Q: Do you know of a more robust test for hardness than the pencil hardness test? We recently had four different labs test a basic black TGIC powder and got four very different results. The results ranged from HB to 6H; the product should have been rated 2H.
A: The pencil hardness test is the most common but also the most inconsistent technique to measure the hardness of a coating. Depending on your needs and budget, you have a few options to consider. The easiest solution is to purchase a Wolf-Wilburn testing sled from an instrument company. They are not too expensive and can be found easily on the internet. This device eliminates most of the operator error inherent to the free-hand method most commonly used. Regardless, pencils have a fundamental variability in hardness due to lot-to-lot inconsistency and differences in pencil manufacturers.
If you would like a more sophisticated and reproducible hardness test for a coated surface, you may want to consider the Sward Rocker hardness tester or the Konig Pendulum hardness tester. The Sward Rocker method employs a technique that relies on the physics that a softer surface affects the amplitude of oscillation of a rocking body. Basically, the softer surface dampens a rocking disk more quickly than a harder surface does. Be advised that minor differences in coating thickness can affect your data. ASTM D2134 covers this technique in detail.
The Konig Pendulum hardness method works on a similar principle. With this technique, the hardness of a coated surface is measured by tracking the dampening effect it has on an oscillating pendulum. This method consists of placing two stainless steel balls on the coated surface. The balls act as the pivot point at the top of a pendulum. The pendulum is swung from a fixed start position, and the number of swings is counted. More elastic or softer surfaces will dampen more quickly and thus swing fewer times. This technique is described in detail is ASTM D4366.