3.1 Microstructure
Composite structure Ti(C,N) based cermets as shown in Figs.2a consists of cobalt enriched area,cobalt poor area and coarse granule, whichis different from conventional Ti(C,N) based cermets as shown in Figs.2b. The microstructure of cobalt enriched area are lots of Co and small amount of core-rim structure, while the microstructure of cobalt poor area has an opposite trend as shown in Fig.3. The formation of the composite structure can be explained as follows. Firstly, the fine granules as shown with white arrow in Fig.4 and Co mixed during milling process, the mixed powders after mill have more Co element in some area as shown in Fig.5, and the Co and fine granules form as a cobalt enriched area at sintering stage. Secondly, the more raw granule is reduced to form fine granules with the increase of milling time, the Co and the fine granules mixed to form cobalt poor area at sintering stage. In addition,a little raw granules retain after mills.
3.2 Mechanical properties
Basic mechanical properties of composite structure Ti(C,N) based cermets are compared to conventional Ti(C,N) based cermets as shown in Table 3. It can be seen that the fracture toughness of composite structure Ti(C,N) based cermets is higher than that of conventional Ti(C,N) based cermets, while the transverse rupture strength has an opposite trend, and the hardness has no change obviously. The higher fracture toughness of Cermet A can be mainly contributed by composite structure effect. The reason why lower TRS of cermet A is explained based on Hall-Petch formula. The nominal composites of the cermets A are the same as that of the cermet B, therefore the hardness changes little.
Table 3 Mechanical properties of each cermet system
Specimen
|
HRA
|
TRS(MPa)
|
KIC( )
|
A
|
86.6
|
807.4
|
13.83
|
B
|
88.1
|
892.4
|
11.84
|
3.3 Fracture morphology
SEM images obtained from the fractured surface of the cermet are shown in Fig.6. Compared to the fracture surface of conventional Ti(C,N) based cermets, the tearing ridge and cleavage of coarse granule can be observed obviously in composite structure Ti(C,N) based cermets. The BSE/SE of the fracture surface are shown in Fig.7. It can be seen that the tearing ridge as shown in zone A in Fig.7a consists of cleavage of coreless(subordinate) and tear of binder (primary) fracture (see Fig.7b).
3.4 Toughening Mechanisms
Main toughnening mechanisms of the composite structure Ti(C,N) based cermets are the tear ridge and large granule of cleavage of coarse granule induced by composite structure. The fracture process of the cermets consists of elastic loading, ductile loading and crack initiation[17,19]. In order to beeter undrsand the toughening mechanisms,the fracture surfaces of the cermets obtained by fracture toughness testing were shown in Fig.8, crack deflection, branch and bridge can be found. Crack bridge of composite structure Ti(C,N) based cermets at the starting stage of crack propagation are shown with black arrow in Fig.8. With the increase of load, the microstructure of cobalt poor area fracture, while the microstructure of cobalt enriched area occur plastic deformation, the process of load transfer misfit between the cobalt enriched area and cobalt poor area, and then crack bridge of composite structure Ti(C,N) based cermets at the starting stage of crack propagation occur. It is concluded that the load transfer between the cobalt enriched area and cobalt poor area during loading process can induce crack deflection, branch and bridge and dissipate the energy during elastic loading stage, which is advantageous to toughness.