As-sprayed Coatings Research Articles

Mathematical modelling and validation of the mechanical properties of HVOF-sprayed WC-12Co coatings considering the phase composition and defects generated at different process parameters

The thermal spraying process involves many machine-based parameters or independent variables (fuel flow rate, oxygen flow rate, powder feed rate, stand-off distance and many others in the HVOF process). Any combination of these parameters produces only two measurable responses, namely, particle temperature and particle velocity. In this current work, first, the machine-based spray parameters are varied in specific ways to produce different particle temperatures at near-constant velocities and vice-versa. This approach has reduced the number of independent variables to only two. These two factors further influence the in-flight particle reactions, thereby affecting the phase composition and the microstructural defects in the as-sprayed coatings. The X-ray diffraction patterns of the as-sprayed coatings revealed the presence of W2C, W, Co3W9C4, and amorphous Co-W-C phases apart from the WC phase. The W2C and W originated owing to the decarburization of the WC phase. On the other hand, the dissolution of the carbide in molten binder led to the formation of Co3W9C4 and the amorphous Co-W-C phase. The nano-hardness of both the carbide and binder phases increased with an increase in particle temperature owing to a higher degree of decarburization and carbide dissolution respectively. The porosity was the most significant micro-structural defect present in the as-sprayed coatings. An increase in either particle temperature or velocity reduced the porosity present in the as-sprayed coatings. The mechanical properties (microhardness, elastic modulus and indentation fracture toughness) tend to improve as porosity decreases. A comprehensive mathematical model was proposed to predict the mechanical properties of as-sprayed coatings as a function of composition and defects. Finally, the process maps of mechanical properties were plotted in temperature-velocity space.

Strong-bonding self-lubricating nickel-matrix coatings at a wide temperature range

The weak bond strength that affects self-lubricating coatings across a wide temperature range severely limits their application on the surfaces of high-temperature mechanical parts. To solve this problem, the strong-bonding self-lubricating NiCrAlY-Ag-BaF2/CaF2 coatings were produced by detonation spraying. The as-sprayed coatings exhibit high bond strengths of 65 – 75 MPa by controlling the formation of semi-molten state particles and in-situ oxides. The composite coating with 10 wt% fluoride eutectic and 20 wt% Ag shows the favorable tribological properties at a wide temperature range from room temperature to 900 °C. The combined effects, including the formation of the lubricating Ag film, the synergistic lubricating effect of BaF2/CaF2 and CaCrO4 formed by tribo-chemistry at high temperature, tribo-glaze of oxides, and tribo-transfer film, contribute to favorable lubrication at a wide temperature range.

The influence of the elemental valence on the thermophysical properties of high entropy (La0.2Sm0.2Eu0.2Yb0.2Y0.2)2(Ce0.5Zr0.5)2O7 coatings for thermal barrier application

Commercial yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs) are no longer sufficient for the stringent demands in the high operating temperature and excellent thermophysical properties of the high-performance gas turbines and aviation engines. High entropy ceramics, distinguished by their superior thermophysical properties, have risen as a potential substitute for TBCs. However, the realm of atmospheric plasma spraying (APS) in the creation of these advanced TBCs remains relatively unexplored. In this work, three distinct high-entropy (La0.2Sm0.2Eu0.2Yb0.2Y0.2)2(Ce0.5Zr0.5)2O7 coatings were developed through the meticulous alteration of spraying power. The effects of spraying power on the microstructure, valence states of constituent elements and thermophysical properties of the coatings were deeply investigated. The results indicated that all the as-sprayed coatings exhibit a defect fluorite structure. Spraying power does have a significant impact on the thermophysical properties of the coatings. With the decrease in spraying power, the thermal conductivity of the coatings shows a progressive reduction, whereas the coefficient of thermal expansion (CTE) exhibits a gradual increase. This trend is closely related to valence states of constituent elements in the coatings. The resulting coating exhibits a low thermal conductivity of 0.54 W⋅m-1⋅K-1, a high CTE of 11.22×10-6·K-1 and a superior phase stability at elevated temperatures.

