## 1 Introduction  
During the lifetime of art pieces, the pieces' surfaces experience a plethora of negative effects, including corrosion, friction, and abrasive wear. In particular, cast brass is susceptible to oxidation, scratching and loss of luster. One way to protect such art pieces and expand their lifespan is the application of two-layered protective ion plasma-deposited coatings. In such coatings, the lower layer provides mechanical stability, and the upper layer serves the decorative purpose. In terms of mechanical properties and wear resistance, transitional metal-nitride coatings doped with boron are particularly attractive. The development of novel coatings based on transition metal nitrides (including the boron-alloyed ones) is relevant for the increase of performance and lifespan of various items, including cutting tools [1; 2; 3], pressing tools [4; 5], frictional units [6; 7], microelectronic elements [8; 9; 10], optical devices [11; 12], medical implants [13], magnetic recording devices [14], etc. A variety of techniques can be used for the deposition of coatings based on nitrides and borides of transitional metals. Chemical vapor deposition (CVD) provides the benefit of high uniformity of coatings even on complex-shaped parts [15]. However, the application of CVD is restricted if the substrate material is sensitive to overheating. Since the heating of the substrate is necessary for the CVD processes, this deposition technique cannot be efficiently used for parts made of brass, bronze, aluminum-based alloys, etc. Atomic-layer deposition (ALD) produces coatings with outstanding depth uniformity, even in holes, grooves, and other hard-to-access areas [16; 17; 18]. However, the ALD technique is incapable of depositing thick coatings. Closed field unbalanced magnetron sputtering (CFUBMS) is one of the most suitable techniques for the deposition of transitional metal nitride-based coatings, especially on temperature-sensitive substrates [19; 20]. Increased field strength enhances the plasma ionization and provides higher ion currents on the substrate [21]. As a result, dense coatings are formed at lower substrate temperatures and offset voltage. TiN is the most ubiquitous industrial coating due to its high corrosion resistance, thermophysical properties, and hardness (20-40 GPa depending on deposition parameters) [22]. CrN coatings are also frequently used due to their lower wear rate and higher fracture toughness, which result in superior wear resistance [23]. Ternary nitride coatings can provide even higher mechanical properties and wear resistance, as exemplified by Ti-Cr-N (40 GPa), Ti-Al-N (25-35 GPa), Cr-Ti-N (16 GPa), and Cr-Al-N (17 GPa) [24]. Gradient Cr/CrN/Cr-Ti-Al-N coatings demonstrated especially high hardness (35 GPa) and adhesion strength [25]. Doping of nitride-based coatings by boron produces nanocomposite (nc) structures with anomalously high hardness (50 GPa) [26], good wear resistance and low friction coefficient [27; 28], high oxidation resistance, and thermal stability [29; 30]. Max hardness (50 GPa) for nc-TiN/a-BN and nc-TiN/a-BN/a-TiB\({}_{2}\) coatings was achieved when TiN grains (25-30 and \(\leq\)10 nm, correspondingly) were separated by near-monoatomic layers of a-BN [31]. Doping of CrN by boron produces two types of nanocomposite structures: nc-CrN/a-BN [28; 32] and nc-CrB2/a-BN [33]. Seventeen to 43 GPa hardness was reported for Cr-B-N coatings [34; 35; 36; 37; 38; 39]. Such coatings are usually deposited by reactive PVD in Ar + N\({}_{2}\) atmosphere using CrB\({}_{2}\) or CrB targets. Boron doping for CFUBMS coatings was investigated mainly for the ternary Ti-B-N and Cr-B-N coatings [40; 41; 42; 43]. Presumably, boron doping of more complex Cr-Al-Ti-N coating might provide considerable improvement in microstructural and mechanical aspects. When applied to cast art pieces, the coating should provide suitable coloration of the upper layer. This might be conveniently achieved by the introduction of oxygen in the deposition chamber near the end of the deposition cycle.