Influence of Titanium in Nickel-Base Superalloys on the Performance of Thermal Barrier Coatings Utilizing γ-γ^′ Platinum Bond Coats

Titanium is a key element in nickel-base superalloys needed with aluminum to achieve the desired volume fraction of the strengthening γ^′-phase. However, depending upon its concentration, titanium can degrade the adherence of aluminum oxide by forming TiO₂ particles near the oxidemetal interface. This effect is extended to thermal barrier coating systems where in this case, the bond coat replaces the superalloy as the underlying substrate. Noting that the onset of failure of thermal barrier coating systems coincides with the first spall of the thermally grown oxide, titanium level in the superalloy can have an important effect on the useful life of the coating. Therefore, this study was undertaken to examine the effect of titanium on the performance of a thermal barrier coating system. Included in the study were two Ni-base superalloys with similar chemical composition except for the Ti content, and a Pt-containing bond coat consisting of γ^′+γ-phases all top coated with zirconia stabilized by 7 weight % yttria. Coating performance was evaluated from thermal exposure tests at 1150 °C with a 24-hour cycling period to room temperature. Various electron-optical techniques were used to characterize the microstructure. The coating system on the low-Ti alloy was found to outperform that on the high-Ti alloy. However, for both alloys, failure was observed to occur by loss of adhesion between the thermally-grown oxide and underlying bond coat.