Evaluation of Adhesion of Coatings Obtained Through the Thermal Spraying Technique by the Interfacial Indentation Method

Abstract

This work evaluates the adhesion strength of FeCoCrNi, FeCr, and FeCoCr coatings produced by electric-arc thermal spraying on carbon steel, using the interfacial indentation test. Microstructural analysis revealed low porosity and oxide content. The test yielded toughness (K) values of 1.05, 1.10, and 1.17 MPa·m^1/2 for FeCoCrNi, FeCr, and FeCoCr, respectively, with FeCoCr showing the highest resistance to crack propagation. Interface microhardness ranged from 808 to 978 HV, and Young’s modulus from 129 to 183 GPa. Residual compressive stresses up to -240 MPa at the interface favored adhesion, consistent with hardness data. Finite element modeling using ANSYS Workbench 2021R closely matched experimental results, with deviations between 1% and 11%. Compared to the ASTM C633 tensile test, the interfacial indentation method provided equal or superior reliability, simpler execution, and lower cost. Results confirm the influence of alloy composition and microstructure on adhesion and validate interfacial indentation as a robust tool.

Keywords:
Thermal spray; Electric Arc; Interfacial indentation test; Adhesion

1. Introduction

The adhesion between thermal spray coatings and substrates is a critical factor for ensuring the structural integrity and performance of coated components, especially in demanding industrial environments. However, the complex nature of the substrate/coating interface, affected by residual stresses, porosity, and microcracks, makes the reliable assessment of adhesion strength a persistent challenge for engineers and researchers¹-³. Traditional adhesion evaluation methods, such as the tensile test (ASTM C 633), are widely used but present significant limitations, including high costs, operational complexity, and potential result distortions due to the use of auxiliary materials like epoxy⁴-⁶. These drawbacks highlight the urgent need for alternative methods that are simple, cost-effective, and capable of providing reliable and representative results.

Among the available techniques, the interfacial indentation test has emerged as a promising alternative for evaluating the adhesive strength of metallic coatings produced by thermal spraying. This method, which can be applied at macro, micro, and nanoscale levels, enables the measurement of key material properties such as elastic modulus and hardness. In this work, the Vickers macroindentation test was employed to assess the adhesion of coatings, offering a relatively simple and direct approach that avoids the introduction of new materials at the interface and minimizes potential artifacts in the results⁷,⁸.

The quality of the coating is intrinsically linked to the bond between the sprayed particles and the substrate, as failures such as cracking and delamination can compromise the safety and reliability of coated parts⁹. Achieving high adhesion typically requires mechanical interlocking, often enhanced by substrate surface roughness created through blasting. Additionally, residual stresses developed during the deposition process can significantly influence both the adhesion and the microstructure of the coating¹⁰.

In this study, the adhesion strength at the substrate/coating interface was evaluated for three electric arc-sprayed coatings based on FeCoCrNi, FeCr, and FeCoCr alloys. These materials are of particular interest due to their expected resistance to wear and corrosion, making them suitable for applications such as bearings and components exposed to aggressive environments, including pre-salt oil fields.

To complement the experimental analysis, a finite element model was developed using ANSYS Workbench 2021R to simulate the interfacial indentation test and predict the propagation of cracks at the coating/substrate interface. The simulation considered a semicircular initial crack, in line with experimental observations, and allowed for a detailed comparison between numerical and experimental results regarding adhesion strength.

This combined experimental and numerical approach aims to validate the interfacial indentation test as a reliable and practical method for characterizing the adhesion of thermally sprayed coatings, addressing the limitations of traditional techniques and contributing to the advancement of coating technology.

2. Material and Methods

2.1. Sample preparation

The coatings were prepared by the electric arc thermal spraying process on SAE 1020 carbon steel sheets. All process parameters, including the spraying distance, were kept constant throughout the coating process, see Table 1¹¹, to standardize the test and analyze the adhesion strength, independently of the variation of the thermal spraying process parameters¹².

Table 1
Parameters used during thermal spraying.

