Effects of substrate constraint on micro-indentation testing of polymer coatings

A flat tipped cylindrical micro-indenter with a tip diameter of 76.2 μm has been used to generate interfacial failure in a soft polymer coating adhering to a rigid substrate. The tip is driven normal to the polymer coating at constant displacement rate until interfacial fracture is observed. The stress and strain fields which develop in the coating during penetration have been calculated using finite element analysis. The interfacial stresses are found to be strongly affected by the constraint of the rigid substrate. At the point of interface failure show the largest stress at the interface was an in-plane shear near the projected locus of the perimeter of the cylindrical tip. In addition, a small but significant tensile stress acting normal to the interface develops in a region just outside the zone directly under the tip. The failure most likely initiates near this region under the combined action of the large shear stress and the opening normal stress.