Microscopic contouring with low coherence interferometry endoscopy

Low coherence interferometry (LCI) methods have been investigated for the detection of damage on coated and uncoated airfoils in advanced gas turbines, and in particular methods for implementing LCI for in situ inspection using borescope-type instrumentation. The work reported in this paper includes design of prototype instrumentation and some test results, as well as results using commercial instruments obtained on TBCs. LCI techniques can provide significant advance over currently employed visual inspection of gas turbine airfoils. The instrumentation provides a significant advance over currently employed visual inspection of gas turbine airfoils. For instance, with thermal barrier coatings (TBCs), these techniques allow the detection and quantification of incipient spalls, delamination, and changes in TBC porosity which typically go unnoticed with visual inspection methods. The methods are well suited for use with borescopes and thus provide a large potential to be developed into commercial optical diagnostics instruments for use during maintenance and inspection of on-wing airfoils in advanced gas turbines.