High-velocity Air Fuel Process Research Articles

Role of high-speed nanoindentation mapping to assess the structure-performance correlation of HVAF-sprayed Cr3C2-25NiCr coating

Cr3C2-NiCr coatings deposited by high-velocity air-fuel (HVAF) spray can potentially exhibit superior wear performance. However, the microstructural variations caused by altering HVAF process parameters and secondary heat treatment considerably impact coating characteristics and wear performance. The effective characterization of such microstructural variations at the micrometer length scale using conventional techniques is limited. In this study, we employ a high-speed nanoindentation mapping technique to spatially map the properties of HVAF sprayed Cr3C2-NiCr coating deposited by varying particle size-combustion mode combination, which was in the as-deposited as well as heat-treated state. Despite the significant microstructural heterogeneity at a small scale, the hardness maps show excellent correlation with the microstructure. The data extracted from the nanoindentation maps estimate the area fraction and average hardness of individual phases (carbide, binder, interacted region) in the coatings. Insights on the role of individual constituents on the coating microstructure and mechanical property evolution are presented. The hardness evolution of the individual constituents with processing conditions and secondary heat treatment is correlated with the solid particle erosion performance of the coatings. Overall, Cr3C2-NiCr coatings deposited through the correct choice of particle size-combustion mode combination and heat treatment improved the desired phase distribution, resulting in superior erosion resistance.

Numerical and Experimental Investigation for Application of CoNiCrAlY Coatings by HVAF

AbstractA relatively small change in oxide content and microstructure in MCrAlY coatings (M = Co, Ni) can affect the functionality of the coating for oxidation protection or as a bond coat. The objective of this study is to fabricate a CoNiCrAlY coating with low porosity and low oxide content. The high velocity air-fuel (HVAF) process, with its relatively low process temperature, is particularly suitable for the deposition of spray materials that are susceptible to oxidation or degradation at high temperature. A CFD simulation model of HVAF process is developed to determine the process parameters for fabricating CoNiCrAlY coating. The simulation results are validated by particle diagnostics, thereby establishing a comprehensive understanding of the underlying process. To assess the coating microstructure, XRD and EDS analyses as well as observation of cross sections of the coatings are conducted. The results highlight the influence of various factors, such as the variation of carrier gas and particle size distribution, on the quality of the coatings. Consequently, the utilization of simulation-based process parameter development is well supported by the coating fabrications, offering valuable insights into the processes prior to implementation.

An Investigation into the Relative Efficacy of High-Velocity Air-Fuel-Sprayed Hydroxyapatite Implants Based on the Crystallinity Index, Residual Stress, Wear, and In-Flight Powder Particle Behavior.

Due to its resemblance to the bone, hydroxyapatite (HA) has been widely used for bioactive surface modification of orthopedic implants. However, it undergoes significant thermal decomposition and phase transformations at a high operating temperature, leading to premature implant failure. This investigation uses high-velocity air-fuel (HVAF) spray, an emerging low-temperature thermal spray technique, to deposit HA over the Ti-6Al-4V substrate. Coating characteristics, such as the crystallinity index and phase analysis, were measured using X-ray diffraction, Raman analysis, and Fourier transform infrared spectroscopy, residual stress using the sin2ψ method, and tribological performance by a fretting wear test. The coating retained an over 90% crystallinity index, a crystallite size of 41.04 nm, a compressive residual stress of -229 ± 34.5 MPa, and a wear rate of 1.532 × 10-3 mm3 N-1 m-1. Computational in-flight particle traits of HA particles (5 to 60 μm) were analyzed using computational fluid dynamics; it showed that 90% of particles were deposited at a 700 to 1000 m/s velocity and a 900 to 1450 K temperature with a 2.1 ms mean residence time. In-flight particle oxidation was minimized, and particle impact deformation was maximized, which caused severe plastic deformation, forming crystalline, compressive residual stressed coatings. The thermal decomposition model of low-temperature HVAF-sprayed HA particles helped to understand the implants' crystallinity index, residual stress, and tribological characteristics. Hence, this experimental and computational analysis shows that the HVAF process can be a promising candidate for biomedical applications for having strong and durable implants.

