Non-destructive Microwave Detection Of The Structure And Properties In Thermal Barrier Coatings

Thermal barrier coatings (TBCs) have been developed in order to increase theoperating temperature in gas turbine engines and some components operating inhigh-temperature condition. TBCs are thin ceramic layers with durability whichdeposited onto the hot-section components. In the process to produce TBCs, processparameters affect coating thickness profiles and morphological features, which affectthermal conductivity of TBCs. Besides, the delamination is induced by thermalstresses under the environment of high temperature and TGO. Therefore, it isnecessary to detect TBCs in order to ensure the quality of TBCs.Although not as widely known or understood as other NDE techniques,microwave non-destructive evaluation (NDE) has proven to be very useful in certainapplications. Non-contact inspection and the ability to penetrate into dielectricmaterials are two of most import attributes of microwave NDE and make it suitablefor the non-destructive inspection of TBCs. Due to the structure features of TBCs, inthis paper microwave NDE is simply based on wave reflection from a dielectric mediainterface.The objective of this paper is to investigate feasibility of microwave NDE todetect or monitor TC thickness, TC porosity, crack on BC, delamination between TCand BC and thermally grown oxide (TGO) growth. Three main works are setting upthe theory model, optimizing detecting parameters and detecting TBCs usingmicrowave NDE. Main works are as follows:(1) The theory model to detecting TBCs using wave reflection of microwaveNDE was established. An effective reflection coefficient is determined in this waywhose phase characteristics are used in the detection and evaluation of TC thickness,TC porosity, crack on BC, delamination between TC and BC and thermally grownoxide (TGO) growth. The characteristics of this phase as a function of severalparameters such as the frequency of operation, standoff distance and probe featuresare investigated.(2) The optimization of waveguide probe was carried out. According to thetransmission condition of microwave in waveguide, the relation of frequency andprobe size was presented in order to predigest model analysis of microwave in probe.How to select the appropriate probe shape was showed according to the transmissioncharacter of microwave in probe. Besides, the influence of probe flange wasdiscussed.(3) The optimization of detect parameter was carried out in microwave NDE byusing computer simulation technology-microwave studio (CST–MS). The detectparameters include frequency and standoff distance (in non-contact detection). The best parameters would be used to detect TBCs, in order to improve sensitivity ofmicrowave NDE.(4) TC thickness measurements and TC porosity monitor in TBCs system hasbeen carried out using microwave NDE. The measurement system used in this paperconsisted of a network analyzer (Agilent E8363C,100MHz-40GHz) feeding signalsto a Ka-band rectangular waveguide (Agilent R281A). The results showed that thephase difference can be used to evaluate TC thickness and porosity. The results werevalidated using destructive detection.(5) The defects of TBCs were detected using millimeter wave NDE. Four kindsof rectangular waveguides were used to detect TBCs. Changes in the phase ofreflection coefficient were primarily studied. The results showed the crack, thedelamination and TGO in TBCs, can be detected effectively using millimeter wavenondestructive testing techniques. Besides, the sensitivity is greater in high frequencythan low frequency.