Hardness testing is the procedure used to determine the hardness of a material. The results allow properties of the material such as strength, wear-resistance and ductility to be empirically determined. This means the operator’s expertise and knowledge of the material are needed to determine these properties.

Categorising these properties is of importance when considering the suitability of the material for its chosen application.

What are the types of hardness testers? There are 7 main types of hardness tester:

1. Rockwell
2. Brinell
3. Vickers
4. Knoop
5. Shore
6. Webster
7. Leeb

In this article, we cover the seven main types of hardness testers in detail. Explaining the scales and techniques used by each to calculate the hardness of materials and their suitability for different materials.

What are Hardness Testers?

Hardness testers are complex in design but simple in principle, operation and application. They measure the hardness of a material as its ability to resist permanent deformation from an applied force. Different types of hardness testers vary in how they do this, some equate the depth and shape of the indentation created in the material by a pre-determined force, applied to the surface for a set amount of time. Others calculate the resistance the material offers to a spring-loaded force, or measure the velocity of a rebounded object propelled onto the surface.

Several factors determine the hardness testing method that is most suitable for your use, including:

• Expected ‘Hardness’ of the material
• The Density of the material
• The Flatness of the material
• The Ductility/brittleness of the material
• The Smoothness of the material

Below we look at the different types of hardness testers and their scales, describing how they work and their suitability for differing applications.

1. Rockwell Hardness Testers

Rockwell hardness testers are the most commonly used of all the types of hardness testers. The Rockwell method offers a quick and accurate measure of hardness on almost all metals and some plastics (see Rockwell scales below).

The wide range of scales used within the Rockwell method (30 in total!) makes it suitable for many applications. However, despite its versatility, it does lack practicality when measuring smaller objects, for which other types of hardness testers are needed.

Rockwell Method

Rockwell hardness method is a two-phase process involving the ‘Pre-load‘ phase and the ‘main load‘ phase. Initially, the pre-load force is applied. This small force breaks the surface, slightly indenting the material to create a measurement baseline. The Main load force is then applied, creating the indention to be measured.

The measurement starts from the baseline created by the pre-load. This two-phase process ensures highly accurate measurements, with the pre-load force negotiating any irregularities in resistance created by the surface.  

The measure of hardness is shown immediately after permanently indenting the material. 

Rockwell Scales

There are 30 scales used by the Rockwell method, allowing its use across a wide range of material types. Here are some of the most commonly used Rockwell scales: 

• A scale measures Tungsten carbide
• B scale measures Aluminium, brass, and soft steels  
• C scale measures Harder steels >B100
• H scale measures Aluminium, zinc and lead
• K scale measures Bearing alloy, tin, hard plastic materials

A Rockwell scale reading looks like this: 80 HR30TW. The reading has 4 sections:

Section 1. ’80’ represents the hardness value

Section 2. ‘HR’ = Hardness Rockwell

Section 3. ’30’ is the main load of 30KGF (kilograms of force) 

Section 4. ‘TW’ is the ball indenter made of tungsten carbide

The Rockwell system is universally accepted as the standard measurement in the steel manufacturing industry. It has also become a common standard in reporting test results.

2. Brinell Hardness Testers

Originating from Sweden, the Brinell Hardness testing method was invented in the 1900s by Dr Johan August Brinell. It is the oldest method still in use today in different types of hardness testers. It often used to measure the thickness of items with a coarse surface structure, such as castings and forgings, where it produces the most accurate readings. 

The Brinell hardness testers are known to be Macro. They are more suitable for measuring harder materials that require more force to penetrate the surface.

Brinell Method

Brinell hardness test is an indentation hardness test. It uses a hard spherical ball (usually around 10mm in diameter). An applied force (a typical test will use 3,000 kilograms) pushes the ball against the surface of the material for a set amount of time (between 10 – 30 seconds, known as the dwell time).

The diameter of the resulting indentation in the surface is then measured at least twice on perpendicular angles. 

