Knoop indenters

In time most commercially available microhardness testers accepted both Vickers and Knoop indenters. The Vickers remained almost universally used in Europe but shared acceptance with the Knoop in the United States. 

Hardness. The Knoop indentation hardness of vitreous sihca is in the range of 473—593 kg/mm and the diamond pyramidal (Vickers) hardness is in the range of 600—750 kg/mm (1 4). The Vickers hardness for fused quartz decreases with increasing temperature but suddenly decreases at approximately 70°C. In addition, a small positive discontinuity occurs at 570°C, which may result from a memory of quartz stmcture (165). A maximum at 570°C is attributed to the presence of small amounts of quartz microcrystals (166). Scanning electron microscopic (sem) examination of the indentation area indicates that deformation is mainly from material compaction. There is htfle evidence of shear flow (167). 

Knoop indenters are particularly useful for studies of the anisotropies of indentations on surfaces because of their elongated shape which gives an indenter two-fold symmetry. For studies of minerals this is quite useful and has been discussed in some detail by Winchell (1945). 

N. Vaidya and B. S. Shah, Absolute Hardness of Phenanthrenedoped Anthracene Single Crystals by Knoop Indentation Technique, Indian J. Phys., 77A, 59 (2003). 

G-17216, and in Victoria Land by John Mulligan, also on an NSF grant to the Bureau of Mines. Each sample was crushed by hand with a mortar and pestle to approximately 0.625 by 0.25 inch. A split of each crushed sample was embeaaed in epoxy resin, polished, examined microscopically, photographed, and then its reflectance was determined. Knoop indention hardnesses were also determined by using a 20-gram load on the diamond indenter. The indenter was kept in contact with the sample for 15 seconds. Additional splits of each sample were crushed to minus 60 mesh, dried at 100°C. for 24 horns, and their electrical resistivity was determined at 20,000 p.s.i. The sample locations, chemistry, petrography, reflectance, electrical resistivity, and hardness of the 36 Antarctic coal samples studied are included in Table I. 

Knoop indenter (pyramid)—longer diagonal of a rhombus,... 

The dimensional ratios of indentations obtained with the first three, most common, indenters, taking indentation depth to be 1, are for the measured parameters 7—with the Vickers indenter 30.5—with the Knoop indenter 7.4—with the Berkovich indenter. 

Fig. 4.3.12. Steel Cl 5, Knoop indenter, load 981 mN, filter 546 mm, measurement magnification 260 x, photograph enlargement 400 x. (After OPTON Feintechnik, Oberkochen). 

As noted above, either a Vickers or Knoop indenter may be used as required, and in some cases, one of the two proves absolutely indispensable. 

Noting the advantages of both Vickers and Knoop indenters, leading manufactures supply the market with hardness testers equipped with both types of indenter. Discussed below are hardness testers, in which to avoid... 

The Micromet M-ll hardness tester supplied by A. J. Buehler Inc. of the United States is fitted out with two types of Vickers and Knoop indenters, an automatic load feeder and a light signalling device, allowing measurement to be made only after elastic recovery of the sample (Fig. 4.3.25). 

A clear crystallochemical interpretation of hardness anisotropy measurement results, especially for monocrystals, makes it possible to estimate the structural homogeneity of crystal. Button et al. (1979) testing the hardness of cubic sodium tungsten bronzes Na W03 (where 0.4 < x < 0.75) with the Knoop indenter, found the hardness of W03 to rise from 450 to 844 within a highly differentiated hardness anisotropy for various values of the Na+ ion. This variation is the outcome of differences in atomic spacings in crystals. 

The problem of recovery leads to the question of hardness. Hard substances have a high number of strongly directed, covalent chemical bonds per unit volume. Soft substances generally have fewer bonds per unit volume or bonds that are weak or weakly directed, such as ionic or dipole attractive forces. Bond energy per unit volume has the same dimensions as pressure (force per unit area), and a plot of hardness measured by the Knoop indenter versus the bond energy per molar volume for various substances is essentially linear, provided that one chooses substances for which the bonding is predominantly of one type (i.e., not mixed, as in graphite or talc). 

Cook suggests that water entry under the load imposed by the indenter is similar to water entry under the load imposed by the abrasive particle. As the abrasive particle moves across the surface, a strain field develops in the glass surface due to the load and velocity of the particle, with compressive strain in front of the particle and tensile strain behind the particle. In front of the particle, hydrostatic pressure leads to water entry into the oxide as the abrasive pushes water into the surface. However, diffusion of water into the oxide is inhibited by the compressive strain occurring in front of the particle. The diffusion coefficient of water decreases exponentially with compressive strain and increases exponentially with tensile strain. The difference between the traveling indenter (i.e., the abrasive particle) and the static indenter (i.e., the Knoop indenter) is the tensile strain that occurs in back of the traveling indenter leads to accelerated diffusion of water into the oxide. Thus, one of the functions of the abrasive particle is to pump water into the oxide surface. Water enters the oxide under the influence of the hydrostatic pressure in front of the particle and diffuses further into die oxide in back of the particle. The depth to which water diffuses into the oxide is a function of abrasive particle... 

Infrared (IR) orbance spectra at various locations of a Knoop indentation mark made in air. (From Ref. (4).)... 

The brazed Joints were mounted in epoxy, ground, polished, and examined using Field Emission Scanning Electron Microscopy (FESEM) (model Hitachi 4700) coupled with energy dispersive x-ray spectroscopy (EDS). Microhardness scans were made with a Knoop indenter across the joint interfaces on a Struers Duramin-A300 machine under a load of 200 g and loading time of 10 s. Multiple (4 to 6) hardness scans were made across each joint to check the reproducibility. 

In situ conductivity measurements by Clarke et al. [6] (Vickers and Knoop indenters) and by Pharr et al. [8] (Berkovich indenter) showed a drop in Ge resistance during indentation with sharp tips and thus provided the first experimental support to the notion of indentation-induced metallization in germanium. The TEM analysis of the indented Ge samples by Clarke et al. [6] revealed amor-phized material within the indentations, similar to the results obtained on silicon. 

C1326-99 Test Method for Knoop Indentation Hardness of Advanced Ceramics... 

Knoop hardness number, KHN n. The indentation hardness determined with a Knoop indenter, and calculated as follows ... 

Knoop indenter n. Pyramidal diamond of prescribed dimensions used for testing the indentation hardness of organic coatings. In a more restricted sense, a type of diamond hardness indenter having edge angles of 172°, 30°, and 130°. www.astm.org. Brown R (1999) Handbook of physical polymer testing, vol 50. Marcel Dekker, New York. 

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