High Velocity Oxy-fuel Technique Research Articles

Physical and microstructural properties of YSZ + Cr2O3 thermal barrier coatings developed using HVOF technique

A composite coating material of Yttria-Stabilized Zirconia (YSZ) and Chromium Oxide (Cr2O3) was developed on the surface of Al-6061 substrates. The coating consisting of equal measures of the composite material of YSZ and Cr2O3 was successfully obtained using the High-Velocity Oxy Fuel (HVOF) technique. The surface roughness and coating thickness of the composite coatings are evaluated. The surface roughness (Ra) was evaluated showing an average of 6.0848 µm using the Surface Roughness Tester (SRT). The coating thickness was evaluated showing an average of 220.8 µm using the Coating Thickness Tester (CTT). The surface roughness and the coating thickness obtained are comparable to typical physical properties of Thermal Barrier Coatings (TBC’s). The microstructural evaluation of the coated specimens was performed through the Scanning Electron Microscope (SEM), showing the clear unification of materials The determination of the elemental composition of surface coatings was performed through the EDAX tests, which clearly established the elements in the coatings. This article is indicative of the achievement of TBC’s consisting of composite materials though HVOF technique as an alternative to TBC’s obtained through surface coats and bond coats.

Experimental and computational investigation of FGM coated cylinder liners prepared using HVOF technique

In the present work, to improve the wear resistance of the cylinder liner, without affecting the heat dissipation, cast steel cylinder liners are coated with five-layer Functionally Graded Material (FGM) coatings prepared using High-velocity oxy-fuel (HVOF) technique. The powders were designated as C1 (WC-10Co-4Cr) which is Tungsten Carbide with 10% Cobalt and 4% Chromium, C2 (WC-12Co) which is Tungsten Carbide with 12% Cobalt and C3 (Ni-25Cr) which is Nickel with 25% Chromium. The results revealed that the FGM coating designated as Functionally Graded Coated Sample-1 (FGCS-1) wears less compared to Functionally Graded Coated Sample-2 (FGCS-2) and Functionally Graded Coated Sample-3 (FGCS-3), due to the presence of wear resistive elements in the composition and uniform material homogeneity. Numerical analysis of FGCS-2, conducted using Ansys Fluent software, revealed a negligible temperature gradient across the coating thickness. This is due to the highly conductive coating material and the extremely thin coating layers, making it suitable for engine cylinder liner applications where less wear and high heat transfer rate is desirable.

Corrosion behaviors of single-layer and double-layer nickel alloy coatings fabricated by arc spraying and high-velocity oxy-fuel

This study investigates single-layer and double-layer thermally sprayed nickel alloy coatings (bond and top coatings). Ni-5 wt% Al and Ni-20 wt% Cr were used as bond coatings, whereas highly alloyed Ni-based coatings, Hastelloy C276 or Inconel 625 were used as top coatings. Arc spraying (A) and high-velocity oxy-fuel (HVOF) techniques were used to create the coatings on a 304 stainless steel substrate. The samples were studied by performing potentiodynamic polarization tests in a 20 vol% H2SO4 solution at room temperature. The scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS) and wavelength dispersive X-ray spectroscopy (WDS) techniques were used to investigate the microstructure and chemical composition of the coatings. The arc sprayed coatings are composed of a lamellar microstructure with low porosities, unmelted particles, and high oxides at the intersplats. A uniform and compact coating with minimal oxides and porosity was achieved for all coatings via HVOF technique. In single-layer coatings, arc spray coating surpasses HVOF coating owing to its oxide layers, which impede sulfuric solution penetration and bolster resistance against electrolyte infiltration. Double-layer coatings, comprising a bonding layer and a top coating, using the same thermal spray method have shown to enhance corrosion resistance.