Effect of laser remelting treatment on the tribological properties of plasma sprayed WC–Cr3C2–Ni reinforced NiCrBSi coatings

The incorporation of single hard phases, such as tungsten carbide (WC), and chromium carbide (Cr3C2), into NiCrBSi coatings effectively enhances their performance, thereby fulfilling the tough requirements of high-temperature friction condition. To investigate the collaborative reinforcing effect of multiple hard phases and further enhance the high-temperature wear resistance of the coating, a novel WC–Cr3C2–NiCrBSi binary hard phase reinforced coating was developed and subjected to laser remelting treatment. The as-sprayed coatings demonstrate a distinct lamellar structure that diminishes upon remelting, resulting in significant densification of the coatings and a reduction in porosity to 0.3 %. The microhardness of the remelting coatings is reduced by about 10 % due to dilution of the coating by the lower hardness of the base material. However, during the laser remelting process, WC and Cr3C2 decompose and dissolve into the alloying binder phase of the coating, serving as solid solution agents. In cases where the concentration of W elements is elevated, a considerable precipitation of W-rich carbide is observed within the coating. Notably, N30 coatings demonstrate remarkable were resistance at both ambient and 700 °C, resulting in a significant reduction in wear volume by 86.5 % and 55.2 % respectively in comparison to original NiCrBSi coatings. The remelting treatment enhances the densification of the coating, improves the homogeneity of the microstructure, and reduces the coefficient of friction of the coating. The laser-remelted coating exhibits a decreased wear volume, attributed to a transition in its wear mechanism from fatigue-dominated to abrasive wear at room temperature. Conversely, under frictional conditions at 700 °C, the remelted coating experiences an augmented wear volume, attributed to severe abrasive wear.

Molten CMAS resistance strategy for PS-PVD TBCs based on laser textured and Al-modified bionic structure

Plasma spray-physical vapor deposition (PS-PVD) is a promising third-generation thermal barrier coatings (TBCs) technique. Feather-like columnar TBCs with excellent strain tolerance and low thermal conductivity can be achieved using PS-PVD. However, molten CMAS (CaO–MgO–Al2O3–SiO2) can penetrate coatings and accelerate PS-PVD TBCs failure due to the feather-like columnar structure. Hence, a strategy is proposed to alleviate molten CMAS corrosion. The super-hydrophobicity structure is fabricated via laser texturing on the surface of PS-PVD TBCs to repel molten CMAS wetting and spreading. Then, a thin layer of the Al-film is deposited on the laser-textured surface. Next, the Al-modified layer is in situ synthesized after vacuum heat treatment, preventing the infiltration of molten CMAS into the TBCs and reducing the coating damage. The results show that the contact angle of laser textured and Al-modified PS-PVD TBCs (LT-Al) at room temperature increased from 12.3° to 168.8°. The wetting and spreading behavior of molten CMAS of as-sprayed (AS), laser textured (LT), and LT-Al coatings is observed in situ at 1230 °C for 1800 s. The LT-Al coatings exhibited excellent CMAS corrosion resistance, attributed to the laser-textured micro-nano structures and Al-modified layer protection. The findings may be an effective approach for solving the disadvantage of PS-PVD feather-like columnar structure TBCs.

Microstructural evolution and high-temperature oxidation behavior of plasma sprayed SiC-YSZ TBCs

In this work, the effect of SiC addition on the microstructure, porosity, phase composition, weight gain, and thickness of TGO of the coatings after repeated oxidation at 1300 °C for various time of APS SiC-YSZ TBCs have been investigated. The phase of the as-sprayed coatings is consisted of t’-ZrO2, SiC and very little SiO2. With the increase of SiC addition, the diffraction peak intensity of SiC and SiO2 are both increased. SiC and SiO2 can fill into the pores in YSZ effectively. However, the porosity in the coatings has little changed with the increased of SiC addition. The addition of SiC during repeated oxidation at 1300 °C can limit the development of the m-ZrO2 phase while causing no ZrO2 lattice deformation. Increasing the SiC addition can impede the growth of TGO between the bonding layer and the ceramic layer, lowering the thermal weight gain rate of the SiC-YSZ coating. The lowest thermal weight gain rate occurs when the SiC composite concentration is 15 wt%. When the composite content of SiC is increased by 10 wt%, the percentage of O element at the interface between the bonding layer and the ceramic layer decreases by approximately 6.56 wt%. Increasing the SiC concentration will improve the compactness of the interface between the bonding and ceramic layers, and limit grain formation in the bonding layer. However, the SiC composite composition must match the ceramic coating's porosity. When the SiC percentage is too low, the ceramic layer's porosity increases and TGO development accelerates. When the SiC concentration is too high, it agglomerates and connects within the coating, causing the coating to fail off during high temperature service.

