航空材料学报

2022, 42(1): 1-2.

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Abstract:

Failure mechanism and cracking-resistant design of thermal barrier coatings with long life span

ZHANG Bo, LI Guangrong, XU Tong, YANG Guanjun

2022, 42(1): 1-14.

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Abstract:
Thermal barrier coatings (TBCs) are deposited on metallic components to prevent heat flux due to their excellent thermal insulation function. Nowadays, TBCs have become the key core technology of the new generation gas turbines. TBCs prepared by plasma spraying method are more readily to be failed, which negatively affect the thermal insulation and may cause substrate erosion. Therefore, long life span is an important guarantee for TBCs to achieve thermal barrier function. This paper described the failure mechanism of plasma sprayed TBCs and crack-resistant designs. To begin with, the essential characteristics of plasma sprayed TBCs were revealed. Plasma sprayed TBCs appeared to be lamellar structure with connected 2D pores. As a result, the plasma sprayed TBCs have excellent thermal insulation and strain tolerance at as-deposited state. By thermal exposure, the density of 2D pores is decreased significantly, which dominantly account for the degradation of strain tolerance and thermal insulation. Subsequently, the failure mechanism of plasma sprayed TBCs is revealed. The degradation of strain tolerance leads to increase of driving force for cracking. Consequently, the micro-scale cracks are extended and connected to form large-scale cracks, which is responsible for the failure of plasma sprayed TBCs. Finally, cracking-resistant designs are reviewed from two aspects: decreasing driving force and increasing fracture toughness. It is worth noting that the current cracking-resistant design often has poor performance on thermal insulation. In future research, how to ensure high thermal insulation and long life of the coating while considering the economy is the key direction of the development of a new generation of high-performance thermal barrier coatings.

Metallic bond coats for thermally-sprayed thermal barrier coatings applied to aero-engines and industrial gas turbines: review and prospect

CHEN Weijie, SONG Peng, GAO Dong, WANG Chao

2022, 42(1): 15-24.

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The MCrAlY bond coats used for thermally-sprayed thermal barrier coatings applied to aeroengines and industrial gas turbines are reached their temperature limits. The further development is aiming to extend service life, low production cost and compatibility with new fuels. The new MCrAlYs have to be oxidation resistant and spallation resistant, and able to prevent SRZ formation caused by the diffusion with superalloy substrate, and to avoid the damage of thermo-mechanical properties. This paper reviewed recent developments in this area, and proposed a composite structured low-β / near-γ′ type MCrAlY with a very low ϕ value and CTE closed to Al₂O₃ to achieve the target. This approach may be cost effective, and more attractive to aeroengine and industrial gas turbine manufacturers, as well as coating developers and research institutions. Meanwhile, big data analysis will help to design new coating composition, speed up the development process and reduce R&D cost, lead to the findings of more durable thermal barrier coatings for aeroengine and industrial gas turbine applications.

Effects of air plasma spraying parameters on microstructure and bonding strength of GdPO₄thermal barrier coatings

LI Haoyu, CHENG Yuxian, LIU Lixiang, WANG Lu, WANG Bo

2022, 42(1): 25-32.

[Abstract](170) [FullText HTML] (77) [PDF 1419KB](45)

Abstract:
With the increase of the turbine inlet temperature of gas engines, the widely used Y₂O₃ partially stabilized ZrO₂（YSZ）thermal barrier coatings (TBCs) have been unable to meet the requirements, and new generation TBCs that can survive ultra high-temperatures are urgently needed. Among many TBC candidates, GdPO₄ has a great application prospect. In this study, GdPO₄/YSZ TBCs were prepared by air plasma spraying, and the effects of preparation parameters, especially spraying power on the phase composition, surface morphology, microstructure and bond strength of GdPO₄ coatings were investigated. The results show that the P loss takes place during spraying, and both GdPO₄ and Gd₃PO₇ phases are detectable in the final topcoats, the content of the latter decreases by reducing the spraying power. The GdPO₄ coating microstructure is mainly composed of piles of fully melted spray particles, among which there is porous micro-zone consisting of unmelted particles. With the decrease of the spraying power, the content of the micro-zone increases, and the coating thickness significantly decreases. The bonding strength of GdPO₄/YSZ TBCs decreases with the decrease of spraying power, which is mainly because the cohesion of coating decreases with the increase of unmelted micro-zone. Therefore, low spraying power is not beneficial to the coating bonding strength.

