Synthesis of boron carbide films by ion beam sputtering
Introduction
There is currently a great deal of interest in the formation of boron carbide, where bulk single crystals, polycrystalline samples [1], [2], [3], [4] and thin films [5], [6], [7], [8], [9], [10], [11] have been studied. Boron carbide has many attractive properties such as high melting point, outstanding hardness, good mechanical properties, low specific weight, excellent resistance to chemical agents, and high neutron absorption cross-section. Its hardness is only surpassed by diamond and cubic boron nitride. Since it is stable at high temperature, boron carbide is considered to be a good candidate for high temperature thermoelectric energy converters. In addition, its low specific weight and high impact resistance are used for armoring purposes [12], [13].
Recently, large efforts have been made in order to understand the phase formations, microstructure, and chemistry of the material. B4C films have always been synthesized by chemical vapor deposition [14], [15], [16], [17]. The deposition temperature is usually very high (>1000°C) and it will limit the choice of different substrate materials.
In the present work, an ion beam sputtering system was used to synthesize boron carbide films. In this system, deposition can be achieved at relatively low substrate temperature (below 400°C). This greatly increases the choice of substrate materials. Film structure and property can also be easily tailored by adjusting the substrate temperature. The aim of this work was to investigate the influence of substrate temperature on the structure and mechanical properties of B4C films.
Section snippets
Experimental procedure
B4C films were prepared by an innovative multi-function IBAD system [18], [19], which was equipped with four Kaufman ion sources, two water-cooled rotatable target holders. The base vacuum of the equipment was 1×10−3 Pa and the working pressure was approximately (1.0–1.2)×10−2 Pa. Single crystal Si(100) wafer was used as the substrate to the film, which was cleaned with acetone and methanol, and then etched in a 20% HF solution prior to loading inside the chamber. Prior to the deposition, the
AES and XPS analysis
Atom concentration and bonding state of boron carbide films were studied using AES and XPS spectroscopy. Fig. 1 shows the atomic concentration along with the film depth. Atomic concentration was stable for boron and carbon and was found to be 57% and 30%, respectively. The concentration of oxygen in the films might come from the deflation of the sintered B4C target during the sputtering procedure.
The X-ray photoelectron technique is an effective method to analyze the bonding state of atoms in
Conclusion
We have investigated the structural and mechanical properties of boron carbide films deposited by IBAD system at relatively lower substrate temperature. The formation of B4C was found to strongly depend on the deposition temperature. It was shown that a higher deposition temperature was beneficial for the boron carbide synthesis. Mechanical properties of these films were also thoroughly studied by a micro-indentation device. With an increase of substrate temperature, the hardness increased and
Acknowledgements
The work was supported by the State Key Laboratory of Tribology at Tsinghua University.
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