Mechanical Properties and Thermal Shock Behavior of Al₂O₃-YSZ Ceramic Layers Obtained by Atmospheric Plasma Spraying

Abstract

Ceramic coatings have many advantages for industrial and medical applications due to their exceptional properties. Ceramic coatings with a thickness of approximately 45 μm, after grinding, were grown using a robotic arm that used the atmospheric plasma spraying procedure. The thermal shock stresses—a common situation in applications but difficult to reproduce under laboratory conditions—of the ceramic layers on top of the metal substrate was achieved using solar energy focused by a concentrating mirror, based on experiments conducted in the CNRS-PROMES laboratory, UPR 8521, belonging to the French National Centre for Scientific Research (CNRS). The ceramic layers showed excellent stability at 1000 °C, even at high heating or cooling rates. At high temperatures (above 1800 °C), the exfoliation of the complex ceramic layer was observed. No differences in the structural, phase, mechanical or adhesion properties of the ceramic layer were observed after the thermal shock cycles (in the literature, there have been quite few reports regarding the properties of the ceramic layers after the thermal shock application). Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), atomic force microscopy (AFM) and X-ray diffraction (XRD) techniques were used to characterize the complex ceramic coating and the effects of thermal shock cycling. The phases and chemical composition of the complex coatings remained similar, insensitive to thermal shock at 1000 °C, consisting of a mixture of crystalline yttrium zirconium oxide and α and γ alumina. For all cases, the main residual stress state was tensile. After 5 or 10 cycles of thermal shocks, a smoothing of the residual stress state was observed in the investigated area. A higher temperature (above 1800 °C), applied as thermal shock, led to higher residual stresses and resulted in large cracks and the spallation of the coating layer.

1. Introduction

Low-carbon steels are prone to corrosion and wear because they are often exposed to aggressive environments [1,2]. To protect steel from the stresses occurring in service and in the situations mentioned above, ceramic coatings are widely used [3,4,5]. The advantages of these coatings are to preserve the toughness characteristic of low carbon steels and to obtain a surface with high hardness, chemical and thermal inertness and thermal shock resistance. Of the wide range of ceramic materials developed over the last 50 years, alumina (Al₂O₃) and zirconia (ZrO₂) are preferred in many applications, due to their exceptional properties (superior hardness, excellent chemical stability, excellent corrosion and wear resistance) and the advanced technologies for producing coatings from these materials [6,7,8].

Gao et al. [9] obtained ZrO₂/Al₂O₃ coatings on a stainless-steel substrate using the electrolytic deposition process and observed a significant improvement in oxidation resistance and interface adhesion. Adhesion studies by Chang and Yen [10] showed that a mixture of ZrO₂ with Al₂O₃ can be considered an important material for producing ceramic coatings. Jakovljevic et al. [11] reported that the pores generated in the plasma-sprayed Al₂O₃ layer can be sealed by depositing a ZrO₂ layer using the immersion method, so it can be considered a viable solution in obtaining combined alumina-zirconium coatings to remedy some deficiencies of ceramic coatings composed of two individually used ceramic materials.

Liu et al. [1] demonstrated that the incorporation of metal-like anchor layers of the substrate ceramic layer, such as Ni₆₀, NiAl and FeAl, for plasma-sputtered Al₂O₃ coatings resulted in higher substrate corrosion resistance. Additionally, previous work on bistratified (ZrO₂/Al₂O₃-13TiO₂) coatings has led to a remarkable improvement in the wear and corrosion resistance of biomedical Ti-13Nb-13Zr [12,13].

