黏结层粗糙度对热障涂层高温氧化及力学性能的影响

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1 1674 硅酸盐学报 第 41 卷第 12 期 12 月 硅酸盐学报 JOURNAL OF THE CHINESE CERAMIC SOCIETY Vol. 41,No. 12 December,2013 DOI: /j.issn 黏结层粗糙度对热障涂层高温氧化及力学性能的影响 张小锋 1,2, 周克崧 1,2, 宋进兵 2, 张吉阜 2, 邓春明 2, 刘敏 (1. 华南理工大学材料科学与工程学院, 广州 ;2. 广州有色金属研究院新材料研究所, 广州 ) 2 摘要 : 采用超音速火焰喷涂 (HVOF) 在高温合金 K4169 上制备了 NiCoCrAlYTa 黏结层, 使用不同粒度的氧化铝对黏结层进行喷砂预处理制备出了不同粗糙度的黏结层, 再通过大气等离子喷涂 (APS) 在不同粗糙度的黏结层上制备了 ZrO 2 7%Y 2 O 3 (7YSZ) 热障涂层 (TBCs) 对不同粗糙度的黏结层表面采用体视镜进行观察分析 ; 并对不同粗糙度下制备的热障涂层的高温氧化性能 热震性能 表面硬度 结合强度等进行了测试以及对它们的变化规律进行了对比分析 此外, 对热震失效后热障涂层的黏结层与陶瓷层界面进行了残余应力分析, 计算了热生长层 (TGO) 裂纹尖端应力场强度 (K) 研究结果表明 : 黏结层粗糙度越大, 热障涂层中黏结层的氧化质量增加, 黏结层与陶瓷层界面残余应力增加 ; 当黏结层粗糙度为 3.52 µm 时, 热障涂层有最优的综合性能, 其中结合强度和热震次数分别为 57 MPa 和 52 次 (950 到 22 循环 ) 关键词 : 热障涂层 ; 黏结层 ; 粗糙度 ; 高温氧化 ; 力学性能中图分类号 :TB174 文献标志码 :A 文章编号 : (2013) 网络出版时间 : :42:00 网络出版地址 : Effect of Bond Coating Surface Roughness on Properties of High Temperature Oxidation and Mechanical Properties in Thermal Barrier Coatings ZHANG Xiaofeng 1,2,ZHOU Kesong 1,2,SONG Jinbing 2,ZHANG Jifu 2,DENG Chunming 2,LIU Min 2 (1. School of Materials Science and Engineering, South China University of Technology, Guangzhou , China; 2. Institute of New Materials, Guangzhou Research Institute of Non-Ferrous Metals, Guangzhou , China) Abstract: The paper was to investigate the effect of bond coating roughness on the high-temperature behavior and mechanical properties in ZrO 2 7%Y 2 O 3 (7YSZ) thermal barrier coatings (TBCs) prepared by air plasma spray (APS). The bond coating surface of NiCoCrAlYTa prepared by high velocity oxygen flame (HVOF) was blasted by different sized Al 2 O 3 grits to form different surface roughnesses before the ceramic deposition. The bond coating surfaces were observed by a 3-D optical microscope. The properties of TBCs like oxidation resistance, thermal shock property, bond strength, and residual stress were measured. The roughness of the bond coating affects the lifetime of TBCs. The mass gain increases evidently with increasing the roughness of the bond coating. When the bond coating roughness is 3.52 µm, the TBCs have the longest lifetime (52 thermal cycles), and the bond strength is 57 MPa. Besides, the stress intensity factor of crack tip (K) at interface between bond coating and ceramic coating in TBCs was calculated. Key words: thermal barrier coating; bond coating; roughness; high temperature oxidation; mechanical property 1 Introduction The importance of increased turbine entry temperature has led to rapid developments in blade cooling technology over the past few decades. The technology depends on internal cooling. The use of thermal barrier coatings (TBCs) has received much attention. [1 2] Plasma sprayed ZrO 2 7%Y 2 O 3 (7YSZ) coatings have a low thermal conductivity and are effective to provide thermal protection for turbine aerofoils and retain light mass. A possible side effect, however, is that plasma sprayed TBCs show that failure results from cracks and delamination of the ceramic coating parallel to and near the interface between bond coating 收稿日期 : 修订日期 : 基金项目 : 国家 973 计划 (2012CB625100) 资助项目 第一作者 : 张小锋 (1986 ), 男, 博士研究生 通信作者 : 周克崧 (1941 ), 男, 教授 Received date: Revised date: First author: ZHANG Xiaofeng (1986 ), male, Doctorial candidate. zxf200808@126.com Correspondent author: ZHOU Kesong (1941 ), male, Professor. kesongzhou2004@163.com

2 第 41 卷第 12 期 张小锋等 : 黏结层粗糙度对热障涂层高温氧化及力学性能的影响 1675 and ceramic coating. [3 4] In general, a rough bond coating surface can improve high temperature behaviors, bond strength and lifetime in a turbine engine because it provides mechanical interlocking at the interface between the ceramic and bond coating. However, the stress field in the TBCs depends on the roughness of interface, which may induce the premature failure of the TBCs system. The experiments and numerical results indicate that the periodic undulations of the surface of the bond coating could provide oscillatory residual stress release that causes cracking in the ceramic coating to be arrested locally, which, in