PSI - Issue 32

Ilya A. Morozov et al. / Procedia Structural Integrity 32 (2021) 131–136 I.A. Morozov et al./ Structural Integrity Procedia 00 (2021) 000–000

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14 nm after 300 s of treatment. Such reduction of roughness could be attributed to inhomogeneous etching of modified soft/hard areas of the polymer. The thickness of the soft layer covering the subsurface structure (see Fig. 3a) falls to 4 nm after 30 s of treatment reaching 2 nm for plasma > 60 s. The stiffness of the outer layer did not change for the treatment > 60 s (the indentation force corresponding on the depth of soft layer remained unchanged ~1 nN). The subsurface stiffness grows; this is indicated by the depth of indentation (see inset in Fig. 3a) which decreases from 40 to 10 nm (all experiments were performed with the same force ~3 nN). A similar indentation experiment was performed on a clean hard substrate of silicon oxide; in this case, the indentation depth was < 0.5 nm. The presence of the external soft layer even after significant time of treatment is explained by volatile molecules that are evaporated from the polymer surface under action of plasma; a certain portion of them sticks back, forming this soft layer. Etching of the polymer occurs predominantly in the areas with high concentration of the soft phase: the roughness of the treated surfaces increases due to growth of cavities, which correspond to softer areas of the surface. In fact, a two-layer structure is formed in the material. The outer layer is due to etching of the soft phase and deposition of volatile structures back onto the surface. This soft layer passes into a stiffer one associated with the modified subsurface structure of the polyurethane. The total thickness of the plasma-modified layer (Fig. 3b) has a significant scattering, which is explained by inhomogeneities of the original substrate. The thickness increases from 4±0.5 nm to 4.5±0.5 nm during the first 120 s of treatment and further does not change. These measurements were carried out by the indentation method. A part of the surface with indentation marks and a cross-section of one of them are shown in Fig. 3c. There are no cracks or delamination of the coating in vicinity of the imprints, the layer is plastically destroyed with non-uniform buckling of the edges of the hole.

Fig. 3. Thickness of the outer soft layer (a), the inset shows the depth of indentation; total thickness of the plasma-modified layer (b); surface fragment with indentation imprints and the section of one of them (c).

After 30 s of treatment, only the outer soft layer of polyurethane is plasma-modified: the total thickness of the plasma-modified layer coincides with the upper soft layer ~4 nm. In fact, stiff coating is separated from the internal polymer structure by an unmodified soft interphase. This has an impact of the deformation properties of the surface: Fig. 4 shows images of the materials stretched by 50%: cracks cover the surface of the stretched material after 30 s of treatment. They have different surface and the subsurface structure: the surface image (Fig. 4a (II)) indicates bumps of height 4...6 nm in the area of cracks. This is a portion of unmodified soft polymer layer which is squeezed out of the cracks. In the subsurface area (the AFM-probe penetrates deep into the soft material of the crack) the cracks have trench-like shape of depth 2...4 nm (Fig. 4a (I)). The cracks are irregular due to heterogeneous subsurface of the original polymer (Fig. 2b).