Difference Between Hardness and Toughness

Main Difference – Hardness vs Toughness

Hardness and toughness are properties related to materials which are generally used in material engineering. Together, they define the strength of a given material. These two properties are inversely proportional to each other. While hardness increases, toughness decreases. This is the key difference between hardness and toughness.

This article explains,
1. What is Hardness
– Definition, Characteristics, Tests, Examples
2. What is Toughness
– Definition, Characteristics, Tests, Examples
3. What is the difference between Hardness and Toughness

What is Hardness

Hardness is defined by the resistance of a material to plastic deformation, usually by indentation. This also refers to the resistance to scratching, abrasion or cutting. There are several globally approved tests to measure hardness.

Tests to Measure Hardness

Rockwell Hardness test
Brinell Hardness Test
Vickers Hardness Test
Knoop Hardness Test
Shore Hardness Test
Mohs Hardness Test

Hard materials are scratchproof. Hardness depends on the strength and the plasticity of the material. Higher the hardness, longer the lifetime of the material.

Examples of Hard Materials

Diamond which is an allotrope of carbon was considered as the hardest material on earth. It was used not only in jewelry manufacturing but also for various machinery. Diamond is also used to cut glasses, ceramic, etc.

However, a group of scientists from North Carolina State University has declared that they have come up with an even harder material called Q-Carbon.

Difference Between Hardness and Toughness

Figure 1: Diamond is one of the hardest material on earth.

What is Toughness

Toughness relates to the resistance of a material to fracturing; this depends on the energy absorbed during fracturing, which in turn depends on the size of the material. The amount of energy absorbed per unique area is characteristic of the material. Tough material like mild steel is not easy to be cracked or broken.

Toughness depends on the ability of the material to be deformed under pressure, which is known as ductility. However, not all ductile materials are strong. Toughness is a combination of strength and ductility. For a material to be tough, both ductility and strength should be high. Material toughness has the units of energy per volume.

Factors that Affect Toughness

The rate of loading- Toughness decreases with the decrease of rate of loading
Temperature – When temperature is decreased, ductility decreases, hence toughness decreases
Notch effect – When force is applied on one axis a certain material may be able to withhold it, however, when force is applied multi-axially the material may fail to do so.

There are several toughness tests and the toughness is measured by the following.

Impact toughness
Notch toughness
Fracture toughness

Some materials can be made tough by heating it to a certain temperature, maintaining that temperature for a given time and rapidly cooling the material. Steel is one such material.

Figure 2: Steel can be made tough by heating and then cooling rapidly

Definition

Hardness: Hardness is the resistance to scratching, cutting or abrasion.

Toughness: Toughness is the resistance to fracturing and this quality depends on the maximum energy that can be absorbed before fracturing.

Properties

Hardness: Hard materials are scratchproof.

Toughness: Tough materials are not easily breakable and can withstand high pressures.

Factors that Affect Hardness and Toughness

Hardness: This is affected by the strength and plasticity of the material.

Toughness: Rate of loading, temperature, notch effect affect toughness.

Testing Methods

Hardness: Rockwell hardness test, Brinell hardness test, Vickers hardness test, Knoop hardness test, Shore hardness test, Mohs hardness test are tests that measure the hardness.

Toughness: Impact toughness, Notch toughness, and Fracture toughness are tests to measure the toughness.

Reference:
1. “Material Hardness.” CALCE. University of Maryland, n.d. Web. 16 Feb. 2017.
2.”Property Information – Toughness.” Department of Engineering. University of Cambridge, n.d. Web. 16 Feb. 2017.
“Toughness.”
3. NDT Resource Center. N.p., n.d. Web. 16 Feb. 2017.

Image Courtesy:
1. “Apollo synthetic diamond” By Steve Jurvetson – (CC BY 2.0) via Commons Wikimedia
2.”Stainless Steel Braids (3054915298)” By włodi from London, UK – Stainless Steel BraidsUploaded by Yarl (CC BY-SA 2.0) via Commons Wikimedia