Mechanical Properties of the Thermal Barrier Coatings Made of Cobalt Alloy MAR-M509

This manuscript presents the microstructure, geometrical product specification, and results of scratch tests performed on the interlayer of thermal barrier coating (TBC) with Rockwell’s intender. The TBC was provided by depositing two layers, metallic interlayer and external ceramic layer, onto a plate coating made of cobalt alloy MAR-M509 in plasma spraying process. Based on measurements of microhardness made with Berkovitz’s indenter using Nano Scratch-Tester (CSM Instruments), it was stated that elastic (Ee) to total energy (Ec) parameters (M IT = Ee/Ec), γ phase matrix of alloy MAR-M509 (M ITγ ), metallic interlayer (45% Ni–22% Co–17% Cr–16% Al–0.3% Y) (M ITM ), and ceramic layer (M ITZrO2 ), are proportion, that is, 0.29:0.22:0.50. The surface of the casting was sandblasted with Al 2 O 3 powder in an air stream before the TBC was introduced. Scratches were made along the cross section from a mould material (MAR-M509) through metallic interlayer and external ceramic layer in the TBC. Friction force, friction factor, and acoustic emission were recorded during the test. It has been proven that metallic interlayer in the TBC of ca. 200 μm thickness forms tough coating without pores with good cohesion values and very good adhesion values to the mould.


Introduction
Thermal barrier coatings (TBCs) are applied via plasma spraying technology (air plasma system [APS]) and build up usually with two layers: metallic interlayer and external ceramic layer [1][2][3]. For special purposes, when a high density of coating together with its good bonding with the mould is demanded, the TBC is applied in vacuum plasma spraying process (vacuum plasma spray [VPS]) or low-pressure plasma spraying technique (low-pressure plasma spray [LPPS]) [4]. The TBC has an excellent heat resistance and a small thermal conductivity. They are characterized by an erosion and abrasion resistance and resistance to aggressive chemicals [5]. Therefore, TBCs are becoming more commonly used as coatings of jet-propelled parts of aircraft engines (combustor, blade outer air seal [BOAS], and blades of turbine), valves, and parts in chemical reactors [6][7].
Materials with MeCrAlY group single oxides that belong to ceramic materials are usually applied on metallic interlayers. These are most often used on the external layer in the TBC: Al 2 O 3 , ZrO 2 , ThO 2 , BeO, MgO, CeO 2 , Cr 2 O 3 i Y 2 O 3 [5,6]. Thermal barrier coatings are the layers that separate the surface of metallic material from the stream of hot gases. Simultaneously, they reduce the temperature of metallic items [8]. In the end, such an influence of the TBC causes an improvement of performance characteristic of jet turbine engine, a reduction of fuel consumption, an increase of inlet gas temperature (ca. 100-170°C) ( Fig. 1) [9], and a decrease of toxic substance emission from outlet gases because of better fuel consumption [7]. ZrO 2 oxide stabilized by Y 2 O 3 is most often used for the external layer in the TBC [10][11][12][13][14][15]. Functional properties, adhesion, cohesion, and crack resistance depend on the phase composition of this oxide. Solid solutions prepared on the basis of regular ZrO 2 (C) have low resistant to changeable heat load. A considerable improvement of the crack resistance of these solutions is noticed during the presence of tetragonal phase of ZrO 2 (T) in the TBC. Factors that favor the tetragonal phase in the TBC after cooling down to the room temperature from the temperature of plasma spraying are small ZrO 2 -8% Y 2 O 3 irregular grain sizes, with the introduction of about 8 wt% of Y 2 O 3 to the solid solution of ZrO 2 -Y 2 O 3 ( Fig. 2) [16].
The aim of this article was to describing the cohesion and adhesion of the metallic interlayer made of 45% Ni-22% Co-17% Cr-16% Al-0.3% Y alloy and to present the geometrical product specification of the ceramic external layer made of ZrO 2 oxides deposited on the casting of plate made of MAR-M509 cobalt alloy.
The main criterion to accommodate metallic interlayer is to compensate stress between the substrate and the external ceramic layer. TBC with plastic interlayer decreases its own stress and enhances connection between harder substrate and fragile hard external ceramic layer [17][18][19]. Useful properties of TBC (erosion invulnerability, thermal fatigue, scale off, and mastication) depend on fat of layers and their spraying thermal parameters [20][21][22][23].
The mechanical properties of TBC layers defined were by microhardness made with Berkovitz's indenter when Nano Scratch-Tester (CSM Instruments) was used. The coating was obtained by single-spraying metallic powder made of 45% Ni-22% Co-17% Cr-16% Al-0.3% Y alloy and triple-spraying ceramic powder made of ZrO 2 -8% Y 2 O 3 oxides onto the surface of the casting with a MultiCoat Plasma Coating System (SulzerMetco).   Two spray powders were used in the experiment: one 45% Ni-22% Co-17% Cr-16% Al-0.3% Y powder alloy, which shows fine spherical morphology with particle granulation, ca.75 ± 15 μm, and ZrO 2 -Y 2 O 3 oxide powder, which shows irregular morphology with particle granulation, ca.53 ± 15 μm.
A deposition of all layers in the TBC was done using the following: primary plasma gas, Ar (40 l/ min); secondary plasma gas, H 2 (10 l/min); electric current, about 600 A; powder feet rate

