Characterization of Mar-Scratch Resistance of Polymeric Coatings – Part l

By Weidian Shen

This two-part article briefly reviews the development of mar/scratch characterization techniques, and focuses on single-probe tests with nano instruments which have been widely used recently. Quantitative measurements of micro mar resistance (MMR), different responses of the coatings to the marring stress (i.e., elastic response, plastic deformation, and abrasive wear), and critical forces for rough trough, cracking, delamination, and chipping are described, as are some complementary test methods.

Statistical investigation of damage on samples used in real environments, combined with laboratory mar/scratch tests on these samples, could determine the force distribution curve in the lab which is approximately equivalent to the field conditions. The curve is useful for development of new coatings, and it can predict the weights of different damage modes that are likely to occur at the surface of the coatings in the field.

The weights, combined with the quantification of the damage levels of different modes, allow calculation of a quantitative index to characterize the mar/scratch resistance of a coating in a specific environment comprehensively. To better understand the mar/scratch resistance behavior of tested materials, a detailed stress-strain study is needed, utilizing theoretical analysis and finite element modeling to complement the experimental measurements described here as an integrated approach.

Mar/scratch resistance is an important and highly desired property for coatings in many applications, such as polymeric dear topcoats applied on automobile bodies, which customers expect to have a long-lasting glossy look while providing protection, and paints on thermoplastic olefin (TPO) that is being used more frequently as interior and exterior materials to replace metals. Consequently, techniques for characterizing mar/scratch resistance have been pursued for years in order to establish reliable laboratory test methods that can predict the performances of coatings against marring/scratching in their real-world applications and direct their further improvement. Mars and scratches are made by external stresses along the surfaces of coatings with a tangential component as well as a normal component.

Conventional hardness measured by well-established indentation tests is not a proper characterization of mar/scratch resistance. Instead, it is just a measurement of a material’s ability against a normal compressive stress. Some polymeric coatings are very hard, but they may have a poor mar/scratch resistance due to their brittleness and/or lack of toughness. Usually, mars/marring refers to the light surface damages encountered in the field; they are usually shallow and narrow, while scratches/scratching refers to medium to severe damages.

The majority of the damage to topcoats applied on automobile bodies belong in the mar category. The depth of most mars ranges from a couple of dozen nanometers to several hundred nanometers, *Surface Science and Nano-Tribology Laboratory, Coatings Research Institute, Ypsilanti, MI 48197. 54 March 2006 while the width ranges from a couple of hundred nanometers up to 2-3 micrometers. A single mar may not be readily noticeable.

However, the existence of a group of such mars does visibly degrade the appearance of coatings. Scratches are more visible, and they may cause fracture and cracking, or even delamination and chipping. Early mar/scratch measurement tests range from very basic ones, such as the pencil test, which ranks the coatings as B, F, H, 2H, etc., to instrumental ones, such as the Taber test, Crockmeter test, etc.

The Taber test, described in ASTM D 1044, employs abrasives of hard alumina particles embedded in a pair of rubber wheels weighted against a spinning test panel ( see Figure l). Although it is still used in many applications, such as in window tests for the auto industry, it was thought to be too harsh for many applications of polymeric coatings, e.g., dear topcoats. The Crockmeter test was commonly accepted by the auto industry to test dear topcoats. Atlas Materials Testing manufactures the devices for this test.