Improvement of high velocity oxy-fuel spray coatings by thermal post-treatments: A critical review

Thin Solid Films 678 (2019) 42–52

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Critical Review

Improvement of high velocity oxy-fuel spray coatings by thermal posttreatments: A critical review F. Ghadami, A. Sabour Rouh Aghdam

T

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Department of Materials Engineering, Tarbiat Modares University, P.O. Box: 14115-143, Tehran, Iran

A R T I C LE I N FO

A B S T R A C T

Keywords: High-velocity oxy-fuel Thermal post-treatment Structure Residual stress Adhesive strength

The high velocity oxy-fuel (HVOF) spraying process has been employed for applying engineering coatings on various industrial parts and components. Recently, post-spray treatments have been widely used to improve the overall quality of sprayed coatings, especially those applied by the HVOF process. In the latter case, thermallyassisted surface treatment of as-sprayed coatings is a critical factor to enhance the properties of the coated parts for specific applications. The present work is a review of the improvement of HVOF spray coatings, focused on different types of post-spray treatments.

1. Introduction In recent years, High Velocity Oxy-Fuel (HVOF) spraying has undergone considerable improvement in industries related to thermal spraying technology and led to the development of other thermal spraying techniques such as cold spraying. The main performance of the deposits applied by the HVOF process is superior to other thermal spraying methods. HVOF spraying is compatible for applying coatings of materials such as cemented carbides. Coatings with higher density and uniformly are achieved as a result of the impact of the high velocity particles on the surface of the substrate to form a collection of compressed splats [1–10]. However in the HVOF spraying technique, excess heating energy can transform the surface. Thus, special cooling methods, or utilizing a discontinuous spraying process, are very important for preventing coating defects. Due to the restriction of process temperature, the HVOF spraying method cannot be recommended for coating of refractory or ceramic materials. More research is still essential to attain a better knowledge of the physical and thermal characteristics of the HVOF technique, to develop diagnostic methods to measure the in-flight velocity and temperature histories of the droplets or particles, and to simulate interactions between droplets and the oxy-fuel flame [11–16]. Table 1 lists general applications of HVOF coatings in different industries. The multiscale characteristics and related operative parameters of a typical HVOF spray process are presented in Fig. 1. The overall microstructure of sprayed coatings depends on the degree of particle deformation, sintering of the splats, and the solidification rate of the coated layer, all of which are related to the properties of the substrate

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(e.g., substrate temperature) as well as the physical and chemical state (temperature, velocity, melting ratio, and oxidant content, spraying distance, etc.) of the particles at the point of impact on the substrate [8,17]. The in-flight behavior of particles is related to the dynamic state of the gas mixture, which can be adjusted by spraying parameters such as the fuel-to-oxygen ratio and the rate of total gas flow. The flow and thermal field can be readily analyzed by continuum-type simulation methods [17,18]. Some researchers believe that the corrosion resistance of HVOF coatings cannot be increased by thermal post-treatments [19]. However, shrouding of the in-flight particle with inert gas appears to reduce the oxygen content (oxide or solution) in the coating. Fig. 2 shows the main advantages and characteristics of post-spray treatments for different types of HVOF coatings. Lee et al. [20] investigated the corrosion resistance of Ni-Cr-W-Mo-B alloy coatings prepared by the HVOF process. They concluded that annealing of the coatings after spraying in a vacuum furnace at temperatures of 550, 750, and 950 °C for 2 h, enhances the corrosion properties due to improved chemical and microstructural homogeneity, as well as a reduction of porosity (densification), and reduction of the eutectic structures. The effect of various post thermal treatments on the electrochemical corrosion properties of HVOF Cr3C2-NiCr coatings has been investigated by Guilemany et al. [21]. According to their work, the first coating layers were HVOF-sprayed, followed by thermal treatment with a torch, and finally, the remaining layers were sprayed accordingly. The results showed that the thermal treatment with the same torch during the spraying process could enhance the degree of protection of the coatings against corrosive electrolytes.

Corresponding author. E-mail addresses: [email protected] (F. Ghadami), [email protected] (A. Sabour Rouh Aghdam).

https://doi.org/10.1016/j.tsf.2019.02.019 Received 12 February 2019; Accepted 12 February 2019 Available online 13 February 2019 0040-6090/ © 2019 Elsevier B.V. All rights reserved.

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Table 1 Example applications of HVOF coatings in a variety of industries. Application

Protection mechanism

Coating material

Ref

Steam turbines Solid oxide fuel cells Food industry Automotive industry Hydro turbines Biomedical industry Marine Aeronautic industry

Erosion/Corrosion High Temperature/Corrosion Wear/Corrosion Wear Erosion/Wear Bio-corrosion Erosion/Corrosion Wear/Corrosion

Diamalloy 4006, MCrAlY La0.8Sr0.2Ga0.8Mg0.2O3 TiC-FeCrAl TiC-FeCrAl, Stellite6 WC-CoCr, 16Cr5N, WC-Co-Cr Nano-Hydroxyapatite Nickel–aluminum bronze WC-Co

[84] [95] [96] [96,97] [82,98] [71] [99] [38,100]

Fig. 1. Multiscale characteristics of the HVOF spray process (Reproduced with permission from [18]).

oxy-acetylene flame, an electron beam, and/or a laser beam. 2. Heat treatment Compared to other types of post spray treatments, furnace heat treatment is usually low-cost and more available. Atmosphere-controlled heat-treatment processing of Fe-based amorphous coatings can affect their corrosion properties positively or negatively by crystallization [20]. Thus, corrosion resistance depends on the microstructure and phase composition of heat-treated coatings [25,27–29]. Fig. 3 schematically indicates some structural aspects of the heat treatment of HVOF coatings. When the HVOF-coated specimens are heat treated by isothermal or cyclic methods, the state of the as-sprayed coating residual stress can be dramatically affected. The change is due to relaxation of stresses at relatively high temperatures and formation of secondary stress fields in the coating layer during cooling at the end of heat treatment process (the latter is a result of thermal expansion coefficient difference between a sprayed coating and the substrate). In addition, heat treatment of HVOF coatings can improve the adhesive strength of the coating due to interdiffusion between the coating layer and the metal substrate [25,30]. Some research also indicates that heat-treated metallic HVOF coatings have a higher hardness compared to as-sprayed coatings [13,25,29,31]. Eaton and Novak [32] studied the effect of annealing treatments in vacuum atmospheres on the microstructure and adhesive strength of zirconia sprayed on stainless-steel substrates, with and without a ternary Ni-Cr-Al bond coat. According to the results, an enhancement in adhesive strength was observed. However, a positive response to annealing at higher temperatures for HVOF coatings may vary depending on the coating and substrate materials used. Other investigations, such

Fig. 2. Characteristics and primary advantages of post-spray treatments.

