Influence of Bond Coat Roughness on Adhesion of Thermal Barrier Coatings Deposited by the Electron Beam–Physical Vapour Deposition Process

Abstract

Thermal barrier coatings (TBCs) are effective protective and insulative coatings on hot section components of turbine engines. The quality and subsequent performance of the TBCs are strongly dependent on the adhesion between the coating and the metal substrate. The adhesion strength of TBCs varies depending on the substrate materials and coating, the coating technique used, the coating application parameters, the substrate surface treatments, and environmental conditions. Therefore, the roughness of the substrate surface has a significant effect on the performance of the TBC system. In this work, the roughness and microstructure of the 7YSZ (7 wt.% yttria-stabilised zirconia) top coat under different bond coat roughness treatments were studied. The purpose of this paper was to investigate the influence of the roughness of the bond coat on the adhesion of 7YSZ TBCs prepared by the electron beam–physical vapour deposition (EB-PVD) process. The VPA (vapour phase aluminium) bond coat was deposited on Inconel 718 nickel superalloy substrate using the above-the-pack technique. The ceramic top coat was applied to the bond coat using the EB-PVD process. The dependence between the TBC coating roughness and the bond coat roughness was determined. Adhesion strength measurements were performed according to the ASTM C 633 standard test method. The highest adhesion value observed in the tensile adhesion tests was 105 MPa. However, it was not determined whether the surface roughness of the bond coat affects the adhesion of the 7YSZ top coat.

1. Introduction

Thermal barrier coatings (TBCs) have been used for the protection of turbine engine hot section components for several decades [1]. The development of TBCs has helped improve the durability and high-temperature performance of turbine blades, thus improving the integrity of the system and increasing the efficiency of the engine [2].

Thermal barrier coatings typically consist of different layers of metallic bond coat and ceramic oxide top coat on a metallic substrate [3]. The top coat typically consists of YSZ (yttria-stabilised zirconia), composed of zirconia (ZrO₂) partially stabilised with 6–8% by weight yttria (Y₂O₃). Although the YSZ layer has low thermal conductivity and provides thermal insulation to the components, the metallic bond coat improves the adhesion of the top coat and also provides protection against oxidation and corrosion of the metallic substrate [4]. Because the TBC is oxygen-transparent, a thermally grown oxide (TGO) layer develops between the bond coat and the ceramic top coat during service. The TGO provides protection to the underlying component against high-temperature oxidation and corrosion, but as it thickens, it is also the main source of strain that ultimately drives the spallation of the ceramic top coat [5]. The TGO layer is the main location of failure of TBC systems [6] caused by thermal cyclic stresses generated during service due to the coefficient of thermal expansion mismatch at higher temperatures (1000–1200 °C) [7].

Electron beam–physical vapour deposition (EB-PVD) columnar coatings are the most widely used TBCs. EB-PVD involves the vapourisation of the coating material using an electron gun. The coatings deposited by EB-PVD have been favoured because their unique microstructure offers the advantage of good bonding between the coating and the substrate, low roughness of the coating leading to aerodynamically smooth surfaces, good erosion resistance [8], and, above all, superior tolerance to mechanical strain and thermal shock during thermal cycling at the high temperatures at which gas turbines operate [9].

In recent years, advances in materials science have led to the development of novel TBC materials that offer improved performance under extreme conditions. One significant breakthrough is the introduction of pyrochlore-structured materials as alternatives to YSZ [10]. These materials have been shown to possess higher thermal stability and lower thermal conductivity, which could potentially extend the life of turbine components at higher temperatures. Additionally, the use of multilayered TBC systems has gained attention [11]. These systems involve the application of multiple layers of different materials to achieve a gradient of properties that can more effectively manage thermal and mechanical stresses. High-Entropy Alloys (HEAs) have also been explored as potential bond coat materials in TBCs [12]. These alloys are composed of multiple principal elements that provide a unique balance of properties that can improve the overall performance of TBCs. HEAs are known for their exceptional mechanical strength, corrosion resistance, and stability at high temperatures, making them ideal candidates for challenging environments such as those found in gas turbines [13]. Therefore, it is extremely important to conduct research aimed at assessing the impact of the bond coat surface quality on the properties of columnar grain growth, not only in standard materials such as YSZ but also in new materials that will play a larger role in future aerospace applications. Conducting research on standard materials may prove crucial in preparing R&D efforts related to the implementation of EB-PVD processes on already quality-tested layers that ensure high adhesion of top coats made from new intermetallic or ceramic materials.

