The Hardness of Cobalt

The binder in WC-Co hardmetal is not pure cobalt but a solid solution of carbon and tungsten in cobalt. When cooling the material from the sintering temperature 

The hardness of pure cobalt has been reported as being between 140 and 240 HV, depending on the method of sample preparation [11]. However, a solid solution of C and W in cobalt can be up to 100% harder than pure cobalt [12], depending on the composition. It has been dilEcult to measure direetly the hardness of the binder in WC o because the width of the binder regions in the hardmetal is typically of the order of 0.1 pm. 

However, Fig. 4 summarizes measurements by a number of investigators of the in situ hardness of the cobalt binder [9]. The hardness is plotted versus A being the thiekness of the cobalt layer where the hardness was measured, usually called the cobalt mean free path . The results in Fig. 4 satisfy the following Hall-Petch type relationship 

From Fig. 4 it has been calculated that = 149kgfmm and A oco = 16 kgfmm /. AToco is related to the ease of slip transfer across Co-WC interfaces. The contribution of the Co-Co grain boundaries to the hardness of the material can be neglected because in WC-Co the size of the cobalt grains is of the order of 10 pm while the size of the WC grains is of the order of 1 pm. Thus the number of Co-Co boundaries is negligible compared to the number of Co-WC interfaces. 

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Figure 11. Band including possible values of the hardness of cobalt at various temperatures. Adapted from reference [11].

The hardness of cobalt was related to the mean free pafo(X) of the cobalt matrix in foe composites by the following empirical relation ... 

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