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The purpose of this work was to carry out comparative studies of WC-Co-Cr coatings deposited using the high velocity oxy fuel (HVOF) method onto two types of substrate material: structural steel S235 and magnesium alloy AZ31. The influence of the substrate material type on the microstructure, phase composition, crystallite size, porosity, Vickers microhardness, instrumental hardness (HIT), Young's modulus (EIT), and fracture toughness was investigated. For both substrates, the deposited coatings deposited were characterized with fine-grained and compact microstructure. The X-ray diffraction (XRD) revealed presence of following phases: WC, W2C, Co0.9W0.1, and Co3W9C4. The WC phase was the most desirable and stable one with crystallites were below 100 nm. On the other hand, the size of the W2C crystallites was below 30 nm. The coatings obtained showed porosity values equal to 2.3 ± 0.4 vol% and 2.8 ± 0.7 vol% for AZ31 and S235, respectively. The average Vickers microhardness for both types of sample was appproximately 1200 HV0.3. The average HIT values for carbide particles and metallic matrix were around 29 GPa and 6.5 GPa, respectively. In the case of EIT, it was around 620 GPa and 190 GPa for WC and Co-Cr, respectively. The differences between coatings were negligible. The EIT value for both coatings was equal to 344 ± 11 GPa. The fracture toughness was around 4.5 MPa · m1/2 in both cases. The investigations revealed that it is possible to replace steel substrate material with a much lighter equivalent, in this case AZ31 alloy, without deterioration of the coating properties. [ABSTRACT FROM AUTHOR]

As modern air engines’ working temperatures are increasing, materials and coatings' hot corrosion resistance characteristics gain significant attention. Hot corrosion is a type of degradation at high temperatures that involves oxidizing or sulphidation of the substrate behind a layer of salt melt deposit, which either causes the development of a thick layer of sulphide scale or the penetration of sulphur via grain boundaries into the matrix to a deeper depth. It may notably change the microstructure, phase composition, and characteristics of the thermally sprayed coating. In recent years, thermally sprayed cermet coatings on steel have been more well-liked as a possible method for enhancing hot corrosion resistance. This review paper qualitatively summarizes the recent development of thermal sprayed coatings to improve hot corrosion performance.

The article describes the influence of a solid lubricant such as graphite on the coating-substrate adhesion, susceptibility to cracking during three-point bending tests and tribological properties of the cermet coating-substrate systems. Two types of deposits Cr3C2-25(Ni20Cr) and (Cr3C2-25(Ni20Cr))-5(Ni25C), cold-sprayed on the Al 7075 alloy substrate, were analyzed. The Cr3C2-25(Ni20Cr) coatings showed a homogeneous microstructure with evenly distributed ceramic particles in a Ni20Cr matrix. The (Cr3C2-25(Ni20Cr))-5(Ni25C) deposits also contained graphite placed both between metallic particles and near the crushed ceramic Cr3C2 particles. The force required for the crack that appeared in the coating-substrate system during the three-point bending test under constant velocity was significantly higher in the case of (Cr3C2-25(Ni20Cr))-5(Ni25C) deposit than in that without the solid lubricant. The cracks were observed perpendicular to the coating-substrate interface. The graphite embedded in the cermet coating structure prevented the formation of crack nuclei during three-point bending test under cyclic load at room temperature and reduced the size of cracks in the deposit at 200 °C. Both cermet coatings revealed the same adhesion. The addition of graphite not only did not deteriorate the adhesion of the deposits and thus their quality but also improved their other properties, such as flexural strength and wear resistance. Coatings containing the solid lubricant showed a lower wear index than the Cr3C2-25(Ni20Cr) deposits examined at both room and elevated temperatures. This recommends their use in industry as deposit working in heavy wear conditions. The presented results of mechanical tests effectively fill the gap regarding the properties of the cold-sprayed cermet coatings. [ABSTRACT FROM AUTHOR]

Four plasma cermet coatings with similar compositions based on TiC carbide and NiCrMo matrices with additional Cr3C2, WC carbides, and carbon have been studied. For four cermets, the average oxygen content increases from 0.51 wt % for powders to 0.86 wt % for coatings at the minimum plasma power and does not increase with its increase. The maximum average nitrogen content in coatings is 0.34 wt % and is determined by the nitrogen content in the plasma. During the spraying of coatings, the loss of carbon relative to the initial composition of powders is 2.79–3.76 wt %, which is less than in the manufacture of powders for spraying (4.3–6.6 wt %). Carbides Cr3C2 and WC, matrix elements, and the content of additional carbon determine the formation of an annular zone around the TiC carbide and increase the total content of carbides in the coating from 60 to 74–83 wt %. With an indenter load of 200 g, the microhardness of the coating is 18 GPa. [ABSTRACT FROM AUTHOR]

