PS-PVD deposition of thermal barrier coatings

SCT-18876; No of Pages 5 Surface & Coatings Technology xxx (2013) xxx–xxx

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PS-PVD deposition of thermal barrier coatings Marek Goral ⁎, Slawomir Kotowski, Andrzej Nowotnik, Maciej Pytel, Marcin Drajewicz, Jan Sieniawski Research and Development Laboratory for Aerospace Materials, Rzeszow University of Technology, Powstancow Warszawy 12, 35-959 Rzeszow, Poland

a r t i c l e

i n f o

Available online xxxx Keywords: Thermal barrier coatings Low pressure plasma spraying Plasma spray physical vapor deposition PS-PVD TBC LPPS hybrid

a b s t r a c t In this study, the influence of the deposition parameters on the coating structure during the ‘quasi-PVD’ process was investigated. This type of coating could be deposited at powder feed rates between 10 and 20 g/min using He/Ar plasma gasses. The microstructure of the ceramic coating obtained using these parameters is unique because the evaporation of the ceramic powder was not complete. The deposition was conducted by the LPPS-Hybrid system produced by Sulzer Metco. Rene 80 nickel superalloy was used as a base material. A Zr-modified aluminide coating deposited by the CVD (Chemical Deposition) method, and a MeCrAlY coating deposited by the APS (Air Plasma Spraying) method were used as bond coats. Metco 6700 yttria-stabilized zirconia powder was used as a coating material. An increase in the coating thickness was triggered by increasing the powder feed rate. The pressure inside the working chamber exercised a strong influence on the structure and thickness of the coatings. In coatings deposited under a pressure of 200 Pa, unevaporated powder particles were observed along with a significantly lower thickness. The same effect was rendered by decreasing the power current of the plasma gun to 1800 A. The PS-PVD method provides an alternative process to APS and EB-PVD (Electron Beam Physical Vapor Deposition) technologies. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The development of civilian air transport demands improvements in the efficiency of aircraft engines. To increase the temperature in the hottest section of an aircraft engine, new types of hightemperature nickel superalloys and thermal barrier coatings (TBCs) have been applied. For deposition of these types of coatings, electron beam — physical vapor deposition (EB-PVD) and Air Plasma Spraying (APS) are used. These technologies have been applied for over 30 years by aircraft engine manufacturers [1,2]. Since the 1980s, new types of plasma spray processes, such as LPPS (low pressure plasma spraying) or VPS (vacuum plasma spraying), have been introduced to the intent of reducing oxidation during the deposition process. The last decade has seen the development of advanced vacuum plasma spraying processes, including LPPS-thin film, PS-PVD (plasma spraying physical vapor deposition) and PS-CVD (plasma spraying chemical vapor deposition). The characteristic properties of these processes are a low pressure in the working chamber (N100 Pa) and gas-phase, rather than liquid-phase, deposition of the feedstock material. Newly developed technologies come close to being used in applications where conventional CVD and PVD are typically used; the new methods, however, facilitate quicker deposition [3–5]. The PS-PVD process is based on conventional methods of TBC deposition by low pressure plasma spraying. Improved efficiency of the vacuum pumps in PS-PVD devices results in pressures between 50 and ⁎ Corresponding author. E-mail address: [email protected] (M. Goral).