Sprinkler parameters	Values used
Voltage	40 V
Chain	100 A
Projection distance	~ 100 mm
Number of passes	3 a 6
Deposition rate	3.24 kg/h
Wire diameter	2.6 mm
Atomizing gas	argon

The chemical compositions of the substrate and wires used are presented in Tables 2 and 3, respectively. Three wire combinations were chosen, as shown in Table 4. The intermediate alloy used in all three combinations was 95Ni5Al. Each wire combination was identified, as illustrated in Table 4. Condition 1 will hereinafter be referred to as FeCoCrNi; condition 2, FeCr; and condition 3, FeCoCr, considering the main alloying elements present in the coatings.

Table 2
Chemical composition of substrate SAE 1020 (% wt).

Fe	Mn	C	Si	P	S
Bal.	0.30 – 0.60	0.17 – 0.23	≤ 0.40	≤ 0.040	≤ 0.050

Table 3
Chemical composition of wires (% wt).

Wire	Fe	Co	Cr	Ni	B	Mn	W	Mo	C	Si
A	66.1		27		3.5	1.8				1.6
B	65.7		25.7	2.9		1.9		0.8	1.6	1.4
C	3.6	58.4	28.8	1.9		0.9	4.9	0.02	1.1	0.3
D	86 to 88		12 to 14	1		1			0.3	0.08

Table 4
Experimental combination of wires and intermediate bond.

Conditions (Main elements)	Combination of wires	Intermediate bond
1 (FeCoCrNi)	B + C
2 (FeCr)	D	95Ni-5Al
3 (FeCoCr)	A + C

2.2. Adhesion test by interfacial indentation method

The specimens used in the test and the details observed under the microscope, Figure 1, show the indentations at the substrate/coating interface. The microstructure of the as-received powders and the sprayed deposits was analyzed using scanning electron microscopy (SEM) and optical microscopy (OM) techniques.

Figure 1
Test specimens prepared for the interfacial indentation test, showing the arrangement of indentations at the substrate/coating interface.

A sequence of indentations on the sample surface is shown in Figure 2. Three indentations at the substrate/coating interface and one in the substrate area for hardness evaluation.

Figure 2
Optical micrograph of the sample surface, illustrating the sequence of Vickers indentations at the substrate/coating interface and in the substrate region for hardness evaluation.

The adhesion test using the interfacial indentation method was performed according to the protocol proposed by Marot¹³,¹⁴, following the analytical model by Demarecaux et al.¹⁵. The principle of the test is to perform a series of “Vickers” indentations on a section of the sample, perpendicular to the cross section of the substrate/coating system, Figure 3. This method, originally proposed and validated by Demarecaux et al.¹⁵, allows the determination of the adhesive properties of thermal spray coatings by analyzing the crack propagation at the interface after indentation.

Figure 3
Schematic diagram of the interfacial indentation test, highlighting the alignment of the Vickers indenter at the substrate/coating interface¹⁶.

In this test, stable cracks are mechanically introduced at the substrate/coating interface using a conventional Vickers indenter¹⁶. A diagonal line of the Vickers indenter is first aligned along the substrate/coating interface, Figure 2, and an increasing indentation load is applied. Upon penetration of the indenter into the substrate/coating system, a crack with a semi-circular shape is generated and located in the plane of the interface¹⁷.

In the experimental test, after calibrating the machine, the hardness of the coating and substrate of the samples was measured. Five measurements were made for each sample.

The actual procedure for the interfacial indentation test was then started. The diagonal of the indenter was positioned in line with the substrate/coating interface. Initially, a load of 30 N was applied, followed by 40 N, 50 N and 60 N, until a crack appeared. Once the appearance of a crack at the interface was observed, the load was fixed and the experiment was repeated 4 more times, totaling 5 valid points. When the alignment of the diagonal of the indenter with the interface between the substrate/coating was not aligned or the crack propagated outside the interface, the point was invalidated. Figure 4a shows an invalidated point where the indenter impression is completely in the substrate area and in Figure 4b, the correct position for the interfacial indentation test.

Figure 4
(a) Example of an invalid indentation point due to total misalignment, with the impression entirely in the substrate; (b) Valid indentation point correctly positioned at the substrate/coating interface.

The only parameter required for the test, according to Marot's model, is the applied load, since all other values are results of the experiment, Table 5.

Table 5
Parameters used during the interfacial indentation test.

