Hardness measurements

Hardness is a measurement of material resistance to plastic deformation in most cases. It is a simple nondestructive technique to test material indentation resistance, scratch resistance, wear resistance, or machinability. Hardness testing can be conducted by various methods, and it has long been used in analyzing part mechanical properties. In reverse engineering, this test is also widely used to check the material heat treatment condition and strength, particularly for a noncritical part, to save costs. The hardness of a material is usually quantitatively represented by a hardness number in various scales. The most utilized scales are Brinell, Rockwell, and Vickers for bulk hardness measurements. Knoop, Vickers microhardness, and other microhardness scales are used for very small area hardness measurements. Rockwell superficial and Shore scleroscope tests are used for surface hardness measurements. Surface hardness can also be measured on a nanoscale today. 

Vickers microhardness is most widely used for microhardness fests of thin coatings. The Shore scleroscope hardness test is a dynamic test that measures rebound height/energy as an indicator of surface hardness by dropping a test hammer onto the surface. The rebound height/energy is heavily dependent on the material elasticity therefore, the Shore hardness should only be 

In light of the complexity and variance of hardness measurement, it is essential that the hardness numbers are measured on the same scale as specified in the reference material specification for direct comparison. If not 

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Hardness can also be calculated by summation of the individually deterrnined alkaline earths by means of atomic absorption analysis. Basic samples must be acidified, and lanthanum chloride must be added to minimise interferences from phosphate, sulfate, and aluminum. An ion-selective electrode that utilizes ahquid ion exchanger is also available for hardness measurement however, this electrode is susceptible to interferences from other dissolved metal ions. 

Two pieces of work-hardened 5000 series aluminium alloy plate were butt welded together by arc welding. After the weld had cooled to room temperature, a series of hardness measurements was made on the surface of the fabrication. Sketch the variation in hardness as the position of the hardness indenter passes across the weld from one plate to the other. Account for the form of the hardness profile, and indicate its practical consequences. 

There is space here for a brief account of only one technique, that is, hardness measurement. The idea of pressing a hard object, of steel or diamond, into a smooth surface under a known load and measuring the size of the indent, as a simple and... 

As also discussed by Maugis and Pollock, the hardness of the material is related to its yield strength Y by H = 2>Y. The factor of 3 is a consequence of the deformation constraints of the indentor geometry used in hardness measurements. In the absence of an applied load, the MP theory predicts that... 

Pethica, J.B., Hutchings, R. and Oliver, W.C., Hardness measurement at penetration depths as small as 20 nm. Philos. Mag. A-Phys. Condens. Matter Struct. Defects Mech. Prop.,A%, 593-606(1983). 

Tests on a wide range of alloys at temperatures varying from 704 to 927°C have been made by Bernsen et al." to determine the temperature limits beyond which engineering materials carburise when held in contact with graphite. Table 7.27 lists the maximum penetrations of the carburised zones while nickel in general showed no visible evidence of carburisation the associated hardness measurements indicated solution of carbon even at 704°C. At this temperature the chromium-containing alloys showed little tendency to carburisation, but at 816°C carburisation leading to the formation of chromium carbide was rapid. 

There are still other surface analysis techniques including ellipsometry, surface enhanced Raman scattering, light scattering, nano-hardness measurements etc. which are used for specific investigations. It is, however, already evident from this discussion that many new and powerful techniques now are available which offer the capability of investigating various aspects of polymer surfaces on a molecular level. Some of those techniques are surface specific while others can be used for the analysis of buried interfaces, too. 

The first section involves a general description of the mechanics and geometry of indentation with regard to prevailing mechanisms. The experimental details of the hardness measurement are outlined. The tendency of polymers to creep under the indenter during hardness measurement is commented. Hardness predicitions of model polymer lattices are discussed. The deformation mechanism of lamellar structures are discussed in the light of current models of plastic deformation. Calculations... 

The Vickers hardness measurement uses a square based pyramid of about 100 pm height as the indenter. 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 ball1 . The hardness value is equal to the applied force P in newtons divided by the actual area of impression in mm2. 

