Marcus P. Borom

Role of environment deposits and operating surface temperature in spallation of air plasma sprayed thermal barrier coatings

Spallation of air plasma sprayed (APS) thermal barrier coatings (TBCs) was investigated on power generation combustors, military turboshaft engines, and commercial turboprop engines. In each case, irrespective of operating conditions or geographic location, spallation was linked to the presence and infiltration of high temperature molten phases of similar composition. Electron microprobe analysis found that, from all the possible oxides available in the external environment, only CaO, MgO, Al2O3 and SiO2 (CMAS) are incorporated in the molten phase that infiltrates the TBC microstructure. Fe and Ni oxides from metallic components and zirconia and yttria from the TBC were also found in varying amounts in the molten phase. The melting and recrystallization behavior of CMAS deposits was carefully defined by differential thermal analysis.

Thermomechanical behavior and phase relationships of plasma-sprayed zirconia coatings

Abstract The performance of plasma-sprayed zirconia coatings, used as thermal barriers in gas turbine applications, is influenced by residual stresses. Coating stresses are induced by the plasma spray process and are affected by thermophysical properties of the system. Stress alteration as a function of temperature and time was evaluated using bimaterial dilatometry. Reversible changes in stress associated with ΔαΔT relationships and phase transformations were observed along with irreversible changes owing to oxidation, metastable phase transformations and sintering of the coating materials. Dilatometric studies of yttria (8 wt.% (-stabilized zirconia coatings which had been chemically removed from the substrate indicated that a stable phase transformation occurs at around 1200 °C. The presence of this phase transformation suggests that changes are required in the generally accepted phase diagram by Scott. The changes and supporting evidence will be presented.