	FeCoCrNi	FeCr	FeCoCr
P(N)	392.26	196.13	392.26

Once the interfacial indentation procedure was completed, the samples were taken to the MO and SEM for evaluation and analysis.

In Figure 5, the alignment between the indenter diagonal and the substrate/coating interface is highlighted, one of the most relevant conditions of the interfacial indentation test.

Figure 5
Scanning electron microscopy image showing the alignment of the Vickers indenter diagonal with the substrate/coating interface for the FeCr coating.

According to Demarecaux, Chicot and Lesage¹⁸,¹⁹, there is a relationship between the applied critical load Pc, the crack length ac, generated by the load at the interface, which represents the attractive resistance between substrate and coating K, expressed by the Equation 1:

$K=\mathrm{0,015}\frac{P_c}{\sqrt[2]{a_c^3}}\sqrt{{\left(\frac{E}{H}\right)}_I}$ (1)

where,

$\sqrt{{\left(\frac{E}{H}\right)}_I}=\frac{{\left(\sqrt{\left(\frac{E}{H}\right)}\right)}_S}{1+\sqrt{\frac{H_S}{H_R}}}+\frac{{\left(\sqrt{\left(\frac{E}{H}\right)}\right)}_R}{1+\sqrt{\frac{H_R}{H_S}}}$ (2)

The indices I, S and R mean, respectively: interface, substrate and coating.

Equation 2 characterizes the global behavior of the substrate/coating system, where E is the Young's Modulus, H is the hardness and the indices I, S and R mean, respectively: interface, substrate and coating. This relationship determines the mechanical properties of the interface, called the apparent toughness of the interface. In the analytical model, the term (E/H)^1/2 incorporates the elastic modulus (E) and hardness (H) of the coating/substrate system. In the present study, the hardness values were obtained from the Vickers indentation tests, while the elastic modulus was independently determined by instrumented nanoindentation.

The critical indentation load P_c characterizes the cracking capacity of the interface. If the apparent toughness of the interface represents well the crack resistance of the interface, then K, necessarily, must vary in the same way as P_c.

The apparent toughness of the interface can then be seen as a criterion that reflects the adhesive properties of a coating on its substrate.

2.3. Nanoindentation test

Instrumented nanoindentation tests were performed as a complementary technique to the interfacial indentation test in order to determine the elastic modulus required for the analytical evaluation of interfacial toughness. The experimental procedure followed the principles established in ISO 14577, ISO 19278 and ASTM E2546, ensuring methodological transparency, reproducibility, and traceability of the measured mechanical properties. Instrumented nanoindentation is a well-established technique for determining the elastic modulus of coatings and substrates, provided that the indenter geometry, loading protocol, and data analysis method are properly defined²⁰.

The experiments were carried out using a HIT 300 nanoindenter (Anton Paar) equipped with a Berkovich diamond indenter, whose geometry was explicitly considered in the data analysis. The Berkovich indenter was selected due to its widespread use in metallic materials and its compatibility with the Oliver and Pharr analytical method.

For all measurements, the maximum applied load was fixed at 250 mN, and the Poisson’s ratio of the tested materials (νₛ) was assumed to be 0.30, a value commonly adopted for carbon steel substrates and Fe-based thermally sprayed coatings. The loading protocol consisted of three stages: loading up to the maximum force and subsequent controlled unloading. All tests were conducted under identical loading conditions to ensure comparability among the studied samples.

The elastic modulus and hardness were determined using the Oliver and Pharr method, based on the initial slope of the unloading segment of the load–displacement curve. The reduced elastic modulus (E*) was calculated from the contact stiffness, defined as the slope of the unloading curve at maximum load. The elastic modulus of the material (E) was then obtained by correcting the reduced modulus for the elastic contribution of the diamond indenter, in accordance with the Oliver and Pharr formulation. The projected contact area was calculated assuming an ideal Berkovich geometry, using the standard area function A = 24.5h²_c​, where h_c​ is the contact depth. Hardness was determined as the ratio between the maximum applied load and the projected contact area.

To ensure statistical reliability, multiple independent measurements were performed for each experimental condition. For Sample 1C, nine nanoindentation measurements were carried out at different locations. For Sample 2B, ten measurements were performed, equally divided between the coating and the substrate, allowing a direct comparison of their mechanical responses. For Sample 3D, fourteen nanoindentation measurements were conducted at different surface positions. For all conditions, mean values and standard deviations were calculated and used in the subsequent analyses.

