Introduction

The thermal spraying technique is one of the type of coating processes in which material in molten or heated state is sprayed over the work piece surface [1,2,3]. In this process, coating materials generally are metals, alloys, plastics, ceramics, etc., melted or heated by electrical (plasma or arc) or chemical (combustion flame) source. This type of coating techniques can provide coating on a large area with higher coating thickness compared to other deposition techniques (e.g. chemical vapour deposition (CVD), physical vapour deposition (PVD) and electroplating etc.). Coating materials generally used for this process are fed in the form of powder or wire in a molten or semi-molten state. These materials are then accelerated towards the substrate surface in the form of micrometre size particles. Among the several varieties of thermal spraying techniques, HVOF coating is one of the most prominent ones [4, 5]. The HVOF coating technique utilizes the combustion of hydrogen and natural gases or liquid fuel like kerosene and then particles of fuel along with oxygen particles mixed and atomizes within the combustion area under the condition that monitors the correct combustion mode and pressure. Within the combustion chamber of the HVOF system, oxygen combusts with gaseous or liquid fuel to produce high kinetic energy under the controlled heat input. The coating materials are now introduced into the chamber and heated by the hot gas stream up to a molten or semi-molten state. After this, high-velocity gas stream propels the coating material particles near supersonic speed before impact on to the substrate’s surface [6,7,8]. The flame with the powder accelerated up to supersonic velocity due to its passing through the converging–diverging nozzle. Powder particles are fallen on the surface of the substrate with a very high velocity, which results in very high-quality coating (e.g. highly dense, 0.5–2% porosity, etc.) [4,5,6,7,8]. The developed film is also less oxidized due to particles spending less time within the heat source as its speed is the very high and lower flame temperature (around 3000 °C) compared to other processes. So all together high particle velocity, low dwell time and uniform heating produce HVOF coating a very tightly bonded and dense coating process. Coating chemistries are very predictable, and coatings have fine, homogeneous microstructures. Coatings produced by the HVOF system have some outstanding properties. The HVOF coating has less than 2% porosity, and even some of the coating has 0.5% of porosity. Carbide coating sprayed with a HVOF system can exhibit bond strength up to 69 MPa. This is significantly higher than other coatings materials and coating processes. Micro-hardness: a typical micro-hardness within the range of 1100–1350 Diamond Pyramid Hardness (DPH) 300 can be achievable with the help of 12% Tungsten Carbide/Cobalt coating [5,6,7]. Factors like short dwell time and lower temperature of HVOF produce significant wear and impact-resistant coating. The HVOF produces higher coating thickness than combustion, plasma or wire coatings of the same materials. Because of the higher thickness, the stress relieving 'short peeing' effect due to high-velocity particles is withstanding. Some of the Tungsten Carbide (WC) coatings can achieve a thickness higher than 6.4 mm [1,3,4,5,6,7]. Significant compressive residual stress and very low tensile stresses enhance the fatigue life of coated components at the same time while reducing the chances of cracking and permitting greater coating thickness. By choosing proper combination of process parameters and selecting suitable materials, this process can impart significant amount of wear, fretting, erosion and cavitation resistance properties [3,4,5]. The properties like higher density and bond strength make HVOF to create self-protection ability against acidic and alkaline solution. Additionally, due to have some significant metallurgical properties HVOF coating can generate resistance against oxidation and corrosion. Superiority in the surface finish makes HVOF coatings materials enable to use for different purposes. Even coatings can be machined, ground, lapped or honed to improve their surface finish [6,7,8,9,10]. Mainly there are two types of HVOF coatings, e.g. liquid and gas, and a comparison table is presented in Table 1.

Table 1 Comparison between gas fuel and liquid fuel HVOF technology
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Brief Historical Viewpoint

The evolution of the thermal spraying technique took place around 1900s when Dr. Max Ulrich Scoop of Zurich realized that coating can be developed with the help of a stream of molten particles impinging upon it. This is the time when engineers used to break the fine particles into super-fine particles or powders with the help of high-pressure gas. Dr. Schoop’s evolution ultimately helped to develop the thermal spraying technique [8, 9]. Then in the year 1910 Dr. Schoop along with some of his German associates developed one composed of equipment by which coating can be made from molten and powder metallic. In the year of 1912, Dr. Schoop and his associates used the metalizing principle to develop one instrument for spraying wire from solid metal [8, 10, 11]. Nowadays, metalizing principle is known as oxy-fuel or flame spraying. At present, a wide range of oxy-fuel techniques are available, e.g. ceramic powder, metallic powder, ceramic rod, HVOF etc. Different thermal spraying techniques are highlighted below [11]. The schematic diagram of thermal spray coating is shown in Fig. 1.

