Fracture analysis of Thermal Barrier Coating Delamination under Thermal Shock

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1 Available online at ScienceDirect Procedia Engineering 00 (2014) APISAT2014, 2014 Asia-Pacific International Symposium on Aerospace Technology, APISAT2014 Fracture analysis of Thermal Barrier Coating Delamination under Thermal Shock Chen Ping-wei a,wang Shao-ming b,wang Feng-Hui a,b, * a AVIC Shenyang engine design and research institute, Shenyang , China b Northwestern Polytechnical University, Xi an , China Abstract Thermal barrier coatings are widely used in section of turbine engines. The effect of a transient thermal load on a coating which is bonded to a square substrate is analyzed using FEM method. Coatings are assumed to contain embedded cracks and they are thermally loaded according to a transient low temperature environment. Thermal barrier coating mixed I/II mode fracture criterion was established, Our models take into account various locations of cracks, crack delamination instances including different crack positions between ceramic coating and bond coating were analyzed. It s found that under thermal shock the crack tip fields are shear-dominant. The total energy release rate and the stress intensity factor of mode II of interfacial crack is greater than that of ceramic coating, indicating that once the unilateral interfacial crack appears, the unstable propagation of interfacial crack is more likely to occur subsequently The Authors. Published by Elsevier Ltd. Peer-review under responsibility of Chinese Society of Aeronautics and Astronautics (CSAA). Keywords: Thermal barrier coating; Finite element method; Delamination; cracks; Thermal Shock 1. Introduction Thermal barrier coating (TBC) has been widely used in the hot section of aero-turbine engines to protect a base material from severe chemical corrosion, or high temperature oxidation. Advanced coatings made of ceramic materials have been developed to improve the efficiency of internal combustion engines and gas turbines. *Corresponding author. Tel: address: nwpu_cpw@126.com The Authors. Published by Elsevier Ltd. Peer-review under responsibility of Chinese Society of Aeronautics and Astronautics (CSAA).

2 2 Chen Ping-wei/ Procedia Engineering 00 (2014) Under the condition of thermal shock, the typical failure mode of TBC is crack initiation, Crack propagation and spallation[1-3]. Research on fracture problem of TBC materials in the thermal shock has important reality. Crack problems of Gradient materials under transient thermal loading were studied by Jin, Noda, Batra using theory method [4-5]. Hutchinson [6] studied the energy release rate of delamination crack in the cooling process. In this paper, the finite element software was used to calculate energy release rate of TBC system with cracks under thermal shock, we also studied fracture behavior of TBC. 2. Fracture parameters Usually, thermal barrier coating system consists of three layers, the ceramic and substrate, the bond coating between substrate and ceramic. The Ceramic is a heat insulation material; the bond coating protects the substrate from High temperature oxidation; the super alloy substrate is mainly exposed to mechanical load. As the overall processing of the material and the working environment, micro cracks and porous microstructures generally can be found in the ceramic and bond coating. The cracks can be divided into the crack in the ceramic layer and Interface crack between the ceramic and bond coatings. These internal cracks may generate stress concentration and grow to large-sized cracks under thermal shock conditions. Thus, the fracture behavior of coatings due to their microstructural defects is important in formulating a comprehensive understanding of the toughness of TBCs. One is a homogeneous crack located somewhere within the ceramic coating and the other is an interface crack lying between the ceramic and bond coats as shown in Fig. 1. The thicknesses of the ceramic coating and bond coat are chosen as t C =450 um and t B =50 um, respectively, and represent typical of thicknesses of TBCs. Fig.1 Two locations of embedded cracks shown in the cross-sections of axisymmetrical models. The location of the crack d is measured from the surface of the ceramic coating. (a) Homogeneous crack within the ceramic coating. (b)interface crack between the ceramic and bond coatings. Because of the complex and constantly changing stress field distribution of TBC, the structure of TBC with cracks suffer deformation in mode I (the opening mode)and mode II(the shearing mode) type, the mismatch of the adjoining material properties generates normal stress and shear stress near the crack tip, this causes the so-called mixed-mode fracture even under symmetrical loading conditions. In order to characterize the crack driving force and toughness of cracks in layered or biomaterial structures under mixed-mode loading conditions, various studies have established that the energy release rate G and the phase angle are the most suitable set of fracture parameters. When employing models based on either the linear fracture mechanics or the elastic plastic fracture mechanics without crack growth or large elastic unloading, the energy release rate is equivalent to the path-independent J- integral. The MCCI [7] method is used to calculate the strain energy release rate G, this method was first used by Rybicki and Kanninen in solving mixed-mode problems.g I and G II were calculated by the stress and displacement of crack tip.