The nanoindentation results provided the elastic modulus values used as input parameters for the analytical calculation of apparent interfacial toughness in the interfacial indentation test. Hardness values used in the analytical model were obtained from Vickers indentation measurements (Section 2.2), whereas nanoindentation hardness values were used only for comparison and methodological cross-validation.

2.4. Stress analysis by x-ray diffraction method

Residual stresses were evaluated by X-ray diffraction, at the Stress Analysis Laboratory (LAT) of Fluminense Federal University (UFF) using an XStress3000 stress analyzer produced by StressTech. The sin²ψ method was used with CrKα radiation (λCrKα = 2.29092 Å), diffracting the (211) plane of ferrite. The XTronic V1-0 Standard software was used to calculate the stress. The measurement parameters used are detailed in Table 6.

Table 6
Residual stress measurement parameters.

Parameters	Values
Collimator diameter (mm)	2
Diffraction angle 2θ (º)	156.41
Diffraction plane (hkl)	(211)
Radiation	CrKα
Wavelength λCrKα (Å)	2.291
X-ray tube current (mA)	6.7
X-ray tube voltage (kV)	30
Inclination angles ψ (º)	0, 21, 30, 38, 45
Exposure time (s)	15

2.5. Mathematical model

For the construction of the mathematical model, developed in ANSYS, the behavior of the materials: substrate and coating, was assumed to be perfectly elastic-plastic due to the lack of information on the true curve of each material. The indenter was assumed to be a rigid material.

For the discretization, the SOLID186 type element was used, due to its adequate characteristics for application in the analysis. The solid element is a continuous (C), three-dimensional (3D), with eight nodes (8) and with reduced integration (R) and presents only three degrees of translational freedom in each node.

To allow for different mesh sizes, the surfaces were subdivided into several semi-areas (20). In the ANSYS Workbench interface, this submodeling feature allows the construction of a partial model, corresponding to a specific region of a larger geometry - global, thus allowing a more detailed analysis in the area of interest of the study. In almost the entire sample area, an orthogonal mesh was used and in the area object of this study, that is, in the substrate/coating interface, a tetrahedral mesh was used, Figure 6.

Figure 6
Finite element mesh used in the ANSYS simulation, illustrating the indenter, coating, and substrate regions with different mesh densities.

The boundary conditions in the numerical model were considered fixed at the lower base and a displacement was applied to the indenter, Figure 7. A load was applied to the center of the indenter.

Figure 7
Boundary conditions applied in the finite element model, with the indenter displacement and fixed lower base.

A third test was performed, “Stress-Strain”, to calculate the yield stress. The equipment used was the Anton Paar NHT³ Nanoindenter.

The results obtained in this experiment confirmed the results obtained by the mathematical model developed in finite elements, corroborating the reliability of the interfacial indentation test, providing a simple and inexpensive alternative to replace the traditional pull-off test.

3. Results and Discussions

3.1. Interfacial indentation test

The main objective of the test for evaluating the adhesion strength between the substrate/coating, thermally sprayed, by the interfacial indentation test, is to cause the propagation of cracks at the substrate/coating interface. The length of the cracks generated was measured, considering the average of the diagonals of the indenter as a pre-crack, see Figure 8.

Figure 8
Optical micrograph of the FeCoCr coating, showing the dimensions of the Vickers indentation diagonals and the generated interfacial cracks.

Table 7 shows the microhardness obtained on the substrate, coating and at the interface. The results were in agreement with those reported in the literature²¹-²³.

Table 7
Comparison between the microhardness.

SAMPLES	HVs	HVr	HVi
FeCoCrNi	150.33	260.06	977.63
FeCr	177.00	270.00	911.28
FeCoCr	148.06	581.05	808.41

One of the modes of sample fracture is adhesion failure. The most critical failure is the fracture at the substrate/coating interface, the result of a poor thermal spraying process, as shown in Figure 9, 10 and 11.

Figure 9
Optical micrograph of a fracture at the substrate/coating interface for the FeCr coating, indicating adhesion failure.

