Influence of microstructural parameters on thermal cycling behavior of DVC-TBC systems

Abstract

This study presents a novel approach to characterizing crack distribution and bond coat roughness in Thermal Barrier Coatings (TBCs) with dense vertically cracked (DVC) top coats and MCrAlY bond coats, aiming to correlate microstructural features with durability and failure mechanisms. A MATLAB®-based image analysis routine was developed to extract microstructural and morphological features from BSE-SEM micrographs. A novel parameter, the equivalent through-the-thickness crack density (ρ*_ttc), was introduced to provide a more accurate representation of crack distribution compared to conventional crack density. Additionally, standard (Ra, Rsm) and advanced (Rdq, Rdr) surface descriptors were calculated directly from SEM micrographs. TBCs with CoNiCrAlY bond coats were deposited on single-crystal and polycrystalline nickel-based superalloys and thermal cycling resistance was investigated with furnace cycle tests (FCT) at 1150 °C and 1100 °C. FCT at 1150 °C revealed that higher ρ*_ttc correlated with improved thermal cycling resistance due to enhanced strain tolerance, while conventional crack density showed no clear link to durability. Similarly, bond coat roughness analysis demonstrated that higher surface tortuosity, quantified by Rdr, associates with extended TBC lifespan by improving mechanical interlocking and stress dissipation. Additionally, a new non-destructive technique for real-time damage assessment using automatic thermographic image analysis was introduced. FCT at 1100 °C confirmed that coatings with higher ρ*_ttc and Rdr exhibit superior resistance to delamination cracks propagation, whereas lower values result in less effective strain tolerance and stress dissipation mechanisms.