*3.2. FTIR-ATR Analysis*

The modification of the PE surface led to significant changes in atomic structure, which was confirmed by the FTIR-ATR method. Obtained results are shown in Figure 3.

The IR spectra of unmodified PE (PE\_0 series) showed two large peaks at 2920 and 2852 cm<sup>−</sup>1, which correspond to C–H asymmetric and symmetric stretching vibrations in the CH2 group, respectively. Two smaller absorption peaks at 1466 and 723 cm<sup>−</sup><sup>1</sup> can be identified as C–H symmetric and C–C bonds. It can be clearly concluded that this spectra is characteristic for unmodified polyethylene [2]. Deposition of DLC coating (see Figure 3—PE\_1) noticeably changed the IR spectra of pure PE, the peaks assigned to C–H and C–C vibrations decreased, and the reordered spectra are typical for DLC structures. This modification also caused the appearance of a spectral line at 1641 cm<sup>−</sup><sup>1</sup> that was assigned to vibrations in C=C groups.

In the case of the next modification (PE\_2 series), due to the obtained N-DLC layer, the new spectra lines were centered at 3325 cm<sup>−</sup><sup>1</sup> (assigned to NH and NH2 groups, in the energy range 3300–3400 cm<sup>−</sup>1) [34,35] and additionally confirmed by a peak at ca. 1373 cm<sup>−</sup><sup>1</sup> [36]. Furthermore, the relatively wide peak (in comparison to spectra for PE\_1 series, centered at 1641 cm<sup>−</sup>1) was also attributed to C=N bonds vibrations [37,38], while two weak spectral lines (for 2185 and 2238 cm<sup>−</sup>1) were attributed to stretching vibrations in C–N groups [39].

In the case of PE substrate modification with the deposition of two layers (N-DLC/DLC, PE\_3 series), the highest intensity (about two times higher than the IR spectra for PE\_2 series) of spectral lines was assigned to C–H, C=C, and C–C vibration groups.

The last surface modification of polyethylene substrate (after deposition of N-DLC/Si-DLC layers, PE\_4 series) resulted in significant changes to the obtained IR spectra. The spectra were dominated by various atomic groups containing Si atoms, including Si–H (2120 cm<sup>−</sup>1) [40], Si–CH3 (1250 cm<sup>−</sup>1) [41], Si–N (750–1050 cm<sup>−</sup>1) [36], and Si–CH2–Si (1090–1020 cm<sup>−</sup>1) vibrations [42]. In addition, in the range of 600 cm<sup>−</sup><sup>1</sup> to 850 cm<sup>−</sup>1, many vibration modes, Si–C stretching, Si–N–Si asymmetrical stretching, CH3–Si rocking-stretching, and Si–H bending [42,43] can be noticed. The high value of oxygen content in this case (ca. 21 at.%, based on EDS analysis) is probably associated with vibrations in the Si–O in Si–O–Si groups, which was assigned to the 1035 cm<sup>−</sup><sup>1</sup> spectral line [41]. It is noteworthy that in the case of modification with Si-DLC layers, the high dissociation energy of the Si–O bonding (798 kJ/mol) [44] resulted in a significant increase in the mechanical resistance of the modified surface.

#### *3.3. Contact Angle and Surface Energy Analysis*

Unmodified polyethylene is a low-energy hydrophobic material which must be modified in order to be useful in biomedical applications. Wettability is one of the most important surface parameters for biomedical applications, because hydrophilic material with higher surface energy favors cell adhesion and biocompatibility [4]. The contact angle value of untreated polyethylene (PE\_0 series) was determined to be high (ca. 85◦) for two measuring liquids (water and diiodomethane), see Figure 4.

**Figure 4.** Contact angle values for water (black) and diiodomethane (blue) of PE surface before and after plasma modification.

A significant decrease in contact angle values after the deposition of DLC-based structures was observed for all tested series, while the contact angle for diiodomethane was lower than for water. A similar effect after the deposition of two different diamond-like carbon structures (flexible-DLC and robust-DLC) was described by Catena et al. [1]. This shows that in most cases plasmochemical treatment causes the contact angle to decrease, which was discussed broadly in many papers [2,10,45,46]. Polyethylene with N-DLC coating (PE\_2 series) is probably the best for biomedical applications, because it exhibits low and comparable water and diiodomethane contact angles.