O Neill, H. Hardness Measurement of Metals and Alloys, Chapman Hall, London 1967... 

Elastomer samples are cast in molds, the size and shape of which depend on its purpose. Samples for physical properties can be produced using a custom-made book mold designed to create a thin sheet (0.1 in.) containing premolded test parts, such as those for die-C tear, flexural modulus, and so on. Alternatively, a flat plaque mold may be used to create a 6 x 6 x 0.1-in. sheet from which may be cut samples for testing. Thicker samples for hardness measurements may... 

Hardness measurement, 243 Havriliak and Negami (HN) function, 388 HCFCs. See Hydrochlorofluorocarbons (HCFCs)... 

A lack of an explicit concentration value indicates, that the value of iip c did not depend upon the concentration. DR = Double layer repulsion method, H = Hardness measurements, T = Tensammetiy (for details see text). See also list of symbols. 

The research work on carbon nitride materials was first motivated due to its hypothetical super-hardness. The tribological properties of the CNx films are, of course, the emphasis of the succeeding research works. Although the expected structure and hardness have not been achieved up to the present, the potential interesting properties still stimulate many approaches on the tribological behavior of CNx films. Figure 8 gives the results of hardness measurement reported by the... 

Indentation hardness using modified tests based on Brinell hardness measurements have been used by some researchers [148] to provide information on the surface hardness of tablets. In addition, these tests are capable of providing a measure of a tablet s plasticity or elasticity. For the most part, such tests have been confined to basic research applications in a few laboratories, but their value is beginning to be more widely recognized. 

There are at least four types of chemical bonding. Some crystals have open atomic structures, while others are close-packed. Also, many crystals are anisotropic. Therefore, although making hardness measurements is relatively simple, understanding the measured values is not simple at all. 

This model is not precise, but does identify some of the factors that are important to indentation. Like the model, the hardness measurement process is not precise. At the micro-hardness level, the projected areas of indentations are measured, but this can only be done with about 10% accuracy. At the nano-indentation level, relative values can determined accurately, but absolute values are probably only about 10% accurate. 

The flow stress in Figure 2.6 is given approximately by Y = Y0 + h8 where h is the deformation-hardening coefficient. It is assumed that the elastic strain is fully recovered in a hardness measurement, so it need not be considered further in this approximate treatment. Then H = 2.2h8, and since 8 = 2x/L = tan 0 = 0.414 then H = 0.89 h. Hence H — h. That is, the indentation hardness number approximately equals the deformation-hardening coefficient. 

Other semiconductor crystals for which photoplasticity has been observed during hardness measurements include III-V compounds (such as GaAs— Koubaiti et al., 1997), and II-VI compounds (such as ZnS and ZnO—Klopfs-tein et al., 2003). Flowever, since the effect declined in these studies with the depth of indentation, it is likely that the observations are artifacts associated with changes of the indenter/specimen friction coefficients. An extensive review of photoplastic effects in II-VI compounds is given by Osip yan et al. (1986). 

Hardness measurements of non-metallic solids are influenced by environmental factors. These have been studied extensively by Westwood (Westwood et al., 1981) and others. However, the evidence is that most, if not all, of the observed effects result from changes in the indenter/specimen friction coefficient caused by adsorption. Under ambient conditions, water vapor is commonly adsorped (Hanneman and Westbrook, 1968). In the presence of various liquids both solvents and solutes are adsorped. Since the effects are not intrinsic to the specimens, they will not be discussed further here. 

This is an example of the use of hardness measurements for interpreting other properties. 

Because of the sensitivity to impurities, hardness measurements within the 20 member set tend not to be systematic. One trend that is clear, however, is that hardness decreases with increasing polarizabilty (Figure 9.1). 

Hardness values for the prototype diborides are listed in Table 10.1. Most hardness measurements for diborides have been made for sintered specimens thus, they vary from one author to another. The values listed are the highest ones reported. Average values have little meaning in this case. 

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