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Mechanics of Advanced Materials and Structures
ISSN: 1537-6494 (Print) 1537-6532 (Online) Journal homepage: https://www.tandfonline.com/loi/umcm20
Solid particle erosion behavior of thermal barrier
coatings produced by atmospheric plasma spray
technique
Mustafa Kaplan, Mesut Uyaner, Egemen Avcu, Yasemin Yildiran Avcu &
Abdullah Cahit Karaoglanli
To cite this article: Mustafa Kaplan, Mesut Uyaner, Egemen Avcu, Yasemin Yildiran Avcu & Abdullah Cahit Karaoglanli (2019) Solid particle erosion behavior of thermal barrier coatings produced by atmospheric plasma spray technique, Mechanics of Advanced Materials and Structures, 26:19, 1606-1612, DOI: 10.1080/15376494.2018.1444221
To link to this article: https://doi.org/10.1080/15376494.2018.1444221
Published online: 08 Mar 2018.
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ORIGINAL ARTICLE
Solid particle erosion behavior of thermal barrier coatings produced by atmospheric
plasma spray technique
Mustafa Kaplana, Mesut Uyanerb, Egemen Avcuc, Yasemin Yildiran Avcud, and Abdullah Cahit Karaoglanlie
aDepartment of Metallurgical and Materials Engineering, Graduate School of Natural and Applied Sciences, Selcuk University, Konya, Turkey; bDepartment of Aeronautical Engineering, Faculty of Aeronautics and Astronautics, Necmettin Erbakan University, Konya, Turkey;cDepartment of
Machine and Metal Technologies, Ford Otosan Ihsaniye Automotive Vocational School, Kocaeli University, Kocaeli, Turkey;dDepartment of Mechanical
Engineering, Faculty of Engineering, Kocaeli University, Kocaeli, Turkey;eDepartment of Metallurgical and Materials Engineering, Faculty of
Engineering, Bartin University, Bartin, Turkey
ARTICLE HISTORY Received November Accepted February KEYWORDS
Thermal barrier coatings (TBCs); atmospheric plasma spray (APS); solid particle erosion; erosion mechanisms; surface roughness; surface topography ABSTRACT
Thermal barrier coatings (TBCs) are commonly applied specifically for aerospace applications in which they are subjected to air-borne particles. Therefore, solid particle erosion behavior of all coating layer has been an important phenomenon and erosion behavior of various TBCs has been widely investigated in literature. In the present study, CoNiCrAlY and yttria stabilized zirconia (ZrO2+ 8% Y2O3) powders were deposited on
Inconel 718 nickel based super alloy substrate. Atmospheric plasma spraying technique was applied for the deposition of the metallic bond coat and the ceramic top coats. Erosion tests were carried out under various particle impingement angles with an air jet erosion tester. Afterwards, eroded surfaces of the specimens were investigated with a three-dimensional (3D) optical surface profilometer (noncontact) and scanning electron microscope. The erosion rates, the areal surface roughness values, the 3D surface topographies, and the surface morphology of the specimens were evaluated based on the particle impingement angle to understand the solid particle erosion behavior of the produced coatings. The maximum erosion rates occurred at 60° impingement angle which is an indication of semi-ductile/semi-brittle erosion behavior. Furthermore, the surface roughness values and surface topographies also dramatically varied depending on the impingement angle. Deeper and wider erosion craters formed at 60° impact angle and the erosion craters were visualized by profilometer analysis.
1. Introduction
Thermal barrier coatings (TBCs) are commonly applied in aerospace applications as a means to increase the efficiency and prolong the life of the turbine components like combus-tion chamber, turbine blades, and vanes such as parts of gas turbine engines[1]–[3]. TBCs also enhance the oxidation and corrosive resistance and solid particle erosion characteristics of materials such as titanium and super alloys used in aerospace applications [4].
A typical TBC deposited on a metal substrate consists of three layers: a metallic bond coat, a ceramic top coat, and thermally grown oxide layer that is formed between the metallic bond and the ceramic top coat layers. MCrAlY (M= Ni and/or Co) bond coat has been commonly used to restrain thermal expansion mismatch between the ceramic top coating and the substrate [5]. The ceramic top coat is usually composed of yttria-stabilized zirconia, which is directly exposed to the aggressive conditions as hot corrosion and oxidation and it also ensures high thermal insulation and improves the solid particle erosion characteristics of the aerospace material under service conditions [1], [6].
Both bond and top coat in TBCs can be deposited utilizing many coating processes such as thermal spray family or electron beam physical vapor deposition techniques. Plasma spraying
CONTACT Mustafa Kaplan mkaplan@gmail.com Department of Metallurgical and Materials Engineering, Faculty of Engineering, Selcuk University, Konya , Turkey.
Color versions of one or more of the figures in the article can be found online atwww.tandfonline.com/umcm.
process has been commonly applied for its low cost and it is more effective compared to other thermal spraying methods such as detonation gun (D-gun), vacuum plasma spray (VPS), high velocity oxygen fuel (HVOF), or cold gas dynamic spraying (CGDS) techniques. Plasma sprayed TBCs are widely applied in several applications of gas turbine engine components, espe-cially turbine blades and vanes [7]. Atmospheric plasma spray (APS) technique provides a porous structure, comprising cracks, discontinuity openings, and lamellar inter-pore formations [8]. TBCs are exposed to a large variety of degradation and failure mechanisms such as oxidation, hot corrosion, thermal cycling, and calcium oxide–magnesium oxide–aluminum oxide–silicon dioxide attack. These failure mechanisms have adverse effects on the integrity and mechanical properties of the TBCs system [9], [10]. Erosion-based failures also have an important place under environmental condition. Erosion can be defined as a crucial problem in many engineering components, including jet tur-bines and steam systems. Atmospheric conditions such as dust storms, volcanic ashes, and ice particles can lead to the erosion and failure of aerospace components [11], [12]. Solid particle erosion is described as material removal from the surface as a result of the mechanical interaction of the material surface with air-borne solid particles [13], [14]. In aerospace applications,
Figure .As-deposited TBC cross-sectional SEM microstructure.
TBCs are subjected to air-borne particles, rendering the solid particle erosion behavior of TBCs an important phenomenon in these applications, which has led to several researches on the study [8], [13]–[15].
The major purpose of this study was to investigate the solid particle erosion behavior of CoNiCrAlY bond coat and yttria stabilized zirconia top coat TBCs which were produced with APS process. The surface topography and the morphology of eroded specimens of the TBCs were investigated based on the impingement angle of the particles to highlight the solid particle erosion behavior.
2. Materials and methods
2.1. Production of coatings
Both CoNiCrAlY and yttria stabilized zirconia powders were sprayed using APS technique. Inconel 718 nickel based super alloy was used as the substrate material. The diameters and thicknesses of the disk-shaped specimens were 25.4 mm and 4 mm, respectively. Inconel 718 substrates were subjected to grit blasting process. The substrates were then subjected to ultra-sonic cleaning. Grit blasting process is applied on the substrate
material surface to increase its adhesion capability via rough-ening, and it is followed by the cleaning process to prepare the substrate for the following coating processes. Commercial CoNiCrAlY metallic bond coat (Amdry 9951, Sulzer Metco, USA, 5–37µm) and yttria stabilized zirconia ceramic top coat (Metco 204NS, Sulzer Metco, USA, −45 +20 µm) powders were deposited on grit-blasted Inconel 718 super alloy substrates (Figure 1).
2.2. Methods
The major purpose of this paper is to examine the solid particle erosion behaviors of the produced TBCs. The specimens were eroded using a specifically designed air jet erosion tester and the eroded surfaces of the specimens were examined via three-dimensional (3D) optical profilometer and scanning electron microscopy. The 3D surface topographies were characterized by an optical profilometer (Nanovea ST400 Profilometer, USA). The surface roughness of the eroded specimens was measured in accordance with ISO 25178-2012 standard. An area of 5× 5 mm from the center of the eroded regions of the specimens was scanned with the profilometer, followingly the 3D surface topographies and areal surface roughness values were analysed via a surface analysis software (MountainsMap
®
Premium software, FR).Particle size analyses were conducted with a Microtrack S3500 laser to determine the size distribution. In-depth investi-gation of erosion rates, surface roughness, 3D surface topogra-phy, and surface morphology of the specimens were conducted. The used experimental procedure is explained step by step in this section.