200 Pa in the working chamber. Due to the lower pressure during the process, the plasma plume length can reach over 2 m, and its diameter is 200–400 mm. For powder spraying, a modified O3CP Sulzer Metco plasma gun is used. Its construction guarantees a plasma gas flow up to 200 NLPM (normal liters per minute) and a power of 180 kW. Although the pressure in the PS-PVD process is higher than in the typical PVD processes, a high plasma plume temperature allows for powder evaporation. For the deposition of columnar TBCs, finer powders are used to enable their vaporization in the plasma plume. The grain size of the powder used in PS-PVD is less than 25 μm, and the morphology of the PS-PVD coatings depends on the deposition parameters [6–9]. Factors that enable a columnar structure of the ceramic layer include: lower feed rates, precise plasma gas composition and high power of plasma gun. Recent reports by G. Mauer et al. [10] have systematized the influence of the PS-PVD parameters on the microstructure of yttria-stabilized zirconia (YSZ) coatings. Using an Ar/He2 mixture and high feed rates (40–80 g/min) results in a dense ‘splat-like’ coating. Application of mixed helium and argon as plasma gasses in combination with a feed rate of 20 g/min results in a columnar structure made up of nano-sized solid clusters, described as ‘quasi-PVD’. At a feed rate of 2 g/min, it is possible to obtain coatings with a similar structure by EB-PVD. The coating structure obtained in the feed rate range between 10 and 20 g/min is a unique columnar structure. More details regarding the influence of YSZ deposition parameters on the structure of PS-PVD coatings were described in a report prepared by Harder et al. (NASA) [6]. In their experiments, they used a short-time deposition process. Special plasma gun movement was used to protect the samples from overheating. Additionally, a lower power of the plasma gun

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Please cite this article as: M. Goral, et al., Surf. Coat. Technol. (2013), http://dx.doi.org/10.1016/j.surfcoat.2013.09.028

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was available (up to 2000 A). Based on these results, an extended experimental procedure was prepared. A full-time deposition process was conducted, with a higher plasma torch power and new types of bond coats. The impact of these process parameters is described in this paper.

a pressure of 150 Pa, and an Ar/He ratio of 35/60 NLPM. The process parameters are shown in Table 2. The coating thickness was measured using a NIKON Epiphot 300 light microscope equipped with a digital camera and image analysis software (NIS Elements), and the microstructure was observed using a scanning electron microscope (Hitachi S-3400 equipped with an EDS analyzer by Thermo).

2. Experimental 3. Results The influence of the deposition parameters on the coating structure during the ‘quasi-PVD’ process was investigated. This type of coating could be deposited at powder feed rates between 10 and 20 g/min using He/Ar plasma gasses. The microstructure of the obtained ceramic coatings is unique because the evaporation of the ceramic powder was incomplete [10]. The deposition processes were conducted by the LPPS Hybrid system produced by Sulzer Metco. The PS-PVD deposition process was conducted in the Research and Development Laboratory for Aerospace Materials in Rzeszow University of Technology using a system dedicated to this type of process. The plasma system used in the tests was a cutting-edge semi-industrial machine specially designed for hybrid processes (LPPS-TF, PS-PVD, PS-CVD) with a vertical working chamber and load lock. A single cathode O3CP-type plasma gun and 60C powder feeders were used during the deposition trials. Rene 80 nickel superalloy, whose chemical composition is presented in Table 1, was used as the substrate. Cylindrical samples were used, linked by truncated cones. Two types of bond coats were selected for the experiments: - a diffusion NiAl zirconium-modified aluminide coating, deposited by chemical vapor deposition (CVD) on BPX Pro 325S by IonBond - a typical NiCoCrAlY coating (AMDRY 997 powder by Sulzer-Metco, chemical composition shown in Table 1) deposited by atmospheric plasma spraying (APS, F-4 Sulzer-Metco plasma gun). The roughness of the sample was Ra = 7 μm. Prior to deposition, the samples were pre-heated by the plasma gun for approximately 10 min, in accordance with the process parameters developed by NASA [6]. The rotation of the manipulator and the sweeping motion of the plasma gun were maintained throughout the process to control surface temperature and prevent overheating of the samples. A special holder was designed to ensure a small sample-holder surface area, which guaranteed a uniform temperature distribution in the substrate during the process. The average sample temperature during the heating and spraying process was measured as 900 °C by a thermal camera (Pyroview 380 Compact). Based on the recently published results of YSZ deposition, an experimental plan was devised. Metco 6700 yttria-stabilized zirconia oxide powder and a typical helium-argon plasma gas mixture were used. The blocking of powder feeding nozzle in the plasma gun was observed during the process when oxygen and hydrogen were used as plasma gasses. The spraying distance was 1200 mm. Based on research results obtained by NASA and the Institute of Energy and Climate Research in Jülich, the influence of four factors was investigated: -

the power of the gun during the process, the chemical composition of the plasma gasses, the sample motion rates, the processing pressure.

The tests confirmed the influence of the selected deposition parameters on the properties of the ceramic TBC by PS-PVD. As expected, accelerating the feed rate caused an increase in the coating thickness — from b 100 μm to over 220 μm (at the feed rate amounting to 20 g/min, Fig. 1a). However, the formed crystals were observably more ‘split’ at the highest feed rate (Fig. 1b–d). XRD analysis showed the presence of cubic and tetragonal types of yttria oxide only. Upon increasing the feed rate to 15 and 20 g/min, the presence of monoclinic phase was observed. Increasing the pressure to 200 Pa during the PS-PVD procedure led to a reduction in the thickness of the produced ceramic layer and the presence of unvaporized powder particles (Fig. 2a, c). Reducing the pressure to 140 Pa caused no changes in the morphology or thickness (Fig. 2a, b). Only two types of yttria oxide were observed in the XRD pattern of the coating deposited at a pressure of 140 Pa. The monoclinic phase was found at higher pressures (1.5 and 2.0 mbar). Changing the rotation speed had no significant effect on the thickness or microstructure of the PS-PVD coatings (Fig. 3a–c). Increasing the sample rotation caused an insignificant increase in the thickness (approx. 10%). No difference was observed in the phase composition (monoclinic, tetragonal, cubic). The conducted experiments showed that the chemical composition of the gasses used in PS-PVD has an insignificant impact on the microstructure of the ceramic coatings. An increase in the amount of Ar resulted in a reduction of the coating thickness and an increased crystal sparsity (Fig. 4a–c). Differences in the phase analysis were also observed. In the coatings with the base parameters (Ar:He ratio 1:2), monoclinic, tetragonal, and cubic phases were observed. The same phase components were found when using an Ar:He ratio of 1:1. An increase in the amount of Ar affected the tetragonal and cubic phases only. Changes in the plasma gun power were observed to exert a large influence on the microstructure (Fig. 5a–d). At a current intensity of 1600 A, the thickness of the coating did not exceed 100 μm. At the same time, numerous unvaporized grains were observed on the surface of the ceramic coating (Fig. 5d). The presence of monoclinic, tetragonal and cubic yttria oxide was observed. Increasing the current intensity from 2200 A to 2400 A did not have any influence on the thickness or morphology of the ceramic coating (Fig. 5a, c). Under these spraying conditions, the presence of tetragonal and cubic phases was confirmed by XRD analysis. 4. Summary The initial test proved the PS-PVD process to be a promising technology for the deposition of thermal barrier coatings. In all deposition processes, a “quasi-PVD” microstructure was obtained. This type of structure could be deposited using an Ar/He plasma gas mixture, as a

The following fundamental (base) parameters were used: a current intensity of 2200 A, a sample rotation of 20 rpm, a feed rate of 10 g/min, Table 1 Chemical composition of the substrate material: Rene 80 high-temperature nickel super alloy and powder used for the bond coat: AMDRY 997 (wt.%). Material

Ni

Co

Cr

W

Mo

Al

Ti

Zr

C

Ta

Y

Rene 80 AMDRY 997

Bal. Bal.

9.5 23

14 20

4 –

4 –

3 8.5

5 –

0.06 –

0.17 –

– 4

– 0.6

Table 2 Parameters varied during the ceramic coating deposition by the PS-PVD method. Variable

Values

Pressure[Pa] Current intensity [A] Sample rotations [rpm] Feed rate [g/min] Ar/He ratio

140, 150, 200 1600, 2200, 2400 2, 10, 20 10, 15, 20 2:1, 1:1, 1:2



YSZ Coating Thickness [µm]

a

3

b 240 220 200 180 160 140 120 100 5

10

15

20

Feed Rate [g/min]

c

d

Fig. 1. The thickness (a) and microstructure of a thermal barrier coating deposited via PS-PVD; the feed rate amounts to (b) 10 g/min, (c) 15 g/min, (d) 20 g/min.

consequence of its high enthalpy (838,965 J/mol) and temperature (~15,550 K). A powder feed rate between 10 and 20 g/min was too high to achieve full evaporation of the YSZ powder. These results confirm a recent report from Mauer et al. [10]. For the deposition of the ceramic coating from the gas phase, the lowest powder feed rate (b2 g/min) is required. In comparison with ceramic coatings deposited by the EB-

PVD method, PS-PVD allows the production of a similar columnar crystal structure; however, the coating deposited by PS-PVD at a feed rate of 10–20 g/min exhibited wider and more irregular columnar grains. An increase in the coating thickness was triggered by an increased powder feed rate. For bond coats, a roughness Ra b 2 μm was necessary [9]. The APS-deposited MeCrAlY coating used in this investigation


a 120 110 100 90 80 70 60 50 40 130 140 150 160 170 180 190 200

Chamber Pressure [Pa]

b

c

Fig. 2. The thickness (a) and microstructure of a thermal barrier coating deposited via PS-PVD; the work chamber pressure amounts to (b) 140 Par and (c) 200 Pa.


4



a 120 110 100 90 80 70 60 0

2

4

6

8 10 12 14 16 18 20

Sample Rotation Speed [rpm]

b

c

Fig. 3. The thickness (a) and microstructure of a thermal barrier coating deposited via PS-PVD; the sample rotation rate amounts to (b) 2 rpm and (c) 10 rpm.

should be polished prior to the PS-PVD process. The structure of the ceramic coating deposited onto Zr-modified aluminide coatings suggests that special grinding or polishing of the bond coat is not necessary. This type of bond coat with PS-PVD coatings displays good erosion resistance [11]. The pressure inside the working chamber exercised a strong influence on the structure and thickness of the coating. In coatings deposited under a pressure of 200 Pa, unevaporated powder particles and a significantly lower coating thickness were observed. The same

effect was observed when the power current of the plasma gun was decreased to 1800 A. Increasing the power current to 2400 A did not influence the structure of the coatings. The sample rotation speed did not have any significant effect on the coating structure. In this experiment, a sweeping movement of the plasma gun was used to protect against overheating, and consequently, a zig-zag columnar structure was not observed [7]. In comparison with NASA experiments [6], the tests were terminated after a significantly longer period of time, and a


a 120 110 100 90 80 70 60 50 1:2

1:1

2:1

Ar/He Plasma Gas Ratio

b

c

Fig. 4. The thickness (a) and microstructure of a thermal barrier coating deposited via PS-PVD; the plasma gas Ar/He ratio is (b) 1:1 and (c) 2:1.




a

5

b 120 110 100 90 80 70 60 50 40 1400 1600 1800 2000 2200 2400

Current [A]

c

d

Fig. 5. The thickness (a) and microstructure of a thermal barrier coating deposited via PS-PVD; the current intensity amounts to (b) 1600 A, (c) 2200 A, 2400 A (d).

substantially higher current intensity was applied (exceeding the 2000 A available for the device installed at NASA). An analysis of the influence of other parameters on the structure of the coating is necessary. The gas composition (addition of hydrogen or oxygen) could change the morphology of the deposited columnars. The relationship between the coating structure and the spray distance should also be investigated. Further research must be performed to examine the influence of the ceramic coating morphology on its operational properties: how it affects the adhesiveness, oxidation resistance, hot corrosion, and erosion. In this way, the coating may be compared with APS- and EB-PVD thermal barrier coatings.

Acknowledgments This research has been funded by the National Science Centre as a part of 2011/01/D/ST8/05228 research project.

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