Fig. 1
figure 1

Thermal spray coating process [11]

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Spray Guns and Coating Parameters

The typical operating pressure is in the range of 0.3–0.6 MPa. Oxygen serves as the oxidizing gas and fuels such as hydrogen, propylene, propane, methane and acetylene may achieve nozzle exit velocities of up to 1600 m/s. Shock diamonds and oblique shock waves can be generated by this flow. The jet is under-expanded since the flow is supersonic and the pressure is above atmospheric, as it has the barrel, pressure is reduced below atmospheric, and temperature decreases at a faster rate because of the increased velocity [12]. The water-cooled HVOF concept has evolved into the axial flow device depicted in Fig. 2b. Figure 2c depicts the last stage of development [13]. There are two ways in which powder is injected: either via throat of the barrel or through its diverging part. Low pressure is caused by an under-expansion situation downstream of the neck. Consequently, powders can be injected at this point to facilitate injection and containment and multiple-port powder injection is conceivable for more uniform loading of the exit stream and effective utilisation of the available heat. As a rule, operational data show that radial injection can produce spray rates at least twice as high as axial injection per unit of energy. This design has also allowed for a rise in combustion gas pressure [14].

Fig. 2
figure 2

Design changes that are typical with HVOF: a the “Jet Kote principle”, b the combustion chamber and axial injection, and c the axial powder injection chamber with radial chamber

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By using liquid petroleum instead of flammable gas and air in place of oxygen, J. W. Browning developed the high-velocity air fuel (HVAF) method [11]. Speeds up to 2000 m/s may be reached even at lower temperatures because of the high pressure in the chamber of combustion (above 0.8 MPa) and the amount of air (about 8–10 m3). Air cooling is another option for the gun. It was not long ago when kerosene was used as an alternative to flammable gases and oxygen was used in its place of air, which enabled extraordinarily high dissipated powers to be achieved (almost 300 kW). Components of the gun are subjected to high temperatures and oxidation, which need a precise water cooling design for the combustion chamber and nozzle [11,13,14,15].

Flow modelling has been the subject of several studies. The transition is from basic isentropic one-dimensional flow to three-dimensional supersonic flow [15,16,17,18]. In such hot flows, it is clear that it is important to figure out the temperature and speed of the particles. As a result of this, traditional equations, numerically influence the drag force on the particle [16,17,18,19,20]. For supersonic speeds, the coefficient of drag is recalculated. The MCrAlY particles smaller than 25 µm in MCrAlY are largely affected by shock diamonds and bow shocks produced near the substrate [19]. Spray guns (Fig. 2b) used in this study produces stainless steel particles (mean diameter 28 µm) at almost same velocity regardless of injector position and this velocity increases linearly as the pressure within the chamber increases (0.5–0.95 MPa). As a result, the range of temperatures lies within the range of 1610–1750 K. But the position of the radial injector (Fig. 2c), which can be changed from upstream to downstream of the nozzle, has a huge effect on the temperature. For particles between 1700 and 1800 K, the oxygen concentration in the coating ranges from 0.25 to 0.8%, depending on the particle temperature. Oxidation occurs in the jet core [13] in the area of mixing air with the jet and finally in the production of oxidation products.

In a high-pressure chamber, it is the deflagration of a hydrocarbon molecule (CxHy) with either oxygen or air as the oxidizer at a pressure between 0.24 and 0.82 MPa. When taking into consideration the local pressure and temperature, the nozzle is capable of producing gas velocities up to 2000 m/s [13]. The reaction energy or enthalpy, in highly compressed explosive medium (about 2 MPa), sent out a shock wave that moved the hot particles [16]. Gas velocities above 2000 m/s are not uncommon. Instead of being fed continuously into the gun as in the previous two devices, this device cycles through flammable gases and powders at a frequency of 3–8 Hz [13, 15, 19, 20].