2.2.1. Solid particle erosion testing
The erosion tests were carried out via an air jet erosion tester specifically designed in accordance with ASTM G76-13 stan-dard (Figure 2). Alumina particles were carried by static pressure and accelerated along nozzle (50-mm length with 5-mm diameter). Alumina particles are not round and have sharp corners (Figure 3). The average particle size was deter-mined as d50= 76.25 µm.
Figure .Solid particle erosion test rig.
Figure .Scanning electron microscopy image of alumina particles.
The erosion tests were carried out under various particle impingement angles by using adjustable specimen holder (Figure 2). The mass flow rate of the alumina particles was found as 13 g/s by calculating the amount of used particles in a period of time. Furthermore, the average velocity of the impingement particles was measured as 27 m/s using double disc (rotating disc) method, which was described elsewhere in detail [16], [17]. Before and after the erosion tests, TBC specimens were cleaned with air to remove the remaining alumina particles. Afterwards, the specimens were weighed in an electronic scale with±0. 1 mg-accuracy. Finally, the erosion rates were calculated with the given Eq.1.
E = m1·− m2
m t (1)
where
E: Erosion rate,
m1: Mass of the specimen before exposure to erosion, m2: Mass of the specimen after exposure to erosion,
·
m: Mass flow rate of erodent particles, t: Erosion time. The erosion tests were repeated for three times according to the specified parameters (Table 1).
2.2.2. Surface roughness, topography, and morphology
analysis
The surface roughness and topography of eroded specimens were measured by a 3D optical profilometer (Nanovea PS50)
Table .Solid particle erosion test parameters.
Erodent type Alumina particles (AlO)
Erodent size mesh
Particle impingement angle °, °, ° Acceleration/blast gun pressure . bar Impingement particle velocity m/s
Mass flow rate g/s
Test temperature °C
Nozzle diameter mm
Nozzle length mm
and the effects of the impingement angle of particles on the 3D surface topographies as well as the surface roughness of the TBCs were investigated. The surface roughness parameters of the specimens were calculated according to ISO 25178:2-2012 standard.
Additionally, SEM analyses of eroded surfaces were per-formed for characterization of eroded surface morphologies to understand the material removal mechanisms. The eroded surfaces of the coated specimens were analyzed via scanning electron microscopy (SEM MAIA3 Tescan) and energy dis-persive spectroscopy was performed to determine possible particle embedding during the erosion tests. Back scattered elec-tron images in addition to secondary elecelec-tron were taken with energy dispersive spectroscopy analysis to clarify the erosion mechanisms depending on the particle impingement angle.
3. Results and discussion
3.1. Variation of erosion rates
In the present study, the erosion behavior of TBCs was inves-tigated in detail. The maximum erosion rate occurred at 60° particle impingement angle followed by 30° and 90° impinge-ment angles (Figure 4). Solid particle erosion behavior is com-monly classified in the literature as follows: ductile, brittle, and semi-ductile or semi-brittle [18]. The ductile erosion behavior is characterized by material removal through erosion mechanisms of microploughing and microcutting, and maximum erosion rates are generally exhibited at low impingement angles (15°– 30°). The brittle erosion behavior is characterized by material removal via microcracking and plastic deformations and maxi-mum erosion rates occur at normal impingement angles (75°– 90°) [13]. Finally, semi-ductile/erosion behavior causes maxi-mum erosion rates between 40° and 70° impingement angles [13]. The maximum erosion rates occurred at 60°
impinge-ment angle which corresponds to the semi-ductile/brittle ero-sion behavior (Figure 4).
The erosion behaviors of the physical vapor deposited and HVOF sprayed TBCs were discussed and ductile, semi
ductile and mostly brittle erosion behavior was reported for various TBCs in the literature [15]. Semi-brittle/ductile erosion behavior with maximum erosion at 45° impinge-ment angle was reported for calcia-stabilized zirconia and plasma-sprayed alumina coatings deposited on aluminum 6061 alloy [19]. The erosion behavior of HVOF sprayed tungsten carbide–cobalt/nickel–chromium–iron–silicon–boron (WC-Co/NiCrFeSiB) coatings using an-air jet erosion tester and they reported brittle erosion behavior in which maximum erosion rate occurred at 90° [20]. Nickel–chromium coating exhibited ductile behavior while Stellite-6 showed brittle erosion behavior [15]. Higher erosion rates at 90° impingement angle compared
to 30° for TBCs deposited by thermal spray and physical vapor deposition [21]. It can be summarized that the erosion rate of TBCs significantly varied with varying impingement angle of particles and therefore, it is difficult to generalize the erosion behavior of TBCs [15]. It can be highlighted that the variety of the materials used to form TBCs and the complex structure of these coatings could be the major result of the variation of ero-sion behavior for various kinds of TBCs. The eroero-sion behavior of the TBCs used in this study can be defined as semi-ductile behavior.