3 Chen Ping-wei/ Procedia Engineering 00 (2014) a 1 GI lim yy ( r,0) ( a r, ) dr a 0 2 a (1) 0 a 1 GII lim xy ( r,0) u( a r, ) dr a 0 2 a 0 Total energy release rate G=GⅠ+GⅡ. Here yy (r,0), yy (r,0) are the stress distribution of the crack tip; (Δ a -r, ),u (Δ a -r, )are the displacement distribution of the crack tip;gⅠ and GⅡ are energy release rate of mode I and mode II respectively;δ a is the length of crack propagation. Since the strain energy release rate G is strongly dependent upon the ratio of the mode I and II stress intensity factors, it is essential to evaluate the mixed-mode KⅠ and KⅡ of a crack in the layered medium. In general, most cracks show higher critical energy release rate when KⅡ is relatively larger than KⅠ. In order to extract the stress intensity factors, a computational approach involving an interaction energy integral is used for cracks in linearly elastic materials. The interaction energy release rate is based on the principle of superposition, and it can be used to numerically separate KⅠ and KⅡ. The no dimensional phase angle Ψ is used to quantify the ratio of mode I and mode II. It is defined as Ψ=tan -1 (KⅡ/ KⅠ). 3. Linear elastic model (2) In this paper, various conditions which affect fracture behavior are investigated towards minimizing the TBC failure due to internal crack growth. It is assumed that a penny-shaped crack exists within the coating system and the TBCs are exposed to a low temperature environment. We assume that the initial temperature of the entire model is 1050 degrees, with no stress field in the modal, then Heat flux boundary conditions are imposed to model. The convection heat transfer coefficient of cold water is 3000W/m 2 ºC; the temperature of water is 25 degrees. The Substrate material is K 3 alloy, with the bond coat material of NiCoCrAlY and ceramic material of ZrO 2 + 8% w.t Y 2 O 3. The properties of the three materials are listed in Table 1. Here E is the Young s modulus, ρis the density, c is the specific heat, νis poison s ratio, αis the coefficient of thermal expansion and кthe thermal conductivity of the materials. We have assumed the materials to remain linearly elastic for the range of temperature considered and no temperature dependent effect is included throughout our analysis. Table 1. Summary of material parameters. Materials K/W m -1 ºC -1 ρ/kg m -3 c/j kg -1 C -1-1 E/GPa ν α/10-6 ºC Substrate Bond coat Ceramic coat Computational results In the finite element analysis, the energy release rate is shown as a function of the Thermal Shock. We analyzed the changes of energy release rate and phase angle under different crack length (a = 0.5, 1, 1.5) and different thermal shock time (t = 0.2, 2, 5, 10, 15) conditions. Here, two separate analyses are carried out, one model with the interface crack between the ceramic and bond coats and the other with the homogeneous crack within the ceramic layer. The crack radii of both models are set at a=1mm.the figure 2 shows G increasing with Thermal Shock time. The energy release rate G of the interface crack and the crack in the ceramic layer increases rapidly, and then come to a balance. In addition, the energy release rate of the interface crack is higher than that of the crack in the ceramic layer, because the higher shear loading due to the mismatch of materials around the ceramic-bond interface.

4 4 Chen Ping-wei/ Procedia Engineering 00 (2014) We have also investigated the effect of crack radius using various models as shown in figure 3. Here only the results at t=2s are shown for both the interface crack and the crack in the ceramic. In general, G of both cracks increases with the crack radius but at different rates. Fig.2 Energy release rate as a function of the thermal shock time Fig.3 Energy release rate as a function of the crack length The result of phase angle is shown in figure 4. Within thermal shock t=2s the phase angle increases rapidly. When t=2s, the computed phase angles show Ψ=91ºfor the interface crack while that of the crack in the ceramic isψ=53º, the crack tip fields are strongly shear-dominant. Both phase angles decrease for longer thermal shock time but Ψis always larger for the interface crack because of its geometrical condition. It notes that the phase angles of the interface crack and crack in ceramic approach the steady-state value of about 84ºand 49ºrespectively. KⅡ increases quickly when transient thermal shock occurs, the crack tip fields are strongly shear-dominant. The stress intensity factor of mode II of interface cracks is larger than that of the ceramic. With the increase of thermal shock time, the trend of the opening mode and the shearing mode come to a balance, which can be seen in Fig5. Fig.4 Corresponding phase angle as a function of the thermal shock time Fig.5 Amplified Corresponding phase angle when t>2s The figure 6 shows that when thermal shock time t=0.2s the phase angle grows with the increase of crack length. However the phase angle grows slowly when crack length is greater than a=1. The figure 7 shows When thermal shock time t=2s, the phase angle grows very slowly.

5 Chen Ping-wei/ Procedia Engineering 00 (2014) Fig.6 Corresponding phase angle when t=0.2s Fig.7 Corresponding phase angle when t=2s 5. Conclusions In this paper, under thermal shock conditions, we discussed the fracture parameters of cracks with different position and different length in the thermal barrier coatings. From the analysis results, Strain energy release rate at the interface crack is larger than that of ceramic coating. Due to the mismatch in the material, the energy release rate of Interface crack increases greatly. The phase angle of interface crack has a greater change. The stress intensity factor of mode II of interface cracks is larger than that of the ceramic. Interface crack is more likely to be damaged under thermal shock. This result provides a foundation for calculating coating failure analysis under thermal shock. References [1] ZHOU Bin, Kokinik K. Effect of pre-existing surface crack morphology on the interfacial thermal fracture of thermal barrier coating: a numerical study [J]. Materials Science and Engineering A, 2003, 348(1-2): [2] Madhwal M, Jordan E H, Gell M. Failure mechanism s of dense vertically-cracked thermal barrier coating [J]. Materials Science and Engineering A, 2004, 384(1-2): [3] WANG Yongjun, WANG Fenghui, WU Yingxi. Stress analysis of thermal barrier coatings failure under thermal shock [J]. Journal of Materials Science and Engineering,2007, 25( 3) : (in Chinese) [4] Jin Z H, Noda N. Transient thermal stress intensity factors for a crack in a semi- infinite plane of a functionally gradient materials [J]. Int J of Solids and Structures, 1994, 31: [5] Jin Z H, Batra. Stress intensity relaxation at the tip of an edge crack in a functionally graded material subjected to a thermal shock [J]. J of Thermal Stress, 1996, 19: [6] Evans A G, Hutchinson J W. The mechanics of coating delamination in thermal gradients [J]. Surface & Coatings Technology, 2007, 201: [7] Aktaa J, Sfar K, Munz D. Assessment of TBC systems failure mechanisms using a fracture mechanics approach [J]. Acta Materialia, 2005, 53: