Hardness Tests in Ceramics

As is known, hardness , H, is defined as the ratio of applied load to the projected area of indentation and is generally expressed as  

The indenters and their indentation shapes vary widely, leaving different impressions. The appropriate dimensions of these impressions on the surfaces are measured to obtain interesting research data. Conversion charts, from hardness to other properties of interest, appear in the literature. Basically, glass and ceramic hardness tests are carried out using static methods, usually by means of a diamond indenter and low test loads, because brittle materials tend to propagate cracks. 

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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... 

Most ceramics have enormous yield stresses. In a tensile test, at room temperature, ceramics almost all fracture long before they yield this is because their fracture toughness, which we will discuss later, is very low. Because of this, you cannot measure the yield strength of a ceramic by using a tensile test. Instead, you have to use a test which somehow suppresses fracture a compression test, for instance. The best and easiest is the hardness test the data shown here are obtained from hardness tests, which we shall discuss in a moment. 

The common tests are shown in Fig. 17.2. The obvious one is the simple tensile test (Fig. 17.2a). It measures the stress required to make the longest crack in the sample propagate unstably in the way shown in Fig. 17.3(a). But it is hard to do tensile tests on ceramics - they tend to break in the grips. It is much easier to measure the force required to break a beam in bending (Fig. 17.2b). The maximum tensile stress in the surface of the beam when it breaks is called the modulus of rupture, o for an elastic beam it is related to the maximum moment in the beam, M by... 

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),... 

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). 

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. 

This relation is shown in Fig. 6.3.4 for the example of Si ceramics. The threshold parameters of plastic deformation in hardness tests can be determined to a good approximation according to Evans and Law (1977) from... 

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... 

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). 

In practice, tensile tests on ceramic objects are hardly ever done because it is difficult to clasp them between the jaws of the testing equipment. The object is damaged by the jaw and a crack is very likely the result and this could be the place where the sample will break. The price of the test dumbbells is also a drawback of this type... 

H—Hardness. There are different types of hardness. Why Because the value of a material s hardness depends on how it is tested. The hardness of a material is its resistance to the formation of a permanent surface impression by an indenter. You will also see it defined as resistance of a material to deformation, scratching, and erosion. So the geometry of the indenter tip and the crystal orientation (and therefore the microstructure) will affect the hardness. In ceramics, there tends to be wide variations in hardness because it involves plastic deformation and cracking. Table 16.4 lists hardness values on the Mohs hardness scale, a scratch test that can be used to compare hardness of different minerals. For example, quartz has a Mohs hardness of 7, which made flint (a cryptocrystalline quartz) particularly useful in prehistoric times for shaping bone (the mineral component is apatite with hardness 5) and shell (the mineral component is calcite with hardness 3). Mohs hardness scale was not the first scratch hardness technique. As long ago as 1690, Christian Huygens, the famous astronomer, had noticed anisotropy in scratch hardness. 

Measuring the hardness of a ceramic is important and this is usually done using an indentation test. The basic idea is that a permanent surface impression is formed in the material by an indenter. We then measure the actual or... 

How does the hardness depend on the surface preparation Do ground ceramics (which are important in technical use) exhibit the same hardness as polished samples (often preferred for tests in development procedures) ... 

In ceramics, the dependence of the strength on flaws is a well-known phenomenon associated with stress concentrations that cannot relax by plastic deformation in these brittle materials. Does hardness testing and the development of sintered materials with an increased hardness also have to consider flaw populations ... 

Figure 3. Effect of the crystallite size in the Vickers hardness of AI2O3 ceramics (HVIO testing load = 10 kg).

The grain size-hardness relationship in Fig. 3, published first in 1995 [9], has opened new prospects for improved tool materials. Until that time it had not been clear whether reducing the grain size of sintered alumina ceramics would cause a technically significant increase in hardness in the USA it was accepted that the hardness increases down to crystallite sizes of 2-4 xm, whereas smaller grain sizes do not contribute to an improved hardness [10]. But even people with a different opinion did not have a chance to test it because more fine grained microstructures could not be prepared until the early nineties. 

The cutting performance of these new materials was tested in a direct comparison with similarly hard but single phase (absolutely oxidation resistant) pure alumina ceramics. The results are discussed in Section 4.4.4. 

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