The lifetime of a TBC system is dependent not only on microstructure and chemistry but also on the interface topology between the bond coat and the ceramic top coat, as it influences the growth dynamic [14], locations of high stress concentration locations [15], interfacial stress distribution, and crack propagation behaviour [16], which determines the TBC degradation mechanism and the overall performance of the thermal barrier coatings as a consequence [17]. Therefore, the surface roughness and the influence of the bond coat on TBC lifetime are of special interest [18]. Recently, it has been proven that the nanosecond pulsed laser texturing technique used to manufacture micro-hole texture on the surface of the bond coat leads to the design of high-durability TBCs [16]. Surface roughness also plays a key role in cyclic oxidation in the presence of CMAS (calcium-magnesium-alumino-silicate) [19]. The wettability and spreading dynamics of CMAS on different materials and coatings depend on their chemistry and surface roughness [20]. The pores and gaps between the columnar structure provide a convenient corrosion channel for molten CMAS [21]. However, the results demonstrate that the wettability and the actual contact area between the coatings and CMAS can be reduced by mechanical sanding and decreasing surface roughness, leading to delayed CMAS [22].

Furthermore, the roughness of the substrate influences the adhesion of the coating, an important parameter to consider for the use of thermal spray coatings during thermomechanical loading conditions [23]. The main mechanisms of adhesion can be described as mechanical locking, physical, diffusive, and chemical [24]. The adhesion strength of TBCs varies depending on the substrate materials and coating, the coating technique used, the coating application parameters, the substrate surface treatments [25], environmental conditions, and subsequent heat treatment [26]. In TBCs, the chemical adhesion mechanism of YSZ top coat to alumina bond coat is based on the formation of solid solutions between Al₂O₃ and ZrO₂ [6]. Adhesion strength is an important parameter that characterises the resistance of the ceramic top coat to spallation [27]; therefore, adhesion testing is one of the crucial methods available to depict the durability of TBCs [28].

Unfortunately, little research has been conducted on the relationship between the surface roughness of the bond coat and the adhesion of 7YSZ EB-PVD TBCs. In this paper, 7YSZ EB-PVD TBCs were deposited on VPA bond coats with four roughness treatments. The roughness and microstructure of the 7YSZ top coat under different bond coat roughness treatments were studied. The purpose of this paper was to investigate the influence of the roughness of the bond coat on the adhesion of 7YSZ TBCs prepared by EB-PVD.

2. Materials and Methods

Inconel 718 nickel superalloy was selected as the substrate material in the form of cylindrical samples (ф25.4 mm × 38.1 mm long) according to the ASTM-C633 standard requirements [29]. Before depositing the bond coats, samples were grit blasted with corundum at a 45° incidence angle with a blasting distance of 100 mm and degreased with isopropanol. The VPA (vapour-phase aluminium) bond coat was deposited on the substrate using the above-the-pack technique with an average thickness of 60 μm. The surface of the bond coats was prepared by rough grinding (320# mesh silicon carbide sandpaper), grinding (SiC paper 500#), or polishing (diamond suspensions with a particle size of 3 μm) in order to differentiate the surface roughness. In addition, as-coated samples without any treatment were used for comparison. Subsequently, the bond coats were activated with shot pinning (400–600 μm glass beads) and degreased with isopropanol. To address potential inconsistencies, all samples were subjected to a heat treatment at 1050 °C for 30 min before the deposition process. This step was designed to ensure complete stress relaxation and to eliminate any residual stresses induced by the different polishing materials. The 7% yttria-stabilised zirconia (7YSZ) ceramic top coat was deposited on each bond coat group by the EB-PVD process (Smart Coater, ALD Vacuum Technologies GmbH) with the specific parameters shown in

Table 1. EB-PVD process trial parameters.

