## 4 Discussion  
GDOES data reveals the presence of minor amounts of oxygen (5 at.%) and carbon (0.5 at.%) in the Cr-Al-Ti-B-N coatings. Presumably, these impurities originate from residual gas in the deposition  
Figure 11: Decorative Cr–Al–Ti–B–O–N coatings on SS substrates. Specimens denotation (D1–D9) corresponds with the compositions in Table 3.  
Figure 12: Uncoated brass pendants (**a**); pendants with Cr–Al–Ti–B–N coating deposited in P4 mode (**b**); pendants with Cr–Al–Ti–B–O–N coatings deposited in D5 and D3 modes (**c**). chamber as well as from impurities in working gas and SHS targets. The depth-derived coating growth rate was 30 nm/min. Boron-free coatings were characterized by columnar growth of the FCC phase and high surface roughness, which are both detrimental for the coating's mechanical properties and performance [45]. The introduction of boron inhibits the formation of columnar grains of chromium and titanium nitrides due to the deformation of their crystal lattices, the prolific formation of crystallization centers, and formation of boron-based thin interlayers (BN or CrBx). The chemical composition of the coatings (Table 1) suggests that chromium nitride should be the main constituting phase. The shift of CrN peaks in XRD patterns is presumably caused by the dissolution of Al atoms in the CrN lattice [46]. Since the atomic radius of Al (0.125 nm) is somewhat lower than that of Cr (0.14 nm), the dissolution of Al in CrN lattice decreases its lattice parameter. The same effect leads to decreasing the lattice parameter of the TiN phase. Boron-based phases were not detected in coatings with 0-2.3 at.% B either because the B amount was insufficient for the precipitation of the related phases or because the content of borides was below the detection threshold of the XRD method. Coatings with 7.3 at.% B were amorphous, so the peaks of discrete phases could not be discerned. A further increase of boron content to 10 at.% evidently leads to the formation of the fine-grained CrB\({}_{2}\)-based phase. The XRD-based estimation of grains size was impossible due to the overlapping peaks; however, the refinement of the structure of CrN-based coatings by boron doping is evident. Mechanical testing revealed that the coatings containing 2 and 2.3 at.% B possess the highest hardness and elastic-plastic characteristics. It should be noted that both the mechanical properties of boron-containing and boron-free coatings were partially compromised by the deliberate use of low deposition currents--a sacrifice made to prevent the brass substrates from overheating. However, it should be noted that even in the case of sub-optimal deposition conditions, the boron-containing quinary film was considerably more mechanically advantageous (hardness 15 GPa as compared to \(\sim\)10 GPa in the base coating). One can reasonably expect that at higher sputtering currents, the mechanical properties of the quinary film would rise accordingly. Hardness often defines the wear resistance of bulk materials; in the case of coatings, the \(H\)/\(E\) ratio provides a more accurate estimate than the hardness alone [47]. \(H^{3}\)/\(E^{2}\) ratio is characteristic of the type of localized deformation [48]. According to these ratios, the P3 and P4 coatings were the most promising in terms of their wear resistance. This supposition was then confirmed by tribological testing. The friction coefficient of P3 coating containing c 2 at.% B was unstable and rose gradually due to the local wear. Coatings with max boron content (10 at.%) demonstrated a drastic rise of friction coefficient due to complete wear. The initial friction coefficient of all boron-rich coatings was similar, but the discrepancy arose once the coatings were worn enough for the counter-body to touch the substrate. Coating with 2.3 at.% B endured the wear up to 200 m, presumably due to its higher mechanical and elastic-plastic properties. The abrasion resistance of this coating was also the highest among the investigated ones.