High-velocity Thermal Spraying Research Articles

Understanding the Influence of High Velocity Thermal Spray Techniques on the Properties of Different Anti-Wear WC-Based Coatings

This work analyzes the differences found in hard metal coatings produced by two high velocity thermal spray techniques, namely high velocity oxy-fuel (HVOF) and high velocity air-fuel (HVAF). Additionally, the effect of the metallic matrix and ceramic composition and the original carbide grain size on coating properties is compared to the most studied standard reference material sprayed by HVOF, WC-Co. For this evaluation, the physical properties of the coatings, including feedstock characteristics, porosity, thickness, roughness, hardness, and phase composition were investigated. Several characterization methods were used for this purpose: optical microscopy (OM), scanning electronic microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDS), and X-ray Diffraction (XRD), among others. The final performance (abrasive wear and corrosion resistance) shown by the coatings obtained by these two methodologies was also analyzed. Thus, the abrasive wear resistance was analyzed by the rubber-wheel test, while the corrosion resistance was characterized with electrochemical methods. The characterization results obtained clearly showed that the coatings exhibit different microstructures according to feedstock powder characteristics (carbide grain size and/or composition) and the thermal spray process used for its deposition. Thus, the incorporation of WB to the cermet composition led to a high hardness coating, and the complementary hardness and toughness of the WC-Co coatings justify its better abrasion resistance. The presence of Ni on the metal matrix increases the free corrosion potential of the coating to more noble region. However, the WC-Co coatings show a lower corrosion rate and hence a higher protective performance than the rest of the coatings.

Creation of modified Cr3C2–NiCr hardmetal coating microstructures through novel processing

High velocity thermal spray Cr3C2–NiCr coatings are widely used to mitigate wear at high temperatures and in corrosive environments due to their superior oxidation and corrosion resistance relative to WC-Co coatings. This work explored a novel two–step process of first depositing a coating from feedstock particles having a high liquid phase content by shrouded plasma spraying (SPS), followed by controlled precipitation of the carbide grains through heat treatment. The appearance of additional melt from peritectic decomposition of Cr3C2, protection from oxidation by the shroud, as well as absence from water vapour of hydrocarbon combustion, created a new quality of microstructure. Heat treatment generated uniquely novel coating microstructures of continuous carbide within which pools of Ni alloy formed, in direct contrast to the conventional coating microstructure of isolated carbide grains encapsulated within a continuous Ni binder. The variation in the as-sprayed chemical and phase compositions between the SPS and HVOF coatings played a direct role in the dramatic microstructural variation seen with heat treatment. The mechanism of compositional and microstructural development with heat treatment in the SPS coating is contrasted to that in conventional HVOF coatings. Microhardness values above 1000 VHN300 demonstrate the potential of this new type of coating.

Erosion wear performance of WC-10Co4Cr and Cr3C2-25NiCr coatings sprayed with high-velocity thermal spray processes

Thermally sprayed hardmetal coatings are widely used to protect components and surfaces against wear in various applications. Hard and wear resistant coatings increase the component lifetime and can generate significant savings promoting ecological manufacturing. This study focuses on the performance of tungsten carbide (WC-10Co4Cr) and chromium carbide (Cr3C2-25NiCr) based hardmetal coatings sprayed with gaseous and liquid fuelled high-velocity oxygen-fuel (HVOF) spray processes and a modern high-velocity air-fuel (HVAF) spray process. The coating characterisation revealed reduced carbide dissolution with decreasing process temperature and denser feedstock powder particles. Smaller carbide size in the Cr3C2-25NiCr material significantly reduced the carbide rebounding leading to higher carbide content in the sprayed coating and improved erosion wear resistance. Most significant improvements were observed in cavitation erosion for HVAF sprayed WC-10Co4Cr coatings (0.4 μm/h) compared to the HVOF sprayed coatings (1.5–3.7 μm/h). The cavitation erosion resistance of the HVAF sprayed coatings was almost at the level of the WC-10Co sintered bulk (0.2 μm/h).

