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

Environmental barrier coatings (EBC) are essential to protect silicon nitride (Si3N4) from hot gas corrosion, enabling the utilization of Si3N4 in gas turbines. Otherwise the initially formed silica (SiO2) layer degrades through reactions with the water vapor from fuel combustion. Therefore, hafnia (HfO2) 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 HfO2-Yb2SiO5/Yb2Si2O7-Si3N4 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.