The thermal expansion of plasma sprayed Y2SiO5 coating was measured using a non-contact thermal expansion measuring system in the temperature range from 25°C to 1500°C. The change in the crystal structure of the coating was also investigated by X-ray diffraction and differential thermal analysis. When first heated up to 1500°C, the Y2SiO5 coating showed irreversible expansion; small expansion at around 400°C and large expansion at about 1200°C. This irreversible expansion is attributable to crystallization of the Y2SiO5 coating, which changes from a dense randomly packed structrue as in the sprayed state to the B-type Y2SiO5 structure (space group: C2/c) as a stable state. The annealed Y2SiO5 coating showed no irreversible expansion and the thermal expansion coefficient of the coating was 6.5×10−6 °C−1. We suggest a new oxidation-resistant coating for use on C/C composites which consists of an inner SiC layer serving as an oxygen barrier and an outer refractory oxide layer for erosion protection. The Y2SiO5 coating is well-suited for use as the outer layer of out oxidation-resistant coating, because the degree of expansion of the annealed Y2SiO5 coating is close to that of the inner SiC layer.
