Introduction

Thermal spray metal coatings were born with thermal spray technology in the year 1910 (Ref 1). In the following 110 years, different thermal spray processes, including very early flame spraying, arc spraying, plasma spraying, and high-velocity oxy-fuel (HVOF) spraying, have been gradually developed to deposit different metal alloy coatings to fulfill all kinds of requirements for various of industrial applications (Ref 2, 3). All these technologies have been developed for effectively heating and accelerating spray particles to generate high velocity and high temperature molten particles stream, which are sprayed onto a substrate for coating deposition. A thermal spray coating is primarily deposited by the successive impact of molten droplets followed by their spreading, rapid cooling and solidification on a rough substrate surface. Therefore, thermally sprayed coatings generally present a lamellar structure packed by multiple flattened splats. The quality of the bonding between coating and substrate, and between the adjacent splats determines the adhesion and cohesion, respectively, and thus is the central concern regarding effective coating applications. Due to insufficient filling of impacting molten droplet into cavities on a rough surface and very limited wetting time of melt to substrate surface less than 10 μs before the solidification of the melt starts, the coatings in the as-sprayed state are characterized as porous microstructure with a relatively low density. Moreover, the coating presents lower adhesive strength and limited cohesive strength in comparison with the bulk counterpart (Ref 4). Taking mild steel coating as an example, its adhesive strength of 116 J/m2 is lower than the cohesive strength, and its cohesive fracture toughness of 261 J/m2 is remarkably lower than 600-60000 J/m2 for brittle fracture of bulk steel (Ref 4). This is because the adhesive strength is mainly determined by the mechanical interlocking (Ref 5, 6), and usually limited contact is formed between adjacent splats within a coating (Ref 7). However, the coating mechanical properties and transportation properties are positively proportional to the interlamellar bonding ratio (Ref 8). Quantitative characterizations revealed that interlamellar bonding ratio is typically less than one-third (Ref 9, 10). Such a low interlamellar bonding ratio sets up a low top celling for thermal spray coating properties compared to their counterpart bulk materials. For example, Young’s moduli of different coatings are lower than one-third bulk materials (Ref 11). The abrasive wear resistances of the as-sprayed coatings taking flame-sprayed NiCrBSi coating (Ref 12) and HVOF-sprayed WC-Co coatings (Ref 13) as examples are relatively lower than their corresponding fused bulk and sintered bulk, respectively. Therefore, one biggest challenge born with thermal spraying is how to improve the adhesion and cohesion of the coating, to reduce the porosity, and consequently deposit a bulk-like coating, although in many applications the coatings with a porous microstructure are required such as for applications to abradable coatings (Ref 14), catalytic coatings (Ref 15) and so on.

Another concern adherent to thermal spray metal coatings is how to control the oxidation during spray deposition to reduce oxide inclusions since the resultant oxide inclusions in most cases degrade the coating performance by lowering coating adhesion and cohesion (Ref 16) although they may improve wear performance by increasing coating hardness (Ref 5, 17, 18). Gan and Berndt (Ref 16) presented the oxidation mechanisms, the effect of oxidation on coating properties and oxidation control approaches in their review paper. While the use of a shroud or/and shield is effective to protect metal coating against oxidation, it is still tough to make effective protection for the parts with complex geometry. It is interesting how to utilize the sacrificial elements in alloy powders as deoxidizer elements as reported by Zeng et al. (Ref 19, 20) for protection of in-flight molten droplets from oxidation throughout the whole spray distance range.

