Guide to Material Hardness: Types and Testing Methods

What is Material Hardness?

Hardness, simply defined, is a material’s ability to resist permanent deformation when acted upon by an external force. This feature aims to prove how well a material will withstand destruction when used for consumer and industrial products to determine durability and strength. In engineering terms, hardness measures the resistance to indentation, scratching, or cutting.

A material’s hardness is determined by its composition, how it was processed, and its molecular structure. For instance, ceramics, polymers, and different types of metals exhibit various hardness levels resulting from differences in their atomic arrangements.

Types of Material Hardness

Materials hardness is generally categorized into three main types: scratch, indentation, and rebound hardness. All three distinct hardness types have different applications and characteristics.

Scratch Hardness

This is the measure of a material’s resistance to scratches. This is defined as a material’s ability to resist surface deformation. They are those narrow, continuous lines you find on the surface of a material due to contact with a harder, sharper material.

Scratch testing is mainly used on brittle materials like ceramics because they do not undergo the plastic deformation phase. As a result, they are highly sensitive to scratches.

To best explain scratch hardness, picture instances when two materials brush against each other; the sharp, more rigid one tends to leave lines on the surface of the lighter material.

Indentation Hardness

Indention hardness valuates resistance to deformation from a specific indenter under controlled conditions. It measures a material’s resistance to permanent deformation after continuous pressure.

The indention hardness test reveals how well a material can withstand a permanent change in shape when acted upon continually by an external force. Typically, the test is achieved by applying pressure to a material, and the indentation level gives insight into its mechanical properties. The lesser the indentation, the stronger the material is.

Tests like Vickers, Brinell, and Rockwell are accepted methods for ascertaining this hardness type. Industries like engineering, manufacturing, and construction depend on this hardness test to consider a material’s strength and durability.

Rebound Hardness

Rebound hardness is a material’s elastic response and action when impacted by an external force. It is typically used to test metal strength as these material types undergo stress and shock.

During the test, the rebound height of a material is measured when a standardized object is dropped onto the material’s surface. A greater rebound height means a harder material, as it absorbs and ejects more energy than deforming.

This testing method is alternatively called dynamic hardness, and it’s widely adopted for material parts that require repeated exposure to vibrations. A good example is the design and manufacture of automobile parts, mechanical tools, and bridge columns.

The Leeb rebound hardness tester is the general go-to device for performing the test because of its handiness and ease of use. The core benefits of rebound hardness tests are their accurate evaluation of materials like metal to ensure prolonged performance and structural integrity.

Measured Units of Material Hardness

Most machinists frequently mix SI units of hardness with those of pressure. The Standard Internation (SI) unit used is N/mm², also known as the Pascal unit, while the latter uses the SI unit (N/m², or kg·m⁻¹·s⁻²). Each of the previously stated hardness classes has a different measurement scale.

The units of hardness are derived from each measurement method; therefore, they cannot be directly compared. Nonetheless, there is a conversion table that you can use to compare. We use “manage” since the comparison cannot be completely accurate, although it gives a suitable indication.

Leeb Hardness Value (HLD, HLS, HLE, etc.), Brinell Hardness Number (HB), Vickers Hardness Number (HV), and Rockwell Hardness Number (HRA, HRB, HRC, etc.) are the most often used units for hardness measurement. The table below compares several measurement units and their conversions.

Why is the Hardness of Materials Important?

The hardness of the material is important because it influences the lifespan, durability, and performance of the products. Below are some reasons why material hardness is important:

Design Considerations

Choosing the right material hardness for your product is important to ensure compatibility with your application needs. Industries like automobiles, aeronautics, construction, and several others rely on material hardness values to ensure the safety of lives and property when conceptualizing various designs.

Wear Resistance

The harder a material is, the more resistant it is to erosion or wear from friction, water, or oil. Materials that are resistant to scratching and abrasion enhance performance in high-stress environments.

Durability

Harder materials are often more durable, ensuring products can withstand wear and tear over time. This helps to identify materials that can withstand certain environmental conditions and improve a product’s lifespan.

Predicting Performance

The hardness of a material is essential in predicting how the material will perform in a specific environment and setting. With a known hardness value, you can determine how the material will perform during an accelerated aging test. Regardless of the industry, the right material hardness will improve the efficiency and functionality of the end product.

