Use of a Scanning Probe Microscope to Measure Marring Mechanisms and Microhardness of Crosslinked Coatings

By Weidian Shen, Stanley M. Smith, Frank N. Jones, Caigui Ji

A scanning probe microscope (SPM) was equipped with a high-modulus probe to indent coating surfaces when normal force is applied. A method for measuring microhardness with this probe is described. The high-modulus probe was also used to mar coating surfaces under controlled conditions by application of normal force plus lateral motion.

Dimensions of the mars were measured by conventional scanning probe microscopy. The data were analyzed in terms of a “three response, two mechanism model” of mar-ring in which three types of responses of polymeric materials: elastic, plastic deformation, and fracture, are measured. Of the three responses, only plastic deformation and fracture result in marring, and the two mechanisms can be quantified.

A fourth quantity which combines plastic deformation and fracture is suggested as a method of comparing “micro mar resistance” of materials under specified conditions. Three crosslinked polymeric coatings were studied in detail. Two had hard crusts of material near their surfaces that responded quite differently than the bulk of the material.