Figure 10
Representative images of the nanoindentation and interfacial indentation tests performed on the coatings.

Figure 11
Optical micrographs of the interfacial indentation test, highlighting crack propagation at the substrate/coating interface.

Table 8 shows the results of the interfacial indentation test. The applied force P, the crack length a and the material properties at the interface, the interfacial toughness (E/H)_I^1/2.

Table 8
Results of apparent interface toughness calculation.

SAMPLES	FeCoCrNi	FeCr	FeCoCr
P(N)	392.26	196.13	392.26
ac(mm)	1.47	0.82	1.21
Hs (MPa)a	1,474.30 ± 75	1,735.80 ± 85	1,509.50 ± 80
Hr (MPa)b	2,550.40 ± 130	2,647.90 ± 140	5,614.10 ± 260
Es (GPa)c	183.70 ± 7.5	172.89 ± 6.9	158.24 ± 6.4
Er (GPa)d	156.56 ± 6.8	160.99 ± 7.2	129.18 ± 5.9
(E/H) I1/2	0.32	0.28	0.27
K (MPa.m1/2)	1.05	1.10	1.17

• Note:
• ^a
  hardness of the substrate obtained by Vickers indentation (MPa);
• ^b
  hardness of the coating obtained by Vickers indentation (MPa);
• ^c
  elastic modulus of the substrate obtained by instrumented indentation (GPa); e
• ^d
  elastic modulus of the coating obtained by instrumented indentation (GPa).

For all samples, elastic modulus values correspond to the mean of multiple independent nanoindentation measurements, with standard deviations calculated from 9 to 14 indents depending on the experimental condition. Hardness values used in the analytical evaluation were obtained by Vickers indentation, while nanoindentation hardness values were used solely as a consistency and cross-validation check.

The hardness values measured in the indentation test and in the nanoindentation test indicate that the interfacial indentation test is very promising. Their values are in the same order of magnitude, as shown in the graph below, Figure 12.

Figure 12
Comparison between hardness values obtained by the interfacial indentation test and nanoindentation for the studied coatings.

In addition to the hardness and Young's Modulus values obtained through the nanoindentation test, the yield stress was also obtained, Table 9 which were necessary for the numerical modeling of the experiment.

Table 9
Stress Strain Analysis.

Sample	Yield Stress [MPa]
FeCoCrNi	1,264.05
FeCr	1,860.54
FeCoCr	2,097.70

Indirect evidence indicated that adhesion is limited both by insufficient particle velocities and thermal energies for coarse particles. The limitation for fine particles is probably caused by the high oxide content. These characteristics of the coatings obtained by thermal spraying (presence of defects such as oxides and lamellar microstructure) directly affect the accuracy of the results obtained for adhesion in the present work²⁴. The broad range of industrial applications and the technological importance of thermal spray coatings have been extensively reviewed by Gaur and Kamari²⁵.

Sobolev et al.⁷ performed an analysis of various effects that affect coating adhesion. The oxide content and the magnitude of residual stresses proved to be detrimental to adhesion. Furthermore, it was pointed out that the adhesion increases with the higher velocity of the particles and with the increase of the thermal energy of the droplets. Furthermore, the weak inter-splat interactions found in thick coatings are related to the low particle velocity. So, poor adhesion of the coating occurs when this condition is present in the thermal spraying process.

According to Berndt and Lin²⁶, adherence can be expressed in several ways. For example, 'basic adhesion' represents interfacial bonding and is the sum of all intermolecular or interatomic interactions. Actual adherence testing, as performed in this article, is commonly referred to as 'practical adherence'. In this case, it reflects basic adhesion and the factors that resist the work required to detach a film or coating from the substrate. In the context of practical adherence, the microhardness values measured for the coatings and interfaces are in line with the findings of Berndt et al., who also reported that the presence of oxides and a lamellar microstructure can influence the mechanical response. The low porosity and small oxide content observed in the present work likely contributed to the favorable hardness and adhesion results, as similarly discussed by Sobolev et al.⁷.

