Mp. Taylor et al., Observations of the spallation modes in an overlay coating and the corresponding thermal barrier coating system, MATER HIGH, 17(2), 2000, pp. 219-224
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.