To improve auto coatings, new tests do more than scratch the surface

Data from these tests could eventually help your vehicle’s exterior better defend itself against dings, dents, scratches and things that go bump on the highway.

Schematic of the coating layers in a typical automobile composite body. Mar and scratch damages from a variety of object impacts are shown. Credit: Eastman Chemical Co./ K. Irvine, NIST
Schematic of the coating layers in a typical automobile composite body. Mar and scratch damages from a variety of object impacts are shown. Credit: Eastman Chemical Co./ K. Irvine, NIST

Scientists at the National Institute of Standards and Technology (NIST) have developed a new suite of tests for simulating scratching processes on automobile clearcoats. Data from these tests could, in the end, enable your vehicle’s outside better to safeguard itself against dings, dents, scratches and things that go bump on the highway.

The tests are designed to give manufacturers a better understanding of the mechanisms behind those processes so that future coating materials can be made more scratch resistant and resilient.

Right now, automobile coating manufacturers utilize two simple test techniques to assess clearcoat scratch obstruction and foresee field execution: the crockmeter and the Amtech-Kistler car wash. The former is a gadget that uses a robotic ‘finger” moving back and forth with shifting degrees of power to impersonate damage from human contact and rough surfaces. The last is a rotating wheel of brushes that recreate the effect of auto washes on clearcoats.

NIST physicist Li Piin Sung, one of the authors of the new paper said, “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. We demonstrated a test method that characterizes scratch mechanisms at the molecular level because that’s where the chemistry and physics happens … and where coatings can be engineered to be more resilient.”

For their test method, the analysts previously tapped a diamond-tipped stylus over the surface of a polymer composite sample to outline morphology, at that point utilized the stylus to make a scratch lastly, retapped and remapped the surface. Three distinct sizes of scratch tests—nano, small-scale and full scale—were led utilizing diverse size tips and diverse scopes of power.

Photomicrograph showing the result of a NIST nanoscale scratch test on a sample of automobile clearcoat material. The scratch, which shows fractures radiating from the line of impact, is 20 micrometers wide, 150 micrometers long and 2 micrometers deep (A micrometer is a millionth of a meter or about half the length of an average E. coli bacterium.).  Credit: NIST
Photomicrograph showing the result of a NIST nanoscale scratch test on a sample of automobile clearcoat material. The scratch, which shows fractures radiating from the line of impact, is 20 micrometers wide, 150 micrometers long and 2 micrometers deep (A micrometer is a millionth of a meter or about half the length of an average E. coli bacterium.).
Credit: NIST

The quantitative contrasts between the pre-scratch and post-scratch profiles, alongside infinitesimal investigations of the scratches, gave significant data on vulnerability to deformation, fracture resistance, and resilience.

Scientists performed the scratch test using a tip radius of 1 micrometer and a force range between 0 and 30 micronewtons. Anton Parr did the micro-scratch test with a 50-micrometer tip and a force range between 25 micronewtons and 5 newtons, while Eastman Chemical performed the macro-scratch test with a 200-micrometer tip and a force range between 0.5 and 30 newtons.

Sung said, “When scratches in the clearcoat are a few micrometers in depth and width and occur without fracture, they are referred to as mars. These shallow, difficult-to-see deformations are most often the result of car washing.”

“The nano-scratch test performed at NIST provided the best data on the mechanisms of marring and light scratches while the micro- and macro-scratch tests conducted by NIST’s partners were better at yielding detailed information about the larger, deeper and more visible deformations known as fracture scratches—the injuries caused by keys, tree branches, shopping carts and other solid objects.”

“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. 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.”

The researchers concluded that to get the truest evaluation of clearcoat performance, the nano-, micro- and macro-scratch tests should be conducted in conjunction with the current industry standard methods.

Scientists have published their paper in the journal Progress in Organic Coatings.