High-temperature erosion behavior of Al2O3 reinforced Inconel625 high velocity oxy-fuel sprayed and direct-aged composite coatings

In this study, Inconel625+30%Al2O3 (IN30AL) blended powders were sprayed on a commercially used boiler steel ASTM SA210 GrA1 using high-velocity oxy-fuel approach (HVOF). The microstructure, mechanical properties and elevated temperature erosion behavior of post-processed coatings and as-sprayed IN30ALcoatings were investigated. The elevated temperature erosion studies on IN30AL coatings both as-sprayed and post-processed were investigated at 900 °C temperature at two impact angles 30° and 90° using standard high temperature erosion test rig. Phase composition, interface of substrate/coating, and morphologies of eroded coatings were examined using x-ray diffraction, x-ray maps, scanning electron microscope, & energy dispersive spectrometer techniques. Compared with the as-sprayed IN30AL coatings, the heat-treated coatings have refined microstructure, better mechanical properties which improve the elevated temperature erosion resistance of IN30ALcoatings.

Effect of Heat Treatment on Mechanical Properties and Corrosion Response of HVOF Sprayed High Entropy Alloy Coatings

Surface and sub-surface related degradation of steels can be minimized using suitable surface coatings. High entropy alloys (HEA) are prominent and emerging materials among many coating materials. The current study investigates the effect of heat treatment of HEA coating on mechanical, metallurgical, and corrosion properties. The HEA coatings on SS304 steel were deposited using a High-Velocity Oxy-Fuel (HVOF) thermal spray process. The developed coatings were furnace heat treated at 700 °C, 900 °C, and 1100 °C, respectively, and their performance was benchmarked with the as-sprayed coatings. The metallurgical, mechanical, and microstructural analyses were performed using X-ray diffraction (XRD), Nanoindentation, Scratch test, and Field Emission Scanning Electron Microscope (FESEM) techniques. The corrosion response of the as sprayed and heat-treated coatings were recorded using a Potentiostat. The results indicated that as-sprayed coatings consisted of a single-phase BCC solid solution; however, the single-phase changed to a dual dual-phase system after heat treatment (BCC+FCC). The 900 °C heat-treated HEA coating exhibited superior mechanical and corrosion properties. But those characteristics started diminishing when the heat treatment temperature exceeded 900 °C. The introduction of the new FCC phase softened the coating, thereby leading to the evolution of microcracks in the coating. These micro-cracks acted as channels for electrolyte diffusion and further corroded the coatings. The current study surmised that HVOF-sprayed HEA coating should not be heat treated at above 900 °C.

Synchrotron X-ray spectromicroscopy analysis of wear tested graphene-containing alumina coatings

Thermally sprayed Al2O3 coatings containing graphene nano platelets (GNP) have been shown to exhibit improved wear resistance. To understand the positive influence of GNP on wear properties, scanning transmission X-ray microscopy (STXM) analyses were performed to determine the structural and chemical changes that occur on the GNP and alumina matrix after wear tests. STXM results acquired at the C K-edge showed that the GNP are aligned parallel to the specimen surface in the as-sprayed coatings. GNP flakes are also observed at the tribo-surface of the wear tested sample. The results obtained at the Al K-edge show that the aluminum oxide below the wear track becomes amorphous during the wear test and carbon is dissolved in it. Wear tests performed on thermally sprayed pure alumina samples (without GNP) and on sintered bulk alumina prove that neither the presence of GNP nor the porosity in the coating are responsible for the amorphization of the alumina matrix. The results discussed in this work advance the fundamental understanding of graphene-containing composites, which is considered very important to exploit the unique advantages of graphene in technological applications.