When HVOF-coated parts are subjected to compressive loadings, for example operating under cavitation, the coatings exhibit a high amount of porosity, the existence of an oxide phase, and un-melted particles in HVOF-sprayed coatings [18,19,22,23]. Furthermore, the coatings have relatively poor adhesion to their substrates, which limits the suitability of the method [24–26]. However, it has been shown that, subsequent post-treatment can be used, in order to refine the layers and enhance the adhesion of coatings to the substrate [25,26]. In this regard, some thermally-based treatments assure the remelting of the coatings by an 43

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Fig. 3. Schematic illustration of a structural comparison between a) as-sprayed and b) heat-treated HVOF-sprayed coatings.

disk wear tests, the wear rate of 600 °C heat-treated coatings decreased by two orders of magnitude. Guilemany et al. [39] reported that the formation of fine distributed Cr3C2 hard carbides caused by heat treatment at 1033 K for 1 h in an argon gas inert atmosphere, enhances the wear resistance HVOF Cr3C225 wt% NiCr coatings. Heat treatment of HVOF coatings can cause softening of the binder matrix phase in composite Cr3C2-NiCr coatings. This mechanism is thought to be related to recrystallization and grain coarsening of the NiCr alloy matrix phase, and particularly to a change from solid-solution to precipitation strengthening as result of the precipitation of secondary carbide phases after heat treatment. This claim is verified by the increasing coating microhardness during the early stages of precipitation hardening as analyzed by Janka et al. [28] (Fig. 6). The sharp hardness decrease after 5 min at 400 °C is attributed to a significant decrease in the amount of solid-solution hardening by the formation of nano-scale secondary carbide phases. Increasing the heat treatment temperature from 400 to 600 °C results in higher microhardness values for the Cr3C2-NiCr coatings. Additional growth of the precipitates at 800 °C increase dislocation migration, and therefore decreases the microhardness compared to 600 °C heat treatment [28]. Murthy et al. [40] claims that the maximum precipitation hardening in Ni occurs for particle sizes of 10–15 nm, in agreement with the nanohardness maximum in the NiCr matrix phase for secondary carbides after heat treatment. Some studies have focused on applying hydroxyapatite coatings on Ti-6A1-4 V alloy substrates using the HVOF method and investigating the effect of heat treatment on the microstructure and properties of the coatings. Fernandez et al. showed that after heat treatment, recrystallization of the hydroxyapatite coatings occurs, which improves the coating hardness and adhesion toTi-6Al-4 V substrates [41].

as by Lim et al. [33], Ghadami et al. [25], and Frodelius et al. [34], confirmed improvement in adhesive strength and hardness of sprayed coatings by post spray annealing treatments. Microstructure and the sliding wear behavior of Cr3C2–CoNiCrAlY sprayed coatings, before and after heat treatment, on hot-worked steel substrate were investigated by Picas et al. [29]. According to the results, hardness and wear properties of the coating after heat treatment were increased, especially at 800 °C. Fig. 4 shows the microstructure of Cr3C2-CoNiCrAlY coatings before and after post heat treatment. After heat treatment at 900 °C, precipitation of fine carbide phases (small points with grey color in Fig. 4a) occurred throughout the upper coating. The creation of precipitated carbides reveals that the binder phase loses Cr content and gains Co and Ni to yield a brighter contrast in the backscatter electron micrographs (Fig. 4a). The precipitates from supersaturated phases form after heat treatment following the dissolution of the carbide phase during the HVOF spraying process. This phenomenon is in accordance with other related reports [5,35]. After precipitation, the carbide particles grew during post-treatment processing at higher temperatures (Fig. 4b). Finally, carbide particles tend to coalesce and create larger carbide phases with complex morphologies and the presence of black chromium oxide (Cr2O3) regions (Fig. 4c). According to Picas et al. [29]. the coating porosity before and after heat treatment are not very different, less than 1% for all types of as-sprayed and heat treated coatings. Regarding furnace heat treatment, some reports are focused on the formation of complex phases during heat treatment of HVOF-sprayed coatings as a function of time and temperature. According to the results, hardness and wear resistance of post-heat-treated coatings can be improved due to the structural transformation of amorphous to crystalline phases together with the formation of complex phases or precipitates [33,36]. These findings are in agreement with Stewart et al. [5], Ghadami et al. [25,30], and Nerz et al. [37] who studied post treatment of plasma sprayed WC-Co coatings. The hardness and wear resistance of heat treated WC-Co coatings increased due to the formation of η-carbides (complex carbide precipitates consisting of W3Co3C, W6Co6C, and W4Co2C). Comparison of x-ray diffraction patterns of the WC-12wt%Co powder feedstock, as-sprayed coating, and heat treated samples after 650, 900, and 1150 °C for 1 h, explains the formation of the complex ηcarbides (see Fig. 5) [38]. Pin-on-disk wear testing and the tribological behavior of HVOFsprayed Co-28% Mo-17%Cr-3%Si coatings before and after heat treatment have been investigated by Bolelli et al. [31]. Results showed that, after heat treatment at 600 °C for 1 h, recrystallization of the coating layer, with the formation of sub-micron crystalline phases, appears and causes an increase in hardness and wear resistance. Based on pin-on-

3. High-energy-beam surface treatments 3.1. Laser surface treatment Recently, the use of laser processing of thermal spray coatings has been growing in industry. Research and development, rather than focusing on classical applications such as surface hardening and cladding is moving toward more advanced technologies, e.g., surface remelting and alloying [11,42–47]. In addition, the combination of two or more different surface post treatments methods has the potential for further improvement of the primary characteristics of thermally sprayed coatings [4,8,48]. Compared to laser-treated coatings, the density of conventional HVOF coatings is relatively low and there is usually a clear 44

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Fig. 5. X-ray diffraction patterns of WC–12%Co powders as a reference, and sprayed coatings on steel substrates before and after heat treatment at 650, 900, and 1150 °C. (Reproduced with permission from [38]).

Fig. 6. Vickers hardness values from cross-sections of Cr3C2-NiCr coatings applied by HVOF process on steel substrates, after different heat treatments at 400, 600, and 800 °C (Reproduced with permission from [28]).

significantly enhanced chemical and physical properties. Several studies have demonstrated improved thermal spray coatings using laser irradiation [4,11,14,43,45,47,52–54]. For the case of HVOF coatings, the laser-beam irradiation results in heating and cooling rates of by 103–1010 K/s, while the mean total energy is between 103 and 104 J/cm2. Furthermore, ultra-fast quenching of the treated HVOF layer may produce different types of amorphous or metastable crystalline solid solutions and result in major transformations in both the coating and substrate materials [55]. The laser-beam scan rate and power are critical. Because the microstructure of HVOF coatings usually consists of lamellar and solidified particles, laser post-spray treatment is useful to obtain a more homogenous microstructure and to enhance the adhesion of the coating to the substrate. At high-speed laser processing, the influence of the latent heat is important for determining the structure in HVOF-sprayed coatings. Improvements to the wear resistance by post laser treatment were successfully demonstrated for the model systems Ti-Ni, Ti-Al, and Al-Ni by Wilden and Frank [56]. A number of researchers have analyzed the effect of laser processing of HVOF-sprayed cermet coatings for enhancing wear resistance and hardness [6,42,52,54,57–60]. The abrasive wear properties of WC–17 wt% Co coatings formed by plasma spraying increased following laser surface treatment due to a reduction in microporosity [51,60]. A study of nickel-chrome based carbide coatings showed similar results [25]. In contrast, Chen et al. found that by laser surface remelting of HVOF WC-Co coatings, both hardness and wear resistance

Fig. 4. Backscattered electron cross-sectional micrographs of the microstructure of HVOF-sprayed Cr3C2–CoNiCrAlY top-coating after heat treatment in atmospheric conditions at: (a) 900 °C; (b) 1000 °C, and (c) 1100 °C (Reproduced with permission from [29]).

mechanical interface between the HVOF coating and the substrate material. Chemical and/or metallurgical bonding is formed at the coating-substrate interface during laser post-treatment in addition to a decrease in holes, cracks, and other defects in the coating microstructure. Houdkova et al. concluded that laser post-treatment influences microstructure, phase composition, and consequently the abrasive wear resistance of Stellite 6 coatings [49]. Uusitalo et al. [50], as well as Lee et al. [20], found that laser remelting of HVOF-sprayed Ni-based coatings enhances the corrosion properties. Laser treatment of the coating can reduce or remove microporosity and oxide layers at splat boundaries in HVOF coatings. Laser processing results from ultra-rapid interlayer mixing by controlled convection and diffusion in the melted coating zone, followed by rapid solidification to form an alloy layer with higher density [26,42,43,51]. Laser-treated layer microstructures can have 45

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Fig. 7. SEM backscattered image micrographs of the cross section of an Inconel 625 HVOF sprayed coating on a 316 L austenitic stainless steel substrate before and after post laser treatment (Reproduced with permission from [53]).

