1 Introduction

Engineering components require graded properties, affecting surfaces through frictional, thermal, mechanical, and chemical interactions. Monitoring Tribological and corrosion phenomena is crucial for recovery. India experiences an economy loss of $6500 US$ due to corrosion. Materials used in industrial applications must satisfy specific needs, such as strength and fracture resistance. The interaction between the environment and the material, particularly the surface, is critical [1,2,3,4,5]. Coatings have broadened design possibilities by combining bulk properties with surface capabilities. Examples include corrosion-resistant coatings for offshore structures and thermal barrier coatings (TBC) for turbine blades [6].

High-temperature-resistant materials and coatings are essential for power generation, shielding parts from oxidation and corrosion. Surface coating and alloy formation are strategies for mechanical strength in fossil fuel energy systems, with carbide-based cermets being popular due to their strength and stability [7,8,9]. Thermal spraying is a popular way for applying protective coatings and repairing large shafts in turbines and pumps, addressing metal degradation due to hot corrosion in high-temperature environments [10]. It is further classified as shown in Fig. 1 with features of various thermal spray methods as represented in Table 1. Whereas, HVOF coating is a prominent thermal spraying technique, using hydrogen and natural gases for controlled heat input [11,12,13]. One of the most prominent thermal spraying techniques is HVOF coating, which uses the combustion of hydrogen and natural gases or liquid fuel, producing high kinetic energy under controlled heat input [14]. The characteristics of various thermal spray methods are shown in Fig. 2a–d.

Fig. 1
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Thermal spray process flowchart

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Table 1 Features of various thermal spray methods [15, 97,98,99,100,101,102]
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Fig. 2
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Different thermal spray methods characteristics. a Spray gun temperature (°C), b Particle velocity (m/s), c Porosity volume (%), and d Hardness (Rh and Rc scale)

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1.1 High-Velocity Oxy-Fuel (HVOF) Spray

HVOF is a thermal spray technology developed by Browning and Witfield in the 1980s using rocket engine technologies. It uses blend of oxygen and fuel gases to generate high temperatures and pressure, facilitating a supersonic gas flow through nozzle. The process of spot melting is influenced by factors, such as flame temperature, dwell time, material melting point, and thermal conductivity [15,16,17]. HVOF differs from conventional flame spray using a supersonic jet, improving coating characteristics, especially for materials, like tungsten carbide coatings. The HVOF technique is a unique and alternative method of deposition, and optimum process parameters are evaluated for each composition [18]. The schematic representation of HVOF method is shown in Fig. 3.

Fig. 3
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HVOF spray process schematic view [18]

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HVOF process is thermal spray technique that uses high velocities to produce higher bond strength and lower porosity. HVOF offers advantages over other techniques, like uniform heating, shorter flight exposure time, lower surface oxidation, lower flame temperature, lower capital cost, and easier use. Additionally, it permits thicker coatings with increased density, impact energy, improved corrosion resistance, reduced porosity, hardness grades, improved bonding, and improved wear resistance. HVOF also offers smoother surfaces, thicker coatings, and shorter times at higher temperatures, and better chemical retention [19,20,21,22]. HVOF coating process involves setting up a machine according to manufacturer’s instructions, with parameters clustered based on the coating material application. The coating process is influenced by input factors such as temperature, melting phase, and particle velocity [23]. The characteristics of in-flight particles impact the adhesive strength and microstructure of coatings, with temperature and velocity having an impact on adhesive strength. Higher particle velocity reduces porosity and increases oxide content in the link between coating microstructure and particle in-flight characteristics [24].

1.2 Significance of HVOF Process Parameters

It was possible to create distinct coating layers with varying chemical compositions without stopping the spraying operation by modifying a conventional powder feed hopper to deposit two powders concurrently. In order to confirm that mixed composition particles are available, a process model was created to mimic the movement of nitrogen gas and powder. We built, commissioned, and calibrated a multi-powder feed device. Onto aluminium substrates, multi-layer coatings made of aluminium tool steel were sprayed [25,26,27].

To evaluate the coatings of the HVOF spraying technique, the learning used factorial design experiments. For combined coatings, the ideal set of spray parameters was similar to that for aluminium powder alone, maybe because of the powders’ different temperatures. Altered types of composite coatings were placed using optimised spray parameters and coatings with thicker layers showed higher residual stress but improved hardness [27, 28].

The varying spray parameters of HVOF for various combinations of coatings to substrates are displayed in Table 2. Whereas, in spray process, standoff lengths, temperature, feed rate, and particle velocity all play a significant effect. Exceptional process parameters for hardness is shown in Fig. 4.