Tribological and corrosive degradation of differently surface engineered 17-4 PH steel

In the present work, a high-velocity oxy-fuel (HVOF) thermal spray process was used to deposit ceramic coatings containing alumina (Al2O3)− 0.8 wt% ceria (CeO2)− 0.2 wt% reduced graphene oxide (rGO) on 17–4 PH steel. The results were compared with the nitrided 17–4 PH (N17–4 PH) steel. The mechanical characteristics of the bare substrate, nitrided substrate, and coating were evaluated regarding micro (Vickers) hardness, scratch hardness, nanoindentation, and high-temperature nanoindentation (300 °C). The tribological behavior of all the specimens mentioned earlier was studied in a universal mechanical tester in a ball on flat reciprocating wear test mode for different loads. Electrochemical responses regarding corrosion properties in 3.5 wt% aqueous salt solution were recorded and analyzed. The results showed that the ceramic coating performed better than the other two specimens regarding mechanical, tribological, and electrochemical properties. For example, the coating was nearly ten and six times more wear resistant than 17–4 PH steel and N17–4 PH steel, respectively, at 50 N load. The performance enhancement in the coating was ascribed to the coating hardness, dense microstructure, and introduction of rGO in the coatings, which participate in the tribological process. Further detailed analysis of the bare steel, ceramic coating, and the nitrided steel properties reveals that the nitriding process can be replaced with the proposed ceramic coating through the HVOF technique for tribological, corrosive, and extreme environmental applications for 17–4 PH steel.

EFFECTS OF ALLOYING ELEMENTS AND SURFACE PROPERTIES ON THE CORROSION BEHAVIOR OF HVOF-DEPOSITED WC COATINGS

The main purpose of this study is to investigate the corrosion behavior of nickel-aluminum buffer and tungsten-carbide-based ceramic-metal composite coated materials on EN 1.4404 quality stainless steels in a sulfuric acid (H2SO4) environment for petrochemical industry applications. For this purpose, tungsten-carbide-based coatings were produced on nickel-aluminum-deposited 1.4404 stainless-steel substrates using the HVOF (High Velocity Oxy-Fuel) technique. In the characterization of coatings, X-ray diffraction (XRD) for phase analysis, optical microscope and scanning electron microscope (SEM) for surface morphology, image analyzer for coating thickness measurements, energy distribution spectroscopy (EDS) for elemental analysis, and roughness device for surface structures, were used. WC-, Co-, Ni-, and NiAl-based phases were observed in the coatings. According to metallographic studies, all the coatings had a similar coating microstructure and made good contact with the substrate. Potentiodynamic polarization measurements and corrosion tests were carried out to determine the corrosion behavior of HVOF plasma-sprayed coatings using a potentiostat/galvanostat. The results showed that the WCCo-NiAl-coated stainless-steel substrate had a higher corrosion resistance to the H2SO4 environment than the NiAl and WCNi-NiAl samples.

Phase evolution and high-temperature wear behavior of non-equiatomic metastable CoCrNiTiMox HEA coatings fabricated by high-velocity oxy-fuel technique

The current research aims to enhance the tribological performance of maraging steels at high temperatures by surface modification techniques. CoCrNiTiMox (x; molar fraction, x = 0.5, 1.5) high-entropy alloy (HEA) coatings with dense lamellar microstructures were deposited onto maraging steels using high-velocity oxy-fuel spray (HVOF). In order to achieve a uniform distribution of constituent elements for thermal spray deposition, mechanical alloying was employed to synthesize the HEA feedstock. The phases and microstructure of the synthesized HEA powder, as-sprayed coatings, and worn surfaces were examined using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The as-sprayed HEA coatings exhibited metastability, with a BCC phase solid solution, NiTiO3 spinel, and an intermetallic MoNi phase for CoCrNiTiMo0.5 and Co2Mo3 phase for CoCrNiTiMo1.5. The average microhardness of CoCrNiTiMo0.5 and CoCrNiTiMo1.5 HEA coatings were 841 ± 62 HV0.3 and 952 ± 23 HV0.3, respectively. The specific wear rate and friction coefficients of CoCrNiTiMox HEA coatings exhibited a decreasing trend with an increase in temperature, owing to the formation of tribofilms on the worn surface. X-ray diffraction studies revealed the formation of NiMoO4 spinel for CoCrNiTiMo0.5 and MoO2, Co3O4 phases for CoCrNiTiMo1.5 HEA at a wear temperature of 600 °C. The investigation of worn surfaces showed a transformation in wear mechanisms from abrasive wear at room temperature to oxidative wear with mild fatigue at elevated temperatures.

