Hardness and Scratch Resistance

[flawbolt]

From time-to-time there is discussion on this forum and others regarding “hard” clear coats. Just the other day there was a question that asked why hard clear coats seem to get swirls so easily. This is a difficult question to answer…and part of the difficulty lies in the language being used.

Hardness=the resistance of a material to permanent plastic deformation

In layman’s terms this simply means…take a pointed hard object and push it vertically against a horizontal surface with a certain force and then measure the indentation that you make in the surface…’hard’ materials will have less indentation.

To state whether a clear coat is ‘hard’ or ‘soft’ is really a difficult statement to make. In order to really know, you would have to perform calibrated hardness tests on different clear coats…in other words, you would have to take a known hard object…for example, a diamond tip…and you would need a number of clear coat samples prepared…the tip would then be pressed into the clear coats with known force and the indentation depths would be measured. The results can then be compared…but I am sure that no one has performed such calibrated tests.

So…what are we really asking and is the question even worth asking?

For simplicity I will treat swirls, marring, and scratches as the same thing. In many ways they are…but detailer’s often talk about them differently. The difference between them has to do with the density, number, and appearance…marring is often described as very high density, shallow scratches over a large area. Swirls are similar but have a curved nature to them. Scratches are low density and deep. Whether we are talking about marring, swirling, or scratching…the question is the same…why do hard clear coats mar, swirl, and scratch easily. I contend that this is not a question we should even ask – there is not a clear well-defined relationship between ‘hardness’ and ‘scratching’.

Some of you may be familiar with the Mohs scale - an effective tool for identifying minerals and their hardness (talc=1 and diamond=10 on the Mohs scale). BUT, measuring scratch resistance is different from measuring hardness. And herein lays the confusion. Scratching involves two components: loading and shearing. "Absolute" hardness involves one component: loading (as shown below).




Are we at fault? No. Hardness is commonly equated to scratch resistance on many websites and in many textbooks. What I am trying to convey here is that we shouldn’t do this. One knows from experience that glass is hard…and is naturally scratch resistant. BUT some of us also know that a dog’s claws can scratch glass…and glass is harder than a dog’s claw.

The following statement is from a mineralogy textbook: "The resistance that a smooth surface of a mineral offers to scratching is its hardness…" Another one states: "Hardness is a mineral's resistance to abrasion or scratching". No wonder there is so much confusion!! Neither of these statements is generally true.

So, we are left to ask: Can hardness be used to predict scratch resistance? Well…experiments show that quartz and topaz (that have the same hardness) have different scratch resistance. Thus, one can conclude that hardness and scratch resistance are different and that there are factors other than just ‘hardness’ that influence scratch resistance. Some of these ‘other’ factors are related to the chemical bonds in the material we are measuring. But, on the flip side, there are some relationships that can be made between hardness and scratch resistance.

Are you confused? Sorry. The point I am trying to make is…you can’t speak of hard and soft clears and their relation to scratch resistance as if they are directly related. Actually, speaking of hard and soft clears shouldn’t even be done. Treat the surface of your car as you would your skin…baby it…and it shall reward you in return.