Metal Hardness Testing: Methods & Scales

Metal hardness is a characteristic of metals the describes how well the material resists local plastic deformation.

It is easy to assume that all metals are hard, but there are some oddballs like mercury – which is liquid at room temperature and doesn’t even have a hardness value – as well as sodium metal, which you can cut with a kitchen knife.

There are several different scales and systems that measure metal hardness. In this article, we will provide a general overview of these measuring systems.

Mohs hardness

When comes to hardness, most people are familiar with the Mohs hardness scale. This system determines the hardness based upon surface wear – scratching one material with other materials. If material A can scratch material B, then material A is harder than B.

This scale is almost exclusively used for minerals and gemstones but can also be applied to metals.

Rockwell hardness

The Rockwell hardness method compares two indentations made in a material. One made with a small load, and the other with a large load.

A unique feature of the Rockwell scale is that it correlates linearly with material tensile strength; Rockwell hardness is generally reserved for harder materials.

Brinell hardness

The Brinell hardness scale is a widely accepted measure of hardness in materials. It involves pressing a ball of steel (or tungsten carbide for harder materials) into the test piece at a constant and known force. The softer the material, the deeper the ball will penetrate and vice versa.

The next step is to take a measurement of the diameter of the resulting impression, followed by a calculation, typically in megapascals, to determine the Brinell hardness scale.

Typical Brinell hardness values for a few popular materials and metals are as follows:

Vickers hardness

The Vickers hardness scale uses a square-based pyramid shaped diamond to impress into the material. The impression is then measured; the size of the impression determines how far it was pushed into the material. A formula is then applied to determine the hardness of the material.

One benefit here is that the width of a square impression is much easier to measure than a circle. This means that you can use the same formula (no matter the size of the indenter) as well as the same indenter for all material types, unlike many other methods.

Challenges of measuring metal hardness

Hardness is not an intrinsic material property.

What does this mean? Unlike melting points, it can change from one material sample to another. This is especially true for a metal like iron, which can appear in many different forms.

As such, different methods will produce different results for the same material. It’s not uncommon to see a disparity in the values of metal hardness on the internet; many values for metal hardness under these different tests aren’t always published or even available.

We should note that there are conversion tables available that can be used to convert between one hardness scale method to another.

Other prominent hardness tests not specifically outlined here include the Knoop hardness and Shore hardness scales.