Failure analysis of suspension plasma sprayed thermal barrier coatings

Abstract: Thermal barrier coatings (TBC) are proven to protect the hot section of components operated under elevated temperature. Suspension Plasma Spray (SPS) is an emerging manufacturing process that is used to produce TBCsI, It has the ability to deposit a thinner TBC that has finer microstructure than coatings deposited using traditional Atmospheric Plasma Spray (APS). The coating spray parameters have a significant role in developing the coating properties and thereby the coatings failure. In this thesis work, the parameters such as the spray distance, the feed rate, and the surface velocity were varied to deposit six sets of TBC samples. The as-sprayed samples were analysed and next tested at different temperatures (1000°C, 1050°C and 1100°C) making thermal cyclic fatigue test (TCF) and thermal shock test also known as Burner Rig Test (BRT). These investigations aimed at trying to find an explanation for the effect of each varied process parameter on the deposited TBC. However the cases at 1100°C were not reliable enough to conclude. It was found that most porous coating was produced in Run 1. The porosity decreased gradually from Run 1 to the last run (Run 6) by changing the spray parameters. The failure in all cases was mainly due to the Thermally Grown Oxide (TGO) which causes cracking near the topcoat-bond coat interface. According to many cross sectional and top surface morphology SEM images taken during this work, the sintering of TBCs during thermal cycling also played a significant role in the top coat failure.