Thermal Cycling Exposure Research Articles

Testing method for measuring corrosion resistance of surface mount chip resistors

Surface mount chip resistors are amongst the simplest and most inexpensive of all components used in electronic circuits and systems. Typically, resistor failure modes include open circuits, resistive shorts or variations in resistance indicating parametric drift or intermittent failure, which in some applications result in overall system failure. Corrosion is currently believed to be the number one failure mechanism for chip resistors deployed in developing markets such as Central and Latin America, Asia, India and Pacific regions where aggressive corrosive conditions are prevalent. The objective of this study is to develop a test to identify or screen out corrosion-susceptible parts. Ten precision chip resistors types representative of resistors in contemporary printed circuit board assemblies are subjected to a well-defined multi-stress screen which comprises thermal cycling and mixed flowing gas exposure. The combination of thermal cycling and corrosive gas exposure is shown to provide an acceptable acceleration test to identify corrosion-susceptible parts by replicating field failures. Currently, no other test method exists which is capable of replicating field failures.

Combined Space Environmental Exposure Test of Multijunction GaAs/Ge Solar Array Coupons

The purpose of this test program is to understand the changes and degradation of the space solar array panel components, including its electrostatic discharge (ESD) mitigation design features in their integrated form, after multiple years (up to 15) of simulated Earth geosynchronous (GEO) space environment. A set of multijunction GaAs/Ge solar array test coupons was subjected to five-year increments of combined environmental exposure tests. These tests consisted of the following: simulated ultraviolet (UV) radiation, ESD, electron/proton particle radiation, thermal cycling, and simulated ion thruster exposures. The solar radiation simulation was produced using a mercury-xenon lamp with wavelengths in the UV spectrum ranging from 230 to 400 nm. The ESD test was performed in the inverted-gradient mode using a low-energy electron (3-6 keV) beam exposure. The ESD test also included a simulated panel coverglass flashover for the primary arc event. The electron/proton radiation exposure included 1.0-MeV electrons, 100-keV electrons, and 40-keV protons. Thermal cycling included simulated transient Earth eclipse for satellites in geosynchronous orbit. With the increasing use of ion thruster engines on many satellites, the combined environmental exposure test also included ion thruster exposure to determine the impact on solar array performance, as well as the ion thruster interaction to ESD events. Before and after each increment of combined environmental exposures, the coupons underwent visual inspection using high power magnification and electrical tests that included characterization by large-area pulse solar simulator, dark I -V, insulation resistance, and electroluminescence. This paper discusses the test objective, test methodologies, and preliminary results after five and ten years of simulated combined environmental exposure tests.

Effect of Cyclic Oxidation Exposure on Tensile Properties of a Pt-Aluminide Bond-Coated Ni-Base Superalloy

The tensile behavior of a directionally solidified (DS) Ni-base superalloy, namely, CM-247LC, was evaluated in the presence of a Pt-aluminide bond coat. The effect of the thermal cycling exposure of the coated alloy at 1373 K (1100 °C) on its tensile properties was examined. The tensile properties were evaluated at a temperature of 1143 K (870 °C). The presence of the bond coating caused an approximately 8 pct drop in the strength of the alloy in the as-coated condition. However, the coating did not appreciably affect the tensile ductility of the substrate alloy. The bond coat prevented oxidation-related surface damage to the superalloy during thermal cycling exposure in air at 1373 K (1100 °C). Such cyclic oxidation exposure (up to 750 hours) did not cause any further reduction in yield strength (YS) of the coated alloy. There was a marginal decrease in the ultimate tensile strength (UTS) with increased exposure duration. Because of the oxidation protection provided by the bond coat, the drastic loss in ductility of the alloy, which would have happened in the absence of the coating, was prevented.

Laser cycling and thermal cycling exposure of thermal barrier coatings on copper substrates

Thermal barrier coatings (TBCs) are successfully applied in turbines and could also protect combustion chambers in rocket engines. Apart from different loading conditions, the main difference between these applications is the substrate material, which is nickel-based for turbines and copper-based for rocket engines. To optimize the coating system, more knowledge of possible failure modes is necessary.In this work a standard coating system was applied by atmospheric plasma spraying to copper specimens. These specimens were exposed to thermal cycling with different cooling rates and to laser shock testing. A laser-cycling set-up was developed to qualify different coating systems. This set-up consists of a high-power diode laser (3kW) which provides high heating rates to up to 1500°C. Laser shock testing has proven to be a suitable alternative to burner rig testing.The results were different to the common failure modes for TBCs on nickel substrates as the coatings system does not fail at the interface between top coat and bond coat, but at the interface between substrate and bond coat. Two failure modes were observed: copper oxide was undermining the coatings at the substrate/bond coat-interface in the case of thermal cycling experiments, and complete delamination occurred at the same interface in the case of laser shock testing. Consequently, this interface is critical in the investigated material system.