75Cr3C2-25NiCr and 86WC-10Co-4Cr High Wear- and Corrosion-Resistant Cermet Coatings Deposited on A356 Substrate by High-Velocity Oxy-Fuel Technique

In this research, Cr3C2-NiCr and WC-Co-Cr cermet coatings were developed on A356 aluminum-based alloy substrate by the high-velocity oxy-fuel (HVOF) technique for use in wear and corrosion applications. The substrate and coatings were characterized using a field emission scanning electron microscope (FESEM) equipped with the energy dispersive spectroscope (EDS), microhardness, wear, and corrosion test instruments. Microstructural observations revealed that the coatings with an average thickness of about 250 μm were well bonded with the substrate. The microhardness of the Cr3C2-NiCr (~930 HV) and WC-Co-Cr (~1300 HV) coatings were about eleven and sixteen times higher than that of the A356 substrate (~80 HV), respectively. Cermet coatings showed significantly lower mass losses, wear rates, and friction coefficients in comparison with the A356 substrate. WC-Co-Cr coating illustrated higher tribological performance in comparison with Cr3C2-NiCr coating. The mass loss and friction coefficient of the WC-Co-Cr coating under an applied load of 10 N was about 0.2 mg and 0.13 (about 99.5% and 79.7% lower than that of the A356 substrate, e.g., 41.5 mg and 0.64), respectively. Rising applied load increased the wear characteristics of the A356 substrate with the more pronounced degrees. FESEM observations on wear test specimens illustrated the different wear mechanisms on the surfaces. The results illustrated significant improvements in the corrosion performances of the coated samples.

Influence of WC Particle Size on the Mechanical Properties and Residual Stress of HVOF Thermally Sprayed WC-10Co-4Cr Coatings.

Cermet coatings deposited using high-velocity oxy-fuel (HVOF) are widely used due to their excellent wear and corrosion resistance. The new agglomeration–rapid sintering method is an excellent candidate for the preparation of WC–Co–Cr feedstock powders. In this study, four different WC–10Co–4Cr feedstock powders containing WC particles of different sizes were prepared by the new agglomeration–rapid sintering method and deposited on steel substrates using the HVOF technique. The microstructures and mechanical properties of the coatings were investigated using scanning electron microscopy, X-ray diffraction, nanoindentation, and Vickers indentation. The through-thickness residual stress profiles of the coatings and substrate materials were determined using neutron diffraction. We found that the microstructures and mechanical properties of the coatings were strongly dependent on the WC particle size. Decarburization and anisotropic mechanical behaviors were exhibited in the coatings, especially in the nanostructured coating. The coatings containing nano- and medium-sized WC particles were dense and uniform, with a high Young’s modulus and hardness and the highest fracture toughness among the four coatings. As the WC particle size increased, the compressive stress in the coating increased considerably. Knowledge of these relationships enables the optimization of feedstock powder design to achieve superior mechanical performance of coatings in the future.

Assessment of erosive wear performance of Pelton turbine injectors using CFD-DEM simulations

The problem of surface erosion due to sediment-laden water in hydropower stations is quite severe, especially in those located in the Himalayan region. It is one of the major causes of reduction in the operational economy for hydropower generation. In Pelton turbines, the injector assembly is subjected to severe erosion, yielding it inoperable in a relatively short period when compared to other parts of the hydropower plant. The present study aims to contribute to an understanding of hydro-abrasive erosion in Pelton turbine injectors due to changes in design and surface material with hard coating. Three different injector designs, conventional design (nozzle and spear angle of 90 o and 50 o , respectively) and two other designs proposed for better hydraulic performance (nozzle and spear angles of 110 o and 70 o , and 150 o and 90 o , respectively) are chosen for the analysis. The simulations are conducted using the CFD coupled discrete element method (DEM). Multi-size sand particles in the range of 75 μm to 300 μm have been selected to conduct the study. The particle-wall collision characteristics are used to estimate the erosion rate using the semi-empirical erosion models calibrated for martensitic steel, high-velocity air fuel (HVAF), and high-velocity oxy-fuel (HVOF) processed WC-CoCr coatings erosion. The erosion distribution on the nozzle and spear for different injector designs with and without coating at different nozzle opening conditions has been studied. The results show that the erosion of the nozzle and spear is asymmetrical. The increase in design angle enhances nozzle erosion while decreasing spear erosion. The erosion resistance of the Pelton injector improves by an order of 100 with a coating of WC–CoCr. Compared to the HVOF technique, coating with the HVAF technique has superior erosive wear performance. The ratio of maximum to minimum erosion ratio reduces with coating. Also, the uniformity in erosion distribution improves for the coated surface compared to the uncoated surface. The results could provide some theoretical assistance for design optimization, coating performance, and maintenance of the Pelton turbine injector for application in sediment-laden water flows. • Comparison of injector designs for hydraulic and erosive wear performance. • Comparison of the erosion of injectors with and without surface coating. • Erosion model for martensitic steel, and HVAF and HVOF processed WC-CoCr coatings. • Identification of zone of higher erosion on nozzle and spear for different designs. • Analysis of particle collision characteristics on nozzle and spear surface.

