A.G. Evans

Mechanisms controlling the durability of thermal barrier coatings

Abstract The durability of thermal barrier coatings is governed by a sequence of crack nucleation, propagation and coalescence events that accumulate prior to final failure by large scale buckling and spalling. Because of differing manufacturing approaches and operating scenarios, several specific mechanisms are involved. These mechanisms have begun to be understood. This article reviews this understanding and presents relationships between the durability, the governing material properties and the salient morphological features. The failure is ultimately connected to the large residual compression in the thermally grown oxide through its roles in amplifying imperfections near the interface. This amplification induces an energy release rate at cracks emanating from the imperfections that eventually buckle and spall the TBC.

The topological design of multifunctional cellular metals

Abstract The multifunctional performance of stochastic (foamed) cellular metals is now well documented. This article compares such materials with the projected capabilities of materials with periodic cells, configured as cores of panels, tubes and shells. The implementation opportunities are as ultra-light structures, for compact cooling, in energy absorption and vibration control. The periodic topologies comprise either micro-truss lattices or prismatic materials. Performance benefits that can be expected upon implementing these periodic materials are presented and compared with competing concepts. Methods for manufacturing these materials are discussed and some cost/performance trade-offs are addressed.

Fabrication and structural performance of periodic cellular metal sandwich structures

Metallic sandwich panels with periodic, open-cell cores are important new structures, enabled by novel fabrication and topology design tools. Fabrication protocols based on the sheet forming of trusses and shell elements (egg-boxes) as well as textile assembly have allowed the manufacture of robust structures by inexpensive routes. Topology optimization enables control of failure mechanisms at the truss length scale, leading to superior structural performance. Analysis, testing and optimization have demonstrated that sandwich panels constructed with these cores sustain loads at much lower relative densities than stochastic foams. Moreover, the peak strengths of truss and textile cores are superior to honeycombs at low relative densities, because of their superior buckling resistance. Additional benefits of the truss/textile cores over honeycombs reside in their potentially lower manufacturing cost as well as in their multifunctionality.

Failure mechanisms associated with the thermally grown oxide in plasma-sprayed thermal barrier coatings

Abstract The microstructure and durability of a thermal barrier coating (TBC) produced by the thermal spray method have been characterized. Upon exposure, the bond coat chemistry and microstructure change by inter-diffusion with the substrate and upon thickening of the thermally grown oxide (TGO). A wedge impression test, in conjunction with observations by scanning electron microscopy, has been used to probe the failure mechanisms. At short exposure times, when the TGO thickness is less than about 5 μ m, the growth of the TGO does not affect the crack patterns in the TBC and delaminations induced by wedge impression propagate within the TBC about 30 μ m from the interface. An amorphous phase at the splat interfaces promotes this failure mode. As the thickness of TGO increases during exposure, cracks form in the TBC around imperfections at the interface. Moreover, induced delaminations develop a trajectory close to the interface, propagating not only through the TBC but also within the TGO and along the interfaces. A scaling result based on the misfit around imperfections caused by TGO growth has been used to rationalize the critical TGO thickness when the TBC fails.

The structural performance of near-optimized truss core panels

Theoretical studies have indicated that truss core panels with a tetragonal topology support bending and compression loads at lower weight than competing concepts. The goal of this study is to validate this prediction by implementing an experimental protocol that probes the key mechanical characteristics while addressing node eccentricity and structural robustness. For this purpose, panels have been fabricated from a beryllium–copper alloy using a rapid prototyping approach and investment casting. Measurements were performed on these panels in flexure, shear and compression. Numerical simulations were conducted for these same configurations. The measurements reveal complete consistency with the stiffness and limit load predictions, as well as providing a vivid illustration of asymmetric structural responses that arises because the bending behavior of optimized panels is dependent on truss orientation. 2002 Elsevier Science Ltd. All rights reserved.

