Foreign object damage in a thermal barrier system: mechanisms and simulations

Experimental studies have been performed of foreign object damage (FOD) imparted to a thermal barrier system under conditions representative of those found in a turbine engine. The sub-surface damage has been characterized by using the focused ion beam (FIB) imaging system. The characterization reveals changes in the thermal barrier coating (TBC), caused by particle impact, that confirm and elaborate previous observations of FOD. These features include a permanent impression, a zone of densification, shear bands penetrating from the impact site to the interface with the bond coat, and delamination cracks extending away from the impact in the TBC adjacent to the interface. The dimensions of these features have been reported. A simulation procedure has been devised and implemented. The simulations have been performed in conjunction with a new non-dimensional analysis that allows the impact and material variables to be grouped into the smallest possible parameter set needed to characterize the stresses and projectile velocities, as well as the impression and densification zone dimensions. This parameterization provides explicit results for the stresses and displacements that arise as the projectile characteristics and material properties are varied over a range applicable to FOD in gas turbines. A scaling relation has been derived from the stress field and the penetration that relates the length of the interface delamination to the impact and material variables. A comparison of the simulations with the measurements indicates that the unknown impact velocity of the projectile can be ascertained from the penetration depth if the yield strength is known and vice versa. With this information, the scaling relation for the size of the interface delamination indicates consistency with the measured cracks. The implication is that delamination can be suppressed by lowering the high temperature hardness of the TBC and by increasing its toughness.