This paper discusses a novel manufacturing route to produce overlay α-NiPt₂Al coatings on single crystal superalloy AM1, by the sputtering of the three pure metals as a multilayer coating. The metal layers are annealed under argon to form the resulting intermetallic. Typical systems have over 100 layers for a total thickness of only 5µm. These bondcoats are very thin due to the high platinum content of the NiPt₂Al compound compared to typical β-(Ni,Pt)Al. The control of the manufacturing process and the range of deposition parameters will be discussed.

The isothermal oxidation behaviour of these coatings at 1100°C in air is detailed, with oxide scale analysis and microstructure evolution of the coating/superalloy system (XRD, SEM, TEM and FIB picturing) after 1h and 50h exposure.

Complete TBCs with these bondcoats and standard EB-PVD deposited YSZ topcoat were successfully manufactured. Thermal cycling behaviour at 1100°C in air of these coatings (novel bondcoat + ceramic topcoat) is described. The relationship between parameters such as layer periodicity, surface finish and aluminium content with thermal cycling lifetime and behaviour is discussed.

The integrity of thermal barrier coating systems is strongly influenced by the mechanical properties of the different layers constituting them, in particular the metallic bondcoat (NiAl-based or MCrAlY alloy). At high temperature, interdiffusion and oxidation phenomena cause changes in the composition of the bondcoats and as a result their mechanical behaviour is likely to change also.

To determine the mechanical behaviour of such alloys in particular, we have designed and developed a high temperature instrumented microindenter capable of functioning up to 1000°C. Several technological solutions have been implemented to control the position of indents (within a range of a few micrometers) and the loads applied (max. 1N), to ensure the thermal homogeneity within the testing zone, and to limit oxidation effects.

In order to exploit the information derived from high temperature microindentation, a simulation of this test, based on FEM calculations, has been developed to solve the inverse problem. Examples of mechanical behaviour of various Ni(Pt)Al alloys thus determined will be presented.

The oxidation behaviour of two γ-TiAl alloys (02G and 98G received from UES, Dayton) coated with thermal barrier coatings (TBCs) was investigated under isothermal oxidation conditions at 900^oC for up to 1000 h. Samples bare or with a 4 µm thick Ti-Al-Cr protective layer were coated with yttria-stabilized zirconia using electron-beam physical vapour deposition. Prior to TBC deposition, the specimens were pre-oxidized in air or oxygen at low pressure. Mass change of the specimens thermally exposed to laboratory air was measured once every week. Post-oxidation analysis of the coating systems was performed using SEM with EDX detector. Furthermore, the interface between TBC and thermally grown oxide scale was investigated by transmission electron microscopy.

Thermal barrier coatings on samples of alloy 98G did not spall off during the maximum exposure time of 1000 h, whereas failure occurred with alloy 02G specimens pre-oxidized in air and oxygen. Microstructural examinations revealed an outer oxide scale with a columnar structure beneath the TBC. Below this columnar oxide layer, a broad porous zone consisting predominantly of titania was observed. At the transition region between oxide scale and substrate, a discontinuous nitride layer was found. Spallation of the TBC was associated with cracking in the porous titania-rich layer. The zirconia top coat layer exhibited an excellent adherence on the oxide scale. Pre-exposure in oxygen at low pressure did not result in an improvement of the oxidation resistance. Post-oxidation analysis of specimens with Ti-Al-Cr layer revealed that the protective coating was nearly completely oxidized. The degradation of the Ti-Al-Cr coating was associated with dissolution of the Laves phase due to chromium depletion and transformation of the γ phase into the α₂-Ti₃Al phase. On the oxidized Ti-Al-Cr layer, an outer oxide scale with columnar structure formed again beneath the TBC.