Session BB8.1

1:30 PM BB8.1
MECHANICAL PROPERTIES OF THERMAL BARRIER COATINGS: EFFECTS OF FRACTURE, RESIDUAL STRESSES AND VOID DISTRIBUTIONS. Toshio Nakamura, Gang Qian, Dept of Mechanical Engineering, Christopher C. Berndt, Dept of Materials Science and Engineering, SUNY at Stony Brook, NY.

Driving mechanisms which lead to internal crack growth and failure in the thermally sprayed coatings are identified using detailed finite element models. Coatings are assumed to contain embedded cracks and they are thermally loaded according to a typical high temperature environment. In order to determine the accurate stress state, thermal gradient within the coating is calculated from the steady-state heat transfer analysis. Our models take into account various locations of cracks, temperature-dependent and -independent plasticity, thermal conductivities of different layers and thermal insulation across crack surfaces. The results indicate that the energy release rate of large cracks can reach close to fracture toughness of ceramic coatings. We have also studied the effect of residual stresses. For a penny-shaped crack located parallel to the coating layers, a limited influence of residual stresses is observed. The effect is more pronounced when the crack orientation is perpendicular to the coating layers where it has shown a beneficial influence. The implications of this work to internal crack initiation and growth, which can lead to coating failure, are addressed. In addition, we have modeled ceramic coating containing many micro-voids. Initially, the effects of void distribution as well as shapes on the overall elastic moduli are investigated. In this analysis, voids are randomly distributed to simulated pores and cracks observed in actual TBC's. The results show if the distribution is sufficiently random, the elastic moduli can be approximated as a function of void volume fraction. Furthermore we have used the random models to study the residual stress field within multilayered TBC's. Here a bond coat is modeled as elastic-plastic material and the influence of void size and distribution on the magnitude of residual stresses are studied.