Analyzing Microstructural Features, Surface Topography, and Scratch Resistance of Innovative Nano-Composites Coated with High Velocity Air-Fuel Technology

New developments in thermal spraying processes may offer higher-quality alternatives to hard chrome plating and possibilities for hard chrome plating in a range of coating applications. These include spraying with high-velocity air fuel (HVAF) and new spray consumables. The low operating temperatures and accelerated particle velocity of the HVAF process enable investigation and development of a wide range of novel coating materials and applications. The High-velocity Air Fuel Process' quality and efficiency are primarily due to the broad combustion chamber and axial injection of the feedstock through it, as well as the relatively low combustion temperature of an air-fuel mixture and the low gas velocity that provides enough time for the mild heating of the powder particles. The current work discusses the inventive thermal spray procedure used for SAE 1008 carbon steel, a cost-effective substrate material. All of the compositions that were treated have undergone microstructure investigations. A scratch test is conducted in accordance with ASTM guidelines. Assessment of surface morphology clearly demonstrates the relationship between the evaluated parameters. According to the occurrence, scratch methods such as delamination, cracking, plastic deformation, and elastic deformation are highlighted. However, the findings of the scratch test showed that the samples' scratch resistance increased as the coating thickness rose. In comparison to samples with thinner coating, those with thicker coating demonstrated a stronger resistance to scratching. This is explained by the fact that coatings with a higher thickness and density can support the subsurface more effectively and stop cracks from scattering. This can retain the coating's integrity and stop more damage from occurring, improving scratch resistance. Better scratch resistance was displayed by the samples with denser microstructures and smoother surface morphologies. The outcome is greater scratch resistance because a higher density covering can withstand deformation and fracture better than a lower density layer. This is due to the mechanism of deformation and fracture in the coating material. This improvement in scratch resistance can be due to the composites' increased HVAF coating's hardness and adherence. The findings imply that using an HVAF coating to increase the scratch resistance of new nanocomposites may constitute a successful strategy.

Advanced diamond-reinforced metal matrix composite (DMMC) coatings via HVAF process: Effect of particle size and nozzle characteristics on tribological properties

High-velocity air fuel (HVAF) spraying is a versatile and cost-effective platform to fabricate wear resistant coatings. In this work, deposition of Nickel–Phosphorus cladded diamond feedstock is explored as a greener alternative to realize highly wear resistant large-area coatings. To the best of authors knowledge. this is the first study that has utilized HVAF technique for developing wear resistant Ni–P coatings reinforced with diamond (NCD). This work also aims to understand the effect of particle size by using coarse (20–30 μm) and fine NCD (10–15 μm) particles as feedstock. The importance of utilizing appropriate processing conditions was also highlighted by using two different nozzle configurations, for which the two powder particle sizes exhibited considerable differences in terms of microstructure, phase characteristics and mechanical properties. Further, the effect of annealing on the above coating characteristics was also examined, and it is shown that optimal spraying conditions can preclude the need for post-treatment. Furthermore, the as-deposited and annealed coatings were subjected to sliding wear tests to assess their tribological performance. Post-wear analysis performed on worn surfaces revealed the associated wear mechanisms. The results ensuing from this work lay the foundation for realizing new generation of HVAF sprayed wear resistant Ni–P/diamond composite coatings for diverse applications.

Effect of spray angle and substrate material on formation mechanisms and properties of HVAF sprayed coatings

Thermally sprayed coatings are often used to enhance the surface properties (wear resistance, corrosion resistance, etc.) of engineering components in order to extend their performance and service lifetime. Typically, the industrial components to be coated possess complex geometries and are fabricated using different materials, which can influence the deposited coating's microstructure and performance. High-velocity air fuel (HVAF) process is a relatively new thermal spray processing technique that has shown tremendous potential to deposit high performance coatings for durable industrial components. However, no detailed studies have been reported on HVAF sprayed coating formation mechanisms so far in relation to the spray angle and substrate properties, and the influence of coating material on the above. The objective of this work was to study the influence of spray angles and substrate materials on splat characteristics, coating microstructure evolution, properties and performance for two distinct coating materials.In this study, one cermet (WC-CoCr) and one metallic (Inconel 625) feedstock were deposited onto three different substrates (aluminium alloy, carbon steel and Hastelloy-X) utilising different spray angles (40°, 60° and 90°). The coating evolution was analysed utilising SEM/EDS, image analysis, and micro-indentation. To determine the tribological performance, coatings were subjected to dry sliding wear test utilising alumina ball as counter surface and specific wear rates were obtained.The results showed that initial splat characteristics were substantially altered on changing the substrate and the spray angle. However, the final coating properties were not affected significantly even though the deposition rate was reduced significantly at lower spray angle, suggesting the versatility of the HVAF process.

