244 Prakt. Met. Sonderband 38 (2006) 
several steps, which consist of low-energy planes formed during sintering [15]. Our results 
indicated that rounded WC hard phases might be retained by selecting a suitable binder 
metal. 
The binder system influences the metal-to-carbon bond relationship and thus, determines 
the growth behaviour of the WC hard phases [16]. Therefore, we would expect that Fe 
addition to the binder might strongly influence coarsening of WC hard phases during the 
solid state sintering process, as already shown by other authors [16], since Fe has the 
highest affinity to C in comparison to Co and Ni. However, our results show that the 
alloying of Fe with Co or Ni did not produce such effect. The addition of a rather high 
amount of carbides e.g. TiC, TaC, and NbC might be the main factor in determining the 
size of the WC hard phases in all hardmetals investigated. Part of the carbides dissolves in 
the binder and segregates at the WC/binder interface during sintering [16]. 
y-phase: SEM micrographs of the nitrided Co-binder hardmetal (sample B) taken at higher 
magnification (Fig. 4) show a darker rim around the y-phase grain, which is essentially 
composed of the (Ti, Ta,Nb)(C,N) solid solution. 
The core-rim structure (Fig.1) was observed in all hardmetals investigated. This kind of Fig. 
structure is generally found in Ti(C,N)-based hardmetals after solid state sintering due to 
the diffusion of N into the y-phase [17]. The darker core is often composed of undissolved Ik 
Ti(C,N) with different carbon-to-nitrogen ratio [10] surrounded by an outer-rim formed by a 
mixture of carbides e.g. TiC, WC, TaC, depending on the raw materials present in the 
starting powders [10,18]. 
Fig. 4. In sample B (a) the binder mean free path decreases from the bulk towards the surface and in (b) 
the effect of the nitridation at the interface bulk-layer zone leading to the formation of the yN-phase is shown. 
Elements such as Nb (as well as Ta, due to their similar physical-chemical properties) are Fi 
mainly associated with the Ti content (Fig.5). This is attributed to the higher affinity of TiC oe 
to form solid solutions with all other transition metal carbides of groups IVB and VB [10]. 
These elements are especially concentrated in the outer-surface zone due to their high 
affinity with N. Similar element distributions are observed for all other specimens. ; 
yN-phase: The yN-phase formed at the surface is harder than the WC and y-phases. The 5 
average grain size of the (Ti,Ta,Nb)(C,N) grains was estimated from TEM micrographs as Tob 
~500 nm, which is in good agreement with synchrotron X-ray diffraction measurements 
performed in grazing incidence [19]. A finer grain size combined with a change in The 
morphology of the hard phases in the outer-surface zone (i.e., from rounded to irregular ana 
shapes) may account for the improved plastic deformation resistance observed in nitrided rem 
hardmetals. It is known that increase in hardness is usually associated with decreasing in hig! 
grain size. Besides, smaller grains provide larger surface area, which improves machining reir 
and wear of working pieces by increasing the contact area. res: