Application of Thermal Barrier Coating in High Temperature Resistance

Application of Thermal Barrier Coating in High Temperature Resistance

Mrinmoy Saha

Mechanical Engineering Department JIS College of Engineering

Kalyani, India

Souradip Mondal

Mechanical Engineering Department JIS College of Engineering

Kalyani, India

Supriya Bakshi

Department of Mechanical Engineering JIS College of Engineering

Kalyani, India

Sandip Ghosh

Mechanical Engineering Department JIS College of Engineering

Kalyani, India

AbstractImprovement of gas turbine performance includes prolonged operating life and high operating efficiency which necessitates use of very high temperature. Due to higher thermal conductivity at elevated temperature very high thermal stress and permanent deformation is developed reducing the blade life and efficiency. Therefore, thermal barrier coating is extremely essential to reduce this effect. In this paper mechanical and thermal properties of gas turbine blade materials have been discussed and various thermal barrier coating techniques along with their effects on material properties have been reviewed. It has been observed that such coatings considerably increase the service life of gas turbine components by reducing the heat penetration into the metallic components. The microstructure of thermal barrier coatings has been found to depend on the manufacturing techniques.

KeywordsCoating, TBC, Temperature, Barrier Coating, Gas Turbine, Turbine blade

1. INTRODUCTION

   To design and develop the best gas turbine engine components exposed to various operating conditions, has been quite challenging in last few decades. Gas turbine components have been traditionally made of nickel, Chromium, Cobalt, Titanium and aluminum-based superalloys. Nickel based super alloys are found to be the best materials to design turbine components. Recently various fields have been researched to improve efficiency of gas turbines operating in high temperature and harsh environments. Improvement of gas turbine performance includes prolonged operating life, high operating efficiency and reduced emissions. To satisfy these conditions, increasing thermodynamic efficiency by increasing turbine entry temperature is the most effective. To improve the turbines performance and lifetime, perfect material must be selected. Initially the gas turbine engines were designed using various types of stainless steels, but they dont exhibit better heat resistance. So, stainless steels were replaced by nickel based super alloys. Such as Nimonic or Inconel that exhibit better heat resistance [1]. A single crystal (SC) casting technology has been introduced to produce single crystal blades. Single crystal blades can be operated at high temperature compared to directional solidification blades. Table 1 shows the representative alloys and their chemical compositions.

   However, at very high temperature core parts will damage if exposed directly without any thermal barrier. So, a Thermal Barrier Coating (TBC) is applied to the gas turbine parts that are directly exposed to the high temperature environment from 1370°C-1590°C. By applying low thermal conductivity coating on gas turbine blades, the temperature of the components can be reduced by approximately 100°C-300°C [2]. By using this technique, the gas turbine can be operated at a combustion gas temperature 250°C higher than the melting temperature of nickel based super alloys. TBCs also increase the service life because they reduce the temperature of metal components. Figure 1 shows a high-pressure gas turbine blade before and after TBC coating. The cooling holes are important attributes to observe in the picture.

   a)
   b)

   Fig. 1. High Pressure Gas Turbine Blade (a) Without Coating (b) With TBC Coating

   TABLE 1 COMPOSITIONS OF ALLOYS USED IN GAS TURBINE BLADES [3]

