Mechanical characterization of polymeric films using depth-sensing instrument: Correlation between viscoelastic-plastic properties and scratch resistance

The scratch behavior of polymer films deposited on PMMA substrate by three different coating techniques is investigated by scratch tests using a depth-sensing instrument. Using an improved measurement technique, we develop an advanced methodology based on a more appropriated model that includes an estimation of realistic stress and strain states during scratch tests. The scratch resistance is evaluated by comparing the average contact pressure for which the coating cracks, by taking into account the elastic-plastic behavior of the layer. The proposed model allows the determination of the true contact depth and the elastic recovery at the rear side of the elastic-plastic contact, and thus the true projected contact area between the moving tip and the polymeric surface. This determination depends first on a rheological factor estimated from standard load-displacement curves obtained by nanoindentation and then on the tip geometry. The viscoplacticity index and the activation volume of each type of coating are then determined by nanoscratch. The viscoplasticity index determined during an elastic-plastic contact and the activation volume related to the ductile-brittle transition are discussed as reliable criteria for determining which bilayer system (polymeric film on PMMA substrate) will truly exhibit better wear and scratch resistances in service. (C) 2007 Elsevier B.V. All rights reserved.