The last few years have witnessed the extensive development of methods used for the characterization of mechanical properties of automotive polymeric clearcoats, [1-3] eventually resulting in the establishment of a dedicated ASTM standard  which is based on the Nano Scratch Tester.
This standard follows on from earlier methods employed in the evaluation of mar and scratch, where the objective was to physically scratch a surface and visually inspect it to assign a ranking. Although useful, some techniques do not have the reproducibility and accuracy of the nano scratch test method, which allows investigation of the relationship between damage shape and size, and external input (such as applied load, indenter geometry, speed, etc.)
Mar resistance characterizes the ability of the coating to resist damage caused by light abrasion. The difference between scratch and mar resistance is that mar is related only to the relatively fine surface scratches which deteriorate the appearance of the coating. Therefore, mar resistance can be directly related to the level of gloss retention in the coating after exposing it to harsh environmental conditions. Typical conditions include salt and acid rain exposure, daily and seasonal fluctuations of humidity and temperature, road grit and car washing. Standard nano scratch testing techniques have focused on simulation of mar-type damage, which comprises of shallow depths (typically < 10 µm) and small width scratches (typically < 10 µm), which both require small indenter radii (typically < 5 µm) to achieve.
The scratch test method is very useful for the characterization of mar-type damage, but can also be employed to simulate other types of damage experienced by an automotive topcoat during the lifespan of the vehicle. Such types of damage might be occurred when keys or jewelry are scraped across the surface, or due to large particle impact (gravel from the road), both of which lead to large-scale damage that can be easily noticed regardless of paint color and light level. Larger sized spherical indenter tips and higher applied loads are required for simulation of larger scale damage.
This article discusses the influence of indenter tip radius on the scratch resistance by changing tip radius and examining the resultant differences in measured signals and the effect on the physical damage of the polymeric coating, especially the critical point at which fracture of the coating takes place. It is a widely known fact that the critical point can rely on the temperature, scratch speed, the amount of deformation (strain) and the deformation history. The indenter tip produces a complex distribution of strains and stresses around its contact which are directly affected by the shape and size of the tip and the rheological properties of the polymer.
ASTM D7187 describes the onset of fracture as the point where the tangential force, normal force and penetration depth start to fluctuate uncontrollably, which is also commonly referred to as the critical failure load (Lc1). However, it is a challenging task to determine the exact fracture point in some polymer topcoats due to the particular formulation and the way that fracture initiates. Observing the normal load measured during the post-scan phase of the test is one way to “amplify” this transition from plastic deformation to fracture.
The post-scan comprises of running the indenter along the scratched track with a very low load to determine the residual depth remaining after the scratch test. This normal load signal, called FnP, can serve as a good indicator of fracture even when no marked transition is observed for the depth signals. Figure 1 illustrates a progressive load scratch on an automotive polymer topcoat where the FnP signal is a good indicator of coating fracture.
Figure 1. Nano Scratch Tester results for a progressive load scratch (0.1 – 15 mN) on a GEN III automotive polymer topcoat showing the penetration depth (Pd), residual depth (Rd) and normal load during post-scan (FnP) signals. The onset of fracture (Lc1) is clearly visible (shown here as a dotted line) and corresponds exactly with the optical micrograph shown of the scratch around the fracture point.
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This information has been sourced, reviewed and adapted from materials provided by Anton Paar GmbH.
For more information on this source, please visit Anton Paar GmbH.