Geometrical Constraint

Regardless of the spraying method employed, it's a line of sight approach. Only those areas that can be reached by the particle stream are coated. The spray pattern on the component can be reduced up to 200 µm in size using rotating masks, but more work needs to be done. The smallest diameter that can be covered sensibly during external spraying is approximately 1 mm [19]. When it comes to internal thermal spraying, the diameter of the cylinder that may be sprayed is around 30 mm. The height-to-diameter ratio is the limiting element in cavities (i.e. blind holes). A diameter of 8 mm is required for a ratio of around 1 [20]. Thermal spray guns are equipment used to feed, accelerate, heat and direct the spray pattern's material flow. According to the method of energy production, they can be divided into three distinct groups [21].

Spray Material

Powders, wires, rods, and cords can be put into the enthalpic jet to change the speed and temperature distribution of the particles to match the materials being sprayed and the qualities of the coatings that are wanted. The size distribution (d10–d90) must match the spray gun melting capabilities (d50) of the material (mass enthalpy of the emitting jet and interaction time) [18]. The powder feeder device must also be able to handle the flow of the powder (several technologies exist). It is important to keep the feedstock moisture content and particle size distribution as low as possible before spraying to maximize flow ability. A minimum of 24 min at temperatures ranging from 60 to 80 °C must be used to remove the moisture in the feedstock. When using agglomerated composite materials, the distribution of each particle's components must be as consistent as feasible [20]. The powdered feedstock is commonly accelerated, heated and liquefied using spray gases. The usage of ductile materials is implied by the use of wire. Cored wires can be made from non-ductile materials as well as ductile ones. Powdered metals or ceramics that don't bend easily are surrounded by a coating of a material that does. Ceramics are the primary use for rods and cords [15,17,18,19]. The molten material is atomized and moved by auxiliary gas that is pumped into the spray gun. At the same time, the wires, rods, and cords are constantly moved forward at a speed that lets the jet that comes out of the spray gun under investigation melt the ends of the wires, rods, and cords [22].

The d10 and d90 dia. (i.e. 10 percent volume of elements below d10 and 10 percent vol. percent of particles above d90) and the average diameter of the particle size distribution, d50, are used to describe powders (e.g. 22–45 m). Spools of ductile-material wires are offered for purchase. In terms of diameter, they measure between 1.20 and 4.76 mm. It is more difficult to feed larger, stiffer wires (d > 2 mm) due to the increased effort and strain required to uncoil them. There are a few millisecond spray times for ceramic rods with diameters of 3.16, 4.75, 6.35 and 7.94 mm [17, 21]. Cords are made by encasing ceramic particles in cellulose or polyester. In the processing of ceramic particles, either an organic binder is used, which begins to degrade at around 250 °C and decomposes completely at around 400 °C or a mineral glue like “bohemite (Al(OH)3)”, which binds the ceramic particles together near their melting point temperatures. Cords provided on spools have a 100 m length, allowing for several hours of spraying, compared to rods. Between 3.16 and 6.35 mm in diameter, they are the most common [15, 16, 21].

Coating Adhesion

Figure 3 depicts the fundamental design of the HVOF spray system using the diamond Jet gun as an illustration [22]. HVOF process possesses higher bond strength, less porosity compared to other thermal spray processes (e.g. plasma spraying, flame spraying, and electric arc spray etc.) as high velocity of particles creates more impact over the substrate surface [23,24,25,26]. Particle velocity, flame temperature, and spray atmosphere are the primary distinctions between spraying techniques. The coating procedure typically affects coating properties such as porosity, bond strength, and oxide content. Table 1 demonstrates the characteristics of tungsten carbide-cobalt (WC-Co) coatings sprayed using various techniques [27]. Figure 4 is a layout and distance diagram of a carbon dioxide nozzle.

Fig. 3
figure 3

Schematic cross section of HVOF gun [28]

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Fig. 4
figure 4

Schematic of carbon dioxide nozzle layout and distances used [28]

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As shown in the literature [29] a copper base, 25 s after the spray method began (Helium, 2 MPa 300 K), immediately before the formation of the coating on the copper surface [30]. Particle concentration and velocity in the flow have a direct impact on the induction time. Plastic deformation occurs when the particle is subjected to a tension larger than its yield stress due to a collision with sufficient energy. In the area of the contact zone, high plastic strain rates are seen upon impact, leading to adiabatic heating and localized material softening. It causes shear instabilities, which are responsible for the ejection of particle–substrate interface solid-state jets of material [28, 31]. Allowing for direct metal-to-metal contact, the oxide layer has been partially eliminated. Low-velocity particles abrade the substrate, whereas high-velocity particles deposit and create a coating on the surface. This is shown schematically in Fig. 5 [32].

