Indentation tests pyramid indenters

Vickers Hardness. The Vickers or diamond pyramid hardness (DPH) developed in 1924 was an improvement over the Brinell test. The Vickers test used a pyramidal diamond as the indenter. This permitted the hardness testing of much harder materials, and the constant 136° angle of the indenter eliminated the problem of variable indentation shape encountered using spherical indenters (1). 

For erosive wear. Rockwell or Brinell hardness is likely to show an inverse relation with carbon and low alloy steels. If they contain over about 0.55 percent carbon, they can be hardened to a high level. However, at the same or even at lower hardness, certain martensitic cast irons (HC 250 and Ni-Hard) can out perform carbon and low alloy steel considerably. For simplification, each of these alloys can be considered a mixture of hard carbide and hardened steel. The usual hardness tests tend to reflect chiefly the steel portion, indicating perhaps from 500 to 650 BHN. Even the Rockwell diamond cone indenter is too large to measure the hardness of the carbides a sharp diamond point with a light load must be used. The Vickers diamond pyramid indenter provides this, giving values around 1,100 for the iron carbide in Ni-Hard and 1,700 for the chromium carbide in HC 250. (These numbers have the same mathematical basis as the more common Brinell hardness numbers.) The microscopically revealed differences in carbide hardness accounts for the superior erosion resistance of these cast irons versus the hardened steels. 

Knoop hardness It is a measure of hardness is measured by a calibrated machine that forces a rhomb-shape, pyramidal diamond indenter having specified edge angles under specific small loading conditions into the surface of the test material the long diagonal in the material is measured after removal of the load. 

The present review shows how the microhardness technique can be used to elucidate the dependence of a variety of local deformational processes upon polymer texture and morphology. Microhardness is a rather elusive quantity, that is really a combination of other mechanical properties. It is most suitably defined in terms of the pyramid indentation test. Hardness is primarily taken as a measure of the irreversible deformation mechanisms which characterize a polymeric material, though it also involves elastic and time dependent effects which depend on microstructural details. In isotropic lamellar polymers a hardness depression from ideal values, due to the finite crystal thickness, occurs. The interlamellar non-crystalline layer introduces an additional weak component which contributes further to a lowering of the hardness value. Annealing effects and chemical etching are shown to produce, on the contrary, a significant hardening of the material. The prevalent mechanisms for plastic deformation are proposed. Anisotropy behaviour for several oriented materials is critically discussed. 

We test the hardness of polymers by applying an indenter to their surface with a known force and noting the depth to which the tip penetrates the sample. These tests typically fall into one of two categories. In the first, the depth of penetration is read directly from a dial on the instrument, calibrated in arbitrary hardness units. The farther the tip penetrates the sample, the lower is its hardness. The second type of test involves impressing a pyramidal indenter tip against the sample with a known force and measuring the depth to which it penetrates. In practice we measure the dimensions of the indentation and calculate the depth of penetration and compressive modulus based on the tip geometry. 

Hardness is measured by the Rockwell A-scale diamond cone indentation test (HRA) or by the Vickers diamond pyramid indentation test (HV). Although the Rockwell scale has been used for decades in the carbide industry as a measure of hardness, a true indication of the resistance of the tool to deformation in metal-cutting operations can be obtained only by measuring hardness at elevated temperatures. The hardness of cemented carbides decreases monotonically with increasing temperatures. 

The physical meaning of hardness determined by Bierbaum s method is exactly equal to load P divided by the projected contact surface of pyramid with material under test in the scratch process. It should be mentioned here that the result obtained under Bierbaum s method is roughly directly proportional to the hardness number obtained by the pyramid indenter method, and the relation has the form... 

Other formulae describing hardness tested by the scratch-with-edge-to-the-front method are also based on the ratio of load to contact surface of indenter with material under test, or to that surface projected to the sample plane. For the tetrahedral pyramid in the widely used Vickers hardness testers and in PMT-3... 

Lawn et al. (1975, 1978), and Lawn and Marshall (1978) distinguish two types of indenter whose action on the tested surface differs significantly (1) a blunt indenter (e.g., a hard ball) distinguished by an ideal elastic contact, so that the crack initiation is controlled by previously present defects (usually on the sample surface), and (2) a sharp indenter (e.g., a cone or pyramid) distinguished by partially plastic contact, so that the original defects start to grow as the result of the indentation process itself. In practice, the contact situations can therefore be seen as intermediate between the two cases. Within this area all typical indenters used for hardness measurement are contained. 

Observations of the specificity of indentations, excluding measurement of the magnitude of Vickers pyramid penetration, enable a description of the properties of minerals, ceramic materials, or other brittle bodies (Vigdo-rovich and Yelenskaya, 1967 A. Szymanski et al., 1969). The action of elastic-recovery forces after removal of the pressure often causes perturbation in the structure of the test surface, around the site subjected to loading (Fig. 6.3.1). This is described in Section 6.2. 

A microscope hardness tester, also equipped with Vickers and Knoop pyramid ensuring observation immediately after measurement and after indenter retraction (without shifting the sample under test), is manufac-... 

Palmquist (1957) was the first to make use of the deformation developing around the Vickers pyramid indentation as an aid in the interpretation of results. The Palmquist test consists of determining the resistance of brittle materials to propagation of the cracks appearing at the corners of the Vickers pyramid mapping on a polished surface. The measured value, defined as Palmquist toughness, is given by the formula... 

