Indentation hardness

Indentation hardness tests are used to determine the resistance 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.

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Material hardness

As the direction of materials science continues towards studying the basis of properties on smaller and smaller scales, different techniques are used to quantify material characteristics and tendencies. Measuring mechanical properties for materials on smaller scales, like thin films, can not be done using conventional uniaxial tensile testing. As a result, techniques testing material "hardness" by indenting a material with an impression have been developed to determine such properties.

Hardness measurements quantify the resistance of a material to plastic deformation. Indentation hardness tests compose the majority of processes used to determine material hardness, and can be divided into two classes: microindentation and macroindentation tests. Microindentation tests typically have forces less than 2 newtons (N). Hardness, however, can not be considered to be a fundamental material property. Instead, it represents an arbitrary quantity used to provide a relative idea of material properties.^[1] As such, hardness can only offer a comparative idea of the material's resistance to plastic deformation since different hardness techniques have different scales.

Macroindentation tests

There are various differently used macroindentation tests. The most frequently used include the Brinell hardness test, the Rockwell hardness test, and the Vickers hardness test.

The Brinell hardness test involves pressing a steel sphere against the material surface for a given while, usually 10–15 seconds. The surface of indentation is measured, and the load divided by the area of the impression surface upon the material gives the hardness, HB. Since different spheres produce different impressions, to maintain the same HB the ratio of the load to the square of sphere diameter needs to be constant.

The Rockwell hardness test is the simplest, and most popular, hardness test. In this test, there is no indentation depth or width measured. Instead, a preload is applied before the main load, and the machine dial gives a number related to the main load. The machine dial provides a number related to the indentation depth, using one of many Rockwell scales. The limitations of the Rockwell test depend on how much the test piece sags. Sagging in the sample causes changes in the hardness measurements.

The Vickers test is one of the most widely applicable hardness tests. It uses a pyramidal indenter mounted on a square diamond base. The Vickers test usually uses forces that vary from 10 to 1,200 N. Requirements include having the surface ground and polished, and making sure work-hardening has not affected the material. Additionally, the mechanism of flow in the material under the indenter affects the shape of the indentation—the sides of the square impression sometimes deform into concave or convex curves, which can result in reading errors.

Microindentation tests

The term "microhardness" has been widely employed in the literature to describe the hardness testing of materials with low applied loads; however, microhardness implies that the hardness is very small rather than the load. 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 test specimen using a known applied force (commonly called a “load” or “test load”) of 1 to 1000 gf. Microindentation tests typically have forces of 2 N and produce indentations of about 50 μm. Due to their specificity, microhardness testing can be used to observe changes in hardness on the microscopic scale. Unfortunately, it is difficult to standardize microhardness measurements; it has been found that the microhardness of almost any material is higher than its macrohardness. Additionally, microhardness values vary with load and work-hardening effects of materials.^[1] Regardless, the two most commonly used microhardness tess are the Knoop and Vickers tests.

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

The Vickers microindentation test is carried out in a similar manner to the Vickers macroindentation tests, using the same pyramid. The Knoop test uses an elongated pyramid to indent material samples. This elongated pyramid creates a shallow impression, which is beneficial for measuring the hardness of brittle materials or thin components. Both the Knoop and Vickers indenters require prepolishing of the surface to achieve accurate results.

Scratch tests at low loads, such as the Bierbaum microcharacter test, performed with either 3 gf or 9 gf loads, preceded the development of microhardness testers using traditional indenters. In 1925, Smith and Sandland of the UK developed an indentation test that employed a square-based pyramidal indenter made from diamond.^[2] They chose the pyramidal shape with an angle of 136° between opposite faces in order to obtain hardness numbers that would be as close as possible to Brinell hardness numbers for the specimen. The Vickers test has a great advantage of using one hardness scale to test all materials.The first reference to the Vickers indenter with low loads was made in the annual report of the National Physical Laboratory in 1932. Lips and Sack describes the first Vickers tester using low loads in 1936.^{[citation needed]}

There is some disagreement in the literature regarding the load range applicable to microhardness testing. ASTM Specification E384, for example, states that the load range for microhardness testing is 1 to 1000 gf. For loads of 1 kgf and below, the Vickers hardness (HV) is calculated with an equation, wherein load (L) is in grams force and the mean of two diagonals (d) is in micrometers:

For any given load, the hardness increases rapidly at low diagonal lengths, with the effect becoming more pronounced as the load decreases. Thus at low loads, small measurement errors will produce large hardness deviations. Thus one should always use the highest possible load in any test. Also, in the vertical portion of the curves, small measurement errors will produce large hardness deviations.

References

1. ^ ^{a b} Meyers and Chawla (1999): "Mechanical Behavior of Materials", 162–168.
2. ^ R.L. Smith and G.E. Sandland, “An Accurate Method of Determining the Hardness of Metals, with Particular Reference to Those of a High Degree of Hardness,” Proceedings of the Institution of Mechanical Engineers, Vol. I, 1922, p 623–641.