The development of modern aeroengines is associated with superalloys that can withstand high temperatures approaching 1,150°C. The turbines in these engines operate at much higher temperatures due to the cooling air distributed within advanced monocrystalline airfoils and the use of thermal barrier coatings (TBCs) [1]. The aim of achieving the highest possible rotor inlet temperature is to increase the thermal efficiency of the engine for economic reasons. However, despite the development of new aeroengine designs, their efficiency is still below the ideal efficiency determined by the combustion of the fuel under stoichiometric conditions, mainly because of the need to increase cooling air flow. Therefore, new materials and coatings for high-pressure turbines are required for further progress [2].
Multilayer TBCs are still the most effective way to reduce the substrate temperature of the hot components of aircraft engines. At the same time, they have led to an increase in rotor inlet temperature and have directly improved the performance of aeroengine turbines [3]. TBCs are characterized by a multilayer structure: a metallic bond coat (multicomponent NiCoCrAlY alloy or diffusion aluminide layer) and an outer ceramic layer (usually yttria-stabilized zirconia [YSZ; ZrO2
Currently, in the aerospace industry, the outer ceramic layer of the TBC is most commonly produced by the following processes: air plasma spraying (APS) and electron beam physical vapor deposition (EB-PVD) [6]. APS is commonly used to produce a protective ceramic layer in applications, such as combustion chamber components or stationary blades as well as wear-resistant coatings [7]. The ceramic layer produced by the APS process is characterized by a lamellar structure, a porosity of 4%–15%, and many microcracks. This structure provides good heat transfer insulation to the bond coat and substrate material due to the thermal conductivity coefficient equaling 0.83 W/(m·K). However, it is not very resistant to changes in thermal and mechanical stresses [5]. Ceramic layers with greater durability under these conditions are deposited using the EB-PVD process [8]. The high cost of the EB-PVD process, however, limits its application only to the critical components of aeroengines, the rotating blades of the first and second stages of a high-pressure turbine. This is because of the requirement of low surface roughness and high resistance to thermal and mechanical loads. The ceramic coatings produced by this process are characterized by a columnar structure, which provides an increase in resistance to cracking with a low thermal conductivity coefficient of 1.71 W/(m·K) [5].
Currently, the development of deposition processes for protective coatings includes plasma spray physical vapor deposition (PS-PVD) [9] and suspension plasma spraying (SPS) [10]. Literature data [9, 11,12,13] and the results of our own research [14, 15] indicate that in both processes, viz., PS-PVD and SPS, it is possible to deposit a ceramic coating with good thermal and physical properties using YSZ powder with small particle sizes (<10 μm). It is also possible to control its porosity [16, 17] and obtain thermal conductivity values similar to coatings obtained with APS [18, 19]. Several studies [11, 20] have also found that the coatings obtained with PS-PVD and SPS are characterized by higher erosion resistance and resistance to high-temperature oxidation and thermal fatigue compared to the layers in the EB-PVD process, which are characterized by particularly high costs. However, the use of powders with small particle sizes (<10 μm) in the SPS and PS-PVD processes is associated with higher implementation costs compared to the conventional APS process. The SPS process requires the use of a suspension feeder and changes in the design of the nozzle that feeds the suspension into the plasma stream [12]. On the other hand, to carry out the PS-PVD process, it is necessary to achieve low pressure in the working chamber (about 1.50 mbar) and to use a high-powered plasma torch >150 kW [21].