The specific characteristics of each oxidized ceramic material (alumina and zirconia) can be combined to overcome the limitations of the two monolithic materials, so excellent results have been obtained from the preparation of alumina-zirconia composites [14,15,16], which are currently used successfully in the femur extremity [17,18]. The advantages of these oxide composites are based on the limited transition from the tetragonal to the monoclinic phase. The tetragonal phase preservation over time maintains the mechanical performance of the composite [19] and ensures that the material’s toughness is preserved [20,21]. This alumina-zirconia composite allows us to complement the moderate hardness of alumina, but also to reduce the aging effect of zirconia. It has been shown that when the percentage of ZrO₂ is kept below 22 wt% [22], aging phenomena do not occur, regardless of the zirconia grain size.

Zhong [23] tested a dual ceramic layer for thermal barrier applications consisting of strengthened gadolinium zirconate (Gd₂Zr₂O₇, GZ) as the upper ceramic layer and partially stabilized 4.5 mol% Y₂O₃ ZrO₂ (4.5YSZ) as the lower ceramic layer. The plasma sputtering and thermal shock resistance technique used for the coatings was significantly improved compared to the GZ-3YSZ single layer ceramic (SCL) coatings. The improvement in properties was attributed to the increase in fracture toughness by incorporating YSZ nanostructured particles into the ceramic layer and due to the improvement in deformation tolerance by using 4.5YSZ material as the lower ceramic layer [23].

The adhesion of ceramic coatings to the steel substrate is generally influenced by the shape, nature and roughness of the metal surface. Most deposition methods using plasma spraying techniques involve a preliminary preparation of the surface by various physical, chemical or mechanical methods to achieve a suitable surface roughness, depending on the size of the deposition compounds. Slurry plasma spraying can be used as a preliminary technique to obtain an anchor layer for APS ceramic coatings, provided it is maintained at a temperature of approximately 400 °C for better adhesion [24].

Concentrated solar flux, using natural sunlight, on the surface of the complex ceramic layer (Al₂O₃-YSZ–APS) was used to achieve thermal shocks based on a very high rate of surface heating in a short time with rapid surface cooling. The structural and mechanical properties of the coatings were evaluated using scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction and scratch tests.

2. Materials and Methods and Thermal Shock Experimental Set-Up

Commercial steel used for custom applications was coated with a complex ceramic material consisting of Metco: 105NS-1 alumina with zirconium 204B-NS, using an industrial deposition system (Sulzer Metco, Sulzer Management Ltd, Winterthur, Switzerland) with a robotic arm, Figure 1a. The plasma jet was coated with Ar (5.2 bar pressure and 39 NLPM gas flow) and H (3.4 bar pressure and 6.6 NLPM gas flow). Five passes (approximately 12–15 µm per layer) were applied to cover the entire exposed metal surface. The ceramic powder mixture consisted of 87.5% alumina with 12.5% YSZ (zirconia with yttria for stabilization), and structural and chemical aspects were presented in reference [25]. The experimental equipment shows a high surface coverage with good potential for industrial applications. The chamber was partially cleaned using a pump to extract dust and main impurities from the deposition chamber.

For many applications, there is the possibility of exposing metallic materials to high temperatures (e.g., 1000 °C) in a very short time and strongly affecting the microstructure of steels, especially low carbon steels [26]. This thermal shock can irreparably break the material or induce microcracks, acting as pores in the microstructure. The use of a thin ceramic coating can greatly improve the steel’s service life by eliminating the possibility of thermal shock.

The ceramic layers were subjected to thermal shocks using solar energy at the Promes Facility, Font-Romeu, France, using a solar concentrator. An image of the schematic configuration of the thermal shock system is shown in Figure 1b,c. The experiments were conducted using a 200 cm diameter concentrator using an experimental cart with a water-cooling system for the sample holder. The experiments were conducted in the presence of designated supervisors from the facility, under completely safe conditions [26].

Thermal shocks were applied for the short-term exposure of the ceramic-coated samples under a concentrated solar flux for short periods (10–12 s), followed by rapid cooling in air (experiments were performed under a glass flask, Figure 1c). The median positioning of the sample under the focused solar beam was achieved using a motorized translation system under the concentrator.

