Impact of cyclic thermal shocks on the electrochemical and tribological properties of Fe-based amorphous coating

Fe-based amorphous coatings hold immense potential for marine industries due to their remarkable properties, including high hardness, exceptional corrosion resistance, and outstanding wear resistance. However, their performance under thermal shock conditions, particularly in high-temperature applications, remains a topic requiring further investigation. In this work, a Fe-based amorphous coating with a composition of Fe48Mo14Cr15Y2C15B6 was successfully developed using High-velocity oxygen fuel thermal spraying. To assess the thermal shock resistance of the amorphous coating, we subjected them to thermal cycles at 300 °C for 150 times, followed by cooling in two different mediums: saltwater quenching and air cooling. The results revealed that the coating maintained excellent contact with the substrate and preserved mainly amorphous structure both in the as-sprayed condition and after thermal shocks. Interestingly, the differential scanning calorimetry (DSC) results indicated that the air-cooled samples exhibited greater structural relaxation and crystallization compared to the brine-quenched samples. This microstructure changes in the air-cooled samples resulted in inferior mechanical properties, such as wear resistance and hardness, compared to the brine-quenched and as-sprayed samples.


Introduction
Various industrial applications, including automotive parts, aerospace, energy sector, and electronic components need efficient protective coatings.The protective coatings can be deposited by thermal spray techniques on different substrates [1,2].Otsubo et al [3] prepared the Fe-based coatings on mild steel (MS) in 1996 by thermally spray techniques, including high-velocity oxy-fuel (HVOF) and atmospheric plasma (APS) process.They found that the HVOF coating exhibited better corrosion resistance than the APS coating.In recent years, HVOF thermal spraying technology has achieved much attention for its novel advantages.HVOF-sprayed amorphous coatings always demonstrate high amorphous content, low porosity, dense structure, high hardness, and superior adhesion strength to the substrate due to the supersonic velocity of in-flight molten particles and moderate operating temperature [4].HVOF-sprayed coatings are extensively used in the repairing and surface protection of industrial production components and improve their wear and corrosion resistance and extend their service life [5].
Among various amorphous coatings deposited by the HVOF technique, Fe-based amorphous coatings have significantly attracted researchers and industrialists [6][7][8].Fe-based amorphous coatings are widely used in marine environments in ships, spent nuclear fuel containers, powder stations, and gas and oil industries [9], attributed to their microdefect-free microstructure without grain boundaries, dislocations or twins along with superior corrosion resistance, excellent wear resistance, high hardness, and low cost [10][11][12][13].Various factors, such as feedstock powders, substrate materials, processing variables, and variations in environmental conditions, including temperature, pressure, and humidity, affect the properties of Fe-based amorphous coatings [14][15][16].
The sudden variations in environmental temperature develop thermal shocks, resulting in the failure of many industrial components due to the failure of the coating.Thermal shock resistance is one of the important properties of the coating for longer service life [17].Therefore, it is mandatory for the coating to have superior thermal shock resistance to survive in complex working conditions.
Only a few studies have been reported in this research field.For instance; Zhu et al [18] studied the thermal shock behavior of CoMoCrSi coatings, deposited by HVOF and APS spraying processes and found that the microstructure and phase of both the coatings remained unaltered even after 50 thermal cycles at 500 °C.But thermal shock treatment at 900 °C caused spallation of APS coatings and segmentation cracking of HVOF coatings.Similarly, Li et al [19] investigated the response of Fe-based amorphous coatings against thermal shock under cryogenic cycling treatment.They found that the interfacial oxidation and porosity are the main influencing factors in the thermal shock resistance of the Fe-based amorphous coatings.The presence of pores slowed down the crack initiation and propagation by dissipating strain energy, while interfacial oxidation caused the cracking of the coating by a sudden increase in the maximum thermal stress in the oxidation regions.Although this is a very important and novel research area, only few studies are available on this topic.The impact of cyclic thermal shocks on the electrochemical and tribological properties of Fe-based amorphous coatings is still a less explored research area.
Therefore, this work aims to investigate the impact of cyclic thermal shocks on the electrochemical and tribological properties of Fe-based amorphous coatings.The Fe-based amorphous coating was successfully deposited onto an AISI 1045 steel substrate by HVOF spraying process and subjected to 150 cyclic thermal shocks at 300 °C by rapidly cooling in air and brine solution.Despite undergoing 150 thermal cycles, the coating did not delaminate, and its amorphous behavior remained unchanged.This indicates its potential to be used in many industrial applications.

