Hardness microindentation

Bulk ceramics are made into the desired shape by reaction in situ, or by "forming" powders into the desired shape, and then sintering to form a solid body. However, ceramic thin films can be made by chemical or physical deposition. Grains, secondary phases, grain boundaries, pores, micro-cracks, structural defects, and hardness microindentions consist of the microstructure of the ceramics, which are generally indicated by the fabrication method and process conditions. 

Measurement of hardness (qv) at room temperature is relatively easy however, it is the hot hardness at the temperature of cutting that is of importance for tool materials. Figure 3 shows the variation of hot (microindentation) hardness of various tool materials measured at different temperatures. The various suppHers of tool materials can be found in References 11—13 and other trade Hterature. 

The present review is devoted to recent advances, mainly concerning the characterization of semicrystalline polymers, specifically polyethylene (PE) by means of microindentation hardness. The author has chosen to organize the chapter as follows ... 

Microindentation hardness normally is measured by static penetration of the specimen with a standard indenter at a known force. After loading with a sharp indenter a residual surface impression is left on the flat test specimen. An adequate measure of the material hardness may be computed by dividing the peak contact load, P, by the projected area of impression1. The hardness, so defined, may be considered as an indicator of the irreversible deformation processes which characterize the material. The strain boundaries for plastic deformation, below the indenter are sensibly dependent, as we shall show below, on microstructural factors (crystal size and perfection, degree of crystallinity, etc). Indentation during a hardness test deforms only a small volumen element of the specimen (V 1011 nm3) (non destructive test). The rest acts as a constraint. Thus the contact stress between the indenter and the specimen is much greater than the compressive yield stress of the specimen (a factor of 3 higher). 

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... 

B. W. Mott, Microindentation Hardness Testing, Btterworths, London (1957). 

In the view of the present authors, it is necessary to consider the degree of brittleness in hardness measurements with microindenters and to specify the results as corrected hardness (Hy cor). 

Rowe, R.C. Microindentation—a method for measuring the elastic properties and hardness of films on conventionally coated tablets. J. Pharm. Pharmacol. 1976, 28, 310-311. 

The microindentation hardness technique has been used for many years for the characterization of such classical materials as metals, alloys, inorganic glasses, etc. Its application to polymeric materials was developed in the 1960s. The potential of this method for structural characterization of polymers was developed and highlighted to a large extent by the studies carried out in the Instituto de Estructura de la Materia, CSIC, Madrid. 

In the last two decades the value of microhardness measurement as a technique capable of detecting a variety of morphological and textural changes in crystalline polymers has been amply emphasized leading to an extensive research programme in several laboratories. This is because microindentation hardness is based on plastic straining and, consequently, is directly correlated to molecular and supermolecular... 

The question of whether microhardness is a property related to the elastic modulus E or the yield stress T is a problem which has been commented on by Bowman Bevis (1977). These authors found an experimental relationship between microhardness and modulus and/or yield stress for injection-moulded semicrystalline plastics. According to the classical theory of plasticity the expected microindentation hardness value for a Vickers indenter is approximately equal to three times the yield stress (Tabor s relation). This assumption is only valid for an ideally plastic solid showing sufficiently large deformation with no elastic strains. PE, as we have seen, can be considered to be a two-phase material. Therefore, one might anticipate a certain variation of the H/ T 3 ratio depending on the proportion of the compliant to the stiff phase. 

Microindentation hardness has been used to characterize the surface mechanical behaviour of two series of sintered PPP/PPS composites over a wide range of compositions. 

The results of using microindentation hardness to characterize injection-moulded PE and PET samples prepared using a range injection (processing) temperatures Tp and mould temperatures, Tmould will be presented. 

ASTM E384-07. (2007) Standard Test Method for Microindentation Hardness of Materials, American Society for Testing Materials. 

