240 Prakt. Met. Sonderband 38 (2006) 
formed. After nitridation, the outer-surface layer of the hardmetals consists mainly of fcc 3 uw 
(Ti,Ta,Nb)(C,N) solid solution. The layer thickness amounts to 5um up to 50 um. Since the (ML 
transition from the modified microstructure at the surface towards the original hardmetal Moi 
microstructure in the bulk occurs gradually, these nitrided hardmetals usually are referred frac 
to as graded (also as functionally graded) hardmetals. The graded hard layer produced by anc 
the nitridation treatment has been shown to be beneficial to the wear resistance of the Cro: 
hardmetals [6,10]. It further improves the toughness of ceramic coatings depositioned e.g. sele 
by CVD processes on the hardmetals, because it decreases the thermal mismatch strain dia! 
between the hardmetal and the ceramic coating, thus improving the adherence of the con 
coating [12]. The nitrided near-surface layer in addition has been shown to provide an Cro 
effective barrier against aggressive media [13]. Besides, the outer-surface of the nitrided pre 
hardmetals has a lesser amount of binder than the bulk, and since the binder is the most face 
vulnerable component of the hardmetal, wear and corrosion resistance are supposed to fror 
improve. me: 
The binder is the main transport medium for the formation of diffusion layers. Thus, san 
surface layer formation strongly depends on the thermodynamical properties of the trar 
system, especially those of the binder metal-carbide system, the solubility of all carbides in par 
the binder as well as on the C and N activity [14]. In consequence the thickness, grain TE! 
size, and porosity of the near-surface layer can be tailored by an appropriate choice of the ord 
binder phase in combination with the process parameters e.g. nitrogen pressure and diss 
temperature. Previous works e.g. [10,13] have shown that hardmetals with alternative det: 
binder phases (e.g. Ni, Co+Ni, Co+Fe, and Ni+Fe) to the dominating Co-based one could 
also be attractive for the production of cutting tools. 
The aim of this work is a characterization of the microstructure changes induced by a 
nitridation treatment in (W,Ti)C+(Ta,Nb)C hardmetals. In particular, the influence of binder 
phase composition on the formation of diffusion layers in hardmetals is studied. The 
morphology of the different surface layers and their dependence on hardmetal composition 
and binder phase is assessed by a combination of SEM/EDX and TEM/STEM/EDX 
techniques. 
2. EXPERIMENTAL DETAILS 
21 Materials 
The cutting tool inserts investigated were produced by industrial methods at BOEHLERIT Tab 
GmbH&Co.KG. The samples had a parallelogram shape: lpase sidey= 12,90mm, 
N(heighy=12,70mm, Wwigtny=4,76mm, diam=5,16mm. Binder powders and hard phase 
powders were mixed, pressed and dewaxed at about 600°C. Afterwards the samples were 3. 
sintered in vacuum at 1450°C and then heat-treated in a nitrogen atmosphere according to 
the procedure described in [6]. Samples containing the same composition and volume 3.1 
fraction of hard phases, but different binder compositions were produced (Table 1). 
The 
2.2 Experimental is « 
The microstructure of the specimens was investigated using a scanning electron Ja 
microscope (SEM) equipped with an energy dispersive X-ray (EDX) analyses unit. SEM gra 
analyses were performed on cross-sections of polished samples. The samples were me. 
embedded and then polished on grinding discs using diamond particles sized 9 um and