Based on thermal spray coating formation theory, to deposit a bulk-like metal coating by reducing coating porosity, the first measure for most spray processes using powder as feedstock is to increase the temperature and velocity of spray particles by optimizing the processing parameters. Effective heating is generally attempted by using high temperature spray processes and increasing the thermal power of heat sources (Ref 1). Since heating of spray particles takes place during in-flight, any effort to enhance the heating ability of the heat source leads to the increase of in-flight particle velocity and subsequently shortening of heating time on the other hand. The above contradictoriness involving in-flight spray particle heating in any flame jet limits the heating up ability of spray particles (Ref 21). Moreover, the vaporization of molten spray particles also limits the maximum particle temperature (Ref 22). Thus, the processes are usually optimized to achieve sufficient melting of spray particles. On the other hand, since higher velocity impact results in higher transient contact pressure at the particle-substrate interface and spreading of the molten droplet (Ref 23), it could enhance the filling ability of the spreading melt to surface cavities. It is generally accepted that increasing the particle velocity of droplets is effective to deposit a dense coating (Ref 1, 2). Accordingly, high power plasma spraying and HVOF processes have been developed. By optimizing spray conditions, the apparent porosity of a metal coating deposited by HVOF can be reduced to a low level of about or less than 1% (Ref 24, 25). However, despite the apparent dense microstructure of HVOF coatings, the electrochemical tests generally reveal that the as-sprayed metal coatings could not provide the substrate with effective and reliable protection from corrosion due to the existence of unbonded lamellar interfaces (Ref 25, 26). Moreover, limited bonding also degrades the wear performance of hard alloy coatings since lamella spalling occurs easily through the propagation of sub-surface cracks along the lamellar interface at high stress conditions (Ref 24, 27), especially as the coatings are subjected to dynamic loading (such as cavitation erosion) (Ref 28). Hiraga et al. reported that compared with vacuum plasma-sprayed NiTi coating the laser remelting treatment leading to the formation of bulk-like NiTi presented the enhanced cavitation erosion resistance by a factor of magnitude from one order to over two orders (Ref 29, 30). This makes certain traditional thermal spray applications to be possibly replaced by other surface engineering processes such as diffusion processes, cladding processes for more reliable protection (Ref 26). Therefore, many efforts have also been made to increase the cohesion of metal coating toward bulk-like ones, since a limited cohesion does not make metal coating corrosion-resistant (Ref 25) and wear-resistant (Ref 24, 25).

Due to limitations of current spraying processes to directly deposit bulk-like metal coatings, post-spray treatments are used to eliminate pores in the coating and achieve metallurgical bonding to the substrate. Main processes include spray-fusing, high temperature annealing treatment, laser remelting, and so on. The spray-fusing process was invented along with self-fluxing alloys in the early of last 1950s, applying only to self-fluxing alloys (Ref 1, 12, 31). Its main features will be mentioned in the later section. Since atom diffusion occurs across the intersplat and coating/substrate boundaries at high temperature, the post-spray heat treatment usually leads to healing of unbonded interface boundaries and reducing of small pores in the coating (Ref 32, 33). For metallic coatings especially MCrAlY bond coats for thermal barrier coatings, high temperature heat treatment in vacuum or inert gas atmosphere not only leads to full densification of the bond coat itself but also the formation of metallurgical bonding between the superalloy substrate and the bond coat (Ref 34, 35). The HIP (hot isostatic pressing) treatment is more effective to densify metallic coatings (Ref 32, 33). The heat treatment needs to be carried out at a high temperature either in a vacuum chamber or in a controlled inert atmosphere. Moreover, the part size is practically limited by the size of the chamber. Meanwhile, the heat treatment does not apply to the parts sensitive to high temperature.

Laser remelting has been applied to different types of thermal spray coatings (Ref 36, 37). For metallic coatings, pores in the coating can be removed after laser remelting to form a fully dense coating for complete protection of substrates from corrosive media (Ref 38,39,40). Moreover, the potential high performance of metallic coating materials under dynamic wear conditions such as cavitation can be fully taped out by producing bulk-like coating rather than lamellar-structured coatings (Ref 29, 30). For ceramic coatings, despite laser densification of coatings presenting bulk-like microstructure, rapid cooling of remelted coating inevitably produces cracks perpendicular to the coating surface to form crack-networks (Ref 41). Fortunately, such vertical crack network perpendicular to coating surface could be effectively utilized to enhances strain tolerance of YSZ thermal barrier coatings during thermal cycling (Ref 42, 43).

Despite the effectiveness of post-spray heat treatment to convert the porous coating to bulk-like one, the ultimate challenge is to directly deposit bulk-like metallic coating by thermal spraying. A recent investigation showed that NiCr alloy coating with a lamellar bonding ratio higher than 80% can be deposited with powder design by shrouded plasma spraying (Ref 44). Since enhanced adhesion and cohesion with low oxide inclusion makes the coating ductile, the post-spray mechanical shot-peening was applied to alloy coating to achieve fully dense coating with pore sealing and to prevent the penetration of the aqueous solution. It was also found that by taking advantages of low melting point of self-fluxing alloys and utilizing in-situ deoxidizing effect of in-flight molten droplets bulk-like metal coatings are achieved by atmospheric plasma spraying (Ref 45, 46).

In conclusion, the present paper will focus on recent research progresses in the metallurgical bonding development in plasma spraying for depositing bulk-like metal coatings. The general strategies to develop metallurgical bonding will be reviewed. Then, the approach to create the spread-fusing effect for a self-bonding mechanism by highly over-heated molten droplets will be introduced with the illustration of typical coating properties. The development of the in-situ deoxidizing mechanism of in-flight molten droplets in an open ambient atmosphere will also be presented suggesting the possibility to deposit the bulk-like metal coatings by APS. In the interest of brevity, this review concentrates mainly on the bonding formation in the like-on-like system of metal coating, i.e., the cases that the metal droplet impacts on the metal substrate with the same compositions as that of droplet.

