Understanding and Life Modeling

Efforts to understand coating behavior and failure mechanisms are helping to guide the development of new coating concepts and are serving as the basis for life prediction model development. Research at NASA has indicated that the thermal expansion mismatch strains encountered on cooling are more deleterious than the strains encountered on heating in a burner rig or turbine engine (refs. 15,16). Also, time-at-temperature effects -- most notably bond coat oxidation — were shown to play a crucial role in coating failure. These experimental observations were cast in the form of a laboratory life prediction model which employed the simplifying assumptions that only bond coat oxidation and thermal expansion mismatch strains controlled life (refs. 17,18). Life prediction development was then extended to design-capable models through three contracts (refs. 13,19,20). The models developed under these con- : tracts will enable engine designers to evaluate current coating system capabilities in any given engine design. The life model development programs have also been very valuable in that they were the first thorough investigation of coating system thermomechanical properties.

Other work at NASA showed that the major crystallographic phase present in zirconia-yttria thermal barrier coatings is a non-equilibrium high yttria tetragonal phase (refs. 21,22). This phase forms as a result of the rapid quench encountered during plasma spraying. The X-ray diffraction techniques used in reference 21 are now commonly employed to characterize zirconia-yttria coatings. Another body of research was aimed at evaluating, understanding, and improving the behavior of thermal barrier coatings exposed to hot salt condensates. Salt deposition can occur when land- or sea-based engines are fired on dirty fuels or when an engine ingests salt-rich aerosols. This work, much of which was DOE-supported, has recently been summarized (ref. 23). Briefly, thermal barrier coatings were found to be prone to premature failure when exposed to molten ■salt condensates, and efforts to produce corrosion resistant coatings have only been partially successful.

0 0

Post a comment