Thermal Swing Evaluation of Thermal Barrier Coatings for Diesel Engines

Abstract

“Thermal swing” coatings have recently been of great interest to automotive researchers for their potential to insulate internal combustion engines, reduce cooling requirements, and increase their efficiency. Plasma-sprayed yttria-stabilized zirconia, ceramics in the MgO-Al2O3-SiO2 system (cordierite, mullite, and steatite), and silicate-yttria-stabilized zirconia composite thermal barriers have been investigated in this context with thermophysical properties measured using a thermal flash method and further screened by thermal swing using a custom laboratory developed functional test. Correlations between microstructure and thermal properties are developed and their combined impact on thermal swing assessed. The coatings thermal effusivity dominates these considerations for coatings above the characteristic thermal diffusion length of periodic exposure, with substrate thermal properties quickly becoming important below this thickness. Plasma-sprayed cordierite and its composites are determined to be the most promising materials for this application, exhibiting the highest thermal swing and effusivities as low as 373 Ws1/2/m2K. Performance testing in a heavy-duty single-cylinder diesel engine is ongoing with preliminary data suggesting that thermal swing alone may not provide efficiency benefits. Coating thickness, roughness, and dynamic interactions in the engine may play a role in this, highlighting that many potential nuances need consideration for the successful applications of these coatings.