turn, prevent or postpones the TBCs spallation. [5 6] In such two layered TBCs, a thermally grown oxide (TGO) layer often appears on the surface of the bond coating caused by oxygen through the 7YSZ layer and prior to active with Al of bond coating for its high affinity. [7] The TBCs with different surface roughnesses of bond coating have different high-temperature behaviors. It is well recognized that the failure of TBCs almost occurs at the TGO layer formed on the bond coating. The formation of TGO layer of the bond coating will increase residual stress and brittleness of the TBCs. [8 9] The purpose of this paper was to investigate the effect of bond coating roughness on the high-temperature behaviors and residual stress in plasma sprayed TBCs. 2 Experimental Two layered TBCs were prepared by air plasma spray (APS) with 7YSZ (AMPERITTM 827, H. C. Starck), in which the bond coatings NiCoCrAlYTa (Amdry 997, Sulzer-Metco) were fabricated by high velocity oxygen flame (HVOF, Model K2, GTV, Germany). The specimens of Ni-based superalloy K4169 with the dimensions of φ 25.4 mm 5 mm were used as substrate materials. The specimens were carried out by grit blasting with alumina under 0.4 MPa. The substrate roughness R a was 3.38 µm. Before the deposition of 7YSZ coatings, the surfaces of bond coating were blasted by different sizes of Al 2 O 3 grit, producing the value of surface roughness R a of 0.98, 1.52, 2.73, 3.52 and 4.41 µm, respectively. Finally, the thickness of bond coating NiCoCrAlYTa and ceramic coating 7YSZ was about 100 and 300 µm, respectively, in which the three-dimensional (3D) surfaces topography of bond coatings were observed by an optical microscope (Model LEICA-DVM5000HD, Germany). The TBCs samples of isothermally oxidation resistance were carried out in a furnace at 950 for 345 h with recorded weigh gain. The thermal shock test was conducted in air furnace. These samples were treated at 950 for 2 h, and followed by quenching into water (22 ) for one cycle. When the TBC has a 5% spallation in area, the number of thermal cycles was recorded and adopted as a criterion for the coating failure. Besides, the microhardness of TBCs was measured during the thermal shock test. A TGO layer at the interface between ceramic and bond coating was characterized by a field emission scanning electron microscope (FE-SEM, Model Nova- Nano-430, The Netherlands). The bond strength of TBCs was tested with pull-off method, which was carried out by Instron universal material test system (ASTM C633-79). The stress intensity factor of crack tip (K) at TGO layer in TBC with thermal cycles of 52 was calculated, in which the ratio of new TGO volume to consumed bond coating volume and radius of an individual splat of grain, etc. were also attained by a software named IQ Materials using scanning electron microscope (SEM). 3 Results and discussion 3.1 Influence of bond coating roughness on mass gain of TBCs Figure 1 shows the 3D surface topographies of bond coatings with different roughnesses measured by an optical microscope. It is clearly indicated that the distance of crest and trough of surface increases with the increase of roughness. In the unit area of bond coating, a higher roughness has more surface area. Figure 2 shows the TGO mass gain of TBCs with isothermal oxidation time of 245 h and different surface roughnesses. It is seen that the mass gain increases with the increase of surface roughness. A higher roughness gives a greater surface area, leading to a greater mass gain during isothermal oxidation test. In general, the rougher surface will increase the net area of surface. Therefore, at the same oxidation rate, the mass gain per unit area will become greater since the area used to calculation was fixed according to the shape of specimen. This reflects that the thickness of TGO does not increase with the increase of surface roughness significantly. [10] 3.2 Influence of bond coating roughness on microhardness, lifetime and bond strength of TBCs Figure 3 shows the relation of average surface microhardness