CHARACTERIZATION OF ZRO2-7 WT-PERCENT Y2O3 THERMAL BARRIER COATINGS WITH DIFFERENT POROSITIES AND FEM ANALYSIS OF STRESS REDISTRIBUTION DURING THERMAL CYCLING OF TBCS

Superallog samples (IN 738) were coated with thermal barrier coatings (TBC). This TBC-system consisted of two layers. The first layer was a vacuum-plasma-sprayed, corrosion-resistant layer (MCrAlY) which also a cted as a bond coat. The ceramic top layer was atmospheric-plasma-spra yed Y2O3 partially stabilized ZrO2. in order to produce different micr ostructures, the plasma-spraying parameters for the production of the ceramic coatings were varied. The different ceramic coatings were char acterized in terms of porosity and mean elastic modulus. The porosity distribution was also investigated due to its influence on the measure d elastic modulus. One series of TBC-coated specimens was cyclically o xidized at a maximum temperature of 1100 degrees C. After 500 h of the rmal cycling, creep within the MCrAlY-bond coat led to a coating failu re at both the internal beveled edge and free edge around the specimen . A finite element analysis study of the cyclic oxidation experiment w as performed to gain insight into the stress redistributions within th e bond coat as a function of time. During the initial temperature incr ease, critical tensile normal stresses developed within the zirconia c oating at the free edge. However, these normal stresses became compres sive for all following cooling cycles. On the other hand, large tensil e normal stresses developed within the zirconia coating at the beveled edge during all the cooling cycles. Therefore, high normal stresses r esponsible for debonding were present within the zirconia coating duri ng all cooling cycles, with the most critical stresses occurring at th e free edge during the first cooling cycle and near the beveled edge f or all the following cooling cycles. (C) 1997 Elsevier Science S.A.