Diamond pyramid

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. 

Most instmments make use of a probe geometry which gives an increasing area of contact as penetration proceeds. In this way, at some depth of penetration, the resisting force can become sufficient to balance the appHed force on the indentor. Unfortunately, many geometries, eg, diamonds, pyramids, and cones, do not permit the calculation of basic viscoelastic quantities from the results. Penetrometers of this type include the Pfund, Rockwell, Tukon, and Buchholz testers, used to measure indentation hardness which is dependent on modulus. 

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 measured by the Rockwell A-scale diamond cone iadentation test (HRA) or by the Vickers diamond pyramid iadentation test (HV). Although the Rockwell scale has been used for decades ia the carbide iadustry as a measure of hardness, a tme iadication of the resistance of the tool to deformation ia metal-cuttiag operatioas can be obtained only by measuting hardness at elevated temperatures. The hardness of cemented carbides decreases monotonicaHy with increasing temperatures. 

Nine strips of pure, fully annealed copper were deformed plcistically by being passed between a pair of rotating rollers so that the strips were made thinner and longer. The increases in length produced were 1,10, 20, 30, 40, 50, 60, 70 and 100% respectively. The diamond-pyramid hardness of each piece was measured after rolling. The results were... 

Assuming that a diamond-pyramid hardness test creates a further nominal strain, on average, of 0.08, and that the hardness value is 3.0 times the true stress with this extra strain, construct the curve of nominal stress against nominal strain, and find ... 

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. 

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... 

Position (a)—sample under objective, choice of test place or measurement of impression diagonal position (b)—setting of sample before or after making the impression position (c)—making the impression by loading. A—diamond pyramid, H—load-acting lever, C—rotation lever of turntable, G—deadweight, P—sample, 0-0—rotation axis of turntable. (After Blazewski, 1953)... 

Ainsworth L., 1954, The diamond pyramid hardness of glass in relation to the strength and structure of glass, Jour. Soc. Glass Techrt., 38, 479-500. 

Smith R. L., Sandland G. E., 1925, Some notes of the use of diamond pyramid for hardness testing, J. Iron, and Steel Inst., Ill, 285-304. 

A No known as Diamond Pyramid Hardness. Indcnter is a square-hased diamond pyramid with included angle between faces of 136°. Loads may vary1 from 1 to I 20 kilograms with 10. 30. and 50 kilograms in common use. Hardness is equal to load (kilogramsi divided hy surface area (square millimeter) of the permanent indentation, ll is determined directly from oplical measurements of the diagonals of the indentation, which appear square at Ihe surface of the metal. 

Also used lor very light loads. Consists of a spring-loaded. Vickers-type diamond pyramid indenter arranged lor use on a metallurgical microscope. 

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.

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. 

As we have seen in Section 1.1, Vickers microhardness measurement uses as the indenter a square-based diamond pyramid about 100 qm in height. The included angles between opposite faces are a = 136°. This corresponds to the tangential angle of an ideal ball impression, considered to have a diameter equal to 0.375 times that of the ball (Tabor, 1951). The pyramid is pressed under a load P into the polymer surface to form a plastic deformation. When the indenter is removed the diameter of the indentation is measured and its area determined. The mean pressure p over the indentation is then... 

According to Tabor, the ratio between the indentation pressure Pm and Y for a Vickers diamond pyramid is about 3.3 H = 0.921 Pm) (Tabor, 1979). This relation applies for materials which behave as ideally plastic solids, but fails for materials in which the elastic strains are non-negligible (March, 1964 Hirst Howse, 1969). 

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. 

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

On the other hand, Fein s [49] extensive study of transition temperatures and scuffing in pure sliding with the four-ball machine showed the same type of response to the velocity/load ratio that he found with the two-disk machine [44]. The sliding speeds ranged from 0.0002 to 68.6 cm/s, the loads from 19.6 to 88.2 N (2-90 kg). Two kinds of steel specimen sets were used hardened AISI 52100, diamond pyramid hardness 740, and heat-treated AISI 4140, hardness 270. There were 13 different lubricants, with the properties shown in Table 15-7. At low rubbing speeds the interfacial flash temperatures were negligible and the bulk temperatures of the lubricant were taken to be the transition temperatures. At speeds above 0.359 cm/s a particularized form of the flash temperature equation was used to calculate the contribution from interfacial rubbing to be added to the bulk temperature to obtain the transition temperature. 

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