Microstructure and thermal cycling life of LaZrCeO thermal barrier coatings deposited by electron beam physical vapor deposition

MU Rende, LIU Guanxi, XIE Xiaochang

2022, 42(1): 33-39.

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Abstract:
Advanced thermal barrier coatings (TBCs) have attracted extensive attention in the field of advanced aero-engine. The thermal cycling life and failure behavior of advanced TBCs still remain a challenge. This work focuses on crystal structure, microstructure and failure behavior of the LaZrCeO/YSZ double ceramic layers TBCs. A composite of pyrochlore and fluorite is formed in LaZrCeO/YSZ coating. The microstructure of LaZrCeO/YSZ coating is composed of feathery nanostructure and intra-columnar pores. The LaZrCeO/YSZ DCL TBCs exhibit good thermal cycling life at 1100 °C. After thermal cycling test, the cracks initiated and propagated in TGO layer due to the stress concentration, including horizontal cracks and vertical cracks, led to the instability of the whole coating system, and finally caused the failure of TBCs.

Improving molten CMAS resistance of thermal barrier coatings by modified laser remelting method

ZHAO Changhao, YANG Lingwei, XIAO Xueren, ZHANG Jun, PAN Wei

2022, 42(1): 40-49.

[Abstract](175) [FullText HTML] (79) [PDF 1418KB](36)

Abstract:
7% yttria stabilized zirconia (7YSZ) thermal barrier coatings (TBCs) prepared by air plasma spray were laser-remelted, and subsequently pre-heated and Al₂O₃ sol-gel repaired for restraining the crack growth in the remelted coatings. The as-prepared coatings were exposed to high temperature molten CaO-MgO-Al₂O₃-SiO₂ (CMAS) to explore their corrosion resistance. The results show that both the laser remelted and the pre-heated, the laser-remelted coatings are densified by CMAS attack. In addition, the thickness of the densified layers is on the same order of that of the non-remelted coatings. Despite of this, the densified layer in the laser remelted coating repaired by Al₂O₃ sol-gel is much thinner than the other coatings. This indicates that Al₂O₃ sol-gel repairing coupling with laser-remelted method can effectively improve the CMAS resistance of 7YSZ TBCs due to the refractory anorthite generated during corrosion process. This refractory compound produced between CMAS and Al₂O₃ sol-gel is capable to decrease the mobility and corrosivity of the CMAS.

Preparation of CeO₂-doped 8YSZ agglomerated powder for PS-PVD and its coating

NIU Shaopeng, LI Xin, DENG Chunming, SUN Yaoning, SONG Jinbing, WANG Chao, HUANG Yicong, ZENG Wei, DAI Hongliang, HUANG Ke, LIU Min

2022, 42(1): 50-58.

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CeO₂-8YSZ (CYSZ) composite agglomerated powder was synthesized by spray drying of 20% micron-scale CeO₂ powder doped with nano-ZrO₂-8 mol% Y₂O₃ (8YSZ) powder. The effect of the binder (carboxymethyl cellulose, CMC) ratio on the properties of the composite agglomerated powder was investigated with the aid of a laser particle size tester, scanning electron microscopy (SEM) and incidental energy spectrometry (EDS). A CYSZ thermal barrier coating with a columnar structure was prepared by PS-PVD, and EDS analysis of the coating cross-section and surface was carried out. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to analyze the physical phases of the coating. The results show that agglomerated powder with high sphericity, good flowability and uniform particle size distribution can be obtained with a binder ratio of 2%, the prepared coating has a uniform distribution of Ce element, and the coating phase is basically a t-phase structure, in which Ce⁴⁺ replaces Zr⁴⁺ and enters into the ZrO₂ lattice to form a homogeneous solid solution structure, showing the inhibition of the transition from t-phase to m-phase by CeO₂ doping. In addition, the prepared CYSZ coating remained intact after 100 cycles of water cooling at 1100 °C, showing high thermal shock resistance.

