Development of Method for Applying Multilayer Gradient Thermal Protective Coatings Using Detonation Spraying

: In this work, multilayer gradient coatings obtained by detonation spraying were studied. To obtain a multilayer gradient coating by detonation spraying, two modes with different numbers of shots of NiCrAlY and YSZ were developed. The presented results demonstrate the effectiveness of creating a gradient structure in coatings, ensuring a smooth transition from metal to ceramic materials. Morphological analysis of the coatings confirmed a layered gradient structure, consisting of a lower metallic (NiCrAlY) layer and an upper ceramic (YSZ) layer. The variation in the contents of elements along the thickness of the coatings indicates the formation of a gradient structure. X-ray analysis shows that all peaks in the X-ray diffraction patterns correspond to a single ZrO 2 phase, indicating the formation of a non-transformable tetragonal primary (t ′ ) phase characteristic of the thermal protective coatings. This phase is known for its stability and resistance to phase transformation under changing operating temperature conditions. As the thickness of the coatings increased, an improvement in their mechanical characteristics was found, such as a decrease in the coefficient of friction, an increase in hardness, and an increase in surface roughness. These properties make such coatings more resistant to mechanical wear, especially under sliding conditions, which confirms their prospects for use in a variety of engineering applications, including aerospace and power generation.


Introduction
In recent years, research interest in ceramic thermal protective coatings (TPCs) has been steadily increasing.These coatings play a key role in protecting structural elements operating at high temperatures, such as gas turbine blades, components of aircraft and space engines, and parts of power equipment.Ceramic TPCs provide a significant reduction in heat flow to the protected surfaces, increasing their service lives and reliability in extreme operating conditions [1][2][3].One of the most promising directions in this field is the use of multilayer gradient structures, which include a substrate, an oxidation-resistant metal binder layer based on nickel (MeCrAlY), and a ceramic layer of ZrO 2 -Y 2 O 3 (YSZ) stabilized with 6%-8% yttrium oxide.This ceramic layer serves as an external thermal insulation top layer [4,5].However, despite the successes achieved, the main challenge for TBC coatings remains to ensure stability and durability under high-temperature and aggressive environmental conditions.According to numerous studies, the weak link of the TPC (NiCrAlY/ZrO 2 -Y 2 O 3 ) is the boundary between the ceramic layer and the heat-resistant sublayer [6][7][8][9].During the high-temperature testing or operation of the blades, acid from the oxidizing medium (air, fuel combustion products) penetrates to the 'ceramic-sublayer' boundary.Its penetration to the surface of the sublayer occurs in two ways-by gas transfer through the open porosity of the ceramic layer and by diffusive movement of oxygen ions in the zirconium dioxide lattice.The penetration of oxygen through the ceramic layer leads to the oxidation of the metal heat-resistant sublayer.Oxides are formed on its surface, the composition and structure of which depend on the amount of oxygen coming to the surface and the composition of the sublayer [10,11].The formation and growth of oxides at the ceramic-sublayer interface creates additional stresses, reduces the adhesion of the ceramic layer, and eventually leads to its chipping.After a certain period of operation at high temperature, the chipping of the ceramic layer occurs even in the complete absence of stresses from external forces.Their appearance is due to the fact that the coefficient of thermal expansion of ceramics is significantly less than the coefficient of thermal expansion of a heat-resistant alloy.
Thus, a decrease in thermal conductivity and an increase in stability at high temperatures make the task of introducing new highly effective thermal protective coatings extremely urgent.One way to solve this task is to create gradient coatings in which the composition gradually changes from the metal sublayer to the outer ceramic layer [12][13][14][15][16].It is believed that the transition cermet layers worsen the thermal stability of the coating at temperatures above 1170-1220 K, which is due to the intensive oxidation of the metal component of the transition layer [17].This leads to additional compressive stresses within the coating and premature peeling of the ceramic layer.Coatings with a smooth gradient of chemical composition allow better damping of thermal compression and expansion of the coating under thermal cyclic loads.It is possible to form a gradient structure in coatings during gas-thermal spraying by gradually changing the composition of the supplied powder from the feeder to the plasma torch, as well as by using several feeders simultaneously.These coatings can be created using both electron beam physical vapor deposition (EB-PVD) methods [18][19][20] and plasma sputtering (PS) [5,[21][22][23][24].For example, plasma spraying (PS) is widely used to create ceramic coatings with high temperature and thermal cycling resistance.In this process, the powder material is melted in a plasma torch and applied to the substrate to form dense and even layers.Another method, electron beam physical vapor deposition (EB-PVD), produces coatings with a characteristic columnar microstructure that provides high thermal stress resistance.This method is particularly effective for thin but tough coatings.Vacuum arc spraying (VAC) is also used for metal and ceramic coatings, providing high adhesion and layer density.This method is particularly useful for wear and corrosion resistant coatings.The detonation spraying (DS) method for obtaining ceramic heat protective coatings is also of interest [25].
In our opinion, detonation spraying has great prospects, since the method allows for controlling the temperature of the sprayed powder and the applied coating over wide intervals by varying technological parameters and alternating the supply of powders during spraying.This makes it possible to control the structural and phase state of the coating material and, therefore, obtain coatings with specified properties [26][27][28][29].The advantages of this method include the low porosity of the coating, high bond strength with the workpiece base, and minimal thermal impact, which avoids undesirable thermal stresses and warping even in thin-walled parts of complex construction [30][31][32][33].Due to the low porosity of the coatings, the preservation of the chemical composition of the initial powder in the coatings, and the use of two powder dispensers, the detonation method shows promise for obtaining TPC.It is noted in [12,32] that the optimization of the detonation spraying process can improve adhesion between coating layers, reduce the effect of thermal stresses, and improve oxidation resistance in aggressive environments.
Thus, in this paper, we proposed to develop a method for applying multilayer gradient heat-protective coatings with a layered gradient structure, providing effective protection for heat-stressed parts and assemblies from the effects of high-temperature gas flow.Gradient heat-protective coatings added by detonation spraying are formed as follows: coatings are applied to samples that have been previously sandblasted, with a gradual change in the composition of the powder from the feeders into the barrel while using two feeders simultaneously.At the same time, by varying the composition of the powder, the supply of ceramic powder from the dispenser gradually increases, while the supply of metal powder gradually decreases.Thus, it is possible to obtain coatings with a gradient structure.The technical result is achieved by obtaining a gradient coating on the surface of the sample, consisting of a lower metal layer based on nickel (NiCrAlY) and an upper ceramic layer of zirconium oxide powder (ZrO 2 ) stabilized with yttrium oxide (Y 2 O 3 ), where the ceramic content increases smoothly from the metal layer to the ceramic layer.The coating obtained by the proposed method will have a layered structure characterized by a smooth transition (gradient) of chemical composition between the main coating zones.A gradual change in the microstructure and a smooth transition in the physical and mechanical properties lead to an improvement in the performance characteristics of thermal protective coatings.Such coatings have the necessary specified properties for the outer layers directly exposed to the external environment.In addition, multilayer gradient coatings reduce the difference in the physical and mechanical characteristics between the coating material and the substrate, thereby reducing the stress surge that occurs during loading at the interface of the mating layers.
The novelty of this work lies in the development and implementation of a technique for obtaining multilayer gradient heat-protective coatings through detonation spraying using two powder dispensers, which allows for the control of the structure and composition of the coating.In contrast to conventional deposition methods, detonation spraying provides high adhesion, low porosity, and the ability to fine-tune the coating structure.
The motivation for this work is the need to improve the reliability and durability of structural elements operating at high temperatures and extreme loads.The development of effective methods of surface protection from thermal effects will significantly increase the service lives of parts and reduce costs for their maintenance and repair.
The purpose of this work is as follows: the development of a method for applying multilayer gradient thermal protective coatings by detonation spraying, featuring a smooth change in chemical composition and coefficient of thermal expansion.

