The hard tests
Determining material hardness is a process that has been around for over 250 years, and it shows no signs falling down the pecking order. Materials World speaks to Sean Malloy who examines some of the most common tests being used in materials development.
The importance of hardness testing, as applied to most materials, is difficult to understate, despite there being various mechanical and optical tests used to determine the characteristics of a material and its suitability for a given application.
Many new products entering today’s marketplace are generally based on new materials and processes. These new materials are supporting innovation as well as boosting competitiveness within technology-driven industries and, as a result, are playing increasingly important roles.
Hardness is the property of a material that enables it to resist plastic deformation, usually by penetration. However, the term hardness may also refer to resistance to bending, scratching, abrasion or cutting.
‘Hardness testing is a process for analysing component properties and can be achieved through a multitude of methods and techniques,’ says Sean Malloy, Hardness Application Testing and Technical Specialist at Tinius Olsen, UK.
‘Determining material hardness can give a valuable insight into its performance, including durability, strength and flexibility. It can also determine the capabilities of a variety of component types, from
raw materials to carefully prepared specimens and finished goods.
‘Today’s industries have high expectations on accuracy and quality, with errors in production having potentially serious consequences. More than ever, research, manufacturing and quality control depend heavily on new and evolving testing techniques to stay ahead of the game.’
External loadings, such as pressures and extreme temperatures in manufacturing, can potentially affect the performance of a material, es
pecially metals and metal alloys.
The complex specimen geometry and linear correlation between hardness and tensile strength in metals means hardness testing is often the best way of ensuring components will survive and perform as intended. It has, therefore, become a vital part of the quality control process.
‘Accurately determining the hardness of a material for any given application involves several factors. These include the type of material, surface conditions, specimen geometry, exposure to heat treatment processes and production requirements,’
‘A range of different hardness tests may be applied to determine different hardness values for the same test specimen. The type of test used is often guided by customer requirements, as hardness measurements are commonly reported values on the spec sheets of the manufactured goods.’
Different materials, different methods
The most common tests used to determine material hardness are the Rockwell, Brinell, Vickers and the Knoop techniques.
All four measure hardness by determining resistance to the penetration of a non-deformable ball, cone or four-sided pyramidal diamond indenter. Also, each determines the depth to which the indenter will sink into the material, at a defined load, within a specific period of time.
While the Rockwell and Brinell tests apply primarily to metals and ceramics, there is an ISO standard for Rockwell hardness testing of plastics (ISO 2039-2). The Vickers test may be used to measure hardness of metals, some ceramics, polymers and even some biomaterials. While Knoop testing is designed to measure the hardness of the thinnest material, and soft materials like paint and plated jewellery.
‘The Rockwell hardness test is the most commonly used hardness test method. It is a measurement based on the net increase in depth of impression as a load is applied. Simply put, when using a fixed force – load – and a known indenter, the smaller the indentation, the harder the material.’
This test method can be used on all metals, except in conditions where the test metal structure or surface conditions would introduce too much variation – where the indentations would be too large for the application, or where the sample size or sample shape prohibits its use.
‘Rockwell measures the permanent depth of indentation produced by a force/load on an indenter. As an example, an indenter may be either a tungsten carbide ball of some specified diameter or a spherical diamond-tipped cone of 120° angle and 0.2mm tip radius, commonly referred to as a Brale indenter. The combination of indenter and the test load determine the hardness scale, which is expressed in letters such as A, B and C,’ said Malloy.
When selecting a Rockwell scale, a general guide is to choose the scale that specifies the largest load and indenter combination possible. This would be without exceeding defined operation conditions and accounting for conditions that may influence the test result. These conditions include, but are not limited to, staying within the hardness range of the utilised Rockwell scale, test specimens that are below the minimum thickness for the depth of indentation, or a test impression that falls too close to the edge of the specimen or another impression. This also includes testing on cylindrical specimens.
‘Brinell hardness, on the other hand, is determined by applying a tungsten carbide sphere of a specified diameter at a specified load into the surface of a material and optically measuring the diameter of the residual indentation post-test.’