Superalloys
(40 g/min); and spray distance, about 130 mm. The traverse speed of a spraying gun was constant at 2 mm/min, and the speed of rotating holder was also constant (60 rot/min) (Fig. 3).  The state of surface of plate castings was assessed with the Talyscan 150 (Taylor-Hobson) tool working with the Mountains Map Universal program before and after the ceramic layers was deposited. Six amplitude parameters of geometrical product specification (GPS) were analyzed: S a (arithmetic mean), S z (mean of the 5 highest peaks and the 5 lowest points), S t (total height), S q (quadratic mean), S p (highest peak over the mean), and S v (lowest valley under the mean).
X-ray examination surface of the TBC was performed with the Siemens Kristalloflex D500 Xray diffractometer. The testing of microstructure layers in the TBC was conducted with a Berkovitz's indenter and Nano Scratch-Tester (CSM Instruments).

Geometrical structure of a surface
The geometrical product specification (GPS) and the surface load-capacity curve of external ceramic layer in the TBC are presented in Figure 4.

Microstructure and X-ray diffraction pattern
The microstructure of the plate casting made of alloy MAR-M509 with the TBC at the cross section is presented in Figures 5 and 6. The X-ray diffraction patterns of the external ceramic layer in the TBC surface are shown in Figure 7.

Measurements of microhardness
Nano Scratch Tester (CSM Instruments) equipped with a Berkovitz's indenter (a pyramid with equilateral triangle as the base area) is to measure microhardness and to determine other mechanical properties of single grains, thin layer, and phase boundaries when loading spreads from 0 to 500 mN [26].
A ratio of elastic strain energy (Ee) to total energy (Ec) when forcing the indenter of the hardness tester is a very important feature of the tested materials. Figure 8 shows example courses of changes of energy plastic deformation (Ep) and energy of elastic deformation (Ee), which describe the forcing of Berkovitz's indenter into a γ phase, into the metallic interlayer (45% Ni-22% Co-17% Cr-16% Al-0.3% Y), and into the external ceramic layer (ZrO 2 ) [27]. Table 1 shows values of energy of plastic deformation (Ep), energy of elastic deformation (Ee), total energy (Ec), ratio Ee/Ec, longitudinal modulus of elasticity (Young's modulus E), and microhardness HV0.05 for γ phase, metallic interlayer, and external ceramic layer (the average value from five measurements).

Cohesion and adhesion
The control was scratched with the diamond Rockwell's indenter on the surface of a crosssectional MAR-M509 mould and TBC (Fig. 9).   Figure 10 shows the surface scratches at the cross section: mould (MAR-M509)-metallic interlayer 45% Ni-22% Co-17% Cr-16% Al-0.3% Y in the TBC together with diagrams of friction force, friction factor, and acoustic emission changes.
The scratches of samples in the direction from the mould (MAR-M509 cobalt alloy) through the metallic interlayer in the TBC at the cross section, which is made at constant intender loads, reveal the presence of typical plastic strain cones (Fig. 10b), crack cones in the coating (Fig.  10a), and "pushes-P" of mould material into this interlayer. The surfaces of longitudinal sections of A ca cones, which characterize the cohesion of the interlayer, are described by a product l x l y (Fig. 10e) [28] and are plotted as a function of loading force (Fig. 11). The "pushes" of mould material into a metallic interlayer and the lack of cracks at a boundary-mould interlayer (Fig. 11) indicate a very good interlayer adhesion to the mould.