Several studies were also carried out to investigate the effect of laser processing parameters on the properties HVOF Ni-based coatings. Nd:YAG laser post-treatment of Ni-based superalloy coatings was analyzed by Tuominen et al. [62]. They showed that the treatment resulted in full homogenization of the coating structure. In addition, surface laser remelting of thermally sprayed metallic coatings was investigated by Oksa et al. [63]. They indicated that laser-treated coatings had higher corrosion resistance during short-term corrosion tests in fluidized bed boiler applications. In addition, sealing of metallic coatings by silicates, silicon paints, aluminum oxide and aluminum phosphate sealers [10], sol-gel solution methods [64] and/or using impregnated sealers [65] decreases the negative effect of structural voids and oxides in HVOF-sprayed coatings. The adhesive strength of HVOF-sprayed Fe12.4Cr0.6Ni0.4Mn0.4Si0.36C, WC-17Co, and 80Cr3C2–16Ni4Cr coatings on low-alloyed steel substrates before and after laser treatment was studied by Hjornhede et al. [26]. They concluded, for the coatings after laser treatment, no detachment occurs for tensile strains up to 15%. In addition, the adhesive properties of laser post-treated HVOF Inconel 625 coatings on mild steel substrates during three-point bending tests were analyzed by Yilbas et al. [66]. They showed that locally distributed splats with high oxygen content acted as crack initiation points due to structural brittleness. The microstructure of laser post-treated thermally sprayed WC12%Co coatings was studied by Afzal et al. [42,57]. They indicated that microhardness of the treated layer increased due to laser remelting, but the amount of the increase was related to the scan rate as well as the coating thickness. At higher laser scan of rates, (e.g., 300–900 mm/ min), the surface of the specimens was not fully melted due to insufficient energy density from the laser beam. At lower speeds, the WCCo layer starts to remelt, and the melting depth increases with decreasing scan rates. In this case, it was shown that the WC-Co carbide coating remelts and starts to intermix with substrate material to form a new hard composite material with excellent surface properties and improved adhesion. The properties of laser treated coatings depend upon the thickness of the treated (remelted) layer and laser energy density. For thick WCCo samples (450 μm), the depth variation in the compositions of the treated layer is shown in Fig. 9a. A CO2 laser with 2.5 kW and a spot size of 3 mm was used for treatment of coatings. An inert shielding gas (N2) prevented oxidation during laser treatment. The compositions of major components with coating depth changed dramatically, revealing uneven mixing of WC-Co with the base metal. Clearly, larger energy densities were needed so the fluctuation in composition avoided clustering in the laser-treated zone. Fig. 9b shows the depth distribution of the composition in the laser treated zone of a 260-μm-thick coating using the same laser energy density. Near the surface, the composition

properties was reduced due to a reduction in WC carbide size and fracture of large carbide particles [52]. Fig. 7, shows scanning electron microscopy (SEM) images of Inconel 625 HVOF-sprayed coating before and after laser treatment. Laser remelting removed the lamellar splatstructure and microporosity in the as-sprayed coating and enhanced metallurgical bonding of the coating with the metallic substrate. Laser treatment by means of surface remelting has been applied to decrease and/or eliminate defects and inhomogeneity in HVOF-sprayed coatings. Zhang et al. [61] analyzed WC-Cr3C2-Ni before and after laser remelting treatment. They showed that interlamellar porosity was essentially removed after laser post-treatment, except for a few microcracks and the microstructure was much more homogeneous. However, only limited investigations have been carried out on the effect of laser post-treatment on the sliding wear and corrosion behavior of HVOFsprayed composite coatings [43,53,54,62]. Depth-dependent microhardness profiles of as-sprayed and heat treated WC-Cr3C2-Ni coatings are presented in Fig. 8. The results show that laser treatment has produced a significant improvement in the hardness value of the treated coatings, especially in the near-surface layers where the mean microhardness value (Vickers test under load of 1.96 N and loading time of 10 s) increases from HV0.2 = 1036.7 in the as-sprayed HVOF coating to HV0.2 = 1421.9 in the laser-treated coatings, an enhancement of 37.2%. This can be partly explained the decreased oxide formation due to the laser remelting treatment [59,61]. The corrosion resistance also improved considerably because of the decrease in the size and number of the structural pores, which are active sites for local and/or pitting corrosion [54,62].

Fig. 8. Microhardness profile along the cross-section of WC-Cr3C2-Ni coatings before (as-sprayed HVOF on Inconel 718 substrates) and after laser remelting treatment (Reproduced with permission from [61]). 46

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Fig. 10. Schematic diagram of the electron-beam remelting process in a sprayed coating (Reproduced with permission from [69]).

Mitsubishi Electronic Model EBW-110 and the beam input power was 110 kW, sufficient to remelt the upper coating surface. Magnetic coils are used to focus and guide the beam. Fig. 11 shows a typical crosssection of an electron-beam treated coating with structural defects: interlayer porosity, solid and incompletely melted portions, micro- and macro-scale cracks and a rough surface [69]. Optimization of electronbeam treatment was necessary to obtain best results. The optimized electron-beam parameters are indicated in Table 2. Experimental conditions for electron-beam surface melting of HVOF-sprayed Ni-based (Inconel 625) coatings on copper substrates (Reproduced with permission from [69]). Optimization of electron-beam treatment was necessary to obtain best results. The optimized electron-beam parameters are indicated in Table 2. The beam scan rate, the input electron-beam power, the sample translation speed, the working distance, and the electron-beam shape were the major experimental factors controlling the depth of penetration of the electron beam onto the HVOF coatings. Other factors such as the lateral scan rate, frequency, and beam width were also optimized to minimize the concentration of coating defect (e.g., porosity) and decrease surface roughness. In addition, the coating adhesive strength was determined by a mechanical shear strength test. Fig. 12 shows the relationship between the penetration depth and the density of the electron-beam energy for HVOF-sprayed Ni-12.4Cr2.5Fe-4.5Si-3B-0.6C alloy coatings on copper substrates. Before HVOF spraying, Ni plating, with a thickness of 0.5 mm, was applied to the substrate surface and after that, a 0.7-mm Ni-12.4Cr-2.5Fe-4.5S1-3B0.6C coating was sprayed. The Figure shows that the penetration depth of the electron beam increases with increasing electron-beam energy density. It was concluded that an electron-beam power of 1.8 kW, the penetration depth begins to decrease with increasing electron-beam energy density. In this condition, the heat conduction loss is increased at lower sample moving speeds by the focused electron beam. Therefore, at the higher power of the electron beam (3.6 kW), the depth of penetration is much higher than with lower powers (1.8 kW) under the same electron-beam energy density.

Fig. 9. Variation of coating composition as a function of the depth in the lasermelted zone in: a) 450-μm-thick WC-Co sprayed coating and b) a 250-μm-thick WC-Co sprayed coating on AISI-321 stainless steel substrates (Reproduced with permission from [57]).

variation was smooth indicating homogenous intermixing of WC-Co with the base metal. However, some variations were detected near the interface of the melted zone and substrate indicating that the temperature was insufficient for complete homogenization. Laser-induced functionally graded WC coatings on high-speed tool steel substrates were analyzed by Riabkina-Fishman et al. [46]. They showed that laser treatment for coatings consisting of 58 wt% tungsten provided enhanced wear resistance over the untreated (as-sprayed) coating. Laser surface treatment of HVOF and plasma sprayed Ni-based Inconel 625 coatings and the effects of Nd:YAG laser treatment on the corrosion resistance of both as-sprayed and laser treated coatings was studied by Tuominen et al. [62]. They concluded that laser remelting of Ni-based Inconel 625 coating enhanced the high corrosion resistance by increasing adhesion and improving the structural homogeneity of the coating. 3.2. Electron-beam surface treatments The electron-beam process as a post-treatment of HVOF-sprayed alloys was investigated by Utu et al. [67] and Gavendová et al. [68]. Furthermore, Hamatani et al. [69] used electron-beam surface remelting for HVOF-sprayed Stellite 6, WC-CO, and Cr3C2-NiCr coatings on copper substrates with a pre-electroplated Ni layer. They reported that in order to decrease the coating porosity and surface roughness, very low homogeneous heating and electron-beam scan rates were preferable. Fig. 10 is a schematic illustration of the experimental scheme for the electron-beam surface treatment for Ni-based HVOF coatings on copper substrates [69]. The electron-beam equipment was

4. Hot isostatic pressing (HIP) Hot isostatic pressing (HIP) is one of the best post-treatments which provides an interesting combination of properties for thermally-sprayed coatings [5,52,70,71]. However, there are only a limited number of investigations carried out on the HIP post-treatment of HVOF coatings [72]. Generally, the density and hardness of the coating improved, while the structural porosity decreasd considerably after HIP posttreatment [71,72]. Abdel-Samad [70] and Rajasekaran [72] concluded 47

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Fig. 11. Cross-sectional optical microscopy image of a remelted Ni-12.4Cr-2.5Fe-4.5Si-3B-0.6C alloy on a copper substrate (Reproduced with permission from [69]).

substrate zones. By means of the HIP mechanism, the substrate material starts to penetrate, and interdiffuse into the coating layer and vice versa. The mechanical interlocking between coating and substrate increases the adhesive strength of the HIP-treated coating. After tensile testing, it was observed that coating detachment often occurs via cohesive, rather than adheive, failure which is decresed by better interlamellar contact after treatment. The failure mode changed from cohesive to adhesive failure after the HIP process. The HIP posttreatment not only influences the structure, but also changes the residual stress state of the sprayed coating. Fig. 14 shows the residual stresses in plasma sprayed Mg2Zr5O12 coatings before and after HIP treatment. The residual stress was reduced at the bond-coat/substrate interface after HIP post-treatment. The surface-to-interface residual stress in as-sprayed coatings vary from +230 (tensile) to −35 MPa (compression) depending upon depth within the coating. The degree of residual stress in the coatings after HIP post-treatment decreased with low tensile stress near the interface region which gradually decreased and became compressive near the upper surface coating.

Table 2 Experimental conditions for electron-beam surface melting of HVOF-sprayed Ni-based (Inconel 625) coatings on copper substrates [69]. Fuel (Kerosene) Oxygen flow rate Carrier gas flow

15–40 (l/min) 50–60 (l/min) 20–30 (l/min) Ar

Spray distance Surface roughness Particle size

150–350 (mm) 40–60 (μm) Ra 20–50 (μm)

5. Spark-plasma sintering (SPS) The SPS process is a relatively new post-spray treatment based on sintering and densification. As compared to conventional sintering techniques, SPS can be used for fast sintering of metal and ceramic coatings at lower temperatures. Heating rates are between 100 and 200 °C/min for SPS treatment of high-velocity sprayed coatings. In the SPS method, a graphite die set, that may also be packed with the coating material and/or additive alloying powders, are located between the upper and lower electrodes. To provide heating and sintering of sprayed coatings, external pulsed electrical power source is applied. Both lower and upper graphite electrodes apply pressure during the treatment. Because of the direct heating of the graphite dies at large spark pulse currents, the SPS treatment results in very high thermal efficiency with uniform heating. Thus, SPS provides a homogenous, adherent coating with lower porosity. Grosdidier et al. [76] investigated the characteristics of the SPS technique for HVOF-sprayed Y2O3-reinforced Fe-40Al (at.%) to obtain a bulk nanostructured layer. SPS treatment at low temperature shows a thin oxide layer (less than 1 μm) at the surface of the powder particle coating. In contrast, SPS treatment at higher temperature tends to break up of the oxide layer by plastic deformation. Therefore, an acceptable layer density can be achieved using combined SPS post-treatment through both plastic flow and local liquid sintering at different processing temperatures.

Fig. 12. The effect of the electron-beam energy density on the penetration depth of a 650-μm-thick sprayed Ni-12.4Cr-2.5Fe-4.5Si-3B-0.6C alloy on a copper substrate (P = beam power) (Reproduced with permission from [69]).

that the microstructure of the as-sprayed coatings can be changed from lamellar to granular due to HIP treatment. They also reported that good metallurgical bonding can be obtained between splats and coating/ substrate interfaces [70,72]. Industrially, HIP post-treatment has been applied for modifying the surfaces of as-sprayed coatings. In this process, a constant compression from all directions is applied to the as-sprayed coated part at high temperatures using a hot fluid medium leading to the creation of a dense coating. The HIP process results in improved metallurgical bonding at the interface, decreased residual compressive stress, structural phase changes from amorphous to crystalline, and elimination of coating anisotropy to provide homogeneous coating properties [72–75]. The effect of the HIP process is clearly noticeable when comparing the coating microstructure before and after treatment. Fig. 13 illustrates the effect of HIP treatment on structure and porosity of NbC-reinforced X220CrVMo13–4 steel composite coatings applied by HVOF [72]. Results indicated that the HIP treatment reduced the structural porosity and resulted in a homogenous and dense microstructure in the coatings. In addition, interlamellar microstructure in the as-sprayed coating is transformed to granular after the HIP process [72]. Abdel-Samad et al. [70] investigated the effect of HIP post-treatment on the high temperature fatigue behavior of plasma-sprayed Mg2Zr5O12 coatings. After the HIP process, considerable plastic deformation occurs at the interfaces of coating/bond-coat and bond-coat/

6. Some drawbacks of post treatments Post-treatment processing of thermally-sprayed coatings may cause some problems for sprayed layers. Some disadvantages such as cracking of the coated layer, formation of oxide phases, detachment of the coating, and inhomogeneity in coating composition have been reported [5,39,77]. Guilemany et al. [39] examined the abrasive wear behavior of 48

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Fig. 13. (a) Microstructure of as-sprayed, and HIP-treated for 2 h at1150 °C, NbC reinforced X220CrVMo13–4 steel composite coatings with 30 vol% NbC. (b) assprayed coating; (c) and (d) HIP-treated coating without encapsulation; (e) and (f) HIP treated coating with encapsulation (Reproduced with permission from [72]).

Cr3C2-25wt%NiCr coatings before and after furnace heat treatment for 1 h at 723, 1033, and 1153 K. During the heat treatment, metal oxide formed on the coating. The existence of these oxides may improve the hardness of the coating heat treated at 1033 K. The main problem is that heat treating in oxidizing atmospheres can lead to structural decohesion and have a negative effect on abrasive wear resistance compared to as-sprayed Cr3C2-25wt%NiCr coatings. Stewart et al. [5] concluded that HVOF-sprayed WC-17wt%Co coatings heat treated above 550 °C for 1 h in an argon atmosphere can cause structural cracks in the sprayed layer. They showed that the main reason for crack formation is recrystallization of an amorphous phase in the coating which affects the stress state. Vostrak et al. [78] showed that to prevent coating deformation and cracking during laser posttreatment process of HVOF-sprayed Stellite coatings, preheating of the steel substrate is necessary. 7. Some advances in HVOF and related treatments 7.1. Laser surface alloying Laser alloying involves near-surface melting and/or alloying of the coating using a high-energy laser beam [79–81]. Laser surface alloying is a modern and effective process which is used to improve properties of sprayed coatings, especially for wear-resistant cermet coatings. The purpose of this method is to melt, totally or partially, the ceramic/ cermet coating with the laser beam while adding alloying elements or compounds to the melted region to form a homogeneous alloy coating with less porosity [56,57]. Researchers have reported positive results

Fig. 14. Residual stress (σ) profile as a function of depth for a) plasma-sprayed and b) HIP post-treated Mg2Zr5O12 coatings on a hot-worked steel H11 substrate. The HIP post-deposition treatment was at 1100 °C at a pressure of 100 MPa for 6 h under argon atmosphere (Reproduced with permission from [70]).

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based upon laser surface alloying of carbide coatings [59].

possibility for new application markets.

7.2. Laser hybrid spraying

References

Laser-assisted thermal spraying, also named laser hybrid spraying, is a single-stage spray-coating technique, in which a relatively high power laser beam is appended to a thermal spraying apparatus. The goal of adding the laser beam is to enhance the heating level of the thermal spray gun and to create a deposit with higher density and adhesion than achieved by conventional thermal spraying [82,83]. The supplemental heating supplied by the laser beam provides good metallurgical bonding throughout the coating layer and at the coating/substrate interface [82,84]. In most of the early experiments, the thermal spray spot and the laser beam have been adjusted to be very near to each other. However, in recent studies, the distance between spray spot and laser beam can be adjusted in order to better focusing on treated zone [85,86]. Laser-assisted spraying and post laser-treatment of HVOF-sprayed Ni- and Crbased coatings has been investigated by Suutala et al. [87]. They indicated that structural cracks propagating perpendicular to the laser scanning direction during laser-assisted spraying were reduced during post-deposition.

[1] M. Benegra, A.L.B. Santana, O. Maranho, G. Pintaude, Effect of heat treatment on wear resistance of nickel aluminide coatings deposited by HVOF and PTA, J. Therm. Spray Technol. 24 (6) (2015) 1111–1116. [2] G. Bolelli, L. Lusvarghi, M. Barletta, Heat treatment effects on the corrosion resistance of some HVOF-sprayed metal alloy coatings, Surf. Coat. Technol. 202 (19) (2008) 4839–4847. [3] G. Bolelli, L. Lusvarghi, T. Varis, E. Turunen, M. Leoni, P. Scardi, C.L. AzanzaRicardo, M. Barletta, Residual stresses in HVOF-sprayed ceramic coatings, Surf. Coat. Technol. 202 (19) (2008) 4810–4819. [4] J.R. García, J.E. Fernández, J.M. Cuetos, F.G. Costales, Fatigue effect of WC coatings thermal sprayed by HVOF and laser treated, on medium carbon steel, Eng. Fail. Anal. 18 (7) (2011) 1750–1760. [5] D.A. Stewart, P.H. Shipway, D.G. McCartney, Influence of heat treatment on the abrasive wear behaviour of HVOF sprayed WC–Co coatings1, Surf. Coat. Technol. 105 (1–2) (1998) 13–24. [6] Z.Y. Taha-al, M.S. Hashmi, B.S. Yilbas, Effect of WC on the residual stress in the laser treated HVOF coating, J. Mater. Process. Technol. 209 (7) (2009) 3172–3181. [7] J.C. Tan, M.S.J. Hashmi, High velocity oxygen fuel (HVOF) thermal spray: prospect and limitation for engineering application, in: A.S. Wifi (Ed.), Current Advances in Mechanical Design and Production VI, Oxford, Pergamon, 1995, pp. 27–33. [8] B.S. Yilbas, I. Al-Zaharnah, A. Sahin, HVOF Coating and Characterization, in Flexural Testing of Weld Site and HVOF Coating Characteristics, Springer Berlin Heidelberg, Berlin, Heidelberg, 2014, pp. 103–156. [9] J. Yuan, C. Ma, S. Yang, Z. Yu, H. Li, Improving the wear resistance of HVOF sprayed WC-Co coatings by adding submicron-sized WC particles at the splats' interfaces, Surf. Coat. Technol. 285 (2016) 17–23. [10] G.-J. Yang, C.-J. Li, F. Han, A. Ohmori, Microstructure and photocatalytic performance of high velocity oxy-fuel sprayed TiO2 coatings, Thin Solid Films 466 (1) (2004) 81–85. [11] C.-R. Ciubotariu, D. Frunzăverde, G. Mărginean, V.-A. Șerban, A.-V. Bîrdeanu, Optimization of the laser remelting process for HVOF-sprayed Stellite 6 wear resistant coatings, Opt. Laser Technol. 77 (2016) 98–103. [12] P. Fauchais, Suspension and solution plasma or HVOF spraying, J. Therm. Spray Technol. 17 (1) (2008) 1–3. [13] C. Lyphout, P. Nylén, A. Manescu, T. Pirling, Residual stresses distribution through thick HVOF sprayed inconel 718 coatings, J. Therm. Spray Technol. 17 (5) (2008) 915–923. [14] B.S. Mann, High power diode laser-treated HP-HVOF and twin wire arc-sprayed coatings for fossil fuel power plants, J. Mater. Eng. Perform. 22 (8) (2013) 2191–2200. [15] L. Gil, M.H. Staia, Influence of HVOF parameters on the corrosion resistance of NiWCrBSi coatings, Thin Solid Films 420-421 (2002) 446–454. [16] A.M. Venter, O.P. Oladijo, V. Luzin, L.A. Cornish, N. Sacks, Performance characterization of metallic substrates coated by HVOF WC–Co, Thin Solid Films 549 (2013) 330–339. [17] S. Bose, High Temperature Coatings, Butterworth-Heinemann, 2017. [18] M. Li, P.D. Christofides, Modeling and control of high-velocity oxygen-fuel (HVOF) thermal spray: a tutorial review, J. Therm. Spray Technol. 18 (5) (2009) 753. [19] T.S. Sidhu, S. Prakash, R.D. Agrawal, State of the art of HVOF coating investigations: a review, Mar. Technol. Soc. J. 39 (2) (2005) 53–64. [20] C.H. Lee, K.O. Min, Effects of heat treatment on the microstructure and properties of HVOF-sprayed Ni–Cr–W–Mo–B alloy coatings, Surf. Coat. Technol. 132 (1) (2000) 49–57. [21] J.M. Guilemany, J. Fernandez, J. Delgado, Influence of thermal treatments in the corrosion behaviour of HVOF Cr3C2-NiCr coatings, in: C.C. Berndt, K.A. Khor (Eds.), Thermal Spray 2001: New Surfaces for a New Millennium, ASM International: Materials Park, Ohio, 2001, pp. 1165–1169. [22] J. Lesage, D. Chicot, Role of residual stresses on interface toughness of thermally sprayed coatings, Thin Solid Films 415 (1) (2002) 143–150. [23] M.H. Staia, E. Ramos, A. Carrasquero, A. Roman, J. Lesage, D. Chicot, G. Mesmacque, Effect of substrate roughness induced by grit blasting upon adhesion of WC-17% Co thermal sprayed coatings, Thin Solid Films 377-378 (2000) 657–664. [24] C.M. Nygårds, K.W. White, K. Ravi-Chandar, Strength of HVOF coating–substrate interfaces, Thin Solid Films 332 (1) (1998) 185–188. [25] F. Ghadami, M.H. Sohi, S. Ghadami, Effect of bond coat and post-heat treatment on the adhesion of air plasma sprayed WC-Co coatings, Surf. Coat. Technol. 261 (2015) 289–294. [26] A. Hjörnhede, A. Nylund, Adhesion testing of thermally sprayed and laser deposited coatings, Surf. Coat. Technol. 184 (2) (2004) 208–218. [27] B. Gudmundsson, B.E. Jacobson, The effect of silicon and zirconium additions on the microstructure of vacuum-plasma-sprayed Co-Ni-Cr-AI-Y coatings, Mater. Sci. Eng. A A 108 (1989) 73–80. [28] L. Janka, J. Norpoth, R. Trache, L.-M. Berger, Influence of heat treatment on the abrasive wear resistance of a Cr3C2NiCr coating deposited by an ethene-fuelled HVOF spray process, Surf. Coat. Technol. 291 (2016) 444–451. [29] J.A. Picas, M. Punset, S. Menargues, E. Martín, M.T. Baile, Microstructural and tribological studies of as-sprayed and heat-treated HVOF Cr3C2–CoNiCrAlY coatings with a CoNiCrAlY bond coat, Surf. Coat. Technol. 268 (2015) 317–324. [30] F. Ghadami, S. Ghadami, H. Abdollah-Pour, Structural and oxidation behavior of atmospheric heat treated plasma sprayed WC–Co coatings, Vacuum 94 (2013)