3.2. Residual stress at interface

Residual stresses play a crucial role in the performance and adhesion of thermally sprayed coatings. In this study, residual stress measurements were performed on three samples, evaluating the coating (R), interface (I), and substrate (S) regions. The results revealed compressive residual stresses ranging from -160 to -240 MPa (Table 10), with the highest compressive values consistently observed at the interface. These findings are consistent with the literature, which indicates that the interface region is critical for adhesion and that compressive residual stresses can increase the mechanical integrity of the coating-substrate system.

Table 10
Residual Stresses Measured in Samples after Thermal Spraying.

Sample	Coating (R) [MPa]	Interface (I) [MPa]	Substrate (S) [MPa]
1	—	-210 ± 10	-210 ± 10
2	-160 ± 2	-240 ± 3	-180 ± 8
3	—	-230 ± 4	-190 ± 4

The measured compressive residual stresses at the interface reached up to -240 MPa (Figure 13), a value consistent with ranges reported in the literature, typically between -150 and -300 MPa²⁷,²⁸, which emphasize the importance of maintaining moderate compressive stresses to enhance adhesion without causing premature failure. These variations may be due to differences in alloy composition, coating thickness, and processing parameters. However, as discussed by Araujo et al.²⁹, excessive tensile or compressive stresses at the interface can be detrimental, potentially leading to microcracks or other adverse effects. While such compressive stresses may indicate good initial adhesion, it is advisable to monitor for possible negative consequences associated with excessive stress levels. The experimental data support the theoretical model proposed by Araujo et al.²⁹, where the distribution and intensity of residual stresses depend on processing parameters and coating thickness. The observed trend of higher compressive stresses at the interface reinforces the hypothesis that optimizing the residual stress profile is essential to maximize adhesion and mechanical performance of coatings.

Figure 13
Residual stress values at the interface obtained in this work.

These results highlight the importance of controlling the residual stress profile during the thermal spraying process to obtain coatings with better adhesion and performance. Future studies are recommended to correlate these stress profiles with direct adhesion measurements and to explore the influence of different process parameters on the residual stress distribution.

3.3. Finite element simulation

In this study, the finite element method (FEM) was employed as a complementary tool to the experimental approach, aiming to deepen the understanding of crack propagation mechanisms and to validate the experimental findings. FEM simulations were performed to model crack propagation at the substrate/coating interface during the indentation test, enabling the calculation of the energy release rate and, consequently, the determination of interfacial toughness. The FEM results showed excellent correlation with the experimental data (Figure 14), with percentage deviations ranging from 1% to 11% under the different conditions tested. The finite element model was validated by the experimental results obtained in the interfacial indentation tests. This validation not only reinforces the reliability of the experimental method but also provides a predictive tool for optimizing thermal spray process parameters and understanding the mechanisms underlying coating adhesion. The close agreement between numerical predictions and experimental data confirms the reliability of the FEM approach for simulating crack propagation and evaluating interfacial toughness in thermally sprayed coatings.

Figure 14
Correlation between analytical and finite element models for interfacial crack propagation and toughness evaluation.

The evaluation of the adhesion and toughness of metallic coatings by interfacial indentation is a complex and constantly evolving field of research. The presentation of exact numerical results is challenging due to the large number of variables that influence the results. However, trend analysis and comparison of different studies can provide valuable insights into the factors that influence the adhesion and toughness of these coatings.

Chicot et al.²⁹ presented a similar analytical model using a coating composed of a Cr3C2 chromium carbide powder with 7% of a NiCr alloy, sprayed by the high-speed flux-dispersion (HVOF) process. To evaluate the adhesion between the substrate and the coating, an interfacial indentation test was performed, arriving at the following results for the different substrates used, Table 11.

Table 11
Results of apparent interface toughness calculation¹.

	Ft25	FGS 700-2	XC 10
K (MPa.m1/2)	4.8	5.5	9.8

3.4. Comparative discussion and methodological contributions

The interfacial toughness values obtained in this study (1.05–1.17 MPa·m¹/²) are consistent with those reported by Chicot et al.²⁹ and Liu³⁰, who found similar ranges for thermally sprayed coatings using the interfacial indentation method. This agreement reinforces the robustness of the technique and suggests that the adhesion performance observed here is representative of high-quality coatings produced by electric arc spraying.

Other researchers, using related methods, found results in the same order of magnitude as the results obtained by this work, increasing the reliability of the study of the evaluation of the adhesion of coatings obtained through the thermal spraying technique by the interfacial indentation method²⁸.