Strategy for Bulk-Like Dense Metal Coating Formation Through Deposition Temperature Control

Effect of Substrate Preheating on the Characteristics of Metallic Splat Formation and Coating Adhesion

Three types of bonding mechanisms contribute to the adhesion and cohesion of thermal spray metal coatings. They are known as mechanical bonding in the form of mechanical interlocking of the roughened surface, physical bonding due to Van der Waals force, and metallurgical bonding through an alloying or chemical reaction (Ref 5, 6, 47). It should be pointed out that when oxides are included in the metallic coating the chemical bonding via a covalent bond or an ionic bond may also exist at the local oxide-metal interface (Ref 48). However, in most cases oxide inclusions hinder the bonding formation (Ref 4, 49). It should be noted that metallurgical bonding is the strongest one among all three bonding mechanisms for metallic coatings. Thus, how to create an effectively metallurgical bonding at the interface between the substrate and the coating, and interfaces among splats during molten droplet impact is the main challenge accompanying the development of thermal spray processes up to date.

As a first mechanism, the metallurgical bonding can be achieved by the wetting of the molten metal to the substrate surface at a low interface temperature as those observed in soldering and brazing processes (Ref 66). Then, the third effect is to activate the surface and thus promote the reaction between splat and substrate (Ref 59, 67, 68). Accordingly, the adhesion of thermal spray coatings increases with increasing the preheating temperature (Ref 58,59,60). However, since the preheating also promotes the oxidation of the metal substrate, it was believed that a higher preheating temperature leads to a thicker oxide scale. It not only reduces adhesion through fracture inside substrate metal oxide layer (Ref 69), but also reduces the possibility to form the metallurgical bonding (Ref 49). The surface oxidation influences little on the formation of regular disk type splats when the oxidation-induced surface roughness increment (Ref 70) and gas entrap** effects (Ref 71) can be neglected on the spreading. The higher adhesive strength obtained at a higher preheating temperature is attributed to improved intimate contact and lower solidification rate which gives melt a longer time to flow into surface cavities (Ref 59). On the other hand, it was also noticed that the oxidation of the substrate surface to a certain degree benefits the improvement of adhesive strength of thermally sprayed ceramic coatings (Ref 58). Thus, increasing the deposition temperature in the ambient atmosphere enhances coating adhesion at the expense of excluding the metallurgical bonding formation for metal coatings. The adhesion of metal splats deposited on a preheated substrate up to 327oC still mainly depends on mechanical interlocking since the adherent splat can be simply detached from the substrate for the microstructure examination from the bottom of the splat surface (Ref 72, 73). Even using the substrate with a gold coating or depositing splat in an N2 atmosphere to prevent the substrate from oxidation the adhesion of splat was almost the same as that sprayed on the bare substrate in the open air (Ref 74). Although the possible interaction layer formation at splat/substrate interface in the local central region by melting or diffusion for like-on-like splat/substrate system was also reported (Ref 75), it can be inferred that to remarkably enhance the adhesion through metallurgical bonding, a much higher deposition temperature is required. However, most researches investigating the effect of preheating temperature on splat formation were concerned only with the disk-like splat formation by suppression of splashing through removing evaporable adsorbates and very few investigations reported the formation of good interface bonding across the boundary (Ref 76). This is because most investigations on splat formation were carried out at ambient atmosphere and its effect was limited by the inevitable substrate oxidation. Consequently, the splat formation investigations up to now still did not reveal at what preheating temperature a strong metallurgical bonding can be achieved through liquid/solid reaction at the interface. This can be partially attributed to a great number of materials combinations of substrate and splat, which leads to complexity of problems. Therefore, to make it simple, the following sections will more focus on the droplet/substrate combinations of the same material, which is similar to what happens in adjacent splats within the coating.