Improve Efficiency

Regarding bending and cutting, harder materials are more difficult to work with. With knowledge of the hardness of a material, you can select the right tools and equipment needed during the production process. This is important in industries where precision is paramount in creating designs and structures.

Different Material Hardness Test Methods

Material testing is a series of processes designed to measure properties like ductility, strength, reactivity, and hardness for plastics, metals, and ceramics when exposed to different conditions. The collated results from these tests guide designers and product engineers on material compatibility.

Different industries have various types of hardness tests, however, here are the standard methods.

Brinell Hardness Number (BHN)

As one of the earliest scale units for measuring hardness, the Brinell Hardness Number is a globally used method for quantifying the hardness ability of metal alloys.

The Brinell test uses a carbide ball or hard steel to press a specific diameter into the material’s surface using a standard load. The resulting indentation is then measured to obtain the BHN by dividing the applied load by the surface area of the indentation.

The BHN is mostly used for materials with even or very rough surfaces, like coarse-grained materials, castings, and forgings, primarily because it provides accurate readings when compared to other scale unit methods.

Rockwell Scale (HRB, HRC)

The Rockwell hardness test is a readily used method for quantifying material hardness, primarily because it is fast to conduct and offers precise results.

It is carried out by using both a minor and major load; the minor load is applied first, followed by the major load, then acted upon by an indenter. The difference in depth levels of penetration in both loaded materials is used to determine the hardness value.

There are several types of Rockwell scales, however, the only two popular variants used in measuring material hardness are the Rockwell B scale (HRB) and the Rockwell C scale (HRC). The HRB scale is employed when quantifying the hardness level of lighter metals like aluminum, and the HRC scale is used for heavier metals like steel.

Vickers Hardness Number (VHN)

Unlike the first two discussed methods, the Vickers hardness test requires a diamond pyramid indenter. The indenter is applied to the material with a specific load, and the diameter of the indentation is used to obtain the VHN. To calculate the VHN, the applied load is divided by the indentation area.

The advantage of this hardness testing method is its applicability to various material types, from lighter to heavier materials, whether plastics or metals. The Vickers scale is popular among researchers and uses cases where precise hardness values are pertinent primarily because of the diamond indenter’s ability to quantify indentation accurately.

Mohs Hardness Scale

Named after Friedrich Mohs, a mineralogist from the 19th century, the Mohs Hardness Scale differs from all other discussed scales because it is a qualitative test that studies the ability of a material to impact scratches on another.

Friedrich Mohs devised a scale in 1812, starting from 1 and ending at 10, with mark 1 as the softest and mark 10 as the hardest. The scaling system establishes that diamonds are the hardest material and also shows that materials on the scale with a greater number will impact scratches on those with a lower number.

The Mohs scale is popular mostly among jewelers and geologists and is used primarily for testing gemstones and minerals to identify materials from scratch impact. This test method isn’t as accurate as the other three; however, it remains the simplest field hardness test.

Quick Chart for Material Hardness

Using a material hardness scale or chart is one of the easiest ways to determine the hardness of several common material kinds. This chart displays the hardness value for several materials with the aid of many measurement methods. For example, the values for hardness of metals like aluminum and titanium are shown in the above chart using Brinell’s scale and further testing methods.

Difference Between Hardness and Hardenability

Hardness and hardenability are similar terms with varying meanings. The major difference between both terms is that hardness refers to the strength and ability of a material to resist damage, whereas hardenability is the ability of a material to be hardened.

• Definition: Hardness measures a material’s resistance to indentation or scratching, whereas hardenability is a material’s ability to be hardened when exposed to heat and quenched rapidly.
• Nature: The nature of hardness is in its material strength and ability to resist damage; however, hardenability is in a material’s ability to be hardened.
• Measurement: One way to measure hardness is through the depth of indenter penetration or the size of the impression left by the indenter. This differs from hardenability as it is only measured using a Jominy test.
• Method: For hardness, its method is on how hard the actual steel is based on its resistance to penetration. The hardenability method is the potential the steel has to be hardened by thermal treatment.