Thermal Wave Imaging and Ultrasonic Characterization of Defects in Plasma Sprayed Coatings

One of the primary concerns in thermal spray coatings is bond integrity, as disbonds at the coating/ substrate interface could cause premature failure of a component. Current quality control test practices use empirical destructive mechanical test methods that do not provide direct correlation between test results and coating performance. Nondestructive evaluation (NDE) techniques used for bulk materials are not readily adaptable for the evaluation and characterization of coatings. This paper reports on the use of thermal wave imaging and ultrasonic immersion C-scan to characterize artificial disbonds in plasma spray coatings on steel substrates. This work demonstrates the capability of these techniques in characterizing total disbonds and presence of ' kissing bonds ” (where the interface surfaces are in such close contact that they give a appearance of a good bond). The effects of ultrasonic frequency on the C-scan images were also studied. A critical evaluation of both thermal wave imaging and ultrasonic techniques is presented along with their relative advantages and disadvantages.     

Characterization of the degraded microstructures of a platinum aluminide coating

The degradation of a platinum modified aluminide (PtAl) coating and a CMSX-4 superalloy substrate were investigated for cyclic and quasi-isothermal heating to 1200 °C. To accelerate the oxidation of the specimens, the thermally grown oxide (TGO) was removed at 10 h intervals. For up to 80 h of exposure, comparisons of specimens with periodic oxide removal and those without oxide removal were made. Qualitatively, the major changes to the bond coat were associated with phase changes from β-(Ni,Pt)Al to γ′-(Ni₃Al) and precipitate coarsening. This evolution was quantified through backscatter scanning electron microcopy and image analysis. With instrumented indentation, the room temperature coating modulus was also measured at 10 h intervals. Additional results include observations of differences in waviness of the bond coat surface for cyclic, quasi-isothermal, and true-isothermal heating, and observations of rafting near the substrate/coating interface. The differences between cyclic and quasi-isothermal heating indicate that stresses associated with cooling and heating significantly alter microstructural evolution.     

NDI of interfaces in coating systems using digital interferometry

This paper describes a new application of two laser interferometry techniques to the non-destructive inspection (NDI) of coated surfaces. The purpose is to detect interfacial disbond between the coating and the substrate. Debonding is detected by properly exciting the surface of the object under inspection in such a way that the interference fringe pattern is modified rendering the disbond readily visible. The fringe patterns resulting from the associated images were captured using electronic speckle pattern interferometry (ESPI) and shearography. Image processing techniques are applied to enhance the detection and better definition of the debonded layer. Some results and discussions are presented to illustrate the applicability of these two optical techniques to thermal barrier coatings.     

Observations of the spallation modes in an overlay coating and the corresponding thermal barrier coating system

Abstract

The oxidation dynamics of an overlay coating and the corresponding thermal barrier coating system are presented. The particular systems examined are composed of a nickel-based superalloy with an air plasma-sprayed NiCrAlY bond coat and the thermal barrier coating system consists of air plasmasprayed yttria stabilized zirconia layer. Failure can occur in these systems by crack propagation within the ceramic outer layer at the interface with the bond coat. Defects, such as microcracks and pores, are common in plasma-sprayed coatings and within the thermally grown oxide scales. These can act as initiation sites for cracks. The subsequent growth of these cracks can lead to loss of the outer protective materials. Considerable information is available by microscopic examination of sections through test specimens that have been held at temperature for varying amounts of time. By careful sample preparation it is possible to monitor the development of the oxide scale formed during high temperature testing and the sites of failure. Identification of the initiation sites and growth of cracks is important in understanding the spallation process. In this study, scanning electron microscopy is used to provide evidence of the processes involved in the two systems. A comparison of the two coating systems reveals the effect the outer ceramic layer has on the oxide scale growth, and the spallation processes crucial to the understanding of the failure mechanisms of these coating systems.     

Optimization of heating protocol in thermal NDT,short and long heating pulses: A discussion

Flash and continuous heating are widely used in transient thermal NDT. Choice of the most appropriate heating protocol leads to multiparametric optimization involving temperature resolution of an IR imager, injected energy, noise on the surface being tested, temperature signal, and contrast over defect as well as times when maximum signals occur. A theoretical analysis of the thermal inspection of glass fiber-steel and carbon epoxy plastic specimens is given. Experimental illustration is performed by testing Plexiglass specimens using both flash and continuous heating techniques. Decision-making rules and limits of the thermal NDT are discussed as well.     

