Abstract
High-temperature oxidation resistant amorphous Hf6B10Si31C2N50 and Hf7B10Si32C2N44 films were deposited by reactive pulsed dc magnetron sputtering. To investigate the oxidation mechanism, the films were annealed up to 1500 °C in air. The evolved microstructures were studied by X-ray diffraction and transmission electron microscopy. A three-layered microstructure was developed upon exposure to high temperature. An oxidized layer formed at the top surface for both films consisting of monoclinic and/or orthorhombic m-/o-HfO2 nanoparticles embedded in an amorphous SiOx-based matrix. The as-deposited bottom layer of the films remained amorphous (Hf6B10Si31C2N50) or partially recrystallized (Hf7B10Si32C2N44) exhibiting a h-Si3N4 and HfCxN1−x distribution along with formation of t-HfO2 at its top section. The two layers were separated by a partially oxidized transition layer composed of nanocrystalline h-Si3N4 and tetragonal t-HfO2. The oxidation process initiates at the bottom/transition layer interface with oxidation of Hf-rich domains either in the amorphous structure or in HfCxN1−x nanoparticles resulting in t-HfO2 separated by Si3N4 domains. The second stage occurs at the oxidized/transition layer interface characterized by densely packed HfO2, Si3N4 and quartz SiO2 nanostructures that can act as a barrier for oxygen diffusion. The small t-HfO2 nanoparticles merge and transform into large m-/o-HfO2 while h-Si3N4 forms amorphous SiOx matrix. A similar oxidation mechanism was observed in both films despite the different microstructures developed.
Highlights
Ultra-high temperature ceramics (UHTCs) have been extensively studied in the recent years as they usually possess desirable properties such as high hardness, high melting point, superior oxidation and corrosion resistance at high temperatures and good thermal stability [1,2,3,4,5,6,7,8,9,10]
We found out that nitrogen can play an important role in thermal stability of Si–B–C–N
1010Si we have studied the structure of the various interfaces using cross-section we have studied the structure of the various interfaces using cross-section high-resolution transmission electron microscopy (HRTEM)
Summary
Ultra-high temperature ceramics (UHTCs) have been extensively studied in the recent years as they usually possess desirable properties such as high hardness, high melting point, superior oxidation and corrosion resistance at high temperatures and good thermal stability [1,2,3,4,5,6,7,8,9,10].
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