Gas-turbine engines used in transportation, energy, and defense sectors rely on high-temperature thermal-barrier coatings (TBCs) for improved efficiencies and power. The promise of still higher efficiencies and other benefits is driving TBCs research and development worldwide. An introduction to TBCs—complex, multi-layer evolving systems—is presented, where these fascinating systems touch on several known phenomena in materials science and engineering. Critical elements identified as being important to the development of future TBCs form the basis for the five articles in this issue of MRS Bulletin. These articles are introduced, together with a discussion of the major challenges to improved coating development and the rich opportunities for materials research they provide.

Thermal-barrier coatings (TBCs) are complex, defected, thick films made of zirconia-based refractory ceramic oxides. Their widespread applicability has necessitated development of high throughput, low cost materials manufacturing technologies. Thermal plasmas and electron beams have been the primary energy sources for processing of such systems. Electron-beam physical vapor deposition (EBPVD) is a sophisticated TBC fabrication technology for rotating parts of aero engine components, while atmospheric plasma sprays (APS) span the range from rotating blades of large power generation turbines to afterburners in supersonic propulsion engines. This article presents a scientific description of both contemporary manufacturing processes (EBPVD, APS) and emerging TBC deposition technologies based on novel extensions to plasma technology (suspension spray, plasma spray-PVD) to facilitate novel compliant and low thermal conductivity coating architectures. TBCs are of vital importance to both performance and energy efficiency of modern turbines with concomitant needs in process control for both advanced design and reliable manufacturing.

Thermal-barrier coatings are complex systems with properties that largely depend on their specific microstructure. Their properties change during operation, typically leading to degradation. A further difficulty arises from the fact that this degradation also depends on specific loading conditions that can be rather complex. Different laboratory setups are described that simulate, at least partially, the actual loading conditions. In addition, sensing and nondestructive methods are described that are targeted toward reliable operation of a gas-turbine engine with thermal-barrier coated components.

Oxides hold great promise as new and improved materials for thermal-barrier coating applications. The rich variety of structures and compositions of the materials in this class, and the ease with which they can be doped, allow the exploration of various mechanisms for lowering thermal conductivity. In this article, we review recent progress in identifying specific oxides with low thermal conductivity from both theoretical and experimental perspectives. We explore the mechanisms of lowering thermal conductivity, such as introducing structural/chemical disorder, increasing material density, increasing the number of atoms in the primitive cell, and exploiting the structural anisotropy. We conclude that further systematic exploration of oxide crystal structures and chemistries are likely to result in even further improved thermal-barrier coatings.

The complex thermochemical and thermomechanical environments in high temperature propulsion and energy generation systems often demand the use of suites of materials with disparate properties. Unique combinations of materials that simultaneously function to optimize mechanical, thermal, and environmental properties can enable breakthroughs in design and system capability. This article focuses on interlayers that function as environmental barriers and promote adhesion of the ceramic thermal barriers to metallic substrates. The structure, composition, processing, and performance of major classes of bond coatings are briefly reviewed. Challenges for the development of new coating systems and for prediction of their performance in service are addressed.

Molten deposits based on calcium-magnesium alumino-silicates (CMAS), originating from siliceous debris ingested with the intake air, represent a fundamental threat to progress in gas turbine technology by limiting the operating surface temperature of coated components. The thermomechanical and thermochemical aspects of the CMAS interactions with thermal-barrier coatings, as well as the current status of mitigating strategies, are discussed in this article. Key challenges and research needs for developing adequate solutions are highlighted.

This article shows how the scanning transmission electron microscope provides a Z-contrast image (where Z is atomic number) that is often directly interpretable and can show higher resolution than a phase-contrast image. It represents an incoherent mode of imaging, similar to that described by Lord Rayleigh for the optical microscope over a century ago. Today, resolution has reached a half Ångstrom, and spectroscopic analysis of individual atomic columns, even of individual atoms in two-dimensional materials, has become possible.