On the Interplay Between Adhesion Strength and Tensile Properties of Thermal Spray Coated Laminates—Part I: High Velocity Thermal Spray Coatings

Adhesion of thermal spray (TS) coatings is an important system level property in coating design and application. Adhesive-based pull testing (ASTM C633) has long been used to evaluate coating/substrate bonding. However, this approach is not always suitable for high velocity spray coatings, for example, where adhesion strengths are routinely greater than the strength of the adhesive bonding agent used in the testing. In this work, a new approach has been proposed to evaluate the adhesion of TS coatings. A systematic investigation of the effects of substrate roughness on both the uniaxial tensile yield strength and traditional bond pull adhesive strength of HVOF Ni and Ni-5wt.%Al, as well as cold-sprayed Ni-coated laminates revealed a strong correlation between these two test methodologies for the respective materials and processes. This approach allows measurement of the adhesion response even where the adhesive method is not applicable, overcoming many of the issues in the traditional ASTM C633. Analysis of cracking patterns of the coatings after 10.5% strain was used to assess the adhesion and cohesion properties. The mechanisms which determine the load transfer between the substrate and the coating are also briefly discussed.

In-situ observation of strain and cracking in coated laminates by digital image correlation

Overlay coatings are widely used in engineering components to impart a range of surface functionalities including thermal, wear, and corrosion protection, as well as material reclamation. In most surface engineering applications, the coating's role is restricted to the surface, with limited integration with the underlying substrate. However, the situation is changing: there is an emerging need for so-called structurally integrated coatings, where the coating and substrate are intimately bonded, resulting in a coupled system. Of interest are the emerging applications of thermal spray and cold spray overlay coatings applied on loaded engineering components such as landing gear, heavy machinery hydraulics, and steel infrastructure. These coatings, even metals or cermets, respond in a brittle manner associated with their layered processing and ultra-fine grain structures resulting from rapid quenching. As such, it is of importance to understand their coupled mechanical response, especially stress-strain behavior and strain to fracture.In this study, an approach involving strain monitoring of coated steel via digital image correlation has been developed. Both elastic response and strain beyond the yield point of the system are assessed to examine load transfer between the coating and substrate and onset of cracking. Three different coating materials (Ni, WC-CoCr and Al2O3) were deposited to near full density via high velocity thermal spray. The results point to a powerful new approach for understanding mechanical behavior of heterogeneous composites using advanced imaging techniques.

Numerical analysis of flame and particle behavior in an HVOF thermal spray process

In this study, a kerosene-fueled high-velocity thermal spraying model is used to examine the flame and particle behavior in a thermal spray process. The gas flow characteristics (including temperature, velocity, and pressure), mass fraction of the gas components, and the particle characteristics (including the particle temperature, velocity, and in-flight trajectory) are successfully simulated. Besides, the effects of particle diameter, particle shape, and particle injection velocity on the particle behavior in the spraying flame are also investigated. The optimal particle size range is found to be between 20 and 40μm. In this case, most of the particles are located towards the center of the gun and are in a semi-solid state before impacting the substrate. When the shape factor falls from 1 to 0.6, the non-spherical particles gain more momentum and less heat than spherical particles in the process owing to their higher drag coefficient and shorter dwell time within the flame flow. A comprehensive consideration of the injection velocity on the spray particle dynamics revealed that the optimal particle injection velocity should be greater than 5m/s but no more than 20m/s.

Microstructure and Selected Properties of WC-Co-Cr Coatings Deposited by High Velocity Thermal Spray Processes

Selected information about high velocity thermal spraying method were presented in this article. Three generations of thermal spraying processes based on oxygen with propane mixture, liquid fuel with oxygen and propane with air were characterized. A powder of WC-Co-Cr 86-10-4 was used for coatings deposition on a steel substrate. Four coatings were deposited by different thermal spraying systems such as Diamond Jet, JP 5000, Micro HVOF and HVAF.

