Diamond Vickers hardness

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). 

The Vickers hardness test, developed in the United Kingdom, is more popular there than in the United States. VHN (Vickers hardness number) and DPH (diamond pyramid hardness) are synonymous terms. 

Hardness. The Knoop indentation hardness of vitreous sihca is in the range of 473—593 kg/mm and the diamond pyramidal (Vickers) hardness is in the range of 600—750 kg/mm (1 4). The Vickers hardness for fused quartz decreases with increasing temperature but suddenly decreases at approximately 70°C. In addition, a small positive discontinuity occurs at 570°C, which may result from a memory of quartz stmcture (165). A maximum at 570°C is attributed to the presence of small amounts of quartz microcrystals (166). Scanning electron microscopic (sem) examination of the indentation area indicates that deformation is mainly from material compaction. There is htfle evidence of shear flow (167). 

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... 

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 .)... 

Superhard materials implies the materials with Vickers hardness larger than 40 GPa. There are two kinds of super-hard materials one is the intrinsic superhard materials, another is nanostructured superhard coatings. Diamond is considered to be the hardest intrinsic material with a hardness of 70-100 GPa. Synthetic c-BN is another intrinsic superhard material with a hardness of about 48 GPa. As introduced in Section 2, ta-C coatings with the sp fraction of larger than 90 % show a superhardness of 60-70 GPa. A typical nanostructured superhard coating is the heterostructures or superlattices as introduced in Section 4. For example, TiN/VN superlattice coating can achieve a super-hardnessof56 GPa as the lattice period is 5.2 nm[101]. 

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... 

The Mackensen blower method is found to be harder to relate to Vickers hardness determined by diamond indenter, although analysing the results of tests carried out by the two methods on ceramic grinding wheels using alundum as the abrasive (A. Szymanski, 1968c), results qualitatively similar are observable (Fig. 4.5.12). Alundum grains, size 8 (3.15-2.50 mm), were moulded in a ceramic binder and fired at various temperatures ranging from 1470 to 1630 K, with one-hour time delay. Polished sections were... 

Table 10.4 Comparison of Vickers hardness and fracture toughness for cemented carbide sintered with and without SiC-coated diamond particles...

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. 

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. 

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

Due to its simplicity, its nondestructive nature, and the fact that minimal machining is required to prepare the sample, the use of the Vickers hardness indentations to measure Ki. has become quite popular. In this method, a diamond indenter is applied to the surface of the specimen to be tested. Upon removal, the sizes of the cracks that emanate (sometimes) from the edges of the indent are measured, and the Vickers hardness H in GPa of the material is calculated. A number of empirical and semiempirical relationships have been proposed relating Ki., c, Y, and //, and in general the expressions take the form... 

Bulk density of specimens was measured by the Archimedes principle. The Vickers hardness as well as indentation fracture toughness were determined at room temperature using a Vickers diamond indenter at a 10 kg load for 10 s. Microstructure observation was performed under a transmission electron microscope (TEM JEOL, Tokyo, Japan JEM-2010/200CX/21 OOF). Optical transmissions of the translucent samples in wavelength of 2.5-6.5pm were measured by FTIR spectrometer (EQU1NOX55, Bruker, Billerica, MA). 

Very hard CtjOj deposits (—1900 Vickers hardness) with excellent wear resistance and low friction coefficient are obtained by plasma spraying. The deposits have low porosity and may be diamond ground to an excellent surface finish. 

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... 

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). 

Figure 1. Vickers hardness of selected materials in comparison with that of the heterostructures and nanocrystalline composites. Notice that the hardness of diamond depends on the quality and purity of the crystal (see text).

The relation between Mohs hardness, M, and the Vickers hardness, V, is not a clear one. However, if diamond (M = 10) is omitted from the Mohs scale, the relationship... 

The Vickers hardness of polished CS-SPSed P-SiAlON compacts was measured using a Vickers microhardness tester with a diamond indenter of regular pyramid with an opposite angle of 136°. The experiments were performed under the loads between 0.981 N (0.1kg) and 19.614N (2.0kg) at room temperature. The dwell time for each load was 20 s. An average of at least five readings at different locations of the specimen surfaces was taken for each specimen. The Vickers hardness GPa) was calculated according to Equation 4.4... 

Some 3D Cgo polymers have been reported to be extremely hard and have bulk moduli even larger than those of diamond. Blank et al. [73-75] reported that the graphitized Cgo at high pressures and temperatures >700 °C could scratch the (111) crystallographic plane of diamond, and called it ultrahard fulleride. The micro-Vickers hardness Hv, kg/mm ) measured on various Cgo polymers are shown in Fig. 7.27. The monomer Ceo crystal is as soft as Hv = 15, and the 2D polymers have the values of the order of Hv = 200-300. The 3D polymer crystals having a density of 2.6-2.8 g/cm are in a range of Hv = 3,500, 500, comparable to c-BN values. The R3 3D polymer having 12 coordination units is the hardest polymer. 

Vickers hardness n. A test similar to that of brinell hardness using an indenter in the form of a square-based diamond pyramid, with a vertex angle of 136° between the opposite faces. The result is expressed as the applied load divided by the projected area of the impression. 

There are a variety of types of hardness tests, but the most popular in the case of rare earth metals seems to have been the Brinell test and the Vickers, or Diamond Pyramid Hardness (DPH) test as it is most commonly called. The DPH test is perhaps the most reliable hardness test and, in fact, should give hardness values in close agreement with the results of properly conducted Brinell and Knoop (when loads are >500 g) hardness tests. Consequently, the emphasis in this section is on DPH data, but Brinell data are also presented where they are instructive. Frequently, microhardness data are quoted in the rare earth literature. Since they are determined with a diamond pyramid indenter using lower loads than macrohardness, the author includes them with DPH values when they are in agreement with macroscopic values although it is recognized that microhardness results can be load sensitive. 

HV measurement of bulk nano-twinned cubic boron nitride (henceforth nt-cBN) samples with a standard square-pyramidal diamond indenter is shown in Fig. 9.19. An explanation of the connection between this figure and observations of the inverse H-P effect are found below. Reliable hardness values are best determined from the asymptotic hardness region. In Fig. 9.19, the variations in Vickers hardness are recorded by applying a series of loads. The asymptotic hardness value obtained at loads above 3 N was extremely high, 108 GPa, which... 

The hardness of a material quantifies its resistance to permanent shape changes induced by applied mechanical forces such as fiiction or indentation by a sharp object. Scratch resistance is often measured on the Moh 1-10 scale and indentation hardness on the Vickers scale. Single crystal nonporous silicon has a Moh scale hardness of 6 (for comparison, diamond is 10, quartz is 7, calcium carbonate is 3, talc is 1). It has a Vickers hardness of 11.5 GPa. There have now been a few studies of the indentation hardness of porous silicon, with typical values for different types of porous silicon listed in Table 2. 

Hardness is the resistance that a body offers to penetration by a harder body. Polished sections of ceramic materials are primarily tested by the Vickers or Knoop methods, both of which can be described as indentation hardness testing methods (Fig. 143). The indenter used in Vickers hardness testing is a four-sided diamond pyramid with an included apical angle of 136°. The diamond indenter used in the Knoop method is also a four-sided pyramid, but this indenter has two different included apical angles, measuring 130° and 172°30. This causes an elongated indentation in the material being tested. 

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