Research [22, 23] is on to develop process conditions for the spraying of ceramic coatings using powders with micro- and nanometric particle sizes <20 μm in the APS process. Dwivedi et al. [22] determined the effect of the size of the YSZ powder particles on the fracture resistance of the film coatings. They found that fracture toughness depended on the porosity of the coating and that the introduction of powder with smaller particle sizes (10–45 μm) resulted in coatings with lower porosity and twice the fracture toughness compared to coatings produced with powder particles of larger diameters (79–117 μm). Huang et al. [23], in their fractographic studies of the YSZ coating, considered different lamellar interface morphologies. Coatings were produced from powder with particle sizes in the range of 15–25 μm and 45–60 μm. The influence of lamellar interface roughness on coating fracture resistance under thermal fatigue and erosion conditions was investigated. They found that the introduction of powders with smaller particle sizes leads to a lower lamellar interface roughness and that the roughness of the lamellar interface also affects crack initiation and crack propagation speed in the coating. Coatings made from coarse-grained powders were shown to have a higher lamellar interface roughness. Compared to coatings made from small particle size powders, these coatings were found to have higher resistance to thermal fatigue and erosion.
A few studies [24, 25] have explored APS processes using micrometric (<10 μm) and nanometric (<1 μm) particles introduced into the plasma stream through a carrier gas rather than in suspension, enabling the deposition of ceramic coatings with good properties and replacing PS-PVD and SPS processes, which are more expensive.
Inconel 625 Ni superalloy with the following chemical composition (wt%) was chosen as the base material: Al: 0.4; C: 0.1; Cr: 22; Co: 1; Fe: 5; Mn: 0.5; Mo: 9; Nb: 3.75; P: 0.015; Si: 0.5; S: 0.015; Ti: 0.4; Ni: balance. This material is characterized by good tensile strength (
The TBC bond coat was made of a multicomponent NiCoCrAlY alloy plasma sprayed in the APS process on the Inconel 625 superalloy substrate. To deposit the bond coat, Amdry 386 powder (Oerlikon Metco, Wohlen, Switzerland) with the following chemical composition (wt%) was used: Co: 21.5; Cr: 16.3; Al: 14.8; Y: 0.1; Hf: 0.2; Ni: balance.
The outer ceramic layer of the TBC was also produced by APS using three types of Oerlikon Metco YSZ oxide powders. Metco-6609 powder with the smallest particle size
The APS process was carried out using the thermal spraying equipment in the Research and Development Laboratory for Aerospace Materials at the Rzeszow University of Technology, Rzeszów, Poland. An A60 single-electrode torch (Thermico, Dortmund, Germany) with radial powder injection into the plasma plume was used. The powder was fed with a Thermico CPF2 feeder (Thermico, Dortmund, Germany). It allows gravimetric control of the flow rate of the powder independently of the grain size.
The data [31] and the results of previous research [25] clearly show that the properties of the deposited ceramic layer in the TBC are most influenced by the current and the chemical composition and flow rate of the plasma gas mixture, especially H2. Therefore, these parameters were changed in the research.
Characterization of the temperature and velocity of the particles moving in the plasma stream was done using the DPV eVolution (Tecnar, Saint-Bruno-de-Montarville, Canada). Measurements were made at a distance of 100 mm from the plasma torch nozzle (the spraying distance used in the experimental processes). The DPV system enables the displacement of the measuring point in the x- and y-axes, perpendicular to the axis of the torch nozzle, within the range of ±50 mm. Thus, it is possible to measure the temperature and velocity of particles in the zone of the plasma plume with the highest flow rate. The distribution of temperature and particle velocity values on the cross section of the plasma plume with dimensions of 20
The characterization of the powder particles moving in the plasma plume using a DPV system was the basis for determining the range of conditions for ceramic layer deposition in the APS process using powders of different grain sizes, as provided in Table 1.
Microscopic examination of the fabricated TBC was performed using a Phenom XL scanning electron microscope (SEM) (Thermo Fisher Scientific, Waltham, MA USA) to determine the thickness and porosity of the outer ceramic layer. An electron beam acceleration voltage of 15 kV and a backscattered electron (BSE) mode were used. The samples were cleaned in isopropyl alcohol solution using an ultrasonic cleaner.
