TMMOB Metalurj i ve Malzeme Mühendisleri Odas ıBildir i ler Kitab ı

15318. Uluslararas ı Metalurj i ve Malzeme Kongresi | IMMC 2016

Th e Eff ects of Particle Size on the Microstructural Properties of YSZ 50% wt – LZ 50% wt Composite Th ermal Barrier Coating

Utku Orçun Gezici, Muhammet Karabaş, Ayşe Kılıç, İsmail Yılmaz Taptık

İstanbul Technical University - Türkiye

Abstract Thermal barrier coatings (TBC), are used to protect the base material against elevated temperature and other environmental impacts in the high-temperature applications such as gas turbine blades. The microstructure design of thermal barrier coatings directly affects the coating properties. In the microstructure of TBC; cracks, gaps, semi–melted phases are valid. Microstructural differences occur in the variation of the process parameters. Spray distance, powder particle size, feed angle and rate, base material type and temperature are the effective parameters of microstructural differences. In this study, the thermal barrier coating material used as yttria stabilized zirconia (YSZ); examined in three classes according to their particle size as finer, medium and coarse, respectively. Other process parameters are kept constant. Also commercial YSZ (Sulzer AMDRY 6643) is used as reference in the study for comparison. YSZ is stirred with 50 wt% Lanthanum zirconate (LZ). Mixed powders are coated on stainless steel with Atmospheric Plasma Spray (APS) method using 9MB plasma gun. The effects of different powder particle size on the porosit, and surface roughness are investigated with The Scanning Electron Microscopy (SEM) analyses.

1. Introduction

Thermal barrier coatings (TBC), are used to protect the base material against elevated temperature and other environmental impacts in the high-temperature applications such as gas turbine blades. Thermal barrier coatings allow higher gas temperature in gas turbine. Due to the high gas temperature, the efficiency of the gas turbine engines increase. A TBC consists of a heat-resistant ceramic top coat and a metallic bond coat. The main purpose of bond coat is to attach the top coat to the base material. Also the bond coat prevents the substrate from oxidation and reduces the thermal expansion differences of ceramic layer and substrate. Bond coat, MCrAlY(M= Co, Ni), is coated between ceramic layer and base material with HVOF process. The top coat of TBCs is a ceramic layer, known as thermal insulation layer, is applied on the bond coat with Atmospheric Plasma Spray process [1-3].

The microstructure of TBC depends on many parameters such as spray distance, powder particle size, feed angle and rate, base material type and temperature. These parameters change the characteristics (cracks, gaps, semi–melted phases) of the microstructure. There are two kinds of cracks at the microstructure of the TBC. One of them is vertical and the other one is horizontal. Horizontal cracks are perpendicular to heat flux. It means, horizontal cracks block the heat transfer. But the horizontal cracks are easily affected by residual stress. Vertical cracks, segmentation cracks are parallel to the heat flux. It means, vertical cracks allow heat transfer. But vertical cracks handle the thermal expansion mismatch between top coat and substrate [1-3].

Feedstock powder particle size play a primary role in the microstructure of TBC. Finer particle sizes cause a decrease in porosity, roughness and crack lengths. On the other hand, finer micro structure causes a decline in the thermal expansion coefficient of the TBC. As this leads to a mismatch between top coat and substrate, residual stress occurs and this damages TBCs [4].

In this study, three different particle sizes of YSZ is used A composite TBC is obtained with YSZ powders mixed with LZ.

2. Experimental Procedure

The study consists of two steps. The first one is the powder preparation and particle size analyses. And the second one is production and characterization of TBC.

2.1. Powder Preparation

At the beginning of the process, commercial YSZ powder Sulzer Metco 204NS (ZrO2–8 wt.%Y2O3) is divided into three classes. Two different sieve were used for the classification of the YSZ powder. One of the sieves has 45 um mesh and the other one has 75 um meshes. After the separation of the powders, particle size distribution is analyzed (Table1). The powder classes were mixed with commercial LZ via turbula mixer for 4 hours. The mixture of powders comprises of wt %50 YSZ and wt %50 LZ.