Effects of process parameters on microstructure and properties of aluminum alloy fabricated by friction extrusion additive manufacturing

TANG Wenshen, YANG Xinqi, TIAN Chaobo, XU Yongsheng

2022, 42(1): 59-67.

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Abstract:
The friction extrusion additive manufacturing (FEAM) process of aluminum 6061-T651 cylindrical bar was successfully achieved by using independently developed solid-state friction extrusion additive equipment. The forming characteristics, microstructure features and mechanical properties of the final specimen obtained under different rotational speeds were comparatively analysed and discussed. The results show that for a given transverse movement speed of 300 mm/min, a fully dense single-channel double-layer specimen with thickness of 2 mm and 4 mm without any internal defects can be obtained by using the rotational speed of 600 r/min and 800 r/min respectively. The final specimen achieved under the higher rotational speed presents a flat interface, a narrower deposition layer, and a rougher surface because the effects of friction and extrusion experienced by the rotational shoulder are weakened during the deposition process. The plastic deformation and thermal cycle experienced by the bonding interface under 600 r/min are more significant than those under 800 r/min, and the grains are refined to 6.0 μm. The softening degree of the interface obtained under 600 r/min is more serious, and the hardness in this region is only 52.7%-56.2% of the value of the as-received feed rod, while this value can reach 56.0%-61.3% of the hardness of the base material. The final specimen attains a good comprehensive mechanical property. The ultimate tensile strength of the final specimen obtained under rotational speeds of 600 and 800 r/min can reach 66% and 70% of the value of the as-received feed rod respectively, while the percentage elongation after the break can reach 212% and 169% of the value of the base material respectively. The tensile properties of 6061 aluminum alloy prepared in this paper have obvious advantages compared with those of other Al-Mg-Si alloys fabricated by other well-developed additive manufacturing processes.

Microstructure characteristics and formation mechanism of linear friction welded TC4-DT titanium alloy joint

GUO Zhenguo, MA Tiejun, LI Ju, LI Wenya, ZHOU Hui

2022, 42(1): 68-73.

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The microstructure characteristics and formation mechanism of linear friction welded (LFW) TC4-DT damage-tolerant titanium alloy joint were investigated . The microstructure of each zone of the joint was analyzed in detail by using an optical microscopy and a scanning electron microscopy; the microhardness distribution of the joint was tested by means of a microhardness tester. The results show that dynamic recrystallization occurs in weld zone (WZ); the WZ temperature exceeds the β-transus temperature during welding, and both β→α′ and β→α phase transformation occur in WZ under rapid cooling after welding, resulting in a large number of α′ martensite and secondary lamellar α are formed. Due to the high deformation resistance of TC4-DT titanium alloy, the thermo-mechanically affected zone (TMAZ) of this joint is relatively narrow. The structure of the TMAZ is elongated, deformed and broken seriously under the strong thermo-mechanically coupling effect. Moreover, a few α′ martensite and a large number of secondary lamellar α are formed in TMAZ under the condition of rapid cooling after welding. The microstructure characteristics of α colony with different orientation of the base metal (BM) is basically preserved in the heat affected zone (HAZ). However, due to the influence of heat, the mutual diffusion of elements occurs at the α/β phase boundary in α colony, the interlayer β is consumed, and the primary α grows up. The refined crystalline strengthening and second phase strengthening of the WZ microstructure, the strain strengthening and second phase strengthening of the TMAZ microstructure, and the growth of α phase in the HAZ make the microhardness of above zones higher than that of the BM.