Investigation of oxidation and hot corrosion behavior of molybdenum coatings produced by high-velocity oxy-fuel coating method

Thermal spray coatings methods are frequently used in automotive, aerospace, and marine vehicles. The most obvious damage mechanisms of coating systems, which are exposed to many damages during use, are high-temperature oxidation and corrosion. While the oxygen element coming from the atmosphere creates the high-temperature oxidation damage in the coating systems, the molten salts such as Na2SO4 and V2O5 coming from low-quality fuels constitute the high-temperature hot corrosion damage mechanism. In this study, its behavior against high-temperature oxidation and hot corrosion damage mechanisms was investigated by coating Molybdenum (Mo) on a 316 L stainless steel substrate with the High-Velocity Oxy-Fuel technique, which is an innovative thermal spray coating method. Both damage mechanisms were carried out isothermally at 650 °C. Oxidation tests were carried out for 5, 25, and 50 h, while hot corrosion tests were carried out for 1, 3, and 5 h. X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), Elemental mapping, and Energy Distribution Spectroscopy (EDS) analysis was performed on molybdenum-coated samples before and after both tests. The results obtained were evaluated and discussed in light of the literature.

Mechanical and microMechanical and microstructural characterization of yttria-stabilized zirconia (Y2O3/ZrO2; YSZ) nanopartstructural characterization of yttria stabilized zirconia (Y2O3/ZrO2; YSZ) nanoparticles reinforced WC-10Co-4Cr coated turbine steel

The aim of this paper is to investigate the WC-10Co-4Cr coatings reinforced with 5 % and 10 % of yttria-stabilized zirconia (Y2O3/ZrO2; YSZ) nanoparticles deposited on the CA6NM turbine steel by using the high-velocity oxy-fuel (HVOF) thermal spraying technique. In the HVOF technique, the hot jet of the semi-solid particles strikes against the workpiece and creates a layer of coating of varying thickness on the substrate material. The coatings fabricated with HVOF were analyzed by scanning electron microscope (SEM) / energy-dispersive x-ray spectroscopy (EDS). The phase identification of a crystalline material was made with the x-ray diffraction (XRD) technique. The mecha­nical properties in terms of porosity, surface roughness and microhardness of the nanocomposite coatings were also evaluated. The SEM/EDS analysis showed that dense and homogeneous coatings were developed by the reinforcement of YSZ nanoparticles. The peaks of XRD graphs of WC-10Co-4Cr coating reinforced with 5 and 10 % of YSZ nanoparticles revealed that the WC was present as a major phase and W2C, Co3W3C, Co, Co6W6C, Co6W and Y2O3/ZrO2 nanoparticles were observed as a minor phase. The porosity level decreased up to 42 and 56 % by the addition 5 and 10 % of YSZ nanoparticles as compared with conventional WC-10Co-4Cr coating. The surface roughness values for WC-10Co-4Cr conventional coating, 95 % (WC-10Co-4Cr) + 5 % YSZ and 90 % (WC-10Co-4Cr) + 10 % YSZ nanocomposite coated samples were found to be 5.03, 4.89 and 4.28 respectively. The nanocomposite coatings reinforced with 10 % YSZ nanoparticles exhibited the highest microhardness value (1278 HV). The WC-10Co-4Cr coatings reinforced with 10 % of YSZ nanoparticles resulted in low porosity, low surface roughness and high microhardness. During the coating process, the nanoparticles of YSZ flow into the pores and are dispersed in the gaps between the micrometric WC particles and provide a better shield to the substrate material. The WC-10Co-4Cr with 10 % of YSZ nanoparticles showed better results in terms of mecha­nical and microstructural properties during the investigation.

