Scratch test failure modes and performance of organic coatings for marine applications

Although the scratch test is widely used for the assessment of thin hard coatings, its use for the testing of organic coatings is much less widespread. This is partly because these organic layers are much thicker than the hard coatings and hence interfacial loadings are much smaller and partly because the low hardness of the coatings means that they will plastically deform considerably during the test. Organic coatings for the protection of marine structures are often based on epoxy-resins or similar materials which are brittle compared to other organic resins. Furthermore, in service these materials undergo photo-oxidation which can increase their brittleness or introduce thin modified surface layers which have poorer properties than the as-received material. Corrosion of the underlying substrate may also lead to changes in coating adhesion. The scratch test is an ideal way of making quantitative comparisons between these materials both in the as-received state and after exposure to UV or salt water. In this study a number of organic coatings and base resins used for marine protection have been assessed by the scratch test. Both through-thickness cracking and interfacial detachment have been observed when the coatings are less than about 200micron thick with mainly plastic deformation for thicker coatings. Interfacial detachment is usually observed as a consequence of through-thickness cracking and initiates behind the moving indenter, contrary to what is usually observed for hard coatings. The critical loads for through-thickness cracking and interfacial detachment are reduced after exposure to salt water. The use of the scratch test to monitor performance of these coatings will be discussed.

This page was last modified on 18th January 2002.

S.J.Bull@ncl.ac.uk

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