Ytterbium silicate-based environmental barrier coatings with hafnia via polymer-derived ceramic technology

Environmental barrier coatings (EBC) are essential to protect silicon nitride (Si 3 N 4 ) from hot gas corrosion, enabling the utilization of Si 3 N 4 in gas turbines. Otherwise the initially formed silica (SiO 2 ) layer degrades through reactions with the water vapor from fuel combustion. Therefore, hafnia (HfO 2 ) as doping additive in combination with ytterbium silicates was used to reduce the growth rate of silica and to enhance adhesion of EBCs without a bond coat. Finally, a uniform and gradient coating was generated in one stage, applied via spraying and sedimentation of a respective slurry. The in situ formed composite gradient coatings composed of HfO 2 -Yb 2 SiO 5 /Yb 2 Si 2 O 7 -Si 3 N 4 without Si bond coat, exhibited excellent adhesion (42.6 ± 6.4 MPa) and high microhardness (up to 4.3 ± 0.4 GPa). The durability of these coatings was confirmed through more than 20 thermal cycling tests in the temperature range of 1200-20 °C, in which self-healing ability of cracks was proven.