Interface Temperature Requirements for the Intersplat Bonding Formation

For oxide ceramics, it was found through like-on-like experimental design that the critical interface temperature for spreading melt to form a strong bonding is equal to the glass transition temperature of spray material (Ref 68, 77). Only when the interface temperature exceeds the critical interface temperature, the spreading melt could bond to the underlying splat in solid-state chemically since the atoms in melt have sufficient activity due to its low viscosity. Supposing that Al2O3 spray particles are heated to a temperature just over its melting point, a critical deposition temperature of approximately 300 °C is required to form the chemical bonding at the splat boundary following the above theory (Ref 68). This is consistent with the reported preheating temperature of Al2O3 substrate temperature over which the deposited Al2O3 splats were bonded to the plasma-sprayed alumina substrate with γ-phase (Ref 78). Moreover, through the controlled oxidation in the CO2 atmosphere to form an FeO layer on carbon steel substrate (Ref 79) with a thickness of about one ceramic splat, from the results reported by Valette et al. (Ref 79) it can be found that the alumina coating deposited at the deposition temperature higher than the critical bonding temperature presented the adhesion higher than 60 MPa. Recently, Valetter et al further reported an adhesion of 105 MPa for alumina coating deposited in the same fashion on the flat pure nickel substrate with an NiO layer (Ref 80). The fact that the deposition temperatures in the investigations mentioned above (Ref 79, 80) are higher than the critical bonding temperature reported (Ref 68) reveals the dominant effect of the interface temperature on the interface bonding formation between the similar materials. Since the critical bonding temperature here is referred to as the critical deposition temperature and is proportional to the melting point of oxide ceramic spray materials (Ref 68), the critical preheating temperature for certain ceramics can be simply estimated.

Although with common metal alloys the critical interface temperature was not defined yet for the bonding formation, it can be supposed that the critical bonding temperature may be related to the lowest interface temperature for splat to maintain the liquid state. If the interface temperature is lower than such temperature, the bonding formation needs to take place at the solid/solid interface from which a strong metallurgical bonding is not expected. Thus, the critical bonding temperature can be estimated by the maximum undercooling temperature of molten metals. It has been reported that the maximum undercooling temperature of most metals except refractory metals is less than 400 °C (Ref

Strategy for Develo** Bulk-Like Metal Coating by Spread-Fusing Effect Through Creating Ultra-High Temperature Droplets

Metallurgical Bonding Formation at the Coating/Substrate Interface

When a metal coating is thermally sprayed in an ambient atmosphere, an oxide scale is inevitably formed on the splat surface (Ref 16, 87, 88). Since the formation of a metallurgical cohesive bonding can be achieved only when the coating surface layer is instantaneously melted by spreading molten melt, this mechanism is referred to here as the spread-fusing bonding. Such spread-fusing bonding mechanism was reported for the metallurgical bonding formation at the interfaces between molybdenum coating and different substrates of a lower melting point than molybdenum including steel, cobalt and Ni-Cr alloy in early last 60 s (Ref 89). Later, during the early development of Ni/Al composite powder (Ref 90), it was pointed out that the impact of NiAl molten droplets on a steel substrate caused the formation of metallurgical bonding. It was demonstrated that the metallurgical bonding was created through melting of the substrate by the ultra-hot droplet due to an exothermic reaction between Ni and Al. Kitahara (Ref 91) investigated the microstructures of the interface between a substrate and a plasma-sprayed coating for different substrate/coating combinations. The results revealed that when high melting point molten droplets impact on a substrate of a low melting point such as the splat-substrate combinations like W-Fe, Mo-Fe, Mo-Ni, Ta-Ni, Ni-Al, the spreading of molten droplets can cause the local substrate to melt, resulting in fusing to bond the splat metallurgically with the underlying substrate. The formation of the intermetallic compounds at the interface provided direct evidence of such spread-fusing bonding effect (Ref 92). All those early works demonstrated the possibility of forming metallurgical bonding at the interface between the substrate and the coating by molten droplet impact-induced melting. This phenomenon was also proved by molten crater generated on the steel substrate by impact of molten refractory metal Mo droplets (Ref 93).

Dallaire established the conditions for the incipient melting for different particle-substrate systems by analytically calculating the splat-substrate interface temperature for the materials of different types (Ref 94). Numerically, Li et al. (Ref 95) reported the interface temperature between the molten metal splat and the substrate for different droplet/substrate combinations. The numerical simulation was made by a simple one-dimensional heat transfer model through supposing that a thin layer of molten metal is instantaneously brought on a flat metal surface with the intimate contact. Theoretically, they showed that the impact of molten refractory metal droplet such as Mo, W, and Ta enables the melting of low melting point substrate such as iron and nickel. Those results agree with that reported by Dallaire and are also consistent with the experimental results reported previously by Kitahara (Ref 91). However, it was also pointed out that the spread-fusing does not occur to the like-on-like material combinations. For example, the impact of nickel or iron droplets does not cause incipient melting of nickel substrate or iron substrate, respectively (Ref 94). Consequently, the metallurgical bonding by incipient melting of impacting molten droplet could not be formed at the interfaces between the splats through the spread-fusing mechanism at ambient condition although few evidences for spread-fusing at a local central area with the like-on-like materials were reported (Ref 96). However, if a Ni alloy molten droplet can be over-heated to a temperature near the melting point of a refractory metal such as Mo its impact on the identical Ni alloy substrate could also cause the substrate surface melting although generating an ultra-hot molten droplet is another challenge.

