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Figure from:Comparative Analysis of the Friction and Microstructural Properties of WC-10Co-4Cr and Cr3C2-25NiCr Coatings Sprayed by High-Velocity Oxy-fuel (HVOF)

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Table 3. Thermal Sprinkler Process Parameters 2.3. Characterization Tests

Table 3 Thermal Sprinkler Process Parameters 2.3. Characterization Tests

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Table 1. Comparison between coatings The wear behavior of the WC-based coatings with Co deposited by different thermal spraying processes were studied by several researchers [5, 6, 9, 10]. The studies developed by the aforementioned authors report that the hardness, fracture strength and adhesion strength between the hardened layer and the die are the most important factors while the Co, Ni, and Cr materials provide toughness [6]. The main phenomena that occur during thermal spraying processes, which influence coating performance, are the oxidation and the high temperature decomposition of tungsten carbide [7]. Experiments carried out by Mateen et al. (2011) revealed that WC-Co coatings have high wear resistance, good mechanical properties and, in addition, good control of the tribological properties can be obtained [8].
Table 1. Comparison between coatings The wear behavior of the WC-based coatings with Co deposited by different thermal spraying processes were studied by several researchers [5, 6, 9, 10]. The studies developed by the aforementioned authors report that the hardness, fracture strength and adhesion strength between the hardened layer and the die are the most important factors while the Co, Ni, and Cr materials provide toughness [6]. The main phenomena that occur during thermal spraying processes, which influence coating performance, are the oxidation and the high temperature decomposition of tungsten carbide [7]. Experiments carried out by Mateen et al. (2011) revealed that WC-Co coatings have high wear resistance, good mechanical properties and, in addition, good control of the tribological properties can be obtained [8].
A summary of the process main characteristics are presented in Table 3. These configurations define characteristics of the coating, such as: layer thickness, lamellar structure, porosity, etc. The expected thickness of the coating is 200 um. After application of the coating the parts are cooled in the air. Figure 1. Application of the coating on the specimens
A summary of the process main characteristics are presented in Table 3. These configurations define characteristics of the coating, such as: layer thickness, lamellar structure, porosity, etc. The expected thickness of the coating is 200 um. After application of the coating the parts are cooled in the air. Figure 1. Application of the coating on the specimens
Table 2. Characteristics and properties of the materials used in the spraying 2.2. Spraying HVOF
Table 2. Characteristics and properties of the materials used in the spraying 2.2. Spraying HVOF
Figure 2. XRD patterns of coatings and powder. (a) Cr3C2-25NiCr and (b) WC-10C04Cr
Figure 2. XRD patterns of coatings and powder. (a) Cr3C2-25NiCr and (b) WC-10C04Cr
powder and the coated surface. In the standard obtained for the powder several peaks of the Cr3C2 phase and an intense Ni peak are present. After the powder is sprayed the Ni peak becomes less intense and closer to one of the peaks of Cr3C2, becoming wider, characterizing an amorphous phase. Similar results were also obtained by Murthy & Venkataraman [19]. Other peaks defined in the powder were less evident after the sprinkling process. The increase in the formation of non-crystalline amorphous phases occurs due to very fast cooling during the spraying process [20]. Figure 3. Morphology of the powder used in the coating based. (a) Cr3C2- 25NiCr and (b) WC-10Co04Cr
powder and the coated surface. In the standard obtained for the powder several peaks of the Cr3C2 phase and an intense Ni peak are present. After the powder is sprayed the Ni peak becomes less intense and closer to one of the peaks of Cr3C2, becoming wider, characterizing an amorphous phase. Similar results were also obtained by Murthy & Venkataraman [19]. Other peaks defined in the powder were less evident after the sprinkling process. The increase in the formation of non-crystalline amorphous phases occurs due to very fast cooling during the spraying process [20]. Figure 3. Morphology of the powder used in the coating based. (a) Cr3C2- 25NiCr and (b) WC-10Co04Cr
Figure 4. Microestrutura dos revestimentos. (a) WC-10Co4Cr. (b) Cr3C2- 25NiCr Fig. 3 (a) shows the scanning electron microscopy (SEM) analysis of the morphological characteristics of WC-10Co4Cr powder. This material has a dense grain, which forms a more homogeneous layer, because material spreads better on the surface [22]. This morphology is desirable for the Thermal spraying y process by HVOF, as it allows a better flow of the powder in the spray gun during the application of the coating [23]. The average diameter of the beads of WC-10Co04Cr powder is 30 um. In Fig. 3 (b) it is observed that the morphology of Cr3C2-25NiCr powder presents a hollow sphere shaped structure with porosities and spaced particles, the grain diameter varies around 40 um.