Structurally Integrated, Damage-Tolerant, Thermal Spray Coatings

Thermal spray coatings are used extensively for the protection and life extension of engineering components exposed to harsh wear and/or corrosion during service in aerospace, energy, and heavy machinery sectors. Cermet coatings applied via high-velocity thermal spray are used in aggressive wear situations almost always coupled with corrosive environments. In several instances (e.g., landing gear), coatings are considered as part of the structure requiring system-level considerations. Despite their widespread use, the technology has lacked generalized scientific principles for robust coating design, manufacturing, and performance analysis. Advances in process and in situ diagnostics have provided significant insights into the process–structure–property–performance correlations providing a framework-enhanced design. In this overview, critical aspects of materials, process, parametrics, and performance are discussed through exemplary studies on relevant compositions. The underlying connective theme is understanding and controlling residual stresses generation, which not only addresses process dynamics but also provides linkage for process-property relationship for both the system (e.g., fatigue) and the surface (wear and corrosion). The anisotropic microstructure also invokes the need for damage-tolerant material design to meet future goals.

The Influence of High Velocity Thermal Spraying Methods on Top Surface Topography of WC-Co Coatings

Characterization of top surfaces of WC-Co coatings obtained during deposition process of high velocity thermal spraying type was showed in this article. The feedstock powder was so called “superfine” powder of WC-17Co types produced by Inframat. In first part of article short characterization of powder was presented, especially characterization of it morphology including shape, internal microstructure and grain size description. The second part of article is dedicated to characterization of microstructure and topography of top surface of coatings deposited by two different methods of high velocity thermal spraying. In this case analysis of top-surface condition by scanning microscopy with chemical analysis in microareas was used and as well as a qualitative and quantitative analysis of topography of carbides surface by traditional non-contact and laser profilometry.

Material, Mechanical, and Tribological Characterization of Laser-Treated Surfaces

Laser treatment under nitrogen assisting gas environment of cobalt-nickel-chromium-tungsten-based superalloy and high-velocity oxygen-fuel thermal spray coating of nickel-chromium-based superalloy on carbon steel was carried out to improve mechanical and tribological properties. Superalloy surface was preprepared to include B4C particles at the surface prior to the laser treatment process. Material and morphological changes in the laser-treated samples were examined using scanning electron microscopy, energy-dispersive spectroscopy, and x-ray diffraction (XRD) analysis. Residual stresses present at the surface region of the laser-treated layer were determined from the XRD data. The microhardness of the laser-treated surface was measured by indentation tests. Fracture toughness of the coating surfaces before and after laser treatment were also measured using overload indentation tests. Macrowear and macrofriction characterization were carried out using pin-on-disk tests.

Properties of WC–FeCrAl coatings manufactured by different high velocity thermal spray processes

WC–FeCrAl coatings deposited by High Velocity Air-Fuel (HVAF) spraying and by High Velocity Oxygen-Fuel (HVOF) spraying with a liquid-fuelled and a gas-fuelled torch were studied and compared to a conventional HVOF-sprayed WC–CoCr coating.During HVOF spraying, WC–FeCrAl had a tendency to oxidise and decarburise. The gas-fuelled torch, operated with excess fuel, could restrain these phenomena, compared to the liquid fuelled one. In the HVAF coating, oxidation and decarburisation were further reduced, but porosity was >1%. As a consequence, the HVAF-sprayed coating was less hard than the HVOF-sprayed ones.Of all the WC–FeCrAl coatings, the gas fuelled-HVOF sprayed one exhibited the best wear resistance both in dry sliding conditions (at room temperature as well as at 400°C) and in dry particle abrasion testing. It indeed achieves the best compromise between density, hardness and low oxidation, so that it can reduce the extent of the brittle cracking phenomena affecting all WC–FeCrAl samples. Its dry sliding wear rate of ≈3∗10−8mm3/(Nm) at room temperature is even slightly lower than that of the WC–CoCr reference. Its performance under high temperature sliding and dry particle abrasion conditions is, however, somewhat poorer than that of the reference.

High Velocity Thermal Spraying of Powders and Suspensions Containing Micron, Submicron and Nanoparticles for Functional Coatings

Thermal Spraying of submicron and nanoscaled powder materials require the application of agglomerated powders that can be processed with standard HVOLF (High Velocity Oxy Liquid Fuel Flame Spraying) equipment, or the conversion of these powders into a finely dispersed suspension together with an organic solvent, appropriate for suspension spraying (HVSFS: High Velocity Suspension Flame Spraying). Both methods are suitable for the manufacturing of finely structured and dense coatings and offer new possibilities in functional coating development for new application fields. Due to their refined microstructure, mechanical and physical coating properties can differ significantly from their conventionally sprayed counterparts. The paper gives an overview of HVOLF sprayed WC-Co cermets containing submicron and nanocarbides and HVSF sprayed nanooxide ceramics used in numerous technical applications.

Advancements in High-Velocity Thermal Spray Gun Design

Abstract A high-velocity oxyfuel (HVOF) spray gun that operates on conventional (gas or kerosene) fuels as well as liquid-gas combinations is shown to be capable of producing MCrAlY bond coats consisting of a rough surface layer (for adhesion) on a dense base (for oxidation protection).

COLMONOY NO. 63HV

Abstract Colmonoy No. 63HV is a nickel-base alloy recommended for hard surfacing parts to resist wear, corrosion, heat, and galling. The hardness of Colmonoy 63HV ranges to 58–61 HRC, and it is resistant to abrasion. Colmonoy 63HV is sized exclusively for high-velocity oxygen-fuel thermal spray systems. This datasheet provides information on composition. It also includes information on heat treating, surface treatment, and powder metal forms. Filing Code: Ni-665. Producer or source: Wall Colmonoy Corporation.

Electrical properties of pressure quenched silicon by thermal spraying

High velocity thermal spray deposition of polycrystalline silicon film onto single crystal substrates, yields metastable high pressure forms of silicon in nanocrystalline form within the deposit. The phases observed in the deposit include hexagonal diamond-Si, R-8, BC-8 and Si-IX. The peculiar attribute of this transformation is that it occurs only on <100> orientation silicon substrate. The silicon deposits containing the high pressure phases display a substantially higher electrical conductivity. The resistivity profile of the silicon deposit containing shock induced metastable silicon phases identified by X-ray diffraction patterns. The density of the pressure induced polymorphic silicon is higher at deposit/substrate interface. A modified two-layer model is presented to explain the resistivity of the deposit impacted by the pressure induced polymorphic silicon generated by the thermal spraying process. The pressure quenched silicon deposits on the p - silicon substrate, with or without metastable phases, display the barrier potential of about 0.72 eV. The measured hall mobility value of pressure quenched silicon deposits is in the range of polycrystalline silicon. The significance of this work lies in the fact that the versatility of thermal spray may enable applications of these high pressure forms of silicon.

Effect of Substrate Hardness on Splat Morphology in High-Velocity Thermal Spray Coatings

In this study, Ni-chrome alloy particles were thermally sprayed onto a variety of substrate materials using the high-velocity air fuel (HVAF) technique. Although the various substrate materials were sprayed using identical powder material and thermal spray conditions, the type and variation of splat morphologies were strongly dependent on the substrate material. Predominantly solid splats are observed penetrating deeply into softer substrates, such as aluminum, whereas molten splats were observed on harder substrates, which resisted particle penetration. The observed correlation between molten splats and substrate hardness could be due a dependency of deposition efficiencies of solid and molten splats on the substrate material. However, it was found that conversion of particle kinetic energy into plastic deformation and heat, dependent on substrate hardness, can make a significant contribution towards explaining the observed behavior.