Damage Evolution in Thermal Barrier Coatings with Thermal Cycling

Thermal barrier coatings typically fail on cooling after prolonged thermal cycling or isothermal exposure. The mechanics of spalling requires that first a critical sized portion of the coating separates from the underlying material, then buckles and finally spalls away. The critical size for buckling depends on the thickness of the coating but is several millimeters for typical zirconia coatings 150 μm thick. As‐deposited coatings do not have interface separations but they form on thermal cycling as described in this work based on observations of coating cross‐sections combined with the stress redistribution in the thermally grown oxide imaged using a piezospectroscopic luminescence method. Analysis of the images reveals that small, isolated regions of damage initially form and then grow, linking up and coalescing to form percolating structures across the coating until the buckling condition is attained, the buckle extends and failure occurs by spallation. The piezospectroscopic imaging of the stresses in the thermally grown oxide formed by oxidation beneath thermal barrier coatings provides a form of “stress tomography” enabling the subcritical separations to be monitored.

Finite element simulation of tensile bond strength of atmospheric plasma spraying thermal barrier coatings

Before thermal cycling or thermal exposure, tensile failure of atmospheric plasma spraying (APS) thermal barrier coatings (TBCs) usually occurs by spallation of the ceramic coating at or near the bond coating, due to the accumulation of microcracks damage. In the current paper, finite element geometric models based on the real microstructural image of TBCs are generated, and the microcracks growth induced by the uniaxial tensile loading is simulated by employing LS-DYNA code with a failure criterion determined by maximum tensile stress of yttria partially stabilized zirconia (YSZ). Additionally, the modified Brazilian disc specimens with a central hole are used to obtain the intrinsic static failure criterion of non-defective YSZ. By means of a statistical method in conjunction with finite element method (FEM) results, the tensile bond strength of APS TBCs is calculated as 40 MPa in this paper. Meanwhile, damage accumulation and microcrack growth can be observed vividly by simulation processing. The numerical simulation result agrees well with the corresponding experimental result. It is shown that the methodology developed in this paper is very efficient in understanding damage evolution in TBCs.

In vitro aerosol characterization of Staccato® Loxapine

Medicinal aerosol products (metered dose and dry powder inhalers) require characterization testing over a wide range of use and pre-operating stress scenarios in order to ensure robust product performance and support submissions for regulatory approval. Aerosol characterization experiments on Staccato® Loxapine for inhalation (Staccato Loxapine) product (emitted dose, particle size, and purity) were assessed at different operating settings (flow rates, ambient temperature and humidity, altitude, and orientation) and at nominal test conditions following exposure to various stresses on the device (mechanical shock, vibration, drop, thermal cycling, and light exposure). Emitted dose values were approximately 90% of the coated dose at every condition, meeting target specifications in each case. Aerosol purity was consistently >99.5% for every test setting, with no reportable impurities according to ICH standards (>0.1%). Particle size averaged 2μm (MMAD) and was independent of the different test conditions with the exception of different airflow rates. Particle size decreased slightly with airflow, which may assist in maintaining constant deep lung deposition. The combination of high emitted dose efficiency and a particle size range ideally suited for lung deposition, along with the consistency of these key aerosol attributes, suggests that the Staccato system has distinct advantages over more traditional aerosol systems.

Silver Surface Iodination for Enhancing the Conductivity of Conductive Composites

AbstractThe electrical conductivity of a silver microflake‐filled conductive composites is dramatically improved after a filler surface treatment. By a simple iodine solution treatment, nonstoichiometric silver/silver iodide nanoislands form on the silver filler surface. Evidence of the decrease of surface silver oxide species is provided by TOF‐SIMS and the redox property of the nanoclusters is studied using cyclic voltammetry and TOF‐SIMS depth profile analyses. The redox property of the nanoclusters on silver flakes helps enhance the electrical conductivity of the conductive composites. The electrical resistivity of the improved conductive composites is measured by four‐point probe method; the reliability of the printed thin film resistors is evaluated by both the 85 °C/85% relative humidity moisture exposure and the −40 ∼ 125 °C thermal cycling exposure. The conductive composite printed radio frequency identification (RFID) antennas with 27.5 wt% of the modified silver flake content show comparable performance in the RFID tag read range versus copper foil antennas, and better than those commercial conductive adhesives that require much higher silver content (i.e., 80 wt%). This work suggests that a surface chemistry method can significantly reduce the percolation threshold of the loading level of the silver flakes and improve the electrical conductivity of an important printed electronic passive component.

