Who scratched my car?

Seriously, who or what did??

Have you experienced this?

You come out of the store, back to your car with arms full of Christmas gifts and you see a long scratch on your car door! ARRRGH! Who or what did that (expletives!!!)?

Any car owner (at least most!) hates damages that affect the look of their car. At least JD powers associates agree with that! (The JD Power 2017 U.S. Initial Quality Study found that 50% or more of consumer complaints regarding the aspect of cars were associated with scratch, mar and chip imperfections.)

The culprits can be: the shopping cart that “escaped”, the jacket zipper of your friend (at least he used to be), the keys that you dropped on the hood, the branches that you were supposed to trim last month, and even the “touchless” carwash brushes! So, how are car paint manufacturers supposed to formulate their paints to resist all of that? The problem comes from the variety and severity of scratches.

I was curious to see if I could compare some typical damages on car paints to some controlled mechanical test that would reproduce the scratches observed.

My thought process

So, I went to the body shop that is close to my house and asked if there were some “standard” paint panels that could represent the “average” car paint. After the funny look I received, this guy explained that there are too many combinations of base coat and topcoat, depending on the car brands, to have a real standard. He recommended a couple of combinations that he sees most often, and I found somebody to spray those examples on test panels.

Next, I was all too happy to take branches, keys, my jacket, and some buffing compound and “illustrate” the damages on those panels!

A little background on car paints!

Cars are painted a little differently than your house walls. To simplify it, you have a piece of metal (or polymer) that constitutes the body of the car. On top of this substrate, some layer(s) of primer is first sprayed to ensure adhesion of what comes next. This is followed by the application of the basecoat, which contains the color of the car and all the “addons” like metallicized particles, for the different aspects of the car. Finally, the topcoat or “clearcoat” is sprayed to provide the “shine” and protect the layers underneath.

This topcoat has 2 functions: protection and shine. It is easy to understand that any permanent damage to this topcoat will affect the visual esthetics of the car. If the damage goes beyond that clearcoat, it will be much more difficult to “Just buff out!”

Results

My first step is just a comparison of the scratches obtained by “real world” situations (I was happy to scratch those panels!) and the scratches obtained through mechanical testing instrumentation.

Real world scratches

Different objects were used to scratch the paint panel. After all scratches were created, the Rtec Instruments profilometer UP-3000 with lambda head was used to image all scratches. Mountain Map software from Digital surf was then used to quantify the deformation and sizes of the scratches.

I started with a simple car washing brush (similar material used in automatic car washes) and obtained multiples small scratches. Each of them is smooth and seemed to only deform the clearcoat without breaking it.

Then, I used a jacket zipper, a branch, a car key and some dirt to create damages on the same paint panel. Here are some images of the type and shape of scratches obtained.

With the jacket zipper sliding:

With the tree branch:

With the car key (light pressure):

Finally, sliding dirt on the panel and focusing on a single asperity scratch:

Controlled scratches on Mechanical tester.

Using the SMT-5000, I tried to reproduce some of the scratches observed on the panel. The same UP-3000 profilometer was used to acquire confocal pictures of the scratch grooves. Increasing load scratches provided the best results as they present damages at different loads.

Using a sharp tip (2 μm) with loads around 100 mN, I was able to get scratches that looked like the car wash brush and the tree branch. These were shallow, smooth grooves.

Increasing the tip size to 50 μm and using loads around 10 N, I was able to see damages looking like the ones done by the key and the most sever ones done with the tree branch.

Finally, the largest damages were done using 200 μm tip combined with loads around 30 N. Here, the damages span from the ones seen with the key at load loads, to the sand / dirt at high loads.

Conclusion

First, let me say that scratching paint with different objects is very cathartic!

In order to keep this post short, I didn’t include all the measurements of depth and width for all the grooves. But the order of magnitudes gave the following equivalence:

Therefore, scratch testing can be used to reproduce many damages seen by surfaces. The combination of different tip radii and different forces provide a wide range of possibilities to study the deformations of surfaces and quantify the resulting damages.

To finish my original story: as I loaded the gifts in my car, I saw this on a car not far from me:

I made me feel a little better, but I realized that this failure is full delamination of the clearcoat with age. Not something I will have the time to illustrate! But this could also be studied via scratch testing … for next time!

A more involved application note is available that shows more involved testing:

https://tinyurl.com/2p8w3r3r