Phase identification on infiltrated thermal barrier materials by 2D Raman spectroscopy

Thermochemical degradation of thermal barrier coatings generated by the infiltration of siliceous debris at high temperatures is considered a serious threat by aircraft industry due to phase destabilization of the thermal barrier material induces changes on its properties. Most materials-based strategies to mitigate the infiltration aim to promote the reactive crystallization of phases such as apatite. In this work, 2D Raman spectroscopy was carried out over the cross sections of lanthanum-gadolinium zirconate ceramic samples infiltrated by Colima and Popocatepetl volcanic ashes at 1250 °C for 10 h to identify the phases reprecipitated after infiltration. Raman mappings showed the characteristic peaks and the distribution of reprecipitated phases such as rare-earth apatite, monoclic and tetragonal zirconia. Additionally, rare-earth zirconate ceramics were identified by the characteristic F2g band of the pyrochlore structure. At the reaction layer, two zones were observed. Zirconia phases reprecipitating right at the upper zone while rare-earth apatite reprecipitating at the lower zone. For apatite, the peak corresponding to stretching vibrations of Si-O tetrahedra shows shifting to higher wavenumber values as gadolinium content increases in the rare-earth zirconate infiltrated. The 2D Raman spectroscopy was very effective to observe the distribution of the reprecipitated phases in addition to correlate the influence of gadolinium in the formed apatite. A correlation between infiltration depth and bands were confirmed.