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

Written By

Zenon Aleksander Opiekun

Submitted: October 20th, 2014 Reviewed: June 23rd, 2015 Published: November 25th, 2015

DOI: 10.5772/61100

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Abstract

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.

Keywords

  • Cobalt alloy MAR-M509
  • geometrical product specification (GPS)
  • thermal barrier coating (TBC)
  • microstructure
  • microhardness
  • interlayer cohesion
  • adhesion
  • nanoscratch test

1. 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 [1–3]. 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 [67].

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: Al2O3, ZrO2, ThO2, BeO, MgO, CeO2, Cr2O3i Y2O3 [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].

Figure 1.

Effect of TBC on reducing temperature of turbine blade [9] (a), and BOAS made of MAR-M509 alloy (b) (schemes).

ZrO2 oxide stabilized by Y2O3 is most often used for the external layer in the TBC [1015]. Functional properties, adhesion, cohesion, and crack resistance depend on the phase composition of this oxide. Solid solutions prepared on the basis of regular ZrO2 (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 ZrO2 (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 ZrO2–8% Y2O3 irregular grain sizes, with the introduction of about 8 wt% of Y2O3 to the solid solution of ZrO2–Y2O3(Fig. 2) [16].

Figure 2.

Phase equilibrium system ZrO2–Y2O3.

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 ZrO2 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 [1719]. Useful properties of TBC (erosion invulnerability, thermal fatigue, scale off, and mastication) depend on fat of layers and their spraying thermal parameters [2023].

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 ZrO2–8% Y2O3 oxides onto the surface of the casting with a MultiCoat Plasma Coating System (SulzerMetco).

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2. Materials and methods of testing

Plate castings made of cobalt alloy MAR-M509 with a dimension of 4 × 20 × 90 mm were used for tests. The composition of plates was as follows: 0.60% C, 28.83% Cr, 10.0% Ni, 7.12% W, 3.77% Ta, 0.36% Zr, 0.18% Ti, 0.007% B, 0.43% Fe, 0.02% Mn, 0.03% Si, 0.002% S, and the rest is Co. The plates were casted in the multilayer ceramic mould (MCM) at initial temperature ca. 1000°C. The liquid alloy MAR-M509 at temperature ca. 1495 ± 5°C [24,25] was poured over MCM in a medium-frequency induction vacuum furnace. The surface of plate castings was at first sandblasted with Al2O3 powder of 200 ÷ 250 μm grain diameter in an air stream of pressure of ca. 0.35 MPa for a few seconds. After a single layer of 45% Ni–22% Co–17% Cr–16% Al–0.3% Y powder alloy was covered, triple layers of ZrO2–8% Y2O3 powder oxides in an argon-hydrogen plasma beam were applied. The plates were heated at a temperature of about 120°C before the layers were covered.

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 ZrO2–Y2O3 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, H2 (10 l/min); electric current, about 600 A; powder feet rate (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).

Figure 3.

Multicoat plasma coating system. View of system (a), spraying gun F4 MB-HBS type (b), scheme of spraying (c).

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: Sa (arithmetic mean), Sz (mean of the 5 highest peaks and the 5 lowest points), St (total height), Sq (quadratic mean), Sp (highest peak over the mean), and Sv (lowest valley under the mean).

X-ray examination surface of the TBC was performed with the Siemens Kristalloflex D500 X-ray diffractometer.

The testing of microstructure layers in the TBC was conducted with a Berkovitz’s indenter and Nano Scratch-Tester (CSM Instruments).

The examination of cohesion and adhesion of the TBC was carried out with the Revetest Scratch Tester CSM Instruments tool. The surface of sample with the TBC was scratched with the diamond Rockwell’s intender of a 200-µm top radius. During the test, six different forces were applied: 2, 4, 8, 16, 32, and 40 N onto the surface of a cross section (MAR-M509 mould and TBC). It has been assumed that each scratch is 0.5 mm long. In the interlayer of the TBC and in the MAR-M509 mould, scratches were made at a 5-N intender load along 0.5 mm. All scratches were made at a rate of 1 mm/min—the speed of the intender. The Revetest Scratch Tester tool was calibrated according to reference sample covered with TiN coating. Friction force, friction factor, and acoustic emission were recorded during the test; 100% of acoustic emission scale equals 65-dB value.

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3. Results

3.1. 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.

Figure 4.

Surface topography of the external ceramic layer in the TBC. 3D view (a), plate surface with amplitude parameters (b), coating map (c), load-capacity curve of GPS and its parameters (d), surface profile (e), and geometrical parameters Sk, Spk, Svk, Sr1, Sr2.

