Hardness

Hardness is the measure of how well a solid object can resist shape change when being exposed to compressive forces.

The Main Idea

Properties of Matter

Properties of matter are divided into two categories, chemical and physical. Hardness falls in the physical category; it can be determined without altering the matter. It is an unusual property because it is not an intrinsic property which can be defined in terms of fundamental SI units. Hardness can be difficult to calculate.

Hardness

Hardness is the measure of how well a solid object can resist shape change when being exposed to compressive forces. There is a link between hardness and chemical composition. This is due to the solid matter's crystal structure.

Why it matters

Solid matter generally has 3 responses to forces, depending on the force type and amount. Elasticity, plasticity, and fracture. Elasticity is the ability to return to original shape after force has been applied. Plasticity is the solid matter's ability to remain one piece. Fracture is when the solid matter splits into two or more pieces. Stress versus strain graphs show how these responses are related. Hardness matters because it is important to know what matters will be able to withstand certain forces and be resistant to deformation, indentation, or penetration.

Calculating Hardness

There is no one way of calculating hardness. In fact, there are many hardness tests, such as Brinell, Knoop, Rockwell, and Vickers.

Sensitivity Coefficient

These tests can be improved by the introduction of a sensitivity coefficient. Sensitivity coefficients are used to determine the factor that different parameters, such as force, diameter, and depth have on hardness.

The Sensitivity coefficient ci, is defined as the change in hardness H, over the input parameter xi:

ci = ΔH/Δxi

Examples and History

Mohs hardness scale

There is no standard hardness scale, but of the Mohs scale is the most commonly used. The Mohs scale of mineral hardness organizes the scratch resistance of various minerals. It does this by determining and ranking the ability of a harder material to scratch a softer material. It is named after its creator, Friedrich Mohs, a German mineralogist.