Synthesis and characterization of Yb and Er based monosilicate powders and durability of plasma sprayed Yb2SiO5 coatings on C/C–SiC composites

Rare-earth silicates such as Yb2SiO5 and Er2SiO5 are promising environmental barrier coating materials for ceramic matrix composites. In this work, Yb2SiO5 and Er2SiO5 ceramic powders have been synthesized by solid-state reaction using Yb2O3, Er2O3 and SiO2 as starting materials. The fabricated powders were subjected to spray drying treatment for subsequent synthesis of coatings by plasma spraying. The spray drying resulted in well-dispersed and spherical powder particles with good flowability. Analytical techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetry and differential scanning calorimetry (TGA/DSC) and dilatometry were applied to study the microstructural and thermal characteristics of the powders. Ultra-high purity monosilicate powders formed as a result of heating treatments at 1400 °C in a box furnace for 20 h. TG/DSC revealed the genesis temperatures of the silicate formation (low temperature polymorphs) and also showed that the solid-state reactions to form Yb and Er based monosilicates proceeded without any weight-loss in the tested temperature range. The values of coefficients of thermal expansion (CTE) of the fabricated compounds are found to be 7.1 ppm/°C for Yb2SiO5 and 7.5 ppm/°C for Er2SiO5 by dilatometric measurements. Besides these studies, coating formation by plasma spraying of spray-dried Yb2SiO5 powders on the ceramic matrix composite specimen such as C/C–SiC has also been evaluated. Well-adhered and uniformed coatings result on composite specimens whose durability is tested by thermal cycling from ∼400 °C to 1500 °C in a gas burner rig.