Silver Surface Iodination for Enhancing the Conductivity of Conductive Composites

AbstractThe electrical conductivity of a silver microflake‐filled conductive composites is dramatically improved after a filler surface treatment. By a simple iodine solution treatment, nonstoichiometric silver/silver iodide nanoislands form on the silver filler surface. Evidence of the decrease of surface silver oxide species is provided by TOF‐SIMS and the redox property of the nanoclusters is studied using cyclic voltammetry and TOF‐SIMS depth profile analyses. The redox property of the nanoclusters on silver flakes helps enhance the electrical conductivity of the conductive composites. The electrical resistivity of the improved conductive composites is measured by four‐point probe method; the reliability of the printed thin film resistors is evaluated by both the 85 °C/85% relative humidity moisture exposure and the −40 ∼ 125 °C thermal cycling exposure. The conductive composite printed radio frequency identification (RFID) antennas with 27.5 wt% of the modified silver flake content show comparable performance in the RFID tag read range versus copper foil antennas, and better than those commercial conductive adhesives that require much higher silver content (i.e., 80 wt%). This work suggests that a surface chemistry method can significantly reduce the percolation threshold of the loading level of the silver flakes and improve the electrical conductivity of an important printed electronic passive component.

Microstructure and oxidation resistance of nanocrystalline 304 SS-Al coatings

The long-term oxidation behavior of nanocrystalline 304 SS and 304 SS-4Al coatings has been investigated. The coatings were deposited on 304 SS samples using a plasma enhanced magnetron sputtering process. Cyclic oxidation tests were conducted on the coated and uncoated samples at a peak temperature of 750 °C for up to 1000 one-hour thermal cycles between the peak and room temperatures. The crystal structure of the as-deposited 304 SS coating consisted of σ phase in a αFe matrix. The addition of 4%Al to the 304 SS coating stabilized the αFe structure and prevented the formation of the σ phase. Thermal cycling exposure led to formation of external oxide scale of mixed oxide on the uncoated sample, Cr 2O 3 on the 304 SS coating and Al 2O 3 on the 304 SS-Al coating. The coating's fine grain structure had promoted selective oxidation of Cr and Al in the 304 SS and 340 SS-4 wt.% Al coatings, respectively. The external oxide scale on the coated samples exhibited good spallation resistance compared to the scale on the uncoated samples. After thermal cycling exposure, both coatings exhibited internal oxidation along the columnar boundaries. Inward diffusion of Al into the substrate during thermal cycling resulted in precipitation of iron-aluminide particles in the interdiffusion zone. The Al content in the coating dropped from 4 wt.% to 1.8 wt.% after 990 thermal cycles due to the inward and outward diffusion of Al. The improvement in oxide scale spallation resistance and accelerated depletion of aluminum are believed to be related to the fine grain structure of the coating.

The Oxide Scale Growth Mechanism on Fe20Cr5Al+RE Alloy in SO<sub>2</sub>+O<sub>2</sub>

The oxidation mechanism of FeCrAl (+RE), RE: reactive elements: Y and Hf) thin foils was studied at temperatures ranging from 1093 K to 1173 K in SO2+1%O2 atmosphere. Materials were subjected to isothermal and thermal cycling exposures as well as to the so-called two-stage-oxidation. In the latter, an oxygen isotope 18O2 was used as a tracer. Starting materials and scales were characterized using Grazing Angle X-Ray Diffraction (GA-XRD), EDX, SEM, XPS and High Spatial Resolution Secondary Ion Mass Spectrometry (HSR-SIMS). The obtained results showed within the studied range of exposure conditions the scales on all the studied alloys grow via outward mechanism typical for transient oxides and not for the -Al2O3 which is consistent with phase composition results and scale morphology and/or microstructure. It was also found that ‘as received’ foils are not bare metals but complex oxide-on-metal systems resulting from their manufacturing procedure. The obtained results are discussed in terms of the diffusion-related transport properties of the scale and of the scale phase composition.

