Effect of the Si content on the microstructure of hard, multifunctional Hf–B–Si–C films prepared by pulsed magnetron sputtering

Abstract

Pulsed magnetron sputtering in pure Ar was used to deposit multifunctional Hf–B–Si–C films on (001) silicon substrates using a single B4C–Hf–Si target. The effect of Si content in the microstructure evolution of the films was studied by X-ray photoelectron spectroscopy, X-ray diffraction, electron diffraction and high-resolution transmission electron microscopy. Hf–B–Si–C films with a chemical composition in at% Hf27B57C8, Hf23B55Si2C11, Hf22B54Si9C9 and Hf21B28Si35C7, respectively were produced using a fraction of 0%, 1%, 7.5% and 30% Si in the target erosion area. The Hf27B57C8 film is composed of hexagonal HfB2 nano-columnar structures (∼50–60nm long, and ∼5–10nm wide). The nano-columnar structures exhibit a preferred orientation with the (001) tilted ∼30° away from the film surface. The Hf23B55Si2C11 film consists of finer HfB2 nanocrystal columnar structures (∼20–30nm long and <5nm wide) concealed by ∼1nm thick amorphous boundaries. All nano-columns are uniquely oriented with the (001) parallel to the film surface. The Hf22B54Si9C9 film consists of refined nano-needle structures (∼2–3nm wide) composed of randomly oriented multi nanocrystals and the Hf21B28Si35C7 film consists of a nanocomposite structure of HfB2 nanocrystals (1–2nm) embedded in an amorphous matrix. The incorporation of Si into the films results in microstructural refinement and promotes the formation of an amorphous structure. As the Si content increases, the size of the HfB2 nanocrystals decreases from 50–60nm down to 1–2nm, the nanocrystals become randomly oriented and the fraction of the amorphous structure at the boundaries increases. This is consistent with the observed reduction in the residual stresses and increase in the high temperature oxidation resistance observed for these films.