Thermal fatigue failure induced by delamination in thermal barrier coating

The paper presents the experimental and theoretical investigation on the thermal fatigue failure induced by delamination in thermal barrier coating system. Laser heating method was used to simulate the operating state of TBC (thermal barrier coating) system. The non-destructive evaluation such as acoustic emission (AE) detect was used to study the evolution of TBC system damage. Micro-observation and AE detect both revealed that fatigue crack was in two forms: surface crack and interface delamination. It was found that interface delamination took place in the period of cooling or heating. Heating or cooling rate and temperature gradient had an important effect on interface delamination cracking propagation. A theoretical model on interface delamination cracking in TBC system at operating state is proposed. In the model, a membrane stress P and a bending moment M are designated the thermal loads of the thermal stress and temperature gradient in TBC system. In this case, the coupled effect of plastic deformation, creep of ceramic coating as well as thermal growth oxidation (TGO) and temperature gradient in TBC system was considered in the model. The thermal stress intensity factors (TSIFs) in non-FGM (functional gradient material) thermal barrier coating system is analytical obtained. The numerical results of TSIFs reveal some same results as obtained in experimental test. The model is based on fracture mechanics theory about heterogeneous materials and it gives a rigorous explanation of delaminations in TBC system loaded by thermal fatigue. Both theoretical analysis and experimental observation reveal an important fact: delaminations are fatigue cracks which grow during thermal shocks due to compressive stresses in the loading, this loads the delaminations cracks in mixed I and II mode.     

Black oxide coating and its effectiveness on prevention of hydrogen uptake

A simple thermal immersion test with constant heating rate is shown to be an effective method for investigating hydrogen uptake from the decomposing lubricant. It is proven that the black oxide coating can be used as a barrier to hydrogen uptake. Detailed microstructure and composition characterisation of the black oxide layer were also performed.     

Prediction of optical fiber coating thickness

A simple analysis of the fluid dynamics of the optical fiber coating process is used to predict the resulting coating thickness as a function of draw velocity, coating cup pressure, and resin viscosity for a cylindrical coating die. The effects of surface tension forces and viscous heating are assessed. The analysis is compared to experimental data and found to give good predictions.     

Ultrasonic inspection of adhesively bonded CFRP/aluminum joints using pulsed laser scanning

In this study, we propose an ultrasonic inspection technique for detecting disbonds in adhesively bonded carbon fiber-reinforced plastics (CFRP)/aluminum joints using pulsed laser scanning. A specimen with artificially induced square disbonds was scanned by a pulsed laser for ultrasound generation, and the propagating waves were received by a transducer placed on its surface. A series of images of the traveling waves were obtained by processing the received signals. An initial, quick inspection was performed using low-frequency Lamb waves. Changes in the propagation of the Lamb waves were observed at the disbond regions, and disbonds larger than 5 × 5 mm² were successfully detected. A second, detailed inspection of the detected disbond regions was performed using the high-frequency through-transmission ultrasonic method. The shape of the disbond was precisely imaged, and the evaluated size of the disbond matched well with the actual size. We thus demonstrated the efficiency and the feasibility of the proposed technique for the inspection of adhesively bonded CFRP/Al joints.     

Thermal wave testing of plasma-sprayed coatings and a comparison of the effects of coating microstructure on the propagation of thermal and ultrasonic waves

A thermal wave technique for the non-destructive examination of plasma-sprayed coatings is described. Measurements of molybdenum NiAl and aluminium coatings are presented to demonstrate that the technique can be used for defect detection, coating thickness determination and thermal property evaluation. The effects observed are shown to be attributable to thermal wave interference in the coating material. A semiquantitative model of the coating microstructure is presented to compare its effects on the propagation of ultrasonic and thermal waves. The model predicts the coating material's very high attenuation of ultrasonic waves and a very low attenuation for thermal waves. This is taken to explain the success of thermal waves, and the failure of ultrasonic waves, in producing measurable interference effects in plasma-sprayed coatings.     