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Table 1. Powder Distribution of YSZ Surface

Weight Mean

Vol. Weight Mean

d(0.5)

Coarse Powder

78.090 82.970 80.134

Medium Powder

57.208 60.518 58.208

Finer Powder 46.638 52.280 49.650

2.2. TBC Preparation Disc shaped 316L stainless steel samples with a diameter of 25.4 mm and thicknesses of 2mm were used as a substrate material. Prior to bond coat production, the substrate was grit blasted with using 50-80 grain mesh alumina. Commercial Sulzer Metco Amdry 997 (Ni- 23Co-20Cr8 .5Al-4Ta-0.6Y) powders were used for the bond coats. The spray torches (APS and DJ2700 HVOF gun) were fastened on a three-axis CNC robot and gun speed is 600 mm/min. Grit blasted samples were clamped on the turntable and the number of passes was 20. Four types of coating were produced by atmospheric plasma spray process with Sulzer Metco 9MB plasma spray gun. The nozzle of plasma gun was Sulzer Metco 730C. The cross section microstructure and thickness of the coatings were inspected with Scanning Electron Microscopy (SEM) (JSM-7000F Model Field Emission SEM). Porosity analysis was carried out by cross section SEM micrographs of the coatings in compliance with ASTM 2109 standard. 3. Results and Discussion

Microstructure and porosity of TBC showed differences on powder particle sizes.

3.1. Microstructure of TBC

The cross sections of YSZ + LZ TBCs including bond coat and substrate were given at Figure 1. Characteristics of TBC’s microstructure, porosity and cracks were investigated.

Figure 1. Layers of Thermal Barrier Coating System

In the microstructure two different sections were observed. One of them is dark and the other one is light. EDS results show that the dark side is YSZ and the light side is LZ. YSZ and LZ are distributed homogenously in microstructure. Adhesive bonding between YSZ and LZ was observed. In that way, composite coating of YSZ and LZ is produced.

Figure 2. LZ and YSZ distribution in TBC

The decrease in particle size on microstructure causes a decline in porosities. It was observed that the coats which used coarse powder has more porous structure. The finer powders lead to a decrease in porosity. These results are consistent with the literature [5]. Porosity measurement values of the TBC are summarized at Table 2.

Table 2. Porosity Measurement Sample Porosity (%) Coarse 17.705 Medium 11.665 Finer 5.852 Commercial 9.540

3.2. Surface Roughness

Surface roughness of engine turbine blades provides an increase in erosion, corrosion, and deposition during working under high pressure and temperature circumstances. Also, due to the increase in roughness value and also at high Reynolds numbers, aerodynamically losses increase [6].

TMMOB Metalurj i ve Malzeme Mühendisleri Odas ıBildir i ler Kitab ı

15518. Uluslararas ı Metalurj i ve Malzeme Kongresi | IMMC 2016

Figure 3. Surface roughness measurement

Surface roughness measurement summary is given at table 3. Maximum height difference (Rt) and average roughness (Ra) of the coating was observed at commercial powder. That means large particle distribution cause high roughness. Also coarse powder causes high height differences. In addition, the medium size powder had the second highest surface roughness after commercial powder. There is any study in the literature to compare these results.

Table 3. Surface roughness measurement results Ra Rq Rz Rt Coarse Powder

9.30 11.75 90.04 127.05

Medium Powder

10.15 13.02 94.95 103.51

Finer Powder

8.40 10.52 76.71 83.33

Commercial Powder

13.89 17.72 142.96 179.54

4. Conclusion

As a result of this study, following important conclusions could be drawn:

• Powder particle size has a great influence on porosity and surface roughness of TBC.

• Finer particles cause less porosity and surface roughness.

• Large particle size distribution causes higher porosity and surface roughness.

• LZ and YSZ bounded adhesively and distributed homogenously in microstructure.

In the further studies, surface roughness and porosity will be simulated to examine the effects on the heat transfer of the TBC.

References

[1] R.B. HERMAN, Plasma Spray Coating, Principles and Applications, VCH, USA, 1-100, 1996.

[2] D. MATEJKA., B. BENKO, Plasma spraying of metallic and ceramic materials, John Wiley and Sons, UK, 11-51, 1989. [3] J.R. DAVIS (ed.), Introduction to thermal spray technology and surface science, Handbook of Thermal Spray Technology, ASM International: Materials Park, OH, 06994G, 1-14, 2004. [4] E. Ciftyurek, %8 Ysz ( triyum le Stabilize Edilmi Zro2) Termal Bariyer Kaplamalarin (Tbk) Üretilmesi Ve Proses Parametreleri Optimizasyonu, M.Sc. Thesis, Istanbul Technical University, 2009, stanbul, Turkey

[5] Chao Zhang, Wen-Ya Li, Marie-Pierre Planche, Cheng-Xin Li, Hanlin Liao , Chang-Jiu Li b and Christain Coddet, Surface & Coatings Technology 202 (2008) 5055–5061

[6] Tao Bain , Jingyuan Liu, Weihao Zhang, Zhengping Zou, Propulsion and Power Research (2014) ;3(2):82–89