The Droplet Temperature Conditions for the Spread-Fusing Self-Bonding Formation at Ni-Based Alloy Intersplat Interfaces

It is natural to consider that the substrate surface layer will be melted when the substrate surface region is heated to a temperature higher than its melting point by the impact of the molten droplet (Ref 94, 95). Therefore, whether the maximum interface temperature between the spreading melt and the substrate is higher than the melting point of the substrate materials can be taken as the criterion for estimating whether the spread-fusing takes place or not. The evolution of the interface temperature between the spreading molten metal and the substrate after a molten metal droplet impact can be numerically estimated by a simple one-dimensional heat transfer model through supposing that a thin layer of molten metal is instantaneously brought on a flat metal surface (Ref 95). Accordingly, as shown in Fig. 3 for Ni-based alloy droplets at different temperatures (Ref 45), only when the droplet temperature is higher than 2400 °C, the maximum temperature of the Ni substrate surface can be heated to a temperature higher than its melting point. Then, the Ni-based droplets having a temperature higher than 2400 °C are bonded to the identical underlying Ni-based splats through a liquid molten phase in the same fashion as that in metal welding. As a result, a metallurgical bonding is formed between the identical material splats through the spread-fusing self-metallurgical bonding mechanism. These results indicate that the formation of the self-bonding requires the generation of the molten droplets having approximately 1000 °C higher temperature than its melting point. To reveal the metallurgical bonding formation at the interlamellar interface by high temperature molten droplet by spread-fusing, the splat deposition test was carried out by a Ni-based alloy molten droplet at a temperature above 2620 °C on the polished flat In738 alloy substrate. This is because In738 alloy has similar thermophysical properties with NiCrMo alloy. The examination into a cross section of splat revealed that a crater in the substrate was present underneath the splat revealing that the splat was welded to the substrate (Fig. 4) (Ref 97).

Fig. 3
figure 3

The effect of droplet temperature on the simulated temperature evolution at the interface between molten splat and solid substrate for Ni-based alloy (Ref 29)

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Fig. 4
figure 4

Cross section of NiCrMo splat deposited on the polished flat In738 nickel alloy substrate by NiCrMo droplet having a temperature higher than 2620 °C, showing the crater on the substrate generated by impacting NiCrMo droplet and welding of the splat to the substrate (Ref 97)

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For low melting point materials with a high boiling point, it can be expected to acquire molten droplets with much higher overheating temperatures. For pure Al, the simulation shows that when the droplet temperature is higher than 1800 °C the Al substrate surface can be heated to a temperature higher than its melting point by Al molten droplets (Fig. 5) (Ref 98). The measurement of Al particle temperatures during plasma spraying reveals that most Al particles are heated to a temperature higher than 1800 °C (Fig. 6) (Ref 98), which is a minimum temperature necessary for creating the spread-fusing self-bonding. Although the Al splat was deposited on a well-polished Al alloy substrate the interface reveals a wave profile (Fig. 7) (Ref 98) indicating the mixing of splat in a molten state with the melted substrate surface layer. Accordingly, during Al coating deposition, the self-bonding through the liquid phase demonstrates that droplet temperature fulfills the requirement for generating spread-fusing. This fact further reveals that the molten droplet temperature for the spread-fusing self-bonding can be estimated theoretically and self-bonding between like-on-like intersplat interfaces can be created through controlling droplet heating. Paredes et al investigated the effect of the substrate preheating on the adhesion of thermally sprayed Al coatings (Ref 99). Their results showed that plasma-sprayed Al coating at an ambient atmosphere without preheating fulfilled the standard requirements for application, while flame spraying must be performed with the substrate preheating to comply with the adhesion standard. Thus, the incipient melting of the substrate surface depends on the temperature of both the droplet and the substrate. A lower molten in-flight particle temperature requires a higher preheating temperature provided that preheating does not cause adverse effect due to oxidation. However, practically, the oxidation of in-flight spray molten particles hinders the formation of metallurgical bonding in a large part of the splat interface in the coating. One challenge to apply the spread-fusing self-bonding mechanism is how to develop oxide-free droplets in the ambient atmosphere.

