sensors Article
An Electromagnetic/Capacitive Composite Sensor for Testing of Thermal Barrier Coatings Yuan Ren *, Mengchun Pan, Dixiang Chen and Wugang Tian * College of Intelligence Science and Engineering, National University of Defense Technology, Changsha 410073, China;
[email protected] (M.P.);
[email protected] (D.C.) * Correspondence:
[email protected] (Y.R.);
[email protected] (W.T.)
Received: 10 April 2018; Accepted: 17 May 2018; Published: 19 May 2018
Abstract: Thermal barrier coatings (TBCs) can significantly reduce the operating temperature of the aeroengine turbine blade substrate, and their testing technology is very urgently demanded. Due to their complex multi-layer structure, it is hard to evaluate TBCs with a single function sensor. In this paper, an electromagnetic/capacitive composite sensor is proposed for the testing of thermal barrier coatings. The dielectric material is tested with planar capacitor, and the metallic material is tested with electromagnetic coils. Then, the comprehensive test and evaluation of thermal barrier coating system can be realized. The sensor is optimized by means of theoretical and simulation analysis, and the interaction between the planar capacitor and the electromagnetic coil is studied. The experimental system is built based on an impedance analyser and multiplex unit to evaluate the performance of the composite sensor. The transimpedances and capacitances are measured under different coating parameters, such as thickness and permittivity of top coating as well as bond layer conductivity. The experimental results agree with the simulation analysis, and the feasibility of the sensor is proved. Keywords: thermal barrier coatings; eddy current; planar capacitor; composite sensor
1. Introduction In order to elevate the thermal efficiency of the engine, it is necessary to improve the temperature of the combustion chamber. The thermal barrier coating (TBC) technology is a surface protection technology developed to meet the high temperature operating state of the engine hot end parts [1]. The conventional TBC structure includes a ceramic layer with low thermal conductivity and a cushioning metal bonding layer. It can not only be used in the aviation field, but also in ship gas turbines, as well as energy and other fields. With the extensive application of TBCs, its life span is concerned as well. Once the peeling and break occurs in the TBCs, the hot end parts of the engine will expose directly to high temperature environments, and the results are very serious. Therefore, it is very important to evaluate the performance of the TBCs during service. Several non-destructive evaluation (NDE) techniques for the assessment of TBCs have been developed, mainly including infrared thermograph, acoustic emission, ultrasonic, impedance spectroscopy, microwave and eddy current testing. An infrared thermograph is widely used to assess TBCs [2–6], which can be employed to test not only the thickness of the ceramic top coating but also the porosity content and thermal conductivity of the TBCs. Palumbo and Tamborrino [7] used an empirical thermographic method to evaluate the thickness of the thermal coating and to discriminate between an unevenness of the thickness and a defect zone. Park [8], Renusch [9] and Yang [10] used the acoustic emission technique to test the thermal fatigue damage of the TBCs during the thermal cycle. The results show that the acoustic emission technique can test the thermal growth Sensors 2018, 18, 1630 ; doi:10.3390/s18051630
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oxide and micro cracks of the TBCs. The initiation and propagation of the crack in the TBCs can be detected with AC impedance spectroscopy [11,12], which is sensitive to defects such as cracks within yttria-stabilized zirconia coatings and growth of thermally grown oxide (TGO) film between the top coating and the bond coating. Ultrasonic technology was used to investigate the TGO layer, the voids, delamination and porosity in TBCs [13–16]. Microwave was used to investigate the feasibility of TBC durability monitoring and non-destructive evaluation of surface crack on the substrate beneath the TBCs [17,18]. Khan [19] and Sabbagh [20] used the eddy current testing method to evaluate the life of TBCs and measure its thickness. Fahr [21] and Roge [22] used an eddy current combined with ultrasound methods to evaluate the performance of TBCs. It can be seen from the above introduction that almost all of the existing methods can only test and evaluate a part of the TBC system. In order to achieve comprehensive evaluation of TBCs, JENTEK Corporation (Waltham, MA, USA) proposed a method combining Meandering Winding Magnetometer (MWM) sensor with Interdigitated Electrode Dielectrometer (IDED) sensor [23]. However, due to the two sensors not having been integrated, the test effect needs to be improved. This paper proposes a composite sensor to test the TBCs, which is based on the principle of eddy current and planar capacitor. In the composite sensor, eddy current testing is used to detect the conductive layer in the TBC system, and planar capacitive testing is used to detect the nonconductive layer in the TBC system. Compared with traditional MWM and IDED sensors, this composite sensor can be utilized to obtain abundant transimpedance and capacitance information from exactly the same test region because of its integrated structure. Information fusion under the two operating modes and different frequencies can be implemented to evaluate the thermal barrier coating accurately and comprehensively. 2. Principle of Electromagnetic/Capacitive Composite Sensor Typically, the TBC is composed of dielectric top coating (TC) and metallic bond coating (BC). The TC is a thin ceramic layer with low thermal conductivity. It is located on the metallic substrate using either plasma spraying or electron beam physical vapor deposition processes. The BC is fabricated to minimize the thermal expansion mismatch between the metallic substrate and the ceramic coating, which is a layer of modified aluminide alloy such as MCrAlY (where M is Co, Fe, Ni, or a mixed combination) [24,25]. It serves to improve the bonding of the ceramic to metallic substrate and to protect the substrate from oxidation. The thickness of the BC is typically 75–150 µm, and the TC is in the range of 80–500 µm. It can be understood that the TBC is a complex coating system that contains both a nonconductive ceramic layer and conductive metal layer, which is hardly to be evaluated with a single method. The combination of electromagnetic and planar capacitive testing can provide a feasible approach for comprehensive assessment of the TBCs. Electromagnetic testing is a non-destructive testing method for conductive materials, which is based on the Faraday’s law of electromagnetic induction. The damnification in material under test (MUT) can be judged with the impedance change of the induction coil. Within the service cycle of the TBCs, the thickness of the ceramic layer or metal bonding layer may change, and cracks may appear in the metal bonding layer or the superalloy substrate. These phenomena will lead to impedance change of the electromagnetic coils. Planar capacitive testing is based on the edge effect of electric field, which is used to test the property of dielectric material beneath the planar capacitor. Within the service cycle of the TBCs, the degradation or shedding of the ceramic layer will result in permittivity change, which can be tested with a planar capacitor. Based on the above principles, this paper presents a composite sensor to test either conductive or dielectric materials in the TBCs, which has two different operating modes: electromagnetic testing mode and capacitive testing mode. A representative scenario showing the unknown properties to be estimated by such a hybrid method is shown in Figure 1. In the electromagnetic testing mode, the output signals of the sensor are affected by the thickness of the top coating (Ht ) and the bond coating (Hb ) as well as the conductivities of the bond coating and the substrate (σb and σs , respectively).