APS process parameters for deposition of YSZ ceramic outer layer of TBC
Plasma torch | A60 |
Plasma gases Ar/H2 [NLPM] | 54/6; 48/12; 42/18 |
Current [A] | 600; 700; 800 |
Substrate radial velocity [rev/min] | 150 |
Plasma torch feed rate [mm/s] | 3 |
Time of spraying [s] | 1,110 |
Spray distance [mm] | 100 |
Spray angle [°] | 90 |
Carrier gas [NLPM] | 6 |
Powder feed rate [g/min] | 10 |
Angle of powder injection to plasma plume [°] | 90 |
APS, air plasma spraying; NLPM, normal liter per minute; TBC, thermal barrier coating; YSZ, yttria-stabilized zirconia
The porosity of the ceramic layer was determined by digital image analysis. For a single layer, 15 images were taken at a magnification of 9,000
The hardness measurement was carried out using the Vickers method with the Innovatest Nexus 4303 (Innovatest, Maastricht, Netherlands) micro-hardness tester. A measured force load of 200 gf and a spraying time of 10 s were selected to measure the coatings of various thicknesses. For each plasma-spraying condition, five measurements were taken for the deposited coating.
Measurements of the average temperature and velocity of the ZrO2
Increased plasma energy at a flow rate of H2 > 6 NLPM (normal liter per minute) was found to cause the melting of powder particles. The measurement results also show that as the flow rate of H2 increases, the mean particle velocity decreases because a higher energy of the plasma stream enables the melting of particles with larger sizes that are characterized by lower velocity. However, the lack of H2 in the plasma gas mixture leads to a lower temperature. Thus, it is impossible to deposit the ceramic layer with good properties. As a result, it is possible to exclude these processes in experimental APS processes.
The results show that the average temperature of the Metco-6700 powder particles that radially feed into the plasma stream in the A60 plasma torch depends more on the current than the flow rate of H2, as shown in Figure 3A. Increasing the flow rate of H2 has little or no effect on the temperature of the melted particles. This effect can occur due to the small difference in the mean particle diameter of the Metco-6700 powder and sufficient plasma stream energy to melt them at the plasma gas flow rate of H2 ≥ 6 NLPM. Higher H2 flow rates could cause difficulties with the radial feed method of highly dispersed powder particles injected into the plasma plume zone with the highest temperature, due to increased plasma enthalpy [32]. In Figure 3B, a linear dependence of the melting particle velocity on the H2 flow rate was also found in the range of 6–18 NLPM. What is noticeable here is that both the highest temperature (approximately 2,900°C) and the highest particle displacement velocity (370 m/s) were obtained for the H2 flow rate of 6 NLPM and the current intensity of 800 A. Similarly for Metco-204NS powder measurement, the lack of H2 in the plasma gas mixture resulted in temperature values that were too low to deposit a ceramic coating with good physical and functional properties. Consequently, for the Metco-6700 powder, these processes were excluded in the experimental tests.
In Figure 4, the comparison of example DPV measurement results can be seen for standard ceramic powder Metco-204NS and micropowder Metco-6700 with H2 flow rate accordingly of 18 NLPM and 6 NLPM and the current intensity of 800 A (for both powders), which results in the highest-obtained temperature. The temperature was the highest (>2,900°C) in the center of the plasma plume, which proves that Metco-6700 powder particles are transported and melted in the center of the plasma plume despite their average size (approximately
In the investigation, an attempt was also made to measure the melted powder particles with the smallest diameter
The main objective of this research was to establish the influence of APS process parameters on the thickness and porosity of ceramic coatings. To evaluate the properties of the coatings deposited with micro and nanopowders (Metco-6700 and Metco-6609), a comparison with the standard Metco-204NS YSZ powder is required.