Fig. 5
figure 5

Effect of molten Al droplet temperature on the interface temperature evolution between Al molten splat and Al solid substrate (Ref 98)

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Fig. 6
figure 6

Change of Al in-flight spray particle temperature in plasma jet against spray distance (Ref 98)

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Fig. 7
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Morphology of Al splat deposited on a polished flat Al alloy substrate at a low magnification (a), showing flat reference surface profile (b), and showing wavy interface between splat and substrate with an evident contrast (Ref 98)

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Bulk-Like Coating Deposition Through Creating Ultra-Hot Molten Droplets

When thermal spray particles are heated to a molten state, vaporization occurs. With the increase of the droplet temperature the vaporization rate increases and reach the maximum until the evaporation starts when the surface temperature reaches to the boiling point (Ref 100). On the other hand, vaporization or evaporation brings out a strong cooling effect to particle heating and limits the heating up of molten droplet. The modeling results show that the effective heat transfer coefficient decreases with the increase of molten droplet temperature. The reduction of heat flux from the plasma jet to the particle due to evaporation can exceed 50% when the temperature is close to the boiling point of spray material (Ref 101). For most nickel base alloy droplets generated by by HVOF or by plasma spraying, it is hard to find out the experimental data for molten Ni alloy droplets up to now to have a temperature higher than 2200 °C. As a result, only a limited splat interface bonding is present in a thermal spray Ni-based coating, especially at the interface between two adjacent passes. Within one spray pass, the critical droplet temperatures for incipient melting could be reduced because the splat may not cool down to room temperature during coating deposition.

Recently, a novel powder design approach was proposed to ensure the generation of molten Ni-based alloy droplets of a temperature higher than 2400 °C by suppressing element vaporization induced cooling effect (Ref 97). In this approach, Ni-based alloy powder particle is cladded with a Mo surface layer to construct a core-shell-structured particle. Figure 8 shows typical results of droplet temperature evolution during in-flight for Mo-clad Ni20Cr particles (Ref 102). It can be found that these novel structured Ni-based alloy powder particles are heated to a temperature of over 2650 °C, higher than the critical temperature of 2400 °C for spread-fusing, being over 500 °C higher than the temperatures of traditional Ni20Cr particles. It should be noticed that this high temperature is even close to the boiling point of alloying element Cr (2670 °C) (Ref 103), which can be considered to be the highest temperature that molten Ni20Cr particles can reach. Accordingly, such droplets are referred to as ultra-high temperature droplets (UHTD). When this powder design was applied to stainless steel, the Mo-cladded stainless steel powder particles could be also heated to a similar temperature to that of Mo-cladded NiCr powder particles (Ref 104).

Fig. 8
figure 8

Significantly enhanced heating effect on the molten Ni-based alloy droplet with plasma jet by Mo cladding on Ni20Cr powder particles, showing more than 500 °C increase compared to bare Ni20Cr particle (Ref 102)

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The microstructure of NiCrMo alloy coating deposited using Mo-cladded Ni20Cr powders by shrouded plasma spraying is shown in Fig. 9. The coating has an apparent porosity of only 0.16% and is much denser than the conventional NiCr coating (Ref 97). The microstructure characterization at the regions near the interface between the stainless steel substrate and the NiCrMo coating and intersplat interfaces within the coating by EBSD revealed the metallurgical bonding formation (Fig. 10) (Ref 97). Since both the stainless steel substrate and the Ni-based alloy coating have the same FCC crystalline structure, the epitaxial grain growth of splats based on the grains of the polished stainless steel substrate is well observed based on the EBSD analysis results (Fig. 10b). Thus, the adhesive strength of the coating is higher than 70 MPa with a fracture that occurred in adhesives during the tensile test. The EBSD analysis also reveals the continuous grain growth across several splats (Fig. 10d) within the coating (Fig. 10c), which indicates the metallurgical bonding formation at the intersplat boundaries.

Fig. 9
figure 9

Typical dense microstructure of NiCrMo coating shrouded plasma-sprayed by using Mo-clad Ni20Cr powders (Ref 97)

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Fig. 10
figure 10

The grain structure near the interface between the NiCrMo coating and stainless steel substrate analyzed by EBSD (b) at the interface region marked in (a), showing the epitaxial grain growth across the interface by impact-induced substrate fusing; (c) and (d) show the microstructure within the coating and the grain morphology presenting the grain growth state across multiple of splats indicating the metallurgical bonding formation within the coating by EBSD, respectively