Microstructural Evolution in Nanocrystalline Ni Coatings

Gas-atomized Ni powders were mechanically milled under liquid nitrogen and subsequently sprayed using a high-velocity oxygen-fuel thermal spraying system. The resultant coatings were evaluated using various analytical methods, including scanning electron microscopy (SEM), transmission electron microscopy, and atom probe field ion microscopy (APFIM). The results indicated that the majority of the grains in nanocrystalline Ni coating were equiaxed with an average grain size of 50 ± 23 nm. SEM elemental dot mapping analysis suggested that oxide particles were distributed along the prior droplet boundaries and interpass boundaries in the coatings. The APFIM analysis indicated that the distribution of oxygen was very inhomogeneous in the nanocrystalline coating samples studied. Mostly small NiO precipitates with a size range of 1–7 nm were found throughout the grains, and the average size of these precipitates was approximately 4 nm. However, large oxide precipitates with a size up to 55 nm were also observed. In addition, increased amounts of oxygen were found at the grain boundaries and at structural defects.

High Velocity Pulsed Plasma Thermal Spray

The quality and durability of coatings produced by many thermal spray techniques could be improved by increasing the velocity with which coating particles impact the substrate. Additionally, better control of the chemical and thermal environment seen by the particles during flight is crucial to the quality of the coating. A high velocity thermal spray device is under development through a Ballistic Missile Defense Organization Small Business Innovation Research (SBIR) project, which provides significantly higher impact velocity for accelerated particles than is currently available with existing thermal spray devices. This device utilizes a pulsed plasma as the accelerative medium for powders introduced into the barrel. Recent experiments using a particle imaging diagnostic system showed that the device can accelerate stainless steel and WC-Co powders to velocities ranging from 1500 to 2200 m/s. These high velocities are accomplished without the use of combustible gases and without the need of a vacuum chamber, while maintaining an inert atmosphere for the particles during acceleration. The high velocities corresponded well to modeling predictions, and these same models suggest that velocities as high as 3000 m/s or higher are possible.

Evaluation of Four High Velocity Thermal Spray Guns Using WC-10% Co-4% Cr Cermets

Four high velocity thermal spray guns were evaluated in the production of 10% Co-4% Cr tungsten carbide (WC) cermets. Three high velocity oxygen fuel guns (JP-5000, JP-5000ST, and Diamond Jet [DJ]-2700) and one plasma gun (Axial III) were used to spray the same angular, agglomerated, and crushed WC-10Co-4Cr powder. The DPV-2000 was used to monitor the in-flight velocity and temperature of the WC cermet-sprayed particles. From those values, spray conditions were selected to produce coatings that were evaluated in terms of porosity, hardness, and deposition efficiency. Results show that the plasma Axial III provides the highest particle temperature, between 2000 °C and 2600 °C, depending on the spray conditions. The JP-5000 imparts the highest velocity to the particles, between 550 and 700 m/s, depending on the spray conditions. The ST version of the JP-5000 provides the same velocity as the standard version but with lower particle temperature. The DJ-2700 sprays particles with temperature and velocity between those of the JP-5000 and the Axial III. Minimum porosity values of 2.1%, 3.7%, and 5.3%, respectively, were obtained for the JP-5000, the DJ-2700, and the Axial III guns. The porosity and carbide degradation are found to depend mostly on the particle velocity and temperature, respectively. The values for the Vickers microhardness number (200g) ranged from 950 to 1250. Measurements of the deposition efficiency indicated a variation between 10% and 80%, depending on the spray conditions and the gun used.

Role of oxides in high velocity thermal spray coatings

Thermal spray coatings applied with high velocity techniques such as high velocity air fuel (HVAF), produce coatings with superior quality in comparison to other traditional techniques such as plasma spraying. To date, our knowledge of the bonding processes and the structure of the particles within thermal spray coatings is very subjective. To improve our understanding of these materials, especially of the surface oxide layer, Ni80/Cr20 HVAF thermally sprayed coatings were studied with scanning electron microscope, nuclear reaction analysis (NRA) and X-ray photoelectron spectroscopy (XPS). In particular, NRA and XPS were used to characterise the oxide composition that gives the coatings their excellent oxidation resistance. The surface oxide on the Ni80/Cr20 particles was found to be only 7 nm thick and enriched in SiO 2. A surprising finding was that the composition of the Ni80/Cr20 powder remained unchanged during the coating process despite the high velocity application with the HVAF method.