and thermal cycle number for TBCs from as-deposited state to failure, which was carried out from 950 to 22 (water). It is seen that the average surface microhardness of ceramic coating decreased gradually with the increase of thermal cycle number. At 950, there does not exist apparent sintering and aggregation in ceramic coating. However, the residual stress and defects of TBCs caused by original preparation process will decrease evidently, leading to a reduction of surface microhardness with the increase of thermal cycle number. For instance, the surface of ceramic coating contains many microcracks, which will release the stress of ceramic coating after some thermal cycles. [11] To some extent, the surface microhardness is correlated to the porosity of ceramic coating. The TBCs with a higher porosity have a lower

3 1676 硅酸盐学报 Fig. 2 Mass gain curves of isothermal oxidation for different roughnesses of bond coating Fig. 3 Surface microhardness of ceramic coating with different roughnesses of bond coating Fig. 1 3D surface topographies of bond coatings microhardness. The surface roughness of bond coating has an effect on the lifetime of TBCs. For bond coating with a low roughness of 0.98 µm, the average thermal cycle number of TBCs was 7 (from as-deposited state to failure). The highest average thermal cycle was 52 when bond coating was 3.52 µm. When bond coating with the roughnesses of 1.52, 2.73 and 4.41 µm, the average thermal cycle number was 47, 49 and 50, respectively (see Fig. 4). The surface roughness of bond coating could be also related to the bond strength of TBCs. Figure 4 shows the relation between average bond strength and average thermal cycle of TBCs with different roughnesses of bond coating. The average bond strength increases with increasing the roughness of bond coating. The roughness of bond coating contributes to the increase of average bond strength, because the deposition of TBCs mainly results from mechanical combination and part of metallurgical combination. When the roughness of bond coating increases from 0.98 µm to 4.41 µm, the average bond strength increases from 27 MPa to 58 MPa. However, a high roughness of bond coating will lead to a great residual stress at the interface crest between TGO layer and ceramic coating, which easily lead to the spallation of TBCs during thermal shock test. Therefore, the TBCs with the roughness

4 第 41 卷第 12 期 张小锋等 : 黏结层粗糙度对热障涂层高温氧化及力学性能的影响 1677 Fig. 4 Influence of roughness of bond coating on average bond strength and average thermal cycle of TBCs of bond coating of 3.52 µm have a higher average thermal cycle number of 52, compared to the TBCs with bond coating roughness of 4.41 µm (average thermal cycle of 50). 3.3 Analysis of residual stress at interface between bond coating and ceramic coating In the TBCs with the NiCoCrAlYTa bond coating prepared by HVOF, an oxide layer of predominantly Al 2 O 3 formed in the initial thermal cycling, along with some mixed oxide clusters between this oxide layer and the ceramic coating (see Fig. 5(a)). [12] The mixed oxide clusters are comprised of chromic, spinel and nickel oxide (CSN). The TGO formed at the interface between bond and ceramic coating is eventually composed of an Al 2 O 3 layer and CSN clusters. [13] When the residual stress resulted from TGO and coefficient of thermal expansion (CTE) mismatch of TGO and ceramic coating exists, the interface will have a crack propagation, which is a important factor of spallation on TBCs. [14] The effect of the thickness of TGO layer on failure of TBC due to local growth stresses is well known. [15 16] Furthermore, a higher surface roughness of bond coating will result in a greater stress concentration at peak region of TGO, leading to a crack propagation at the interface between TGO layer and ceramic coating. This contributes to the recommendations of a critical TGO thickness for crack initiation. Figure 5(a) shows the crack initiation at peak region of TGO layer. The CTE mismatch between the ceramic and bond coating gives the ceramic