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The erosion wear of thermal spray coating, especially under the test condition by the normal impact of erodent, is very sensitive to the lamellar interface bonding. The shockwave effect induced by the repeated impacts of the erodent causes crack propagation along the interface boundaries between the splats exposed directly to erodent and the subsurface splats. Figure 11 shows a typical example of cracks observed in HVOF cobalt coating with an initial thickness of about 250 μm, experienced the erosion test under the normal impact. Such cracking results in debonding of the deposited splats exposed to the top surface, making the coating eroded in a layer by layer fashion. Thus, the erosion wear resistance is well linearly related to the mean bonding ratio of plasma-sprayed brittle Al2O3 coatings (Ref 105). During erosion wear test of bulk materials, the dependency of the wear rate of ductile metallic materials on the impact angle of erosive particles presents a completely different characteristic from typical brittle ceramic materials (Ref 106). It is well known that the ductile materials present a peak erosion wear rate in an impact angle range from 15° to ~ 30° and then decrease with the increase of the impact angle. On the other hand, the erosion rate of brittle materials increases with the impact angle and reaches the maximum at the angle of 90°, i.e., the normal impact of erodent on the material surface. For most thermal spray brittle ceramic coatings, it is reasonable that the erosion rate increases with the impact angle. For thermal spray metal coatings, when they are subjected to normal erodent impact they would be expected to present excellent wear resistance with less erosion rate than that a high impact angle. However, most investigations into the impact angle effect reveal completely different results (Ref 107,108,45, 46), which meets the requirement for initiation of spread-fusing mechanism. At such high temperature, the deoxidizing mechanism works well to generate oxide-free molten NiCrBSi droplets throughout whole the in-flight in ambient atmosphere. Thus, since all requirements are fulfilled, the coating with few oxide inclusions and sufficiently metallurgical bonding at splat interfaces is deposited in open ambient atmosphere. Figure 19 shows a typical microstructure of plasma-sprayed NiCrBSi coating. Apparently, it is remarkably dense. The etching test of cross section of the coating, from which the unbonded splat interfaces can be usually visualized, further confirmed that the fully dense coating is deposited by sufficiently bonded lamellae.

Fig. 19
figure 19

Microstructure of the as-plasma-sprayed NiCrBSi coating with little trace of lamellar structure (Ref 45)

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The abrasive wear against emery paper was conducted under different applied loads to examine the abrasive wear behavior of the coating. For thermal spray coating with lamellar structure, the wear behavior at low stress can be much different from that at high stress. This is because the wear mechanism transition from bulk-like uniform wear to lamella spalling accelerated wear occurs as observed for other types of coatings such as HVOF metal coatings (Ref 24) and WC-Co coatings (Ref 13). Therefore, the wear performance of HVOF WC-Co coating is lower than that of sintered bulk (Ref 13). Compared to the flame-sprayed lamellar-structured NiCrBSi coating that presents a significant loading dependency of wear mechanisms (Fig. 20), the plasma-sprayed NiCrBSi coating reveals the same linear wear behavior as that of the spray-fused coating. This fact indicates that the mechanical behavior of the plasma-sprayed NiCrBSi coating with the spread-fusing mechanism is comparable to the spray-fused coating. Moreover, this result also means that the lamellae in the plasma-sprayed NiCrBSi coating are sufficiently bonded together with each other to behave bulk-like. However, when the spread-fusing mechanism is not developed through heating control of spray particles, plasma-sprayed NiCrBSi coating still presents a conventional lamellar structure. The abrasive wear test of such coating yielded a much higher abrasive wear rate even by a factor of three than those of spray-fused bulk-like NiCrBSi (Ref 12).

Fig. 20
figure 20

Comparison of load-dependence of the abrasive wear weight loss of the as-plasma-sprayed NiCrBSi coating with those of as-flame-sprayed NiCrBSi and flame-spray-fused NiCrBSi coating. A critical load for the as-flame-sprayed coating is presented over which the accelerated wear occurs through lamella-peeling dominant mechanism. The APS NiCrBSi presented the same abrasive wear behavior as that of spray-fused bulk NiCrBSi (Ref 45)

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The investigation into the electrochemical behavior also revealed that the APS dense NiCrBSi coating presents comparable behavior with that of spray-fused coating in terms of both the open circuit potential and the dynamic polarization (Fig. 21) (Ref 45). Similar results were also acquired using boron-alloyed Ni20Cr alloy coating (Ref 46). As a result, it is clear that through materials design to control their physical properties along with the control of overheating temperature of spray molten droplets, bulk-like metal coatings of the same mechanical and electrochemical performance as the bulk counterparts can be simply deposited by plasma spraying in an open ambient atmosphere.

