NanoTesting of Automobile Paint

Application Note

NanoTesting

of Automobile Paint

Small-scale mechanical property testing of paint films can provide detailed information on indentation response, abrasive wear behaviour, coating marring and multiple impact response. Since the tests performed often simulate mechanical interactions which are experienced by the painted surface, the results can indicate in an unambiguous manner optimum processing conditions and formulations.

The following results were obtained from a weathered automobile paint system consisting of a steel substrate, primer, metallic paint and clear laquer overcoat. The penetration depths employed mean that the results pertain to the overcoat layer specifically.

A sequence of Depth vs. Load indentation curves provides information on the variation in hardness and modulus with depth into the surface. The small loops are due to an elasticity and the gaps at each maximum load are due to creep during a 10s hold period. A 3-faceted Berkovitch diamond indenter was used.

Variation in hardness with depth into the surface. These results were derived from the indentation data shown above combined with a similar sequence for a lower load range. As can be seen, the hardness of the paint increased towards the surface but reached a constant, lower value at greater depths into the material.

Transverse scratching across a pre-existing defect shows its influence on material wear. Here, 8 consecutive scratches were performed using a spherical diamond under a load of 200 mN. The final topography scan shows only the resulting plastic deformation. As can be seen, a substantial initial defect had little influence on long-term wear.

Using a low load, the topography of soft surfaces can be mapped. The above result shows a transverse topography measurement performed across 3 scratches made with a 25µm radius spherical diamond indenter. The 3 parallel scratches were produced by 5, 10 and 20 passes under a load of 200mN. Pile-up along each scratch can be clearly seen.