7.3. Optimized duplex coating systems An optimized duplex coating, consisting of a thick interlayer applied by thermal spraying (e.g., HVOF spraying) and a thin ceramic top layer using a high-energy-beam surface coating method (e.g., PVD) has been proposed [88–93]. HVOF spraying is primarily useful for the deposition of the thick interlayer coating because the method is extremely flexible in material selection for both coating and substrate and hence the HVOF coating process has several industrial applications on a wide range of simple and complex-shaped industrial parts [91]. Specifically, HVOFsprayed cermet coatings seem highly promising as a thick layer, on account of their high hardness, higher toughness, appropriate elastic modulus, and high deposition rate [25,30,60,69,83,94]. In addition, the wear and toughness properties of the HVOF-sprayed coating can be enhanced by the PVD top coating [91]. Picas et al. [90] reported that a thick HVOF spray interlayer WC-Co-Cr coating provides good support for a harder top TiN or TiAlN coating, allowing a good distribution of contact coating stresses and preventing delamination. 8. Summary and conclusions The HVOF spraying method is widely used for many industrial applications and has advantages over other thermal-spray technologies for deposition of many materials such as carbide cermet coatings. Based on the present study, it was concluded that post-deposition heat treatment typically improves wear properties and the adhesive strength of HVOF carbide coatings. Post heat treatment is generally applied to change and modify the phase composition and microstructure of HVOF-sprayed coatings. Heat treatment of as-sprayed coatings is also used for stress relief. It has been reported that many types of HVOF coatings can be modified by remelting, using high-energy laser or electron beam postdeposition surface treatment processes. Such post surface treatments can also be used to intermix the coating with substrate material to form new hard dense coating material which has enhanced and improved coating/substrate adhesion. Both the beam energy density and the scan speed must be controlled to achieve the desired amount of remelting. HIP post-treatment for HVOF coatings leads to a decrease in structural porosity and as a result, a dense coating with a uniform microstructure in which interlamellar boundaries in as-sprayed HVOF coatings disappear. The SPS post-spray process is also useful for improving coating microstructure. The growth of new generations of HVOF and properly chosen post-spray techniques will undoubtedly open the 50