Table 2 Various process parameters of HVOF for different coating materials [103,104,105,106,107,108,109,110,111,112,113,114,115,116]
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Fig. 4
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Exceptional process parameters for hardness

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2 Electrochemical Oxidation (EO)

Electrochemical reactions involve oxidation and reduction at the anode and cathode, primarily used for heavy metal remediation. These procedures remove pollutants through redox reactions at both the anode and cathode [29, 30]. Electro-oxidation is a wastewater treatment technique primarily used for industrial effluents. It involves two electrodes connected to a power source, forming strong oxidising types that degrade contaminants. Popular for its ease of setup and effectiveness, combining it with other technologies reduces operational costs whilst achieving high degradation standards.

Because anodic oxidation processes may result in partial or complete mineralization, the electrocatalytic properties of the anodic materials utilised have an impact on how well electrochemical procedures remove carbon-based pollutants [31,32,33,34]. The two different processes are indirect oxidation (ii) and direct anodic oxidation (i).

In order to stop combustion, carbon-based pollutants go through charge transfer processes in direct anodic oxidation or electrolysis. Applying potentials lower than the potential of the water oxidation process results in inhibition and surface poisoning. Similar to this, in situ electro-generation of a highly oxidant type mediates the indirect EO activities at the electrode surface [35]. Mixed metal oxides (MMOs) have been the subject of much heterogeneous catalysis research. Recent years have seen a significant increase in interest in MMOs as anode materials for the electrochemical treatment of waste waters, including refractory organic components [36]. There are two categories of MMOs: supported metal oxide anodes and bulk mixed metal oxide anodes. Different metal oxides may be deposited concurrently in bulk mixed metal oxide anodes using techniques, such as electro-deposition, chemical vapour deposition, physical vapour deposition, and thermochemical degradation. However, by combining metal oxides in the surface layer, supported MMO anodes seek to increase electrocatalytic performance and prolong service life [37, 38]. The surface composition of a binary metal oxide anode system is conceptually schematically shown in Fig. 5a. When all of the mixed MMO components are present in a bulk mixed metal oxide system, the MMO layer provides active sites for electrocatalytic processes. In the supported metal oxide anode, the layered structure of the supported oxide layer, dispersion layer, and active oxide layer is shown in Fig. 5b [39, 40].

Fig. 5
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Speculative diagram and surface oxides structures of a binary bulk mixed metal oxide anode and b supported metal oxide anode

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2.1 Examining the Effects of Oxidation on HVOF Process Coatings

Hot oxidation is a process where salt contaminants, like NaCl, Na2SO4, and V2O5, combine to form molten deposits, destroying the protective surface oxide [41]. A number of variables, including contaminant, temperature, velocity, flux rate, erosion process, temperature cycle, and thermo-mechanical conditions, might affect the classification of it into hot- and low-temperature varieties [42]. High-temperature oxidation occurs between 850 and 950 °C, where fused alkali metal salt condenses to high temperature, causing chemical reaction that lowers the substrate materials chromium content. This results in rapid oxidation, proliferous scale, and the breaking of metallic components. Low-temperature oxidation occurs in the temperate region between 650 and 800 °C, causing pitting and sulphidation [43,44,45,46]. When the shielding oxide layer fails and liquid salt comes into contact with the substrate material, high-temperature oxidation takes place. Salt fluxing and sulphidation oxidation are two methods for producing hot oxidation [47, 48]. Researchers examined oxidation conditions and mechanical properties for coatings, discussing coating materials and substrates for HVOF process.

The illustration explains the oxidation mechanism of metal oxide nanostructures, where electrons are withdrawn from an anode, resulting in the formation of metal hydroxide and metal oxide. Thermal oxidation is a simple and high-yielding technique for growing metal oxide nanostructures, producing highly crystalline materials, easy patterning, scalability, and operating at atmospheric pressure. However, the main drawback is the long growth process time [49]. In Fig. 6a, the model for producing oxide scales in gaseous settings involves atomic oxygen adsorption on the metal surface, followed by the formation of a thin oxide coating in Fig. 6b. Metal oxidation occurs as shown in Fig. 6c, releasing electrons that move through the oxide coating and react with atomic oxygen. Defects like porosity, voids, and micro cracks are caused by growing stresses and thickening of the oxide scale as shown in Fig. 6d.

