Research On The Microstructure And Properties Of Fine Grade Ti(C,N) Based Cermets

Ti(C, N) based cermets with different compositions and powder sizes were made by powder metallurgical method. The effects of chemical composition, powder size and boronizing on the microstructure and mechanical properties of Ti(C, N) based cermets were studied. Thermal shock resistance and cutting performance of nano modified Ti(C, N) based cermets were also studied.Firstly, the development history, fabricating method, microstructure, mechanical properties, thermal shock resistance and cutting performance of Ti(C, N) based cermets were introduced. The fabricating methods of cermets, including the methods of preparing ceramic phase powder, formation, sintering and so on, were outlined specially; the advantage and disadvantage of each method were also compared. The conducted methods and influenced factors on the microstructure and mechanical properties of Ti(C, N) based cermets were summarized. Simultaneously, the detailed influences of chemical composition, fabricating process, powder size on the microstructure and mechanical properties were pointed out. Furthermore, the purpose and significance of this desertation, as well as the application future of Ti(C, N) based cermets, were analyzed.Secondly, the effect of molybdenum (Mo) content on the microstructure and mechanical properties of ultra-fine grade Ti(C, N) based cermets were researched. It shows that Mo exists in Ti(C, N) based cermets as the form of (Ti, Mo, W)(C, N) solid solution, and the solid solution has the same crystal structure and similar lattice parameter as Ti(C, N). The addition of Mo refines grains, improves the relative density, transverse rupture strength and hardness. The solid solution strengthening and refining strengthening are responsible for the strengthening mechanisms. Moreover, the effect of zirconium carbide (ZrC) content on the microstructure and mechanical properties were studied. It is found that the relative density of materials declines due to the imperfect wettability of metallic phase to ZrC. Zr-rich carbide solid solution is gradually formed in TiC based cermets with increasing ZrC content, and the microstructure gets finer, strength and hardness declines. However, the results calculated according to the full density show that transverse rupture strength can be improved significantly with proper addition of ZrC. Bright and gray spherical grains, as well as micro pores and cracks, are found in cermets with the chemical constitute of 10%TiC-40%ZrC-14%Mo-15%WC-20%Ni-1%C. The fracture toughness of the cermets is 16.6 MPa·m^1/2, which is 51% higher than those without ZrC. Coreless grain toughening, micro crack toughening and micro pore toughening are responsible for the improved toughness. Furthermore, the influence of Zr on microstructure and fracture toughness was also studied. It is found that coreless grains increase and grains get finer. Fracture toughness of Ti(C, N) based cermets is improved due to coreless grain toughening, crack deflection and branching toughening mechanisms.Thirdly, the effect of different metallic phases (20%Ni,10%Co-10%Ni,20%Co) on the microstructure, mechanical properties and thermal shock resistance of nano modified Ti(C, N) based cermets were studied. It is found that nano TiN can improve mechanical properties of cermets, and a new structure which can be named grey core structure is found in cermets. In addition, cobalt can exhibit higher hardness and nickel can offer higher transverse rupture strength and hardness. Quenching-strength test, indentation-quench test and notches pre-made in the end of samples are operated to study the thermal shock resistance of nano TiN modified Ti(C, N) based cermets. All the results obtained by these three methods show that the thermal shock resistance of cermets with 10%Co-10%Ni is better and that of cermets with 20%Ni is the best compared with cermets with 20%Co. The thermal shock resistance of cermets depends on strength and fracture toughness of materials, the higher strength and fracture toughness are, the better thermal shock resistance is. The experimental results have a good agreement with the theoretical calculated results. Moreover, the decline rate of hardness of nano modified Ti(C, N) based cermets after thermal shock is less than 1% even though the strength decreases sharply. This result shows that the effect of thermal shock on hardness of cermets is very slight.Finally, boronizing treatment was operated on the surface of nano modified Ti(C, N) based cermets. The experimental results show that a series of borides are presented on the surface of cermets and the microhardness of surface increases significantly. Furthermore, the cutting performances of Ti(C, N) based cermet cutters were compared before and after boronizing at the various cutting speeds. The results show that the tool life of boronized Ti(C, N) based cermet cutters exhibit the longer tool life than those without boronizing at lower speed.