Structural analysis of Fe-Al coatings applied by gas detonation spraying

The article analyzes the formation of oxide phases in the structure of intermetallic FeAl coatings applied by gas detonation spraying (GDS). The structural properties of powder charge particles and FeAl coating formed during GDS were determined. The effect of the GDS process on phase changes in FeAl coating applied under controlled conditions was examined. The results indicate that at specific process parameters, FeAl powder particles are strongly oxidized in a hot stream of gases produced during supersonic combustion. Powder particles undergo very strong plastic deformation during the process, and the resulting multiphase coating contains oxide phases that form thin membranes along grain boundaries. The results of structural analyses and microanalyses of chemical composition (SEM/EDS) and phase composition (XRD) indicate that strongly heated FeAl particles undergo surface oxidation during GDS and are transformed into grains (splats) when they collide with a steel substrate. The produced FeAl coating has a multilayered and multi-phase structure characteristic of the sprayed material, and it contains thin oxide layers, mainly


Introduction
Intermetallic materials have numerous practical applications due to their unique properties [NIEWIELSKI and JABŁOŃSKA 2007]. FeAl alloys can be applied by the gas detonation spraying (GDS) method to form protective coatings [HEJWOWSKI 2013]. The GDS method is characterized by supersonic flow of the two-phase (gas-powder) metallizing stream.
Strongly heated powder particles have very high kinetic energy when they collide with the substrate [ASSADI et al. 2016, FIKUS et al. 2019, LIU et al. 2007]. FeAl coatings produced in the GDS process have many advantages, including considerable resistance to high temperature in chemically aggressive environments and very high resistance to tribological wear [BOJAR et al. 2002].
The thermal energy of gaseous detonation products directly influences the metallurgical quality of sprayed FeAl coatings [CHROSTEK et al. 2019]. Research has demonstrated that even minor changes in a single parameter of the GDS process, including the fuel-oxidizer ratio, the amount of gas transporting powder particles, spraying frequency, spraying distance, and powder position inside the barrel at the time of detonation, significantly affect the kinetic and thermal energy of the stream of gaseous detonation products, which influences the quality of the sprayed coatings [NIKOLAEV et al. 2003, SADEGHIMERESHT et al. 2017].
The rate of chemical reactions in strong shockwaves is extremely high during thermal spraying. These reactions produce free radicals which significantly affect the oxidation of strongly heated powder particles. Powder particles undergo partial surface melting, and their oxidation is further exacerbated during transport by gaseous detonation products [SENDEROWSKI et al. 2016].
The aim of this study was to compare the structural parameters of FeAl powder and FeAl protective coating applied by the GDS method. The results of the comparison were used to evaluate the degree of oxidation and phase stability of intermetallic coating sprayed with a detonation gun.

Materials and Methods
The analyzed intermetallic protective coating was produced by GDS of powder composed of a mixture of Fe and Al elemental powders with a composition of Fe40Al0.05Zr-50 ppm B, at%. The powder was manufactured by LERMPS-UTBM with the use of the Vacuum Inert Gas Atomization (VIGA) method. The substrate was a sample of 15HM (13CrMo4-5) boiler steel measuring 50x50x5 mm which was blasted with alumina directly before spraying. The surface roughness after sandblasting the substrate was Ra = 18,98 µm.
The coating in the form of circular deposit was sprayed by placing the substrate material in a fixed position relative to the barrel of the detonation gun operating at a frequency of 6.66 Hz ( Fig. 1). The barrel had a length of 1090 mm, and it was positioned at a distance of L=110 mm from the sprayed substrate. The powder with 540 µm particle size was loaded into the gun, and it was located at a distance of 412.5 mm from the barrel outlet at the time of detonation (powder injection position -PIP). Spraying parameters, the composition of the explosive detonation mix, and the flow rate of powder-transporting air are presented in Table 1. The variations in the geometric dimensions of the sprayed FeAl coating after 100 gun shots were determined with a PG10 profilometer (Fig. 1b).  X-ray diffraction analysis was conducted with the Rigaku Ultima IV diffractometer with focused monochromatic CoK radiation and a spectral wavelength of λ=0.178897 nm. CoK filtering was applied, and the operating parameters of the radiation lamp were set at 40kV/40mA. Data were collected within the angular range of 20° to 120° at a scanning speed of 1 mm/min.