Five cermet coatings based on carbides 45TiC–10Cr3C2–5WC with different contents of additional carbon (0, 1.4, 2, and 2.8 wt %) were formed by plasma spraying with local protection. In four cermets, the matrix was based on Ni–20Cr. In one cermet, the alloy used was 38.5Co–32Ni–21Cr–8Al–0.5Y. All matrices were supplemented with Mo. Powders for spraying were obtained by crushing cakes. In the particles of the obtained powders, carbides were distributed relatively uniformly; in coatings, this was noticeable to a lesser extent. After liquid-phase sintering, WC and Mo were not fixed in cermets; part of the Cr3C2 carbide transformed into another structural state. The initial carbides in the cake and coating partially dissolved and, upon solidification and together with matrix elements and additional carbon, formed an annular zone around the initial TiC carbide, decreasing its lattice period. XRD fixed TiMoC2 carbide in annular zone, the content of which was higher than the content of TiC carbide in the initial mixture. The content of the initial carbides in the coatings, measured by optical microscopy, decreased from 71 vol % in the powder to 48 vol % at the minimum plasma power and to 36 vol % at the maximum power. The average total TiMoC2 content of carbides in coatings according to X-ray data in four cermets was 76 wt %, higher than their content in spraying powders (72 wt %) owing to higher spray hardening rates. The average microhardness for all coatings was 22.01 GPa at an indenter load of 20 gf, which was lower than the average microhardness for all powders (23.51 GPa). At an indenter load of 200 gf; the average microhardness for all coatings of 15.88 GPa corresponded to the average microhardness for all powders (15.17 GPa). [ABSTRACT FROM AUTHOR]

There is an excessive material loss in steels components due to sliding wear in different industrial applications. The SS 316 steel is extensively used in the power generation industry for boilers, induction fans, ducts, etc., and counter high sliding wear. The studies have shown that protective coatings deposited by thermal spray methods are successful in controlling the wear and enhancing the service life of steels. In this work, the nickel-alumina coating was deposited on SS 316 by cold spray technique to understand the effectiveness of coatings to resist wear. The wear behavior of coatings was analyzed by conducting the wear test on a pin-on-disc apparatus at different loads, i.e., 30, 40 and 50 N keeping the speed constant. The wear trends and variation in friction coefficient due to wear were observed. The mechanical and microstructural characterization was done by FE-SEM/EDS and XRD techniques. The coatings were found effective in resisting the wear on SS 316 steel. The results indicate that the wear rate of coatings increased with an increase in normal load.

Results of studies of the physical and mechanical properties of cermet coatings formed during carbo-vibro-arc hardening (CVAH) using a multicomponent paste containing nanosized aluminum oxide Al2O3 in its composition are presented. It was found that the maximum microhardness of such coatings was 1110 HV at the coating surface and 835 HV at the interface with the base metal. The microstructure was very fine with uniform distribution of strengthening ceramic phases B4C and Fe2B throughout the coating. The wear resistance was 1.9 times higher than in coatings that did not contain nanosized components in the multicomponent paste for CVAH. The results of the studies showed that these coatings are promising for hardening of machine parts of various functional purposes. [ABSTRACT FROM AUTHOR]

Results of studies of the physical and mechanical properties of cermet coatings formed during carbo-vibro-arc hardening (CVAH) using a multicomponent paste containing nanosized aluminum oxide Al2O3 in its composition are presented. It was found that the maximum microhardness of such coatings was 1110 HV at the coating surface and 835 HV at the interface with the base metal. The microstructure was very fine with uniform distribution of strengthening ceramic phases B4C and Fe2B throughout the coating. The wear resistance was 1.9 times higher than in coatings that did not contain nanosized components in the multicomponent paste for CVAH. The results of the studies showed that these coatings are promising for hardening of machine parts of various functional purposes. [ABSTRACT FROM AUTHOR]

• An air-stable hybrid-structure solar selective absorber (hs-SSA) was made on Si. • The ultra-thin textured Si (ut-Si) showed excellent flexibility and absorptivity. • The hs-SSA performed good thermal stability in the air and vacuum. • The Δα/α and Δε/ε were only 0.31% and 1.18% for air-annealing at 250 °C for 120 h. • Its α/ε keet at 0.962/0.352 after annealing at 500 °C in a vacuum for 72 h. A kind of air-stable hybrid-structured solar selective absorber (hs-SSA) with enhanced thermal stability is prepared by combining ultra-thin textured silicon (ut-Si) with cermet coating. The ut-Si obtained by the wafer thinning and fast texturization method shows excellent flexibility and light-trapping ability. Then the W-WO x coatings are deposited onto the ut-Si substrate in a spreading manner on the pyramid-like structure. The as-deposited hs-SSA exhibits good light-harvesting capability with spectral selectivity (α/ε) of 0.968/0.340. Additionally, the ut-Si shows its superiority in oxidation resistance and chemical stability compared with the traditional metal substrate. After 120-h air-annealing at 250 °C and 72-h vacuum-annealing at 500 °C, the performance losses are within a tiny and reasonable range, showing excellent chemical, structural and optical stability, exhibiting excellent application potential in medium-temperature solar harvesting field. [ABSTRACT FROM AUTHOR]