A Rigaku Ultima IV diffractometer was used to identify the crystalline phase of the investigated samples. The XRD pattern acquisition conditions were as follows: θ-θ Bragg-Brentano geometry, CuKα radiation, D/teX Ultra 1D detector with graphite monochromator, [20θ–80θ] 2θ range, 0.02θ step and a scan rate of 0.5θ/min. The PDXL2 suite (Rigaku, Tokyo, Japan) and the ICDD PDF4+ 2022 database were used to determine the phase composition and microstructural characteristics of the coatings using the Rietveld method.

A specialized micro-area X-ray stress measurement system (Automate II from Rigaku, Tokyo, Japan) was used to map the residual stresses in the central areas of the coatings. The experimental parameters were as follows: iso-inclination scanning method, CrKα radiation, Kβ filter, parallel beam geometry and 1 mm diameter collimator. The relative values of the residual stresses present in the coatings were evaluated using the sin2Ψ method, where Ψ is the angle between the normal surface of the sample and the [hkl] direction under analysis.

Microindentation and scratch tests were performed on the CETR UMT-2 microtribometer and the test conditions were as follows: two types of microscratch tests were performed applying a constant load and an increasing linear force. The tests were performed for a vertical force of 10, 15 and 19 N by moving the table at a 10 mm distance within 60 s at a speed of 0.167 mm/s. For the microscratch tests, an Nvidia cutting blade with a radius at the tip of 0.4 mm was used. The results were interpreted using the Test Viewer software version 2.16. The software performed the test automatically and recorded the vertical force Fz, the horizontal force Fx, the acoustic emission signal AE, the time and the travel distance in the horizontal direction Y.

The surface of the ceramic layer before and after the heat shock cycles were investigated using a VegaTescan LMH II scanning electron microscope (SEM), SE detector, 30 kV heating gun voltage and an EasyScan Nanosurf atomic force microscope (AFM) with CETR10 tip and non-contact scanning mode. Chemical composition analysis was performed with a Bruker EDS detector using the mapping mode.

3. Experimental Results

3.1. Thermal Shock Behavior of the Experimental Samples

The 80 × 20 × 5 mm steel substrate samples with a ceramic layer on top (45 μm), were positioned under the solar beam using a mechanical system, and a K thermocouple (in a metal sheath) was used under the sample in the central part to retrieve the training data connected to the Graphtec Corporation type GL220 equipment on channel two. On the first channel, we recorded the surface temperature of the ceramic layer (Figure 2a). The thermal conductivity of the substrate was high (Figure 2b), and as the surface (5 mm thick) heated up, heat was rapidly transferred to the underside of the sample. During the first heating cycle of the coated sample, at a temperature of approximately 350–400 °C, evaporation was observed, which can be attributed to the removal of connected water from the ceramic layer [27]. In the first heating cycle, from room temperature to 1000 °C, an accommodation period was required to reach the proposed temperature. Typically, ceramic materials exhibit low thermal conductivity, especially compared to alloys. Various ceramic materials have a low heat transfer coefficient, so they are used as thermal insulators (e.g., ceramic or glass fibers, asbestos). Of these, zirconium (cubic phase), a well-known imitator of diamond as a gemstone, has the lowest thermal conductivity values. Thermal transfer through materials is supported by electron motion and lattice vibration transfer. For metals (alloys) with low electrical resistance and for crystals, in which structure vibrations are easily transferred (e.g., crystals with atoms or ions of similar masses at lattice points and covalent crystals with strong bonds) exhibit high thermal conductivity. For oxide ceramics, the influence of pore spreading, particle boundaries or impurities can alter this property. For fine ceramics, the level of thermal conductivity (higher or lower) can be controlled by these factors. The decrease in thermal conductivity will decrease with the increasing temperature because structural disorder prevents the wave vibrations of atoms/lattices. For some ceramic materials, radiative heat transfer will dominate mechanical conduction, i.e., thermal conductivity will increase at higher temperatures. Electrons, phonons and magnons are responsible for heat transfer. In the case of metals, it is the electrons moving through the lattice that perform the heat transfer, whereas ceramic materials do not possess free electrons, and in this case, the heat transfer is performed by phonons. Like photons, phonons have high vibrational velocities that can transfer large amounts of heat.