   Casting Type	Alloy	Compositions (weight %)
   		Cr	Co	Mo	w	Al	Ti	Ta	Nb	Hf	C	B	Zr
   Conventional cast (CC)	IN-713LC	12	–	4.5	–	5.9	0.6	–	2	–	0.05	0.01	0.1
   	IN-738LC	16	8.5	1.75	2.6	3.4	3.4	1.75	0.9	–	0.11	0.01	0.04
   	Rene 80	14	9	5	5	4	5	–	–	0.8	0.17	0.016	0.01
   	MR2747	8	11	0.7	11	5.6	2	4	–	1.5	0.15	0.015	0.03
   Directional Solidified (DS)	Mar-M200HF	8	9	–	12	5	1.9	–	1	2	0.13	0.015	0.03
   	CM247LC	8.1	9.2	0.5	9.5	5.6	0.7	3.2	–	1.4	0.07	0.015	0.007
   	CM186LC	6	9.3	0.5	.4	5.7	0.7	3.4	–	1.4	0.07	0.015	0.005
   	PWA1426	8	5	0.6	8	5.6	1	6	–	—	–	–	–
   Single crystal superalloys (SC)	René N4	9.79	7.39	2	6	3.7	4.2	4.73	0.5	–	–	–	–
   	CMSX-4	6.4	6.4	0.1	5.6	2.9	0.6	6.4	–	–	–	–	–
   	CMSX-10	2.4	3.3	0.4	5.3	5.7	0.2	8.2	0.08	0.03	–	–	–
   	TMS-162	2.9	5.8	3.9	5.8	5.8	–	5.6	–	0.09	–	–	–

2. PREDICTION OF BLADE LIFE AND EFFECT OF METAL PROPERTIES

   1. Gas Turbine Blade Life Analysis

      If high temperature and stress is applied to any material for a long time, the matrial undergoes permanent deformation, such as creep. At this time, the stress is less than the yield strength and the temperature is below the melting temperature. When creep deformation occurs, the material can mechanically degrade or break. Therefore, the study of creep is important to improve safety and reliability. However, it is difficult to experimentally obtain long-term creep data and thats why it is insufficient. Thus, the importance of research for predicting long-term creep life from relatively short-term creep experimental data is gradually increasing [4]. Commonly used creep life prediction models use approaches to predict long-term creep life by extrapolation using time-temperature parameters. This approach has the advantage of requiring a small amount of experimental data to derive the creep life prediction curve. Eq. 1 represents the power law model for steady-state creep rate ss, as a function of temperature T and stress [5].

      (1)

      Where, Qc is the creep activation energy, R is the ideal gas constant, A is a proportionality constant, and n is the creep exponent.

      Fig. 2. Schematic representation of steady-state strain rate versus stress

      [6].

      In the case of power law, Qc and n are assumed to be constant. In a creep deformation, when the temperature is

      increasing, dependence of stress on steady state creep rate is increasing shown in the change of n value (Fig. 2). For predicting long-term creep life from short-term creep tests, the most widely used creep life prediction method for metals is the Larson-Miller parameter (LMP) method [7]. It predicts creep life based on empirical fitting and constant stress assumption as given by the following equation

      (2)

      where, tr is the rupture time in the creep test, PLM is the Larson-Miller parameter and CLM is the Larson-Miller constant. CLM typically has a value between 10 and 50. However, it has limitations in explaining the change of deformation mechanism when the stress and temperature is changing. In recent years, many studies have been conducted to improve the predictability of creep life by solving the above problem. Normalized power law and Wilshire model are creep models normalizing with tensile strength. All creep data are represented in the stress range from /TS = 0 to /TS = 1, and the temperature dependence of creep activation energy (Qc) is improved [8]

      (3)

      Where, M is Monkman-Grant constant, TS is the tensile strength. With this model, it can be seen that the predictability is superior to the LMP model when predicting long-term creep life. Another method of gas turbine creep life prediction is The Wilshire model [9] as given in Eq. 4. It examines the long-term rupture strength by extrapolating the short-term creep measurements.

      (4)

      Where k1 is constant and derived by linear relationship. The Wilshire model shows better long-term creep life prediction accuracy compared to LMP model.

   2. Effect of Temperature on Superalloys properties

   There has been a lot of improvement in manufacturing process of superalloys. Conventional investment casting, directional solidification and single crystal casting is used to cope up with the improvement of the gas turbine inlet

   temperature. However, the higher the thermal conductivity of the super alloys, the easier the heat dissipation and cooling in the blade design. On the other hand, the uniform temperature distribution inside the blade has the advantage of reducing the thermal stress during operation by reducing the temperature gradient. The thermal conductivity of super alloys is influenced by electrons because of the free electrons in the metal than those happening in case of nonmetals by phonons due to lattice vibration as shown in Figure 3.