Experimental work 2.1. Materials
The gas-atomized Fe-based amorphous powder, also known as SAM-1651 with chemical composition Fe 48 Mo 14 Cr 15 Y 2 C 15 B 6 and particle size of 33-55 μm, was used as a feedstock powder.The majority of the powder particles exhibit a spherical shape while a minority of the particles have irregular shape, as shown in figure 1, which was proved to be completely amorphous.AISI 1045 steel plate of dimensions 150 mm × 150 mm × 5 mm was used as substrate material.The process flow chart of complete experimental work is depicted in figure 1.

Coating preparation and thermal shock treatment
To deposit the Fe-based amorphous powder on the substrate, a high-velocity oxygen fuel (HVOF) spraying process was used, due to its several advantages over the other spraying techniques, such as good metallurgical bonding, low processing temperature (~2500-3000 °C), large compressive stresses, and low porosity percentage.The powder was sprayed on the AISI 1045 steel substrate according to the parameters given in table 1.
Cyclic thermal shocks were applied to the as-sprayed Fe-based amorphous coatings and their impact on the morphology, structure, electrochemical, and tribological properties of the Fe-based amorphous coatings was investigated.For this purpose, HVOF sprayed Fe-based amorphous coating was cut into samples of dimensions 10 mm × 10 mm × 10 mm by wire arc cutting process.Cyclic thermal shocks were applied to the coated samples by heating to 300 °C at a heating rate of 10 °C/min and holding at this temperature for 10 min for each cycle.After the holding time, the samples were taken out of the furnace and cooled in two media, including air and brine solution (10 wt% NaCl), separately and then put back into the furnace for another 10 min for another thermal cycle.The samples were subjected to continuous 150 thermal cycles and the obtained three coatings were designated as AS, TS-air, and TS-brine coatings.

Characterization
The structures of powder and coatings were characterized by x-ray Diffraction (XRD) (GNR EXPLORER 1879619), by Cu Kα (λ = 1.5418Å) radiations, using Ni filter and solid-state detector in the 2θ range of 20°-80°.To observe the cross-sectional morphologies, all the samples were prepared by grinding and polishing, according to the ASTM E3 standard.The cross-sectional morphology of the three coatings was evaluated by scanning electron microscopy (SEM, MIRA 3, TESCAN), according to the ASTM E7 standard.The thermal stability of all three coatings was also analyzed by differential scanning calorimetry (TGA/DSC 1 STAR System GC 200), as per ASTM E794.
Micro-Vickers hardness was measured, using a micro-hardness tester (NABEYA VLS 3858) by applying a load of 2 N for a dwell time of 10 s for all the coatings, according to the ASTM E92 standard [20,21].Each test was repeated five times and the final value was obtained by taking the average.For electrochemical analysis, coated samples of dimension 10 mm × 10 mm × 10 mm were cold mounted in epoxy resin by exposing only one side (10 mm × 10 mm) to the electrolyte.3.5 wt% NaCl solution was used as the electrolytic media to measure the corrosion resistance of all the coatings in the seawater environment.The Potentiodynamic polarization tests were then performed on a potentiostat electrochemical workstation (PARSTAT 3000A), using a three-electrode cell system in the potential range of -1 to 1.5 V and scanning rate of 1 mV s −1 .The coated and thermal shocked treated samples, a Standard Calomel Electrode (SCE), and a graphite rod were used as working, reference, and auxiliary electrodes respectively.
Wear tests of all AS, TS-air, and TS-brine coated samples were performed, using a Pin-on-disk Tribometer (MT/60/NI/HT/L, Microtest S.A.) at room temperature, according to the ASTM G99-05 standard.A diamond ball of 3 mm diameter was used as a counterpart.Wear tests were carried out in dry sliding conditions under a load of 10 N with 150 rpm rotational speed of disc-shaped coated samples for a rotation sliding distance of 50 m.ranging from 35-55 μm.A very homogeneous distribution of all elements was observed in the Fe-based powder particle as illustrated in figure 2(b).Figure 3 shows the cross-sectional SEM images of AS, TS-air, and TS-brine coatings, respectively.It was observed that the AS coating of thickness about 300 ± 50 μm exhibits a highly compact and homogeneous structure with excellent bonding to the substrate, as illustrated in figure 3(a).Figures 3(b) and (c) show that TS-air and TS-brine coatings exhibit good morphology with no severe cracks or delamination from the surface of the substrate even after 150 thermal shocks cycles.