A promising method of hardening the working bodies of agricultural machines by soldering is considered. The method is based on the hardening and restoration of parts by soldering cermet plates. A VK-8 hard alloy (tungsten–cobalt alloy with 8% cobalt and 92% tungsten carbide) is used as the material of the plates. The dependence of the adhesion strength of the metal plowshare–cermet joint on the angle of wetting with a solder, the area of its spreading, and the gap is studied. Under rational soldering conditions, the adhesion strength between the cermet plates and plowshare steel is 120–125 MPa. The experimental research methodology also includes comparative laboratory and operational tests of commercial and experimental plowshares restored by soldering the cermet plates. The experimental plowshares restored according to the developed technology are found to have high abrasive and corrosion-mechanical wear resistance. The use of cermet plates allows us to increase the resource of the restored plowshares by 4.6–5.0 times as compared to commercial plowshares. The developed technology is recommended for implementation at repair and technical enterprises engaged in the repair and restoration of agricultural machinery. [ABSTRACT FROM AUTHOR]

The paper presents the computational studies on the microstructure of WC-Co-Cr coatings deposited by High Velocity Oxy Fuel spraying (HVOF). The study covers the porosity assessment according to ASTM E2109-01 standard, carried out in ImageJ software, in terms of volume porosity, size and shape of the pores. The evaluation was preceded by scanning electron microscope (SEM) observations at magnifications of 2000x and 5000x. Additionally, topography analysis has been performed by confocal laser scanning microscope (CLSM), and the surface roughness Ra was evaluated by the contact method with use of a stylus profilometer. Finally, the influence of porosity was observed for coatings microhardness HV0.3. According to the results, the total closed porosity was found to be in the range of 5.01 vol.% and 5.38 vol.%. The dominated pores in the coatings were of size 0.1-1.0 μm. Studies showed that HVOF process enabled deposition of dense coatings, characterized by homogenous distribution of pores and low roughness. [ABSTRACT FROM AUTHOR]

Many machinery parts operating in contact with rapidly flowing fluid flows (for instance, turbine blades of hydroelectric power plants, valves, pump impeller blades, ship propellers, cooling systems of various assemblies, and so on) are exposed to wear such as cavitation erosion. The elimination or reduction of cavitation erosion is an urgent task, because it makes it possible to achieve significant cost efficiency. This work describes a procedure that has been developed and patented for estimating resistance against the cavitation erosion of cermet thermal sprayed coatings (WC–10Co4Cr and WC–20CrC–7Ni). The cermet coatings were produced by high velocity air fuel (HVAF) spraying. The aim of this work is to test the new estimation procedure of coating resistance against cavitation impact, which differs from the standard procedure by position of tested specimen with regard to fluid used for testing. In addition, the structure of the coatings has been analyzed in the initial state before tests and their behavior after cavitation impact has been studied using scanning electron microscopy. The coating resistance has been estimated by the criterion of material volume loss. The experimental results have demonstrated that the cavitation resistance of the WC–20CrC–7Ni coating is somewhat higher in comparison with WC–10Co4Cr, despite its lower average hardness (850 ± 90 HV0.5 against 950 ± 60 HV0.5). An analysis of surface and transversal cross sections of the coatings demonstrates that they are characterized by different mechanisms of erosion destruction. It can be concluded that the existence of defects (pores) in the coating structure is the main reason promoting a decrease in their resistance against cavitation erosion. Therefore, this procedure has proven its efficiency upon obtaining experimental data for an analysis of the cavitation wear of cermet thermal sprayed coatings. [ABSTRACT FROM AUTHOR]

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The NTC20 and NTC40 composite powders were produced by conglomeration from the NiCrBSiC self-fluxing alloy with additions of 20 and 40 wt.% TiCrC, respectively, using an organic binder. Since the organic binder burnt out in the plasma spray process, components of the composite powders segregated and a certain amount (up to 15–20 vol.%) of the TiCrC particles was lost. The plasma-sprayed NTC20 and NTC40 coatings showed a heterophase lamellar structure, consisting of a nickel-base matrix in which fine chromium boride and carboboride grains (1–2 μm) and TiCrC particles (5–8 μm) were evenly distributed. Electron microprobe analysis revealed a higher content of oxides in the NTC20 and NTC40 coatings compared to the NiCrBSiC coating, which was associated with the oxidation of TiCrC particles in the plasma spray process. The introduction of TiCrC additions into the NiCrBSiC self-fluxing alloy increased porosity of the plasma-sprayed NTC20 and NTC40 coatings (up to 8%) versus the NiCrBSiC coating (5%). Wear tests of the plasma-sprayed NTC20 and NTC40 coatings were performed in dry sliding friction conditions using 65G steel as a counterface. For comparison, a plasma-sprayed coating consisting of the serial NiCrBSiC self-fluxing alloy was tested. Additions of 20 and 40 wt.% TiCrC particles to the NiCrBSiC self-fluxing alloy increased the wear resistance of the plasma-sprayed coatings by 2 to 2.3 times. As the sliding speed increased from 4 to 12 m/sec, the NiCrBSiC coating underwent catastrophic wear (I ≈ 60 μm/km), while the wear rate of the NTC20 and NTC40 coatings remained constant (I ≈ 12–22 μm/km). [ABSTRACT FROM AUTHOR]

Superhydrophobic surfaces that are durable and can be easily manufactured are of high interest for many industrial applications. Measuring and understanding roughness in the context of superhydrophobicity is the first step in creation of a surface that does not require activation to be hydrophobic. In this study, the as sprayed surface of different cermet (WC-10Co4Cr and Cr 3 C 2 -25Ni20Cr) coatings produced by High Velocity Air Fuel (HVAF) spraying – have been investigated to assess their wetting ability. In order to address the challenges raised by the specific roughness profile of thermal spray surfaces, two routes have been adapted and used for surface characteristics analysis i.e. statistical and fractal. Results show that both methods have a strong correlation to wettability. Roughness parameters S dq and S dr show good correlation with advancing contact angle. Hausdorff Dimension of a sub-micrometer profile shows good relation with the contact angle and provides information for state of the droplet. To determine how to increase the contact angle of the coating surface, coating parameters such as CGS Density have been correlated with Hausdorff Dimension. Both methods provide good understanding in terms of wettability of rough cermet surfaces. Unlabelled Image • High performance cermet coatings produced by High Velocity Air Fuel Spraying. • Hydrophobic properties of the coatings without surface activation • Roughness – wettability correlations by statistical analysis and fractal analysis • Wetting characteristics predictability [ABSTRACT FROM AUTHOR]

Abstract Thermally sprayed carbide-based coatings are nowadays extensively considered as an alternative to electrolytic hard chrome (EHC) coatings to reduce the environmental impact and the overall cost associated with EHC process. In this investigation, high-velocity oxy-fuel (HVOF) spray process was employed to prepare coatings using the traditional carbide powders namely the WC-10Co4Cr, the Cr 3 C 2 -25NiCr and a new type of mixed carbide powder WC-40Cr 3 C 2 -25NiCr. The Powder deposition rate, basic mechanical properties, abrasive wear, slurry erosion and corrosion resistance of the three coatings were then compared with the EHC coating. The results show that WC-10Co4Cr coating exhibited the highest hardness, abrasive wear and slurry erosion resistance followed by WC-40Cr 3 C 2 -25NiCr, EHC, and Cr 3 C 2 -25NiCr coating. The deposition efficiency of the powders as per hierarchy was found to be WC-40Cr 3 C 2 -25NiCr > WC-10Co4Cr > Cr 3 C 2 -25NiCr and all the HVOF sprayed coatings exhibited higher corrosion resistance than EHC coating. The highest powder deposition efficiency coupled with low density, acceptable tribo-corrosion performance, as well as low post processing cost makes the HVOF sprayed WC-40Cr 3 C 2 -25NiCr coating a potential candidate to replace the EHC coating. Highlights • A new type WC-40Cr 3 C 2 -25NiCr coating used for replacement of EHC was explored. • Wear, erosion and corrosion resistance of HVOF sprayed coatings and EHC were compared. • HVOF sprayed WC-40Cr 3 C 2 -25NiCr coating showed moderate performance with low cost. • The new coating deposited on rotor showed excellent performance in saturated saline. [ABSTRACT FROM AUTHOR]

Abstract In this study, 4 types materials, i.e., WC-12Co, WC-10Co4Cr, NiCr-Cr 3 C 2 and Ni60 were coated on the bare copper to simulate the wear resistant materials on the surface of strip casting rolls. Then, these coatings were characterized by the field emission scanning electron microscopy (FE-SEM), energy-dispersive spectrometer (EDS), X-ray diffraction (XRD), and laser scanning confocal microscope (LSCM). Next, the heat transfer performance of those coatings was evaluated through the droplet solidification tests. The results indicated that the maximum interfacial heat flux and the total heat-removed is varying with different coatings, in which the highest values of 6.25 MW/m2 and 7.21 MJ/m2 in the case of bare copper system could be reduced to 2.62 MW/m2 and 4.22 MJ/m2 with the application of WC-10Co4Cr coating. The heat transfer performance of different coatings is determined by the comprehensive effects of the characteristics of coatings, such as microstructure, porosity, phase composition, adhesion property, and thickness, etc. Furthermore, the results demonstrate that the Ni60 coating with a low porosity (0.78%), good adhesion property and thin thickness (14.1 μm), etc., shows a better heat transfer capability, through which the maximum heat flux is 5.24 MW/m2 and total heat removed is 5.78 MJ/m2, and shows great potential to be used in the strip casting process. Graphical abstract Image 1 Highlights • Characteristics of the coatings of strip casting rolls were investigated. • Heat transfer performance was evaluated by droplet solidification test. • Coating property exhibits significant effects on heat transfer performance. • The Ni-contained coatings show a better heat transfer performance than WC-contained coatings. [ABSTRACT FROM AUTHOR]

WC-12 Co coatings were prepared on copper matrix composites by high velocity oxygen-fuel flame(HVOF) thermal spraying. Their microstructure,surface and cross-section hardness,composition and content of phases,and tribological behavior were studied. The results show that the coatings appear uniform microstructure and high hardness. During the wear,two kinds of tribofilms form. The most suitable condition for the coatings is 300-500 r·min-1 and 2-3 N. The wear rate increases with increasing load. While the rotating speed increases,the wear rate presents a trend of first decreasing and then increasing. The fitting equations between wear rate and sliding speed and between wear rate and normal load are quadratic and exponential,respectively.

Effect of structure and phase composition of cermet coatings of Al2O3-Ti (Al) system produced using a unit for multichamber detonation spraying was investigated. Analysis of structure peculiarities of investigated coatings was performed applying optical metallography, analytical scanning as well as transmission microdiffraction electron microscopy. It is shown that dispersion of grain and subgrain structures as well as distribution of forming hardening phases of dispersion size make the most significant contribution in the indices of strength, ductility and crack resistance of the investigated coatings. [ABSTRACT FROM AUTHOR]

The ability to deposit nanostructured feedstock by using high-velocity oxygen-fuel (HVOF) spray offers potential improvements in coating hardness, wear resistance and toughness for applications in the oil sands industry. In this study, the wear behavior of a near-nanostructured coating was compared under dry and slurry abrasive wear test using an uncoated AISI-1018 low-carbon steel substrate as a reference. The coating microstructures were analyzed in the as-sprayed, dry and slurry test conditions using scanning electron microscopy, x-ray diffraction and microhardness measurements. Wear behavior of the steel and coating surfaces were assessed using a pin-on-plate wear test under various loads. The results showed that a coating could be successfully deposited using the HVOF spraying technique and with retention of the near-nanosized WC dispersion within the coating structure. The wear rate under dry test conditions was greater for the steel and coating compared to tests performed under slurry conditions. Examination of the wear tracks revealed that the wear mechanism was different for the two test conditions. Wear in the dry test condition resulted from 2-body abrasion, while 3-body abrasion dominated wear in slurry conditions. The latter showed lower wear rates due to a lubricating effect of the oil. [ABSTRACT FROM AUTHOR]

Cr/Al 2 O 3 coatings were deposited by low-power plasma spray process with dry-crushed powders. The coatings exhibited a microstructure formed by fully molten splat-like metallic lamellae and partially molten or un-molten submicron Al 2 O 3 particles, surround by a fully molten alumina matrix. The content and size of metallic Cr in the coatings played a dominant role in the formation of metallic clusters. Complex permittivity measurement showed that the coatings with higher Cr content and larger Cr particle size exhibited higher ε′ and ε″ values due to the enhanced interfacial polarization and conductance loss. Furthermore, strong frequency dependency was observed for the coatings with 15 wt% Cr addition and 50–74 μm Cr particle size. [ABSTRACT FROM AUTHOR]

[Display omitted] • Cavitation erosion of HVOF cermet coatings was investigated. • Sliding wear and erosion mechanisms of HVOF cermet coatings were analyzed. • Microstructure of the cermet coatings by HVOF spraying onto AZ31 magnesium alloy. This work studies the effect of mechanical properties and microstructure on sliding wear and cavitation erosion behaviors of cermet coatings deposited via high velocity oxy-fuel (HVOF) spraying on AZ31 magnesium substrate. Cavitation tests indicated that WC-Co-Cr cermet had superior resistance to cavitation erosion than WC-Cr 3 C 2 -Ni, WC-Co coatings, and far better resistance than the AZ31 expressed by the volume material loss 3.74 mm3, 6.99 mm3, 10.30 mm3, and 108.82 mm3, respectively. WC-Co-Cr coating's erosion mechanism exhibited uniform material removal, which slows the erosion rate. The CoCr binder prevented severe surface pitting and the detachment of the WC-Co-Cr cermet material in massive chunks which was observed for WC-Co and WC-Cr 3 C 2 -Ni coatings. Cermet coatings microstructure discontinuities such as pores and ceramic-metallic phase interfaces are the centers of material erosion. No clear correlation between the erosion behavior and mechanical properties of coatings was revealed. Contrary to that, sliding wear results were strongly related to the mechanical properties. The WC-Co-Cr and WC-Co samples exhibited higher hardness and higher values of Young's modulus than the WC-Cr 3 C 2 -Ni one. The generated stresses with lower values of hardness and Young's modulus for the WC-Cr 3 C 2 -Ni sample resulted in a higher wear factor, approx. 1·10−7 mm3·N−1·m−1. For the harder samples, which exhibited higher stiffness (WC-Co-Cr and WC-Co), this type of wear resulted in lower values of the wear factor: 6·10−8 mm3·N−1·m−1 and 9·10−8 mm3·N−1·m−1, respectively. Deposition of the cermet coatings effectively protected the magnesium substrate, which shows low resistance to cavitation erosion and sliding wear when uncoated. [ABSTRACT FROM AUTHOR]

Low-power plasma-sprayed FeCrAl/AlO composite coatings with 1.5 mm thickness have been fabricated for radar absorption applications. The effects of FeCrAl content on the coating properties were studied. The FeCrAl presents in the form of a few thin lamellae and numerous particles, demonstrating relatively even distribution in all the coatings. Results show that the micro-hardness and porosity decrease with the increase in FeCrAl content. With FeCrAl content increasing from 28 to 47 wt.%, the bonding strength of the coatings with 1.5 mm thickness increases from 10.5 to 27 MPa, and the failure modes are composed of cohesive and adhesive failure, which are ascribed to the coating microstructure and the residual stress, respectively. [ABSTRACT FROM AUTHOR]

Optimized processing windows for spraying high-quality metal carbide-based coatings are developed using particle diagnostic technology. The cermet coatings were produced via the high-velocity oxygen fuel (HVOF) spray process and are proposed for service applications such as marine hydraulics. The traditional 'trial and error' method for developing coating process parameters is not technically robust, as well as being costly and time consuming. Instead, this contribution investigated the use of real-time monitoring of parameters associated with the HVOF flame jets and particles using in-flight particle diagnostics. Subsequently, coatings can be produced with knowledge concerning the molten particle size, temperature, and velocity profile. The analytical results allow identification of optimized coating process windows, which translate to coatings of lower porosity and improved mechanical performance. [ABSTRACT FROM AUTHOR]

Cr/Al 2 O 3 coatings were fabricated by plasma spraying technology from ball milled and spray dried feedstock powders, and their microstructure, complex permittivity and microwave absorption properties were investigated. The results show that Cr particles were uniformly distributed throughout the coating with the evidence of pores, cracks and a few light contrast stringers. The complex permittivity of Cr/Al 2 O 3 coatings increased with increasing Cr volume fraction, and the coating with higher chrome content exhibited visible frequency-dependent dielectric response. The minimum reflection loss ( RL min ) shifted to a lower frequency region with the increase of chrome content and coating thickness. RL min of the coating with 15 vol% Cr reached –33.3 dB at 8.8 GHz and the reflection loss values below –10 dB could be obtained in the frequency range from 8.2 to 9.7 GHz when the coating thickness was 1.7 mm. These results imply that the microwave absorption properties can be manipulated by tuning the coating thickness and permittivity values. [ABSTRACT FROM AUTHOR]

Cold spray is a severe plastic deformation-based processing technique. The material undergoes extreme environments and a large energy can be assumed to be stored in material in form of strain, dislocations, and vacancies. The thermal stability and structural integrity of cermet coatings processed by cold spray for elevated temperature applications are rather important but not well understood. Here, commercially pure-Ni (CP-Ni) and Ni-Y 2 O 3 cermet coatings were fabricated and annealed by an in-situ hot-stage transmission electron microscopy (TEM) technique to investigate and compare the microstructural evolution of these two materials including strain relaxation, sub grain boundary and grain boundary motion which as otherwise impossible to probe using ex situ annealing. The as-deposited CP-Ni exhibits crystallized ultra-fine grains (UFG), while the Ni-Y 2 O 3 coatings show a combination of nanocrystalline (NC) and UFG grains and a non-uniform distribution of Y 2 O 3 in the Ni matrix. The hot-stage TEM results show a decreased strain contrast and limited grain growth for both materials annealed at 350 °C for 2 h. When the materials are continuously heated from 25 °C to 800 °C in 19 min, the presence of nano Y 2 O 3 particles shows pinning effects at grain boundaries, resulting in improved thermal stability of Ni-Y 2 O 3 coatings compared with CP-Ni coatings. • The as-deposited cold sprayed CP-Ni exhibits crystallized ultra-fine grains (UFG). • Ni-Y 2 O 3 coatings show a combination of nanocrystalline and UFG grains and a non-uniform distribution of Y 2 O 3 in the Ni matrix. • Grain growth in the two-dimensional specimen is more sluggish compared to that in the three-dimensional specimen. • Nano Y 2 O 3 addition shows pinning effects at grain boundaries, resulting in improved thermal stability of Ni-Y 2 O 3 coatings. [ABSTRACT FROM AUTHOR]

This study deals with some structural characteristics, mechanical properties and corrosion behaviour of the chromium carbide coatings produced on microalloy steel by using electrospark deposition (ESD) process. The present study was primarily focused on the electrical parameters of deposition process to achieve a hard and thick coating. Thus, the electrical parameters of process and material transfer were investigated systematically and then the average thicknesses of deposited layers were determined. Using electrical pulses with 0.032 C electricity (current amplitude of 300 A, pulse duration of 100 μs), it has been possible to obtain a uniform coating layer with an average thickness of 80 μm after the third coating pass. The results indicate that the coating consists of fairly hard Cr 7 C 3 and Cr 0.19 Fe 0.70 Ni 0.11 phases and the wear resistance of steel substrate, which is coated by using ESD process, is noticeably enhanced under dry sliding wear conditions. Also, while the microhardness of steel substrate is 227 Hv on average, the microhardness of coating reaches 1381 Hv onto the surface. In addition to that, the corrosion resistance of steel substrate has been improved by approximately 30%. [ABSTRACT FROM AUTHOR]

The paper presents the influence of substrate preparation on low pressure cold sprayed (LPCS) coatings adhesion strength and microhardness. The LPCS process parameters such as: (i) gas temperature; (ii) gas pressure; (iii) gun linear speed; (iv) powder feeding rate; and (v) spray distance were kept constant during deposition. The coatings were sprayed using metallic powders having different morphology, namely spherical (Al, Zn, Sn, Cu) and dendritic (Ni, Cu) with the grain size of about − 50 + 10 μm. These powders were mixed with alumina before spraying in a weight ratio of 50:50. Copper and aluminium alloy were used as substrates. The substrate preparation included sand-blasting and grinding. The coating microstructures were characterized by scanning electron microscopy (SEM) and X-ray diffractometry (XRD). The adhesion strengths of coatings were determined by pull-off method and showed that coating adhesion reached 65 MPa. The measurements of coatings microhardness were carried out also. Finally, the values of adhesion strength and microhardness of coatings were correlated with the substrate preparation and the microstructure of coatings. The microhardness of LPCS coatings was in the range of 20 to 201 HV0.02 and depended mostly on sprayed powder material. [ABSTRACT FROM AUTHOR]

It is a well-known fact that application of supersonic methods of thermal spray deposition results in significant increase of service properties of parts. However, up to moment no trails were made on producing of carbide or cermet coatings with the help of supersonic methods of plasma spraying. Present work is dedicated to production of cermet coatings from chromium carbide-nichrome compositions using a method of supersonic plasma gas air spraying (SPGAS). SPGAS technology has series of advantages in comparison with well-known technology of HVOF spraying. First of all, it concerns efficiency, economy and characteristics (temperature, speed) of gas jet. Structure and phase composition of produced coatings were investigated. It is shown that spraying using supersonic gas jets promotes for increase of content of Cr7C3 carbide and reduction of NiCr in the coatings as a result of oxidation and formation of oxides. Coating from composite powder differs by higher level of density and homogeneity, has layered fine-laminated structure with inclusions of fine carbides, contains lower amount of oxide phase, but larger quantity of chromium carbide Cr3C2 in comparison with coating from mechanical mixture Cr3C2 + NiCr. These coatings can be recommended for application as wear-resistant at increased temperatures. [ABSTRACT FROM AUTHOR]

The tribological performance of High Velocity Air-Fuel (HVAF) sprayed WC-based cermet coatings with binders containing no or very limited amount of cobalt was evaluated under dry sliding, erosion, and abrasion wear conditions. The wear and corrosion behaviors of WC-NiMoCrFeCo, WC-FeNiCrMoCu and WC-FeCrAl HVAF sprayed coatings were investigated and compared to standard WC-CoCr coatings as benchmark. Microstructure characterization along with XRD analysis was conducted on all powders as well as the corresponding coatings. Comprehensive post wear analysis was conducted on all coatings subjected to ball-on-disk, gas jet erosion and dry sand-rubber wheel abrasion tests. Moreover, all coatings were exposed to 3.5% (wt./vol.) NaCl aqueous solution to evaluate their corrosion performance through electrochemical testing. XRD results showed negligible phase transformation between the powders and the deposited coatings. The WC-NiMoCrFeCo coating exhibited the best sliding wear and electrochemical corrosion performance, with an average specific wear rate value of 3.1 × 10−8 (mm3·N−1·m−1) and a corrosion current density of 1.9 μA/cm2. This coating also showed comparable abrasive wear resistance to the WC-CoCr coating. Under erosive wear conditions, too, the WC-FeNiCrMoCu and WC-FeCrAl coatings showed a comparable performance to the benchmark. Dominant wear mechanisms for the reference WC-CoCr coating, under sliding wear conditions, were abrasion (deep grooving) and surface fatigue (crack propagation and pitting). On the contrary, no pitting was observed in WC-NiMoCrFeCo and WC-FeCrAl coatings during the sliding wear test. No considerable difference was identified in the wear mechanisms of the different coatings under abrasion and erosion wear conditions. The results highlight the promise of some of the environment friendly binders studied to replace Co. • HVAF-sprayed coatings showed very little decarburization and oxidation. • WC-NiMoCrFeCo coatings exhibit the best sliding wear performance, comparatively poorer jet erosion resistance. • WC-FeNiCrMoCu and WC-FeCrAl coatings showed slightly lower sliding and abrasive wear resistance. • The NiMoCrFeCo matrix gives the best corrosion protection due to its better pseudo-passivation ability. • Fe-based matrices are comparatively less protective against corrosion. [ABSTRACT FROM AUTHOR]

Cermets coatings are extensively used in energy applications both because of their high wear resistance as required, for example, in components like gas turbine sealants, and because of their specific functionality as required in solar absorbers. So far, high-temperature thermal spraying and physical vapor deposition have traditionally been used to deposit this kind of coatings. In this study, Ni-AlO coatings have been deposited using a Kinetic3000 cold-spray system starting from Ni and AlO powders blend; five blends have been prepared setting the alumina content in the feedstock to 10, 25, 50, 75, and 90 wt.%. The embedded alumina ranges between a few percent weight up to 16 and 31 wt.%, while the microhardness shows a deep increase from 175 Vickers in the case of pure Ni coatings up to 338 Vickers. The spray and coating growth mechanism have been discussed, with special attention to the fragmentation of the ceramic particles during the impact. Finally, the coating behavior at high temperature was analyzed by oxidation tests performed in air at 520 °C emphasizing a good oxidation resistance that could represent a very promising basis for application in power generation systems. [ABSTRACT FROM AUTHOR]

This work demonstrated the feasibility of cold spray for the achievement of industrial electrical contacts to be used in contactors and circuit breakers. This resulted from the study of cold spray conditions when applied to two types of silver-based particulate composites. Starting materials, primarily powder characteristics and addition content, and spraying conditions were optimized from thorough studies of microstructures and mechanical properties of the cold-sprayed composites. These involve x-ray microtomography and transmission electron microscopy in particular, in addition to tensile tests. Moreover, thermomechanical laser testing of series of cold-sprayed microstructures was successfully developed to investigate into damage mechanisms in coatings. The whole led to a novel phenomenological approach to damage in cold-sprayed materials. Durability tests of the best coatings were carried out using industrial facilities. Cold-sprayed contacts compared more than well with those from conventional powder metallurgy (P/M), which opens a new and cost-attractive production route for electrical contacts. [ABSTRACT FROM AUTHOR]

Soldering and erosion are two of the biggest serious problems faced in the die-casting industries. Cermet coatings utilized by high-velocity oxy-fuel (HVOF) spray technology have been developed in an attempt to overcome these problems. MoB-based cermet feedstock powders (MoB/NiCr and MoB/CoCr) were deposited on SKD61 (AISI H-13) substrates used as a preferred die (mold) material. Microstructural and mechanical properties of the coatings have been characterized by scanning electron microscopy, x-ray diffraction, Romulus bond strength test, and Vickers microhardness test. The durability of these coatings on cylindrical specimens against soldering also has been investigated by immersing in molten aluminum alloy (ADC-12) for 25 h at 670 °C and subsequently, compared with that of NiCr and CoMoCr coatings. Both types of MoB-based cermet coatings have shown high soldering resistance as negligible intermetallic formation occurred during the immersion test. This result is attributed to the existence of multiple inert borides in the coatings. The coatings also showed excellent mechanical properties. MoB/NiCr, in particular, showed higher bond strength, hardness, and wear resistance than MoB/CoCr. This suggests that MoB/NiCr will show higher durability than MoB/CoCr, NiCr, and CoMoCr during high pressure die-casting of aluminum alloys. [ABSTRACT FROM AUTHOR]

High velocity oxy-fuel (HVOF) spray process has grown into a well-accepted industrial technology for obtaining coatings resistant to significant surface degradation processes. In the present study, HVOF process was used to deposit Ni-based hardfacing NiCrFeSiB alloy powder on kinds of boiler tube steels designated as SA210 grade-A1, SA213-T11, and SA213-T22. The microstructures and several properties of the as-sprayed coatings have been investigated. Thermocyclic oxidation studies were performed in static air at 900 °C. NiCrFeSiB-coated steels showed slow oxidation kinetics and considerably lower weight gains than that of uncoated steels. The superior performance of NiCrFeSiB coatings can be attributed to continuous and protective thin oxide scale of amorphous SiO and CrO formed on the surface of the oxidized coatings. The combined technique of x-ray diffraction, scanning electron microscopy, energy dispersive x-ray analysis, and electron probe microanalysis are used to characterize reaction products of the oxidized surfaces. [ABSTRACT FROM AUTHOR]