Trial Number	Samples Coated	Emission Current [A]	Pressure [mbar]	Temperature [°C]	Feed Rate [mm/min]	Duration [s]
01	I, II, III	3.02	0.00925	985.8	1.5	900
02	IV, V, VI	3.02	0.00923	988.9	1.5	900
03	VII, VIII, IX	3.01	0.00920	992.6	1.5	900
04	X, XI, XII	3.02	0.00925	996.5	1.5	900

. The SMART Coater features a single electron beam gun with a maximum output of 160 kW, capable of evaporating from two crucibles. These crucibles are arranged perpendicular to the sample rotation axis and aligned with the main telescope sting. During the run, the crucibles can be moved parallel to the main sting axis while evaporation continues to form the top coat. For this study, no movements of the samples were applied. Samples before and after the EB-PVD process can be seen in Figure 1. The EB-PVD coating equipment used in the current study is shown in Figure 2. The process parameters in each trial were as similar as possible to obtain coatings with properties that were closely identical. All top coat depositions in this study were performed at a substrate temperature of approximately 990 °C using single-source evaporation. The temperature range observed during the deposition process was within the optimal range for the deposition of the ceramic coating, specifically between 950 °C and 1000 °C. This temperature range is well known to provide excellent conditions for depositing ceramic coatings. In each EB-PVD process, the sample holder was mounted in a middle rake position, centred above the evaporation pool (Figure 1b) to ensure the most uniform thickness possible. Ingots from the same manufacturer (Phoenix Coating Resources Inc, now a subsidiary of Saint-Gobain Coating Solutions, Worcester, MA, USA) with diameters of 62.5 to 62.9 mm were used. The average thickness of the prepared 7YSZ coatings was approximately 175 μm.

A surface roughness measuring system (Wave System, Hommelwerke, Villingen-Schwenningen, Germany) was used to measure the surface of the bond coat after different treatments and the corresponding surface of the deposited 7YSZ coating for each sample. The surface roughness values quoted are an average of 10 measurements for each coating. A scanning electron microscope (SEM, Phenom XL, Thermo Scientific, Norristown, PA, USA) was used to observe the microstructure of the 7YSZ coatings with different surface roughness bond coats. The adhesion of the TBCs was measured according to the ASTM-C633 standard method. The coated samples were glued to similar uncoated, grit-blasted counterparts. The adhesive used in the adhesion test was the HTK Ultra Bond 100 (HTK Hamburg GmbH, Hamburg, Germany) cured for 1.5 h at 180 °C with the contact pressure of 70 N/cm² used for bonding. The tensile adhesion test of the glued sets was carried out on a universal testing machine (Instron 8801, Instron, Norwood, MA, USA), as shown in Figure 3.

3. Results and Discussion

3.1. Roughness Analysis

Table 2. Average surface roughness of bond coats and corresponding top coats.

Sample	Bond Coat Preparation	Bond Coat Roughness Ra [µm]	Top Coat Roughness Ra [µm]
I	As-coated	1.124	1.100
V	As-coated	1.057	0.920
IX	As-coated	0.945	0.828
II	Rough grinding	0.545	0.582
VI	Rough grinding	0.521	0.515
X	Rough grinding	0.529	0.541
III	Grinding	0.236	0.562
VII	Grinding	0.286	0.460
XI	Grinding	0.276	0.398
IV	Polishing	0.018	0.405
VIII	Polishing	0.023	0.447
XII	Polishing	0.020	0.390

presents the average surface roughness of the as-coated, grinded, and polished bond coats, as well as the corresponding roughness of the 7YSZ top coats deposited on these bond coats. For better visualisation of the dependence of bond coat surface roughness on top coat roughness, the data are shown in graphical form in Figure 4.