Effect of Yttria-Stabilized Zirconia (Y2O3/ZrO2) Nanoparticles Reinforced Cr3C2-25NiCr Coatings on the Microstructural and Mechanical Properties of Turbine Steel

Abstract The present work is mainly focused on the investigation of Cr3C2-25NiCr coatings reinforced with 5 % and 10 % of yttria-stabilized zirconia (YSZ) nanoparticles deposited on the CA6NM turbine steel by using the high-velocity oxy-fuel technique. The coatings were analyzed by scanning electron microscope (SEM)/energy-dispersive X-ray spectroscopy (EDS). The phase identification of a crystalline material was done with the X-ray diffraction (XRD) technique. The SEM/EDS analysis showed that dense and homogeneous coatings were developed by the reinforcement of YSZ nanoparticles. The peaks of XRD graphs of Cr3C2-25NiCr coating reinforced with 5 % and 10 % of YSZ nanoparticles revealed that the chromium and carbon were present as a major phase, and the presence of nickel, yttrium, and zirconium was observed as a minor phase. The porosity level decreased up to 32 % and 45 % by the addition 5 % and 10 % of YSZ nanoparticles as compared with conventional Cr3C2-25NiCr coating. The surface roughness values for coated samples were found to be 5.03, 4.89, and 4.28. The nanocomposite coatings reinforced with 10 % YSZ nanoparticles exhibited the highest microhardness value (1,251 HV). The Cr3C2-25NiCr coatings reinforced with 10 % of YSZ nanoparticles resulted in low porosity, low surface roughness, and high microhardness. During the coating process, the nanoparticles of YSZ flow into the pores and gaps that exist in the coatings and provide a better shield to the substrate material. The Cr3C2-25NiCr with 10 % of YSZ nanoparticles showed better results in terms of mechanical and microstructural properties during the investigation.

Tribological behaviour of composite coatings for sliding wear resistance

The present work is based on the development of hybrid composite coating for sliding wear resistance, where coating was developed using the HVOF (High Velocity Oxy-fuel) technique. The carbon has grown using an eco-friendly agricultural waste product that consists of excellent tribological properties and improved mechanical properties. The developed coating was tested for sliding wear resistance applications at elevated temperatures. The obtained result showed significant improvement in test results at evaluated temperatures where the value of hardness and residual stress coating has increased as temperature increases. The micro-hardness and residual stress was about 540HV and −12 MPa respectively. These changes were found due to the development of oxides and carbides in the developed coatings. The experimental test results showed that developed composite coating is temperature dependent. The experimental results shows that at 450 °C, composite coating shows lowest coefficient of friction (0.06), 95 µm value of wear. The presence of micron size hard particles along with Cr3C2 (chromium carbide), MoO2 (Molybdenum dioxide), and Mo2C (Molybdenum carbide) hard phase particles, which cause a barrier and is responsible for the wear reduction.

ELECTROCHEMICAL CORROSION BEHAVIOR AND MICROSTRUCTURAL CHARACTERIZATION OF HVOF SPRAYED INCONEL718-Al2O3COMPOSITE COATINGS

In the current experimental study, grey cast iron (CI) substrate was coated with Inconel718-Al2O3based composite coating with a high-velocity oxy-fuel technique. The effect of changing the Al2O3content (10, 20 and 30 wt.%) on the microstructure, hardness, porosity and electrochemical corrosion performance of Inconel (INC718) coating was studied. Investigations on the corrosion behavior of uncoated and HVOF-coated substrates were carried out at room temperature at 3.5[Formula: see text]wt.% sodium chloride solution (NaCl) with the help of the potentiodynamic polarization approach. The surface morphologies and compositions of HVOF as-sprayed and electrochemically corroded coatings were studied through SEM and EDS techniques. The various phases existing in the INC718 and Al2O3feedstock powders and HVOF-deposited composite coatings were determined by XRD analysis. The microhardness of INC718-based coatings was found to be increased with the increase in Al2O3content. The highest average microhardness value of about [Formula: see text]HV[Formula: see text] was observed in INC718-30[Formula: see text]wt.% Al2O3coating. The deposited coatings exhibited an increased porosity level with the increased amount of Al2O3contents. However, the coating with 10[Formula: see text]wt.% Al2O3content exhibited the maximum corrosion resistance. Its improved corrosion performance is attributed to low porosity levels, which causes the penetrating pathways of Cl−ions to be blocked completely.

Investigation on Microstructure, Mechanical and Wear Properties of HVOF Sprayed Composite Coatings (WC-Co + CR) On Ductile Cast Iron.

Recent work indicates that the high-velocity oxy-fuel (HVOF) thermal spraying WC–Co coatings have been used to enhance the wear resistance of various engineering components in a variety of industrial environments. In the present work, WC–Co powder, containing Cr particles in an amount of 10%, was deposited on ductile cast iron with the HVOF thermal spray coating technique. An investigation was conducted to determine the role of Cr particles in the WC–Co coating produced with the HVOF technique on microstructure, mechanical, and wear properties in a system of type: WC-Co coating/ductile cast iron. The microstructure of the HVOF-sprayed WC–Co + Cr coating was characterised by light microscopy, X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), and energy-dispersive X-ray spectroscopy (EDS). The analysis of the microstructure showed the formation of a coating with low porosity, compact structure, and good adhesion to the substrate with a typical lamellar structure composed of fine molten Cr particles and finely fragmented WC grains embedded in a Co matrix, reaching the size of nanocrystalline. The scratch test was applied for the analysis of the adhesion of coatings to the substrate. The erosion behaviour and mechanism of material removal was studied and discussed based on microstructural examinations. Moreover, the results were discussed in relation to the bending strength test, including cracks and delamination in the system of the WC–Co + Cr/ductile cast iron, as microhardness and erosion resistance of the coating. It was found that the addition of Cr particles to the WC–Co powder, which causes hardening of the binder phase is a key influence on increased mechanical and wear properties in the studied system. Additionally, due to the construction of nanostructured coatings, suitable proportion of hard and soft phases, the technique sprayed HVOF coatings have advantageous properties such as high density and good slurry erosion resistance.

High temperature corrosion behavior of Ni and Co base HVOF coatings exposed to NaCl-KCl salt mixture

The high-temperature corrosion behavior of NiMoCrW and CoNiCrAlY base coatings produced by high velocity oxy-fuel technique (HVOF) has been analyzed in dry air atmosphere and in presence of NaCl-KCl salt mixture at 650 °C for 360 h. Their behavior was compared with that of NiCr coating used as reference. Their corrosion resistance in presence of NaCl-KCl was measured by dimensional metrology analysis and the corroded samples were characterized by XRD and SEM techniques. The coatings exhibited excellent oxidation resistance in dry air atmosphere due to the formation of Cr2O3 and Cr-rich spinels, as well as the formation of Al2O3 in CoNiCrAlY coating. Their corrosion resistance in presence of NaCl-KCl was significantly lower due to chlorine-induced active oxidation and due to the formation of chromates, identified by XRD as K2CrO4. However, both coatings presented higher corrosion resistance in presence of the alkali salts compared to that of NiCr coating. The presence of Mo in the NiMoCrW coating improved the high-temperature corrosion resistance in presence of the alkali salts and the formation of a stable Al2O3 oxide scale on CoNiCrAlY coating reduced the penetration of chlorides and the degradation of the protective oxide scale by chromates formation.

Wear properties of WC–Co and WC–CoCr coatings applied by HVOF technique on different steel substrates

Abstract Low alloy and stainless steel are the most used types of iron-based materials world wide. Their use against in machine element work, reclamation, corrosion and wear resistance are still challenging. To overcome this problem, many steel alloys are coated with cermet coatings to protect the parts from wear and corrosion. In the present study, WC-Co and WC-CoCr coatings were applied by means of a high velocity oxy-fuel (HVOF) technique on AISI 304, AISI 1040, and AISI 4340 steel alloys used as substrates. The aim was to investigate surface properties and wear resistance of the coatings and to determine their relationship with the type of coating and substrate. In accordance with this purpose, hardness and thickness of the coatings were measured, sliding wear tests were performed, scanning electron microscope (SEM) images and X-ray diffractions (XRD) were taken, surface roughness and friction coefficients were determined. The results showed that the WC-CoCr coatings had higher hardness and lower thickness than the WC-Co coatings. Maximum hardness was obtained in the WC-CoCr coating applied to AISI 4340 steel, which was also the hardest alloy among those studied. After wear resistance tests, it was revealed that the wear resistance of the WC-CoCr coatings was better than that of the WC-Co coatings for each steel substrate. During the coating, the new phases resulting from the decomposition of the WC phase in the WC-CoCr coatings contributed more to wear resistance than those of the WC-Co coatings. A lower friction coefficient and lower surface roughness of the WC-CoCr coatings during wear were obtained, resulting in higher wear resistance. A WC-CoCr coating on AISI 4340 alloy which has the highest hardness, lowest surface roughness and lowest friction coefficient resulted in the highest wear resistance among all types studied.

Wear properties of WC–Co and WC–CoCr coatings applied by HVOF technique on different steel substrates

Abstract Low alloy and stainless steel are the most used types of iron-based materials world wide. Their use against in machine element work, reclamation, corrosion and wear resistance are still challenging. To overcome this problem, many steel alloys are coated with cermet coatings to protect the parts from wear and corrosion. In the present study, WC-Co and WC-CoCr coatings were applied by means of a high velocity oxy-fuel (HVOF) technique on AISI 304, AISI 1040, and AISI 4340 steel alloys used as substrates. The aim was to investigate surface properties and wear resistance of the coatings and to determine their relationship with the type of coating and substrate. In accordance with this purpose, hardness and thickness of the coatings were measured, sliding wear tests were performed, scanning electron microscope (SEM) images and X-ray diffractions (XRD) were taken, surface roughness and friction coefficients were determined. The results showed that the WC-CoCr coatings had higher hardness and lower thickness than the WC-Co coatings. Maximum hardness was obtained in the WC-CoCr coating applied to AISI 4340 steel, which was also the hardest alloy among those studied. After wear resistance tests, it was revealed that the wear resistance of the WC-CoCr coatings was better than that of the WC-Co coatings for each steel substrate. During the coating, the new phases resulting from the decomposition of the WC phase in the WC-CoCr coatings contributed more to wear resistance than those of the WC-Co coatings. A lower friction coefficient and lower surface roughness of the WC-CoCr coatings during wear were obtained, resulting in higher wear resistance. A WC-CoCr coating on AISI 4340 alloy which has the highest hardness, lowest surface roughness and lowest friction coefficient resulted in the highest wear resistance among all types studied.

Microstructure Characteristics, Tribology and Nano-Hardness of HVOF Sprayed NiCrRe Coating

The high-velocity oxy-fuel technique (HVOF) was used to produce dense NiCrRe coating on boiler steel substrate with a minimal amount of oxide impurities and low porosity. Microstructure analysis, tribology and nano-hardness measurement have been realized with the aim to characterize the systems. The microstructure was studied using scanning electron microscopy and Energy-dispersive X-ray spectroscopy. Tribological characteristics have been studied under the dry sliding condition at applied loads of 5, 10 and 20 N using the ball-on-flat technique with SiC ball at room temperature. Nano-hardness was investigated in continuous stiffness measurement (CSM) mode, the indentation depth limit was 1500 nm. Microstructure analyses proved that the HVOF sprayed layer, with a thickness approximately 870 µm, contains a relatively low volume fraction of porosity with a chemical composition based on Nickel, Chromium, with white areas of Rhenium. The wear rate of the coating is significantly lower than the wear rate of 16Mo3 steel. The average values of indentation modulus and hardness were EIT = 237.6 GPa and HIT = 6.3 GPa, respectively.

Facile and Green Engineering Approach for Enhanced Corrosion Resistance of Ni-Cr-Al2O3 Thermal Spray Coatings.

Thermal spray coatings (TSCs) are widely utilized for limiting degradation of structural components. However, the performance of TSCs is significantly impaired by its inherent non-homogeneous microstructure, comprising of splat boundaries, porosities, secondary phase-formation, and elemental segregation. Herein, we report a simplistic approach for significantly enhancing the corrosion resistance of TSCs. Ni–Cr–5Al2O3 coatings were deposited on stainless steel using high-velocity oxy-fuel technique. The microstructure of as-sprayed coating showed significant inhomogeneities in the form of isolated splats and elemental segregation. The microstructure of developed coatings was modified using a novel processing technique, known as stationary friction processing (SFP). The SFP treatment resulted in complete refinement of coating microstructure with elimination of splat boundaries and pores along with elemental homogenization. The corrosion behavior of as-sprayed and SFP treated coating was evaluated in 3.5% NaCl solution using potentiodynamic polarization and electrochemical impedance spectroscopy. The SFP treatment reduced the corrosion rate of as-sprayed coating by an order of magnitude. Long-time immersion studies showed continuously decreasing impedance of the as-sprayed coating due to the penetration of the electrolyte along the splat boundaries. In contrast, impedance for the SFP treated coating increased with the immersion time due to the removal of all microstructural defects.