Figure 5 shows results of surface free energy obtained for the tested samples, including polar and dispersive components of SFE. The influence of plasma treatment for LDPE on surface free energy was also described by Pandiyaraj et al. [47,48].

**Figure 5.** Surface free energy (γtot.—total surface energy, <sup>γ</sup>d.—dispersive surface energy, <sup>γ</sup>p.—polar surface energy) of unmodified and modified polyethylene, calculated using two different measuring liquids.

The authors concluded that usually in such processes oxygen flow results in an increase in the polar component (by incorporation of polar functional groups), without significantly changing the dispersive component. In our case, the performed experiments demonstrated that after deposition of DLC-based structures, the dispersive component increased, while the polar component decreased in relation to unmodified PE (PE\_0 series). For example, in the case of modification with undoped DLC coatings (PE\_1 series), the dispersive and polar components of surface free energy increased up to 37.5 mJ/m<sup>2</sup> (γd.) and decreased to 1.0 mJ/m<sup>2</sup> (γp.), respectively. Despite the fact that the total surface energy of all modified samples increased considerably, the best results (the highest γtot. value and the lowest contact angle value) were obtained for the PE\_2 series. This leads to the conclusion that the deposition of DLC layers (mainly N-DLC) can improve biocompatibility by increasing the surface energy of the substrate.

## *3.4. Mechanical Analysis*

Surface modification of the LDPE surface under plasmochemical conditions improves its mechanical properties. The hardness and Young's modulus profiles in relation to displacement into the surface are shown in Figure 6.

Deposition of DLC-based coatings generally improves hardness (by up to nine times), especially at a distance of 600 nm from the surface (Figure 6a). The unmodified polyethylene (sample PE\_0) was characterized by increased hardness only up to about 50 nm displacement into the surface (hardness of 0.3 GPa) and then stabilized at a value of ca. 0.1 GPa. In the case of the DLC layer obtained on the PE substrate (PE\_1 series), we observed a hardness increase of up to ca. 2 GPa. The DLC structure doped with N atoms (N-DLC coating, PE\_2 series) was characterized by lower hardness than the previous one (PE\_1), the surface hardness achieved a value of ca. 1 GPa, and the strengthening remained at ca. 550 nm distance from the surface. The PE\_3 series, corresponding to N-DLC/DLC multilayer, exhibited the highest surface hardness of up to 2.3 GPa, at a similar distance from the surface as in the case of the PE\_2 series (ca. 600 nm). As a result, the addition of N to the structure of DLC caused a decrease in layer hardness compared to undoped DLC coatings. A comparable relationship was also observed for the addition of Si, but only at a distance of about 50 nm from the surface. Similar dependencies were observed by Ruijun et al. [49] and Wang et al. [50]. For the tested samples, the

highest strengthening range (up to ca. 1750 nm from the surface) was shown in the PE\_4 series, after deposition of the N-DLC/Si-DLC coating. In this case, the hardness increased to ca. 1.8 GPa.

**Figure 6.** (**a**) Hardness and (**b**) Young's modulus profiles of tested samples.

Similar relationships were observed for the Young's modulus of tested samples (Figure 6b). Unmodified polyethylene reached a maximum value of 5 GPa near the surface, while in the interior it was about 2 GPa, which is a typical value for polyethylene. First modifications (DLC, N-DLC, and DLC/N-DLC corresponding to PE\_1–PE\_3) caused significant alterations in the Young's modulus of the samples (18 GPa for PE\_1, 12 GPa for PE\_2, and 16 GPa for PE\_3), but these modifications increased these values only up to about 200 nm displacement into the surface. Again, the best mechanical properties were exhibited by the PE\_4 series (N-DLC/Si-DLC modification), with a maximum Young's modulus value of 25 GPa on the surface. Increased E modulus in relation to unmodified PE remained escalated for about 1000 nm. This value is substantial, because it is the closest result to bone stiffness, which can be related to the enhanced biocompatibility required for implants.