Materials and Methods
Steel 12Kh18N10T (X6CrNiTi18-10 (ISO 15510:2014 [34])) with a size of 20 mm was used as a substrate for obtaining gradient thermal protective coatings by detonation spraying.Before coating, the substrate surface was sandblasted to create roughness.For this purpose, a corundum with a particle size of 60-80 µm was used at an air pressure of 0.6 MPa.
A powder mixture of the PNH20K20Y13 brand (NiCr20AlY) was used as a metal binder layer, the chemical composition of which is shown in Table 1. Figure 1a,b shows the microstructures of NiCr20AlY powder obtained by scanning electron microscopy (SEM).The initial NiCr20AlY powder is represented by irregularly shaped (comminuted) particles forming conglomerates ranging in size from 4 to 30 µm (Figure 1c).The phase composition includes a NiAl intermetallic compound (β-phase), which accounts for 93.8% of the total composition, with small contents of γ ′ -Ni 3 Al (3.9%) and NiCoCr (2.3%) phases (Figure 1d).A ceramic powder made of yttrium-stabilized zirconium dioxide Metco 233B (8YSZ) was used to apply the top coat, the chemical composition of which is shown in Table 2. Commercial agglomerated and sintered zirconium dioxide powder stabilized with yttrium, 8YSZ, had a spherical shape with porous powder particles, and the average particle diameter was about 30 µm (Figure 2a-c) and, according to the results of X-ray phase analysis, contained 89.52 vol.% of the tetragonal (t ′ -ZrO 2 ) phase and 10.48 vol.% of the monoclinic (m-ZrO 2 ) phase (Figure 2d).A ceramic powder made of yttrium-stabilized zirconium dioxide Metco 233B (8YSZ) was used to apply the top coat, the chemical composition of which is shown in Table 2. Commercial agglomerated and sintered zirconium dioxide powder stabilized with yttrium, 8YSZ, had a spherical shape with porous powder particles, and the average particle diameter was about 30 µm (Figure 2a-c) and, according to the results of X-ray phase analysis, contained 89.52 vol.% of the tetragonal (t′-ZrO2) phase and 10.48 vol.% of the monoclinic (m-ZrO2) phase (Figure 2d).The mechanism of gradient coating formation during detonation spraying included several key steps.The surface of the substrate was prepared by mechanical and chemical treatment, after which the powder materials were applied.In the detonation spraying process, a mixture of gas and powders detonated, creating a high-temperature plasma and shock wave that heated and accelerated particles to high temperatures and velocities.Powder particles hit the substrate surface at high velocity, forming dense coating layers.The mechanism of gradient coating formation during detonation spraying included several key steps.The surface of the substrate was prepared by mechanical and chemical treatment, after which the powder materials were applied.In the detonation spraying process, a mixture of gas and powders detonated, creating a high-temperature plasma and shock wave that heated and accelerated particles to high temperatures and velocities.Powder particles hit the substrate surface at high velocity, forming dense coating layers.To obtain a gradient coating by detonation spraying, two modes were developed with different amounts of NiCrAlY and YSZ shots.In the first mode, starting from the base, the number of NiCrAlY shots gradually decreased from 5 to 1, while the number of YSZ shots gradually increased from 1 to 20 (Table 3).This mode provided a smooth transition from the metal to the ceramic material, creating a gradient structure in the coating.In the second mode, the number of NiCrAlY shots also started at 10 and decreased to 2, while the number of YSZ shots increased from 2 to 40.This mode also provided a smooth transition from metal to ceramic material, but with higher YSZ shot values, which could lead to a higher concentration of ceramic material in the coating.Both modes provided the ability to control the structure and properties of the gradient coating, allowing it to be adapted to specific requirements and operating conditions.The phase composition of the coatings was determined using an X'PertPro X-ray diffractometer (Philips Corporation, Amsterdam, The Netherlands) with CuKa radiation.To quantify the phases, the Rietveld method was applied using the COD1525706 database cards, and the material analysis was conducted using Diffraction (MAUD) software (version 2.999) [35].
The morphology of the surfaces and cross-sections of the coatings were studied using a MIRA 3 LMU scanning electron microscope (Tescan, Brno, Czech Republic) equipped with an energy-dispersive spectrometer (EDS) for local microanalysis.Micrographs were obtained using SEM in both electron backscattering (BSE) and secondary electron (SE) modes.
The surface roughness (Ra) was determined using a profilometer 130 in accordance with GOST 25142-82 [36].When determining the roughness of gradient coatings, the speed was 0.25 mm/s, the measuring scale was 500 µm, and the measuring length was 10 mm.
The hardness and modulus of elasticity of the coatings were measured using the FISCHERSCOPE HM2000S device with WIN-HCU (version 7.1) software.This computercontrolled measuring system is designed to evaluate the microhardness and characteristics of materials in accordance with the international standard ISO 14577 [37].For all hardness and modulus of elasticity tests, the holding time was set to 10 s at a load of 1 N.The average hardness values were obtained from the results of 10 measurements.
Coatings 2024, 14, 899 6 of 14 Tribological tests were carried out on the Anton Paar TRB 3 tribometer (international standards ASTM G 133-95 [38] and ASTM G99 [39]) under dry friction conditions at room temperature using the standard "disc-ball" method.A 100Cr6 ball with a diameter of 6 mm was used as a contour.The tests were carried out at a load of 10 N and a linear velocity of 10 cm/s, the radius of curvature of wear was 3 mm, and the friction path was 400 m.