Fig. 1 Single cell elasticity measurements with microindentation technique. (A) AFM allows both live cell imaging and mechanical testing (e.g., microindentation) in near physiological conditions. The AFM tip attached to a cantilever descents slowly to a surface and causes an indentation. The depth of indentation is detected by laser light diffraction pattern. (B) Typical force-distance curves are obtained for hard and soft surfaces and usually analyzed with the classical Hertz model that relates the applied force to the indentation depth.

UO2 has a surprisingly low brittle-ductile transformation. The only observed slip system at low temperatures is lll (110), and this does not depend on stoichiometry. Sources of mobile dislocation are an issue, however, and in order to achieve deformation at temperatures below 600 °C the crystals must be pre-deformed at 600 °C. With such pre-deformed crystals, deformation to plastic strains >1% is possible with modest yield stresses, typically 80 MPa at 450°C, 110 MPa at 400 °C, and 120 M Pa at 250 °C. Attempts to deform these crystals at room temperature were not successful, although perhaps a more careful alignment of the load train might have allowed plastic deformation at temperatures below 250 °C. Microindentation at room temperature is always possible, however, and the Knoop hardness anisotropy at room temperature is also consistent with 111 slip [74]. The yield stress at 600 °C was variable, but surprisingly was not a function of the 0/ U ratio the plastic deformation... 

Thus, toughness, measured and expressed by Kic, is dependent on the elastic modulus, E, of the material, its hardness, H, (microindentation is often preferable for the proper evaluation of the indentation crack), crack length, c, and the applied load. Anstis et al. [11] employed a two-dimensional fracture mechanics analysis. The crack length, c, is measmed from the center of the impression to the crack tip in meters E is in GPa and H is the Vickers hardness in GPa. The height of the opposite triangular faces is h. It is clear that under small indentation loads, only small cracks form, as indicated schematically in Fig. 2.12. Actual Vickers indentation cracks are shown in Fig. 2.13. Equation (2.8) is often also expressed as ... 

Westrich, R.M., 1986. Use of the scanning electron microscope in microhardness testing of high-hardness materials, microindentation techniques. Materials Science and Engineering ASTM STP 889, Philadelphia, p. 196. 

The indentation test is one of the simplest ways to measure mechanical properties of a material. The micromechanical behavior of polymers and the correlation with microstrnctnre and morphology have been widely investigated over the past two decades (23). Conventional microindentation instruments are based on the optical measnrement of the residual impression produced by a sharp indenter penetrating the specimen surface under a given load at a known rate. Microhardness is obtained by dividing the peak load by the contact area of impression. From a macroscopic point of view, hardness is directly correlated to the yield stress of the material, ie, the minimnm stress at which permanent strain is produced when the stress is snbseqnently removed. 

Althongh hardness derived from residual impression measurements is an indicator of the reversible plastic deformation processes, information about elastic release of the indentation depth is mostly lost. Continuous load-displacement monitoring (as the indenter is driven into and withdrawn from the film) sub-stitntes the imaging method nsed in conventional microindenters. The need to characterize the snrface of very thin films and near surfaces has led to the development of nltra- and nanoindentation testers with indentation depths within the submicrometer scale... 

P. J. Blau. Microindentation Hardness Testing of Coatings Techniques and Interpretation of Data. In American Vacuum Society Series 2 Physics and Chemistry of Protective Coatings. (G. Lucovsky, Series Ed. W. D. Sproul,... 

Microindentation data reveals hardness profile of LDPE/EPDM blends, staying in agreement with structural data for the surface layer of systems studied. 

Lancasta microindentation tester, 50 Lanthanide dicarbides pendulum hardness, 295 Vickers hardness, 304 Lanthanum boride, LaB , 297 Lanthanum oxide (1 03) effect on silica hardness, 238 as network modifier, 238 Lanthanum silicate (La2 i207) hardness, 242 precipitate in silica, 238 Lateral vent crack, 55, 126, 148, 154, 158-161, 235 analysis of, 159-161 circular contours of, 159 critical flaw size, 154 length as a function of load, 161 and surface distortion, 159 at thin film interfaces, 205 Lattice energy and fracture energy, 198 and hardness, 22, 24 of silicon, 24... 

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