3.2. 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.

Figure 5.

Microstructure at the cross section: mould (MAR-M509) coating (TBC) (a, b), external ceramic layer (ZrO2) (d), metallic interlayer (45% Ni–22% Co–17% Cr–16% Al–0.3% Y) (c), and SEM micrograph.

Figure 6.

Microstructure of TBC (a), graph of roentgen dispersion radiation from 1 area (b), share of elements in 1 area (c), chemical composition in microareas (1–4) of TBC (table).

Figure 7.

Diffraction patterns of the external ceramic layer in the TBC surface obtained for a grazing incident geometryα= 2°, 5°, and 9° and in the Bragg–Brentano geometry (BB). ZrO2 (C)—regular, ZrO2 (T)—tetragonal.

3.3. 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 (ZrO2) [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).

PhaseEp, pJEe, pJEc, pJMIT = Ee/EcE, GPaHV0.05
γ226739 ± 3%94733 ± 3%321458 ± 3%0.29 ± 0.1122 ± 5425 ± 5
Metallic interlayer (45% Ni–22% Co–17% Cr–16%
Al–0.3% Y)
257307 ± 5%72309 ± 3%333381 ± 5%0.22 ± 0.1103 ± 7540 ± 8
Ceramic external layer (ZrO2)16187 ± 5%16124 ± 5%32311 ± 5%0.5 ± 0.1125 ± 161220 ± 45

Table 1.

The values of energy of plastic deformation (Ep), energy of elastic deformation (Ee), total energy (Ec), ratio Ee/Ec, Young’s modulus E, and microhardness HV0.05. The average value from five measurements.

3.4. Cohesion and adhesion

The control was scratched with the diamond Rockwell’s indenter on the surface of a cross-sectional MAR-M509 mould and TBC (Fig. 9).

Figure 8.

The influence of loading on depth of forcing in the Berkovitz’s indenter and energy change for γ phase (a), metallic interlayer (45% Ni–22% Co–17% Cr–16% Al–0.3% Y) (b), and external ceramic layer (ZrO2) (c).

Figure 9.

Macrostructure with the picture of surface scratches (a), example of the scratch Rockwell’s indenter for the a loading force 8 N of a cross section; MAR-M509 mould, metallic interlayer (45% Ni–22% Co–17% Cr–16% Al–0.3% Y) and external ceramic layer (ZrO2–8% Y2O3) (b), SEM micrograph.

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.

Figure 10.

Examples of scratches along the cross section: mould (MAR-M509) interlayer (45% Ni–22% Co–17% Cr–16% Al–0.3% Y) in the TBC, together with diagrams of friction force, friction coefficient, and acoustic emission changes for a loading force2 N (a), 4 N (b), 8 N (c), 16 N (d), 32 N (e), and 40 N (f).

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 Aca cones, which characterize the cohesion of the interlayer, are described by a product lx ly (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.

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4. 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 ZrO2–8% Y2O3 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 ZrO2 (C) and tetragonal ZrO2 (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 M23C6 [27].

A gradient of elastic strain energy to total energy (MIT = 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 MIT parameter equals 0.29 for the γ phase (MITγ), 0.22 for the metallic interlayer (MITM) 45% Ni–22% Co–17% Cr–16% Al–0.3% Y, and 0.50 for the external ceramic layer (MITZrO2) (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 Al2O3 particles and by heating at about 120°C. The geometrical product specification (GPS) of the sandblasted plate is isotropic. Its amplitude parameters Sa, Sq, Sp, Sv, St, and Sz are 3.1 μm, 3.9 μm, 16.2 μm, 18.3 μm, 34.5 μm, and 31.4 μm, respectively, and functional parameters Sk, Spk, Svk, Sr1, and Sr2 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 St of the layer has increased by ca. 50%, and the arithmetic mean Sa increased by ca. 60% in comparison to St and Sa for the sandblasted surface of the mould. The roughness of the core (Sk) 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 (Spk) (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).

Figure 11.

Surfaces of longitudinal sections of Aca cones plotted as a function of loading force in the metallic interlayer 45% Ni–22% Co–17% Cr–16% Al–0.3Y.

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].

Figure 12.

Macrostructure at the cross section: mould (MAR-M509) coating (TBC) with MIT parameters.

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).

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Written By

Zenon Aleksander Opiekun

Submitted: October 20th, 2014 Reviewed: June 23rd, 2015 Published: November 25th, 2015