Results and discussion
XRD spectra demonstrated only a broad hump at a 2θ value of 43.48°without any characteristic Bragg diffraction peaks, presenting fully amorphous nature of AS, TS-air, and TS-brine coatings (figure 4).As the selected temperature was below glass transition temperature therefore no change in the amorphous nature was observed.

Thermal behavior of coatings
Differential scanning calorimetery (DSC) analysis was employed to evaluate the thermal behavior of the AS, TSair, and TS-brine coatings and the obtained temperature versus heat flow curves are plotted in figure 5.The DSC analysis results were observed to be fully in agreement with the XRD results.The exothermic profiles in DSC curves of all three AS, TS-air, and TS-brine coatings are almost identical, presenting the crystallization process in the temperature range 620 °C to 750 °C.Structural relaxation is the most important phenomenon, occurring due to thermal shock treatment.Sudden cooling from an elevated temperature below glass transition temperature (T g ) results in the diffusion of species as they tend to shift toward equilibrium.This structural relaxation can be seen in the DSC curves of both TS-air, and TS-brine coated samples.From figure 5, it can be seen that the area under the crystallization peak of the TS-air coating is larger than that of the TS-brine coating.The difference in this area indicates that the crystallization will be more prominent in TS-air.This can be attributed to the slower cooling rate of TS-air sample, as slow cooling process facilitates the diffusion process and promotes structural relaxation and crystallization.

