PSI - Issue 40

I.A. Morozov et al. / Procedia Structural Integrity 40 (2022) 314–320 I.A. Morozov et al. / Structural Integrity Procedia 00 (2022) 000 – 000

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2.3. Mechanical tests Plasma-treated specimens (working area 30x4x2 mm) were subjected to uniaxial fatigue loading using servohydraulic Bi-100 machine: initial tension – 10%; amplitude of deformation – 50%, frequency – 1 Hz and 10000 cycles of tension/compression. Possible field of application of these materials – biomedical products. The amplitude of the strain was chosen due to the fact that the maximum strain in the interphalangeal prosthesis is 40% (Beliaev et al. (2016)); the strain of the blood vessel walls is units of percent. 2.4. Atomic force microscopy The surfaces were examined by an atomic force microscope (AFM) Ntegra Prima (NT-MDT BV) in dynamic and static indentation modes. Probes with calibrated radius R of the tip and stiffness k of the cantilever were used. The initial surfaces, specimens after fatigue loading, and materials in tensile state were investigated in this work. In the latter case, the samples were stretched to a strain of 50% after multi-cycle loading, fixed on the substrate with adhesive, and then examined by the AFM. In the dynamic indentation regime, the probe ( k = 0.5 nN / nm, R = 4 nm) is pressed the surface with high frequency (the vertical component of the scanner speed is constant, 5 nm / ms) with a given load: ~3 nN for untreated polymer) and ~20 nN for plasma-treated polymer. This load is not damages the coating. High indentation frequency allows obtaining arrays of force curves with high lateral resolution: surface areas 3х3 μm, 10х10 μm with step of 4...30 nm were investigated; each point of the surface has its own interaction curve d ( z ): deflection d of the cantilever vs. distance z between sample and the probe (interaction force is F = kd ). Interaction curves give various physical and mechanical surface properties (elastic modulus, dissipation of energy, hardness, adhesion, etc). In this paper, the elastic modulus was calculated using the Johnson-Kendall-Roberts model. Static indentation regime was applied to evaluate the thickness of the coatings. Stiff probes ( k = 4 nN / nm, R = 10 nm) with a significant in dentation load (~150 nN) were used in this case (10x10 points for 5x5 μm areas, indentation frequency ~50 Hz). The subsequent study of these surfaces in semi-contact mode showed that such indentation left imprints. The surface of untreated polymer recovers elastically after indentation to the same depth. Thus, the depth of the imprint can be used to estimate the thickness of the coating (Morozov and Kamenetskikh (2019)). 3. Results and discussion The structure of the untreated polyurethane surface is shown in Fig. 1a – a soft (elastic modulus ~4 MPa) layer 4...6 nm thick (Morozov (2021)), hiding the internal structure of the polyurethane (Fig. 1b) – stiffer fibrils (11 MPa) inhomogeneously woven into a soft matrix (4 MPa).

Fig. 1. Surface (a) and subsurface (b) of the untreated polyurethane (Morozov (2021)). The inset shows the map of elastic modulus the subsurface structure: stiffer (bright) fibrils in the soft matrix.