Fracture Behavior of Plasma-Sprayed Thermal Barrier Coatings with Different Bond Coats upon Cyclic Thermal Exposure

The effects of bond coat nature in thermal barrier coating (TBC) systems on the delamination or fracture behavior of the TBCs with different bond coats prepared using two different processes—air plasma spray (APS) and high velocity oxyfuel (HVOF)—were investigated by cyclic thermal fatigue tests. The TBCs with the HVOF bond coat were delaminated or fractured after 3–6 cycles, whereas those with the APS bond coat were delaminated after 10 cycles or show a sound condition. These results indicate that the TBC system with the APS bond coat has better thermal durability than the system with the HVOF bond coat under long-term cyclic thermal exposure. The hardness values of the TBCs (top coats) in both systems are dependent on applied loads, irrespective of the hardness of the bond coats and the substrate. The values are not responded to the bond coat nature or the exposure time. Thermally grown oxide (TGO) layers in both cases consist of two regions with the inner TGO layer containing only Al2O3 and the outer TGO layer of mixed-oxide zone containing Ni, Co, Cr, Al in Al2O3 matrix. The outer TGO layer has a more irregular shape than the inner TGO layer, and there are many pores within the outer layer. At failure, the TGO thickness of the TBC system with the HVOF bond coat is 9–13 m, depending on the total exposed time, and that of the TBC system with the APS bond coat is about 20 m. The both TBC systems show the diffusion layer on the side of substrate in the interface between the bond coat and the substrate. The relationship between the delamination or fracture behavior and the bond coat nature has been discussed, based on the elemental analysis and microstructural evaluation.

IMC Growth Mechanisms of SAC305 Lead-Free Solder on Different Surface Finishes and Their Effects on Solder Joint Reliability of FCBGA Packages

Intermetallic compound (IMC) growth behavior of lead-free solder plays an important role in ball grid array (BGA) solder joint reliability for flip chip BGA (FCBGA) packaging applications. The growth mechanism of IMC is reported based on a diffusion model. Thermal treatment such as accelerated thermal cycling (ATC) and isothermal aging exposure also contribute to the growth rate and morphology of lead-free solder IMC. Among the lead-free solder alloys, Sn-3.0wt.%Ag-0.5wt.%Cu (SAC305) solder is a promising substitute for Sn-Pb because of its good mechanical properties and wettability with current surface finishes. After thermal exposure, BGA solder joint reliability is degraded due to IMC formation and growth. In this study, two different thermal treatments, ATC and isothermal aging, and two different pad surface finishes, solder on pad (SOP) and electroless Ni immersion gold (ENIG), are considered in terms of IMC growth rate and mechanical solder joint reliability. An SOP finished interface forms a thin ε-phase Cu3Sn layer and a scallop-like η-phase Cu6Sn5 layer. In contrast, the ENIG finished interface forms a thick (Cu,Ni)6Sn5 IMC layer and prevents overall IMC growth. Different surface finished test vehicles are evaluated in an ATC test in a 0°C to 100°C temperature range and the Ni diffusion layer shows a longer solder joint fatigue lifetime than the nondiffusion barrier interface based on the micro cross-section and dye penetration analysis results. In an isothermal aging test at 100°C and 150°C, the aging temperature and time are valid factors to decide mechanical shock reliability. Interfacial fractures are found in the 100°C aged test vehicle due to easier crack propagation at the interface between the thin Cu3Sn layer and the scallop-like Cu6Sn5 layer based on SEM microstructure analysis results. Finally, this investigation proposes how to improve solder joint reliability and prevent interfacial fracture for SAC305 lead-free application.

Performance and testing of glass-ceramic sealant used to join anode-supported-electrolyte to Crofer22APU in planar solid oxide fuel cells

This work describes the performance and testing of a glass-ceramic sealant used to join the ceramic electrolyte (anode-supported-electrolyte (ASE)) to the metallic interconnect (Crofer22APU) in planar SOFC stacks. The designed glass-ceramic sealant is a barium and boron free silica-based glass, which crystallizes by means of the heat-treatment after being deposited on substrates by the slurry technique. Joined ASE/glass-ceramic seal/Crofer22APU samples were tested for 500 h in H 2–3H 2O atmosphere at the fuel cell operating temperature of 800 °C. Moreover, the joined ASE/glass-ceramic seal/Crofer22APU samples were submitted to three thermal cycles each of 120 h duration, in order to evaluate the thermomechanical stability of the sealant. The microstructures and elemental distribution at Crofer22APU/glass-ceramic and ASE/glass-ceramic interfaces were investigated. SEM micrograph observations of joined samples that underwent cyclic thermal tests and exposure for 500 h in H 2–3H 2O atmosphere showed that the adhesion between the glass-ceramic and Crofer22APU at either interface was very good and no microstructural changes were detected at the interfacial boundaries. The study showed that the use of the glass-ceramic was successful in preventing strong adverse corrosion effects at the Crofer22APU/glass-ceramic sealant interface.

Validation of Alternative RoHS Compliant Au Pastes for the DuPont LTCC 951 System

The restriction of the use of certain hazardous substances (RoHS) came into force on July 1, 2006. Equipment producers will need to obtain materials declarations or certificates of compliance from their suppliers. Certain exemptions from this regulation, for example, RF ports and certain medical applications, still allow for the manufacture of LTCC products with non-RoHS-compliant materials. However, there is a continuously growing demand for LTCC products without cadmium or lead. Since the end of 2006 DuPont has been offering Cd-free Au pastes as possible replacements for established products. For cofiring internal and external conductors, Au paste type 5734 is frequently used by LTCC manufacturers. The alternative RoHS compliant paste is type CDF34. For pin, tab, and frame brazing, a combination of Au pastes 5062D (adhesion layer) and 5063D (barrier layer) are currently the first choice for according LTCC parts at MSE. The Cd-free versions are types 5062E and 5063E. MSE is validating all three new pastes according to a comprehensive test plan that includes five different test layouts for CDF34 and another two for 5062E and 5063E. Simultaneously, corresponding reference parts are built to have exact comparison possibilities. The validation of CDF34 includes the investigation of bondability and the reliability of resistor terminations, vias, and adhesive bonded components. Furthermore, the lateral migration (insulation resistance) is tested. The 5062E and 5063E validation includes the reliability of soldered components, vias in soldered pads, and soldering of frames and base plates. The different test cells include visual inspection, cross sections, initial measurements, and results after thermal cycling, thermal shock, temperature, and humidity exposure.

Thermal failure of thermal barrier coating with thermal sprayed bond coating on titanium alloy

A thermal spray technology high-velocity oxygen fuel (HVOF) was used to deposit NiCoCrAlY as a bond coating between the titanium alloy substrate and top 8 wt% yttria-stabilized zirconia thermal-barrier coating (TBC) deposited by electron beam-physical vapor deposition (EB-PVD). The thermal cycling and isothermal exposure tests were conducted to evaluate the durability of the TBC. Investigations using OM, SEM, EPMA, and XRD revealed that the thermal-sprayed BC makes the TBC more durable in isothermal exposure tests but more short-lived in thermal cycling tests, in comparison to our previous study in which the BC was prepared by EB-PVD. This is because the thermal-sprayed imperfections, such as microcracks and voids, elevate the diffusion resistance and degrade the mechanical properties of the BC, simultaneously. To current TBC systems in which the BC is deposited by HVOF, thermal failure behaviors—such as the formation of the Ti/Al mixture oxides at some individual places in the BC, and the Ti2Ni gaps formed around the BC/substrate interface—were also discussed.

The Influence of Specimen Thickness on the High Temperature Corrosion Behavior of CMSX-4 during Thermal-Cycling Exposure

CMSX-4 is a single-crystalline Ni-base superalloy designed to be used at very high temperatures and high mechanical loadings. Its excellent corrosion resistance is due to external alumina-scale formation, which however can become less protective under thermal-cycling conditions. The metallic substrate in combination with its superficial oxide scale has to be considered as a composite suffering high stresses. Factors like different coefficients of thermal expansion between oxide and substrate during temperature changes or growing stresses affect the integrity of the oxide scale. This must also be strongly influenced by the thickness of the oxide scale and the substrate as well as the ability to relief such stresses, e.g., by creep deformation. In order to quantify these effects, thin-walled specimens of different thickness (t = 100−500 μm) were prepared. Discontinuous measurements of their mass changes were carried out under thermal-cycling conditions at a hot dwell temperature of 1100 °C up to 300 thermal cycles. Thin-walled specimens revealed a much lower oxide-spallation rate compared to thick-walled specimens, while thin-walled specimens might show a premature depletion of scale-forming elements. In order to determine which of these competetive factor is more detrimental in terms of a component’s lifetime, the degradation by internal precipitation was studied using scanning electron microscopy (SEM) in combination with energy-dispersive X-ray spectroscopy (EDS). Additionally, a recently developed statistical spallation model was applied to experimental data [D. Poquillon and D. Monceau, Oxidation of Metals, 59, 409–431 (2003)]. The model describes the overall mass change by oxide scale spallation during thermal cycling exposure and is a useful simulation tool for oxide scale spallation processes accounting for variations in the specimen geometry. The evolution of the net-mass change vs. the number of thermal cycles seems to be strongly dependent on the sample thickness.

