The Effect of Platinum on Diffusion Kinetics in β‐NiAl: Implications for Thermal Barrier Coating Lifetimes

Abstract

Platinum is added to thermal barrier coatings (TBCs) as it is observed empirically to extend their lifetime, but the mechanism by which Pt acts is unknown. Since Pt has been proposed to alter diffusivities in NiAl, a key component of TBCs, we use first-principles quantum mechanics calculations to investigate atomic level diffusion mechanisms. Here, we examine the effect of Pt on five previously proposed mechanisms for Ni diffusion in NiAl: next-nearest-neighbor jumps, the triple defect mechanism, and three variants of the six jump cycle. We predict that Pt increases the rate of Ni diffusion by stabilizing point defects and defect clusters that are diffusion intermediates. Previously, we predicted the triple defect mechanism to be a dominant Ni diffusion mechanism; it simultaneously results in long-range Al diffusion in the opposite direction. Since Pt increases the rate of Ni diffusion, it also increases Al diffusion in NiAl, which may be key to extending the coating lifetime.

Ni and Al diffusion in Ni-rich NiAl and the effect of Pt additions

“…The migration energy of step II, case A represents an upper bound since the Ni atom is actually moving to lattice site that is in close proximity to a periodic image of the Pt atom. We have previously seen that when a Ni or Al atom moves toward a Pt atom, the migration energy increases [5,19], and if the Pt atom was in a different location further from the migrating atom, the migration energy would not Table 4 Al hop starting with the ASB [100] defect cluster 2 in Fig. 2: Defect formation energies (E d ), migration energies (E m ), activation energies (Q), hTST pre-exponential factors (n 0 ); entropic factors and Arrhenius pre-exponential factors (D 0 ) for 1200 K and 1500 K for pure NiAl, and with Pt in site A or site B in Fig.…”

“…Table 3 Antistructure bridge mechanism [111] jump: Defect formation energies (E d ), migration energies (E m ), activation energies (Q), hTST pre-exponential factors (n 0 ); entropic factors, and Arrhenius pre-exponential factors (D 0 ) for 1200 K and 1500 K for (a) pure NiAl, (b) with Pt in site A or (c) site B, as depicted in Fig. 4. Step I: [19]. Our earlier work showed that Pt had no effect on the pre-exponential factor to the diffusion constant, so only Pt's effect on activation energies will be discussed here, but the calculated values of the entropic factors (Equation (4)) and pre-exponential factors are presented in Tables 1e3 for completeness.…”

“…Due to the dynamic nature of the coatings at high temperature, it is possible that Pt affects diffusivities in the alloy [14]. To test this hypothesis, we undertook an extensive DFT-based study of the effect of Pt on diffusion of Ni [19] and Al [5] in stoichiometric NiAl. We predicted that Pt enhances both diffusion rates by stabilizing point defects and defect clusters involved in the diffusion mechanisms by decreasing their formation energy [3].…”

Atomic-scale insight and design principles for turbine engine thermal barrier coatings from theory

“…Indeed, density difference plots (Fig. 4) reveal an increase in electron density around the Pt atom such that a s 2 d 9 -like Pt anion is formed, due to its higher electronegativity (52). In turn, the more diffuse Pt anion destabilizes nearby Ni and Al atoms, favoring Ni vacancy formation in particular.…”

“…However, the substantial decreases in the defect formation energies produce large increases in the diffusion rate because these energies appear in the exponential of the diffusion coefficient. Therefore, Pt enhances Ni and Al diffusion by stabilizing the point defects and defect clusters that are intermediate minima along the diffusion pathways (52,55,57).…”