热冲击作用下基底/涂层结构的应力分析及结构优化

分析了基底/涂层结构的动态热应力, 并对涂层的厚度进行了优化。采用Fourier 变换法,在L aplace 域内求解了问题的控制微分方程, 然后利用界面连续条件、边界条件以及L ap lace 数值反演, 得出了结构中的动态温度场和热应力场。数值算例表明, 加热速度对结构中的最大热应力、破坏模式以及涂层厚度的优化值都会产生影响。      

Warm spraying—a novel coating process based on high-velocity impact of solid particles

Abstract

In recent years, coating processes based on the impact of high-velocity solid particles such as cold spraying and aerosol deposition have been developed and attracting much industrial attention. A novel coating process called ‘warm spraying’ has been developed, in which coatings are formed by the high-velocity impact of solid powder particles heated to appropriate temperatures below the melting point of the powder material. The advantages of such process are as follows: (1) the critical velocity needed to form a coating can be significantly lowered by heating, (2) the degradation of feedstock powder such as oxidation can be significantly controlled compared with conventional thermal spraying where powder is molten, and (3) various coating structures can be realized from porous to dense ones by controlling the temperature and velocity of the particles. The principles and characteristics of this new process are discussed in light of other existing spray processes such as high-velocity oxy-fuel spraying and cold spraying. The gas dynamics of particle heating and acceleration by the spraying apparatus as well as the high-velocity impact phenomena of powder particles are discussed in detail. Several examples of depositing heat sensitive materials such as titanium, metallic glass, WC–Co cermet and polymers are described with potential industrial applications.     

EB-PVD process and thermal properties of hafnia-based thermal barrier coating

Thermal barrier coatings (TBCs) are being developed for the key technology of gas turbine and diesel engine applications. In general, 8 mass% Y₂O₃–ZrO₂ (8YSZ) coating materials are used as the top coating of TBCs. The development of hafnia-based TBC was started in order to realize the high reliability and durability in comparison with 8YSZ, and the 7.5 mass% Y₂O₃–HfO₂ (7.5YSH) was selected for coating material. By the investigation of electron-beam physical vapor deposition (EB-PVD) process using 7.5YSH ceramic ingot, 7.5YSH top coating with about 200 µm thickness could be formed. The microstructure of the 7.5YSH coated at coating temperature of 850 °C showed columnars of laminated thin crystals. On the other hand, the structure of the 7.5YSH coated at coating temperature of 950 °C showed solid columnars. From the result of sintering behavior obtained by heating test of 7.5YSH coating, it was recognized that the thermal durability of 7.5YSH coating was improved up to about 100 °C in comparison with 8YSZ coating. This tendency was confirmed by the experimental result of the thermal expansion characteristics of sintered 7.5YSH and 8YSZ.

©2003 Elsevier Science Ltd. All rights reserved.     

一体化膨胀型防火涂料的热解机理研究

为解决传统防火涂料的装饰性问题,设计并合成了一种一体化膨胀型防火涂料.采用热红联用技术对该防火涂料的热解行为进行了研究,发现其热解过程可以分为4个阶段:第1阶段后期,炭化反应开始;第2阶段不稳定磷酸酯结构的断裂释放出CO2等气体,使熔融炭化物发生初步膨胀;第3阶段气体产物使炭化层进一步膨胀,最后硬化形成多孔炭化层;最终阶段炭化层高温热解.研究发现该防火涂料的热解反应发生顺序符合膨胀防火体系的要求,可以顺利地形成具有多孔结构的炭化层来保护基材.     

Tests of NASA ceramic thermal barrier coating for gas turbine engines

A two-layer thermal barrier coating system with a bond coating of nickel-chromium-aluminum-yttrium (Ni-16Cr-6Al-0.6Y, in wt.%) and a ceramic coating of yttria-stabilized zirconia (ZrO₂12Y₂O₃, in wt.%) was tested for corrosion protection, thermal protection and durability. Full-scale gas turbine engine tests demonstrated that this coating eliminated burning, melting and warping of uncoated parts. During cyclic corrosion resistance tests made in marine diesel fuel products of combustion in a burner rig, the ceramic cracked on some specimens. However, metallographic examination showed no base metal deterioration.     

