Role of Si Segregation in the Structural, Mechanical, and Compositional Evolution of High-Temperature Oxidation Resistant Cr-Si-B2±Z Thin Films

Abstract

Alloying concepts targeting improved high-temperature oxidation resistance in protective coatings are valued in their simultaneous influence on phase stability and mechanical properties. Within this study, we investigate the influence of Si-alloying up to 17 at.% on the structural, mechanical, and oxidation properties of magnetron sputtered CrB2±z-based thin films. Density-functional theory calculations combined with atom probe tomography reveal the preferred Si occupation of Cr-lattice sites and an effective solubility limit between 3 to 4 at.% in AlB2-structured solid solutions. The addition of Si results in refinement of the columnar morphology, accompanied by enhanced segregation of excess Si along grain boundaries. The microstructural separation leads to a decrease in both film hardness and Young’s modulus from H ~ 24 to 17 GPa and E ~ 300 to 240 GPa, respectively, dominated by the inferior mechanical properties of the intergranular Si-rich regions. Dynamic thermogravimetry up to 1400 °C proves a significant increase in oxidation onset temperature from 600 to 1100 °C above a specific Si content of 8 at.%. In-situ X-ray diffraction correlates the protective mechanism with thermally activated precipitation of Si from the Cr-Si-B2±z solid solution at 600 °C, enabling the formation of a stable, nanometer-sized SiO2-based scale. Moreover, high-resolution TEM analysis reveals the scale architecture after oxidation at 1400 °C – consisting only of ~20 nm amorphous SiO2 beneath ~200 nm of nanocrystalline Cr2O3. In summary, the study underpins the promising capabilities of Cr-Si-B2±z coatings applied in high-temperature oxidative environments and provides detailed guidelines connecting the chemical composition to the resulting thin film properties.