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.

https://link.springer.com/content/pdf/10.1557/mrs.2012.232.pdf

Thermal-barrier coatings for more efficient gas-turbine engines

Since the mid-1990s, phosphors have played a key role in emerging solid-state white-lighting technologies that are based on combining a III-nitride-based near-UV or blue solid-state light source with downconversion to longer wavelengths. Almost all widely used phosphors comprise a crystalline oxide, nitride, or oxynitride host that is appropriately doped with either Ce3+ or Eu2+. These ions, with [Xe] 4fn5d0 configurations (n = 1 for Ce3+ and 7 for Eu2+) have proximal excited states that are [Xe] 4fn−15d1. Optical excitation into these states and concomitant reemission can be tuned into the appropriate regions of the visible spectrum by the crystal these ions are hosted in. In this article, we review the current needs and key aspects of the conversion process. We describe some currently used families of phosphors and consider why they are suitable for solid-state lighting. Finally, we describe some empirical rules for new and improved host materials.

Phosphors for Solid-State White Lighting

A comprehensive and integrated review of thermal barrier coatings (TBCs) applied to turbine components is provided. Materials systems, processes, applications, durability issues, technical approaches and progress for improved TBC, and our understanding of the science and technology are discussed. Thermal barrier coating prime reliance and further advances have been hampered by TBC loss by particle impact and erosion in certain locations of the turbine blades. Accumulation of low melting eutectic containing calcia, magnesia, alumina and silica resulting in TBC spallation limits maximum surface temperature. Design methodologies to address durability and data scatter issues are discussed. Compositions, morphology, characteristics and performance data for new bonds to achieve longer TBC life are described. Further reduction in the thermal conductivity of the top layer to minimise the parasitic mass of the coating on the component is being sought via top layer composition and processing modifications a...

Thermal barrier coatings technology: critical review, progress update, remaining challenges and prospects

Zirconia and alumina have been successfully incorporated into turbines used for propulsion and power generation. They exert a crucial influence on the fuel efficiency. The roles of these oxides within the overall system are described, relative to those for the other constituents, and their most important properties are outlined. The mechanisms that govern their properties are presented and approaches for adjusting them in desirable directions are discussed. Opportunities for new materials with potential for superior performance are assessed.

The influence of oxides on the performance of advanced gas turbines

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.

/pdf/environmental-degradation-of-thermal-barrier-coatings-by-1ehpxl8r4o.pdf

Environmental degradation of thermal- barrier coatings by molten deposits

The measurement of high temperatures in oxides and oxide-based structures in practical applications often presents challenges including steep thermal gradients, the presence of flames or chemically aggressive environments, and the transparency or translucency of most oxides For turbine engines, oxide coatings are of great commercial importance, and the rapid motion of parts prohibits contact thermometry Luminescence thermometry offers a number of advantages for measuring temperature in such systems and has been the subject of ongoing study for many years Recent work on rare-earth-doped thermal barrier coatings, environmental barrier coatings, and related oxides has demonstrated the feasibility of luminescence thermometry to temperatures well in excess of 1000°C The luminescent properties of these materials and the analytical techniques used to extract reproducible temperature measurements from the measured luminescence are reviewed

Doped Oxides for High-Temperature Luminescence and Lifetime Thermometry

An analysis has been conducted that characterizes the susceptibility to delamination of thermal barrier coated (TBC) hot-section aero-turbine components when penetrated by calcium-magnesium-alumino-silicate (CMAS). The assessment has been conducted on stationary components (especially shrouds) with relatively thick TBCs after removal from aero-engines. In those segments that experience the highest temperatures, the CMAS melts, penetrates to a depth about half the coating thickness, and infiltrates all open areas. Therein the TBC develops channel cracks and sub-surface delaminations, as well as spalls. Estimates of the residual stress gradients made on cross-sections (by using the Raman peak shift) indicate tension at the surface, becoming compressive below. By invoking mechanics relevant to the thermo-elastic stresses upon cooling, as well as the propagation of channel cracks and delaminations, a scenario has been presented that rationalizes these experimental findings. Self-consistent estimates of the stress and temperature gradients are presented as well as predictions of channel cracking and delamination upon cooling.

Mechanisms of cracking and delamination within thick thermal barrier systems in aero-engines subject to calcium-magnesium-alumino-silicate (CMAS) penetration

Decay pathway and high-temperature luminescence of Eu3+ in Ca2Gd8Si6O26

The use of the luminescence from europium doped yttria-stabilized zirconia thermal barrier coatings for non-contact thermometry has been previously studied up to 850 °C [Choy, K.L. et al., Surface Engineering, 16 [6] (2000) 496] and 1150 °C [M.M. Gentleman and D.R. Clarke, Surface and Coatings Technology, 188–189 (2004) 93]. In this contribution we examine the effect of long-term, high temperature aging and martensitic phase transformation on both the details of the luminescence spectrum and the temperature-dependent lifetime of the luminescence. It is found that even after prolonged aging, the wavelength of the 5D0 → 7F2 emission peak shifts only very slightly with increasing percentage of transformation to the monoclinic phase. Preliminary data suggests that the effect on the decay lifetime up to 1150 °C is very slight, expressed in a slight shift of the onset of thermal quenching to lower temperatures; the slope of the characteristic decay time-constant versus temperature appears relatively unaffected.

Effect of long term, high temperature aging on luminescence from Eu-doped YSZ thermal barrier coatings

Previous work has demonstrated the utility of europium doping into 7YSZ for luminescence sensing of temperature in TBC systems by the luminescence decay time of the europium chromophore after pulsed laser excitation, with sensitivity up to 1100 °C Based on relative energy gaps and a multi-phonon relaxation model, terbium is predicted to yield longer decay times than europium and allow extension of the temperature sensitivity to higher temperatures Therefore, terbium doping as a chromophore for luminescence sensing of current thermal barrier coatings is investigated Terbium doping is found to be ineffective in both yttria-stabilized zirconia and gadolinium zirconate (GZO) because of the formation of Tb 4+  However, terbium doping in gadolinium aluminum perovskite (Tb:GaAlO 3 ) gives rise to strong luminescence bands Luminescence decay measurements indicate that Tb:GaAlO 3 has potential as a thermographic sensor to at least 1250 °C Further, the brightness of Tb:GdAlO 3 at moderate and high temperatures is found to exceed that of Eu:7YSZ The phase compatibility of Tb:GaAlO 3 with alumina and gadolinium zirconate is suitable for luminescence temperature sensing both at a TGO interface and on the surface of a GZO coating

M.D. Chambers

Papers

Doped Oxides for High-Temperature Luminescence and Lifetime Thermometry

Mechanisms of cracking and delamination within thick thermal barrier systems in aero-engines subject to calcium-magnesium-alumino-silicate (CMAS) penetration

Decay pathway and high-temperature luminescence of Eu3+ in Ca2Gd8Si6O26

Effect of long term, high temperature aging on luminescence from Eu-doped YSZ thermal barrier coatings

Terbium as an alternative for luminescence sensing of temperature of thermal barrier coating materials