Abrasion & Hardness Testing

Floors carry forklifts, truck beds take shovel loads of gravel, chutes and hoppers handle abrasive slurries, and every coated surface is scratched, scuffed and knocked during its life. Abrasion and hardness tests give a numerical way to compare how coatings resist that wear — on data sheets, in specifications and on site as a quality check. Understanding what each test actually measures is essential, because the numbers are easy to misread.

Taber abrasion (ASTM D4060)

In the Taber Abraser test, a coated flat panel rotates on a turntable beneath two abrasive wheels. The wheels are pressed onto the surface by a set load and turn as the panel rotates, producing a ring-shaped wear track by a combination of rolling and sliding. ASTM D4060 and ISO 7784-2 describe the method for organic coatings.

• Wheels. CS-10 wheels are resilient and relatively mild; CS-17 wheels are harsher and are commonly quoted for industrial floor and lining coatings.
• Load. Typically 500 g or 1,000 g per wheel.
• Cycles. Often 1,000 cycles, though some specifications use more, or run until the film wears through.
• Results. Reported as weight loss in milligrams for a stated number of cycles, as a wear index (weight loss per 1,000 cycles), or as cycles to wear through. Lower weight loss means better abrasion resistance.

Wheels must be resurfaced on a refacing disc at specified intervals and debris removed by vacuum during the test, or results drift. A data-sheet value such as “CS-17 wheel, 1,000 g, 1,000 cycles” is meaningful only against another value measured the same way.

Other abrasion methods exist. ASTM D968 drops a stream of sand onto an inclined coated panel and reports the volume of abrasive needed to wear through a unit thickness of film. Industry-specific tests for slurry erosion, gouging and impact are used where the service demands them.

Hardness test methods

“Hardness” is not a single property. Each test measures a different response of the film, which is why the scales cannot be converted.

Running a pencil hardness test

Pencil hardness is cheap, fast and surprisingly repeatable when done carefully. The basic procedure under ASTM D3363 is:

1. Prepare the leads. Use a calibrated set of drawing pencils. Expose the lead without sharpening it to a point, then square the tip by rubbing it on fine abrasive paper held flat.
2. Hold at 45°. Grip the pencil firmly at about 45° to the surface, lead pointing away from you.
3. Push forward. Push the pencil away with uniform, firm pressure for a stroke of about ¼ in (6.5 mm).
4. Work down the scale. Start with a hard pencil and step down until a lead no longer cuts or gouges the film.
5. Report two values. Record the gouge hardness (hardest lead that does not cut into the film) and, if required, the scratch hardness (hardest lead that does not leave a scratch mark).

Hardness as a cure indicator

As a coating cross-links, its hardness rises, quickly at first and then more slowly until it plateaus. That makes hardness a useful practical check of cure. Applicators of polyurea and other thick elastomers often take Shore A or Shore D readings over the first days after application and compare them with the product data sheet. On epoxy floors and linings, a coating that remains well below its expected hardness can signal off-ratio mixing, low temperature or incomplete cure.

Why harder is not always tougher

Wear in service happens by different mechanisms: cutting by sharp particles, sliding abrasion, rolling contact, impact and gouging. A hard, rigid film resists fine scratching well but may chip or crack under impact. A resilient elastomer can deform under a sharp particle and recover, absorbing energy instead of being cut. That is why elastomeric coatings with modest Shore hardness are widely used in truck bed liners, chutes and hopper linings, while hard, high-gloss finishes are chosen where scratch and mar resistance or cleanability matter.

Choosing tests for the service

Match the test to the expected wear mechanism:

• Industrial floors: Taber (CS-17) for foot and wheeled traffic, plus impact resistance where heavy items may drop.
• Liners, chutes and hoppers: Shore hardness, tensile and elongation properties, and service-specific abrasion or impact tests.
• Finishes on machinery and architectural metal: Pencil hardness and mar resistance.
• Quality control on site: Shore or pencil hardness against data-sheet values at a stated cure age and temperature.

Whatever the test, compare products only on identical test conditions and confirm the results against the manufacturer’s product data sheet and real-world references in similar service.

Frequently asked questions

What is a good Taber abrasion result?

There is no universal benchmark. Lower weight loss is better, but values depend on wheel type, load and cycles. Compare only results obtained under identical test conditions.

Can Shore D be converted to pencil hardness?

No. Shore measures indentation of a spring-loaded indenter, while pencil hardness measures resistance to scratching and gouging. The tests respond to different properties.

Why is my coating softer than the data sheet says?

Common reasons include incomplete cure, low application or ambient temperatures, off-ratio mixing, or testing before the stated cure age. Check temperature and cure time before assuming a product problem.

Is polyurea harder than epoxy?

Usually not. Most polyureas are elastomeric with Shore A or low-to-mid Shore D hardness, while cured epoxies are typically harder and more rigid. Polyurea’s abrasion resistance comes largely from its resilience rather than hardness.