Y2SiO5 coatings fabricated by RF magnetron sputtering

Yttrium monosilicate (Y2SiO5) is considered a promising material for environmental barrier coatings (EBCs) for ceramic matrix composites (CMCs). Y2SiO5 coatings were deposited on all-oxide and non-oxide CMCs by magnetron sputtering, respectively. As deposited Y2SiO5 is X-ray amorphous, homogenous, and virtually free of cracks and macropores. Annealing performed between 1273 and 1473 K induces crystallization of metastable X1-Y2SiO5, apatite-type Y4.67(SiO4)3O and finally stable X2-Y2SiO5. Annealing also produces significant pore coalescence and re-crystallization. Experiments performed with Al2O3-based CMCs prove the fundamental instability of the Y2SiO5/Al2O3 interface at high temperatures, however no detrimental melting is observed and a well-bound Y2Si2O7/YAG double layer is developing. Experiments with CVD-SiC coated C/SiC CMCs in different atmospheres revealed that SiC oxidation to SiO2 favors adhesion of Y2SiO5 coatings. The SiO2 layer on top of SiC reacts with Y2SiO5 to highly porous Y2Si2O7 which is able to bridge gaps at the Y2SiO5/SiC interface. However, the thin SiO2 interphase may be critical with respect to thermal cycling. As a preliminary conclusion, sputtered Y2SiO5 coatings are considered more advantageous for all-oxide CMCs.