Investigation of Critical Issues in Thermal Barrier Coating Durability

Download or read book Investigation of Critical Issues in Thermal Barrier Coating Durability written by Hyungjun Kim and published by . This book was released on 2005 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Abstract: To more fully understand microstructural evolution and the development of damage in thermal barrier coating (TBC) systems, thermocyclic experiments were conducted on TBC specimens with and without top coats. The degradation of a PtAl (platinum modified aluminide) bond coat and a CMSX superalloy substrate were investigated for cyclic and quasi-isothermal heating to 1200 0C. To accelerate the oxidation of the specimens, the thermally grown oxide (TGO) was removed at 10 hour intervals. Scanning electron microscopy (SEM) and instrumented indentation were employed to investigate microstructural evolution and material property changes of the bond coat. The microstructural evolution of a PtAl bond coat is strongly affected by the type of thermal exposure and the presence of TGO. The differences between cyclic and quasi-isothermal heating indicate that stresses associated with cooling and heating significantly alter microstructural evolution. Damage to TBC specimens with EB-PVD (electron beam physical vapor deposition) processed YSZ (yttria stabilized zirconia) top coats, PtAl bond coats, and René N5 superalloy substrates was assessed during thermal cycling to 1200 0C. Acoustic emission (AE) techniques were used to temporally identify damage to the TBC. For automated, continuous, and high temperature AE detection, a custom made experimental setup with either a nickel-chrome alloy wire wave guide or an alumina wave guide was used. It was found that there are four distinct regions of AE activity during the life of a TBC. Throughout the cooling cycles, images of the top coat were collected with darkfield-type lighting. These images showed how undulations developed in the top coat. In addition, digital image correlation (DIC) was used to identify regions with interfacial damage. Finally, the images were used to analyze the spallation of the top coat. Cycling of an additional specimen was interrupted periodically for analysis with profilometry and SEM. The profilometry and SEM images were used to further investigate the failure mechanisms in TBCs. The experiments and analysis performed in this dissertation work have elucidated how various failure mechanisms develop during the life of TBCs. This work has improved the scientific understanding of TBCs and could aid in the development of improved life prediction models.