Damage of the hard coating on the soft polyurethane substrate under the uniaxial deformation

Abstract

One of the ways of improving surface characteristics of a polymeric material is plasma treatment. However, deformation of the elastic substrate can lead to undesirable damage of the created coating. In this work, the effect of uniaxial loading (amplitude of deformation – 40%) on the surface of soft polyurethane treated with plasma immersion ion implantation of nitrogen (energy – 1 or 3 keV) was investigated. As the result of plasma treatment, a hard wrinkled nanolayer was formed on the surface; its thickness (22 or 40 nm) and the elastic modulus (150 or 300 MPa) depend on the energy of ions. Samples were loaded with short-term or long-term cyclic deformation. In the first case, after 10 uniaxial loading cycles, a grid of short cracks was observed on the material with the thin coating; extended cracks were found in the second material. Folds are also appear on the surfaces – the result of compression of the material in the transverse direction. The depth of cracks after a short-term load was comparable with the thickness of the coating. An increase of the number of loading cycles resulted in the crack propagation deeper into the material by an order of magnitude greater than the thickness of the layer. In the material with a thicker and stiffer layer, the crack edges cut the soft polymer in the cracks that squeezes to the surface. It seems that, despite the positive effects of similar coatings widely presented in the literature, the actual working conditions of such materials may have negative effect on the product itself and on the environment.