Brinell Scale

Unlike Rockwell, Brinell has a single scale that measures the indentation created by the hardened steel ball, taking into account the amount of force applied across the dwell time. These factors will result in a single number called ‘hardness Brinell‘, which represents the hardness.

Brinell is used on a wide variety of materials. Here are some examples of materials and their expected Brinell readings.

• Softwood (e.g., pine): 1.6 HB
• Hardwood: 2.6–7.0 HB
• Lead: 5.0HB
• Pure Aluminium: 15HB
• Copper: 35HB
• Hardened Aluminium: 75HB
• Mild steel: 120HB
• 18–8 (304) stainless steel annealed 200 HB
• Hardox wear plate 400-700 HB
• Hardened tool steel 600–900
• Glass 1550 HB
• Rhenium diboride 4600 HB

3. Vickers Hardness Testers

Developed as an alternative to Brinell, the Vickers method was created by Smith and Sandland while working at Vickers Ltd in 1924. Vickers hardness testers are known as microhardness testers. They are types of hardness testers that have a wide-ranging scale, while low forces and a shallow indentation enable the measurement of soft metals and thin materials such as foil. They are also used to measure small individual parts or small sections of larger samples.

Vickers Method

The Vickers measure consists of applying a force (load) onto the test material through a diamond-shaped indenter. The distances between the horizontal and vertical points of the resulting indentation are used to calculate the indentation size and determine the Vickers measurement. These horizontal and vertical distances must be within 5% of each other. Generally speaking, the smaller the indentation, the harder the material.

Vickers Scale

As Brinell, Vickers has a single scale that measures the indentation created by the hardened pyramid indenter. Both lengths of the diamond-shaped indentation are measured before the average is calculated to establish the hardness rating. Below, you will see the types of materials and their expected value.

• 316L stainless steel: 140HV30
• 347L stainless steel: 180HV30
• Carbon steel: 55–120HV5
• Iron: 30–80HV5
• Martensite: 1000HV
• Diamond: 10000HV

In the 316L stainless steel (140HV30) reading, you will see it has 3 parts. The 140HV value represents the hardness value, whilst HV represents the measure being used, Hardness Vickers. The final number following HV represents the amount of force applied to the test load. The load is measured in KGF (kilograms of force), 30KGF.

Vickers testers are suitable for testing a variety of materials. The indentation produced by the Vickers test is relatively small. This means it is extremely useful in testing the hardness of small items, sections of larger parts, and thin objects, such as foils.

4. Knoop Hardness Testers

An alternative to Vickers, the Knoop hardness test is a microhardness test used particularly for very brittle materials or thin sheets. The small size and shape of the indenter on these types of hardness testers allow this. The diamond shape is elongated, increasing the surface area of the indentation. This spreads the force out, and a less concentrated force area means the brittle materials are less likely to break under test. Thin ceramics are an example of this.

Knoop Method

The Knoop hardness test uses a rhombohedral-shaped diamond indenter. This is pressed into the polished surface of the test material (100g load) for a specified dwell time. The indentation created is more elongated than the square diamond produced by the Vickers method.

An advantage of the test is that only a small sample of material is required. It is also valid for a wide range of test forces. The main disadvantage is the difficulty of using a microscope to measure the indentation (with an accuracy of 0.5 micrometres)

Knoop Scales

Knoop uses the Vickers scale for its measurements of the indentation created by the hardened pyramid shape. It measures the length of both diagonals formed on the test surface, and the average calculates the hardness. However, even though it is similar, Knoop uses a slightly different calculation than Vickers. This hardness value is expressed HK for short.

Below you will see some readings found in more brittle materials.

• Dentin: 68 HK
• Gold foil: 69 HK
• Tooth enamel: 343 HK
• Quartz:  820 HK
• Silicon carbide: 2480 HK
• Diamond: 7000 HK

Although the Knoop method uses the Vickers scale, these types of hardness testers are more suited to measuring objects that are brittle such as ceramics, due to their elongated indenter.