Novel suspension route to incorporate graphene nano-platelets in HVAF-sprayed Cr3C2–NiCr coatings for superior wear performance

Graphene nano platelets (GNP) have several attractive properties, including excellent lubricity that can be used to develop wear-resistant coatings. Thermally sprayed chromium carbideenickel chromium (Cr3C2eNiCr) coatings are widely employed to impart wear resistance to engineering components. This work attempts to improve the wear resistance of high velocity air fuel (HVAF) sprayed Cr3C2eNiCr coatings by incorporating GNP using a hybrid approach in which Cr3C2eNiCr (powder) and GNP (suspension) are co-axially injected. Two different powder-to-suspension delivery ratios were employed in this study that utilizes a liquid feedstock in tandem with a HVAF system. Furthermore, for comparison, a pure (without graphene) Cr3C2eNiCr reference coating was deposited by the HVAF process using identical spray parameters. The as-sprayed coatings were characterized for their microstructure and phase constitution by SEM/EDS and X-Ray Diffraction. Mechanical properties such as hardness and fracture toughness were evaluated using micro-indentation technique. The hybrid coatings were subjected to dry sliding wear tests and wear performance was compared with reference Cr3C2eNiCr. The GNP incorporated hybrid coatings exhibited lower CoF and lower wear rates than the reference Cr3C2eNiCr coating. Post wear SEM/EDS analysis revealed different wear mechanisms predominant in the investigated coatings. Utilizing the above as a case study, this work provides key insights into a new approach to produce GNP incorporated coatings for mitigating wear.

Novel utilization of powder-suspension hybrid feedstock in HVAF spraying to deposit improved wear and corrosion resistant coatings

Deployment of a suspension feedstock has been known to alleviate problems associated with using sub-micron and nanosized powder feedstock for thermal spraying of monolithic as well as powder-suspension ‘hybrid’ composite coatings. However, a powder-suspension hybrid feedstock has never been previously used in high-velocity air-fuel (HVAF) spraying. In this work, for the very first time, a chromium carbide (Cr3C2) suspension has been co-sprayed along with an Inconel-625 (IN-625) powder by the HVAF process as an illustrative case study. Two variants of the IN-625 + Cr3C2 hybrid coatings were produced by varying relative powder-suspension feed rates. For comparison, pure IN-625 coating was also deposited utilizing identical spray parameters. Detailed microstructural characterization, porosity content, hardness measurement and phase analysis of the as-deposited coatings was performed. The suspension-derived carbides were retained in the bulk of the coating, resulting in higher hardness. In the dry sliding wear test, the hybrid coatings demonstrated lower wear rate and higher coefficient of friction (CoF) compared to the conventional, powder-derived IN-625 coatings. Furthermore, the wear rate improved slightly with an increase in Cr3C2 content in the hybrid coating. Post-wear analysis of the worn coating, worn alumina ball and the wear debris was performed to understand the wear mechanisms and material transfer in the investigated coatings. In the potentiodynamic polarization test, higher corrosion resistance for hybrid coatings than conventional IN-625 coatings was achieved, indicating that the incorporation of a secondary, carbide phase in the IN-625 matrix did not compromise its corrosion performance. This work demonstrates a novel approach to incorporate any finely distributed second phase in HVAF sprayed coatings to enhance their performance when exposed to harsh environments.

