Low load hardness

US that microstructural effects must be removed or standardized before using a simple comparison of ISE indices to draw conclusions about ceramic systems. However, the following general relationships hold for the low-load hardness region, as defined in Figure 1.3 ... 

A third indentation test parameter of fundamental importance is temperature. In several studies of the temperature dependence of hardness, an activated process becomes apparent from Arrhenius-type behavior. Calculation of the activation energies associated with thermal softening is characteristic of the activated processes governing plastic flow in crystals. " Thus, the role played by the crystal structure in determining micro- and low-load hardness values must be a major one since, apart from its effect on lattice energy, the actual arrangement of the ions in a ceramic crystal is important in determining the ease of plastic flow. This aspect of ceramic hardness and its potential applications is discussed in Chapter 3. 

It should be noted at this stage that variations in micro- and low-load hardness observed as functions of indenter geometry and crystal orientation are in fact only reflections of different distributions of shear stress within the bulk of the crystal. In this respect, too, the variations in chemical bond type found in ceramics must be important since ionic bonding proves less of a barrier to plastic flow than the strictly directed covalent bonds. 

Figure 2.2. Typical Durimet micros and low-load hardness apparatus as supplied by Leitz.

Indentation has been used for over 100 years to determine hardness of materials [8J. For a given indenter geometry (e.g. spherical or pyramidal), hardness is determined by the ratio of the applied load to the projected area of contact, which was determined optically after indentation. For low loads and contacts with small dimensionality (e.g. when indenting thin films or composites), a new way to determine the contact size was needed. Depth-sensing nanoindentation [2] was developed to eliminate the need to visualize the indents, and resulted in the added capability of measuring properties like elastic modulus and creep. 

Analysis of Table II shows discrepancies in the hardness and stress behavior of a-C(N) H films. Although all the works reported a clear stress reduction upon nitrogen incorporation, the hardness sometimes is quoted as almost constant, or on the other hand clearly decreasing. In addition to the possible effect of different deposition methods and conditions, it can be easily seen that the differences in hardness testing methods are the major source for discrepancies. Constant hardness behavior is only reported with the use microindentation methods, like Vickers and Knoop microhardness. On the other hand, the use of low-load nanoindentation methods always led to a nitrogen-induced decrease in hardness. This is basically the consequence of two factors. The first one is the higher penetration... 

The Knoop test is a microhardness test. In microhardness testing the indentation dimensions are comparable to microstructural ones. Thus, this testing method becomes useful for assessing the relative hardnesses of various phases or microconstituents in two phase or multiphase alloys. It can also be used to monitor hardness gradients that may exist in a solid, e.g., in a surface hardened part. The Knoop test employs a skewed diamond indentor shaped so that the long and short diagonals of the indentation are approximately in the ratio 7 1. The Knoop hardness number (KHN) is calculated as the force divided by the projected indentation area. The test uses low loads to provide small indentations required for microhardness studies. Since the indentations are very small their dimensions have to be measured under an optical microscope. This implies that the surface of the material is prepared approximately. For those reasons, microhardness assessments are not as often used industrially as are other hardness tests. However, the use of microhardness testing is undisputed in research and development situations. 

Pt-catalyzed hydration of various aliphatic and aromatic alkynes under phase transfer conditions in (CH2C1)2/H20 in the presence of Aliquat 336 led to either a Markovnikov product, mixtures of two ketones, or ketones with the carbonyl group positioned away from the bulky side.72 In the absence of the phase transfer reagent, Aliquat 336, hardly any reaction took place. Recently, a hydrophobic, low-loading and alkylated polystyrene-supported sulfonic acid (LL-ALPS-SO3H) has also been developed for the hydration of terminal alkynes in pure water, leading to ketones as the product.73 Under microwave irradiation, the hydration of terminal arylalkynes was reported to proceed in superheated water (200°C) without any catalysts.74... 

It is observed that indentations made with low loads on an indenter are smaller than expected from the sizes made with high loads. Thus the apparent hardness of a specimen increases as the indentation size decreases. This is known as the indentation size effect (ISE). It has been given a variety of interpretations, but the most simple is that it is associated with friction at the interface between the indenter and the specimen (Li et al., 1993). 

Khrushchev (1957) considers that the need to measure the force T has not been sufficiently well substantiated, nor has a sufficiently precise and easy in service hardness tester been developed yet for determinations of this type. However, he appreciates the usefulness of scratch hardness tests, especially at low loads, as a non-destructive technique. He recommends these methods as very useful for hardness determination of metallic layers or of materials exposed to abrasive wear under operating conditions (plastics, organic coatings, such as varnishes and paints, etc.). Scratch methods are especially important in tests of anisotropic materials where a change in scratch width is the measure of anisotropy. In static indentation methods, the indentations obtained in anisotropic materials are misformed, varying... 

The possibility of testing the hardness of brittle materials at low loads has created conditions for the application of these methods to mineralogy (Bernhardt, 1941 Winchell, 1945 Taylor, 1949 Khrushchev, 1949 Tabor and Bowden, 1964). 

The instantaneous efficiency of the stack and FCS are shown in Fig. 6.29 for R47 cycle in hard hybrid conhguration and in Fig. 6.30 for R40 cycle in soft hybrid configuration. According to the results obtained in steady-state operation, during the low load phases the stack efficiency reaches the value of about 0.7, whereas during the power variations required by the electric drive the efficiency decreases down to the lowest value of about 0.6, in correspondence with the most... 

Suitable for machine parts for high mechanical loads, hard bushes and sliding surfaces, worm gears for low velocities, and for propeller sockets and screw shaft bushes. 

From the engineering perspective, a hardness test is an ideal method for monitoring the mechanical properties of hard materials, since minimal sample preparation is required and the test can be performed on actual components using simple apparatus operated at low loads. The hardness test may also be considered nondestructive , since components can often be put into service after testing. The mechanical performance of hard materials is often ranked by their indentation hardness, which in itself accounts for both the popularity and the technological success of this simple, cost effective test. 

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