Laser remelting of high-velocity arc-sprayed Fe-based amorphous coating for improving corrosion resistance in 3.5 % NaCl solution with varying Na2S concentrations

In this study, high-velocity arc-spraying (HVAS) was employed to produce Fe-based amorphous coatings, and subsequent to that, laser remelting (LR) technology was applied on the as-sprayed coatings (ASC) to prepare laser remelted coatings (LRC). The corrosion resistance of both coatings was tested in 3.5 wt% NaCl solutions containing varying Na2S concentrations (0, 20, 50, 100, and 200 ppm) to accurately simulate the corrosion environment of offshore components. The corrosion behavior was investigated using electrochemical workstation, scanning electron microscopy, energy dispersive spectroscopy and X-ray photoelectron spectroscopy. The experimental results revealed that LR resulted in the elimination of defects in Fe-based amorphous coatings, a substantial reduction in porosity, the disappearance of the laminar structure, and the achievement of a homogeneous composition. The corrosion tendency of ASC and LRC both exhibited a pattern of initially decreasing, then increasing and finally decreasing with increasing sulfide concentration. This was attributed to the deposition of corrosion products on the surface and the dissolution of the corrosion products. When the sulfide concentration exceeded 50 ppm, the corrosion current density of both coatings slowly and was lower than that at 20 ppm, indicating that both coatings possessed strong corrosion resistance at higher sulfide concentrations. Highly polarized anions in solution (e.g. HS−, S2−) occupied vacancies in the passivation film, destabilizing the coating's passivation film and consequently diminishing its corrosion resistance. From the electrochemical analysis, it was evident that LRC exhibited greater corrosion resistance than ASC at all sulfide-containing solution concentrations, which was attributed to its smoother surface, denser structure and more stable passivation film.

Role of carbide grit size and in-flight particle parameters on the microstructure, phases, and nanoscale properties of HVOF sprayed WC-12Co coatings

Effects of particle temperature and velocity on crystallinity and phase composition of WC-Co coating are successfully decoupled. The effect of grit size on the dissolution and decarburization process is established analytically. The nanostructured coatings were found to be less crystalline than their conventional counterpart for similar in-flight parameters. The cooling rate and the degree of carbide dissolution in the binder phase govern the crystallinity of the as-sprayed coatings. An increase in either of these factors results in a higher degree of amorphization of as-sprayed coatings. The phases present in the coating were quantified, and correlated with particle temperature and velocity independently. The degree of decarburization increased with an increase in particle temperature. The calculated change in Gibbs free energy justified these observations. The effect of particle velocity on crystallinity or phase composition was not significant. The nano hardness of the carbide grits was increased with particle temperature owing to the formation of a hard W2C phase. Similarly, the dissolution of carbides in the binder and the formation of the Co3W9C4 phase increased the binder hardness. The energy analysis revealed brittleness of the binder matrix increased significantly with deposition temperature owing to the formation of the Co3W9C4 phase.

A novel plasma-sprayed Cr/FeCrAl dual-layer coating on Zr alloy for potential high‐temperature applications

To better meet the requirements in loss-of-coolant accidents (LOCA) and conventional operating conditions in nuclear applications, this work prepared Cr, FeCrAl, and Cr/FeCrAl composite coatings on Zr alloys using plasma spraying technology. The microstructures, hardness, and high-temperature oxidation resistance of coatings are examined. The high-temperature oxidation of coatings was conducted at 1200 °C in air for 20, 40, and 60 min. All coatings exhibited characteristic lamellar microstructures accompanied by small pores. The inner layers of the coatings, benefiting from the “compacting effect” of subsequently deposited particles, displayed lower porosity and higher hardness. The order of high-temperature oxidation resistance among the coatings was determined as follows: Cr/FeCrAl coating > Cr coating > FeCrAl coating. Notably, the Cr/FeCrAl composite coating demonstrated exceptional durability even after a 60-min exposure, with no ZrO2 formation and negligible Fe and Cr diffusion. However, oxygen could still reach the Zr substrate due to inherent pores and the lamellar structure in the as-sprayed coatings. Therefore, post-treatments are suggested to mitigate or eliminate these pores, thereby improving high-temperature oxidation resistance.

