Carbides composition dependent hardness

Niobium boride (NbB2), composition dependent hardness, 300 Niobium carbide (Nb Cj) etch pit observations, 139 single-crystal hardness, 83, 303 Nitrides, 296-297... 

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. 

In earlier work, it was found for borides, silicides and nitrides that specific activity, expressed as total rate of methane consumption per unit surface area, plummeted with increasing surface area of the catalyst samples.1718 The same relationship was also found for transition metals carbides (Figure 16.4). It should be noted the dependence of specific activity on surface area rather than catalyst composition is unusual for heterogeneous catalytic reactions. In addition, it can be found that the reaction order in the oxidant is perceptibly in excess of 1 (Tables 16.8 and 16.9). Such an order is hard to explain in terms of common mechanism schemes for heterogeneous catalytic oxidative reactions. 

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