Liu's³⁰ analysis, using the same substrates and coatings as Chicot and Lesage³¹, obtained results very close to those obtained by Chicot and Lesage³¹, Table 12.

Table 12
Results of apparent interface toughness calculation².

	Ft25	FGS 700-2	XC 10
K (MPa.m1/2)	3.8	4.4	9.4

Some differences in adhesion and toughness values compared to other studies may be attributed to variations in alloy composition, spraying parameters, and substrate preparation. For instance, the use of FeCoCr alloys in this work, which are less common in the literature, may explain the slightly higher toughness observed for certain samples.

Considering the diversity of thermal spraying methods employed in previous studies, variations in adhesion results were expected. However, this study demonstrates that adhesion values, evaluated using the interfacial indentation method, consistently fall within the same order of magnitude across different methods. This finding strongly suggests the robustness and reliability of this evaluation technique.

The use of new combinations of alloying elements in the coatings, as well as the use of the electric arc thermal spraying process, expand the use of this simple and reliable method, as shown by the results obtained in this study. Consequently, the analytical result found confirmed the result obtained from an unprecedented finite element analysis, since the coating and the spraying process are different from previous studies.

One of the most important contributions of this article is the application of a simple technique such as microhardness measurement to obtain mechanical properties with reliability when compared, for example, with nanoindentation, as explained by Liu et al.³¹.

4. Conclusions

The main conclusions of this study are presented below:

1. Coating Quality: The coatings produced by electric arc thermal spraying exhibited good uniformity, low porosity, and small oxide content, which favorably contributed to the reliability of the interfacial indentation test results.

2. Adhesion and interfacial crack resistance: The interfacial indentation results showed apparent interfacial toughness values of 1.05–1.17 MPa·m¹ᐟ², with the FeCoCr coating exhibiting the highest calculated toughness (K = 1.17 MPa·m¹ᐟ²). This indicates greater resistance to crack propagation at the interface.

3. Residual Stresses: Residual stress measurements indicated compressive values up to -240 MPa at the interface, which favor coating adhesion. A strong correlation was observed between residual stress and microhardness results, reinforcing the robustness of the experimental findings.

4. Influence of Alloy Composition: The use of unusual alloy combinations, including cobalt-containing alloys, broadened the scope of the study. Although cobalt alloys are more expensive, they may be suitable for aggressive industrial environments due to their superior performance.

5. Experimental Validation of FEM: The experimental results validated the finite element model developed for this study, with deviations between 1% and 11%, confirming its predictive capability for the tested materials and process parameters.

6. Methodological Advantages: The interfacial indentation method proved to be a simple, reliable, and thickness-independent alternative for adhesion evaluation, offering advantages over the traditional ASTM C 633 tensile test in terms of cost, simplicity, and result consistency.

7. Limitations: This study was limited to three specific alloy combinations and a single thermal spraying technique. The influence of other process parameters and substrate types was not evaluated.

8. The results suggest that the evaluation of the adhesion strength between substrate and coating by the interfacial indentation method is simple and reliable, as it is independent of the coating thickness. However, recent reviews, such as that by Xiaozhen et al.³², emphasize that the interpretation of indentation test results in coatings depends on several experimental and methodological factors, highlighting the importance of standardization and critical analysis to ensure comparability between different studies.

9. Future Perspectives: Further research should focus on standardizing the interfacial indentation methodology, exploring its applicability to other coating systems and process conditions, and investigating the long-term performance of coatings under different service environments.

5. Acknowledgment

The authors are grateful for the financial support of the Brazilian agencies FAPERJ, CNPq and CAPES.

• Data Availability
  The dataset that supports the results of this study is not publicly available and is therefore not applicable to SciELO Data.

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Edited by

• Associate Editor:
  Ana Sofia de Oliveira.

• Editor-in-Chief:
  Luiz Antonio Pessan.

Data availability

The dataset that supports the results of this study is not publicly available and is therefore not applicable to SciELO Data.

Publication Dates

• Publication in this collection
  13 Mar 2026
• Date of issue
  2026

History

• Received
  03 Mar 2025
• Reviewed
  17 Jan 2026
• Accepted
  28 Jan 2026