Fig. 5
figure 5

Particle velocity and deposition efficiency are shown to be correlated in the following diagram. The critical velocity, Vc, is defined by the transition between abrasion and deposition at low velocities

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Adhesion and elastic deformation energy created by particle impact may be compared using an easy-to-understand model devised by B. R. Marple [33]. At first coating layer is formed when the adhesion energy exceeds the elastic deformation energy, and this has been researched. It has been discovered that the best spraying circumstances are dependent on the qualities of sprayed materials and surfaces as well as spray particle sizes and temperatures [28]. Various spray materials and substrates have had their critical velocities calculated. Gartner and colleagues [34] looked at the critical velocities of several spray materials on a copper substrate. Figure 6 sums up the findings.

Fig. 6
figure 6

Various spray material critical velocities were experimentally determined. The error bar takes into consideration the wide range of powder purities that are accessible

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A study by Gartner et al. [32] found that particle’s particular mass and temperature affect their critical velocities. The hitting particle will cause significant erosion if its velocity exceeds the range of hydrodynamic penetration. The erosion velocity (Verosion) was calculated by Schmidt et al. [35, 36]. It is faster than the device's critical speed observed (Fig. 7).

Fig. 7
figure 7

The erosive velocity of different metals was calculated for 25 m particles

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Spray Specifications

When adhering a coating to the surface and forming the following layer, the most important impact factors are particle velocities and materials, together with the substrate's material qualities. Gas stagnation pressure, temperature, molecular weight, and particle shape and size all affect particle velocity [37, 38].

When injecting particles axially with a DJ-2700 gun, using a JP-5000 gun and injecting radially increases the oxygen content of the coating only slightly (316L). As a result, while using JP-5000, where the particles are at a lower temperature (about 800 K lower), O2 in the flame cools them even further, lowering their oxidation content even further. A gas shroud attachment for commercial HVOFs has been developed by Ishikawa et al. [14] to minimize oxidation using a shroud gas, although at a considerably lower flow rate. WC–Co particle velocities rise from 760 to 850 m/s with the gas shroud arrangement, but particle temperatures fall from 1950 to 1830 °C due to a decreasing flame at a pressure of 0.72 MPa. The density rose, decomposition of WC was 6 to 2.5%, and corrosion resistance and good wear resistance greater than without a shroud were also achieved. High-density coatings with little oxidation and no carbide breakdown can be achieved by raising the particle velocity while reducing the temperature of the powders used in HVOF or HVAF. Because of this, Kawahita et al. [37] modified the pistol by including a mixing chamber between the combustion chamber and the nozzle. Nitrogen is added to the combustion gas in the combustion chamber to its lesser temperature. Operating parameters for the altered JP-5000 include kerosene consumption of 0.29–0.35 l/min, N2 consumption of 0.5–2 Nm3/m, and O2 consumption of 0.55–0.73 Nm3/m. Thus, titanium (Ti) coatings with a high density that are resistant to corrosion were developed [38].

When it comes to HVOF gun development, researchers have concentrated on reducing particle temperature and boosting their velocity for more than two decades already. Up to 1 MPa chamber pressure can produce spray particle speeds of up to 650 m/s [39]. Hydrogen pilot flame (88 Litres /min, air 500 L/min) ignites combustion chamber fire when power rises up to 300 Kilo Watt (Kerosene 31 l/h, O2 965 l/min). For example, Trompetter et al. [40] used an HVAF gun to spray NiCr powder (melting point Tm = 1400 °C), which resulted in a particle velocity of 670 m/s. In comparison with hardness of NiCr, spraying soft surfaces produced mostly deeply penetrated solid splats, whereas spraying hard substrates resulted in more heavily molten splats. They hypothesized that the quantity of plastic deformation owing to substrate hardness would explain this occurrence by converting particle kinetic energy into heat. Peening and persistent compressive stresses in coatings are the second results of high-velocity impacts [32, 41, 42]. Studies show that HVOF coatings have minimal surface residual stress and typically have tensile stress, but the stress within thin films is substantial and mostly in compression, according to the measurements. Pumping on a stainless steel substrate results in a 100-m-long hardening zone, which is why this peening effect is so important for metals like stainless steel.

