| ABSTRACT: | NONDESTRUCTIVE EVALUATION OF SUPERALLOY SPECIMENS WITH A THERMAL
BARRIER COATING
J.H. Heida
National Aerospace Laboratory NLR, Amsterdam, The Netherlands.
This paper describes an investigation into the applicability of NDE techniquesfor the inspection of
superalloy specimens with a thermal barrier coating(TBC), both in unloaded condition and during constant
load exposure at hightemperatures (creep). More specifically, the eddy current and ultrasonictechnique were
investigated for the characterisation of the coating, while theacoustic emission technique was examined for
the monitoring of the degradationincurred in the material during creep. Both uncoated and TB coated
specimensof single crystal and directionally solidified material were used in theinvestigation.
The main conclusions of the investigation are:
- The eddy current technique can be effectively used in the measurement of thecoating thickness. The
phase and the length of the lift-off response giveinformation about the thickness of the bond coat and top
coat, respectively.Differences in the bond and top coat thickness of about 10 µm can bedistinguished.
- The ultrasonic technique does not provide enough information about the
thickness or the quality of the coating.
- The acoustic emission (AE) technique can be effectively used during creeptesting to monitor the
initiation and accumulation of damage, particularly inthe coating layer of the TBC specimens. A distinctly
different AE response wasobserved in the TBC specimens when creep tested at a different temperature(750
to 950 ¼C). A correlation between the number of AE events and the numberof coating cracks was
demonstrated.
|