Evaluating the durability of silicone duplex coatings using a scratch tester

Scratch tests were used to evaluate the durability of duplex elastomeric coatings under single pass, continuous loading, low speed sliding contact. The coatings consisted of a softer elastomer top layer (effective modulus = 3 MPa) on a stiffer elastomer bond layer (effective modulus = 23 MPa) on epoxy-coated steel. Duplex coating thicknesses ranged from 0.1 to 1 mm. Two different scratch styli were used: a 1.6 mm radius steel ball and a conical pin with a 0.8 mm radius diamond tip. The tangential force increased monotonically with normal force, then dropped suddenly; the onset of this drop is defined as the critical tangential force, T-c. T-c increased linearly with increases in both bond and top coat thicknesses, but the thickness dependence of the bond coat was 2 to 3 times more than that of the top coat. Optical and Raman microscopy of scratch tracks associated the damage at T-c with either tearing of the top coat behind the slider or debonding of the bond coat ahead of the slider or a combination of both. The thickness dependence of T-c is discussed qualitatively in terms of both stress and fracture mechanics models that account for damage to elastomer-coated substrates.