The HVOF technique creates coatings with a high density because of the particles with high kinetic energy (400–650 m/s) [37] (porosity less than 3%). As a result, materials like WC may be deposited at temperatures below their melting point, minimizing oxidation and degradation. It is now possible to use spray guns to apply materials that lack the extraordinary copper's ductility or aluminium and they are more vulnerable to oxidation, than Ti [40].

Influence of HVOF Process Parameters

The qualities of the coating, resistance to corrosion and erosion, and other factors are often improved by optimising the spraying parameters. The distance of spray, powder feed rate, oxygen flow rate, fuel flow rate and carrier gas used are few factors that affect coating qualities. Brezinová et al. [43] examined the impact of milling time (36 h) on the mechanical properties of SAE 1020 steel that had been sprayed with HVOF and coated with WC/Co and WC/Co-CNT. The findings demonstrated that the inclusion of Carbon Nano Tube (CNT) enhanced the coating's mechanical properties, including hardness, fracture toughness, and apparent yield stress. The effects of heat treatment (using oxyacetylene flame) on the adhesion resistance, microstructure, micro-hardness and erosive wear of HVOF-sprayed NiCrBSiW coated plain carbon steel were studied by Rodriguez et al. [44]. After the post-spray heat treatment, the coating's porosity was reduced. Additionally, better metallurgical bonding was created after the post-spray heat treatment, which improved the coating's cohesion and adhesion strength. Using a factorial experimental approach, Gil et al. [45] investigated the effects of process factors on the adhesion strength and microstructure of NiWCrBSi coatings formed by HVOF process. The coating's adhesion strength and porosity are most significantly impacted by the spray distance and equivalency ratio. Table 2 makes it evident that Cr2O3-4%CNT, NiCrBSi, Cr3C2-NiCr and Ni-20Cr coatings are sprayed with HVOF and deposited under ideal conditions offer higher mechanical properties.

Table 2 Process parameters of HVOF-sprayed coatings [46,47,48,49,50,51,52,53,54,55]
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Effect of Residual Stresses in HVOF Coatings

The process of thermal spray coating encompasses a broad variety of other processes, some of which include wire-arc, HVOF, plasma spraying, and many others that are utilised in a variety of various applications. One of the most important things to think about when it comes to coating performance is how the stresses left over from the coating process affect the life and behaviour of the coating layer [56]. It could be very important to the evaluation process to measure these stresses. Because of this, a lot of research has been done on how to measure and calculate the quantitative and qualitative residual stresses. During the coating process, most of the work was done using pure experimental methods to measure the residual stresses. Others get the stress values using a method that is a mix of experiments and math [57].

Sources of Residual Stresses

Residual stresses in coatings that are sprayed with heat come from two main places. The first is the sudden temperature decrease from the melting point to the substrate temperature experienced by the sprayed particles. This kind of stress is called intrinsic, quenching, or deposition, and it is tensile. The second source, which may be the most important, is that the coating and the substrate have different coefficients of thermal expansion (CTE). This happens when the average temperature of the coating system after the deposition process starts to drop towards room temperature. This is called secondary cooling [57]. Then, the difference in how much the substrate and the coating shrink when heated leads to the residual stresses. Aside from the two main causes, there may be other things that indirectly cause residual stress. For example, the amount of residual stress left after deposition depends on how fast the coating particles are moving. This has to do with the speed of the coating and the size of the particles.

Literature shows that in HVOF coating systems, residual stresses come from [56, 57]

  1. 1.

    The process of coating deposition. When particles hit something, they freeze and shrink, which creates tensile stresses that are mostly relieved by the formation of tiny cracks in the coating.

  2. 2.

    Stress from the wrong materials. When the parts that have been coated cool down to room temperature.

  3. 3.

    Differences in the thermal expansion coefficients of the coating and the material underneath it.

Parameters Affecting Residual Stresses

The generation of residual stresses is influenced by a number of process variables, including deposition speed, spray gun velocity, coating temperature, coating thickness, etc. However, these parameters cannot be improved purely based on residual stresses; it is also necessary to take the final coating's quality into account.

