M. Attia, R. David, J. Thornton, DSTO, Melbourne, Australia; W. R. Chen, C. Mandache, Institute for Aerospace Research, National Research Council of Canada, Ottawa, ON, Canada; J. R. Nicholls, Cranfield University, Cranfield, United Kingdom; R. Sikorski, Air Force Research Laboratory, Wright Patterson AFB, OH; M. Winstone, DSTL Porton Down, Salisbury, Wiltshire, United Kingdom; B. Withy, Defence Technology Agency, NZDF, Auckland, New Zealand
The aim of this work was to use non-destructive evaluation (NDE) to identify precursors to failure in thermal barrier coatings (TBCs), and link this to changes in the microstructure. The work compares the performance, under thermal cycling, of two types of electron-beam physical vapour deposited (EBPVD) TBCs on the nickel superalloy CMSX-4. The difference between the two types was the bond coats: either a platinum modified aluminide bond coat or a platinum diffused bond coat. Samples were examined visually to determine on which cycle they failed, or if they survived intact, and then subjected to non-destructive evaluation techniques: thermography, ultrasound, eddy current and piezospectroscopic imaging. Subsequently, the discs were cross-sectioned and examined with electron microscopy using backscattered imaging and energy dispersive X-ray spectroscopy. The TBCs with the diffused bond coat survived longer than those with the aluminide bond coat. Both thermography and piezospectroscopic imaging showed anomalies scattered across some of their images. The anomalies were not in images of the lower durability aluminide TBCs, as might be expected, but in those of the more durable platinum diffused TBCs. Cross-sectioning showed that the platinum diffused TBCs contained patches of internal oxides within their bond coats and beneath the usual layer of thermally grown oxide on the bond coat surface. There are indications that thermography and piezospectroscopic imaging can see the patches of internal oxide that form in the platinum diffused bond coats before the delamination of the thermal barrier layer. If further work can establish the relationship between the degree of this internal oxidation and remaining life, and if thermography and piezospectroscopic imaging can reliably assess the degree of this internal oxidation, then these NDE techniques could help reduce maintenance costs.
Summary: The aim of this work was to use non-destructive evaluation (NDE) to identify precursors to failure in thermal barrier coatings (TBCs), and link these to changes in the microstructure. The work compares the performance, under thermal cycling, of two types of electron-beam physical vapour deposited (EBPVD) TBCs on the nickel superalloy CMSX-4. The difference between the two types were the bondcoats: a platinum modified aluminide bondcoat or a platinum diffused bondcoat. The TBCs with the diffused bondcoat survived longer than those with the aluminide bondcoat. Both thermography and piezospectroscopic imaging showed anomalies scattered across their images of the TBCs with platinum diffused bondcoats. These anomalies may be due to the patches of internal oxide observed with electron microscopy within the bondcoat and below the usual layer of thermally grown oxide on the bondcoat surface. There may be potential to use the NDE detection of the anomalies to assess the remaining life of these TBCs, and hence reduce maintenance costs.