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[61] S.H. Zhang, J.H. Yoon, M.X. Li, T.Y. Cho, Y.K. Joo, J.Y. Cho, Influence of CO2 laser heat treatment on surface properties, electrochemical and tribological performance of HVOF sprayed WC–24%Cr3C2–6%Ni coating, Mater. Chem. Phys. 119 (3) (2010) 458–464. [62] J. Tuominen, P. Vuoristo, T. Mäntylä, S. Ahmaniemi, J. Vihinen, P.H. Andersson, Corrosion behavior of HVOF-sprayed and Nd-YAG laser-remelted high-chromium, nickel-chromium coatings, J. Therm. Spray Technol. 11 (2) (2002) 233–243. [63] M. Oksa, E. Turunen, T. Varis, Sealing of thermally sprayed coatings, Surf. Eng. 20 (4) (2004) 251–254. [64] S. Armada, B.G. Tilset, M. Pilz, R. Liltvedt, H. Bratland, N. Espallargas, Sealing HVOF thermally sprayed WC-CoCr coatings by sol-gel methods, J. Therm. Spray Technol. 20 (4) (2011) 918–926. [65] J. Knuuttila, P. Sorsa, T. Mäntylä, J. Knuuttila, P. Sorsa, Sealing of thermal spray coatings by impregnation, J. Therm. Spray Technol. 8 (2) (1999) 249–257. [66] B.S. Yilbas, A.F.M. Arif, M.A. Gondal, HVOF coating and laser treatment: threepoint bending tests, J. Mater. Process. Technol. 164–165 (2005) 954–957. [67] I.D. Utu, G. Marginean, Effect of electron beam remelting on the characteristics of HVOF sprayed Al2O3-TiO2 coatings deposited on titanium substrate, Colloids Surf. A Physicochem. Eng. Asp. 526 (1) (2017) 70–75. [68] P. Gavendová, J. Čížek, J. Čupera, M. Hasegawa, I. Dlouhý, Microstructure modification of CGDS and HVOF sprayed CoNiCrAlY bond coat remelted by electron beam, Procedia Mater. Sci. 12 (2016) 89–94. [69] H. Hamatani, Y. Miyazaki, Optimization of an electron beam remelting of HVOF sprayed alloys and carbides, Surf. Coat. Technol. 154 (2–3) (2002) 176–181. [70] A. Abdel-Samad, E. Lugscheider, K. Bobzin, M. Maes, The influence of hot isostatic pressing on plasma sprayed coatings properties, Surf. Coat. Technol. 201 (3–4) (2006) 1224–1227. [71] R.S. Lima, K.A. Khor, H. Li, P. Cheang, B.R. Marple, HVOF spraying of nanostructured hydroxyapatite for biomedical applications, Mater. Sci. Eng. A 396 (1) (2005) 181–187. [72] B. Rajasekaran, G. Mauer, R. Vaßen, A. Röttger, S. Weber, W. Theisen, Development of cold work tool steel based-MMC coating using HVOF spraying and its HIP densification behaviour, Surf. Coat. Technol. 204 (23) (2010) 3858–3863. [73] V. Stoica, R. Ahmed, M. Golshan, S. Tobe, Sliding wear evaluation of hot isostatically pressed thermal spray ceramet coatings, J. Therm. Spray Technol. 13 (1) (2004) 93–107. [74] V. Stoica, R. Ahmed, T. Itsukaichi, S. Tobe, Sliding wear evaluation of hot isostatically pressed (HIPed) thermal spray cermet coatings, Wear 257 (11) (2004) 1103–1124. [75] L.Y. Zheng, W.H. Xiong, X.M. Yan, Microstructure and mechanical properties of coated cermets modified by hot isostatically pressing, Surf. Coat. Technol. 201 (9) (2007) 5198–5202. [76] T. Grosdidier, G. Ji, F. Bernard, E. Gaffet, Z.A. Munir, S. Launois, Synthesis of bulk FeAl nanostructured materials by HVOF spray forming and spark plasma sintering, Intermetallics 14 (10−11) (2006) 1208–1213. [77] V.V. Sobolev, J.M. Guilemany, Oxidation of coatings in thermal spraying, Mater. Lett. 37 (4–5) (1998) 231–235. [78] M. Vostřák, J. Tesař, Š. Houdková, E. Smazalová, M. Hruška, Diagnostic of laser remelting of HVOF sprayed Stellite coatings using an infrared camera, Surf. Coat. Technol. 318 (2017) 360–364. [79] D. Ravnikar, R.S. Rajamure, U. Trdan, N.B. Dahotre, J. Grum, Electrochemical and DFT studies of laser-alloyed TiB2/TiC/Al coatings on aluminium alloy, Corros. Sci. 136 (2018) 18–27. [80] Z.K. Chen, C. Meng, J.X. Dong, T. Zhou, X. Tong, H. Zhou, The comparative study of W/Cr addition in Fe-base by laser surface alloying on fatigue wear resistance of GCI, Surf. Coat. Technol. 286 (2016) 25–35. [81] Y. Wu, A.H. Wang, Z. Zhang, R.R. Zheng, H.B. Xia, Y.N. Wang, Laser alloying of Ti–Si compound coating on Ti–6Al–4V alloy for the improvement of bioactivity, Appl. Surf. Sci. 305 (2014) 16–23. [82] R.K. Kumarr, M. Kamaraj, S. Seetharamu, S. Anand Kumar, A pragmatic approach and quantitative assessment of silt erosion characteristics of HVOF and HVAF processed WC-CoCr coatings and 16Cr5Ni steel for hydro turbine applications, Mater. Des. 132 (2017) 79–95. [83] S.C. Tjong, Performance of laser-consolidated plasma-spray coatings on Fe-28Mn7A1-1C alloy, Thin Solid Films 274 (1) (1996) 95–100. [84] J. Morales-Hernández, A. Mandujano-Ruiz, F. Castañeda-Zaldivar, R. Antaño-López, J.T. González, High temperature corrosion resistance of coatings deposited by Hvof for application in steam turbines, Procedia Chem. 12 (2014) 80–91. [85] G. Antou, G. Montavon, F. Hlawka, A. Cornet, C. Coddet, F. Machi, Modification of thermal barrier coating architecture by in situ laser remelting, J. Eur. Ceram. Soc. 26 (16) (2006) 3583–3597. [86] H. Hiraga, T. Inoue, A. Matsunawa, H. Shimura, Effect of laser irradiation condition on bonding strength in laser plasma hybrid spraying, Surf. Coat. Technol. 138 (2) (2001) 284–290. [87] J. Suutala, J. Tuominen, P. Vuoristo, Laser-assisted spraying and laser treatment of thermally sprayed coatings, Surf. Coat. Technol. 201 (5) (2006) 1981–1987. [88] E. Bemporad, M. Sebastiani, D. De Felicis, F. Carassiti, R. Valle, F. Casadei, Production and characterization of duplex coatings (HVOF and PVD) on Ti–6Al–4V substrate, Thin Solid Films 515 (1) (2006) 186–194. [89] W. Chen, T. Mao, B. Zhang, S. Zhang, X. Meng, Designs and preparation of advanced HVOF-PVD duplex coating by combination of HVOF and arc ion plating, Surf. Coat. Technol. 304 (2016) 125–133. [90] J.A. Picas, S. Menargues, E. Martin, C. Colominas, M.T. Baile, Characterization of duplex coating system (HVOF + PVD) on light alloy substrates, Surf. Coat. Technol. 318 (2017) 326–331. [91] F. Pougoum, J. Qian, M. Laberge, L. Martinu, J. Klemberg-Sapieha, Z. Zhou, K.Y. Li,

