Vickers loads

The Vickers hardness test is a macrohardness test in which loads are commonly varied from 9.8 to 1180 N (1 to 120 kgf). Vickers hardness numbers are invariant with load within the stated limits. 

The Vickers hardness test is commonly made on a flat specimen on which the indenter is hydrauhcaHy loaded. When the desired number of indentations have been made, the specimen is removed and both diagonals of the indentations, measured using a caUbrated microscope, are then averaged. The Vickers hardness number may be calculated, or for standard loads taken from a precalculated table of indentation size vs VHN. The preferred procedures are described in ASTM E92 (2). 

Surface finish requirements for the Vickers test vary with the test load. Heavy load tests can be made on a 120 grit ground surface. At low loads increasingly finer surface preparation is required, approaching that for metaHographic specimens, to permit accurate diamond indentation measurements. 

Hardness is determined by measuring the penetration (depth or area) when a harder material, such as diamond, is pushed into the surface of the material of interest under a specified load. Tme hardness is defined as the force divided by the projected area. Vickers hardness tests, which employ a pyramid-shaped indentor, are frequently used to characterize ceramics however, Vickers hardness calculations normally employ total surface area rather than projected area (43). Measurements are made on the diamond impression shown in Figure 6. Vickers hardness is calculated using... 

Appleton and Waddington [40] present experimental evidence that pulse duration also affects residual strength in OFHC copper. Samples shock loaded to 5 GPa for 1.2 ps pulse duration exhibit poorly developed dislocation cell structure with easily resolvable individual dislocations. When the pulse duration is increased to 2.2 ps (still at 5 GPa peak stress) recovered samples show an increase in Vickers hardness [41] and postshock electron micrographs show a well-developed cell structure more like samples shock loaded to 10 GPa (1.2 ps). In the following paragraphs we give several additional examples of how pulse duration affects material hardness. 

This consists of loading a pointed diamond or a hardened steel ball and pressing it into the surface of the material to be examined. The further into the material the indenter (as it is called) sinks, the softer is the material and the lower its yield strength. The true hardness is defined as the load (F) divided by the projected area of the indent, A. (The Vickers hardness, H , unfortunately was, and still is, defined as F divided by the total surface area of the indent. Tables are available to relate H to Ff .)... 

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. 

Fig. 20. Vickers indentations of oriented CEPE along the fiber axis for various loads showing the typical anisotropic impressions...

Flowever, in order to probe the fracture toughness of thin films or small volumes using ultra low load indentation, it is necessary to use special indenters with cracking thresholds lower than those observed with the Vickers or Berkovich indenters (for Vickers and Berkovich indenters, cracking... 

Analysis of Table II shows discrepancies in the hardness and stress behavior of a-C(N) H films. Although all the works reported a clear stress reduction upon nitrogen incorporation, the hardness sometimes is quoted as almost constant, or on the other hand clearly decreasing. In addition to the possible effect of different deposition methods and conditions, it can be easily seen that the differences in hardness testing methods are the major source for discrepancies. Constant hardness behavior is only reported with the use microindentation methods, like Vickers and Knoop microhardness. On the other hand, the use of low-load nanoindentation methods always led to a nitrogen-induced decrease in hardness. This is basically the consequence of two factors. The first one is the higher penetration... 

A Vickers diamond scratch test diamond load of the first visible scratch (by microscope)... 

Quantitative hardness tests slowly apply a fixed load to an indentor that is forced into the smooth surface of the specimen. After the load is removed, either the diameter across the impression or the depth of the impression is measured. The size of the penetration is proportional to the material s hardness. Rockwell, Brinnell, Vickers, and Knoop are well-known indentation hardness testing instruments. 

The hardness tests are classified according to the magnitude of the load. Macrohardness tests apply major loads that are greater than 1000 g (1 kg). Microhardness tests employ loads less than 1000 g and are commonly used to measure the hardness of a single grain or a very small area. Brinnell, Rockwell, and Vickers are examples of macrohardness tests, and Knoop is an example of a microhardness test. 

Hardness is essentially a measure of stiffness and in principle can be related to modulus. For plastics, the term hardness refers to resistance to indentation but depending on the test method the measurement is made either with the indentation load applied or after its removal when elastic recovery has taken place. The standard methods are given in ISO 868 (Shore) [6] and ISO 2039 (Ball indentation and Rockwell) [7]. However, Vickers microhardness is more satisfactory for monitoring degradation of rigid materials. 

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... 

The Vickers method consists of replacing the steel ball in the Brinell hardness tester by a tetrahedral diamond pyramid with a dihedral angle 2y = 136 + 0.5° (Fig. 4.3.3, Table 4.3.1). Measurement involves applying the following loads to the pyramid as required 9.8, 19.6, 24.5, 29.4, 49, 98, 196, 291, 490 or 980 N and in measuring the diagonal of the indent obtained. Vickers hardness is the ratio of load P to lateral surface of indent... 

Fig. 4.3.14. Plexiglass, differential interference contrast, grey of 1st order, Vickers indenter, load 122.5 mN, filter 546 mm, measurement magnification 260 x, photograph enlargement 400 x. (After OPTON Feintechnik, Oberkochen)...

The advantages of the MHP hardness tester can be summarized in the following points (Gahm, 1968). 1. Unvariable feed of load in three switch-able load ranges, with an accuracy better than 2% 2. A fully exploitable aperture of the objective—0.9 3. Exchangeable Vickers and Knooppyra-... 

Vickers hardness with various applied loads P... 

Fig. 4.3.29. Nomogram for determination of hardness HV. load P, diagonal d and indent t of Vickers pyramid. (After Glazov and Vigdorovich, 1969)...

Figure 6.1.3 gives as an example relevant data regarding the influence of load applied to a Vickers pyramid, for the changes occurring on the surface of brittle materials around the indentation (Srebrodolski and Yushkin, 1966). Under analysis were four faces of sulphur crystals at... 

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. 

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