Results and Discussion
The size, morphology and phase composition of powder particles significantly influence the metallurgical quality of coatings produced by thermal spraying, including GDS. These parameters considerably affect the thermophysical properties of powders and, consequently, the performance of the produced protective coatings.
A structural analysis (VIGA) of the original FeAl powder (as supplied by the manufacturer) revealed considerable differences in particle size. Powder particles were spherical in shape regardless of their size (Fig. 2a).  The XRD analysis of phase composition revealed the single-phase structure of FeAl powder particles with a characteristic reflection {100} of the B2 superstructure, which confirms that the original FeAl powder particles (as supplied by the manufacturer) were not oxidized ( Fig. 4). A very small half-width of X-ray reflections with highly intense peaks, which is characteristic of a given family of FeAl phase lattice planes within a specified range of Bragg angles, can be attributed to the homogeneous chemical composition of individual powder particles and very low residual stress values. The above preserves the structural integrity of the crystal structure of the FeAl phase, which is a secondary solid solution of Al in Fe() and constitutes the basis of the single-phase powder without oxide phases. The structural analysis of FeAl coating revealed significant changes in the physicochemical and mechanical properties of FeAl powder particles sprayed with a frequency of 6.66 Hz (Fig. 5). The observed changes resulted from particle heating and their plastic deformation.
The particle deformation analysis and the SEM/EDS microanalysis of the surface of FeAl coating revealed partial melting of powder particles in selected regions (Fig. 5). Particle melting leads to very strong oxidation of diffused aluminum and the formation of oxide phases on the surface of partially melted FeAl particles that form dark grains with a varied morphology in SEM/BSE images (Fig. 5). The SEM/EDS point microanalysis confirmed considerable variations in the chemical composition of grains in the FeAl coating, where aluminum content was estimated at 4-45 at.% and oxygen content was determined at up to 52 at.% in the region of dark grains (Fig. 5, Table 2). These results clearly indicate that the intermetallic phases of FeAl alloys differ considerably in aluminum content and are characterized by a high content of oxygen which forms complex aluminum oxides Al2O3 and oxide spinels.  The SEM/EDS structural analysis of FeAl coating (GDS) performed at the cross-section of the metallographic specimen revealed lamellar grains with a multi-phase structure and varied chemical composition. In the images acquired with the use of the BSE detector, differences in the chemical composition of different regions on the surface of FeAl coating were presented in shades of gray (Fig. 6a).
The observed variations in the chemical composition of plastically deformed FeAl powder particles (with a single-phase structure in the original state) can be attributed to chemical reactions that take place in the stream of gaseous detonation products and the in situ formation of oxide phases in the form of oxide membranes in the GDS process (dark layers in Fig. 6a). When analyzing the oxidation of the FeAl coating produced by the GDS method, special attention should be paid to the preferential sites for the formation of oxide phases. These sites represent partially melted FeAl powder particles where thin oxide layers are formed, undergoing strong non-dilatational strain when the particles collide with the substrate material in a supersonic detonation wave. The produced coating has a mosaic structure (Fig. 6).
Aluminum is depleted, and the intermetallic Fe3Al phase and the secondary Al solution in Fe are formed in regions adjacent to strongly oxidized phases with a chemical composition of Al2O3, Fe(Al2O4) and Fe3O4.
The content of alloy elements and oxygen mapped in the SEM/EDS microanalysis of chemical composition at the cross-section of FeAl coating (GDS) is presented in Table 3.   6 and Table 3).
The X-ray diffraction analysis of FeAl coating (GDS) confirmed that the FeAl phase is the main structural component that is inherited from FeAl powder (VIGA). The analysis also confirmed the presence of the Fe3Al phase and the following oxide phases: aluminum oxide -Al2O3, spinel -Fe(Al2O4), magnetite -Fe2O3 and ferrous oxide -FeO (Fig. 7).
A comparison of the XRD image of the FeAl coating (GDS) (Fig. 7) with the XRD image of the original FeAl powder (Fig. 4) indicates that an increase in the half-width of FeAl coating reflections and a decrease in reflection intensity probably resulted from the high dispersity of Al2O3 phases and oxide spinels identified as Fe(Al2O4). The oxide phases identified in XRD analysis contribute to the formation of pseudo-composite coating and increase residual stress in the structure of the intermetallic FeAl coating, which also widens the reflections of diffraction peaks.
The behavior of the superstructure peak {100} indicates that the sprayed coating is based on the FeAl (B2) phase which has a less ordered structure and contains the aluminum-deficient Fe3Al phase without superstructure reflection (Fig. 7).

Conclusions
During the GDS of intermetallic FeAl coating with the use of single-phase FeAl alloy powder produced by the VIGA method, the detonation wave and gaseous detonation products lead to the oxidation of particle surfaces and the formation of oxide membranes which are an integral part of coatings with a layered structure. The supersonic metallizing stream causes strong volumetric deformation of powder particles when they collide with the substrate material. Thin oxide membranes are formed along the boundary of strongly flattened grains without impairing their cohesiveness. Aluminum is depleted in the region where oxide phases are formed, and a solid solution of the secondary Fe3Al phase is formed in microregions.
The pseudo-composite structure of intermetallic FeAl coating with oxide phases is characterized by a less ordered structure and higher residual stress which is exacerbated by dispersive oxide phases. Residual stress generated during GDS does not cause microcracks in the multi-phase structure of FeAl coatings containing ceramic oxides which stabilize the structure during high-temperature heating, including in aggressive corrosive environments.