Electrochemical properties
To evaluate the effects of thermal shock on the corrosion resistance of AS, TS-air, and TS-brine coatings, potentiodynamic polarization tests were carried out and the obtained PDP curves are illustrated in figure 6.The polarization behavior was evaluated in the potential range of -1 V to 1.5 V with respect to open circuit potential with a scan rate of 1 mV s −1 .The kinetic parameters were calculated by a Tafel extrapolation method using NOVA 2.1 software as illustrated in table 2. According to the Butler-Volmer relation [22]; Where a b ¢ ¢ and c b ¢ ¢ are the anodic and cathodic Tafel constants.a h ¢ ¢ and c h ¢ ¢ represent the anodic and cathodic polarization of the metal surfaces during Tafel extrapolation.
The obtained results indicate that the values of I corr for AS, TS-air and TS-brine coatings were 2.39 μA cm −2 , 4.57 μA cm −2 , and 6.42 μA cm −2 , respectively.Similarly, the width of the passive region is highest for AS coating (~1250 mV), followed by TS-air coating (~900 mV) and lowest in TS-brine coating (~500 mV).It was found that the corrosion resistance of AS coating was superior to both the samples, subjected to thermal shock treatment.Whereas, the corrosion resistance of the TS-air was much better than the TS-brine coating.The passivation current density of the AS coating is lower than the TS-air and TS-brine, as illustrated in figure 6.The AS coating  has a wider passive region, followed by the TS-air and lowest for TS-brine.Low passivation current density of the AS coating ensures that the passive layer, formed on its surface is stable and thicker, providing excellent corrosion protection similar to the work of Zhu et al [23].The ease of formation of a stable and thicker passive layer depends on the chemical composition and microstructure of the deposited amorphous coatings.As the chemical compositions of the three coated samples were same, therefore, we can say the difference in I corr values is only due to the difference in microstructure, generated due to cyclic thermal shocks treatments.
Similarly, it can be seen in both TS coated samples that the passivation region is relatively narrow ~900 mV for TS-air and ~500 mV for TS-brine samples, followed by reactivation and re-passivation regions as clearly indicated in figure 6.This indicates pitting corrosion behavior of these samples during reactivation region as shown in figure 6.This phenomenon rose due to depletion of chromium out of the matrix in a corrosive environment.The difference in passivation regions of the sample can be explained by the phenomena induced because of thermal shocks.At elevated temperature, the diffusion kinetics play a vital role.Diffusion of species results in two main processes; (1) It accelerates corrosion by promoting corrosion reaction kinetics, (2) It also promotes the rate of formation of passive film.Overall, the net effect of elevated temperature treatment is dependent upon the dominance of one of the above two factors [24].From figure 6 the narrow region of passivation regions of thermal shock-induced sample indicates that the corrosion kinetics are dominant over the kinetics of formation of passive layer.

Micro-hardness
The micro-hardness value of the AS coating was the highest (1150 HV 0.2 ), followed by the micro-hardness value of TS-brine (1078 HV 0.2 ), and the lowest for the TS-air (888 HV 0.2 ) ass illustrated in figure 7.These microhardness values are in correlation to the mechanical properties of amorphous coatings.The micro-hardness values of the TS-air and TS-brine were lesser than that of the AS coating because of softening induced in amorphous coatings as a result of structural relaxation phenomena at elevated temperatures.The lowest value of micro-hardness in the TS-air is due to the effect of softening in this amorphous-coated sample is being more prominent, this is because of slower cooling rate during air cooling results in a greater degree of structural relaxation.These results of micro-hardness are in complete correlation with the DSC scans that predict the degree of structural relaxation highest in the TS-air coating followed by TS-brine and lowest in the AS coating.

