patterns of the CWC and SWC samples. Based on the diffraction interferences, it is revealed that both coatings exhibit almost the same phase composition due to the use of comparable feedstocks. Thus, both coatings consist of Fe, WC, and W2C, which are also present in the feedstock used within this study. It turns out that in the case of the SWC sample, the intensity of the Fe reflection is greater than for the CWC sample, which supports the assumption that the SWC sample consists of a significantly higher amount of Fe when compared to the CWC sample. The additional reflections in the diagram, obtained from the SWC and CWC samples versus those obtained from the feedstock (data not shown), indicate that new phases were formed. With respect to the XRD pattern of the SWC sample, the coating is mainly composed of eta carbides such as FeW3C, Fe2W2C, Fe3W3C, and Fe6W6C. Moreover, the 159
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Table 2 Hardness and Young's modulus of relevant carbides and other phases.
Eta carbides Fe2W2C [40] Fe3W3C [38–40] Fe6W6C [38,40] Fe21W2C6 [40] Others Fe [46,47] W [45,47] Fe2W [47] Fe3C [39,40,43]
Young's modulus E [GPa]
Hardness [GPa]
275 446–470 447 438
7.6 15.6–16.8 14.73–15.6 15.0
Tungsten carbides h-WC [40–42] WC [43,44] h-W2C [42] o-W2C [42] t-W2C [42] W2C [43–45]
141 400–410 370–373 306–311
Young's modulus E [GPa]
Hardness [GPa]
691–697 710 480 437 427 420–444
16–22 23.5
29.4
h-WC: hexagonal WC, h-W2C: hexagonal W2C, o-W2C: orthorhombic W2C, t-W2C: tetragonal W2C.
carbides such as Fe3W3C and Fe6W6C using first-principle calculations. Their calculations showed that all eta carbides are thermodynamically stable. However, the authors concluded that the most thermodynamically stable compound among the investigated eta carbides is Fe6W6C. This eta carbide is detected in both samples, but the formation is more pronounced in the CWC sample. At the same time, it is reported that all examined eta carbides are less stable than tungsten mono-carbides which consists of hexagonal WC (h-WC). Moreover, the authors studied the formation energies of serval eta carbides for some possible reaction routes with participation of h-WC and tungsten semi-carbide W2C. It was found out that the formation of eta carbides in reactions with the participation of W2C is preferable. As opposed to that, reactions that included the participation of h-WC are unlikely due to the fact that h-WC is highly stable. Thus, the replacement of W atoms by Fe is energetically unfavorable. Liu et al. [40] employed first principles calculations in order to investigate the mechanic and electronic properties of Fe2W2C, Fe3W3C, Fe6W6C, and Fe21W2C6 compounds. According to this study, the cohesive energy and formation enthalpy were used to evaluate the stability of these carbides. The calculations implied that all compounds are thermodynamically stable, and the stability sequence of the examined phases from the Fe–W–C ternary system forms the following order: Fe21W2C6 > Fe6W6C > Fe3W3C > Fe2W2C. The aforementioned Fe3W3C and Fe2W2C eta carbides occurred in both samples, whereby their formation is more pronounced in the CWC sample. This can be due to a higher amount of WC and W2C in this sample, conditioned by the manufacturing process, and the embedment of CTC. Concerning the CWC sample, a higher amount of partially molten CTC is observed compared to the SWC sample. Accordingly, the implementation of CTC insertion, either as an external injection or as filling of the electrode tips, affects the metallurgical interactions. Mechanical properties of the eta carbides mentioned above as well as other relevant phases of the Fe-W-C-system can be found in literature [38–47]. Accordingly, the