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In the capacitive testing mode, the output signals of the sensor are determined by the thickness and the permittivity of the coating Informationfusion fusionunder underthe thetwo two operating permittivity of the toptop coating (H(H t and εt,εrespectively). Information t and t , respectively). implemented to evaluate the thermal barrier coating accurately modesSensors can be accurately and comprehensively. comprehensively. 2018, 18, xFOR PEER REVIEW 3 of 14 permittivity of the top coating (Ht and εt, respectively). Information fusion under the two operating modes can be implemented to evaluate thermal barrier accurately and comprehensively. Capacitive Testing Mode Composite Sensorcoating Electromagnetic Testing Mode the HtElectromagnetic (Eddy Current Lift-off) Testing Mode Hb
b Current Lift-off) Ht (Eddy
Hb
s
b
s
Top coating Composite Sensor Bond coating Top coating
Mode Ht t Capacitive Testing Ht
t
Bond coating Substrate Substrate
Figure Figure 1. 1. Schematic Schematic diagram diagram of of thermal thermal barrier barrier coating coating (TBC) (TBC) system system testing testing with with composite composite sensor. sensor. Figure 1. Schematic diagram of thermal barrier coating (TBC) system testing with composite sensor.
3. 3. Design Design of of the the Composite Composite Sensor Sensor
3. Design of the Composite Sensor As sensor, a spatially periodic winding serves as both the As shown shown in in Figure Figure2,2,ininthe thecomposite composite sensor, a spatially periodic winding serves as both excitation coil of the electromagnetic unit and the driving electrode of the planar capacitor. The As shown in Figure 2, in the composite sensor, a spatially periodic winding serves as both the the excitation coil of the electromagnetic unit and the driving electrode of the planar capacitor. excitation coil of athe electromagnetic unit and the driving of theuniform planar capacitor. The excitation coil iscoil a meandering structure, which can produce aelectrode moreauniform electromagnetic field in The excitation is meandering structure, which can produce more electromagnetic excitation coil istest a meandering structure, which can producecoils a more uniform electromagnetic field incoil, the material under (MUT). The multi-turn induction locate close to the excitation field in the material under test (MUT). The multi-turn induction coils locate close to the excitation the material under test (MUT). The multi-turn induction coils locate close to the excitation coil, which is designed to detect the the change of of eddy current field. coil, which is designed to detect change eddy current field.The Thesensing sensingelectrode electrodeof ofthe the planar planar which is designed to detect the change of eddy current field. The sensing electrode of the planar capacitor is designed as a U-shaped structure, which can increase the equivalent capacitance and capacitor is designed as aasU-shaped structure, theequivalent equivalent capacitance capacitor is designed a U-shaped structure,which which can can increase increase the capacitance and and testing sensitivity of the planar capacitor. testing sensitivity of the planar capacitor. testing sensitivity of the planar capacitor.
Excitation Excitation coil/coil/ Driving electrode Driving electrode
... ...
Induction coilcoil Induction
... ...
Sensing electrode
Sensing electrode
Figure 2. Schematic diagramofofthe theplanar planar composite Figure 2. Schematic diagram compositesensor. sensor. Figure 2. Schematic diagram of the planar composite sensor. 3.1. Finite Element Modeling and Analysis
3.1. Finite Element Modeling and Analysis In order Modeling to evaluate theAnalysis performance of the composite sensor, a three-dimensional simulation 3.1. Finite Element and Inmodel orderhas to been evaluate the performance of the composite5.2a sensor, a three-dimensional simulation established using COMSOL Multiphysics software (COMSOL Inc., Stockholm, In order to evaluate the performance of the composite sensor, a three-dimensional simulation modelSweden). has beenInestablished using COMSOL Multiphysics 5.2afield software (COMSOL Inc., Stockholm, the electromagnetic testing mode, the physical was selected as magnetic field model has been established using COMSOL Multiphysics 5.2a software (COMSOL Inc., Stockholm, Sweden). InAC/DC the electromagnetic testing mode, the physical field was selected as magnetic fieldthe under under module and solved with frequency domain solver. In the capacitive testing mode, Sweden). In the electromagnetic testing mode, physical wasand selected as magnetic field physical field was selected electrostatic fieldthe under AC/DC module solved mode, with stationary AC/DC module and solved withasfrequency domain solver. In thefield capacitive testing the physical under AC/DC module and solved with frequency domain solver. In the capacitive testing mode, the solver. The geometric dimension of the whole simulation area is 20 mm (length) × 12 mm (width) × field was selected as electrostatic field under AC/DC module and solved with stationary solver. 7.516 mm (height), in which the material under test is the TBC, as shown in Figure 3. The area over physical field was selected as electrostatic field under AC/DC module and solved with stationary The geometric dimension of the whole simulation area is 20 mm (length) × 12 mm (width) × 7.516 mm composite sensor is set toofbethe air with the thickness,area conductivity, relative permeability and × solver.the The geometric dimension simulation 20 mm (length) 12 mm (width) (height), in which the material under test whole is the TBC, as shown inisFigure 3. The area ×over the composite relative permittivity of 5 mm, 0 MS/m, 1 and 1, respectively. The parameters of each layer in the TBC 7.516 (height), which material under test is the TBC, as shown and in Figure 3. permittivity The area over sensormm is set to be airinwith thethe thickness, conductivity, relative permeability relative of system are listed in Table 1. the composite sensor is set to be air with the thickness, conductivity, relative permeability and relative permittivity of 5 mm, 0 MS/m, 1 and 1, respectively. The parameters of each layer in the TBC system are listed in Table 1.
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5 mm, 0 MS/m, 1 and 1, respectively. The parameters of each layer in the TBC system are listed in Sensors 2018, 18, xFOR PEER REVIEW 4 of 14 Table 1.