A novel high-temperature FeSiAl/CaO–B2O3–SiO2 microwave absorption coating prepared via atmospheric plasma spraying

In this paper, CaO–B2O3–SiO2 (CBS) is utilized as matrix material of microwave coating for the first time to achieve high-performance microwave absorption under high-temperature. A series of FeSiAl (FSA)/CBS composite coatings were prepared via atmospheric plasma spraying. The impact of spraying power on the microstructure, electromagnetic properties, and high-temperature stability of as-sprayed coatings were studied. The experimental results reveal that CBS could effectively suppress the formation of the lamellar structure of FSA as the low initial melting temperature, also form unexpected Fe2SiO4 structure with FSA during the spraying process. The increase in complex permittivity of the coating is primarily attributed to enhanced coating density and the development of Fe-rich regions within the CBS matrix. Furthermore, the dense coating prevents the oxidation of FSA and exhibits high-temperature stability. Our coating of 1 mm thickness has a reflection loss below −5 dB with frequency range of 7–14.8 GHz at 500 °C.

Enhanced CMAS and hot corrosion degradation of YSZ thermal barrier coating with nano powders

Thermal barrier coatings are used to protect hot section parts of gas turbine engines. These coatings are subject to corrosion due to impurity elements in the fuel and dust absorbed from the atmosphere into the engine. The rough and porous surface nature of plasma-sprayed coatings facilitates the penetration of these factors into the coating. Coatings are damaged prematurely as a result of corrosion. In this study, nano-structure YSZ, YSZ + Al2O3, YSZ + TiO2, and YSZ + Al2O3 + TiO2 coatings were deposited on plasma sprayed YSZ coating by electrophoretic deposition method. Hot corrosion and CMAS resistance were investigated. The coatings were characterized by SEM, XRD, AFM, and microhardness before corrosion tests. The changes in the coatings after corrosion tests were characterized by SEM and XRD. The penetration behavior of corrosive factors was examined by elemental mapping with EDS. Nanostructure coatings deposited on APS YSZ made the surfaces of as-sprayed coatings more stable. The ability of these nanolayers to prevent pure penetration into coatings during hot corrosion and CMAS is limited. However, the Al2O3 + TiO2-doped nanostructure YSZ reduced the bond-layer oxidation. In addition, the addition of Al2O3 + TiO2 to the EPD deposited layer also reduced the tetragonal-monoclinic phase transformation that occurred in YSZ as a result of corrosion. Al2O3 and TiO2 additives alone are not effective in improving the corrosion behavior of coatings. Nanostructured coatings deposited with the EPD technique can help protect plasma-sprayed thermal barrier coatings against corrosion.

Deposition characteristics, sintering and CMAS corrosion resistance, and mechanical properties of thermal barrier coatings by atmospheric plasma spraying

Atmospheric plasma spraying (APS) was used to fabricate 4.5 mol% Y2O3 stabilized ZrO2 (YSZ) and 4.0 mol% Yb2O3 and 0.5 mol% Y2O3 co-stabilized ZrO2 (YbYSZ) series coatings with different spraying parameters. This study presents the effects of particle size of feed powder and substrate temperature on the microstructure and performance of thermal barrier coatings. Single-pass and multi-pass experiments were conducted to explore the relationship between the splat and the coatings. The experimental results indicate that adjusting particle size of feed powders and deposition temperature can alter the splat behaviour and microstructure, improve anti-corrosion and anti-sintering abilities, and enhance the hardness and elastic modulus. Changes in parameters can cause changes in the microstructure of as-sprayed coatings. Lower substrate temperatures can lead to vertical cracks, finer powders can introduce more microdefects, and conventional sized powders can form larger pores. A coating deposited using a conventional feed powder at a substrate temperature of 800 °C shows excellent sintering resistance. A coating deposited using a fine feed powder on 800 °C substrate exhibits the best calcium‑magnesium-alumina-silicate (CMAS) corrosion resistance.