Following is a broad breakdown of the processes in the coating process that have an impact on coating quality [58]:

Spray particle movement

figure a

particle collision and deformation

figure b

coating formation

figure c

splat interactions

figure d

pore formation

figure e

coating build-up

figure f

and final coating layer.

Importance of Residual Stresses Calculation

In general, compressive residual stresses are preferable, and unless they are present in relatively high quantities, their existence usually has no negative effects on the coating system. Tensile residual stresses are typically more detrimental and cause numerous coating issues, including fatigue failure and the initiation and spread of cracks, which can result in interface delamination. Drilling holes is one of the most popular methods for evaluating residual stresses [59, 60]. Typically, drilling in this procedure is done subtly at the micro-level until the substrate is reached [57, 61, 62]. It is reasonable to infer that this procedure produces minimal stresses because it uses moderate feed rates, high cutting rates, and low drilling depths for each phase. A cooling time is also used in between every other drilling increment to ensure that there are no thermally induced stresses brought on by the drilling operation [62].

HVOF Wire Spraying

Due to the low impact velocities for the particles, wire flame spraying produces very porous coatings (over 10%). The high rate of wire flame spraying (up to 10 kg/h) makes it a major issue in terms of process economics, which is often the case. The automotive industry has been studying the HVOF wire spraying technique since 1995. There has also been spraying of molybdenum. Compared to plasma spray coatings, wire flame-sprayed thin films have lower friction resistance but are tougher and more wear-resistant [63]. Methane and oxygen were also sprayed on low-carbon steel. [64] Such thin films are also applied to nickel–chromium wires [65] and nickel–chromium alloys such as Cr3C2-NiCr [66]. Those treated guns with a fuel-oxygen had a strong resistance to heat corrosion, notably Ni-20Cr, with less than one percent porosity. As a last note, an HVOF/plasma jet gun and arc spray hybrid method [67] has been devised Table (3).

Table 3 Mechanical properties of different coating materials and substrates [47,68,69,70,71,72,73] 
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Mechanical Properties of HVOF Coating

Tribological Properties of HVOF Coating

The average coefficient of friction and volume lost by the coatings against various counter bodies are summarised in Table 4 [74]. It's interesting to note that while sliding against the identical counterpart, HVOF coatings had a lower coefficient of friction than other types of coatings. In comparison with other counter bodies, the volume lost by both coverings because of the Al2O3 ball is significantly higher. In comparison with Si3N4 and steel balls, the volume lost by coatings due to Al2O3 balls is precisely 2.7 and 20 times higher, respectively. Additionally, among the three counter bodies, the volume loss of the Al2O3 ball is the least. As a result, the Al2O3 ball was able to keep its initial curve throughout the test period. As a result, the Al2O3 ball has effectively pierced both coating surfaces [74].

Table 4 Tribological properties for different counter bodies in HVOF process
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Comparative Analysis in Between Different Coating Processes

Innovations in different coating processes, thermal spraying, CVD, PVD and other processes mainly rely over the evolution of their past developments. To deposit uniform thin film coating, many coating techniques are available in the market. Each techniques have their own pros and cons along with some important parameters. So utmost care should have been taken over key parameters during the process to get the desired coatings. In thermal spray process, by conveying gas stream, feed stock material will be heated, atomized and ultimately deposit over the prepared substrate surface [75]. As the materials are in heated state, they will transform into plastic or molten state, they become atomized and confined before accelerated towards the substrate’s surface. Then the stroked particles over the substrate surface become flat and develop thin platelets. These flat particles then match up to the irregularities of the substrate surface. So it will provide considerable barrier protection over the surface. In PVD process, the developed exceptional properties in coating are due to the presence of highly ionized metallic vapours. In CVD process, it is possible to homogeneously coat 3D samples, whereas in physical vapour deposition PVD processes mainly have command over growth of coating. In Tables 2 and 3, comparison between different thermal spraying techniques and a comparison of HVOF with CVD and PVD have been highlighted [75,76,77] (Tables 5, 6, and 7).

Table 5 Comparison in between different thermal spraying techniques [78,79,80] 
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Table 6 Comparison in between HVOF and CVD PVD process [80,81,82,83,84,85,86,87,88,89,90,91,92,93,94]
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Table 7 Applications of different materials coated by HVOF technique [95,96,97,98,99,100,101,102,103,104]
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Applications HVOF Process

Engineers use HVOF to coat technical components with cermet, metal, and some ceramic thin films that are generally 100–300 µm in thickness, enabling them to operate under harsh circumstances. Aside from wear resistance, they're being examined extensively for their higher corrosion and resistance to oxidation in comparison with different spraying methods. They are widely used in a variety of industries, including maritime, aviation, and automobiles. With their help, a broad range of petrochemical system elements can be recovered.