Figure 4. Microestrutura dos revestimentos. (a) WC-10Co4Cr. (b) Cr3C2- 25NiCr Fig. 3 (a) shows the scanning electron microscopy (SEM) analysis of the morphological characteristics of WC-10Co4Cr powder. This material has a dense grain, which forms a more homogeneous layer, because material spreads better on the surface [22]. This morphology is desirable for the Thermal spraying y process by HVOF, as it allows a better flow of the powder in the spray gun during the application of the coating [23]. The average diameter of the beads of WC-10Co04Cr powder is 30 um. In Fig. 3 (b) it is observed that the morphology of Cr3C2-25NiCr powder presents a hollow sphere shaped structure with porosities and spaced particles, the grain diameter varies around 40 um.
Figure 5. Pores and voids in coatings. (a) WC-10Co4Cr. (b) Cr3C2- 25NiCr It is possible to visualize in Fig. 5 (a) and (b) that both coatings have pores, voids and a layer with lamellar structure, containing in their outline a discrete oxide film, a common feature of coatings applied via HVOF. This occurs due to the overlapping of high-speed melt and semi-fused particles deposited on the substrate [24]. The ImageJ program was used to measure porosity. The Cr3C2-25NiCr coating had a higher pore concentration, around 1.2% and_ the WC-10Co04Cr coating had 0.9% of its pore area. Thus, both layers are well constituted and within acceptable limits for this type of coating, as described in the literature [15].
Figure 5. Pores and voids in coatings. (a) WC-10Co4Cr. (b) Cr3C2- 25NiCr It is possible to visualize in Fig. 5 (a) and (b) that both coatings have pores, voids and a layer with lamellar structure, containing in their outline a discrete oxide film, a common feature of coatings applied via HVOF. This occurs due to the overlapping of high-speed melt and semi-fused particles deposited on the substrate [24]. The ImageJ program was used to measure porosity. The Cr3C2-25NiCr coating had a higher pore concentration, around 1.2% and_ the WC-10Co04Cr coating had 0.9% of its pore area. Thus, both layers are well constituted and within acceptable limits for this type of coating, as described in the literature [15].
The average Vickers microhardness results and the standard deviation of each coating are shown in the graph of Fig. 6 (a). The microhardness values of the layer sprayed with Cr3C2-25NiCr are lower than WC-10Co4Cr values. This behavior is compatible with both researched literature and material supplier indications [28]. It will be appreciated that both coatings have high hardness variations along the cross section, this is due to non-uniformity of the layer. Fig. 6 (a) and (b) shows that each indentation point is located in different microstructures, such as carbides, oxides, inclusions and finally the substrate. The substrate measurements do not have values as discrepant as those found in the sprayed layer, which confirms the variation due to the coating characteristics. Figure 6. Microhardness profiles of the coatings
The average Vickers microhardness results and the standard deviation of each coating are shown in the graph of Fig. 6 (a). The microhardness values of the layer sprayed with Cr3C2-25NiCr are lower than WC-10Co4Cr values. This behavior is compatible with both researched literature and material supplier indications [28]. It will be appreciated that both coatings have high hardness variations along the cross section, this is due to non-uniformity of the layer. Fig. 6 (a) and (b) shows that each indentation point is located in different microstructures, such as carbides, oxides, inclusions and finally the substrate. The substrate measurements do not have values as discrepant as those found in the sprayed layer, which confirms the variation due to the coating characteristics. Figure 6. Microhardness profiles of the coatings
The behavior of the coefficient of friction for both coatings is shown in Fig. 7. It is observed that the friction coefficient of the sample coated with WC-10Co4Cr is greater than the sample coated with Cr3C2-25NiCr. Initially there is a zone of instability, which occurs due to the detachment of particles from the sphere and the coating [15]. Then, the coefficient of friction reaches a steady value, but there is a maximum and minimum fluctuation that remains constant for both coatings, this phenomenon indicates the wear stabilization on the coating [29, 30].
The behavior of the coefficient of friction for both coatings is shown in Fig. 7. It is observed that the friction coefficient of the sample coated with WC-10Co4Cr is greater than the sample coated with Cr3C2-25NiCr. Initially there is a zone of instability, which occurs due to the detachment of particles from the sphere and the coating [15]. Then, the coefficient of friction reaches a steady value, but there is a maximum and minimum fluctuation that remains constant for both coatings, this phenomenon indicates the wear stabilization on the coating [29, 30].
It is possible to observe that both tracks have a scattered appearance and that even after sanding the roughness remains high, it happens because during the finishing process the most pronounced peaks are eliminated, but the pores remain. In general, coatings applied by HVOF have a low wear rate [29]. Figure 8. Profilometry of worn tracks after pin-on-disc tests
It is possible to observe that both tracks have a scattered appearance and that even after sanding the roughness remains high, it happens because during the finishing process the most pronounced peaks are eliminated, but the pores remain. In general, coatings applied by HVOF have a low wear rate [29]. Figure 8. Profilometry of worn tracks after pin-on-disc tests
4. Conclusions
4. Conclusions