The 7YSZ top coat surface deposited on the smoother bond coat is also smoother, indicating that the roughness of the bond coat has a certain effect on the roughness of the 7YSZ coating prepared by EB-PVD. The lower the roughness of the bond coat, the lower the overall roughness of the top coat prepared by EB-PVD. These observations are consistent with those presented in reference [21].

However, an exponential dependence was observed between the roughness of the ceramic layer and the roughness of the substrate. This relationship is approximated by the equation:

$$y=0.5328x^2-0.0531x+0.4277$$

(1)

where y—Top coat roughness Ra; x—Bond coat roughness Ra.

It was found that the grinding of the bond coat significantly affects the surface roughness of the ceramic top coat, whereas polishing does not have such a significant impact. Therefore, further reduction of surface roughness through bond coat polishing does not noticeably influence the roughness of the 7YSZ coating. This is evidently due to the columnar structure of the ceramic top coat, as best illustrated by the surface profiles shown in Figure 5. The shape of the ceramic columns, particularly their tips, determines the surface roughness of the ceramic top coat. Although the roughness of the bond coat was reduced by an order of magnitude through polishing, the roughness of the ceramic top coat remains at a level similar to that after the bond coat grinding (compare Figure 5e–h).

Based on the data analysed, it can be concluded that the most significant impact on the roughness of the top coat is seen when the roughness of the bond coat ranges from Ra of 1.1 to 0.5 µm. Within this range, the bond coat roughness directly influences the top coat roughness because of the larger surface irregularities that are transferred during the deposition process. However, when the roughness of the bond coat is below Ra of 0.2 µm, its influence on the roughness of the top coat becomes minimal. The lowest achievable surface roughness of the 7YSZ top coat can oscillate around Ra of 0.4 µm. Thus, knowing that bond coat polishing does not significantly improve the surface roughness of the ceramic top coat, it is crucial to assess whether it has a significant impact on other properties of the 7YSZ coating or its morphology.

3.2. Characterisation of Microstructure

The microstructure of cross-sections of 7YSZ TBC coatings deposited on samples I-IV with four different bond coat roughness treatments is presented in Figure 6. Analysing Figure 6a–d, it was observed that with the reduction in the bond coat roughness, the columnar structures of the 7YSZ top coat become more uniform, and the growth direction is more consistent, perpendicular to the bond coat. Meanwhile, the columnar gaps become smaller, and the linear density of the columns increases. This indicates that the columnar structure is correlated with the roughness of the bond coat surface. This leads to the conclusion that TBCs prepared on smoother bond coats exhibit a more uniform columnar structure. Therefore, the initial treatment of the bond coat roughness is more beneficial for the nucleation and growth of perpendicular, uniform columns.

Figure 6e–h illustrate the cross-sectional morphology of the coating at higher magnification. It was observed that the columnar crystals growing on the polished bond coat appear to be denser and that the columnar gaps between the columns are narrower compared to those of the rougher bond coat. The columnar crystals growing on the as-coated bond coat surface exhibit uneven and irregular growth, and the columnar gaps seem to be quite large. There is also a noticeable lack of perpendicularity of the columnar crystals to the substrate. In summary, surface preparation has a significant impact on the microstructure of TBC coatings, and to improve their quality, the bond coat should be polished prior to the deposition of the 7YSZ top coat in the EB-PVD process.

3.3. Adhesion Evaluation

Table 3. The results of tensile adhesion strength tests.

Sample	Bond Coat Preparation	Maximal Load [N]	Adhesion Strength [MPa]
V	As-coated	33,305.2	65.7
VI	Rough grinding	46,368.4	91.5
VII	Grinding	46,410.6	91.6
VIII	Polishing	53,185.2	105.0
IX	As coated	39,892.1	78.7
X	Rough grinding	41,785.7	82.5
XI	Grinding	33,559.1	66.2
XII	Polishing	37,795.3	74.6

presents the results of the tensile adhesion test for TBC coatings produced by the EB-PVD process on samples V-XII. The highest recorded adhesion strength was 105 MPa. However, all adhesion tests resulted in cohesive failure within the adhesive. The sets after the adhesion test, consisting of a TBC-coated sample and a counterpart sample, are shown in Figure 7. Each of the tested coatings exhibited greater adhesion than the tensile strength of the adhesive used. This is confirmed by the analysis of the tensile stress–strain curves obtained during the tests, shown in Figure 8. The behaviour of the curves and the mode of failure are characteristic of epoxy materials.