(a)
(b)
A
B Hair
Induction coil
Air
10 turns w2
w1 h4
Hsensor h5
h1 h2 h3
Htop
Sensing electrode
Hbond
Excitation coil / Driving electrode
w3
w4
Composite sensor w5 Top coating Bond coating Substrate
Hsubstrate
(c) Figure 3. Simulation geometry model. (a) overall structure; (b) vertical view of overall structure; (c)
Figure 3. Simulation geometry model. (a) overall structure; (b) vertical view of overall structure; cross-section of composite sensor for a half wavelength, where Hair = 5 mm; Hsensor = 0.116 mm; Htop = 0.3 (c) cross-section of composite sensor for a half wavelength, where Hair = 5 mm; Hsensor = 0.116 mm; mm; Hbond = 0.1 mm; Hsubstrate = 2 mm; h1 = h3 = 0.0275 mm; h2 = 0.025 mm; h4 = h5 = 0.018 mm; w1 = w2 = 0.1 = w0.1 mm; H = 2 mm; h1 = h3 = 0.0275 mm; h2 = 0.025 mm; Htop = mm; 0.3 mm; w3 = wH 4=bond 1 mm; 5 = 2.5 mm. substrate h4 = h5 = 0.018 mm; w1 = w2 = 0.1 mm; w3 = w4 = 1 mm; w5 = 2.5 mm. Table 1. Parameters of the thermal barrier coating (TBC) under simulation.
Table 1. Parameters of the thermal barrier coating (TBC) under simulation. Thickness Top coating (mm) Bond coating 0.3 Substrate 0.1
Thickness Conductivity Relative (mm) (MS/m) Permeability Conductivity (MS/m) Relative Permeability 0.3 0 1 0.1 0.2 0 1 1 2 1.6 1 0.2 1
Relative Permittivity Relative 12.5 Permittivity 1 12.5 1 1
Top coating Bond coating SubstrateThe composite 2 sensor consists of a1.6 1 two-layer structure, as shown in Figure 3b. The1 vertical
distance is 25 μm between the induction coil plane and the excitation coil plane. The material of the coil and electrode is set to be copper, whose electrical conductivity, relative permeability and The composite sensor consists of a two-layer structure, as shown in Figure 3b. The vertical relative permittivity is 59.98 MS/m, 1 and 1, respectively. Apart from the coil and electrode, the distance is 25 µm between the induction coil plane and the excitation coil plane. The material of the coil material of other part of the sensor is to set to be polyimide, whose electrical conductivity, relative and electrode is set be copper, whoseiselectrical relative permeability and relative permeability andtorelative permittivity 0.004 S/m,conductivity, 1 and 4 respectively. In the simulated model, the permittivity is 59.98the MS/m, 1 andand 1, respectively. theconsidered. coil and electrode, material liftoff between top coating the compositeApart sensorfrom was not Since the the thickness of of other part of the is to set polyimide, whoseit electrical relative permeability the wire andsensor the electrode in to thebe sensor is very small, is set to beconductivity, a surface without thickness in the physical field of electromagnetic, reduce the difficulty of grid dividing the and relative permittivity is 0.004 S/m,which 1 andcan 4 respectively. In the simulated model, and the shorten liftoff between computation time. the top coating and the composite sensor was not considered. Since the thickness of the wire and
the electrode in the sensor is very small, it is set to be a surface without thickness in the physical field of electromagnetic, which can reduce the difficulty of grid dividing and shorten the computation time.
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3.2. Structure Optimization of the Composite Sensor Sensors 2018, 18, xFOR PEER REVIEW
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In order to to improve improvethe thetesting testingperformance, performance,ititisis necessary optimize composite sensor. necessary to to optimize thethe composite sensor. In 3.2.capacitive Structure Optimization of the Sensor In testing mode, the width and spacing of electrodes influence on thethe capacitive testing mode, theComposite width and spacing of electrodes havehave greatgreat influence on the the sensitivity of the sensor. One of the units in the sensor is shown incomposite Figure 4,sensor. in which W1 sensitivity the sensor. One oftesting the units in the arrayed sensor isoptimize shown in 4, in which is Inof order to improve the performance, itarrayed is necessary to theFigure InW1 is width the driving electrode, W2 thewidth width thesensing electrode, G1 is is influence the spacing spacing thethe width ofof the driving the ofofthe electrode, G1 the the capacitive testingelectrode, mode, theW2 width and spacing ofsensing electrodes have great on between the the driving electrode and thethe sensing G2 isG2theisdistance between the two ofends the driving electrode and sensing electrode, the distance between the of the sensitivity of the sensor. One of theelectrode, units in the arrayed sensor is shown in Figure 4,ends intwo which W1sensing is the width of the driving electrode, W2 the width of the sensing electrode, G1 is the spacing between electrode, and L is the of length the linear segment electrode. In order toIn maximize sensitivity, sensing electrode, andlength L is the of the linearof segment of the electrode. order to the maximize the the driving electrode and the sensing electrode, G2 capacitor istothe theinitial two ends the parameters each part of the planar capacitor need be distance optimized, the parameters are sensitivity, the of parameters each part of the planar needbetween to and be optimized, and of thetheinitial sensing electrode, and the==length of Lthe linear electrode. In order to maximize the selected as: W1 = W2 = G1 G2 1 mm, 5 mm. parameters are selected as:L=is W1 W2 = G1 ==G2 = 1 segment mm, L =of5 the mm. sensitivity, the parameters of each part of the planar capacitor need to be optimized, and the initial parameters are selected as: W1 = W2 = G1 = G2 = 1 mm, L = 5 mm.
G1 G1
L
L G2
L
L
G2
W2 W2
W1
W1
W1
W1
Figure 4. Structure of of planar planar capacitor. Figure capacitor. Figure4.4.Structure Structure of planar capacitor.