hardness and Young's modulus of several eta carbides and other possible phases in the deposits are listed below (Table 2). With respect to the findings observed by load and displacement sensing indentation (Figs. 7 and 9) in this study, it seems obvious that WC makes the largest share of the embedded W-rich hard phase (bright phase). The obtained mechanical properties (Fig. 7: ident no. 356 and 358, and Fig. 9: ident no. 174 or 207) are in good accordance with the values (E, and H) found in the literature (Table 2). With respect to the CWC sample, the measured values for H and E, taken from the surrounding matrix (ident no. 185, 211, and 276) correspond well with the measurements of eta carbides such as Fe3W3C and Fe6W6C as reported in other studies (Table 2). With respect to their shear and bulk modulus, Fe3W3C is more brittle than other eta carbides in the examined compounds [40]. Summarized, the CWC sample shows a larger transition area between CTC and the iron matrix, which is due to a greater dwell time next to the arc burning zone and stronger melting degree of the CTC. Thus, a change of the blocky shaped starting-off CTC to a more
to a wide varying non-equilibrium state. Thus, the phase transformation processes could not be completely analyzed. 4. Discussion As observed from the XRD patterns, both coatings contain Fe, WC, and W2C, which serve as feedstock components. Since the solid wire configuration provides a higher Fe content, the XRD pattern of the SWC sample exhibits a greater intensity of the Fe reflection than for the CWC sample. Besides the eutectic WC and W2C phases, traces of elementary W were identified. It can be stated that the occurrence of elementary W is caused by the dissolution of eutectic carbides. Moreover, only a small amount of WC was detected. Within a study on the microstructure of arc sprayed Fe-FeB-WC coatings, He et al. [4] reported that during spraying the C-deficient W2C phase is formed as a result of oxidation and decarburization processes of WC. Compared to the deposit sprayed with the use of the cored wire using eutectic tungsten carbides as filling (sample CWC), the layer sprayed with the use of the solid wire and external injection of eutectic tungsten carbides (sample SWC) is less to form ternary carbides, which can be assigned to FeW3C, Fe2W2C, Fe3W3C, and Fe6W6C. WC-Fe composites were already investigated by numerous researchers [35–37]. Their work show different melting interactions of WC particles with the surrounding molten matrix. Research concerning the production of WC-reinforced iron based alloys by means of laser melt interjection conducted by Nascimento et al. [35] showed that small WC and W2C isles formed around the WC particles. It is reported that the partial dissolution of WC particles forms new microstructural features containing Fe-(Fe,W)xC eutectics with different amounts of W. In another study, Nascimento et al. [36] examined the different types of reaction zones formed close to the interface between the steel matrix and WC particle in more detail. In this observation, it is recognized that the metallurgical interactions depend on the trajectories of injected particles and their interaction with the laser beam. The authors therefore concluded that the interface created in a reaction between the steel melt and WC particle surface is impaired by the WC particle temperature. According to Zhou et al. [37], the solubility of W in Fe is also influenced by the WC particle size. Thus, the solubility of W to the surrounding Fe-matrix increases with a decreasing WC particle size. The process kinetics of the abovementioned laser assisted manufacturing techniques differs to that of an arc spraying process. In particular, such kinetics play a significant role in interdiffusion processes. The formation of eta carbides as found for both sample types within this study can be also estimated taking thermodynamic aspects into account. Thus, several authors have reported about the ground state properties of Fe–W–C ternary compounds either by theoretical calculations or with empirical methods [38–40]. Suetin et al. [38] examined the stability for eta
