Light alloys are being investigated as an alternative to ferrous-based engineering components. The manufacturing of such components requires a surface modification step necessary to eliminate the top surface's poor wear and corrosion response for improved functionality. Thermally sprayed cermet coatings offer improved surface resistance to wear and/or corrosion. This work presents a customized composition of WC-CoCr feedstock cut in fine and coarse powder size distribution (PSD) to fabricate different coatings on aluminium alloy and steel substrates using two high velocity spray techniques. The WC-CoCr coatings sprayed using the high velocity air-fuel (HVAF) technique at varied parameters consist of six different coatings (four thick, ~ 200 μm and two thin ones, 60-80 μm) to investigate the relationship between processing conditions, microstructure, and performance. Using scanning electron microscopy (SEM) and electro-dispersive X-ray spectroscopy (EDX) offered a comprehensive characterization of the respective coatings. Micro indentation, dry sliding wear, dry sand abrasion, and cavitation erosion tests conducted on the samples show the performance of the coatings based on the processing techniques and spray conditions. However, despite the similarities in the microstructural makeup of the coatings and the measured micro indentation hardness of the coatings (1000-1300 HV0.1), their respective specific wear rate (SWR) varied based on spray processing techniques and the substrate on which the coatings were deposited. Three of the HVAF coatings showed ~ 60 % more wear on the aluminium alloy substrate compared to the same coating deposited on a steel substrate. However, irrespective of the substrate used the HVAF coatings showed better wear resistance than the HVOF coating. The dry sand abrasion wear results of the two thick HVAF coatings show them superior to the HVOF coating in the three-body wear experiment conducted. The cavitation erosion resistance of the coatings varied based on the processing conditions and the driving mechanisms but the best two were the AF-2 and AF-6 samples.

Silicon coatings have been developed for environmental barrier coatings by thermal spraying. Until now, these coatings have been produced almost exclusively by Atmospheric Plasma Spraying (APS). High Velocity Oxy-Fuel (HVOF) spraying is commonly used to produce dense metallic and carbide-based coatings due to high particle velocities. However, there have been no scientific reports on HVOF-sprayed silicon coatings in the literature. This study was conducted to investigate the feasibility of fabricating silicon coatings by HVOF using a DJ2600 spray system. Both the spray powders and the parameters were varied. The coatings were investigated on their surfaces and cross-sections using scanning electron microscopy (SEM) and X-ray diffraction analysis (XRD). The hardness and indentation modulus of the silicon coatings were also determined. The results show that the particle size distribution and the stand-off distance are important influencing factors. Dense coatings could be produced by HVOF spraying, confirming the feasibility.

Twin wire arc is a commonly used thermal spray technology for application of steel coatings to cast iron components. Hardness and adhesion strength are critical properties of such coatings, and significant research is available reporting these properties. However, residual stresses and the anisotropic structure of the coatings leads to significantly different behavior in bending applications than in the purely tensile loading of the standard adhesion test. In addition, microstructural features that are controlled by certain process parameters during deposition of the coating can have a significant effect on these properties. This work seeks to relate the hardness and pull-off adhesion strength to the coating microstructure, and to assess the related bending strength and failure mode. Comparisons between bend tests and pull-off adhesion tests show significant differences to consider when evaluating twin wire arc coatings.

This study assesses the influence of particle size and spray parameters on the structural, mechanical, and electrical insulation properties of alumina coatings deposited by atmospheric plasma spraying. It has been found that the combination of a relatively fine feedstock powder and high velocity plasma spraying promotes the formation of denser coatings with high dielectric strength. Correlations between dielectric strength and deposition efficiency, coating hardness, crystal structure, and surface roughness are also assessed.

This study compares the performance of an ordinary plasma spray gun with that of recently developed gun designed for spraying feedstock suspensions on inner diameter surfaces. To evaluate the guns, yttrium oxide was deposited on 304 stainless steel while varying supply pressure, spraying distance, and gun traverse speed. Different methods of delivering suspension spray material to the gun were also investigated. Although Y 2 O 3 inner-diameter coatings were successfully formed, hardness and cross-sectional porosity need improvement. Based on the findings, it may be necessary to increase substrate temperature, readjust spraying parameters, and optimize feedstock materials.

In this investigation, NiTi coatings are applied by atmospheric plasma spraying. Surface and interface morphology of the as-deposited material is studied using scanning electron microscopy, the presence of the different phases are revealed by X-ray diffractometry, and microhardness is determined by Vickers hardness testing. The as-deposited coatings are exposed to air-borne particle erosion to investigate their wear properties while varying erodent impact pressure and angle. It was found that the plasma sprayed NiTi splats are well formed with significant amounts of intermetallic and oxide phases at the surface and interface, contributing to dense splat formation and higher hardness.

This study investigates the influence of particle size, nozzle diameter, gas flow rate, and stand-off distance on the microstructure and density of suspension plasma sprayed yttrium-oxide coatings and the intermediate effect of particle characteristics. Three ethanol suspensions were prepared, one with coarse Y 2 O 3 , one with fine Y 2 O 3 , and one with submicron YSZ. The suspensions were injected vertically into the plasma jet downstream of the nozzle and a thermal spray sensor was used to measure in-flight velocity and temperature. The coatings were found to have columnar and dense vertically cracked (DVC) microstructure, varying in hardness and density. Text results and examination findings are presented and correlated with spray parameters, particle properties, and possible coating formation mechanisms.

In this study, an internal injection plasma torch is used to deposit nano-agglomerated YSZ feedstock powders on superalloy substrates at low ambient pressures ranging from 5000 Pa to 6000 Pa. Coatings with unique fully nano-equiaxed structures were obtained when operating the torch below 300 A. With increasing current, up to 700 A, coatings with mixed-grain and eventually large-grain structures were produced. Experimental results suggest that the equiaxed nanoscale structure derives from the original agglomerated nanoparticles that had undergone melting while inside the nozzle of the plasma torch and were subsequently solidified or sintered in the coating. Coating hardness and elastic modulus were also measured and are shown to correspond with microstructure.

This paper examines the microstructure and morphology of zirconia coatings and demonstrates the calculation of elastic modulus and Martens hardness based on instrumented indentation test results. Coatings samples varying in microstructure, phase content, and chemical composition were deposited by suspension plasma spraying using different torches and different suspension formulations. Coatings produced from low-concentration suspensions with submicron-size powders had a columnar structure with long vertical pores between the columns and fine spherical pores within the columns. Coatings made from suspensions with high concentrations of solids and coarser, more irregular powders, on the other hand, were more uniform and their surfaces smoother. They are also shown to be harder and have higher elastic modulus based on indentation test results.

This work demonstrates the fabrication of a hydroxyapatite (HA) composite material for potential use in biomedical implant applications. A composite powder is prepared by introducing graphene oxide (GO) and F- ions, which are incorporated in the HA crystal structure via in-situ chemical synthesis. The powder is consolidated through spark plasma sintering, resulting in a biocomposite (GO-FHA) material that is mechanically stronger and more chemically stable after implantation than HA. The addition of GO and partial substitution of F- also promote osteoblast proliferation as in-vitro bioactivity tests show.

In this study, Vickers indentation testing is used to determine the fracture toughness of hard flame spray coatings produced from cored-wire feedstocks, including WC-10Co4Cr, 75CrCo-25NiCr, and WC-12Co. Average measured values are compared with fracture toughness values calculated from seven different equations found in the literature in order to validate the experimental results and to better understand the relationship between fracture toughness and coating hardness for each of the tested materials.

A new nanoparticle plasma spray process has been developed that uses conventional powder feeders and injectors to produce fine ceramic coatings at high deposition rates. This paper explains how powder feedstocks are prepared and how the resulting coatings compare with coatings by other methods. The feedstock used in the demonstration was produced by adding YSZ nanoparticles to an acrylic liquid resin, which was then solidified, crushed, and screened. SEM images show that the nanoparticles are well dispersed throughout the resin fragments. In the plasma flame, the resin fragments burn down as the nanoparticles are heated and accelerated toward the substrate, producing fine zirconia layers free of microcracks and pores as observed by SEM. The presence of carbon deriving from the resin material is dealt with by post-process heating at different temperatures, the effect of which is assessed by means of thermogravimetry. Vickers hardness of the YSZ phase was measured to estimate the sintered density.

The mechanical, tribological, and corrosion properties of two hybrid coating systems were assessed: 1) a tungsten–tungsten carbide (W-WC) top layer and a laser cladded cobalt– chromium (Co-Cr) interlayer (Stellite 6 superalloy) applied to a 316 stainless steel substrate; and 2) the same W-WC top layer and an HVOF spray-and-fused Ni-W-Cr-B interlayer (Colmonoy 88 superalloy) applied to an Inconel 718 substrate. X-ray diffraction, energy dispersive spectroscopy, and scanning electron microscopy were used to analyze the microstructure of the coating layers. Microindentation was used to measure surface hardness and the hardness profile of the coating systems. Rockwell indentation was used to assess coating adhesion according to CEN/TS 1071-8. Surface load-carrying capacity was also assessed by measuring micro- and macrohardness at high loads. Tribological properties were assessed with a linear reciprocating ball-on-flat sliding wear test, and corrosion resistance was measured by potentiodynamic polarization and electrochemical impedance spectroscopy. W-WC layers showed class I adhesion to both the SS 316 and Inconel 718 substrates, with and without an interlayer. Hardness profile measurements on cross section showed hardness of 13.6 GPa and 7.0 GPa for W-WC and Co-Cr, respectively, with average hardness of 9.7 GPa for Ni-W-Cr-B. Furthermore, hardness measurements at different high loads revealed that the addition of an interlayer increases surface hardness by up to 200% compared to the same coating system provided without an interlayer, quantifying the additional load-carrying capacity provided by the supplementary interlayer. The tribological measurements show that, except for the Inconel 718 / Ni-W-Cr-B / W-WC system, the hardest interlayer or substrate leads to the highest wear rates. In addition, the W-WC layer showed excellent corrosion protection, with no pitting observed after potentiodynamic polarization testing.

