*3.1. Morphological Analysis*

SEM and AFM techniques were used to obtain detailed information about morphological and topographical changes of polyethylene induced by plasma treatment and deposition of DLC-based coatings. It is worth noting that air and oxygen plasma treatments are more aggressive than argon plasma. Even though the ions of these gases are reactive and aggressive in contact with the surface layer of the polymeric substrate [25], these atmospheres are also used to clean the surface prior to the coating deposition. For instance, Rohrbeck et al. [26] applied an oxygen plasma cleaning process (10 min, 200 W), and after such treatment the initially smooth polymer surface turned out to be considerably rougher, and trenches and holes were more pronounced. However, in our work, the etching process with application of argon (less-reactive gas than oxygen and air) was carried out under the plasma power of 8 W, and in these conditions no significant negative influence of temperature on LDPE was observed. SEM analysis (Figure 1) revealed that each surface modification resulted in the formation of continuous and homogenous structures on the surface, without any cracks. Only in the case of modifications with DLC layer deposition (PE\_1 series), could a more diverse microstructure be observed, with visible heterogeneities in the micro scale.

**Figure 1.** SEM images of unmodified polyethylene (PE) (PE\_0 series) and selected modified PE surface after plasma processes: (PE\_1) DLC deposition; (PE\_2) N-DLC deposition; (PE\_3) N-DLC and DLC deposition; (PE\_4) N-DLC and Si-DLC deposition.

The new formed structures showed more details with atomic force microscopy, at the nanometric scale (Figure 2).

**Figure 2.** Atomic force microscopy (AFM) images of unmodified PE (PE\_0 series) and modified polyethylene surface modification: (PE\_1) DLC deposition; (PE\_2) N-DLC deposition; (PE\_3) N-DLC and DLC deposition; (PE\_4) N-DLC and Si-DLC deposition.

Analysis of AFM images of modified substrates showed granular-like structures, which in the case of Si-DLC coatings were composed of agglomerated clusters (see Figure 2, sample PE\_4).

A similar effect was observed Catena et al. when DLC layers were deposited on polyethylene [1]. In this respect, it is worth mentioning that plasma treatment of PE surfaces (with coating deposition) caused an increase in the surface area of tested samples, which is also beneficial for cell adhesion. The characteristic bulges (observed in sample PE\_3 and PE\_4) are similar to those presented by Catena et al. [27], caused by intrinsic stress release phenomena. More details concerning the surface roughness values of all samples, their chemical composition, and layer thicknesses are presented in Table 2.

It is important that determined roughness values (*R*a) for samples after coatings deposition were similar in the case of one-layer modifications (ca. 24–30 nm) as well as for the two-layer ones (ca. 13–16 nm). These values were two to over three times higher than the value of this parameter for the unmodified polyethylene (ca. 9 nm). On one hand, the plasma treatment contributed to the

increase in surface roughness of the PE substrate, and on the other hand it influenced the surface structure of the modified samples. The increases of surface roughness value after plasma processes are in good agreemen<sup>t</sup> with results obtained by Novotná et al. [28].

The chemical composition of tested samples (series PE\_1–PE\_4 in Table 2) confirms that the obtained coatings consisted of C, N, and Si elements, depending on the chemical composition of the gas mixture during plasma processes in the RF reactor. In the case of PE\_2 series (with the N-DLC coating), nitrogen was incorporated into the structure to ca. 8 at.%, while Si atoms (for the PE\_4 series) in the Si-DLC structure to ca. 27 at.%. It is worth noting that the addition of N and Si atoms to the diamond-like carbon structure caused a decrease in the value of internal stresses inside the obtained coatings as well as their hardness, which was also observed in other works [29–31]. However, the presence of silicon above (ca. 16 at.%) positively affected the anti-bacterial properties of the DLC coatings as well, which was also confirmed by Bociaga et al. [19]. The chemical composition studies of the tested samples revealed the presence of oxygen atoms in the structure, up to ca. 3 at.% in the case of the PE\_1, PE\_2, and PE\_3 series.

**Table 2.** Layer thickness (*d*), surface roughness ( *R*a), and chemical composition of unmodified (PE\_0 series) and modified polyethylene with obtained coatings.


*R*a: arithmetic average roughness (nm), measured using AFM; \*: the thickness of Si-DLC and N-DLC layers was ca. 1.55 μm and ca. 0.40 μm, respectively; \*\*: N-DLC layer was out of range of EDS analysis (thickness of Si-DLC above 1 μm).

The growth of oxygen concentration after plasma treatments was strongly affected by the creation of polar oxygen groups, which was also concluded by Novotná et al. [28]. In the case of PE\_4, the thickness of Si-DLC layer was above 1 μm and the N-DLC layer was out of range of EDS analysis. This can be explained by the absence of nitrogen in the average content (at.%). In the case of PE\_1, PE\_2 and PE\_3 series oxygen appeared in EDX analysis (up to ca. 3 at.%), possibly as the result of the adsorption of this element after the coating deposition process at ambient air conditions. In the case of the series with a Si-DLC layer (PE\_4), the content of oxygen atoms was much higher (ca. 20 at.%), which can be associated with a large silicon content in the DLC structure, and therefore increased compliance for the incorporation of oxygen into the top surface of the modified PE substrate. The presence of oxygen was attributed to the surface oxidation. This process was also observed by Batory et al. [32]. The confirmation of this fact was by the presence of the Si-O atomic groups in the IR spectra as well as the highest range of surface hardness for obtained Si-DLC layers (the tested PE\_4 sample, vide infra Figure 3 and see Section 3.4). This is mainly due to the very high binding energy for Si–O (ca. 532 eV), compared to the value for C–H (ca. 338.5 kJ/mol) and Si–H (ca. 298.7 kJ/mol) [33]. Additionally, Si–H bonds were less stable than C–H bonds, which also confirms the incorporation of oxygen into the Si-DLC structure.

**Figure 3.** FTIR-ATR spectra of unmodified (PE\_0) and surface-modified low-density polyethylene: (PE\_1) DLC deposition; (PE\_2) N-DLC deposition; (PE\_3) N-DLC and DLC deposition; (PE\_4) N-DLC and Si-DLC deposition.
