Indentation hardness

Indentation hardness, used in mechanical engineering to determine the hardness of a material to deformation. Several such tests exist, wherein the examined material is indented until an impression is formed; these tests can be performed on a macroscopic or microscopic scale.

Nanoindentation, or nanopenetration, uses very small loading forces (down to nanonewtons) and is used to measure the surface hardness of materials (very thin layers).

When testing metals, hardness can be correlated to tensile strength. This important relationship makes it economically feasible to perform tests that are considered non-destructive (to the specimen, but not to the point of measurement) for distributions (supplies) of light, massive metals, even with portable equipment such as Rockwell hardness testers.

As the direction of materials science continues to move toward the fundamental study of properties at smaller and smaller scales, several techniques are used to quantify material trends and properties. Measuring the mechanical properties of materials at smaller scales, such as thin films, cannot be done using conventional uniaxial tensile testing. As a result, indentation “hardness” testing techniques have been developed to determine such properties.

Hardness measurements quantify a material’s resistance to plastic deformation. Indentation hardness tests represent the majority of methods used to determine material hardness and can be divided into two classes: micro-penetration and macro-penetration. Micropenetration tests typically use forces less than 2 N. However, hardness cannot be considered a fundamental material property. Instead, it is an arbitrary quantity used to give a relative idea of the material property. As such, hardness can only provide a comparative idea of the material’s resistance to plastic deformation because different hardness techniques have different scales.

The main source of error in penetration testing is the effect of work hardening in the process. However, it has been found experimentally using “stress-free hardness testing” that this effect is minimal for smaller indentations.

The surface finish of the workpiece and the indenter have no effect on the hardness measurement, provided the penetration is large relative to the surface irregularity. This is useful when measuring the hardness of actual surfaces as well as when leaving shallow indentations, as a finely etched indenter makes the impression much easier to read than a smooth one.

The indentation produced after the indenter and load are removed is called “recovery” or “bounce back”. This effect is more properly known as shallowing. For spherical penetrators, it is known that the penetration remains symmetrical and spherical, but with a larger radius. For very hard materials, the radius can be three times the radius of the penetrator. This effect is due to the release of elastic stresses. Because of this effect, the diameter and depth of penetration will cause errors. It is also known that the error due to diameter change is only a small percentage, while the error due to depth is much larger.

Another effect of loading on penetration is the “piling up” or “sinking in” of the surrounding material. If the metal is annealed, it will have a tendency to pile up and form a “crater”. If the metal is annealed, it will sink in around the penetration. Both of these effects add to the error of the hardness measurement.

Macroindentation testing

The term “macroindentation” is used to refer to tests with a larger test load, such as 1 kgf or more. There are several macroindentation tests, including

  • Vickers Hardness Test (HV), which has one of the widest scales. Widely used to test the hardness of all kinds of metal materials (steel, non-ferrous metals, tinsel, cemented carbide, sheet metal, etc.); surface layer / coating (carburizing, nitriding, decarburizing layer, surface hardening layer, galvanized coating, etc.).
  • Brinell hardness test (HB) BHN and HBW are widely used.
  • Knoop hardness test (HK), for small area measurement, widely used to test glass or ceramic material
  • Janka hardness test, for wood
  • Meyer hardness test
  • Rockwell hardness test (HR), mainly used in the USA. HRA, HRB and HRC scales are most commonly used.
  • Shore hardness test, for polymers, widely used in the rubber industry
  • Barcol hardness test, for composites

In general, there is no simple relationship between the results of different hardness tests. Although there are practical conversion tables for hard steels, for example, some materials behave qualitatively differently under the various measurement methods. However, the Vickers and Brinell scales correlate well over a wide range, with Brinell giving overestimated values only at high loads.

Indentation methods, however, can be used to extract true stress-strain relationships. Certain criteria must be met to obtain reliable results. These include the need to deform a relatively large volume and therefore use large loads.

Microindentation testing

The term “microhardness” has been widely used in the literature to describe hardness testing of materials with low applied loads. A more precise term is “microindentation hardness testing”. In microindentation hardness testing, a diamond indenter of specific geometry is impressed into the surface of the specimen using a known applied force (commonly referred to as “load” or “test load”) of 1 to 1000 gf. Microindentation tests typically have forces of 2 N (approximately 200 gf) and produce indentations of approximately 50 μm. Because of their specificity, microhardness tests can be used to observe changes in hardness at the microscopic level. Unfortunately, it is difficult to standardize microhardness measurements; it has been found that the microhardness of almost any material is greater than its macrohardness. In addition, microhardness values vary with load and work hardening effects of materials[3] The two most commonly used microhardness tests are tests that can be used with higher loads than macroindentation tests:

  • Vickers hardness test (HV)
  • Knoop hardness test (HK)

In microindentation testing, the hardness value is based on measurements of the indentation made in the surface of the specimen. The hardness number is based on the applied force divided by the surface area of the indent itself, giving hardness units in kgf/mm². Microindentation hardness testing can be performed using both Vickers and Knoop indenters. In the Vickers test, both diagonals are measured and the average is used to calculate the Vickers pyramid number. In the Knoop test, only the longer diagonal is measured and the Knoop hardness is calculated by dividing the projected area of the indent by the applied force, also giving test units in kgf/mm2.

The Vickers microindentation test is performed in a similar manner to the Vickers macroindentation test, using the same pyramid. The Knoop test uses an elongated pyramid to indent material specimens. This elongated pyramid produces a shallow indentation, which is advantageous for measuring the hardness of brittle materials or thin components. Both the Knoop and Vickers indenters require polishing of the surface to obtain accurate results.

Nanoindentation testing

Nanoindentation was developed in the mid-1970s to measure the hardness of small volumes of material.

In a traditional indentation test (macro or micro indentation), a hard tip whose mechanical properties are known (often made of a very hard material such as diamond) is pressed into a sample whose properties are unknown. The load applied to the indenter tip increases as the tip penetrates further into the specimen and soon reaches a user-defined value. At this point, the load can be held constant for a period of time or removed.

This technique is limited by the large and varied range of indenter tip shapes, and by indenter rigs that do not have very good spatial resolution (the location of the area to be indented is very difficult to specify accurately). Comparison between experiments, typically performed in different laboratories, is difficult and often meaningless. Nanoindentation improves on these macro- and micro-indentation tests by indenting at the nanoscale with a very precise tip shape, high spatial resolutions for indentation placement, and by providing real-time load-displacement data (into the surface) while the indentation is in progress.

Nanoindentation uses small loads and tip sizes, so the indentation area may be only a few square microns or even nanometers. This creates problems in determining hardness because the contact area is not easy to find. Atomic force microscopy or scanning electron microscopy techniques can be used to image the indentation, but can be cumbersome. Instead, an indenter with a known high-precision geometry is used (usually a Berkovich tip, which has a three-sided pyramid geometry). During the instrumented indentation process, the depth of penetration is recorded and then the area of the indent is determined using the known geometry of the indenter tip. During the indentation process, various parameters such as load and depth of penetration can be measured.

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