The main difference between electrons and phonons is considered to be that phonons remain localized along the material, while electrons can move more freely throughout the material.

Both thermocouples used had metal protection to avoid melting during the experiments, and we considered an inertial difference of approximately 30 °C between the actual surface temperature and the temperature recorded by the thermocouple (Figure 2c). On the substrate, thermal shocks were performed by repetitive heating up to 600 °C, and a rust layer was observed after the first rapid heating–cooling cycle (Figure 2b). Twelve heating cycles were performed with an average heating rate of approximately 75 °C/s and an average cooling rate of 60 °C/s. The small variations in the heating peaks were attributed to the solar flux, which, even though it appeared to be clear skies, exhibited small variations that translated into temperature variations. The growth of the Al₂O₃/YSZ microstructure is considered a method of establishing the high-temperature phase of zirconia at room temperature. During the heating and cooling cycles in the metal substrate, the temperature does not exceed 200 °C, with a continuous temperature increase with each cycle (Figure 2c).

During the thermal shock experiments, a small change in the color of the ceramic layer was observed after six heating/cooling cycles. No visible structural changes were observed in or near the heated area (Figure 3a). The SEM image of the ceramic layer grains (Figure 3a image of the layer surface and Figure 3b,c on the layer section) shows pores, as the layers were not completely dense. Generally, the density of Al₂O₃ decreased with the addition of YSZ on the basis that the ZrO₂ particles prevented the diffusion of the alumina matrix particles [28]. After the delamination of the ceramic complex layer on the metal surface, its thickness was evaluated (Figure 3b) by 20 determinations (a 28.96 minimum value, 58.19 maximum value and 43.85 mean value, with a 6.85 standard deviation). The layer structure, in Figure 3c, shows plate grains with thicknesses between 2 and 4 μm, longitudinally oriented with respect to the spray direction of the ceramic material. Micropores and the misorientation of some grains can also be observed on the cross-sectional images. At the macroscale, the layer shows excellent compactness and structural homogeneity.

However, during the experiments, based on a slight difference between the solar flux concentrated at the surface and the area from where the surface temperature was recorded (the thermocouple was fixed in the middle of the sample), at the edge of the sample the temperature was much higher than recorded (Figure 2c), the ceramic layer was exfoliated and the metal substrate melted (Figure 3d). The ceramic layer exhibited macroscopic cracks near the exfoliation zone (Figure 3e), affecting the integrity of the coating layer along a long line. The chemical element distribution in Figure 3f shows a homogeneous ceramic layer near the exfoliation zone and a small heat-affected area in the ceramic layer.

The ceramic layer before (Figure 4a) and after (Figure 4b) thermal shock cycles were analyzed by microscanning using AFM, and the results are shown in Figure 4 with 2D, 3D and line representations over an area of 38.31 μm². Larger particle spacings can be observed before the coating thermal shocks, confirmed on both 2D and 3D images. Roughness determinations: before Sa (arithmetic mean height): 146.75 nm; Sq (root mean square heights): 172.84 nm and after thermal shock: Sa: 148.89 nm; Sq: 188.27 nm. Only a small difference in the thickness of the ceramic layer is observed, which can be associated with the released vapor observed at approximately 350 °C.

3.2. X-ray Diffraction Analyze of the Ceramic Layer

The XRD patterns (Figure 5) show the phase composition and associated PDF4+ database cards of the plasma sprayed (PS) coating and the coatings subjected to thermal shock cycles (1000_5c for 5 cycles at 1000 °C and 1000_10c for 10 cycles at the same temperature).

