Functionally Graded Thermal Barrier Coatings with Improved Reflectivity and High-Temperature Capability

Abstract

Conventional thermal barrier coating (TBC) systems consist of a duplex structure with a metallic bondcoat and a ceramic, heat isolative topcoat. In modern TBCs the ceramic topcoat is further divided into layers with different functions. One example is the double layer system in which conventional yttria stabilized zirconia (YSZ) is used as bottom and new materials as pyrochlores or perovskites are used as topcoat layers. These systems demonstrated an improved temperature capability compared to standard YSZ. Examples of such systems will be shown. In modern gas turbines the increased temperatures and gas pressures lead to an increased fraction of radiative heat flow. Coatings with increased reflectivity can be used to avoid the direct heating of the metallic substrates by this radiation. An effective method to produce such coatings is suspension plasma spraying. These reflective coatings are deposited on top of the TBC system and will lead to a further grading and improved performance of the coating.

A study of suspension plasma-sprayed insulated pistons evaluated in a heavy-duty diesel engine

“…The temperature reduction for the piston with the APS coating was the lowest, despite having the lowest thermal conductivity (0.43 W m −1 K −1 ) 27 of the three coatings tested. It has been shown by Vaßen et al 24 that SPS coatings have a higher hemispherical reflectance than APS coatings. A higher radiative heat transfer through the APS coating is thus a potential reason for the low reduction in oil temperature for the APS YSZ piston.…”

“…However, there is a risk that this benefit may be counteracted by gas and fuel entrapment in the intercolumnar gaps that typically exist in SPS coatings. Small grain size and large amounts of scattering defects, such as pores, have also been shown to improve the hemispherical reflectance of a TBC, 23,24 which may result in lower heat transfer from thermal radiation through the coating. SPS coatings with columnar microstructure or vertical cracks have also been shown to survive longer in thermal cyclic tests than APS coatings.…”

A study on nanostructured in-situ oxide dispersed NiAl coating and its high temperature oxidation behavior

“…For this reason, the application of appropriate coating could be an excellent achievement. There have been many efforts to improve thermally sprayed metallic coatings including the use of various thermal spray methods, post-spray treatments, grain size control, oxide dispersoides, new alloys development [1] and/or new designs of coating systems, such as functionally graded coatings [2,3].…”

SUPERMAT – A virtual center for sustainable development of advanced materials operating under extreme conditions

“…spark plasma sintering, field assisted sintering, gel casting) with controlled surface/interface properties have been extensively studied to obtain different materials solutions required in many fields of application. Complex systems such as Nano-laminates, Nano-composites (granules, fibers, whiskers, nano-plates reinforced, nano-whiskers reinforced) and oxide dispersion strengthen steels (ODS) systems have been succesfully developed for solar power systems, parts working under high corrosion and erosion, friction parts for automotive or cutting tools [1][2][3][4][5][6][7][8][9][10][11]. On the other hand, modern coatings methods (physical, chemical, electrochemical and combined) feature advantages such as: versatility of materials range to be coated, properties independent of thermodynamic compositional constraints, the ability to control defects states and concentrations, the controllable production of different microstructures and morphologies (including nanosized architechtures), the ability to accommodate large residual strains associated with thermal cycles, ecological benefits and reduced critical raw materials consumption [12,13].…”