5. Shore Hardness Testers

Shore hardness testers (shore durometers) are types of hardness testers used for measuring the hardness of rubbers and plastics. There are generally two further subcategories of Shore hardness types that include Shore A and Shore D.

Shore Method

The Shore hardness instrument works by a spring-loaded steel rod with a bevelled cone at the tip. The test sample is positioned straight under the bevelled cone. The hardness tester is then pressed against the material until the flat metal plate in the bottom is level with the elastomer sample.

Shore Scales

There are 12 shore scales, each designed to measure different types of material, depending on their intended use. The scales are A, B, C, D, DO, E, M, O, OO, OOO, OOO-S, and R. Each scale results in a value between 0 and 100, with higher values indicating a harder material.

• Scale OO: Bicycle gel seat 15–30
• Scale OO: Chewing gum 20
• Scale OO: Sorbothane 30–70
• Scale A: Rubber band 25
• Scale A: Door seal 55
• Scale A: Automotive tire tread 70
• Scale A: Soft wheels of roller skates and skateboard 78
• Scale A: Hydraulic O-ring 70–90
• Scale A: Pneumatic O-ring 65–75
• Scale A: Hard wheels of roller skates and skateboard 98
• Scale A: Ebonite rubber 100
• Scale D: Solid truck tires 50
• Scale D: Hard hat (typically HDPE) 75
• Scale D: Cast urethane plastic 80

This means the least alterations / impact made from material then the higher material hardness. Alternatively, lesser resistance to indentation and softer materials.

6. Webster Hardness Testers

The Webster hardness testing method originated in the United States. It is used in small hand-held devices, where force is applied from a spring-loaded handle. Webster testers are types of hardness testers that are easy and quick to use, with the hardness result read directly from the indicator with a simple clamp.

Used commonly for on-site quality inspections, it can test aluminium alloys, brass, copper and soft steel. Although, its primary use is measuring sheet material.

Webster Scale

The indenter is used to determine the hardness of the material. The depth of the indenter is 0.01mm, which uses the Wechsler hardness unit (referenced as HW). The full range has only 20 graduations so a typical instrument is limited when testing metallic materials.

7. Leeb Hardness Testers

The Leeb hardness testing method, also known as the Equotip method, was first invented by a swiss company in 1975. This method measures the velocity of rebound when a carbide ball or diamond-tipped head is propelled onto a surface.

Standardised in ASTM A956-17, Leeb hardness testers are suitable for testing larger workpieces, over 5KG. Suitable for use on most metallic materials, the sample must have a prepared, smooth finish for an accurate reading.

Leeb Method

You will find the Leeb method in both Handheld (portable) and Bench (fixed) hardness testers.

Leeb Scale

After taking a reading, the Leeb device will return a value of the rebound velocity, a unit of measure known as HL (hardness Leeb). This value will be read as the hardness rating which is a standard across all Leeb devices. Leeb calculates the energy loss from an impact, which is equivalent to the measure of that energy loss due to deformation. The impact body rebounds faster from harder samples than the softer ones, resulting in a larger value.

It is important to note that some of these types of hardness testers will automatically convert the unit from HL to one of the more common scales.

Conclusion

In this article, we covered the 7 essential types of hardness testers. The most important thing to consider is the scale of hardness most suited to your material. For example,
· If you are using harder materials, such as metals then Rockwell hardness testers are used
· Brinell is best for coarse surface structure, such as castings and forgings.
· Vickers can measure small individual parts or small sections of larger samples.
· Knoob testers spread the force and can be used on more brittle materials, such as thin ceramics.
· For plastics or rubbers, Shore hardness may be a good choice
· Webster testers are used to measure sheet material
· Leeb hardness testers are good for not permanently damaging the surface of what you’re measuring. However you must have a smooth solid surface with sufficient mass and thickness to prevent reverberation of the sample

Check out our range of Hardness Testers here.

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