# Room-Temperature Formation of Hard BC\({}_{\rm x}\) Films by Low Power Magnetron Sputtering
## 3 Results and Discussion
### Functional Properties of Films  
The refractive index of the samples of series A varied in the range of 1.75-2.91 (Figure 7) and increased with increasing B4C target power. The rise of the refractive index is, apparently, associated with an increase in the proportion of B-C bonds in the films. The refractive index values of B75 and B100 films were equal to 2.43 and 2.62, respectively. Transmittance of the BC\({}_{\mathrm{x}}\) film/fused silica structures was in the range of 60-75% in the visible region of spectra and increased with the decreasing B4C target power (Figure 8). Carbon-rich The BC\({}_{\mathrm{x}}\) films exhibited a redshift of the absorption edge with increasing carbon incorporation. This obviously deals with the absorbance properties of carbon, the concentration of which is higher at higher power as was shown in Table 3. The optical band gap E\({}_{\mathrm{g}}\) was estimated as a result of absorption edge processing according to the Tauc law [73]. The E\({}_{\mathrm{g}}\) value decreased from 2.8 to 2.1 eV with increasing B4C target power. The lowest band gaps equal to 1.4 and 1.1 eV were reached for the B75 and B100 samples, respectively. These results are in good agreement with the literature data [20,74]. The hardness (\(H_{f}\)) and Young's modulus (\(E_{f}\)) of the films of series A were 7-25 GPa and 100-230 GPa, respectively, and increased with increasing RF target power (Figure 9a). As seen from Figure 6 (B1s XPS spectra), higher values of sputtering power led to an increase in the proportion of B-C bonds. This suggests that in the films, the B-C bonding plays an important role in determining the mechanical property of the network due to the B-C bond  
Figure 8: Effect of (**a**) B4C target power and (**b**) C target power on transmittance of samples. Figure 7: Refractive index of samples versus B4C target power (series A). being a more rigid covalent bond [75]. Unfavourable chemical bonds involving oxygen may be the possible origins of decreasing hardness and elastic modulus values with decreasing sputtering power values for the BC\({}_{\rm x}\) films. It should be noted that, according to [37,76], the hardness and Young's modulus of crystalline B\({}_{\rm 4}\)C are 25-40 GPa and 441-472 GPa, respectively, while the hardness and elastic modulus of amorphous BC\({}_{\rm x}\) films with carbon concentrations of 21-44 at.% prepared by pulse DC MS at room temperature and a power of 130 W were comparable to our results and were equal to 18-29 GPa and 198-247 GPa, respectively [1]. Also, similar hardness values (20-28 GPa) were obtained for BC\({}_{\rm x}\) films with carbon concentration changing from 19 at.% to 76 at.% deposited at 200 \({}^{\circ}\)C by pulse DC MS using B (880 W) and C (220-1750 W) targets [23], while boron carbide film prepared at 250 \({}^{\circ}\)C by DC MS (500 W) had a higher value equal to 34 GPa [21]. Unlike the films of series A, films of the series B had lower hardness values equal to 13-16 GPa (Table 4). Apparently, in the case of dual sputtering, the appearance of an additional phase of carbon, which was recorded by Raman spectroscopy, led to a decrease in the hardness value. According to the conclusions of the review [77], changes in hardness, as well as in other functional properties, depend on the carbon content in the homogeneity region of boron carbide. It should be emphasized that the maximum hardness was observed at a stoichiometric composition, and, for nonstoichiometric B\({}_{\rm 4}\)C materials (B/C > 4), decreased with increasing boron content. On the other hand, the hardness of BC\({}_{\rm x}\) (B/C < 4) also decreased in the presence of free carbon in the films [78]. As an effective way to determine the elastic limit for surface contact, the elasticity index (H/E) is widely used [79]. Also, the material resistance to plastic deformation is described by the plasticity index (H\({}^{3}\)/E\({}^{2}\)), which means that the higher the value of H\({}^{3}\)/E\({}^{2}\), the more difficult will be the shear steps [80].