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

Abstract

To maximize energy efficiency, gas turbine engines used in airplanes and for power generation operate at very high temperatures, even above the melting point of the metal alloys from which they are comprised. This feat is accomplished in part via the deposition of a multilayer, multicomponent thermal barrier coating (TBC), which lasts up to approximately 40,000 h before failing. Understanding failure mechanisms can aid in designing circumvention strategies. We review results of quantum mechanics calculations used to test hypotheses about impurities that harm TBCs and transition metal (TM) additives that render TBCs more robust. In particular, we discovered a number of roles that Pt and early TMs such as Hf and Y additives play in extending the lifetime of TBCs. Fundamental insight into the nature of the bonding created by such additives and its effect on high-temperature evolution of the TBCs led to design principles that can be used to create materials for even more efficient engines.A ircraft and power plants share a common source of usable energy: Both employ turbine engines that combust fuel to either propel airplanes or produce electricity. At a time in which efficient use of energy is paramount, improving the efficiency of turbine engines is one means to contribute to this global challenge. Turbine engines operate via the Brayton cycle, which offers lower carbon dioxide emissions and lower cost for power generation than other possible alternatives. Their efficiency can be increased by increasing the inlet temperature, which allows more expansion of gas that creates more pressure to drive the turbine. However, high-temperature operation, under oxidizing conditions, poses serious demands on the materials used to construct jet engine components. Materials must be found that are robust under such harsh operating conditions. Engineers over the past few decades have improved greatly the thermomechanical properties of the metal alloy comprising, e.g., the turbine blades, and have created a multilayer coating for the blades that protects against both heat and corrosion, referred to as a thermal barrier coating (TBC). These materials advances, along with internal component cooling, have been astonishingly successful, allowing the gas temperature to exceed the melting point of the metal alloy from which the engine components are constructed! Despite these advances, more robust TBCs are desired, either to extend TBC service lifetime under present-day operating conditions or to operate at even higher temperatures to achieve more efficient energy conversion. As a result, characterization and optimization of TBC properties have continued to be active areas of research. As is the case for most materials development, the usual path to improve TBCs relies on trial and error. Many materials compositions are fabricated, characterized, and tested. Unfortunately, characterization typically is performed postmortem, as virtually no instruments exist to characterize a TBC in situ during operation. The lack of in situ probes makes...

Mixed-mode mechanical responses of Ni(111)/α-Al2O3(0001) interface by first-principle calculations

“…[1][2][3][4][5][6][7][8] The service lifetime of the TBCs is strongly affected by thermal cycling of the engine, which eventually causes the coating to spall. Often the failure appears to occur along the bond coat/TGO interface, by nucleation of small cracks and voids at defects, which then coalesce, leading eventually to large-scale buckling and spallation.…”

Generalized Debye-Peierls/Allen-Feldman model for the lattice thermal conductivity of low-dimensional and disordered materials

“…Of the downto-zero modes, the LA and the TW modes clearly exhibit an acoustic nature, while the degenerate TA 1 ,TA 2 modes exhibit a more quadratic nature. For the latter set, the transition from parabolic to linear dispersion only becomes complete in the limit of vanishing radius (truly 1D systems); the (13,13) CNT dispersion shown in Fig. 5 therefore shows remnants of 2D dispersion and in some sense this CNT can be considered a quasi-1D system.…”

“…As thermal science expands into the realm of lowdimensional (low-D) materials 1-8 , a variety of intriguing effects are being revealed [6][7][8][9][10] . The unique physics of thermal transport in low-D has inspired many potential applications in the real world [11][12][13] . This physics is pushed to its limits when materials are genuinely atomically thin, such as two-dimensional graphene 14 and one-dimensional carbon nanotubes (CNTs) 1,3,4,15 .…”

“…For instance, nanoporous graphene 16 and glassy diamond nanothreads 17,18 are recent examples of materials systems in which both disorder and low dimensionality are simultaneously present. Ample applications of these materials are in incubation 16,17 , including thermoelectrics 12 and thermal barrier coatings 13 . However, while the structural, electronic 19,20 , and mechanical properties [16][17][18] of low-D and disordered materials have received comparably greater attention, their thermal properties are less established.…”

Large scale Molecular Dynamics simulation of microstructure formation during thermal spraying of pure copper

“…Several studies have been conducted in order to investigate formation processes using various modeling techniques ranging from the atomistic to the continuum scale, including ab-initio techniques [28], Molecular Dynamics (MD) [29], [30], [31], [32], [33], [34], kinetic Monte Carlo [12], Smoothed Particle Hydrodynamics [35] and multiple methods on the continuum scale [10], [36], [37]. Especially the required coupling of solidification and thermo-mechanical phenomena, e.g.…”