64–68. [31] G. Bolelli, V. Cannillo, L. Lusvarghi, M. Montorsi, F.P. Mantini, M. Barletta, Microstructural and tribological comparison of HVOF-sprayed and post-treated M–Mo–Cr–Si (M = Co, Ni) alloy coatings, Wear 263 (7–12) (2007) 1397–1416. [32] H.E. Eaton, R.C. Novak, Sintering studies of plasma-sprayed zirconia, Surf. Coat. Technol. 32 (1) (1987) 227–236. [33] L.C. Lim, S.C. Lim, M.O. Lai, S.F. Chong, S. Alli, Annealing of plasma-sprayed WCCo coating, Surf. Coat. Technol. 79 (1) (1996) 151–161. [34] J. Frodelius, E.M. Johansson, J.M. Córdoba, M. Odén, P. Eklund, L. Hultman, Annealing of thermally sprayed Ti2AlC coatings, Int. J. Appl. Ceram. Technol. 8 (1) (2011) 74–84. [35] S.S. Chatha, H.S. Sidhu, B.S. Sidhu, The effects of post-treatment on the hot corrosion behavior of the HVOF-sprayed Cr3C2–NiCr coating, Surf. Coat. Technol. 206 (19–20) (2012) 4212–4224. [36] M.J. Filiaggi, R.M. Pilliar, N.A. Coombs, Post-plasma-spraying heat treatment of the HA coating/Ti-6Al-4V implant system, J. Biomed. Mater. Res. 27 (2) (1993) 191–198. [37] J. Nerz, B. Kushner, A. Rotolico, Microstructural evaluation of tungsten carbidecobalt coatings, J. Therm. Spray Technol. 1 (2) (1992) 147–152. [38] M. Heydarzadeh Sohi, F. Ghadami, Comparative tribological study of air plasma sprayed WC–12%Co coating versus conventional hard chromium electrodeposit, Tribol. Int. 43 (5) (2010) 882–886. [39] J.M. Guilemany, J.M. Miguel, S. Vizcaı́no, C. Lorenzana, J. Delgado, J. Sánchez, Role of heat treatments in the improvement of the sliding wear properties of Cr3C2–NiCr coatings, Surf. Coat. Technol. 157 (2–3) (2002) 207–213. [40] J.K.N. Murthy, S. Bysakh, K. Gopinath, B. Venkataraman, Microstructure dependent erosion in Cr3C2–20(NiCr) coating deposited by a detonation gun, Surf. Coat. Technol. 202 (1) (2007) 1–12. [41] J. Fernández, M. Gaona, J.M. Guilemany, Effect of heat treatments on HVOF hydroxyapatite coatings, J. Therm. Spray Technol. 16 (2) (2007) 220–228. [42] M. Afzal, A.N. Khan, T.B. Mahmud, T.I. Khan, M. Ajmal, Effect of laser melting on plasma sprayed WC-12wt.%Co coatings, Surf. Coat. Technol. 266 (2015) 22–30. [43] C. Cui, F. Ye, G. Song, Laser surface remelting of Fe-based alloy coatings deposited by HVOF, Surf. Coat. Technol. 206 (8–9) (2012) 2388–2395. [44] G. Jin, Z. Cai, Y. Guan, X. Cui, Z. Liu, Y. Li, M. Dong, D. Zhang, High temperature wear performance of laser-cladded FeNiCoAlCu high-entropy alloy coating, Appl. Surf. Sci. 445 (2018) 113–122. [45] D. Kong, B. Zhao, Effects of loads on friction–wear properties of HVOF sprayed NiCrBSi alloy coatings by laser remelting, J. Alloys Compd. 705 (2017) 700–707. [46] M. Riabkina-Fishman, E. Rabkin, P. Levin, N. Frage, M.P. Dariel, A. Weisheit, R. Galun, B.L. Mordike, Laser produced functionally graded tungsten carbide coatings on M2 high-speed tool steel, Mater. Sci. Eng. A 302 (1) (2001) 106–114. [47] R. Shoja-Razavi, Laser surface treatment of stellite 6 coating deposited by HVOF on 316L alloy, J. Mater. Eng. Perform. 25 (7) (2016) 2583–2595. [48] S.-H. Zhang, T.-Y. Cho, J.-H. Yoon, M.-X. Li, P.W. Shum, S.-C. Kwon, Investigation on microstructure, surface properties and anti-wear performance of HVOF sprayed WC–CrC–Ni coatings modified by laser heat treatment, Mater. Sci. Eng. B 162 (2) (2009) 127–134. [49] Š. Houdková, Z. Pala, E. Smazalová, M. Vostřák, Z. Česánek, Microstructure and sliding wear properties of HVOF sprayed, laser remelted and laser clad Stellite 6 coatings, Surf. Coat. Technol. 318 (1) (2017) 129–141. [50] M.A. Uusitalo, P.M.J. Vuoristo, T.A. Mäntylä, High temperature corrosion of coatings and boiler steels in reducing chlorine-containing atmosphere, Surf. Coat. Technol. 161 (2–3) (2002) 275–285. [51] B.S. Yilbas, S.S. Akhtar, Laser re-melting of HVOF coating with WC blend: thermal stress analysis, J. Mater. Process. Technol. 212 (12) (2012) 2569–2577. [52] H. Chen, C. Xu, Q. Zhou, I.M. Hutchings, P.H. Shipway, J. Liu, Micro-scale abrasive wear behaviour of HVOF sprayed and laser-remelted conventional and nanostructured WC–Co coatings, Wear 258 (1–4) (2005) 333–338. [53] Z. Liu, J. Cabrero, S. Niang, Z.Y. Al-Taha, Improving corrosion and wear performance of HVOF-sprayed Inconel 625 and WC-Inconel 625 coatings by high power diode laser treatments, Surf. Coat. Technol. 201 (16–17) (2007) 7149–7158. [54] M. Rakhes, E. Koroleva, Z. Liu, Improvement of corrosion performance of HVOF MMC coatings by laser surface treatment, in: S. Hinduja, L. Li (Eds.), Proceedings of the 36th International MATADOR Conference, Springer London, London, 2010, pp. 531–534. [55] Y. Yue, J. Hu, Rapidly solidified microstructure in laser alloyed Ni-Al layer by TEM, STEM z-contrast and HRTEM techniques, J. Mater. Sci. Technol. 30 (12) (2014) 1301–1303. [56] J. Wilden, H. Frank, Formation of intermetallic phases by laser alloying of thermally sprayed Ti and Al coatings for enhanced wear resistance of lightweight materials, in: R. Basil, C. Marple, C. Moreau (Eds.), Thermal Spray 2003: Advancing the Science and Applying the Technology, ASM International, Orlando, Florida, USA, 2003, pp. 475–483. [57] M. Afzal, M. Ajmal, A. Nusair Khan, A. Hussain, R. Akhter, Surface modification of air plasma spraying WC–12%Co cermet coating by laser melting technique, Opt. Laser Technol. 56 (2014) 202–206. [58] A. Gisario, M. Puopolo, S. Venettacci, F. Veniali, Improvement of thermally sprayed WC–Co/NiCr coatings by surface laser processing, Int. J. Refract. Met. Hard Mater. 52 (2015) 123–130. [59] J. Mateos, J.M. Cuetos, E. Fernández, R. Vijande, Tribological behaviour of plasmasprayed WC coatings with and without laser remelting, Wear 239 (2) (2000) 274–281. [60] M.F. Teixeira, V.A. Veit Schmachtenberg, G. Tontini, G.D. Lana Semione, W.L. Weingaertner, V. Drago, Laser composite surfacing of A681 steel with WC + Cr + Co for improved wear resistance, J. Mater. Res. Technol. 6 (1) (2017) 33–39.

51

Thin Solid Films 678 (2019) 42–52

F. Ghadami and A. Sabour Rouh Aghdam

[92]

[93]

[94]

[95]

[96] G. Bolelli, A. Colella, L. Lusvarghi, P. Puddu, R. Rigon, P. Sassatelli, V. Testa, Properties of HVOF-sprayed TiC-FeCrAl coatings, Wear 418-419 (2019) 36–51. [97] P. Sassatelli, G. Bolelli, M. Lassinantti Gualtieri, E. Heinonen, M. Honkanen, L. Lusvarghi, T. Manfredini, R. Rigon, M. Vippola, Properties of HVOF-sprayed stellite-6 coatings, Surf. Coat. Technol. 338 (2018) 45–62. [98] B.S. Mann, V. Arya, Abrasive and erosive wear characteristics of plasma nitriding and HVOF coatings: their application in hydro turbines, Wear 249 (5) (2001) 354–360. [99] K.S. Tan, J.A. Wharton, R.J.K. Wood, Solid particle erosion–corrosion behaviour of a novel HVOF nickel aluminium bronze coating for marine applications—correlation between mass loss and electrochemical measurements, Wear 258 (1) (2005) 629–640. [100] R.G. Bonora, H.J.C. Voorwald, M.O.H. Cioffi, G.S. Junior, L.F.V. Santos, Fatigue in AISI 4340 steel thermal spray coating by HVOF for aeronautic application, Procedia Eng. 2 (1) (2010) 1617–1623.

S. Savoie, R. Schulz, Investigation of Fe3Al-based PVD/HVOF duplex coatings to protect stainless steel from sliding wear against alumina, Surf. Coat. Technol. 350 (2018) 699–711. F. Pougoum, J. Qian, L. Martinu, J. Klemberg-Sapieha, Z. Zhou, K.Y. Li, S. Savoie, R. Lacasse, E. Potvin, R. Schulz, Study of corrosion and tribocorrosion of Fe3Albased duplex PVD/HVOF coatings against alumina in NaCl solution, Surf. Coat. Technol. 357 (2019) 774–783. W. Tillmann, D. Stangier, L. Hagen, P. Schröder, M. Krabiell, Influence of the WC grain size on the properties of PVD/HVOF duplex coatings, Surf. Coat. Technol. 328 (2017) 326–334. M.S. Zoei, M.H. Sadeghi, M. Salehi, Effect of grinding parameters on the wear resistance and residual stress of HVOF-deposited WC–10Co–4Cr coating, Surf. Coat. Technol. 307 (Part A) (2016) 886–891. S.-L. Zhang, C.-X. Li, C.-J. Li, G.-J. Yang, M. Liu, Application of high velocity oxygen fuel flame (HVOF) spraying to fabrication of La0.8Sr0.2Ga0.8Mg0.2O3 electrolyte for solid oxide fuel cells, J. Power Sources 301 (2016) 62–71.

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