Tribological properties
The thermal shock resistance of a coating highly depends on the substrate type and mechanical properties.It can also be influenced by thermal expansion coefficient, heat transfer coefficient, the specimen size fracture stress, thermal conductivity, modulus of elasticity, Poisson's ratio, and thermal diffusivity [25].In current work, the effects of cyclic thermal shocks on the tribological properties of AS, TS-air, and TS-brine coatings were also studied in dry sliding conditions.The accumulated wear rates of all the three coatings are illustrated in figure 8.
The results of the accumulative wear rate are shown in table 3. Figure 8 (according to table 3) suggests that the wear rate of AS coating was the lowest followed by the TS-brine and the highest wear rate was observed in TSair coating.Wear rate is in inverse relation to the wear resistance, the lower the wear rate, higher is the wear resistance of the material, and vice versa [26].The AS coating had the highest wear resistance due to the high amorphous behavior of the coating.As wear is associated with the phenomena of plastic deformation, highly amorphous structure resists plastic deformation and contributes to high wear resistance.For thermal shock-treated samples, TS-brine coating has a lower wear rate than that of TS-air coating.This is due to the softness induced by structural relaxations in TS-air being more prominent as compared to TS-brine coating.This softening decreases the resistance to plastic deformation of the amorphous coating.As previously mentioned, wear is a plastic deformation-dependent process, ease of plastic deformation will result in a decrease in wear resistance of the coating.SEM coupled with EDS analysis was employed on the wear tracks of the three samples to examine the worn surface morphology of these coatings.Figures 9(a), (b) shows the SEM and EDS analysis of the AS coating.Worn surface morphology at the wear track of AS coating can be seen in figure 9(a).It can be clearly seen that there are very few cracks associated with fatigue wear present in the wear track.This indicates that fatigue wear is not prominent in sprayed samples.This gives a valid explanation for the high wear resistance of as sprayed sample.Similarly, the EDS analysis of the wear track in figure 9(b) can be clearly seen.High oxygen content in the wear track indicates the presence of oxides responsible for the oxidation wear of the coating.A few cracks and high oxygen content in the wear track indicate that in as-sprayed coating oxidation wear was a prominent wear mechanism present.
Figures 9(c), (d) illustrates SEM and EDS analysis of TS-brine coating.It can be seen in figure 9(c) that fatigue wear cracks are present in abundance in the wear track as compared to the AS coating.The presence of fatigue wear cracks in abundance indicates that fatigue wear is prominent.This analysis corresponds with the high accumulated wear rate of TS-brine as compared to the AS coating.The presence of fatigue wear cracks is also an indicator of softening induced due to structural relaxations because of thermal shock treatment.Similarly, figure 9(d) indicates the EDS analysis of the worn surface in the wear track.High oxygen content in the wear track indicates the presence of oxides responsible for oxidation wear.
SEM and EDS analysis of the worn surface in the wear track of TS-air as illustrated in figures 9(e), (f).It can be clearly seen in figure 9(e) that the worn surface in the wear track has a significant number of fatigue cracks.This indicates fatigue wear is more prominent in the TS-air coatings and gives an explanation for the lowest wear resistance.Softening is induced due to structural relaxations because of thermal shock treatment.Similarly, EDS analysis of the wear track shown in figure 9(f) indicates lesser Oxygen content as compared to the other two coatings.This indicates oxidation wear was not as prominent as fatigue wear in TS-air employing the highest wear rate for this sample in comparison with the TS-brine and the AS coating which has the lowest wear rate.

Conclusions
The Fe-based amorphous powder, with a composition of Fe 48 Mo 14 Cr 15 Y 2 C 15 B 6 , was successfully deposited on AISI 1045 steel substrate, using the HVOF thermal spraying process.The effect of cyclic thermal shock treatments on the electrochemical and tribological properties of Fe-based coating was investigated.For this purpose, the Fe-based coatings were heated to 300 °C and then rapidly cooled in two different media, including brine solution and air.The samples were subjected to 150 heating/cooling cycles.It was concluded that the AS coating exhibited superior properties, compared to the TS-air and TS-brine samples.This decrease in properties of the TS-air sample was attributed to structural relaxation and softening effect upon air cooling.Additionally, corrosion resistance was found to be minimal for the TS-brine coatings, mainly due to the corrosive nature of the cooling medium.Importantly, both before and after the thermal cycles, the coatings maintained their amorphous behavior, as confirmed by XRD analysis.DSC results also confirmed this behavior, showing that the coatings did not crystallize up to temperatures of around 650-680 °C.This indicates significant potential for these coatings in high-temperature industrial applications as a thermal barrier coating.

3. 1 .
Morphological and structural analysis SEM image and corresponding EDS maps of Fe-based amorphous powder (Fe 48 Mo 14 Cr 15 Y 2 C 15 B 6 ) are illustrated in figure 2. It can be seen in figure 2(a) that most of the particles are spherical with particle size,

Figure 1 .
Figure 1.Process flow chart of experimental work.

Figure 8 .
Figure 8. Wear rate of AS, TS-air, and TS-brine coatings.

Table 1 .
Process parameters employed for HVOF spraying.

Table 2 .
Corrosion kinetic parameters of the AS, TS-air, and TS-brine coatings.

Table 3 .
Wear rate of the AS, TS-air, and TS-brine coatings.