Using HVOF, lower oxygen and fuel ratios allowed for the production of ultra-thin, pure aluminium films [105]. The HVOF process was used to spray iron-aluminide with 7–15% oxide [106]. With gaseous or liquid fuel, Ni-20wt% Cr spray creates less oxide, lower porosity, and lower melting evidence [107]. HVAF spraying on aluminium substrates results in a strong connection between the substrate and the Ni Cr particles, but there is no chemical bonding [108]. Waste-to-energy boilers use protective Ni Cr or Ni Cr Si B coatings to avoid corrosion [109]. To avoid corrosion, Ni Cr Mo Nb can also be used [110]. The HVOF coating made of super alloys can also be used as thermal barrier bonds [111, 112].

For HVOF and HVAF procedures, substantial research has been done on cermet from the start [113,114,115,116,117,118,119]. Spray cermet most typically employs WC-Co and Cr3C2NiCr. Avoiding or at least reducing carbide breakdown as much as feasible is a vital part of coatings quality. Because of their corrosion and wear resistance (especially those containing NiCr), these coatings are commonly employed. A variety of other materials, like MoB and CoCr [120], which protect against erosion with liquefied Al–Zn alloy, a variety of Nickel–Titanium–Carbon (Ni–Ti–C) [121], which contains Ni-rich solution, Ni-rich spraying Ti-in addition to Ni-rich solution, and a variety of silicon nitride-based coatings [117] all of which are sprayed.

When it comes to coatings made using HVOF or HVAF technology, high temperatures are less of a rarity than fast speeds. Propylene devices may easily melt Ti [122]. High Weibull modulus values and thick and homogenous coatings, with rutile as the dominating phase, are achieved in contrast to previous spray processes. Chromium stabilizes the phase in most alumina [123]. Zirconia stabilized with yttria particles smaller than 10 m can be sprayed, and the production of adhesive and cohesive coatings may be influenced by sintering [124].

Without ceramic doping, nylon 11 is the most widely used polymer [125,126,127]. Degradation is usually limited to a change in colour due to the HVOF residence time being short enough (1 m/s). Abrasion and fatigue wear may be taken into account without doping powders in these coatings. Materials sprayed with a mixture of liquid fuel and oxygen are nearly identical to those sprayed with gaseous fuel. Higher velocities and lower temperatures are the primary distinctions.

When copper was heated to a temperature where it was highly deformable just below its melting point [128] extremely dense coatings could be produced. Coatings, on the other hand, were more durable than wrought iron [129].

Furthermore, the adhesion of Ni–Cr–9Cr–1Mo steel exceeded 70 MPa and had outstanding resistance to steam oxidation [127, 128]. With these HVOF process guns and a noble gas shroud, the thin film oxygen levels attained with vacuum plasma spraying (VPS) were almost equal [129]. A 600 °C treatment of the CoMCrSi coatings increases hardness and decreases friction against an alumina pin [130].

Abrasion resistance is better for WC-Co + CoMCrSi coatings in two-body sliding situations than for electrolytic hard chrome (EHC) coatings, but the mass loss in three-body abrasion conditions is equivalent to or even larger than that for EHC coatings [130]. A coating of WC-NiCrFeSiB is extremely resistant to oxidation and hot corrosion at temperatures as high as 800 °C [131].

For example, iron sulphide can be sprayed with HVOF liquid fuel that contains lubricants [132]. Through spraying SiO2/Ni/Al–Si–Mg powder, resulting in MgAl2O4 and Mg2Si in an AlSi matrix, for HVOF process self-propagating high-temperature synthesis (SHS) may be achieved [133]. Flight and splat stacking are the two stages of the reaction. These materials are extremely strong [134,135,136,137].

