Vickers microhardness indenter

In the second approach shown in Fig 3.12(b), a force is applied continuously using a Vickers microhardness indenter to compress the fiber into the specimen surface (Marshall, 1984). For ceramic matrix composites where the bonding at the interface is typically mechanical in nature, the interface shear stress, Tf, against the constant frictional sliding is calculated based on simple force balance (Marshall, 1984) ... 

Fig. 3.12. Schematic drawings of indentation (or liber push-out) techniques using (a) a spherical indenter (b) a Vickers microhardness indenter (c) on a thin slice. After Grande ct al. (1988).

Microhardness of both particulate and platelet composites was evaluated at ambient temperature with a Vickers microhardness indenter with an indent load of 9.8 N using five indents for each composite in accordance with ASTM C 1327 [28]. Figure 16 shows the results of Vickers microhardness measurements for both composites. Microhardness increased linearly with increasing alumina content for the particulate composites up to 20 mol% alumina and then leveled off above 20 mol%. Microhardness of the platelet composites remained almost unchanged up to 10 mol% and then decreased appreciably at 30 mol%, resulting in a significant difference in hardness at 30 mol% between the two composites. Individual microhardness data for both composites are also summarized in Tables 1 and 2. 

Hardness is essentially a measure of stiffness and in principle can be related to modulus. For plastics, the term hardness refers to resistance to indentation but depending on the test method the measurement is made either with the indentation load applied or after its removal when elastic recovery has taken place. The standard methods are given in ISO 868 (Shore) [6] and ISO 2039 (Ball indentation and Rockwell) [7]. However, Vickers microhardness is more satisfactory for monitoring degradation of rigid materials. 

Deformation induced by indentation. The deformation induced in single crystals of San Carlos (Arizona, USA) olivine around the in-denter in a Vickers microhardness test has been studied by Gaboriaud... 

As we have seen in Section 1.1, Vickers microhardness measurement uses as the indenter a square-based diamond pyramid about 100 qm in height. 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 ball (Tabor, 1951). The pyramid is pressed under a load P into the polymer surface to form a plastic deformation. When the indenter is removed the diameter of the indentation is measured and its area determined. The mean pressure p over the indentation is then... 

The Vickers microhardness test gives a smaller indentation for a given force than the Knoop test and is less sensitive to material anisotropy. The Knoop test is easier to use because the impression is longer for a given load and usually only one measurement per test is required. The microhardness measurement with the Knoop diamond is quite sensitive to material anisotropy because of the twofold symmetry of indentation. Nevertheless, the Vickers diamond, as we shall show below, also detects anisotropy conveniently (Balta Calleja Bassett, 1977 Balta Calleja et al, 1980a). 

Fahad Hasan, Wang and Berndt (2014) investigated the Vickers microhardness of the cross- section as well as the top surface of plasma-sprayed polished hydroxyapatite coatings. The effects of applied load, measurement direction and indent location (Figure 7.55) on the microhardness were investigated. Indentation was performed on dense and porous areas of the coatings. After Vickers indentation,... 

Nitrides and carbides are also considered among the hardest materials. Table 5.2 shows data for the measured Vickers microhardness for these compounds. Such measurements are performed using a diamond indenter with square geometry. The indenter is forced towards the surface of the material and the diagonal of the microindentation is measured. In all cases, carbides and nitrides are significantly harder than the pure metals and are also comparable or superior to that of ceramic materials. 

The Vickers hardness of polished CS-SPSed P-SiAlON compacts was measured using a Vickers microhardness tester with a diamond indenter of regular pyramid with an opposite angle of 136°. The experiments were performed under the loads between 0.981 N (0.1kg) and 19.614N (2.0kg) at room temperature. The dwell time for each load was 20 s. An average of at least five readings at different locations of the specimen surfaces was taken for each specimen. The Vickers hardness GPa) was calculated according to Equation 4.4... 