A literature survey on the stability and significance of residual stresses during fatigue

ABSTRACTMany manufacturing processes can induce residual stresses in components. These residual stresses influence the mean stress during cyclic loading and so can influence the fatigue life. However, the initial residual stresses induced during manufacturing may not remain stable during the fatigue life. This paper provides a broad and extensive literature survey addressing the stability of surface and near‐surface residual stress fields during fatigue, including redistribution and relaxation due to static mechanical load, repeated cyclic loads, thermal exposure and crack extension. The implications of the initial and evolving residual stress state for fatigue behaviour and life prediction are addressed, with special attention to fatigue crack growth. This survey is not a critical analysis; no detailed attempt is made to evaluate the relative merits of the different explanations and models proposed, to propose new explanations or models or to provide quantitative conclusions. Primary attention is given to the residual stresses resulting from four major classes of manufacturing operations: shot peening and related surface treatments, cold expansion of holes, welding and machining.

Fatigue Crack Propagation Behavior of Thermally Degraded SiCp/Al Composites

Silicon carbide particle reinforced aluminum 6061 composites (SiCp/Al) with 5 and 10% SiC particle volume fractions as well as aluminum 6061 alloy (AA6061) were used to examine their fatigue crack propagation behavior after degraded by thermal cycling and isothermal exposure. Fracture surfaces were observed under a scanning electron microscope. Thermally cycled specimens (TC-SiCp/Al and TC-AA6061) were prepared through 100 thermal cyclings between 200 C and 300 C and isothermally exposed specimens (IE-SiCp/Al and IE-AA6061) was prepared by isothermal exposure at 300 C for 80 h. Difference of fatigue crack propagation behavior in the IE- and TC-AA6061 was small with increase of stress intensity factor range, iK even if da/dN of the both types of AA6061 were different at the initial stress intensity factor range. Fatigue crack propagation behavior of the IE-SiCp/Al was similar regardless of SiC particle volume fractions. While for the TC-SiCp/Al, fatigue crack propagation behavior was different in the SiC particle volume fractions and to the TC-AA6061. Besides, it notes that there was good relation between exponent m and coefficient C of the Paris equation regardless of thermal cycling and isothermal exposure and SiC particle volume fractions. The relation between m and C could be described as C 1/4 AΘm where constants A and B were 1.668 E-5 and 0.165, respectively.

Electrochemical impedance spectroscopy of thermal barrier coatings as a function of isothermal and cyclic thermal exposure

Electrochemical impedance spectroscopy (EIS) is being developed as a non-destructive evaluation technique of thermal barrier coatings (TBCs) for life-remaining assessment and quality control. In this investigation, EIS was employed to non-destructively examine TBCs, as a function of thermal exposure during isothermal and cyclic oxidation in air at 1121 °C. TBCs examined in this study include electron beam physical vapor deposited (EB-PVD) and air-plasma sprayed ZrO 2–7wt.%Y 2O 3 (YSZ) on NiCoCrAlY or (Ni,Pt)Al bondcoat. Electrochemical impedance response of TBC specimens at room temperature were acquired and analyzed with an AC equivalent circuit based on the multi-layered micro-constituents of TBCs. Microstructural characteristics of TBCs were also examined by optical and electron microscopy. Initial increase in the resistance of YSZ with thermal exposure was observed and related to the high temperature sintering of YSZ. The parabolic growth of TGO during high temperature oxidation was inversely proportional to the capacitance of TGO. An explanation based on electrolyte penetration into sub-critical damage is proposed for the gradual decrease in the resistances of YSZ and TGO with prolonged thermal exposure. Observation of exposed metallic bondcoat surface on the fracture surface, which provides conduction readily through electrolyte contact, was related to the abrupt and large increase in the capacitance of YSZ and TGO.