3.2. Investigation of surface roughness and topography
Eroded surfaces were examined using 3D surface topographies and areal surface parameters. The areal surface parameters of the specimens before and after erosion at 30°, 60°, and 90° impingement angles were obtained (Figure 5). Sa, Sv, and Sp are the arithmetical mean height of the surface, the maximum height of valleys, and the maximum heights of the peaks, respectively. Surface roughness values of the eroded specimens greatly increased specifically at 60° and 30° impingement angles compared to untreated specimens. The maximum surface parameters were observed at 60° impact angle in which maxi-mum erosion occurred. It can be concluded that higher erosion rates cause higher surface roughness due to large amount of material removal. However, at 90° impingement angle, maxi-mum height of valleys (Sv) dramatically increased compared to height of the surface (Sa) and maximum heights of the peaks (Sp) unexpectedly.
The impact of the particles at normal angles caused the formation of deep valleys on the coating. Normal velocity com-ponent of the impinging particles induce plastic deformations on the surface which leads to formation of deeper valleys during
Figure .Variation of the areal surface parameters depending on the particle impingement angle.
erosion. However, this usually leads to lower erosion rates specif-ically for ductile and semi-ductile materials [19], [22], [23]. In conclusion, the variation of surface roughness parameters is compatible with the variation of erosion rates (Figures 4and5). The 3D surface topographies are good mean for evaluating eroded TBCs (Figure 6). The maximum surface roughness value occurred at 60° impingement angle, which supports the above discussion. At 60° impingement angle, a deeper and wider ero-sion crater formed compared to 30° and 90° (Figure 6). The depth of the erosion crater reached up to 230µm and the coat-ing was almost completely removed, while the depths of the ero-sion craters reached approximately 120µm and 80 µm at 30° and 90° impingement angles, respectively. Thus, it is concluded that at 30° and 60° impingement angles the erodent particles roughened up the surface more effectively compared to 90° impingement angles. The main reason of this situation is the dominant erosion mechanisms depending on the particle impingement angle. These mechanisms are discussed with SEM analysis in the next section.
3.3. Investigation of erosion mechanisms
In general, the material removal mechanism on a coating under erosion could be brittle, ductile, or semi-brittle/ductile. Ductile erosion generally results from ploughing and cutting mecha-nisms, whereas brittle erosion occurs as a result of chipping and cracking of the loosened and fractured coating [15]. The SEM images of the eroded surfaces of TBCs are presented in secondary electron mode (Figure 7) and backscattered elec-tron mode (Figure 8). The microcutting, microploughing, and microcracking mechanisms are clearly indicated in the SEM images. The coatings exposed to erosion generally undergo plas-tic deformation at first and are then removed by the impact of subsequent erodent particles on to the surface [11], [15]. Micro-cutting mechanism occurs in the form of lips or ridges because of the impacts of particles at the edges of the grooves (Figure 7). With further erosion, such lips are removed from the grooves.
SEM observations were conducted to describe the influences of particle impingement angles on the dominant erosion mecha-nisms and damages such as scratch, removal of material, pit plas-tic deformation mechanisms and to trace the morphologies of the eroded coatings.
Wider and deeper erosion craters formed at 60° impinge-ment angle which shows the magnitude of the erosion damage (Figure 7). A typical fracture and spalling feature is observed on the eroded surface. Ceramic-based coatings can exhibit erosion at the impact angle of 90° which is normally referred to as brittle manner [24]. However, at 30° and 90° impingement
angles shallow craters occur compared to 60° impingement angle.
The erodent particles roughened up the surface more effec-tively at 60° compared to 30° and 90° impingement angles (Figures 7and8). Formations of lips and craters resulting from the plastic deformation of the surfaces are visible in the image of the coating eroded at 90°. The groove sites and microcracks induced by individually impacting erodent particles via micro-cracking mechanism can be also seen in the figure. Such type of intense and localized plastic deformation occurred as a result of at 90° impact angle whereas erosion occurred in the form of microfracture or pull-out.