Based on the results obtained, it cannot be determined whether the surface roughness of the bond coat affects the adhesion of the 7YSZ top coat, as cohesive failure within the adhesive occurred in all cases. However, it can be unequivocally stated that the adhesion strength of the thermal barrier coatings produced by the EB-PVD process exceeds 90 MPa, demonstrating excellent mechanical properties. The adhesion of the TBC produced on a polished bond coat exceeds 105 MPa. Considering that the roughness of the bond coat significantly influences the microstructure of the deposited top coat, as demonstrated in this article, it can be assumed that it also affects the adhesion of the TBC coating. Unfortunately, currently, no adhesives available on the market have a tensile strength that significantly exceeds 100 MPa, making it impossible to obtain adhesion strength values of TBC coatings produced by the EB-PVD process according to ASTM-C633. Therefore, it is necessary to develop a new adhesive or use a different testing method to determine the adhesion of these coatings. This issue will be continued in future work. In the meantime, it can only be assumed that the adhesion of EB-PVD TBC coatings significantly exceeds the 105 MPa obtained in this study.

4. Conclusions

In this study, EB-PVD TBC coatings were successfully deposited on bond coats subjected to various roughness treatments. The main conclusion is that the roughness of the bond coat affects the roughness of the 7YSZ top coat deposited by the EB-PVD method. The lower the roughness of the bond coat, the lower the overall roughness of the ceramic coating. In addition, the columnar structure is correlated with the surface roughness of the bond coat. EB-PVD TBCs prepared on smoother bond coats exhibit a more uniform columnar structure. Therefore, to improve the quality of TBC coatings, the bond coat should be polished before applying the 7YSZ top coat in the EB-PVD process. However, it was not determined whether the surface roughness of the bond coat affects the adhesion of the 7YSZ top coat. Nevertheless, it was found that the adhesion strength of thermal barrier coatings produced by the EB-PVD surely exceeds 90 MPa. With the properly treated bond coat, the adhesion of the EB-PVD TBC coating may exceed the 105 MPa obtained in this study.

These preliminary results form the basis for ongoing research aimed at evaluating TBC coatings with different current–voltage parameters to verify the impact of bond coat preparation on the functional properties of TBC layers, including the cyclic oxidation resistance. Initial findings indicate that the preparation of the bond coat significantly influences the microstructure of the ceramic top coat, which, in turn, affects its performance characteristics. For instance, smoother bond coats lead to more uniform columnar structures, which can enhance the thermal barrier properties of the coating. This is particularly important for applications in high-temperature environments, such as jet engines and gas turbines, where TBCs are critical for protecting components from thermal degradation. Improved TBC coatings can lead to a longer lifetime and better efficiency of these engines, reducing maintenance costs and improving overall performance. Moreover, understanding the relationship between bond coat roughness and top coat adhesion is crucial for the development of more durable coatings. Although current adhesives used in tensile adhesion tests limit the measurable adhesion strength to 105 MPa, ongoing research aims to develop new testing methods or adhesives that can provide more accurate measurements. This will help to optimise coating processes to achieve the best possible adhesion and durability. Future work will also focus on assessing the impact of bond coat roughness on other properties of the TBC, such as thermal conductivity, resistance to thermal shock, and overall durability. These studies are essential for advancing the technology and ensuring that coatings meet the rigorous demands of industrial applications.

In summary, the findings of this study are significant for manufacturers, particularly in the aerospace and energy sectors, where the quality and performance of TBCs can have a profound impact on the reliability and efficiency of the product. By optimising the surface preparation of bond coats, manufacturers can produce superior TBCs that offer enhanced protection and performance, ultimately leading to more efficient and durable engines and turbines.