This work focuses on the properties of Cu-Ag alloys deposited by cold spraying. Helium was used as the carrier gas, accelerating particles to 823 m/sec, which is in the middle of the deposition window for Cu alloys. To avoid oxygen contamination, the gun was placed in a helium-filled chamber and a closed-loop circulating system was used to minimize helium loss. Deposition parameters were varied during spraying and their effect on hardness, tensile properties, residual stress, and porosity was assessed in as-sprayed and heat-treated samples. Ultimate tensile strengths of 450 MPa and yield strengths of about 420 MPa were obtained for the as-sprayed samples and it was shown that strength and ductility can be tailored by heat treating, reaching elongation values higher than 45%. An increase in deposition rate from 55 to 142 g/min was also achieved without a significant decrease in mechanical properties.

In this study, a design of experiment (DOE) approach is used to investigate the influence of powder size and mixing ratio on the microstructure and properties of thermal spray coatings. Nano and micro sized WC-12Co powders combined in different proportions with Inconel 625 were deposited on carbon steel substrates by HVOF spraying. The resulting coatings are examined and the effect of different powder combinations on hardness and yield strength is assessed. Spraying procedures and test methods are described and the findings are discussed. Of the various coatings produced, one shows great promise for wear protection, particularly in oil and gas applications.

This work assesses the feasibility of using a high-velocity airfuel (HVAF) gun both to grit blast and spray substrate surfaces. A design of experiments (DoE) approach was used to establish relationships between grit blasting variables, substrate surface conditions, and coating properties. Alumina was selected as the abrasive media, the substrates were HSLA steel, and CrC-NiCr and Fe-based powders were used to form the coatings. Uncoated and as-sprayed substrates were characterized based on hardness, residue levels, surface roughness profiles, and adhesion strength, which are correlated with mesh size, feed rate, offset angle, and standoff distance.

In this study, MCrAlY-Al 2 O 3 composite powders were produced by ball milling and deposited by plasma, HVOF, and cold spraying. The results show that Al 2 O 3 fractions can be well controlled using composite powder due to non-preferential impact debonding of the matrix and Al 2 O 3 . The microstructure of spray powders is well retained in HVOF and cold-sprayed coatings due to the unmelted or partially molten condition of the spray particles. In the case of plasma-sprayed coatings, however, most Al 2 O 3 particles segregate at lamellar interfaces, forming a continuous oxide scale on the splat. The cold-spray coatings exhibit the highest hardness due to the work hardening effect of kinetic deposition.

Alumina-zirconia ceramic material has been plasma sprayed using a water stabilized plasma torch (WSP) to produce free standing coatings. The as-sprayed coatings have very low porosity and are mostly amorphous. The amorphous material crystallizes at temperatures above 900 °C. A spark plasma sintering apparatus has been used to heat the as-sprayed samples to temperatures above 900 °C to induce crystallization while at the same time a uniaxial pressure of 80 GPa has been applied to the their surface. After such post-treatment, the ceramic samples are crystalline and exhibit very low open porosity. The as-sprayed amorphous materials also exhibit high hardness and high abrasion resistance. Both properties are significantly improved in the heat-treated samples whose microstructure is best described as nanocomposite with the very small crystallites embedded in an amorphous matrix.

This work assesses the influence of powder characteristics on the deposition efficiency, microstructure, and tribological properties of Cr 3 C 2 -NiCr coatings. Four commercial powders prepared by different methods were used for the study. All have a spherical morphology but vary in terms of porosity, carbide grain size, and flowability. The feedstocks were deposited on flat low-carbon steel substrates using a liquid-fueled HVOF torch mounted on an industrial robot. Deposition efficiency was measured along with coating hardness, Young’s modulus, and abrasive wear resistance. In addition, some of the coatings were heat treated and changes in microstructure and hardness were recorded.

In this study, cold spraying is used to develop WC-Co coatings with a WC-10Co core as reinforcement and a Co-rich WC-Co matrix as the binder. Core-shell structured powders were prepared by mechanical milling and coating samples were deposited by cold spraying. Post-spray annealing was carried out to further modify the coating microstructure. WC-Co coating microstructure and mechanical properties were investigated along with various structure-property relationships. It was found that a WC-Co layer with a porosity of only 0.7 % was realized by cold spraying the mechanically milled powder and that annealing at 900 °C for 2 h resulted in a remarkable improvement in fracture toughness with no evident change in hardness.