This article covers the concept, origins, development and application of hardness testing, and the main testing methods in use today.
What is hardness testing?
Hardness testing is an essential element of materials science and engineering; providing vital insights into the mechanical properties of materials, namely strength, wear resistance and ductility. ‘Hardness’ refers to a material’s ability to resist being permanently deformed by a load (see footnote), and this resistance will also apply to the effects of scratching, cutting, or abrasion. Understanding a material’s hardness is crucial for applications ranging from construction and manufacturing to product design and quality control and is especially important in industries such as aerospace and defence, transport, and oil and gas, where the failure of a component due to it being either too brittle or too soft can have catastrophic consequences. This article covers the history, technical developments, different hardness scales, and modern applications of hardness testing methods. Overwhelmingly, hardness testing is used to determine whether any particular metal is fit for purpose. Metals are the primary components of mechanical engineering and industrial production.
The origins of hardness testing
The need to know the approximate hardness of materials would have been apparent to those who worked with flint and wood tens of thousands of years ago. As metal production developed, the need to cut, to hammer, to bend and to shape was essential, both to the craftsmen who worked with it and to the toolmakers who supplied them. These needs led to the development of the various alloys and to annealing, case-hardening and so on, with much of the technological progress surely being by chance, or through ‘hit-and-miss’ approaches, as there was no way for our ancestors to understand what was happening at a molecular level. A very good example of non-scientific hardening was the production of metal arrowheads in Medieval England, which were heated and beaten to the point where they could penetrate the personal armour of the day (with dramatic effect at the battle of Agincourt, 1415, for example). There can be little doubt that, over the first few thousand years of the use of metals, “hardness” was probably a more significant characteristic, in the mind of the craftsman-metalworker, than compressive or tensile strength even if, at the most basic level, it was not much more sophisticated than “metal A will dent metal B but not vice-versa”.
The concept of hardness testing dates back to antiquity, but the testing – as far as we are aware – was rudimentary and probably plagued with subjectivity. Simple scratch tests were used to compare the hardness of materials – which metal could scratch which other metal etc. By the eighteenth century a great deal had been learnt about alloy production and about how metal hardness could be changed by heating and/or beating, and a straightforward scratch test was employed using a long metal bar that had been subjected to progressively greater hardening processes along its length, so one end was relatively easy to scratch and the other not, and the hardness of particular materials was tested by seeing how far along this bar the material was able to leave a scratch mark.
The Mohs scale of mineral hardness, developed by the German geologist Friedrich Mohs in 1812, was one of the earliest formalised hardness testing methods. This qualitative scale ranks minerals based on their ability to scratch one another, with talc assigned the lowest hardness value of 1 and diamond the highest value of 10. On this scale a human fingernail would rank about 2.5!
The industrial revolution (developed, of course, from the steam engine) and the subsequent advances in metallurgy and materials science necessitated more precise and quantitative methods of hardness testing, not least to avoid explosions and other catastrophes, as metal tools were now being driven by greater forces than ever before and metals were being used to construct the very steam pressure vessels that these machines depended on. Iron bridges, the mere existence of which would have astounded the Romans, were constructed which subjected component parts to tension loads that no stone bridge ever had to withstand.
Technical developments in hardness testing
1. The Brinell Test (1900). As mentioned above, the Industrial Revolution placed demands upon manufacturing materials as never before and the 19th century saw the introduction of indentation-based hardness testing (a significant advance on scratch testing) in which an indenter is pressed into the surface of the test material by a specific load for a specific time, and the width of the resulting indentation is measured to assess the hardness (the wider the indentation, the softer the material). Indentation-based measurement achieved widespread acceptance following the launch of Swedish metallurgist Johan August Brinell’s eponymous test method. The Brinell test achieved levels of accuracy, repeatability and applicability that were previously unheard of, through the utilisation of different sizes of spherical, tungsten-carbide indenters, in combination with forces ranging from 1 kgf to 3,000 kgf. The Brinell Hardness number (suffixed HBW for ‘Hardness Brinell Wolfram’ (Wolfram being tungsten carbide)) is the result of a force divided by area calculation:
The Brinell test is especially useful when measuring the hardness of rough-finished castings and similar coarse-surfaced materials, as the indentations are large enough to be unaffected by the grain structure of the metal. It is, therefore, used extensively in forges and foundries. It suffers from one weakness: the indentation is often measured by a technician using a low-powered microscope with a graticule. Operator error / interpretation is therefore a risk, though since the 1980s extremely fast and accurate automatic microscopes have been available. For a much more in-depth look at Brinell testing, see our article here. For more information on the development of Brinell microscopes and other automation options in Brinell testing, see our article here.