All investigated samples show a similar phase composition, with tetragonal yttrium zirconium oxide (YZO—Y0.077Z0.923O1.962), corundum (α-Al₂O₃), a mixture of crystalline and amorphous gamma alumina phase (γ-Al₂O₃) and the 100% relative intensity diffraction line of the substrate material (S–Feα). This agrees with other reports [29,30,31] dealing with the thermal sputtering of alumina and alumina composite coatings, with metastable and amorphous phases being associated with the rapid solidification of molten sputtered species [29,32]. The results of the phase composition and the ratio of the main crystalline phases of the layer are summarized in

Table 1. Phase composition and phase ratio of the analyzed coatings.

Sample	YZO		α-Al2O3		γ-Al2O3
	DB Card, PDF4+	wt%	DB Card, PDF4+	wt%	DB Card, PDF4+	wt%
PS	04-018-4055	12.67 ± 0.12	04-013-1687	9.10 ± 0.30	00-010-0425	77.40 ± 1.40
5c	04-018-4055	13.24 ± 1.66	04-013-1687	10.80 ± 0.40	00-010-0425	75.90 ± 1.80
10c	04-018-4055	12.36 ± 1.50	04-013-1687	11.30 ± 0.60	00-010-0425	74.28 ± 1.90

. Quantitative phase analysis was performed based on the WPPF (whole powder pattern fitting) method without considering the substrate contribution. Due to the inhomogeneity of the ceramic layer structure, the weight percentage values represent an approximation.

Qualitative and quantitative phase analysis indicated that no phase transformation or formation of new phases occurred after solar irradiation, the composition of the composite coatings being the same regardless of the number of thermal shock cycles.

The crystallinity of all the coatings, defined as:

$$C\%=\frac{scatterd intensity from crystalline phases}{scatterd intensity from crystalline phases+scatterd intensity from amrphous phase} \times 100$$

was approximately 95% for all the coatings.

The microstructural parameters of the identified polycrystalline phases, calculated by the Rietveld method, are shown in

Table 2. Calculated microstructural parameters of the coating’s main crystalline phases.

Sample	YZO		α-Al2O3		γ-Al2O3
	Lattice Parameter (Å)	Crystallite Size (nm)	Lattice Parameter (Å)	Crystallite Size (nm)	Lattice Parameter (Å)	Crystallite Size (nm)
PS	a = 3.616	91.2 ± 0.7	a = 4.758	68.3 ± 3.9	a = 7.900	17.6 ± 0.1
	c = 5.161		c = 12.990
5c	a = 3.616	75.5 ± 3.0	a = 4.760	86.6 ± 0.9	a = 7.901	25.3 ± 0.6
	c = 5.162		c = 12.991
10c	a = 3.615	56.6 ± 2.2	a = 4.759	73.8 ± 0.9	a = 7.902	13.4 ± 0.2
	c = 5.162		c = 12.996

.

Microstructural Rietveld analysis indicated the nanocrystalline composition of the coatings, with gamma alumina having the smallest crystallite size. From Table 1, a reduction in the size of YZO crystallites can be observed with the number of thermal shock cycles, indicating a fragmentation of the yttrium zirconia oxide block under the influence of thermal loading [33]. On the other hand, for both alumina crystalline phases (α and γ), after five cycles of thermal shock, a slight increase in crystallite size can be observed, probably due to a recrystallization of aluminum oxides in the irradiated areas [34]. Subsequent thermal loading led to the fragmentation of alumina crystallites due to increased thermal stresses. The calculated Rietveld fit parameters, i.e., the March coefficient [35] of the crystal planes [400] corresponding to a gamma alumina phase of approximately 0.6, indicate plate-like oriented alumina crystals. The γ-Al₂O₃ (440) family of crystallographic planes was subsequently used to estimate the residual stress state in plasma-sprayed and thermally shocked coatings. Figure 6 shows the elemental distribution of Al, O and Zr, Y and Hf, with the chemical percentages (wt and at%) determined at selected points. The chemical composition confirms the XRD results and the phase distribution along the layer.