Mechanical properties of a low-pressure-plasma- applied Co-Cr-Al-Y coating

The low pressure plasma spray process was used to deposit coatings of a Co- 29Cr-6A1-1Y alloy onto IN-738 test specimens and to make free-standing deposits from which specimens were machined. Tensile tests, stress rupture tests and high cycle fatigue test were run on the coated specimens from room temperature to 870°C and results showed no significant effect of the coating process. A strain-to-coating-cracking test was also run on coated specimens and a coating ductile-to-brittle transition was found between 760 and 870°C. Free-standing Co- 29Cr-6Al-1Y tensile specimens were machined from the thick low-pressure- plasma-sprayed deposits. The tensile properties of these deposits were measured from room temperature to 980°C and results showed that the tensile strength of the deposits ranged from 50% above to 100% below the strength of cast IN-738, while elongation went from 0% to 130%. It is shown that the low pressure plasma process produced clean low defect coatings which perform well in mechanical cycling. However, it is also shown that the Co-29Cr-6Al-1Y coating, at low temperatures (less than 540°C), has relatively low ductilities which could cause early cracking in high strain level testing or service conditions. Some scanning electron microscopy replica and metallographic evaluations are also presented to illustrate the tensile cracking behavior of the coating.     

Retarding of corrosion processes on reinforcement bar in concrete with an FBE coating

Fusion-bonded epoxy-coated rebar (FBECR) was studied by casting in a poor quality concrete and subjecting the specimens to a cyclic wet/dry exposure (monthly) using either 3.5% NaCl or demineralised water over a period of 12 months. On-going measurements of half-cell potentials were made together with regular visual observations. At the end of the test, the bars were removed for visual inspection and assessed for corrosion rate using weight loss. Variables examined included the presence or absence of a coating holiday (3 mm ∅) and the presence or absence of a chromate conversion coating. It was concluded that the FBECR provided a significant improvement in corrosion resistance that was further enhanced by the presence of the chromate conversion coating.     

Modified zirconia abradable seal coating for high temperature gas turbine applications

The results and development of a new full ceramic abradable turbine seal coating material prepared by thermal spraying are presented. The main objective was to achieve high temperature abradability and low mating part wear using an erosion-resistant coating with high temperature stability and thermal shock resistance. The new coating was successfully laboratory tested at temperatures of at least 1100°C (2012°F). Commercial metal-based abradable coatings which are currently available are limited to lower operating temperatures. Typical plasma- sprayed ceramic coatings, because of inherent high particle velocities, are normally to dense to permit abrading without experiencing high turbine blade tip wear damage. In contrast, lower velocity combustion-sprayed ceramic coatings frequently have lower toughness and cohesive particle strength for resistance to abrasive erosion. The new coating material is designed to react exothermically, during combustion spraying, to produce a coating both with high interparticle cohesive strength for resistance to abrasive particle erosion and with controlled porosity for low turbine blade tip wear and effective abradability. Adjustment of spraying conditions gives flexibility to alter the coating hardness and porosity and permits the tailoring of abradability and erosion resistance properties for specific operating requirements.Based on specially developed test methods for high temperature abradability, high temperature particle erosion and thermal cycling, the modified zirconia coating showed superior performance to high performance baseline materials tested in the program. Industrial evaluation of this coating is presently being conducted.     

Design and characterization of bioceramic coating materials for Ti6Al4V

Novel bioceramics used as coating materials for Ti6Al4V were designed and characterized by adjusting the thermal expansion coefficient. The results show that the thermal expansion coefficient (α) of 6PM-B5-F4 coating is 10.1×10⁻⁶/°C, which matched that of Ti6Al4V. The bonding strength between the alloy and 6PM-B5-F4 coating was further measured by the longitudinal pull-off test. The in vitro response of the bioceramic was studied by immersing the specimens in simulated body fluid (SBF). The bioceramic morphology and structure were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier transformed infrared spectroscopy (FTIR).     

冷喷涂CuO/ZnO/Al_2O_3催化涂层微结构

采用冷喷涂方法在铝基板上制备了CuO/ZnO/Al2O3催化涂层,并利用扫描电镜、能谱分析仪对涂层进行了表征。结果表明,冷喷涂前后催化剂颗粒和涂层的组成基本相同;涂层与基板和涂层之间以机械咬合结合为主;加热气体温度较低时,涂层呈现疏松扁平层叠和多孔结构;涂层形貌受加热气体温度与颗粒破碎的影响。在该涂层上进行甲醇水蒸气重整制氢试验,结果表明冷喷涂技术可以成功应用于制备催化功能涂层材料。