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(2001) 176–177. [11] H.C. Starck, AMPERIT Thermal Spray Powder Brochure, Available online at: https://www.hcstarck.com/hcs-admin/file/ae23e4b2316f58850131b30f364743f9. de.0/akt_04_2017_web.pdf , Accessed date: 10 October 2017. [12] Commission Regulation (EC) No 790/2009: amending, for the purposes of its adaptation to technical and scientific progress, Regulation (EC) No 1272/2008 of the European Parliament and of the Council on classification, labelling and packaging of substances and mixtures, Off. J. Eur. Union L235/1 (5.9.2009) Available online at: http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri= OJ:L:2009:235:0001:0439:EN:PDF. [13] S. Zimmermann, B. Gries, J. Fischer, E. Lugscheider (Eds.), Thermal Spray 2008: Crossing Borders: Proceedings of the International Thermal Spray Conference 2008, DVS Deutscher Verband für Schweißen, Düsseldorf, 2008. [14] U.S. Department of Health and Human Services, Cobalt–Tungsten Carbide: Powders and Hard Metals, 12th Report on Carcinogens, Available online at: http://ntp.niehs. nih.gov/?objectid=03C9AF75-E1BF-FF40-DBA9EC0928DF8B15. [15] A. Scrivani, N. Antolotti, S. Bertini, R. Groppetti, T. Gutema, A. Mangia, S. Mucchino, G. Viola, C. Coddet (Eds.), Thermal Spray: Meeting the Challenges of the 21st Century, ASM International, 1998, p. 1019 Ed.. [16] J.C. Farmer, et al., Corrosion resistance of thermally sprayed high-boron iron-based amorphous-metal coatings: Fe49. 7Cr17. 7Mn1. 9Mo7. 4W1. 6B15. 2C3. 8Si2. 4, J. Mater. Res. 22 (2007) 2297–2311. [17] W.H. Liu, F.S. Shieu, W.T. Hsiao, Enhancement of wear and corrosion resistance of iron-based hard coatings deposited by high-velocity oxygen fuel (HVOF) thermal spraying, Surf. Coat. Technol. 249 (2014) 24–41. [18] V. Lovelock, Powder/processing/structure relationships in WC-Co thermal spray coatings: a review of the published literature, J. Therm. Spray Technol. 7 (1998) 357–373. [19] I.-T. Baumann, Hochverschleißfeste und konturnahe Werkzeugoberflächen durch Hochgeschwindigkeitsflammspritzverfahren, Dissertation TU Dortmund, 2012. [20] A.S. Kurlov, A.I. Gusev, Phase equilibria in the W–C system and tungsten carbides, Russ. Chem. Rev. 75 (2006) 617–636. [21] Y. Li, et al., First-principles study on the stability and mechanical property of eta M3W3C (M = Fe, Co, Ni) compounds, Physica B 405 (2010) 1011–1017. [22] Y. Liu, Y. Jiang, R. Zhou, J. Feng, First-principles calculations of the mechanical and electronic properties of Fe–W–C ternary compounds, Comput. Mater. Sci. 82 (2014) 26–32. [23] S. Kaierle, et al., Review on laser deposition welding: from micro to macro, Phys. Procedia 39 (2012) 336–345. [24] S.W. Huanga, et al., Abrasive wear performance and microstructure of laser clad WC/Ni layers, Wear 256 (2004) 1095–1105. [25] M. Jones, et al., The influence of carbide dissolution on the erosion-corrosion properties of cast tungsten carbide/Ni-based PTAW overlays, Wear 271 (2011) 1314–1324. [26] Z. Li, Y. Jiang, R. Zhou, D. Lu, R. Zhou, Dry three-body abrasive wear behavior of WC reinforced iron matrix surface composites produced by V-EPC infiltration casting process, Wear 262 (2007) 649–654. [27] R. Zhou, Y. Jiang, D. Lu, The effect of volume fraction of WC particles on erosion resistance of WC reinforced iron matrix surface composites, Wear 255 (2004) 134–138. [28] J. Wilden, J. Bergmann, S. Jahn, A new method for the production of particle reinforced coatings, Thermal Spray 2007: Global Coating Solutions, 2007, pp. 359–364. [29] G. Barbezat, C. Warnecke, United States Patent, No. US 7,019,249 B2, 2006. [30] G. Paczkowski, et al., Der neue alte Weg-Hochgeschwindigkeitslichtbogenspritzen eröffnet neue Perspektiven & Prozessdiagnostik beim offenen Drahtlichtbogenspritzen, Tagungsunterlagen 9. HVOF-Kolloquium, 2012, pp. 139–150. [31] A.N. Dubovoj, S.A. Prokudin, A.A. Karpetschenko, Therm. Spray Bull. 6 (2) (2013) 102–108. [32] W.C. Oliver, G.M. Pharr, An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments, J. Mater. Res. 7 (1992) 1564–1583. [33] C. Krywka, C. Sternemann, M. Paulus, N. Javid, R. Winter, A. Al-Sawalmih, S. Yi, D. Raabe, M. Tolan, The small-angle and wide-angle x-ray scattering setup at beamline BL9 of DELTA, J. Synchrotron Radiat. 14 (2007) 244–251. [34] A. Hammersley, S. Svensson, M. Hanfland, A. Fitch, D. Hausermann, Two-dimensional detector software: from real detector to idealised image or two-theta scan, Int. J. High Pressure Res. 144 (1996) 6235–6248. [35] A. Do Nascimento, V. Ocelik, M. Ierardi, J. De Hosson, Wear resistance of WC p/ Duplex Stainless Steel metal matrix composite layers prepared by laser melt injection, Surf. Coat. Technol. 202 (2008) 4758–4765. [36] A. Do Nascimento, V. Ocelik, M. Ierardi, J. De Hosson, Microstructure of reaction zone in WC p/duplex stainless steels matrix composites processing by laser melt injection, Surf. Coat. Technol. 202 (2008) 2113–2120. [37] S. Zhou, X. Dai, Microstructure evolution of Fe-based WC composite coating prepared by laser induction hybrid rapid cladding, Appl. Surf. Sci. 256 (2010) 7395–7399. [38] D. Suetin, I. Shein, A. Ivanovskii, Structural, electronic and magnetic properties of eta carbides (FE3W3C, FE6W6C, Co3W3C and Co6W6C) from first principles calculations, Physica B 404 (2009) 3544–3549.
roundly shape in the CWC sample is observed. In addition, the molten or partially molten state promotes the formation of new phases with the iron based matrix. The phase transformation processes (i.e. formation of eta carbides) in turn can lead to a degradation of the mechanical properties such as hardness. 5. Conclusion Within the scope of this study, an external injection of eutectic tungsten carbides was applied in a TWAS process in order to analyze the embedment of eutectic tungsten carbides in arc sprayed low alloyed steel. The embedment of eutectic tungsten carbides was compared with that obtained for deposits, manufactured by using cored wires and eutectic tungsten carbides as fillings. Both sample types are formed from a lamellar microstructure. W-rich hard phases scatter throughout the coating and are surrounded by a Fe-based matrix. Concerning the embedded W-rich hard phases which were inserted by means of external injection, electron microscopy and EDX analyses revealed a slight dissolution of eutectic carbides. Moreover, the interface of the eutectic carbide to the surrounded matrix is characterized by a planar transition zone. With respect to the embedded W-rich hard phases, which were inserted as filler material by using a cored wire, electron microscopy and EDX analyses showed a significant dissolution of eutectic tungsten carbides as a reactive layer was generated. Globular clusters that can be assigned to WC, W2C, or WC1-x precipitates were formed, typically at the interface between the polygonal shaped eutectic carbide and surrounding matrix, which comprise of composite carbides. XRD analyses confirmed that both sample types are mainly composed of WC, W2C as well as of eta carbides (M4C, M6C, and M12C), elementary W, and some other Fe-rich carbides. However, the formation of M4C, M6C, and M12C phases such as FeW3C, Fe2W2C, Fe3W3C, and Fe6W6C was less pronounced when inserting eutectic tungsten carbides via external injection during spraying. Acknowledgement The authors gratefully acknowledge the financial support of the DFG (German Research Foundation) within the Collaborative Research Centre SFB 708 subproject A1. The contributions of DURUM Verschleissschutz GmbH are gratefully acknowledged for supplying the feedstock material. The authors thank the DELTA machine group for providing the synchrotron radiation. References [1] A.P. Newbery, P.S. Grant, R.A. Neiser, The velocity and temperature of steel droplets during electric arc spraying, Surf. Coat. Technol. 195 (2005) 91–101. [2] I. Gedzevicius, A.V. Valiulis, Analysis of wire arc spraying process variables on coatings properties, J. Mater. Process. Technol. 175 (2006) 206–211. [3] P. Sheppard, H. Koiprasert, Effect of W dissolution in NiCrBSi–WC and NiBSi–WC arc sprayed coatings on wear behaviors, Wear 317 (2014) 194–200. [4] D.J. He, B.J. Fu, J.M. Jiang, X.J. Li, Microstructure and wear performance of arc sprayed Fe-FeB-WC coatings, J. Therm. Spray Technol. 17 (2008) 757–761. [5] W. Tillmann, B. Klusemann, J. Nebel, B. Svendsen, Analysis of the mechanical properties of an arc-sprayed WC-FeCSiMn coating: nanoindentation and simulation, J. Therm. Spray Technol. 20 (2011) 328–335. [6] W. Tillmann, W. Luo, J. Nebel, der Einfluss, Hartstoffkorngröße auf die tribologischen Eigenschaften gewalzter und geschliffener WSC-FeCSiMn-Schichten, Therm. Spray Bull. 4 (2011) 56–63. [7] W. Tillmann, W. Luo, U. Selvadurai, Wear analysis of thermal spray coatings on 3D surfaces, J. Therm. Spray Technol. 23 (2014) 245–251. [8] B. Xu, Z. Zhu, S. Ma, W. Zhang, W. Liu, Sliding wear behavior of Fe–Al and Fe–Al/ WC coatings prepared by high velocity arc spraying, Wear 257 (2004) 1089–1095. [9] P. Niranatlumpong, H. Koiprasert, Phase transformation of NiCrBSi–WC and NiBSi–WC arc sprayed coatings, Surf. Coat. Technol. 206 (2011) 440–445. [10] D.G. Atteridge, R. Davis, M. Scholl, G. Tewksbury, J. Therm. Spray Technol. 10
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Surface & Coatings Technology 331 (2017) 153–162
W. Tillmann et al.
[44] M. Yandouzi, L. Ajdelsztajn, B. Jodoin, WC-based composite coatings prepared by the pulsed gas dynamic spraying process: effect of the feedstock powders, Surf. Coat. Technol. 202 (2008) 3866–3877. [45] H. Taimatsu, S. Sugiyama, Y. Kodaira, Synthesis of W2C by reactive hot pressing and its mechanical properties, Mater. Trans. 49 (2008) 1256–1261. [46] H. Berns, Stahl und Gusseisen, Springer Verlag, Berlin Heidelberg, 2013 (in german). [47] Q.Q. Ren, J.L. Fan, Y. Han, H.R. Gong, Structural, thermodynamic, mechanical, and magnetic properties of FeW system, J. Appl. Phys. 116 (2014) (093909-1 093909-9).
[39] Y. Li, et al., First-principles study on the stability and mechanical property of eta M3W3C (M = Fe, Co, Ni) compounds, Physica B 405 (2010) 1011–1017. [40] Y. Liu, Y. Jiang, R. Zhou, J. Feng, First-principles calculations of the mechanical and electronic properties of Fe–W–C ternary compounds, Comput. Mater. Sci. 82 (2014) 26–32. [41] T. Dash, B. Nayak, Preparation of WC–W2C composites by arc plasma melting and their characterisations, Ceram. Int. 39 (2013) 3279–3292. [42] Y. Liu, Y. Jiang, J. Feng, Mechanical properties and chemical bonding characteristics of WC and W2C compounds, Ceram. Int. 40 (2014) 2891–2899. [43] T. Gerthsen, Chemie für den Maschinenbau, Universitätsverlag Karlsruhe, Karlsruhe, 2006 (in german).
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