The sensitivity of the planar capacitoris defined as: The sensitivity of the planar capacitor isisdefined defined as:
C∆C C (1) (1) sensitivity _ C = SSsensitivity Ssensitivity_C , (1) _C ∆ε , , ΔC represents the change of capacitance, Δε represents the of change of permittivity. relative wherewhere ∆C represents the change of capacitance, and ∆εand represents the change relative where ΔC represents the change of capacitance, and Δε represents the change of relative permittivity. It can be seen from Figure 5 that the capacitive testing sensitivity will decrease with the It can be seen from Figure 5 that the capacitive testing sensitivity will decrease with the spacing between permittivity. It can bethe seen from Figure 5 that the capacitive testing(G1) sensitivity will decrease with the spacing between driving electrode and the sensing electrode while increase with other the driving electrode the sensing electrode (G1) while electrode increase with other parameters, especially spacing between theand driving electrode the sensing (G1)electrode. while increase parameters, especially the width of the and driving electrode and the sensing However,with smallother the width of the driving electrode and the sensing electrode. However, small electrode spacing of parameters, width of the will driving and the sensing However, electrodeespecially spacing ofthe planar capacitor resultelectrode in low penetration depth ofelectrode. electric field [26], andsmall planar capacitor will result in low penetration depth of electric field [26], and the increase of the other the increase theplanar other parameters will enlarge sizepenetration of the testingdepth unit, leading to decrease of and electrode spacingofof capacitor will result inthelow of electric field [26], parameters will enlarge the the sizetrade-off of the testing unit, leading to decrease ofsensitivity, spatial resolution. Thus, spatial resolution. Thus, should be made among the testing penetration the increase of the other parameters will enlarge the size of the testing unit, leading to decrease of the trade-off should be made among the testing sensitivity, penetration depth and spatial resolution. depth and spatial resolution. spatial resolution. Thus, the trade-off should be made among the testing sensitivity, penetration depth and spatial resolution.
Figure 5. Influence geometricalparameters parameters on of of capacitive unit.unit. Figure 5. Influence of of geometrical onthe thesensitivity sensitivity capacitive
Figure 5. Influence of geometrical parameters on the sensitivity of capacitive unit.
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3.3. Performance Analysis of the Composite Sensor
When testing the thermal barrier coated blades with an electromagnetic/capacitive composite 3.3. Performance Analysis of the Composite Sensor sensor, either the sensor output or its change caused by coating defect is very small and easy to be When the thermal barrier blades an electromagnetic/capacitive affected by testing other factors. Therefore, it coated is necessary towith analyze the interaction between thecomposite induction sensor, the sensor output change caused by coating defect is very small and easy to be coil andeither the sensing electrode of or theits planar capacitor. affected by other factors. Therefore, it is necessary to analyze the interaction between the induction 3.3.1.and Influence of theelectrode Sensing of Electrode on capacitor. the Electromagnetic Testing coil the sensing the planar the electromagnetic mode, theElectromagnetic transimpedanceTesting of electromagnetic coil is used to 3.3.1.In Influence of the Sensingtesting Electrode on the represent the property of sensor, which is defined to be the ratio of induced voltage and excitation In the electromagnetic testing mode, the transimpedance of electromagnetic coil is used current: to represent the property of sensor, which is defined to be the ratio of induced voltage and V excitation current: Z transfer VS (2) (2) Ztrans f er = I DS ,, ID where IIDD isis the coil, and VS VisS the induced voltage of the where theexcitation excitationcurrent currentflowing flowingininthe theexcitation excitation coil, and is the induced voltage of induction coil.coil. When 20 mA current withwith 100 100 kHzkHz frequency is applied to the excitation coil,coil, andand the the induction When 20 mA current frequency is applied to the excitation thickness of the ceramic layer is changing from 0.07 mm the thickness of the ceramic layer is changing from 0.07 mmtoto0.7 0.7mm, mm,the thetransimpedance transimpedance (Figure (Figure 6) 6) can be be calculated calculated with with Equation Equation (2). (2). can
Figure 6. Variation of transimpedance imaginary part with top coating thickness.
As the the thickness thickness of of the the ceramic ceramic layer layer increases, increases, the the eddy eddy current induced on bond As current induced on the the metal metal bond coating and substrate will become weaker. Thus, the intensity of secondary magnetic field induced coating and substrate will become weaker. Thus, the intensity of secondary magnetic field induced by eddy eddy current current will will decrease, decrease, whose whose direction direction is opposite to the main main magnetic magnetic field field generated generated by by is opposite to the by the excitation excitation coil. the coil. Because Because the the excitation excitation current current and and main main magnetic magnetic field field intensity intensity remain remain constant, constant, the total well as the module of the total magnetic magnetic field field strength strengthwill willincrease, increase,and andthe theinduced inducedvoltage voltageVSVas S as well as the module the transimpedance will increase accordingly. Since the imaginary part of the transimpedance is the of the transimpedance will increase accordingly. Since the imaginary part of the transimpedance is main concern in electromagnetic induction [27], it is mainly studied in the following simulation and the main concern in electromagnetic induction [27], it is mainly studied in the following simulation experiment. and experiment. It can can be be seen seen from from Figure Figure 66 that that the the imaginary imaginary part part of of the the transimpedance transimpedance is is approximately approximately in in It linear relationship with the thickness of the ceramic layer when the sensor contains sensing electrode. linear relationship with the thickness of the ceramic layer when the sensor contains sensing electrode. However, the the relationship relationship is is nonlinear nonlinear when when the the sensor sensor is is working working without without aa sensing sensing electrode. electrode. As As However, shown by ofof coating thickness is shown by the the slopes slopes of ofthe thecurves curvesininthe theFigure Figure6,6,the themeasurement measurementsensitivity sensitivity coating thickness is 3.588 Ω/m when the sensor with sensing electrode, smaller than 4.763 Ω/m when the sensor
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without sensing electrode. The reason is that the eddy current induced on the sensing electrode will 3.588 Ω/m when the sensor with sensing electrode, smaller than 4.763 Ω/m when the sensor without influence the eddy current induced on the bond coating and substrate. However, the improvement of sensing electrode. The reason is that the eddy current induced on the sensing electrode will influence the thickness achieved in the and capacitive testing. the eddy testing current sensitivity induced onisthe bond coating substrate. However, the improvement of the The electromagnetic simulations were carried out under thickness testing sensitivity is achieved in the capacitive testing. eleven different conductivity values of the bond 0.1, 0.2, 0.5, 1, 2,were 5, 10, 15, 25, and 60 MS/m. operating frequency The coating: electromagnetic simulations carried out 40, under eleven differentThe conductivity values of bondascoating: 0.1,in 0.2, 0.5,earlier 1, 2, 5, 10, 15, It 25,can 40, and 60 MS/m. operating frequency is 100 kHz, is 100the kHz, same as the case. be seen fromThe Figure 7 that the imaginary part of as same as in will the earlier case.the It can be seen fromof Figure the When imaginary of transimpedance transimpedance fall with conductivity bond7 that layer. the part conductivity of the bond will fall with the conductivity of bond layer. When the conductivity of the bond coating increases, coating increases, the eddy current induced on the metal bond coating and substrate will become the eddy inducedof onsecondary the metal bond coating and substrate become stronger. Thus, thewhich stronger. Thus,current the intensity magnetic field induced bywill eddy current will increase, intensity of secondary magnetic field induced by eddy current will increase, which will reduce the will reduce the total magnetic field strength, finally leading to the decrease of induced voltage VS total magnetic field strength, finally leading to the decrease of induced voltage VS and the imaginary and the imaginary part of transimpedance. When the sensing electrode exists in the sensor, the eddy part of transimpedance. When the sensing electrode exists in the sensor, the eddy current will be current will be on the sensing the on eddy on the metalis bond coating is induced oninduced the sensing electrode andelectrode the eddy and current thecurrent metal bond coating smaller. In smaller. In general, the sensing electrode will influence the sensitivity of the composite sensor more or general, the sensing electrode will influence the sensitivity of the composite sensor more or less less under electromagnetic testing mode. under the electromagnetic testing mode.
Figure 7. Variation transimpedance imaginary imaginary part of bond layer. Figure 7. Variation of of transimpedance partwith withconductivity conductivity of bond layer.
3.3.2. Influence of the Induction Coil on the Capacitive Testing
3.3.2. Influence of the Induction Coil on the Capacitive Testing In the capacitive testing mode, when a given voltage U is applied between the driving electrode
In the testing of mode, a given voltage U is applied the driving and thecapacitive sensing electrode planarwhen capacitor, a certain amount of chargebetween Q will produce on theelectrode two and the sensing electrode of planar capacitor, a certain amount of charge Q will produce on the two electrodes. The capacitance can be calculated using the equation C = Q/U. When the physical electrodes. Theof capacitance can be calculated the equation C = Q/U. the physical properties properties the dielectric materials inside using the planar capacitor change, theWhen electric field distribution the planar capacitor will also change, thus affecting the the charge produced the electrodes of thearound dielectric materials inside the planar capacitor change, electric field on distribution around and the capacitance of the planar capacitor. the planar capacitor will also change, thus affecting the charge produced on the electrodes and The planar in the composite sensor for TBCs testing can be described with an the capacitance of thecapacitor planar capacitor. equivalent circuit diagram as shown in Figure 8, where Cair is the capacitive contribution of the air, The planar capacitor in the composite sensor for TBCs testing can be described with an equivalent Csf is the fringe capacitive contribution of the insulating layer in the composite sensor, Ctcf is the circuit diagram as shown in Figure 8, where Cair is the capacitive contribution of the air, Csf is the fringe fringe capacitive contribution of the top coating, Cec is the overlap capacitance between the driving capacitive contribution of the insulating layer in thecapacitive composite sensor, Ctcf fringe capacitive electrode and the induction coil, Csp is the overlap contribution of is thethe insulating layer contribution of the top coating, C is the overlap capacitance between the driving electrode and ec the metallic bond coating, and Ctcp is the overlap capacitive between the driving electrode and the induction coil,ofCthe thecoating overlapbetween capacitive of the insulating layer between theThe driving sp istop contribution the contribution driving electrode and the metallic bond coating. electrode and the metallic bondcapacitor coating,can andbeCexpressed is the overlap capacitive contribution of the top total capacitance of the planar by: tcp coating between the driving electrode and the metallic bond coating. The total capacitance of the planar capacitor can be expressed by: C f = Cair +
Csp Ctcp Cec + Cs f + Ctc f + . 2 2(Csp + Ctcp )
(3)
thickness of the top coating increases, Ctcp will decrease and Ctcf will increase, while Cair, Cec, Csf and Csp remains constant, and, moreover, the decrease of Ctcp is about two orders larger than the increase of Ctcf [28–32]; thus, the total capacitance Cf will fall with the thickness of top coating. As shown in Figure 8, the capacitance variation against the thickness of the top coating is nonlinear. This is, however, Sensors 2018,within 18, 1630 expectation considering the non uniform field distribution of the planar capacitor 8 of 14 since most of the field energy is concentrated around the sensor electrodes.
Cf Induction coil
Air
Insulating layer Cair
Driving electrode
Csp
Composite sensor
Cec
Cec Csf
Ctcp
Csp Ctcp
Sensing electrode Top coating
Ctcf
Bond coating (a)
Cair Cec
Cec Csf
Cf Ctcf Csp
Ctcp
Ctcp
Csp (b)
Figure 8. Planar Planar capacitor capacitorininthe thecomposite composite sensor TBC testing: (a) schematic sketch in a view; side Figure 8. sensor forfor TBC testing: (a) schematic sketch in a side view; (b) equivalent circuit diagram. (b) equivalent circuit diagram.
It can be seen from Figure 9 that the planar capacitor with the induction coil has higher In this capacitor model, the capacitor Ctcp is considerably larger than the capacitor Ctcf . When equivalent capacitance and testing sensitivity to the thickness of ceramic layer than that of the the thickness of the top coating increases, Ctcp will decrease and Ctcf will increase, while Cair , Cec , instance without induction coil, which can be explained as following. The voltage applied on the Csf and Csp remains constant, and, moreover, the decrease of Ctcp is about two orders larger than driving electrode will produce charges on the induction coil and the metal bond coating, which will the increase of Ctcf [28–32]; thus, the total capacitance Cf will fall with the thickness of top coating. As accordingly induce charges on the sensing electrode. The change of charge on the sensing electrode shown in Figure 8, the capacitance variation against the thickness of the top coating is nonlinear. This and the driving electrode due to the variety of the ceramic layer thickness is larger than the instance is, however, within expectation considering the non uniform field distribution of the planar capacitor without induction coil. Since the potential difference between the driving electrode and the sensing since most of the field energy is concentrated around the sensor electrodes. electrode is constant, the capacitance change and thickness testing sensitivity of planar capacitor is It can be seen from Figure 9 that the planar capacitor with the induction coil has higher equivalent higher when the induction coil exists. capacitance and testing sensitivity to the thickness of ceramic layer than that of the instance without induction coil, which can be explained as following. The voltage applied on the driving electrode will produce charges on the induction coil and the metal bond coating, which will accordingly induce charges on the sensing electrode. The change of charge on the sensing electrode and the driving electrode due to the variety of the ceramic layer thickness is larger than the instance without induction coil. Since the potential difference between the driving electrode and the sensing electrode is constant, the capacitance change and thickness testing sensitivity of planar capacitor is higher when the induction coil exists.