Fig. 6
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Mechanism of oxidation. a Absorption, b Nucleation and progress, c Film growth, and d Defects

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In arrears to the detached and unprotected oxide scales on the steel surface, the mass gain of the SS304 sample was four times more than that of the NiCrSiB/Al2O3 sample sprayed with HVOF as shown in Fig. 7. The behaviour of oxidation deteriorated with time, reaching its maximum mass increase after 20 h. The coatings oxidative mass gain significantly increased after 20 h, showing the production of oxide at the surface, splat boundaries, and open pores. Oxides produced regularly on the surface, which resulted in constant rate of oxidation. On the other hand, the gradual increase in weight in the next cycles points to mass loss via carbon oxidation [50].

Fig. 7
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Experimental evaluation of high-temperature oxidation reactions for HVOF coatings

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As part of valuation when oxidation occurs, the behaviour of microhardness was ascertained. Ni3Ti and Ni3Ti + (Cr3C2 + 20NiCr) coatings on AISI 420 stainless steel and Ti-15 titanium alloy are produced using the HVOF technique. Figures 8 and 9 clarify the hardness line for substrates and coatings. When compared to Ni3Ti + (Cr3C2 + 20NiCr) coating, Ni3Ti coating demonstrated greater microhardness on Ti-15 substrate. The strong cohesive strength, low porosity, and high density amongst individual splats are responsible for the enhanced microhardness value [51].

Fig. 8
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Microhardness line for Ti-15 and MDN 420 substrates coated with Ni3Ti, sprayed using HVOF

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Fig. 9
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Microhardness line Ni3Ti + (Cr3C2 + 20NiCr) coating sprayed by HVOF for MDN 420 and Ti-15 substrate

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After 500 h of isothermal oxidation at 1273 K, the NiCoCrAlY-1W% nano-CeO2 coatings show the formation of oxide scale, as explained by the scanning electron microscopy (SEM) picture. The layer of thermally graded oxides (TGO) has a compact structure and fully occupies the coated surface. TGO has an average thickness of around 2.0 μm, according to research on TGO growth. Phases may have a greater contrast if there are nano-CeO2 clusters dispersed throughout the coating and inside the TGO layer. Because Ce has a limited solid solubility in MCrAlY, oxidation at 1273 K does not affect the chemical stability of nanoscaled CeO2 oxide phases. [52] (Fig. 10).

Fig. 10
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FESEM cross-sectional micrograph of NiCoCrAlY-1W% nano-CeO2 nanocomposite coatings

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A grey cast iron (GCI) substrate was successfully coated with a bi-layer of alloy-718/NiCrAlY utilising a high-velocity oxy-fuel technique. The microstructure of the coating was found to be more dense and low porosity than that of the untreated substrate, and it also had a higher microhardness value. The coating also showed reduced oxidation rate and little weight gain compared to the uncoated substrate. The development of protective phases like NiCr2O4, Al2O3, and Cr2O3 may contribute to the enhanced high-temperature oxidation resistance of the Alloy-718 coating. [53].

The microstructural properties of completely densified WC-Co particles in HVOF thermal covering on steel substrates. The feedstock powder, which lacks W2C, contains Co6W6C and a minor amount of W2C. The coating inhibits decarburization due to its densified microstructure. Low oxygen concentration of thick particles also prevents oxidation-induced decarburization. The porous feedstock powder’s carbon interacts with oxygen to produce CO/CO2 products. The completely densified feedstock powder allows most W and C atoms to precipitate as WC [54]. Thermally sprayed Cr3C2-NiCr coatings used to protect components from increased temperature wear because of coating resistance towards wear and high-temperature oxidation. These coatings are frequently used in boiler applications even though high temperatures marginally impair their strength and hardness. This study used an HVOF technique to mix a feedstock containing 70% FeNiCrMo and 30% SiC using ball milling in order to deposit the feedstock on ASTM-SA213-T-11. Because strong carbide phases formed to give microhardness and strength at high temperatures, the coating exhibited the lowest wear rate when compared to the substrate [55].

The weight increases for coated and uncoated items made of various coating materials with distinct substrates has been listed. Table 3 illustrates the assessment of HVOF approaches oxidation performance for different coated and uncoated substrates at 800 °C. In contrast, the oxidation performance of the HVOF approaches is valued for a range of coated and uncoated substrates at temperatures between 550 and 800 °C, as shown in Table 4 [56, 57].