   Fig. 3. Variation of thermal conductivity of nickel-based superalloys

   with temperature [11]

   The inclusion of various chemical compositions improves the mechanical properties of superalloys; however, they result in point defects in the nickel lattice, which reduces the thermal conductivity. The change in thermal conductivity depends on how many types of alloying materials affecting the conductivity. The melting temperatures of the base superalloy elements are nickel at 1452°C, cobalt at 1493°C and iron at 1535°C [10]. Due to higher thermal conductivity at elevated temperature very high thermal stress and permanent deformation is developed reducing the blade life and efficiency. Therefore, thermal barrier coating is extremely essential to reduce this effect.

3. THERMAL BARRIER COATING MATERIALS & METHODS

   Thermal Barrier coating is a plasma-sprayed ceramic layer applied over a metallic bond-coat layer for the protection of gas turbine blades and other components. These coatings are made of yttria-stabilized zirconia (YSZ), which is used to protect and insulate hot-section metal components of advanced gas-turbines used in aircrafts and power generation. TBCs consist of a top coating of ceramic material that directly blocks the flames and reduces heat transfer and a metal material that binds the top coating and the base material, as shown in the figure 4. Bond coating is a layer of MCrAlY alloy which is used to prevent flame penetration into the superalloy base material [12]. It also enhances corrosion resistance and relieves the thermal stress developed due to the differences in thermal expansion coefficient between the superalloy and top coating. Because of the high oxygen permeability of the top coating, the Al component present in the bond coating reacts with the penetrated oxygen to form a thermally grown oxide (TGO). Therefore, in the design of TBCs, consideration should be given to the TGO as

   well as the heat resistance, high temperature properties, and the techniques used for the deposition of coatings.

   Fig. 4. Scanning Electron Microscope (SEM) images of thermal barrier

   coating (TBC) [12]

   The failure mechanism and corresponding behavior prediction of TBC Coatings in high temperature environment is still under research. The microstructure of TBCs depends on the material used as well as the manufacturing techniques. The microstructure of the coating affects the required performances, such as oxidation resistance and coating durability. Therefore, it is necessary to understand the coating manufacturing techniques. TBCs have different deposition techniques depending on the temperature and material used. TBC methods include Electron beam physical vapor deposition (EB-PVD), Chemical vapor deposition (CVD) and Atmospheric plasma spray (APS). Aspects to be considered before choosing a deposition technique are thickness, composition, surface roughness requirements, cooling hole and cost per part. For coating bond layer CVD is used to coat thin films where deposition rate is slow [13]. APS technique uses ultra-high temperature high-speed plasma jet made of air, He and N2 to deposit the coating powder. It prevents the regeneration and premature delamination of the TBC [14]. In the Electron Beam-Physical Vapour Deposition method, YSZ ingot is evaporated in vacuum chamber by using an electron beam, which is deposited on a preheated substrate with a thin layer of TGO. EB-PVD and APS methods are used to make top coating. CVD method is used to provide excellent thermal barrier performance. The use of TBCs can result in a temperature decrease of as much as 300°C at the metal surface, thereby improving the durability of the metal component and enhancing engine performance. However, there is a growing demand for even higher engine efficiency and longer durability, providing great motivation for developing alternate TBC ceramics with decreased high temperature-thermal conductivity.

