# Room-Temperature Formation of Hard BC\({}_{\rm x}\) Films by Low Power Magnetron Sputtering
## 3 Results and Discussion
### Film Structure, Composition, and Bonding Types  
According to the SEM study, the surface of all the films was smooth, and no features were revealed, indicating that a change in power does not affect the morphology of the deposited layers and the films could be amorphous. This result was confirmed by the XRD data. The XRD patterns of all the films did not contain any reflexes (Figure S1). It is known that BC\({}_{\rm x}\) films obtained by magnetron sputtering have an amorphous structure when synthesized up to 900 \({}^{\circ}\)C. [48]. The EDX analysis showed that the samples contained in their composition atoms of boron, carbon, and some oxygen. A small amount of nitrogen embedded in the films was observed; its amount does not exceed 3%. The oxygen probably came from the residual water or oxygen adsorbed in the reaction chamber during the deposition process and may be due to the film surface contamination after deposition during storage of the samples. The content of the elements in the as-grown films of series A is shown in Figure 3. It can be observed that an increase in the RF power applied to the B4C target resulted in a slight increase in the content of boron and carbon, and a decrease in the oxygen content. The same tendency of oxygen decreasing with increasing applied RF power was obtained by the authors of [20]. A decrease in the oxygen content in the film with an increase in the  
Figure 1: Dependence of (**a**) hardness and (**b**) Young’s modulus on the indentation depth for the BC\({}_{\rm x}\) films. Figure 2: Deposition rate of the boron carbide films as a function of B4C target power. plasma power is explained by an increase in the particle energy, at which the probability of the collision of particles decreases [49]. As seen from Figure 3, the atomic ratio of boron to carbon is lower than the stoichiometry of B4C, which is consistent with the results of other investigations [49; 50; 51]. It should be noted that most authors of works where boron carbide films were deposited using a B4C target do not provide the data on the total elemental composition of these films or the ratio of B/C atoms, and do not mention the content of oxygen and nitrogen [48; 52; 53]. Table 3 presents atomic concentrations of dual-target sputtered samples of series B prepared using B4C and C targets in comparison with the A150 sample. The use of dual sputtering led to a significant change in chemical composition. The expected sharp increase in the carbon content in the film was observed. The atomic ratio of B/C equal to about 2/1 was obtained at a single sputtering of the B4C target, whereas at dual sputtering of B4C and C targets, the ratio of B/C equal to about 1/2 was obtained. FTIR-spectroscopy was used for the study of chemical bonding. According to literature data, the spectrum of polycrystalline B4C has two bands near 1090 cm\({}^{-1}\) (E\({}_{\text{u}}\)) and 1580 (A\({}_{\text{2u}}\)) that are associated with vibrations of the icosahedron and chains connecting the icosahedra to each other, respectively [54; 55; 56]. The IR spectrum of amorphous boron carbide has a band at 1100 cm\({}^{-1}\), which shifts to more than 1200 cm\({}^{-1}\) for carbon-enriched a-B\({}_{\text{1-x}}\)C\({}_{\text{x}}\) films [32]. The absorbance bands associated with boron oxide appear in the IR-spectra at 1420 cm\({}^{-1}\) (B-O stretching), 1280 cm\({}^{-1}\) (B-O deformation), and 724 cm\({}^{-1}\) (B-O deformation) [57]. The evolution of the FTIR spectra of the samples of series A presented in Figure 4. The broad absorption bands with maximums in the region of 1152-1254 cm\({}^{-1}\) are exhibited. With an increase of RF power applied to the B4C target from 50 to 150 W (Figure 4a), the maximum of the main band shifted to the region of low-frequency vibrations. This is apparently due to an increase in the proportion of B-C bonds. According to the EDX data, the concentration of boron and carbon rose with increasing power.