In Situ Generated β‐Yb2Si2O7 Containing Coatings for Steel Protection in Extreme Combustion Environments

The rapid permeation and degradation of silazane‐based coatings by water vapor limit their application in combustion environments. Hence, this work reports on the reaction of the oligosilazane Durazane 1800 with an appropriate Yb‐complex by a molecular approach and its application as protective thin coatings (1.2 µm) for AISI 304 in comparison with the benchmark Durazane 2250 in combustion‐like environments. Fourier transform infrared‐ and nuclear magnetic resonance‐spectroscopy elucidate the reaction mechanism and the chemical structure of the resulting Yb‐modified silazanes, whereas elemental and X‐ray diffractometry analyses confirm the formation of crystalline β‐Yb2Si2O7 and SiO2 after pyrolysis at 1000 °C in air. Energy dispersive spectroscopy and X‐ray photoelectron spectroscopy profile analyses show the enhanced diffusion of Fe, Cr, and Mn from the substrate into the Yb50 coating, which is responsible for a better adhesion (23.7 MPa), scratch tolerance (38 N), and a decreased coefficient of thermal expansion‐mismatch to the substrate, resisting 9 thermal cycles between 1000 and 20 °C. Despite the low Yb‐silicate content (6.8 wt%), only minor damage is caused to the Yb50 coating after wet oxidation in moist flowing air (1000 °C for 15 h), whereas the Durazane 2250 coating spalled‐off. This is a clear indication of the potential of the Yb2Si2O7‐containing coatings to protect metals and ceramics in extreme combustion environments. The high sensitivity of the synthesized Yb‐modified silazanes to air leads to the generation of crystalline β‐Yb2Si2O7 and SiO2 after pyrolysis at 1000 °C. Hence, corresponding thin coatings effectively protect AISI 304 steel against oxidation in wet environments (1000 °C, 15 h) besides their excellent adhesion and damage tolerance caused by the enhanced diffusion of substrate elements into the coating.