Study of Cavitation Erosion-Corrosion Resistance of Thermally Sprayed Ni-Based Coatings Prepared by HVAF Process

In this study, two Ni-based coating materials, namely NiCrMoNb and NiCrBSi, have been applied using High Velocity Air Fuel (HVAF) thermal spraying process. The performance of the coated surface in resistance to cavitation erosion-corrosion of both coating materials, has been evaluated using an ultrasonic vibratory method. The cumulative material loss and erosion rate curves of the two coatings have been discussed. Surface topography, microhardness, macroscopic images, and scanning electron microscope (SEM) micrographs were used to characterize the coatings before and after the cavitation test. The cavitation results showed that the NiCrMoNb coating surface has exhibited better performance than the NiCrBSi coating surface under the same test conditions. The total cumulative weight loss of the NiCrMoNb coating was about 1/3 that of the NiCrBSi coating. SEM micrographs of the eroded surfaces showed that the surface layer of the NiCrBSi coating was more damaged, compared to layer of the NiCrMoNb coating. Overall, the NiCrMoNb coating can be effectively used against the cavitation wear, due to its superior performance.

Study of the corrosion and cavitation resistance of HVOF and HVAF FeCrMnSiNi and FeCrMnSiB coatings

Cavitation and corrosion on hydrodynamic components and systems reduces the operational efficiency. The use of wear resistant coatings has been studied as a solution to the problem of corrosion and cavitation in industrial environments. The high velocity oxy-fuel process (HVOF) can produce coatings with high density and bond strength. High velocity air-fuel (HVAF) is an alternative process, which can deposit coatings with higher velocity and lower temperatures, compared to the HVOF process, resulting in lower oxide content. This paper analyzes the HVOF and HVAF processes to deposit FeCrMnSiNi and FeCrMnSiB coatings, comparing their cavitation and corrosion resistance. HVAF coatings presented lower porosity and oxide levels, as well as higher hardness values. The HVAF process presented better cavitation and corrosion resistance, due to lower porosity and oxide contents of the coatings. The amount of oxides and pores in the coatings was crucial in their corrosive behavior, by facilitating the penetration of the chloride ions through the pores, leading to a higher corrosion rate and pitting formation.

A Study of Cr3C2-Based HVOF- and HVAF-Sprayed Coatings: Microstructure and Carbide Retention

The research on high-velocity air-fuel (HVAF)-sprayed Cr3C2-based materials has mostly focused on conventional Cr3C2-25NiCr composition. In this paper, two alternative compositions (Cr3C2-50NiCrMoNb and Cr3C2-37WC-18NiCoCrFe) were sprayed with high-velocity oxy-fuel (HVOF) and HVAF spray processes to evaluate the material behavior during spraying and to provide characterization of the microstructures and mechanical properties of the coatings. For comparison, coatings from the Cr3C2-25NiCr composition were sprayed with both processes. Spray diagnostics were carried out to obtain average particle velocity and temperature for each material and process combinations. The measured average in-flight particle data were 1800 °C and 700 m/s for HVOF process, and 1450 °C and 900 m/s for HVAF process. Characterization of the coating microstructures was carried out by scanning electron microscopy and X-ray diffraction. In addition, the carbon content of the feedstock powders and sprayed coatings was measured with carbon analyzer. The results show that carbide rebounding or selective deposition of particles with higher metal matrix content is the dominating reason for carbide loss during HVAF spraying, while carbide dissolution is an additional source for the HVOF spraying. Higher particle velocities and controlled temperature measured for the HVAF process produced dense coatings with improved toughness and more homogenous coating structure.

Application of TiC reinforced Fe-based coatings by means of High Velocity Air Fuel Spraying

In the field of hydraulic applications, different development trends can cause problems for coatings currently used as wear and corrosion protection for piston rods. Aqueous hydraulic fluids and rising raw material prices necessitate the search for alternatives to conventional coatings like galvanic hard chrome or High Velocity Oxygen Fuel (HVOF)-sprayed WC/Co coatings.In a previous study, Fe/TiC coatings sprayed by a HVOF-process, were identified to be promising coating systems for wear and corrosion protection in hydraulic systems. In this feasibility study, the novel High Velocity Air Fuel (HVAF)-process, a modification of the HVOF-process, is investigated using the same feedstock material, which means the powder is not optimized for the HVAF-process. The asserted benefits of the HVAF-process are higher particle velocities and lower process temperatures, which can result in a lower porosity and oxidation of the coating. Further benefits of the HVAF process are claimed to be lower process costs and higher deposition rates. In this study, the focus is set on to the applicability of Fe/TiC coatings by HVAF in general. The Fe/TiC HVAF coating could be produced, successfully. The HVAF- and HVOF-coatings, produced with the same powder, were investigated using micro-hardness, porosity, wear and corrosion tests. A similar wear coefficient and micro-hardness for both processes could be achieved. Furthermore the propane/hydrogen proportion of the HVAF process and its influence on the coating thickness and the porosity was investigated.

