Open AccessNON-DESTRUCTIVE TECHNIQUES FOR THE EVALUATION OF OVERLAY AND THERMAL BARRIER COATINGS ON GAS TURBINE COMPONENTS
Author(s) -
M. Cybulsky
Publication year - 1998
Language(s) - English
Resource type - Reports
DOI - 10.2172/778009
Subject(s) - thermal barrier coating , coating , spall , materials science , service life , forensic engineering , process (computing) , gas turbines , superalloy , layer (electronics) , nondestructive testing , turbine blade , overlay , turbine , process engineering , composite material , mechanical engineering , computer science , engineering , operating system , medicine , microstructure , radiology , programming language
There is currently no satisfactory method for the non-destructive examination (NDE) of coatings on gas turbine parts and determination of coating thickness, for example, has to be carried out by sectioning of the component and subsequent metallographic analysis. This method, which is both time-consuming and expensive, has nevertheless been used extensively for aero-engine parts to monitor coating quality and to gather statistical information for process control. For large components from utility size gas turbines costs are high and compared with aero-engines, only a limited number of parts can be examined so that the destructive method becomes less attractive both as an inspection technique and for obtaining process control data to measure part to part variations in coating thickness, for example. During engine service protective coatings slowly degrade and this degradation process effectively controls the life of the part, particularly in situations where a thermal barrier coating (TBC) is used to protect against excessive metal temperatures. In this case growth of the oxide at the interface between the bond coat and the TBC leads to a build-up of stress in the TBC which can be relieved by a spalling of the ceramic layer and loss of the protection from the thermal barrier. In situations where the integrity of the TBC system is critical to the survival of the part, some non-destructive method of determining the degradation condition of the bond coat would clearly be advantageous. In this report the results are described of recent progress in a program to develop non-destructive methods to measure coating quality and to monitor the condition of coatings in service. The work which has formed part of the Advanced Turbine Systems (ATS) Project funded by DOE, has involved the use of eddy-current (ET) and ultrasonic (US) methods developed by SouthWest Research Institute (SwRI) who have been responsible for development of the technique