Table 3 Covering Evaluation of HVOF techniques oxidation performance for various layered and base materials at 800 °C [117,118,119,120,121,122,123,124]
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Table 4 Evaluation of various layered and base materials oxidation performance at temperature 550–800 °C [117,118,119,120,121,122,123,124]
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3 Performance of Coatings Against Hot Corrosion and Erosion Using HVOF Technique

Hot corrosion is a complex, accelerated phenomenon affecting materials in industries, like aerospace, energy, and chemical processing [58]. It is caused by salt deposits, typically sodium sulphate, dissolving the protective oxide layer and exposing it to aggressive oxidation. Deterioration is natural process of material weakening and loss due to oxides, sulphides, and hydroxides [58,59,60]. Whereas, erosion is surface deprivation caused by mechanical actions. Erosive wear is significant degradation mechanism in engineering systems, like gas turbine engines, thermal power plants, and coal slurry pipe lines [61]. To improve resistance, coatings can be used on superalloy components to address erosion problems and strengthen them at elevated temperatures [62].

Samples were subjected to hot corrosion testing after the deposition of NaCl at 750 °C, which produced ideal conditions for hot corrosion at rapidly varying temperatures [63,64,65]. A minimum of 3 specimens were analysed in order to guarantee the reproducibility of the results. The corrosion dynamics of the alloys were examined using mass gain measurements. Figure 11 depicts the alloys’ weight change kinetics after 15 cycles. The A1, A2, and A3 alloys clearly lost weight when exposed to NaCl corrosion, whilst the A4 alloy did not lose weight even after 150 h [66]. The A4 alloy showed a weight increase of 0.56 mg cm −2 and its kinetic curve began to decline after three cycles. A1 alloy had a weight change that was similar to A2, whereas A3 alloy had a weight change of − 10 mgcm −2. In the heated corrosion test, the A3 alloy demonstrated greater stability, indicating that the mass loss for alloys reduced as the Mo concentration increased [67].

Fig. 11
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Kinetic curves of A1–A4 hot corrosion caused by NaCl at 750 °C

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The study assesses the lifetime and failure mechanisms of metal link coats thermal barrier coatings (TBC) systems based on titanium and CoNiCrAlY, which are generated on nickel-based Inconel 718 superalloy substrates using Atmospheric plasma spray (APS) and HVOF procedures. An APS method cross-sectional micrograph of YSZ TBCs with HVOF CoNiCrAlY tie coat is shown in Fig. 12. The APS approach produced microstructures for TBC that are porous, cracked, and had discontinuous apertures. On the other hand, microstructures of TBC produced by the HVOF process are evaluated when TBC is sprayed. They are less oxide- and porosity containing [68].

Fig. 12
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The cross-sectional micrograph for YSZ TBC created using APS method shows an as-sprayed coat of HVOF CoNiCrAlY

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The cavitation erosion mechanism of the HEA coating in a 3.5-wt% NaCl solution is illustrated in Fig. 13, with deep craters appearing on pits and interfaces. The main mechanism is lamellar spalling, increasing cracks and accelerating local spalling. Under micro-jet impact, the coating’s surface deforms, causing stress concentration and crack growth. Corrosion damage is aggravated by the interface between the FCC phase and BCC phase. Pitting corrosion is more common on the eroded surface of 06Cr13Ni5Mo steel. [69].

Fig. 13
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Schematic diagram of cavitation erosion mechanism of the HEA coating in 3.5-wt% NaCl solution

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In molten salt environment of Na2SO4–60% V2O5, at 900 °C, a hot corrosion investigation was conducted on the uncoated and Ni–20% Cr-coated superalloy 825. Optical microscope and SEM/EDS on behalf of elemental enquiry were used to study the cross-sectional morphology of hot-corroded, hot-coated, and uncoated superalloy following 50 cycles of exposure to molten salt at 900 °C. At 63.09 µm and 8.64 µm in thickness, respectively, the oxide scales on the untreated and HVOF-coated specimens were thicker. There were also visible cracks and the depth of attack as depicted in Fig. 14a and b. Vital information on the characteristics of hot-corroded superalloy is provided by the study [70].

Fig. 14
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Optical microscope cross-section picture of Ni-based superalloy after 50 cycles of exposure to a Na2SO4-60%V2O5 atmosphere at 900 °C. a Bare 825 and b Ni-20Cr-coated Superalloy [70]

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Using HVOF and low vacuum plasma spray (LVPS) method, a hot corrosion performance test was performed on an Inconel-738 substrate coated with CoNiCrAlYSi. Using molten film containing 20-weight percent NaV2O3, samples of various coating processes were evaluated for roughly 560 h at high temperature 880 °C. The study evaluated hot corrosion performance using mass gaining analysis. LVPS coatings experienced weight change in three stages, but HVOF spray method shielded hot corrosion for the entire duration, proving superior to LVPS [71].