Results and Discussion
Using a scanning electron microscope, images of secondary electrons (SE) were obtained from cross-sections of detonation coatings (Figures 3a and 4a), along with corresponding EDS maps of elements (Zr, O, Y, Ni, Cr, and Al), enabling a more detailed analysis of their structure and composition (Figures 3c-h and 4c-h).Microstructural images show that detonation coatings included layers of NiCrAlY (binder coating), YSZ (top coating), and graduated intermediate layers consisting of a mixture of NiCrAlY and YSZ.To obtain a more detailed understanding of the morphology of the gradient surface of the coating, cross-sectional images were obtained using a scanning electron microscope with electron backscattering (BSE) (Figures 3b and 4b).In BSE images, it was revealed that the area with light contrast corresponds to the ceramic phase YSZ, while the area with dark (gray) contrast represents the metallic phase NiCrAlY.The mechanism for forming a multilayer gradient coating involves the sequential spraying of layers, with a gradual change in the contents of metallic and ceramic components.When the number of detonation spray shots changed, an increase in the light contrast layer of YSZ was observed from the substrate to the coating surface, while the dark layer of NiCrAlY, conversely, decreased, which indicated a high degree of control over the formation process and the thickness of each layer.The coatings had a typical layered structure with the tight adhesion of all layers to each other and the substrate surface, which indicated a high quality of adhesion.In the detonation spraying process, the high kinetic energy of the particles ensured their deep penetration into the substrate and the formation of a tight bond between the layers.In this study, there was no evidence of cracks or unequal dimensions in the area of the top layer.Black spots concentrated in the angular areas at the interface between the binder coating and the substrate may have represented voids formed as a result of sandblasting (Figures 3 and 4).Comparison of the obtained results with the literature data showed that the developed method of applying multilayer gradient TBCs showed improved performance compared to conventional methods.In particular, the samples obtained by detonation spraying had higher densities and less porosity.change in the contents of metallic and ceramic components.When the number of detonation spray shots changed, an increase in the light contrast layer of YSZ was observed from the substrate to the coating surface, while the dark layer of NiCrAlY, conversely, decreased, which indicated a high degree of control over the formation process and the thickness of each layer.The coatings had a typical layered structure with the tight adhesion of all layers to each other and the substrate surface, which indicated a high quality of adhesion.In the detonation spraying process, the high kinetic energy of the particles ensured their deep penetration into the substrate and the formation of a tight bond between the layers.In this study, there was no evidence of cracks or unequal dimensions in the area of the top layer.Black spots concentrated in the angular areas at the interface between the binder coating and the substrate may have represented voids formed as a result of sandblasting (Figures 3 and 4).Comparison of the obtained results with the literature data showed that the developed method of applying multilayer gradient TBCs showed improved performance compared to conventional methods.In particular, the samples obtained by detonation spraying had higher densities and less porosity.Special attention was given to the analysis of the thickness of coatings from various modes of detonation spraying.The uneven thickness of the coatings was revealed, being due to variations in the detonation spraying parameters, such as the number of detonation shots.It was found that the layer thickness in sample 2D2 (273.72 = 1.26 µm) was less than that in sample 1D1 (963.67 = 13.59 µm).Special attention was given to the analysis of the thickness of coatings from various modes of detonation spraying.The uneven thickness of the coatings was revealed, being due to variations in the detonation spraying parameters, such as the number of detonation shots.It was found that the layer thickness in sample 2D2 (273.72 = 1.26 µm) was less than that in sample 1D1 (963.67 = 13.59 µm).
Figure 5a shows a linear scan of EDS, illustrating the distribution of elements in the detonation coating obtained in 1D1 mode across the cross-section.The EDS lines were drawn at a distance of 0-800 µm from the coating surface to the substrate, as shown in  Figure 5a shows a linear scan of EDS, illustrating the distribution of elements in the detonation coating obtained in 1D1 mode across the cross-section.The EDS lines were drawn at a distance of 0-800 µm from the coating surface to the substrate, as shown in Figure 5a.EDS linear analysis confirmed that the coating structure was a multilayer gradient structure consisting of a mechanical mixture of metal and ceramics.The distribution of the elements varied along the thickness of the coating, reflecting a multilayered gradient change in the content of the elements.The total spectrum along the lines confirmed the content of elements in accordance with the expected components, where the contents of elements was set (by weight) as follows: Zr-43.8%,O-18.1%,Ni-16.8%,Cr-11.4%,Al-4.8%,Y-3.5%, and Fe-1.7%.
To analyze the distribution of elements along the EDS line of detonation coatings obtained by mode 2 in cross-section, measurements from 0 to 27 µm were carried out (Figure 5b).This analysis also showed that the YSZ content gradually increased from the substrate to the coating surface, and it also revealed a high YSZ content in the outer layer of the coating.The total spectrum along the lines showed the following element contents (by weight): Zr-36.5%,Ni-22.8%,O-14.5%,Cr-13.8%,Al-6.4%,Fe-3.2%, and Y-2.9%.Thus, this study showed that both types of detonation coatings exhibited a multilayer gradient change in the contents of elements along their thickness, emphasizing the importance of this structural feature for their properties and applications.Figure 6 shows the X-ray diffraction patterns of the sprayed coating, as well as the atomic structures of the ideal and refined phases using VESTA (version 3) and MAUD (version 2.999) software.The figure shows the original X-ray structure patterns (Iobs), the calculated patterns after Rietveld refinement (Icalc), and the residual data (Iobs-Icalc), where | represents Bragg reflections corresponding to zirconium dioxide in the tetragonal phase (t′).It was found that all peaks on the X-ray images for samples of both modes corresponded to one phase, which indicates the formation of an exclusively nontransformable tetragonal primary (t′) phase of ZrO2.This was due to the rapid solidification and cooling of raw material particles, which is characteristic of heat protective coatings.It is noted in [9] that the presence of non-transforming tetragonal t′-ZrO2 plays an important role in preserving the structures of coatings and their functional characteristics under thermocyclic exposure.The identified peaks on the X-ray images corresponded