A comparative study on wear and corrosion behaviour of HVOF- and HVAF-sprayed WC–10Co–4Cr coatings

In this work, WC–10Co–4Cr coatings were separately deposited on 300M steel by high-velocity oxygen fuel (HVOF) and high-velocity air fuel (HVAF) spraying processes. Microstructure, porosity, micro-hardness, bonding strength, wear and corrosion resistance of the coatings after different processing were investigated and compared. The result shows that dense and homogeneous microstructure existed in both two WC–10Co–4Cr coatings. Owing to lower temperature flames and higher particle velocity in the HVAF process, the HVAF-sprayed WC–10Co–4Cr coating exhibited less decarburisation, lower porosity (0.74%), higher micro-hardness (1162 HV0.3) and bonding strength (74.68 MPa) than HVOF-sprayed WC–10Co–4Cr coating. In addition, wear and corrosion resistance of these two coatings were evaluated by sliding wear test and electrochemical corrosion test. The HVAF-sprayed WC–10Co–4Cr coating also exhibited excellent wear resistance and superior corrosion property than HVOF-sprayed WC–10Co–4Cr coating.

Microstructure Effect of Intermediate Coat Layer on Corrosion Behavior of HVAF-Sprayed Bi-Layer Coatings

The inherent pores and carbides of Cr3C2-NiCr coatings significantly reduce the corrosion resistance, the former by providing preferential paths for ion diffusion and the latter by forming cathodic sites in galvanic couples (between NiCr and Cr3C2). Adding a dense intermediate layer (intermediate coat layer) between the Cr3C2-NiCr coating (top coat) and substrate increases the corrosion protection of the coating if the layer acts as cathode in connection to the top coat. In the present work, NiCr, NiAl, and NiCoCrAlY layers were deposited by high-velocity air-fuel process as intermediate coat layers for the Cr3C2-NiCr top coat. Effects of coating microstructure on corrosion behavior of single- and bi-layer coatings were studied by open-circuit potential and polarization tests in 3.5 wt.% NaCl at room temperature. A zero resistance ammeter technique was used to study the galvanic corrosion of the coupled top and intermediate coat layers. Methods such as SEM and XRD were employed to characterize the as-sprayed and corroded coatings and to investigate the corrosion mechanisms. The results showed that the NiCoCrAlY coating not only presented a more positive corrosion potential (Ecorr) than the Cr3C2-NiCr coating, but also provided a better passive layer than the single-phase NiCr and NiAl coatings.

A Comparative Study on Ni-Based Coatings Prepared by HVAF, HVOF, and APS Methods for Corrosion Protection Applications

Selection of the thermal spray process is the most important step toward a proper coating solution for a given application as important coating characteristics such as adhesion and microstructure are highly dependent on it. In the present work, a process-microstructure-properties-performance correlation study was performed in order to figure out the main characteristics and corrosion performance of the coatings produced by different thermal spray techniques such as high-velocity air fuel (HVAF), high-velocity oxy fuel (HVOF), and atmospheric plasma spraying (APS). Previously optimized HVOF and APS process parameters were used to deposit Ni, NiCr, and NiAl coatings and compare with HVAF-sprayed coatings with randomly selected process parameters. As the HVAF process presented the best coating characteristics and corrosion behavior, few process parameters such as feed rate and standoff distance (SoD) were investigated to systematically optimize the HVAF coatings in terms of low porosity and high corrosion resistance. The Ni and NiAl coatings with lower porosity and better corrosion behavior were obtained at an average SoD of 300 mm and feed rate of 150 g/min. The NiCr coating sprayed at a SoD of 250 mm and feed rate of 75 g/min showed the highest corrosion resistance among all investigated samples.

