Interfacial reactions between B2O3 and spark plasma sintered Yb2Si2O7

Reactions between boria (B₂O₃) and Yb₂Si₂O₇ were studied via a series of idealized interfacial “well” tests. Boria oxidizes out of SiC/SiC ceramic matrix composites (CMCs) where BN is used as a fiber/matrix interphase and boron-rich inclusions often serve as aids in the melt infiltration process. Borate phases are highly reactive and can react with the rare earth silicates currently being utilized as environmental barrier coatings (EBCs) for these CMC systems. Ytterbium disilicate substrates for these well tests are prepared via spark plasma sintering. The well is then drilled into the substrates and filled with a boria glass plug. Exposures in a stagnant-air box furnace show that the boria is reacting with the disilicate via a substitution reaction leaving YbBO₃ and amorphous silica glass as the product phases. This phase was characterized with scanning electron microscopy and elemental dispersive spectroscopy (SEM/EDS), micro-focus X-ray diffraction, and selected-area electron diffraction (SAED). Inductively coupled plasma optical emission spectroscopy (ICP-OES) was also used to analyze water-soluble glassy phases left on the surface of the substrates post-exposure, indicating that the boron content of the glass was decreasing with both increasing exposure times and temperatures. There are few data on the borate product phase properties, however the results of this study suggest that the boria formed via oxidation from the SiC/BN/SiC composites could be detrimental to the performance of Yb₂Si₂O₇ environmental barrier coatings via formation of the borate phase and silica.