Scratch Testing of Automotive Paints Introduction
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Scratch Test Problematic
Despite the high quality of existing clear coats, there is an increase demand to improve the performances of those materials to resist mechanical damage seen by the cars during their life. To this day, Original equipment manufacturers (OEMs) use simple test methods such as Crockmeter and Amtek-Kistler carwash, to evaluate clear coat resistance to scratches and other mechanical stresses. But as clear coats increase in quality, these tests are unable to distinguish the improvements made to materials because of their large standard deviations and relatively subjective results.
More recently, scratch testing has shown the ability to reproduce real life mechanical damages seen by the automotive clear coats and provide a better differentiator for minor clear coat material improvements. The automotive clear coats are subjected to different types of damage:
- A carwash brush will create small scratches with relatively sharp and small particles. This is often referred to as mar.
- Fingernails and tree branches present a larger size contact and will generate deeper scratches characterized as micro-scratches.
- Keys and shopping carts can create larger and deeper scratches that will sometimes see the removal of the clear coat.
Therefore, clear coat formulators must test their pro- ducts for mar resistance (nano-scratch) along with resistance to micro-scratch and macro-scratch. Rtec Instruments are perfectly fitted for this type of scratch tests, being the only company to provide ex- changeable heads from nano to macro scales with high precision on a single platform.
Scratch Test Methodology
Figure 2: Scratch Testing principle
Figure 3: Confocal image of Scratch Failures
Scratch Test Conditions
Three different tips were used to simulate the different types of damages automotive paints can incur in real life. The test parameters are summarized in Table 1.
|Car wash, buffering…
|Nails, keys, tree branches…
|Shopping cart, belt buckle, coat zipper…
|Load Application Profile
|Rockwell 2 µm radius
|Rockwell 50 µm radius
|Rockwell 200 µm radius
Scratch Testing of Automotive Paint Test Results
The use of a sharp diamond (2 µm) and small forces (< 1 N) simulates the damage caused by brushes in carwash for example. In this case only one type of failure is observed (cohesive) as shown in Figure 3.
A larger radius tip (50 µm) with normal forces in the 10 to 20 N, are used to simulate scratches caused by branches or fingernails on the clear coats. In this case, both cohesive and removal failure can be observed as shown in Figure 4.
The large tip radius (200 µm) along with large normal forces are used to reproduce large scratches sustained by clear coats when scratched aggressively with Keys or the scrapping of a shopping cart.
For these scratches, the removal of clear coat happens relatively quickly after the cohesive failure but also yields deeper damage to the base coat as shown in Figure 5.
Lc1: First cracks in the clear coat (cohesive)
Lc2: First signs of removal of the clear coat
Lc3: Full removal of the clear coat and damage to underlying base coat
Two paint panels were tested to compare two different clearcoats. For each force range, the results of critical loads and scratch depth at Lc are presented in Table 2 and plotted on Figure 12.
The stresses generated during the scratch test provide information on the strength of the clear coat first, and on the resistance to removal of the clear coat from base coat and underlying coatings. The careful analysis of all signals recorded during a scratch test in addition to the robust imaging provided by confocal and bright field imaging allows for the most advanced under- standing of automotive paint systems behaviors at different scales (mar, micro and macro damages).
“Standard Test Method for Measuring Mechanistic Aspects of Scratch/Mar Behavior of Paint Coatings by Nano-scratching”
“Paints and varnishes – scratch test”
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