Hardness testing of ceramic materials

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. 

The Vickers hardness is calculated from the load P in kgf (1 kgf = 9.81 N) and the mean d of the two diagonals d and J2 of the indentation, expressed as load per unit area of the indentation  

The Vickers hardness is specified with the calculated numerical value, followed by the abbreviation HV and the load P in kgf. Decimal values are omitted from hardness values greater than 100. Because the Vickers hardness is dependent on the load, the load should always be specified along with the hardness value. 

Examples A ceramic macrohardness of 1600 HVIO has a value of 1600 tested under a load of 10 kgf (98.1 N). 

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Hardness is determined by hardness tests which involve the measurement of a material s resistance to surface penetration by an indentor with a force applied to it The indentation process occurs by plastic deformation of metals and alloys. Hardness is therefore inherently related to plastic flow resistance of these materials. Brittle materials, such as glass and ceramics at room temperature, can also be subjected to hardness testing by indentation. This implies that these materials are capable of plastic flow, at least at the microscopic level. However, hardness testing of brittle materials is frequently accompanied by unicrack formation, and this fact makes the relationship between hardness and flow strength less direct than it is for metals. 

The Rockwell hardness tester is designed for measurement of fine- and medium-grained rocks and ceramic products of moderate hardness. In engineering, excluding metallurgy, it has wide uses in the hardness testing of files and grinding wheels with ceramic binder, as well as soft materials (scale Hrd and others). 

The comparability of results has prompted the use of the Grindo-Sonic method in hardness tests of other ceramic materials and has led to attempts... 

Fig. 8.5..,EfFects of dynamic abrasion hardness testing of some ceramic materials with Si02 grit.

A practical and relatively easy method for obtaining mechanical properties is by indentation tests. For ceramics, the most common methods of performing such tests are the Vickers and the Knoop hardness tests, which are very attractive, practical and relatively less expensive than the other tests discussed in Chap. 1. Assessing the toughness of ceramic materials by means of indentation testing is often done, also due to the ease of performance and low cost of conducting the measurements. Vickers hardness can be expressed [7] as ... 

Ceramic materials are expected to be applied to the structural parts, especially to tribological application, as they have high hardness and high elastic constants. For this application, it is important to know the properties of mechanical behavior on the surface of ceramics. Instrumented indentation is one of the tests that can explain the mechanical properties on the surface, as it expresses the relation between the penetration depth of the indenter and the applied force.For metallic materials, the standard test method in instrumented indentation is already published. We have tried the instrumented indentation tests on ceramic materials. Figure 1 is a typical example of the loading-unloading curve by this test on alumina ceramics. 

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... 

As well as being a good way of measuring the yield strengths of materials like ceramics, as we mentioned above, the hardness test is also a very simple and cheap nondestructive test for (Ty. There is no need to go to the expense of making tensile specimens, and the hardness indenter is so small that it scarcely damages the material. So it can be used for routine batch tests on materials to see if they are up to specification on without damaging them. 

Tests for indention under load are performed basically like the ASTM measure the hardness of other materials, such as metals and ceramics. There are at least four popular hardness scales in use. Shore A and Shore D is for soft to relatively hard plastics and elastomers. Barcol is used from the mid-range of Shore D to above it as well as RPs. Rockwell M is used for very hard plastics (Chapter 5, MECHANICAL PROPERTY, Hardness),... 

Other methods of determining the hardness of a material include a variety of "penetration" tests that yield hardness values measured in scales known as the Brinell, Rockwell (B or C), and scleroscope scales. These scales provide reliable hardness values for most materials, including ceramics, glass, metals and alloys, and wood (see Table 21). Unfortunately, as can be seen in the table, the various tests provide somewhat different hardness values for the same materials. 

Although in practice the formation of inner cracks requires a certain threshold loading, for most brittle ceramic materials this threshold is negligibly small (usually less than 1 newton, seen clearly in hardness tests). It is thought that cracks make well defined spheres entirely beneath the contact zone, and that they grow downwards as the load is applied. Such a system presents a complicated elastic-plastic problem. 

There are many methods of hardness testing, fairly widely used, depending on tradition and laboratory equipment. Moreover, the materials to be tested vary in nature owing to their particular structure, chemical properties and texture. A proper choice of test method for a given type of materials is therefore a fundamental problem in petrotechnical estimation of materials, rocks and products of their technological processing, chiefly ceramic materials (Katz and Lenoe, 1976). 

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