# Room-Temperature Formation of Hard BC\({}_{\rm x}\) Films by Low Power Magnetron Sputtering
## 1 Introduction  
In recent years, boron carbide films have attracted great interest as hard, protective, and oxidation-resistant coatings [1; 2]. At present, there is an increase in the use of boron carbide as X-ray optical coating elements (X-ray mirrors) for beam transport for current and future free-electron lasers [3]. Currently, several thermal, plasma, or ion-beam-based physical and chemical vapor deposition (PVD and CVD) methods are available for the production of boron carbide coatings. CVD boron carbide films are deposited using toxic and flammable volatile precursors such as B\({}_{\rm 2}\)H\({}_{\rm 6}\)[4; 5; 6], B\({}_{\rm 5}\)H\({}_{\rm 9}\)[7], B\({}_{\rm 10}\)H\({}_{\rm 14}\)[8], BF\({}_{\rm 3}\)[9], BCl\({}_{\rm 3}\)[10], and BBr\({}_{\rm 3}\)[11] with carbon-containing substances CH\({}_{\rm 4}\), C\({}_{\rm 3}\)H\({}_{\rm 6}\), and C\({}_{\rm 6}\)H\({}_{\rm 6}\), or single-source precursors such as B(CH\({}_{\rm 3}\))\({}_{\rm 3}\)[12], B(C\({}_{\rm 2}\)H\({}_{\rm 5}\))\({}_{\rm 3}\)[13; 14], B(OCH\({}_{\rm 3}\))\({}_{\rm 3}\)[15], and C\({}_{\rm 2}\)B\({}_{\rm 10}\)H\({}_{\rm 12}\)[16; 17] at high or elevated process temperatures. The composition and structure of the formed boron carbide layers greatly depend on the gas phase composition, boron-to-carbon ratio, the type of volatile precursors, and the deposition conditions (temperature, pressure, type, and quantity of additional gas). In the case of PVD processes, the composition of the layers is influenced by the experimental parameters of radio frequency (RF) [18; 19; 20] and direct current (DC) [21; 22; 23] magnetron sputtering (MS), pulsed laser (PLD) [24; 25], and ion-beam deposition [26; 27]. It should be emphasized that, unlike CVD processes, the deposition of the films in PVD processes can be carried out at a low temperature and without using hazardous gases. In these processes, the most commonly used starting material is B\({}_{\rm 4}\)C target, and in some works, B and C targets are sputtered. It should be noted that crystalline boron carbide is a phase of variable composition with a wide homogeneity region, and its properties depend on the ratio of boron/carbon. Although it is nominally denoted B\({}_{4}\)C, the carbon concentration of the compound, according to different authors, can be the following: 9-19 at.% C [28], 9-20 at.% C [6], 8.6-21.6 at.% C [29], and 10-24 at.% C [30], where boron carbide exists as a stable single phase. Boron carbide has a complex structure based on B\({}_{12-x}\)C\({}_{x}\) (x = 0-2) icosahedron and linear X-Y-Z groups (X, Y, and Z is B or C) [30] or B\({}_{12}\) or B\({}_{11}\)C icosahedra and C-B-C, C-B-B, or B\(\square\)B (\(\square\)-vacancy) chains [31]. In the literature, as rule, the terms boron-rich and carbon-rich boron carbide are used to denote the edges of the homogeneity region. At present, the films with higher carbon content than the carbon-rich limit of the homogeneity range are also called carbon-rich boron carbide films [32]. Now, attention is paid to the synthesis of both boron-rich [14; 15; 25; 33; 34; 35] and carbon-rich [14; 20; 23; 25; 36; 37; 38] BC\({}_{\rm x}\) materials. Selected literature data on synthesis conditions of boron- and carbon-rich boron carbide films and their composition are presented in Table 1.  
It was shown that the hardness and elastic modulus of boron-rich boron carbides decrease with the increase of boron concentration [34] while the highest boron substitution shows relatively high hardness and modulus [35]. On the other hand, according to the recent data of quantum mechanics simulations, the maximum shear stress of the B\({}_{13}\)C\({}_{2}\) structure is higher than the strength limit of regular B\({}_{4}\)C [33]. Thus, the mechanical properties of B-rich boron carbide explored in the literature remain controversial. Authors have reported that the undoubted advantage of boron excess is the densification of boron carbide structure and the corresponding reduction in porosity.