Study of scratch resistance of a hard-on-soft polymer bilayer: Combination of in situ vision, X-ray tomography and numerical simulations

The deposition of a hard coating is a commonly used solution to increase the scratch resistance of soft polymeric materials at low and moderate normal loads. But the major risk for a coating subjected to tribological stresses is to crack and spall off. In this study, the influence on scratch behavior of a hard polyurethane coating on a plasticized PVC substrate has been investigated. At low load, the hard coating allows decreasing the plastic deformation of the substrate, and a geometrical interpretation is proposed. At increasing loads, this coating shows three fracture mechanisms: first, cracks oriented at 45° to the scratch direction starting near the detachment point behind the tip, on the edge of the scratch; then, in addition to this first mechanism, tearing under the tip forming a central crack parallel to the scratch direction; finally, in addition to the first two mechanisms, a circular crack forms ahead of the indentor by a bending effect at the top of the frontal bulge. These mechanisms are observed in situ thanks to a microscope coupled to a camera. Then, the scratches are observed by X-Ray tomography in order to analyze crack propagation in the depth of the material. Finally a numerical model of the scratch test has been developed. The combination of the three techniques gives a precise insight into the local mechanical conditions that lead up to these different kinds of scratches.