Novel Al 2 CoCrFeNi high-entropy alloy coating produced using suspension high velocity air fuel (SHVAF) spraying

Metallic coatings of Al 2 CoCrFeNi high entropy alloy (HEA) were deposited using the suspension high velocity air fuel spray (SHVAF) process, towards exploring its viability as a bond coat in thermal barrier coatings. The relatively high Al content promoted a BCC + B2 phase-dominated coating structure, leading to enhanced mechanical properties. The oxidized microstructure exhibited a protective Al 2 O 3 layer with characteristics comparable to conventional bond coat alloys.


Introduction
Thermal barrier coatings (TBCs) are composed of a ceramic topcoat and a metallic bond coat that are overlaid onto, typically a superalloy substrate.Bond coat materials of MCrAlY-alloys (M = Ni,Co) are generally deposited using the vacuum plasma spray (VPS) [1].The high velocity air-fuel (HVAF) process is also gaining popularity to achieve a denser bond coat microstructure without exposing the feedstock to high plasma temperatures [2].Furthermore, suspension-based thermal spray processes, such as suspension plasma spray (SPS), are increasingly being used to produce the topcoat in TBCs due to their ability to engineer coating microstructures with desirable properties [3].Although SPS is traditionally used to deposit ceramic coatings, there is merit in combining the suspension feedstock method with the HVAF processing route to deposit metallic coatings.This combination allows the use of finer, nanocrystalline metallic feedstock particles without incurring the risk of thermal degradation.To this end, this study explores/documents the use of suspension-HVAF (SHVAF) technology [4] to deposit bond coats of a new high entropy alloy composition.
High entropy alloys (HEAs) have demonstrated promising thermal stability and oxidation resistance [5,6], identifying them as frontrunners in the search for next-generation bond coat materials.The wide applicability of thermal spray and its establishment as a prime coating technique for producing TBCs has propelled the development of potential thermal spray HEA bond coats [7,8].Al 2 CoCrFeNi is one such bond coat HEA candidate, exhibiting slow oxidation kinetics at par with conventional MCrAlY alloys in bulk state [9].However, coatings of this alloy have not yet been reported.As well, the combination of suspension feedstock route and HVAF for HEA feedstocks has also not been reported yet.
Therefore, the present work focuses on developing Al 2 CoCrFeNi HEA coatings using a novel suspension feedstock followed by HVAF processing towards producing potential bond coats.Further, various aspects of the consequent coating such as microstructure, mechanical properties and high temperature oxidation behaviour have been assessed.

Materials and methods
Al 2 CoCrFeNi HEA powder, with particle size between 1 and 5 μm produced using mechanical alloying (MA), was mixed in water with a solid load content of 20 wt% HEA to produce an aqueous suspension.It was used as a feedstock to manufacture a coating using an HVAF spray system as per the spray parameters listed in Table 1.Hastelloy®X coupons, 6 mm in thickness and 25.4 mm in diameter, were used as substrates.Isothermal air oxidation was performed at 1100 • C for 100 h and oxidized samples were air-cooled after exposure.
The phase composition of the powder and coating was determined using powder X-ray diffraction (XRD) (Bruker D8).The cross-sectional microstructure of the coating were evaluated using a scanning electron microscope (SEM) (Zeiss SUPRA™ 40VP FESEM system) equipped with an energy dispersive X-ray spectrometer (EDS).Microindentation and nanoindentation were performed using a Hysitron TI 980 triboindenter at loads of 3 N (n = 50 1 ) and 3000 μN (n = 400), respectively.The coating was oxidized isothermally at 1100 • C for 100 h, followed by microstructural characterization.

Microstructural characterization
XRD analysis (Fig. 1(a)) reveals that the MA Al 2 CoCrFeNi HEA powder is composed of a BCC phase (marked as B) including an ordered BCC (B2) phase.Previous reports suggest that higher Al contents (mole fraction >1.3) favour the formation of BCC + B2 phases in Al x CoCrFeNi [10], which corroborates the present case, with 33.3 at.%Al.This is due to the high negative formation enthalpies of Al-Ni, Al-Co and Al-Fe B2 phases [11].Similar results were reported for as-cast Al 2 CoCrFeNi HEAs [12,13].
Rietveld analysis of the XRD data indicates a significant reduction in B2 phase content from 52 wt.% in the powder to 34 wt.% in the coating, along with the generation of an FCC phase (10 wt.%).These changes are attributed to (i) supersaturation in the BCC and B2 phases characteristic of MA processing, which transformed to a more stable combination of BCC + B2 + FCC upon spraying [14]; and (ii) increased reactivity of nanocrystalline MA powder during the SHVAF process.
The as-sprayed coating has a thickness of 70 ± 5 μm (Fig. 1(b)) and porosity of 7.6 ± 1.1%, measured using image analysis.Point EDS analysis on the coating cross-section (Table 2) revealed two discrete phases (Fig. 1(c)): (i) a HEA phase, which practically retained the feedstock composition; and (ii) a 'depleted HEA' phase, with a reduced Al content (24 at.%).These phases could not be readily distinguished by compositional contrast, but the depleted HEA phase likely corresponds   to BCC + FCC, while the HEA phase comprises the B2 phase.Further, the oxygen content in both phases (see Table 2) can be explained by in-flight oxidation (IFO), albeit much less than that seen in APS.The heated powder particles may have experienced surface oxidation and fragmented sub-microscopic oxide pieces could be embedded in the alloy phases, contributing to the oxygen content [15].