Summary
A TBC with ca. 920 μm thickness was obtained as a result of spraying melted powder with particle granulation ca. 75 ± 15 μm made of 45% Ni-22% Co-17% Cr-16% Al-0.3% Y alloy and ceramic powder made of ZrO 2 -8% Y 2 O 3 oxides with particle irregular granulation ca. 53 ± 15 μm. The surface of a plate casting was made of the MAR-M509 cobalt alloy in a plasma beam (Figs. 5 and 6). Such a coating consists of two layers of fine grain metallic interlayer 45% Ni-  22% Co-17% Cr-16% Al-0.3% Y (ca. 200 μm thickness) and external ceramic layer as a mixture of two phases, regular ZrO 2 (C) and tetragonal ZrO 2 (T) (Fig. 7). The microstructure of the metallic interlayer in the TBC is tough, homogeneous, and deprived of porosity (total porosity less than 2%) (Fig. 5c). The thin layer of the γ phase (about 0.015 μm thick), which adheres to the metallic interlayer, is characterized by similar values of microhardness as the metallic interlayer, ca. 520 HV0.05. This is an effect of hardening of the γ phase with secondary carbides of M 23 C 6 [27].  A gradient of elastic strain energy to total energy (M IT = Ee/Ec) when forcing the Berkovitz's indenter is a very good parameter that describes elastic-plastic properties of materials (Fig.  8, Table 1). The M IT parameter equals 0.29 for the γ phase (M ITγ ), 0.22 for the metallic interlayer (M ITM ) 45% Ni-22% Co-17% Cr-16% Al-0.3% Y, and 0.50 for the external ceramic layer (MIT ZrO2 ) (Fig. 12). The metallic interlayer, which has better plastic than the substratum γ phase, was deposited on the plate surface, which was activated by sandblasting with Al 2 O 3 particles and by heating at about 120°C. The geometrical product specification (GPS) of the sandblasted plate is isotropic. Its amplitude parameters S a , S q , S p , S v , S t , and S z are 3.1 μm, 3.9 μm, 16.2 μm,  18.3 μm, 34.5 μm, and 31.4 μm, respectively, and functional parameters S k , S pk , S vk , S r1 , and S r2 are 10 μm, 4.0 μm, 3.5 μm, 9.6%, and 91.4%, respectively. The GPS of the external ceramic layer in the TBC is also isotropic, but it is rougher than the mould. The total height S t of the layer has increased by ca. 50%, and the arithmetic mean S a increased by ca. 60% in comparison to S t and S a for the sandblasted surface of the mould. The roughness of the core (S k ) has increased by ca. 35%. The only functional parameter of GPS of ceramic layer, which is lower by ca. 100% in comparison to the mould, is the roughness of peaks (S pk ) (Fig. 4).
The scratches of samples along the cross section from the mould (MAR-M509 cobalt alloy) through the metallic interlayer and external ceramic layer (Fig. 9a,b) in the TBC, which were made with Rockwell's cone at different intender loads (Fig. 10a,f), reveal the presence of typical strain and crack cones in the interlayer and "pushes-P" of mould material (γ phase) into the interlayer. A linear surface increase of longitudinal sections of these cones indicates good cohesion of the metallic interlayer (Fig. 11). A load increase of the Rockwell's intender from 2 N to 40 N ( Fig. 10a-f) causes an increase of friction force from ca. 0.3 N to 10 N, together with a minor increase of friction coefficient from ca. 0.1 to 0.2. For all scratches, no increase of acoustic emission effect is observed at boundary mould coating. The constant value of acoustic emission at 3% level (ca. 2 dB) shows that strains at boundary MAR-M509 cobalt alloy-metallic interlayer in the TBC are plastic and do not appear in this crack area (Fig. 10 a-c). The shape of "pushes" of the γ phase is similar to the cones, and its depth increases linearly together with the increase of intender loading force from ca. 3 μm for 2 N force to ca. 53 μm for 40 N force (Fig. 11). Such a course of changes happening at boundary mould (MAR-M509 cobalt alloy), that is, the metallic interlayer in the TBC during an attempt at scratching, proves a very good adhesion for this interlayer made of 45% Ni-22% Co-17% Cr-16% Al-0.3% Y alloy [29]. Cracks in "cohesion cones" of the metallic interlayer in the TBC are noticed when the loading force of the intender exceeds 16 N (Fig. 10d). Huge differences (even 6.5 dB) between values of acoustic emission are the effect of the interlayer cracking [29] (Fig. 10e,f).