Influences of flame remelting and WC-Co addition on microstructure, mechanical properties and corrosion behavior of NiCrBSi coatings manufactured via HVOF process

We investigated the microstructures, mechanical properties, and corrosion behavior of NiCrBSi coatings upon the addition of the WC-Co powder in various amounts (0, 5, 10, 15, and 20 wt%) using the high velocity oxy-fuel process on low-carbon steel. Additionally, flame remelting was performed at 1100 °C for 1 min following thermal spraying of the coatings. The microstructures of the coatings were examined using X-ray diffractometry and scanning electron microscopy, and the mechanical properties of the coatings were assessed via Vickers microhardness and ball-on-disk tests. The corrosion behavior of the coatings was evaluated using potentiodynamic polarization testing in a 20 vol.% sulfuric acid solution. The results demonstrated that an increase in the WC-Co amount enhanced the hardness and wear resistance of the coatings. For 10–20 wt% WC-Co addition, small cracks were observed in the as-sprayed coatings owing to the heat applied during thermal spraying and rapid cooling, respectively. The presence of porosity directly affects the corrosion behavior of the coatings. Flame remelting led to better cohesion and the distribution of the γ-Ni and WC phases, reducing microdefects, such as pores and cracks. The mechanical properties of the coatings improved as the WC-Co amount increased after flame remelting, resulting in the formation of intermetallic compounds (FeB, CoSi2, and Ni2Si) along with the WC phase. The flame-remelted NiCrBSi/ 10 wt% WC-Co coating demonstrated high hardness and low wear and corrosion rates (1072.26 HV, 4.8 × 10−6 mm3/m, and 0.560 mm/year, respectively). Therefore, WC-Co addition and the flame remelting process significantly enhanced the properties of NiCrBSi coatings. This study highlights the substantial contributions of WC-Co addition and the flame remelting process in enhancing the mechanical properties and corrosion resistance of NiCrBSi coatings. These findings offer valuable insights for optimizing the performance of commercially available coatings in various real-world applications in maintaining power plants and industrial components like high-pressure pump piston rods, acid-resistant pumps in oil refineries.

Effect of laser remelting on the microstructure and corrosion behavior of high-velocity arc-sprayed FeNiCrBSiNbW amorphous coating

Fe-based amorphous coating was produced using high-velocity arc-spraying (HVAS), and then laser remelting (LR) technology was used to treat the as-sprayed coatings (ASC) to prepare the laser remelted coatings (LRC). The morphology, phase composition, microhardness, porosity, and corrosion property of the Fe-based amorphous coatings before and after LR were investigated. The outcomes demonstrated that the internal defects of the ASC were eliminated and the porosity was significantly reduced after LR, which was significant in dramatically enhancing corrosion resistance. After the LR procedure, the ASC's free corrosion potential (Ecorr) in 3.5 wt% NaCl solution increased, and the corrosion current density (Icorr) almost reduced by a factor of ten. Therefore, LR helped to strengthen the corrosion characteristics of Fe-based amorphous coating and improve its microstructure.

The Influence of Spraying Parameters and Powder Sizes on the Microstructure and Mechanical Behavior of Cold-Sprayed Inconel®625 Deposits

Cold spraying (CS) of high-strength materials, e.g., Inconel®625 is still challenging due to the limited material deformability and thus high critical velocities for achieving bonding. Further fine-tuning and optimization of cold spray process parameters are required, to reach higher particle impact velocities and temperatures, while avoiding nozzle clogging. Only then, sufficiently high amounts of well-bonded particle–substrate and particle–particle interfaces can be achieved, assuring high cohesive strength and minimum amounts of porosities. In this study, Inconel®625 powder was cold sprayed on carbon steel substrates, using N2 as propellant gas under different spray parameter sets and different powder sizes for a systematic evaluation. Coating microstructure, porosity, electrical conductivity, hardness, cohesive strength, and residual stress were characterized in as-sprayed condition. Increasing the process gas temperature or pressure leads to low coating porosity of less than 1% and higher electrical conductivity. The as-sprayed coatings show microstructures with highly deformed particles. X-ray diffraction reveals that powder and deposits are present as γ-solid-solution phase without any precipitations. The deposits show high microhardness and compressive residual stresses, which is attributed to work hardening and peening effects. The optimized deposits reach almost bulk material properties and are thus well suited for industrial applications.