Fig. 21
figure 21

Comparison of the dynamic polarization behavior of plasma-sprayed NiCrBSi coating on Q235 with those of the flame spray-fused NiCrBSi on Q235 steel and bare Q235 steel substrate in 3.5% NaCl suggesting little trace of solution penetration during the electrochemical test and that the coating behaviors electrochemically in the same way like bulk-NiCrBSi (Ref 45)

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Conclusions and Prospective

Depositing thermal spray metallic coatings with bulk-like microstructure is the most effective way to tape out the full potential of metal coating materials for corrosion and wear protection. This requires sufficiently metallurgical bonding between splat interfaces. Using vacuum plasma spraying in a reduced inert atmosphere, bulk-like metal coatings can be deposited by increasing deposition temperature using preheating substrate. However, under an ambient atmosphere, the oxidation of substrate and previously deposited coating layer hinders the applicability of the strategy to increase deposition temperature. Instead, the spread-fusing self-bonding mechanism can be proposed for producing bulk-like thermal spray metallic coatings. To realize such a self-bonding mechanism, ultra high-temperature molten metal droplets free of oxide need to be generated. The minimum droplet temperature for the self-bonding can be estimated by one-dimensional heat transfer model. Thus, the first challenge is to generate ultra-high temperature droplets. For Fe-based and Ni-based coatings, Mo-cladded powder design was proposed to create ultra-high temperature droplets, which can fulfill the requirements for spread-fusing self-bonding. Using shrouded plasma spraying, dense bulk-like Ni-based alloy coatings with limited oxides and sufficient lamellar bonding can be deposited, and the coatings present the same mechanical behavior as that of casted IN738 bulk. The fully dense coating for complete protection of metal from corrosion can also be fabricated.

A second challenge is to generate oxide-free molten metal droplets in an open ambient atmosphere. It was shown that three conditions need to be fulfilled. (I) The powders should be designed to have self-deoxidizing potential with a certain amount of deoxidizing alloying elements (B, C, etc.). (II) Ultra-high temperature droplets should be generated to achieve rapid evaporation of the oxides formed by deoxidizing and spread-fusing self-bonding mechanism. It was clearly shown that the generation of molten metal droplets of a temperature higher than 1860 °C leads to the formation of oxide-free Cu-based, oxide-free Ni-based alloy molten droplets containing deoxidizer boron. For Cu-based and Ni-based alloys, the addition of a de-oxidizer such as B can also decrease the melting point of the alloys. This not only makes the droplet temperature requirement be easily fulfilled for self-bonding, but also provides the alloy droplets with an in-situ in-flight deoxidizing effect, by which oxide-free metal droplets can be generated in open ambient atmosphere. Consequently, the deposition of bulk-like dense metal coatings with sufficiently bonded lamellae is realized by APS.

Although the recent research progresses emphasize the possibility to deposit the bulk-like dense metallic coatings with fully metallurgical intersplat bonding, many fundamental research works are still required to well understand the concerned physical and chemical phenomena. The main topics can be raised as follows. (I) How to effectively heat metal powder particles to the temperature near boiling point of the main alloying elements is the first challenge for the application of the present strategies for different applications by APS. This involves the control and enhancement of heat transfer between spray metal particles and high temperature flame, although it is supposed that the physical phenomena behind heat transfer have been well understood. (II) Rapid mass transfer through Hill’s vortex flow mechanism within in-flight molten droplet is required to maintain the continuously sufficient supplying of deoxidizer elements to the molten droplet surface from its inner to acquire oxide-free molten metal droplets along the whole spray distance range in an open air. (III) The physical absorption phenomenon of gas species by molten droplets at the ultra-hot state needs to be clarified since the gas species solubility in molten droplets increases with the droplet temperature and absorbed gas species may not be released during rapid spreading process. (IV) How the wetting of spreading melt at a high temperature to the underlying splat surface contributes to the formation of metallurgical bonding between adjacent splats within one spray pass is an important topic since with spreading of molten melt the temperature of melt at the spreading front may decrease significantly. Once a tiny fraction of the interface at the periphery of the splat is not bonded effectively, the corrosive media in a liquid or gas state could penetrate through the coating and cause corrosion at the substrate/coating interface. Thus, complete wetting throughout the whole splat interface during spreading is required. Accordingly, the modeling of the splat formation process to clarify the melt/substrate interface temperature evolution across the whole contact interface will help to understand such phenomena. (V) How the spreading melt makes self-adapting to a wavy splat surface profile to achieve the full intersplat metallurgical bonding is not taken into account yet but is important to be clarified. (VI) The control approaches for the coating surface oxidation between each scanning spray pass should be developed to avoid the serious effect on the bonding formation and how their effect on the bonding formation affects the performance of the coatings may challenge the successful application of the present strategies. (VII) The deposition of the metallic coatings by ultra-hot droplets for sufficiently bonded lamellae may change the residual stress distribution in the coating. How the residual stress evolves and affects the coating performance would also need to be clarified.