PSI - Issue 37

2

I. Shardakov et al. / Structural Integrity Procedia 00 (2019) 000 – 000

I. Shardakov et al. / Procedia Structural Integrity 37 (2022) 1065–1072

1066

1. Introduction Due to their mechanical and chemical properties, polymers are actively used as a material for the manufacture of implants (Ricklefs, et al. (2017)). Biocompatibility is a general requirement for all materials to be implanted into living tissue. Such materials should perform their functions for a long time and should not have a negative effect on surrounding tissues and on the body as a whole. Ion-plasma treatment of polymer materials is a promising method that can significantly improve the biocompatibility of the polymer surface (Nedela et al. (2017), Jacobs et al. (2012)). This effect is achieved due to the appearance of a carbonized layer on the polymer surface. When plasma ions are introduced into the polymer surface, a set of complex physical and chemical processes and phenomena occurs. Among them, 2 main groups of chemical reactions can be identified: • fractionation or cutting of the polymer molecular chain; • cross-linking due to the interaction of the formed free radicals. As a result of plasma treatment, carbon clusters are formed in the surface layer of the polymer. The number and size of these clusters depends on the energy density and the energy of the implanted ions, as well as on the type of polymer (Kondyurin and Bilek (2008)). So, the mechanical properties of the carbonized layer will depend on these parameters. In this work, we investigate the mechanical properties of the carbonized layer on the surface of low-density polyethylene obtained as a result of surface treatment with nitrogen ions with an energy of 20 keV. The ion fluence varied in the range from 10 15 ion/cm 2 to 10 16 ion/cm 2 . The elastic modulus of the carbonized layer was determined from the results of uniaxial tension of the samples according to our proposed method. The process of the appearance of cracks in the carbonized surface layer was monitored with an optical microscope. Deformation of the samples was carried out using a specially designed miniature stretching device. 2. Materials and methods The test pieces were made from high-density polyethylene (LDPE) film. The size of the samples was 100 mm  10 mm, the thickness was 40 μm. For experiments on uniaxial tension and investigation of the process of cracking, 12 samples were prepared. Polyethylene samples were treated on an ion-plasma system "VSIO-20KV-100NS" by high energy nitrogen ions (Chudinov et al. (2020)). The thickness of the resulting carbonized layer was estimated from the results of calculations in the framework of molecular dynamics, which made it possible to estimate the depth of penetration of the ion flux into the surface layer of the polyethylene film. The calculation performed using the TRIM software showed that the average depth of ion penetration into polyethylene is 70 nm (Chudinov, et al. (2018)). During the subsequent processing of the experimental data, this value was taken as the average thickness of the carbonized layer. The essence of the proposed method for determining the elastic modulus of the carbonized layer is largely based on the results of uniaxial deformation of polyethylene film samples. In the experiment, the tensile force corresponding to a given level of sample deformation was determined (Fig. 1). We assume that the deformation of the sample along the direction of the load P is uniform and the same for all layers. For an arbitrary cross section of the sample, the equilibrium equations can be written as follows:

/ l l P E F L L E F L L =   +    2 / s s

(1)

(

)

1 2 2 s F h h d = −

where,

l E and s E are the elastic moduli of the carbonized layer and the substrate,

2 l F h d = , and

are the cross-sectional areas of the carbonized layer and the substrate.