Indentation hardness load effect

The susceptibility of hardness measurements of silica and silicate glasses to environmental factors is consistent with the effects of water on the deformation of quartz. The load effect and indentation size effect appear to be a result of the frictional forces at the indenter-specimen interfaces. 

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. 

As other mechanical properties also the hardness shows a strong dependence on temperature (Fig. 4.195) and loading time (Fig. 4.196). With increasing temperature and holding time for example the ball indentation hardness HB is decreasing due to the influence of relaxation and retardation effects. 

Clearly all indents made with a pyramidal indenter should have the same shape regardless of their size. Thus, since we take pressure used to make this shape to be a measure of hardness—see equations (1.6) and (1.7)—we would expect hardness to be the same and there to be no load effect. Therefore when hardness increases as the applied load decreases, as shown in Figure 1.3, it must be because the volume of material used to yield is smaller and the mechanism for yielding is dependent on a volume term which becomes more significant as the indent size decreases. The most obvious development of this idea is that the shallow near-surface volume of the deformation zone can become a significant fraction of the total affected volume when a very small load is used to make the indent. Thus, work hardened layers, surface compressed layers, ion-implanted layers, and the possibility of chemical reactions between the atmosphere and the surface can dominate the yielding mechanism to produce nonstandard hardness values. Conversely we can say that these phenomena could be studied by measuring the ISE of a ceramic. 

As equation (5.91) shows, the threshold load can be estimated from a knowledge of Hy, K, and all of which are found by the indentation hardness technique. In the case just considered, with Kc = 0.75 MN Hy = 6 GN m, ( c = 130 MN m , and A = 0.076, an estimate for P is 0.43 N. This small load over the area represented by a, is of course a large local stress, but the estimate does emphasize that small particles can generate critical flaws beneath the surface. The effect of the surface compression layer can be seen by removing the -1.6critical load decreases to 0.068 N for this glass. 

An alternative to the measurement of the dimensions of the indentation by means of a microscope is the direct reading method, of which the Rockwell method is an example. The Rockwell hardness is based on indentation into the sample under the action of two consecutively applied loads - a minor load (initial) and a standardised major load (final). In order to eliminate zero error and possible surface effects due to roughness or scale, the initial or minor load is first applied and produce an initial indentation. The Rockwell hardness is based on the increment in the indentation depth produced by the major load over that produced by the minor load. Rockwell hardness scales are divided into a number of groups, each one of these corresponding to a specified penetrator and a specified value of the major load. The different combinations are designated by different subscripts used to express the Rockwell hardness number. Thus, when the test is performed with 150 kg load and a diamond cone indentor, the resulting hardness number is called the Rockwell C (Rc) hardness. If the applied load is 100 kg and the indentor used is a 1.58 mm diameter hardened steel ball, a Rockwell B (RB) hardness number is obtained. The facts that the dial has several scales and that different indentation tools can be filled, enable Rockwell machine to be used equally well for hard and soft materials and for small and thin specimens. Rockwell hardness number is dimensionless. The test is easy to carry out and rapidly accomplished. As a result it is used widely in industrial applications, particularly in quality situations. 

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). 

H. Li and R. C. Bradt, The Indentation Load/Size Effect and the Measurement of the Hardness of Vitreous Silica, Jour. Non-Cryst. Sol. 146,197 (1992). 

The measured hardness decreases with time of load application because rubbers are not perfectly elastic, hence results at different times will not be in agreement. Furthermore, the effect will be material dependent. The effect of time was investigated in some detail for Shore A58 several decades ago by filming both the durometer on a stand and the stopwatch. This work demonstrated the considerable creep that occurs and concluded that manual measurements should be taken at 30 sec. Measurements were also made with a dead load tester which showed less creep than the Shore because the load is constant and not increasing with indentation. 

The type and engineering quality of the measuring device actually used may have vital effect on the results. In the case of automatic feed of indenter loading, the measuring accuracy is higher than for hand-operated feed (Fig. 6.5.1). As a result, for hardness testers with hand-operated loading, it is advisable to take more measurements. 

Investigation of the effect of particulate properties during CMP of W showed a significant increase in the polish rate in the presence of ferric nitrate compared to the polish rate in de-ionized water, at all alumina bulk density values (shown in figure 4). Kaufmann et al. , attributed the increase in the polish rate in the presence of ferric nitrate to the "softness" of the passivating oxide film compared to W. Potentiodynamic experiments and open circuit potential measured as a function of time indicate passivation of W surface. However, the hardness values of tungsten films exposed for 5 min to 0.1 M ferric nitrate, even at the lowest load (300p,N), were the same as those of as-deposited W films within experimental error. Since a 10 nm indentation depth was observed at the lowest load, it is possible that the thickness of the oxide film is smaller and its effect does not manifest itself on the hardness measurement. 

Pellicle and tea-immersed pellicle were analyzed using nanoDMA (dynamic mechanical analysis) to see if the tannins had an effect on the viscoelasticity of the pellicle. NanoDMA is a technique used to study and characterize mechanical properties in viscoelastic materials. The method is an extension of nanoindentation testing [58, 59], An analysis of the nanoindentation load-depth curve gives the hardness (H) and reduced elastic modulus (E ), provided the area of contact, A, between the indenter tip and the sample is known [ 13]. By... 

Hardness is not a fundamental property. Its measurement is subjected to the effects of temperature, time and other test variables. Hardness measurement of plastics is similar to the traditional methods applied to metals. It usually employs a standard indenter (often a hardened steel ball), forcing it under known load into a flat surface of the plastic, then measuring the resultant degree of indentation. 

It may be argued [3] that the goal hardly attainable in the experiments using high-pressure diamond anvil cells could be more easily achieved in as simple an experiment as a conventional hardness test. The well-documented indentation size effect (ISE) [189] reveals itself in the following relation between the Meyer hardness HM (equivalent to the mean contact pressure) and the applied load P [190] ... 

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