coating in overall compression at room temperature. [17] However, these stresses are an order of magnitude lower than the residual stresses in the TGO primarily because the porous and cracked ceramic coating is much more compliant relative to the TGO, and it has a relatively lower CTE mismatch with the bond coating. [18] Also, the out-of-plane stresses result in the vicinity of the interface between TGO layer and ceramic coating and the tension at the crests and compression at the troughs due to the highly undulating nature of the interface between ceramic and bond coating. [19] The stress Fig. 5 Cross-sectional SEM photograph of a failed TBC with thermal cycles of 52 and schematic diagram of the crack propagation in the peak region at crests and toughs will increase at a higher roughness of bond coating. Thus, the TBCs with the roughness of 4.41 µm have a shorter lifetime than that of TBCs with the roughness of 3.52 µm. The tension causes the fracture propagation along the interface between TGO layer and ceramic coating at the crest (see Fig. 5(a)). The analysis by an analytical model proposed by Evans and co-worker [20] indicated that the stress intensity factor K at the crack tip in a TBC can be expressed as shown in equation (1) (see Fig 5 (b)). The values of E and ν can be obtained (i.e., E = 34 GPa, ν = 0.26, ). [14] R, a, m, h can be acquired by TBCs with the roughness of 3.52 µm, and the thermal cycle number of 52 (when R = 10 µm, h = 4 µm, m = 1.3, a = µm). Finally, the calculated value of K of 1.56 MPa can be obtained by the equation (1). For crack propagation in a brittle material of TGO, the crack tip at the interface between TGO layer and ceramic coating will continue expanding and lead to the delamination when K > K IC (critical fracture toughness). [21] The relation between the crack length and TGO thickness would follow the equation (2). [20] The equation (2) indicates that the critical crack size for initiating the TBC spallation is somehow correlated to the TGO thickness. Thus, the determination of the relationship between crack

5 1678 硅酸盐学报 length and TGO thickness may predict the TBC life. The crack length could be measured by industry computed tomography (ICT). [22] 3 R E( m 1) h K = 2(1 + ν ) π a 3(1 ν ) m R 3 2 3/2 a = λh (2) where K is a stress intensity factor, h is TGO thickness, a is crack length, R is radius of an individual splat of grain at the interface, m is the ratio of new TGO volume to consumed bond coating volume, E is elastic modulus of the TBC, ν is Poisson s ratio, λ is a constant. 4 Conclusions Two-layered TBCs with different surface roughnesses of bond coating were prepared. The properties of high temperature oxidation and mechanics were investigated. 1) The mass gain increased significantly with the increase of bond coating surface roughness from 0.89 µm to 4.41 µm during the isothermal oxidation. However, the thickness of TGO did not increase with the surface roughness. 2) The surface microhardness of TBCs decreased with the increase of thermal cycle. The bond strength of TBCs increased from 27 MPa to 58 MPa with increasing the surface roughness from 0.89 µm to 4.41 µm. When the surface roughness is 3.52 µm, the TBCs have the optimum properties that the lifetime is 52 (thermal cycle number) and the bond strength is 57 MPa. 3) The stress at crests of TGO layer could increase at a higher roughness of bond coating. The crack propagation thus appeared at the crest of TGO layer. The lifetime of TBCs is somewhat correlated to the crack length and the TGO thickness. The stress intensity factor of crack tip (K) at TGO layer in TBC after thermal cycles of 52 is 1.56 MPa. Acknowledge This work was supported by TBCs research team at Guangzhou Research Institute of Non-ferrous Metals. The authors also thank Bo-yu Chen from Katholieke Universiteit Leuven for her help. References: [1] KHAN A N, LU J. Thermal cyclic behaviour of air plasma sprayed thermal barrier coatings sprayed on stainless steel substrates [J]. 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