to different planes of the t′-ZrO₂ crystal lattice, such as (101), (002), (110), (102), (112), ( 220), ( 103), (211), (202), and (004).This confirms the crystal structure of the samples and the correspondence of the obtained data with that of the literature on tetragonal zirconium dioxide.For the non-transformable tetragonal phase (t′-ZrO2), the ratio c/a√2 is known to approach 1.010.In this study, the measured values of the cell parameters showed 1.008 and 1.009, respectively, which demonstrates the good agreement of the experimental data with the expected values for this crystal structure.The parameters of the lattice cells and the tetragonality (c/a√2) of the samples are presented in Table 4.To analyze the distribution of elements along the EDS line of detonation coatings obtained by mode 2 in cross-section, measurements from 0 to 27 µm were carried out (Figure 5b).This analysis also showed that the YSZ content gradually increased from the substrate to the coating surface, and it also revealed a high YSZ content in the outer layer of the coating.The total spectrum along the lines showed the following element contents (by weight): Zr-36.5%,Ni-22.8%,O-14.5%,Cr-13.8%,Al-6.4%,Fe-3.2%, and Y-2.9%.Thus, this study showed that both types of detonation coatings exhibited a multilayer gradient change in the contents of elements along their thickness, emphasizing the importance of this structural feature for their properties and applications.
Figure 6 shows the X-ray diffraction patterns of the sprayed coating, as well as the atomic structures of the ideal and refined phases using VESTA (version 3) and MAUD (version 2.999) software.The figure shows the original X-ray structure patterns (Iobs), the calculated patterns after Rietveld refinement (Icalc), and the residual data (Iobs-Icalc), where | represents Bragg reflections corresponding to zirconium dioxide in the tetragonal phase (t ′ ).It was found that all peaks on the X-ray images for samples of both modes corresponded to one phase, which indicates the formation of an exclusively non-transformable tetragonal primary (t ′ ) phase of ZrO 2 .This was due to the rapid solidification and cooling of raw material particles, which is characteristic of heat protective coatings.It is noted in [9] that the presence of non-transforming tetragonal t ′ -ZrO 2 plays an important role in preserving the structures of coatings and their functional characteristics under thermocyclic exposure.The identified peaks on the X-ray images corresponded to different planes of the t ′ -ZrO 2 crystal lattice, such as (101), (002), ( 110), ( 102), ( 112), ( 220), ( 103), ( 211), (202), and (004).This confirms the crystal structure of the samples and the correspondence of the obtained data with that of the literature on tetragonal zirconium dioxide.For the non-transformable tetragonal phase (t ′ -ZrO 2 ), the ratio c/a √ 2 is known to approach 1.010.In this study, the measured values of the cell parameters showed 1.008 and 1.009, respectively, which demonstrates the good agreement of the experimental data with the expected values for this crystal structure.The parameters of the lattice cells and the tetragonality (c/a √ 2) of the samples are presented in Table 4.The surface roughness (Ra) measurements showed a significant difference between the coating samples obtained through different modes of detonation spraying.Figure 7a,b shows the results of the surface roughness measurements of the coating layers.The surface roughness represented in Ra was obtained by measuring the profile along the coating surface.Sample 1D1 was characterized by a higher roughness (3.76 µm) compared to sample 2D2 (1.21 µm), which may be due to heterogeneity and the presence of a molten Coatings 2024, 14, 899 9 of 14 region in the coating 1D1.The higher roughness of the 1D1 coating was associated with the formation of a rougher surface, whereas the 2D2 coating probably had a more uniform and compact structure (Figure 7c,d).
ZrO2 plays an important role in preserving the structures of coatings and their functional characteristics under thermocyclic exposure.The identified peaks on the X-ray images corresponded to different planes of the t′-ZrO₂ crystal lattice, such as ( 101), ( 002), ( 110), ( 102), ( 112), ( 220), ( 103), ( 211), (202), and (004).This confirms the crystal structure of the samples and the correspondence of the obtained data with that of the literature on tetragonal zirconium dioxide.For the non-transformable tetragonal phase (t′-ZrO2), the ratio c/a√2 is known to approach 1.010.In this study, the measured values of the cell parameters showed 1.008 and 1.009, respectively, which demonstrates the good agreement of the experimental data with the expected values for this crystal structure.The parameters of the lattice cells and the tetragonality (c/a√2) of the samples are presented in Table 4.   Table 4.The results of X-ray phase analysis.The surface roughness (Ra) measurements showed a significant difference between the coating samples obtained through different modes of detonation spraying.Figure 7a,b shows the results of the surface roughness measurements of the coating layers.The surface roughness represented in Ra was obtained by measuring the profile along the coating surface.Sample 1D1 was characterized by a higher roughness (3.76 µm) compared to sample 2D2 (1.21 µm), which may be due to heterogeneity and the presence of a molten region in the coating 1D1.The higher roughness of the 1D1 coating was associated with the formation of a rougher surface, whereas the 2D2 coating probably had a more uniform and compact structure (Figure 7c,d).Figure 8 shows a graph of the indenter penetration depth relative to the load.The measurement results were obtained using the Martens method to evaluate the mechanical properties of the material and the defect structure occurring during the application process.The results show that both coatings have comparable values for the modulus of elasticity (E), indicating the similar mechanical characteristics of the coating materials.Figure 8 shows a graph of the indenter penetration depth relative to the load.The measurement results were obtained using the Martens method to evaluate the mechanical Coatings 2024, 14, 899 10 of 14 properties of the material and the defect structure occurring during the application process.The results show that both coatings have comparable values for the modulus of elasticity (E), indicating the similar mechanical characteristics of the coating materials.That is, it indicates their similar behavior when exposed to mechanical loads.However, sample 1D1 has a slightly higher hardness compared to sample 2D2.This difference may be due to several factors.Firstly, the higher roughness of the coating 1D1 may contribute to the hardness due to the increased surface area impacted by the indenter, resulting in a load distribution on the rougher surface.Second, the presence of irregularities and molten regions in the coating 1D1 can result in denser and harder areas that enhance the overall hardness of the coating.Third, detonation spraying processes can induce internal stresses in the coating material.In the case of 1D1, these stresses may be higher, which also affects the increase in coating hardness.