Figure .D surface topographies of the specimens (a) before erosion and eroded at (b) °, (c) °, and (d) ° impingement angles.
Figure .Scanning electron microscopy images of the specimens in backscattered electron mode eroded at (a) °, (b) °, and (c) ° impingement angles. At 30° impingement angle, the surface morphology exhibits
intense plastic flow with numerous scratch formations due to the microploughing and microcutting mechanisms. The brittle
Figure .Energy dispersive spectroscopy analysis of the surfaces of the specimens eroded at ° impingement angle.
behavior of the coating surface is characterized by chipping mechanism or typical scratch formation, rather than intense and localized plastic flow which induces the formation of lips nearby the groove [22], [25]. Therefore, it is confirmed that the studied TBCs exhibit semi-ductile/brittle behavior.
At the impingement angle of 90°, erodent particles may penetrate and embed into ductile structures such as coated sur-faces. Therefore, energy dispersive spectroscopy and energy dis-persive X-ray analyses were also performed for the detection of embedded erodent particles on eroded specimen surfaces [16]. The existence of alumina particles on the eroded surfaces is clearly indicated as aluminum and oxygen peaks in the energy dispersive spectrum (Figure 9). Therefore, the microcracks and plastic deformation regions were observed as a result of embed-ding of the accelerated powder particles into the specimen sur-faces eroded at 90° impingement angle (Figure 7). This also explains the formation of deeper erosion craters and the increase of maximum height of valleys (Sv) value at 90° as presented in the former sections [16]. However, this type of erosion mecha-nism leads to effective removal of material in brittle materials as mentioned before.
4. Conclusions
Solid particle erosion behavior of CoNiCrAlY bond and yttria stabilized zirconia top coatings produced by APS TBC system MECHANICS OF ADVANCED MATERIALS AND STRUCTURES 1611
was investigated. The erosion tests were implemented at 30°, 60°, and 90° impingement angles. Erosion rates of the speci-mens, surface topographies, and morphologies of the eroded surfaces were investigated. The major conclusions of the study can be summarized as follows:
r
CoNiCrAlY and yttria stabilized zirconia coatings were successfully deposited on Inconel 718 substrates by APS technique.r
The maximum erosion rates occurred at 60° impingement angle which corresponds to semi-ductile/brittle erosion behavior.r
The surface roughness of the coatings was significantly affected by the impingement angle of the particles. The maximum surface roughness was observed at 60° impinge-ment angle.r
3D visualization of the eroded surfaces clearly highlighted the erosion craters and damage. The deepness of the ero-sion crater increased greatly at 60° and the coating was removed almost completely.r
The surface topographies and the erosion rates were well correlated with erosion behavior of the studied TBC system.r
The erosion mechanisms of the coatings also significantly varied depending on the impingement angle. Micro-cutting and microploughing erosion mechanisms were clearly observed at 30° impingement angle whereas microcracking mechanism was observed at 90° impingement angle. The combination of microcutting, microploughing, and microcracking mechanisms was observed at 60° impingement angle.r
Erodent particles were embedded in the TBC surfaces specifically at 90° impingement angle.In conclusion, the solid particle erosion behavior of the CoNiCrAlY metallic bond and yttria stabilized zirconia ceramic top coatings produced by APS technique was suc-cessfully highlighted. It is suggested that investigation of the surface topographies of the coatings via surface roughness and 3D surface topographies could be beneficial for the investigation of TBCs for future studies.
Acknowledgments
The present study was derived from Ph.D. thesis of Mustafa Kaplan in the University of Selcuk Graduate School of Natural and Applied Sciences, Konya, Turkey. The study was also supported by the Surface Treatment Laboratory, Kocaeli University. The authors thank the Institute of Mate-rials Science and Engineering, Chemnitz University of Technology. The authors would like to thank M. Sc. Mech. Eng. candidates Okan Yetik from Kocaeli University and Berzah Yavuzyegit from Manchester University for their great support throughout the experimental and writing processes of the paper.
Funding
The study was also supported by Selcuk University Scientific Research Projects under the Grant Number of 15201071.
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