Study On Strengthening-toughening And Manufacturingtechnology Of Graded Structure Of Ti(c,n)-based Cermets

In order to further improve the performance of Ti(C,N)-based cermets, in this dissertation, the strengthening-toughening technology and related theories of Ti(C,N)-based cermets were studied. Based on above studies, the plasma carburizing fabrication technology of graded structure Ti(C,N)-based cermets was investigated. The diffusion characteristics and related mechanism of carbon atoms during carburization of graded structure Ti(C,N)-based cermets were discussed.In the first part of dissertation, the vacuum sintering process on Ti(C,N)-based cermets have been studied systematically. The results showed that the Ti(C,N)-Ni-based cermets with 23 wt.%Ni sintered at 1450 ℃for 60 minutes had the moderate thickness of rim phase and the homogeneous microstructure, resulting in better mechanical properties. When Ni was partly replaced by 4.6wt.% Co, the Ti(C,N)-Ni-Co-based cermets sintered at 1410 ℃for 60 minutes had better microstructure and properties.The influence of the chemical composition on the microstructure and properties has been studied. The results showed that with Ni addition increasing, the microstructure of the cermets became even gradually, the hard phase got rounded, and the dissolution of Mo、W and Ti in binder phase decreased gradually. When Ni was partly replaced by Co, the core size of hard particle and the thickness of rim phase changed. When the ratio of Co to Ni+Co was 0.2, the cermets had better transverse rupture strength(TRS). The dominant precipitation sites for the complicated(Ti, Mo, W, Ta)(C,N) rim phase changed from the surface of the Ti(C,N) to the surface of Ta C with increasing Ta C addition. The higher TRS for cermets with 5wt.% Ta C addition was mainly attributed to the finer grain size, the homogeneous microstructure and the moderate thickness of rim phase.The influence of the multi-walled carbon nanotubes(MWCNTs) addition on the microstructure and performance of cermets has been studied. The conclusions were obtained as follows: after plasma treatment, most of the amorphous carbons and impurities were peeled from the surface and bundle of MWCNTs, and some reactive groups were successfully introduced on the surface of MWCNTs, resulting in the improvement of dispersibility. The increase of MWCNTs addition decreased the dissolution of Ti, W and Mo in the binder phase. However, too high MWCNTs addition resulted in agglomeration of the grains. When the MWCNTs addition was 0.5wt.%, the cermets had better mechanical properties, resulting from the finer grain size, the homogeneous microstructure and the higher volume fraction of binder phase. The toughening mechanisms were characterized by crack deflection, bridging and pulling-out.The influence of the Si C whisker addition on the microstructure and performance of cermets has been studied. Experimental results showed that the porosities of the cermets increased with increasing Si C whisker addition because of the bad wettability between the binder and Si C whiskers. No new phase in the cermets was formed with the Si C whisker addition. When the Si C whisker addition was 1.0wt.%, the TRS and fracture toughness of the cermets increased by 24% and 29%, respectively. The strengthening mechanisms were attributed to the finer grain size, the moderate thickness of rim phase and the homogeneous microstructure. The toughening mechanisms were characterized by crack deflection, whisker bridging and whisker pulling-out.The graded structure Ti(C,N)-based cermets were prepared by plasma carburizing technology. The diffusion coefficient of atoms increased, and the diffusion rate of the elements increased with increasing carburizing temperature, resulting in the increase of binder-deficient layer. As the carburizing time increased, the surface binder content reduced, and the crystal defects from surface to inside of the cermets decreased, which resulted in the growth rate of the binder-deficient layer deviating from the parabolic law. When the cermets were treated at 1200 ℃for 180 min, a 20μm binder-deficient layer, enriched C and Ti was formed, the particle size became finer. A binder-rich area was formed in the near surface area between about 20?m and 35?m depth. The surface hardness and wear resistance of the cermets improved greatly,The diffusion characteristics and related mechanism of carbon atoms during carburization of graded structure Ti(C,N)-based cermets were investigated. The semi-empirical formula calculating the surface carbon concentration of the cermets was obtained. The calculated results were consistent with experimental results. As the chemical affinities between carbon element and other elements in the cermets were different, Ti, W and Mo moved to the surface of the cermets, Ni was forced to transport inwards. The diffusion coefficient of plasma carburizing technology was much higher than the diffusion coefficient of common carburization, which resulted from more crystal defects on the surface of the cermets.