Coatings
Coatings 2024, 14, x FOR PEER REVIEW 11 of 16 That is, it indicates their similar behavior when exposed to mechanical loads.However, sample 1D1 has a slightly higher hardness compared to sample 2D2.This difference may be due to several factors.Firstly, the higher roughness of the coating 1D1 may contribute to the hardness due to the increased surface area impacted by the indenter, resulting in a load distribution on the rougher surface.Second, the presence of irregularities and molten regions in the coating 1D1 can result in denser and harder areas that enhance the overall hardness of the coating.Third, detonation spraying processes can induce internal stresses in the coating material.In the case of 1D1, these stresses may be higher, which also affects the increase in coating hardness.
The ratio between hardness (H) and the modulus of elasticity (E), defined as the plasticity index (H/E), indicates a material's ability to deform elastically and can be used to predict its wear characteristics.The obtained values of the plasticity index (H/E) for samples 1D1 and 2D2 were 0.036 and 0.027, respectively.Generally, the higher the H/E value, the better the wear resistance.Thus, the sample 1D1 has a higher wear resistance compared to the sample 2D2.These results are consistent with the tribological tests carried out and indicate the importance of the structure and mechanical properties of coatings for their behavior under the influence of mechanical loads and wear.Figure 9 shows the distribution of microhardness from the substrate to the surface of the NiCrAlY/YSZ coating.A gradual increase in the microhardness values is observed from the substrate to the surface of the NiCrAlY/YSZ coating.This gradient character of the microhardness distribution plays an important role in reducing transient stresses between various materials such as ceramics and metal [40,41].This gradient distribution of microhardness contributes to a more uniform distribution of stresses in the coating, which in turn increases the strength of the joints between the coating and the substrate.This is especially important to prevent the possibility of cracking and fracture of joints under various types of mechanical stresses, such as bending or tensile.The gradient distribution of microhardness also contributes to increasing the strength and durability of coatings, making them more resistant to the effects of mechanical loads and increasing their effectiveness at protecting against high temperatures [40].It should be noted that measuring the hardness of a coating composed of individual microstructures can result in significant errors, as the Vickers pyramid may hit either the microstructure itself or an empty space.The ratio between hardness (H) and the modulus of elasticity (E), defined as the plasticity index (H/E), indicates a material's ability to deform elastically and can be used to predict its wear characteristics.The obtained values of the plasticity index (H/E) for samples 1D1 and 2D2 were 0.036 and 0.027, respectively.Generally, the higher the H/E value, the better the wear resistance.Thus, the sample 1D1 has a higher wear resistance compared to the sample 2D2.These results are consistent with the tribological tests carried out and indicate the importance of the structure and mechanical properties of coatings for their behavior under the influence of mechanical loads and wear.
Figure 9 shows the distribution of microhardness from the substrate to the surface of the NiCrAlY/YSZ coating.A gradual increase in the microhardness values is observed from the substrate to the surface of the NiCrAlY/YSZ coating.This gradient character of the microhardness distribution plays an important role in reducing transient stresses between various materials such as ceramics and metal [40,41].This gradient distribution of microhardness contributes to a more uniform distribution of stresses in the coating, which in turn increases the strength of the joints between the coating and the substrate.This is especially important to prevent the possibility of cracking and fracture of joints under various types of mechanical stresses, such as bending or tensile.The gradient distribution of microhardness also contributes to increasing the strength and durability of coatings, making them more resistant to the effects of mechanical loads and increasing their effectiveness at protecting against high temperatures [40].It should be noted that measuring the hardness of a coating composed of individual microstructures can result in significant errors, as the Vickers pyramid may hit either the microstructure itself or an empty space.Figure 10 shows the dependence of the friction coefficient of detonation multilayer gradient coatings at different time.The average value of the sample friction coefficient in the first mode is 0.215 ± 0.048, and no coating destruction was observed until the sliding distance reached 400 m (or 4000 s).For the second sample, the friction coefficient was 0.584 ± 0.130.This value is higher than that of the first sample, which may indicate its less effective thermal protection ability or greater susceptibility to wear during sliding.Roughness and microhardness may affect the tribological characteristics of the samples [42].An increase in the surface roughness of sample 1D1 compared to sample 2D2 leads to a decrease in the actual contact area of the interacting bodies, which also leads to a decrease in the friction coefficient.In Figure 11a,c, there is no significant fracture or wear on the surface of the coating, whereas in Figure 11b,d, there is obvious excessive wear and damage to the counterbody made of a 100Cr6 steel ball.This indicates the possibility of material transfer from the counterbody to the coated samples due to the intense heating of the surface layers of the rubbing bodies at sliding speeds of 10 cm/s.One of the reasons for the wear of the counterbody is the presence of high thermal stresses in its surface layers caused by Figure 10 shows the dependence of the friction coefficient of detonation multilayer gradient coatings at different time.The average value of the sample friction coefficient in the first mode is 0.215 ± 0.048, and no coating destruction was observed until the sliding distance reached 400 m (or 4000 s).For the second sample, the friction coefficient was 0.584 ± 0.130.This value is higher than that of the first sample, which may indicate its less effective thermal protection ability or greater susceptibility to wear during sliding.Roughness and microhardness may affect the tribological characteristics of the samples [42].An increase in the surface roughness of sample 1D1 compared to sample 2D2 leads to a decrease in the actual contact area of the interacting bodies, which also leads to a decrease in the friction coefficient.Figure 10 shows the dependence of the friction coefficient of detonation multilayer gradient coatings at different time.The average value of the sample friction coefficient in the first mode is 0.215 ± 0.048, and no coating destruction was observed until the sliding distance reached 400 m (or 4000 s).For the second sample, the friction coefficient was 0.584 ± 0.130.This value is higher than that of the first sample, which may indicate its less effective thermal protection ability or greater susceptibility to wear during sliding.Roughness and microhardness may affect the tribological characteristics of the samples [42].An increase in the surface roughness of sample 1D1 compared to sample 2D2 leads to a decrease in the actual contact area of the interacting bodies, which also leads to a decrease in the friction coefficient.In Figure 11a,c, there is no significant fracture or wear on the surface of the coating, whereas in Figure 11b,d, there is obvious excessive wear and damage to the counterbody made of a 100Cr6 steel ball.This indicates the possibility of material transfer from the counterbody to the coated samples due to the intense heating of the surface layers of the rubbing bodies at sliding speeds of 10 cm/s.One of the reasons for the wear of the counterbody is the presence of high thermal stresses in its surface layers caused by In Figure 11a,c, there is no significant fracture or wear on the surface of the coating, whereas in Figure 11b,d, there is obvious excessive wear and damage to the counterbody made of a 100Cr6 steel ball.This indicates the possibility of material transfer from the counterbody to the coated samples due to the intense heating of the surface layers of the rubbing bodies at sliding speeds of 10 cm/s.One of the reasons for the wear of the counterbody is the presence of high thermal stresses in its surface layers caused by temperature gradients.This leads to the formation of cracks and the development and peeling of wear particles.It is also possible that the insufficient hardness of the counter body contributes to its rapid wear upon contact with the hard coatings of the samples.
Coatings 2024, 14, x FOR PEER REVIEW 13 of 16 temperature gradients.This leads to the formation of cracks and the development and peeling of wear particles.It is also possible that the insufficient hardness of the counter body contributes to its rapid wear upon contact with the hard coatings of the samples.

Conclusions
This study of multilayer gradient detonation coatings has shown that the developed modes make it possible to effectively create a smooth transition connection from a metal to a ceramic material, forming a multilayer gradient structure in coatings.Morphological and chemical analysis confirmed the layered structure of the coatings consisting of NiCrAlY/YSZ layers with a gradient change in element content along the coating thickness.It is found that all peaks in the X-ray diffraction patterns correspond to a single ZrO2 phase, indicating the formation of a non-transformable tetragonal primary (t′) phase characteristic of heat-shielding coatings.It was found that sample 1D1, with a higher coating thickness (963.67 ± 13.59 µm), showed higher roughness (3.76 ± 0.24 µm), hardness (4 GPa), elastic modulus (112 GPa), and wear resistance (CF 0.215 ± 0.048) compared to sample 2D2, which had a lower coating thickness (273.72 ± 1.26 µm).This makes sample 1D1 more resistant to mechanical wear and sliding failure.
Thus, this study of multilayer gradient coatings has confirmed the effectiveness of the developed modes at creating gradient thermal protective coatings by detonation spraying.The mode 1 (1D1) allows us to obtain a coating with higher thickness, roughness, hardness, and modulus of elasticity; high wear resistance; a low friction coefficient; and better resistance to slip failure, which confirms its superiority in operating conditions.The main future prospects include an in-depth study of the adhesion properties of the layers and their behavior under cyclic thermal loads, an analysis of the influence of porosity and roughness on adhesion, and the determination of the optimum ratio of metal to ceramic powders.It is also important to study the behavior of gradient coatings under thermocyclic testing to evaluate their durability and thermal shock resistance, investigate microstructure changes after repeated heating and cooling cycles,

Conclusions
This study of multilayer gradient detonation coatings has shown that the developed modes make it possible to effectively create a smooth transition connection from a metal to a ceramic material, forming a multilayer gradient structure in coatings.Morphological and chemical analysis confirmed the layered structure of the coatings consisting of NiCrAlY/YSZ layers with a gradient change in element content along the coating thickness.It is found that all peaks in the X-ray diffraction patterns correspond to a single ZrO 2 phase, indicating the formation of a non-transformable tetragonal primary (t ′ ) phase characteristic of heat-shielding coatings.It was found that sample 1D1, with a higher coating thickness (963.67 ± 13.59 µm), showed higher roughness (3.76 ± 0.24 µm), hardness (4 GPa), elastic modulus (112 GPa), and wear resistance (CF 0.215 ± 0.048) compared to sample 2D2, which had a lower coating thickness (273.72 ± 1.26 µm).This makes sample 1D1 more resistant to mechanical wear and sliding failure.
Thus, this study of multilayer gradient coatings has confirmed the effectiveness of the developed modes at creating gradient thermal protective coatings by detonation spraying.The mode 1 (1D1) allows us to obtain a coating with higher thickness, roughness, hardness, and modulus of elasticity; high wear resistance; a low friction coefficient; and better resistance to slip failure, which confirms its superiority in operating conditions.The main future prospects include an in-depth study of the adhesion properties of the layers and their behavior under cyclic thermal loads, an analysis of the influence of porosity and roughness on adhesion, and the determination of the optimum ratio of metal to ceramic powders.It is also important to study the behavior of gradient coatings under thermocyclic testing to evaluate their durability and thermal shock resistance, investigate microstructure changes after repeated heating and cooling cycles, and evaluate the effect of differences in thermal expansion coefficients on the stress state of the substrate/layer system.Numerical modeling will help us to predict the stress distribution under different

