Multiple-scale modeling of Pt effect on durability of aluminide coatings

The beneficial effect of Pt on the durability of aluminide coatings subjected to thermal cycling was studied by first-principles density functional theory (DFT) calculations, thermal physical analysis, atom diffusion simulation and mechanics-based modeling. This presentation will highlight these analytical approaches and research findings in three subject areas. First, the effect of doping elements (S, Pt, Cr, Y, Zr, Hf) on work adhesion, Wad, of the β-NiAl coating/Al2O3 scale interface was investigated. S was found to significantly reduce, while Y, Zr and Hf to largely increase Wad of the interface, with Pt having a minimal effect. The bonding characteristics of the interface associated with the elemental doping were used to explain the trend of Wad. Furthermore the X-S (X=Pt, Y, Zr, Hf) co-doping effects were also examined to elucidate the potential of these dopants in supressing the detrimental effects of S. Second, the beneficial effect of Pt on reducing the β-NiAl/Al2O3 interfacial tensile stress was assessed. The coefficients of thermal expansion (CTE) of Pt, β-NiAl and β-NiAl+Pt were obtained using DFT calculations and thermal physical formulas. The calculated CTE of the coatings, along with the experimentally measured CTE of Al2O3, were then incorporated to evaluate thermal cyclic tensile stress at the undulated β-NiAl/Al2O3 interface. The results showed that the addition of Pt to b-NiAl coating reduced the interfacial tensile stress, thus contributing to the improvement of thermal cyclic durability of the coating. This beneficial effect of Pt was more pronounced with a thicker oxide scale and a larger ratio of interfacial wave amplitude over wavelength. Third, the beneficial effect of Pt on lowering the diffusivity of S in b-NiAl coating was evaluated. The apparent activation energy and the pre-exponential factor of diffusivity via the next nearest neighbour (NNN) atom transportation as well as via interstitial jumps were analyzed, and the bonding characteristics of S with its surrounding atoms were calculated and compared with experimental results in the literature to elucidate the diffusion process of S. The addition of Pt in b-NiAl was found to significantly reduce the diffusivity of S, thus supressing the detrimental segregation of S to the β-NiAl coating/Al2O3 scale interface. Please click Additional Files below to see the full abstract