The Brinell hardness test is most commonly used to test materials that have too course a structure or that have to rough a surface too be tested using another test method, such as castings and forgings. Brinell testing often use a very high-test load – 3,000 kilogram-force (kgf) – and a 10mm diameter indenter, so that the resulting indentation averages out most surface and sub-surface inconsistencies. Lighter load/ball diameter combinations can be utilised when sample thickness does not support the heavier loads
‘The Brinell hardness number is obtained by dividing the load used, in kilograms, by the actual surface area of the indentation in square millimetres,’ added Malloy.
The Vickers and Knoop hardness test methods, also referred to as a micro-hardness tests due to the light loads being applied, are mostly used for small parts, thin sections, microstructures, weld heat-affected areas, fracture testing or case depth work. Although Vickers and Knoop are very different methods based on the indenter design and the way they measure the resulting indentation, they both share the same methodology for applying the test and are commonly performed on the same hardness tester.
These are very useful for testing on a wide type of materials but samples must be highly polished in order to measure the size of the impressions in microns. Typically loads for Knoop or Vickers testing are very light, ranging from 1g to 1kgf, although Macro Vickers loads can range up to 120kg or more.
‘The Vickers test method subjects the sample surface to a pre-defined load, exerted by a four-sided pyramidal diamond indenter. This gives a standardised length of time referred to as the dwell time. The diagonal of the resulting indentation is measured under a microscope and the Vickers hardness value is then calculated.’
Knoop differs from Vickers testing by elongating two of the four sides of the diamond indenter and only using the dimensions of the elon
gated impression to determine the Knoop hardness value. This is to be able to test the hardness of softer materials with the least amount of penetration.
‘This test is used in the most demanding quality control applications. This includes the inspection of welds subjected to extreme pressures and heavy-duty machinery utilised in manufacturing operations,’ said Malloy.
‘Basically, the type of material and expected hardness will determine the test methods. Materials such as hardened bearing steels have small grain size and can be measured using the Rockwell scale due to the use of diamond indenters and high PSI loading. Materials such as cast irons and powder metals will need a much larger indenter such as those used with Brinell scales. Very small parts or small sections may need to be measured on a micro-hardness tester using the Vickers or Knoop Scale.’
Use and design
The Tinius Olsen Testing Machine Company was founded in 1880 by Norwegian engineer Tinius Olsen, specialising in the manufacturer and supply of static tension and compression materials testing machines.
Malloy has been with the company for over eight years, establishing himself as the company’s resident expert on materials hardness testing and is responsible for producing a series of testing machines.
He says that a range is required because manufacturer needs vary by budget, frequency of testing and specific loads of Vickers/Knoop or low force Brinell testing as examples.
The size of a manufacturing business can affect choices in hardness testing. Small family businesses may use it occasionally, larger firms offer required degrees of automation and advanced technologies.
‘Despite the strength of this technology, new testing machines, such as the Tinius Olsen FH-14 and FH-3 Series, are still easy to navigate through all the functionality making it easy for an operator to learn the machine operating routine. Therefore training is simplified. There is also the ability to store results with pictures, which enables recall of those pictures for re-evaluation the measuring process,’ said Malloy.
The FH series has an optical path that allows a microscope to be used in partnership with a digital camera which, when used in conjunction with a dual optical path, allows an operator to see through the microscope as well as digitally. ‘This technology is ideal for training students and university research departments,’ claims Malloy.
Machines such as the FH are also designed to be incorporated with an automated system, which collects data from multiple machines performing tests simultaneously. This can be a combination of tensile, dimensional, hardness and flexure testing, with all samples being robot-fed.
The automated system ensures precise sample positioning and imaging, enhanced accuracy, an extremely high level of repeatability and overall increased productivity. It also eliminates operator-related errors caused by eyestrain, fatigue and inevitable inconsistencies, which can be a common problem in micro-hardness testing.
In short, having the ability to automate high-volume testing processes reduces the amount of idle time invested by laboratory technicians, resulting in cost-effective quality control.