To provide a commercial return from Combined Cycle Gas Turbine (CCGT) technology for power generation, a balance has to be struck between efficiency, maintenance costs and component life. The parts that significantly influence this are collectively known as Hot Gas Path (HGP) components and generally consist of blades, vanes and heat shields. To achieve high thermal efficiencies, these components are subject to high temperatures, centripetal forces and erosive/corrosive atmospheres and therefore need to be protected by complex coatings and through internal cooling. The drive for higher efficiencies, however, often means that the protection put in place is only a partial solution and experience shows that these components have a relatively short lifespan and require regular inspection. Inspections to date have consisted of visual examination and fluorescent penetrant inspections and, therefore, are limited to the detection of surface damage usually associated with 'teething' problems during early years of operation; and are often inadequate for detecting in-service degradation or for coating qualification. Furthermore, as CCGT technology is becoming more established, there is a requirement to assess the remnant life of components of which there is currently no suitable technique. This paper describes the inspection technique developed using a multi-frequency eddy current instrument capable of distinguishing between coatings and substrates for the thickness gauging of TBCs to help qualify coatings prior to engine runs. This is a crucial inspection as poor control during coating application can lead to premature failure of the component by thermo-mechanical fatigue, as was the case at a UK CCGT power station. Here, several blades in one particular row which were coated by two different vendors exhibited TMF cracking, however through a thorough investigation with a multi-frequency instrument, it was possible to identify the poorly coated blades and establish that they were all from the single vendor.