3.3. XRD—Residual Stress Mapping

The elastic coefficient values used in the residual stress calculations were taken from [25] as an approximation of Young’s modulus and Poisson’s ratio for the whole composite layer. The values of the elastic coefficients do not take into account the multiphase composition of the coatings and the elastic anisotropy of the selected [hkl] direction. A value of 75.4 GPa for Young’s modulus (E) and a Poisson’s ratio (ν) of 0.26 were the values used to estimate the residual stress state of the investigated coatings [25]. The relative values presented were used only to assess the residual stress state evolution of ceramic coatings under thermal shock conditions. For this purpose, a number of 33 measuring points were used to construct a 10 mm (length) × 3 mm (width) measuring grid on each sample surface. The coordinate (x,y) = (0,0) represents the center of each sample, with a distance of 1 mm in the x and y directions between the measurement points of each grid. The residual stress measurement parameters are shown in

Table 3. Residual stress scan parameters.

PDF Card	00-010-0425, γ-Al2O3
[hkl]	440
2θ range (°)	104–116
Step size (°)	0.1°
Scan duration (s)	200
Tilt angle range (°)/number of points	0–30/8
2θ without strain (°)	110.306
Scanning direction	x
X range/scan step (mm)	[−5:+5]/1
Y range/scan step (mm)	[−1:+1]/1

.

The resulted residual stress mappings are presented in Figure 7a–c.

The residual stress state of the thermally sputtered coating is due to the high thermal and kinetic energies involved in the process and the different thermophysical and mechanical properties of the substrate and raw material powders and can be explained by the combination of two stress generating mechanisms: the quenching and cooling stresses [36]. The XRD residual stress mapping (Figure 7a) of the PS sample shows a tensile residual stress state with an average value of 117.80 ± 42.18 MPa. Some relaxation zones with low residual stresses can correspond to stress relaxation by the microcracking of the plasma spray coating [36]. With 5 and 10 cycles of thermal shock (Figure 7b), a uniformity of residual stresses can be observed in the investigated area, with an increase in the average value of residual stress up to 140.67 ± 31.76 MPa. Near the area where we performed 10 thermal cycles, where there was an increase in temperature caused by the variation of solar flux (Figure 7c), the tensile stress values increased to an average value of 166.74 ± 82.86, inducing large cracks and a transition to the compressive stress state in the spallation zones, which explains the high value of the calculated standard deviation.

3.4. Mechanical Characterization of Ceramic Layer at Scratch Test

The use of a coating significantly reduces the temperature of metal substrate elements and protects their properties for long-term use cycles. Ceramic coatings and metal substrates have different coefficients of thermal expansion, brittleness and elastic modulus, and defects such as cracking or spalling can occur because of thermal and mechanical loading. It becomes important to study, in addition to the thermal shock behavior of the coating, the mechanical properties for high temperature applications.

Microscratch tests are used to assess the adhesion and cohesive strength of thin coatings and surface treatments. It involves using a sharp stylus to produce a controlled and continuous microscratch on the material’s surface under test, while monitoring the force required to produce the microscratch and the resulting deformation and damage to the surface. Additionally, AE signals can provide information about the load-displacement behavior of the material during the microcracking test, which can be used to identify the occurrence of damage and failure mechanisms.

Figure 8a,b show the scratch test patches at three different forces. In the macroscopic optical image, Figure 8a, the difference between the response of the ceramic coating layer to different stress forces (10, 15 and 19 N) is observed, from a narrower trace for the lowest force to a wider and more impregnated trace in the ceramic layer for 19N force. It is also observed that at 10 N force, the ceramic layer was penetrated much later compared to the other two traces. This shows that the ceramic composite layer behaves better at lower perpendicular stress forces.