Hardness measurements such as Rockwell or Vicker s indentation properties are time-dependent as a result of the viscoelastic flow and relaxation processes (236) (see Hardness). Microhardness measurements have been used to correlate with other properties such as Yoimg s modulus and compressive yield stress in polyethylenes (237) and glass-transition temperature of amorphous plastics (238). Scratch resistance in polyproplyene studies was found to have shear yielding as the main cause of plastic flow scratch pattern with tensile tear effects on the surface and shear-induced fracture on the subsurface (239). 

Many techniques have been developed to measure the Young s modulus and the stress of the mesoscopic systems [12, 13]. Besides the traditional Vickers microhardness test, techniques mostly used for nanostructures are tensile test using an atomic force microscope (AFM) cantilever, a nanotensile tester, a transmission electron microscopy (TEM)-based tensile tester, an AFM nanoindenter, an AFM three-point bending tester, an AFM wire free-end displacement tester, an AFM elastic-plastic indentation tester, and a nanoindentation tester. Surface acoustic waves (SAWs), ultrasonic waves, atomic force acoustic microscopy (AFAM), and electric field-induced oscillations in AFM and in TEM are also used. Comparatively, the methods of SAWs, ultrasonic waves, field-induced oscillations, and an AFAM could minimize the artifacts because of their nondestructive nature though these techniques collect statistic information from responses of all the chemical bonds involved [14]. 

In this experiment, wear and hardness tests were performed on the disc surface facing the spacer. The hardness was measured using Vickers microhardness tester. The indenter is a square shaped diamond pyramid with a surface angle of 136°. The applied load was 0.9807 N for 15 seconds. Two distinct regions, the treated and untreated regions, were... 

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. 

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. 

The question whether hardness is a property related to modulus (E) or yield stress (Y) is a problem which has been commented before by Bowman and Bevis 13). These authors found an experimental relationship between microhardness and modu-lus/yield-stress for injection-moulded semicrystalline plastics. According to the clasical theory of plasticity the expected indentation hardness value for a Vickers indenter is approximaterly equal to three times the yield stress (Tabor s relation). This assump-... 

Diameter of indentation Brinell hardness. Length of diagonal Vickers and Knoop microhardness. 

Up to today it has been impossible to measure reproducible microhardness data of non-metallic crystals. This was due to the too high indentation forces of commercial available hardness testing devices. Today with the new ultra-microhardness devices, which allow to reduce the indentation force down to 5 - 10 N, it is no longer the limiting problem to measure the Vickers hardness of such brittle crystals. 

All experiments were carried out with an ultra-microhardness measuring device (Vickers-hardness) produced by Anton-Paar-Company. It is attached to a microscope with an adapted photo camera. The force of indentation, the time of indentation and the time-gradient of the indentation force can be selected and adjusted separatly. 

Although Vickers and DPH microhardness tests should yield the same numerical results on a given material, such is not always the case. Much of the observed variance maybe a function of differences in the volume of sample material displaced by the macro and micro indentations. 

The effects of very high stresses and strain-rates have been investigated in microhardness experiments. In these experiments, loads of 50-500 g (corresponding to stresses as high as 2 GPa) are exerted by a diamond or sapphire Vickers indenter for about 20 seconds at temperatures up to 1,(X)0°C. Clearly, steady-state flow is never achieved but such experiments have provided important information about the dislocations involved in the deformation of olivine, for example. 

Mechanical and Chemical Characterization Enamel has often been viewed as a homogeneous solid [2, 3], but Knoop microhardness tests [4, 5] and compression tests [6] have shown that the Young s modulus (E) and hardness (H) are higher for cusp (or surface) enamel than for side (or subsurface) enamel. Depth-sensing Vickers indentation [7] has shown that the H and E obtained from an occlusal section of enamel are higher than those for an axial section. The variations in mechanical properties with location have been explained in terms of the degree of tissue mineralization. Notably,... 

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