Evaluating the Durability of Silicone Duplex Coatings Using a Scratch Tester

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Abstract
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, Tc. Tc 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 Tc 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 Tc is discussed qualitatively in terms of both stress and fracture mechanics models that account for damage to elastomer-coated substrates.

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