A photomicrograph shows a NIST nanoscale scratch, 20µm wide, 150µm long, and 2µm deep.
Photo courtesy of: NIST

A new suite of tests developed by the National Institute of Standards and Technology (NIST) and three industry partners could enable vehicle exteriors that resist dings, dents, and scratches.

Researchers at NIST, Hyundai America Technical Center, Eastman Chemical Co., and Anton Paar USA developed three versions of a fast, reliable laboratory method for simulating scratching processes on automobile clearcoats – the surface layer of an exterior polymer composite coating. The tests give manufacturers a better understanding of the mechanisms behind those processes so future coatings can be made more scratch resistant and resilient.

Automobile coating manufacturers use two test methods to evaluate clearcoat scratch resistance and predict field performance:

  • Crockmeter – A device that moves a robotic finger back and forth, with varying degrees of force, mimics damage from human contact and abrasive surfaces
  • Amtech-Kistler car wash – A rotating wheel of brushes simulates car washes

“Unfortunately, both methods only assess clearcoat performance based on appearance, a qualitative measure where the results vary from test to test, and they don’t provide the quantitative data that scientifically helps us understand what happens to auto finishes in real life,” says NIST physicist Li Piin Sung.

Researchers instead focused on molecular-level changes to surface finishes. In the initial test, researchers tapped a dia-mond-tipped stylus across the surface of a polymer composite sample to map its morphology, used the stylus to create a scratch, then retapped and remapped the surface. Researchers repeated the test three times:

  • Nano – NIST; 1µm tip radius; 0 micronewtons (µN)-to-30mN force range
  • Micro – Anton Paar; 50µm tip radius; 25µN-to-5N force range
  • Macro – Eastman Chemical; 200µm tip radius; 0.5N-to-30N force range
Schematic of the coating layers in a typical automobile composite body. Mar and scratch damages from a variety of object impacts are shown.
Chart courtesy of: Eastman Chemical Co./ K. Irvine, NIST

Scratches that are a few micrometers in depth and width, and occur without fracture, are referred to as mars. The shallow, difficult-to-see deformations are most often the result of car washing, Sung says.

“Data from the nano-scratch test also proved best for determining how well the coating responded to physical insult based on its crosslink density, the measure of how tightly the polymer components are bound together,” Sung says. “With this molecular-level understanding, clearcoat formulas can be improved so that they yield materials dense enough to be scratch resistant and resilient but not so hard that they cannot be worked with easily.”

Researchers recommend nano-, micro-, and macro-scratch tests to be run in conjunction with the standard industry methods to qualitatively and quantitatively characterize coating performance.

Anton Paar USA

Eastman Chemical Co.

Hyundai America Technical Center

National Institute of Standards and Technology