Hydroxyapatite (HAp) Coatings

For a variety of applications, including total hip and knee replacement, thermally sprayed HAp coatings enable a natural fixation on metal implants. These coatings enhance patient biomechanics, prolong the life of the implant, and improve the patient's quality of life [138]. The only methods for creating HAp coatings that have received FDA approval are thermal plasma spray techniques [139]. However, this group of operations can result in the development of secondary phases, including amorphous calcium phosphate (ACP), tricalcium phosphate (TCP), and tetracalcium phosphate (TTCP), among others [139] since HAp is unstable at high temperatures (i.e. T > 1500 °C). The formation of these phases implies that they will be more soluble than HAp in a physiological environment, which will compromise the coating's long-term stability [140]. The HVOF thermal spray is an alternative method to generate bioactive coatings of a quality that is equivalent to or even superior to that obtained by plasma spray [139, 141]. The HVOF enables the creation of thick, crystalline, and adherent coatings by letting the powder to travel at greater speeds and lower temperatures [142]. A technique for enhancing the Osseo integration of metal implants [143]. Metal implants have a high mechanical strength, but because they are bio-inert, they have a poor ability to connect to bone tissue. Based on the favourable bioactive characteristics of several bulk ceramic materials seen in physiological conditions, bioactive coatings have been created [144]. In particular, it has been shown that the mineral HAp, Ca10(PO4)6(OH)2) stimulates bone formation. Due to its chemical resemblance to the mineral component of bone, which is composed of carbonated apatite, calcium-deficient apatite, and ions such as potassium, magnesium, sodium, chloride, and fluoride, HAp has been successful as a biological material [145, 146].

Challenges and Merits of HVOF Thermal Coatings

Merits

  1. 1.

    As a coating material, a wide variety of substances (metal, ceramics, polymers, alloys, plastic, etc.) in different forms (powder, wire/rod) that melt without disintegrating can be used.

  2. 2.

    Almost any material can be coated with thermal spray paint without considerably heating the base material.

  3. 3.

    A thermal spray method can be used to produce the distinctive surface characteristics, such as resistance to corrosion, wear, abrasion, and dimensional restoration.

  4. 4.

    Compared to other processes, the thermal spray process produces less coating material waste. Therefore, when using more expensive coating materials, it is preferable to use the thermal spray procedure.

  5. 5.

    Effective coating can lengthen the component's lifespan.

Challenges

  1. 1.

    Use a line-of-sight technique.

  2. 2.

    During coating deposition, the material's structure and composition change.

  3. 3.

    The surface of the coating is uneven; therefore, some surface finishing is required to improve the surface of things.

  4. 4.

    Fewer process approaches for thermal spray coatings necessitating more expensive equipment result in a higher initial setup cost.

  5. 5.

    In certain processes (such as wire arc spray), only electrically conducting substances can be sprayed [147, 148].

Conclusions

HVOF thermal spraying is a process in which powder material is melted and propelled at high velocity towards a surface using oxygen and fuel gas mixtures. In the combustion zone, the powdered material enters the flame and becomes molten or semi-molten, depending on the melting temperature and feed rate. It is the deflagration of a CxHy using an oxidizing agent that is either O2 or air at the pressure of between 0.24 and 0.82 MPa in a chamber at high velocity. Spray particle mass and temperature have a substantial influence on the critical velocities of sprayed materials and substrates, and the ideal spraying conditions have been identified based on these factors. The hitting particle will cause significant erosion if its velocity exceeds the range of hydrodynamic penetration. Compared to plasma spray coatings, wire flame-sprayed coatings have lower friction resistance but are tougher and more wear resistant. Lastly, it should be stated that a hybrid thermal spray process combining an arc spray process with an HVOF process/plasma jet process gun has been developed. In comparison with other coating processes, such as CVD, PVD and electroplating, thermal spray processes are distinguished by their high coating rates. Other advantages of thermal spray processes include the ability to deposit thick coating sections and the simplification of waste disposal. Materials sprayed with a mixture of liquid fuel and oxygen are nearly identical to those sprayed with gaseous fuel. Spraying substances containing solid fluids, such as iron sulphide, is another application for HVOF liquid fuel. In HVOF process, SHS reactions may be generated by spraying SiO2/Nickel/Al-Silicon-Mg fine particles, arising in MgAl2O4, Mg2Si in AlSi matrix, as in the case of flame or gaseous fuel. Using thermal spray methods, you can work with alloys, ceramics, polymers, cermet, and composite materials. It’s also possible to create coatings with a wide range of architectural features, from thick to porous to some micrometres or millimetres dense, as well as nano- or microscale architecture.