Vickers testing

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). 

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. 

Minimum thickness requirements are 11/2 times the indentation diagonal measurement, and there should be no visible marking or bulge visible on the side opposite the indentation.The Vickers test is based on a plane surface however, correction tables are available for both convex and concave surfaces (see ASTM E92) (2). 

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 Vickers test for hardness, which is the most quantitative, the indentation left by a diamond stylus under a fixed load is measured. The hardness number can be expressed in pressure units, usually kg/mm. This test, and the scratch test, are irreversible. That is, the sample does not return to its original state. The deformation is said to be plastic, rather than elastic. 

The transition pressure at which semiconductors become metals has been measured for a number of 4-4, 3-5 and 6-2 solids. Gilman showed that these numbers are a linear function of the hardness numbers measured by the Vickers test. This test measures the resistance to a compressional force, acting in one... 

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... 

As noted for rubbers, micro tests using scaled-down indentor sizes and smaller loads are being used, and special mention should be made of the micro Vickers test [18]. which has been applied successfully to plastics (sec Ref. [19] for a review) in the determination of crystallization effects, anisotropy, the effect of weathering and heat aging, the study of polymer blends, etc. 

FIGURE 3.42 Indentation hardness tests, (a) Brinell test, (b) Vickers test, (c) Knoop test. 

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. 

It is the Vickers test which is most commonly used to specify the hardness of hard materials, with results often quoted in kgmm , but more correctly in SI units, GPa. 

Hardness was found by resistance to indentation of diamond pyramid into a material (Vickers test). The said procedure is notable for its relative simplicity, reproducibility and is supported with standard industrial instruments. Samples were tested with the help of PMT-3 hardness tester. Measured for each sample was the diagonal of the diamond pyramid indent in the material, its length depending on the sample hardness. It is the diagonal length, expressed in relative units (tied up with the use of a specific instrument), that is denoted as hardness index, Kh. 

The Vickers Hardness Test is similar to the BrineU method, with a square based pjrramid used as the indentor. As in the BrineU test, the Vickers number is the ratio of the load to the surface area of the indentation in kUograms per square miUimeter. An advantage of the Vickers test is the increased accuracy in determining the diagonal of a square as opposed to the diameter of a circle. 

Although Vickers test method is different than BrineU, the scales are identical up to about a hardness of 300. The Vickers test is less prone to the errors produced by the BrineU system because a diamond square based pyramid is used, which does not deform as easily as a baU. 

Fig. 2.12 Schematic indentation of a Vickers test before and after indentation. Crack resulting from the indentation are shown...

For normal Vickers testing, the layer thicknesses should be at least 5-10 pm. Better conditions are obtained with Knoop pyramids, which feature a more favorable ratio of diagonal length to penetration depth. One particular problem is the difiiculty in recognizing the features of the indentations, which can be severely exacerbated by the roughness and morphology of the surface (Fig. 158). The individual hardness testing technique to be applied will depend on the types of layers involved and their dimensions. 

An outstanding property of DLC is its hardness. Vickers hardness ranges from 2000 to 9000 kg/mm. The large spread is due in part to the difficulty of testing thin coatings by indentation such as the Vickers test, since it is difficult to eliminate the substrate effect. Hardness also varies with the structure and composition. 

The two microindentation hardness testing techniques are the Knoop and Vickers tests. Small indenters and relatively light loads are employed for these two techniques. They are used to measure the hardnesses of brittle materials (such as ceramics) and also of very small specimen regions. 

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