Microstructural and electrochemical characterization of Ni-based bi-layer coatings produced by the HVAF process

Bi-layer coating systems comprising a Cr3C2-NiCr coating with different underlying coatings of Ni, NiCr, NiCoCrAlY, and CoNiCrAlY were deposited on 304L stainless steel substrates using the high-velocity air fuel (HVAF) process. The corrosion behavior of the coating systems was studied using electrochemical and immersion tests in 3.5 wt% NaCl at 25 °C. The higher open-circuit potential (OCP) value of the NiCoCrAlY coating (− 120 mV/SCE) compared to the value of the Cr3C2-NiCr coating (− 230 mV/SCE) revealed that the underlying NiCoCrAlY coating was sacrificially protected by Cr3C2-NiCr coating. The polarization resistance (Rp) of the bi-layer coatings with the different underlying coatings of Ni, NiCr, NiCoCrAlY and CoNiCrAlY was approximately 77, 189, 487, and 74 kΩ·cm2 respectively, while the value was 101 kΩ·cm2 for the single-layer Cr3C2-NiCr coating and 30 kΩ·cm2 for the 304L substrate, which confirmed a higher corrosion protection of the NiCoCrAlY coating. The electrochemical impedance spectroscopy (EIS) results showed that the corrosion resistance of the 304L substrate was significantly improved by adding an intermediate layer of NiCoCrAlY to the Cr3C2-NiCr coating. Results of the immersion tests confirmed that the underlying coatings in the different bi-layer coating systems acted as protective barriers. Moreover, the NiCoCrAlY coating showed the best corrosion protection among the investigated underlying coatings.

Microstructure and Sliding Wear Behavior of Fe-Based Coatings Manufactured with HVOF and HVAF Thermal Spray Processes

The microstructure and micromechanical behavior of thermally sprayed Fe-based coatings manufactured with high-velocity oxygen fuel (HVOF) and high-velocity air fuel (HVAF) processes were investigated. Fe-Cr-Ni-Si-B-C and Fe-Cr-Ni-Mo-Si-B-C powders were used as the feedstock materials. The coatings showed a highly dense microstructure with near-zero oxidation. The microstructure of the feedstock powders was better retained when sprayed with HVAF process. Differential scanning calorimetry revealed two small exothermic peaks at about 600 °C for the HVOF-sprayed coatings, without any increase in weight in thermogravimetric analysis. It suggested the re-precipitation of carbides that were dissolved during spraying due to the higher particle temperature reported by spray diagnostics system during the HVOF process (≈1800 °C) compared to the HVAF one (≈1400 °C). Micro- and nano-indentations helped to show the difference in inter-lamellar cohesive strength and, in turn, in the particle deposition mechanism. Coatings sprayed with Fe-Cr-Ni-Mo-Si-B-C composition possessed higher sliding wear resistance than that of Fe-Cr-Ni-Si-B-C due to higher nano-hardness. More specifically, HVOF-sprayed Fe-Cr-Ni-Mo-Si-B-C coating showed the largest intra-lamellar hardness, the largest elasticity, and high quality of particle interfaces which resulted in lower sliding wear rate.

Tribological Properties of Hard Metal Coatings Sprayed by High-Velocity Air Fuel Process

Lowering the thermal energy and increasing the kinetic energyof hard metal particles sprayed by the newly developed HVAF systems can significantly reduce their decarburization, and increases the sl ...

Comparative Study of WC-Cermet Coatings Sprayed via the HVOF and the HVAF Process

The high velocity air fuel (HVAF) system is a high-velocity combustion process that uses compressed air and kerosene for combustion. Two WC-cermet powders were sprayed by the HVAF and the high-velocity oxyfuel (HVOF) processes, using an AeroSpray gun (Browning Thermal Systems Inc., Enfield, New Hampshire) and a CDS-100 gun (Sulzer Plasma Technik, Wohlen, Switzerland) respectively. Several techniques, including x-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy, were used to characterize the microstructures and phase distribution of the powders and coatings. In addition, mechanical properties such as hardness and wear resistance (pin-on-disk) were investigated. A substantial amount of W2C was found in the HVOF coatings, as well as a high concentration of tungsten in the binder phase, indicating that oxidation and dissolution processes change the composition and microstructure from powder to coating during spraying. This was in contrast to the HVAF coatings in which composition and microstructure were unchanged from that of the powder. Additionally, the wear resistance of the HVAF coatings was superior to that of the HVOF coatings.