Specialized Hardness Testers

Specialized Hardness Testers

The Hardness Laboratory at Touchstone is well equipped to measure the hardness of virtually any material including: ceramics, metals, polymers, adhesives and paints. The following is a brief description of two of the more common hardness tests used for paints and other coatings.

Film Hardness-Pencil Test – ASTM D3363

Test method ASTM D3363 covers a procedure for rapid, inexpensive determination of the film hardness of an organic coating on a substrate in terms of drawing leads or pencil leads of known hardness.

A coated panel is placed on a firm, horizontal surface. The pencil is held firmly against the film at a 45º angle (point away from the operator) and pushed away from the operator in a 0.256-in. (6.5-mm) stroke. The process is started with the hardest pencil and continued down the scale of hardness to either of two end points; one, the pencil that will not cut into or gouge the film (pencil hardness), or two, the pencil that will not scratch the film (scratch hardness).

ASTM D3363 is especially useful in developmental work and in production control testing in a single laboratory. It should be recognized that the results obtained may vary between different operators and laboratories. Every effort should be made to standardize the hardness of the lead used and the technique followed. If used as a basis for purchase agreement, this test method will achieve maximum precision if a given set of referee pencils are agreed upon between the purchaser and the seller.

Reference: ASTM D3363 Standard Test Method for Film Hardness by Pencil Test

Tukon Hardness / Microhardness (ASTM D 1474)

“Standard Test Methods for Indentation Hardness of Organic Coatings”

ASTM D1474 covers the determination of the indentation hardness of organic materials such as dried paint, varnish and lacquer coatings, when applied to an acceptable plane rigid surface, for example, metal or glass.

A hardness tester consisting of a load applicator, a Knoop indenter, and a microscope fitted with a movable micrometer stage is required for these determinations. The Knoop indenter is a pyramidal diamond and provides hardness values in terms of Knoop Hardness Number (KHN).

In its use for organic coatings, a load of 25g is applied for 18s, after which time the indenter is removed from the coating and the length of the long diagonal of the impression remaining in the coating is measured and converted into a KHN measurement.

In addition with both the light microscope and SEM techniques, additional information about the coating can be assessed. The coating bond, coating uniformity and coating defects such as porosity and inclusions can be evaluated. In the case of metallic coatings, intermetallics such as the brittle Iron/Zinc layer which forms in galvanized steel can also be analyzed.