2. Rockwell Hardness Test (1919): takes its name from Hugh and Stanley Rockwell, American engineers working in the ball bearing (and bearing race) industry prior to World War One. They built on the conceptual work (1908) of Viennese Professor Paul Ludwik, whose work Die Kegelprobe showed that a metal hardness test methodology involving a very small conical indenter could provide very accurate results, with minimal damage to the material being tested. The test would also overcome any problems due to surface imperfections by calculating the hardness using the difference in depth achieved by an indenter when pressed into the metal under a light, initial load then a much heavier full load.
Driven by the need to test the hardness of small ball-bearing races, Hugh and Stanley Rockwell invented a depth differential machine in 1914. Stanley Rockwell made significant improvements after World War One and the test, as used today, is essentially regarded as his. Rockwell collaborated with Charles Wilson of the Wilson-Maulen Company to produce a suitable testing machine and the test was in use from the mid 1920s.
The Rockwell test works as follows: a very small indenter is driven vertically downwards into the test material by a ‘minor’ load of 10 Newtons (N). The depth achieved by the indenter under this minor load establishes the datum. A much higher load (not less than 45 N and as much as 150 N) is then applied for several seconds then removed, leaving the minor load applied. The difference in depth achieved by the indenter under the minor and major loads is then used to calculate the hardness. Obviously the harder the material being tested, the less the penetration depth but the harder the material, the higher the Rockwell hardness number (see below) so the Rockwell hardness number is inversely proportional to the penetration depth. The Rockwell test has two very significant advantages over the Brinell test: First, the depth measurement is performed and displayed by the test machine itself. No additional optical measuring equipment is required (operator error in optical measurement by manual microscope is still the most problematic element in the use of Brinell testing by some organisations, as discussed earlier) and second, the indentations are much, much smaller than in Brinell testing, so much smaller components and samples can be tested in a non-destructive manner.
There are a number of Rockwell hardness scales, in order to cater for the widest range of materials, and they are named Hardness Rockwell A (HRA), Hardness Rockwell B (HRB) and so on. There are also a variety of indenters (tungsten carbide spheres of various diameters or a sphero-conical diamond). Most general testing requirements in mainstream manufacturing and engineering are covered by the HRB (for copper alloys, aluminium, soft steel) and HRC (for steel, cast iron and hardened steel) scales, so these are very widely used. Sometimes the Rockwell hardness number even appears on household items such as kitchen knives or scissors.
The disadvantages of the Rockwell system are due to the very small indenter – the slightest surface contamination on the test material or the indenter itself can affect the result, and there is also a risk of errors when the test is used on very coarse grained material.
3. Vickers Hardness Test (1921): Developed by Robert Smith and George Sandland, this test uses just one diamond indenter for every type of test – in the shape of an equilateral pyramid with an angle of 136° between the pyramid’s opposing faces. As with Brinell and Rockwell testing, the indenter is pressed into the test material under a specific load for a specific period but the measurement made is of the length of the diagonal marks left by the indenter. The area of the ‘rectangular’ indentation’s sloping surface is established by the limits of the diagonal marks. The Vickers hardness value (HV) formula is:
where F is the force in kgf and d is the mean length of the two diagonal indentations (as viewed from directly above, not their lengths on the material’s surface, of course).
Vickers test machines are large and significantly more expensive than their Brinell and Rockwell counterparts and the surface of the test material must be clean and smooth to ensure accurate measurements, but the results are very accurate indeed and the test can be used on all manner of materials, whatever their hardness, by varying the load (from 1 kgf to 100 kgf) as appropriate. The Vickers test is therefore extremely useful for soft and thin materials.
4. Knoop Hardness Test (1939): Created by Frederick Knoop, this is a micro hardness testing method, that is, it’s designed for measuring the hardness of thin and/or brittle materials or layers. This test method also uses a pyramid diamond indenter but it is a ‘rhombic’ indenter with a long diagonal around seven times the length of the short diagonal. The indenter tip is less pointed than on a Vickers indenter and the loads are 1 kgf or less, so the risk of cracking or deforming materials is very much lower than with the Vickers test. The test obviously creates indentations with markedly different lengths of diagonal and normally only the long diagonal is measured. With delicate materials and hence low forces, the indentations can be very small, so precise measurement of the diagonal is absolutely critical to obtaining an accurate Knoop hardness value (HK), which is calculated based upon the ratio of the applied load to the projected area of the indentation. Knoop hardness testing is particularly valued in aerospace, electronics and materials science when great precision and surface integrity are critically important.