4. MECHANICAL PROPERTIES OF TBC COATED SUPERALLOYS

   Important attributes of TBCs are low thermal conductivity, mechanical stability with temperature, and the effect of pores inside the material. Polycrystalline oxides used as TBCs have a thermal conductivity of 1-30 W/m-K, depending on the temperature [13]. Among hem zirconia shows very small thermal conductivity (2 W/m-K). For Yttria stabilized zirconia, as the content of yttria in zirconia increases, the thermal conductivity decreases by more than 50% [14]. Zirconia with rare earths elements increase the durability of

   TBCs, thereby increasing the acceptable temperature of a) turbine entry temperature, and use smaller coating thicknesses than conventional TBCs. A coating thickness of

   250 m including 4% rare earth metals like Gadolinia, Neodymia, or Ytterbia, have thermal conductivities of approximately 1W/m-K at room temperature and approximately 0.88-1.02 W/m-K at 500°C [15].

   1. Effect of Coating Thickness

      Thickness of the top coating and the bond Coating can be

      adjusted when manufacturing the TBC. Figure 5 shows the variations in the interfacial stress as the thickness of the top coating (TC) changes from 200800 m. As the thickness of the TBC increases, the thermal barrier performance improves and the thermal stress generated between the top coating and the bond coating interface is reduced. However, excessively thick coatings cannot be applied to high pressure turbine blades and nozzles in commercial and military jet engines because of the strong centrifugal forces at high speeds [16].

      Fig. 5. Stress variation with TBC coating thickness [17]

   2. Effect of Cracks and Pores developed during coating process

      Although cracks present in the top coating may cause the TBC to fail, but vertical cracks artificially inserted during the coating manufacturing process increases the durability of the TBC. Such cracks improve the life of the coatings by reducing the thermal stresses caused by repeated thermal expansion and contraction. As the vertical crack approached TGO, the stress intensity factor and energy rate decreased because the TGO layer reduces stress concentration. As the crack length increases stress intensity factor (kI) and energy release rate (G) increases, which is consistent with the theory of linear elastic fracture mechanism. Increasing the number of pores decreases the thermal conductivity of top coating and reduces the heat transfer to the base material. It also decreases interfacial thermal stress, thus increases thermal fatigue life of TBC. Excessive pores reduce the adhesion between the top and bond coating.

      b)

      Fig. 6. Cross-sectional view of TBC (a) EB-PVD deposited [17], (b)

      CVD deposited [18]

   3. Effect of coating process on mechanical properties of TBC

   The Coating through the EB-PVD process makes columnar microstructures, resulting in a low planar modulus. This is an advantage in the generation of strain because of the mismatch in thermal expansion coefficients between the metal base material and the ceramic top coating. CVD is used to coat thin films as the deposition rates are slow. Although for non-oxides the process parameters can be optimized, for oxides it is difficult to increase the deposition rate by appropriate selection of the precursor and CVD chamber [18], [19]. Figure 6 shows a microstructure comparison of YSZ coating deposited using EB-PVD and laser CVD. APS based Thermal spraying is used for high melting point metals and ceramic coatings where sprayed materials collide with the substrate at a high speed, allowing the coating to exhibit high adhesion strength and density. During the APS process Al2O3 is formed from the bond coating during preheating, deposition and subsequent heat treatments, which is important to prevent the premature delamination of the TBCs. In this method, the powder has a layered structure which gets stacked on the superalloys. They contain several pores, which causes the thermal conductivity of the TBC to be 0.8-1.1 W/m-K depending on the temperature [20]. The parameters during deposition vary with plasma gas power, nozzle diameter, grain size, base material temperature and the roughness.

5. CONCLUSION

A review has been done on performance of various superalloys used for gas turbine blade manufacturing. A comparative analysis on the need and performance on thermal barrier coating has been discussed. The various Thermal Barrier Coating techniques and their applicability for various need of development has been mentioned. The study reflects that to protect turbine components from damage due to high temperature TBC coating is a must. Since the thermal barrier coating has varying mechanical and thermal properties, it is essential to select the appropriate coating process when applying on existing materials or developing a novel coating. The pores present in the coating play a role in reducing the thermal conductivity, but excessive amounts of pores may deteriorate mechanical properties. Therefore, the thermal barrier coating plays an important role in enhancing durability of gas turbine blades and other components.

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