Microhardness carbides

Many of the transition metal carbides such as TiC are very hard compounds. In 5-TiCi x the microhardness increases with increasing carbon content, a phenomenon that is probably closely related to the VEC, with the maximum stability at VEC = 8 at the composition TiC. Figure 11 shows this behavior for S-TiCi-, other data are contained in Table 1. Both microhardness and nanohardness were recently measured for Ti, Zr, and Hf carbonitrides as a function of the C/N ratio. The data are... 

On single crystals of transition metal carbides it has been shown for hexagonal WC and also for fee NbCi that the microhardness is dependent of orientation. ... 

Single-crystal and poly crystalline transition metal carbides have been investigated with respect to creep, microhardness, plasticity, and shp systems. The fee carbides show slip upon mechanical load within the (111)plane in the 110 direction. The ductile-to-brittle transformation temperature of TiC is about 800 °C and is dependent on the grain size. The yield stress of TiC obeys a Hall Petch type relation, that is, the yield stress is inversely proportional to the square root of the grain size. TiC and ZrC show plastic deformation at surprisingly low temperatures around 1000 °C. 

Properties Orthorhombic crystals. D 6.65, microhardness 2700 kg/sq mm Hg (load 50 g), mp 1890C, bp 3800C, resistivity 95 pro cm (room temperature). Highest oxidation resistance at high temperature of all metal carbides also resistant to acids and alkalies. 

Fig. 3 Profiles of microhardness on Co content graded cemented carbide-steel by Spark Plasma Sintering (1283K) compared with conventional sintering(1673K)...

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. 

Silicon carbide is noted for its extreme hardness [182-184], its high abrasive power, high modulus of elasticity (450 GPa), high temperature resistance up to above 1500°C, as well as high resistance to abrasion. The industrial importance of silicon carbide is mainly due to its extreme hardness of 9.5-9.75 on the Mohs scale. Only diamond, cubic boron nitride, and boron carbide are harder. The Knoop microhardness number HK-0.1, that is the hardness measured with a load of 0.1 kp (w0.98N), is 2600 (2000 for aAl203, 3000 for B4C, 4700 for cubic BN, and 7000-8000 for diamond). Silicon carbide is very brittle, and can therefore be crushed comparatively easily in spite of its great hardness. Table 8 summarizes some typical physical properties of the SiC ceramics. 

A wide carbide grain size distribution obviously leads to variations in the microhardness of the hardmetal, but it has not been established what effect it has on the macrohardness. However, if variations in grain size distribution lead to variations in the binder mean free path the effect on hardness can be substantial, as shown in Fig. 8 [17]. Figure 8 shows that at equal mean free path (and so equal binder hardness, according to Eq. (2)), finer grades have lower hardness than coarser grades. 

Carbide eoatings found their apphcations to improve the working properties of cutting tools. Their microhardness and plasticity are basically determined by the composition of the carbide whereas the fracture toughness is determined by the structure of the coating [2000Bya]. 

Bya] SEM, XRD, metallography Microhardness, plasticity and fracture thoughness of carbide coatings... 

The apparent hardness of these carbides naturally depends on the composition and purity if cast materials are measured, and on the annealing temperature if the pieces were sintered from powders. Because these variables cannot be easily related to the fundamental property of the material, only microhardness values of reasonably well-characterized material have been listed. 

The introduction of carbon vacancies produces a marked reduction in the microhardness in the Group 4 cubic carbides. A maximum has been observed in the TaC phase and if the latter behavior is typical of Group 5, the high value for VCq.ss (see Table 80) could result because the phase boundary composition happens to fall near the maximum hardness value. If NbC has a maximum, a composition near NbCo.9 could be the hardest carbide. Clearly, there are theoretical and practical reasons to pursue this observation further. 

Figure 1.2. Grain size effects in microhardness testing, (a) Results from tungsten carbide cutting-tool compositions, (b) Apparent hardness changing as grain size to indent size decreases.

The microhardness of coatings was 18-19 GPa for molybdenum carbide [6], 29-31 GPa for tungsten carbide, and 31-32 GPa for zirconium diboride. The presence of a diffusion zone that ensured the adhesion of coatings to the substrate, was confirmed by qualitative and semiquantitative X-ray spectral microanalyses of microsections which were performed with a MS-46 Cameca electronic probe. The continuity of the coating-substrate transition was confirmed by stereoscopic images of the cross-sections of the electroplated samples. 

DL Kohlstedt. The temperature dependence of microhardness of the transition-metal carbides. J Mater... 

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