Multi-layered ruthenium-containing bond coats for thermal barrier coatings
Advances in thermal barrier coating (TBC) technology for Ni-base superalloys have shown that B2 Pt-modified NiAl-based bond coatings outperform conventional NiAl bond coat layers for high temperature TBC multilayer systems. This thesis addresses the potential improvement in the high temperature capability of a 132 Ru-modified aluminide bond coat layer due to improved high temperature properties of RuAl over NiAl. The objectives of this research have been to define a processing path for fabrication of a multi-layered Ru-modified aluminide bond coating and to investigate its performance within a TBC system.
Microstructural development and the oxidation behavior of Ru-modified and Ru/Pt-modified bond coatings have been studied in detail. Two types of Ru-modified bond coatings have been fabricated: one by means of high temperature, low activity chemical vapor deposition (CVD) processing, and one via high temperature, high activity pack-aluminization. The location of the RuAl-rich layer has been shown to be process dependent with a low activity Ru-containing bond coating producing an exterior B2 NiAl layer with an interior B2 RuAl layer and a high activity Ru-containing bond coat producing the reverse arrangement of B2 layers.
While all bond coating systems studied offer some oxidation protection by forming α-Al2O3, the low activity Ru/Pt-modified bond coatings exhibited a higher resistance to oxidation-induced failure compared to Ru-modified bond coatings. Through 1000 cyclic oxidation exposures, the Ru/Pt-modified coatings with an initial Ru deposition of 3μm are comparable to conventional Pt-modified aluminide coatings.
The Ru-Al-Ni ternary system is the basis for Ru-modifed aluminide coating systems. An experimental assessment of the Ru-Al-Ni phase diagram at 1000°C and 1100°C has been produced via a series of diffusion couple experiments. A continuous solid-solution has been shown to exist between the RuAl and NiAl phases in the ternary system at the temperatures of 1000°C or 1100°C. Regarding the Ru-modified bond coatings, it has been observed that higher order elemental additions appear to stabilize the miscibility gap between B2 NiAl and RuAl phases in TBC systems. Results of studies on phase equilibria in the ternary system are consistent with the microstructure evolution observed in the more chemically complex bond coatings.