Investigation and development of transient thermography for detection of disbonds in thermal barrier coating systems

Abstract

This thesis has explored the use of transient thermography for the detection of disbonds of minimum diameter 2mm located in a thermal barrier coating (TBC) system whose surface may be unpainted. The technique, the type/size of the defect and also the condition of the TBC system for the inspection has been specified by Alstom Power Switzerland, the sponsor of the EngD project. As for other Non Destructive Testing (NDT) techniques, reference test specimens are required for calibration, but unfortunately, real disbonds are very difficult to use because it is difficult to control their size, and larger ones tend to spall. Flat bottomed holes are commonly used, but these over-estimate the thermal contrast obtained for a defect of a given diameter. The thesis quantifies the differences in thermal response using finite element analysis validated by experiments, and proposes a form of artificial disbond that gives a better representation of the thermal responses seen with real defects. Real disbonds tend to have a non-uniform gap between the disbonded surfaces across the defect, and the effect of this on the thermal response is evaluated using finite element simulations. It is shown that the effect can be compensated for by adjusting the diameter of the calibration defect compared to the real defect. Surfaces of inspected specimens are usually covered by a black, energy absorbing paint before the transient thermography test is carried out. Unfortunately, this practice is not acceptable to some turbine blade manufacturers (including the project sponsor) since thermal barrier coatings are porous so the paint is difficult to remove. Unpainted TBC surfaces have very low emissivity, and after period of service their colour changes unevenly and with which also absorptivity and emissivity changes. The low emissivity gives low signal levels and also problems with reflections of the incident heat pulse, while the variation in emissivity over the surface gives strong variation in the contrast obtained even in the absence of defects. The thesis has investigated the effects of uneven discolouration of the surface and of Infra Red (IR) translucency on the thermal responses observed by using mid and long wavelength IR cameras. It has been shown that unpainted blades can be tested satisfactorily by using a more powerful flash heating system assembled with an IR glass filter and a long wavelength IR camera. The problem of uneven surface emissivity can be overcome by applying of the 2nd time derivative processing of the log-log surface cooling curves.