Research On The Manufacturing Technique Of Ti(c,n)-based Cermets With High Strength And Toughness

In this thesis, Ti(C,N)-based cermets were prepared by vacuum sintering. The relationship among composition, manufacturing process, microstructure and mechanical properties of Ti(C,N)-based cermets has been systematically investigated.In the first part of the thesis, the development and present research situation of Ti(C,N)-based cermets have been critically overviewed, which includes the influence of composition, additive, manufacturing process on the microstructures and mechanical properties of Ti(C,N)-based cermets. The developmental of the toughening technology by using SiC whisker and carbon nano-tubes (CNT) has also been summarized, Based on above work, the purpose and significance of the dissertation have been pointed out.The influence of the soaking time on the microstructures and mechanical properties of Ti(C,N)-based cermets with Ni and Ni-Co as the binder has been studied respectively. The conclusions were obtained as follows: the degree of the dissolution and precipitation was different with different soaking time, which affected the structure and thickness of the rim phase, and further influenced the mechanical properties. When the sintering temperatures were 1450℃and 1410℃respectively for the two systems mentioned above, the suitable soaking time was 60min, which could result in the cermets with good microstructures and high mechanical properties.The effects of of Ni and Co/(Ni+Co) additions on the microstructure and mechanical properties of cermets have been studied. The results showed that the grain size of the hard phase increased with decreasing Ni addition, which resulted in the reduction of TRS and fracture toughness, and the increase of hardness. When a part of Ni was replaced by Co, the grain size of the hard phase decreased, which resulted from the higher solubility of TiC in Co than in Ni during solid-phase sintering stage. As the proportion of Co/(Ni+Co) increased, the volume fraction of the rim phase increased. However, the wettability of metal liquid phase to hard grains decreased, which resulted in the agglomeration of the grains. When the proportion of Co/(Ni+Co) was 0.2, the transverse rupture strength of the cermet was increased and the fracture toughness was reduced slightly.The influence of grain growth inhibitors Cr3C2, TaC and NbC on the microstructures and mechanical properties of Ti(C,N)-based cermets has been studied. The conclusions were obtained as follows: with the addition of grain growth inhibitors, the content of the alloying element dissolved in the binder increased and the thickness of the rim phase became thinner. When the amount of Cr3C2, TaC and NbC were 0.6 wt%,5wt% and 3wt% respectively, the cermets had the smallest grain size. As the addition of grain growth inhibitors further increased, the wettability of the binder to the hard phase decreased greatly, which resulted in the reduction of the transverse rupture strength.The effects of SiC whisker and CNT content on the microstructures and mechanical properties of Ti(C,N)-based cermets have been studied systematically. The conclusions were obtained as follows: when the addition of SiC whisker was 1.0wt%, the grain size decreased, which resulted in the obvious increase of TRS and fracture toughness. Comparing with the cermets with no whisker addition, the transverse rupture strength and the fracture toughness of the cermets were 2279MPa and 17.7MPa·m1/2, which increased by 24% and 29% respectively. The toughening mechanisms were characterized by crack deflection, whisker bridging and whisker pulling-out. The content of the dissolved W,Mo and Ti element in the binder decreased, and the toughness of the binder increased with increasing CNT additions. When the amount of CNT was 0.5wt%, the cermets had the highest transverse rupture strength and fracture toughness. The toughening mechanisms were characterized by CNT pulling-out. As the SiC whisker and CNT were difficult to be dispersed homogeneously, and the wettability between the binder and SiC whisker/CNT was poor, the porosity of the cermets increased with excessive additions, which resulted in the sharp reduction of mechanical properties.