Effect of Flame Remelting on the Microstructure, Wear and Corrosion Resistance of HVOF Sprayed NiCrBSi Coatings

As a post treatment, thermal remelting is an effective method to eliminate pores and establish a metallurgical bonding for thermal sprayed coatings. However, it is rather difficult to obtain simultaneously high corrosion and wear resistance, since additional energy input usually leads to more homogeneous microstructure in coatings, which deteriorates mechanical hardness. In this work, flame remelting has been imposed to high velocity oxygen-fuel sprayed self-flux NiCrBSi coatings. The remelting effects on microstructure were characterized in terms of porosity and phase analysis. The microhardness, wear resistance and corrosive behaviors were compared among substrate steel, as-sprayed and as-remelted coatings. Results show that the lamellar boundaries and internal defects in the as-sprayed coatings have been eliminated by remelting. The coating porosity has substantially reduced from 7.36% to 0.75%, and a metallurgical bonding at the coating/substrate interface has been formed. Comparing with the as-sprayed coatings, the microhardness of the remelted coatings increases about 21% and the wear weight loss reduces about 42%. By flame remelting, the wear mechanism changes from furrow and abrasive wear to micro-cutting and local fracture. The remelted coatings have also exhibited better corrosion resistance by means of salt spraying and potentiodynamic tests.

Elevated temperature tribological performance of non-equiatomic CoCrNiTiWx high entropy alloy coatings developed by mechanical alloying and high-velocity oxy-fuel spray

High entropy alloys (HEA) have applications in multiple fields owing to their exceptional mechanical and physical properties. In the current study, mechanical alloyed CoCrNiTiWx (x; a molar fraction, x = 0.5 and 1.5) HEA feedstock powders were deposited on maraging steel substrate using high-velocity oxy-fuel spray (HVOF). The phase evolution and the microstructure of the milled powders and as-sprayed coatings were analysed by X-ray diffraction (XRD) and scanning electron microscope (SEM). The tribological behaviour of CoCrNiTiW0.5 and CoCrNiTiW1.5 HEA coatings at elevated temperatures was studied extensively using a Pin-on-Disc tribometer. The CoCrNiTiW0.5 and CoCrNiTiW1.5 HEA coatings retained the BCC solid solution phases formed during the milling stage. However, additional oxide and intermetallic phases were formed owing to the in-flight oxidation and high temperatures experienced during the HVOF deposition. The deposited coatings exhibited a lamellar structure and good mechanical bonding with the substrate. The porosities of CoCrNiTiW0.5 and CoCrNiTiW1.5 HEA coatings were found to be 1.69 ± 0.32 % and 1.51 ± 0.37 % respectively.Consequently, the CoCrNiTiW0.5 and CoCrNiTiW1.5 HEA coatings displayed average microhardness values of 863 ± 52 HV0.3 and 1025 ± 39 HV0.3, respectively. Further, the wear rates of coatings exhibited a significant reduction at elevated temperatures, owing to the formation of TiO2, NiCr2O4 oxide tribofilms for CoCrNiTiW0.5, and CoCr2O4, NiWO4, WO3 oxides for CoCrNiTiW1.5. The specific wear rate of CoCrNiTiW0.5 HEA coating dropped by 73.6 % from 22.7 ± 2.6 × 10−6 mm3/N-m to 5.99 ± 1.9 × 10−6 mm3/N-m, while CoCrNiTiW1.5 dropped by 78.8 % from 11.86 ± 3.5 × 10−6 mm3/N-m to 2.51 ± 1.5 × 10−6 mm3/N-m, with a rise in the temperature from RT to 600 °C. Likewise, The frictional coefficients of CoCrNiTiW0.5 HEA dropped from 0.504 ± 0.015 to 0.397 ± 0.005, while CoCrNiTiW1.5 HEA dropped from 0.578 ± 0.025 to 0.471 ± 0.004, with a rise in temperature from RT to 600 °C. At room temperature, the wear mechanisms of the as-sprayed CoCrNiTiWx coatings were dominated by adhesive wear. However, at elevated temperatures, a shift towards oxidative wear was observed.