Boyarskaya (1972) carried out comparative tests to determine hardness anisotropy by indentation with a Vickers pyramid and by scratch in a number of minerals and crystals (Table 7.7, Figs. 7.4, 7.5). It is seen from the table that for most of the crystals tested the results for scratch hardness are clearer. Interesting is her finding that where K,1V is lowest, KIiR is highest. This appears to be due to a more legible test, which no doubt facilitates correct hardness measurement. 

When the indentation hardness is measured, a small sphere or the tip of a pyramid-shaped crystal of a material which is harder than the one to be tested is pressed in the surface. The depth of the imprint is a measure for the hardness. Especially in the case of porous material it is vital that the determination is carried out carefully to avoid that... 

Fig. 8. Result of hardness tests on a series of TiX alloys (where X is a combination of Fe, Co, and Ni). Diamond pyramid indenter in a standard Tukon micro hardness tester was used.

Another criterion is used with the Vickers hardness test after penetration of a pyramid-shaped diamond under stress, the diameter of the indent is measured after removal of the diamond. The hardness is defined as the applied force divided by the area of the indent. This is again a measure of the permanent deformation, or, possibly, of the yield stress. 

Indentation hardness is a very common determination in materials testing. In this test a very hard indenter (a hard steel sphere in the Brinell test, a diamond pyramid in the Vickers test) is pressed under a load into the surface of the material. 

In the Brinell test (Brinell, 1900 Meyer, 1908) the indenter consists of a hard steel ball, though in examining very hard metals the spherical indenter may be made of tungsten carbide or even of diamond. Another type of indenter which has been widely used is the conical or pyramidal indenter as used in the Ludwik (1908) and Vickers (see Smith Sandland (1925)) hardness tests, respectively. These indenters are now usually made of diamond. The hardness behaviour is different from that observed with spherical indenters. Other types of indenters have, at various times, been described, but they are not in wide use and do not involve new principles. 

Polymers showing a viscoelastic behaviour occupy the intermediate range. Out of all the existing hardness tests, the pyramid indenters are best suited for research on small specimens and microstructurally inhomogeneous samples (Tabor, 1951). Pyramid indenters provide, in addition, a contact pressure which is nearly independent of indent size and are less affected by elastic release than other indenters. 

The hardening and embrittlement of polyimides by ion implantation has been also studied (Pivin, 1994). Nanoindentation tests performed using a sharp diamond pyramid of apical angle 35° provided very quantitative depth profiles of microhardness in polyimides implanted with C, N, O, Ne or Si ions. In all cases the microhardness increased steeply when the amount of deposited energy reached the order of 20 eV atom". For energies of 200 eV atom" the polymer is transformed into an amorphous hydrocarbon and the microhardening factor saturates at a value of 13-20. However, the carbonized layer is poorly adherent, as is evidenced by reproducible discontinuities in the depth vs load curves, when the indenter tip reached the interface. 

An alternative method of determining the plasticity and elasticity of a material is indentation hardness testing. The principle of indentation hardness testing is that a hard indenter of specified geometry, either a sphere or square-based pyramid, is pressed onto the surface of the test... 

The Vickers Hardness test uses a square-based diamond pyramid as the indenter. The Vickers Hardness, Hv, is calculated by... 

The standard methods to determine the coating microhardness by indentation of a pyramid-shaped diamond indenter are described in ASTM E384-07 (2007). Depending on the shape of the indenter, Knoop- and Vickers-type diamonds as well as Berkovich indenters can be distinguished. The reported hardness number expressed in N mm-2 (MPa) is the force exerted on the specimen surface by the diamond indenter used to produce the impression. In principle, the technique is less affected by porosity than the scratch tests based on measuring the indenter travel caused by a specific increase in load. Microhardness tests are usually made... 

In principal, microhardness determination is rather simple. The indenter may be a square based pyramid of diamond or sapphire with a face angle of a = 136° for the Vickers test, or a rhombic-based pyramid with angles between the edges at the top of P = 130° and = 172° 30 for the Knoop test. The load P between 2 and 200 pond is... 

The Vickers hardness test differs from the Brinell test in that the indentor is a diamond (square-based) pyramid (Figure 3.42) having an apex angle of 136°. If the average diagonal of the indent is d, the hardness number is calculated from... 

Indentation Test. Indentation hardness is measured by two methods. A spherical or pyramidal indentor is applied to the paint surface for a specified period. The penetration depth under load is measured (ISO 6441, ASTM D1474, Pfund hardness number). In the other method an indentor with a specified load is applied for a... 

Other common hardness tests involve the use of diamond pyramids. In the Vickers hardness test, a square pyramid is used and in the Knoop hardness test, the pyramid is elongated. The area term in the former test is the actual indentation area and in the latter, the projected area. From the impression geometries, shown in Fig. 6.30, the Vickers Hardness Number (VHN) and Knoop Hardness Number (KHN) can be shown to be VHN=1.854F/a and KHN=14.2F/L, respectively. A common hardness test in the USA is the RockweO hardness test, which uses various indenter types and loads. The result of these tests is a dimensionless number and leads to the use of various hardness scales (e.g., Rockwell B, Rockwell C). 

Hardness refers to the resistance of steel to indentation. The three important methods to determine the hardness are (1) the Brinell test, (2) the Vickers test, and (3) the Rockwell test. All these methods use the same basic principle wherein a ball or a pointed indenter is forced onto the material surface under a given load and the area of indentation thus created is measured. The Brinell test uses a 10-mm-diameter tungsten ball indenter under a load of 29,420 N. The Rockwell C hardness test uses a diamond cone indenter under a load of 1471 N. The Vickers test uses a diamond pyramid indenter, and the load varies. The greater the hardness of a material, the smaller the area of indentation. 

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