For the duration for coating process, the HVOF deposition spraying parameters were kept constant. Based on the ASTM G76-02 specification, Figures 15 and 16 depict the balanced state, volume erosion, and its rate as a function of rate of impact angle and cumulative mass for erodent, respectively. The graph shows that 90° is the highest impact angle and 30° is the lowest angle, and the rate of degradation is stabilised. Elevated surface roughness played a major role in initial transient. The balanced volume erosion rate for coatings remained larger at 90° than 30° and erosive loss for brittle materials stayed greater at 90° than 30° [72,73,74,75].

Fig. 15
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Variation in uncoated SS316 steel’s rate of incremental erosion at impact angles of 30°, 60°, and 90°

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Fig. 16
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Variation in incremental erosion rate at 30°, 60°, and 90° for coatings with WC-Co/Ni Cr Al Y Si (35–65%)

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The HVOF spraying process effectively deposited Inconel 625 and Inconel 718 coatings for T-22 boiler steel. However, significant weight growth and oxide layer spalling were observed, possibly due to high iron content in the steel. Fe2O3 and chlorine gas were produced when environmental chlorine created volatile metal chlorides. The main phases identified from HVOF-sprayed Inconel 625 and Inconel 718 were Cr2O3, NiCr2O4, K2CrO4, Ni, and NiS, with Na2CrO4 peaks in Inconel 625 and Fe2O3 in Inconel 718 [76,77,78,79,80,81,82].

AISI 316 austenitic steel plates were coated using the HVOF spray technique with a Cr3C2/NiCr composition in three different weight ratios, i.e. A (85/15) %, B (90/10) %, and C (95/10) %. The erosion wear test was conducted in an atmosphere with a high temperature of approximately 650 °C and three different impact angles: 60°, 75°, and 90°. Because sample angles composition has a smaller amount of carbide, it exhibits excellent erosion resistance qualities. Additionally, sample erosion wear rate is also lower at 75° angles of impingement for every sample [83,84,85,86,87,88,89].

A material will naturally weaken and lose some of its properties due to oxide, sulphide, and hydroxide. This process is called corrosion. Erosion is the mechanical deterioration of a surface caused usually by liquid impinging, abrasion through a slurry, elements deferred from gas or fluid that flows quickly, foams, droplets, etc. [90,91,92]. Table 5 presents the corrosion and erosion performance conducted by multiple researchers.

Table 5 The study evaluates the corrosion and erosion of various coating compositions under various operating conditions [125,126,127,128,129,130,131,132]
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An analysis of the erosion and erosion-corrosion characteristics of MoNbTaTiZr and SS316L high-entropy alloys (HEA) under oblique lighting circumstances. In erosive circumstances, the HEA exhibited greater resilience and lower rates of erosion than stainless steel. Under typical impact situations, however, erosion rates somewhat increased. Additionally, the HEA showed far greater resistance to erosion and corrosion—more than 3.5 times better than that of stainless steel. Its increased hardness, which restricts material removal by reducing the mobility of abrasive particles during shearing action and offers protection against slurry erosion and corrosion, is principally responsible for its superior erosion and corrosion resistance [93,94,95,96].

4 Conclusion

This literature included insights on how the HVOF spray method was used to change the surface of several components from a number of applications, including the paper, aerospace, chemical, gas turbine, automobile, and nuclear power plant sectors, via its characteristics and spray parameters. This technology is adaptable and may lower coating costs, according to ongoing research and development.

The authors have derived their conclusions from the literature.

  • HVOF spraying method enhances component surface qualities in aggressive environments, is cost-effective, compact, and has low porosity, achieving 200-micron coating thickness without oxide formation.

  • The study compared the oxidation, corrosion, and erosion performance of HVOF at high temperatures. The HVOF-sprayed coating showed greater protection, whilst adhesion properties varied depending on coating method and post-treatment. The heat-treated HVOF coating method achieved superior adhesion properties, as per previous research.

  • HVOF spray technique improves metal component surface properties with mixture of nano- and micro-sized particle, overcoming the cost and carbon-repellent issues of nano-sized particles alone.

  • Investigations on mixed compositions using HVOF spray technique are ongoing. Impending studies have to consider altered weight percentages and post-treatment compositions.

  • Important parameters that affect the qualities of coatings and have an influence on the HVOF spray process. Different spray settings compress the features of the coating.