Effect of solution cooling methods on properties and residual stress of GH4096 alloy

BAI Yunrui, FU Rui, LI Zuojun, LI Fulin, MENG Lingchao, BI Zhongnan

2022, 42(1): 74-80.

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In order to ensure the main mechanical properties of GH4096 alloy, reduce the internal residual stress of forgings for thin-walled parts and improve the dimensional control accuracy of parts machining, the effect of air cooling (AC), wind cooling (WC), oil cooling (OC) and salt bath cooling (SC) after solution heat treatment on the size, morphology of precipitated phase, mechanical properties and internal residual stress of GH4096 alloy ring forgings was studied. The results show that the secondary and tertiary γ′ phases of the air-cooled sample are coarser than those of the traditional oil-cooled sample, and the creep and high-temperature tensile properties are greatly reduced. The size of the secondary γ' phase is slightly increased, the size of the third γ' phase is equivalent of wind cooled sample, and the creep and high-temperature tensile properties are slightly reduced. The size of the secondary and tertiary γ' phase of the salt bath cooled sample is equivalent, so the difference of creep and high-temperature tensile properties is small. The residual stress in the samples by wind cooling could be controlled within ±100 MPa, which was more than 50% lower than that in the samples by oil cooling and salt bath cooling. The residual stress of the ring forgings by air cooling is lower than that of the ring forgings by oil cooling and salt bath cooling, and the main mechanical properties are only slightly reduced. It is helpful to improve the deformation of GH4096 alloy thin-walled parts during cold machining and improve the dimensional accuracy.

Impact compression characteristics and failure mechanism of 3D five-directional braided composites

CUI Can, MAO Xianbiao

2022, 42(1): 81-91.

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The dynamic compressive properties of 3D five-directional braided composites were investigated by using the Split Hopkinson pressure bar (SHPB) and the high-speed photography device. Through the impact compressive tests along the longitudinal and transverse directions of the sample with the braided angle of 22.3°, the stress-strain curves within the range of 200~1200/s strain rate were obtained respectively. And combined with the dynamic compressive process, the progressive failure law of the materials under high-speed deformation under different strain rates were analyzed. Meanwhile, the failure mode and failure mechanism of the material were further analyzed by combing the macroscopic failure characteristics and microscopic fracture morphology characteristics of the sample. The results show that the material is sensitive to strain rate in both longitudinal and transverse directions, among which the strain rate strengthening effect in transverse directions is more significant. The progressive failure process, stress-strain curve characteristics and the failure modes of the material under different loading directions are obviously different, and change with the increase of strain rate.

Effect of PVA concentration on microwave absorption properties of ZnO nanofibers prepared by electrospinning

CHEN Dan, ZHOU Yingying, WANG Fan, WANG Zehua, YANG Jilong

2022, 42(1): 92-99.

[Abstract](149) [FullText HTML] (66) [PDF 1040KB](14)

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ZnO nanofibers were prepared by electrospinning method in this work. The effects of PVA concentration on the morphology, dielectric properties and microwave absorption properties of ZnO nanofibers were studied. Results show that with the concentration of PVA increases from 6% to 10%, the diameter of ZnO nanofibers becomes smaller, but the knots increase and the thickness is uneven. When the concentration of PVA is 8%, ZnO nanofibers show the best morphology with fine and uniform diameter, smooth surface and less knots. At this point, the complex permittivity of it reaches the highest value with a real part of 15.4-20.8 and imaginary part of 3.6-4.7. It also has an optimum microwave absorption property in a thinner thickness. When the thickness of 70%ZnO nanofiber/paraffin sample is 1.3 mm, the microwave absorption bandwidth of reflection loss below –5 dB reaches 5.4 GHz (12.6-18 GHz), the minimum reflection loss is –16.6 dB. In addition, the content of paraffin also has an important effect on the dielectric property and microwave absorption property of the sample. The complex permittivity of the sample decreases as increasing the content of paraffin. When the content of paraffin is 30% and 20%, the samples have good microwave absorption properties.

2022 Vol. 42, No. 1