Thickness apropos stoichiometry in Pt-aluminide (PtAl) coating: Implications on the tensile properties of a directionally solidified Ni-base superalloy

The effect of thickness and stoichiometry of a PtAl coating on the tensile properties of a directionally solidified superalloy has been examined in the temperature range between room temperature (RT) to 1000 °C. The coating thickness was varied as 60, 90 and 180 µm. A concomitant change in the inherent stoichiometry, from Ni-rich to Al-rich, occurred for the constituent B2-(Ni,Pt)Al phase of the coating with the increase in thickness. The brittle-to-ductile-transition-temperature (BDTT) of the coating increased relatively with increase in thickness and, correspondingly, ductile deformation in the coatings occurred at higher temperatures of 700, 870 and 1000 °C. The high BDTT and brittle fracture in the thick coating till high temperatures had a negative bearing on the tensile properties of the superalloy substrate. In the brittle fracture regime for the respective coatings, the tensile properties of the substrate deteriorated more with the application of thick coatings. The formation of long through-thickness cracks and higher crack penetration depths in thick coating induce greater deterioration in tensile properties of the superalloy. On the other hand, ductile failure in the coatings retards the formation of through-thickness cracks, preserves the tensile strength, and improves the ductility of the substrate.