5. Shore Hardness Test (1920): Developed by Albert F. Shore, this method uses a durometer to measure the resistance of a material to indentation by a spring-loaded indenter. The Shore hardness test, like the Rockwell test, measures indentation depth. Shore hardness testing is widely used on polymers, elastomers, and rubbers, with different scales (A to D) catering to varying material hardness ranges. The Shore A scale covers the softer materials like flexible rubbers and elastomers, while the Shore D scale covers the harder plastics such as nylon or polycarbonate. Shore hardness testing is fast and non-destructive.
Modern application of hardness testing
Hardness testing is integral to various industries and research fields, providing essential data for material selection, quality control, and failure analysis. Modern applications of hardness testing include:
1. Metallurgy and Manufacturing: Hardness testing ensures that metals and alloys meet the required specifications for strength and wear resistance. It is used in the production of industrial machinery, aerospace and defence components, transport (e.g. engine and transmission systems, railway locomotive wheels) and oil and gas extraction. The oil and gas industry has particularly narrow hardness tolerances as components are subjected to extreme pressures.
2. Quality Control: Hardness tests are routinely performed in manufacturing; both at the ‘goods in’ stage, for determining the acceptability of raw materials, and the ‘goods out’ stage, to prevent the despatch of defective products. The worst ever high-speed train disaster, the notorious ‘Eschede Derailment’ of June 1998 in which 101 people died, was due to the incorrect hardness of a steel ‘tyre’ on a single wheel in the first passenger carriage. See also ‘Failure Analysis’ below.
3. Product Design and Development: Understanding the hardness/ flexibility of materials aids in the design of products with desired mechanical properties. This is crucial for developing reliable and durable components and equipment / machinery. An extremely simple but vivid example is a climbers’ carabiner. If a climber slips from a rock face the elasticity in the safety rope reduces the shock to the human body but the ultimate load involved in arresting the fall is frequently taken by a carabiner. This slim piece of metal must not be so brittle that it shatters nor so soft that it stretches beyond breaking point.
4. Failure Analysis: Hardness testing plays a key role in investigating material failures and understanding the causes of wear, deformation, and fracture. This information is vital for improving material performance and preventing future failures.
5. Research and Development: Hardness testing is a fundamental tool in materials science research. It helps in studying the effects of alloying, heat treatment, and surface treatments on material properties.
6. Nanotechnology and Thin Films: With the advent of nanotechnology, hardness testing methods such as nano-indentation have been developed to measure the hardness of thin films, coatings, and nanoscale materials. These techniques provide insights into the mechanical properties of materials at the micro and nano scale.
Conclusion
Hardness testing has evolved significantly since the early scratch tests, with numerous standardized methods developed to meet the diverse
needs of modern industries. The Brinell, Rockwell, Vickers, Knoop, and Shore hardness tests each offer unique advantages, with the result that virtually any material can be hardness tested. In today’s technological landscape, hardness testing remains a vital tool in
ensuring materials and machinery are fit for purpose (that is, they meet appropriate standards of performance, reliability and durability). As materials science continues to advance, hardness testing methods will undoubtedly evolve further.
Note for non-engineers: deformation is classed by engineers as either elastic or plastic. Elastic deformation is temporary and the material pulls itself back, as it were, to its previous shape when a load to which it has been subjected is removed. Plastic deformation is permanent, and the reason that indenters are held in position under full load for several seconds during the testing cycle is to ensure the indentation is a plastic deformation. You can see elastic and plastic deformation in action with a rubber band and a spoon, respectively: If you apply ‘fingertip’ force to extend the length of a rubber band it will return to its original length when the force is no longer applied. If, however, you apply firm thumb and finger force with both hands, to bend the handle of a teaspoon, it will be permanently deformed. The elastic limit of a material is the maximum amount of stress it can withstand but still return to its original shape. Those who are interested in materials science and the behaviour of materials under load will find much of interest in the two outstanding works by Professor James E Gordon: The New Science of Strong Materials and Structures, or Why Things Don’t Fall Down, both published by Penguin.
Alex Austin, author
Alex is a member of the ISE/101/105 Indentation Hardness Testing Commitee at the British Standards Institution. He has been part of the delegation to the International Standards Organisation advising on the development of the standard ISO 6506 Metallic Materials – Brinell Hardness Test and is the chairman and convenor for the current ISO revision of the standard.
,