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Figure 9. Variation of capacitance with ceramic layer thickness. Variation of of capacitance capacitance with with ceramic ceramic layer layer thickness. thickness. Figure 9. Variation
It can be seen from Figure 10 that the capacitance of the planar capacitor will increase It can canbewith be seen from Figure 10 the that of and the planar capacitor will nonlinearly the relative permittivity ofthe the capacitance ceramic layer, the planar capacitor hasincrease higher It seen from Figure 10 that capacitance of the planar capacitor will increase nonlinearly nonlinearly with the relative permittivity of the ceramic layer, and the planar capacitor has higher testing to the permittivity of thelayer, ceramic layer when the induction coil testing exists. When the with thesensitivity relative permittivity of the ceramic and the planar capacitor has higher sensitivity testing sensitivity to the permittivity of the ceramic layer when the induction coil exists. When the induction coil exists and the potential difference between the driving electrode and the sensing to the permittivity of the ceramic layer when the induction coil exists. When the induction coil exists and induction coil exists and the potential difference between the driving electrode and the sensing electrode is constant, the charge change on the sensing electrode caused by the change of the the potential difference between the driving electrode and the sensing electrode is constant, the charge electrode the charge change onthan the sensing caused the coil. change the permittivity ofsensing the ceramic layer is larger theofinstance without Thus, change on is theconstant, electrode caused by the change the electrode permittivity ofinduction the by ceramic layer is of larger of the ceramic layer is larger than the instance without induction coil. Thus, the permittivity testing sensitivity of capacitor is higher. than the instance without induction coil. Thus, the permittivity testing sensitivity of capacitor is higher. permittivity testing sensitivity of capacitor is higher.
Figure Figure 10. 10. Variation Variation of capacitance with relative permittivity of ceramic layer. Figure 10. Variation of capacitance with relative permittivity of ceramic layer.
In word, the the existence existence of of induction induction coil is beneficial beneficial to to the the sensitivity sensitivity in in capacitive capacitive testing testing In aa word, coil is In and a word, the existence of induction coilon is beneficial to theofsensitivity in capacitive testing mode, the influence of sensing electrode the sensitivity the electromagnetic testing is mode, and the influence of sensing electrode on the sensitivity of the electromagnetic testing is weaker. mode, and the influence of sensing electrode on the sensitivityunit of the electromagnetic testing is weaker. Therefore, it is feasible to integrate the electromagnetic and the planar capacitor in a Therefore, it is feasible to integrate the electromagnetic unit and the planar capacitor in a sensor and to weaker. Therefore, it it is in feasible to integrate sensor operate time-sharing mode.the electromagnetic unit and the planar capacitor in a operateand it intotime-sharing mode. sensor and to operate it in time-sharing mode.
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4. Experimental Validation
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Sensors 2018,to 18,verify xFOR PEER 10 of 14 out. In order theREVIEW effect of the composite sensor, benchmark experiments were carried 4. Experimental Validation Figure 11 shows the measurement system, which consists of the composite sensor, impedance analyser, 4. Experimental In order toValidation verify the effectand of the composite sensor, benchmark experiments wereprecise carried impedance out. multiplex unit, personal computer a LabVIEW program. A Wayne Kerr 6510B Figure 11 shows the measurement system, which consists of the composite sensor, impedance In order Kerr to verify the effect Chichester, of the composite benchmark experiments and weretransimpedance carried out. analyser (Wayne Electronics, UK)sensor, was utilized for capacitance analyser, multiplex personal computer and a LabVIEW A Wayne Kerr 6510B precise Figure 11 shows theunit, measurement system, which consistsprogram. of the composite sensor, impedance measurement. The operating frequency of the impedance analyser was set to be 100 kHz. This impedance analyser (Wayne Kerr Electronics, Chichester, UK) was utilized for capacitance and analyser, multiplex unit, personal computer and a LabVIEW program. A Wayne Kerr 6510B precise particular frequency measurement. ensured thatThe theoperating measurement error of impedance the impedance analyser less than transimpedance frequency of the analyser was set towas be 100 impedance analyser (Wayne Kerr Electronics, Chichester, UK) was utilized for capacitance and 0.05% for a 1This pF particular capacitance, whileensured at the same time giving a good the electrostatic kHz. frequency that the measurement error approximation of the impedancefor analyser was transimpedance measurement. The operating frequency of the impedance analyser was set to be 100 0.05% a 1 pF capacitance, while The at themultiplex same timeunit giving a good of approximation for the arrays case in less the than model for for capacitive testing mode. consists reed relay switch kHz. This particular frequency ensured that the measurement error of the impedance analyser was electrostatic case in the model for capacitive testing mode. The multiplex unit consists of reed relay to extend the 0.05% number and to enable different terminal configurations less than for of a 1measurement pF capacitance,channels, while at the same time giving a good approximation for the for switch arrays to extend the number of measurement channels, and to enable different terminal various measurement protocols. The multiplex unit is connected to H , L , H CURconsists CUR of reed POT and electrostatic case in the model for capacitive testing mode. The multiplex unit relay LPOT configurations for various measurement protocols. The multiplex unit is connected to HCUR, LCUR, terminals ofarrays WKLPOT 6510B and can switch between the 4-terminal-pair (4TP) and the mutual inductance switch toterminals extend the number measurement and to enable different terminal HPOT and of WK 6510Bofand can switch channels, between the 4-terminal-pair (4TP) and the configurations for various measurement protocols. The multiplex unit is connected to H CUR, LCUR, configurations [33]. mutual inductance configurations [33].