There are no additional cracks or exfoliation along the scratch mark, either at the macro or micro level (Figure 8). It can be seen in the SEM image that there is a clean peeling of the ceramic material, there are no cracks in the coating and no defects near the exfoliation area. At the end of the scratch, the substrate is partially damaged by the pulling out of the metallic material. In Figure 9, sequential images of the area, the mapping of the main chemical elements of the substrate and coating, the scratch test and the evolution of the scratch mark on the coating are highlighted. At the beginning, only the plastic deformation of the ceramic layer is observed, without additional cracks among the stressed areas, and after more than 4–5 mm, the ceramic layer breaks and the substrate, represented by the Fe element, is observed. After penetration into the layer, there are alternating areas with and without the ceramic layer. After 6–7 mm of scratching, there is no ceramic layer and the tip rests only on the metal substrate.

The Fx (lateral force recorded as a function of the scratching distance during the test), COF (coefficient of friction) and AE (acoustic emission) evolution on the ceramic layer along the 10 mm scratching distance is shown in Figure 10a. All parameters show variations in three distinct zones, the first mainly on the ceramic layer for the first 5–6 mm, a second zone between 6 and 7.5 mm where the ceramic layer alternates with the substrate and the final one only on the substrate up to 10 mm. The force and friction coefficient are lower on the ceramic layer and increase on the metal substrate. Acoustic emission shows the breakage of the ceramic layer after 6–7 mm, increasing the vibration signal on the metal. The area between 6 and 7.5 mm showed the highest AE variation, based on the alteration of the ceramic with metallic zones. Figure 10b shows the COF variations on the ceramic layer for different scratch forces (10, 15 and 19 N). On the ceramic layer, all variations are similar and the surface response to external stress continues for the lowest force of 10 N and increases considerably on the metal for higher forces (15 and 19 N, respectively).

The differences between the parameters of the ceramic layers before and after the thermal shocks (Figure 10 and

Table 4. Mechanical characteristics of the ceramic layer before and after the thermal shocks.

Material	Fx [N]		AE [Volt]		COF
	Before	After	Before	After	Before	After
Ceramic coating + metallic substrate (10N)	3.52	3.58	−0.001	0.003	0.61	0.59
Ceramic coating + metallic substrate (15N)	6.62	6.49	−0.001	0.007	0.73	0.68
Ceramic coating + metallic substrate (19N)	9.20	9.05	0.001	0.007	0.79	0.74

) are insignificant, which means that the influence of thermal shocks is minimal on the mechanical characteristics of the ceramic layer or the adhesion to the substrate under these conditions.

However, the COF variations before and after the thermal shocks are similar on the ceramic layer. They behave differently on the substrate, which is also the same, but on the sample after the thermal heating cycles, it probably undergoes heat treatment under thermal flow cycles (Figure 2c).

4. Conclusions

• Solar energy can be used to heat different materials with repetitive high rates, exposure time and controlled parameters to simulate a possible thermal shock.
• The composition of the complex coatings remained the same regardless of the number of thermal shock cycles, being a mixture of crystalline yttrium zirconium oxide and α and γ alumina. In all cases, the main residual stress state for all samples was tensile. After five thermal shock cycles, a uniformity of the residual stress state was observed on the investigated surface. Ten thermal shock cycles led to higher residual stresses and resulted in large cracks and the spallation of the coating.
• Even though increasing the temperature to a very high temperature (above 1800 °C) will cause the exfoliation of the ceramic layer, heating to 1000 °C at a high rate and cooling at the same conditions does not change the microstructure of the ceramic layer or the composition of the phases and retains mechanical and adhesion properties similar to the initial state. Al₂O₃-YSZ ceramic layers with a thickness of approximately 45 μm are stable to thermal shocks at 1000 °C and under atmospheric conditions.