Figure 1 .
Figure 1.The results of analyses of phase and elemental composition of NiCr20AlY: SEM micrographs (a,b), particle size (c), and X-ray phase analysis (XRD) of NiCr20AlY (d).

Figure 1 .
Figure 1.The results of analyses of phase and elemental composition of NiCr20AlY: SEM micrographs (a,b), particle size (c), and X-ray phase analysis (XRD) of NiCr20AlY (d).

Figure 2 .
Figure 2. The results of analysis of phase and elemental composition of 8YSZ: SEM micrographs (a,b), particle size, (c) and X-ray phase analysis (XRD) of 8YSZ powder (d).

Figure 2 .
Figure 2. The results of analysis of phase and elemental composition of 8YSZ: SEM micrographs (a,b), particle size, (c) and X-ray phase analysis (XRD) of 8YSZ powder (d).
Figure 5a.EDS linear analysis confirmed that the coating structure was a multilayer gradient structure consisting of a mechanical mixture of metal and ceramics.The distribution of the elements varied along the thickness of the coating, reflecting a multilayered gradient change in the content of the elements.The total spectrum along the lines confirmed the content of elements in accordance with the expected components, where the contents of elements was set (by weight) as follows: Zr-43.8%,O-18.1%,Ni-16.8%,Cr-11.4%,Al-4.8%,Y-3.5%, and Fe-1.7%.Coatings 2024, 14, x FOR PEER REVIEW 8 of 16

Figure 7 .
Figure 7.The results of surface roughness measurements of multilayer gradient coatings and surface microstructure: (a-c) sample 1D1; (b-d) sample 2D2.

Figure 8 .
Figure 8. Dependence of the penetration depth of the indenter into samples 1D1 and 2D2 depending on the applied load.

Figure 8 .
Figure 8. Dependence of the penetration depth of the indenter into samples 1D1 and 2D2 depending on the applied load.

Figure 9 .
Figure 9. Change in microhardness from the substrates to the surfaces of multilayer gradient coatings.

Figure 10 .
Figure 10.A graph of the dependence of the friction coefficient on the friction path of multilayer gradient coatings.

Figure 9 .
Figure 9. Change in microhardness from the substrates to the surfaces of multilayer gradient coatings.

Coatings 2024 , 16 Figure 9 .
Figure 9. Change in microhardness from the substrates to the surfaces of multilayer gradient coatings.

Figure 10 .
Figure 10.A graph of the dependence of the friction coefficient on the friction path of multilayer gradient coatings.

Figure 10 .
Figure 10.A graph of the dependence of the friction coefficient on the friction path of multilayer gradient coatings.

Figure 11 .
Figure 11.The morphology of wear of the surface layer of samples and the counterbody: (a) sample 1D1; (b) the counter body applied to the 1D1 coating; (c) sample 2D2; (d) the counter body applied to the 2D2 coating.

Figure 11 .
Figure 11.The morphology of wear of the surface layer of samples and the counterbody: (a) sample 1D1; (b) the counter body applied to the 1D1 coating; (c) sample 2D2; (d) the counter body applied to the 2D2 coating.

Table 1 .
Composition and contents of NiC20rAlY powders.

Table 2 .
Composition and contents of YSZ powders.

Table 2 .
Composition and contents of YSZ powders.

Table 4 .
The results of X-ray phase analysis.