HPOT and LPOT terminals of WK 6510B and can switch between the 4-terminal-pair (4TP) and the mutual inductance configurations [33]. Personal Computer Personal Computer
Electromagnetic/Capacitive Composite Sensor
Impedance Analyser
Multiplex Unit
Impedance Electromagnetic/Capacitive Multiplex Figure 11. Schematic diagram of the sensor testing system. Analyser Composite Sensor Unit Figure 11. Schematic diagram of the sensor testing system. According to theFigure numerical modeling results, we optimized and fabricated the composite 11. Schematic diagram of the sensor testing system.
According numerical modeling results, and fabricated composite sensor forto allthe measurements. As shown in Figure we 12, aoptimized flexible composite sensor is the fabricated with sensor flexible printed circuit board (FPCB) technology. The meandering driving electrode/excitation coilflexible for all measurements. As shown in Figure 12, a flexible composite sensor is fabricated with According to the numerical modeling results, we optimized and fabricated the composite extends a half wavelength at each end of the array, and a pair of dummy sensing elements printed circuit (FPCB) technology. meandering driving electrode/excitation coil are extends a sensor for board all measurements. As shownThe in Figure 12, a flexible composite sensor is fabricated with formed within those final meander half wavelengths to maintain the periodicity of the field as flexible printed circuitend board (FPCB) technology. The drivingelements electrode/excitation half wavelength at each of the array, and a pair ofmeandering dummy sensing are formedcoil within viewed by the end sensing elements. The dummy elements are not closed and not connected to form a half wavelength at each end of the array, a pair ofof dummy sensing elements thoseextends final meander half wavelengths to maintain theand periodicity the field as viewed by are the end a loop so that the net current flowing through the windings is minimized. The purpose of dummy formed within those final meander half wavelengths to maintain the periodicity of the field sensingelements elements. dummy elementsofare connectedsensing to form a looptoso that as the net is toThe extend the periodicity thenot fieldclosed beyondand the not last connected element reduce viewed by the end sensing elements. The dummy elements are not closed and not connected to form currentthe flowing through theeffects windings minimized. The is to unmodeled “edge” at the is end of the sensor. Thepurpose shape of of thedummy compositeelements sensor array is extend a loop so that the net current flowing through the windings is minimized. The purpose of dummy T-shapedof and sizebeyond of sensorthe is 133 × 142 mm. The sensitive area about the 38 mm × 10 mm, “edge” the periodicity thethe field lastmm connected sensing element to is reduce unmodeled elements is to extend the periodicity of the field beyond element the last connected sensing element to reduce including four pairs of detection units and two dummy structures. effects at the end of the sensor. The shape of the composite sensor array is T-shaped and the size of the unmodeled “edge” effects at the end of the sensor. The shape of the composite sensor array is sensor is 133 mm × 142 The issensitive area 10 mm, including pairs of Driving electrode /about Excitation coilmm ×area T-shaped and the sizemm. of sensor 133 mm × 142 is mm. The 38 sensitive is about 38 mm ×four 10 mm, Dummy winding Dummy electrode detection unitsfour andpairs twoof dummy element structures. including detection units and two dummy element structures. Driving electrode / Excitation coil Dummy electrode Sensing electrode
Dummy winding Induction coil
Induction coil
Sensing electrode The front of sensing area
The back of sensing area
The front of sensing area
The back of sensing area
Shielded terminal
Figure 12. A composite sensor fabricated with flexible printed circuit board (FPCB). Shielded terminal
Figure A composite sensorfabricated fabricated with circuit board (FPCB). Figure 12. 12. A composite sensor withflexible flexibleprinted printed circuit board (FPCB).
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4.1. Thickness Testing of Top Coating
A three-layer structure is employed in the experiments as test specimens. The layered structure 4.1. Thickness Testing of Top Coating consists of a plastic slice (as top coating), an Aluminum slice (as bond coating) and a Brass plate (as substrate). The parameters test specimens are listed inas Table A three-layer structureofisthe employed in the experiments test 2. specimens. The layered structure consists of a plastic slice (as top coating), an Aluminum slice (as bond coating) and a Brass plate (as Table Parametersare of the testin specimens. substrate). The parameters of the test 2.specimens listed Table 2. Thickness Conductivity Relative Table 2. Parameters of the(MS/m) test specimens. (mm) Permeability Plastic slice (top coating) 0.07–0.7 0 1 Thickness (mm) Conductivity (MS/m) Relative Permeability Aluminum slice (bond coating) 0.2 38.1 1 Plastic slice (top coating) 0.07–0.7 0 1 Brass plate (substrate) 2 58.1 Aluminum slice (bond coating) 0.2 38.1 1 1 Brass plate (substrate)
2
58.1
Relative Permittivity 4 Relative Permittivity 1 4 11
1
1
The thickness of plastic slice was changed by stacking one to ten thin plastic slices with the sameThe thickness of of 0.07 mm.slice When composite sensorone operates in the electromagnetic thickness plastic wasthe changed by stacking to ten thin plastic slices with thetesting same mode, 20 mA current is applied to the excitation coil with frequency of 100 kHz and the thickness of 0.07 mm. When the composite sensor operates in the electromagnetic testing mode, 20 mA transimpedance was obtained withfrequency the impedance 13 shows the variety of current is appliedoftosensor the excitation coil with of 100analyser. kHz andFigure the transimpedance of sensor transimpedance imaginary part with top coating thickness, which shows good agreement with the was obtained with the impedance analyser. Figure 13 shows the variety of transimpedance imaginary simulation data. part with top coating thickness, which shows good agreement with the simulation data.
Figure 13. Variety of transimpedance imaginary part with top coating thickness.
