242 Prakt. Met. Sonderband 38 (2006) 
3.2 Microstructure of nitrided hardmetals B ¢ 
col 
3.2.1 Dependence of layer thickness on binder composition 
The nitridation treatment (samples B, D, F, H and J, Table 1) results in the development of 
a near-surface layer visible both in optical and SEM micrographs (Fig.2). The layer in the 
pure Co-binder hardmetal is about twice as thick as the one in the pure Ni-binder 
hardmetal. The alloying of both metals produces a layer with a thickness of ~6um. This 
value is an average of the values obtained in the pure Co and Ni binder hardmetals, 
suggesting that a combination of the effect of both metals on the layer growth takes place. 
In contrast, the thickness of the layers in Co/Fe binder and Ni/Fe binder hardmetals is 
significantly larger, with values of 27um and 14pm, respectively, which is 3 to 4 times 
larger compared to the layers in hardmetals containing a pure Co- and Ni-binder. This is 
probably due to the higher solubility of N in Fe and the high activity of C in the Fe- 
containing binder [12], which increases ion mobility and, considering that all sintering © 
parameters such as nitrogen pressure, temperature, and time were kept constant, thus 
increases the diffusion rate and diffusion layer thickness. Nevertheless, practically the Fig 
same layer thickness ratio observed between the pure Co- and Ni-binder hardmetals is B( 
kept. J 
Fig. 1. SEM micrographs of the cross-section of sample A (Co-binder) and sample C (Ni-binder) 
Sample Foinder fwe f, dwc (um) d, (um) A (um) 
A 0.13+0.01  0.19+0.02  0.62+0.04 1.50.7 3+2 1.1+0.9 
B 0.12£0.01  0.17+0.01  0.63+0.01 1.4+0.7 2.60.9 1.4+1.1 
0.10£0.01 0.18+0.01 0.65+0.03 1.30.8 341 1.241 
D 0.12£0.01  0.17+0.01  0.63+0.03 1.40.7 3+1 1.4+1.1 
Table 2. Metallographic values of volume fraction (f;) of all phases, average grain size of the hard phases . 
(dwe and d,), and binder mean free path (A) in the bulk determined by image analysis (according to a Fig 
previous work [11]). B ( 
san 
3.2.2 Dependence of layer morphology on binder composition 3: 
‚4 
The microstructure of the nitridation layer is characterized by a homogeneous and " 
compact phase appearing dark in the micrographs (Fig.2, Fig.3) in which embedded WC the 
hard phases as well as remains of the binder are present. The dark phase produced by the ‘ 
nitridation treatment is a (Ti,Ta,Nb)(C,N) solid solution, referred here as yN-phase. The a 
nitridation treatment does not affect the binder mean free path and the average grain size > | 
of the hard phases in the bulk (Table 2). In the region near the interface between the 2 
nitridation layer and the bulk of the samples a sponge-like structure formed by the or 
(Ti, Ta,Nb)(C,N) surrounding the binder inclusions is observed in the Co-binder hardmetal