Amorphous Si-B-C-N alloys were deposited by reactive magnetron sputtering, and their high-temperature stability was investigated using a combined approach of experiment and molecular-dynamics simulations. We show that both a higher Si/C ratio and the addition of boron improve the thermal stability of the materials. We find that lifetimes of bonds of the same type are significantly longer at the higher Si/C ratio. The addition of boron results in a conversion of some of the electrons in lone pairs associated with nitrogen to bonding electrons. This increases the network's average coordination number. In both cases, the higher network coordination number and resulting lower diffusion, expressed in terms of longer bond lifetimes, shift decomposition reactions in materials to higher temperatures.

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Novel quaternary Si-B-C-N materials are becoming increasingly attractive because of their possible high-temperature and harsh-environment applications. In the present work, amorphous Si-B-C-N films were deposited on Si and SiC substrates by reactive dc magnetron cosputtering using a single C-Si-B or B 4 C -Si target in nitrogen-argon gas mixtures. A fixed 75% Si fraction in the target erosion areas, a rf induced negative substrate bias voltage of −100 V, a substrate temperature of 350°C, and a total pressure of 0.5 Pa were used in the depositions. The corresponding discharge and deposition characteristics ͑such as the ion-to-film-forming particle flux ratio, ion energy per deposited atom, and deposition rate͒ are presented to understand complex relationships between process parameters and film characteristics. Films deposited under optimized conditions ͑B 4 C -Si target, 50% N 2 +50% Ar gas mixture͒, possessing a composition ͑in at. %͒ Si 32-34 B 9-10 C 2-4 N 49-51 with a low ͑less than 5 at. %͒ total content of hydrogen and oxygen, exhibited extremely high oxidation resistance in air at elevated temperatures ͑even above 1500°C͒. Formation of protective surface layers ͑mainly composed of Si and O͒ was proved by high-resolution transmission electron microscopy, Rutherford backscattering spectrometry, and x-ray diffraction measurements after oxidization. Amorphous structure of the Si-B-C-N films was maintained under the oxidized surface layers after annealing in air up to 1700°C ͑a limit imposed by thermogravimetric analysis in oxidative atmospheres͒.

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