Why are there so many different hardness scales?

Background

From pure metals like zinc, aluminum, steel and tungsten, to plastics, cork, a fingernail and everything in between, there’s a hardness scale associated with its chemical makeup. But not every metal or material can be tested the same.

The Different Scales

A very simple way of explaining why there are so many different hardness scales to choose from is that the hardness value of a particular object is the ability to resist scratches of indentations on its surface. Usually, there will be an indenter used and the measurement of the indentation. The shape and hardness is determined by the properties of the substrate.

These scales include:

The MOHs hardness scale

Developed by Fredrich Mohs in 1812, this testing method involves progressively attempting to scratch one material with another, above or below its own scale. The Mohs hardness scale goes from the softest material TALC, at a level (1), to the hardest material, diamond, at level (10). While diamond is largely known to be hard to procure, materials like a fingernail (2.5) and a piece of steel (6.5) might be used in testing.

The KNOOP hardness scale (HK)

The Knoop hardness scale is intended for small and thin materials utilizing a microscope in its process. It is calculated by using a rhombus shape indenter and force onto the substrate being tested. One of the benefits of The Knoop hardness scale is very small and a limited amount of test material is needed to obtain an accurate measurement.

The BRINELL scale

Developed by Johan August Brinell in 1900, an indenter the shape of a ball is pressed into the material being measured for hardness. This testing is slow and measures hardness based on the applied force and size of indentation made.

The VICKERS scale

Developed by Robert L. Smith and George E. Sandlord at Vickers Ltd. was an alternative to the Brinell scale. Vickers is an alternative method that uses a diamond inventor and force to calculate hardness. This process is for very small or thin materials.

The ROCKWELL scale (HR_)

Rockwell is the most widely used hardness scale. The Rockwell scale has 3 stages and is based on indentation hardness. Using a ball indenter or diamond, a preliminary load is first applied and measured. Then a Major load is applied, increased and then released. Finally, the preliminary load is re-applied for another short amount of time, and after Rockwell hardness is calculated from the difference between the first and last indentation depths.

SHORE hardness scales

Shore utilizes a spring-loaded indenting machine to measure and compare hardness, usually in plastics, rubbers and other polymers.

Transmet uses the KNOOP scale

Historically, Transmet has used independent labs to determine the hardness of our shot and flake products. Since the test requires a very small sample, using Knoop was easy to obtain and utilize, due to our products small and thin nature (Our original Aluminum flake product was 1 mm square x 25 micron thick!) After the first measurements were done, we kept things consistent for our customers and stuck with Knoop.