Figure 5A shows a strong correlation between the thickness of the ceramic coatings deposited with the Metco-204NS powder and the increase in H2 flow rate. The thickest ceramic coatings (100–120 μm) were obtained only for the flow rate of H2 ≥ 12 NLPM and current >700 A. Thermal energy has been shown to be sufficient to melt powder particles with average diameter
Until now, in the aerospace industry, ZrO2
The Metco-6609 YSZ ceramic powder is generally used mixed with ethanol in suspension plasma spray [27]. In the present research, this powder was fed into a plasma plume using a carrier gas. Due to the fact that the particle diameter is below the detection threshold of the DPV eVolution measuring system, the influence of APS process parameters on thickness and porosity was studied only by metallographic examination.
The layers produced were <40 μm thick irrespective of the process parameters, as shown in Figure 9A. This phenomenon is likely caused by the large dispersion of Metco-6609 powder particles (
The lowest porosity and the highest hardness of the examined APS process parameters examined were found in the ceramic layer deposit with 12 NLPM H2 flow and 600 A current, making them the most suitable values when using radial powder fed into the plasma plume. As with Metco-6700 powder, the use of nanometric Metco-6609 powder fed radially into the plasma plume increases the proportion in surface morphology with visible recrystallized particles compared to well-melted areas, as seen in Figure 10C.
In summary, results show that the use of the same ceramic material (YSZ) causes a different lamellar structure depending on the size of the powder particles. The Metco-6700 micrometric powder resulted in a less-porous coating with significantly smaller pore sizes and a higher number in the interlayer boundaries compared to coatings deposited with standard Metco-204NS powder with the same thickness (approximately 100 μm) and only for processes with a low H2 flow rate ≤ 6 NLMP. The coatings obtained by such a process were characterized by a similar hardness >500 HV0.2. The use of Metco-6609 nanometric powder resulted in a coating with spherical particles and remelted areas; therefore, the porosity of these coatings was higher than that of the other powders used in the study. The coatings were characterized by hardness about twice smaller than that of the Metco-204 and Metco-6700 coatings. Such a result indicates that this powder is not suitable for spraying in the APS process with its radial feed method. In further research, the axial introduction of this powder in the Axial III multielectrode torch will be studied, where problems with introducing a powder particle into the plasma plume can be avoided, and the properties of the coatings, e.g., resistance to oxidation, thermal properties, and bond nature of the coatings, can be tested.
Based on the results obtained from the conducted research and their analysis, it can be concluded that it is possible to produce ceramic coatings using ZrO2 The determination of the thermal and physical properties, viz., temperature and velocity of the YSZ particles moving in the plasma jet, enables one to reduce the number of experimental technological processes necessary to determine the correct plasma spraying process conditions. The developed plasma spraying process conditions made it possible to establish the influence of torch current (in the range of 600–800 A) and H2 flow rate (in the range of 6–18 NLMP) on the thickness, porosity, and hardness of the ceramic layer of the TBC. The outer ceramic layer of the TBC produced by plasma spraying of Metco-6700 micropowder and Metco-6609 nanopowder is characterized by a thickness and porosity of approximately 100 μm and <8%, respectively, for Metco-6700 and 40 μm and 16%, respectively, for Metco-6609 when optimal APS process conditions are used. The hardness of the ceramic coatings deposited by Metco-6700 micropodwer is similar to that of the coatings produced by standard Metco-204NS powder (>500 HV0.2) for coatings with equivalent thickness (approximately 100 μm). The deposited ceramic layers are characterized by a lamellar structure comparable to that obtained with the standard Metco-204NS powder. The average temperature and velocity of the melted particles depend linearly on the H2 flow rate for the Metco-204NS powder; the thickness of the deposited coating depends more on the H2 flow rate, while porosity depends more on the current. The highest temperature (approximately 2,900°C) and the highest particle displacement velocity (370 m/s) were obtained for the lowest flow rate of H2 (6 NLPM) and the highest current (800 A) for the Metco-6700 powder, resulting in the highest thickness of the obtained coating (approximately 100 μm) and the lowest porosity.