Executive Summary

This report describes progress during the just completed second six-month period of the first year of the program. The previously submitted report describes the results of the initial, first six-month period of the program. Work has initiated in the major areas of study aimed towards developing a science-based approach to enhancing the properties of zirconia-based materials for advanced thermal barrier applications in gas turbine engines. This includes: a) the development of refined models for thermal cycling induced damage in plasma sprayed coatings; b) investigation of alternate stabilizers of the tetragonal-prime (t') phase for higher temperature operation; c) the effect of different dopants on the optical absorption properties of zirconia in the infra-red; d) the chemical compatibility between monazite and sodium vanadate and sodium sulfate and the suitability of monazite as a superficial, protective layer against vanadate and sulfate attack; e) the effect of TBC sintering on life.

Substantial progress has been made in both modeling thermal cycling induced damage in plasma sprayed coatings and in the development of phase equilibria for alternate rare-earth element stabilizers and this is described in the attached, more detailed technical summaries. In the area of the effects of different dopants on infra-red absorption, series of samples with different potential absorber ions have been prepared by sintering. Initial characterization has largely been completed but more detailed characterization awaits the (delayed) delivery of a solid-state detector which has high-sensitivity from 1 mm to 6 mm, the infra-red range of interest for TBCs exposed to the high-temperatures in current and future engines.

Systematic experiments to quantify the extent of sintering of thermal barrier coatings, attached to a bond-coated superalloy, as a function of time and temperature are underway. For comparison, the sintering behavior of thermal barrier coatings deposited on sapphire is also being investigated. As described in the following more detailed technical summary, as sintering occurs locally between adjoining columns, "mud-cracks" appear in the coating because of the constraint imposed on the overall TBC sintering by the underlying substrate, whether it be a bond-coated superalloy or a sapphire substrate. A number of distinct sintering mechanisms have been identified, some specific to morphological changes occurring in the underlying bond-coat. With the identification of what are believed to be the principal sintering mechanisms, the mechanics issues related to predicting "mud-cracking" will be pursued in the coming contract period with the objective of establishing quantitatively the development of "mud-cracking", their spacing and depth as a function of time at temperature.

Our experiments have revealed that thin-section parts distort upon cyclic oxidation. To guide further experimentation, an analysis of the stresses and thermal strains has been undertaken through a thermal cycle. New insight has been gained from this combined analytical and finite element analysis concerning the conditions for shape distortion on cycling. The new insight is that the fundamental ie-

quirement for distortion upon cyclic oxidation is that both the bond-coat and the TGO must experience yielding during at least one of the thermal stages in each thermal cycle.

Studies on the potential use of alternate stabilizers have been initiated, seeking improved understanding of the effects of gradually substituting rare-earth cations for Y. Work during this period has focused on three main issues: (1) Establishing the basic features of the ZrO2-REO phase equilibrium for the systems and regions of interest, with an emphasis on Gd additions as specified in the proposal; (2) Exploring the relative stabilizing efficiency of Y and Gd, and combinations of them; (3) Developing a methodology for growing TBCs with mixtures of stabilizers by EB-PVD. This work, described in the following technical summary, indicates that there is considerable flexibility - within the fundamental constraints afforded by thermodynamic equilibrium - for the use of alternate stabilizers. This important area will be continued in the second year of the program.

Experiments to investigate thermal cycling induced "rumpling" of CoNiCrAlY bond coats that will be used in support of the models of thermal cycling have still not begun. We await delivery of a series of samples promised by our industrial partners. They themselves, however, are awaiting the requested materials from their vendors. Despite the delays, we prefer to study the bond coats promised rather than make up our own so that the results will be of direct value to our partners.

The education and training aspects of the program continue. Three graduate students are now working exclusively on the program, one having begun five months ago. An undergraduate student, who worked on the program during the summer, is also continuing to be heavily involved in the program and is actively involved in several of the experiments. The weekly meetings in which issues related to TBC are discussed, students present up-dates of their research and faculty present tutorials on specific topics are continuing.

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