SEGREGATION ASPECTS AT THE FRACTURE SURFACES OF 8 WT-PERCENT YTTRIA-ZIRCONIA THERMAL BARRIER COATINGS

Small-area x-ray photoelectron spectroscopy (SAXPS), small-area x-ray-induced Auger electron spectroscopy (SAXAES) and secondary ion mass spectrometry (SIMS) have been used to investigate segregation phenomena at fracture surfaces of 8 wt.% Y2O3-ZrO2 plasma-sprayed thermal barrier coatings (TBCs) as a function of temperature up to 1620 K. The results show the complex microchemistry of the fracture surfaces, where segregation of bulk dissolved impurities such as Si, Al, Fe and Na and of the stabilizing yttrium oxide takes place from 1170 K. The SAXPS and SAXAES results indicate that the chemical composition of the segregated phase is ascribable to an infinite chain silicate of sodium with a variable presence of iron, yttrium and aluminium as a function of temperature. The SIMS ion images suggest, further, that aluminium and yttrium segregate both at the silicate segregated regions as well as separately. The effect of impurity segregation phenomena on the thermal shock resistance of the TBCs is discussed.