When the thecomposite compositesensor sensor operates in the capacitive testing mode, the capacitance of the When operates in the capacitive testing mode, the capacitance of the planar planar capacitor is measured with the impedance analyzer. Figure 14 shows the variety of capacitor is measured with the impedance analyzer. Figure 14 shows the variety of capacitance with capacitance thickness ofshows top coating, which shows rather good agreement the the thicknesswith of topthe coating, which rather good agreement with the simulation data.with Because simulation data. Because the simulation model does not consider the influence of parasitic and stray the simulation model does not consider the influence of parasitic and stray capacitors in the testing capacitors the testing circuit, the capacitance slightly larger circuit, the in measured capacitance is measured slightly larger than theissimulation data.than the simulation data. It can can be be seen seen from from Figures Figures 13 13 and and 14 14 that that the the top top coating coating thickness thickness can can be tested by both It be tested by both transimpedance and capacitance with impedance analyzer. It is obvious that the composite sensor transimpedance and capacitance with impedance analyzer. It is obvious that the composite sensor has higher higher thickness thickness testing has testing sensitivity sensitivity in in capacitive capacitive mode mode when when the the thickness thickness is is less less than than 0.21 0.21 mm, mm, and the sensitivity is higher and more constant in electromagnetic mode when the thickness and the sensitivity is higher and more constant in electromagnetic mode when the thickness is is larger larger than 0.21 under two modes cancan be implemented to improve the than 0.21 mm. mm. InInaddition, addition,information informationfusion fusion under two modes be implemented to improve reliability of the non-destructive testing (NDT) system. the reliability of the non-destructive testing (NDT) system.
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Figure 14. 14. Variety Variety of of capacitance capacitance with with the the thickness thickness of of top top coating. coating. Figure
4.2. Conductivity Conductivity Testing Testing of of Bond Bond Coating Coating 4.2. The conductivity conductivity variety variety of of bond bond coating coating will will change change the the voltage voltage of of the the induction induction coil, coil, but but has has The no influence on the planar capacitor. Thus, in order to investigate how the sensor signal depends on no influence on the planar capacitor. Thus, in order to investigate how the sensor signal depends conductivity of the bond layer, thethe imaginary partpart of transimpedance is tabled as aasfunction of the on conductivity of the bond layer, imaginary of transimpedance is tabled a function of conductivity for a closely spaced set of thicknesses. When the thickness of the top layer and the conductivity for a closely spaced set of thicknesses. When the thickness of the top layer and substrate are are 0.35 0.35 mm of 0.1 0.1 mm mm thick thick metal metal sheet sheet with with substrate mm and and 22 mm, mm, respectively, respectively, and and two two kinds kinds of different conductivities conductivities are are selected selected as asbond bondcoating, coating,the thecalculated calculatedand andmeasured measuredimaginary imaginarypart part of of different transimpedance is listed in Table 3, which shows good agreement between the measured value and transimpedance is listed in Table 3, which shows good agreement between the measured value and the calculated calculated value value with with simulation simulation model model for for two two different differentvalues valuesof ofbond bondlayer layerconductivity. conductivity. the Table imaginary part part between between experimental experimentaland andsimulation simulationresults. results. Comparison of of transimpedance transimpedance imaginary Table 3. 3. Comparison Conductivity of Bond Layer (MS/m) 11.4 11.4 37.74 37.74
Calculated Value (mΩ) Value (mΩ) Conductivity of Bond Layer (MS/m) Calculated Value (mΩ)Measured Measured Value (mΩ) 3.62 3.21
3.62 3.21
3.74 3.74 3.31 3.31
Relative RelativeError Error (%) (%) 3.31 3.31 3.12 3.12
In In the the real real coatings, coatings, the the conductivity conductivity change between the bond coating and the substrate substrate is often less than 10%. Therefore, it is difficult to detect and evaluate the changes in the thickness less than 10%. to detect and evaluate in the thickness and the the conductivity conductivity of of the the bond bond coating coating when when its its conductivity conductivity is is so so close closeto tothe thesubstrate. substrate. 4.3. 4.3. Permittivity Permittivity Testing Testing of of Top TopCoating Coating The increase with dielectric permittivity of topofcoating when its thickness remains The capacitance capacitancewill will increase with dielectric permittivity top coating when its thickness constant. order toIn investigate the permittivity affects the sensor affects signal, the is tabled remains In constant. order tohow investigate how the permittivity thecapacitance sensor signal, the as a function of the dielectric permittivity for a closely spaced set of thicknesses. When the thickness of capacitance is tabled as a function of the dielectric permittivity for a closely spaced set of thicknesses. top layer substrate mm and 2 mm, respectively, calculated and measured When theand thickness of are top0.35 layer and substrate are 0.35 mmthe and 2 mm, respectively, thecapacitance calculated are listed in Table 4, which shows good agreement between the measured data and the and measured capacitance are listed in Table 4, which shows good agreement between thecalculated measured value model for two different valuesmodel of top layer permittivity. data with and simulation the calculated value with simulation for two different values of top layer
permittivity. Table 4. Comparison of capacitance between experimental and simulation results. Permittivity of Top Layer 2 4.5
Calculated Value (pF) 2.08 2.91
Measured Value(pF) 2.17 3.02
Relative Error (%) 4.32 3.78
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Table 4. Comparison of capacitance between experimental and simulation results. Permittivity of Top Layer
Calculated Value (pF)
Measured Value (pF)
Relative Error (%)
2 4.5
2.08 2.91
2.17 3.02
4.32 3.78
5. Conclusions In this paper, an electromagnetic/capacitive composite sensor for comprehensive testing of TBCs is proposed. The ceramic layer can be tested with the planar capacitor, and the metal substrate material can be tested with the electromagnetic coils. The sensor is optimized by simulation analysis, and the interaction between the planar capacitor and the electromagnetic coils is studied. It can be found that the existence of induction coil can improve the sensitivity of planar capacitor to some extent, and the existence of a sensing electrode has a slight negative influence on the sensitivity of the electromagnetic coils. Three kinds of experiments were carried out and the experimental results are in good agreement with the simulation results, which shows the feasibility of integrating the two testing methods. Future work will deal with an attempt to use the experiment data, which measured with the composite sensor in two operating modes, in order to achieve parametric inversion of TBCs. Author Contributions: Yuan Ren proposed the idea. Yuan Ren and Dixiang Chen arranged the methods and also contributed to the writing and editing the manuscript. Wugang Tian ensured the realization of the experimental device and prepared the data. Mengchun Pan and Wugang Tian guided the research and were charged with the critical revisions of the manuscript. All of the authors analyzed the results and revised the paper. Funding: This paper was supported by The National Key R&D Program of China (Program Number: 2016YFF0203400) and National Natural Science Foundation of China (Project Number: 61171460). Conflicts of Interest: The authors declare no conflict of interest.
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