Observations of the spallation modes in an overlay coating and the corresponding thermal barrier coating system

The oxidation dynamics of an overlay coating and the corresponding thermal barrier coating system are presented. The particular systems examined are c omposed of a nickel-based superalloy with an air plasma-sprayed NiCrAlY bon d coat and the thermal barrier coating system consists of air plasma-spraye d yttria stabilized zirconia layer. Failure can occur in these systems by c rack propagation within the ceramic outer layer at the interface with the b ond coat. Defects, such as microcracks and pores, are common in plasma-spra yed coatings and within the thermally grown oxide scales. These can act as initiation sites for cracks. The subsequent growth of these cracks can lead to loss of the outer protective materials. Considerable information is ava ilable by microscopic examination of sections through test specimens that h ave been held at temperature for varying amounts of time. By careful sample preparation it is possible to monitor the development of the oxide scale f ormed during high temperature testing and the sites of failure. Identificat ion of the initiation sites and growth of cracks is important in understand ing the spallation process. In this study, scanning electron microscopy is used to provide evidence of the processes involved in the two systems. A co mparison of the two coating systems reveals the effect the outer ceramic la yer has on the oxide scale growth, and the spallation processes crucial to the understanding of the failure mechanisms of these coating systems.