Characterization of a cyclic displacement instability for a thermally grown oxide in a thermal barrier system

Abstract The mechanism responsible for the performance of a commercial thermal barrier system upon thermal cycling has been investigated. It comprises an electron beam physical vapor deposited (EB–PVD) yttria-stabilized zirconia thermal barrier coating (TBC) on a (Ni,Pt)Al bond coat. At periodic interfacial sites, the thermally grown oxide (TGO) that forms between the TBC and the bond coat at high temperature displaces into the bond coat with each thermal cycle. These displacements induce strains in the superposed TBC that cause it to crack. The cracks extend laterally as the TGO displaces, until those from neighboring sites coalesce. Once this happens, the system fails by large scale buckling. The displacements are accommodated by visco-plastic flow of the bond coat and “vectored” by a lateral component of the growth strain in the TGO. They depend upon the initial morphology of the metal/oxide interface. The observed responses are compared with the predictions of a ratcheting model.

Mechanics-based scaling laws for the durability of thermal barrier coatings

The durability of thermal barrier systems is governed by a sequence of crack nucleation, propagation and coalescence events that accumulate prior to final failure by large scale buckling and spalling. This sequence is governed by the σzz stresses that develop normal to the substrate, around imperfections, as the thermally grown oxide (TGO) thickens. Their effect is manifest in the stress intensity factor, K, caused by the σzz stresses acting on cracks emanating from them. In turn, these events are governed by scaling laws, ascribed to non-dimensional groups governing σzz and K. In this article the basic scaling relations are identified and used to gain some understanding of the relative importance of the various mechanisms that arise for application scenarios with minimal thermal cycling. These mechanisms are based on stresses that develop because of TGO growth strains in combination with thermal expansion misfit. The results are used to identify a critical TGO thickness at failure and express it in terms of the governing material variables. The changes in behavior that arise upon extensive thermal cycling, in the presence of TGO ratcheting, are elaborated elsewhere.

Mechanics of materials: Top-down approaches to fracture

The utility and robustness of the mechanics of materials is illustrated through a review of several recent applications to fracture phenomena, including adhesive failures, the role of plasticity in enhancing toughness in films and multilayers, and crack growth resistance in ductile structural alloys. The commonalty among the approaches rests in a reliance on experiments to provide calibration of the failure process at the smallest scale.

A numerical model for the cyclic instability of thermally grown oxides in thermal barrier systems

Morphological instability of the thermally grown oxide (TGO) is a fundamental source of failure in some thermal barrier systems. The instabilities occur when initial non-planarity in the TGO grows in amplitude as the system experiences thermal cycling. By numerical means, this study explores how these instabilities are linked to constituent properties. The associated phenomena involve oxidation of the TGO, plastic flow of the bond coat, thermal expansion misfit between the TGO, bond coat and substrate, and stress relaxation in the TGO at high temperature. A key implication of the simulations is that the incidence of reverse yielding upon reheating differentiates between systems that exhibit a systematic increase in imperfection amplitude upon thermal cycling (ratcheting) and those that exhibit shakedown.

The characterization of telephone cord buckling of compressed thin films on substrates

The topology of the telephone cord buckling of compressed diamond-like carbon (lms (DLC) on glass substrates has been characterized with atomic force microscopy (AFM) and with the focused ion beam (FIB) imaging system. The pro(les of the several buckles have been measured by AFM to establish the symmetry of each repeat unit, revealing similarity with a circular buckle pinned at its center. By making parallel cuts through the buckle in small, de(ned locations, straight-sided buckles have been created on the identical (lms, enabling the residual stress in the (lm to be determined from the pro(le. It has been shown that the telephone cord topology can be e;ectively modeled as a series of pinned circular buckles along its length, with an unpinned circular buckle at its front. The unit segment comprises a section of a full circular buckle, pinned to the substrate at its center. The model is validated by comparing radial pro(les measured for the telephone cord with those calculated for the pinned buckle, upon using the residual stress in the (lm, determined as above. Once validated, the model has been used to determine the energy release rate and mode mixity, G( ). The results for G( ) indicate that the telephone cord con(guration is preferred when the residual stress in the DLC is large, consistent with observations that straight-sided buckles are rarely observed, and, when they occur, are generally narrower than telephone cords. Telephone cords are observed in many systems, and can be regarded as the generic morphology. Nevertheless, they exist subject to a limited set of conditions, residing within the margin between complete adherence and complete delamination, provided that the interface has a mode II toughness low