Mechanical properties
The microhardness value of the coating is 9.4 ± 0.6 GPa, higher than that of other AlCoCrFeNi-based HEA coatings fabricated using other thermal spray processes and laser deposition methods [16][17][18] and as-cast Al 2 CoCrFeNi HEA [12,13].The increased hardness is attributed to the high Al content, and its larger atomic radius, which not only promotes the harder BCC and B2 phases, but also initiates severe lattice distortion and inhibits dislocation movement.Furthermore, IFO during deposition promotes reinforcement by the oxide dispersion, contributing to higher hardness.The nano-hardness (H) and reduced elastic modulus (E r ) of the coating are 9.8 ± 3.1 and 146 ± 28 GPa, respectively.The H/E r and H 3 /E r 2 ratios, which represent the ability of a material to resist wear and plastic deformation, are 0.067 and 0.044, respectively.These ratios are higher than thermal sprayed AlCoCrFeNi coatings produced using mechanical alloyed (0.059 and 0.031, respectively) [19] and gas atomized feedstock (0.046 and 0.014, respectively) [20].This substantiates that the higher aluminium content enhances mechanical properties at both micro and nano levels.Table 3 summarises the mechanical properties of SHVAF Al 2 CoCrFeNi HEA coatings.

Oxidation behaviour
SEM micrographs of the oxidized coating cross-section, along with corresponding EDS maps are presented in Fig. 2. The cross-sectional microstructure of the oxidized coating revealed the development of a thin continuous Al 2 O 3 layer, along with minor mixed oxides of Co, Fe and Ni.Together, these form the thermally grown oxide layer (TGO).
The average thickness of the TGO after 100 h of oxidation at 1100

Conclusions
The SHVAF technology has been used to successfully develop metallic Al 2 CoCrFeNi HEA coatings.The process allowed the usage of fine (1-5 μm) feedstock without observable in-flight oxidation.
Furthermore, it allows retention of the majority of feedstock phases as

Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig. 1 .
Fig. 1.(a) XRD patterns of Al 2 CoCrFeNi HEA powder and coating and, (b and c) cross-sectional SEM micrographs of the Al 2 CoCrFeNi HEA coating.

• C was 4 . 7 ±
1.1 μm (n = 45).Since the oxidation kinetics is only represented for 100 h at 1100 • C, a comparison of oxide layer thickness was made with open literature reports relating to the oxidation of conventional MCrAlY bond coats and other HEAs developed for potential bond coat applications, which develop oxide layers at the similar oxidation parameters.They are listed in Table 4.The oxidation kinetics were estimated based on the oxide scale thickness (Δh) and oxidation time (t) using the expression Δh 2 = 2k h t.The calculated oxidation rate constant (k h ) is 3.1 × 10 − 13 cm 2 /s and the parabolic oxidation rate constant, based on weight gain [21] is 1.1 × 10 − 12 g 2 /cm 4 /s.Detailed oxidation kinetics studies on SHVAF Al 2 CoCrFeNi coatings are currently under investigation.

Fig. 2 .
Fig. 2. Cross-sectional microstructure with corresponding EDS elemental maps of oxidized Al 2 CoCrFeNi SHVAF coating showing the oxide layer developed after 100 h at 1100 • C on the Al 2 CoCrFeNi HEA coating.
1'n' in this manuscript refers to the number of individual readings/measurements taken, whose average with standard deviation is then reported.A.Meghwal et al.

Table 3
Micro and nano mechanical properties of SHVAF Al 2 CoCrFeNi coating.

Table 4
Oxide layer characteristics of SHVAF HEA coating compared with conventional MCrAlY bond coats and potential HEA bond coats from open literature.as their nanocrystalline grain size, thereby resulting in coatings with higher hardness than bulk material of the same composition.Additionally, the high Al content of the HEA SHVAF coating allows the formation of a protective alumina layer comparable to conventional coatings for high temperature applications.These initial results demonstrate the potential offered by the SHVAF process to develop novel metallic coatings without degradation of the feedstock, resulting in properties favourable for high temperature applications.Further studies on optimization of feedstock and SHVAF deposition parameters which will lead to improved coating characteristics and consequently oxidation resistance are underway.Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writingoriginal draft, Visualization.Ameey Anupam: Validation, Formal analysis, Investigation, Writingoriginal draft.Michael Boschen: Formal analysis, Investigation.Surinder Singh: Investigation, Writingreview & editing.Stefan Björklund: Investigation, Writingreview & editing.Shrikant Joshi: Validation, Investigation, Resources, Writingreview & editing.Paul Munroe: Validation, Writingreview & editing.Christopher C. Berndt: Conceptualization, Writingreview & editing, Visualization, Supervision, Funding acquisition.Andrew Siao Ming Ang: Conceptualization, Methodology, Writingreview & editing, Visualization, Supervision, Project administration, Funding acquisition.